CORNELL UNIVERSITY. THE THE GIFT OF ROSWELL P. FLOWER FOR THE USE OF THE N. Y. STATE VETERINARY COLLEQE. 1897 Cornell University Library QH 309.P24 1897 Lessons In elementary biology 3 1924 001 017 650 LESSONS ELEMENTARY BIOLOGY T PQQOMQ Cornell University Iibrar\^ T. jEFTjiBKJjigmkcKfSlBi? bcstK is^JR.s. PKOFESSOE OF BIOjjolS lfe»Gl|En®WEyi|rH V6fSJ4iy lUiWSriSr^W ZEALAND There are no known copyright restrictions in the United States on the use of the text. WITH ONE HUNDRED AND TWENTY-SEVEN ILLUSTRATIONS Hontion MACMILLAN AND CO., Limited NEW YORK: THE MACMILLAN COMPANY 1897 The Right of Translation and Rcprodiiction is Rcsei'ved ■ http://www.archive.org/details/cu31924001017650 Richard Clay and Sons, Limited, LONDON and BUNGAY. First Edition, 1891. Second Edition, Reinsed, 1893. Third Edition, Revised atid Enlarged, 1897. PREFACE TO THE FIRST EDITION In his preface to the new edition of the well-known Practical Biology^ Professor Huxley gives his reasons for beginning the study of organised nature witli the higher forms of animal life, to the abandonment of his earlier method of working from the simpler to the more complex organisms. He says in effect that experience has taught him the unwisdom of taking the beginner at once into the new and strange region of microscopic life, and the advan- tage of making him commence his studies with a subject of which he is bound to know something — the elementary anatomy and physiology of a vertebrate animal. Most teachers will probably agree with the general truth of this opinion. The first few weeks of the beginner in natural science are so fully occupied in mastering an un- familiar and difficult terminology and in acquiring the art of using his eyes and fingers, that he is simply incapable for a time of grasping any of the principles of the science ; and, this being the case, the more completely his new work can vi PREFACE TO FIRST EDITION be connected with any knowledge of the subject, however vague, he may already possess, the better for his progress. On the other hand, the advantage to logical treatment of proceeding from the simple to the complex — of working upwards from protists to the higher plants and animals — is so immense that it is not to be abandoned without very good and sufficient reasons. In my own experience I have found that the difficulty may be largely met by a compromise, namely, by beginning the work of the class by a comparative study of one of the higher plants (flowering plant or fern) and of one of the higher animals (rabbit, frog, or crayfish). If there were no limitations as to time, and if it were possible to avoid alto- gether the valley of the shadow of the coming examination, this preliminary work might be extended with advantage, and made to include a fairly complete although elementary study of animal physiology, with a minimum of anatomical detail, and a somewhat extensive study of flowering plants with special reference to their physiology and to their relations to the rest of nature. In any case by the time this introductory work is over, the student of average intelligence has overcome pre- liminary difficulties, and is ready to profit by the second and more systematic part of the course in which organisms are studied in the order of increasing complexity. It is such a course of general elementary biology which I have attempted to give in the following Lessons, my aim having been to provide a book which may supply in the study the place occupied in the laboratory by " Huxley and Martin," by giving the connected narrative which would be TREFACE TO FIRST EDITION vii out of place in a practical handbook. I also venture to hope that the work may be of some use to students who have studied zoology and botany as separate subjects, as well as to that large class of workers whose services to English science often receive but scant recognition — I mean amateur microscopists. As to the general treatment of the subject I have been guided by three principles. Firstly, that the main object of teaching biology as part of a liberal education is to familiarise the student not so much with the facts as with the ideas of science. Secondly, that such ideas are best understood, at least by beginners, when studied in connection with concrete types of animals and plants. And, thirdly, that the types chosen should illustrate without unnecessary complication the particular grade of organisation they are intended to typify, and that exceptional cases are out of place in an elementary course. The types have therefore been selected with a view of illustrating all the more important modifications of structure and the chief physiological processes in plants and animals ; and, by the occasional introduction of special lessons on such subjects as biogenesis, evolution, &c., the entire work is so arranged as to give a fairly connected account of the general principles of biology. It is in obedience to the last of the principles just enunciated that I have described so many of the Protozoa, omitted all but a brief reference to the development of Hydra and to the so-called sexual pro- cess in Penicillium, and described Nitella, instead of Chara, and Polygordius instead of the earthworm. The last-named substitution is of course only made possible by the book viii PREFACE TO FIRST EDITION being intended for the study and not for the laboratory, but I feel convinced that the student who masters the structure of Polygordius, even from figures and descriptions alone, will be in a far better position to profit by a practical study of one of the higher worms. Lessons XXVII. and XXX.^ are mere summaries, and can only be read profitably by those who have studied the organisms described, or allied forms, in some detail. Such abstracts were however necessary to the plan of the book, in order to show how all the -higher animals and plants may be described, so to speak, in terms of Polygordius and of the fern. For many years I have been convinced of the urgent need for a unification of terminology in biology, and have now attempted to carry out a consistent scheme, as will be seen by referring to the definitions in the glossary. Many of Mr. Harvey Gibson's suggestions are adopted, and three new words are introduced — phyllula, gamobium, and agamo- bium. I expect and perhaps deserve to be criticised, or, what is worse, let alone, for the somewhat extreme step of using the word ovary in its zoological sense throughout the vegetable kingdom ; and for describing as the venter of the pistil the so-called ovary of Angiosperms. I would only beg my critics before finally pronouncing judgment to try and look at the book, from the point of view of the begin- ner, as a graduated course of instruction, and to consider the effect upon the entire scheme of using a term of funda- mental importance in two utterly different senses. A large proportion of the figures are copied either from ^ See Preface to the Third Edition, p. xi. PREFACE TO FIRST EDITION ix original sources or from my own drawings — the latter when no authority is mentioned. The majority, even of those which have previousl)' appeared in text-books, have been specially engraved for the work, the draughtsman being my brother, Mr. M. P. Parker. In order to facilitate reference the illustrations referring to each subject have, as far as possible, been grouped together, so that the actual is considerably larger than the nominal number of figures. Full descriptions are given instead of mere lists of reference- letters : these will, I hope, be found useful as abstracts of the subjects illustrated. I have to thank my friends Mr. A. Dillon Bell and Pro- fessor J. H. Scott, M.D., for constant and valuable help in criticising the manuscript. To Dr. Paul Meyer, of the Zoological Station, Naples, I am indebted for specimens of Polygordius ; and to Professor Sale, of this University, Professor Haswell, of Sydney, Professor Thomas, of Auck- land, and Professors Howes and D. H. Scott, of South Kensington, for important information and criticism on special points. My brother, Professor W. Newton Parker, has kindly promised to undertake a final revision for the press. DUNEDIX, N.Z., August 1S90. PREFACE TO THE THIRD EDITION In the two former editions the " Lessons '' practically concluded with Polygordius as an example of tripoblastic animals, and with the Fern as an example of vascular plants, and the merest sketches of the higher groups of both kingdoms were added (see Preface to the first edition, p. viii). It has, however, been suggested to me from more than one source, that the usefulness of the book would be increased by expanding these sketches into somethmg more comprehensible to the beginner. This I have done in the present edition, with the result that Lesson XXVII. of previous editions has been expanded into Lessons XXVI. — XXIX., and Lesson XXX. into Lessons XXXII. — XXXIV. The new matter is illustrated vi'ith forty additional figures. I have again to thank my brother, Prof. W. N. Parker, for sacrificing much time in the labour of proof correcting. September 1896. TABLE OF CONTENTS PREFACE TO THE FIRST EDITION PREFACE TO THE THIRD EDITION LIST OF ILLUSTRATIONS AMCEBA LESSON I. H^MATOCOCCUS LESSON n. HETEROMITA LESSON in. 36 EUGLENA LESSON IV. 4 + LESSON V. PROTOMYXA AND THE MYCETOZOA 49 xiv TABLE OF CONTEXTS LESSON VL I'AGE A COMPARISON OF THE FOREGOING OEGAXISMS WITH CERTAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS AND PLANTS . . . 56 ANIMAL AND PLANT CELLS 5^ MINUTE STRUCTURE AND DIVISION OF CELLS AND NUCLEI ... 62 OVA OF ANIMALS AND PLANTS . . . , . 69 LESSON VIL SACCHAROMYCES . . . 71 LESSON VIIL BACTERIA . . .... 82 LESSON IX. BIOGENESIS AND ABIOGENESIS .... . 95 HOMOGENESIS AND HETEROGENESIS 102 LESSON X. PARAMCECIUM , . . . . . . . I06 STYLONYCHIA ... . . I16 OXYTRICHA . ... 120 LESSON XL OPALINA LESSON XIL VORTICELLA 126 EOOTHAMNIUM ... • • '135 TABLE OF CONTENTS xv LESSON xin. PAGE SPECIES AND THEIR ORIGIN : THE PRINCIPLES OF CLASSIFICA- TION 137 LESSON XIV. THE FORAMINIFERA . . I4S THE RADIOLARIA . . ... 152 THE DIATOMACE^ . , . . ISS LESSON XV. MUCOR .... . . 158 LESSON XVL VAUCHERIA 169 CAULERPA 175 LESSON XVIL THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS . . 176 LESSON XVIIL PENICILLIUM . .... 184 AGARICUS .... 191 LESSON XIX. SPIROGYRA 194 LESSON XX, MONOSTROMA . . 201 ULVA 203 NITELLA . . 203 TABLE OF CONTENTS LESSON XXL PAGE HYDRA . ... . 2l8 LESSON XXIL HYDROID POLYPES . ... . 234 BOUGAINVILLEA, &C . . 234 DIPHYES . . ... 248 PORPITA . . . . . ... 249 LESSON XXIIL SPERMATOGENESIS AND OOGENESIS . • . . . 253 THE MATURATION AND IMPREGNATION OF THE OVUM 258 THE CONNECTION BETWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS ... . 261 LESSON XXIV. POLYGORDIUS . 268 LESSON XXV. POLYGORDIUS {continued) . . ... 290 LESSON XXVI. THE CHIEF DIVISIONS OF THE ANIMAL KINGDOM . . . 304 THE STARFISH . . 306 LESSON XXVII, THE CRAYFISH . . ... . j'S LESSON XXVIII. THE FRESH-WATER MUSSEL . 348 THE DOGFISH TABLE OF CONTENTS LESSON XXIX. LESSON XXX. PAGE 400 LESSON XXXL FERNS 412 LESSON xxxn. THE CHIEF DIVISIONS OF THE VEGETABLE KINGDOM 431 EQUISETUM 434 SALVINIA . 438 SELAGINELLA . 442 LESSON XXXIIL GYMNOSPERMS . ,447 LESSON XXXIV. ANGIOSPERMS ... . . 461 SYNOPSIS , 477 INDEX AND GLOSSARY . . . : . . . . 4^7 LIST OF ILLUSTRATIONS FIG. 1. Amo;ba, various species . 2 2. Protamaba prinutiva . 9 3. HamaiococciLs pluvialis and H. laatstris 24 4. Heteroniita rostrata 38 5. Eitglena viridis . 45 6. Protoinoyxa atirantiaca . . 50 7. Badhamia and Physaruin . . 53 8. Typical animal and vegetable cells 57 9. Animal and plant cells, detailed structure . . 62 10. Diagram illustrating the process of indirect cell-division 64 11. Ova' of Carniarina and Gyinnadenia 69 12. Saccharomyces cerevisia 72 13. Bacterium terino 83 14. Bacleriuni iermo, showing flagella . 84 15. Murococass 86 16. Bacillus sui tills 87 1 7. Vibrio serpens. Spirillum tenue, and S. volutans . 88 18. Bacillus anthracis 90 19. Beaker with culture-tubes 100 20. Paramcecitim caudatum . loS 21. Paraincecium caudatum, conjugation . . 115 22. Stylonychia inytihis . 117 XX LIST OF ILLUSTRATIONS FIG. PAGE 23. Oxytricha Jlava . 120 24. Opalina ranarzim . ■ 122 25. Vortkella . 127 26. Zoothaniniiun arhuscnla . . . 1 34 27. ZoothaninuiDi, various species . . 13^ 28. Diagram illustrating the Origin of the Species of Zootham- nium by Creation 142 29. Diagram illustrating the Origin of the Species of Zoothani- iiiiiui by Evolution I44 30. Kotalia . . 149 31. T)\a%x7s.\Xii, ':>l Foraminifera . . 15° 32. Alveolina quoii 151 33. Lithocirais anmdaris 152 34. Actinomma asteracantltion . 153 35. Diagrams of a Diatom and shells of Naviada and Aulaco- discics 156 36. JMiicor viucedo and il/. stolonifer . 159 37. Moist Chamber . 163 3S. Vaucheria , . 170 39. Cazderpa scalpeUlformis 175 40. Penicillium glaiiciwi . 186 41. Agarictts campestris . 192 42. Spirogyra . . 195 43. MoHostroma bidlosum and Vl/. laceratum . 202 44. Nitella^ general structure 204 45. Nitella, terminal bud 209 46. Nitdla, spermary 212 47. NiieUa^ ovary . 214 48. Chara, pro-embryo 216 49. Hydra viridis and H. fusca, external form . 219 50. Hydra, minute structure 223 51. Hydra, nematocyst and nerve-cell 225 52. Hydra viridis, ovum . 232 53. Bougainvillea ramosa 235 54. Eiiiopella, portion of tentacle 237 LIST OF ILLUSTRATIONS xxi FIG. PAGE 55. Diagrams illustrating derivation of Afisa';«a and A5'i^rfl«^/2 . 241 56. Eiuopella campamilaria, muscle fibres and cells . 243 57. Laomedeaflexuosa&Ti&.Etidendriiim7'amosum, development. 247 58. Diphyes cainpanulata 250 59. Porpila pacifica a.nA P. mediterranea 251 60. Spermatogenesis in the Mole-cricket 254 61. Ovum of Toxopnetisles lividus . . . 257 62. Maturation and impregnation of the animal ovum . 258 63. Pandorina morum . 263 64. Volvox gloiator . 265 65. Volvox globator . 266 66. Polygordius neapolitanns , external form . 269 67. Polygordhts neapolitmius^ anatomy . . 271 68. PolygOTdius ^zeapoUtanus , nephridium . . 282 69. Polygordius, diagram illustrating the relations of the nervous- system . 284 70. Polygordius neapolitanus, rejaroductive organs . 291 7 1 . Polygordius neapolitanus, larva in the trochosphere stage 293 72. Diagram illustrating the origin of the trochosphere from the gastrula . 295 73. Polygordius neapolitanus, advanced trochosphere . 297 74. Polygordius neapolitanus, larva in a stage intermediate be- tween the trochosphere and the adult 300 75. Starfish, ventral aspect . . 307 76. Starfish, diagrammatic sections . 309 77. Starfish, digestive organs . . 311 78. Starfish, water vascular system 313 79. Starfish, early stages in development . 3 '6 80. Starfish, development of bipinnaria larva 3^7 81. Crayfish, side view . . 3^9 82. Crayfish, principal appendages 322 83. Crayfish, diagrammatic sections 328 84. Crayfish, action of abdominal muscles 330 85. Crayfish, leg, with muscles 331 86. Crayfish, dissection 333 LIST OF ILLUSTRATIONS FIG. PAGE 87. Crayfish, gills . . 356 88. Crayfish, diagram of circulation of blood . 34° 8g. Crayfish, early development 344 90. Crayfish, early embryo in nauplius stage 345 91. Crayfish, later embryo 34° 92. Mussel, side view, and shell . . 35 ' 93. Mussel, diagrammatic sections . • 353 94. Mussel, dissection . 35° 95. Mussel, structure of gill 35° 96. Mussel, circulatory system . ■ 361 97. Mussel, advanced embryo and free larva 3^4 98. Dogfish, side view . 3^7 99. Dogfish, diagrammatic sections 37° 100. Dogfish, skull . . 373 loi. Dogfish, vertebrae . 376 102. Dogfish, pectoral arch . 37^ 103. Dogfish, dissection ... . 3^° 104. Dogfish, vascular system 385 104a. Dogfish, diagram of circulation . . 389 105. Dogfish, brain . . 392 106. Dogfish, early embryo . 398 107. Dogfish, advanced embryo 399 108. Mosses, various genera, anatomy and histology . 402 109. Funaria, reproduction and development . , 406 no. Pterzs STi^ Aspidiwn^ anatomy and histology . 414 111. Ferns, various genera, reproduction and development . 424 112. Eqiiisetum, aerial shoot and spores . 435 113. .£i7JK>i;/«OT, reproduction and development 437 1 14. Salvinia, part of plant . 439 115. i'a/z'wzM, reproduction and development . 441 116. ^e/a^K^^/Za, part of plant and sporangia . . 443 117. &/flj2'«£//a, reproduction and development 445 118. /'zHzw, sections of stem . 449 119. Gymnosperms, reproduction and development . 453 1 20. Piiitis, stamen . . . 455 LIST OF ILLUSTRATIONS xxiii FIG. PAGE :2i. Pimis, carpel . . 456 122. Zainia and Cycas, reproductive organs 457 123. Lily, section of stem . . 462 1 24. Buttercup, structure of flower 465 125. Transition from petal to stamen . 467 126. Angiosperms, reproduction and development 470 127. Helleborus, Campanula, and Ribes, flower 472 LESSONS IN ELEMENTARY BIOLOGY LESSON I AMCEEA It is hardly possible to make a better beginning of the systematic study of Biology than by a detailed examination of a microscopic animalcule often found adhering to weeds and other submerged objects in stagnant water, and known to naturalists as Amceba. Amoeba are mostly invisible to the naked eye, rarely exceeding one-fourth of a millimetre (jinj- inch) in dia- meter, so that\it is necessary to examine them entirely by the aid of the kiicroscope. They can be seen and re- cognised under the low power of an ordinary student's microscope which magnifies from twenty-five to fifty dia- meters ; but for accurate examination it is necessary to employ a far higher power, one in fact which magnifies about 300 diameters. Seen under this power, an Amoeba appears like a little 3E c AAKJiliA A psfl Fig. 1. — A. Ainaihi qiiajla, a Ining ^pLcimLii, ihuwing gianular ciulosarc surrounded by clear eclosare, and bcveral p^eudopodb (/Ji/), t GENERAL CHARACTERS } some formed of eclosarc only, others containing a core of endosarc. The larger bodies in the endosarc are mostly food-particles ( x 300).' B. The same species, killed and stained with carmine to show the numeious nuclei {uii) ( x 300). c. Ama'da proteits^ a living specimen, showing large irregular pseudopods, nucleus [nil), contractile vacuole (c. vac), and two food vacuoles (/ mic), each containing a small infusor (see Lesson X.) which has been ingested as food. The letter a to the right of the figure in- dicates the place where the protoplasm has united round the prey to inclose the food vacuole. The ' contractile vacuole in this figure is sujDposed to be seen through a layer of granular protoplasm, whereas in the succeeding figures (d, e, and G) it is seen in optical section, and therefore appears clear. D. An encysted Amceba, showing cell-wall or cyst {cy), nucleus {/lu), clear contractile vacuole, and three diatoms (see Lesson XIV. ) ingested as food. E. Ama-ba froletis, a living specimen, showing several large pseudo- pods (psd), single nucleus {nu), and contractile vacuole {c. vac], and numerous food-particles embedded in the granular endosarc ( x 330). F. Nucleus of the same after staining, showing a ground substance or nuclear sap, containing deeply-stained granules of chromatin, and surrounded by a distinct membrane ( x loio). G. Amceba verritcosa, living specimen, showing wrinkled surface, nucleus (««), large contractile vacuole (c. vac), and several ingested organisms ( x 330). H. Nucleus of the same, stained, showing the chromatin aggregated in the centre to form a nucleolus ( x loio). I. Amoeba proicus, in the act of multiplying by binary fission (x 500). (A, B, E, F, G, and H after Gruber ; c and I after Leidy. ; D after Howes.) shapeless blob of jelly, nearly or quite colourless. The central part of it (Fig. i, a, c, and e) is granular and semi- transparent — something like ground glass — while surround- ing this inner mass is a border of perfectly transparent and colourless substance. So clear, indeed, is this outer layer that it is easily overlooked by the beginner, who is apt to take the granular internal substance for the whole Amceba. If in any way the creature can be made to turn over, or if a number of specimens are examined in various positions, these two constituents will always be found to have the 1 A number preceded by the sign of multiplication indicates the number of diameters to which the object is magnified. B 2 4 AMCEBA LEs.s. same relations, whence we conclude that an Amoeba con- sists of a granular substance the endosarc, completely surrounded by a clear transparent layer or ectosarc. One very noticeable thing about Amoeba is that it is never of quite the same shape for long together. Often the changes of form are so slow as to be almost imperceptible, like the movements of the hour-hand of a watch, but by examining it at successive intervals the alteration becomes perfectly obvious, and at the end of half an hour it will probably have altered so much as to be hardly like the same thing. In an active specimen the way in which the changes of form are brought about is easily seen. At a particular point the ectosarc is pushed out in the form of a small pimple-like elevation (Fig. i, a, left side) : this increases in size, still consisting of ectosarc only, until at last granules from the endosarc stream into it, and the projection or pseudopod (a, c, e, psd^) comes to have the same structure as the rest of the Amceba. It must not be forgotten that the animal does not alter perceptibly in volume during the process, every pseudopod thus protruded from one part of the body necessitating the withdrawal of an equal volume for some other part. This peculiar mode of movement may be illustrated by taking an irregular lump of clay or putty and squeezing it between the fingers. As it is compressed in one direction it will elongate in another, and the squeezing process may be regulated so as to cause the protrusion of comparatively narrow portions from the solid lump, when the resemblance to the movements described in the preceding paragraph will be fairly close. Only it must be borne in mind that in Amceba there is no external compression, the " squeezing " being done by the animalcule itself. 1 COMPOSITION OF PROTOPLASM 5 The occurrence of these movements is alone sufficient to show that Amoeba is an organism or living thing, and no mere mass of dead matter. The jelly-like substance of which Amoeba is composed is called protoplasm. It is shown by chemical analysis ^ to consist mainly of certain substances known as proteids, bodies of extreme complexity in chemical constitution, Ihe most familiar example of which is white of egg or albumen. They are compounds of carbon, hydrogen, oxygen, nitrogen, and sulphur, the five elements being combined in the following proportions : — ■ Carbon . . from 51 '5 to ^a,':, per cent. Hydrogen . ., 6-9 „ 7^3 „ „ Oxygen . „ 20-9 „ 23-5 „ „ Nitrogen . „ 15-2 „ 17-0 „ „ Sulphur . ., o"3 ,, 2'o ,, ,, Besides proteids, protoplasm contains small proportions of mineral matters, especially phosphates and sulphates of potassium, calcium, and magnesium. It also contains a considerable quantity of water which, being as essential a constituent of it as the proteids and the mineral salts, is called water of organization. Protoplasm is dissolved by prolonged treatment with weak acids or alkalies. Strong alcohol coagulates it, i.e.., causes it to shrink by withdrawal of water and become comparatively hard and opaque. Coagulation is also produced by raising the temperature to about 40° C. ; the reader will remember how the famihar proteid white of egg is coagulated and rendered hard and opaque by heat. ' Accurate analyses of the protoplasm of Amoeba have not been made, but the various micro-chemical tests which can be applied to it leave no doubt that it agrees in all essential respects with the protoplasm of other organisms, the composition of which is known (see p., 7). 6 AMCEBA LKSS. There is another important property of proteids which is tested by the instrument called a dialyser. This consists essentially of a shallow vessel, the bottom of which is made of bladder, or vegetable parchment, or some other organic (animal or vegetable) membrane. If a solution of sugar or of salt is placed in a dialyser and the instrument floated in a larger vessel of distilled water, it will be found after a time that some of the sugar or salt has passed from the dialyser into the outer vessel through the membrane. On the other hand, if a solution of white of egg is placed in the dialyser no such transference to the outer vessel will take place. The dialyser thus allows us to divide substances into two classes ; crystalloids — so called because most of them, like salt and sugar, are capable of existing in the form of crystals — which, in the state of solution, will diffuse through an organic membrane ; and colloids or glue-like substances which will not diffuse. Protoplasm, like the proteids of which it is largely composed, is a colloid, that is, is non- diffusible. It has a slightly alkaline reaction. Another character of proteids is their iiistability. A lump of salt or of sugar, a piece of wood or of chalk, may be preserved unaltered for any length of time, but a proteid if left to itself very soon begins to decompose ; it acquires an offensive odour, and breaks up into simpler and simpler compounds, the most important of which are water (HgO), carbon dioxide or carbonic acid (COg), ammonia (NHg), and sulphuretted hydrogen (HgS).! In this character of instability or readiness to decompose protoplasm notoriously agrees with its constituent proteids ; any dead organism will, ' For a more detailed account of the phenomen.-i of piUrefaction see Lesson VIII., in which it will be seen that theabove statement as to the instabiUty of (dead) proteids requires qualification ; as a matter of fact they decompose only in the presence of living Bacteria. I CHARACTERS OF THE NUCLEUS 7 unless special means are taken to preserve it, undergo more or less speedy decomposition. Many of these properties of protoplasm can hardly be verified in the case of Amoeba, owing to its minute size and the difficulty of isolating it from other organisms (water- weeds, &c.) with which it is always associated; but there are soime tests which can be readily applied to it while under observation beneath the microscope. One of the most striking of these micro-chemical tests depends upon the avidity with which protoplasm takes up certain colouring matters. If a drop of a neutral or slightly alkaline solution of carmine or logwood, or of some aniline dye, or a weak solution of iodine, is added to the water con- taining Amoeba, the animalcule is killed, and at the same time becomes more or less deeply stained. The staining is, however, not uniform. The endosarc, owing to the granules it contains, appears darker than the ectosarc, and there is usually to be seen, in the endosarc, a rounded spot more brightly stained than the rest. This structure, which can sometimes be seen in the living Amoeba (Fig. I, c, E, and G, ntc), while frequently its presence is revealed only by staining (comp. a and b), is called the nuckus. But when viewed under a sufficiently high power, the nucleus itself is seen to be unequally stained. It has lately been shown, in many Amoebae, to be a globular body, en- closed in a very delicate membrane, and made up of two constituents, one of which is deeply stained by colouring matters, and is hence called chromatin^ while the other, the nuclear sap or achromatin, takes a lighter tint (Fig. i, f). The relative arrangement of chromatin and sap varies in different Amcebse : sometimes there are granules of chromatin in an achromatic ground substance (f) ; some- 8 AMCEBA '-E^^- times the chromatin is collected towards the surface or periphery of the nucleus; sometimes, again, it becomes aggregated in the centre (g, h). In the latter case the nucleus is seen to have a deeply-stained central portion, which is then distinguished as the nucleolus. When it is said that Amcebee sometimes have one kind of nucleus and sometimes another, it must not be inferred that the same animalcule varies in this respect. What is meant is that there are found both in fresh and salt water many kinds or species of Amceba which are distinguished from one another, amongst other things, by the character of their nuclei, just as the various species of Felts — the cat, lion, tiger, lynx, &c. — are distinguished from one another, amongst other things, by the colour and markings of their fur. According to the method of binomial nomenclature intro- duced into biology by LinnEus, the same generic name is applied to all such closely allied species, while each is specially distinguished by a second or specific name of its own. Thus under the genus Amoeba are included Amceba profeus (Fig. i, c, E, and f), with long lobed pseudopods and a nucleus containing evenly-disposed granules of chromatin; A. quarta (a and e), with short pseudopods and numerous nuclei ; A. verrucosa (g and h) with crumpled or folded surface, no well-marked pseudopods, and a nucleus with a central aggregation of chromatin, or nucleolus ; and many others. Besides the nucleus, there is another structure frequently visible in the living Amceba. This is a clear, rounded space in the ectosarc (c, e, and g, c. vac), which periodically dis- appears with a sudden contraction and then slowly reappears, its movements reminding one of the beating of a minute colourless heart. It is called the contractile vacuole, and consists of a cavity in the ectosarc containing a watery fluid. I MORniOLOCV AND rin^SIOLOGY 9 Occasionally Amccba; — or move strictly Amfcba-like organisms — are met with which ha\e neither nucleus ^ nor contractile vacuole, and are therefore placed in the separate genus Protaimiihi (Fig. 2). They may be looked upon as the simplest of living things. T\ ^.-"SKi ^'^.i A B D Vic 2 — Proia'ihvha priinllh^a , A, B, llic same specimen diawn at slioit inler\als of lime, showing changes of form. C— E. Three stages in tlie process of l:unary fission. (After Ilaeekel.) The preceding paragraphs may be summed uj) by saying that Amceba is a mass of protoplasm produced into tempo- rary processes or pseiidopcds, divisible into ectosarc and endosarc, and containing a nucleus and a contractile vacuole : tliat the nucleus consists of two substances, chromatin and nuclear sa]), enclosed in a distinct membrane : and that the contractile vacuole is a mere ca\-ity in the protoplasm con- taining fluid. All these facts come under the head of Morpholoi^^y, tlie division of biology which treats of ibrm and structure : we must now study tlie P/ivsio/oiy of our animalcule — that is, consider the actions or fn/uiioiis it is capable of performing. First of all, as we have already seen, it moves, the nio\'e- ment consisting in the slow protrusion and withdrawal ot pseudopods. This may be expressed generally by saying ^ jLi'-iging from tlie analogy of the Infusiii-ia it seems very prol)al)le lliat sucli apparently noi-i-nucleate forms as I'rotannelia contain chroma- tin cliffusc'l in the form of minute granules throughout their suh^tance (see end of Less(-in X.. p. 120), (U" that they are forms which ha\"e lost their nuclei. lo AMCEBA x.F,ss. that Amceba is contractile, or that it exhibits contractility. But here it must be borne in mind that contraction does not mean the same thing in biology as in physics. When it is said that a red-hot bar of iron contracts on cooling, what is meant is that there is an actual reduction in volume, the bar becoming smaller in all dimensions. But when it is said that an Amoeba contracts, what is meant is that it diminishes in one dimension while increasing in another, no perceptible alteration in volume taking place : each time a pseudopod is protruded an equivalent volume of protoplasm is withdrawn from some other part of the body. We may say then that contractility is a function of the protoplasm of Amoeba — that is, that it is one of the actions which the protoplasm is capable of performing. A contraction may arise in one or other of two ways. In most cases the movements of an Amoeba take place without any obvious external cause ; they are what would be called in the higher animals voluntary movements — movements dictated by the will and not necessarily in response to any external stimulus. Such movements are called spontaneous or automatic. On the other hand, movements may be in- duced in Amoeba by external stimuli, by a sudden shock, or by coming into contact with an object suitable for food : such movements are the result of irritability of the proto- plasm, which is thus both automatic and irritable — that is, its contractility may be set in action either by internal or by external stimuli. Under certain circumstances an Amoeba temporarily loses its power of movement, draws in its pseudopods, and becomes a globular mass around which is formed a thick, shell-like coat, called the cyst or cell-wall (Fig. i, d, rr). The composition of this is not known ; it is certainly not I MODE OF FEEDINC; ii protoplasmic, and very probably consists of some nitrogenous substance allied in composition to horn and to the chitin which forms the external shell of Crustacea, insects, &c. After remaining in this encysted condition for a time, the Amceba escapes by the rupture of its cell-wall, and resumes active life. Very often an Amoeba in the course of its wanderings comes in contact with a still smaller organism, such as a diatom (see Lesson XIV., Fig. 35) or a small infusor (see Lessons X. — XIL). When this happens the Amoeba may be seen to extend itself round the lesser organism until the latter becomes sunk in its protoplasm in much the same way ■ as a marble might be pressed into a- lump of clay (Fig. i, c, a). The diatom or other organism becomes in this way completely enclosed in a cavity or food-vacuole {f. vac), which also contains a small quantity of water necessarily in- cluded with the prey. The latter is taken in by the Amoeba as food : so that another function performed by the animal- cule is the reception of food, the first step in the process of mitrition. It is to be noted that the reception of food takes place in a particular way, viz. by ingestion — i.e. it is enclosed raw and entire in the living protoplasm. It has been noticed that Amoeba usually ingests at its hinder end — that is, the end directed backwards in progression. Having thus ingested its prey, the Amoeba continues its course, when, if carefully watched, the swallowed organism will be seen to undergo certain changes. Its protoplasm is slowly dissolved ; if it contains chlorophyll — the green colouring matter of plants — this is gradually turned to brown ; and finally nothing is left but the case or cell-wall in which many minute organisms, such as diatoms, are enclosed. Finally, the Amoeba, as it creeps slowly on, leaves this empty cell-wall behind, and thus gets rid of what it has no further 12 AMCEBA LESS. use for. It is thus able to ingest living organisms as food ; to dissolve or digest their protoplasm ; and to egest or get rid of any insoluble matorials they may contain. Note that all this is done without either ingestive aperture (mouth), digestive cavity (stomach), or egestive aperture (anus) ; the food is simply taken in by the flowing round it of protoplasm, digested as it lies enclosed in the protoplasm, and got rid of by the Amoeba flowing away from it. It has just been said that the protoplasm of the prey is dissolved or digested : we must now consider more particu- larly what this means. The stomachs of the higher animals — ourselves, for instance — produce in their interior a fluid called gastric juice. When this fluid is brought into contact with albumen or any other proteid a remarkable change takes place. The proteid is dissolved and at the same time rendered diffusible, so as to be capable, like a solution of salt or sugar, of passing through an organic membrane (see p. 6). The diffusible proteids thus formed by the action of gastric juice upon ordinary proteids are called peptones : the transformation is effected through the agency of a constituent of the gastric juice called pepsin. There can be little doubt that the protoplasm of Amoeba is able to convert that of its prey into a soluble and diffusible form by the agency of some substance analogous to pepsin, and that the dissolved matters diffuse through the body of the Amceba until the latter is, as it were, soaked through and through with them. Under these circumstances the Amoeba may be compared to a sponge which is allowed to absorb water, the sponge itself representing the living proto- plasm, the water the solution of proteids which permeates it. It has been proved by experiment that proteids are the only class of food which Amoeba can make use of ; it is unable to 1 GROWTH 1^ digest either starch or fot — two very important constiluenls of the food of the higher animals. Mineral matters must, however, be taken with the food in the form of a weak watery solution, since the water in which the animalcule lives is never absolutely pure. The Amoeba being thus permeated, as it were, with a nutrient solution, a very important process takes place. The elements of the solution, hitherto arranged in the form of peptones, mineral salts, and water, become rearranged in such a way as to form new particles of living protoplasm, which are deposited among the pre-existing particles. In a word, the food is assimilated or converted into the actual living substance of the Amceba. One effect of this formation of new protoplasm is obvious : if nothing happens to counteract it, the Amoeba must grow, the increase in size being brought about in much the same way as that of a heap of stones would be by continually thrusting new pebbles into the interior of the heap. This mode of growth — by the interposition of new particles among old ones — is called growth by intussusception, and is very characteristic of the growth of protoplasm. It is necessary to distinguish it, because there is another mode of growth which is characteristic of minerals and occurs also in some organized structures. A crystal of alum, for instance, suspended in a strong solution of the same substance, grows ; but the increase is due to the deposition of successive layers on the surface of the original crystal, in much the same way as a candle might be made to grow by repeatedly dipping it into melted grease. This can be proved by colouring the crystal with logwood or some other dye before suspending it, when a gradually-increasing colourless layer will be deposited round the coloured crystal : if growth took place by intussusception we should have a gradual weakening 14 AMCEBA LESS. of the tint as the crystal increased in size. This mode of growth — by the deposition of successive layers — is called growth by accretion. It is probable that the cyst of Amoeba referred to above (p. ii) grows by accretion. Judging from the analogy of other organisms it would seem that, after rounding itself off, the surface of the sphere of protoplasm undergoes a chemi- cal change resulting in the formation of a thin superficial layer of non-protoplasmic substance. The process is re- peated, new layers being continually deposited within the old ones until the cell-wall attains its full thickness. The cyst is therefore a substance separated or secreted from the protoplasm ; it is the first instance w-e have met with of a product of secretion. From the fact that Amoeba rarely attains a greater dia- meter than \ mm., it follows that something must happen to counteract the constant tendency to grow, which is one of the results of assimilation. We all know what happens in our own case : if we take a certain amount of exercise — walk ten miles or lift a series of heavy weights — we undergo a loss of substance manifested by a diminution in weight and by the sensation of hunger. Our bodies have done a certain amount of work, and have undergone a proportional amount of waste, just as a fire every time it blazes up consumes a certain weight of coal. Precisely the same thing happens on a small scale with Amoeba. Every time it thrusts out or w-ithdraws a pseudo- pod, every time it contracts its vacuole, it does a certain amount of work — moves a definite weight of protoplasm through a given space. And every movement, however sHght, is accompanied by a proportional waste of substance, a cer- tain fraction of the protoplasm becoming oxidized, or in other words undergoing a process of low temperature combustion. 1 POTENTIAL AND KINETIC ENERe.\' 15 When we say that any combustible body is burnt what we usually mean is that it has combined with oxygen, forming certain products of combustion due to the chemical union of the oxygen with the substance burnt. For instance, when carbon is burnt the product of combustion is carbon dioxide or carbonic acid (C + 02 = C0,); when hydrogen is burnt, water (H2 + = H20j. The products of the slow com- bustion which our own bodies are constantly undergoing are these same two bodies — carbon dioxide given off mainly in the air breathed out, and water given off mainly in the form of perspiration and urine — together with two com- pounds containing nitrogen, urea (CH^NjO) and uric acid (CjH^N^Oj), both occurring mainly in the urine. In some animals urea and uric acid are replaced by other compounds such as guanin (CjHjNgO), but it may be taken as proved that in all living things the products of combustion are carbon dioxide,, water, and some nitrogenous substance of simpler constitution than proteids, and allied to the three just mentioned. With this breaking down of proteids the vital activities of all organisms are invariably connected. Just as useful mechanical work may be done by the fall of a weight from a given height to the level of the ground, so the work done by the organism is a result of its complex proteids falling, so to speak, to the level of simpler substances. In both instances potential energy or energy of position is converted into kinetic or actual energy. In the particular case under consideration we have to rely upon analogy and not upon direct experiment. We may, however, be quite sure that the products of combustion or waste matters of Amoeba include carbon dioxide, water, and some comparatively simple (as compared with proteids) compound of nitrogen. i6 AMfEBA T.ESN. These waste matleis or excrctury producis are gixcn off partly from the general surface of the body, but partly, it would seem, through the agency of the contractile \ncuole. It appears that the water taken in with the food, together in all probability with some of that formed by oxidation of the protoplasm, makes its way to the vacuole, and is ex- pelled by its contraction. AVe have here another function, performed by Amoeba, that of excretion, or the getting rid of Avaste matters. In this connection the reader must be warned against a possible misunderstanding arising from the fact that the word excretion is often used in two senses. We often hear, for instance, of solid and liquid "excreta." In Amceba the solid excreta, or more correctly faces, consist of such things as the indigestible cell-walls, starch grains, &c., of the organisms upon which it feeds ; but the rejection of these is no more a process of excretion than the spitting out of a cherry-stone, since they are simply parts of the food which have never been assimilated — never formed part and parcel of the organism. True excreta, on the other hand, are invariably products of the waste or decomposition of protoplasm.! The statement just made that the protoplasm of Amreba constantly undergoes oxidation presupposes a constant sup- ply of oxygen. The water in which the animalcule lives invariably contains that gas in solution : on the other hand, as we have seen, the protoplasm is continually forming carbon dioxide. Now when two gases are separated from one another by a porous partition, an interchange takes place between them, each diffusing into the space occupied by the 1 In the higher animals the distinction between excret.a and feces is complicated by the fact that the latter always contain true excretory products derived from llie epithelium of the intestine and its glands. I METABOLISM 17 Other. The same process of gaseous diffusion is continually going on between the carbon dioxide in the interior of Amoeba and the oxygen in the surrounding water, the proto- plasm acting as the porous partition. In this way the carbon dioxide is got rid of, and at the same time a supply of oxygen is obtained for further combustion. The taking in of oxygen might be looked upon as a kind of feeding process, the food being gaseous instead of solid or liquid, just as we might speak of "feeding" a fire both with coals and with air. Moreover, as we have seen, the giving out of carbon dioxide is a process of excretion. It is, however, usual and convenient to speak of this process of exchange of gases as respiration or breathing, which is therefore another function performed by the protoplasm of Amoeba. The oxidation of protoplasm in the body of an organism, like the combustion of wood or coal in a fire, is accompanied by an evolution of heat. That this occurs in Amoeba can- not be doubted, although it has never been proved. The heat thus generated is, however, constantly being lost to the surrounding water, so that the temperature of Amoeba, if we could but measure it, would probably be found, like that of a frog or a fish, to be very little if at all above that of the medium in which it lives. We thus see that a very elaborate series of chemical pro- cesses is constantly going on in the interior of Amoeba. These processes are divisible into two sets : those which begin with the digestion of food and end with the manufac- ture of hving protoplasm, and those which have to do with the destruction of protoplasm and end with excretion. The whole series of processes are spoken of collectively as metabolism. We have, first of all, digested food diffused through the protoplasm and finally converted into fresh c i8 AMCEBA I.F.SS. living protoplasm : this is the process of constructive meta- bolism or anahoiism. Next we have the protoplasm, gradually breaking down and undergoing conversion into excretory products : this is the process of destructive metabolism or katabolism. There can be little doubt that both are pro- cesses of extreme complexity : it seems probable that after the food is once dissolved there ensues the successive formation of numerous bodies of gradually increasing complexity {anabolic mesostates or attastates), culminating in protoplasm ; and that the protoplasm, when once formed, is decomposed into a series of substances of gradually diminishing complexity {katabolic mesostates or katastates), the end of the series being formed by the comparatively simple products of excretion. The granules in the endosarc are probably to be looked upon as various mesostates imbedded in the protoplasm proper. Living protoplasm is thus the most unstable of substances ; it is never precisely the same thing for two consecutive seconds: it "decomposes but to recompose," and recom- poses but to decompose ; its existence, like that of a water- fall or a fountain, depends upon the constant flow of matter into it and away from it. Tt follows from what has been said that if the income of an Amoeba, i.e., the total weight of substances taken in (food plus oxygen plus water) is greater than its expenditure or the total weight of substances given out (faeces plus excreta proper //z^j carbon dioxide) the animalcule will grow : if less it will dwindle away ; if the two are equal it will remain of the same weight or in a state of physiological equilibrium. We see then that the fundamental condition of existence of the individual Amoeba is that it should be able to form new protoplasm out of the food supplied to it. But some^ I REPRODUCTION 19 thing more than this is necessary. Amcebae are subject to all sorts of casualties ; they may be eaten by other organ- isms or the pool in which they live may be dried up ; in one way or another they are constantly coming to an end. From which it follows that if the race of Amcebae is to be preserved there must be some provision by which the individuals composing it are enabled to produce new in- dividuals. In other words Amoeba must, in addition to its other functions, perform that of reprodudioji. An Amoeba reproduces itself in a very simple way. The nucleus first divides into two : then the whole organism elongates, the two nuclei at the same time travelling away from one another : next a furrow appears across the middle of the drawn-out body between the nuclei (Fig. i, i ; fig. 2, c, d) ; the furrow deepens until finally the animalcule sepa- rates into two separate Amcebae (Fig. 2, e), which hence- forward lead an independent existence. This, the simplest method of reproduction known, is called simple or binary fissiofi. Notice how strikingly dif- ferent it is from the mode of multiplication with which we are familiar in the higher animals. A fowl, for instance, multiplies by laying eggs at certain intervals, in each of which, under favourable circumstances, and after a definite lapse of time, a chick is developed : moreover, the parent bird, after continuing to produce eggs for a longer or shorter time, dies. An Amoeba, on the other hand, simply divides into two Amoebae, each exactly like itself, and in doing so ceases to exist as a distinct individual. Instead of the successive production of offspring from an ultimately dying parent, we have the simultaneous production of offspring by the division of the parent, which does not die, but becomes simply merged in its progeny. There can be no better instance of the fact that reproduction is discontinuous growth. c 2 zo AMCEBA LESS. From this it seems that an Amoeba, unless suffering a violent death, is practically immortal, since it di\ides into two completely organised individuals, each of which begins life with half of the entire body of its parent, there being therefore nothing left of the latter to die. It would appear, however, judging from the analogy of the Infusoria (see Lesson X.) that such organisms as Amoeba cannot go on multiplying indefinitely by simple fission, and that occasion- ally two individuals come into contact and undergo complete fusion. A conjugation of this kind has been observed in Amoeba, but has been more thoroughly studied in other forms (see Lessons III. and X.). Whether it is a necessary condition of continued existence in our animalcule or not, it appears certain that "death has no place as a natural recurrent phenomenon" in that organism. Amoeba may also be propagated artificially. If a speci- men is cut into pieces each fragment is capable of develop- ing into a complete animalcule provided it contains a portion of nuclear matter, but not otherwise. From this it is obvious that the nucleus exerts an influence of the utmost importance over the vital processes of the organism. If an Amoeba does happen to be killed and to escape being eaten it will undergo gradual decomposition, becoming converted into various simple substances of which carbon dioxide, water, and ammonia are the chief (See p. gi.) In conclusion, a few facts may be mentioned as to the conditions of life of Amceba — the circumstances under which it will live or die, flourish or otherwise. In the first place, it will live only within certain limits, of temperature. In moderately warm weather the temperature to which it is exposed may be taken as about 15° C. If gradually warmed beyond this point the movements at first I CONDITKJNS OF LIKE 21 show an increased activity, then become more and more sluggish, and at about 30° — 35° C. cease altogether, re- commencing, however, when the temperature is lowered. If the heating is continued up to about 40° C. the animal- cule is killed by the coagulation of its protoplasm (see p. 5) : it is then said to suffer heat-rigor or death-stiffening pro- duced by heat. Similarly when it is cooled below the ordinary temperature the movements become slower and slower, and at the freezing point (0° C.) cease entirely. But freezing, unlike over-heating, does not kill the pro- toplasm, but only renders it temporarily inert ; on thawing, the movements recommence. We may therefore distinguish an optimum temperature at which the vital actions are carried on Avith the greatest activity ; maximum and minimum tem- peratures above and below which respectively they cease ; and an ultra- maximum temperature at which death ensues. There is no definite ultra-minimum temperature known in the case of Amoeba. The quantity of water present in the protoplasm — as water of organization (see p. 5) — is another matter of importance. The water in which Amoeba lives always contains a certain percentage of salts in solution, and the protoplasm is affected by any alteration in the density of the surrounding medium ; for instance, by replacing it by distilled water and so reducing the density, or by adding salt and so increasing it. The addition of common salt (sodium chloride) to the amount of two per cent, causes Amoeba to withdraw its pseudopods and undergo a certain amount of shrinkage . it is then said to pass into a condition of dry-rigor. Under these circumstances it may be restored to its normal con- dition by adding a sufficient proportion of water to bring back the fluid to its original density. In this connection it is interesting to notice that the dele- 22 A^SIfEBA i.ESS. I terious effects of an excess of salt are produced only when the salt is added suddenly. By the very gradual addition of sodium chloride Amoeba have been brought to live in a four per cent, solution, i.e., one twice as strong as would, if added suddenly, produce dry-rigor. From what has been said above on the subject of respira- tion (p. 17) it follows that free oxygen is necessary for the existence of Amoeba. Light, on the other hand, appears to be unnecessary, amoeboid movements having been shown to go on actively in darkness. LESSON II H^MATOCOCCUS The rain-water which collects in puddles, open gutters, &c., is frequently found to have a green or red colour. The colour is due to the presence of various organisms — plants or animals — one of the commonest of which is called Hamatococcus (or as it is sometimes called Proto- coccus or Sphczrelld) phivialis. Like Amoeba, Haematococcus is so small as to require a high power for its examination. Magnified three or four hundred diameters it has the appearance (Fig. 3, a) of an ovoidal body, somewhat pointed at one end, and of a bright green colour, more or less flecked with equally bright red. Like Amoeba, moreover, it is in constant movement, but the character of the movement is very different in the two cases. An active Hiematococcus is seen to swim about the field of the microscope in all directions and with considerable apparent rapidity. AVe say apparent rapidity because the rate of progression is magnified to the same extent as the organism itself, and what appears a racing speed under the microscope is actually a very slow crawl when divided by 300. It has been found that such organisms as Hsematococcus travel at the rate of one foot in from a quarter of an hour to an hour : or, to express 24 11.1',.MAT(jC<)(,'CUS the fact in anotlicr and Hili-cr way, tliat tlicy Iravc! a distance equal to two and a half times their own diameter in one second. In swimming the ii(jinted end is always directed FUt. 3. — A. JItcindlui"])uscle) of a crayfish showing amceboid movements: A'"', a'', the same, killed and stained, showing the nucleus {nu), b', leucocyte of the frog, }ii( the nucleus ; B-, two leucocytes beginning to conjugate : v>-\ the same after conjugation, a bnnicleate Plasmodium being formed . ?/', a leucocyte undergoing binary fisbion : B'"', surface view and b", edge view of a red corimscle of the same, nit, the nucleus. c\ C-, leucocytes of the newt ; in c^ particles of vcrniiliun, repre- sented by black dots, have been ingested. C'', surface view and C"*, edge view of a red corpuscle of man. d\ columnar epithelial cells from intestine of frog : !•", a similar S8 EPITHELIAL CELLS less. cell showing strinted distal border from which in 1)'* pseudoi^ods are protruded. e\ ciliated epithelial cell from mouth of frog ; e", e^, similar cells from windpipe of dog. f\ parenchyma cell from root of lily; showing nucleus {mi), vacuoles [vac), and cell-wall : F-, a similar cell from leaf of bean, showing nucleus, vacuoles, cell-wall and chromatophores (chr). (B, D^, and e', after Howes : c, E^, and E^, after Klein and Noble Smith : d", d', after Wiedersheim : f', after Sachs : F^, after Behrens. ) coloured by a pigment called hcemoglobin, and provided each with a large nucleus (jiu) which, when the corpuscle is seen from the edge (b"), produces a bulging of its central part. These bodies may be compared to Ainoebse which have drawn in their pseudopods, assumed a flattened form, and become coloured with hfemoglobin. In the blood of mammals, such as the rabbit, dog, or man, similar leucocytes occur, but their red-blood corpuscles (c^c*) have the form of biconcave discs, and are devoid of nuclei. In many animals the leucocytes have been observed to ingest solid particles (c^), to multiply by siinple fission (b*), and to coalesce with one another forming plasmodia (b^, b^) (P- 52). The stomach and intestines of animals are lined with a sort of soft slimy skin called mucous jiiemhrane. If a bit of the surface of this membrane — in a frog or rabbit for instance — is snipped off" and " teased out," i.e., torn apart with needles, it is found when examined under a high power to be made up of an immense number of microscopic bodies called epithelial cells, which in the living animal, lie close to one another in the inner layer of mucous mem- brane in something the same way as the blocks of a wood pavement lie on the surface of a road. An epithelial cell (d\ D-) consists of a rod-like mass of protoplasm, contain- ing a large nucleus, and is therefore comparable to an VI PARENCHYMA CELLS 59 elongated Amoeba without pseudopods. In some animals the resemblance is still closer : the epithelial cells have been observed to throw out pseudopods from their free surfaces (d'), that is, from the only part where any such movement is possible, since they are elsewhere in close contact with their fellow cells. The mouth of the frog and the trachea or windpipe of air- breathing vertebrates such as reptiles, birds, and mammals, are also lined with mucous membrane, but the epithelial cells which constitute its inner layer differ in one important respect from those of the stomach and intestine. If ex- amined quite fresh each is found to bear on its free surface, i.e., the surface which bounds the cavity of the mouth or windpipe, a number of delicate protoplasmic threads or cilia (e'' — E^) which are in constant vibratory movement. In the process of teasing out the mucous membrane some of the cells are pretty sure to become detached, and are then seen to swim about in the containing fluid by the action of their cilia. These ciliated epithelial cells remind one strongly of Heteromita, except for the fact that they bear numerous cilia in constant rhythmical movement instead of two only — in this case distinguished as flagella — presenting an irregular lashing movement. Similar ciliated epithelial cells are found on the gills of oysters, mussels, &c., and in many other situations. The stem or root of an ordinary herbaceous plant, such as a geranium or sweet- pea, is found when cut across to consist of a central mass of pith, around which is a circle of woody substance, and around this again a soft greenish material called the cortex. A thin section shows the latter to be made up of innumerable polyhedral bodies called 6o PARENCHYMA CELLS i f.ss. parenchyma cells, fitting closely to one another like the bricks in a wall. A parenchyma cell examined in detail (f^) is seen to consist of protoplasm hollowed out internally into one or more cavities or vacuoles {vac) containing a clear fluid. These vacuoles differ from those of Amceba, Heteromita, or Euglena in being non-contractile ; they are in fact mere cavities in the protoplasm containing a watery fluid : the layer of protoplasm immediately surrounding them is denser than the rest. Sometimes there is only one such space occupying the whole interior of the cell, sometimes, as in the example figured, there are several, separated from one another by delicate bands or sheets of protoplasm. The cell contains a large nucleus (jiu) and is enclosed in a moderately thick cell-wall composed of cellulose. The above description applies to the cells composing the deeper layers of the cortex, i.e., those nearest the woody layer : in the more superficial cells, as well as in the internal cells of a leaf, there is something else to notice. Imbedded in the protoplasm, just within the cell wall, are a number of minute ovoid bodies of a bright green colour (f-, chr). These are clu-omatophores or chlorophyll corpuscles ; they consist of protoplasm coloured with chlorophyll which can be proved experimentally to have the same properties as the chlorophyll of Haematococcus and Euglena. Such a green parenchyma cell is clearly comparable with an encysted Hjematococcus or Euglena, the main differences being that in the plant cell the form is polyhedral owing to the pressure of neighbouring cells and that the chromato- phores are relatively small and numerous. Similarly a colourless parenchyma cell resembles an encysted Amoeba. The pith, the epidermis or thin skin which forms the outer surface of herbaceous plants, the greater part of the VI MINUTE STRUCTURE OF CELLS 6i leaves and other portions of the plant may be shown to consist of an aggregation of cells agreeing in essential respects with the above description. We come therefore to a very remarkable result. The higher animals and plants are built up — in part at least — of elements which resemble in their essential features the minute and lowly organisms studied in previous lessons. Those elements are called by the general name of cells . hence the higher organisms, whether plants or animals, are multicellular or are to be considered as cell aggregates, while in the case of such beings as Amoeba, Hsematococ- cus, Heteromita, or Euglena, the entire organism is a single cell, or is imicellular. Note further that the cells of the higher animals and plants, like entire unicellular organisms, may occur in either the amoeboid (Fig. 8, a, b^, cS) the ciliated (e), or the encysted (f) condition, and that a plasmodial phase (b-) is sometimes produced by the union of two or more amoeboid cells. One of the most characteristic features in the unicellular organisms described in the preceding lessons is the con- stancy of the occurrence of binary fission as a mode of multiplication. The analogy between these organisms and the cells of the higher animals and plants becomes still closer when we find that in the latter also simple fission is the normal mode of multiplication, the increase in size of growing parts being brought about by the continual division of their constituent cells. The process of division in animal and vegetable cells is frequently accompanied by certain very characteristic and complicated changes in the nucleus to which we must now 62 MINUTi: STRUCTUUK ()!■ CKLLS 1 K^s. direct our attention. First of all, however, it will be neces- sary to describe the e\act microscopic structure of cells and their nuclei as far as it is known at present. ctir ru.m Fjg. 9. — A, Cell from the i^enital ridge of a >"oung sal.imanJer, show'ini:; eell-niembrane (r. ///), protoplasm or cell-In:)'!}' ((". /') with asliosphere {s) and centrosome (<"), and nucleus willi memliranc {f//f. Ill) and irregular network c)f ehromatin {ihr). le Cell from the immature .stamen of a lil}', allowing celI-^\"all ((■, re), protoi")la.sin with two astrospheres {.0, and nucleus as in A. Both figures ^'ery highly magnified. (.A, from a drawing by Mr. J, E. S. Moore : B, after GuignarJ. ) There seems to be a good deal of variation in the precise structure of various animal and plant cells, but the more recent researches show that in the celbbody or protoplasm (Fig. 9, c, h) two constituents may be distinguished, a clear semi-fluid substance, tra\'ersed by a delicate sponge-work. Now under the microscope the whole cell is not seen at once but only an optical section of it, that is all the parts which are in focus at one time : by altering the focus we view the object at successi\'e depths, each ^iew being [jraetically a slice parallel to the lenses of the instrument. This being the case, protoplasm presents the microscopic appearance of a clear or slightly granular VI MINUTE STRUCTURE OF NUCLEI 63 matrix traversed by a delicate network. In the epithe- lial cells of animals the protoplasm is bounded exter- nally by a cell-membrane (Fig. 9, a, c. m) of extreme tenuity, in plants by a cell-wall (e, c. w) of cellulose : in amoeboid cells the ectosarc or transparent non-granular portion of the cell consists of clear protoplasm only, the granular endosarc alone possessing the sponge-work. In the majority of full-grown plant cells (Fig. 8, r) and in some animal cells the protoplasm is more or less exten- sively vacuolated, but in the young growing- parts as well as in the ordinary cells of animals the foregoing description holds good. It is quite possible that the reticular character of the protoplasm may be merely the optical expression of an extensive but minute vacuolation, or may be due to the presence of innumerable minute granules developed in the protoplasm as products of metabolism. The nucleus is usually spherical in form : it is enclosed in a delicate nuclear membrane {n.m) and contains, as in Amoeba (p. 7) two constituents, the nuclear sap and the chromatin which exhibit far more striking differences than the two constituents of the cell-body. The nuclear sap is a homogeneous semi-fluid substance which forms the ground-work of the nucleus : it resembles the clear cell- protoplasm in its general characters, amongst other things in being unaffected by dyes. The chromatin (chr) takes the form of a network or sponge work of very variable form, and is distinguished from all other constituents of the cell by its strong affinity for aniline and other dyes. Frequently one or more minute globular structures, the nucleoli (b, mi), occur in the nucleus either connected with the network or lying freely in its meshes : they also have a strong affinity for dyes although they often differ considerably from the chromatin in their micro-chemical reactions. Fig, 10. — Diagrams illustrating the process of indirect cell division ur mitosis. A, the resting cell : the nucleus shows a nuclear membrane (iiit. in), chromatin [chr) arranged in loop.s united into a network (the latter shown on the right side only), and two nucleoli {nu') : near the nucleus is an astrosphere (s), containing a centrosome (c) and surrounded by radiating protoplasmic filaments. B, The chromatin has resolved itself into distinct loops or chromo- somes (lAr) which have divided longitudinally : the nuclear membrane has begun to disappear : there are two astrospheres and between them is seetr the commenceinent of the nuclear spindle (sp). c, The nuclear membrane has disappeared : the chromosomes are LESS. VI CELL-DIVISION 65 arranged irregularly : the spindle has increased in size and is silualed definitely within the nuclear area. D, The chromosomes are arranged round the equator of the fully formed nuclear spindle. E, The daughter-loops of the chromosomes are passing in opposite directions towards the poles of the spindle, each having a spindle-fibre attached to it. F, Later stage of the same process. G, The chromosomes are now arranged in two distinct groups, one at each pole of the spindle. H, The daughter-cells are partly separated by constriction and the chromosomes of each group are uniting to form the network of the daughter-nucleus. I, Shows the division of a plant cell by the formation of a cell-plate (f. //) : the daughter nuclei are fully formed. (Altered from Flemraing, Rabl, &c.) In the body of some cells and possibly of all there is found a globular body, surrounded by a radiating arrange- ment of the protoplastii and called the astrosphere {s) : it lies close to the nucleus, and contains a minute granule known as the central particle or centrosoine (c). In many cells two astrospheres and two or more centrosomes have been found in each cell (b, s). The precise changes which take place during the fission of a cell are, like the structure of the cell itself, subject to considerable variation. We will consider what may probably be taken as a typical case (Fig. 10). First of all, the astrosphere, with its centrosome, divides (b) and the products of its division gradually separate froin one another (c), ultimately passing to opposite poles of the nucleus (d). At the same time the network of chromatin divides into a number of separate filaments called chromosomes (b, chr), the number of which appears to be constant in any given species of animal or plant, although it may vary in different species from two to twenty-four. Soon after this the nuclear membrane and the free nucleoli disappear (b, c) and the 66 MINUTE STRUCTURE OF CELLS less. nucleus is seen to contain a spindle shaped body {sp) formed of excessively delicate fibres which converge at each pole to the corresponding astrosphere. The precise origin of this nuclear spindle is uncertain : it may arise either from the nuclear matrix or, more probably, from the protoplasm of the cell . it is not affected by colouring matters. At the same time each chromosome splits along its whole length so as to form two parallel rods or loops in close contact with one another (b) : in this way the number of chromosomes is doubled, each one being now represented by a couple. The divided chromosomes now pass to the equator of the spindle (d) and assume the form of more or less V-shaped loops, which arrange themselves in a radiating manner so as to present a star-like figure when the cell is viewed in the direction of the long axis of the spindle. Everything is now ready for division to which all the foregoing processes are preparatory. The two chromosomes of each couple now gradually pass to opposite poles of the spindle (e, f), two distinct groups being thus produced (g) and each chromosome of each group being the twin of one in the other group. Perhaps the fibres of the spindle are the active agents in this process, the chromosomes being dragged in opposite directions by their contraction : on the other hand it is possible that the movement is due to the contractility of the chromosomes themselves. After reaching the poles of the spindle the chromosomes of each group unite with one another to form a network (h) around which a nuclear membrane finally makes its appear- ance (i). In this way two nuclei are produced within a single cell, the chromosomes of the datcghter-nticlei, as well VI CELL-DIVISION 67 as their attendant astrospheres, being formed by the binary fission of those of the mother-nudeus. But pari passu with the process of- nuclear division, fission of the cell-body is also going on. This may take place by a simple process of constriction (h) — in much the same way as a lump of clay or dough would divide if a loop of string were tied round its middle and then tightened — or by the formation of what is known as a cell-plate. This arises as a row of granules formed from the equatorial part of the nuclear spindle (i) : the granules extend until they form a complete equatorial plate dividing the cell-body into two halves : fission then takes place by the cell-plate split- ting into two along a plane parallel with its flat surfaces.^ In plants the cell-plate gives rise to a partition wall of cellulose which divides the two daughter-cells from one another. In some cases the dividing nucleus, instead of going through the complicated processes just described, divides by simple constriction. AVe have therefore to distinguish between direct and indirect nuclear division. To the latter very elaborate method the name mitosis or karyokinesis is applied : direct division is then distinguished as amitotic. In this connection the reader will not fail to note the extreme complexity of structure revealed in cells and their nuclei by the highest powers of the microscope. When the constituent cells of the higher animals and plants were discovered, during the early years of the present century, by Schleiden and Schwann, they were looked upon as the ultima Thule of microscopic analysis. Now the demonstration of ■■ It must not be forgotten that the cells,' which are necessarily repre- sented in such diagrams as Fig. 10 as planes, are really solid bodies, and that consequently the cell-plate represented in the figures as a line is actually a plane at right angles to the plane of the paper. F 2 68 COMPLEXITV OF CELL STRUCTURE less. the cells themselves is an easy matter, the problem is to make out their ultimate constitution. What would be the result if we could -get microscopes as superior to those of to-day as those of to-day are to the primitive instruments of eighty or ninety years ago, it is impossible even to conjecture. But of one thing we may feel confident — of the enormous strides which our knowledge of the constitution of living things is destined to make during the next half century. The striking general resemblance between the cells of the higher animals and plants and entire unicellular organisms has been commented on as a very remarkable fact : there is another equally significant circumstance to which we must now advert. All the higher animals begin life as an egg, which is either passed out of the body of the parent as such, as in most fishes, frogs, birds, &c., or undergoes the first stages of its development within the body of the parent, as in sharks, some reptiles, and nearly all mammals. The structure of the egg is, in essential respects, the same in all animals from the highest to the lowest. In a jelly-fish, for instance, it consists (Fig. ii, a) of a globular mass of protoplasm {gd), in which are deposited granules of a pro- teinaceous substance known as yolk-spherides. Within the protoplasm is a large clear nucleus {g.v) the chromatin of which is aggregated into a central mass or nucleolus {g."i-). An investing membrane may or may not be present. In other words the egg is a cell : it is convenient, for reasons which will appear immediately, to speak of it as the ovum or egg-celt. The young or immature ova of all animals present this structure, but in many cases certain modifications are under- gone before the egg is mature, i.e., capable cf development VI STRUCTURE OF THE ECCl ,., 69 into a new individual. For instance, the protoplasm may throw out pseudoiiods, the egg becoming amceboid (see Fig. 52) ; or the surface of the protoplasm may secrete a thick cell-wall (see Fig. 61). The most extraordinary modification takes place in some Vertebrata, sucli as birds. In a hen's egg, for instance, the yolk-spherules increase immensely, swelling out the microscopic ovum until it becomes what we know as the "yolk"' of the egg: around this layers of albumen or " white ' are deposited, and finally the shell membrane and the shell. Hence we ha.\e to distinguish carefully in eggs of this character between the entire " egg " in the ordinar)- acceptation of the term, and the ovum or egg-cell But complexities of this sort do not alter the fundamental Fig, n. — A, ovum of an animal [Carniayina Ims/ala, one of ihe jelly fishes), showing proloplasm (.;,'(/), nucleus (gv), and nucleolus (,;'"<). B, ovum of a plan I [^Gymnadt'uia coiwpsea, one of the orchids), show mg protoplasm (/Aw), nucleus (//;/), and nucleolus ('"/'). (..\, from IJalfour after Hacckel . B, after Marshall Ward.) fact that all the higher animals begin life as a single cell, or in other words that multicellular animals, however large and complex they iriay be in their adult condition, originate as unicellular bodies of microscopic size. The same is the case with all the higher jilants. The pistil or seed-vessel of an ordinary flower contains one or more little ovoidal bodies, the so-called " o\ ules " (more ac- curately megasporangia (see Lesson X.KXIA'., and k'ig. i27\ which, when the flower withers, develop into the seeds. A 70 THE PLANT OVUM less, vi section of an ovule shows it to contain a large cavity, the embryo-sac or megaspore (see Fig. 127, d), at one end of which is a microscopic cell [pv, and Fig. 12 b), consisting as usual of protoplasm {plsm), nucleus {)iu). and nucleolus {nu'). This is the ovum or egg-cell of the plant : from it the new plant, which springs from the germinating seed, arises. Thus the higher plants, like the higher animals, are, in their earliest stages of existence, microscopic and unicellular. LESSON VII SACCHAROMYCES Every one is familiar with the appearance of the ordinary brewer's yeast— the light-brown, muddy, frothing substance which is formed on the surface of the fermenting vats in breweries and is used in the manufacture of bread to make the dough "rise." Examined under the microscope yeast is seen to consist of a fluid in which are suspended immense numbers of minute particles, the presence of which produces the mud- diness of the yeast. Each of these bodies is a unicellular organism, the yeast-plant, or, in botanical language, Sac- charomyces cerevisice. Saccharomyces consists of a globular or ellipsoidal mass of protoplasm (Fig. 12), about y^y- mm. in diameter, and surrounded with a delicate cell-wall of cellulose (c, c.iti). In the protoplasm are one or more non-contractile vacuoles (vac) — mere spaces iiUed with fluid and varying in number and size according to the state of nutrition of the cell. Granules also occur in the protoplasm, some of them being of a proteid material, others fat globules. Under ordinary circumstances no nucleus is to be seen : but by the em- ployment of a special mode of staining, a small rounded 72 SACCIIAKOMYCES nucleus has been shown to exist near the centre of the cell. The cell-wall is so thin that it is difficult to be sure of its presence unless very high powers are employed. It can however be easily demonstrated by staining yeast with Fig. 12. — Sai:charomv\i'cto7>i'/;i viridc, B. chloyiniim, and BacUliis i ireiis) contain chlorophyll, and several others fbrni pigments of \'arying tints and often of great intensity. For instance, there are red, yellow, brown, blue, and violet species of J\licrococcus which grow P^ic. 16. — Baiiilus ^uhlilis, showing various btagcb birlwecn single forms and iong filaments (Leplolhri.x). on slices of boiled potato, hard-boiled egg, &c., fo:mijig brilliantly coloured patches ; and the yellow colour often assumed by milk after it has been allowed to stand for a considerable time is due to the presence of Baihriinii xaiitliiniDit. i\\\ ISacteria iriultiply by simple trans\'erse lission, the process taking place sometimes during the motile, sometimes during the resting condition. Frequently the daughter-ceils do not separate cnmi)letely from one another but remain ss BACTIiRIA loosely aUaclicd, forming chains. These are very common in some siiecies of micrococcus (see Fig. 15). Bacillus \Nhen undergoing fission behaves something like Heteromita . the mother-cell divides transversely across the middle, and the two haU-es gradually wriggle away from one another, but remain connected for a time by a very fine thread Fig. 17. — .\, ]'iVrio. b, SpiriUnfii tiniie. c, ^pirillittn volutaiis. ( prom Klein. ) of protO]ilasm which extends between their adjacent ends. 'J1iis is drawn out by the gradual se])aration of the two cells, until it attains twice the length of a llagellum, when it snaps in the middle, thus providing each daughter-cell with a new flogellum. Lacillus may, however, divide while in the resting condition and, under certain circumstances, the process is repeated again and again, and the daughter-cells, viii NATURE OF GENERIC FORMS 89 remaining in contact, form a long wavy or twisted filament called Leptothrix (Fig. 16) the separate elements of which are usually only visible after staining. Bacillus also multiplies by a peculiar process of spore- formation which may take place either in the ordinary resting form or in a leptothrix filament. A bright dot appears at one place in the protoplasm (Fig. 18) : this increases in size, the greater part of the protoplasm being used up in its formation, and finally takes on the form of a clear oval spore which remains for some time enclosed in the cell-wall of the Bacillus, by the rupture of which it is finally liberated. In other Bacteria spores are formed directly from the ordin- ary cells which become thick walled. The spores differ from the Bacilli in being unstained by aniline dyes. After a period of rest the spores, under favourable cir- cumstances, germinate by growing out at one end so as to become rod-like, and thus finally assuming the form of ordinary Bacilli. There are other genera often included among Bacteria, for the description of which the student is referred to the more special treatises.^ One remark must, however, be made in concluding the present brief account of the morphology of the group. There is a great deal of evidence to show that what have been spoken of as genera (Bacterium, Bacillus, Spirillum, &c.) may merge into one another and are there- fore to be looked upon as phases in the life-history of various microbes rather than as true and distinct genera. But this is a point which cannot at present be considered as settled. The conditions of life of Bacteria are very various. Some live in water, such as that of stagnant ponds, and of these ^ See especially De Bary, Fungi, Myceiozoa, and Bacteria (Oxford, 1887), and Klein, Micro-organisms and Disease (London, 1896). 90 BACTERIA three species, as already stated (p. 87), contain chloropliyll. The nutrition ot' such forms must obviously he holophytic, and in tlie case of Bacterium chlorinum the giving off of oxygen in sunlight lias actuallv been proved. But this mode of nutrition is rare among the Bacteria : nearly all of those to which reference has been made are Fig, iS. — Spoie-foiniaLion in l^aLi/lus. (Krnn\ Klein.) saprophytes, that is, live upon decomposing animal and vegetable matters. They are, in fact, nourished in precisely tlic same way as Heterorjiita (see p. 37). Many of these forms, such as Bacterium termo and species of Bacillus, Vibrio, ivc, will, howe\'er, flourisli in Pasteur's solution, in which they obtain their nitrogen m the form of ammonium vrn BACTERIA AS FERMENTS 91 tartrate instead of decomposing proteid. It has also been shown that some Bacteria can go further and make use of nitrates as a source of nitrogen, and of a carbonate or even of carbon dioxide as a source of carbon : in other words, they are able to live upon purely inorganic matter in spite of the fact that they contain no chlorophyll. Some species may even multiply to a considerable extent in distilled water. But pan passu with their ordinary nutritive processes, many Bacteria exert an action on the fluids on which they live comparably to that exerted on a saccharine solution by the yeast-plant. Such microbes are, in fact, organized ferments. Every one is familiar with the turning sour of milk. This change is due to the conversion of the milk-sugar into lactic acid. QHioO, = 2(C3H,03), Sugar. Lactic Acid. The transformation is brought about by the agency of Bacterium lactis, a microbe closely resembling B. termo. Beer and wine are two other fluids which frequently turn sour, there being in this case a conversion of alcohol into acetic acid, represented by the equation — CgHgO + O2 = H3O + C.,H,0„ Alcohol. Oxygen. Water. Acetic Acid. The ferment in this instance is Bacterhim aceti, often called Mycoderma aceti, or the "vinegar plant." It will be noticed that in this case oxygen enters into the reaction : it is a case of fermentation by oxidation. Putrefaction itself is another instance of fermentation induced by a microbe. Bacterium termo — the putrefactive ferment — causes the decomposition of proteids into simpler compounds, amongst which are such gases as ammonia 92 BACTERIA less. (NH3), sulphuretted hydrogen (H,,S), and ammonium sulphide ( (NH4)jS), the evolution of which produces the characteristic odour of putrefaction. The final stage in putrefaction is the formation of nitrates and nitrites. The process is a double one, both stages being due to special forms of Bacteria. In the first place, by the agency of the nitrous ferment, ammonia is converted into nitrous acid — NH3 + 30 = HgO + HNO., Ammonia. Oxygen. Water. Nitrous Acid. The nitric ferment then comes into action, converting the nitrous into nitric acid — NHO2 + O = HNO3 Nitrous Acid. Oxygen. Nitric Acid. This process is one of vast importance, since by its agency the soil is constantly receiving fresh supplies of nitric acid which is one of the most important substances used as food by plants. Besides holophytes and saprophytes there are included among Bacteria m.2i-a.y parasites, that is, species which feed not on decomposing but on living organisms. Many of the most deadly infectious diseases, such as tuberculosis, diph- theria, typhoid fever, and cholera, are due to the presence in the tissues or fluids of the body of particular species of microbes, which feed upon the parts affected and give rise to the morbid symptoms characteristic of the disease. Some Bacteria, like the majority of the organisms pre- viously studied, require free oxygen for their existence, but others, like Saccharomyces during active fermentation (see p. 80), are quite independent of free oxygen and must there- fore be able to take the oxygen, without which their metabolic vm CONDITIONS OF LIFE 93 processes could not go on, from some of the compounds contained in the fluid in which they Hve. Bacteria are for this reason divided into aerobic species which require free oxygen, and anaerobic species which do not. As to temperature, common observation tells us that Bacteria flourish only within certain limits. We know for instance that organic substances can be preserved from putrefaction by being kept either at the freezing-point, or at or near the boiling-point. One important branch of modern industry, the trade in frozen meat, depends upon the fact that the putrefactive Bacteria, like other organisms, are rendered inactive by freezing, and every housekeeper knows how easily putrefaction can be staved off by roasting or boiling. Simi- larly it is a matter of common observation that a moderately high temperature is advantageous to these organisms, the heat of summer or of the tropics being notoriously favourable to putrefaction. In the case of Bacterium ternio, it has been found that the optimum temperature is from 30° to 35" C , but that the microbe will flourish between 5° and 40° C. Although fully-formed Bacteria, like other organisms, are usually killed by exposure to heat several degrees below boiling-point, yet the spores of some species will withstand, at any rate for a limited time, a much higher temperature — even one as high as 130° C. On the other hand, putrefactive Bacteria retain their power of development after being exposed to a temperature of-iri° C, although during the time of exposure all vital activity is of course suspended. Bacteria also resemble other organisms in being unable to carry on active life without a due supply of water : no perfectly dry substance ever putrefies. The preservation for ages of the dried bodies of animals in such countries as Egypt and Peru depends at least as much upon the moisture- less air as upon the antiseptics used in embalming. 94 BACTERIA less, vni For the most part Bacteria are unaffected by light, since they grow equally well in darkness and in ordinary daylight. Many of them, however, will not bear prolonged exposure to direct sunlight, and it has been found possible to arrest the putrefaction of an organic infusion by insolation, or exposure to the direct action of the sun's rays. It has also been proved that it is the light-rays and not the heat-rays which are thus prejudicial to the life of micro organisms. LESSON IX BIOGENESIS AND AEIOGENESIS : HOMOGENESIS AND HETERO- GENESIS The study of the foregoing living things and especially of Bacteria, the smallest and probably the simplest of all known organisms, naturally leads us to the consideration of one of the most important problems of biology — the problem of the origin of life. In all the higher organisms we know that each individual arises in some way or other from a pre-existing individual : no one doubts that every bird now living arose by a process of development froin an egg formed in the body of a parent bird, and that every tree now growing took its origin either from a seed or from a bud produced by a parent plant, But there have always — until quite recently, at any rate — been upholders of the view that the lower forms of life, bacteria, monads, and the like, may under certain circum- stances originate independently of pre-existing organisms . that, for instance, in a flask of hay-infusion or mutton-broth, boiled so as to kill any living things present in it, fresh forms of life may arise de novo, may in fact be created then and there. We have therefore two theories of the origin of the lower 96 BIOGENESIS AND IIOMOGENESIS less. organisms, the theory of Biogenesis, according to which each living thing, however simple, arises by a natural process of budding, fission, spore-formation, or what not, from a parent organism : and the theory of Ahiogenesis, or as it is some- times called Spontaneous or Equivocal Generation, accord- ing to which fully formed living organisms sometimes arise from not-living matter. In former times the occurrence of ahiogenesis was uni- versally believed in. The expression that a piece of meat has " bred maggots " ; the opinion that parasites such as the gall-insects of plants or the tape-worms in the intestines of animals originate where they are found ; the belief still held in some rural districts in the occurrence of showers of frogs, or in the transformation of horse-hairs kept in water into eels ; all indicate a survival of this belief. Aristotle, one of the greatest men of science of antiquit)', explicitly teaches ahiogenesis. He states that some animals "spring from putrid matter," that certain insects " spring from the dew which falls upon plants," that thread-worms " originate in the mud of wells and running waters,'' that fleas " originate in very small portions of corrupted matter,'' and that " bugs proceed from the moisture which collects on the bodies of animals, lice from the flesh of other creatures." Little more than 200 years ago one Alexander Ross, commenting on Sir Thomas Browne's doubt as to " whether mice may be bred by putrefaction," says, " so may he doubt whether in cheese and timber worms are generated ; or if beetles and wasps in cow's dung ; or if butterflies, locusts, grasshoppers, shell-fish, snails, eels, and such like, be pro- created of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense, and experience. IX PROBLEM LIMITED TO MICROSCOPIC FORMS 97 If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants." As accurate inquiries into these matters were made, the number of cases in which equivocal generation was sup- posed to occur was rapidly diminished. It was a simple matter — when once thought of — to prove, as Redi did in 1638, that no maggots were ever "bred" in meat on which flies were prevented by wire screens from laying their eggs. Far more difficult was the task, also begun in the seventeenth century, of proving that parasites, such as tape-worms, arise from eggs taken in with the food ; but gradually this pro- position was firmly estabhshed, so that no one of any scientific culture continued to believe in the abiogenetic origin of the more highly organized animals any more than in showers of frogs, or in the origin of geese from barnacles. But a new phase of the question was opened with the in- vention of the microscope. In 1683, Anthony van Leeuwen- hoek discovered Bacteria, and it was soon found that however carefully meat might be protected by screens, or infusions by being placed in well-corked or stoppered bottles, putrefaction always set in sooner or later, and was invariably accom- panied by the development of myriads of bacteria, monads, and other low organisms. It was not surprising, considering the rapidity with which these were found to make their appearance, that many men of science imagined them to be produced abiogenetically. Let us consider exactly what this implies. Suppose we have a vessel of hay-infusion, and in it a single Bacterium. The microbe will absorb the nutrient fluid and convert it into fresh protoplasm : it will divide repeatedly, and, its progeny repeating the process, the vessel will soon con- H gS BIOGENESIS AND HOMOGENESIS less. tain millions of Bacteria instead of one. This means, of course, that a certain amount of fresh living protoplasm has been formed out of the constituents of the hay-infusion, through the agency, in the first instance, of a single living Bacterium. The question naturally arises. Why may not the formation of protoplasm take place independently of this insignificant speck of living matter ? It must not be thought that this question is in any way a vain or absurd one. That living protoplasm has at some period of the world's history originated from not-living matter seems a necessary corollary of the doctrine of evolution, and is obviously the very essence of the doctrine of special creation ; and there is no a priori reason why it should be impossible to imitate the unknown conditions under which the process took place. At present, however, we have absolutely no data towards the solution of this fundamental problem. But however insoluble may be the question as to how life first dawned upon our planet, the origin of living things at the present day is capable of investigation in the ordinary way of observation and experiment. The problem may be stated as follows : — Any putrescible infusion — i.e. any fluid capable of putrefaction — will be found after a longer or shorter exposure to swarm with bacteria and monads : do these organisms, or the spores from which they first arise, reach the infusion from without, or are they generated within it ? And the general lines upon which an investigation into the problem must be conducted are simple : given a vessel of any putrescible infusion ; let this be subjected to some process which, without rendering it incapable of sup- porting life, shall kill any living things contained in it ; and let it then be placed under such circumstances that no living particles, however small, can reach it from without. If, IX EXPERIMENTS ON BIOGENESIS 99 after these two conditions have been rigorously compHed with, living organisms appear in the fluid, such organisms must have originated abiogenetically. To kill any microbes contained in the fluid it is usually quite sufficient to boil it thoroughly. As we have seen, protoplasm enters into heat-rigor at a temperature consider- ably below the boiling-point of water, so that, with an exception which will be referred to presently, a few minutes' boiling suffices to sterilize all ordinary infusions, i.e., to kill any organisms they may contain. Then as to preventing the entrance of organisms or their spores from without. This may be done in various ways. One way is to take a flask with the neck drawn out into a very slender tube, to boil the fluid in it for a sufficient time, and then, while ebullition is going on, to close the end of the tube by melting the glass in the flame of a Bunsen-burner or spirit-lamp, thus hermetically sealing the flask. By this method not only organisms and their spores are excluded from the flask but also air. But this is obviously unnecessary : it is evident that air may be admitted to the fluid with perfect impunity if only it can be filtered, that is, passed through some substance which shall retain all solid particles however small, and therefore of course bacteria, monads, and their spores. A perfectly efficient filter for this purpose is furnished by cotton-wool. A flask or test-tube is partly filled with the infusion : the latter is boiled, and during ebullition cotton- wool is pushed into the mouth of the vessel until a long and firm plug is formed (Fig. 19). When the source of heat is removed, and, by the coohng of the fluid, the steam which filled the upper part of the tube condenses, air passes in to supply its place, but as it does so it is filtered of even the H 2 100 BIOGENESIS AND IIOMOGENESIS less smallest solid particles by having to i^ass through the close meshes of the cotton-wool. Experiments of this sort conducted ^Yith projicr care have been known for many years to give negative results in the great majority of cases the fluids remain perfectly sterile for any length of time. But in certain instances, in spite of the most careful precautions, bacteria were found to appear Fig. 19. — A Beaker with a number of test-tubes containing putres- cible infusions and plugged with cotton-wool. (From Klein.) in such fluids ; and for years a fierce controversy raged between the biogenists and the abiogenists, the latter in sisting that the experiments in question proved the occurrence of spontaneous generation, while the biogenists considered that all such cases were due to defective methods — either to iiriperfect sterilization of the fluid or to imperfect exclusion of germ-containing atmospheric dust. The matter was finally set at rest, and the biogenists IX EXPERIMENTS ON BIOGENESIS loi proved to be in the right, by the important discovery that the spores of bacteria and monads are not killed by a tem- perature many degrees higher than is sufficient to destroy the adult forms : that in fact while the fully developed organisms are killed by a few minutes' exposure to a temperature of 70° C. the spores are frequently able to survive several hours' boiling, and must be heated to 130° — 150= C. in order that their destruction may be assured. It was also, shown that the more thoroughly the spores are dried the more difficult they are to kill, just as well-dried peas are hardly affected by an amount of boiling sufficient to reduce fresh ones to a pulp. This discovery of the high thermal death-point or ultra- maximum temperature of the spores of these organisms has necessitated certain additional precautions in experiments with putrescible infusions. In the first place the flask and the cotton-wool should both be heated in an oven to a temperature of 150° C, and thus effectually sterilized. The flask being filled and plugged with cotton-wool is well boiled, and is then kept for some hours at a temperature of 32° — 38° C, the optimum temperature for bacteria. The object of this is to allow any spores which have not been killed by boiling to germinate, in other words to pass into the adult condition in which the temperature of boiling water is fatal. The infusion is then boiled again, so as to destroy any such freshly germinated forms it may contain. The same process is repeated once or twice, the final result being that the very driest and most indurated spores are induced to ger- minate, and are thereupon slain. It must not be forgotten that repeated boiling does not render the fluid incapable of supporting life, as may be seen by removing the cotton-wool plug, when it will in a short time swarm with microbes. Experiments conducted with these precautions .all tell the I02 BIOGENESIS AND HOMOGENESIS less. same tale : they prove conclusively that in properly sterilized putrescible infusions, adequately protected from the entrance of atmospheric germs, no micro-organisms ever make their appearance. So that the last argument for abiogenesis has been proved to be fallacious, and the doctrine of biogenesis shown, as conclusively as observation and experiment can show it, to be of universal application as far as existing conditions known to us are concerned. It is also necessary to add that the presence of microbes in considerable quantities in our atmosphere has been proved experimentally. By drawing air through tubes lined with a solid nutrient material Prof. Percy Frankland showed that the air of South Kensington contains about thirty-five micro-organisms in every ten litres, and by ex- posing circular discs coated with the same substance he was further able to prove that in the same locality 279 micro- organisms fall upon one square foot of surface in one minute. There is another question intimately connected with that of Biogenesis, although strictly speaking quite independent of it. It is a matter of common observation that, in both animals and plants, like produces like : that a cutting from a willow will never give rise to an oak, nor a snake emerge from a hen's egg. In other words, ordinary observation teaches the general truth of the doctrine of Homoge7iesis. But there has always been a residuum of belief in the opposite doctrine of Jleterogettesis, according to which the offspring of a given animal or plant may be something utterly different from itself, a plant giving rise to an animal or vice ve?-sd, a lowly to a highly organised plant or animal and so on. Perhaps the most extreme case in which hetero- genesis was once seriously believed to occur is that of IX HETEROGENESIS 103 the " barnacle-geese." Buds of a particular tree growing near the sea were said to produce barnacles, and these falling into the water to develop into geese. This sounds absurd enough, but, within the last twenty years, two or three men of science have described, as the result of repeated observations, the occurrence of quite similar cases among microscopic organisms. For instance, the blood-corpuscles of the silkworm have been said to give rise to fungi, the protoplasm of the green weed Nitella (see Fig. 45) to Amcebse and Infusoria (see p. 107), Euglenae to thread- worms, and so on. It is proverbially difficult to prove a negative, and it might not be easy to demonstrate, what all competent naturalists must be firmly convinced of, that every one of these sup- posed cases of heterogenesis is founded either upon errors of observation or upon faulty inductions from correct observations. Let us take a particular case by way of example. Many years ago Dr. Dallinger observed among a number of Vorti- cellae or bell-animalcules (Fig. 25) one which appeared to have become encysted upon its stalk. After watching it for some time, there was seen to emerge from the cyst a free- swimming ciliated Infusor called Amphileptus, not unlike a long-necked Paramoecium (Fig. 20, p. 108). Many ob- servers would have put this down as a clear case of hetero- genesis : Dallinger simply recorded the observation and waited. Two years later the occurrence was explained : he found the same two species in a pond, and watched an Amphileptus seize and devour a Vorticella, and, after finish- ing its meal, become encysted upon the stalk of its victim. It is obvious that the only way in which a case of hetero- genesis could be proved would be by actually watching the transformation, and this no heterogenist has ever done ; at 104 BIOGENESIS AND HOMOGENESIS less. the most, certain supposed intermediate stages between the extreme forms have been observed — say, between a Euglena and a thread-worm — and the rest of the process inferred. On the other hand, innumerable observations have been made on these and other organisms, the result being that each species investigated has been found to go through a dehnite series of changes in the course of its development, the ultimate result being invariably an organism resembling in all essential respects that which formed the starting-point of the observations : Euglens always giving rise to EuglenEe and nothing else. Bacteria to Bacteria and nothing else, and so on. There are many classes which imperfect knowledge might class under heterogenesis, such as the origin of frogs from tadpoles or of jelly-fishes from polypes (Lesson XXI. Fig. 53), but in these and many other cases the apparently anomalous transformations have been found to be part of the normal and invariable cycle of changes undergone by the organism in the course of its development ; the frog always gives rise ultimately to a frog, the jelly-fish to a jelly- fish. If a frog at one time produced a tadpole, at another a trout, at another a worm : if jelly-fishes gave rise sometimes to polypes, sometimes to infusoria, sometimes to cuttle- fishes, and all without any regular sequence — that would be heterogenesis. It is perhaps hardly necessary to caution the reader against the error that there is any connection between the theory of heterogenesis and that of organic evolution. It might be said — if, as naturalists tell us, dogs are descended from wolves and jackals and birds from reptiles, why should not, for instance, thread-worms spring from EuglenEe or Infusoria from Bacteria ? To this it is sufficient to answer that the evolution of one form from another takes place by a series IX HETEROGENESIS 105 of slow, orderly, progressive changes going on through a long series of generations (see Lesson XIII.) ; whereas heterogenesis presupposes the casual occurrence of sudden transformations in any direction — i.e., leading to either a less or a more highly organized form — and in the course of a single generation. LESSON X PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA It will have been noticed with regard to the simple uni- cellular organisms hitherto considered that all are not equally simple : that Protamoeba (Fig. 2, p. 9) and Micrococcus (Fig. 15, p. 86) may be considered as the lowest of all, and that the others are raised above these forms in the scale of being in virtue of the possession of nucleus or contractile vacuole, or of flagella, or even, as in the case of Euglena (Fig. 5, p. 45), of a mouth or gullet. Thus we may speak of any of the organisms already studied as relatively " high " or " low " with regard to the rest : the lowest or least differentiated forms being those which approach most nearly to the simplest conception of a living thing — a mere lump of protoplasm : the highest or most differentiated those in which the greatest complication of structure has been attained. It must be remembered, too, that this increase in structural complexity is always accompanied by some degree of division of physiological labour, or, in other words, that morphological and physio- logical differentiation go hand in hand. We have now to consider certain organisms in which this differentiation has gone much further ; which have, in fact. LESS. X GENERAL CHARACTERS 107 acquired many of the characteristics of the higher animals and plants while remaining unicellular. The study of several of these more or less highly differentiated though unicellular forms will occupy the next seven Lessons. It was mentioned above that, in the earlier stages of the putrefaction of an organic infusion, bacteria only were found, and that later, monads made their appearance. Still later organisms much larger than monads are seen, generally of an ovoidal form, moving about very quickly, and seen by the use of a high power to be covered with innumerable fine cilia. These are called ciliate Infusoria^ in contradistinction to monads, which are often known as flagellate Infusoria : many kinds are common in putrefying infusions, some occur in the intestines of the higher animals, while others are among the commonest inhabitants of both fresh and salt water. Five genera of these Infusoria will form the subjects of this and the four following Lessons. A very common ciliate infusor is the beautiful " slipper animalcule," Parammcium, which from its comparatively large size and from the ease with which all essential points of its organization can be made out is a very convenient and interesting object of study. Compared with the majority of the organisms which have come under our notice it may fairly be considered as gigantic, being no less than \ — \ mm. (200 — 26o;u.) in length : in fact it is just visible to the naked eye as a minute whitish speck . Its form (Fig. 20 a) can be fairly well imitated by making out of clay or stiff dough an elongated cylinder rounded at one end and bluntly pointed at the other ; then giving the broader end a slight twist ; and finally making on the side ^UC. f/V Fjg. 20. — Parama-duiu caudatuin. A, the living animal from the ventral aspect, showing the covering of cilia, the buccal groove (to the right) ending posteriorly in the mouth LESS. X MOVEMENTS rog - l^mlh) and gullet [gul) ; several food vacuoles (/ vac), and the two contractile vacuoles (c. vac). B, the same in optical sections showing cuticle [cu), cortex [cort), and medulla (?necl) ; buccal groove (buc. gr), mouth, and gullet (gul) ■; numerous food vacuoles (f. vac) circulating in the direction indicated by the arrows, and containing particles of indigo, which are finally ejected at an anal spot ; meganucleus (mt), micronucleus (pa. nu), and trichocysts, some of which (trch) are shown with their threads ejected. The scale to the right of this figure applies to A and B. C, a specimen killed with osmic acid, showing the ejection of tricho- cyst-threads, which project considerably beyond the cilia. D, diagram of binary fission : the micronucleus (pa. mi) has already divided, the nucleus (nu) is in the act of dividing. (d, after Lankester. ) rendered somewhat concave by the tvcist a wide shallow groove beginning at the broad end and gradually narrowing to about the middle of the body, where it ends in a tolerably deep depression. The groove is called the buccal groove (Fig. 20, a & b, buc. gr) : at the narrow end is a small aperture, the mouth {mth), which, like the mouth of Euglena (Fig. 5), leads into the soft internal protoplasm of the body. The surface of the creature on which the groove is placed is distinguished as the ventral surface, the opposite surface being upper or dorsal ; the broad end is anterior, the narrow end posterior, the former being directed forwards as the animalcule swims. These descriptive terms being decided upon, it will be seen from Fig. 20 a, that the buccal groove begins on the left side of the body, and gradually curves over to the middle of the ventral surface. As the animal swims its form is seen to be permanent exhibiting no contractions of either an amoeboid or a euglenoid nature. It is however distinctly flexible, often being bent in one or other direction when passing between obstacles such as entangled masses of weed. This perma- nence of contour is due to the presence of a tolerably firm though dehcate cuticle {cu) which invests the whole surface. no PARAMQECIUM, STYLONYCHIA, OXYTRICHA less. The protoplasm thus enclosed by the cuticle is distinctly divisible into two portions — an external somewhat dense layer, the cortical layer or cortex {cort), and an internal more fluid material, the medullary substance or medulla {med). It will be remembered that a somewhat similar distinction of the protoplasm into two layers is exhibited by Amoeba (p. 3), the ectosarc being distinguished from the endosarc simply by the absence of granules. In Paramoecium the distinction is a far more fundamental one : the cortex is radially striated and is comparatively firm and dense, while the medulla is granular and semi-fluid, as may be seen from the fact that food particles (/ vac, see below, p. 112) move freely in it, whereas they never pass into the cortex. The medulla has a reticular structure similar to that of the protoplasm of the ordinary animal cell (Fig. 9, p. 62), consisting of a delicate granular network the meshes of which are filled with a trans- parent material. In the cortex the meshes of the network are closer, and so form a comparatively dense substance. The cortex also exhibits a superficial oblique striation, form- ing what is called the myophan layer. The mouth {mth) leads into a short funnel-like tube, the gullet {gul), which is lined by cuticle and passes through the cortex to end in the soft medulla, thus making a free com- munication between the latter and the external water. The cilia with which the body is covered are of approxi- mately equal size, quite short in relation to the entire animal, and arranged in longitudinal rows over the whole outer surface. They consist of prolongations of the cortex, and each passes through a minute perforation in the cuticle. They are in constant rhythmical movement, and are thereby distinguished from the flagella of Hsematococcus, Euglena, &c., which exhibit more or less intermittent lashing move- ments (see p. 25, note, and p. 59). Their rapid motion and X CONTRACTILE VACUOLES in minute size make them somewhat difficult to see wliile the Paramoecium is alive and active, but after death they are very obvious, and look quite like a thick covering of fine silky hairs. Near the middle of the body, in the cortex, is a large oval nucleus (B, nu), which is peculiar in taking on a uniform tint when stained, showing none of the distinction into chroma- tin and nuclear sap which is so marked a feature in many of the nuclei we have studied (see especially Fig. i, p. 2, and Fig. 9, p. 62). It has also a further pecuharity: against one side of it in J^. caudatum is a small oval structure {pa. nu) which is also deeply stained by magenta or carmine. This is the micronudevs : it is to be considered as a second, smaller nucleus, the larger body being distinguished as the meganudeus. In the closely allied P. aurelia, there are \yecic^ of ZonUi.amninm. A, cUcholominn. c, '/.. siiiipley. n, Z. nfline. Saville Kent.) 7. nlltrvans . H, Z. r:, Z. nutans. (After cula may be compared to a standard fruit tree, Z. alternans to an espalier. In this species also the colony is dimorphic. Z. diciiotfliiiuiii (Fig. 27, b) is also dimorphic and presents a third mode of branching. The main stem divides into two, and each of tlie secondary branches does the same, so that a repeatedly forking stem is jjroduced. The branching of this species is said to be diclioioiiwiis, while that of Z. alter- nans is vionopodial, and that of Z. arbuscula iniihi'llaii. Another mode of aggregation of the zooids is found in Z. simplex {Y\g. 27, c), in which the stem is unbranched and 'xm GENUS AND SPECIES 139 bears at its distal end about six zooids in a cluster. The zooids are more elongated than in any of the preceding species, and there are no special reproductive individuals, so that the colony is homomorphic. In Z. affine (Fig. 27, d) the stalk is dichotomous but is proportionally thicker than in the preceding species, and bears about four zooids, all alike. It is found in fresh water attached to insects and other aquatic animals. The last species we shall consider is Z. nutans (Fig. 27, e), which is the simplest known, never bearing more than two zooids, and sometimes only one. A glance at Figs. 26 and 27 will show that these six species agree with one another in the general form of the zooids, in the characters of the nucleus, contractile vacuole, &c., in the arrangement of the cilia, and in the fact that they are all compound organisms, consisting of two or more zooids attached to a common stem, the axial fibre of which branches with it, i.e., is continuous throughout the colony. On account of their possessing these important characters in common, the species described are placed in the single genus Zoothamnium, and the characters summarized in the preceding paragraph are called generic characters. On the other hand the points of difference between the various species, such as the forking of the stem in Z. dichotomum, the presence of only two zooids in Z. nutans, and so on, are called specific characters. Similarly the name Zoothamnium, which is common to all the species, is the generic name, while those which are applied only to a particular species, such as arbuscuta, simplex, &c., are the specific names. As was mentioned in the first lesson (p. 8), this method of naming organisms is known as the Linnean system of binomial nomenclature. It will be seen from the foregoing account that by a I40 SPECIES AXD THEIR ORIGIN less. species we understand an assemblage of individual or- ganisms, whether simple or compound, which agree with one another in all but unessential points, such as the precise number of zooids in Zoothamnium, which may vary con- siderably in the same species, and come, therefore, within the limits of individual variation. Similarly, what we mean by a genus is a group of species agreeing with one another in the broad features of their organization, but differing in detail, the differences being constant. A comparison of the six species described brings out several interesting relations between them. For instance, it is clear that Z. arbuscula and Z. alternans are far more complex, i.e., exhibit greater differentiation of the entire colony, than Z. simplex, or Z. nutans ; so that, within the limits of the one genus, we have comparatively low or generalized, and comparatively high or specialized species. Nevertheless, a little consideration will show that we cannot arrange the species in a single series, beginning with the lowest and ending with the highest, for, although we should have no hesitation in placing Z. nutans at the bottom of such a list, it would be impossible to say whether Z. afifine was higher or lower than Z. simplex, or Z. arbuscula than Z. alternans. It is, however, easy to arrange the species into groups according to some definite system. For instance, if we take the mode of branching as a criterion, Z. nutans, affine, and dichotomum will all be placed together as being dichoto- mous, and Z. simplex and arbuscula as being umbellate — the zooids of the one and the branches of the other all springing together from the top of the main stem : on this system Z. alternans will stand alone on account of its mono- podial branching. Or, we may make two groups, one of dimorphic forms, including Z. arbuscula, alternans, and xm CREATION AND EVOLUTION 141 dichotomum, and another of homomorphic species, including Z. affine, simplex, and nutans. We have thus two very obvious ways of arranging or classifying the species of Zoothamnium, and the question arises — which of these, if either, is the right one ? Is there any standard by which we can judge of the accuracy of a given classification of these or any other organisms, or does the whole thing depend upon the fancy of the classifier, like the arrangement of books in a hbrary ? In other words, are all possible classi- fications of living things more or less artificial, or is there such a thing as a natural classification ? Suppose we were to try and classify all the members of a given family — parents and grandparents, uncles and aunts, cousins, second cousins, and so on. Obviously there are a hundred ways in which it would be possible to arrange them — into dark and fair, tall and short, curly-haired and straight-haired and so on. But is is equally obvious that all these methods would be purely artificial, and that the only natural way, i.e., the only way to show the real connection of the \'arious members of the family with one another would be to classify them according to blood-relationship, in other words to let our classification take the form of a genea- logical tree. It may be said — what has this to do with the point under discussion, the classification of the species of Zoothamnium ? There are two theories which attempt to account for the existence of the innumerable species of living things which inhabit our earth : the theory of creation and the theory of evolution. According to the theory of creation, all the individuals of every species existing at the present day— the tens of thousands of dogs, oak trees, amoebae, and what not — are derived by a natural process of descent from a single indi- 142 SPECIES AND THEIR ORIGIN vidua], or from a pair of individuals — in each case precisely resembling, in all essential respects, their existing descend- ants — which came into existence by a process outside the ordinary course of nature and known as Creation. On this hypothesis the history of the genus Zoothamnium would be represented by the diagram (Fig. 28) ; each of the species being derived from a single individual which came into Existing Individuals Z.arbuscula Z.attenians Z.dichoiomum Z. simplex Z.affine 0666 e Ancestral Individuals Fig. 28. — Diagram illustrating the origin of the species of Zoothamnium by creation. existence, independently of the progenitors of all the other species, at some distant period of the earth's history. Notice that on this theory the various species are no more actually related to one another than is either of them to "Vorticella, or for the matter of that to Homo. The in- dividuals of any one species are truly related since they all share a common descent, but there is no more relationship between the individuals of any two independently created species than between any two independently manufactured XII! EVOLUTION 143 chairs or tables. The words affinity, relationship, &c., as applied to different species are, on the theory of creation, purely metaphorical, and mean nothing more than that a certain likeness or community of structure exists ; just as we might say that an easy chair was more nearly related to a kitchen chair than either of them to a three-legged stool. We see therefore that on the hypothesis of creation the varying degrees of likeness and unlikeness between the species receive no explanation, and that we get no absolute criterion of classification : we may arrange our organisms, as nearly as our knowledge allows, according to their resem- blances and differences, but the relative importance of the characters relied on becomes a purely subjective matter. According to the rival theory — that of Descent or Organic Evolution — every species existing at the present day is derived by a natural process of descent from some other species which lived at a former period of the world's history. If we could trace back from generation to gener- ation the individuals of any existing species we should, on this hypothesis, find their characters gradually change, until finally a period was reached at which the differences were so considerable as to necessitate the placing of the ancestral forms in a different species from their descendants at the present day. And in the same way if we could trace back the species of any one genus, we should find them gradually approach one another in structure until they finally con- verged in a single species, differing from those now existing but standing to all in a true parental relation. Let us illustrate this by reference to Zoothamnium. As a matter of fact we know nothing of the history of the genus, but the comprehension of what is meant by the evolution of species will be greatly facilitated by framing a working hypothesis. Suppose that at some distant period of the world's history 144 SPECIES AND THEIR ORIGIN there existed a Vorticella-like organisjii which we will call A (Fig. 29), having the general characters of a single, stalked zooid of Zoothamnium (compare Fig. 26, F-), and suppose that, of the numerous descendants of this form, represented by the hnes diverging from a, there were some in which both the zooids formed by the longitudinal division of the body remained attached to the stalk instead of one of them swimming off as in Vorticella. The result — it matters Braiichbig dichotonioits JSraiichuig Titcnopociiai DIMORPHIC HOilOMORPHIC Fig. 29. — Diagram illustrating the origin of the species or Zoothamnium by evolution. not for our present purpose how it may have been caused — would be a simple colonial organism consisting of two zooids attached to the end of a single undivided stalk. Let us call this form b. Next let us imagine that in some of the descendants of e, represented as before by the diverging lines, the plane of division was continued downwards so as to include the distal end of the stalk : this would result in the production XIII DIVERGENCE OF CHARACTER 145 of a form (c) consisting of two zooids borne on a forked stem and resembling Z. nutans. If, in some of the descend- ants of c, this process were repeated, each of the two zooids again dividing into two fixed individuals and the division as before affecting the stem, we should get a species (d) con- sisting of four zooids on a dichotomous stem, like Z. aflfine. Let the same process continue from generation to genera- tion, the colony becoming more and more complex ; we should finally arrive at a species e, consisting of numerous zooids on a complicated dichotomously branching stem, and therefore resembling Z. dichotomum. Let us further suppose that, in some of the descendants of our hypothetical form b, repeated binary fission took i place without affecting the stem : the result would be a new form F, consisting of numerous zooids springing in a cluster from the end of the undivided stem, after the manner of Z. simplex. From this a more complicated umbellate form (g), like Z. arbuscula, may be supposed to have originated, and again starting from b with a different mode of branch- ing a monopodial form (h) might have arisen. Finally, let it be assumed that while some of the descend- ants of the forms c, D, and f became modified into more and more complex species, others survived to the present time with comparatively little change, forming the existing species nutans, affine, and simplex : and that, in the similarly surviving representatives of e, g, and h, a differentiation of the individual zooids took place resulting in the evolution of the dimorphic species dichotomum, arbuscula, and alternans. It will be seen that, on this hypothesis, the relative like- ness and unlikeness of the species of Zoothamnium are explained as the result of their descent with greater or less modification or divergence of character from the ancestral form A : and that we get an arrangement or classification L 146 SPECIES AND THEIR ORIGIN LESS, in the form of a genealogical tree, which, on the hypothesis, is a strictly natural one, since it shows accurately the relationship of the various species to one another and to the parent stock. So that, on the theory of evolution, a natural classification of any given group of allied organisms is simply a genealogical tree, or, as it is usually called, a phylogeny. It must not be forgotten that the forms a, b, c, d, e, f, g, and H are purely hypothetical : their existence has been assumed in order to illustrate the doctrine of descent by a concrete example. The only way in which we could be perfectly sure of an absolutely natural classification of the species of Zoothamnium would be by obtaining specimens as far back as the distant period when the genus first came into existence ; and this is out of the question, since minute soft-bodied organisms like these have no chance of being preserved in the fossil state. It will be seen that the theory of evolution has the advantage over that of creation of offering a reasonable explanation of certain facts. First of all the varying degrees of likeness and unlikeness of the species are explained by their having branched off from one another at various periods : for instance, the greater similarity of structure between Z. affine and Z. dichotomum than between either of them and any other species is due to these two species having a common ancestor in d, whereas to connect either of them, say with Z. arbuscula, we have to go back to b. Then again the fact that all the species, however complex in their fully developed state, begin life as a simple zooid which by repeated branching gradually attains the adult complexity, is a result of the repetition by each organism, in the course of its single life, of the series of changes passed through by its ancestors in the course of ages. In other words ontogeny^ XIII HEREDITV AND VARIABILITY 147 or the evolution of the individual, is, in its main features, a recapitulation oi phylogeny or the evolution of the race. One other matter must be referred to in concluding the present lesson. It is obvious that the evolution of one species from another presupposes the occurrence of varia- tions in the ancestral form. As a matter of fact such individual variation is of universal occurrence : it is a matter of common observation that no two leaves, shells, or human beings are precisely alike, and in our type genus Zootham- nium the number of zooids, their precise arrangement, the details of branching, &c., are all variables. This may be expressed by saying that heredity, according to which the offspring tends to resemble the parent in essentials, is modified by variability, according to which the offspring tends to differ from the parent in details. If from any cause an individual variation is perpetuated there is produced what is known as a variety of the species, and, according to the theory of the origin of species by evolution, such a variety may in course of time become a new species. Thus a variety is an incipient species, and a species is a (relatively) permanent variety. It does not come within the scope of the present work to discuss either the causes of variability or those which deter- mine the elevation of a variety to the rank of a species : both questions are far too complex to be adequately treated except at considerable length, and anything of the nature of a brief abstract could only be misleading. As a preliminary to the study of Darwin's Origin of Species, the student is recommended to read Romanes's Evidences of Organic Evolution, in which the doctrine of Descent is expounded as briefly as is consistent with clearness and accuracy. L 2 LESSON XIV FORAMINIFERA, RADIOLARIA, AND DIATOMS In the four previous lessons we have learnt how a uni- cellular organism may attain very considerable complexity by a process of differentiation of its protoplasm. In the present lesson we shall consider briefly certain forms of life in which, while the protoplasm of the unicellular body un- dergoes comparatively little differentiation, an extraordinary variety and complexity of form is produced by the develop- ment of a skektoji, either in the shape of a hardened cell- wall or by the formation of hard parts within the protoplasm itself. The name Foraminifei-a is given to an extensive group of organisms which are very common in the sea, some living near the surface, others at various depths. They vary in size from a sand-grain to a shilling. They consist of variously shaped masses of protoplasm, containing nuclei, and pro- duced into numerous pseudopods, which are extremely long and delicate; and frequently unite with one another to form networks, as at X in Fig. 30. The cell-body of these organisms is therefore very simple, and may be compared to that of a multinucleate Amoeba with fine radiating pseudopods. THE SHELL 1-19 But what gives the Foraminifera their special character is the fact that around the protoplasm is developed a celhwall, sometimes membranous, but usually impregnated with cal- cium carbonate, and so forming a shell. In some cases, as in the genus Rotalia (Fig. 30), this is jjerforated by nume- rous small holes, through which the pseudopods are pro- truded, in others it has onl\- one large aperture (Fig. 31), Fig. 30. — A living Foramiiiiier (Kotalia), showing the fine radialijig pieudopods ]3assing thiougli apertures in the chambered shell : at :-; several of them have united. (From Gegenljaur. ) through which the protoplasm protrudes, sending off its pseudopjods and sometimes flowing over and covering the outer surface of the shell. Thus while in some cases the shell has just the relations of a cell-wall with one or more holes in it, in others it becomes an internal structure, bemg covered externally as well as filled internally by protojjlasm. The mode of growth of Foraminifera is largely determined by the hard and non-distensible character of the cell-wall. 15° FORAMINIFERA, RADIOLARIA, DIATOMS less. which wlien once fouiied is incapable of being enlarged. In the young condition they consist of a simple mass of proto- plasm covered by a more or less globular shell, having at least one aperture. But in most cases as the cell-body grows, it protrudes through the aperture of the shell as a mass of protoplasm, at first naked, but soon becoming CQ\'ered by the secretion around it of a second compartment or chamber of the shell. The latter now consists of two Fig. 31. — A, di.^gram of a F'ornminifer in which new chambers are atlded in a straight line : the smallest first-formed chamber is below, the newest and largest is above and communicates with the exterior. B, diagram of a Foraminifer in which tlie chambers are added in a flat spiral : the oldest and smallest chamber is in the centre, the newest and largest as before comminiicates with the exterior. (After Carpenter.) chambers comrnunicating with one another by a small aperture, and one of them — the last formed — communi- cating with tlie exterior. This process may go on almost indefinitely, the successive chambers always remaining in communication by small apertures through which continuity of the protoplasm is maintained, while the last formed chamber has a terminal aperture placing its protoplasm in free communication with the outer world. XIV COMPLEXITY OF SHELL 151 The new chambers may be added in a straight hne (Fig. 31, a) or in a gentle curve, or in a flat spiral (Fig. 31, b), or like the segments of a Nautilus shell, or more or less irregularly. In this way shells of great variety and beauty Fig. 32. — Section of one of the more complicateil Foianimifera (Alveolina), showing the numerous chambers containing; ]iroloijlasni (dotted), separated by partitions of the shell (white). :■; 60. (From Gegenbaur after Carpenter.) of form are produced, often resembling the shells of Mol- lusca, and soinetimes attaining a marvellous degree of com- ple.xity (Fig. 32). The student should make a point of examining mounted slides of some of the principal genera and of consulting the plates in Carpenter's Introduction to the Study of FoniJuinifiya (Ray Society, 1862), or in Brady's Report on the duirantinifera of the " Challenger^' Expedition, in order to get some notion of the great amount of dif- ferentiation attained by the shells of these extremely simple oriranisms. 152 FOKAMINIKKk.A, RADKJLAKIA, DIATOMS Lliss. The liadiolaria form another group of marine animal- cules, the numerous genera of which are, like the Foram- inifera, amongst the most beautiful of micioscopic objects. They also (Fig. 33) consist of a mass of protoplasm giving off numerous delicate jiseudopocls (['sd) whicli usually ha\-e a radial direction and sometimes unite to form networks. In the centre of the protoplasmic cell-body one or more nuclei (mi) of unusual size and complex structure are found. psd A' Sicel Fig. '}^'^. — Lithocircus annularis, one of the Radiolaria, showing central capbule {cent, caps.), intra- and extra capsular protoplasm {in/. caps. pi'. , tuV. caps, pi: ), nucleus [Jut), pseudopods {psd), silicious skeleton (sic/), and symbiotic cells of Zooxanlhclla (c). (After Liitschli. ) In tlie interior of the protoplastn, surrounding the nucleus, is a sort of shell, called the central capsule (eent. eaps.), formed of a membranous material, and perforated by pores which jilace the inclosed or intfa-eapsular protoplasm (////. caps, pr.) in communication with the surrounditig or e.v/ra- capsu/ar protop)lasiir (e.vt. caps. pr.). But besides this simple membranous shell there is often developed, mainly in the extra-capsular protoplasm, a skeleton (skel) formed in the majority of cases of lutre silica, and often of surpassing beauty and comple-\iiy. One \-er\' exquisite form is shown COMPLEXITY OF SHELL 153 in Fig. 34 ; it consists of three perforated concentric spheres connected by radiating spicules : the material of which it is composed resembles the clearest glass. The student should examine mounted slides of the silicious shells of these organisms — sold under the name of Poly- cvsfineic — and should consult the plates of Haeckel's Die Radiolaricjt . he cannot fail to be struck with the complexity Fig. 34. — Skeleton of a Radiolarian (Aihinviima), conbis.tiiii,' of three concentric perforated spheres— the two outer partly broken a«ay to show the inner— connected by radiating spicules. (From Gegenbaur after Hacckel.) and variety attained by the skeletons of organisms which are tliemselves little more complex than Amrebre. Before leaving the Radiolaria, we must toucli upon a matter of considerable interest connected with the physio- logy of the group. Imbedded usually in the extra-capsular 154 FORAMINIFERA, RADIOLARIA, DIATOMS less. protoplasm are found certain little rounded bodies of a yellow colour, often known as " yellow cells " (Fig. 33, z). Each consists of protoplasm surrounded by a cell-wall of cellulose, and coloured by chlorophyll, with which is asso- ciated a yellow pigment of similar character called diatomin. For a long time these bodies were a complete puzzle to biologists, but it has now been conclusively proved that they are-independent organisms resembling the resting condition of Hsematococcus, and called Zooxanthella ?iuiricola. Thus an ordinary Radiolarian, such as Lithocircus (Fig. 33), consists of two quite distinct things, the Lithocircus in the strict sense of the word plus large numbers of Zooxan- thelte associated with it. The two organisms multiply quite independently of one another : indeed Zooxanthella has been observed to multiply by fission after the death of the associated Radiolarian. This living together of two organisms is known as sym- biosis. It differs essentially from parasitism (see p. 123), in which one organism preys upon another, the host deriving no benefit but only harm from the presence of the parasite. In symbiosis, on the contrary, the two organisms are in a condition of mutually beneficial partnership. The carbon dioxide and nitrogenous waste given off by the Radiolarian serve as a constant food-supply to the Zooxanthella : at the same time the latter by decomposing the carbon dioxide provides the Radiolarian with a constant supply of oxygen, and at the same time with two important food-stuffs — starch andproteids — which, after solution, diffuse from the protoplasm of the Zooxanthella into that of the Radiolarian. The Radiolarian may therefore be said to keep the Zooxanthellse constantly manured, while the Zooxanthellse in return supply the Radiolarian with abundance of oxygen and of ready- digested food. It is as if a Hsmatococcus ingested by an XIV MOVEMENTS OF DIATOMS 155 Amceba retained its vitality instead of being digested : it would under these circumstances make use of the carbon dioxide and nitrogenous waste formed as products of kata- bolism by the Amceba, at the same time giving off oxygen and forming starch and proteids. The oxygen evolved would give an additional supply of this necessary gas to the Amceba, and the starch after conversion into sugar and the proteids after being rendered diffusible would in part diffuse through the cell-wall of the Hsematococcus into the surrounding protoplasm of the Amoeba, to which they would be a valuable food. Thus, as it has been said, the relation between a Radio- larian and its associated yellow-cells are precisely those which obtain between the animal and vegetable kingdoms generally. The Diatomacea or Diatoms, as they are often called for the sake of brevity, are a group of minute organisms, in- cluded under a very large number of genera and species, and so common that there is hardly a pond or stream in which they do not occur in millions Diatoms vary almost indefinitely in form : they may be rod- shaped, triangular, circular, and so on. Their essential structure is, however, very uniform : the cell-body contains a nucleus (Fig. 35, a, nu) and vacuoles {vac), as well as two large chromatophores {chr) of a brown or yellow colour; these are found to contain chlorophyll, the characteristic green tint of which is veiled, as in Zooxanthella, by diatomin. The cell is motile, executing curious, slow, jerky or ghding movements caused by the protrusion of dehcate threads of mucilage from between the valves of the cell-wall : the threads are shot out at intervals in a given direction, and, by the resistance of the water, the diatom is jerked in the opposite direction. .56 FORAMINIFERA, RADIOLAKI/Si, DIATOMS Lf:ss. The most interesting feature in the organisation of diatoms is however tlie structure of the cell-wall : it consists of two parts or valves (i;, c, c. a', c. iJ), each provided with a rim or -^*"^^'- 35-^^1 semi-(.liagrammcitic \'iew of ci diatom from iU flat face, showing cell-wall (f. iv) and protoplasm ^\ ilh nucleus {nu)^ t\\ o vacuoles (e'flt'), and two chromatophores [chr). B, diagram of the shell of a diatom from tlie side, i.e., turned on its long axis at right angles to A, sliowing the two valves (r, tc, i', tc') with then' o\erlapping girdles. c, the same in transverse section, ri, surface \de\v of the silicious shell eif Xariciila Intiuata. li, suiface view of the silicious shell of Anlaiodisiiis sollillianus. (D, after Donkin ; v., after Norman.) :^iydh', and so disposed that in the entire cell the girdle of one \'alve {i. w) fits o\er that of the other {c. 7e') like the lid of a i)ill-box. The cell-wall is imiiregnated with silica, so that diatoms can be boiled in strong acid or exposed to XIV MARKINGS OF DIATOMS 157 the heat of a flame without losing their form : the protoplasm is of course destroyed, but the flinty cell-wall remains uninjured. Moreover, the cell-walls 01 diatoms are remarkable for the beauty and complexity of their markings, which are in some cases so delicate that even now microscopists are not agreed as to the precise interpretation of the appearances shown by the highest powers of the microscope. Two species are shown in Fig. 35, d and e, but, in order to form some con- ception of the extraordinary variety in form and ornamenta- tion, specimens of the mounted cell-walls should be ex- amined and the plates of some illustrated work consulted. (See especially Schmidt's Atlas fiir Diatoinaceenkunde and the earlier volumes of the Quarterly Journal of ]\Jia-o- scopical Science. ) We see then that while Diatoms are in their essential structure as simple as Hsematococcus, they have the power of extracting sihca from the surrounding water, and of forming from it structures which rival in beauty of form and intricacy of pattern the best work of the metal-worker or the ivory-carver. LESSON XV M0COR The five preceding lessons have shown us how complex a cell may become either by internal differentiation of its protoplasm, or by differentiation of its cell-wall. In this and the following lesson we shall see how a considerable degree of specialization may be attained by the elongation of cells into filaments. Mucor is the scientific name of the common white or grey mould which every one is familiar with in the form of a cottony deposit on damp organic substances, such as leather, bread, jam, &c. For examination it is readily obtained by placing a piece of damp bread or some fresh horse-dung under an inverted tumbler or bell-jar so as to prevent evapo- ration and consequent drying. In the course of two or three days a number of delicate white filaments will be seen shooting out in all directions from the bread or manure ; these are filaments of Mucor. The species which grows on bread is called Mucor stolonifei; that on horse-dung, M. mucedo. The general structure and mode of growth of the mould can be readily made out with the naked eye. It first appears, as already stated, in the form of very fine white threads projecting from the surface of the mouldy substance ; and these free filaments (Fig. 36, a, a. hy) can be easily Fig. 36. — Aliicor. A, portion of mycelium of M. mucedo [my) with two aerial hyphaa a. hy), each ending in a sporangium [spg). B, small portion of an aerial hypha, highly magnified, showing pro- toplasm (flsm) and cell-wall (c. w). The scale above B applies to this figure only. c', immature sporangium, showing septum {sep) and undivided pro- toplasm : c^, mature sporangium in which the protoplasm has divided into spores ; the septum (sep) has become very convex distally, forming the columella. I)', mature sporangium in the act of dehiscence, showing the spores (sp) surrounded by mucilage [g) ; D^, small portion of the same, more highly magnified, showing spicules of calcium oxalate attached to wall. E, a columella, left by complete dehiscence of a sporangium, showing the attachment of the latter as a black band. The scale above c^ and d' applies to c^ c'-', d\ and E. i6o MUCOR LESS. F, spores. G^, G-, G^, three stages in the germination of the spores. H, a group of germinating spores forming a small mycelium. I* — r', five stages in conjugation, showing two gametes {gam) uniting to form the zygote {zyg). k\ k", development of ferment cells from submerged hyphee. (a, c^, d, e, f, g, and K, after Howes ; I, after De Bary.) ascertained to be connected with others (tny) \Yhich form a network ramifying through the substance of the bread or horse-dung. This network is called a mycelium ; the threads of which it is composed are mycelial hyphce ; and the fila- ments which grow out into the air and give the characteristic fluffy appearance to the growth are aerial hyphce. The aerial hyphae are somewhat thicker than those which form the mycelium, and are at first of even diameter through- out : they continue to grow until they attain a length, in M. mucedo, of 6-8 cm. (two or three inches). As they grow their ends are seen to become dilated, so that each is termi- nated by a minute knob (a, spg) : this increases in size and darkens in tint until it finally becomes dead black. In its earlier stages the knobs may be touched gently without injury, but when they have attained their full size the slightest touch causes them to burst and apparently to dis- appear — their actual fate being quite invisible to the naked eye. As we shall see, the black knobs contain spores, and are therefore called sporangia or spore-cases. Examined under the microscope, a hypha is found to be a delicate, more or less branched, tube, with a clear trans- parent wall (b, c. w) and slightly granular contents {plsm) : its free end tapers slightly (h), and the wall is somewhat thinner at the extremity than elsewhere. If a single hypha could be obtained whole and unbroken, its opposite end would be found to have much the same structure, and each of its branches would also be seen to end in the same way. XV ASEXUAL REPRODUCTION i6i So that the mould is simply an interlacement of branched cylindrical filaments, each consisting of a granular substance completely covered by a kind of thin skin of some clear transparent material. By the employment of the usual reagents, it can be ascer- tained that the granular substance is protoplasm, and the surrounding membrane cellulose. The protoplasm moreover contains vacuoles at irregular intervals and numerous small nuclei. Thus a hypha of Mucor consists of precisely the same constituents as a yeast-cell — protoplasm, containing nuclei and vacuoles, surrounded by cellulose. Imagine a yeast- cell to be pulled out — as one might pull out a sphere of clay or putty — until it assumed the form of a long narrow cyhn- der, and suppose it also to be pulled out laterally at intervals so as to form branches : there would be produced by such a process a very good imitation of a hypha of Mucor. We may therefore look upon a hypha as an elongated and branched cell, so that Mucor is, like Opalina, a multinucleate but non-cellular organism. We shall see directly, however, that this is strictly true of the mould only in its young state. As stated above, the aerial hyphs are at first of even calibre, but gradually swell at their ends, forming sporangia. Under the microscope the distal end of an aerial hypha is found to dilate (Fig. 36, c^) : immediately below the dilata- tion the protoplasm divides at right angles to the long axis of the hypha, the protoplasm in the dilated portion thus becoming separated from the rest. Between the two a cellulose partition or septum {sep) is formed, as in the ordi- nary division of a plant cell (Fig. 10, p. 64). The portion thus separated is the rudiment of a sporangium. Let us consider precisely what this process implies. Before it takes place the protoplasm is continuous throughout the M i62 MUCOR LESS. whole organism, \Yhich is therefore comparable to the un- divided plant-cell shown in Fig. 9, b. As in that case, the protoplasm divides into two and a new layer of cellulose is formed between the daughter-cells. Only, vifhereas in the ordinary vegetable cell the products of division are of equal size (Fig. 10, i), in Mucor they are very unequal, one being the comparatively small sporangium, the other the rest of the hypha. Thus a Mucor-plant with a single aerial hypha becomes, by the formation of a sporangium, bicellular : if, as is ordi- narily the case, it bears numerous aerial hyphse, each with its sporangium, it is multicellidar. Under unfavourable conditions of nutrition, septa fre- quently appear at more or less irregular intervals in the mycelial hyphte : the organism is then very obviously multi- cellular, being formed of numerous cylindrical cells arranged end to end. The sporangium continues to grow, and, as it does so, the septum becomes more and more convex upwards, finally taking the form of a short, club-shaped projection, the colu- mella, extending into the interior of the sporangium (c^) : at the same time the protoplasm of the sporangium under- goes multiple fission, becoming divided into numerous ovoid masses each of which surrounds itself with a cellulose coat and becomes a spore ij?- D-, sp). A certain amount of the protoplasm remains unused in the formation of spores, and is converted into a gelatinous material {g), which swells up in water. The original cell-wall of the sporangium is left as an exceedingly delicate, brittle shell around the spores : minute needle-like crystals of calcium oxalate are deposited in it, and give it the appearance of being closely covered with short cilia (d-). GERMINATION OF SPORES 163 In the ripe sporangium the slightest touch suffices to rupture the brittle wall and liberate the spores, which are dispersed by the swelling of the transparent intermediate substance. The aerial hypha is then left terminated by the columella (e), around the base of which is seen a narrow black ring indicating the place of attachment of the sporangium. The spores (f) are clear, bright-looking, ovoidal bodies consisting of protoplasm containing a nucleus and sur- ^■.^.^.,..,....^^^.,i EiG. 37. — Moist chamber formed by cementing a ring of glass or metal (c) on an ordinary glass slide (a), and placing over it a cover-slip (b), on the under side of which is a hanging drop of nutrient fluid (p). The upper figure shows the apj^aratus in perspective, the lower in vertical section. (From Klein.) rounded by a thick cell-wall. A spore is therefore an ordinary encysted cell, quite comparable to a yeast-cell. The development of the spores is a very instructive process, and can be easily studied in the following way : A glass or metal ring (Fig. 37, c) is cemented to an ordinary microscopic slide (a) so as to form a shallow cylindrical chamber. The top of the ring is oiled, and on it is placed a cover glass (b), with a drop of Pasteur's solution on its under surface. Before placing the cover-glass in position a ripe sporangium of Mucor is touched with the point of a needle, which is M 2 i64 MUCOR LESS. then stirred round in the drop of Pasteur's solution, so as to sow it with spores. By this method the drop of nutrient fluid is prevented from evaporating, and the changes under- gone by the spores can be watched by examination from time to time under a high power. The first thing that happens to a spore under these con- ditions is that it increases in size by imbibition of fluid, and instead of appearing bright and clear becomes granular and develops one or more vacuoles. Its resemblance to a yeast-cell is now more striking than ever. Next the spore becomes bulged out in one ormore places (g\ Fig. 36), looking not unlike a budding Saccharomyces. The buds, however, instead of becoming detached increase in length until they become filaments of a diameter slightly less than that of the spore and somewhat bluntly pointed at the end (g^). These filaments continue to grow, giving off as they do so side branches (g^) which interlace with similar threads from adjacent spores (h). The filaments are obviously hyphas, and the interlacement is a mycelium. Thus the statement made in a previous paragraph (p. 161), that Mucor is comparable to a yeast- cell pulled out into a filament, is seen to be fully justified by the facts of develop- ment, which show that the branched hyphas constituting the Mucor-plant are formed by the growth of spores each strictly comparable to a single Saccharomyces. It will be noticed that the growth of the mycelium is cen- trifugal : each spore or group of spores serves as a centre from which hyphte radiate in all directions (h), continuing to grow in a radial direction until, in place of one or more spores quite invisible to the naked eye, we have a white patch more or less circular in outline, and having the spores from which the growth proceeded in its centre. Owing to the centrifugal mode of growth the mycelium is always XV CONJUGATION 165 thicker at the centre than towards the circumference, since it is the older or more central portions of the hyphae which have had most time to branch and become interlaced with one another. Under certain circumstances a peculiar process of con- jugation occurs in Mucor. Two adjacent hyphte send out short branches (Fig. 36, i^), which come into contact, with one another by their somewhat swollen free ends (i^). In each a septum appears so as to shut off a separate terminal cell (i^, gam) from the rest of the hypha. .The opposed walls of the two cells then become absorbed (i*) and their contents mingle, forming a single mass of protoplasm (i^, zyg), the cell-wall of which becomes greatly thickened and divided into two layers, an inner dehcate and trans- parent, and an outer dark in colour, of considerable thick- ness, and frequently ornamented with spines. Obviously the swollen terminal cells {gani) of the short lateral hyphse are gametes or conjugating bodies, and the large spore-like structure {zyg) resulting from their union is a zygote. The striking feature of the process is that the gametes are non-motile, save in so far as their growth towards one another is a mode of motion. In Heteromita both gametes are active and free-swimming (p. 41) : in Vorticella one is free-swimming, the other fixed but still capable of active movement (p. 132); here both conjugating bodies exhibit only the slow movement in one direction due to growth. There are equally important differences in the result of the process in the three cases. In Heteromita the proto- plasm of the zygote breaks up almost immediately into spores ; in Vorticella the zygote is active, and the result of conjugation is merely increased activity in feeding and fissive l66 MUCOR r.Ess. multiplication ; in Mucor the zygote remains inactive for a longer or shorter time, and then under favourable conditions germinates in much the same way as an ordinary spore, forming a mycelium from which sporangium-bearing aerial hyphffi arise. A resting zygote of this kind, formed by the conjugation of equal-sized gametes, is often distinguished as a zygospore. Notice that differentiation of a very important kind is exhibited by Mucor. In accordance with its comparatively large size the function of reproduction is not performed by the whole organism, as in all previously studied types, but a certain portion of the protoplasm becomes shut off from the rest, and to it — as spore or gamete — the office of reproduc- ing the entire organism is assigned. So that we have for the first time true reproductive organs, which may be of two kinds, asexual — the sporangia, and sexual — the gametes.^ In describing the reproduction of Amceba it was pointed out (p. 20) that as the entire organism divides into two daughter-cells, each of which begins an independent life, an Amceba cannot be said ever to die a natural death. The same thing is true of the other unicellular forms we have considered, since in the majority of them the entire organism produces by simple fission two new individuals.^ But in Mucor the state of things is entirely altered. A compara- ^ In Mucor no distinction can be drawn between the conjugating body (gamete) and the organ which produces it (gonad). See the de- scription of the sexual process in Vaucheria (Lesson XVI.) and in Spirogyra (Lesson XIX.). - An exception is formed by colonial forms such as Zoothamnium, in which life is carried on from generation to generation by the reproduc- tive zooids only. In all probability the colony itself, like an annual plant, dies down after a longer or shorter time. Moreover the ciliate infusoria are found, as already stated (p. 116), to sink into decrepitude after multiplying by fission for a long series of generations. XV NUTRITION 167 tively small part of the organism is set apart for repro- duction, and it is only the reproductive cells thus formed — spores or zygote — which carry on the life of the species : the remainder of the organism having exhausted the available food supply and produced the largest possible number of reproductive products, dies. That is, all vital manifestations such as nutrition cease, and decomposition sets in, the protoplasm becoming converted into pro- gressively simpler compounds, the final stages being chiefly carbon dioxide, water, and ammonia. Mucor is able to grow either in Pasteur's or in some similar nutrient solution, or on various organic matters such as bread, jam, manure, &c. In the latter cases it appears to perform some fermentative action, since food which has become "mouldy" is found to have experienced a definite change in appearance and flavour without actual putre- faction. When growing on decomposing organic matter, as it often does, the nutrition of Mucor is saprophytic, but in some instances, as when it grows on bread, it seems to approach very closely to the holozoic method. M. stolo- nifer is also known to send its hyphss into the interior of ripe fruits, causing them to rot, and thus acting as a para- site. The parasitism in this case is, however, obviously not quite the same thing as that of Opalina (p. 123) : the Mucor feeds not upon the ready digested food of its host but upon its actual living substance, which it digests by the action of its own ferments. Thus a parasitic fungus such as Mucor, unhke an endo-parasitic animal such as Opahna or a tape- worm, is no more exempted from the work of digestion than a dog or a sheep : the organism upon which it lives is to be looked upon rather as its prey than as its host. It is a remarkable circumstance that, under certain con- i6S MUCOR LRSS. XV ditions, Mucor is capable of exciting alcoholic fermentation in a saccharine solution. When the hyphse are submerged in such a fluid they have been found to break up, forming rounded cells (Fig. 36, k\ k-), which not only resemble yeast-cells in appearance but are able like them to set up alcoholic fermentation. The aerial hyphse of Mucor exhibit in an interesting way what is known as heliotropism, i.e., a tendency to turn to- wards the light. This is very marked if a growth of the fungus is placed in a room lighted from one side : the long aerial hyph» all bend towards the window. This is due to the fact that growth is more rapid on the side of each hypha turned away from the light than on the more strongly illuminated aspect. LESSON XVI VAUCHERIA AND CAULERPA Stagnant ponds, puddles, and other pieces of still, fresh water usually contain a quantity of green scum which in the undisturbed condition shows no distinction of parts to the naked eye, but appears like a homogeneous slime full of bubbles if exposed to sunlight. If a little of the scum is spread out in a saucer of water, it is seen to be com- posed of great numbers of loosely interwoven green filaments. There are many organisms which have this general naked- eye character, all of them belonging to the Algce, a group of plants which includes most of the smaller fresh-water weeds, and the vast majority of sea-weeds. One of these filamentous Algffi, occurring in the form of dark-green, thickly-matted threads, is called Vai/iheria. Besides occur- ring in water it is often found on the surface of moist soil, e.g., on the pots in conservatories. Examined microscopically the organism is found to consist of cylindrical filaments with rounded ends and occasionally branched (Fig. 38, a). Each filament has an outer cover- ing of cellulose (b, c.w) within which is protoplasm con- taining a vacuole so large that the protoplasm has the c Hi c C3 '1 •5y>y .■'/' d6 , . „ ftf D'i- :i;!«5cr 1 1 05 ■#^, :i,^^-f>>-y;i(y-.\:. Fig. 38. — I 'aiicluria. A, tangled filaments of the living plant,. showing mode of branching. B, extremity of n filament, showing cell-wall {c. zv) and protoplasm with chromatophores {c/ir), and oil-drops (0). The scale above applies to this figure only. C^, immature sporangium [s/^) separated from the filament -by a ' septum {sej>) ; c^, mature sporangium with the spore [sp) in the act of escaping ; c^, free-swimming spore, showing cilia, colourless ectoplasm i.Ess. XVI ASEXUAL REPRODUCTION 171 containing nuclei, and endoplasm containing the green chromatophores ; C*, the same at the commencement of germination, n\ early, and d", later stages in the development of the gonads, the spermary to the left, the ovary to the right ; d', the fully-formed spermary {spy) and ovary (pvy), each separated by a septum (sep, sep') from the filament. D**, the ovary after dehiscence, showing the ovum (ov), with small detached portion of protoplasm ; vr>, sperms ; d", distal end of ripe ovary, showing sperms {sp) passing through the aperture towards the ovum {ov), d', the gonads after fertilisation, showing the oosperm {osp) still enclosed in the ovai-y and the dehisced spermary. E^, oosperm about to germinate : E^, further stage in germination. (c^ and c', after Strasburger ; c^ and c*, after Sachs ; D and E, after Pringsheim. ) character of a membrane lining the cellulose coat. Numerous small nuclei occur in the protoplasm, as well as oil-globules {o), and small, close-set, ovoid chromatophores {chr) coloured with chlorophyll and containing starch. Thus a Vaucheria-plant, like a Mucor-plant, is non-cellular : it is comparable to a single multinucleate cell, extended in one dimension of space so as to take on the forin of a filament. Various modes of asexual reproduction occur in different species of Vaucheria : of these we need only consider that which obtains in V. sessiV/s. In this species the end of a branch swells up (c') and becomes divided off by a septum (sep), forming a sporangium (spg) in principle like that of Mucor, but differing in shape. The protoplasm of the sporangium does not -divide but separates itself from the wall, and takes on the form of a single naked ovoidal spore (c^, c^), formed of a colourless cortical layer containing numerous nuclei and giving off cilia arranged in pairs, and of an inner or medullary substance containing numerous chromatophores. The wall of the sporangium splits at its distal end (c^), and the contained spore {sj>) escapes and swims freely in the 172 YAUCIIERIA AND CAULERPA less. water for some time by the vibration of its cilia (c^). After a short active Hfe it comes to rest, develops a cell-wall, and germinates (c*), i.e., gives out one or more processes which extend and take on the form of ordinary Vaucheria-filaments ; so that in the present case, as in Mucor (p. 164), the de- velopment of the plant shows it to be, to all intents and purposes, a single immensely elongated cell, which has become multinucleate without any corresponding division of the protoplasm. In its mode of sexual reproduction Vaucheria differs strikingly not only from Mucor, but from all the organisms we have hitherto studied. The filaments are often found to bear small lateral pro- cesses arranged in pairs (d^), and each consisting of a little bud growing from the filament and quite continuous with it. These are the rudiments of the sexual reproductive organs or gonads. The shorter of the two becomes swollen and rounded (d-), and afterwards bluntly pointed (d^, ovy) : its protoplasm becomes divided from that of the filament, and a septum (d^, sej>') is formed between the two : the new cell thus constituted is the ovary.^ The longer of the two buds undergoes further elongation and becomes bent upon itself (d^), its distal portion is then divided off by a septum (d^, Sep) forming a separate cell {spy), the spermary.^ Further changes take place which are quite different in the two organs. At the bluntly-pointed distal end of the ovary the cell-wall becomes gelatinized and the protoplasm protrudes through it as a small prominence which divides off and is lost (d"^). The remainder of the protoplasm then separates from the wall of the ovary and becomes a naked ' Usually called the oogonium. ^ Usually called the antheridiiim. XVI SEXUAL REPRODUCTION 173 cell, the ovutnS or egg-cell (d*, ov), which, by the gelatiniza- tion and subsequent disappearance of a portion of the wall of the ovary, is in free contact with the surrounding water. At the same time the protoplasm of the spermary under- goes multiple fission, becoming converted into numerous minute green bodies (d^), each with two flagella, called spermsP- These are liberated by the rupture of the spermary (d'') at its distal end, and swim freely in the water. Some of the sperms make their way to an ovary, and, as it has been expressed, seem to grope about for the aperture, which they finally pass through (d'^), and are then seen moving actively in the space between the aperture and the colourless distal end of the ovum. One of them, and prob- ably only one, then attaches itself to the ovum and be- comes completely united with it, forming the oosperinj^ a body which we must carefully distinguish from the ovum, since, while agreeing with the latter in form and size, it differs in having incorporated with it the substance of a sperm. Almost immediately the oosperm (d', osp) surrounds itself with a cellulose wall, and numerous oil-globules are formed in its interior. It becomes detached from the ovary, and, after a period of rest, germinates (e^, e-) and forms a new Vaucheria plant. It is obvious that the fusion of the sperm with the ovum is a process of conjugation in which the conjugating bodies differ strikingly in form and size, one — the megagamete or ovum — being large, stationary, and more or less amoeboid-: the other — the microgamete or sperm — small, active, and ^ Frequently called oosphere. - Often called spermatozooids or antherozooids. ■* Often called oospore. 174 VAUCHERIA AND CAULERPA less. flagellate. In other words, we have a more obvious case of sexual differentiation than was found to occur in Vorticella, (p. 132): the large inactive egg-cell which furnishes by far the greater portion of the material of the oosperm is the female gamete ; the small active sperm-cell, the function of which is probably (see Lesson XXII.) to furnish additional nuclear material, is the male gamete. Similarly the oosperm is evidently a zygote, but a zygote formed by the vmion of the higfily differentiated gametes, ovum and sperm, just as a zygospore (p. 166) is one formed by the union of equal sized gametes. As we shall see, this form of conjugation — often distin- guished as fertilization — occurs in a large proportion of flowerless plants, such as mosses and ferns (Lessons XXX. and XXXI.), as well as in all animals but the very lowest. From lowly water-weeds up to ferns and club mosses, and from sponges and polypes up to man, the process of sexual reproduction is essentially the same, consisting in the conju- gation of a microgamete or sperm with a megagamete or ovum ; a zygote, the oosperm or unicellular embryo, being produced, which afterwards develops into an independent plant or animal of the new generation. It is a truly remark- able circumstance that what we may consider as the highest form of the sexual process should make its appearance so low down in the scale of life. The nutrition of Vaucheria is purely holophytic ; its food consists of a watery solution of mineral salts and of carbon dioxide, the latter being split up, by the action of the chro- matophores, into carbon and oxygen. Mucor and Vaucheria are examples of non-cellular plants which attain some complexity by elongation and branching. XVI CAULERPA 175 The maximum differentiation attainable in this way by a non-cellular plant may be illustrated by a brief description of a sea-weed belonging to the genus Caukrpa. Caulerpa (Fig. 39) is commonly found in rock-pools between lide-marks, and has the form of a creeping stem from which root-like fibres are given off downwards and branched leaf-like organs upwards. These " leaves " may attain a length of 30 cm. (i ft.) or more. So that, on a Fig. -y). — Caulerpa scalpelliformis (i nat. size), showing the stem- like, root-like, and leaf-Uke portions of the non-cellular plant. (After Harvey. ) superficial examination, Caulerpa appears to be as complex an organism as a moss (compare Fig. 39 with Fig. 108, a). But microscopical examination shows that the plant consists of a single continuous mass of vacuolated protoplasm, containing numerous nuclei and green chromatophores and covered by a continuous cell-wall. Large and complicated in form as it is, the whole plant is therefore nothing more than a continuous mass of protoplasm exhibiting no cellular structure. LESSON XVII THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS Hitherto the words "animal" and "plant" have been either avoided altogether or used incidentally without any attempt at definition. We are now however in a jDosition to consider in some detail the precise meaning of the two words, since in the last half-dozen lessons we have been dealing with several organisms which can be assigned without hesi- tation to one or other of the two great groups of living things. No one would dream of calling Paramoecium and Stylonychia plants, or Mucor and Vaucheria animals, and we may there- fore use these forms as a starting-point in an attempt to form a clear conception of what the names//ff«/ and animal really signify, and how far it is possible to place the lowly organisms described in the earlier lessons in either the vegetable or the animal kingdom. Let us consider, first of all, the chief points of resemblance and of difference between the indubitable animal Paramcecium on the one hand, and the two indubitable plants Mucor and Vaucheria on the other. Li the first place, the essential constituents of all three organisms is protoplasm, in which are contained one or more nuclei. But in Paramoecium the protoplasm is invested LESS. XVII DIFFERENCES IN NUTRITION 177 only by a delicate cuticle interrupted at the mouth and anus, while in Mucor and Vaucheria the outer layer is formed by a firm, continuous covering of cellulose. We thus have as the first morphological difference between our selected animal and vegetable organisms the absence of a cellulose cell-wall in the former and its presence in the latter. This is a fundamental distinction, and applies equally well to the higher forms. The constituent cells of plants are in nearly all cases covered with a cellulose coat (p. 60), while there is no case among the higher animals of cells being so invested. Next, let us take a physiological character. In all three organisms there is constant waste of substance which has to be made good by the conversion of food material into proto- plasm : in other words, constructive and destructive meta- bolism are continually being carried on. But when we come to the nature of the food and the mode of its reception, we meet at once with a very fundamental difference. In Para- moecium the food consists of living organisms taken whole into the interior of the body, and the digestion of this sohd proteinaceous food is the necessary prelude to constructive metabolism. In Vaucheria the food consists of a watery solution of carbon dioxide and mineral salts — i.e., it is liquid and inorganic, its nitrogen being in the form of nitrates or of simple ammonia compounds. Mucor, hke Paramoscium, contains no chlorophyll, and is therefore unable to use carbon dioxide as a food : like Vaucheria, it is prevented by its continuous cellulose investment from ingesting solid food, and is dependent upon an aqueous solution. It takes its carbon in the form of sugar or some such compound, while it can make use of nitrogen either in the simple form of a nitrate or an ammonia salt, or in the complex form of proteids or peptones. N 178 CHARACTERS OF yVNIMALS AND PLANTS less. In this case also our selected organisms agree with animals and plants generally. Animals, with the exception of some internal parasites, ingest solid food, and they must all have their nitrogen supplied in the form of proteids, being unable to build up their protoplasm from simpler compounds. Plants take their food in the form of a watery solution ; those which possess chlorophyll take their carbon in the form of carbon dioxide and their nitrogen in that of a nitrate or ammonia salt : those devoid of chlorophyll cannot, ex- cept in the case of some bacteria, make use of carbon dioxide as a food, and are able to obtain nitrogen either from simple salts or from proteids. Chlorophyll-less plants are therefore nourished partly like green plants, partly like animals. This difference in the character of the food is connected with a morphological difference. Animals have, as a rule, an ingestive aperture or mouth, and some kind of digestive cavity, either permanent (stomach) or temporary (food-vacuole). In plants neither of these structures exists. Another difference which was referred to at length in an early lesson (p. 32), is not strictly one betw'een plants and animals, but between organisms with and organisms without chlorophyll. It is that in green plants the nutritive processes result in deoxidation, more oxygen being give'n out than is taken in ; while in animals and not-green plants the precise contrary is the case. There is also a difference in the method of excretion. In Paramcecium there is a special structure, the contractile vacuole, which collects the superfluous water taken in with the food and expels it, doubtless along with nitrogenous and other waste matters. In Vaucheria and Mucor there is no contractile vacuole, and excretion is simply performed by XVII DEFINITIONS 179 diffusion from the general surface of the organism into the surrounding medium. This character also is of some general inportance. The large majority of animals possess a special organ of excretion, plants have nothing of ihe kind. Another difference has to do with the general form of the organism. Paramcecium has a certain definite and constant shape, and when once formed produces no new parts. Vaucheria and Mucor are constantly forming new branches, so that their shape is always changing and their growth can never be said to be complete. Finally, we have what is perhaps the most obvious and striking distinction of all. Paramcecium possesses in a con- spicuous degree the power of automatic movement ; in both Mucor and Vaucheria the organism, as a whole, exhibits no automatism but only the slow movements of growth. The spores and sperms of Vaucheria are, however, actively motile. Thus, taking Paramcecium as a type of animals, and Mucor and Vaucheria as types of plant.?, we may frame the following definitions : — Animals are organisms of fixed and definite form, in which the cell-body is not covered with a cell.ulose wall. They ingest solid proteinaceous food, their nutritive processes result in oxidation, they have a definite organ of excretion, and are capable of automatic movement. Plants are organisms of constantly varying form in which the cell-body is surrounded by a cellulose wall ; they cannot ingest solid food, but are nourished by a watery solution of nutrient materials. If chlorophyll is present the carbon dioxide of the air serves as a source of carbon, nitrogen is obtained from simple salts, and the nutritive processes N 2 i8o CHARACTERS OF ANBTALS AND PLANTS less. result in deoxidation ; if chlorophyll is absent carbon is obtained from sugar or some similar compound, nitrogen either from simple salts or from proteids, and the process of nutrition is one of oxidation. There is no special excretory organ, and, except in the case of certain reproductive bodies, there is usually no locomotion. Let us now apply these definitions to the simple forms described in the first eight lessons, and see how far they will help us in placing those organisms in one or other of the two "kingdoms" into which living things are divided. Amceba has a cell-wall, probably nitrogenous, in the resting condition : it ingests solid proteids, its nutrition being therefore holozoic : it has a contractile vacuole : and it performs amoeboid movements. It may therefore be safely considered as an animal. Hjematococcus has a cellulose wall : it contains chloro- phyll and its nutrition is purely holophytic : a contractile vacuole is present in H. lacustris but absent in H. pluvialis : and its movements are ciliary. Euglena has a cellulose wall in the encysted state : in virtue of its chlorophyll it is nourished by the absorption of carbon dioxide and mineral salts, but it can also ingest solid food through a special mouth and gullet : it has a contractile vacuole, and performs both euglenoid and ciliary move- ments. In both these organisms we evidently have conflicting characters ; the cellulose wall and holophytic nutrition would place them both among plants, while from the con- tractile vacuole and active movements of both genera and from the holozoic nutrition of Euglena we should group them with animals. That the difficulty is by no means XVII DOUBTFUL FORMS i8i easily overcome may be seen from the fact that both genera are claimed at the present day both by zoologists and by botanists. For instance, Prof. Huxley considered Hsema- tococcus as a plant, and expressed doubts about Euglena ; Mr. Saville Kent ranks Hfematococcus as a plant and Euglena as an animal ; Prof. Sachs and Mr. Thiselton Dyer place both genera in the vegetable kingdom ; while Profs. Ray Lankester and Biitschli group them both among animals. In Heteromita the only cell-wall is the delicate cuticle, which in the zygote is firm enough to hold the spores up to the moment of their escape : food is taken exclusively by absorption and nutrition is wholly saprophytic : there is a contractile vacuole, and the movements are ciliary. Here again the characters are conflicting : the probable absence of cellulose, the contractile vacuole, and the cilia all have an " animal " look, but the mode of nutrition is that of a fungus. In Protomyxa there is a decided preponderance of animal characteristics — ingestion of living prey, and both amoeboid and ciliary movements. There is no chlorophyll, and the composition of the cell-wall is not known. In the Mycetozoa, the life-history of which so closely resembles that of Protomyxa, the cyst in the resting stage consists of cellulose, and so does the cell-wall of the spore : nutrition is holozoic, a contractile vacuole is present in the flagellulEe, and both amoeboid and ciliary movements are performed. Here again we have a puzzling combination of animal and vegetable characters, and as a consequence we find these organisms included among plants — under the name of Myxomycetes or " slime-fungi " — by Sachs and Goebel, while De Bary, Biitschli, and Ray Lankester place them in the animal kingdom. i82 CHARACTERS OF ANDIALS AND PLANTS less. In Saccharomyces there is a clear preponderance of vegetable characters. The cell-wall consists of cellulose, nutrition takes place by absorption and proteids are not essen- tial, there is no contractile vacuole, and no motile phase. Lastly, in the Bacteria t!ie cell-wall is composed of cellu- lose, nutrition is usually saprophytic, there is no contractile vacuole, and the movements are ciliary. So that in all the characters named, save in the presence of cellulose and the absence of a contractile vacuole, the Bacteria agree with Heteromita, yet they are universally — except by Prof. Claus — placed among plants, while Heteromita is as constantly included among animals. We see then that while it is quite easy to divide the higher organisms into the two distinct groups of plants and animals, any such separation is by no means easy in the case of the lowest forms of life. It was in recognition of this fact that Haeckel proposed, many years ago, to institute a third "kingdom." called Protista, to include all unicellular organ- isms. Although open to many objections in practice, there is a great deal to be said for the proposal. From the strictly scientific point of view it is quite as justifiable to make three subdivisions of living things as two ; the line between animals and plants is quite as arbitrary as that between protists and plants or between protists and animals, and no more so ; the chief objection to the change is that it doubles the difficulties by making two artificial boundaries instead of one. The important point for the student to recognise is that these boundaries are artificial, and that there are no scientific frontiers in Nature. As in the liquefaction of gases there is a "critical point " at which the substance under experiment is neither gaseous nor liquid : as in a mountainous country it is impossible to say where mountain ends and valley XVII PROTISTA 183 begins : as in the development of an animal it is futile to argue about the exact period when, for instance, the egg becomes a tadpole or the tadpole a frog: so in the case under discussion. The distinction between the higher plants and animals is perfectly sharp and obvious, but when the two groups are traced downwards they are found gradually to merge, as it were, into an assemblage of organ- isms which partake of the characters of both kingdoms, and cannot without a certain violence be either included in or excluded from either. When any given " protist " has to be classified the case must be decided on its individual merits : the organism must be compared in detail with all those which resemble it closely in structure, physiology, and life-history : and then a balance must be struck and the doubtful form placed in the kingdom with which it has, on the whole, most points in common. It will no doubt occur to the reader that, on the theory of evolution, we may account for the fact of the animal and vegetable kingdoms being related to one another like two trees united at the roots, by the hypothesis that the earhest organisms were protists, and that from them animals and plants were evolved along divergent lines of descent. And in this connection the fact that some bacteria — the simplest organisms known and devoid of chlorophyll — may flourish in solutions wholly devoid of organic matter, is very significant. LESSON XVIII PENICILLIUM AND AGARICUS One of the commonest and most familiar of the lower organisms is the " green mould " which so quickly covers with a thick sage-green growth any organic substances ex- posed to damp, such as paste, jam, cheese, leather, &c. This mould is a plant belonging, like Mucor, to the group of Fungi, and is called Penidllium glmicum. Examined with the naked eye a growth of Penicillium is seen to have a powdery appearance, and, if the finger is passed over it, a quantity of extremely fine dust of a sage- green colour comes away. This dust consists, as we shall see, of the spores of Penicillium. The best Avay to study the plant is to sow some of the spores in a saucer of Pasteur's solution by drawing a needle or brush over a growth of the mould and stirring it round in the fluid. It is as well to study the naked eye appearances first. If the quantity of spores taken is not too large and they are sufficiently well diffused through the fluid, little or no trace of them will be apparent to the naked eye. After a few days, however, extremely small white dots appear on the surface of the fluid ; these increase in size and are seen, especially by the aid of a hand-magnifier, to consist of little LESS, xvni MYCELIUM 185 discs, circular or nearly so in outline, and distinctly thicker in the centre than towards the edge : they float on the fluid so that their upper surfaces are dry. Each of these patches is a young Penicillium-growth, formed, as will be seen hereafter, by the germination of a group of spores. As the growths are examined day by day they are found to increase steadily in size, and as they do so to become thicker and thicker in the middle : their growth is evidently centrifugal. The thicker central portion acquires a fluffy appearance, and, by the time the growth has attained a diameter of about 4 or 5 mm., a further conspicuous change takes place : the centre of the patch acquires a pale blue tint, the circumference still remaining pure white. When the diameter has increased to about 6-10 mm. the colour of the centre gradually changes to dull sage-green : around this is a ring of light blue, and finally an outer circle of white. In all probability some of the growths, several of which will most likely occur in the saucer, will by this time be found to have come together by their edges : they then become completely interwoven, their original boundaries remaining evident for some time by their white tint. Sooner or later, however, the white is replaced by blue and the blue by sage- green, until the whole surface of the fluid is covered by a single growth of a uniform green colour. Even when they are not more than 2-3 mm. in diameter the growths are strong enough to be lifted up from the fluid, and are easily seen under a low power to be formed of a tough, felt-like substance, the mycelimti, Fig. 40, a (wy), from the upper surface of which delicate threads, the aerial hyphce {a. hy.), grow vertically upwards into the air, while from its lower surface similar but shorter threads, the sub- merged hyphce. {s. hy.), hang vertically downwards into the fluid. Fig. 40. — Peuicillhim glatuum. A, Diagrammalic vertical section of a young growth ( x 5), showing mycelium {my), submerged hyphce (s. hy), and aerial hyphse (a. hy'\. E, group of spores : I, before commencement of germination ; 2, after imbibition of fluid : the remaining tliree have begun to germinate. c, very young mycelium formed by a small group of germinating spores. LESS, xviii MULTICELLULAR HYPIi^ 187 D, more advanced mycelium : the hyphre have increased in lerglh. and begun to branch, and septa {sep) have appeared. E, germinating spore {sp) very highly magnified, sending out one short and one long hypha, the latter with a short lateral branch, and several septa (sep). Both spore and hyphse contain vacuoles (vac) in their protoplasm. F^-F*, development of the spore-bearing brushes by repeated branch- ing of an aerial hypha : the short terminal branches or sterigmata are already being constricted to form spores, F^, a fully-developed brush with a row of spores developed from each sterigma [stg). f", a single sterigma {slg) with its spores (sf). F', an over-ripe brush in which the structure is obscured by spores which have dropped from the sterigmata. B-i), f'-f', and f' x 150 : F* x 200 : E x 500. As long as the growths are white or blue in colour no powder can be detached by touching the aerial hyphcC, showing that the spores are not yet fully formed, but as soon as the permanent green hue is attained the slightest touch is sufficient to detach large quantities of spores. A bit of the felt-like mycelium is easily teased out or torn asunder with two needles, and is then found, like actual felt, to be formed of a close interlacement of delicate threads (d). These are the mycelial hypha. : they are regularly cylindrical, about y^u mm. in diameter, frequently branched, and differ in an important particular from the somewhat similar hyphee of Mucor (p. 160). The protoplasm is not continuous, but is interrupted at regular intervals by transverse partitions or sej)/a (d, e, Sep). In other words, a hypha of Penicillium is normally what a hypha of Mucor becomes under un- favourable conditions (p. 162), multicellular, the septa dividing it into separate portions, each of which is morphologically comparable to a single yeast-cell. Penicillium shows therefore a very important advance in structure over the organisms hitherto considered. While in these latter the entire organism is either a single cell in i88 PENICILLIUM AND AGARICUS less. the strict sense, or a continuous multinucleate mass of protoplasm not divided into cells ; in Penicillium it is a cell- aggregate — an accumulation of numerous cells all in organic connection with one another. As the cells are arranged in a single longitudinal series, Penicillium is an example of a linear aggregate. Each cell is surrounded, as already described, by a wall of cellulose : its protoplasm is more or less vacuolated (e, vac)^ sometimes so much so as to form a mere thin layer within the cell-wall, the whole interior of the cell being occupied by one large vacuole. Recently, by staining with logwood, numerous nuclei have been found, so that the Penicillium cell, like an Opalina (p. 123) or a filament of Mucor or Vau- cheria, is multinucleate. The submerged hyphse have the same structure, but it is easier to find their actual ends than those of the mycelial hyphK. The free extremity tapers to a blunt point where the cellulose wall is thinner than elsewhere (see e). The aerial hyphs from the youngest (white) part of a growth consist of unbranched filaments, but taken from a part which is just beginning to turn blue they are found to have a very characteristic appearance (f^ — f*). Each sends off from its distal or upper end a larger or smaller number of branches which remain short and grow parallel to one another : the primary branches (f^, f^) form secondary ones (f^), and the secondary tertiary (f*), so that the hypha finally assumes the appearance of a little brush or pencil, or more accurately of a minute cactus with thick-set forking branches. The ultimate or distal branches are short cells called sierig- iiiata (v-', stg). Next, the ends of the sterigmata become constricted, exactly as if a thread were tied round them and gradually tightened (f^, f''), the result being to separate the distal end xvni GERMINATION OF SPORES 189 of the sterigma as a globular daughter-cell, in very much the same way as a bud is separated in Saccharomyces (p. 73). In this way a spore is produced. The process is repeated • the end of the sterigma is constricted again and a new spore formed, the old one being pushed further onwards. By a continual repetition of the same process a longitudinal row of spores is formed {v'\ r*'), of which the proximal or lower one is the youngest, the distal or upper one the oldest. The spores grow for some time after their formation, and are therefore found to become larger and larger in passing from the proximal to the distal end of the chain (f"). Sooner or later they lose their connection with each other, become detached, and fall, covering the whole growth with a fine dust which readily adheres to all parts owing to the some- what sticky character of the spores. In this stage it is by no means easy to make out the structure of the brushes, since they are quite obscured by the number of spores adhering to them (f''). It is at the period of complete formation of the spores that the growth turns green. The colour is not due to the pres- ence of chlorophyll. Under a high power the spores appear quite colourless, whereas a cell of the same size coloured with chlorophyll would appear bright green. The germination of the spores can be readily studied by sowing them in a drop of Pasteur's solution in a moist chamber (Fig. 37, p. 163). The spores, several of which usually adhere together, are at first clear and bright (b^) : soon they swell considerably, and the protoplasm becomes granular and vacuolated (b^) : in this stage they are hardly distinguishable from yeast -cells (compare Fig. 12, p. 72). Then one or more buds spring from each and elongate into hyphae (b, c), just as in Mucor. But the difference between the two moulds is soon apparent : by the time a hypha has grown to a length 190 PENICILLIUM AND AGARICUS i.f.ss. equal to about six or eight times its own diameter, the pro- toplasm in it divides transversely and a cellulose septum is formed (d, e, scp^ dividing the young hypha into two cells (compare Fig. 36, H, p. 159). The distal cell then elongates and divides again, and in this way the hyphae are, almost from the first, divided into cells of approximately equal length. The mode of growth of the distal or apical cell of a hypha is probably as follows. The free end tapers slightly (e) and the cellulose wall thins out as it approaches the apex. The protoplasm performing constructive more rapidly than de- structive metaboHsm increases in volume, and its tendency is to grow in all directions : as, however, the cellulose mem- brane surrounding it is thinner at the apex than elsewhere, it naturally, on the principle of least resistance, extends in that direction, thus increasing the length of the cell without adding to its thickness. Thus the growth of a hypha of Penicillium is apical^ i.e. takes place only at the distal end, the cells once formed ceasing to grow. Thus also the oldest cells are those nearest the original spore from which the hypha sprang, the youngest those furthest removed from it. A process which has been described as sexual, sometimes, but appa- rently very rarely, occurs in Penicillium, and is said to consist essentially in the conjugation of two gametes having the form of twisted hyph^, and the subsequent development of spores in the resulting branched zygote. But as the details of the process are complicated and its sexual character is doubtful, it is considered best to do no more than call attention to it. The student is referred to Brefeld's original account of the process in the Quarterly Journal of Microscopical Science^ vol. xv. , p. 342. The so-called sexual reproduction of the closely-allied Eurotiutii is described in Hu>dey and Martin's Eleniculary Biology (new edition), 13. 419, and figured in Howcs's Alias of Elementary Biology, pi. xix., figs. xxvi. and xxvii. The nutrition of Penicillium is essentially like thatof Mucor (p. 167). But, as it has been remarked, "it is often content xviii PILEUS AND LAAfELL.E 191 with the poorest food which would be too bad for higher fungi. It hves in the human ear ; it does not shun cast-off clothes, damp boots, or dried-up ink. Sometimes it contents itself with a solution of sugar with a very little [nitrogenous] organic matter, at other times it appears as if it preferred the purest solution of a salt with only a trace of organic matter. It will even tolerate the hurtful influence of poisonous solutions of copper and arsenious acid." It flourishes best in a solution of peptones and sugar. This eclecticism in matters of diet is one obvious ex- planation of the universal occurrence of Penicillium ; another is the extraordinary vitality of the spores. They will ger- minate at any temperature between I's" and 43° C, the optimum being about 22" C. They are not killed by a dry heat of 108° C., and some will even survive a temperature of 120°. And lastly, they will germinate after being kept for two years. We have seen that the form of a Penicillium growth is ir- regular, and is determined by the surface on which it grows. There are, however, certain fungi which are quite constant and determinate both in form and size, and are yet found on analysis to be formed exclusively of interlaced hyphse, that is, to belong to the type of linear aggregates. Among the most striking of these are the mushrooms and toad- stools. A mushroom (Agaricus) consists of a stout vertical stalk (Fig. 41, A, si), on the upper or distal end of which is borne an umbrella-like disc ox pileus (/). The lower or proximal end of the stalk is in connection with an underground mycelium (iny), from which it springs. On the under side of the pileus are numerous radiating vertical plates or lamella {I) extending a part or the whole 192 PENICILLIUM AND AGARICUS of the distance from the circumference of the pileus to the stalk. In the common edible mushroom {Agariais cam- pestris) these lamelte are pink in young specimens, and afterwards become dark brown. Fig. 41. — Agariius campesfris. A, Diagrammatic vertical section, showing the stalk {st) springing from a mycelium {my), and expanding into the pileus (/), on the under side of wfiich are the radiating lamella: (/). E, transverse vertical section of a lamella, showing the hyphce (hy) turning outwards to form the layer of club-shaped cells {a) from which the sterigmata spring. c, one of the club-shaped cells {a), highly magnified, showing its two sterigmata {sfo), each bearing a spore (sp). (B and c after Sachs.) The mushroom is too tough to be readily teased out like the mycelium of Penicillium, and its structure is best in- xvni HISTOLOGY OF MUSHROOM 193 vestigated by cutting thin sections of various parts and examining them under a high power. Such sections show the whole mushroom to be composed of immense numbers of closely interwoven, branched hyphae (b) divided by numerous septa into cells. In the stalk the hyphje take a longitudinal direction ; in the pileus they turn outwards, passing from the centre to the circumference, and finally send branches downwards to form the lamellae. Fre- quently the hyphse are so closely packed as to be hardly distinguishable one from another. At the surfaces of the lamellae the hyphse turn outwards, so that their ends are perpendicular to the free surfaces of those plates. Their terminal cells become dilated or club- shaped (b, c, a), and give off two small branches or sterig- mata (c, stg), the ends of which swell up and become constricted off as spores {sp). These fall on the ground and germinate, forming a mycelium from which more or fev/er mushrooms are in due course produced. Thus in point of structure a mushroom bears much the same relation to Penicillium as Caulerpa (p. 175) bears to Vaucheria. Caulerpa shows the extreme development of which a branched non-cellular organism is capable, the mushroom how complicated in structure and definite in form a simple linear aggregate may become. LESSON XIX SPIROGYRA Amongst the numerous weeds which form a green scum in stagnant ponds and slowly flowing streams, one, called Spirogyra, is perhaps the commonest. It is recognised at once under a low power by the long delicate green filaments of which it is composed being marked with a regular green spiral band. Examined under the microscope the filaments are seen to be, like the hyphae of Penicillium, linear aggregates, that is, to be composed of a single row of cells arranged end to end. But in Penicillium the hyphae are frequently branched, and it is always possible in an entire hypha to distinguish the slightly tapering distal end from the proximal end which springs either from another hypha or from a spore. In Spirogyra the filaments do not branch, and there is no distinction between their opposite ends. The cells of which the filaments are composed (Fig. 42, a) are cylindrical, covered with a cellulose cell-wall, {c. k'), and separated from adjacent cells by septa (sef) of the same substance. The protoplasmic cell-body presents certain characteristic peculiarities. It has been noticed in more than one instance that in the Fig. 42. — Spirogyra. A, small portion of a living filament, showing a single cell, with cell- wall {c. w), septa {sep) separating it from adjacent cells, peripheral layer of protoplasm {plsm) connected by threads with a central mass contain- O -> 196 SPIROGYRA less. ing the nucleus {nit), two spiral cliromatophores {chr), and pyrenoids (fyr). B^, E-, middle portion of a cell, showing two stages m binary fission. c, four stages in dicecious conjugation : in tf the gonads [goii^, goit^) are connected by short processes of their adjacent sides : in c- the active or male gamete (gam''-) has separated from the wall of the gonad {gon^) preparatory to passing across the connecting bridge to the stationary or female gamete \ganfl) which has not yet separated from its containing gonad {gon-) : in c' the female gamete [gavfi) has undergone separation, and the male gamete (gatu^) is in the act of conjugating with it : in c* the two have united to form a zygote (g;?) lying in the female gonad. D, two stages in moncecious conjugation : in d' the adjacent cells (gonads) have sent out conjugating processes (a) : in D- conjugation is complete, the male gamete having passed through the aperture between the conjugating processes and united with the female gamete to form the zygote [zyg). E, parthenogenetic formation of zygotes, F, fully developed zygote (zygospore). G, early stage in the germination of the zygote. (b after Sachs : c after Strasburgcr ; F and G from Sachs after Pringsheim. ) larger cells of plants the development of vacuoles is so ex- tensive that the protoplasm is reduced to a thin layer in contact with the cell-wall (see pp. 169 and 188). This state of things is carried to excess in Spirogyra : the central vacuole is so large that the protoplasm (a, p/sm) has the character of a mere delicate colourless membrane within the cell wall : to rnake it out clearly the speciinen should be treated with a fluid of greater density than water, such as a 10 per cent, solution of sodium chloride, which, by absorbing the water in the vacuole, causes the protoplasm to shrink away from the cell-wall and so brings it clearly into view. It is to this layer of protoplasm that the name primordial utricle is applied by botanists, but the student should remember that a primordial utricle is not a special constituent of those cells in which it occurs, but is merely the protoplasm of a vegetable cell in which the vacuole is inordinately large. The protoplasm of the cell of Spirogyra is not, however. XIX INTERSTITIAL GROWTFI 197 confined to the primordial utricle ; towards the centre of the vacuole is a small irregular mass of protoplasm connected to the peripheral layer by extremely delicate protoplasmic strands. Imbedded in this central mass is the nucleus («//), which has the form of a biconvex lens and contains a distinct nucleolus. The chromatophores differ from anything we have yet considered, having the form of green spiral bands ichr), of which each cell may contain one (d^) or two coiled in oppo- site directions (a). Imbedded in the chromatophores are numerous pyrenoids i^pyr, see p. 27), to which the strands of protoplasm proceeding from the central nucleus-containing mass can be traced. The process of growth in Spirogyra is brought about by the binary fission of its constituent cells. It takes place under ordinary circumstances during the night (ir — 12 p.m.), but by keeping the plant cold all night may be delayed until morning. The nucleus divides by the complicated process (mitosis) already described in general terms (p. 67), so that two nuclei are formed at equal distances from the centre of the cell. The cell-body with its chromatophores then begins to divide across the middle (b^), the process commencing near the cell-wall and gradually proceeding inwards : as it goes on cellulose is secreted between the halves of the dividing protoplasm so that a ring of cellulose is formed lying transversely across the middle of the cell, and in con- tinuity externally with the wall (b^). The ring is at first very narrow, but as the annular furrow across the dividing cell- body deepens, so the ring increases in width, until by the time the protoplasm has divided it has become a complete partition separating the newly-formed daughter cells from one another. igS SPIROGYRA LESS. Any of the cells of a Spirogyra-filament may divide in this way, so that the filament grows by the intercalation of new cells between the old ones. This is an example of interstitial growth. Note its difference from the apical growth which was found to take place in Penicilhum (p. 1 90), a difference which explains the fact mentioned above (p. 194) that there is no distinction between the two ends of a filament of Spirogyra, while in Penicillium the proximal and distal ends can always be distinguished in a complete hypha. The sexual reproduction of Spirogyra is interesting, as being intermediate between the very different processes which were found to obtain in Mucor (p. 165) and in Vaucheria (p. 172). In summer or autumn adjoining filaments become arranged parallel to one another and the opposite cells of each send out short rounded processes which meet (Fig. 42, c^), and finally become united by the absorption of the adjacent walls, thus forming a free communication between the two connected cells or g07iads {goii^, g07i^). As several pairs of cells on the same two filaments unite simultaneously a ladder-hke ap- pearance is produced. The protoplasmic cell-bodies (c^, gain^, gam") of the two gonads become rounded ofi and iorva gametes or conjugating bodies (see p. 166, note i) : it is observable that this process of separation from the wall of the gonad always takes place earlier in one gamete (c^, gam'^) than in the other (c-, c^, gam^). Then the gamete which is ready first (gam'^) passes through the connecting canal (c^) and conjugates with the other (gam^), forming a zygote {c*, zyg) which soon surrounds itself with a thick cell-wall. It has been ascertained that the nuclei of the gametes unite to form the single nucleus of the zygote. XIX CONJUGATION 199 Thus, as in Mucor, the gametes are similar and equal- sized, and the result of the process is a resting zygote or zygospore. But while in Mucor each gamete meets the other half way, so that there is absolutely no sexual differentiation, in Spirogyra, as in Vaucheria, one gamete remains passive, and conjugation is effected by the activity of the other. So that we have here the very simplest case of sexual differen- tiation : the gametes, although of equal size and similar ap- pearance, are divisible into an active or male cell, correspond- ing with the sperm of Vaucheria, and a passive or female cell corresponding with the ovum. It will be seen that in Spirogyra the whole of the protoplasm of each gonad is used up in the formation of a single gamete, whereas in Vaucheria, while this is the case with the ovary, numerous gametes (sperms) are formed from the protoplasm of the spermary. In some forms of Spirogyra conjugation takes place not between opposite cells of distinct filaments, but between adjacent cells of the same filament. Each of the gonads sends out a short process (d^, a) Vifhich abuts against a corresponding process from the adjoining cell : the two processes are placed in communication with one another by a small aperture (d^) through which the male gamete makes its way in order to conjugate with the female gamete and form a zygote {::yg). In the ordinary ladder-like method of conjugation the conjugating filaments appear to be of opposite sexes, one producing only male, the other only female gametes : the plant in this case is said to \>^ dioecious, i.e. has the sexes lodged in distinct individuals, and conjugation is a process of cross- fertilization. But in the method described in the preceding paragraph the individual filaments are monmcious, i.e. produce both male and female cells, and conjugation is a process of self-fertilization. 200 SPIROGYRA less, xix Sometimes filaments are found in which the protoplasm of certain cells separates from the wall, and surrounds itself with a thick coat of cellulose forming a body which is quite indistinguishable from a zygote (e). There seems to be some doubt as to whether such cells ever germinate, but they have all the appearance of female cells which for some reason have developed into zygote-like bodies without fertil- ization. Such development from an unfertilized female gamete, although it has not been proved in Spirogyra is known to occur in many cases, and is distinguished as parthenogenesis. When the zygote is fully developed (f) its cell wall is divided into three layers, the middle one undergoing a peculiar change which renders it waterproof: at the same time the starch in its protoplasm is replaced by oil. . In this condition it undergoes a long period of rest, its structure enabling it to offer great resistance to drought, frost, &c. Finally it germinates : the two outer coats are ruptured, and the protoplasm covered by the inner coat protrudes as a club-shaped process (g) which gradually takes on the form of an ordinary Spirogyra filament, dividing as it does so into numerous cells. Thus in the present case, as in Penicillium and the mushroom, the multicellular adult organism is originally unicellular. The nutrition of Spirogyra is purely holophytic : like Hjematococcus and Vaucheria it lives upon the carbon dioxide and mineral salts dissolved in the surrounding water. Like these organisms also it decomposes carbon dioxide and forms starch only under the influence of sunlight. LESSON XX MONOSTROIIA, ULVA, AND NITELLA It \yas pointed out in a previous lesson (p. 193) that the highest and most complicated fungi, such as the mushrooms, are found on analysis to be built up of linear aggregates of cells — to consist of hyphje so interwoven as to form struc- tures often of considerable size and of definite and regular form. This is not the case with the Algs or lower green plants — the group to which Vaucheria, Caulerpa, Spirogyra, the diatoms, and, in the view of some authors, Hsematococcus and Euglena, belong. These agree with fungi in the fact that the lowest among them (e.g. Zooxanthella) are unicellu- lar, and others {e.g. Spirogyra) simple linear aggregates ; but the higher forms, such as the majority of sea-weeds, have, as it were, gone beyond the fungi in point of structure and attained a distinctly higher stage of morphological differen- tiation. This will be made clear by a study of three typical genera. Amongst the immense variety of sea- weeds found in rock- pools between high and low water-marks are several kinds having the form of flat irregular expansions or of bladder- 202 MONOSTROMA, ULVA, AND NITELLA less. like masses, of a bright green colour and very transparent. One of these is the genus Monostroma, of which M. bullosum is a fresh-water species. Examined microscopically the plant (Fig. 43) is found to consist of a single layer of close-set, green cells, the cell-walls of which are in close approximation, so that the cell-bodies appear as if embedded in a continuous layer of transparent cellulose. Thus Monostroma, like Spirogyra, is only one m Fig. 43. — Monostroma. A, surface view of M. bullosum, showing the cells embedded in a common layer of cellulose : many of them are in various stages of division. B, vertical section of M. laceratum, showing the arrangement of the cells in a single layer. (a after Reinke : B after Cooke. ) cell thick (b), but unlike that genus it is not one but many cells broad. In other words, instead of being a hnear it is a supe7-ficial aggregate. To use a geometrical analogy : — a unicellular organism like HfEmatococcus may be compared to a point ; a Hnear aggregate like Penicillium or Spirogyra to a line ; a superficial aggregate like Monostroma to a plane. Growth takes place by the binary fission of the cells (a), but here again there is a marked and important difference from Spirogyra. In the latter the plane of division is always XX SOLID AGGREGATES 203 at right angles to the long axis of the filament, so that growth takes place in one dimension of space only, namely in length. In Monostroma the plane of division may be inclined in any direction provided it is perpendicular to the surface of the plant, so that growth goes on in two dimensions of space, namely in length and breadth. Another of the flat, leaf-like, green sea-weeds is the very common genus Ulva, sometimes called "sea-lettuce." It consists of irregular, more or less lobed expansions with crinkled edges, and under the microscope closely resembles Monostroma, with one important difference : it is formed not of one but of two layers of cells, and is therefore not a superficial but a solid aggregate. To return to the geometrical analogy used above it is to be compared not to a plane but to a solid body. As in Monostroma growth takes place by the binary fission of the cells. But these divide not only along variously inclined planes at right angles to the surface of the plant but also along a plane parallel to the surface, so that growth takes place in all three dimensions of space — in length, breadth, and thickness. Ulva may be looked upon as the simplest example of a solid aggregate, being built up of similar cells, and therefore exhibiting no cell-differentiation. We shall now make a detailed study of a solid aggregate in which the constituent cells differ very considerably from one another in form and size, the result being a degree 01 complexity far beyond anything we have hitherto met with. Nitella (Fig. 44, a) is a not uncommon fresh-water weed, found in ponds and water-races, and distinguished at once int.n'I ' FlO. 44.— XtVc//a\ A, the entire plant (nat. size), showing the segmented stem, each seg- ^ This and the following figures are taken from a New Zealand species closely allied to, if not identical with, the British N. flexilis. LESS. XX EXTERNAL CHARACTERS 205 ment [se^) consisting of a proximal internode {int. tid) and distal node ("^0 • tlie leaves (/) arranged in whorls and ending in leaflets (/') : the rhizoids (?-/;) : and two branches (l>r), each springing from the axil of a leaf and ending, like the main stem, in a terminal bud {term. bud). B, distal end of a shoot with gonads attached to the leaves : ovy, the ovaries ; spy, the spermaries. C, distal end of a rhizoid. D, distal end of a leaf (/) with two leaflets (/), showing the chroma- tophores and the white line. The arrows indicate the direction of rota- tion of the protoplasm. E, distal end of a leaflet, showing the general stractm-e of a typical cell of Nitella in optical section : c. w, the cell-wall ; ph//i^, the quies- cent outer layer of protoplasm containing chromatophores {c/ir) ; plsiiP, the inner layer, rotating in the direction indicated by the arrows, and containing nuclei {nu) ; vac, the large vacuole. F, terminal bud, partly dissected, showing the nodes {nd], internodes {int. nd), and leaf-whorls (/), numbered from i to 4, starting from the proximal end ; gr. pt, growing point. G, distal end of a leaf (/) with two leaflets {[), at the base of which are attached a spermary {spy] and two ovaries {ovy). from such low Alg» as Vaucheria and Spirogyra by its ex- ternal resemblance to one of the higher plants, since it piesents structures which may be distinguished as stem, branches, leaves, &c. A Nitella plant consists of a slender cyUndrical stem, some 15-20 cm. and upwards in length, but not more than about \ mm. in diameter. The proximal end is loosely rooted to the mud at the bottom of the stream or pond by delicate root-filaments or rhizoids (a, rh) : the distal end is free. Springing from it at intervals are circlets or whorls of delicate, pointed leaves {I). Owing to the regular arrangement of the leaves the stem is divisible into successive sections or segments (seg), each consisting of a very short distal division or node (nd) from which the leaves spring, and of an elongated proximal division or internode {int. nd), which bears no leaves. Throughout the greater part of the stem the whorls of leaves are disposed at approximately equal distances from one another, so that the internodes are of equal length, but 2o5 -MONOSTROMA, ULVA, AND NITELLA less. towards the distal end the internodes become rapidly shorter and the whorls consequently closer together, until, at the actual distal end, a whorl is found the leaves of which, in- stead of spreading outwards like the rest, are curled upwards so that their points are in contact, In this way is formed the terminal bud (term, bud), by which the uninjured stem is always terminated distally. The angle between the stem and a leaf, above (distal to) the attachment of the latter, is called the axil of the leaf. There is frequently found springing from the axil of one of the leaves in a whorl a branch or s/ioof (br) which repeats the structure of the main stem, i.e. consists of an axis from which spring whorls of leaves, the whole ending in a ter- minal bud. The axis or stem of a shoot is called a second- ary axis, the main stem of the plant being the primary axis. It is important to notice that both primary and secondary axes always end in terminal buds, and thus differ from the leaves which have pointed extremities. The rhizoids or root-filaments (rJi) arise, hke the leaves and branches, exclusively from nodes. In the autumn the more distal leaves present a peculiar appearance, owing to the development on them of the ^(7/zi7i/.y or sexual reproductive organs (Fig. 44, b and g) : of these the spermaries (antheridia) look very like minute oranges, being globular structures {spy) of a bright orange colour : the ovaries (oogonia) are flask-shaped bodies ipvy) of a yellowish brown colour when immature, but turning black after the fertilization of the ova. Examined under the microscope each intemode is found to consist of a single gigantic cell (f, int. nd^) often as much as 3 or 4 cm. long in the older parts of the plant. A node on the other hand is composed of a transverse plate of small XX HISTOLOGY 207 cells (tid^) separating the two adjacent internodes from one another. The leaves consist each of an elongated proximal cell like an internode (d, /; f, /i), then of a few small cells having the character of a node, and finally of two or three leaflets (d, g, /'), each consisting usually of three cells, the distal one of which is small and pointed. Thus the Nitella plant is a solid aggregate in which the cells have a very definite and characteristic arrange- ment. The details of structure of a single cell are readily made out by examining a leaflet under a high power. The cell is surrounded by a wall of cellulose (e, c.w) of considerable thickness. Within this is a layer of protoplasm (primordial utricle, p. 196), enclosing a large central vacuole {vac), and clearly divisible into two layers, an outer {plsiti^) in im- mediate contact with the cell-wall, and an inner [plsm^) boun.ding the vacuole. In the outer layer of protoplasm are the chromatophores or chlorophyll- corpuscles {chr) to which the green colour of the plant is due. They are ovoidal bodies, about yj^ ram. long, and arranged in obliquely longitudinal rows (d). On opposite sides of the cylindrical cell are two narrow ob- lique bands devoid of chromatophores and consequently colourless (d). The chromatophores contain minute starch grains. The inner layer of protoplasm contains no chlorophyll corpuscles, but only irregular, colourless granules, many of which are nuclei (e, ?iu: see below, p. 211). If the tem- perature is not too low this layer is seen to be in active rotating movement, streaming up one side of the cell and down the other (e), the boundary between the upward and downward currents being marked by the colourless bands just mentioned, along which no movement takes place (d). This rotation of protoplasm is a form of contractility very 2o8 MONOSTROMA, ULVA, AND NITELLA less, xx common in vegetable cells in which, owing to the confining cell-wall, no freer movement is possible. The numerous nuclei (e, 7iu) are rod-Hke and often curved : they can be seen to advantage only after staining (Fig. 45). Lying as they do in the inner layer of protoplasm, they are carried round in the rotating stream. In the general description of the plant it was mentioned that the stem ended distally in a terminal bud (Fig. 44, a, term, hud) formed of a whorl of leaves with their apices curved towards one another. If these leaves (f, /i) are dis- sected away, the node from which they spring {nd'^) is found to give rise distally to a very short internode (int. nd^), above which is a node [nd^) giving rise to a whorl of very small leaves (/^), also curved inwards so as to form a bud. Within these is found another segment consisting of a still smaller internode {i?it. fid^) and node, bearing a whorl of extremely small leaves (/^), and within these again a segment so small that its parts {tftt. ?id-, /*) are visible only under the microscope. The minute blunt projections (/*), which are the leaves of this whorl, surround a blunt, hemispherical projection {gr. pt), the actual distal extremity of the plant — the growing point or punctum vegetationis. The structure of the growing point and the mode of growth of the whole plant is readily made out by examining vertical sections of the terminal bud in numerous specimens (Fig- 45)- The growing point is formed of a single cell, the apical cell (a, ap. c), approximately hemispherical in form and about -/o mm. in diameter. Its cell-wall is thick, and its cell-body formed of dense granular protoplasm containing a large rounded nucleus (tin) but no vacuole. In the living plant the apical cell is continually undergoing binary fission. It divides along a horizontal plane, i.e., a Fig. 45. — Nitella : Vertical sections of the growing point at four successive stages. The nodes {nd), internodes {int. nd), "and leaf- whorls (/) are all numbered in order from the proximal to the distal end of the bud, the numbers corresponding in all the figures. The proximal segment {int. nd^, nd'-, /') in these figures corresponds with the third segment (int. nd^, P) shown in Fig. 45, F. In A, the apical cell {aji. c) is succeeded by a very rudimentary node (nd^) without leaves : int. nd'- is in vertical section, showing the proto- plasm {phm), vacuole {vat:), and two nuclei {i7zi). In B, the apical cell has divided transversely, forming a new apical cell {a/>. c) and a sub-apical cell {s. ap. c) : the leaves {I'-') of nd^ have appeared. In c, the sub-apical cell has divided transversely into the proximally- situated intemode {int. nd^) and the distally-situated node {ltd*) of a new segment ; in the node the nucleus has divided preparatoi-y to cell- division. The previously formed segments have increased in size : int. nd' has developed a vacuole {vac), and its nucleus has divided (comp. int. nop in A) : int. nd' is shown in surface view with three dividing nuclei {nii). In u, nd^ has divided vertically, forming a transverse plate of cells, and is now as far advanced as nd^ in A : the nucleus of int. nd"^ is in the act of dividing, while int. niP; shown in surface view, now contains numerous nuclei, some of them in the act of dividing. 2IO MONOSTROMA, ULVA, AND NITELLA less. plane parallel to its base, into two cells, the upper (distal) of which is the new apical cell (b, ap. c), while the lower is now distinguished as the sub-apical or segmeiital cell [s. ap. c). The sub-apical cell divides again horizontally, forming two cells, the uppermost of which (c, nd"^) almost immediately becomes divided by vertical planes into several cells (d, nd '') ; the lower (c, d, int. nd^) remains undivided. The sub-apical cell is the rudiment of an entire segment ; the uppermost of the two cells into which it divides is the rudiment of a node, the lower of an internode. The future fate of the two is shown at once by the node dividing into a horizontal plate of cells while the internode remains unicellular. Soon the cells of the new node begin to send out short blunt processes arranged in a whorl : these increase in size, undergo division, and form leaves (a — d, Z^, Z^). These processes are continually being repeated ; the apical cell is constantly producing new sub-apical cells, the sub- apical cells dividing each into a nodal and an internodal cell ; and the nodal cell dividing into a horizontal plate of cells and giving off leaves, while the internodal cell remains undivided. The special characters of the fully-formed parts of the plant are due to the unequal growth of the new cells. The nodal cells soon cease to grow and undergo but little altera- tion (comp. nd'^ and nd'^), whereas the internodes increase immensely in length, being quite 3,000 times as long when full-grown as when first separated from the sub-apical cell. The leaves also, at first mere blunt projections (a, Z^), soon increase sufficiently in length to arch over the growing point and so form the characteristic terminal bud . gradually they open out and assume the normal position, their successors of the next younger whorl having in the meantime developed XX MULTIPLICATION OF NUCLEUS 211 sufficiently to take their place as protectors of the growing point. The multinucleate condition of the adult internodes is also a result of gradual change. In its young condition an internodal cell has a single rounded nucleus (a, int. nd^, int. tid ^), but by the time it is about as long as broad the nucleus has begun to divide (d, itit. nd ^ ; c, int nd ^), and when the length of the cell is equal to about twice its breadth, the nucleus has broken up into numerous fragments (c, int. nd'', D, int. nd^), many of them still in active (amitotic) division. This repeated fission of the nucleus reminds us of what was found to occur in Opalina (p. 123). Thus the growth of Nitella like that of Penicillium (p. 1 90), is apical : new cells arise only in the terminal bud, and, after the first formation of nodes, internodes, and leaves, the only change undergone by these parts is an in- crease in size accompanied by a limited differentiation of character. A shoot arises by one of the cells in a node sending off a projection distal to a leaf, i.e., in an axil : the process separates from the parent cell and takes on the characters of the apical cell of the main stem, the structure of which is in this way exactly repeated by the shoot. Tl'.e leaves, unlike the branches, are strictly limited in growth. At a very early period the apical cell of a leaf becomes pointed and thick-walled (Fig. 44, e), and after this no increase in the number of cells takes place. The rhizoids also arise exclusively from nodal cells : they consist of long filaments (Fig. 44, c), not unlike Mucor- hyphae but occasionally divided by oblique septa into linear aggregates of cells, and increase in length by apical growth. The structure of the gonads is peculiar and somewhat complicated. p 2 212 MONOSTROMA, ULVA, AND NITELLA less. As we have seen, the sperraary (Fig. 44, G, spy) is a globular, orange coloured body attached to a leaf by a short stalk. Its wall is formed of eight pieces or shields, which fit against one another by toothed edges, so that the entire spermary may be compared to an orange in which an equa- torial incision and two meridional incisions at right angles to one another have been made through the rind, dividing Fig. 46. — A, diagrammatic vertical section of the spermary of Nitella, showing the stalk [stk], four of the eight shields {sh), each bearing on its inner face a handle (hii), to which is attached a head-cell (hd) : each head-cell bears six secondary head-cells {Ad'), to each of which four spermatic filaments {sp. f.) are attached. B, one of the proximal shields {sh), with handle {hii), head cell {hd), secondary head-cells {hd'), and spermatic filaments {sp. f.). c, a single sperm. d\ D-, D-', three stages in the development of the spermary. (c, after Howes. ) it into eight triangular pieces. Strictly speaking, however, only the four distal shields are triangular : the four proximal ones have each its lower angle truncated by the insertion of the stalk, so that they are actually four-sided. Each shield (Fig. 46, a and b, s/i) is a single concavo- convex cell having on its inner surface numerous orano-e- coloured chromatophores : owing to the disposition of these XX STRUCTURE OF SPERMARY 213 on the inner surface only, the spermary appears to have a colourless transparent outer layer — like an orange inclosed in a close-fitting glass case. Attached to the middle of the inner surface of each shield is a cylindrical cell, the handle {hn), which extends towards the centre of the spermary, and, like the shield itself, con- tains orange chromatophores. Each of the eight handles bears a colourless head-cell {hd), to which six secondary head cells {hd') are attached, and each of these latter bears four delicate coiled filaments {sp.f.) divided by septa into small cells arranged end to end, and thus not unlike the hyphae of a fungus. There are therefore nearly two hundred of these spermatic filaments in each spermary, coiled up in its interior like a tangled mass of white cotton. The cells of which the filaments are composed have at first the ordinary character, but as the spermary arrives at maturity there is produced in each a single sperm (c), having the form of a spirally-coiled thread, thicker at one end than the other, and bearing at its thin end two long flagella. In all probability the sperm proper, i.e., the spirally coiled body, is formed from the nucleus of the cell, the flagella from its protoplasm. As each of the 200 spermatic filaments con- sists of from 100 to 200 cells, a single spermary gives rise to between 20,000 and 40,000 sperms. When the sperms are formed the shields separate from one another and the spermatic filaments protrude between them like cotton from a pod : the sperms then escape from the containing cells and swim freely in the water. The ovary (Fig, 44, g, ovy, and Fig. 47 a) is ovoidal in form, attached to the leaf by a short stalk {stk), and ter- minated distally by a little chimney-like elevation or crown {cr). It is marked externally by spiral grooves which can be 214 MONOStROMA, ULVA, AND NITELI.A traced into the crown, and in young specimens its interior is readily seen to be occupied by a large opaque mass {ov). Sections show that this central body is the ovum, a large cell very rich in starch : it is connected with the unicellular stalk by a small cell («. c), rhizoids (r/i), and leaves (/) of the embryo : and the rudiment of the leafy plant (shaded) ending in the characteristic terminal bud {term. bud). (After Howes, slightly altered. ) Impregnation takes place in the same manner as in Vaucheria (p. 173). A sperm makes its way down the canal in the . chimney-like crown of cells terminating the ovary, and conjugates with the ovum converting it into an oosperm. After impregnation the ovary, with the contained oosperm, becomes detached and falls to the bottom, where, after a XX GERMINATION 217 period of rest, it germinates. The process begins by the division of the oosperm into two cells, a small one nearest the crown and composed almost wholly of protoplasm, and a larger one full of starch granules. The larger cell serves simply as a store of nutriment to the growing plant which is itself developed exclusively from the small cell. The latter divides into two cells one of which grows downwards as a root-fibre, the other upwards as a shoot, consisting at first of a single row of cells (Fig. 48). Soon two nodes [?!d) are formed on the filament, or embiyo, from the lower of which rhizoids (;'/;) proceed, while the upper gives rise to a few leaves (/), and to a small process which is at first uni- cellular, but, behaving like an apical cell of Nitella, soon becomes a terminal bud (term, bud) and grows into the adult plant. It will be seen that the development of Nitella is remark- able for the facts that the adult plant is not formed directly from the oosperm but that the latter gives rise to an embryo, quite different from the adult in structure, and that, from the embryo, the adult is finally developed as a lateral bud. LESSON XXI We have seen that with plants, both Fungi and Algfe, the next stage of morphological differentiation after the simple unicellular or non-cellular organism is the linear aggregate. Among animals there are no forms known to exist in this stage, but coming immediately above the highest unicellular animals, such as the ciliate Infusoria, we have true solid aggregates. The characters of one of the simplest of these and the fundamental way in which it differs from the plants described in the two previous lessons will be made clear by a study of one of the little organisms known as " fresh-water polypes " and placed under the genus Hydra. Although far from uncommon in pond-water, Hydra is not always easy to find, being rarely abundant and by no means conspicuous. In looking for it the best plan is to fill either a clear glass bottle or beaker or a white saucer with weeds and water from a pond and to let it remain undisturbed for a few minutes. If the gathering is successful there will be seen adhering to the sides of the glass, the bottom of the saucer, or the weeds, little white, tawny, or green bodies, about as thick as fine sewing cotton, and 2 — 6 mm. in length. They adhere pretty firmly by one end, and examin- Fig. 49. — Hydra, A, Two living specimens of H. viridis attached to a bit of weed. The larger specimen is fully expanded, and shows the elongated body ending distally in the hypostome {hyp), surrounded by tentacles {{), and three buds (bd^, lid', bd'^) in different stages of development : a small water-flea (a) has been captured by one tentacle. The smaller specimen (to the right and above) is in a state of complete retraction, the tentacles (t) ap|Dearing like papillae. B, H. fusca, showing the mouth {inth) at the end of the hypostome (hyp), the circlet of tentacles (/), two spermaries [spy), and an ovary {ovy). c, a Hydra creeping on a flat surface by looping movements. D, a specimen crawling on its tentacles. (C and D after W. Marshall.) 220 HYDRA LESS. ation with a pocket lens shows that from the free extremity a number of very deHcate filaments, barely visible to the naked eye, are gi\'en off. Under the low power of a compound microscope a Hydra (Fig. 49, b) is seen to have a cylindrical body attached by a flattened base to a weed or other aquatic object, and bearing at its opposite or distal end a conical structure, the hyposiome {hyp), at the apex of which is a circular aperture, the mouth (inth). At the junction of the hypostome with the body proper are given off from six to eight long delicate feti- tades {i) arranged in a circlet or whorl. A longitudinal section shows that the body is hollow, containing a spacious cavity, the enteron (Fig. 50, a, ent. cm!), which communicates with the surrounding water by the mouth. The tentacles are also hollow, their cavities communicating with the enteron. There are three kinds of Hydra commonly found : one, H. vulgaris, is colourless or nearly so ; another, H. fusca, is of a pinkish-yellow or brown colour ; the third, H. viridis, is bright green. In the two latter it is quite evident, even under a low power, that the colour is in the inner parts of the body-wall, the outside of which is formed by a transparent colourless layer (Fig. 49, a, b). It is quite easy to keep a Hydra under observation on the stage of the microscope for a considerable time by placing it in a watch-glass or shallow " cell " with weeds, &c., and in this way its habits can be very profitably studied. It will be noticed, in the first place, that its form is continually changing. At one time (Fig. 49, a, left-hand 'figure) it extends itself until its length is fully fifteen times its diameter and the tentacles appear like long delicate filaments: at another time (right-hand figure) it contracts itself into an almost globular mass, the tentacles then appearing like little blunt knobs. XXI MOVEMENTS 221 Besides these movements of contraction and expansion, Hydra is able to move slowly from place to place. This it usually does after the manner of a looping caterpillar (Fig. 49, c) : the body is bent round until the distal end touches the surface : then the base is detached and moved nearer the distal end, which is again m»oved forward, and so on. It has also been observed to crawl like a cuttle fish (d) by means of its tentacles, the body being kept nearly vertical. It is also possible to watch a Hydra feed. It is a very voracious creature, and to see it catch and devour its prey is a curious and interesting sight. In the water in which it lives are always to be found numbers of " water-fleas," minute animals from about a miUimetre downwards in length, belonging to the class Crustacea, a group which includes lobsters, crabs, shrimps, &c. Water-fleas swim very rapidly, and occasionally one may be seen to come in contact with a Hydra's tentacle. Instantly its hitherto active movements stop dead, and it remains adhering in an apparently mysterious manner to the tentacle. If the Hydra is not hungry it usually liberates its prey after a time, and the water-flea may then be seen to drop through the water like a stone for a short distance, but finally to expand its limbs and swim off If however the Hydra has not eaten recently it gradually contracts the tentacle until the prey is brought near the mouth, the other tentacles being also used to aid in the process. The water- flea is thus forced against the apex of the hypostome, the mouth expands widely and seizes it, and it is finally passed down into the digestive cavity. Hydrse can often be seen with their bodies bulged out in one or more places by recently swallowed water-fleas. The precise structure of Hydra is best made out by cutting 222 HYDRA LESS. XXI it into a series of extremely thin sections and examining them under a high power. The appearance presented by a vertical section through the long axis of the body is shown in Fig. so. The whole animal is seen to be built up of cells, each consisting of protoplasm with a large nucleus (b, c, mi), and with or without vacuoles. As in the case of most animal cells, there is no cell-wall. Hydra is therefore a solid aggre- gate : but the way in which its constituent cells are arranged is highly characteristic and distinguishes it at once from a plant. The essential feature in the arrangement of the cells is that they are disposed in two layers round the central digestive cavity or enteron (a, ent. cai<) and the cavities of tentacles {ent. cav'). So that the wall of the body is formed throughout of an outer layer of cells, the ectoderm {ect), and of an inner layer, the endoderm {end), which bounds the enteric cavity. Between the two layers is a delicate trans- parent membrane, the mesoglcea, or supporting lamella (jnsgl). A transverse section shows that the cells in both layers are arranged radially (b). Thus Hydra is a two-layered or diploblastic animal, and may be compared to a chimney built of two layers of radially arranged bricks with a space between the layers filled with mortar or concrete. Accurate examination of thin sections, and of specimens teased out or torn into minute fragments with needles, shows that the structure is really much more complicated than the foregoing brief description would indicate. The ectoderm cells are of two kinds. The first and most obvious (b, ect and c), are large cells of a conical form, the bases of the cones being external, their apices internal. Spaces ntc Fig. 50. — Hydra. A, Vertical section of the entire animal, showing the body-wall com- posed of ectoderm {ect) and endoderm {end), enclosing an enteric cavity 224 HYDRA LESS. XXI (enl. cav), which, as well as the two layers, is continued (ent. cav') into the tentacles, and opens externally by the mouth {vith) at the apex of the hypostome (hyp). Between the ectoderm and endoderm is the mesogloea [insgl)^ represented by a black line. In the ectoderm are seen large [nlc) ancl small (7itc') nematocysts : some of the endoderm cells are putting out pseudopods ipsd), others fiagella (_/?). Two buds (bd', bd'). In the course of a few hours this prominence enlarges greatly, and near its distal end six or eight hollow buds appear arranged in a whorl (Fig. 49, a, M'^ ; Fig. 50, XXI REPRODUCTION 231 A, bd'''). These enlarge and take on the characters of ten- tacles : a mouth is formed at the distal end of the bud, which thus acquires the character of a small Hydra (Fig. 49, A, bd^). Finally the bud becomes constricted at its base, separates from the parent, and begins an independent ex- istence. Sometimes, however, several buds are produced at one time, and each of these buds again before becoming detached : in this way temporary colonies are formed. But the buds always separate sooner or later, although they frequently begin to feed while still attached. It is a curious circumstance that Hydra can also be mul- tiplied by artificial division : the experiment has been tried of cutting the living animal into pieces, each of which was found to grow into a perfect individual. As in Vaucheria and Nitella, the sexual organs or gonads are of two kinds, spermaries and ovaries. Both are found in the same individual, Hydra being, like the plants just mentioned, hermaphrodite or moncecious. The spermaries (Fig. 49, b, and Fig. 50, a, spy) are white conical elevations situated near the distal end of the body • as a rule not more than one or two are present at the same time, but there may be as many as twenty. They are per- fectly colourless, even in the green and brown species, being obviously formed of ectoderm alone. In the immature condition the spermary consists of a little heap of interstitial cells covered by an investment of some- what flattened cells formed by a modification of the ordinary large cells of the ectoderm. When mature each of the small internal cells becomes converted into a sperm' (Fig. 50, g), consisting of a small ovoid head formed from the nucleus of the cell, and of a long vibratile tail formed from its proto- plasm. By the rupture of the investing cells or wall of the ii2 HYDRA spermary the sperms are liberated and swim freely in the water. The ovaries (Fig. 49, b, and Fig. 50, a, oi'y) are found near the proximal end of the body, and vary in number from one to eight. When ripe an ovary is larger than a spermary, and of a hemispherical form. It begins, like the spermary, as an aggregation of interstitial cells, so that in their earher stages the se.x of the gonads is indeterminate. But while xmm^-^' '«°«iA /"OC^^ ac:—^^ of Fig. 52.— a, Ovum of Bydra viridis, showing pseudopods, nucleus (gv), and numerous chromatophoi-es and yolk spheres. B, a single yolk sphere. (From Balfour after Kleinenberg. ) in the spermary each cell is converted into a sperm, in the ovary one cell soon begins to grow faster than the rest becomes amoeboid in form (Fig. 50, a, ov, and Fig. 52, a), sending out pseudopods amongst its companions and ingest- ing the fragments into which they become broken up, thus continually increasing in size at their expense. Ultimately the ovary comes to consist of this single amoeboid ovum, and of a layer of superficial cells forming a capsule for it. XXI DEVELOPMENT 233 As the ovum grows yolk-spheres (Fig. 52), small rounded masses of proteid material, are formed in it, and in Hydra viridis it also acquires green chromatophores. When the ovary is ripe the ovum draws in its pseudopods and takes on a spherical form : the investing layer then bursts so as to lay bare the ovum and allow of the free access to it of the sperms. One of the latter conjugates with the ovum, producing an oosperm or unicellular embryo. The oosperm divides into a number of cells, the outer- most of which becomes changed into a hard shell or capsule. The embryo, thus protected, falls to the bottom of the water, and after a period of rest develops into a Hydra. As, how- ever, there are certain abnormal features about the develop- ment of this genus which cannot well be understood by the beginner, it will not be described in detail, but the very important series of changes by which the oosperm of a multicellular animal becomes converted into the adult will be considered in the next lesson. LESSON XXII. HYDROID POLYPES : EOUGAINVILLEA, DIPHYES, AND PORPITA. It was stated in the previous lesson (p. 231) that in a budding Hydra the buds do not always become detached at once, but may themselves bud while still in connection with the parent, temporary colonies being thus produced. Suppose this state of things to continue indefinitely : the result would be a tree-like colony or compound organism consisting of a stem with numerous branchlets each ending in a Hydra-like zooid. Such a colony would bear much the same relation to Hydra as Zoothamnium bears to Vorticella (see p. 134). As a matter of fact this is precisely what happens in a great number of animals allied to Hydra and known by the name of Zoophytes or Hydroid polypes. Every one is familiar Avith the common Sertularians of the sea-coast, often mistaken for sea-weeds : they are delicate, much-branched, semi-transparent structures of a horny con- sistency, the branches beset with little cups, from each of which, during life, a Hydra-like body is protruded. iV very convenient genus for our purpose is Bougainvillea, a hydroid polype found in the form of little tufts a few centimetres long attached to rocks and other submarine Fig. 53. — Bougainvillea raniosa. A, a complete living colony of the natural size, showing the branched stem and root-like organ of attachment. B, a portion of the same magnified, showing the branched stem bear- ing hydranths {hyd) and mechiss; (ined), one of the latter nearly mature, the others undeveloped : each hydranth has a circlet of tentacles (/) surrounding a hypostome (hyp), and contains an enteric cavity (ent. cai;) continuous with a narrow canal [en I. cav') in the stem. The stem is covered by a cuticle [cu). c, a medusa after liberation from the colony, showing the bell with tentacles (t), velum (v), manubrium (iiinb), radial {rod. c) and circular {cir. c) canals, and eye-spots ipc). (After AUman.) 236 HYDROID POLYPES LESS. objects. Fig. 53, a, shows a colony of the natural size, b a part of it magnified : it consists of a much-branched stem of a yellowish colour attached by root-like fibres to the support. The branches terminate in little Hydra-like bodies called hydranths (b, hyd), each with a hypostome (Jiyp) and circlet of tentacles (/). Lateral branchlets bear bell-shaped structures or medusa {med) : these will be considered presently. Sections show that the hydranths have essentially the structure of a Hydra, consisting of a double layer of cells — ectoderm and endoderra — separated by a supporting lamella or mesoglcea, and enclosing a digestive cavity [enf. cm') which opens externally by a mouth placed at the summit of the hypostome. The tentacles, however, differ from those of Hydra in two important respects. In the first place they are solid : the endoderm instead of forming a lining to a prolongation of the enteron, consists (Fig. 55, ejid.) of a single axial row of large cells with thick cell-walls and vacuolated protoplasm. Then in the position of the muscle-processes of Hydra there is a layer of spindle-shaped fibres {m.f.), many times longer than broad, and provided each with a nucleus. Such miisde-fibres are obviously cells greatly extended in length, so that the ectoderm cell of Hydra with its continuous muscle- process is here represented by an ectoderm cell with an adjacent muscle-ir^//. We thus get a partial intermediate layer of cells between the ectoderm and endoderm in addition to the gelatinous mesoglcea, and so, while a hydroid polyp is, like Hydra, diploblastic (p. 222), it shows a tendency towards the assumption of a three-layered or triploblastic condition. The stem is formed of the same layers and contains a cavity (ent. cav) continuous with those of the hydranths, and thus the structure of a hydroid polype is, so far, simply STRUCTURE OF COLONY 237 that of a Hydra in which the process of budding has gone on to an indefinite extent and without separation of the buds. nil Fig. 54. — Portion of the tentacle of a Zoophyte (Eucopella), In the lower part of the figure are seen the ectoderm cells (ect) with the nematocxsts (iitc). In the middle part the ectoderm is removed, and the muscle-fibres (m.f) and nerve-cells (iiv. c) axe exposed. ]n the upper part the muscular and nervous layer is removed, and parts of two endoderm cells {end) are shown ; mi, nucleus. (From Parker and Haswell, after von Lendenfeld.) There is however an additional layer added in the. stem for protective and strengthening purposes. It is evident that a colony of the size shown in Fig. S3, a, would, if formed 238 IIYDROID POLYPES less. only of soft ectodermal and endodermal cells, be so weak as to be hardly able to bear its own weight even in water. To remedy this a layer of transparent, yellowish substance of horny consistency, called the cuticle, is developed outside the ectoderm of the stem, extending on to the branches and only stopping at the bases of the hydranths and medusse. It is this layer which, when the organism dies and decays, is left as a semi-transparent branched structure resembling the living colony in all but the absence of hydranths and medusae. The cuticle is therefore a supporting organ or skeleton, not, like our own bones, formed in the interior of the body (endoskeletOTi), but like the shell of a crab or lobster lying altogether outside the soft parts {exo- skelefon). As to the mode of formation of the cuticle : — we saw that many organisms, such as Amoeba and Hsematococcus, form, on entering into the resting condition, a cyst or cell-wall, by secreting or separating from the surface of their protoplasm a succession of layers either of cellulose or of a transparent horn-like substance. But Amceba and Hsematococcus are unicellular, and are therefore free to form this protective layer at all parts of their surface. The ectoderm cells of Bougainvillea on the other hand are in close contact with their neighbours on all sides and with the mesogloea at their inner ends, so that it is not surprising to find the secretion of skeletal substance taking place only at their outer ends. As the process takes place simultaneously in adjacent cells, the result is a continuous layer common to the whole ectoderm instead of a capsule to each individual cell. It is to an exoskeletal structure formed in this way, i.e. by the secretion of successive layers from the free faces of adjacent cells, that the name cuticle is in strictness applied in multi- cellular orffanisms. XXII STRUCTURE OF A MEDUSA 239 The medusae (b, med. and c), mentioned above as occur- ring on lateral branches of the colony, are found in various stages of development, the younger ones having a nearly globular shape, while when fully formed each resembles a bell attached by its handle to one of the branches of the colony and having a clapper in its interior. When quite mature the medusEe become detached and swim off as little jelly-fishes (c). The structure of a medusa must now be described in some detail. The bell or umbrella (c) is formed of a gela- tinous substance (Fig. 55, d, insgl) covered on both its inner surface or sub-umbrella and on its outer surface or ex-umbrella by a thin layer of delicate cells (ect). The clapper-like organ or manubriu7n (Fig. 53, c and Fig. 55 D and d', mnV) is formed of two layers of cells, precisely resembling the ectoderm and endoderm of Hydra, and separated by a thin mesogloea; it is hollow, its cavity (Fig. 55, d, ent. cav) open- ing below, i.e. at its distal or free end, by a rounded aperture, the mouth {mth), used by the medusa for the ingestion of food. At its upper (attached or proximal) end the cavity of the manubrium is continued into four narrow, radial canals (Fig. 53, c, rad. c, and Fig. 54, d and d' rad) which extend through the gelatinous substance of the umbrella at equal distances from one another, like four meridians, and finally open into a circular canal {cir. c) which runs round the edge of the umbrella. The whole system of canals is lined by a layer of cells (Fig. 55, d and d', end) continuous with the inner layer or endoderm of the manubrium ; and extending from one canal to another in the gelatinous substance of the umbrella is a delicate sheet of cells, the endodenn-lamella (d', end. la). From the edge of the umbrella four pairs of tentacles (Fig. 53, c and Fig. 55, d, t) are given off, one pair corres- 240 IIYDROID rOLVrES less. ponding to each radial canal, and close to the base of each tentacle is a little speck of pigment (Fig. 53, oc), the ocellus or eye-spot. Lastly, the margin of the umbrella is continued inwards into a narrow circular shelf, the velum (v). At first sight there appears to be very little resemblance between a medusa and a hydranth, but it is really quite easy to derive the one form from the other. Suppose a simple polype or Hydra-like body with four tentacles (Fig. 55, a, a') to have the region from which the tentacles spring pulled out so as to form a hollow, trans- versely extended disc (b). Next, suppose this disc to become bent into the form of a cup with its concavity towards the hypostome, and to undergo a great thickening of its meso- glcea. A form would be produced like c, I.e. a medusa-like body with umbrella and manubrium, but with a continuous cavity (c', e/if. cav) in the thickness of the umbrella instead of four radial canals. Finally, suppose the inner and outer walls of this cavity to grow towards one another and meet, thus obliterating the cavity, except along four narrow radial areas (d, rad) and a circular area near the edge of the umbrella (d, dr. c). This would result in the substitution for the continuous cavity of four radial canals opening on the one hand into a circular canal and on the other into the cavity of the manubrium (e7it. cav), and connected with one another by a membrane — the endoderm-lamella {end. la) — indi- cating the former extension of the cavity. It follows from this that the inner and outer layers of the manubrium are respectively endoderm and ectoderm : that the gelatinous tissue of the umbrella is an immensely thickened mesogloea . that the layer of cells covering both inner and outer surfaces of the umbrella is ectodermal : and that the layer of cells lining the system of canals, together with the endoderm-lamella. is endodermal. ect Fig. 55'. — Diagrams illustrating the derivation of the medusa from the hydranth. In the whole series of figures the ectoderm [ect] is dotted, the endoderm [end) striated, and the mesoglcea (msgl) black. A, longitudinal section of a simple polype, showing the tubular body with enteric cavity (etit. cav), hypostome {hyp), mouth (mlh), and tentacles [t). R 242 ■ HYDROID POLYPES less. a', transverse section of the same through the plane a b. B, the tentacular region is extended into a hollow disc. c, the tentacular region has been further extended and bent into a bell-like form, the enteric cavity being continued into the umbrella [eiit. cav') : the hypostome now forms a manubrium (iiinb). c', transverse section of the same through the plane a b, showing the continuous cavity [ent, cav') in the umbrella. D, fully formed medusa : the cavity in the umbrella is reduced to the radiating (rad) and circular {cir. c) canals, the velum (v) is formed, and a double nerve-ring (iw, nv') is produced from the ectoderm. d', transverse section of the same through the plane a b, showing the four radiating canals [rad) united by the endoderm-lamella {end. la), produced by partial obliteration of the continuous cavity ent. cav' in C. Thus the medusa and the hydranth are similarly con- structed or homologous structures, and the hydroid colony, like Zoothamnium (p. 136), is dimorphic, bearing zooids of two kinds. Sooner or later the meduste separate from the hydroid colony and begin a free existence. Under these circum- stances the rhythmical contraction — i.e. contraction taking place at regular intervals — of the muscles of the umbrella causes an alternate contraction and expansion of the whole organ, so that water is alternately pumped out of and drawn into it. The obvious result of this is that the medusa is pro- pelled through the water by a series of jerks. The movement is performed by means of the muscle-processes and muscle- fibres of the sub-umbrella and velum, both of which differ from the similar structures in the hydranth in exhibiting a delicate transverse striation (Fig. 57). There is still another important matter in the structure of the medusa which has not been referred to. At the junction of the velum with the edge of the bell there lies, imme- diately beneath the ectoderm, a layer of peculiar branched cells (Fig. 56, B, n. c), containing large nuclei and produced into long fibre-like processes. These nerve-cells (see p. 227) NERVOUS SYSTEM 243 are so disposed as to form a double ring round the margin of the bell, one ring (Fig. 55, d, tiv) being immediately above, the other {tiv') immediately below the insertion of the velum. An irregular network of similar cells and fibres occurs on the inner or concave face of the umbrella, between the ectoderm and the layer of muscle-fibres. The whole consti- FlG. 56. — A, Muscle fibres from the inner face of the bell of the medusa of a hydroid polype {Eiuopella campanularia), showing nucleus and transverse striation. B, portion of the nerve-ring of the same, showing two large nerve- cells («. c) and muscle-fibres («/. c] on either side. (After von Len- denfeid. ) tutes the ttervous system of the medusa ; the double nerve-ring is the central, the network the peripheral nervous system. Some of the processes of the ner^e-cells are connected with ordinary ectoderm-cells, which thus as it were connect the nervous system with the external world : others, in some instances at least, are probably directly connected with muscle-fibres. We thus see that while the manubrium of a medusa has the same simple structure as a hydranth, or what comes to R 2 244 HYDROID POLYPES less. the same thing, as a Hydra, the umbrella has undergone a very remarkable differentiation of its tissues. Its ordinary ecto- derm cells, instead of being large and eminently contractile, form little more than a thin cellular skin or epithelium over the gelatinous mesoglcea : they have largely given up the function of contractility to the muscle processes or fibres, and have taken on the functions of a protective and sensitive layer. Similarly the function of automatism, possessed by the whole body of Hydra, is made over to the group of specially modified ectodermal cells which constitute the central nervous system. If a Hydra is cut into any number of pieces, each of them is able to perform the ordinary move- ments of expansion and contraction, but if the nerve-ring of a medusa is removed by cutting away the edge of the umbrella, the rhythmical swimming movements stop dead : the bell is in fact permanently paralysed. It is not, however, rendered incapable of movement, for a sharp pinch, i.e. an external stimulus, causes a single con- traction, showing that the muscles still retain their irritabilit)'. But no movement takes place without such external stimulus, each stimulus giving rise infallibly to one single contraction : the power possessed by the entire animal of independent!}* originating movement, i.e. of supplying its own stimuli, is lost with the central nervous system. Another instance of morphological and physiological differentiation is furnished by the pigment spots or ocelli (Fig. 53, c, oc) situated at the bases of the tentacles. They consist of groups of ectoderm cells in which are deposited granules of deep red pigment. Their function is proved by the following experiment. If a number of medusa: are placed in a glass vessel of \\ater in a dark rOom, and a beam of light from a lantern is XXII GONADS 245 allowed to pass through the water, the animals are all found to crowd into the beam, thus being obviously sensitive to and attracted by light. If however the ocelli are removed this is no longer the case : the medusre do not make for the beam of light, and are incapable of distinguishing light from darkness. The ocelli are therefore organs of sight. In Zoothamnium we saw that the two forms of zooid were respectively nutritive and reproductive in function, the re- productive zooids becoming detached and swimming off to found a new colony elsewhere (p. 135). This is also the case with Bougainvillea : the hydranths are purely nutritive zooids, the medusfe, although capable of feeding, are specially distinguished as reproductive zooids. The gonads are found in the walls of the manubrium, between the ectoderm and endoderm, some medusa pro- ducing ovaries, others spermaries only. Thus while Hydra is monmdous, both male and female gonads occurring in the same individual, Bougainvillea is diaciotis, certain individuals producing only male, others only female products. In some Hydroids it has been found that the sexual cells from which the ova and sperms are developed do not originate in the manubrium of a medusa, but apparently arise either in the ectoderm or endoderm of the stem of. the hydroid colony, afterwards migrating, while still small and im- mature, to their permanent situation where they undergo their final development. In Bougainvillea, however, the reproductive products are said to originate in the manubrium. The medusae, when mature, become detached and swim away from the hydroid colony. The sperms of the males are shed into the water and carried to the ovaries of the females, where they fertilize the ova, converting them, as usual, into oosperms. 246 IIYDROID POLYPES i-F.ss. The changes by which the oosperm or unicellular embryo of a hydroid polype is converted into the adult are very remarkable. The process is begun by the oosperm, still enclosed within the body of the parent (Fig. 57, a), undergoing binary fission, so that a two-celled embryo is formed (b). Each of the two cells again divides (c), and the process is repeated, the embryo consisting successively of 2, 4, 8, 16, 32, &c., cells, until a solid globular mass of small cells is produced (d, e) by the repeated division of the one large cell which forms the starting-point of the series. The embryo in this stage has been compared to a mulberry, and is called the morula or polyplast. So far all the cells of the polyplast are alike — globular nucleated masses of protoplasm squeezed into a polyhedral form by mutual pressure. But before long the cells lying next the surface alter their form, becoming cylindrical, with their long axes disposed radially (f). In this way a superficial layer of cells, or ectoderm, is differentiated from an internal mass, or e?idoderm. The embryo now assumes an elongated form (g) and begins to exhibit slow, worm-like movements, finally escaping from the parent and beginning a free existence (h). The ectoderm cells are now found to be ciliated, and before long a cavity appears in the previously solid mass of endoderm cells ; this is the first appearance of the enteron or digestive cavity. In this stage the embryo is called a planula : it swims slowly through the water by means of its cilia, the broader end being directed forwards in progression. It then loses its cilia and settles down on a rock, shell, sea-weed, or other submarine object, assuming a vertical position with its broader end fixed to the support (i). The attached or proximal end widens into a disc of attach- XXII DEVELOPMENT 247 ment, a dilatation is formed a short distance from the free or Fig. 57. — Stages in the development of two hydroid polypes, Lao.- niedca flextiosa (a-h) and Eiidendrhim raiiiosnin (i-m). A, oosperm. B, two-celled, and c, four-celled stage. I), E, polyplast. F, G, formation of planula by differentiation of ectoderm and endoderm. In A-G the embryo is embedded in the maternal tissues. H, free swimming planula, showing ciliated ectoderm, and endoderm enclosing a narrow enteric cavity. I, planula, after loss of its cilia, about to affix itself. K, the same after fixation. I., Hydra-like stage, still enclosed in cuticle. M, the same after rupture of the cuticle and liberation of the tentacles. (After Allman.) distal end, and a thin cuticle is secreted from the whole surface of the ectoderm (k). From the dilated portion 248 HYDROID POLYPES I-ess. short buds arise in a circle : these are the rudiments of the tentacles : the narrow portion beyond their origin becomes the hypostome (i.). Soon the cuticle covering the distal end is ruptured so as to set free the growing tentacles (m) ; an aperture, the mouth, is formed at the end of the hypostome, and the young hydroid has very much the appearance of a Hydra with a broad disc of attachment, and with a cuticle covering the greater part of the body. Extensive budding next takes place, the result being the formation of the ordinary hydroid colony. Thus from the oosperm or impregnated egg-cell of the medusa the hydroid colony arises, while the medusa is produced by budding from the hydroid colony. \Ve have what is called an alternation of gejierations, the asexual genera- tion or agamobiuin (hydroid colony) giving rise by budding to the sexual ge?ieration or ganiobium (medusa), which in its turn produces the agamobium by a sexual process, i.e. by the conjugation of ovum and sperm. Two other Hydroids must be briefly referred to in con- cluding the present lesson. Floating on the surface of the ocean in many parts of the world is found a beautiful transparent organism called Diphyes. It consists of a long, slender stem (Fig. 58, a, a), at one end of which are attached two structures called swimmiiig-bells {m, m) in form something like the bowl of a German pipe, while all along the stem spring at intervals groups of structures {e), one of which is shown on an enlarged scale at b. Each group contains, first, a tubular structure (b, n) with an expanded, trumpet-like mouth, through which food is taken this is clearly a hydranth. From the base of the hydranth proceeds a single, long, branched tentacle or xxn DIPIIYES AND PORPITA 249 " grappling-line " (?'), abundantly provided with nematocysts. Springing from the stem near the base of the hydranth is a body called a medusoid (g), very like a sort of imperfect medusa, and like it containing gonads. Lastly, enclosing all these structures, much as the vi'hite petaloid bract of the common Arum-lily encloses the flower-stalk, is a delicate folded membranous plate (/), to which the name drarf, borrowed from botany, is applied. The whole organism is propelled through the water by the rhythmical contraction of the swimming-bells. Microscopic examination shows that the stem consists, like that of Bougainvillea, of ectoderm, mesogloea, and endo- derm, but without a cuticle. The hydranth has a similar structure to that of Bougainvillea, only differing in shape and in the absence of tentacles round the mouth : the grap- pling lines are formed on the polype-type : the medusoids are merely simplified medusse : the swimming-bells are practic- ally medusae in which the manubrium is absent : and the bracts are shown by comparison with allied forms to be greatly modified medusa-like structures. Diphyes is in fact a free-swimming hydroid colony which, instead of being dimorphic like Bougainvillea, is polymorphic. In addition to nutritive zooids or hydranths, it possesses locomotive zooids or swimming-bells, protective zooids or bracts, and tentacular zooids or grappling-lines. Morpho- logical and physiological differentiation are thus carried much further than in such a form as Bougainvillea. Porpita is another free-swimming Hydroid, presenting at first sight no resemblance whatever to Diphyes. It has much the appearance of a flattened medusa (Fig. 59), consisting of a circular disc, slightly convex above and concave below, bearing round its edge a number of close-set tentacles, and on its under side a central tubular organ (hy) with a ter- 250 IIYDROID POLYPES Fig. 5$. — Diphyes campanulata. A, the entire colony, natural size, showing stem (a) bearing groups of zooids (") and two swimming bells (w/, «/), the apertures of which are marked o. B, one of the groups of zooids marked e in A, showing common stem, (a), hydranth (;;), medusoid (g), bract (t), and branched tentacle or grappling line [i). (From Gegenbaur. ) DIPHYES AND TORriTA 251 minal mouth, like the manubrium of a medusa, surrounded by a great number of structures like hollow tentacles i/iy'). Fig. 59. — A, Porplta pacifica (nat. size), from beneath, showing disc- like stem surrounded by tentacles {t\ a single fimctional hydranth {hy), and numerous mouthless hydranths [hy'). B, vertical section of P. viediterranea, showing the relative i^osifions of the functional {hy) and mouthless (hy') hydranths, the tentacles, and the chambered shell {sh). (A after Duperrey ; B from Huxley after Kolliker.) The discoid body is supported by a sort of shell having the consistency of cartilage and divided into chambers which contain air (b, sli). Accurate examination shows that the manubrium-like 252 IIYDROID POLVrES i.ESS. xxii body (/y) on the under surface is a hydranth, that the short, hollow, tentacle-like bodies {hy) surrounding it are mouthless bydranths, and that the disc represents the common stem of Diphyes or Bougainvillea. So that Porpita is not what it appears at first sight, a single individual, like a Medusa or a Hydra, but a colony in which the constituent zooids have become so modified in accordance with an extreme division of physiological labour, that the entire colony has the char- acter of a single physiological individual. It was pointed out in the previous lesson (p. 230) that Hydra, while morphologically the equivalent of an indefinite number of unicellular organisms, was yet physiologically a single individual, its constituent cells being so differentiated and combined as to form one whole. A further stage in this same process of individuation is seen in Porpita, in which not cells but zooids, each the morphological equivalent of an entire Hydra, are combined and differentiated so as to form a colony which, from the physiological point of view, has the characters of a single individual. LESSON XXIII SPKRiMATOGENESIS AND OOGENESIS. THE iMATURATION AND IMPREGNATION OF THE OVUM. THE CONNECTION BE- TWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS In the preceding lessons it lias more than once been stated that sperms arise from ordinary undifferentiated cells in the spermary, and that ova are produced by the enlargement of similar cells in the ovary. Fertilisation has also been de- scribed as the conjugation or fusion of ovum and sperm. We have now to consider in greater detail what is known as to the precise mode of development of sperms {spermatogenesis) ■ and of ova (oogenesis), as well as the exact steps of the pro- cess by which an oosperm or unicellular embryo is formed by the union of the two sexual elements. The following description applies to animals : recent researches show that essentially similar processes take place in plants. Both ovary and spermary are at first composed of cells of the ordinary kind, the primitive sex-cells, and it is only by the further development of these that the sex of the gonad is determined. In the spermary the sex-cells (Fig. 60, a) undergo repeated fission, forming what are known as the sperm-mother-cells (b). These have been found in several instances to be 254 SPERMATOGENESIS AND OOGENESIS distinguished by a peculiar condition of the nucleus. We saw (p. 65) that the number of chromosomes is constant in Fig. 60. — Spermatogenesis in the Mole-Cricket {Gryllotalpa). A. Primitive sex-cell, just preparatory to division, showing twelve chromosomes (('/"') ; f, the centrosome. B. Sperm-mother-cell, formed by the division of A, and containing wenty-four chromosomes. The centrosome has divided into two. c. The sperm-mother-cell has divided into two by a reducing division, each daughter-cell containing twelve chromosomes. D. Each daughter-cell has divided again in the same manner, a group of four sperm-cells being produced, each with six chromosomes. E. A single sperm-cell about to elongate to form a sperm. F. Immature sperm ; the six chromosomes are still visible in the head. G. Fully formed sperm. (After von Rjith.) xxni REDUCING DIVISION 255 any given animal, though varying greatly in different species. In the formation of the sperm-mother-cells from the primitive sex-cells the number becomes doubled : in the case of the mole-cricket, for instance, shown in Fig. 61, while the ordinary cells of the body, including the primitive sex- cells, contain twelves chromosomes, the sperm-mother-cells contain twenty-four. The sperm-mother-cell now divides (c), but instead of its chromosomes splitting in the ordinary way (p. 64 and Fig. 10) half of their total number — in the present instance twelve — passes into each daughter cell : in this way two cells are produced having the normal number of chromosomes. The process of division is immediately repeated in the same peculiar way (d), the result being that each sperm-mother- cell gives rise to a group of four cells having half the normal number of chromosomes — in the present instance six. The four cells thus produced are the immature sperms (e) : in the majority of cases the protoplasm of each undergoes a great elongation, being converted into a long vibratile thread, the tail of the sperm (f, g), while the nucleus becomes its more or less spindle-shaped head and the centrosome takes the form of a small intermediate piece at the junction of head and tail. Thus the sperm or male gamete is a true cell, specially modified in most cases for active movement : its head, representing the nucleus, is directed forwards in progres- sion, its long tail, formed from the protoplasm, backwards. The direction of movement is thus the precise opposite of that of a monad (p. 36) to which a sperm presents a certain resemblance. This actively motile tailed form is, however, by no means essential : in many animals the sperms are non-motile and in some they resemble ordinary cells. The peculiar variety of mitosis described above, by which 2S6 SPERMATOGENESIS ATSID OOGENESIS less. the number of chromosomes in the sperm-mother-cells is reduced by one-half, is known as a reducing division. As already stated, the ova arise from primitive sex-cells, precisely resembling those which give rise to sperms. These divide and give rise to the egg-mother-celh in which, as in the sperm-mother-cells, the number of chromosomes is doubled. The egg-mother-cells do not immediately undergo division but remain passive and increase, often enormously, in size, by the absorption of nutriment from surrounding parts : in this way each egg-mother-cell becomes an ovum. Sometimes' this nutriment is simply taken in by osmosis, in other cases the growing ovum actually ingests neigh- bouring cells after the manner of an Amceba. Thus in the developing egg the processes of constructive are vastly in excess of those of destructive metabolism. We saw in the second lesson (p. 33) that the products of destructive metabolism might take the form either of waste products which are got rid of, or of plastic products which are stored up as an integral part of the organism. In the developing egg, in addition to increase in the bulk of the protoplasm itself, a formation of plastic products usually goes on to an immense extent. In plants the stored-up materials may take the form of starch, as in Nitella (p. 214), of oil, or of proteid substance : in animals it consists of rounded or angular grains of proteid material, known as yolk-granuks. These being deposited, like plums in a pudding, in the protoplasm, have the effect of rendering the fully-formed egg opaque, so that its structure can often be made out only in sections. "When the quantity of yolk is very great the ovum may attain a comparatively enormous size, as for instance in birds, in which, as already mentioned (p. 69), the " yolk " is simply an immense egg-cell. When fully formed, the typical animal ovum (Fig. 61) STRUCTURE OF TME OVUM 257 consists ot a more or less globular mass of protoplasm, generally exhibiting a reticular structure and enclosing a larger or smaller quantity of yolk-granules. Surrounding the cell-body is usually a cell-wall or cuticle, often of con- siderable thickness and known as the vitelliiie membrajie : frequently it is perforated at one pole by an aperture, the micropyh (fig. (it,, microp). The nucleus is large and has Fig. 6i. — Ovum of a Sea-urchin [Toxopieiis/es liviiius), showing the radially-striated cell-wall (vitelline membrane), the protoplasm contain- ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its network of chromatin, and a large nucleolus (germinal spot). (From Balfour after Hertwig. ) the usual constituents (p. 63) — nuclear membrane, nuclear sap, and chromatin. As a rule there is a very definite nucle- olus, which is often known as the germinal spot, the entire nucleus being called the germinal vesicle. Such a fully-formed ovum is, however, incapable of being fertilized or of developing into an embryo : before it is ripe for conjugation with a sperm or able to undergo the first stages of segmentation it has to go through a process known as the maturatio?t of the egg. S J prow Fig. 62. — The Maturation and Impregnation of tlie Animal Ovum. A, the ovum, surrounded by the vitelline membrane {mem), in the act of forming the first yjolar cell {pol) : 9 cenf, centrosome. B, both polar cells (pel) are formed, the female pronucleus ( 9 firoii) lies near the centre of the ovum, and one of several sperms is shown maldng its way into the ovum at the micropyle {micro/'). LESS. XXIII POLAR CELLS 259 C, the head of the spenn has become the male pronucleus ( S pron), its intermediate piece the male centrosome ( i cent) ; other structures as before. D, the male and female pronuclei are in the act of conjugation. E, conjugation is complete and the segmentation nucleus (seg, m(cJ) formed. (From Parker and Haswell's Zoology.) Maturation consists essentially in a twice-repeated process of cell-division. The nucleus (Fig. 62, a,) loses its mem- brane, travels to the surface of the egg, and takes on the form of an ordinary nuclear spindle. Next the protoplasm grows out into a small projection or bud, into which one end ■of the spindle projects. The usual process of nuclear division then takes place (Fig. 10, p. 64), one of the daughter nuclei remaining in the bud {pol), the other in the ovum itself. Nuclear division is followed as usual by division of the protoplasm, and the bud becomes separated as a small cell distinguished as the first polar cell. It was mentioned in a previous lesson (p. 200) that in some cases development from an unfertilized female gamete took place, the process — which is not uncommon among insects and crustaceans — being distinguished as partheno- genesis. -It has been proved in many instances and may be generally true that in such cases the egg begins to develop after the formation of the first polar cell. Thus in partheno- genetic ova it appears that maturation is completed by the separation of a single polar cell. In the majority of animals, however, development takes place only after fertilization, and in such cases maturation is not complete until a second polar cell (b, pol) has been formed in the same manner as the first. The ovum has now lost a portion of its protoplasm together with three-fourths of its chromatin, half having passed into the first polar cell and half of what remained into the second : the remaining one- fourth of the chromatin takes on a rounded form and is dis- tinguished as the female pronucleus (b, ? pron). s 2 26o SPERMATOGENESIS AND OOGENESIS less. The formation of both polar cells takes place by a reducing division, so that, while the immature ovum con- tains double the number of chromosomes found in the ordinary cells of the species, the mature ovum, like the sperm, contains only one-half the normal number. In some animals the first polar body has been found to divide after separating from the egg. In such cases the egg- mother-cell or imm^ature ovum gives rise to a group of four cells — the mature ovum and three polar-cells ; just as the sperm-mother-cell gives rise to a group of four cells, all of which, however, become sperms. Shortly after, or in some cases before maturation the ovum is fertilized by the conjugation with it of a single sperm. As we have found repeatedly, sperms are produced in vastly greater numbers than ova, and it often happens that a single egg is seen quite surrounded with sperms, all apparently about to conjugate with it. It has however been found to be a general rule that only one of these actually conjugates : the others, like the drones in a hive, perish without fulfilling the one function they are fitted to perform. The successful sperm (b) takes up a position at right angles to the surface of the egg, and gradually passes through the micropyle (inicrop) or works its way through the vitelline membrane until its head lies within the egg protoplasm. The tail is then cast off, and the head, ac- companied by the intermediate piece or centrosome, pene- trating deeper into the protoplasm, takes on the form of a rounded nucleus-like body, the male pronudetis (c, $ p-on). The two pronuclei approach one another (d) and finally unite to form what is called the segmentation nucleus (e. seg. nucl), the single nucleus (e) of what is not now the ovum but the oosperm — the impregnated egg or unicellular embryo. The fertilizing process is thus seen to consist of the union xxui UNICELLULAR AND MULTICELLULAR ANIMALS 261 of two nuclear bodies, one contributed by the male gamete or sperm, the other by the female gamete or ovum. It follows from this that the essential nuclear matter or chro- matin of the oosperm is derived in equal proportions from each of the two parents. Moreover, as both male and female pronuclei contain only half the number of chromosomes found in the ordinary cells of the species, the union of the pronuclei results in the restoration of the normal number to the oosperm. In some cases the astrospheres of the sperm and ovum as well as their nuclei appear to unite with one another, but more usually the egg-centrosome degenerates and disappears, the centrosome of the oosperm — and conse- quently of all the cells of the fully-formed animal — being derived from the centrosome of the sperm, i.e. from the male parent. Fertilization being thus effected, the process of segmenta- tion or division of the oosperm takes place as described in the preceding lesson (p. 246). In concluding the present lesson, we shall consider briefly a point which has probably already struck the reader. Among the plant-forms which have come under our notice there has been a very complete series of gradations from the simple cell, through the non-cellular filament, linear aggregate, and superficial aggregate, to the solid aggregate, whilst among the animals already discussed there has so far been no attempt to fill up the ^■ery considerable gap between unicellular and multicellular forms. In Amoeba, Vorticella, y, spy, spy", and Fig. 65, s). By the conjugation of a sperm (f) with an ovum (g) an oosperm (h) is produced, and from this by continued division a new colony arises. ^^olvox is clearly comparable to a hollow polyplast, and further resembles the higher or multicellular animals in that certain of its cells are differentiated to form true sexual products. It is necessary, in conclusion, to remind the reader that xxin UNICELLULAR AND MULTICELLULAR ANIMALS 267 the Mycetozoa and Opalina may be said to take an inter- mediate place between the strictly unicellular and the multi- cellular animals in much thesameway as Mucorand Vaucheria connect unicellular and multicellular plants. The Plas- modium of the Mycetozoa is formed, in the first instance (p. 54), by the fusion of amoebulas : hence it is a many-celled structure, the constituent cells of which have lost their boundaries and are indicated only by their nuclei. Sub- sequently the nuclei multiply by division, and, although the process does not affect the protoplasm, it is allowable to say that the number of virtual cells of which the Plasmodium is composed is thereby increased. The Mycetozoon, in its plasmodial stage, is, in fact, a non-cellular organism, like Mucor or ^'^aucheria. But if this way of looking at the Mycetozoa is correct, it follows that Opalina is to be con- sidered rather as a multinucleate but non-cellular than as a unicellular animal. LESSON XXIV POLYGORDIUS PoLYGORDius is a minute worm, about 3 or 4 cm. in length, found in the European seas, where it lives in sand at a depth of a few fathoms. It has much the appearance of a tangle of pink thread with one end produced into two delicate processes (Fig. 66, a). These, which are the tentacles, mark the anterior end of the animal — the opposite extremity, which in some species also bears a pair of slender processes, is the posterior end. As the creature creeps along, one side is kept constantly upwards and is distinguished as the dorsal aspect ; the lower surface is called ventral. The anterior end is narrower than the rest of the body, and is marked off behind by a groove (b and c) ; this division is called the prostomium {Pr. si) and bears the tentacles (t) already mentioned in front and above and on each side a small oval depression {c. p) lined with cilia. Immediately following the prostomium is a region clearly marked off in front, but ill-defined posteriorly, and known as thf peristomium {Per. st) ; on its ventral surface is a trans- verse triangular aperture the mouth {Milt). The rest of the body is more or less distinctly marked by annular grooves (d and e, gr) into body-segments or melaDieres T, st Fig. 66. — Polygordius neapoKtanus. A, the living animal, dorsal aspect, about five times natural size. B, anterior end of the worm from the right side, more highly magni- fied, showing the prostomium (Pr. st), iDeristomium (Per. si), tentacles (t), with set;e (s) and ciliated pit (c. /). c, ventral aspect of the same : letters as before except Afth, mouth. D, portion of body showing metameres (Mtmr) separated by grooves E, posterior extremity from the ventral aspect, showing the last three metameres (Mtmr) separated by distinct grooves (gr), the anal seg- ment (An, seg) bearing the anus (-■in), and a circlet of papillse (f), (After Fraipont.) 270 POLYGORDIUS less xxiv. {Mtmr), the number of which varies considerably. Poly- gordius is thus the first instance we have met with of a trans- versely segmented animal. The last or anal segment (e, An. seg) differs from the others by its swollen form and by bearing a circlet of little prominences or papilte (p) ; it is separated from the preceding segment by a deep groove and bears at its posterior end a small circular aperture, the anus {All). Polygordius may therefore be described as consisting of a number of more or less distinct segments which follow one another in longitudinal series ; three of these, the prostomium, which lies altogether in front of the mouth, the peristomium, which contains the mouth, and the atial segment, which contains the anus, are constant and are distinguished by special characters ; while between the peristomium and the anal segment are intercalated a variable number of metameres which resemble one another in all essential respects. Polygordius feeds in much the same way as an earth- worm ; it takes in sand, together with the various nutrient matters contained in it, such as infusoria, diatoms, &:c., by the mouth, and after retaining it for a longer or shorter time in the body, expels it by the anus. It is obvious, therefore, that there must be some kind of digestive cavity into which the food passes by the mouth, and from which effete matters are expelled through the anus. Sections (Fig. 67) show that this cavity is not a mere space excavated in the interior of the body, but a definite tube, the enteric canal (a, b), which passes in a straight line from mouth to anus, and is separated in its whole extent from the walls of the body (a, B. fF!) by a wide space, the body cavity or calome [Ccel). So that the general structure of Polygordius might be imi- tated by taking a wide tube, stopping the ends of it with SS V o .S'^ t/3 dj rt , • U T' s P-S ,a ^ .^ c| 1^ > " ^ agra: St), the; ^ 2 c-S t; ° -> « tn f* '-a iC-TS «"" c S g •"l^ g •-,.■"- 5: - g 2 ^cq 4^,-^ ^ tig 2 ht tentac body-wa rface and mouth (. An) ; it i 2^^ b/3 lu :i! Ccelomic epithelium layer • '^^ Mesoderm lesoderm . . layer j / • 11 (rudimentary)j ' ^P^"^ ^ ^l^'' Splanchnic i Ccelomic epithelium ' layer \ (visceral layer). Endoderm Enteric epithelium. I In the majority of the higher animals there is a layer of muscle between the enteric and coslomic epithelia : in such cases the body-wall and enteric canal consist of the same layers but in reverse order, the ccelomic epithelium being internal in the one, external in the other. T 2 276 POLYGORDIUS i.ess. Strictly speaking, this comparison does not hold good of the anterior and posterior ends of the worm : at both mouth and anus the deric passes insensibly into the enteric epithe- lium, and the study of development shows (p. 296) that the cells lining both the anterior and posterior ends of the canal are, as indicated in the diagram (Fig. 68, b), ectodermal. For this reason the terms deric and enteric epithelium are not mere synonyms of ectoderm and endoderm respectively. It is important that the student should, before reading further, understand clearly the general composition of a triploblastic animal as typified by Polygordius, which may be summarised as follows. It consists of two tubes formed of epithelial cells, one within and parallel to the other, the two being continuous at either end of the body where the inner tube (enteric epithelium) is in free communication with the exterior ; the outer tube (deric epithelium) is lined by a layer of muscle-plates within which is a thin layer of coelomic epithelium, the three together forming the body- wall ; the inner tube (enteric epithelium) is covered ex- ternally by a layer of ccelomic epithelium which forms with it the enteric canal ; lastly, the body-wall and enteric canal are separated by a considerable space, the coelome. The enteric canal is not, as might be supposed from the foregoing description, connected with the body-wall only at the mouth and anus, but is supported in a peculiar and somewhat complicated way. In the first place there are thin vertical plates, the dorsal and ventral mesenteries (Fig. 67, A and c, Z>. Mes, V. Mes), which extend longitudinally from the dorsal and ventral surfaces of the canal to the body wall, dividing the ccelome into right and left hahes. The structure of the mesenteries is seen in a transverse section (Pig, 67, c, and Fig. 70, a) which shows that at the middle XXIV ENTERIC CANAL 277 dorsal line the parietal layer of ccelomic epithelium becomes deflected downwards, forming a two-layered membrane, the dorsal mesentery ; the two layers of this on reaching the enteric canal diverge and pass one on either side of it, form- ing the visceral layer of coelomic epithelium ; uniting again below the canal, they are continued downwards as the ventral mesentery, and on reaching the body-wall diverge once more to Join the parietal layer. Thus the mesenteries are simply formed of a double layer of ccelomic epithelium, continuous on the one hand with the parietal and on the other with the visceral la)'er of that membrane. Beside the mesenteries, the canal is supported by trans- verse vertical partitions or septa (Fig. 67, a and b. Sept) which extend right across the body-cavity, each being perforated by the canal. The septa are regularly arranged and correspond with the external grooves by which the body is divided into metameres. Thus the transverse or metameric segmen- tation affects the ccelome as well as the body-wall. Each septum is composed of a sheet of muscle covered on both sides with ccelomic epithelium (e. Sept). Where the septa come in contact with the enteric canal, the latter is more or less definitely constricted so as to pre- sent a beaded appearance (a and b) ; thus we have segmen- tation of the canal as well as of the body-wall and ccelome. The digestive canal, moreover, is not a simple tube of even calibre throughout, but is divisible into four portions. The first or pharynx {Ph) is very short, and can be pro- truded during feeding; the second, called the gullet or cesophagus {Oes), is confined to the peristomium and is distin- guished by its thick walls and comparatively great diameter ; the third or intestine {Int) extends from the first metamere to the last — i.e., from the segment immediately following the peristomium to that immediately preceding the anal 278 POLVGORDIUS less. segment ; it is laterally compressed so as to have an elongated form in cross section (c, and Fig. 70, a) : the fourth portion or rectum {Rc() is confined to the anal seg- ment ; it is somewhat dilated and is not laterally compressed. The epithelium of the intestine is, as indicated in the diagram (b), endodermal ; that of the remaining divisions of the canal is ectodermal. The large majority of the cells in all parts of the canal are ciliated. The cells of the enteric canal and especially those of the gullet are very granular, and like the endoderm cells of the hypostome of Hydra (p. 228) are to be considered as gland cells. They doubtless secrete a digestive juice which, mixing with the various substances taken in by the mouth, dissolves the proteids and other digestible parts, so as to allow of their absorption. There is no evidence of intra- cellular digestion such as occurs in Hydra (p. 229), and it is very probable that the process is purely extra-cellular or enteric, the food being dissolved and rendered diffusible entirely in the cavity of the canal. By the movements of the canal — caused partly by the general mo^■ements of the body and partly by the contraction of the muscles of the septa, aided by the action of the cilia — the contents are gradually forced backwards and the sand and other indi- gestible matters are expelled at the anus. The coelome is filled with a colourless, transparent ccelomk fluid in which are suspended minute, irregular, colourless bodies, as well as oval bodies containing yellow granules. From the analogy of the higher animals one would expect these to be leucocytes (p. 56), but their cellular nature has not been proved. The function of the coelomic fluid is probably to distribute the digested food in the enteric canal to all parts of the XXIV BLOOD-VESSELS 279 body. In Hydra, where the lining wall of the digestive cavity is in direct contact with the simple wall of the body the products of digestion can pass at once by diffusion from endoderni to ectoderm, but in the present case a means of communication is wanted between the enteric epithelium and the comparati\Lly complex and distant body-wall. The peptones and other products of digestion diffuse through the enteric epithelium into the coilomic fluid, and by the con- tinual movement of the latter — due to the contractions of the body-wall — are distributed to all parts. Thus the external epithelium and the muscles, as well as the nervous system and reproductive organs, not yet described, are wholly dependent upon the enteric epithelium for their supply of nutriment. We have now to deal with structures which we find for the first time in Polygordius, namely blood-vessels. Lying in the thickness of the dorsal mesentery is a delicate tube (Fig. 67, A and c, D.Y.) passing along almost the whole length of the body ; this is the dorsal vessel. A similar vc7itral vessel (\^.V) is contained in the ventral mesentery,' and the two are placed in communication with one another in every segment by a pair of commissural vessels {h,Com.i') which spring right and left from the dorsal trunk, pass downwards in or close behind the corresponding septum, following the contour of body-wall, and finally open into the ventral vessel. Each commissural vessel, at about the middle of its length, gives off a recurrent vessel (R.V.) which passes backwards and ^ The statement that the dorsal and ventral vessels he in the thickness o( the mesenteries requires qualification. As ■^ matter of fact, these vessels are simply spaces formed by the divergence of the two layers of epithelium composing the mesentery (Fig. 67, c, and Fig, 70, A) : only thejr anterior ends have proper N\alis. 28o POLYGORDIUS less. ends blindly. The anterior parts of the commissural vessels lie in the peristomium and have an oblique direction, one on each side of the gullet. The whole of these vessels form a single, closed vascular system, there being no communication between them and any of the remaining cavities of the body. The vascular system contains a fluid, the blood, which varies in colour in the different species of Polygordius, being either colourles.s, red, green, or yellow. In one species cor- puscles (? leucocytes) have been found in it. The function of the blood has not been actually proved in Polygordius, but is well known in other worms. In the common earthAvorm, for instance, the blood is red, the colour being due to the same pigment, hcemoglobin, which occurs in our own blood and in that of other vertebrate animals. Haemoglobin is a nitrogenous compound, containing, in addition to carbon, hydrogen, nitrogen, oxygen, and sulphur, a minute quantity of iron. It can be obtained pure in the form of crystals which are soluble in water. Its most striking and physiologically its most important property is its power of entering into a loose chemical combination with oxygen. If a solution of hsemoglobin is brought into contact with oxygen it acquires a bright scarlet colour, and the solu- tion is then found to have a characteristic spectrum distin- guished by two absorption-bands, one in the yellow, another in the green. Loss of oxygen changes the colour from scarlet to purple, and the spectrum then presents a single broad absorption-band intermediate in position between the two of the oxygenated solution. This property is of use in the following way. All parts of the organism are constantly undergoing destructi^-e meta- bolism and gi\ing off carbon dioxide : this gas is absorbed by the blood, and at the same time the htemoglobin gives up XXIV EXCRETORY ORGANS 281 its oxygen to the tissues. On the other hand, whenever the blood is brought sufficiently near the external air — or water in the case of an aquatic animal — the opposite process takes place, ox\'gen being absorbed and carbon dioxide given off. Hfemoglobin is therefore to be looked upon as a respirator}- or oxygen-carrying pigment ; its function is to provide the various parts of the body with a constant supply of oxygen, while the carbon dioxide formed by their oxidation is given up to the blood. The particular part of the body in which the carbon dioxide accumulated in the blood is exchanged for the oxygen of the surrounding medium is called a respiratory organ ; in Polygordius, as in the earthworm and many other of the lower animals, there is no specialised respiratory organ — lung or gill — but the necessary exchange of gases is performed by the entire surface of the body. In discussing in a previous lesson the differences between plants and animals, we found (p. 178) that in the unicellular organisms previously studied, the presence of an excretory organ in the form of a contractile vacuole was a characteristic feature of such undoubted animals as the ciliate infusoria, but was absent in such undoubted plants as Vaucheria and Mucor. But the reader will have noticed that Hydra and its allies have no specialised excretory organ, waste products being apparently discharged from any part of the surface. In Polygordius we meet once more with an animal in which excretory organs are present, although, in correspondence with the complexity of the animal itself, they are very different from the simple contractile vacuoles of Paramoe- cium or A'^orticella. The excretory organs of Polygordius consist of little tubes called nephridia, of which each metamere possesses a pair, one on either side (Fig. 67, a, b, and c, Nphm). Each 282 POLVGORDIUS less. nephridium (Fig. 68) is an extremely delicate tube consisting of two divisions bent at right angles. The outer division is placed vertically, lies in the thickness of the body-wall, and opens externally by a minute aperture, the neplwidiopore (Figs. 68 and 69, Nph. p). The inner division is horizontal and lies in the coelomic epithelium , passing forward it pierces the septum which bounds the segment in front (Fig. 67, A and b), and then dilates into a funnel-shaped extremity or neplirostome {Nph. st), \\\\\c\\ places its ca\"ity in free com- munication with the ccelome. The whole interior of the tube as well as the inner face of the nephrostome is lined with cilia which work outwards. Nph-st^i^ -Njih.p. Fig. 68. — A nephvidium of Polygordius, showing the cilia lining thu tube, the ciliated funnel or nephrostome [Nph. si), and the external aperture or nephridiopore [A^fk. i>). (After Fraipont. ) A nephridium may therefore be defined as a ciliated tube, lying in the thickness of the body-wall and opening at one end into the coelome and at the other on the exterior of the body. In the higher worms, such as the earthworm, the nephridia are lined in part by gland-cells, and are abundantly supplied with blood-vessels. Water and nitrogenous waste from all parts of the body pass by diffusion into the blood and are conve)'ed to the nephridia, the gland-cells of which withdraw the waste products and pass them into the cavities of the tubes, whence they are finally discharged into the surround- ing medium. In all probability some such process as this takes place in Polygordius XXIV NERVOUS SYSTEM 283 In discussing the hydroid polypes we found that one of the most important points of difference between the loco- motive medusa and the fixed hydranth was the presence in the former of a well-developed nervous system (p. 243) con- sisting of an arrangement of peculiarly modified cells, to which the function of automatism was assigned. It is natural to expect in such an active and otherwise highly- organised animal as Polygordius a nervous system of a considerably higher degree of complexity than that of a medusa. The central nervous system consists of two parts, the brain and the ventral nerve-cord. The brain (Fig. 67, a and B, Br) is a rounded mass occupying the whole interior of the prostomium and divided by a transverse groove into two lobes, the anterior of which is again marked by a longitu- dinal groove. The ventral nerve-cord ( V. Nv. Cd.) is a longitudinal band extending along the whole middle ventral line of the body from the peristomium to the anal segment. The posterior lobe of the brain is connected with the anterior end of the ventral nerve-cord by a pair of nervous bands, the oesophageal connectives (CEs. Con) which pass respectively right and left of the gullet. It is to be noted that one division of the central nervous system — the brain — lies altogether above and in front of the enteric canal, the other division — the ventral nerve-cord — altogether beneath it, and that, in virtue of the union of the two divisions by the oesophageal connectives, the enteric canal perforates the nervous system. It is also important to notice that the nervous system is throughout in direct contact with the epidermis or ectoderm, the ventral cord appearing in sections (Fig. 67, c, and Fig. 70, a) as a mere thickening of the latter. Both brain and cord are composed of delicate nerve-fibres 2S4 POLYGORDIUS LESK, (Fig. 69, Nv. F) interspersed with nerve-cells {Nv. C). In the cord the fibres are arranged longitudinally, and the nerve-cells are ventral in position, forming a layer in imme- Der I'lith ner.Eptlim. Fig. 69. — Diagram illiisfratirg the relations of the nervous system of Polygordius. The deric epithelium {Der. Epihm) is either in direct contact with the central nervous system (lower part of figure), or is connected by afferent nerves [af. iiv) with the inter-muscular j^lexus [int. iinisc. p/x) : the latter is connected with the muscle-plates [M. PI) by efferent nerves (Ef. ,w). The central nervous system consists of nerve-fibres {^"1'. F) and nerve-cells (Nv. C) : other nerve cells {A'v. C) occur at intervals in the inter-muscular plexus. The muscle-plates (M. Pl)^ one of which is entire, while only the middle part of the other is shown, are invested by a delicate protoplasmic network, containing nuclei (««), to which the efferent nerves can be traced. (The details copied from Fraipont.) diate contact with the deric epithelium. In the posterior lobe of the brain the nerve-cells are superficial and the central part of the organ is formed of a finely punctate XXIV NERVOUS SYSTEM 2S5 substance in which neither cells nor fibres can be made out. Ramifying through the entire muscular layer of the body- wall is a network of delicate .nerve-fibres {int. muse, fix.) with nerve-cells {jVv. C') at intervals, the inter-muscular plexus. Some of the branches of this plexus are traceable to nerve-cells in the central nervous system, others {af. nv.) to epidermic cells, others {Ef. 7iv.) to the delicate proto- plasmic layer covering the muscle-plates. The superficial cells of both brain and cord are also, as has been said, in direct connection with the overlying epidermis, and from the anterior end of the brain a bundle of nerve-fibres (Fig 67, b, A^v.) is given off on each side to the corresponding tentacle, constituting the nerve of that organ, to the epidermic cells of which its fibres are distributed. ^^'e see then that, apart from the direct connection of nerve-cells with the epidermis, the central nervous system is connected, through the intermediation of nerve-fibres (a) with the sensitive cells of the deric epithelium and (1^) with the contractile muscle-plates. And we can thus distinguish two sets of nerve-fibres, {a) sensory or afferent {af. nv.) which connect the central nervous system with the epidermis, and (1^) motor or efferent {Ef. nv.) which connect it with the muscles. Comparing the nervous system of Polygordius with that of a medusa (p. 243) there are two chief points to be noticed. Firstly, the concentration of the central nervous system in the higher type, and the special concentration at the anterior end of the body to form a brain. Secondly, the important fact that the inter-muscular plexus is not, like the peripheral nervous system of a medusa which it resembles, situated immediately beneath the epidermis (ectoderm) but lies in the muscular layer, or, in other words, has sunk into the mesoderm. 286 POLYGORDIUS LESS. It is obvious that direct experiments on the nervous system would be a very difficult matter in so small an animal as Polygordius. But numerous experiments on a large number of other animals, both higher and lower, allow us to infer ' with considerable confidence the functions of the various parts in this particular case. If a muscle be laid bare or removed from the body in a living animal it may be made to contract by the application of various stimuli, such as a smart tap (mechanical stimulus), a drop of acid or alkali (chemical stimulus), a hot wire (ther- mal stimulus), or an electric current (electrical stimulus). If the motor nerve of the muscle is left intact the application to it of any of these stimuli produces the same effect as its direct application to the muscle, the stimulus being con- ducted along the eminently irritable but non-contractile nerve. Further, if the motor nerve is left in connection with the central nervous system, i.e., with one or more nerve-cells, direct stimulation of these is followed by a contraction, and not only so, but stimulation of a sensory nerve connected with such cells produces a similar result. And finally, stimulation of an ectoderm cell connected, either directly or through the intermediation of a sensory nerve, with the nerve-cells, is also followed by muscular contraction. An action of this kind, in which a stimulus applied to the free sensitive surface of the body is transmitted along a sensory nerve to a nerve-cell or group of such cells and is then, as it were, reflected along a motor nerve to a muscle, is called a }-efiex action ; the essence of the arrangement is the inter- position of nerve-cells between sensory or afferent nerves connected with sensory cells, and motor or efferent nerves connected with muscles. The diagram (Fig. 69) serves to illustrate this matter, The muscle-plate (J/. Fl.) ma)- be made to contract by a XXIV ORGANS OF SENSE 287 stimulus applied (a) to itself directly, {/>) to the motor fibre {Ef. ?ni), (c) to the nerve-cells {JVv. C) in the central nervous system, or to those {Nv. C) in the inter-muscular plexus, {d) to the sensory fibre {af. nv.), or (e) to the epidermic cells {Der. Eptluii). In all probability the whole central nervous system of Polygordius is capable of automatic action. It is a well- known fact that if the body of an earthworm is cut into several pieces each performs independent movements ; in other words, the whole body is not, as in the higher animals, paralysed by removal of the brain. There can, however, be little doubt that complete co-ordination, i.e., the regulation of the various movements to a common end, is lost when the brain is removed. The nervous system is thus an all-important means of communication between the various parts of the organism and bet\\'een the organism and the external world. The outer or sensory surface is by its means brought into connection with the entire muscular system with such perfection that the slightest touch applied to one end of the body may be followed by the almost instantaneous contrac- tion of muscles at the other. In some species of Polygordius the prostomium bears a pair of eye-specks, but in the majority of species the adult animal is eyeless, and, save for the ciliated pits (Fig. 66, B, (T./), the function of which is not known, the only definite organs of sense are the tentacles, which have a tactile function, their abundant nerve-supply indicating that their delicacy as organs of touch far surpasses that of the general surface of the body. They are beset with short, fine pro- cesses of the cuticle called seta (Figs. 66 and 67, s), which probably, hke the whiskers of a cat, serve as conductors of external stimuli to the sensitive epidermic cells. 288 POLYGORDIUS less. There are two matters of general importance in connec- tion with the structure of Polygordius to which the student's attention must be drawn in concluding the present lesson. Notice in the first place how in this type, far more than in any of those previously considered, we have certain definite parts of the body set apart as organs for the performance of particular functions. There is a mouth for the reception of food, an enteric canal for its digestion, and an anus for the extrusion of fseces : a ccelomic fluid for the transport of the products of digestion to the more distant parts of the body : a system of blood-vessels for the transport of oxygen to and of carbon dioxide from all parts : an epidermis as organ of touch and of respiration ; nephridia for getting rid of water and nitrogenous waste : and a definite nervous system for regulating the movements of the various parts and forming a means of communication between the organism and the external world. It is clear that differentiation of structure and division of physiological labour play a far more obvious and important part than in any of the organisms hitherto studied. Notice in the second place the vastly greater complexity of microscopic structure than in any of our former types. The adult organism can no longer be resolved into more or less obvious cells. In the deric, enteric, and ccelomic epithelia we meet with nothing new, but the muscle-plates are not cells, the nephridia show no cell-structure, neither do the nerve-fibres nor the punctate substance of the brain. The body is thus divisible into tissues or fabrics each clearly distinguishable from the rest. A\'e have epithelial tissue, cuticular tissue, muscular tissue, and nervous tissue : and the blood and ccelomic fluid are to be looked upon as liquid tissues. One result of this is that, to a far greater extent than in the foregoing types, we can study the morphology of Polygordius under two distinct heads : XXIV ANATOMY AND HISTOLOGY 289 anatomy, dealing with the general structure of the parts, and histology, dealing with their minute or microscopic structure. One point of importance must be specially referred to in connection with certain of the tissues. It has been pointed out (p. 273) that the epidermis has rather the character of a sheet of protoplasm with regularly-arranged nuclei than of a layer of cells, and that the muscle-plates are covered with a layer of protoplasm with which the ultimate nerve-fibres are continuous (p. 274). Thus certain of the tissues of Polygordius are multinucleate but non-cellular. They are comparable in minute structure to an Opalina or to the Plasmodium of a Mycotozoon, and must therefore be dis- tinguished frorh such definitely cellular tissues as the enteric epithelium. LESSON XXV Poi.VGORDius {Continued) Asexual reproduction is unknown in Polygordius, and the organs of sexual reproduction are very simple. The animal is dioecious, gonads of one sex only being found in each individual. In the species which has been most thoroughly investi- gated {P. neapolitanns) the reproductive products are formed in each metamere from the fourth to the last. Crossing these segments obliquely are narrow bands of muscle (Fig. 70, A, O.M) and certain of the cells of coelomic epithelium covering these bands multiply by fission and form little heaps of cells {Spy), each of which is to be looked upon as a gonad. There is thus a pair of gonads to each segment with the exception of the prostomium, the peristomium, the first three metameres, and the anal segment, the reproductive organs exhibiting the same simple metameric arrangement as the digestive, excretory, and circulatory organs. It will be noticed that the primitive sex-cells, arising as they do from coelomic epithelium, are mesodermal structures, not ectodermal as in hydroids (pp. 231 and 245). In the male the primitive sex-cells divide and sub-divide, the ultimate products being converted into sperms (Fig. 70, Fig. 70. — Polygordius iieapolitamis. A, transverse section of a male specimen to show the position of the immature gonads {spy) and the precise form and arrangement of the various layers represented diagrammatically in Fig. 68, c The body-wall consists of cuticle (Cii), deric epithelium [Der. Eplhtn), muscle-plates (/I/. PI), and parietal layer of coelomic epithelium (Cal. Epthm). The ventral nerve cord { V. Nv. Cd) is shown to be continu- ous with the deric epithelium. The enteric canal consists of ciliated enteric epithelium [Ent. Epthm) covered by the visceral layer of ccelomic epithelium [Ccsi. Epthm') : connecting it with the body-wall are the dorsal and ventral mesenteries formed of a double layer of coelomic epithelium, and containing respec- tively the dorsal (D. V) and ventral ( V. V) blood-vessels. Passing obliquely across the ccelomc are the oblique muscles ( 0. M) tr 2 292 POLYGORDIUS less. covered with ccelomic epithelium : by differentiation of groups of cells of the latter the spermaries {Spy) are formed. B, a single sperm, showing expanded head and delicate tail. c, horizontal section of a sexually mature female. The body-wall (C//, Der. Epthm, M. PI) has undergone partial histological degeneration, and is ruptured in two places to allow of the escape of the ova (oz') which still fill the ccelomic spaces enclosed betw^een the body-wall, the enteric canal {Ent, Epilwi), and the septa {Sep). (After Fraipont.) E : see p. 255) : in the female they enlarge immensely, and take on the character of ova (c, ov). Multiplication of the sexual products takes place to such an extent that the whole ccelome becomes crammed full of either sperms or ova (c). In the female the growth of the eggs takes place at the expense of all other parts of the body, which undergo more or less complete atrophy : the epidermis for instance, be- comes liquefied and the muscles lose their contractility. Finally rupture of the body-wall takes place in each segment (c), and through the shts thus formed the eggs escape. So that Polygordius, like an annual plant, produces only a single brood ; death is the inevitable result of sexual maturity. A\'hether or not the same dehiscence of the body- wall takes place in the male is not certain : it has been stated that the sperms make their escape through the nephridia. Thus while there are no specialized gonoducts, or tubes for carrying off the sexual products, it is possible that the ne- phridia may, in addition to their ordinary function, serve the purpose of male gonoducts or spermiduds. Female gono- ducts or oviducts are however entirely absent. The ova and sperms being shed into the surrounding water, impregnation takes place, and the resulting oosperm undergoes segmentation or division (see p. 246), a polyplast being formed. The cells of the polyplast become differen- tiated, an enteron or digestive cavity is formed, and the THE TROCHOSPHERE m embryo is gradually converted into a curious free-swimming creature shown in Fig. 72, a, and called a trochosphere. The trochosphere, or newly-hatched larva of Polygordius (Fig. 71, a) is about \ mm. in diameter, and has something Fig. 71. — A, larva of Polygordius neapolitanus in the trochosphere stage ; from a living specimen. B, diagrammatic vertical section of the same : the ectoderm is dotted, the endoderm radially striated, the mesoderm evenly shaded, and the nervous system finely dotted. c, transverse section through the plane ab in B. The body-wall consists of a single layer of ectoderm cells, which, at the apex of the prostomium (upper hemisphere) are modified to form the brain {Br) and a pair of ocelli [oc). The enteric canal consists of three parts : the stomodasum (St. dm), opening externally by the mouth {Mth), and lined by ectoderm ; the enteron {Ent) lined by endoderm ; and the proctodaeum [Pre. dm), opening by the anus [An) and lined by ectoderm. Between the body-wall and the enteric canal is the larval body-cavity or blastocoele [Bl. caJ). The mesoderm is confined to two narrow bands of cells (B and c, Msd) in the blastocoele, one on either side of the proctodfeum ; slender mesodermal bands (Msd') are also seen in the prostomium in A. The cilia consist of a prfE-oral circlet (Pr. or. ci) above the mouth, a post-oral circlet (Pt. or. ci) below the mouth, and an anal circlet (An. ci) around the anus. (a after Fraipont.) the form of a top, consisting of a dome-like upper portion, the prostomium, produced into a projecting horizontal rim ; of an intermediate portion or peristomium, having the form of an inverted hemispheret; and of a lower somewhat conical 294 POLVGORDIUS less. ttfial region. Around the projecting rim is a double circlet of large cilia {Fr. or. ci) by means of which the lar\'a is propelled through the water. Beneath the edge of the ciliated rim is a rounded aperture, the mouth {Mth); this leads by a short, nearly straight gullet (.S*^. dm), into a spacious stomach {Ent), from the lower side of which proceeds a short slightly curved intestine {Pre. dm), opening at the extremity of the conical inferior region by an anus {An). Between the body-wall and the enteric canal is a space filled with fluid {Bl. cxl), but, as we shall see, this does not correspond with the body-cavity of the adult. The body-wall and the enteric canal consist each of a single layer of epithelial cells, all the tissues included in the adult under the head of mesoderm (p. 275) being absent or so poorly developed that they may be neglected for the present. Leaving aside all details, it will be seen that the trocho- sphere of Polygordius is comparable in the general features of its organization to a medusa (compare Fig. 56, p. 241), consisting as it does of an outer layer of cells forming the external covering of. the body and of an inner layer lining the digestive cavity. There are, however, two important differences : the space between the two layers is occupied by the mesogloea in the medusa, while in the worm it is a cavity filled with fluid ; and the digestive cavity of the trochosphere has two openings instead of one. But in order to compare more accurately the medusa with the trochosphere, it is necessary to fill up, by the help of other types, an important gap in our knowledge of the development of Polygordius — the passage from the polyplast to the trochosphere. From what we know of the develop- ment of other worms, the process, in its general features, is probably as follows ; — » The polyplast is converted, by the accumulation of fluid FORMATION OF TROCHOSPHERE 295 in its interior, into a hollow sphere, bounded by a single layer of cells and containing a cavity, the blastocxle : this stage of development is called the Mastida. Next, one side of the blastula becomes tucked in or invaginated so as to convert the embryo from a single-layered sphere into a double-layered cup (Fig. 72, a). This process can be sufficiently well imitated by pushing in one side of a hollow india-rubber ball. The resulting embryonic stage —l^rcdm Ca.st.Mlh ^An Fig. 72. — Diagram illustrating the origin of the trochospheie from the gastrula. The ectoderm is dotted, the endoderm striated. A, gastrula, with enteron {£:ni) and gastrula-mouth {Gas/. Mth), and with the ectoderm and endoderm separated by the larval body-cavity or blastocoele {Bl. cccl). B, the gastrula-mouth has closed, the enteron [Ent) becoming a shut sac. c, two ectodermal pouches, the stomodajum (St. dm) and proctodeum (Pre. dm) have appeared. D, the stomodseum (St. dm) and proctodseum (Pir. dm) have opened into the enteron (Ent), forming a complete enteric canal with mouth (Mth) and anus (An). is known as the gastrula : its cavity is the enteron {£fit) and is bounded by the invaginated cells which now con- stitute the endoderm, the remaining cells, forming the outer wall of the gastrula, being the ectoderm. The two layers are continuous at the aperture of the cup, the gastrula- mouth or blastopore (Gast. Mth). Between the ectoderm and endoderm is a space, the greatly diminished blastocoele. The resemblance of the gastrula to a simplified Hydra, devoid of tentacles, will be at once apparent. 296 rOLYGORDIUS Liiss. Before long the mouth of the gastrula closes {B), the enteron {Eni) being thus converted into a shut sac. At about the same time the ectoderm is tucked in or invaginated at two places (C), and the two little pouches {St. dm, Pre. din) thus formed grow inwards until they meet with the closed enteron and finally open into it (Z*), so that a complete enteric canal is formed — formed, we must not fail to notice, of three distinct parts : (i) an anterior ectodermal pouch, opening externally by the mouth, and distinguished as the stomodaum ; (2) the enteron, lined with endoderm ; and (3) a posterior ectoder- mal pouch, opening externally by the anus, and called the prododaum. In the trochosphere (Fig. 71) the gullet is derived from the stomodaeum, the stomach from the enteron, and the intestine from the proctodeum ; so that only the stomach of the worm-larva corresponds with the digestive cavity of a medusa : the gullet and intestine are structures not repre- sented in the latter form. ■ Two or three other points in the anatomy of the trocho- sphere must now be referred to. At the apex of the dome-shaped prostomium the ecto- derm is greatly thickened, forming a rounded patch of cells (Figs. 71 and 73, Br), the rudiment of the brain. On the surface of the same region and in close relation with the brain is a pair of small patches of black pigment, the eye-spots or ocelH (Oc). On either side of the intestine, between its epithelium and the external ectoderm, is a row of cells forming a band which partly blocks up the blastoccele (b and c, Msd). These two bands are the rudiments of the whole of the meso- dermal tissues of the adult — muscle, ccelomic epithelium, &c. — and are hence called mesodermal bands. THE TROCHOSPHERE 297 Finally on either side of the lower or posterior end of the stomach is a delicate tube (Fig. 73, a, NpK) opening by a small aperture on to the exterior, and by a wide funnel-shaped Fig. 73- — a, living specimen of an advanced trochosphere-larva of Polygordius nearpolitanus, showing the elongation of the anal region to form the trunk. B, diagrammatic vertical section of the same : the ectoderm is coarsely, the nervous system finely, dotted, the endoderm radially striated, and the mesoderm evenly shaded. C, transverse section through the plane ab in B. The pre-oral (/V. or. ci), post-oral {Pt. or. ci), and anal {An. ci) cilia, brain {Br), ocelli (Oc), blastocoele {Bl), mouth {Mtii), stomo- djeum {St. dm), proctodasum {Pre. dm), and anus {An) as in Fig. 71, the enteron (Ent) has extended some distance into the trunk. In A, slender mesodermal bands {Msd. bd) in the prostomium, and the branched head-nephridium {Nph) are shown. In B and C the mesoderm {Msd) is seen to have obliterated the blasto- ccele in the trunk-region : the ectoderm has undergone a thickening, forming the ventral nerve-cord ( V. Nv. Cd). (A after Fraipont. ) extremity into the blastocoele : it has all the relations of a nephridium, and is distinguished as the head-nephdridium. As the larva of Polygordius is so strikingly different from the adult, it is obvious that development must, in this, as in 298 POLYGORDIUS Less. several cases which have come under our notice, be accom- panied by a metamorphosis. The first obvious change is the elongation of the conical anal region of the trochosphere into a tail-like portion which may be called the trunk (Fig. 73, a). The stomach (enteron), which was formerly confined to the pro- and peri-stomium, has now grown for a considerable distance into the trunk (b, eni), so that the procto- dseum {Pre. dm) occupies only the portion in proximity to the anus. Important internal changes have also taken place. The deric epithelium or external ectoderm is for the most part composed, as in the preceding stage, of a single layer of cells ; but on that aspect of the trunk which lies on the same side as the mouth — i.e.., to the left in Fig. 73, a and b — this layer has undergone a notable thickening, being now com- posed of several layers of cells. This ectodermal thickening is the rudiment of the ventral nerve-cord ( V. Nv. Cd), and the side of the trunk on which it appears is now definitely marked out as the ventral aspect of the future worm, the opposite aspect — that to the right in the figures — being dorsal. At a later stage two ectodermal cords — the oesopha- geal connectives — are formed, connecting the anterior end of the ventral nerve-cord with the brain. Note that the two divisions of the central nervous system are originally quite distinct. The mesodermal bands, which were small and quite separate in the preceding stage (Fig. 71, b and c, Msd). have now increased to such an extent as completely to sur- round the enteron and obliterate the blastocoele (Fig. 73, b and c, Msd). At this stage therefore there is no' body- cavity in the trunk, but the space between the deric and enteric epithelia is occupied by a solid mass of mesoderm. XXV METAMORPHOSIS 299 In a word, the larva is at present, as far as the trunk is con- cerned, triploblastic but acotlomate. Development continues, and the larva assumes the form shown in Fig. 74, a. The trunk has undergone a great increase in length and at the same time has become divided by a series of annular grooves into segments or metameres, like those of the adult worm but more distinct (compare Fig. 66, D, p. 269). By following the growth of the larva from the preceding to the present stage, it is seen that these segments are formed from before backwards, i.e.^ the seg- ment next the peristomium is the oldest, and new ones are continually being added between the last formed and the extremity of the trunk, or what may now be called the anal segment. By this process the larva has assumed the appear- ance of a worm with an immense head and a very slender trunk. The original larval stomach (enteron) has extended, with the formation of the metameres, so as to form the greater portion of the intestine : the proctodseum (Pre. dm) is confined to the anal segment. Two other obvious changes are the appearance of a pair of small slender processes (a, t) — the rudiments of the tentacles — on the apex of the prostomium, and of a circlet of cilia {Fj-. an. ci) round the posterior end of the trunk. The internal changes undergone during the assumption of the present form are very striking. In every fully formed metamere the mesoderm — solid, it will be remembered, in the previous stage — has become divided into two layers, a somatic layer (b and c, Msd {soni) ) in contact with the ectoderm and a splanchnic la}'er (Msd [spl) ) in contact with the endoderm. The space between the two layers (Cxt) is the permanent body-cavity or ccelome, which is MM An.ci Fig. 74. — A, larva of Polygordius neapolitanus in a condition inter- mediate between the trochosphere and the adult worm, the tnmk-region being elongated and divided into metameres. B, diagrammatic vertical section of the same: the ectoderm is coarsely, the nervous system finely, dotted, the endoderm radially striated, and the mesoderm evenly shaded. c, transverse section along the plane ab in B. The pre-oral {Pi: or. ci), post-oral {PI. or. ci), and anal {An. ci) cilia, the blastoccele {Bl. cccl), stomodteum {St. dm), and proctodajum {Pre. dm) are as in Fig. 71, A and B : the enteron now extends through- out the segmented region of the trunk. A pair of tentacles {t) has appeared on the prostomium near the ocelli {0), and a pre-anal circlet of cilia {Pr. an. ci) is developed. The mesoderm has divided into somatic {Msd {som) ) and splanchnic {Msd {spl) ) layers with the ccelome {Cal) between : the septa {Sep) are formed by undivided plates of mesoderm separating the segments of the ccelome from one another. d'-d'', three stages in the development of the somatic mesoderm. In D^ it {Msd (Soiii) ) consists of a single layer of cells in contact with the deric epithelium (Der. Eplhiii) : in D^ the cells have begun to split up in a radial direction : in d' each has divided into a number of radially arranged sections of muscle-plates {Af. PI) and a single cell of ccelomic epithelium (Cai. Epthni). (A after Fraipont.) XXV METAMORPHOSIS 301 thus quite a different thing from the larval body-cavity or blastoccele, being formed, not as a space between ectoderm and endoderm, but by the splitting of an originally solid mesoderm. The division of the mesoderm does not however extend quite to the middle dorsal and middle ventral lines : in both these situations a layer of undivided mesoderm is left (c), and in this way the dorsal and ventral mesenteries are formed. Spaces in these, apparently the remains of the blastoccele, form the dorsal and ventral blood-vessels. More- over the splitting process takes place independently in each segment, and a transverse vertical layer of undivided mesoderm (b, Sep) is left separating each segment from the adjacent ones before and behind : in this way the septa arise. The nephridia appear to have a double origin, the super- ficial portion of each being formed from ectoderm, the deep portion, including the nephrostome, from the somatic layer of mesoderm. In the ventral nerve-cord the cells lying nearest the outer surface have enlarged and formed nerve-cells, while those on ' the dorsal aspect of the cord have elongated longitudinally and become converted into nerve-fibres. This process has already begun in the preceding stage. But the most striking histological changes are those which gradually take place in the somatic layer of mesoderm. At first this layer consists of ordinary nucleated cells (d^, Msd Som), but before long each cell splits up in a radial direction (d^) from without inwards — i.e., from the ectoderm {£>er. Epthni) towards the coelome — finally taking on the form of a book with four or more slightly separated leaves directed outwards or towards the surface of the body, and with its back — the undivided portion of the cell — bounding 302 POLYGORDIUS less. the coelome. The cells being arranged in longitudinal series, we have a number of such books placed end to end in a row with the corresponding leaves in contact — page one of the first book being followed by page one of the second, third, fourth, &c., page two by page two, and so on through one or more segments of the trunk. Next, what we have compared with the leaves of the books — the divided portions of the cells — become separated from the backs — the undivided portions (d') — and each leaf {M. Ft) fusg.3 with the corresponding leaves of a certain number of books in the same longitudinal series. The final result is that the undivided portions of the cells (backs of the books, Ccel. Epthiii) become the parietal layer of ccelomic epithelium, the longitudinal bands formed by the union of the leaves {M. PI) becoming the muscle-plates, which are thus cell- fusiotis, each being formed by the union of portions of a series of longitudinally arranged cells. At the same time the cells of the splanchnic layer of mesoderm thin out and become the visceral layer of ccelomic epithelium We see then that by the time the larva has reached the stage shown in Fig. 74, it is no longer a mere aggregate of simple, cells arranged in certain layers. The cells them- selves have undergone differentiation, some becoming modi- fied into nerve-fibres, others by division and subsequent fusion with their neighbours forming muscle-plates, while others, such as the epithelial cells, remain almost unaltered. Thus, in the course of the development of Polygordius, cell-multiplication and cell-differentiation go hand in hand, the result being the formation of those complex tissues the presence of which forms so striking a difference between the worm and the simpler types previously studied. XXV SIGNIFICANCE OF DEVELOPMENTAL STAGES 303 It is important to notice that this comparatively complex animal is in one stage of its existence — the oosperm — as simple as an Amoeba ; in another — the polyplast — it is com- parable to a Pandorina, and in a third — the blastula — to a ^^olvox ; in a fourth — the gastrula — it corresponds in general features with a Hydra ; while in a fifth — the trochosphere — ■ it resembles in many respects a Medusa. As in other cases we have met with, the comparatively highly-organised form passes through stages in the course of its individual develop- ment similar in general characters to those which, on the theory of evolution, its ancestors may be considered to have passed through in their gradual ascent from a lower to a higher stage of organization. The rest of the development of Polygordius may be summarized very briefly. The trunk grows so much faster than the head (pro-//w peri-stomium) — that the latter under- goes a relative diminution in size, finally becoming of equal diameter with the trunk, as in the adult. The ciliated rings are lost, the tentacles grow to their full size, the eye-spots atrophy, and thus the adult form is assumed. LESSON XXVI THE CHIEF DIVISIONS OF THE ANIMAL KINGDOM : THE STARFISH The student who has once thoroughly grasped the facts of structure of such typical unicellular animals as Amoeba and the Infusoria, of such typical diploblastic animals as Hydra and Bougainvillea, and of such a typical triploblastic animal as Polygordius, ought to have no difficulty in understanding the general features of the organization of any other members of the animal kingdom. When once the notions of a cell; a cell-layer, a tissue, an organ, body-wall, enteron, stomodseum, proctodeum, ccelome, somatic and splanchnic mesoderm, are fairly understood, all other points of structure become hardly more than matters of detail. If we turn to any text-book of Zoology we shall find that the animal kingdom is divisible into eight primary sub- divisions, called sub-kingdoms, types, or phyla. These are as follows : — Protozoa. Echitiodermata. Porifera. Arthropoda. Cxlenterata. Mollusca. Vermes. Vertebrata. LESS. XXVI GENERAL STRUCTURE 305 With a few exceptions, the discussion of which would be out of place here, the vast number of animals known to us may be arranged in one or other of these groups. , The Protozoa are animals which are either unicellular in the strict sense, or non-cellular, or colonies of unicellular zooids : they have been represented in previous lessons by Amceba and Protamoeba, Hsematococcus, Heteromita, Euglena, the Mycetozoa, Paramoecium, Stylonychia, Oxy- tricha, Opalina, Vorticella, Zoothamnium, the Foraminifera, the Radiolaria, Pandorina, and Volvox. The reader will therefore have no difficulty in grasping the general features of this phylum. The Coslenterafa axe the diploblastic animals, and have also been well represented in the foregoing pages, namely by Hydra, Bougainvillea, Diphyes, and . Porpita. The sea- anemones and corals also belong to this phylum, in which also the Porifera or sponges are sometimes included. The Vermes, or Worms, are a very heterogeneous assem blage. They are all triploblastic, but while some are coelomate, others have no body-cavity ; some, again, are segmented, others not. Still, if the structure of Polygordius is thoroughly understood, there will be little difficulty in understanding that of a fluke, a tape-worm, a round-worm, an earthworm, or one of the ordinary marine worms. Of the remaining four sub-kingdoms we have, so far, studied no example, but a brief description of a single example of each will show how they all conform to the general plan of organisation of Polygordius, being all triplo- blastic and ccelomate. Under the Echmodermata are included the various kinds of starfishes — sand-stars, brittle-stars, and feather-stars, as well as sea-urchins, sea-cucumbers, &c. A starfish will serve as an example of the group. X 3o6 THE STARFISH Less. The phylum Arthropoda includes crayfishes, lobsters, crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions, spiders, and mites ; centipedes and millipedes ; and all kinds of insects, such as cockroaches, beetles, flies, ants, bees, butterflies, and moths. A crayfish forms a very fair example of the group. In the phylum MoUusca are included the ordinary bi- valves, such as mussels and oysters ; snails, slugs, and other univalves or one-shelled forms ; and cuttle-fishes, squids, and Octopi. An account of a fresh-water mussel will serve to give a general notion of the character of this group. Finally, under the head of Vertebrata are included all the backboned animals : the lampreys and hags ; true fishes, such as the shark, skate, sturgeon, cod, perch, trout, &c. ; amphibians, such as frogs, toads, newts, and salamanders ; true reptiles, such as lizards, crocodiles, snakes, and tor- toises ; birds ; and mammals, or creatures with a hairy skin which suckle their young, such as the ordinary hairy quadrupeds, whales and porpoises, apes, and man. The essential structure of a vertebrate animal will be understood from a brief description of a dog-fish. The Starfish. The commonest British starfish is Asterias riibens, but the main features of the following description will apply to any species. The starfish consists of a central disc-like portion, from which radiate five arms or rays. The animal crawls over the rocks with its flat, light-coloured ventral surface downwards, and with its darker, convex, dorsal surface upwards. It can move in any direction, so that, in the ordinary sense of the words, anterior and posterior ex- tremities cannot be distinguished. Radial symmetry such TUBE-FEET 307 as this, i.e., the division of the body into similar parts radiating from a common centre, is characteristic of the Echinodermata generally. In the centre of the disc on the ventral surface is a five- sided depression, at the bottom of which is the large mouth (Fig. 75 and Fig. 76, a, Mtli). From it radiate five grooves Fig. 75. — A Starfish, from the ventral aspect, showing the disc and arms, the central mouth, and the numerous tube-feet. (From Parker and Haswell's Zoology, after Leuckart and Nitsche. ) called the ambulacral grooves, one along the ventral surface of each arm (Fig. 76, a and b). In the living animal numerous delicate semi-transparent cylinders, the tube-feet (Fig. 75 and Fig. 76, T. F), are protruded from these grooves ; they are very extensible and each ends in a sucker. It is by moving these structures in various directions, protruding some and withdrawing others, that the starfish is able to move along 3o8 THE STARFISH Less, xxvi either a horizontal or a vertical surface, and even to turn itself over when placed with the ventral side upwards. Near the middle of the disc, on the dorsal surface, is the very minute anus (Fig. 76 a, An) ; it is situated on a line drawn from the centre of the disc to the re-entering angle between two of the rays, and is therefore said to be inter-radial in position. Near the anus, and also inter-radially situated, is a circular calcareous plate, the madreporite (Mdpr), per- forated by numerous microscopic apertures. The presence of this structure disturbs the radial symmetry of the starfish and gives rise to a bilateral symmetry, since the animal can be divided into two truly equal halves by a single plane only, viz., the plane passing through the middle of the madreporite and of the arm opposite to it. The body, though flexible, is tolerably firm and resistant, owing to the fact that immediately beneath the soft, slimy skin there is a layer of little irregular calcareous bodies, the ossicles (Fig. 76, os), forming a kind of scale armour. Many of them give attachment to spines, and between them are minute apertures, the dermal pores, through which, during the life of the animal, are protruded delicate, glove-finger- like processes, the dermal gills or respiratory cceca (Resp. cci). Both on the dorsal and the ventral surfaces are found curious and characteristic organs called pedicellaricB {Ped). These are minute forceps-like structures, consisting of a basal piece or stalk and of two jaws, each supported by a calcareous plate : the jaws are w^orked by nmscles, and apparently serve to remove faecal matter, foreign bodies, &c., from the surface of the animal. The tube-feet, already referred to, are arranged symme- trically on either side of each ambulacral groove. At the extremity of the groove is a single structure {t) like a tube- foot without the terminal sucker : it is called the tentacle, Tire, Clx Rcsp\ Fig. 76. — Diagrammatic sections of a Starfish. A, vertical section passing on the right through a radius, on the left through an inter-radius. The off-side of the ambulacral groove, with the tube feet ( T. F) and ampullte (Amp), is shown in perspective. B, transverse section through an arm. The ectoderm is coarsely dotted, the nervous system finely dotted, the endoderm radially striated, the mesoderm evenly shaded, the ossicles of the skeleton black, and the coelomic epithelium represented by a beaded line. The body-wall consists of deric epithelium [Der. Eptkm), dermis (Derm), and the parietal layer of ccelomic epithelium (Cccl. Eplhin). To the body-wall are attached pedicellarite (Fed), and the end of the arm bears a tentacle (t) with an ocellus (oc) at its base. The skeleton consists of ossicles (os) imbedded in the dermis : lar^e ambulacral ossicles (Amb. os) bound the ambulacral grooves on the ventral surfaces of the arms. The mouth (3Ith) leads by a short gullet into a stomach (St), which gives off a cardiac ctecum (Cd. cce) and a pair of pyloric cfeca (PyL ca) to each arm, and passes into an intestine (Ini) which gives off intestinal cneca (Int. ca) to the inter-radii, and ends in the anus (An). The pyloric cfeca are connected to the dorsal body-wall by mesenteiies (Mes. in B). The wall of the enteric canal consists of enteric epithelium covered by the visceral layer of ccelomic epithelium (Cccl. Epthin'). From the ccelome are given off respiratory caeca (Resp. cce), which project through the body-wall : the latter contains spaces (p. h) derived from the ccelome. 3IO THE STARFISH less. The circular blood-vessel (C. B. V) surrounds the gullet and gives off radial vessels {liacl B. V) to the arms and an inter-radial plexus connected with a pentagonal ring round the intestine. The circular ambulacral vessel (C. Aiub. V) gives off radial vessels (Racl. Ami). V) to the arms connected with the ampullae {Amp) and tube- feet {T. F): it is also connected with the stone-canal {St. C), which opens externally by the madreporite {Mdpr). The nerve-ring {Nv. R) gives off radial nerves {Rad. Nv) to the arms. The ovary {Ovy) is inter-radial, and opens by a dorsal oviduct {Ovd). and is probably an organ of smell. At the base of the tentacle is a bright red eye-spot (oc). Sections show that there is a well-marked ccelome, separating the body-wall from the enteric canal and contain- ing the gonads, blood-vessels, &c. The body-wall consists externally of a very thin cuticle, then of a layer of deric epithelium or epidermis lyDer. Eptlim), then of a thick, double, fibrous layer {Derm), then of a thin and interrupted layer of muscle, and finally, of a layer of coelomic epithelium (C«/. Epthui) bounding the body cavity. The ossicles with their spines together form an external skeleton or exoskektow: as already mentioned they are, for the most part, small irregular bodies developed in the fibrous layer of the body-wall, and overlapping one another in a scale-like fashion. But the ambulacral grooves are bounded by regularly arranged pairs of large, rod-like a!?il>it- lacral ossicles {Amb. os), arranged like rafters, the dorsal ends of each pair uniting at the summit of the groove, while their ventral ends diverge and are connected with the ordinary ossicles at the edge of the arm. Between each ambulacral ossicle and its predecessor and successor in the row is an aperture, the ambulacral pore, with which one of the tube-feet is connected. The mouth (Fig. 76, a, mtli) leads by a short gullet into a stomach {st) divisible into two portions, called respectively XXVI DIGESTIVE ORGANS 311 the cardiac and pyloric divisions. The cardiac division (Fig. 77, card, si), into which the gullet opens, is a spacious sac, produced into five wide pouches, the cardiac cmca (Fig. 76, A, Cd. cot ; Fig. 77), one of which extends into the pyl. coec Fig. 77. — Digestive organs of a Starfish {Asterias rtihns), seen from the dorsal aspect. The cardiac portion of the stomach (card, st) gives off five short cardiac coeca or pouches and leads into the pyloric division (pyl. st), from which five bifid pyloric coeca (/_j//. car) are continued to the ends of the arms. The short intestine is recognisable by the presence of the intestinal coeca [ziit. cccc) and of the anus (a») : mad?; madreporite. (From Parker and Haswell's Zoology, after Leuckart. ) base of each arm. When the starfish is feeding it can evert this cardiac sac over the shellfish or other object serving as prey, and is thus able to devour animals too large to be taken into the mouth . the everted stomach is afterwards drawn back by means of special muscles. Dorsally the 312 THE STARFISH less. cardiac communicates with the small pyloric division (Fig. 77, />'/. s/), which also gives off five pouches, the pyloric cceca (Fig. 76 and l'],pyl. coi) ; but each of these, instead of extending merely into the base of the correspond- ing arm, divides into two, and both branches extend to the extremity of the arm, giving off as they go small side- branches, so that the whole CKCum has a tufted or sacculated character. The pyloric caeca are lined by gland cells, and in them the digestion of the food takes place. They are connected with the dorsal walls of the arms by mesenteries (Fig. 76, n, 7nes). The pyloric division of the stom.ach leads into a very short intestine which passes upwards in a straight line to the anus (an\ previously giving off two intestinal cceca {int. ex) situated inter-radiall)' — not radially like the blind offshoots of both divisions of the stomach. The whole enteric canal is lined with enteric epithelium (Fig. 76, Ent. Epthni), and is covered by the visceral layer of ccelomic epithelium (Cxi. Epthni') : it has no muscular layer. There is a spacious coelome (Cxi) between the body- wail and the enteric canal filled with a watery fluid contain- ing leucocytes. The ccelomic epithelium is ciliated, the cilia effecting a circulation of the ccelomic fluid. The dermal gills (Resp. co>), already referred to, communicate with the ccelome, and are, in fact, hollow outpushings of the body-wall. They serve to bring the ccelomic fluid into close relation with the surrounding water, and are therefore to be looked upon as organs of respiration. One of the most characteristic structures in the anatomy of the starfish is a peculiar system of vessels called the water-vascular or anibulacral system : it is of great func- tional importance, being connected with the working of the tube-feet. AMBULACRAL SYSTEM 313 The central part of the ambulacral system is a pentagonal tube (Fig. 78, c; Fig. 76, C. Amb. V) which surrounds the gullet, and is called the ambulacral ring-vessel. From each angle of the pentagon is given off a radial ambulacral vessel Fig. 78- — The water vascular system of a Starfish (diagraniatic). The ring-vessel (c) gives off five radial vessels {r), lateral off-shoots of which (/) are connected with the tube-feet (/) and ampullce {a). Inter-radially the ring-vessels give off Polian vesicles (ap) and the madreporic canal {m') ending in the madreporite (vi). (From Gegenbaur. ) (Fig. 78, r ; Fig. 76, Had. Amb. V) which proceeds to the end of the corresponding arm, lying in the dihedral angle included by the double row of ambulacral ossicles, and consequently external to this portion of the skeleton (Fig. 76, b). Each radial vessel sends off side branches (Fig. 78, r) 314 THE STARFISH less. which communicate with the hollow tube-feet (Fig. 78,// Fig. 76, T. F.), and each tube-foot is connected by a narrow canal passing through an ambulacral pore (p. 310) with a bladder-like body, the ampulla (Fig. 78, a ; Fig. 76, Amp) lying in the coelome. The ampulte consequently form a double row of bladders along the ventral region of the interior of the arm. The ring-vessel also gives off inter-radially, i.e., in the intervals between the arms, bladder-like bodies, the Poliati vesicles (Fig. 78, ap), a pair in each inter-radius. In one of the inter-radii there also goes off from the ring-vessel a tube, called the stone-canal (Fig. 78, ;;/' , Fig. 76, St. c) from the fact that its walls are calcified, which passes directly upwards and becomes connected with the madreporite (Fig. 78, ot/ Fig. 76, A, Mdpr). The latter is perforated by minute apertures which are in communication with the cavity of the stone-canal, and in this way the ambulacral system is placed in direct communication with the surrounding water. The whole ambulacral system contains a watery fluid, and its walls consist of a lining of epithelium and an outer muscular layer particularly well developed in the ampulte and tube-feet. Contraction of the muscles of the ampulte forces water into the tube-feet, and causes protrusion of these organs : their withdrawal is brought about by the con- traction of the longitudinal muscles in their walls, by which the fluid is forced back into the ampullae. Thus the whole ambulacral system forms an elaborate locomotory apparatus worked by water-power. It is quite confined to Echinoderms. In all the other higher animals movements are effected by the direct, and not, as in this case, by the indirect action of muscles. A second system of vessels is present and constitutes the blood-system. Surrounding the gullet below the ambulacral XXVI REPRODUCTIVE ORGANS 315 ring-vessel is a ring blood-vessel {Fig. 76, a, C. B. V), send- ing off radial blood-vessels (Jiad. B. V) to the arms. An inter-radial sinus or blood-space lies alongside the stone- canal, surrounding the ovoid gland (see p. 316), and is con- nected below with the ring-vessel and above with a pentagonal vessel or sinus, from which inter-radial branches proceed to the gonads. The nervous system is considerably simpler than that of Polygordius. It consists, in the first place, of a pentagonal nerve-ring (Fig. 76, a, Nv. R) surrounding the mouth, and having the character of a mere thickening of the deric epithelium. From each of its angles goes off a radial nerve {Rad. Nv) which passes along the arm below the ambu- lacral and blood-vessels, and is also nothing more than a thickening of the epidermis, some of the cells of which are modified into nerve cells and fibres. At the end of the arm the radial nerve terminates in the eye-spot. In addition to this superficial nervous system there is a deep nervous system, situated internal to the former, and consisting of a double pentagon round the mouth, sending off double radial nerves to the arms. There are also scattered nervous elements in the dorsal region of the body-wall. Like Polygordius, the starfish is dioecious : there is no external distinction between the sexes, and even the ovaries and spermaries can be distinguished only by microscopical examination. There are five pairs of gonads — ovaries (Fig. 76, A, ovy) or spermaries as the case may be — one pair in each inter-radius. Each gonad has the form of a bunch of grapes, being a much lobed sac lined by epithelium from which the ova or sperms are developed. It is con- tinued into a tube or gonoduct, called spermiduct in the male, oviduct {Ovd) in the female, which opens inter-radially on the dorsal surface close to the bases of the arms. The 3i6 THE STARFISH gonads are all connected by cords of tissne with an organ called the ovoid gland, which lies alongside the stone-canal and is surrounded by a blood sinus. Its function is prob- ably the formation of leucocytes. The ova and sperms are shed into the water, where im- arch. arch i'lfi Fig. 79. — Early stages in the development of a Stavfish. A. The polyplast, surrounded by the vitelline membrane. B. The blastula, in section. c. The gastrula, external view, showing the blastopore (/;/. /). D. The gastrula, in vertical section : arch, enteron. E. More advanced gastrula, with ciliated ectoderm. Arch, enteron ; blastoc, blastocoele ; bl.p. blastopore ; cct, ectoderm ; cud. endoderm. (From Parker and Haswell's Zoology.) pregnation takes place. The oosperm undergoes the usual process of segmentation, forming a polyplast (Fig. 79, a), which is soon converted into a blastula (e) by the cells arrang- ing themselves round a central cavity. One side of the blastula becomes invaginated or tucked in, and a gastrula XXVI DEVELOPMENT 317 (c, D, e) is formed, the cells becoming differentiated into ectoderm and endoderm, and the ectoderm cells acquiring cilia. The gastrula gradually takes on the form of a peculiar free-swimming larva having a certain general resemblance to the trochosphere and called a bipmiaria (Fig. 80) : it differs from the adult starfish in showing no trace of radial symmetry, the body being produced into several ciliated Fig. 80.— Three stages in the develoiDment of the Bipinnaria larva of a Starfish. An, anus ; aor, pre-oral ciliated ring ; mo, mouth ; par, post-oral ciliated ring. (From Parker and Haswell, after Leuckart and Nitsche.) processes or arms, all bilaterally arranged, and the enteric canal having the form of a curved cylindrical tube, consist- ing of gullet, stomach, and intestine lying in the median plane. The bipinnaria lives a free life for a time, swimming by means of its cilia, and finally, by a complex series of changes, undergoes gradual metamorphosis into the adult starfish. LESSON XXVII THE CRAYFISH The Starfish has furnished us with an example of an animal in which an obvious radial symmetry is, as it were, superposed upon an original bilateral symmetry : in which also there is an extremely simple form of nervous system, a unique type of locomotory apparatus, and no trace of metameric segmentation. ^Ve have now to study, in the crayfish, an animal formed upon quite the same general plan of structure as Polygordius as to segmentation, arrange- ment of organs, &:c., but which reaches, in every respect, a far higher grade of organisation. The Common British Fresh-water Crayfish is Astaais fluvialilis : allied species occur in Europe, Asia, and America. The following description ^^'ill apply almost equally well to the Lobster, Homarus vulgaris. The body of the crayfish (Fig. 8i) is divided into two regions, an anterior, the cephalothorax, which is unjointed and is covered by a cuirass-like structure, the carapace, and a posterior, the abdomen, which is divided into distinct seg- ments, movable upon one another in a vertical plane. The cephalothorax is again divided into two regions, an anterior, the head (ctli), and a posterior, the thorax {kd), by a trans- LESS. XXVII EXTERNAL CHARACTERS 319 verse depression, the cervical groove. The carapace is developed from the dorsal and lateral regions of both head and thorax : it is free at the sides of the thorax, where it Fig. 81. — Side view of male Fresh-water Crayfish, natural size. The cephalothorax is covered by the carapace, produced in front into a rostrum {r) and divisible into cephalic {cth) and thoracic [kd) portions separated by an oblique cervical groove. The line from kd points to the gill- cover. The abdomen (aV) is made up of six movably articulated segments (xiv-xix), followed by a telson, the extremity of which is indicated by the lower end of the bracket from ab. The eye-stalk is seen at the base of the rostrum. Of the cephalic appendages the antennule (a') and antenna {cP) are shown ; of the thoracic appendage the third maxilliped (8), the enlarged first leg or cheliped (9), and the four slender walking legs (10-13) ! of the abdominal appendages three pleopods and the uropod (18). (From Lang, after Huxley. ) forms a flap or gill-cover (Fig. 83, li, Brsfg) on each side, separated from the actual body-wall by a narrow space in which the gills are contained. From the ventral surface spring a number of paired limbs 320 THE CRAYFISH less. or appendages, structures which we have not hitherto met with. Both trunlc and appendages are covered with a sort of shell, formed of a substance called chititi, strongly im- pregnated with carbonate of lime so as to be hard and but slightly elastic. The abdomen is made up of seven segments : the first six of these (Fig. 8i, xiv-xix) are to be considered as meta- meres in the sense in which the word is used in the case of Polygordius. Each has a ring-like form, presenting a broad dorsal region or tergum ; a narrow ventral region or sternum , and downwardly directed lateral processes, the pleura. The seventh division of the abdomen is the telson : it is flattened horizontally and divided by a transverse groove into anterior and posterior portions. All seven segments are calcified, and are united to one another by chitinous articular mem- branes : the first segment is similarly joined to the thorax. Thus the exoskeleton of the Crayfish is a continuous structure, but is discontinuously calcified so as to have the character of a hard jointed armour. It has been stated that the abdominal segments are movable upon one another in a vertical plane, i.e., the whole abdomen can be extended or straightened, scoA flexed ox bent under the cephalothorax : the segments are incapable of movement from side to side. This is due to the fact that, while adjacent segments are connected dorsally and ven- trally by flexible articular membranes, they present at each side a joint, placed at the junction of the tergum and pleuron, and formed by a little peg-like process of one seg- ment fitting into a depression or socket in the other. A line drawn between the right and left joints constitutes the axis of articulation, and the only possible movement is in a plane at right angles to this axis. Owing to the presence of the carapace the thoracic region XXVII APPENDAGES 32I is immovable, aiid shows no distinction into segments either on its dorsal (tergal) or lateral (pleural) aspect. But on the ventral surface the sterna of the thoracic segments are clearly marked off by transverse grooves, and the hindmost of them is slightly movable. Altogether eight thoracic segments can be counted. The ventral and lateral regions of the thoracic exoskeleton are produced into the interior of the body in the form of a segmental series of calcified plates, so arranged as to form a row of lateral chambers in which the muscles of the limbs lie, and a median tunnel-like passage or sternal canal, con- taining the thoracic portion of the nervous system. The entire endophragmal system, as it is called, constitutes a kind of internal skeleton (Fig. 83, b). The head exhibits no segmentation : its sternal region is formed largely by a shield-shaped plate, the epistoma, nearly vertical in position. The ventral surface of the head is, in fact, bent so as to face forwards instead of downwards. The cephalic region of the carapace is produced in front into a large median spine, the rostrum (Fig. 81, r) .■ immediately below it is a plate from which spring two movably articu- lated cylindrical bodies, the eye-stalks, bearing the eyes at their ends. The appendages have very various forms, and are all, like the abdomen, jointed or segmented, being divisible into freely articulated limb-segments or podotneres. The observer is at once struck by the long feelers attached to the head, the five pairs of legs springing from the thorax, and the little fin-like bgdies arising from the sterna of the abdomen. It will be convenient to begin with the last-named region. The third, fourth, and fifth segments of the abdomen bear each a pair of small appendages, the swimming-feet or pkopods. A pleopod (Fig. 82, 10) consists of an axis or Y 322 THE CRAYFISH S. S'-'Maxilla 6. 1'^Maxillifed 7. 3'.''Maxilliped 3. J'.' Lee Q.Copulafory Organs lO.Swimming Foor 11. U r o (> o d Fig. 82. — The principal appendages of the Fresh-water Crayfish placed in the same position, with the protopodite (/;-) and epipodite [ep] downwards, the endopodite {en) to the left, and the exopodite {ex) to the right. The protopodite is typically formed of two podomeres {pr, i, fr. 2), the endopodite of five {en. \-cn. 5) : a gill (g) may be attached to the epipodite and a bunch of long setse to the protopodite (7 and 8). The three segments of the antennule are marked 1-3, its flagellar?, i and fl. 2 : at the distal end of the endopodite of the antenna is a flagellum (_/?). (From Parker and Haswell'S Zoology, after Huxley.) xxvii APPENDAGES 323 protopodite having a very short proximal {pr. i), and a long distal (//-. 2) podomere, and bearing at its free end two jointed plates, fringed with setae, the endopodite {en) and exopodite {ex). These appendages act as fins, moving back- wards and forwards with a regular swing, and probably aid- ing in the animal's forward movements. In the female a similar appendage is borne on the second segment, while that of the first is more or less rudimentary. In the male the first and second pleopods (9) are modified into incomplete tubes which act as copulatory organs, serving to transfer the spermatophores to the body of the female. The sixth pair of pleopods (11) are alike in the two sexes : they are very large, both endo- and exopodite having the form of broad flat plates : in the natural position of the parts they lie one on each side of the telson, forming with it a large five-lobed tail fin : they are therefore conveniently called uropods or tail-feet. The telson itself bears no appendages. The thoracic appendages are very different. The four posterior segments bear long slender, jointed legs (Fig. 81, 10-13), upon which the animal walks : in front of these is a pair of very large legs (9) terminating in huge claws or chela, and hence called chelipeds. The three anterior segments bear much smaller appendages (8), more or less leg-like in form, but having their bases toothed to serve as jaws : they are distinguished as maxillipeds or foot-jaws. The structure of these appendages is best understood by a consideration of the third maxilliped (Fig. 82, 7). The main portion of the limb is formed of seven podomeres arranged in a single series, strongly calcified, and, with the exception of the second and third, which are fused, movably articulated with one another. The second podomere, counting from the proximal end, bears a many-jointed 324 THE CRAYFISH less. feeler-like organ (c.v), and from the first springs a thin, folded plate {ep) having a plume-like gill {£) attached to it. The first two segments of the axis form the protopodite, its remaining five segments the endopodite, and the feeler, which is directed outwards, or away from the median plane, the exopodite. The folded plate is called the epipodite : in the natural position of the parts it is directed upwards, and lies in the gill-cavity between the proper wall of the thorax and the gill-cover (Fig. 87, a, pM.). The five legs (8) differ from the third maxillipiedin their greater size, and in having no exopodite : in the fifth or last the epipodite also is absent. The first three of them have undergone a curious modification, by which their ends are converted into pincers or chelm : the fourth segment of the endopodite (sixth of the entire limb, en. 4) is produced dis- tally so as to form a claw-like projection {en. 4'), against which the terminal segment {en. 5) bites. The first leg is much stouter than any of the others, and its chela is of immense size, and forms an important weapon of offence and defence. The second maxilliped resembles the third, but is considerably smaller : the first (6) has its endopodite greatly reduced, the two segments of its protopodite large and leaf-like, and no gill is connected with the epipodite. The head bears a pair of mandibles and two pairs of maxillse in relation with the mouth, and in front of that aperture a pair of antennules and one of antennse. The hindmost appendage of the head is the second maxilla (5), a leaf-like appendage, its protopodite being cut up into lobes, while the exopodite is modified into a boomerang- shaped plate, which, as we shall see, is an important accessory oi-gan of respiration. The first maxilla (4) is a very small organ, having neither exo- nor epipodite. The wandihle (3) is a large, strongly calcified bod)-, toothed along XXVII APrENDAGES 325 its inner edge, and bearing on its anterior border a little three-jointed feeler-like , body, the palp, the two distal seg- ments of which represent the endopodite, its proximal segment, together with the mandible proper, the protopodite. The antenna (2) is of great size, being nearly as long as the whole body. It consists of an axis of five podomeres, the fifth or last of which bears a long, flexible, many-jointed structure, or flagellum {fl), while from the second segment springs a scale-like body or squame (ex). It is fairly obvious that the two proximal segments represent the protopodite, the remaining three, with the flagellum, the endopodite, and the squame the exopodite. The a7itenmile (i) has an axis of three podomeres ending in two many-jointed flagella [fl. i,fl. 2), which are some- times considered as endo- and exopodite. But in all the other limbs, as we have seen, the exopodite springs from the second segment of the axis, and the probabilities are that there is no exact correspondence between the parts of the antennule and those of the remaining appendages. The eye-stalks, already noticed, arise just above the an- tennules, and are formed each of a small proximal and a large distal segment. They are sometimes counted as appendages serially homologous with the antennte and legs, &c., but are more properly to be looked upon as articulated processes of the prostomium. It is possible that the antennules are also prostomial and not metanuric structures : assuming this to be the case, it will be seen that the body of the crayfish consists of a prostomium, eighteen metameres, and a telson, which is probably com- posed of an anal segment plus a post-anal extension. The prostomium bears eye-stalks and antennules : the first four metameres are fused with the prostomium to form the head, and bear the antennse, mandibles, first maxilla, and second 326 THE CRAYFISH less. maxillae : the next eight metameres (fifth — twelfth) consti- tute the thorax, and bear the three pairs of maxillipeds and the five pairs of legs : the remaining six metameres (thirteenth — eighteenth), together with the anal segment, constitute the abdomen, and bear five pairs of pleopods and one of uropods. The articulation of the various podomeres of the append- ages is on the same plan as that of the abdominal segments (p. 320). The podomeres are, it must be remembered, rigid tubes : they are connected with one another by flexible articular membranes (Fig. 85, art. fit), but at two points the adjacent ends of the tubes come into contact with one another and are articulated by peg-and-socket joints (/;), the two joints being at opposite ends of a diameter which forms the axis of articulation. The two podomeres can therefore be moved upon one another in a plane at right angles to the axis of articulation and in no other direction, the joints being pure hinge-joints. As a rule the range of movement is from the perpendicular to a tolerably extensive flexion on one side — the articulations are single-jointed, like our own elbows and knees. The whole limb is, however, capable of universal movement, owing to the fact that the axes of articu- lation vary in direction in successive joints : the first joint of a limb bending, for instance, up and down, the next backwards and forwards, the next obliquely, and so on. In some cases, e.g., in the pleopods, peg-and-socket joints are absent, the articulation being formed merely by an annular articular membrane, movement being therefore possible in any plane. Sections show the body-wall to consist of a layer of deric epithelium (Fig. 83, Der. Epthm) secreting a thick cuticle (C?/), a layer of connective tissue forming the dermis (Demi), and a very thick layer of large and complicated XXVII MUSCULAR SYSTEM 327 muscles {M), which fill up a great part of the interior of the body. Neither on the deric epithelium nor elsewhere are there any cilia, the absence of these structures being gene- rally characteristic of Arthropods. The cuticle {Cu) is of great thickness, and except at the joints between the various segments of the body and Umbs, is impregnated with lime salts so as to form a hard, jointed armour. It thus constitutes a skeleton which, unlike that of the starfish (p. 310), is a cuticular exoskeleton, forming a continuous investment over the whole body but discon- tinuously calcified. It is shed and renewed periodically — once a year during adult life — the process being known as ecdysis. The muscular system shows a great advance in complexity over that of Polygordius, and consists entirely of transversely striated fibres. In the abdomen the muscles are of great size, and are divisible into a smaller dorsal and a larger ventral set. The dorsal muscles (Fig. 86, em; Fig. 84, d. m) are paired longitudinal bands, divided into segments called myomeres, and inserted by connective tissue into the anterior border of each segment : anteriorly they are trace- able into the thorax, where they arise from the side-walls of that region. When these muscles contract they draw the anterior edge of each tergum under the posterior edge of its predecessor, and thus extend or straighten the abdomen. The ventral muscles (Fig. 86, / m) are extraordinarily complex. Omitting details, there is on each side a wavy longitudinal band of muscle (Fig. 84, cm), nearly circular in section, which sends off a slip (ex) to be inserted into each segment above the hinge : the contraction of this muscle must obviously tend to approximate the terga, and so aid the dorsal muscles in extending the abdomen. Around this central muscle is wrapped, in each segment, a band of LESS, xxvii MUSCULAR SVSTEM 329 The body is divided into a head {Hd) and Ihorax (770. together constituting the cephalotliorax (C Th), and seven free abdominal segments (^.bd. seg. I, Alui. scg. 7) : the head is produced in front into a rostrum (A'). The body-wall consists of cuticle [Cu), partly calcified to form the exoskeleton, deric epithelium (Dei: Eptlim), dermis (Derm), and a very thick layer. of muscle (/J/) which in the abdomen is distinctly segmented. The mouth (Mth) leads by a short gullet (Gtd) into a large stomach (Si), from which a short small intestine (S. Int) leads into a large in- testine' (L. Inl), ending in the anus (An). Opening into the small intestine are the digestive glands (D. Gl). The epithelium of the small intestine and digestive glands is endodermal, that of the rest of the canal is ectodermal and secretes a cuticle : the outer layer throughout is mesodermal (connective tissue and muscle). The cavity (B. S) between the enteric canal and the body-muscles is a blood-sinus. The heart (///) is enclosed in the pericardial sinus (Per. S): the chief ventral blood-vessel or sternal artery (St. A) is shown in B. The gills (b. Gill) are enclosed in a cavity formed by a fold of the thoracic body-wall called the branchiostegite (Brstg) : they are formed of the same layers as the body-wall, of which they are offshoots. The kidneys (A, K) are situated in the head. The brain (Br) lies in the prostomium : the ventral nerve-cord [V. Nv. Cd) consists of a chain of ganglia (Gii) united by connectives. The ovary (ovy) is a hollow organ opening by an oviduct (b, ovd) on the base of one of the legs (Leg). muscle {env. m) in the form of a loop, the outer limb of which (fl) turns forwards and is inserted into a sternum, while the inner limb {fl') turns backwards and is inserted into another and more posterior sternum. The contraction of this enveloping muscle produces an approximation of the sterna, and thus flexes the abdomen, the central muscle alwaj-s keeping the middle of the loop in place. The ventral muscles are, like the dorsal, traceable into the thorax, where they arise from the endophragmal system : their various parts are coniiected by a complex sj-stem of fibres extending between the central and enveloping muscles, and connecting both with their fellows of the opposite side. The flexor muscles are immensely powerful, and produce, when acting together, a sudden and violent bending of the 330 THE CRAYFISH a.H m tg LESS. XXVII dm. Fig. 84. — Diagram illustvating the action of the abdominal muscles in the Crayfish. A shows the position in extension, B in flexion. Four abdominal segments are shown in sagittal section ; tg, terga ; st, sterna ; art. m, tergal articular membranes ; ait. m', sternal articular membranes ; h. hinges. The muscles are represented as narrow bands (comp. Fig. 86 for their actual dimensions), and their arrangement is greatly simplified, dm, dorsal muscles ; cm, central muscle giving off extensor slips {ex) ; env. m, enveloping muscles continued into anterior (Ji) and posterior (/?') flexor slips. (From Parker and Haswell's Zoology. ) abdomen upon the cephalothorax, causing the crayfish to dart backwards with great rapidity. en^.S" efz,.'f; A- ,eoc^ en #- -cert m. en^.3- art.; ~exi en-.Z Fig. 85. — A leg of the Fresh-water Crayfish with part of the exo- skeleton removed to show the muscles. en. 2-en. 5, segments of endopodite ; /;, hinges ; art. in, articular membrane; ext, extensor muscles 57?, flexor muscles. (From Parker and Haswell's Zoology. ) 332 THE CRAYFISH less, xxvii It will be seen that the body-muscles of Astacus cannot be said to form a layer of the body-wall, as in Polygordius, but constitute an immense fleshy mass, filling up the greater part of the body-cavity, and leaving a very small space around the enteric canal. In the limbs (Fig. 85) each podomere is acted upon by two muscles situated in the next proximal podomere. These muscles are inserted, by chitinous and often calcified tendons, into the proximal edge of the segment to be moved, the smaller (ext) on the extensor, the larger (_/?) on the flexor side, in each case half-way between the two hinges, so that a line joining the two muscular insertions is at right angles to the axis of articulation. The digestive organs are constructed on the same general plan as those of Polygordius, but present many striking differences. The mouth (Fig. 83, a, Mtli) lies in the middle ventral line of the head, and is bounded in front by a shield- shaped process, the labrum, at the sides by the mandibles, and behind by a pair of delicate lobes, Xhe paragnatha. It leads by a short wide gidkt (Fig. 83, Gul ;¥\g. 86, ce) into a capacious stomach, which occupies a great part of the interior of the head, and is divided into a large anterior or cardiac division (Fig. 83, St ; Fig. 86, cs), and a small pos- terior or pyloric division {ps) : the latter passes into a narrow and very short small intestine (Fig. 83, S. hit; Fig. 86, nid), from which a somewhat wider la?-ge intestine (Fig. 83, L. Lit ; Fig. 86, hd) extends to the anus {an), situated on the ventral surface of the telson. The outer layer of the enteric canal consists of connective tissue containing striped muscular fibres : within this is a single layer of columnar epithelial cells, none of them glandular. In the gullet and stomach, and in the large intestine, the epithelium secretes a layer of chitin, which Fig. 86. — l')issection of Fresli- water Crayfish made by removing the e.xoskeleton with the appendages and the muscles, digestive gland and kidney of the right side (compare with diagrammatic figure 83, a). aa, antennary artery ; ab, abdomen ; an, anus ; b. d, aperture of right digestive duct exposed by removal of gland ; bf. 4, cheliped ; bn. ventral nerve cord ; cs, cardiac division of stamaeh ; cth, cephalo- 334 THE CRAYFISH less. thorax ; a, gullet ; em, dorsal muscles ; f/ii, ventral muscles ; g, brain ; h, heart ; hd, large intestine ; Ir, left digestive gland ; md, small intes- tine ; 0, right lateral ostium of heart ; oa, ophthalmic artery ; oaa, dorsal abdominal artery ; a, gullet ; pi. 1-5, pleopods ; pit 6, uropod ; ps, pyloric division of stomach ; j, a, sternal artery ; I (near heart), testis ; I (below anus) telson ; naa, ventral abdominal artery ; v. d, vas defer- ens ; vds, male genital aiDerture. (From Lang, after Huxley.) thus constitutes the innermost layer of those cavities. It is proved by development that the small intestine, which has no chitinous lining, is the only part of the enteric canal developed from the enteron of the embryo : the gullet and stomach arise from the stomodsum, the large intestine from the proctodfeum. Thus a very small portion of the enteric epithelium is endodermal (see Fig. 83, a). In the cardiac division of the stomach the chitinous lining is thickened and calcified in certain parts, so as to form a complex articulated framework, the gastric mill, on which are borne a median and two lateral teeth, strongly calcified and projecting into the cavity of the stomach. Two pairs of strong muscles arise from the carapace, and are inserted into the stomach ; when they contract they move the mill in such a way that the three teeth meet in the middle line and complete the comminution of the food begun by the jaws. The separation of the teeth is effected partly by the elasticity of the mill, partly by delicate muscles in the walls of the stomach. The pyloric division of the stomach forms a strainer ; its walls are thickened and pro- duced into numerous setse, which extend quite across the narrow lumen and prevent the passage of any but finely divided particles into the intestine. Thus the stomach has no digestive function, but is merely a masticating and strain- ing apparatus. On each side of the cardiac division is found, at certain seasons of the year, a plano-convex mass of calcareous matter, the gasirolith or "crab's-eye."' xsvii GILLS 335 The digestion of the food, and to some extent the absorp- tion of the digested products, are performed by a pair of large glands (Fig. 83, D. Gl ; Fig. 86, /r), lying one on each side of the stomach and anterior end of the intestine. They are formed of finger-like sacs or caca, which discharge into wide ducts opening into the small intestine, and are lined with glandular epithelium derived from the endoderm of the embryo. The glands are often called livers, but as the yellow fluid they secrete digests proteids as well as fat, the name hepato-pancreas is often applied to them, or they may be called simply digestive glands. The crayfish is car- nivorous, its food consisting largely of decaying animal matter. The digestive organs and other viscera are surrounded by a body-'Cavity, which is in free communication with the blood-vessels and itself contains blood. This cavity is not lined by epithelium, and is to be looked upon as an immense blood-sinus, and not as a true coelome. There are well-developed respiratory organs in the form of gills (Fig. 83, b), contained in a narrow branchial chamber, bounded internally by the proper wall of the thorax, externally by the gill-cover or pleural region of the carapace. Each gill consists of a stem giving off numerous branchial filaments, so that the whole organ is plume-like. The filaments are hollow and communicate with two parallel canals in the stem — an external, the afferent branchial vein, and an internal, the efferent branchial vein. The gill is to be considered as an out-pushing of the body-wall, and con- tains the same layers — a thin layer of chitin externally, then a single layer of epithelial cells, and beneath this connective tissue, hollowed out for the blood channels. According to their point of origin the gills are divisible into three sets — first, podobranchice or foot-gills (Fig. 87, A; Fig. 87. — Two dissections showing the gills of the Fresh-water Crayfish. In A the right gill-cover has been removed, but the gills are undis- turbed ; in B the podobranchice {pdb. in A) are cut away, and the outer set of arthrobranchiie (arh^) turned down to show the inner arlhro- branchice {arb) and the pleurobranchitE {pi. h). All the gills are numbered according to the segment from which they spring, the first thoracic segment being numbered 6, the last 13. ep. 5, scaphognathite. ab. I, ab. 2, abdominal segments ; a^, antennule ; a'-, antenna ; 6-8, maxillipeds ; 9-13, legs ; pi. I, first pleopod. (From Lang; after Huxley.) LESS, xxvii CIRCULATORY ORGANS 337 pdb)^ springing from the epipodites of the thoracic appen- dages, from which they are only partially separable ; secondly, arihrobranchim or joint-gills (b, arU), springing from the articular membranes connecting the thoracic appendages with the trunk ; and thirdly, pleurobranchicB, or wall-gills i^plb'), springing from the lateral walls of the thorax, above the attachment of the appendages. The total number of gills is eighteen, besides two filaments representing vestigial or vanishing gills. The excretory organs differ both in position and in form from those of Polygordius. There are no distinct Nephridia, but at the base of ea h antenna is an organ of a greenish colour, the antenna7y or green gland (Fig. 83, a, K), by which the function of renal excretion is performed. The gland is cushion-shaped, and contains canals and irregular spaces lined by glandular epithelium : it discharges its secre- tion into a thin-walled sac or urinary bladder, which opens by a duct on the proximal segment of the antenna. The green glands are to be looked upon as organs of the same general nature as nephridia. The circulatory organs are in a high state of development. The heart (Fig. 83, Ht ; Fig. 86, K) is situated in the dorsal region of the thorax, and is a roughly polygonal muscular organ pierced by three pairs of apertures or ostia (Fig. 86, (?), guarded by valves which open inwards. It is enclosed in a spacious pericardial sinus (Fig. 83, Pcd. S), which contains blood. From the heart spring a number of narrow tubes, called arteries, which serve to convey the blood to various parts of the body. At the origin of each artery from the heart are valves which allow of the flow of blood in one direction only, viz., from the heart to the artery. From the anterior end of the heart arise five vessels — a median ophthalmic artery (Fig. 86, oa), which passes forwards to the Z 33S THE CRAYFISH less. eyes ; paired antennary arteries (ad), going to the anten- nules, antennae, green glands, &c., and sending off branches to the stomach; and paired hepatic arteries, going to the digestive glands. The posterior end of the heart gives off two unpaired arteries practically united at their origin, the dorsal abdominal artery (paa), which passes backwards above the intestine, sending branches to it and to the dorsal muscles ; and the large sternal artery (sa), which passes directly downwards, indifferently to right or left of the intestine, passing between the connectives uniting the third and fourth thoracic ganglia, and then turns forwards and runs in the sternal canal, immediately beneath the nerve- cord, and sends off branches to the legs, jaws, &c. At the point where the sternal artery turns forwards it gives off the median ventral abdomitial artery {v. a. a), which passes backwards beneath the nerve-cord, and supplies the ventral muscles, pleopods, &c. All these arteries branch extensively in the various organs they supply, becoming divided into smaller and smaller off- shoots, which finally end in microscopic vessels called capillaries. These latter end by open mouths which com- municate with the blood-sinuses, spacious cavities lying among the muscles and viscera, and all communicating sooner or later with the sternal sinus (Fig. 83, a, B. S), a great median canal running longitudinally along the thorax and abdomen, and containing the ventral nerve-cord and the sternal and ventral abdominal arteries. In the thorax the sternal sinus (Fig. 88, sf. s) sends an offshoot to each gill in the form of a well-defined vessel, which passes up the outer side of the gill and is called the affere?it branchial vein (af. br. v). Spaces in the gill-filaments place the afferent in communication with the efferent branchial vein (ef. br. v). which occupies the inner side of the gill- XXVII CIRCULATION 339 Stem. The eighteen efferent branchial veins jopen into six bratichio-cardiac veins {br. c. v), which pass dorsally in close contact with the lateral wall of the thorax and open into the pericardial sinus. The whole of this system of cavities is full of blood, and the heart is rhythmically contractile. ^A'hen it contracts the blood contained in it is prevented from entering the peri- cardial sinus by the closure of the valves of the ostia, and therefore takes the only other course open to it, viz., into the arteries, ^^'hen the ^hear.t relaxes, the blood in the arteries is prevented frorn regurgitating by the valves at their origins, and the pressure of blood in the pericardial sinus forces open the valves of the ostia and so fills the heart. Thus in, virtue of the successive contractions of the heart, and of the disposition of the valves, the blood is kept constantly moving in one direction, viz., from the heart by the arteries to the various organs of the body, where it receives carbonic acid and other waste matters ; thence by sinuses into the great sternal sinus ; from the sternal sinus by afferent branchial veins to the gills, where it exchanges carbonic acid for oxygen ; from the gills by efferent branchial veins to the branchio-cardiac veins, thence into the peri- cardial sinus, and so to the heart once more. It will be seen that the circulatory system of the crayfish consists of three sections — (i) the /teari or organ of pro- pulsion ; (2) a system of out-going channels, the arteries, which carry the blood from the heart to the body generally ; and (3) a system of returning channels — some of them, the sinuses, mere irregular cavities, others, the veins, with definite walls — these return the blood from the various organs back to the heart. The respiratory organs, it should be observed, are interposed in the returning current, so that blood is taken both to and from the gills by veins. 7. 2 340 THE CRAYFISH less. Comparing the blood-vessels of Astacus with those of Polygordius, it would seem that the ophthalmic artery, heart, and dorsal abdominal artery together answer to the dorsal vessel, part of which has become enlarged and mus- cular, and discharges the whole function of propelling the af.br V Fig. 88. — Diagram illustrating the course of the circulation of the blood in the Crayfish. Heart and arteries red : veins and sinuses containing non-aerated blood blue : veins and sinuses containing aerated blood pink. The arrows show the direction of the flow. The blood from the pericardial sinus (pcd. j) enters the heart {ht) by a valvular aperture [v^] and is propelled into arteries («), the orifices of which are guarded by valves [v^) : the ultimate branches of the arteries discharge the blood into sinuses {s), and the sinuses in various parts of the body debouch into the sternal sinus {si. s) : thence the blood is taken by the afferent branchial veins (a/, lir. v) into the gills, where it is purified and is returned by efferent branchial veins (ef. br. zi) into the branchio- cardiac veins (br. c. v) which open into the pericardial sinus. (From Parker and Haswell's Zoology. ) blood. The horizontal portion of the sternal artery, together with the ventral abdominal, represent the ventral vessel, while the vertical portion of the sternal artery is a com- miss.ure, developed sometimes on the right, sometimes on the left side, its fellow being suppressed. xxvn NERVOUS SYSTEM 341 The blood when first drawn is colourless, but after ex- posure to the air takes on a bluish-gray tint. This is owing to the presence of a colouring matter called hcemocyanin, which becomes blue when combined with oxygen ; it is a respiratory pigment, and serves, like haemoglobin, as a cairier of oxygen from the external medium to the tissues. The hsemocyanin is contained in the plasma of the blood : the corpuscles are all leucocytes. The nervous system consists, like that of Polygordius, of a brain (Fig. 86, g) and a ventral nerve-cord (/'«), united by oesophageal connectives. But the ventral nerve-cord is differentiated into a series of paired swellings or ganglia to which the nerve-cells are confined, united by longitudinal connectives. The brain supplies not only the eyes and antennules, but the antennae as well, and it is found by development that the two pairs of ganglia belonging to the antennulary and antennary segments have fused with the brain proper. Hence we have to distinguish between a primary brain or archi-cerebrum, the ganglion of the prosto- mium, and a secondary brain or sy?i-cerebrum formed by the union of one or more pairs of ganglia of the ventral cord with the archi-cerebrum. A further case of concrescence of ganglia is seen in the ventral nerve-cord, where the ganglia of the last three cephalic and first three thoracic segments have united to form a large compound sub-asophageal ganglion. All the remaining segments have their own ganglia, with the exception of the telson, which is supplied from the ganglion of the preceding segment. There is a visceral system of nerves supplying the stomach, originating in part from the brain and in part from the oesophageal connectives. The eyes have a very complex structure. The chitinous cuticle covering the distal end of the eye-stalk is transparent. 342 THE CRAYFISH i.Ess. divided by delicate lines into square areas or facets, and constitutes the cornea. Beneath each facet of the cornea is an apparatus called an ommattdeum, consisting of an outer segment or vitreous body having a refractive function, and an inner segment or retinula forming the actual visual portion of the apparatus. The ommatidia are optically- separated from one another by black pigment, so that each is a distinct organ of sight, and the entire eye is called a compound eye. The antennules contain two sensory organs, to which are assigned the functions of smell and hearing respectively. The olfactory orgati is constituted by a number of extremely, delicate olfactory setce, borne on the external flagellum and supplied by branches of the antennulary nerve. The auditory organ is a sac formed by invagination of the dorsal surface of the proximal segment, and is in free communi- cation with the surrounding water by a small aperture. The chitinous lining of the sac is produced into delicate feathered auditory setce, supplied by branches of the antennulary nerve, and in the water which fills the sac are minute sand- grains, which take the place of the otoliths or ear-stones found in most auditory organs, but which, instead of being formed by the animal itself, are taken in after each ecdysis, when the lining of the sac is shed. Many of the setae on the general surface of the body have a definite nerve-supply, and are probably tactile organs. The crayfish is dioecious, and presents a very obvious sexual dimorphism or structural difference between male and female, apart from the actual organs of reproduction. The abdomen of the female is much broader than that of the male : the first and second pleopads of the male are modified into tubular or rather spout-like copulatory organs ; and the reproductive aperture is situated in the male on the XXVII REPRODUCTIVE ORGANS 343 proximal podomere of the fifth leg, in the female on that of the third. The testis (Fig. 86, /) lies in the thorax, just beneath the floor of the pericardial sinus, and consists of paired anterior lobes and an unpaired posterior lobe. From each side goes off a convoluted spermiduct or vas deferens (vd), which opens on the proximal segment of the last leg. The sperms are curious non-motile bodies produced into a number of stiff processes : they are aggregated into vermicelli-like sj>er- tiiatophores by a secretion of the vas deferens. The ovary is also a three-lobed body, and is similarly situated to the testis : from each side proceeds a thin-walled oviduct, which passes downwards, without convolutions, to open on the proximal segment of the third or antepenulti- mate leg. The eggs are of considerable size and contain a great quantity of yolk {see p. 256). Both ovary and testis are hollow organs, discharging their products internally. Their cavities represent the ccelome, and their ducts are organs of the same general nature as nephridia. The ova, when laid, are fastened to the setce on the pleopods of the female by the sticky secretion of glands occurring both on those appendages and on the segments themselves : they are fertilised immediately after laying, the male depositing spermatophores on the ventral surface of the female's body just before oviposition. The process of segmentation of the oosperm presents certain striking peculiarities. The nucleus divides repeatedly (Fig. 89, A, nu), but no corresponding division of the pro- toplasm takes place, with the result that the morula-stage, instead of being a heap of cells, is simply a multinucleate but non-cellular body. Soon the nuclei thus formed retreat from the centre of the embryo, and arrange themselves in a single layer close to the surface (b) : around each of these 344 THE CRAYFISH less. protoplasm accumulates, the central part of the embryo consisting entirely of yolk-material. We thus get a super- ficial segmentation, characterised by a central mass of yolk and a superficial layer of cells collectively known as the blastoderm. On one pole an invagination of the blastoderm takes place, giving rise to a small sac, the enteron, which commu- nicates with the exterior by an aperture, the blastopore. By this process the embryo passes into the gastrula-stage, which, however, differs from the corresponding stage in Polygordius Fig. 89. — Two stages in the early development of the Crayfish. In A the products of division of the nucleus [mi] are seen in the centre of the yolk : in B the nuclei have arranged themselves in a peripheral layer, each surrounded by protoplasm, so as to form the blastoderm. (From Parker and Haswell's Zooiogy, after Morin.) (p. 295) in the immense quantity of food-yolk filling up the space (blastocoele) between ectoderm and endoderm. Very soon the embryo becomes triploblastic, or three-layered, by the budding off of cells from the endoderm in the neigh- bourhood of the blastopore : these accumulate between the ectoderm and endoderm, and constitute the mesoderm. Before long the blastopore closes, and a stomodfeum and proctodeum (p. 296) are formed as invaginations of the ectoderm which e\'entually communicate with the enteron, forming a complete enteric canal. On each side of the mouth DEVELOPMENT 345 or aperture of the stomodreal depression (Fig. 90) three eleva- tions appear, the rudiments of the antennules (a^), antennae (a-), and mandibles {///) : in front of them is another pair of elevations on whieh the eyes (A) subsequently appear. Fi stage 90. — Early Lniljr}0 of X- iLbhw aUr CrasfLsli in tlie ]iau|i]iub stage. A in the upper part of the llgure is the e)e : /, the lahruin o\"erlianL;ing [he mouth, on each side of which are the ruclinicnls of the antennules (a'), antennae {a"), and mandibles (;«) : behind them is the rudiment of the thorax and abdomen (Tl^jwith the anus (.-/). The rudiments of the first three piairs of ganglia {G, ^^a", ^v;/) are seen through the trans- parent ectoderm. (From Lang, after Reichenbaeh. ) An unpaired ele\-ation {T.l) behind the moutli, and ha\i]ig the anus (.-i) or aperture of the proctodreal dejiression at its summit, is the rudiment of the thorax and abdomen. The embryo is now called a naiipliiis. jMaii)" Crustacea are ,U6 THE CRAYFISH LESS. hatched ill ihc form of a frcc-swimniiiit; larva, to which this name is ajiplied, characterised by the presence of three pairs of appendages, used for swimming and becoming the hic. 91, — Later ciiiliiyi of Fresh-water Ciavfisli, from the ventral aspect ; the abdomen {a/j) is fuMed down over the cephalothorax, so that its dorsal surface faces the observer, and the telson (7") reaches nearly to the mouth. The following appendages are indicated : A, eye-slalks ; tz\ anten- nules ; a", antenna;; />/, mandibles; w.v', ///.x'-, maxillae; /. I -A 8, thoracic appendages (maxillipcdes and legs). At the sides of the thorax are seen the edges of the carapace (/s) : in front of the mouth is the labrum (/), in front of the labrum the brain (^), and at the base of the eye-stalk the optic ganglion (^c). (From Fang, after Reichcnbach. ) antennules, antennK, and mandibles of the adult. In the cra)'tish there is no free lar^a, and the nauplius stage is passed through before hatching. The nauplius is gradually transtonned into the crayfish by XXVII DEVELOPMENT 34'? the appearance of fresh appendages, in regular order, behind the first three (Fig. 91); by the elongation of the rudiment of thorax and abdomen {ab) ; and by the gradual differen- tiation of the appendages. "When hatched the young animal agrees in all essential respects with the adult, but its proportions are very different, the cephalothorax being nearly globular and the abdomen small. For some time after hatching the young crayfishes cling in great numbers to the pleopods of the mother by means of the peculiarly hooked chaise of the first pair of legs. LESSON XXVIII THE FRESH-.WATER MUSSEL In the mussel we meet with an entirely new type of structure : the animal is bilaterally symmetrical, with no trace of metameric segmentation ; the power of locomotion is greatly restricted, and food is obtained passively by ciliary action, as in Infusoria, not by the active movements of definite seizing organs — tentacles, limbs, or protrusible mouth — as in most of the higher animal forms. Fresh-water mussels are found in ri\-ers and lakes in most parts of the world. Anodonta cygnea^ the swan-mussel, is the commonest species in England ; but the pearl-mussel, Unio margaritifer, is found in mountain streams, and other species of the same genus are universally distributed. The mussel is enclosed in a brown shell formed of two separate hahes or valves hinged together along one edge. It lies on the bottom, partly buried in the mud or sand, with the valves slight!)- gaping, and in the narrow cleft thus formed a delicate, semi-transparent substance is seen, the edge of the mantle or pallium. The mantle really consists of separate halves or lobes corresponding with the vahes of LESS. XXVIII GENERAL STRUCTURE 349 the shell, but in the position of rest the two lobes are so closely approximated as to appear simply like a membrane uniting the valves. At one end, however, the mantle pro- jects between the valves in the form of two short tubes, one (Fig. 92, B, ex. sph.) smooth-walled, the other {in. sph.) beset with delicate processes ox fimbria:. By diffusing particles of carmine or indigo in the water it can be seen that a current is always passing in at the fimbriated tube, hence called the inhalant siphon, and out at the smooth or exhalaiit siphon. Frequently a semi-transparent, tongue-like body (//) is pro- truded between the valves at the opposite side from the hinge and at the end furthest from the siphons : this is the foot, by its means the animal is able slowly to plough its way through the sand or mud. AVhen irritated the foot and siphons are withdrawn and the valves tightly closed. In a dead animal, on the other hand, the shell always gapes, and it can then be seen that each valve is lined by the corre- sponding lobe of the mantle, that the exhalant siphon is formed by the union of the lobes above and below it and is thus an actual tube, but that the boundary of the inhalant siphon facing the gape of the shell is simply formed by the approximation of the mantle-lobes, so that this tube is a temporary one. The hinge of the shell is dorsal, the gape ventral, the end bearing the siphons posterior, the end from which the foot is protruded anterior : hence the valves and mantle-lobes are respectively right and left. In a dead and gaping mussel the general disposition of the parts of the animal is readily seen. The main part of the body lies between the dorsal ends of the valves : it is produced in the middle ventral line into the keel-Hke foot : and on each side, between the foot and the corresponding mantle-lobe, are two delicate, striated plates, the gills. Thus 35° THE FRESII-WATER MUSSEL less. the whole animal has been compared to a book, the back being represented by the hinge, the covers by the valves, the fly-leaves by the mantle-lobes, the two first and the two last pages by the gills, and the remainder of the leaves by the foot. When the body of the mussel is removed from the shell the two valves are seen to be united, along a straight hinge- line (Fig. 92, A, h. I), by a tough, elastic substance, the hinge-ligament (Fig. 93, b, lig) passing transversely from valve to valve. It is by the elasticity of this ligament that the shell is opened : it is closed, as we shall see, by muscular action : hence the mere relaxation of the muscles opens the shell. In Anodonta the only junction between the two valves is afforded by the ligament, but in Unio each is pro- duced into strong projections and ridges, the hinge-teeth, separated by grooves or sockets, and so arranged that the teeth of one valve fit into the sockets of the other. The valves are marked externally by a series of concentric lines parallel with the free edge or gape, and starting from a swollen knob or elevation, the umbo, situated towards the anterior end of the hinge-line. These lines are lines of growth. The shell is thickest at the umbo, which represents the part originally formed, and new layers are deposited under this original portion, as secretions from the mantle, the shell being, like the armour of the crayfish, a cuticular exoskeleton. As the animal grows each layer projects beyond its predecessor, and in this way successive outcrops are produced giving rise to the markings in question. In the region of the umbo the shell is usually more or less eroded by the action of the carbonic acid in the water. The inner surface of the shell also presents characteristic markings (Fig. 92, a). Parallel with the gape, and at a SHELL 351 Fir,. 92. — A, intcriMV of right valve of AnodonUi, allowing the vaiious i 111 i"ircssions produced by tlic muscles shown in B : //, /, liingedinc ; //. /. 2)allial line. B, the animal removed from the shell and seen from the left side. a. (7;/, anterior adductor ; a. r, anterior retractor ; d. 1^^ digestive gland, .seen through mantle ; ex. sph, exhalant siphon ; fl, foot ; gl, gills, seen through mantle ; in. spli, inhalant siphon ; kd, kidney, seen tlirough mantle ; /'. 0, Keber's organ, seen tlrrough mantle ; w, mantle ; /. ad^ posterior adductor ; pc^ pericardium, seen through mantle ; //. w, pallial muscles ; p. r, posterior retractor ; /;-(■, protractor. (From Parker and Ilaswell's ZooIoq'.) bhort distance from it, is a delicate streak (//. /) caused by the insertion into the sliell of muscular fibres from the edt^e • (jf the mantle : the streak is lience called the pallial line. 352 THE FRESH-WATER MUSSEL less, xxviii Beneath the anterior end of the hinge the pallial line ends in an oval mark, the anterior adductor itnpression (a. ad), into which is inserted one of the muscles which close the shell. A similar, but larger, posterior adductor impression (/. ad) lies beneath the posterior end of the hinge. Two smaller markings in close relation with the anterior adductor impression mark the origin of the anterior retractor (a. r), and of the protractor {pre) of the foot : one connected with the posterior adductor impression, that of the posterior retractor {p. r) of the foot. From all these impressions faint converging lines can be traced to the umbo : they mark the gradual shifting of the muscles during the growth of the animal. The shell consists of three layers. Outside is a brown horn-like layer, the periostracum, composed of conchiolin, a substance alUed in composition to chitin. Beneath this is a prismatic layer formed of minute prisms of calcium carbon- ate, separated by thin layers of conchiolin ; and, lastly, forming the internal part of the shell is the nacre, or " mother-of-pearl," formed of alternate layers of carbonate of lime and conchiolin arranged parallel to the surface. The periostracum and the prismatic layer are secreted from the edge of the mantle only, the pearly layer from the whole of its outer surface. The hinge ligament is continuous with the periostracum, and is to be looked upon simply as a median uncalcified portion of the shell, which is therefore, in strictness, a single continuous structure. By the removal of the shell the body of the animal (Fig. 92, b) is seen to be elongated from before backwards, narrow from side to side, produced on each side into a mantle-lobe {in), and continued ventrally into a keel-like visceral mass, which passes below and in front into the foot {ft). Thus each vahc of the shell is in contact with lal.Jjilhnl. CAEMml- y,„,. r)et:EptJ,m:-^Z^ CJ'.t,) Fig. 93. — Diagrammatic sections of tlie Fresh-water Mussel. A, longitudinal section : right manUe-lobe (Man/) and gills (/. G, O. G) arc shown in perspective, B, transverse section. The cuticular shell [Sh), shown only in B, is black, the ectoderm dotted, the nervous system finely dotted, the endoderm radially striated, the mesoderm evenly shaded, and the ccelomic epitlielium represented liy a beaded line. The dorsal region is produced into the right and left mantleduliLS iMant), attached to which are the val\es of the shell (.S7/)joined dorsalJy by an elastic ligament [Itg). The mantle-lobes are partly united so as to form the inhalant (////;. Af) and exhalant (Exh. Ap) apertures at the posterior end. The body is produced ventrally iirlo the foot [Foot)^ on each side of which arc the gills, an inner (/. G) and an outer (O. G), each formed of an inner and an outer lamella. The body is covered externally by deric epithelium {Dcr. EplJuu), \\'ithin which is mesoderm (Msd) largely differentiated into muscles, of which the anterior (.-/. ./(/) and posteiior {P. ./i/) adductors are indi- cated in A. The mouth [Mtk) leads by tlic short gullLt [u:i/) mlo the stomach {Si), from wdiich proceeds the coiled intestine (Inl), ending in the anus A A 354 THE FRESH-WATER MUSSEL less. {An) : the enteric epithelium is mostly endodevmal. The digestive gland {Z>. Gl) surrounds the stomach. The coelome [Ccel) is reduced to a small dorsal chamber enclosing part of the intestine and the heart ; the parietal {Cal. Epthm) and visceral (Ccel. Epthm^) layers of ccelomic epithelium are shown. The heart consists of a median ventricle ( Vent), enclosing part of the intestine, and of paired auricles {Aur). The paired nephridia [Nphm) open by apertures into the coelome (Nph. st) and on the exterior (Nph. p). The gonads (Goii) are imbedded in the solid mesoderm, and open on the exterior by gonoducts [Gnd). The nervous system consists of a pair of cerebro-pleural ganglia (C. P. Gn) above the gullet, a pair of pedal ganglia {Pd. Gn) in the foot, and a pair of visceral ganglia ( V. Gn) below the posterior adductor muscle. the dorso-lateral region of the body of its own side, together with the corresponding mantle-lobe, and it is from the epi- thelium (Fig. 93, Der. Epthm) covering these parts that the shell is formed as a cuticular secretion. The whole space between the two mantle-lobes, containing the gills, visceral mass, and foot is called the matitle-cavity. A single layer of epithelial cells, the deric epithelium or epidermis {Der. Epthm), covers the whole external surface, i.e., the body proper, both surfaces of the mantle, the gills, and foot; that of the gills and the inner surface of the mantle is ciliated. Beneath the epidermis come connective and muscular tissue, which occupy nearly the whole of the interior of the body not taken up by the viscera, the coelome being, as we shall see, much reduced. The muscles are all unstriped, and a!re arranged in distinct bands or sheets, many of them very large and conspicuous. The largest are the anterior and posterior adductors (Figs. 92, 93, and 94, a. ad, p. ad), great cylindrical muscles which pass trans- versely across the body and are inserted at either end into the valves of the shell, which are approximated by their contraction. Two muscles of much smaller size pass from the shell to the foot, which they serve to draw back : they xxvin DIGESTIVE ORGANS 355 are the anterior (a. r) and posterior {J>. r) retractors of the foot. A third foot-muscle {j>rc) arises from the shell close to the anterior adductor, and has its fibres spread fan-wise over the visceral mass which it serves to compress, thus forcing out the foot and acting as a. protractor o{ that organ. The substance of the foot itself consists of a complex mass of fibres, the intrinsic muscles of the foot, many of which also act as protractors. Lastly, all along the border of the mantle is .a row of delicate pallial muscles (Fig. 92, b, pi. m), which, by their insertion into the shell, give rise to the pallial line already seen. The coelome is reduced to a single ovoidal chamber, the pericardium (Fig. 93, C(el , Fig. 94, pc), lying in the dorsal region of the body and containing the heart and part of the intestine: it is lined by coelomic epithelium (Ca7. Epthm), and does not correspond with the pericardial sinus of the crayfish, which is a blood-space. In the remainder of the body the space between the ectoderm and the viscera is filled by the muscles and connective tissue. The mouth (Fig. 94, mth) lies in the middle line, just below the anterior adductor. On each side of it are two triangular flaps, the internal and external labial palps ; the external palps unite with one another in front of the mouth, forming an upper lip ; the internal are similarly united behind the mouth, forming a lower lip : both are ciliated externally. The mouth leads by a short gullet (Fig. 94, gut) into a large stomach (st), which receives the ducts of a pair of irregular, dark-brown digestive glands {d.gl). The intestine (Jni) goes off from the posterior end of the stomach, descends into the visceral mass, where it is coiled upon itself, then ascends parallel to its first portion, turns sharply backwards, and proceeds, as the rectum (ret), through the pericardium where it traverses the ventricle of the heart, A A 2 356 THE FRESH-WATER MUSSEL ElG. 94.— Dissection of Anodonta, made by removing the mantlc- lobe, inner and outer gill.-;, wall of pericardium, and auricle of the left side, and di-^seeting away the skin, muscles, &c. of the same side down to the level of the enteric canal, kidney, nervous system, Sec. Part o( the enteric canal is laid oiien, as aRo are the kidney {/cd) and bladder {d/}. The connection b^itween the cerebro-pleural [r. ^I. ^ii) and Ni'.CLral {v. i^ii) ganglia is indicated by a dotted line. (2, anus ; a. ad, antLrior adductor ; a. ao, anterior aorta ; u. <■. a/>, auiicido-ventricubr ajierture ; /'/, urinary bladder; c. pi. gn, cerebro- plenial ganglion : iL if, duct of digestive gkand ; d. _sK digestive gland ; (1. p. a. dorsal pcillial apcUure ; l'.v. spi, cxhalant siphun ; //, foot ; g. op, genital apeiture ; ,;■"<-'//, gonad ; ,;7//, gullut ; /. /. /, interdamellar iunc- lion ; ///. sph, inhalant siphon ; ?>//, intestine ; kdy kidney ; /;/, mantle ; //////, mouth ; /. (T^^ posterior aorta ; /. ad, posterior adductor ;/<', peri- cardium ; pd. ,;'7/, pedal ganglion ; r. 7^ t;) o :=; oj " "• -c „ ■-§-i £ V •gas- ' =: £ -^ "5 ^ — -.^ >^ ?■ ^ ^ !z: r; ~:i ^ i-. tea rt r 374 THE DOGFISH less. The entire skeleton consists of separate pieces of cartilage, calcified or not, and connected with one another by sheets or bands of connective tissue called ligaments : it is divisible into the following parts : — A. The skull or skeleton of the head, consisting of — 1. The cranium or brain-case, enclosing the brain and the chief sense-organs. 2. The upper and loiver jaws. 3. The visceral arches, a series of cartilaginous hoops supporting the gills. B. The vertebral column or backbone, a jointed axis extend- ing from the cranium to the end of the tail, and enclosing the spinal cord, c. The skeleton of the median fins. D. The skeleton of the paired fins, consisting of — 1 . The shoulder-girdle or pectoral arch, to which are attached 2. T\iQ pectoral fins. 3. The hip-girdle or pelvic airh, to which are at- tached 4. Ths pelvic fins. The cranium (Fig. 100, Cr) is an irregular cartilaginous box containing a spacious cavity for the brain, and pro- duced into two pairs of outstanding projections : a posterior pair, called the auditory capsules (aud. cp), for the lodgment of the organs of hearing, and an anterior pair, the olfactory capsules {olf. cp), for the organs of smell. Between the olfactory and auditory capsules, on each side, the cranium is hollowed out into an orbit {or) for the reception of the eye. In front the cranium is produced into three cartila- ginous rods {r), which support the snout. On its posterior XXIX BRANCHIAL ARCHES 375 face is a large aperture, "Cor foramen magnum, through which the brain joins the spinal cord, and on each side of the foramen is an oval elevation or condyle for articulation with the first vertebra. In the human and other higher vertebrate skulls the m)per jaw is firmly united to the cranium, and the lower ^one is free. But in the dogfish both jaws {up. j, I. j) are connected with the cranium by ligament Qg, Ig) only, and each consists of strong paired (right and left) moieties, united with one another by fibrous tissue. The posterior end of the upper jaw presents a rounded surface, on which fits a corresponding concavity on the lower jaw, so that a free articulation is produced, the lower jaw working up and down in the vertical plane, not from side to side like the jaws of the crayfish. The visceral arches consist of six pairs of cartilaginous half-hoops, lying in the walls of the pharynx (Fig. 99, b, £r. A), and united with one another below so as to form a basket-like apparatus supporting the gills. The first of these arches is distinguished as the hyoid, and is situated imme- diately behind the jaws. It consists of two parts, a strong, rod-like hyomandibular (Fig. 100, hy. m), which articulates above with the auditory capsule, and is connected below by fibrous tissue with the jaws, thus helping to suspend them to the cranium : and a hyoid cornu, which curves forwards inside the lower jaw, and is connected with its fellow of the opposite side by a median plate which supports the tongue. The remaining five arches (br. a. i — br. «. 5) are called the branchial arches. Each is formed of several separate pieces, movably united by fibrous tissue so as to render possible the distension of the throat during swallowing. Both they and the hyoid give attachment to delicate cartilaginous branchial rays {br. r, br. r : Fig. 99, Br. R) which support the gills. 376 THE DOGFISH The vertebral column has the general charactei of a jointed tube surrounding the spinal portion of the neural canal. L)'ing beneath this ca\-ity, i.e., between it and the ccelome, is a longitudinal row of biconcave discs, the vcr- -y-nf^ ~ir-"^'V'^f"ynr^ yy^' * ^ 'm r^ 'J- ^^^^v v^"" J Jl i( ^Stx^^^^ si^.'M lr.hr Fii;. loi. — A, Three Irunk vertebrae of Sc)lliuin from the side, n, a single trunk vertebra viewed from one end. e, three eaudal vertebra from the side. II, a single caudal vertebra fronr one end. (-, centrum ; /;. a, ha-'mal arch ; ii. a, neural arch ; /r. fr. transverse process. (After Hasse.) Icbral ixittra (Fig. loi, r .• Fig. 99, 7". Cent): they are lornied of cartilage, but ha\ e their anterior and posterior faces strongly calcified. The biconcave intervals between them (Fig. 99, a) are filled with a gelatinous matter or ititer- XXIX VERTEBRAL COLUMN 377 vertebral substance. The centra are united by ligament, so that the whole chain of discs is very flexible. Connected with the dorsal aspect of the series of centra is a cartila- ginous tunnel, arching over the spinal cord : it is divided into segments, corresponding with, but usually twice as numerous as the centra, and called the neural arches (Fig. 101, n. a ; Fig. 99, N. A). In the anterior part of the vertebral column the centra give off paired outstanding processes (Fig. loi, a and b, tr. pr) called transverse processes, to the end of each of which is articulated a short cartilaginous rod, the rib. Further back the transverse processes are directed down- wards, instead of outwards, and in the whole caudal or tail region they unite below, forming hcemal arches (Fig. loi, c and D, h. a ; Fig. 99, A, H. A), which together constitute a kind of inverted tunnel in which lie the artery and vein of the tail. In the region of the caudal fin the haemal arches are produced into strong median hamal spines (Fig. 99, a, H.A to the right), which act as supports to the fin. A centrum, together with the corresponding neural arch and transverse processes or hsmal arch, forms a vertebra or single segment of the vertebral column. It should be noticed that in the vertebral column we have another instance of the metameric segmentation of the vertebrate body. The vertebrae do not, however, correspond with the myomeres, but alternate with them. The myo- commas are attached to the middle of the vertebree, so that each myomere acts upon two vertebrse and thus produces the lateral flexion of the backbone. In the embryo, before the development of the vertebral column, an unsegmented gelatinous rod, the notochord, lies beneath the neural cavity in the position occupied in the adult by the line of centra, by the development of which it 37S THE DOGFISH is largely replaced. Much of it, however, remains as the gelatinous intervertebral substance. The notochord is one of the most characteristic organs of the Vertebrata. The skeleton of the median fins consists of a series of parallel cartilaginous rods, the fiii-rays or pterygiophores '-'/ Fig. 102. — Ventral view of pectoral arch of Scyllium with light pectoral fin. pectoral arch is divisible into dorsal (pet. g) and ventral (pit. g) s separated by the articular facets (art. f) for the fin. pectoral fin is formed of three basal cartilages (lis. 1-3) and .melons radials (rnd) ; its free edge is supported by dermal rays (d.f. r). (Modified from Marshall and Hurst.) portions The (Fig. i>9, J'igpli), the pro.\imal ends of which are more or less fused togetlier to form basal cartilages or hasalia. The free edges of the fins are supported by a double series of delicate horn-like fibres, the dcntial fiii-raxs (Dcfiii. F. R). The shoulder-girdle (I'lg. 102) is a strong, inverted arch of XXIX TEETH 379 calcified cartilage, situated just behind the last branchial arch (Fig. 99, a, Sh. G). On each side of its outer surface it presents three elevations or articular facets (Fig. 102, art.f) for the pectoral fin ; the presence of these allows of the divi- sion of each side of the arch into a narrow, pointed dorsal portion {pet. g), and a broader ventral portion {pet. g) united in the middle line with its fellow of the opposite side. The pectoral fin is formed of pterygiophores {rad), fused proxim- ally to form basals (^.f. i. — 3), which are three in number, and very large and strong. The pelvic girdle is a transverse bar of cartilage situated just in front of the vent (Fig. 99, a, Pelv. G), and present- ing on its posterior edge facets for the pelvic fin. The latter has the same general structure as the other fins, but has a single very large basal cartilage, and its first or anterior radial is also much enlarged. The free edges of both pectoral and pelvic fins are supported by horn-like dermal rays (Fig. 102, d.f.r). It will be noticed that while the skeleton of the crayfish is a series of articulated tubes with the muscles inside them, that of the dogfish is a series of articulated rods with the muscles outside. The joints, formed by two rods applied at their ends and bound together by ligament, are not confined to movement in one plane, like the hinge- joints of the crayfish, but are capable of more or less rotatory movement. The mouth, as we have seen, is a transverse aperture bounded by the upper and lower jaws. In the mucous membrane covering the jaws are imbedded large numbers of teeth, (Fig. 99, T) bony conical bodies, with enamelled tips, arranged in transverse rows. They are to be looked upon as special developments of the placoid scales or dermal teeth, enlarged for the purpose of seizing prey. ^ "" rt en & _aj d ^ Ss o •-M rt •s a O ^Vj 3 § ^ ?; .?2 CU tn •-5 O ^^ V-i O o U) C! d .9, o m CJ (U , spiracle ; i. br. a}-?', br. cv', internal branchial apertures ; cd. st, cardiac, and pyl. st, pyloric portions of stomach ; sp. vl, spiral valve of intestine (int) ; /. Ir, left, and r. Ir, right lobe of liver ; pan, pancreas ; spl, spleen ; ret. gl, rectal gland ; jiie.<:, mesentery. The heart consists of sinus venosus {s. v), auricle {nu), ventricle (v), and conus arteriosus (c. art) : the latter gives off the ventral aorta {v. ao] from which are seen to arise the afferent branchial arteries of the right side. The dorsal aorta (rf. ao) receives anteriorly the efferent branchial arteries, and posteriorly becomes the caudal artery {cd. a), lying above the caudal vein {cd. v). The spinal cord {sp. cd) passes in front into the brain, which con';ists of medulla oblongata {med. obi), cerebellum {crb), optic lobes (opt. I), dien- cephalon {dien), proscephalon {prs), and olfactory lobes {olf. /). To the diencephalon are attached the pineal (pin) and pituitary ipty) bodies. The left kidney {kd) opens by the ureter (k;-) into the urinogenital sinus (;(. g. s) which discharges into the cloaca. The left testis {ts) is connected with the epididymis (epid) from which the vas deferens {v. def) passes backwards, dilates into the vesicula seminalis {vs. sent) and opens into the urinogenital sinus, with which is also connected the sperm-sac {sp. s). Attached to the fold of peritoneum supporting the liver is a small tube (/. ;/. d) representing the oviduct of the female. The mouth (Figs. 99, Mth and 103) leads into an oral cavity (Or. cav), which passes insensibly into the throat or pharynx {ph), a division of the enteric canal distinguished by having its walls perforated by five pairs of slits, the in- fernal branchial apertures {i. br. a 1-5) as well as by the inner opening of the spiracle {sp). The pharynx is continued by a short gullet {gul) into a capacious U-shaped stomach consist- ing of a wide cardiac division {cd. st) and a narrow pyloric {pyl. st) division. The pyloric division communicates by a narrow valvular aperture with the intestine {int), a wide, nearly straight tube having its lining membrane produced into a spiral fold, the spiral valve {sp. vl), which practically 382 THE DOGFISH less. converts the intestine into a very long, closely-coiled tube, and greatly increases the absorbent surface. Finally the intestine opens into a large chamber, the cloaca {ct), which communicates with the exterior by the vent. From the gullet backwards the enteric canal is contained in the abdominal division of the ccelome, to the dorsal wall of which it is suspended by a median mesentery (Fig. 99, c, and Fig. 104, mes). The greater part of the canal is de- veloped from the enteron of the embryo, and is consequently lined by endoderm ; only the oral cavity is formed from the stomodKum, and the cloaca from the proctodaeum (Fig. 99, a). Outside the enteric epithelium are connective and muscular layers, the latter formed of unstriped fibres : it is generally characteristic of Vertebrates that the voluntary muscles are striped, the involuntary unstriped. The digestive glands are characteristic. The largest is an immense liver (Fig. 99, Lr) divided into two lobes (Fig. 103, /. lr, r. lr) and situated below the stomach along the whole length of the abdomen, to the wall of which it is attached by a fold of peritoneum. It discharges its secretion, the bile, into the commencement of the intestine by a tube, the bile-duct (Fig. 99, £. D), which gives off a blind offshoot terminating in a large sac, the gall-bladder {G. BT) ; this serves as a reservoir for the bile, the chief function of which is to act upon the fatty portions of the food. But besides secreting this special digestive juice, the liver-cells produce a substance called glycogen or animal starch, which is passed directly into the blood in the form of sugar. Another gland, of considerably smaller size, is the pan- creas (Fig. 99, Pn Fig. 104, pan); it lies against the anterior end of the intestine, into which it opens by the pa?icreafic duct. It secretes pancreatic juice, which has an action upon all the principal classes of food, converting XXIX GILLS 383 proteids into peptones, starch into sugar, and breaking up fats. Opening into the cloaca is a small finger-like rectal gland (ret. gl), the function of which is uncertain. In addition to these glands the inner surface of the stomach and intestine is dotted all over with microscopic apertures, leading into minute tubular glands sunk in the mucous membrane. These are the gastric and intestinal glands : the former secrete gastric Juice, which digests pro- teids ; the latter intestinal juice, which probably acts upon all classes of food. Thus as compared with the animals pre- viously studied, the dogfish, in common with other Verte- brates, shows an extraordinary differentiation of digestive glands and fluids. There is another characteristic vertebrate organ in close connection with the enteric canal and called the spleen . {spt). It is an irregular dark-red, gland-like body, of con- siderable size, attached by peritoneum to the stomach. It has no duct, and its chief function is probably the manufac- ture of leucocytes and the disposal of worn-out, red blood corpuscles. Other ductless glands are the thyroid in the throat ; the thymus in connection with the dorsal ends of the branchial arches ; and the supra-renal bodies in relation with the kidneys. The respiratory organs or gills consist of five pairs of pouches, each opening by one of the internal branchial apertures (Figs. 99 and 103, a and b. Int. br. ap) into the pharynx, and by one of the external branchial apertures [Ext. br. ap) on the exterior. The walls of the pouches are supported by the visceral arches {Br. A) and branchial rays {£r. R, Br. R), and are lined with mucous membrane raised into horizontal ridges, the branchial filaments {Br. Fit), which are abundantly supplied with blood-vessels, ahd are the actual organs of respiration. As the fish swims water 384 THE DOGFISH less, xxix enters the mouth and passes by the internal clefts into the branchial pouches, and thence outwards by the external clefts, a constant supply of oxygen being thus ensured. The gill- pouches are developed as offshoots of the pharynx, and the respiratory epithelium is therefore endodermal, not ecto- dermal, as in the starfish, crayfish, and mussel. The organs of circulation attain a degree of specialisation not met with in any of our former types. The heart is situated in the pericardial cavity or anterior compartment of the coelome, and is a large muscular organ composed of four chambers. Posteriorly is a small, thin-walled sinus venosus (Figs. 103 and 104, s. v), opening in front into a capacious thin-walled auricle {au) ; this communicates with a very thick- walled ventricle (v), from which is given off in front a tubular chamber, also with thick muscular walls, the conus arteriosus (c. art). There are valves between the sinus and the auricle, and between the auricle and ventricle, and the conus contains three longitudinal rows of valves : all the valves are arranged so as to allow of free passage of blood from sinus to auricle, auricle to ventricle, and ventricle to conus, but to prevent any flow in the opposite direction. The heart, alone among the involuntary muscles, is formed of striped fibres. The conus gives off in front a single blood-vessel (v. ao), having thick elastic walls composed of connective and elastic tissue and unstriped muscle. This vessel, the ventralaorta, passes forwards beneath the gills, and gives off on each side paired lateral branches, the afferent branchial arteries (a. br. a). Each afferent artery passes to the corresponding gill, and there branches out into smaller and smaller arteries, which finally become microscopic, and open into a network of delicate tubes called capillaries, with which the connec- tive tissue of the branchial filaments is permeated. The - ^ 53 § >" %lf-2 i'P 1^ ':; •^ § -D ^'■" - 'Z^ CJ G /J o o •p S C " o i=. rC -^ 'S ■S 2 1 rt - S^-^-r ^2 ^ "^ c o c X rt "^ G C' ^ >> ■-' rt C^ +-. '^ 1 8 ^ -G :/j si ■f. rt , . ■a G ■ ■ OJ 'J^ ^ ^ y^ == i- ■"" G b;] -S g ■^I'S-ii-ji ■^£^ .5 ^ -d S S ^-S ^^•5 "Sn i: d rt ■? " ? - >^ ■5 7= ^- ^ ^ "? ^ G ^ '^"? J^ ?^ OJ OJ S 1^ " " 2 & I'li 1) - CJ C > G OJ .z. S >-' r^ ^ r- ,^ G -.s- sf^ ij i^ ?-!"'"' "^ •^ U rt *^ ,n cl) ._r, -5 ^ "" >^ i "<-o t/ •S ^ S -2 -^ u-j ^ G rt "^ tj '^ 'iJ l.-l o o o ^ ^ -n^ -r- 3< -a u M - — ~ c c 386 . THE DOGFISH less. capillaries, unlike the arteries, have no muscle or confiective tissue in their walls, which are formed of a single layer of epithelial cells. The blood in these respiratory capillaries is therefore brought into close relation with the surrounding water, and as the blood flows through them it exchanges its carbon dioxide for oxygen. From the respiratory capillaries the blood is collected into minute arteries, which join into larger and larger trunks, and finally unite into efferent branchial arteries (ef. br. a) — two to each gill — by which the purified blood is carried from the gills. The efferent arteries of the right and left sides unite in a median longitudinal artery, the dorsal aorta (d. ao), which passes backwards, immediately beneath the vertebral column, to the end of the tail. From the efferent branchial arteries and the dorsal aorta are given off numerous arteries supplying the whole of the body with blood. The most important of these are two pairs of carotid arteries (c. a) to the head, a pair of subclavians iscl. a) to the pectoral fins, unpaired cmliac {cl. a) and mes- enteric arteries {ms. a) to the enteric canal, liver, pancreas, and spleen, numerous paired renals (r. a) to the kidneys, spermatics (sp. a) to the gonads, and a pair of iliacs (il. a) to the pelvic fins. The posterior part of the dorsal aorta, supplying the tail, is contained in the haemal canal of the caudal vertebrae, and is known as the caudal artery (cd. a). All these arteries branch and branch again in the various parts to which they are distributed, their ultimate ramifica- tions opening, as in the case of the gills, into a capillary network with which every tissue, except the cartilages and the epithelia, is permeated. In traversing these systemic capillaries the blood parts with its oxygen and various nutrient matters to the tissues, and receives from therij carbon dioxide and other waste matters. XXIX CIRCULATORY ORGANS 387 From the systemic as from the respiratory capillaries the blood is collected into vessels which join into larger and larger efferent trunks. But these trunks are not thick-walled elastic arteries, but thin-walled, non-elastic, collapsible tubes, having valves at intervals, called veins. As a general rule every part of the body has a vein running alongside its artery, the blood in the artery flowing to the part in question, that of the vein away from it. The blood from the head is brought back by a pair of jugular veins (j. v) : each of these enters a large frecaval vein {pr. cv. v), which passes vertically downwards and enters the sinus venosus. The blood from the tail is re- turned by a caudal vein (cd. v) lying immediately beneath the caudal artery in the hsemal canal : this vessel enters the coelome and then divides into right and left branches, the renal portal veins {r. p. v), which pass to the kidneys and join with the capillaries of these organs, the impure blood brought from the tail mingling with the pure blood of the renal arteries {r. a). From the kidneys the blood is returned into a pair of immense cardinal veins {crd. v), which pass forwards, receiving veins from the reproductive organs {sp. v), muscles, 6z:c., and finally join each with the corresponding jugular to form the precaval vein. From the stomach, intestine, spleen, and pancreas the blood is collected by numerous veins, which all join to form a large hepatic portal vein (h. p. v). This behaves in the same way as the renal portal : instead of joining a larger vein on its way to the heart, it passes to the liver and breaks up to connect with the capillaries of that organ, its blood, deprived of oxygen but loaded with nutrient matters from the enteric canal, mingling with the oxygenated blood brought to the liver by a branch of the coeliac artery. After circulating through the capillaries of the liver the blood C c 2 388 THE DOGFISH less. is taken by a pair of hepatic vehis (h. v) to the sinus venosus. The iliac veins (il. v) from the pelvic fins pour their blood into the lateral veins (lat. v), paired trunks running forwards in the side walls of the body to the sinus venosus, and receiving at their anterior ends the subclavian veins (scl. v) from the pectoral fins. Some of the veins, especially the cardinals and spermatics, are dilated into spacious cavities called sinuses. These are, however, of a totally different nature from the sinuses of the crayfish, which are mere spaces among the tissues devoid of proper walls. In the dogfish, as in ^^ertebrata generally, the blood is confined, throughout its course, to definite vessels, the heart, arteries, capillaries, and veins invariably forming a closed system of communicating tubes. The general course of the circulation will be seen to agree with that already described in the crayfish and mussel : i.e., the blood is driven by the contractions of the heart through the arteries to the various tissues of the body, whence it is returned to the heart by the veins or sinuses (compare Figs. 88, 96, and 104A). But whereas in both crayfish and mussel the respiratory organs are interposed in the returning current, both their afferent and efferent vessels being veins, in the dogfish they are interposed in the out- going current, their afferent and efferent vessels being arteries. An artery, it must be remembered, is a vessel taking blood from the heart to the tissues of the body, and having thick walls tc3 resist the strain of the heart's pulsa- tion ; a vein is a thin-walled vessel bringing back the blood from the tissues to the heart. Moreover, the circulation in the dogfish is complicated by the presence of the two portal systems, renal and hepatic. In both of these we have a vein, renal portal or hepatic CIRCULATION 3S9 portal, which, instead of joining with larger and larger veins, and so returning its blood directly to the heart, breaks up, after the manner of an artery, in the kidney or liver, the blood tinding its way into the ordinary venous channels after having traversed the capillaries of the gland in question. I'hus an ordinar)' artery arises from the heart or from an r^ (ZO e hr Fie. 104A. — Diai^ram illubUaiiiiy ihc cuursc of iIk' tiiculalion in ihe Do-hsh. Vessels containing oxygenated blood, red ; non-oxygenated, lilue. j5, capillaries of the body generally ; E, of the enteric canal ; G', ctf the gills ; A', of the kidneys ; i, of the liver; T, of the tail. a, br. a, afferent branchial arteries , mt, auricle ; c a, conns arteri- osus ; d. ao, dorsal aorta ; e. br. a, efferent branchial arteries ; A. p. i\ hepatic portal vein ; /;. v, hepatic vein ; II, lacteals ; ly, lyniphatici ; /;'. iZ'. V, precaval vein ; ;■. /. Z', renal portal vein ; s. v, sinus venosus ; V, ventricle ; v. ao^ ventral aorta. The arrows show the direction of the current. { From Parker and Flaswell's Zoo/o^y. ) artery of higher order and ends in capillaries , an ordinary vein arises from a capillary network and ends in a vein of higher order or in the heart. ]!ut the hepatic and renal portal veins end in capillaries after the manner of arteries, and the efferent branchial arteries begin in capillaries alter the manner of \'eiris, 390 THE DOGFISH tEss. With regard to the general morphology of the blood-system, the dorsal aorta with the caudal artery may be considered as a dorsal vessel (compare Polygordius, p. 279, and Crayfish, p. 340), the caudal vein, hepatic portal vein, heart, and ven- tral aorta as together representing a ventral vessel, the affer- ent and efferent branchial arteries as commissural vessels, and the lateral veins as lateral vessels. It will be seen that the heart of Vertebrates- is a muscular dilatation of the ventral vessel. The blood is red, the colour being due, as in some species of Polygordius (p. 280), to haemoglobin. The pigment is not, however, diffused in the plasma of the blood, but is confined to the red corpuscles, flattened oval cells with large nuclei, like those of the frog referred to in an early Lesson (p. 56, Fig. 8). Among the red corpuscles, but in much smaller numbers, are leucocytes. AX'hen ' the blood is fully oxy- genated it takes on a bright scarlet colour, and is usually called arterial blood ; when the oxygen has been given up to the tissues the colour becomes dull purple, and the blood is called ve?ious. But the student must avoid the common error that arterial blood is necessarily confined to arteries and venous to veins ; in the dogfish, for instance, the ventral aorta and the afferent branchial arteries contain venous blood. In addition to the blood-vessels the dogfish possesses a set of channels called lymphatics (Fig. 104A, ly), consisting of colourless thin-walled vessels, mostly running alongside the arteries and veins. Traced in one direction they ramify ex- tensively, and end in a set of lymph-capillaries interwoven with, but distinct from, the blood capillaries ; traced in the other direction they join into larger and larger trunks, pro- \ided at intervals with valves, and finally open into the veins. The lymph capillaries take up the drainage from the XXIX NERVOUS SYSTEM 391 tissues and pass it into the veins. The fluid they contain, called lyinph, is practically blood, minus its red corpuscles; its leucocytes are formed in structures called lymphatic glands, which occur in the course of the vessels. The lymphatics of the enteric canal are called ladeals ; they take an im- portant share in the absorption of fats. The nervous system, like the circulatory organs, is vastly in advance of anything we have yet met with. The central nervous system consists of a brain (Fig. 103), contained in the cranial cavity, and continuous posteriorly with a spinal cord {sp. c) contained in the neural canal of the back- bone. Thus the central' nervous system is exclusively dorsal in position, and is not traversed by the enteric canal as in Polygordius, the crayfish, and the mussel. Another characteristic feature of the dogfish's nervous system is that it is not solid, like that of Polygordius and the crayfish, but is tubular, being traversed by a longitudinal canal, the neuroccek (Fig. 99, N'. Coe), lined with epithelium. In the spinal cord the neurocoele has the form of a narrow central canal ; in the brain it expands into a fairly capacious system of cavities, the cerebral ventricles. The brain or anterior expansion of the nervous system is a complex structure divisible into several parts. The hind- most division, continuous with the spinal cord, is the medulla oblongata (Figs. 103 and 105, NH), and has above it the cere- helium {HH). Immediately in front of these two divisions is the mid-brain, produced above into paired elevations, the optic lobes. In front of the mid-brain is a small section called the diencephalon {ZH), and anterior to this again a large prosencephalon ( VJEC), corresponding with the cerebral hemispheres of the higher Vertebrata, and giving off in front paired olfactory lobes (Z. ol). All these divisions of the brain contain ventricles {F. rho), varying considerably in 392 THE DOGFISH less. form and size. Connected with the dorsal region of the '^ff Lol Fig. 105. — Dorsal view of the brain of the Siyllinm canicula. Tile posterior division of tlie brain is the medulla oblongata {NH), on the dorsal surface of which is shown one of the cerebral ventricles {F. rho). The large cerebellum (HIT) nearly covers the optic lobes (A/H). The diencephalon [ZH) shows in the middle one of the cerebral ven- tricles, and the place of attachment of the pineal body (Gp). The prosencephalon ( J'H) gives off the olfactory lobes ( 7>-i', /-. c/. 1. The following nerves are shown : — optic {If), trochlear (/f), tri- geminal ( V), facial ( I'll), auditory ( VIII), glossopharyngeal (IX), and vagus (A'). (From Wicdersheim.) diencephalon is i\\Q pincaH'oJy (l-'ig. 103, ////), rcpresentinij XXIX NERVES 393 the vestige of a sensory organ, and connected with the ventral surface of the same division is the pituitary body ifty). The mode of origin of the nerves is also characteristic. From the spinal cord the nerves arise segmentally, one pair corresponding to each myomere, and pass through aper- tures in the neural arches of the vertebrae. Each arises by two roots, a dorsal and a ventral. The dorsal root is dilated into a ganglion, and contains only sensory fibres ; the ventral root is non-ganglionated, and "is motor. A longitudinal ganglionated sympathetic nerve, extending along the dorsal region of the coelome, is connected with the spinal nerves, and sends branches to the viscera, blood-vessels, &c. From the brain arise ten pairs of nerves, some of which are sensory, others motor, others mixed. Three are the nerves of the principal sense-organs, the first or olfactory supplying the organ of smell ; the second or optic (Fig. 105, 11) the retina of the eye {see below), and the eighth or auditory ( VIII), the organ of hearing. The third or oculomotor, the fourth or trochlear (IV), and the sixth or abducent go to the muscles of the eye ; the fifth or trigeminal ( V) to the snout and jaws ; the seventh or facial ( VII) to the palate, lower jaw, and hyoid arch ; the ninth or glossophary?igeal (IX) to the hyoid and first branchial arches, and the tenth or vagus to the remaining branchial arches, as well as to the heart, stomach, and lateral line. Besides the lateral line, which is probably the seat of a delicate tactile sense, and the tongue, which is presumably an organ of taste, there are three pairs of sensory organs, the structure and position of which is very characteristic of Ver- tebrates. These are the olfactory organs, the eyes, and the auditory organs. The olfactory organs are a pair of cup-like sacs on the under side of the snout, enclosed in the olfactory capsules 394 THE DOGFISH less. and opening externally by the nostrils. They are lined with mucous membrane, which is raised up into ridges so as to increase the surface. The actual organ of smell is the epithelium forming the superficial layer of the mucous membrane ; it is developed as an in-pushing of the ectoderm, and is supplied by the olfactory nerve. The eyes are a pair of nearly globular organs, lying in the orbits and moved each by six muscles. Their structure is, in all essential respects, th'e same as in man. There is an outer capsule, the sclerotic, formed of cartilage, lined by a vascular membrane, the choroid, within which is a delicate membrane, pigmented externally, the retina or actual organ of sight. In the front or exposed part of the eye the sclerotic passes into a transparent, watch-glass-like cornea, and the choroid into a curtain or diaphragm, the iris, having a central aperture, the pupil, to admit the rays of light to the interior of the eye. Behind the pupil is a gelatinous, biconvex crystallitie lens of glassy transparency. The space between the cornea and the iris is called the anterior chamber of the eye, and is filled by a watery fluid, the aqueous humour. The main part of the cavity of the eye, bounded in front by the lens, and for the rest of its extent by the retina, is the posterior chamber, and is filled with a gelatinous substance, the vitreous humour. The cornea, aqueous humour, lens, and vitreous humour together form a series of adjustable lenses serving to focus objects on the retina, and the stimulus thus applied to that mem.brane is conveyed by the fibres of the optic nerve to the brain. The auditory organ is a sac of complex form, the mem- branous labyrinth, enclosed in the auditory capsule of the skull, where it floats in a watery fluid, the perilymph. It consists of a sac called the vestibule, with which are con- nected three tubes, called from their form the semicircular XXIX URINOGENltAL ORGANS 395 canals. Two of these, the anterior and posterior canals, are vertical in position, and are united with one another at their adjacent ends ; at the other end each is dilated to form a bulb-like swelling, the ampulla. The third or horizontal canal opens at each end into the vestibule, and has an ampulla at its anterior end. The vestibule gives off a tube, the endolymphatic duct, which opens on the top of the head. The whole apparatus contains a fluid, the etidolymph, in which is a gelatinous substance enclosing calcareous par- ticles or otoliths. Patches of sensory epithelium are found in the vestibule and in the ampulte, and to these the fibres of the auditory nerve are distributed. There seems little doubt that the membranous labyrinth has not only an auditory, but also an equilibrating function — ?'.?., that the fish is enabled by its means to maintain its equilibrium in the water. The excretory and the reproductive organs of the dogfish are so closely associated as to be spoken of together as the urinogenital organs. The sexes are distinct, and the males are distinguished externally by having a pair of large grooved rods, the claspers, connected with the inner borders of the pelvic fins. They are used, like the peculiarly modified first and second pairs of pleopods in the male crayfish (p. 323), as copulatory organs. The kidneys (Fig. 103, kd) are long, flat, lobulated bodies lying one on each side of the backbone in the posterior part of the abdominal cavity. From the ventral surface of each spring numerous delicate ducts, which unite into a single tube, the ureter (ur), opening directly into the cloaca in the female, in the male into a small unpaired chamber, the urinogenital sinus (u.g.s), which opens into the cloaca. In the embryo the kidneys appear in the form of separate segmen tally arranged tubes (Fig. 99, JVph) having the 396 THE DOGFISH less. general character of nephridia, opening on the one hand by nephrostomes into the coelome, and on the other into a longitudinal duct which discharges into the cloaca. Thus the primitive structure of the kidney furnishes another instance of metamerism in the dogfish. ; In the male there is a single pair of testes (Fig. 103, ts), in the form of large soft organs, united with one another posteriorly. They are suspended by a fold of peritoneum to the dorsal body-wall. From the anterior end of each arise numerous delicate ejferent ducts, which enter a long, convoluted spermiduct or vas deferens (v. def). This passes along the ventral aspect of the kidneys and dilates into a conical pouch, the vesicula seminalis (vs. sem), and the two vesiculse open, along with the ureters and a pair of reser- voirs called sperrh-sacs {sp. s), into the urinogenital sinus. The female has a single ovary (Fig. 99, ovy) suspended to the dorsal body- wall by a fold of peritoneum. In the adult it is studded all over with rounded projections, thp ova, varying in diameter from 12-14 1^1™- downwards. The oviducts {ovd) are paired and extend along the whole length of the dorsal wall of the coelome, below the kidneys. Anteriorly they unite with one another below the gullet and just in front of the liver, and at the point of junction is a single aperture of considerable size {ovd), by which both tubes communicate ^^•ith the coelome : posteriorly they open into the cloaca. About the anterior third of each oviduct is narrow ; its posterior two-thirds is wide and distensible, and at the junction of the two parts is a yellowish, glandular mass, the shell-gland. Internal impregnation takes place, the spermatic fluid of the male being passed, by means of the claspers, into the oviducts of the female. The eggs, when ripe, break loose from the surface of the ovary into the coelome, and thence pass, through the common aperture, into one or other ot XXIX DEVELOPMENT 397 the oviducts, where fertilisation occurs. As it passes into the dilated portion of the oviduct the oosperm of Scyllium becomes surrounded by a horn-like egg-shell or " mermaid's purse " secreted by the shell-gland, and having the form of a pillow-case produced at each of its four angles into a long, tendril-like process. The eggs are laid among sea-weed, to which they become attached by their tendrils. In Acanthias and Mustelus a mere vestige of the egg-shell is formed, and the eggs undergo the whole of their development in the Fig. io6. — Section of the upper part of the embryo of a Dogfish in the blastula stage. The blastoderm is formed of a single layer of ectoderm cells (white) and of several rows of cells (shaded), which subsequently give rise to endoderm and mesoderm : sg., the blastocoele. Below the blastoderm is the unsegmented yolk containing scattered nuclei in). (From Balfour.) oviducts, the young being eventually born alive with the form and proportions of the adult. The great size of the egg is due to the immense quantity of yolk it contains : its protoplasm is almost entirely aggre- gated at one pole in the form of a small disc. AVhen segmentation of the oosperm takes place it affects the protoplasm alone, the inactive yolk, as in the Crayfish (p. 344), taking no part in the process. The polyplast stage consequently consists of a little heap of cells, called the blastoderm (Fig. 106), at one pole of an undivided 39S THE DOGFISH sphere of )olk. The edge of the blastoderm becomes in- vagiiiated at one point, forming the gastrula, and its cells become differentiated into the three embryonic layers — ectoderm, mesoderm, and endoderm. At the same time the blastoderm extends in a peripheral direction so as gradually to eo\er the volk, aiitl its middle part becomes raised up into a ridgedike thickening which is moulded, Fig. 107. — A, embryo of Scyllium with yolk-sac ( ;■; i J) : B, underside of tiead, enlarged hr. y", branchial filainents protruding through gill-clefts ; br. f , branchial filaments protruding through spiracle ; cd.J, caudal fin ; d, f, dorsal fins ; c, eye ; ex. br. ap, external branchial apertures ; nit'h, mouth ; na, nostrils ; pet. f, pectoral fin ; pi: f, pelvic fin ; st, yolk- stalk ; ''. /, ventral fin ; yk. s, yolk-sac. (After Balfour, slightly altered.) step by step, into the form of the embryo fish. The head, trunk, and tail acquire distinctness, and become more and more clearly separated off from the bulk of the egg, the latter taking the form of a _vo/A-sae (Fig. 107, .\, r/c. s) attached by a narrow stalk to the ventral surface of the embryo. In this condition the various parts of the adult fish can XXIX DEVELOPMENT 399 be recognized, but the proportions arc different, and the head presents several peculiarities. The gill-filaments {In-, f) are so long as to project through the external branchial apertures and the spiracle (l>r. /'), in the form of long threads, abundantly supplied with blood-vessels, and apparently serving for the absorption of nutriment — the albumen in the egg-shell in the case of Scyllium, secretions of the oviduct in the viviparous forms. Besides this mode of nutrition the yolk-sac communicates with the intestine by a narrow duct {st), through which absorption of its contents is constantly going on. By the time the young fish is ready to be born or hatched the greater part of the yolk-sac has been drawn into the coelome, a mere vestige of it still dangling from the ventral surface of the body. LESSON XXX MOSSES In the six previous lessons we have traced the advance in organization of animals from the simple diploblastic Hydra to the complicated triploblastic forms which con- stitute the five higher phyla of the animal kingdom, ^^'e have now to follow in the same way the advance in structure of plants. The last member of the vegetable kingdom with which we were concerned was Nitella (Less. XX), a solid aggregate, exhibiting a certain differentiation of form and structure, but yet composed of what were clearly recogniz- able as cells, there being, as in Hydra, none of those well-marked tissues which form so noticeable a feature in Polygordius as in other animals above the Coelenterata. Taking Nitella as a starting point, we shall see that among plants, as among animals, there is an increasing differentia- tion in structure and in function as we ascend the series. The first steps in the process are well illustrated by a considera- tion of that very abundant and beautiful group of plants, the Mosses. In spite of the variations in detail met with in different genera of the group, the essential features of their organization are so constant that the following description will be found to apply to any of the common forms. Fig. ioS. — The Anatomy and Ili^lulogy of Mosses. A, luitive plant of Funaria ln%'romclyiui, showing stem [st], leaves (/), and rhizoids (/■/;). ( x 6. ) B, leaf of the same, showing midrib {iiid. r) and lateral poitions, X 25.) D D 402 MOSSES LESS. c, semi-diagrammatic veilical section of a moss, showing the arrange- ment of the tissues. The stem is formed externally of sclerenchyma {scl), and contains an axial bundle {ax. b) : in some of the leaves (/) the section passes through the midrib, in others (/') through the lateral portion : the stem ends distally in an apical cell (ap. c), from which segmental cells (seg. c) are separated. D, transverse section of the stem of Bryum roseum, showing scleren- chyma (scl), axial bundle (ax. b), and rhizoids (rh). ( x 60.) E, transverse section of a leaf of Funaria, showing the midrib (iiid. r) formed of several layers of cells, and the lateral portions one cell thick, (x 150-) F, small portion of the lateral region of the same, showing the form of the cells and the chroniatophores (c/ir). (x 150.) G, distal end of the stem of Fontinalis aiiHpyretica in vertical section, showing the apical cell (ap. c) giving rise to segmental cells (seg. c), which by subsequent division form the segments of the stem with the leaves : the thick lines show the boundaries of the segments. H, diagram of the apical cell of a moss in the form of a tetrahedron with rounded base abc and three flat sides abd, bed, acd. (d, after Sachs ; G, after Leitgeb. ) The plant consists of a short slender stem (Fig. 108, A, st), from which are given off structures of two kinds, rhizoids or root-hairs (r/z), which pass downwards into the soil, and leaves (;), which are closely set on the stem and its branches. As in Nitella (p. 205) the portion of the stem from which a leaf arises is called a node, and the part intervening between any two nodes an internode, while the name segment is applied to a node with the internode next below it. At the upper or distal end of the stem the leaves are crowded, forming a terminal bud. Owing to the opacity of the stem, its structure can only be made out by the examination of thin sections (c and d). It is a solid aggregate of close-set cells which are not all alike, but exhibit a certain amount of differentiation. In the outer two or three rows the cells {scT) are elongated in the direc- tion of the length of the stem, so as to have a spindle-shape, and their walls are greatly thickened and of a reddish colour. They thus form a protective and supporting tissue, to which the name sclerenchyma is applied. Running longitudinally XXX TERMINAL BUD 403 through the centre of the stem is a mass of tissue (ax. b) distinguished by its small, thin-walled cells, and constituting the axial bundle. The leaves (u) are shaped like a spear-head, pointed distally, and attached proximally by a broad base to the stem. The axial portion (b and e, vid. r, c, T) consists of several layers of somewhat elongated cells and is called the midrib : the lateral portions (e and f : c, /) are formed of a single layer of short cells. Thus the leaf has, for the most part, the character of a superficial aggregate. The cells contain oval chromatdphores (f, cAr). The rhizoids (c and D, rA) are linear aggregates, being formed of elongated cells, devoid of chlorophyll, arranged end to end. In the terminal bud the leaves, as in Nitella (pp. 206 and 208), arch over the growing point of the stem, which in this case also is formed of a single apical cell (c and G, ap. c). But in correspondence with the increased complexity of the plant, the apical cell is not a hemisphere from which new segments are cut off parallel to its flat base, but has the form (h) of an inverted, three-sided pyramid or tetrahedron, the rounded base of which {abc) forms the apex of the stem while segments {seg. c) axe cut off from each of its three triangular sides in succession. The best way to understand the apical growth of a moss is to cut a tetrahedron with rounded base out of a carrot or turnip : this represents the apical cell (h) : then cut off a slice parallel to the side abd, a second parallel to bed, and a third parallel to acd : these represent three successively formed segments. Now imagine that after every division the tetrahedron grows to its original size, and a very fair notion will be obtained of the way in which the successive segments of the moss-stem are formed by the fission in three D D 2 404 MOSSES , LESS. planes of the apical cell. Each segment (c and g, seg. c) immediately after its separation divides and subdivides, pro- ducing a mass of cells from which a projection grows out forming a leaf, and in this way the stem increases in length and the leaves in number. Asexual reproduction takes place in various ways ; all of them are, however, varieties of budding, and the buds always arise in the form of a linear aggregate of cells called a protonema : from this the moss-plant develops in the same way as from the protonema arising from a spore (p. 408). The gonads are developed at the extremity of the main stem or one of its branches, and are enclosed in an involucre or tuft of leaves often of a reddish colour — the terminal bud of the fertile shoot or so-called " flower " of the moss. The spermary (Fig. 109, a^, a^) is an elongated club- shaped body consisting of a solid mass of cells, the outer- most of which form the wall of the organ, while the inner (a^) become converted into sperms. The latter (a*) are spirally coiled and provided with two cilia : they are liber- ated by the rupture of the wall of the spermary at its distal end (a^), and swim in the rain or dew covering the plant. The ovaries ^ (see Preface, p. viii) (b^, b^) may or may not occur on the same plant as the spermaries, some mosses being monoecious, others dioecious. Like the sperm- aries, they consist at first of a solid mass of cells which assumes the form of a flask, having a rounded basal portion or venter (v) and a long neck (n). The outer layer of cells in the neck and the two outer layers in the venter form the wall of the ovary, the internal cells are arranged in a single ^ The ovary of mosses, ferns, &c,, is usually called an anhegonium : the spermary, as in the lower plants, an antheridium. XXX DEVELOPMENT OF SPOROGONIUM 405 axial row at first similar to those of the wall. As the ovary develops, the proximal or lowermost cell of the axial row takes on the character of an ovum (b^, ov) ; the others, called canal cells [en. c), are converted into mucilage, which by its expansion forces open the mouth of the flask and thus makes a clear passage from the exterior to the ovum (b^). Through the passage thus formed a sperm makes its way and conjugates with the ovum, producing as usual an oosperm or unicellular embryo. The development of the embryo is at first remarkably like what we have found to take place in Hydroids (p. 246). The oosperm, having surrounded itself with a cell-wall, divides into two cells by a wall at right angles to the long axis of the ovary : each of these cells divides again re- peatedly, and there is produced a solid multicellular embryo or polyplast {&, spgnin). "Very early, however, the moss-polyplast exhibits a striking difference from the animal polyplast or morula : one of its cells — that nearest the neck of the ovary — takes on the character of an. apical cell, and begins to form fresh seg- ments like the apical cell of the stem. Thus the plant embryo differs almost from the first from the animal embryo. In the animal there is no apical cell : all the cells of the polyplast divide and take their share in the formation of the permanent tissues. In the plant one cell is at a very early period differentiated into an apical cell, and from it all cells thereafter produced are, directly or indirectly, derived. The embryo continues to grow, forming a long rod-like body (c2, spgnm) the base of which becomes sunk in the tissue of the moss-stem, while its distal end projects vertically upwards, covered by the distended venter (w) of the ovary. Gradually it elongates more and more and its distal end t^mt r/t Fir.. log. — Reproduction and Development of Mosses. A"^, A spermary of Funarin in optical section, showing the wall en- closing a central mass of spcrni-cells : A-, the same from the surface discharging iLs sncrms. ( x 300.) LESS. XXX PROTONEMA 407 a', a sperm-cell with enclosed sperm : A^, a free-swimming sperm. ( X 800. ) B^, nn ovary of Funaria, surface view, showini; venter {v) and neck (") : b", the same in optical section, showing ovum {m') and canal cells (f«. f ) : E'', the same after disappearance of the canal cells : the neck is freely open, and the ovum {oz') exposed. ( x 200. ) c^, ovary with withered neck containing an embryo {spgnni) in the polyplast stage ( x 200) : in c- the ovary, consisting of swollen venter {v) and shrivelled neck («), encloses a young sporogonium (spgnm] ; the distal end of the stem is shown with bases of leaves (/) ; in c^ the venter has mptured, forming a proximal portion or sheath and a distal portion or calyptra which is carried up by the growth of the sporogonium. (X 10.) c*, a small plant of Funaria with ripe sporogonium consisting of seta (sf), with urn {u) and lid (/) covered by the calyptra (c). c', diagrammatic vertical section of urn («), showing lid {/), air spaces (a), and spores {sp). d\ a germinating spore of Funaria, showing ruptured outer coat [sp) and young protonema (pr) with rhizoid [rh], ( x 550.) D-, portion of protonema of the same, showing lateral bud {bd), from which the leafy plant arises. ( x 90. ) (a and D, after Sachs ; B, c^, and c^, altered from Sachs. ) dilates : the embryo has now become a sporogoiiium, con- sisting of a slender stalk (c*, st) bearing a vase-like capsule or urn (it) at its distal end. In the meantime the elonga- tion of the stalk has caused the rupture of ■ the enveloping venter of the ovary (c^) : its proximal part remains as a sort of sheath round the base of the stalk, while its distal portion, with the shrivelled remains of the neck (n), is carried up by the elongation of the sporogonium and forms an extinguisher- like cap or calyptra (c*, c) over the urn. As development goes on, the distal end of the urn be- comes separated in the form of a lid (c*, c*, /), and certain of the cells in its interior, called spore-mother cells, divide each into four daughter cells, which acquire a double cell- wall and constitute the spores (c^, sp) of the moss. When the spores are ripe the calyptra falls off or is blown away by the wind, the lid separates from the urn, and the spores are scattered. In germination, the protoplasm of the spore covered by 4o8 MOSSES LESS. the inner layer of the cell-wall protrudes through a split in the outer layer (d'', sp) and grows into a long filament, the protonema {pr.), divided by oblique septa into a row of cells. The protonema — which it will be observed is a simple linear aggregate — branches, and may form a closely-matted mass of filaments. Sooner or later small lateral buds (d^, bd) appear at various places on the protonema . each of these takes on the form of a three-sided pyramidal apical cell, which then proceeds to divide in the characteristic way (p. 403), forming three rows of segments from which leaves spring. In this way each lateral bud of the protonema gives rise to a moss-plant. Obviously we have here a somewhat complicated case of alternation of generations (see p. 248). The gamobium or sexual generation is represented by the moss-plant, which originates by budding and produces the sexual organs, while the agamobium consists of the sporogonium, developed from the oosperm and reproducing by means of spores. The protonema, arising from a spore and producing the leafy plant by budding, is merely a stage of the gamobium. The nutrition of mosses is holophytic ; but there is a striking differentiation of function correlated with terrestrial habits. In Nitella the entire organism is submerged in water and all the cells contain chlorophyll, so that decom- position of carbon dioxide and absorption of an aqueous solution of salts are performed by all parts alike, every cell being nourished independently of the rest. In the moss, on the other hand, the rootlets are removed from the influence of light and contain no chlorophyll : hence they cannot decompose carbon dioxide ; but, being sur- rounded by moist soil, are in the most favourable position for absorbing water and mineral salts. The stem, again, is XXX DISTRIBUTION OF FOOD-MATERIALS 409 converted into an organ of support : the thickness of its external cells prevents absorption and it contains no chlorophyll. Hence the function of decomposing carbon dioxide is confined to the leaves. We have thus as an important fact in the nutrition of an ordinary terrestrial plant that its carbon is taken in at one place, its water, nitrogen, sulphur, potassium, &c., at another. But as all parts of the plant require all these substances it is evident that there must be some means by which the root can obtain a supply of carbon, and the leaves a supply of elements other than carbon. In other words, we find for the first time in the ascending series of plants, just as we did in ascending from the simple Hydra to the complex Polygordius (p. 278) the need for some contrivance for the distribution of food-materials. The way in which this distributing process is performed has been studied chiefly in the higher plants, but its essential features are probably the same for mosses. ^Vater is continually evaporating from the surface of the leaves, its place being as constantly supplied by water — with salts in solution — taken in by the rhizoids. This trans- piration, or giving off of water from the leaves, is one important factor in the process under consideration, since it ensures a constant upward current of water, or, more accurately, of an aqueous solution of mineral salts. The withering of a plucked moss-plant is of course due to the fact that when the roots are not embedded in moist soil or in water, transpiration is no longer balanced by absorption.^ In the higher plants it has been found that the root-hairs have an absorbent action independent of transpiration, so, that water may be absorbed in the absence of leaves. ^ Mosses, however, unlike most higher plants, can absorb water by their leaves. 4IO MOSSES LESS. By the transpiration current, then, the leaves are kept constantly supplied with a solution of mineral salts derived from the soil, and are tlius nourished like any of the aquatic green plants considered in previous lessons : by the double decomposition of water and carbon dioxide a carbo-hydrate is formed : this, by further combination with the nitrogen of the absorbed ammonium salts or nitrates, forrns simple nitrogenous compounds, and from these, probably through a long series of mesostates or intermediate products, proto- plasm is finally manufactured. In this way the food supply of the green cells of the leaves is accounted for, but we have still to consider that of the colourless cells of the stem and rhizoids, which, as we have seen, are supplied by the transpiration current with everything they require except carbon, and this, owing to their possessing no chlorophyll, they are unable to take in in the form of carbon dioxide. As a matter of fact the chlorophyll-containing cells of the leaves have to provide not only their own food, but also that of their not-green fellows. In addition to making good the waste of their own protoplasm they produce large quantities of plastic products (see p. 33) such as grape sugar, and simple nitrogenous compounds like asparagin, and these pass by diffusion from cell to cell until they reach the uttermost parts of the plant, such as the axis of the stem and the extremities of the rhizoids. The colourless cells are in this way provided not only with the salts contained in the ascending transpiration current, but with carbo-hydrates and nitrogenous compounds. From these they derive their nutriment, living therefore like yeast-cells in Pasteur's solution, or like Bacteria in an organic infusion. We see then that the colourless cells of the stem and XXX DISTRIBUTION OF FOOD-MATERIALS 411 rhizoids are dependent upon the green cells of the leaves for their supplies. Like other cells devoid of chlorophyll they are unable to make use of carbon dioxide as a source of carbon, but require ready-made carbo-hydrates, the manufacture of which is continually going on, during daylight, in the chlorophyll-containing cells of the leaves. This striking division of labour is the most important physiological difference between mosses and the more lowly organised green plants described in previous lessons. LESSON XXXI FERNS We saw in the previous lesson that in mosses there is a certain though small amount of histological differentiation, some cells being modified to form sclerenchyma, others to form axial bundles, ^^'e have now to consider a group of plants which may be considered to be, in this respect, on much the same morphological level as Polygordius, the adult organism being composed not of a mere aggregate of simple cells, but of various well-marked tissues. A fern- plant has a strong stem which in some forms, such as the common Bracken {Fteris aquilina) is a horizontal underground structure called a rhizome, often incorrectly considered as a root : in others it creeps over the trunks of trees or over rocks : in others again, such as the tree-ferns, it is vertical, and may attain a height of three or four metres. From the stem are given off structures of two kinds — the leaves, which present an almost infinite variety of form in the various species, and the numerous slender roots. In some cases, such as the tree-ferns and the common Male Shield-fern {Aspidiuni filix-inas), the plant ends distally in a terminal bud, consisting, as in Nitella and mosses, of the growing end of the stem over-arched by leaves : in others LESS. XXXI TISSUES OF T?IE STEM 413 such as Pteris, the stem ends in a blunt, knob-hke extremity quite uncovered by leaves. On the proximal portion of the stem are usually found the withered remains of the leaves of previous seasons, or the scars left by. their fall. The roots are given off from the whole surface of the stem, often covering it with a closely-matted mass of dark brown fibres. When the stem is cut across transversely (Fig. no, a) it is seen, even with the naked eye, to consist of three well marked tissues. The main mass of it is formed of a whitish substance, soft and rather sticky to the touch, and called ground-parenchyma {par) : this is covered by an external layer of very hard tissue, dark brown or black in colour, the hypodermis {hyp) : bands of a similar hard brown substance are variously distributed through the parenchyma, and con- stitute the sclerenchyma {set) : and interspersed with these are rounded or oval patches of a yellowish colour {V.B) harder than the parenchyma but not so hard as the sclerenchyma, and called vascular bundles. The general distribution of these tissues can be made out by making longitudinal sections of the stem in various planes or by cutting away the hypodermis, and then scraping the parenchyma from the vascular bundles and bands of sclerenchyma. The hypodermis is found to form a more or less complete hard sheath or shell to the stem, while the sclerenchyma and vascular bundles form longitudinal bands and rods imbedded in the parenchyma, and serve as a sort of supporting framework Or skeleton. The minute structure of the stem can be made out by the examination either of very thin longitudinal and trans- verse sections, or of a bit of stem which has been reduced to a pulp by boiling in nitric acid with the addition of a few crystals of potassium chlorate ; by this process the various r.c£ Fig, ho. — Anatomy and llislolugy of Ferns. LESS. XXXI GENERAL CHARACTERS 415 A, Transverse section of the stem of Pteris aqiUlina, showing hypo- dermis (hyp), ground-parenchyma (par), sclerenchyma (scl), and vascular bundles (V. B). ( x 2. ) B, transverse section of a vascular bundle, showing bundleshealh (b. sh), sieve-tubes (sv. i), scalariforni vessels (sc. v), and spiral vessels (sp. V). (X 6.) c, semi-diagrammatic vertical section of the growing point of the stem, showing apical cell {ap. c), segmental cells (seg. c), and apical meristem (ap. iiier) passing into permanent tissue consisting of epidermis (ep), hypodermis (hyp), ground parenchyma (par), sclerenchyma (scl), and vascular bundles in which the sheath lb. sh), sieve-tubes (sv. t), scalariform vessels {sc. v), and spiral vessels (sp. v) are indicated. D, a single parenchyma cell, showing nucleus (mi), and vacuole (vac). sL, cell of hypodermis. F, portion of a sieve-tube, showing sieve-plates (sv. pi). G, portion of a spiral vessel with the spiral fibre partly unrolled al the lower end. H, fibre-like cell of sclerenchyma. I, portion of a scalariform vessel, part of the wall being supposed to be removed. K, vertical section of a leaf of Pteris, showing upper and lower epi- dermis (ep), mesophyll cells (ms. ph), with intercellular spaces (i. i. sp), a stoma (st) in the lower epidermis, and hairs (h). L, surface view of epidermis of leaf of Aspidium, showing two stomata (st) with their guard-cells (gd. c). M, vertical section of the end of a root, showing apical cell (ap. c), segmental cells (seg. c), and root-cap (r. cp) with its youngest cap-cells marked cp. c. (A, B, and D-K after Howes ; M from Sachs, slightly altered.) tissue elements are separated from one another, and can be readily examined under a high power. By combining these two methods of sectioning and dissociation, the parenchyma is found to consist of an aggregate of polyhedral cells {c, par ; d) considerably longer than broad, their long axes being parallel with that of the stem itself. The cells are to be considered as right cylinders which have been converted into polyhedra by mutual pres- sure. They have the usual structure, and their protoplasm is frequently loaded with large starch-grains. They do not fit quite closely together, but spaces are left between them, especially at the angles, called intercellular spaces. 4i6 FERNS LESS. The cells of the hypodermis (e) are proportionally longer than those of the parenchyma, and are pointed at each end : they contain no starch. Their walls are greatly thickened, and are composed not of cellulose but of lignin, a carbo- hydrate allied in composition to cellulose, but containing a larger proportion of carbon. Schulze's solution, which, as we have seen, stains cellulose blue, imparts a yellow colour to lignin. Outside the hypodermis is a single layer of cells (c, ep) not distinguishable by the naked eye and forming the actual external layer of the stem : the cells have slightly thickened, yellowish-brown walls, and constitute the epidermis. From many of them are given off delicate filamentous processes consisting each of a single row of cells : these are called hairs. In the sclerenchyma the cells (h) are greatly elongated, and pointed at both ends, so as to have the character rather of fibres than of cells. Their walls are immensely thickened and lignified, and present at intervals oblique markings due to narrow but deep clefts : these are produced by the deposition of lignin from the surface of the protoplasm (see p. 33) being interrupted here and there, instead of going on continuously as in the case of a cell-wall of uniform thickness. The vascular bundles have in transverse section (b) the appearance of a very complicated network, with meshes of varying diameter. In longitudinal sections (c) and in dis- sociated specimens they are found to be partly composed of cells, but to contain besides structures which cannot be called cells at all. In the centre of the bundle are a few narrow cylindrical tubes (b and c, sp. v.) characterised at once by a spiral marking, and hence called spiral vessels. Accurate exam- ination shows that their walls (g) are for the most part thin, XXXI XYLEM AND PHLOEM 417 but are thickened by a spiral fibre, just as a paper tube might be strengthened by gumming a spiral strip of paste- board to its inner surface. These vessels are of considerable length, and are open at both ends : moreover they contain no protoplasm, but are filled with either air or water : they have therefore none of the characteristics of cells. They are shov^Ti, by treatment with Schulze's solution, to be com- posed of lignin. Surrounding the group of spiral vessels, and forming the large polygonal meshes so obvious in a transverse section, are wide tubes (b and c, sc. v) pointed at both ends and fitting against one another in longitudinal series by their oblique extremities. They have transverse markings like the rungs of a ladder, and are hence called scalarifortn vessels. The markings (i) are due to wide transverse pits in the otherwise thick lignified walls : in the oblique ends by which the vessels fit against one another the pits are frequently replaced by actual slits, so that a longitudinal series of such vessels forms a continuous tube containing, like the spiral vessels, air or water, but no protoplasm. In most ferns the terminal walls are not thus perforated, and the elements are then called tracheides. The presence of these vessels — spiral and scalariform — is the most important histological character separating ferns and mosses. The latter group and all plants below them are composed exclusively of cells : ferns and all plants above them contain vessels in addition, and are hence called vas- cular plants. The vessels, together with small parenchyma-cells inter- spersed among them, make up the central portion of the vascular bundle, called the wood or xylein. The peripheral portion is formed of several laj'ers of cells composing the bast or phloem, and surrounding the whole are two layers of E E 4iS FERNS LESS. small cells, the inner called the phloem-sheath or fericyck, the outer, the bundle-sheath or endodermis (b. sh). The cells of the phloem are for the most part parenchy- matous, but among them are some to which special attention must be drawn. These (b and c, sv. t), are many times as long as they are broad, and have on their walls irregular patches ot sieve-plates (f, sv.pl.) composed of groups of minute holes through which the protoplasm of the cell is continuous with that of an adjacent cell. The transverse or oblique partitions between the cells of a longitudinal series are also perforated, so that a row of such cells forms a sieve- tube in which the protoplasm is continuous from end to end. We have here, therefore, as striking an instance of a non- cellular tissue as in the deric epithelium and certain other tissues of Polygordius (see p. 289). The distal or growing end of the stem terminates in a blunt apical cone or punctum vegetationis (c), surrounded by the leaves of the terminal bud in the case of vertical stems, or sunk in a depression and protected by close-set hairs in the underground stem of the bracken. A rough longitudinal section shows that, at a short distance from the apical cone, the various tissues of the stem — epidermis, parenchyma, sclerenchyma, and vascular bundles — merge insensibly into a whitish substance, resembling parenchyma to the naked eye, and called apical meristein (ap. iner). Thin sections show that the summit of the apical cone is occupied by a wedge-shaped apical cell (ap. c) which in vertical stems is three-sided like that of mosses (Fig. 108, h, p. 401), while in the horizontal stem of Pteris it is two-sided. As in mosses, segmental cells {seg. c) are cut off from the three (or two) sides of the apical cell in succession, and by further division form the apical meristem (ap. mer), which consists XXXI APICAL GROWTH 419 of small, close-set cells without intercellular spaces. As the base of the apical cone is reached, the meristem is found to pass insensibly into the permanent tissues, the cells near the surface gradually merging into epidermis and hypodermis, those towards the central region into sclerenchyma and the various constituents of the vascular bundles, and those of the intermediate regions into parenchyma. The examination of the growing end of the stem shows us how the process of apical growth is carried on in a compli- cated plant like the fern. The apical cell is continually undergoing fission, forming a succession of segmental cells ; these divide and form the apical meristem, which is thus being constantly added to at the growing end by the forma- tion and subsequent fission of new segmental cells : in this way the apex of the stem is continually growing upwards or forwards. But at the same time the meristem cells farthest from the apex begin to differentiate : some elongate but slightly, increasing greatly in size, and become parenchyma cells : others by elongation in the direction of length of the stem and by thickening and lignification of the cell-wall become sclerenchyma cells : others again elongate greatly, become arranged end to end in longitudinal rows, and, by the loss of their protoplasm and of the transverse partitions between the cells of each row, are converted into vessels — spiral or scalariform according to the character of their walls. Thus while the epidermis, parenchyma, and sclerenchyma are formed of cells, the spiral and scalariform vessels are cell- fusions, or more accurately cell-wall-fusions, being formed by the union in a longitudinal series of a greater or less number of cell-walls. It will be remembered that the muscle-plates of Polygordius are proved by the study of development to be cell-fusions (p. 302). We thus see that every cell in the stem of the fern was E E 2 420 FERNS LESS. once a cell in the apical meristem, that every vessel has arisen by the concrescence of a number of such cells, and that the meristem cells themselves are all derived, by the ordinary process of binary fission, from the apical cell. In this way the concurrent processes of cell-division, cell- differentiation, and cell-fusion result in the production of the various and complex tissues of the fully-formed stem. The leaves vary greatly in form in the numerous genera and species of ferns : they may consist of an unbranched stalk bearing a single expanded green blade : or the stalk may be more or less branched, its ramifications bearing the numerous subdivisions of the blade, or pinnules. The anatomy of the leaf, like that of the stem, can be readily made out by a rough dissection. The leaf-stalk and its branches have the same general structure as the stem, consisting of parenchyma coated externally with epidermis and strengthened internally by vascular bundles, which are continuous with those of the stem. But the blade, or, in the case of a compound leaf, the pinna, has a different and quite peculiar structure. It is invested by a layer of epidermis - which can be readily stripped off as an extremely thin, colour- less membrane, exposing a soft, green substance, the leaf- parenchyma or mesophyll. The leaf is marked externally by a network of delicate ridges, the veins ; these are shown by dissection to be due to the presence of fine white threads which ramify through the mesophyll, and can be proved by tracing them into the leaf-stalk to spring from its vascular bundles, of which they are in effect the greatly branched distal ends. Microscopic examination shows the epidermis of the leaf (Fig, no, K, ep and l) to consist of flattened, colourless cells of very irregular outline and fitting closely to one another like xx\i LEAVES AND ROOTS 421 the parts of a child's puzzle. Among them are found at intervals pairs of sausage-shaped cells {gd. c) placed with their concavities towards one another so as to bound a narrow slit-like aperture {st). These apertures, which are the only intercellular spaces in the epidermis, are called stomates : the cells bounding them are the guard-cells, and are distinguished from the remaining epidermic cells by the possession of a few chromatophores. The mesophyll, which as we have seen occupies the whole space between the upper and lower epidermis, is formed of thin-walled cells loaded with chromatophores (k, ms.fh) and therefore of a deep green colour. The cells in contact with the upper epidermis are cylindrical, and are arranged verti- cally in a single row : those towards the lower surface are very irregular both in form and arrangement. Large inter- cellular spaces (?'. c. sp) occur between the mesophyll-cells and communicate with the outer air through the stomates. • The leaves arise as outgrowths of the distal or growing end of the stem, each originating from a single segmental cell of the apical cone. The fern is the first plant we have yet considered which possesses true roots, the structures so-called differing funda- mentally from the simple rhizoids of Nitella and the mosses. Instead of being mere linear aggregates of cells, they agree in general structure with the stem from which they spring, consisting of an outer layer of epidermis within which is parenchyma strengthened by bands of sclerenchyma and by a single vascular bundle in the middle. The epidermic cells give rise to unicellular prominences, the root-hairs. The apex of the root, like that of the stem, is formed of a mass of meristem in which a single wedge-shaped apical cell (Fig. no, m, ap. c) can be distinguished But instead 422 FERNS I.KSS. of the base of this cell forming the actual distal extremity, as in the stem (compare c), it is covered by several layers of cells which constitute the root-cap (r. cp). In fact the apical cell of the root divides not only by planes parallel to its three sides, but also by a plane parallel to its base, and in this way produces not only three series of segmental cells {seg. c) which afterwards subdivide to form the apical meristem, but also a series of cap-cells {cp. c) which form a protective sheath over the tender growing end of the root as it forces its way through the soil. Roots are also peculiar in their development. Instead of being, like leaves, prominences of the superficial tissues of the stem, they arise from a layer of cells immediately ex- ternal to the vascular bundles, and in growing force their way through the superficial portion of the stem, through a fissure from which they finally emerge. They are thus said to be endogenous in origin while leaves are exogenous. The nutrition of ferns is carried on in much the same way as in mosses (see p. 408). Judging from the analogy of flowering plants it would seem that the ascending current of water from the roots passes mainly through the xylem of the vascular bundles, while the descending current of nitrogenous and other nutrient matters for the supply of the colourless cells of the stem and roots passes chiefly through the phloem and especially through the sieve-tubes. The absorption of water is effected by the root-hairs. In the autumn there are found on the under surfaces of the leaves brown patches called sari, differing greatly in form and arrangement in the various genera, and formed of innumerable, minute, seed-like, bodies, the sporangia (Fig. Ill, a), just visible to the naked eye. Each sorus or group XXXI REPRODUCTION 423 of sporangia is covered by a fold of the epidermis of the leaf, called the indusium. A sporangium is attached to the leaf by a multicellulaf stalk {sf), and consists of a sac resembling two watch-glasges placed with their concave surfaces towards one another and their edges united by a thick rim (an). The sides are formed of thin flattened cells with irregular outlines, the rim or anniilus of peculiarly shaped cells which are thin and broad at one edge (to the left in a), but on the other (to the right) are thick, strongly lignified, and of a yellowish-brown colour. The whole internal cavity is filled with spores (b, sp) having the form of tetrahedra with rounded edges, and each consisting of protoplasm containing a nucleus, and surrounded by a double wall of cellulose. A spore is there- fore, as in mosses, a single cell. Each sporangium arises from a single epidermic cell of the leaf. This divides repeatedly so as to form a solid mass of cells, of which the outermost become the wall of the sporangium while the inner are the spore-mother-cells. The latter divide each into four spores, as in mosses (p. 407). As the spores ripen, the wall of the sporangium dries, and as it does so the thickened part of the annulus straightens out, tearing the thin cells and producing a great rent through which the spores escape (b). AVhen the spores are sown on moist earth they germinate, the protoplasm, covered by the inner coat, protruding through the ruptured outer coat (c, sp) in the form of a short filament. This divides transversely, forming two cells, the proximal of which sends off a short rhizoid (r/z). The resemblance of this stage to the young protonema of a moss is sufficiently obvious (see Fig. 109, d^, p. 406). Further cell-division takes place, and before long the Fir,, III, — Reproduclion an,l r>LVcIopment of Ferns. A, Spoi-an|^iuni of Ftcris, external \"ie\A', sl^o^^■ing ^lalk {st) and annul us (an). B, tlie same, during dehiscence, ll]e spores [s/:) escaping. c, a germinating spore, sliowing tlie ruptured outer coat (.17^), and a LESS. XXXI THE PROTHALLUS 425 rhizoid (rh) springing from the proximal cell of the rudimentary (two- celled) iDrothallus. D, a young prothallus, showing spore, rhizoid, apical cell [nf. c), and segmental cells {seg. c). E, an advanced prothallus, from beneath, sho\ving rhizoids [r/i), ovaries (ozy), and spermaries [s/iy). F, a mature spermary of Pin-is, inverted {i.e. with its distal end directed upwards) so as to compare with Fig. 109, A. G, a single sperm, showing coiled body and numerous cilia. H, a mature ovary of Aspidium, inverted so as to compare with Fig. 109, b'> showing venter (v), neck (h), ovum Uv), and canal cells (en. c). I, small portion of a prothallus of Aspleniiim in vertical section, showing the venter [v) and part of the neck (n) of a single ovary after fertilisation. The venter contains an embryo just passing from the polyplast into the phyllula stage, and divided into four groups of cells, the rudiments respectively of the foot [ft], stem {sf), root [rt], and cotyledon (ct\ K, vertical section of a prothallus [prth) of Kephrohpis, bearing rhizoids {rh), and a single ovary with greatly dilated venter (v\ and withered neck [ri). The venter contains an embryo in the phyllula stage, consisting of foot (ft), rudiments of stem (st), and root {rt), and cotyledon (cl) beginning to grow upwards. L, prothallus (prth) with rhizoids (rh), bearing a young fern-plant, consisting of foot (ft), rudiment of stem (st), first root (rt), cotyledon (ct), and first ordinary leaf (/). (After Howes.) distal cells divide longitudinall)', a leaf-like body being produced, vyhich is called the prothallus (d). This is at first only one layer of cells thick, but it gradually increases in size, becoming more or less kidney-shaped (e), and as it does so its cells divide parallel to the surface, making it two and finally several cells in thickness. Thus the prothallus is at first a linear, then a superficial, and ultimately a solid aggregate. Root-hairs (rli) are produced in great number from its lower surface, and penetrating into the soil serve for the absorption of nutriment. At an early period a two- sided apical cell (d, ap. c) is differentiated, and gives off segmental cells {seg. c) in the usual way : an abundant formation of chromatophores also takes place at a very early period in the cells of the prothallus, which therefore re- sembles both in structure and in habit some very simple form of moss. 426 FERNS LESS. On the lower surface of the prothallus gonads (e, spy, ovy) are developed, resembling in their essential features those of mosses. The spermaries {spy) make their appearance first, being frequently found on very young prothalli. One of the lower cells forms a projection which becomes divided off by a septum : further division takes place, resulting in the differentiation (f) of an outer layer of cells forming the wall of the spermary, and of an internal mass of sperm-mother- cells in each of which a sperm is produced. The sperm (g) is a corkscrew-like body, probably formed from the nucleus of the cell, bearing at its narrow end a number of cilia which appear to originate from the protoplasm. To the thick end is often attached a globular body, also arising from the proto- plasm of the mother-cell ; this is finally detached. The ovaries (e and h, ovyi) are not usually formed until the prothallus has attained a considerable size. Each arises, like a spermary, from a single cell cut off by a septum from one of the lower cells of the prothallus i the cell divides and forms a structure resembling in general characters the ovary of a moss (see Fig. 109, b, p. 406), except that the venter (h, V) is sunk in the prothallus, and is therefore a less distinct structure than in the lower type. As in mosses, also, an axial row of cells is early distinguished from those forming the wall of the ovary : the proximal of these becomes the ovum (ov\ the others are the canal-cells {en. c), which are converted into mucilage, and by their expansion force open the neck and make a clear passage for the sperm. The sperms swarm round the aperture of the ovary and make their way down the canal, one of them finally conju- gating with the ovum and converting it into an oosperm. The early stages in the development of the embryo remind us, in their general features, of what we found to occur in mosses (p. 405). The oosperm first divides by a XXXI rOLYPLAST AND PHYLLULA 427 plane parallel to the neck of the ovary, forming two cells, an anterior nearest the growing or distal end of the prothallus, and a posterior towards its proximal end. Each of these divides again by a plane at right angles to the first, there being now an upper and a lower anterior, and an upper and a lower posterior cell : the lower in each case being that towards the downwardly directed neck of the ovary. Each of the four cells undergoes fission, the embryo then consist- ing of eight cells, two upper anterior (right and left), two lower anterior, two upper posterior, and two lower posterior. We thus get a multicellular but undifferentiated stage, the polyplast. It will be remembered that in mosses the polyplast forms an apical cell, and develops directly into the sporogonium (p. 405). In the fern the later stages are more complex. One of the upper anterior cells remains undeveloped, the other (Fig. in, i and k, sf) takes on the form of a wedge- shaped apical cell, and, dividing in the usual way, forms a structure like the apex of the fern-stem, of which it is in fact the rudiment. The two upper posterior cells divide and subdivide, and form a multicellular mass called the fooi (ft), which becomes embedded in the prothallus, and serves the growing embryo for the absorption of nutriment. One of the lower posterior cells remains undeveloped, the other (/-/) takes on the form of the apical cell of a root, i.e., of a wedge- shaped cell, which not only produces three sets of segmental cells from its .sides but also cap-cells from its base (p. 422) : division of this cell goes on very rapidly, and d. primary root is produced which at once grows downwards into the soil. Finally the two lower anterior cells undergo rapid fission, and develop into the first leaf of the embryo, called the cotyledon (ct), which soon begins to grow upwards towards the light. 428 FERNS LESS, Thus at a comparatively early stage of its development the fern-embryo has attained a degree of differentiation far beyond anything which occurs in the moss-embryo. The scarcely differentiated polyplast has passed into a stage which may be called the pkyllula, distinguished by the possession of those two characteristic organs of the higher plants, the leaf and root. Notice how early in development the essential features of animal or plant manifest themselves. In Polygordius the polyplast is succeeded by a gastrula distinguished by the possession of a digestive cavity : in the fern no such cavity is formed, but the polyplast is succeeded by a stage dis- tinguished by the possession of a leaf and root. In the one case the characteristic organ for holozoic, in the other the characteristic organs for holophytic nutrition make their appearance, and so mark the embryo at once as animal or plant. We may say then that while the oosperm and the polyplast stages of the embryo are common to the higher plants and the higher animals, the correspond- ence goes no further, the next step being the formation in the animal of an enteron, in the plant of a leaf and root. In other words the phyllula is the correlative of the gastrula. The cotyledon increases rapidly in size, and emerges between the lobes of the kidney-shaped prothallus (l) : the root at the same time grows to a considerable length, the result being that the phyllula becomes a very obvious structure in close connection with the prothallus, and indeed appearing to be part of it. The two are actually, however, quite distinct, their union depending merely upon the fact that the foot of the phyllula is embedded in the tissue of the prothallus like a root in the soil. Hence the phyllula is related to the prothallus in precisely the same way as the XXXI GAMOBIUM AND AGAMOBIUM 429 sporogonium to the moss plant (compare Fig. in, k, with Fig. 109, c^, and Fig. in, l, with Fig. 109, c''). The rudiment of the stem (l, st) continues to grow by the production of fresh segments from its apical cell ; leaves (/) are developed from the segments, and grow upwards parallel with the cotyledon. The leaves first formed are small and simple in structure, but those arising later become succes- sively larger and more complicated, until they finally attain the size and complexity of the ordinary leaves of the fern. In the meantime new roots are formed and the primary root ceases to be distinguishable ; the cotyledon, the foot, and the prothallus wither, and thus the phyllula, by the successive formation of new parts from its constantly growing stem, becomes a fern-plant. We see that the life-history of the fern resembles in essentials that of the moss. In both, alternation of genera- tion occurs, a gamobium or sexual generation giving rise, by the conjugation of ovum and sperm, to an agamobium or asexual generation, which, by an asexual process of spore- formation, produces the gamobium. But in the relative proportions of the two generations the difference is very great. What we know as the moss plant is the gamobium, and the agamobium is a mere spore-producing structure, never getting beyond the stage of a highly differentiated polyplast, and dependent throughout its existence upon the gamobium, to which it is permanently attached. What we know as the fern plant is the agamobium, a large and complex structure dependent only for a brief period of its early life upon the small and insignificant gamobium. Thus while the gamobium is the dominant phase in the life-history of mosses, the agamobium appearing like a mere organ, in ferns the positions are more than reversed — the agamobium may assume the proportions of a tree, while the gamobium is so 43° FERNS LESS, xxxi small that its very existence is unknown to a large propor- tion of fern-collectors. It follows from what has just been said that the various organs of a fern do not severally correspond with those of a moss. The leaves of a moss are not homologous with those of a fern, but are rather comparable to lobes of the pro- thallus : in the same way the rhizoids of a moss correspond, not with the complicated roots of the fern, but with the rhizoids of the prothallus. LESSON XXXII THE CHIEF DIVISIONS OF THE VEGETABLE KINGDOM : EQUISETUM : SALVINIA : SELAGINELLA In the 26th Lesson (p. 320) it was pointed out that a thorough comprehension of the structure and development of Polygordius would enable the student to understand the main features of the organisation of all the higher animals. In the same way the study of the fern paves the way to that of the higher groups of plants, all of which, indeed, differ far less from the fern than do the various animal forms con- sidered in Lessons XXVI — XXIX from Polygordius. We saw that the differences between these included matters of such importance as the presence or absence of segmentation and of lateral appendages, the characters of the skeleton, and the structure and position of. the nervous system. In the higher plants, on the other hand, the essential organs — root, stem, and leaves — are, save in details of form, size, &c., practically the same in all : the tissues always consist of epidermis, ground-parenchyma, and vascular bundles, the latter being divisible into phloem and xylem : the growing point both of stem and of root is formed of meristem, from which the permanent tissues arise ; and the growing point of 432 CHARACTERS OF THE HIGHER PLANTS less. the root is always protected by a root-cap, that of the stem being simply over-arched by leaves. Moreover, an alterna- tion of generations can be traced in all cases. Plants may be conveniently divided into the following chief groups or phyla : 1. Alga. 2. Fungi. 3. Muscinece. 4. Vascular Cryptogams. a. Filicins. b. Equisetacese. c. Lycopodineffi. 5. Phanerogams. a. Gymnosperms. b. Angiosperms. The AlgcR are the lower green plants. They may be unicellular, or may take the form of linear, superficial, or solid aggregates : they never exhibit more than a limited amount of cell-differentiation. This group has been repre- sented in the foregoing pages by Zooxanthella, Diatoms, Vaucheria, Caulerpa, Monostroma, Ulva, and Nitella. The Fungi are the lower plants devoid of chlorophyll : some are unicellular, others are Imear aggregates : in none is there any cell-differentiation worth mentioning. Saccharo- myces, Mucor, Penicillium, and the mushroom belong to this group. The position of some of the lower forms which have come under our notice is still doubtful. Bacteria, for instance, are considered by some authors to be Fungi, by others Algse, while others place them in a group apart. Diatoms also are XXXII CHARACTERS OF THE PHYLA 433 sometimes placed in a distinct group. It must, moreover, be remembered that most botanists include Hsematococcus, Pandorina, and Volvox among Algfe, and place the Myce- tozoa either among Fungi or in a separate group of chloro- phyll-less plants (p. 181). The Musdnea are the mosses and liverworts, the former of which were fully described in Lesson XXX. The Vascular Cryptogams are flowerless plants in which vascular bundles are present. Together with the Phanero- gams they constitute what are known as vascular plants, in contradistinction to the non-vascular Algae, Fungi, and Muscineae, in which no formation of vessels takes place. The group contains three subdivisions. The first division of Vascular Cryptogams, the Filicina; includes the ferns, an account of which has been given in the previous lesson. It will be necessary, however, to devote some attention to an aquatic form, called Salvinia, which differs in certain important particulars from the more familiar members of the group. The EquisetacecR include the common horsetails (genus Equisetuni), a brief account of which will be given, as they form an interesting link in their reproductive processes between the ordinary ferns and Salvinia. The Lycopodineoi, or club-mosses, are the highest of the Cryptogams or flowerless plants. A short description of one of them, the genus Selaginella, will illustrate the most striking peculiarities of the group. The Phanerogams, or flowering plants, are so called from the fact that their reproductive organs take the form of specially modified shoots, called cones or flowers. They are sometimes called by the more appropriate name of Sperma- phytes, or seed-plants, from the fact that they alone among plants reproduce by means of seeds structures which differ F F 434 EQUISETUM less. from spores in the fact that each contains an embryo plant in the phyllula stage. The Gymnosperms, or naked-seeded Phanerogams, include the cone-bearing trees, such as pines, larches, cypresses, &c., as well as cycads and some other less familiar forms. A general account of this group will be given. The Angiosperms, or covered-seeded Phanerogams, include all the ordinary flowering plants, as well as such trees as oaks, elms, poplars, chestnuts, &c. A brief description of the general features of this group will conclude the Lessons, Equisetum The horsetails are common British plants found usually in moist or marshy situations, and reaching a height of I to 3 feet. The plant consists of a branched underground stem or rhizome, lateral branches of which grow vertically upwards, and constitute the aerial shoots. Both stem and branches have a very characteristic appearance : they are distinctly segmented or divided into nodes and internodes, and from each node springs a crown-like structure or leaf-sheath (Fig. 112, A, and Fig. 113, a, /. sh), formed by a whorl of leaves united into a continuous structure. In some cases the aerial shoots also give rise to secondary shoots (Fig. 112, A, sh), arranged in whorls and apparently arising below the leaves : actually, however, they originate in axillary buds, as in Nitella, but, instead of growing out between the stem and the leaf, perforate the base of the latter. The internodes of both rhizome and aerial shoots are hollow, each having a large axial air-cavity (Fig. 112, b, c^) extending throughout its whole length, and formed by the disintegration of the central parenchyma-cells of the young XXXII STRUCTURE 435 stem. At each node is a transverse partition separating the internodal spaces from one anotlier. iVround the central cavity, and corresponding with the longitudinal ribs with which the stem is marked, is a series of smaller air-cavities (/-), arranged in a circle, and alternating with these, between Fig. 112. — A, portion of aerial shoot of Equisetum, showing a node (nJ) from which arise a leaf-sheath (/. s/i) and a whorl of secondary shoots (sh). (Nat. size.) B, transverse section of aerial shoot, showing central ((') and peri- pheral (f-) air-cavities, and ring of vascular bundles with smaller air- cavities (t-*). { X 2.) c, a single sporophyll (,t/. //;) with stalk (s/) and sporangia (s/£-). (X 10.) D, a single spore showing coiled elater (t7). E, the same, with elater (e/) expanded. (A-c, after Goebcl ; D and E, after Lc .Maout and Decaisne. ) them and the central cavity, are the vascular bundles (v. />), each with a small air-cavity (r) in its inner or central portion. The microscopic structure of the plant agrees in essential respects with that of the fern, though differing in many details to which no further reference need be made here. F F 2 436 EQUISETUM less. Each axis — rhizome and shoots— terminates in a tetrahedral apical cell. As in ferns, there is no primary root in the adult, but numerous roots spring from the nodes of the rhizome, and agree in all essential points of structure and development with those of ferns. Some of the aerial shoots bear only leaf-sheaths and branches, and are hence called sterile shoots : others, the fertile shoots, terminate in a cone-like structure (Fig. 113, a), formed of hexagonal scales {sp. ph), at first closely applied to one another at their edges, but afterwards becoming separated. Each scale (Fig. 112, c, and Fig. 113, b, sp.ph) is a mushroom-like body, springing from the axis of the cone by a stalk {st) attached to the centre of the inner surface of its expanded portion. Around the point of attachment of the stalk spring from five to ten elongated sacs, the sporangia The structure and development of these mushroom-like bodies or scales of the cone show them to be peculiarly modified leaves, developed in whorls like the ordinary leaves of the stem, but not cohering into sheaths, and assuming the characteristic form just described in relation with their special function of bearing the sporangia. We have there- fore to distinguish, in Equisetum, between ordinary or foliage-leaves and spore-bearing leaves or sporophylls. The spores are developed in the same way as in mosses and ferns, but have a very distinctive structure. Outside the usual double cell-wall is a third coat, which, as develop- ment proceeds, becomes split up into four bands (Fig. 112, D, E, el), wound spirally round the spore and attached to it by one end, the opposite expanded end being free. These bands or elaters are hygroscopic : when moist they are coiled round the spore (d), when dry they straighten themselves DIMORrHISM OF THE GAMOBIUM 437 and stand out separately from its surface (e). The spores become entangled by their elaters, by the coiling and un- coihng of which they are able to execute slight movements. Fig. 113. — Reproduclion and Development oi Eqiiiscliiiii. A, distal end of a fertile shoot, showing two leaf-sheaths (/. sh), and the cone formed of hexagonal sporophylls \sp. pJi). (Nat. size.) B, diagrammatic vertical section of a portion of the cone, showing the sporophylls (sp.pli) attached hy short stalks to the axis of the cone, and bearing sporangia {sp,^) on their inner surfaces. C, a male prothallus bearing three spermaries {.py). { x 100.) D, portion of a female prothallus bearing three ovaries {ozy), those to the right and left containing ova, that in the middle a polyplast ; ;-/;, rhizoids. ( x 30. ) (a, after Le j\faout and Decaisnc ; C and J"), after llofmeislcr.) The spores are liberated by the bursting of the sporangia, and germinate, giving rise to prothalli. ]3ut instead of the prothalli being all alike in form and size and all monoecious, 438 SALVINIA less. some (c) remain small and simple, and produce only spermaries {spy) ; others (d) attain a complicated form and a length of over a centimetre, and produce only ovaries {ovy). Thus although there is no difference in the spores, the prothalli produced from them are of two distinct kinds, the smaller being usually exclusively male, the larger female. The oosperm develops in much the same way as in ferns : it divides and forms a polyplast, which, by formation of a stem, root, foot, and two cotyledons, becomes a phyllula and grows into the adult plant. As in the fern, the Equisetum plant, reproducing as it does by asexual spores, is the agamobium, the gamobium being represented by the prothallus. The peculiarity in the present case is that the gamobium is sexually dimorphic, some prothalli producing only male, others only female gonads. Salvinia Salvinia is a small fresh-water plant, found floating, like duckweed, on the surface of still water. The stem (Fig. 114, st) is an elongated slender rhizome floating at or near the surface, and distinctly divided into nodes and internodes. Each node gives off three appen- dages, two broad, flat foliage-leaves (/ /. 1-3 ; / /. 1-3'), which lie above the surface of the water, and a branched structure {s. I. 1-3) which has all the appearance of a root, its thread-like branches hanging down into the water and being covered with hairs. The study of their de\elopment show-s, however, that these organs arise exogenousl}- from the node and have no root-cap : they are, in fact, not roots, but submerged leaves, performing the function of roots. XXXII STRUCTURE 439 The latter organs are, quite exceptionally among the higher plants, wholly absent. The stem ends distall)' in a terminal bud (/. bd), the J^la fl3 fl-^ jtL. t.hd ' S.1.3 s.f.a. s.l.f Fig. 114. — IJislal portiun of a .Salvinia plant seen obliciuely from below. The btem (s/) encU in a (cnninal bud (/. ///), and the part figured contains three nodes, each bearing a ]:)air of foliagedeaves ( f. I. 1-3, f. I. 1-3'), and a much-divided rootdike submerged leaf (j-. /. 1-3). < )n the bases of the submerged ]ea\es are borne gi'0ii[is of sfiri {so), containing sporangia. (Slightly enlarged.) (b'rom \'ines, after Sachs. ) growing point of which is tornied l)\' a two-sided apical cell : it is tra^"ersed b)' a single \'ascular bundle, which sends branches into the leaves. 440 SALVINIA LESS. Springing from the bases of the submerged leaves are numerous globular capsules (so), each containing a number of sporangia. The wall of the capsule (Fig. 115, a) corre- sponds with the indusium of a fern, and the contained group of sporangia with a sorus. But the sori of Salvinia, unlike those of ordinary ferns, are dimorphic, some containing a comparatively small number of large sporangia {7ng. spg), others a much larger number of small ones {?ni. spg). The larger kind, distinguished as megasporangia, contain each a single large spore, or megaspore : the smaller kind, or micro- sporangia, contain a large number of minute spores, like those of an ordinary fern, and called microspores. It is this striking dimorphism of the sori, sporangia, and spores which forms the chief distinction betweeti Salvinia and its allies and the true ferns. When ripe the sporangia become detached and float on the surface of the water. The microspores germinate (b), while still enclosed in their sporangium : each sends out a filament, which protrudes through the wall of the micro- sporangium, its extremity {spy) becoming separated off by a septum and then divided into two cells. The protoplasm of each of these divides into four sperm-mother-cells, and from these spirally-twisted sperms are produced in the usual manner. It is obvious that the two cells in which the sperms are developed represent greatly simplified spermaries : the single proximal cell {prtk) of the filament arising from the microspore, a still more simplified prothallus. Both prothallus and spermaries are ^•estigial structures ; the pro- thallus is microscopic and unicellular instead of being a solid aggregate of considerable size, as in the two preceding types ; each spermary forms only four sperm-mother-cells, and the total number of sperms is therefore reduced to eight. REDUCTION OF THE GAMOBIUM 441 The contents of the megaspore are divisible into c. com- paratively small mass of protoplasm at one end, and of starch grains, oil-globules, and proteid bodies, which fill up the rest Fig. 115 — Reproduction and Development oi Sah'iiiia. A, portion of a submerged leaf, showing thiee sori in \erLical sectiiin, two containing micvosporangia {nn. sf^) and one megasjxjrangia (///;''. J-Ai. (x 10.) B, a germinating microspore {mi. sf^), showing tlie vestigia] prothallus [prtli) and its two spermaries (spy). ( x 150.) c, diagrammatic vertical section of a germinating megaspore, showing the otiter [nig. sp) and inner {ni^. sp^) coats of the spore, and its cavity (f) containing plastic products, separated \>y a septum (d) from the ]iro- thallus (prlh), in which two ovaries (oiy) are shown, that to the left containing an ovum, that to the right a polyplast. ( x 50.) 1', megasjiore [nt^. sp) with prothallus [prlh) anil ]ih)llula just begin- ning to develop into the leafy plant : si, stem ; ct, cotyledon ; and /, outermost leaf of the terminal bud. ( v 20.) {a and B, after .Sachs; Ii, after Tringsheim.) (c, c) of the spore. The megaspore has, in fact, attained its large size by the accumulation of great quantities of plastic products, which serve as nutriment to the future [irothallus 442 SELAGINELLA less. and embryo, after the manner of the yolk in the eggs of the crayfish and dogfish. The protoplasm of the megaspore (c) divides and forms a prothallus {prth) in the form of a three-sided multicellular mass projecting from the spore, which it slightly exceeds in size. Three ovaries {ovy) are formed on it, having much the same structure as in ordinary ferns : if neither of these should be fertilised others are developed subsequently. Thus the reduction of the prothallus produced from the megaspore, although obvious, is far less than in the case of that arising from the microspore. ^Ve see that sexual dimorphism has gone a step further in Salvinia than in Equisetum : not only are the prothalli differentiated into male and female, but also the spores from which they arise. Impregnation takes place in the usual way, and the oosperm divides to form a polyplast, which, by differentiation of a stem-rudiment, a cotyledon, and a foot, passes into the phyllula stage : no root is developed in Salvinia. By the gradual elongation of the stem (d, st) and the successive formation of whorls of leaves (/), the adult form is assumed. Thus the life-history of Salvinia resembles that of the fern, but with two important differences : the spores are dimorphic, and the gamobium, represented by the male and female prothalli, is greatly reduced. Selaginella Selaginella, one of the club-mosses, is common on hill- sides in many parts of the world. In the commoner species there is a creeping stem which forks repeatedly in the hori- zontal plane, and bears numerous small, close-set leaves, giving the whole plant much the appearance of a moss. STRUCTURE 443 The leaves (Fig. ii6, a) arise in four longitudinal rows, but, owmg to the horizontal position of the plant, the two rows belonging to the lower side (/-) project laterally, and Fig. ii6. — A, distal end of a shoot of Selaginella, sliowing the two rc.ws of small dorsal leaves (/'), the two laterally placed rows of ventral leaves (/-), and the terminal cone [i). (Xat. size,) B, a microsporangiuiTi bursting to allow of the escape of the micro- spores {/«/. s/). c, a megasporangium, with four megaspores (///;,'■. sj>). (A, after Sacli^ ; 1! and c, after I.e Afaout and Decaisnc.) are mail) times larger tlian tlie t«'0 upi)er rows (/'). i'.arli leaf bears on its upper or distal surface, near the base, a small process called a /if^iilv. The stem usually ends in a two- or three-sided ajiical cell, from which segments are cut off to form the apical 444 SELAGINELLA less. meristem, but in some species no apical cell can be distin- guished. There are from one to three vascular bundles running through the stem, each surrounded by a ring of small air-cavities : from them a single bundle is given off to each leaf. The presence of vascular bundles and of a well- marked epidermis is enough to distinguish our present type from the mosses, to which it bears a superficial resemblance. The peculiar forked branching is due to the development of lateral branches alternately on each side of the stem. The roots arise from peculiar leafless branches, sometimes mis- taken for true roots. The branches terminate in cones (Fig. ii 6, a, c, and Fig. 117, a) formed of small leaves {sp. pK), which overlap in somewhat the same way as the scales of a pine-cone. Each of these leaves is a sporophyll, and bears on its upper or distal side, near the base, a globular sporangium. The sporangia are fairly uniform in size, but some are megasporangia (Fig. 116, c, and Fig. 117, a, mg. spg), and contain usually four megaspores ; others are microsporangia (Fig. 116, b, and Fig. 117, a, tni. sj>g), containing numerous microspores. The microspore (Fig. 117, b) cannot be said to germinate at all. Its protoplasm divides, forming a small cell {prth), which represents a vestigial prothallus, and a large cell, the repre- sentative of a spermary. The latter {spy) undergoes further division, forming six to eight cells in which numerous sperm- mother-cells are developed. The sperms are finally liberated by the rupture of the coats of the microspore. A similar but less complete reduction of the prothallus is seen in the case of the megaspore (c). Its contents are divided, as in Salvinia, into a small mass of protoplasm at one end, and a large quantity of plastic products filling up the rest of its cavity. The protoplasm divides and forms a small prothallus {prtli). and a process of division also takes I'ROTIIALLUS 445 place in the remaining contents (pti^i'^) of the spore, pro- ducing a large-celled tissue, the seiondary frofkalli/s. By the rupture of the double cell-wall of the megaspore Fig. 117. — Reproduction and Development c,{ Si'laginc//ti. A, diagrammatic vertical section of a cone, consisting of an axis bear- ing close-set sporophylls (sp. ph), on the bases of wliich microsporangia (tni. spg) and megasporangia (mg. spg) are borne. B, section of a microspore, showing the outer coat (;;//. sp), protliallial cell (piih), and multicellular spermary [spy). C, vertical section of a megaspore, the wall of which [fug. sp) has been burst by the growth of the prothallus {prlh) : its cavity {prth^\ contains a large-cellecl tissue, the secondary prothallus : in tlie prothallus are three ovaries (ovy), that to the left containing an ovum, that to the right an embryo [emb] in the polyplast stage, and that in the centre an embryo in the phyllula stage, showing stem-rudiment [st], foot (/), and two cotyledons (ct) : both embryos are provided with suspensors (dotted) (spsr), and have sunk into the secondary prothallus. (Altered from Sachs.) the prothallus is exposed to the air, but it never protrudes through the opening thus made, and is, therefore, like the corresponding male structure, purely endogenous. One or 446 SELAGINELLA less, xxxii more ovaries (ovy) are formed on it, each consisting of a short neck, an ovum, and two canal-cells afterwards con- verted into mucilage : there is no venter, and the neck con- sists of only two tiers of cells. The oosperm divides by a plane at right angles to the neck of the ovary, forming the earliest or two-celled stage of the polyplast. The upper cell undergoes further division, forming an elongated structure, the siispensor ispsr) : the lower or embryo proper (emU) is forced downwards into the secondary prothallus by the elongation of the suspensor, and soon passes into the phyllula stage by the differentiation of a stem-rudiment {st), two cotyledons (ct), a foot (/), and subsequently of a root. A further reduction of the gamobium is seen in Selagi- nella : both male and female prothalli are quite vestigial, never emerging from the spores : and the spermary and ovary are greatly simplified in structure. LESSON XXXIII GYMNOSPERMS The commonest Gymnosperms are the evergreen cone- bearing trees such as pines, spruces, larches, cypresses, and yews. They all have a primary axis or trunk from which branches arise in a monopodial manner, i.e., the oldest are near the proximal, the youngest near the distal end. The branches give off, in successive seasons, branches of a higher order, so that the older or lower branches are always them- selves more or less extensively ramified, and the whole plant tends to assume a conical form, the base of the cone being formed by the oldest secondary axes springing from the base of the trunk, the apex by the distal end of the primary axis. The branches are all axillary, each arising from the axil of a leaf, and, like the main stem, ending distally in a terminal bud. The foliage-leaves differ greatly in the various genera of Gymnosperms : in the pines they are long, needle- like structures, borne in pairs on short axillary branches or dwarf-shoots. In correspondence with the size attained by the aerial portion of the plant, the root attains far greater relative dimensions than in any case we have previously studied. 448 GYMNOSPERMS less. The trunk is continued downwards by a great primary root, from which secondary roots arise in regular order, and, these branching again and again, there is produced a root-system of immense size and complexity, extending into the soil to a sufficient depth to resist the strain to which the aerial part oi the tree is subjected by the wind. One remarkable feature about the pines and their allies as compared with the plants previously studied, is their practi- cally unlimited growth. In mosses, ferns, &c., the stem after attaining a certain diameter ceases to grow in thick- ness, so that even in the tallest tree-ferns the stem is always slender. But in pines the trunk, the branches, and the roots continue to increase in thickness for an indefinite period, the trunk in the common Scotch Fir {Finns sylvest?-is) attaining a circumference of four or five metres or even more, and the other parts in proportion. The tree may survive for hundreds of years. The changes undergone during this remarkable process of growth are best studied, in the first instance, by a series of rough transverse sections of branches of different ages. In a first year's branch the middle is occupied by an axial strand of soft tissue, the pith or medulla (Fig. ii8, a and b, med) ; outside this comes a ring of wood {xy), divided into radially arranged wedge-shaped masses ; and this in turn is sur- rounded by the bark or cortex (cor), which can be readily stripped off the wood, and which contains numerous resin- canals (r. c) appearing in the section as rounded apertures with drops of resin oozing from them. In a somewhat older branch the layer of wood is seen to have increased greatly in thickness, and has a well-marked concentric and radial striation (c) : the cortex also has thickened though to a less extent, while the pith is unaltered. The bark, moreover, is clearly divisible into an inner light coloured layer, the I'ast STRUCTURi; OF STEM 449 OX phloem (plil), a middle green layer of cortical pafoicliytiia (cor) containing resin-canals, and an outer brown layer, the cork (cl;). Lastly, in the trunk and larger branches the wood forms by for the greater part of the whole section, the bark being a comparativel\- thin layer, easily stripped off, with no 7ru^l f ntej . Fio. ii8. — Diagramm:itic transverse scctinns of three branches of Piiiiis of different ages. A, very young axis, showing epidermis (tyi\ cortex (coy) with resin- canals (r. c), medulla [i?ied), and ring of vascular bundles, separated by medullary rays (iiied. r), and each consisting of xylem (-rj')i cambium (("Z'), and phloem {phi). B, older axis, in which the cambium forms a complete cylinder, owing to the formation of interfascicular carnbium (ri') between the bundles. C, Axis of the third year, showing xylem of first (xy^), second (.!_)'-), and third (xy'^) year's growth ; cork {ck) ; and cork cambium [,-k. cb.) cortical parenchyma, and with its corky outer layer much thickened, gnarled, and wrinkled. The wood has been stated to exhibit both concentric and radial striations. The radial markings are called medullary ravs (Fig. ii8, c, med. r) and follow the "grain" of the G G 450 GYMXOSPERMS LESS. wood. The concentric markings, which are against the grain, -are the annual 7-ings {xy^, xy^, xy^), and owe their existence to the fact that the wood formed in summer and autumn is denser than that formed in spring, while in winter there is a cessation of wood-production. Thus, by counting the annual rings of the main trunk, the age of the tree may be estimated. The wood, it will be observed, grows from within outwards, a new layer being added each year outside the old. The power of indefinite increase in diameter, which is so striking a feature in the pines and their allies, is connected with a peculiarity in the structure and arrangement of the vascular bundles. In the ^ery young condition, i.e., in the terminal bud, the vascular bundles of the stem (Fig. ii8, a) are wedge-shaped in transverse section and are arranged in a circle, the apex of each being turned towards the axis of the stem, the base towards its periphery. Actually, of course, as in the fern, the bundles are longitudinal strands with pro- longation into the leaves. The arrangeihent of the tissues in the vascular bundles differs in an important respect from the condition we are familiar with in the fern. Instead of the xylem occupying the centre of the bundle and being surrounded by phloem, the xylem (Fig. ii8, a, xy) forms the whole of the in-turned side, z.e., the narrow portion of the wedge in transverse sec- tions, the phloem (pM) the outer portion or broad end of the wedge. In a word, the bundles are not concentric as in the fern, but collateral. Moreover, the phloem and xylem are separated by a layer of small thin-walled cells, called the cambiinn layer {cb). By this arrangement of the vascular bundles the ground- parenchyma of the stem is divisible into three portions, an external layer, the cortex {cor), between the epidermis (ep). xxxiii GROWTH IN THICKNESS 451 and the phloem bundles, an axial cylinder, the pith or medulla {med), internal to the xylem bundles, and a series of radial plates, the f?-i!iiary medullary fuys (med. r) separat- ing the bundles from one another. As development proceeds the parenchyma-cells connecting the cambium of adjacent bundles take on the characters of cambium-cells, the result being the formation of a closed .cambium-cylinder, or, in transverse section, cambium-ring (e, cb, cb'\ In this a distinction is to be drawn between the fascicular cambium {cb) or original cambium of the bundles and inter-fascicular cambium {cb') formed by con- version of cells of the medullary rays. The cambium-cells now begin to divide in a tangential direction, i.e., along a plane parallel to the surface of the stem. If this process went on alone the result would be simply an increase in the thickness of the cambium layer, but as it proceeds the products of division of the cells on the inner face of the cambium-cylinder become con- verted into new xylem-elements, those on its outer face into new phloem-elements, ^^'e have thus a formation of secondary wood and secondary bast, which, being formed from the whole of the cambium-cylinder, show no division into bundles but form a continuous cyhnder (c, xy, phi) of constantly increasing thickness. The phloem now forms the inner layer of the bark, which, as we have seen, can be readily stripped from the wood owing to the delicate cambium-cells being easily torn apart. At the same time a layer of cells of the cortical parenchyma begins to divide tangentially so as to form a cylinder, or in transverse section a ring, of cork-cambium (Fig. 118, c, ck. cb), from the outer face of which layer after layer of cork- cells {ck) is formed. In the cork-cells the protoplasm dis- appears and the cell-walls undergo a peculiar change by G G 2 452 GY.MNOSrERMS less, xxxiii which they become waterproof : this process, besides pro- tecting the interior of the stem from external moisture, prevents the access of nutrient matters to the epidermis and outer layers of cortical parenchyma. Both these layers consequently die and peel off, the outer surface coming to be formed by the cork itself. The wood of pines contains no vessels, i.e., cells joined end to end so as to form a continuous tube, but only tracheides, i.e., elongated spindle-shaped cells with lignified walls and devoid of protoplasm. Radial bands of cells, mostly parenchymatous, are formed between the tracheides of the secondary wood, and give rise to the secondary medullary rays (c, med. r) to which the radial striation of the wood is due : they increase in number with the increase in thickness of the wood. The tracheides formed in autumn have smaller cavities and thicker walls than those formed in spring and summer : hence the formation of annual rings. The tracheides are not scalariform like those of ferns, but their walls have at intervals circular depressions perforated in the centre and called bordered pits. The tracheides of the primary xylem bundles ha\e spirally thickened walls, like the spiral vessels of ferns. The phloem, both primary and secondary, consists of sieve- tubes and parenchyma. The growing point of Gymnosperms presents a striking difference to that of ferns and other fiowerless plants. It consists simply of a mass of meristem cells among which no apical cell is to be distinguished. Pines, Uke horsetails and club-mosses, reproduce by means of cones or flowers. These are of two kinds, male and female, so that sexual differentiation is carried a step further than in Selaginella, in which sporangia of both sexes A //U c sj)jiJi . i ^/'J m m /mcj sjy A/jsy Fig. 119. — Reproduction and DcvclopmL-iiL of Gyinnos^ernis. A, diagrammatic vertical section of male cone, showing axis witli male .spovophylh [sp.ph. 6 ) bearing microsporangia (/«/. spi;) : pcf, scalcdike leaves forming a rudimentary perianth. 454 GYMNOSPERMS i-ESS. B, a single miciospore, showing bladder-lilrtli). As in Vascular Cryptogams, single super- ficial cells of the prothallus are converted into ovaries which are extremely simple in structure, each consisting of a large ovum (pv)., and of a variable number of neck-cells. The pollen, liberated by the rupture of the microsporangia, is carried to considerable distances by the wind, some of it falling on the female cones of the same or another tree. In this way single microspores (pollen-grains) find their way into the micropyle of a megasporangium (d, mi. sp). This is the process known as poHination, and is the necessary antecedent of fertilisation. The microspore now germinates : the outer coat bursts, and the vegetative cell (b, b) protrudes in the form of a filament resembling a hypha of Mucor, and called a pollen- tube (d, /./). This forces its way into the tissue of the nucellus, like a root making its way through the soil, and finally reaches the megaspore in the immediate neighbour- hood of an ovary. A process then grows out from the end of the tube, passes between the neck-cells, and comes in contact with the ovum. In the meantime the nucleus of the \-egetative cell {b) — that from which the pollen-tube grows — has travelled towards the end of the pollen-tube and undergone degeneration. The generative cell at the same time enters the pollen-tube and di\-ides into two spcrin-cells. The end of the pollen-tube becomes mucilaginous and one of the sperm-cells makes its way through it, clown the neck of the ovary and into the ovum. The nucleus of the sperm-cell — called the male xxxui FORMATION OF THE SEED 459 fronuckiis — Ihen conjugates with the nucleus of the ovum, Qx fetnale pronucleus, and thus effects the process of fertOisa- tion, or the conversion of the ovum into the oosperm. The development of the oosperm is a very complicated process, and results in the formation not of a single polyplast but of four, each at the end of a long suspensdr (d, spsr), formed of a linear aggregate of cells, which by its elonga- tion carries the embryo iemb) down into the tissue of the prothallus. As a rule only one of these embryos comes to maturity : it develops a rudimentary stem, root, and four or more cotyledons, and so becomes a phyllula. While these processes are going on the female cone in- creases greatly in size and becomes woody. The mega- sporangia, now called seeds, also become much larger, their integuments (e, t\ becoming brown and hard and constitut- ing the seed-coat or testa, which in the pine is produced into a flattened expansion or wing. The megaspore in each seed enlarges so much as to displace the nucellus : at the same time the cells of the prothallus filling the megaspore develop large quantities of plastic products, such as fat and albumin- ous substances, to be used in the nutrition of the embryo : the tissue thus formed is the endosperm (end). As the cone dries the placental scales separate and expose the seeds, which drop out and may be carried considerable distances by the wind, acting upon their wings, before falling to the ground. Under favourable circumstances the seed germinates. By absorption of moisture its contents swell and burst the seed-coat, and the root of the phyllula {r) emerges, followed before long by the stem {si) and cotyledons {ct). The phyllula thus becomes the seedling plant, and by further growth and the successive formation of new parts is con- verted into the adult. 46o GYMNOSPERMS less, xxxiii In Gymnosperms we see an even more striking reduction of the gamobium than in Selaginella. The female prothallus is permanently inclosed in the megaspore, and the mega- spore in the megasporangium : the ovaries also are greatly simplified. The male prothallus is represented by the smaller cell of the microspore, and no formation of sperms takes place, fertilisation being effected by sperm-cells formed from one of the products of division of . the prothallial cell, which migrate to the extremity of a tubular prolongation of the larger or vegetative cell of the microspore, and finally conjugate with the ova. It is worthy of notice that Phanerogams, alone among the higher organisms, have abandoned the ordinary method of fertilisation by the conjugation of ovum and sperm. In this respect they are the most specialised of living things. LESSON XXXIV AXGIOSPERMS To this group belong all the commoner herbs and shrubs as well as trees other than Gymnosperms, such as palms, oaks, elms, beeches, poplars, &c. There are two sub- divisions of the group which must be mentioned, because of the necessity of referring to them later on : they are the Dicotyledons, so called because of the presence of two coty- ledons or seed-leaves in the phyllula, and the Monocotyledons, in which only a single seed-leaf is present. Among Dico- tyledons are included the large majority of wild and garden flowers, as well as most of the angiospermous trees : the best known Monocotyledons are the lilies and their allies, the various kinds of narcissus, orchids, grasses, and palms. The general relations of the main parts of the plant — stem, root, leaves, &c. — are the same as in Gymnosperms, as may be seen by comparing a wallflower, an elm, a poplar, and a lily, taken as examples of dicotyledonous herbs, of dicotyledonous trees, and of Monocotyledons respectively. In the lily, however, as in Monocotyledons generally, there is no primary root, but a great number of equal-sized root- fibres springing from the base of the stem. In Dicotyledons the arrangement of the tissues is the same as in Gymnosperms (p. 448) : the vascular bundles 462 ANGIOSPERMS less. are arranged in a circle, and there is a closed cambium cylinder from which new xylem is added internally, and new phloem externally. Moreover, in trees and shrubs, i.e., plants which survi^'e from year to year instead of dying down at the end of one or two seasons, a cork-cambium is formed cor fihl Fig. 123. — Diagrammatic transverse section of the stem of a Lily, sliowing the epidermis (ep), cortical parenchyma containing chloro- phyll (cor), and axial cylinder of parenchyma surrounded by the pericycle (pre) and containing vascular bundles, each consisting of phloem (phr) and xylem (xy). in the cortex from which an external layer of cork is pro- duced, the epidermis disappearing. So that the phenomena of growth in thickness can be studied as conveniently in any dicotyledonous tree as in a pine or cypress. In Monocotyledons — in a lily, for instance — the arrange- ment of tissues is different. The vascular bundles (Fig. 123) are arranged in a number of irregular circles scattered throughout the central parenchyma or ground tissue, which XXXIV VENATION 463 is separated from the cortical parenchyma (corf) by a layer of sclerenchymatous cells, the pericyck {pre). The bundles are collateral, the xylem {xy) facing the axis of the stem, the phloem {phi) its periphery : but there is a fundamental difference from the bundles of Gymnosperms and Dico- tyledons in that the fully formed bundle contains no cambium, and is therefore incapable of further growth. The bundles of Monocotyledons are therefore closed, while those of Gymnosperms and Dicotyledons are open. Owing partly to this circumstance, partly to the thick unyielding pericycle, the stems of nearly all Monocotyledons are in- capable, when once their tissues are fully formed, of further increase in thickness. Hence the characteristic slenderness of the trunk of a palm as compared with that of a pine or an oak. The wood of Angiosperms consists of spiral, annular and dotted vessels, of fibres or prosenchymatous cells, and of parenchyma. The phloem contains sieve-tubes, long tough prosenchymatous cells called bast-fibres, and paren- chyma. The growing point, as in Gymnosperms, has no apical cell. The leaves vary indefinitely in form, and all that can be mentioned with regard to them in the present brief sketch is that in most Monocotyledons they are long and narrow, and traversed by numerous parallel veins, while in Dico- tyledons they are generally broad, with a smaller number — one to five — of primary veins from which secondary veins branch out and unite in a network. So that the venation or veining is parallel in Monocotyledons, reticulate in Dico- tyledons. It is in the structure of the flower that the most striking differences from, and the most marked advance upon, 464 ANGIOSPER>rS LESS. Gymnosperms are seen. The modifications of the flower among both groups of Angiosperms are almost infinite, and can be thoroughly understood only by a careful study of numerous forms : all that can be attempted here is to give some idea of the essential points of structure and the lead- ing modifications, by reference to a few selected forms. In a buttercup {Ranunculus), one of the most generalised Dicotyledons, the flower is borne at the end of a long stalk ox peduncle (Fig. 124, a and e, pd), the distal end of which is expanded into a conical Jlofal receptacle (b and c, fl. r), serving for the attachment of the various parts of the flower. From the broad proximal end of the receptacle spring five greenish leaves (a and b, cp), arranged in a whorl : they are the sepals, and together constitute the calyx of the flower. A little higher up arise, alternately with the sepals, five larger leaves (a and b, pi) of a brilliant jellow colour, forming the conspicuous part of the whole flower : they are the petals, and together constitute the corolla. Each petal has at the base of its upper side a little scale called a nectary (f, net), from which a sweet juice, called nectar, is secreted. Both sepals and petals spring from the base of the conical receptacle. From the lower half of the part above their origin arise a large number of stamens (b and c, st), arranged, not in a whorl, but in a close spiral, and together constituting the andrcecaan. Each stamen (d) consists of a stalk or filament {fl), bearing at its distal end an expanded body or anther {an), divided by longitudinal ridges into four lobes. A transverse section (Fig. 125, b^) shows that each lobe contains a pollen-sac or microsporangium {mi. spg), filled, in the ripe condition, with minute pollen-grains or microspores {mi. sp). From the distal portion of the receptacle arise, also in a close spiral, a number of little pod-like bodies, the carpels vlo\\']-:r 465 (b and c, ,-p) together constituting tlie gyiuvciiiin or pisti/. Eacli carpel consists of an expanded, liollow, proximal l^.spl Flc. 124. — Structare of the flo\\"er of the Buttercup. A, the entire flower from Ijelow, sliowing }")eduncle [pd], sepals {■^/'), and petals (//). B, vertical section of Hower, showing peduncle (/(/), floral receptacle {J!, r), sepals (s/)), petals (//), stamen (s/), and carpels {ij>). The carpel cp' is cut vertically, and shows the mcgasporangium. c, floral receptacle (_/?. ?■), with carpels (f/), one stamen {si), and sears left by the removal of the remaming stamens. 11, stamen, showing filament (/?) and anther (irii). E, carpel in vertical section, showing venter (77//) with contained mcgasporangium (wy. sj'i''), and style (j/j. F, petal, with nectary {in'/). (a and c, after \^ines ; B, D, and K, after T^Iaout and Decaisne ; E, after Oliver.) ])ortiun or vciitcr'^ (e, viil), and of a short, hookdike distal extremity (j'Z) covered with stick)' hairs and called the sligina. ^ Commonly called ovai'y. 11 H 466 ANGIOSPERMS less. The venter contains a single ovule or megasporangium {ing. spg), differing from that of the pine in being covered by a double instead of a single coat (Fig. 126, d, Z', /^), both perforated by a micropyle (;«. py), which places the central mass of tissue or nucellus {ncl) in communication with the cavity of the venter (Fig. 126, a). The nucellus, like that of pines, contains a single embryo-sac or megaspore {mg. sp). The fact that the megasporangia are contained in a cavity of the carpel, and so shut off from all direct communication with the exterior, forms a fundamental difference between the angiospermous or covered-seeded, and the gymno- spermous or naked-seeded Phanerogams. A\'e saw that in Gymnosperms, as in the Vascular Crypto- gams, the sporangia were borne on structures, the sporophylls, which were obviously modified leaves. In the buttercup the stamens and carpels have departed so widely from the leaf-type that their true nature becomes obvious only after comparison with other forms. In the White Water-lily (Nymphcea alba) the very numerous petals are arranged, like the stamens, in a spiral, and the two sets of organs pass insensibly into one another. As we trace the petals (Fig. 125, a^) upwards on the floral recep- tacle we find them become narrower in proportion to their breadth (a^), and on the apex two little yellow lobes appear (;/«'. spg). Still passing up the spiral the lobes become more and more pronounced, and the petal narrower (a^), until at last the lobes become aggregated into an undoubted anther (a*, an), while the blade of the petal is narrowed to a filament, its distal end serving to unite the anther-lobes and constituting the co?inective (cor). The same transition from petals to stamens is seen in many " double " flowers, such as the double apple, in which the number of petals becomes greatly increased by the XXXIV MORPHOLOGY OF FLOWER 467 assumption of a petaloid form by the outer stamens, various intermediate stages being present from the typical stamen, through irregular leaves with anther-lobes at their distal ends, to the ordinary broad white petal. We see, then, that a stamen is a leaf on the surface of which four microsporangia (b^, ini. sj>g) are developed : the blade of the leaf is narrowed to form a mere stalk, while the Fig. 125. — A^-A**, transition from petal to stamen : 7ni. spg, micro- sporangia ; _/?, filament ; an, anther. B^, transverse section of male sporophyll in the stage A^ ; mr, mid- rib of staminal leaf ; mi. spg, microsporangia. B^, transverse section of typical anther, showing connective (cor) with vascular bundle or midrib (mr), on the left two microsporangia (mi. spg), and on the right the escape of the microspores (mi. sj>) by dehis- cence of the anther. microsporangia have become so closely aggregated as to form a single four-lobed body, the^ anther (b^). Similarly the carpel can be shown to conform to the leaf- type. The flower of the cherry has a single flask-shaped carpel, consisting of a rounded venter, with an expanded stigma borne on the end of a stalk or style. But when the cherry flower becomes double, the normal carpel is replaced by a little green leaf, quite like a foliage-leaf, except that it is permanently folded upon the midrib so as to bring the two halves of its upper or dorsal surface almost into contact. H H 2 468 ANGIOSPERMS less. Imagine one or more of the marginal lobes of such a leaf to be replaced by megasporangia, as in Cycas (Fig. 122, e), and the edges of its proximal part to come together and unite (Fig. 126, b\ b^). The result will be the enclosure of the ovules in a capsule formed from the proximal part of the leaf, while its distal end forms the style and stigma. The extreme differentiation of both male and female sporophylls is not the only important difference between the angiospermous and the gymnospermous flower. Almost equally characteristic, and equally striking as a sign of advance in organisation, is the fact that the sporophylls are surrounded by two sets — sometimes reduced to one — of floral organs, the sepals and petals, which together form the floral envelope or perianth. In most Gymnosperms the only indication of a perianth is in the form of inconspicuous barren scales, i.e., scales not bearing sporangia, at the base of the cone (Fig. 1 1 9, a and b, per)., while in Angiosperms the perianth has become differentiated into two well-marked and conspicuous sets of leaves. The function of the sepals is usually to protect the other parts of the flower in the bud : they are generally of such a size- as completely to close over the petals, stamens, and carpels until the flower opens, when they often either turn back or fall off". They are therefore to be looked upon as leaves which have been njodified for protective purposes. The petals serve an entirely different function. They are usually large and brightly coloured, forming the most con- spicuous part of the flower : they are also commonly scented, and from them or some adjacent part nectar is secreted. This fluid forms the staple food of many insects, especially butterflies, moths, and bees, which, as soon as a flower is opened, may be seen to visit it and to insert head or proboscis in order to suck the sweet juice. xxxiv MORPHOLOGY OF FL0WP:R 469 By the time this takes place the stamens have dehisced, i.e., split do\Yn each side, so that the two pollen-sacs of each half-anther discharge their pollen by a common slit (Fig. 125, B^). The pollen is usually not dry like that of Gymnosperms but sticky, so that the grains are not readily blown away but tend to adhere to one another and to the ruptured anther. Thus, when the insect inserts its head into the flower a greater or less quantity of pollen is certain to stick to it, and to be carried off as the insect flies to another flower. It will be remembered that the stigma is covered with sticky hairs, the consequence of which is that as the insect flies from flower to flower, the pollen it has collected from the stamens of one is transferred to the stigmas of another, and thus, in all the higher Angiosperms, pollination is effected by the agency of insects and not, as in Gymnosperms, by the chance action of the wind. Thus the corolla serves an attractive purpose : by its colour and scent insects are informed of the store of nectar it contains, and in the search for that food they uncon- sciously benefit the plant by performing the work of pollina- tion. In this way pollination is made more certain than when left to the wind, and the plant is saved the production of the immense quantity of pollen essential to a wind- fertilised plant, in which a very small fraction of the grains produced can possibly find their way to a female cone. Still another striking feature of the angiospermous as compared with the gymnospermous flower is the shortening of its a.xis. A comparison of Fig. 126, a, with Fig. 119, A and c, shows that the floral receptacle {fl. r) of the Angio- sperm is nothing but the axis of the gymnospermous cone shortened and broadened. The natural result is the suppres- sion of the internodes and the consequent approximation ml Fir 111.. 126. — Reproduction and Development of Angiosperms. A, diagrammalic vertical .section of a flower consisting of an abbreviated axis or floral receptacle {_fl. r) bearing .1 proximal (pcr^) and a distal (/(')-) whorl of perianth leaves (sepals and petals), a whorl of male sporophylls or stamens {sp. pJi. c? ), and one of female sporoi"ihylLs or carpels ()/. //;. 9 )■ LESS. XXXIV MORPHOLOGY OF FLOWER 471 The male sporophyU bcivs microsporangia {mi. spg) containing microspores (////. sp). The female sporophyll consists of a solid style {si) terminated by a stigma (slg), and of a hollow venter {v) containing a megasporangiuni {iiig. spg) in which is a single megaspore {mg. sp). On the right side a microspore is shown on the stigma, and has sent off a pollen-tube (p.t) through the tissue of the style to the micropyle of the megasporangiuni. b', diagram of a female sporophyll from the dorsal aspect, and B^, the same in transverse section, showing the folding in of its edges to form the cavity or venter in which the megasporangia {tug. spg) are enclosed : m. r, the midrib. C^, a microspore, showing the two cells {a and b) into which its contents divide ; the larger is the vegetative-cell. c^, the same, sending out a pollen-tube {p. t); 111/ , nu^, the two nuclei ; the generative nucleus has not yet divided. D, diagrammatic vertical section of a megasporangium, showing the double integument (f^tf^), nucellus («r/), micropyle (wi./y), and mega- spore {mg. sp) : the latter contains the secondary nucleus {nu) in the centi'e, three antipodal cells {ant) at the proximal end, and two syner- gidse {sng) and an ovum (ov) at the distal end. A pollen-tube (/. t) is shown with its end in contact with the synergidfe. E, semi-diagrammatic section of the megaspore of a young seed, showing an embryo {emb) in the polyplast stage with its suspensor {spsr) ; also numerous vacuoles {vac) and nuclei {niC). F, diagrammatic vertical section of a ripe seed, showing the seed-coat {t\ micropyle {in. fy), perisperm {per) derived from the tissue of the nucellus, and endosperm {end) formed in the megaspore and containing an embryo in the phyllula stage with stem-rudiment {st), cotyledons {ct), and root {r). (B^, after Behrens ; c\ C^, and E, altered from Howes.) of the nodes, so that all the leaves — sepals, petals, stamens, and carpels — arise close together from a small area. Thus, the angiospermous flower, like the gymno- spermous cone, is a modified shoot of limited growth, having its axis shortened to a floral receptacle and its leaves modified to form the various floral organs. The composition of the flower may therefore be expressed in a diagrammatic form as follows : — fPerinnth /Protective— Sepals (Calyx). Floral Receptaclelj. J ^ '="^"'" \ Attractive— Petals (Corolla). = Axis of Shoot / ' I Sporo- /Male — Stamens (Andrcecium). ^ phylls (_ Female — Carpels (Gynoecium). 472 ANGIOSl'ERMS my.spjf in^.S-pj FlO. 127. — a\ Vertical section of flowei- <-,( ffi/hhonis, sliow ing_/7. r, floral receptacle ; sf, .sepals ; f/, petals ; st, stamens ; and c/, carpels, that to the right cut longitudinally to show the megasporangia (/«;,"'. spg). a", transverse section of gynoecium of Helleborus passing through the venter (i7;/) of the six carpels, each of which has a midrib {»/>) and united edges (e) to which the megasporangia are attached. b', vertical section of flower of Campanula, showing floral recep- tacle [fl. r) enclosing venter of g^'ncecium (?'///), with megasporangia (iiig. spg) ; calyx (cat) ; corolla {cor) ; anthers {aii\ and filaments ( //) of stamens ; and style (.^V)') and stigma [slg). E-, transverse section of gyncecium of Campanula enclosed in lloral receptable (fl. r). Letters as in A-. c, transverse section of gynrccinm of 7\/7u:^. Letters a^ in a-, (a' and b', afler I,c Maiiul and Decaisnc.) XXXIV MODIFICATIONS OF FLOWER 473 There are one or two important modifications of the flower which must be briefly referred to. In the Christmas-rose {Hellehoi-us) the general structure of the flower resembles that of the buttercup except that the petals (Fig. 127, a^, pt) are small and tubular, and the sepals {sp) so large as to form the obvious and attractive part of the flower. But the large carpels (cp) are few — three to six — in number, arranged in a single whorl, and closely applied to one another by their lateral faces (a^). The peripheral or outwardly-facing border of each represents the midrib {inr) of the carpellary leaf, the central border — that facing the axis of the flower — its united edges («). To the latter are attached several megasporangia arranged in a longitudinal row. In the Canterbury-bell {Campanula) there appears at first sight to be a single carpel (b^ vnt) with three stigmas {stg). But a transverse section of the venter (b^) shows it to contain three cavities arranged round a longitudinal axis to which are attached three rows of ovules {mg. spg), one to each chamber. Obviously such a pistil is produced by the three carpels of which it is composed being not simply applied to one another as in the Christmas-rose, but actually fused. In the currant {Ribes) the pistil shows in transverse section a single cavity only (c), but with two rows of ovules {mg. spg) : here the carpellary leaves have united with one another simply by their edges. Campanula illustrates concrescence not of the carpels only but of all the other floral whorls. The sepals have united to form a cup-like calyx (Fig. 127, b"', cat), the petals are joined into a vase-like corolla {cor), and the filaments of the stamens {fl) are united below. Moreover, the floral receptacle {fl. r) instead of being conical, as in the butter- cup, is hollowed into a cup which encloses and is fused with 474 ANGIOSPERMS LESS. the venter of the pistil {imi) : it thus loses all appearance of being a stem-structure and becomes a mere capsule for the gyncecium, giving attachment at its edges to the other floral organs. An extended study of flowers will show how all the main modifications are brought about by the varying form of the floral receptacle, by the concrescence of one part with another, by the enlargement of certain parts, and by the diminution or complete suppression of others. The microspores are at first simple cells, each with a double cell-wall and a nucleus. The nucleus divides into two (Fig. 126, c^), a larger vegetative nucleus, and a smaller which divides again forming two generative nuclei, each of which may become surrounded by a thin cell-wall. No prothallus is formed in the megaspore, but its nucleus divides, the products of division pass to opposite ends of the spore, and each di^ddes again and then again, so that four nuclei are produced at each extremity. Three of the nuclei at the proximal end — that furthest from the micropyle — become surrounded by protoplasm and take on the char- acter of cells (d, qnt) all devoid of cell- wall and called antipodal cells ; the fourth remains unchanged. Similarly, of the four nuclei at the distal or micropylar end, one remains unchanged and three assume the form of cells by becoming invested with protoplasm. Of these three, two lie near the wall of the megaspore and are called synergidce (sng): the third, more deeply placed, is the ovum (ov). The two unaltered nuclei now travel to the centre of the megaspore and unite with one another, forming the secondary nucleus (iiii) of the spore. There is thus a single ovum produced in each megaspore, but no ovary and no prothallus : the female portion of the gamobium is reduced to its simplest expression. XXXIV POLLINATION AND FERTILISATION 475 Pollination may take place, as we have seen, by the agency either of the wind or of insects. The microspores are deposited on the stigma (a), where they germinate, each sending off a pollen-tube (a and c^, p. t), which grows downwards through the tissue of the stigma and style to the cavity of the venter, where it reaches a niegasporangium, and entering at the micropyle (d, p. t), continues its course through the nucellus, finally applying itself to the distal end of the megaspore in the immediate neighbourhood of the synergidfe. In the meantime the nuclei of the microspore (c-, mi, nu^) have passed into the end of the pollen-tube. The vegetative nucleus undergoes degeneration, becoming shrivelled and unaffected by dyes. One of the two generative nuclei also degenerates, the other, probably surrounded by protoplasm containing the astrosphere, passes through the softened cell-wall of the swollen end of the poUen-tubeeand enters the ovum, uniting with its nucleus in the usual way. The ovum is thus converted into an oosperm or unicellu- lar embryo : it acquires a cell-wall and almost immediately divides into two cells, of which that nearest the micropyle becomes the suspensor (e, spsr), the other, or embryo proper {emb), forming a solid aggregate of cells, the poly- plast. By further differentiation rudiments of a stem (f, st), a root {r) and either one or two cotyledons {ct) are formed, and the embryo passes into the phyllula stage. While the early development of the embryo is going on, the secondary nucleus of the megaspore divides repeatedly, and the products of division (e, mi) becoming surrounded by protoplasm, a number of cells are produced, which, by further multiplication, fill up all that part of the megaspore which is not occupied by the embryo. The tissue thus 476 ANGIOSPERMS LESS, xxxiv formed is called the endosperm (f, end), and occupies pre- cisely the position of the vestigial prothallus of Gymnosperms (Fig. 119, p. 453, vi, prth, and e, end: and p. 458), differing from it in the fact that it is formed only after fertili.sation. We have here a case of retarded development : the degenera- tion of the prothallus has gone so far that it arises long after the formation of the ovum which, in both Gymnosperms and Avascular Cryptogams, is a specially modified prothallial cell. As a rule the tissue of the nucellus disappears as the embryo grows, but in some cases, e.g., the water-lily, it is retained, forming an additional store of nutrient material and called the perisperm (Fig. 126, f, per). The phyllula continues to grow and remains inclosed in the megasporangium, which undergoes a corresponding in- crease in size and becomes the seed. One or more seeds also remain inclosed in the venter of the pistil, which grows considerably and constitutes the fi-uit. Finally the seeds are liberated, the phyllula protrudes first its root, and then its stem and cotyledons through the rupture-d seed-coat, and becomes the seedhng plant. We learn from this and the two preceding lessons that there is a far greater uniformity of organisation among the higher plants than among the higher animals, not only in anatomical and histological structure, but also in the fact that alternation of generations is universal from mosses up to the highest flowering plants. But as we ascend the series, the gamobium sinks from the position of a conspicu- ous leafy plant to that of a small and insignificant prothallus, becoming finally so reduced as to be recognisable as such only by comparison with the lower forms. SYNOPSIS A.— AN ACCOUNT OF THE STRUCTURE, PHYSIOLOGY, AND LIFE-HISTORY OF A SERIES OF TYPICAL ORGANISMS IN THE ORDER OF INCREASING COMPLEXITY. I. — The Simpler Unicellular Organisms. 1. Aniccba. PAGE Cell-body amoeboid or encysted : cell- wall nitrogenous (?): nutrition holozoic : reproduction by simple or binary fission . .... I 2. Hicinatococcus. Cell-body ciliated or encysted : cell-wall of cellulose : nutrition holophytic : reproduction by binary fission . 23 3. Heleromita. Cell-body ciliated : nutrition saprophytic : asexual repro- duction by binary fission : sexual reproduction by conju- gation of equal and similar gametes followed by multiple fission of the protoplasm of the zygote, forming spores . 36 4. Eugtena. Cell-body ciliated or encysted : cell-wall of cellulose : mouth and gullet present : nutrition holophytic and holozoic : reproduction by binai7 and multiple fission . . 44 5. Protomyxa. Cell-body amoeboid, ciliated, or encysted : plasmodia formed by concrescence of amoebulae : cell-wall nitro- genous (?) : nutrition holozoic : reproduction by multiple fission of encyste'd Plasmodium 49 478 SYNOPSIS PAGE 6. JMycetozoa. Like Protomyxa, but owing to the presence of nuclei the relation of the green colouring matter of plants, properties of, 26, 31 : occurrence in Bacteria, 87 : in Hydra, 228 Chrom'atin (xpwju.a, a colour), the con- stituent of the nucleus which is deeply stained by dyes, 7, 63 : male and female in nucleus of oosperm, 261 Chrom'atophore (xpoijua, colour : 0e'ptu to bear), a mass of proteid material im- pregnated with chlorophyll or some other colouring matter, 26, 46, 197, 207, 228 Cbromosome (xpw/Lta, colour : cu/Aabody), 65, 66, 261 Cil'ium {ciliwH, an eye-lash), defined, ■zsnoie : comparison of with pseudopod, 34, 52 ; absence of cilia in Arthropoda, 327 Cil'iary movement, 25 : a form of con- tractility, 33 Cil'iate Infusoria, 107 Circulatory organs, Polygordius. 2B2 : Crayfish, 337 : Mussel, 360 : Dogfish, 384 Classification, natural and artificial, 141 : natural, a genealogical tree, 145 Cnid'oblast («vt6>], a nettle : /SAauros, a bud), the cell in which a nematocyst iq.v.) is developed, 227 Cnid'OOil {KviBriauA ciliuin\ the " trigger- hair" of a cnidoblast, 227 Ccelenterata, the, 305 Ccelome {KOL\oiiJ.af a hollow), the body- cavity : — Polygordius, 270 : Starfish, 306 : Crayfish, 33s, 343 '■ Mussel, 355 : JJogfish, 372 : development of, Poly- gordius, 299 Coelom'ate, provided with a coelome, 273 Ccelomic epithelium. See Epithelium. CcBlomiC fluid, Polygordius, 278 Colloids (koAAc, glue ; eiSos, form), pro- perties of, 6 Colony, Colonial organism, meaning of term, 135, 234: formation of temporary colonies. Hydra, 231 Colum.el'la (a little column), 162 Com'missure {cominissilra^ a band), 279 Compoimd organism. See Colony. Concres'cence {cujn, together : cresco, to grow), the union of parts during growth Cone, an axis bearing sporophy lis : — Equi- setum, 436 : Selaginella, 444 : Gymno- sperms, 452 Conyi^B/tion {conj'zegdtio, a coupling), the union of two cells, in sexual reproduc- tion : — Amoeba, 20 : Heteromita, 41 : Paramoecium, 314: Vorticella, 132: Mucor, 165 : Spirogyra, 198 : of ovum and sperm, 260; monoecious and dioe- cious, 199 : comparison wiih plasmodium- formation, 54 Connective, oesophageal, 283, 341 Connective tissue, 329, 369 Contractile vac'uole {vaciiiis, empty) ;— Amoeba, 8, 16 : Euglena, 47 : Paramoe- cium, III Contractility (coniractto, a drawing to- gether), nature of, lo, 34 : muscular. Contraction, physical and biological, 10 Conus artinosus, 384 Cork, 449 Cork-cambium, 451 Corolla (corolla, a little wreath), the inner or distal whorl of the perianth in the flower of Angiosperms, 464, 468, 471, „473 Corpuscles. See Blood-corpuscles, and Leucocytes. Cortex, cor'tical layer {cortex, bark), Flowering plants, 59, 448 : Infusoria, no, 126 Cotton-wool as a germ-fitter, 99 Cotyle'don (kotuAtjSwi', a cup or socket), the first leaf or leaves of the phyllula ig.v.) in vascular plants, 427 Cranium {Kpaviov, the skull), 374 CRAYFISH ;— Figure, 319; general charac- ters, 314, 315 : limited number and con- crescence of metameres (Figure), 320: ap- pendages (Figure), 332 : exoskeleton, 319: enteric canal (Figures), 322 ; gills (Figure), 318: blood-system (Figuie,) 335. 337 - kidney, 337 : nervous system, 319 : Muscles (Figures), 327 : reproduc- tive organs, 343 : development, 343 Creation {creo, to produce), definition of, 141 : illustrated in connecton with species of Zoothamnium (Diagram), 142 Cross-fertilization : applied to the sexual process when the gametes spring from different individuals, 109 492 INDEX AND GLOSSARY Cryst'alloids (KputrraXAos, crystal : elfios form), properties of, 6 Cut'icle {cnticula^ the outer skin), nature of in unicellular animals, 45, log ; in multicellular animals, 238 Cyst («ua-Tis, a bag), used for cell-wall in many cases, 10, 51 D DalUnger, Dr. \V. H , observations on an apparent case of heterogenssis, 103 Daugliter-cellS, cells formed by the fission or gemmation of a mother-cell, 35, 67 Deatli, phenomena attending, 20, 21, 1G6 Decomposition, nature of, 6, 91 Dermal gills. See Respiratory Cceca. Dermis {Sepfia, skin), the deep or connec- tive tissue layer of the skin, 326 Descent, doctrine of. See Evolution. Development, meaning of the term^ 43. For development of the various typos see under their names Dextrin, 113 Diastase, Si Diast'ole (Stao-TeWw, to separate), the phas^ of dilatation of a heart, contractile vacuole, &c., in DIATOMA'CE.ffl (SiaTdfLvu), to cut across, because of the division of the shell into two valves), 155 : Figure, 156 Diat'omin, the characteristic yellow colour- ing matter of diatoms, 154, 155 Dichot'omous {8LxoToixeui, to cut in two), applied to branching in which the stem divides into two axes of equal value, 318 Differ entia'tion {differo, to carry different ways), explained and illustrated, 34, 119 Diges'tion {digSro, to arrange or digest), the process by which food is rendered fit for absorption, 12, ; intra- and extra- cellular, 229: contrasted with assimila- tion, 230 Digestive gland, 335, 355, 382 Dimorph'ism, dimorph'ic (SiV, twice: ju.opi^>7, form), existing under two forms, 35. 136, 242, 438, 442 ^ DiCB'ciOUS (6ts, twice : ot/co?, a dwelling), applied to organisms in which the male and female organs occur in different in- dividuals, 199 DIPH'YES iht^vq';, double) : Figure, 250 : occurrence and general characters, 248 ; polymorphism, 249 Diploblast'ic (SittAoo?, double : jSAao-Tos, a bud), two-layered : applied to animals in which the body consists of ectoderm and endoderm, 236: derivation of diploblas- tic from unicellular animals, 261 Directive sphere, see also Astrosphere Disc, Vorticella, 128 Dispersal, means of ; in internal parasite, 124 : in lixed organisms, 132, 134 Distal, the end furthest from the point of attachment or organic base, 126 Distribution of food-materials :— in a complex animal, 278 : in a complex- plant, 409 Divergence of character, 145 Division of physiological labour, 34 DOGFISH :— Figure, 367 : general charac- ters, 368 : exoskeleton, 369 : endo- skeleton (Figures). 372 ; enteric canal (Figures), 381: gills, 383; blood-.system (Figures), 384 : kidney, 396 : gonads, 396 : nervous system and sense organs (Figure), 391 : development (Figure), 397 Dry-rigor, stiffening of protoplasm due to abstraction of water, 21 Ecdysis (cKSutrt?, a slipping out), 325 EcMnodermata, the, 305 Ect'oderm (ekto?, outside ; fie'p/xa, skin), the outer cell-layer of diploblastic and triploblastic animals, 222, 275 Ect'osarc (cktos, outside : o-dp^, flesh), the outer layer of protoplasm in the lower unicellular organisms, distinguished by freedom from granules, 4 Egest'ion {egero, to expel), the expulsion of waste matters, 12 Egg-cell. See Ovum. Elater, 436 Em'bryo (e^^puor, an embryo or foetus), the young of an organism before the commencement of free existence. Em'bryo-sac. See Megaspore. Encysta'tion, being enclosed in c* cyst {g v.) End'oderm (evSo;/, within; Sepfta, skin), the inner cell-layer of diploblastic and triploblastic animals, 222, 228, 275 Endodermis, 418 End'oderm-lamella, Medusa, 240 Endog'enous (li'Soi', within : yiyvofi-aL, to come into being), arising from within, e.g. the roots of vascular plants, 422 Endopbragmal System (li'Sov, within: <^pa,y/j.a, protection), 321 Endopodite {evBov, within , irovq, foot), 323 End'Osai'C {evBov, within : a-ap^., flesh), the inner, granular protoplasm of the lower unicellular organisms, 4 Endoskel'eton (ef5oi/,within, and skeleton, from 0-Ke'A.Aa), to dry), the internal skele- ton of animals, 372 End'osperm (ei/Sor, within : o-Trepiua, seed), nutrient tissue formed in the megaspore of Phanerogams, 459, 476 Energy, conversion of potential into kinetic, 15 : source of, in chlorophyll- containing organisms, 31 INDEX AND GLOSSARY 493 Enter'ic (ivrepov, intestine), canal, the entire food-tube from mouth to anus : — Polygordius, 270, 276 ; Starfish, 312 : Crayfish, 332 : Mussel, 355 ; Dogfish, 381 Ent'eron or Enteric cavity, the simple digestive chamber of diploblastic ani- mals, 222 Epidermis (eiri upon: Sepfia, the skin) : in animals synonymous with deric epi- thelium {^.7/. , under Epithehum) : in vas- cular plants a single external layer of cells, 416, 420 Epipodiie (en-t, upon : ttov?, foot), 324 Epi'stoma (cTrt, upon : (jTOjLia, month), 321 Epitherial cells: columnar, 58 : ciliated, 59 Epitherium (eirC, upon : fljjA^, the nipple), a cellular membrane bounding a free surface, 244 ; ccelomic, 274 ; cleric, 273 ; enteric, 274 EQUISE'TUM (eg7ius, a horse: seia, a bristle) : — Figures, 435, 437 : general characters, 434 : cone and sporophylls, 436 ; male and female prothalli, 437 : al- ternation of generations, 438 Equivocal generation. See Abiogenesis. EUGLEN'A ( euyAiji/o?, bright-eyed) :— Figure, 45 : occurrence and general characters, 44 : movements, 44 : struc- ture, 45: nutrition, 46: resting stage, 47 : reproduction, 48 : animal or plant ? 180 Euglen'oid. movements, 45 Ev'olution ievolvo, to roll out), organic : definition, 143 : illustration of in connec- tion with species of Zoothamnium (Dia- gram), 144 Excre'tion {excemo, to separate), the separation of waste matters derived from the destructive metabolism of the or- ganism, 16, 281 Exog'enous (ef, out of: ytyrojaai, to come into being), arising from the exterior, e.g. leaves, 422 Exopodite (efw, outside : ttov'?, foot), 323 Exoskereton (efw, outside, and skeleton^ from (TKikkiji to dry), tiie external or skin-skeleton : cuticular, 238, 273 : der- mal, 308, 327, 350, 369 Eye, Crayfish, 342 ; Dogfish, 394 : Eye-spots or Ocel'li :— Medusa, 244 : Polygordius, 296 Eye-Stalks, 321 F Faeces {faex, dregs), solid excrement, consisting of the undigested portions of the food, 16 Ferm'ent {fermenUim^ yeast, fTom/er- veo, to boil or ferment), a substance which induces ferinenta'tion, i.e. a. definite chemical change, in certain sub- stances with which it is brought into contact, without itself undergoing change : unorganized and organized ferments 80 : alcokolic, 76 : ace- tous, 91 : diastatic or amylolytic, 81 ; lactic, 01 : peptonizing or proteolytic, 81 : putrefactive, 91 : ferment cells of Mucor, 168 FERNS : — Figures, 414, 424 : general characters 412 : histology of stem, leaf, ■ and root, 415 : nutrition, 422 : spore- formation, 422 : prothallus and gonads, 425 ; development, 426 ; alternation of generations, 429 Fertiliza'tion {fertilis, _ bearing fruit) ; the process of conjugationof a sperm or sperm-nucleus with an ovum, whereby the latter is rendered capable of develop- ment : a special case of conjugation ig.v,), igg ; details of process, 260 : in Vaucheria, 173 : in Gynmosperms, 458, in Angiosperms, 475 Filtering air, method of, 99 Fins, Dogfish 369 Fiss'ion {Jisszo, a cleaving), simple or binary, the division of a mother- cell into two daughter-cells : in Amoeba, 19 ; Heteromita, 40 : animal- and plant-cells generally, 65 : Paramoeclum, 114 : Vorticella 131 Fission, multiple, the division of a mother-cell into numerous daughter- cells ; — in Heteromita, 42 ; Protomyxa, 51 : Saccharomyces, 74 Fission, process intermediate between simple and multiple, Opalina, 124 Flagellum. See Cilium. Flag'ellate Infusoria, 107 Flagell'ula (diminutive oi flagellum), the flagellate germ of one of the lower organisms (often called zoospores, 51, 54 Flagell'um {fldgelluvi, a whip) : defined; 25 : transition to pseudopod, 52, 22a Floral receptacle, the abbreviated axis of an angiospermous flower 464, 471, 473 Flower, a specially modified cone i^q.v.\ having a shortened axis, which bears perianth-leaves as well as sporophylls, 463 : often applied to the cone of Gymno- sperms, 452 Food-current, Mussel, 359 Food-Vacuole, a tempi-rary space in the protoplasm of a cell containing water and food-particles, 11, 112 Foot : of Mussel, 349 : of phyllula of fern, 427 FORAMINIF'ERA {foramen, ahole :fero to bear), 148 : Figures, 149, 15a, 151 Fragmenta'tion of the nucleus, 120 Fruit of Angiosperms, 476 Func'tion {/undw, a performing), mean- ing of the term, 9 G Gam'ete (yaj^ew, to marry), a conjugating cell, whether of indeterminate or deter- 494 INDEX AND GLOSSARY minate sex : — Heteromlta, 41 ; Mucor, 156 : Spirogyra, 198 :* Vaucheria 173 Gamoblum (ya/j,09, marriage : ^St'of, life), the sexual generation in organisnis ex- hibiting alternation ofgenerations((7.z'.) : progressive subordination of, to agamo- bium in vascular plants, 429, 440, 444, 458, 474 GangUou (ydyy\LOv, a tumour), a swelling on a nerve-cord in which nerve-cells ar>: accumulated, 341 Gastric juice (yau-rqp, the stomach), pro- perties of, 12 Gastric mill, 334 Gastrolith. {yaa-rrjp, stomach : Mdos, stone), 334 .. . Gast'rula (diminutive of yacmjp, the stomach), the diploblastic stage of the animal embryo in which there is a diges- tive cavity with an external opening : characters and Figure of, 295 : contrasted with phyllula, 428 Gemma'tion (^t.v«;«fl, a bud). See Bud- ding. Genera'tion, asexual, See Agamobium. Generation, sexual. See Gamobium. Genera'tions, Alternation of. See Al- ternation of generations. Generative cell, 455 Generative nucleus, 474 Gen'eralized, meaning of term, 140 Ge'nus {^emts, a race), generic name, generic characters, 8, 139 Germ-filter, 99 Ger'minal spot, the nucleolus of the ovum, 257 Germlna'tion {gennmatio, a budding), the sprouting of a spore, zygote, or oosperm to form the adult plant : for germination of the various types see under their names. Gill, an aquatic respiratory organ. 335, 357, 383 Gland {glans, an acorn), an organ of secretion iff-V.) : gland-cells, 228, 278 GlOcMd'ium, larva of Mussel, 365 Gon'ad {yovos, offspring, seed), the essen- tial organ of sexual reproduction, whether of indeterminate or determinate sex, i.e. an organ producing either un- differentiated gametes, ova, or sperms ; see under the various types, and espe- cially 172, 198, 211, 290 Gon'odUCt {gonad and duco, to lead), a tube carrying the ova or sperms from the gonad to the exterior, 292 Grapping-lines, Diphyes, 249 Green gland. See Kidney Growing point : Nitella, 208 : Moss, 403 ; Fern, 418: Gymnosperms, 452 Growth, 13 Guard-cells of stomates, 421 Gullet, the simple food-tube of Infusoria, 47, no : or part of the enteric canal of the higher animals, 277 GYM'NOSPERMS (yu/iAvo?, naked : vT6y, a plant), nutrition, defined, 31 Holozo'ic (ijAo?, whole : ^ioov, an animal), nutrition, defined, 31 Homogen'esis (o/xd? , the same : yeVeo-t? origin), meaning of the term, 102 Homorogous(6ja6Ao7os, agreeing), applied to parts which have had a common origin, 242 Homomorphlsm homomorph'ic (o/xd?, the same : fxoptii-q, form), existing under a single form, 1^9 Host, term applied to the organism upon \y hich a parasite preys, 123 HYDRA (v5pa, a water-serpent) : Figures, 219, 223, 225, 232: occurrence and general characters, 218 : species, 220 : move- ments, 220 : mode of feeding, 221 : micro- scopic structure, 222 : digestion, 229 : asexual, artificial, and sexual reproduc- tion 230 : development, 233 Hydx'antli {v^pa., a water-serpent : a.vQo';, a flower), the nutritive zooid of a hydroid polype, 236 Hydroid (iJBpa, a water-serpent ; eldo^, form) Polypes (ttoAvttous, many footed', compound organisms, the zooids of which have a general resemblance to Hydra, 234 Hyper'tropliy (vir4p, over : rpoi^TJ, nourish- ment), an increase in size beyond the usual limits, iiS Hyph'a (v^aivo}j to weave) applied to the separate filaments of a fungus : they may be mycelial (see mycelium), sub- merged, or aerial : Mucor, 160 : Peni- cillium, 185 Hyp'odermis (bno, under : Sepjxa , skin). Fern, 413, 416 Hypostome (viro, under : a-TOfia, mouth), 320, 236 Insola'tion ijnsolo, to place \\\ the sun), exposure to direct sunlight, 94 Integ'ument {integitmentinn^ a covering) of megaspore : Gymnospenns, 456 : Angiosperms, 466 Inter-cellular spaces, 415 Inter-muscular ploxus (ttAekco, to twine), 285 Internode {inter, between : nodus, a knot), the portion of stem intervening between two nodes, 205 Intersti'tial {intci'stltium^ a space be- tween) cells, Hydra, 224 : growth, Spirogyra, 198 Intestine (intestlnus, internal), part of the enteric canal of the higher animals, 277 IntuS-SUSCep'tion {inins, into : snscipio, to take up), addition of new matter to the interior, 13 Iodine, test for starch, 27 Irritabil'ity {ij-ritabilis, irritable, the property of responding to an external stimulus, 10 J Jaws : Crayfish, 324 : Dogfish, 368, 375 K Karyokines'is (Kapuof, a kernel or nu- cleus : Kt(^(7is, a movement), indirect nuclear division, 67 Katato'oUsm (Kara^oKri, a laying down), iS. See Metabolism, destructive. Kat'astates (KaratrTTJvai, to sink down), 18. See Mesostates, kataholic. Kidney :— Crayfish, 337: Mussel, 359: Dogfish, 396 Immortality, virtual, of lower organisms, 21 Income and expenditure of protoplasm, 18 Individual. See Zooid. Individuation, meaning of the term, 230, 252 IndUS'ium iindusium^ an under-garment, 423 Inflores'cence (^florcsco, to begin to flower), an aggregation of cones or flowers, 454 Infusoria (so called because of their fre- quent occurrence in infusions), 107 Ingesta {ingero, to put into) and Egesta {egero, to expel), balance of, 32 Ingestion {ingero, to put into), the taking in of solid food, ji Labial palps. Mussel, 355 Larva, the free-living young of an animal in which development is accompanied by a metamorphosis, 293 Larval stages, significance of, Polygor- dius, 303 Leaf, structure of: — Nitella, 205, 207 : Moss, 403 : Fern, 420 : limited growth of. 211 Leaflet, Nitella, 207 Leg, Crayfish. 324 Lept'otlirix(Aen-Td?, slender : 5pt^, a hair), filamentous condition of Bacillus, 89 : Figure, 87 Leuc'ocyte (Aev/cds white ; kvto? a hollow vessel, cell), a colourless blo6d corpuscle : — structure of, in various animals (Figures), 57 : ingestion of solid par- INDEX AND GLOSSARY tides by, 58 : fission of, 58 : formation of Plasmodia by, 58 Leuwenhoek, Anthony van, discoverer of Bacteria, 97 Life, origin of. See Eiosenesis. Life-history, meaning of the term, 43 Lignin (//Y«?^;«, wood), composition and properties of, 416 Li^le, 443 Linear agg^regate, an aggregate of cells arranged in a single longitudinal series, 188 Linnaeus, C, introducer of binomial no- menclature, 8, 139 Liver, Dogfish. 382 Lymphatics, Dogfish, 391 M Mad'reporite (from its similarity to a madrepore or stone-coral), 308 Mandihle, 324 Mantle, Mussel, 348 Manub'rium {manub^-iuni^ ^ handle) of Medusa, 239 Matura'tion of ovum, 259 Maxilla, 324 Maxilliped. {maxilla, jaw ; pes, foot), Maximum temperature of amoeboid movements, 21 Medulla or Medullary substance {7;/^"- dulla, marrow): in Infusoria, no: in Gymnosperms, 448 Medullary rays, 449, 452 Medus'a (Me'Souo-a name of one of the Gorgons), the free-swimming reproduc- tive zooid of a hydroid polype, 239 ; derivation of a, from hydranth (Figure), 240 Medus'oid, a reproductive zooid having the form of an imperfect Medusa, Diphyes, 249 Meg'agam'ete (ix4ya<; large : ya/new to marry), a female gamete (^.v.) distin- gui-^hed by its greater size from the male or microgamete, 132 Meg'anucieus (is-eyas large ; muleus, a kernel), in, 128 Meg'asporan^gium {\U-ja.s large : o-Tropa seed : ayyeLOv a vessel), the female sporangium in plants with sexually di- morjphic sporangia, usually distinguished by Its greater size from the male or micro-sporangium : — Salvinia, 440 : Sela- ginella, 41.1 : Gymnosperms, 455 : Angio- sperms, 466. Meg'aspore (/te'yos, large : o-n-opa, a seed), the female spore in plants with sexually dimorphic spores, always distinguished by its large size from the male or micro- spore : — Salvinia, 440 : Selaginella, 441 : Gymnosperms, 455 *■ Angiosperms, 466 Megazo'Oid (fte'yatr. large ; ^wor, animal : eIfio9, form), the larger zooid in unicel- lular organisms with dimorphic zoolds, 35, 132 Mer'iStem (/xeptV-nj/ia, formed from ju-epi'^w, to divide), indifferent tissue of plants from which permanent tissues are differentiated, 418 Mes'entery (/neVos, middle : ei/repoi', in- testine), a membrane connecting the en- teric canal with the body-wallj276 : de- velopment of, 301 Mes'Oderm (jneVoy, middle : Sep^ua, skin), the middle cell-layer of triploblastic animals : Polygordius, 275 : develop- ment of, 296: splitting of toform somatic and splanchnic layers, 299 Mesoglce'a dueVo?, middle : yAota, glue), a transparent layer between the ecto- and endo-derm of Ccel*'nterates : — in Hydra, 222 : in Bougainvillea, 236, 240 Mes'ophyll (/j-eVo?, middle : ^vKkov, a leaf), the parenchyma of leaves, 420 Mes'ostates (jneVo?, middle : a-T-iji/at, to stand), intermediate products formed during metabolism ig.v.) and divisible into {a) anabolic mesostates cr ana- states, products formed during the con- version of food -materials into proto- plasm ; and {b) katabolic mesostates or katastates, products formed during the breaking down of protoplasm, 18 Metabolism {jieTa^oX-q, a change), the entire series of processes connected with the manufacture of protoplasm, and divisible into (a) constructive meta- bolism or anabolism, the processes by which the substances taken as food are converted into protoplasm, and {b) de- structive metabolism or katabolism, the processes by which the protopla>im breaks down into simpler products, ex- cretory or plastic, 17 Met'amere (juera, after : jue'po?, a part), a body-segment in a transversely seg- mented animal such as Polygordius, 268 : development of, 299 : limited num- ber and concrescence of in Crayfish, 320 : in Dogfish, 372, 377, 396 Metamorphosis (/j.eTa,u.dp^tiJtrts), a trans- formation applied to the striking change of form undergone by certain organisms in the course of development after the commencement of free existence : — Vor- ticella, 133 : Polygordius, 298 : Mussel, Microbe (lUtKpb?, small : )3ios, life). See Bacteria MICROCOCCUS (fttKpbs, small : kokkos, a berry) (Figure), 86 Microgam'ete (/xt/cpby, small : ya^e'u, to marry), a male gamete {q.v.\ distin- guished by its smaller size from the female or megagamete, 132 Micro-millimetre, the one- thousandth of INDEX AND GLOSSARY 497 a millimetre, or i-25,oooth of an inch, 84 Micro-organism. See Bacteria. Micronucleus ( fJLiKpo<;, small : nnclens, a kernel), iir, ^78 Micropyle (jLti/cpb?, small : ttvA.!], an en- trance), 457, 466 Micro-sporan'gium(/xtKpbs, small : o-Tropa, a seed : a-y-^etoi', a vessel), the male sporangium m plants with sexuallj' di- morphic sporangia, usually distinguished by its smaller size from the female or mega-sporangium : — Salvinia, 440 : Se- laginella, 441 : Gymnosperms, 455 : An- giosperms, 466 Mic'rospore (ju.tKpb?, small : (mopo., a seed), the male spore in plants with sexually dimorphic spores, always dis- tinguished by its small size from the female or mega-spore : — Salvinia, 440 : Selaginella, 441 : Gymnosperms, 455 : Angiosperms, 466 MicroZO'Oid (/xiKpby, small ; foioi', an ani- mal : et5os, form), the smaller zooid in unicellular organisms with dimorphic zooidSj 35, 132 Midrib of leaf. Moss, 403 Minimimi temperature for amoeboid move- ments, 21 MollUSCa, the, 306 MonOBO'iOllS (ju.oi'o?, single : oiKo?, a house), applied to organisms in which the male and female organs occur in the same individual, 199 Monopod'ial (fiofo?, single : ttous, a foot), applied to branching in which the main axis continues to grow in a straight line and sends off secondary axes to the sides, 138 MONOSTROMA (yadyos, single : o-Tpw/xa, anything spread out), 202 (Figure) Morphol'ogy (juopi^)!}, form : Aoyos a dis- cussion), the department of biology which treats of form and structure, 9 Mor'ula (diminutive of juorian, a mul- berry) See Polyplast, MOSSES : — Figures, 401, 406 : general characters, 402 : structure of stem, 402 : leaf, 403 : rhizoids, 403 : terminal bud, 403 ; reproduction, 404 : development of sporogonium, 405: of leafy plant, 4o3 : alternation of generations, 340 ; nutrition, 408 Mouth, ; — Euglena, 47 ; Paramcecium, 109 : Hydra, 220 : Medusa, 239 : Polygordlus, 768 MUCOR {}}iucor, mould) ;— Figure, 159 : occtirrence and general characters, 158 : mycelium and aerial hyphse, 160 : sporangia and spores, 160 ; transition from uni- to multi-cellular condition, 162 : development of spores, j6^ ; con- jugation, 165 : death, 166 : nutrition, 167 : parasitism, 167 : ferment-cells, 168 Mucous membrane, 58 Multicellular, formed of many cells, 61, 162 Muscle (w?7««^/?/j,a little mouse,a muscle) nature of. 130 Muscle-flbres, Bougainvillea, 236 Muscle-plate, Poiygordius, 273 : develop- ment of, 301 Muscle-process, Hydra, 224, 232 Muscular System, Crayfish, 327: Mussel, 354 Mushroom. See Agaricus. MUSSEL (same root as juiescle). Fresh- water :— Figures, 351, 353, 356, 358, 361, 364 : general characters, 348 : mantle, shell, and foot, 348 : food- current, 359 : enteric canal, 355 : gills, 357: blood-system, 360: muscles, 354: nephrldia, 359 ; gonads, 363 : nervous system, 362 Mycelial hyphee, the hyphse interwoven to form a mycelium. Mycelium {fxvKiqs, a fungus), a more or less felt-like mass formed of interwoven hyphse :— Mucor, 160 : Penicillium, iSc MYCET'OZOA (a^ukt]?, a fungus : ^iov, an animal) : — Figure, 53 : occurrence and general characters. 52 : nutrition, 54 : reproduction and life-history, 54 : animals or plants ? 181 Myomere (/iv?, mouse, muscle : /lepo?, a. part), a muscle-segment, 327 My'ophan (fxv^, mousej muscle : ^aivu), to appear), no Myxomyce'tes (fJiv^a, slime : ynu'/cvj^, a fungus). See Mycetozoa. N Nauplius, embryo of Crayfish, 346 Nem'atocyst {vrftJ-a, a thread : kucttis, a. bag). Figure, 226 Nephrid'iopore (i'6(f)p6?, a kidney : Tropos, a passage), the external opening of a nephridium, 282 Nephrid'ium (vei^pos, a kidney), structure of, Poiygordius, 281 (Figure) : develop- ment of, 301 : Mussel, 359 ; Dogfish, 396 Neph'rOStome (ve^p6?, a kidney : mono., a mouth), the internal or ccElomic aper- ture of a nephridium, 282 Nerve, afferent and efferent, functions of, 286 Nerve-cell, 227, 242 Nervous system, central and peripheral : — Medusa, 243 : Poiygordius. 283 : func- tions of, 287; Starfish, 315: Crayfish, 241 : Mussel, 362 : Dogfish,. 391 Neur'OCOele {vevpov, a nerve ; koiAtj, a hollow), the central cavity of the verte- brate nervous system, 391 NITELL'A {niteo^ to shine) :— Figures, K K INDEX AND GLOSSARY 204, 209, 212, 214: occurrence and general characters, 203 : microscopic structure, 206 : terminal bud, 208 : struc- ture and development of gonads, 206, 21T : development, 216: alternation of generations, 217 NodQ {nodus, a knot), the portion of a stem which gives rise to leaves, 205 Not'ochord (i/wToy, the back : x^P^^j ^ string), 377 Nucel'lus (diminutive of nnclens, the name formerly applied), 456, 466 Nuclear division, indirect : 64 (Figure) : direct, 67 Nuclear membrane, 63 Nuclear sap, 7, 63 Nuclear spindle, 66 Nucle'olus (diminutive of nucleus), 8 Nu'cleUS {nucleus, a kernel), minute struc- ture of, 63 ; Amoiba, 7, 8 : Paramcecium, HI, 114; Opalina, 121: Vorticella, 128 : Nitella, 208, 211 : fragmentation of, 120 Nucleus, secondary, of megaspore, An- giosperms, 474 Nutrient solution, artificial, principles of construction of, 77, Nutrition :— Amoeba (holozoic), 11: Has- matococcus (holophytic), 28 : Hetero- mita (saprophytic), 37 : Opalina (type of internal parasite), 123 : Mucor 167 : Penicillium, igo : Polygordius (type of higher animals), 270, 279: Moss (type of higher plants), 408 Ocellus {ocellus, a little eye), structure and functions of, Medusa, 240, 244 CEsopll'agUS (oio-0(fiaYoy, the gullet). See Gullet. Olfactory organ, Crayfish, 342 : Mussel, 363 : Dogfish, 394 Ommatideum (dim. of ofxfia, eye), 342 OntOg'eny (ovto?, being : -yeVetrt?, origin), the development of the individual ; a recapitulation of phylogeny {q.v.), 146 Oogen'esiS (wrfi', an egg: yeuealpa., a sphere), a name frequently given to the ovum {g.v.) of plants. Oospore (woi', an egg : UTropa, a seed), a name frequently applied to the oosperm {g.v.) of plants, OPALIN'A (from its opalescent appear- ance) : — Figure, 122 ; occurrence and general characters, 121 : structure and division of nuclei, 121 : parasitic nutrition, 123 : reproduction, 124 : means of dispersal, 124: development, 125 Opt'imum {optimus^ best) temperature for amceboid movements, 21 : for sapro- phytic monads, 40 Organ (opyavo;', an instrument), a portion of the body set apart for the performance of a particular function, 2B8 Or'ganism, any living thing, whether animal or plant, 5 Ospliradiujn(6(rii>paiVo)u.a[, to smell), 363 Oss'icle (diminutive of ^i-, a bone), 308 Ov'ary (pvu}7z, an egg), the female gonad or ovum-producing organ ; see under the various types and especially Vaucheria, 172 : atrophy of, in Angiosperms, 474. The name is also incorrectly applied to the venter of the pistil of Angiosperms^ 465 Oviduct {oviun, an egg : d7ico, to lead), a tube conveying the ova from the ovary to the exterior, 292 Ov'um {ovuvz, an egg), the female or megagamete in its highest state of dif- ferentiation : general structure of, 68 : minute structure and maturation of, 256; see also under the various types and ^ especially Vaucheria, 173; formation of, in Angiosperms, 474 Ov'ule (diminutive of ovnin), the name usually applied to the mega■^porangium {g.v.) of Phanerogams. Oxidation of protoplasm, 15 OXYTRICH'A (ofus, sharp : 0pt|, a hair), 120 (Figure) Pallium See Mantle. Palp {palpo, to stroke), Crayfish, 325 : Mussel, 355 Pancreas (TravKpe'a?, sweetbread), 382 PANDORINA (Figure), 262 Param'ylum (n-apd, beside ; afj-vKov. fine meal, starch), 46 PARAMCE'OIUM :— Figures, 108, 115 : structure, 107: mode of feeding, 112; asexual reproduction, 114 : conjugation, 114 Par'asite, parasitism (n-apao-Zro?, one who lives at another's table) : — Bacteria, 92 ; Opalina, 123 : Mucor, 167 Paren'cnyma ( Trape'-yxu/J-a, anything poured in beside, a word originally used to describe the substance of the lungs, liver, and other soft internal organs), applied to the cells of plants the lengthof INDEX AND GLOSSARY 499 which does not greatly exceed their breadth and which have soft non-Hgni- fied walls, 60 ". ground -parenchyma, 413, 415 Pari'etal {paries, a wall), applied to the layer of coslomic epithelium lining the body-wall, 274 Parthenogen'esis (TrapeeVos, a virgin : ■yeVecrt?, origin^, development from an unfertilized ovum or other female gamete, 200 Parthenogenet'ic ova, characteristics of, 259 Pasteur, Louis, researches on yeast, 76 Pasteur's solution, composition of, 76 Pectoral Arch and Fin, 378 Pedal {.pes^ the foot) ganglion, Mussel, Pedlcellaria, 308 Pelvic Arch and Fin, 379 PENICILL'IUM (pefiicillum, a painter's brush, from the form of the fully-deve- loped aerial hyphae) : — Figure, 186: oc- currence and general characters, 184 ; mode of growth, 185 : microscopic structure, 185 : formation and germina- tion of spores, 189: sexual reproduction, 190: nutrition, igo: vitality of spores, 191 Peps'in (ttcVtw, to digest), the proteolytic or pepsonizing ferment of the gastric juice, 12, 80 Peptones, 12 Perianth {n^pC, around : ai/^o?, a flower), the proximal infertile leaves of a flower, 468, 471 Pericardial Sinus, Crayfish, 337 Pericardium (TTcpi, around: xapSta, heart), Mussel. 355 : Dogfish, 372 Pericycle {irepi, around : kvkKos, a circle), 418, 463 PeriSperm (Trepi, around : airipixa^ seed), nutrient tissue developed in the nacellus of the seed, 476 Peristom'e (n-ept, around : UTOfxa, the mouth), Vorticella, 128 Peristom'ium (irepC, around : o-TOfLtov, a little mouth), the mouth-bearing segment of worms, 268 Peritone'um (-rrepLTovaLOv), the membrane covering the viscera, 372 Pet'alS (niraXoi', a leaf), the inner or dis- tal perianth leaves in the flower of Angiosperms, 464 Phar'ynx ((f)apuyf, the throat) :— Poly- gordius, 277 ; Dogfish, 381 Phloem (<^Xotd?, bark or bast), the outer portion of a vascular bundle, 417 Phyla ((^uAoy, a tribe) of the animal king- dom, 304 : of the vegetable kingdom, 432 Phyll'ula (diminutive of (^u'AAoi-, a leaf), the stage in the embryo of vascular plants at which the first leaf and root have appeared, 360 : contrasted with gastrula, 428 Phylog'eny {^v\ov, a race : ydvea-K, origin), the development of the race, 147 Physiol'Dgy {(^vtni;, the nature or property of a thing : \6yos, a -discussion), the de- partment of biology which treats of function, g et seq. Pigment-spot, Euglena, 47 Pileus {pileus, a cap), Agaricus, igi Pinnule (dim. oi pinna, a feather), of leaf, 420 Pistil (pistilhun, a pestle, from pinso, to pound.) See Gyncecium. Pith. See Medulla. Placoid scale, 369 Plan 'Ula (diminutive of TrAdii'O?, a wander- ing about), the mouthless diploblastic larva of a hydroid, 246 Plant, definition of, 179 Plants, classification of, 434 Plas'ma (TrAao-jita, anything moulded), of blood, 56 Plasmo'dium(7rAacrju.a,anything moulded), 52-55 : comparison of with zygote, 54 Plastic (TrAacTTiKo?, formed by moulding) products, products of katabolism which remain an in tegral part of the organism, 33 Pleopod, 321 Pleurobranchia (TrAevpoi', side ; ^p6.yxia, gills)! 337 Pleuron (TrAeupdr, side), 320 PodobrancMa (ttous foot, ^payx"^ gill), 335 Pod'omere (ttovs, a foot : /xe'pos, a part), a limb-segment, 321 . Polar cells, formation of, 259 Pollen grain {pollen, fine flour), a name given to the microspore (g.v. ) of Phanero- grams. PoUen-sac, a name given to the mlcrospo- rangium ig.v.) of Phanerogams, Pollen-tube, 458, 475 PoUina'tion, 4=8, 475 POLYGORD'IUS (ttoAu's, many: TdpSioy, King of Phrygia, inventor of the Gordian knot) : — Figures, z6g, 271, 282, 284, 291, 293, 297, 300 : occurrence and general characters, 268 : metameric segmenta- tion, 270: mode of feeding, 270; enteric canal, 270, 277: cell-layers, 273: ccelome, 270 ;■ distribution of food, 278 : blood-system, 279 : nephridia, 281 : nervous system, 283 : differentiation of definite organs and tissu'^s, 288: repro- duction, 290: development and meta- morphosis, 292 Polymorph'ism (ttoAvs, many : /nop«|)^, form), existing under many forms, 249 Poryplast(7roAvs, many: TrAacTTo?, formed, modelled), the multicellular stage of the embryo before the differentiation of cell- layers or organs : — Hydroids, 246 : Star- fish, 316 : Moss, 405 : Fern, 427 PORPITA(7r6pTr77,a brooch), 249; Figure, 251 500 INDEX AND GLOSSARY Primor'dial utricle, 196, 207 ProctodaB'Tim (ttp^ktos, the anus : oSato?, belonging to a way), and ectodermal pouch which unites with the enteron and forms the posterior end of the enteric canal, its external aperture being the permanent anus, 296 Pro-nucleus, female, 259, 459 ; viale, 260, 459 ; conjugation of male and fe- male, 260, 459 Prostom'ium(7rpd, before : o-ro^toi', a little mouth), the first or pre-oral segment in worm';, &c., 268, 293 PROT'AMCEBA (n-pwro?, first: i/ioi/Sos, changing), 9 (Figure). Prothal'lUS (rrpd, before : 6aK\6s, a twig), the gamobiumof vascular plants : — Fern, 425 : dimorphism of in Equiseium, 438 : reduclion of in Salviiiia, 440, 442 ; Selaginella, 444, and Gymnosperms, 458 ; retarded development of in Angicsperms, 476 Prothallus, secondary, Selaginella, 445 Prot'eidS (TrpwTO?, first), composition of, 5 Protist'a (TrpcoTtcrTo?, the first of all), the lowest organisms, intermediate between the lowest undoubted animals and plants, 182 ProtOCOC'CUS(7rpwTO?, first: KOKKog, berry). See Hsematococcus. PROTOMYX'A (TrpwTo?, first : /jlv^ol, mucus) : Figure, 50 : occurrence and general characters, 49 : life-history, 51 : animal or plant? 181 Protoiieni'a(7rp(I)TO?, first: vrjfia, a thread), iVIoss, 404, 408 Prot'Oplasm (Trpiro?, first ; TrAatTiLta, any- thing moulded), composition of, 5 : pro- perties of, 5 : micro-chemical tests for, 7 : minute structure of, 62 : continuity of in Fern, 418 : in Polygordius, 289 : intra- and extra-capsular, Radiolaria, 152 Protopodite (Trpiuro?, first: n-ov? foot), 323 Protozoa, the, 305 Prox'imal {proximns^ nearest), the end nearest the point of attachment or or- ganic base, e.g. in the stalk of Vorticella, 126 Pseud'opod (t/feuS7j?, false : ttou?, foot), described, 4 : comparison of with cilium, 34, 52: in columnar epithelium, 59: in endoderm cells of Hydra, 228 Pteris. See Ferns. Punctum vegetationis. See Growing point. Putrefac'tlon {putrefado, to make rotten) nature of, 82 : a process of fermentation, gi : conditions of temperature, moisture, &c., 93 Putres'cent {putresco, to grow rotten) solution, characters of, 37. 82 Putres'cible infusion, sterilization of, 99 Pyloric division. See Stomach. Pyron'oid (7rupi]V, the stone of stone-fruit : eISo5 form), a small mass of proteid material invested by starch, 27 R Radial symmetry, starfish, 306 RADIOLARIA {radius, a spoke or ray): — Figures, 152, 153; occurrence and general characters, 152 : central capsule, 152 : intra- and extra-capsular pro- toplasm, 152 : silicious skeleton, 152 : symbiotic relations with Zooxanthella, 154 Reot'um (intestinum rectum, the straiglit gut), the posterior or anal division of the enteric canal, 278 Redi, Francisco (Italian savant), experi' ments on biogenesis, 97 Reducing division, 256, 260 Reflex action, 286 Reproduction, necessity for, ig Reproductive organ. See Gonad. Reservoir of contractile vacuole, Euglena, 47 Respiration : — Amceba, 17 : Po!ygordius. 280 Respiratory casca. Starfish, 308 Rhiz'oid(pt^a, root :elSo?, form): — Nitella, 205, 2n : Moss, 403 : prothallus of Fern, 423. 430 Root, Fern, 421, 427 ; Gymnosperms, 448 Root-cap, 422 Root-hairs, 421 Ross, Alexander, on abiogenetic origin of mice, insects, &c., g6 Rostrum {rps/nnH, beak), 321 Rotation of protoplasm, 207 Rudiment, m^imQUtaxj {rndimottusn, a beginning), the early stage of a part or organ : often used for a structure which has undergone partial atrophy, but in such cases the word vestige {q. v.) is more suitable. SACCHAROMY'CES {(raKxa-pov, sugar : /i.vK>js, fungus): — Figure, 72 : occurrence, 71 : structure, 71 : budding, 73: in- ternal fission, 74 : nutrition, 75 : alco- holic fermentation caused by, 75 : experi- ments on nutrition of, 78, 80 : animal or plant? 182 SALVIN'IA : — Figures, 439, 441 : general characters, 438 : mega- and micro-spor- angia and spores, 440 ; male and female prothalli and gonads, 4.;2 ; development and alternation of generations, 442 SaprOpliyt'iC (o-an-pd?, putrid : ^vtqv, a plant) nutrition, defined, 39 Schulze's solution, test for cellulose, 28 : for iignin, 416 INDEX AND GLOSSARY SOI Sclereu'Cliyma (o-KAijpds, hard : eyxvixa infusion) : — Moss, 402 : Fern, 413, 416 Scyllium. See Dogfish. Secre'tion {secreUis, separate), nature of, 227 : formation of cell-wall a process of, 14 Seed, formation and germination of, 459, 476 Seg'ment {segvienium. a piece cut off), in plants a node together with the next proximal internode, 205 : in animals the name is variously applied. See Meta- mere, Podomere. Segment'al cell: Nitella, 210: Moss, 403 : Fern, 418 Segmentation, metameric. See Meta- mere. SELAGINELL'A (creAaye'w, to shine):— Figures, 443, 445 ; general characters, 442 : cone, sporangia, and spores, 444 : prothalli and gonads, 444 : development and alternation of generations, 446 Self-fertilization, applied to the sexual process when the gametes spring from the same individual, 199 Sep'als {separ, separate), the outer or proximal perianth-leaves in the flower of Angiosperms, 464, 468 Sep'tum {septum, a barrier) : — In Peni- cillium, 187 : in Polygordius, 277 ; development of, 301 Set'a {seta, a bristle), 287 Sex-cells, primitive, 253 : origin of in Hydroids, 245: in Polygordius, 290 Sexual differentiation, illustrated by Vaucheria, 172 : by Spirogyra, 1^9 Sexual generation. See Gamobium. Sexual reproduction, nature of, 42 Shell, Mussel, 350 Shoot, in plants, an axis of the second or any higher order with its leaves, 206 Sieve-tubes and plates, 418 Sinus {sinnSy a hollow) in Crayfish, a spacious cavity containing blood, 338 Sinus venosus, 384 Siphons, in- and ex-halant, 349 Skeleton. See Endo- and Exo-skeleton. Skull, Dogfish, 374 Slime-fungl, See. Mycetozoa. Solid aggregate, 203 Somat'iC (o-w/ia, the body), applied to the layer of mesoderm which is in contact with the ectoderm and with it forms the body-wall, 275 Sor'us (trajpos, a heap), an aggregation of sporangia, 422, 440 Species {species, a kind), meaning of term illustrated, 8, 137; definition of, 139: origin of, 141 Specific characters, specific name, 8, 139 Specialized, meaning of, 140 Sperm (a-irdpfxa, seed), the male or micro- gamete in its highest stage of differentia- tion : structure and development of, 255 : see also under the various types, and especially Vaucheria, 173 Spermatozo'id, spermatozo'on (o-Trepjua, seed : ^wov, animal, from the actively moving sperms of animals having been supposed to be parasites), synonyms of sperm. Spermary (o-jrep/xa, seed), the male gonad or sperm-producing organ ; see under the various types, and especially Vaucheria, 172 Sperm'iduct (a-irepfxa, seed : dttco, to lead), a tube conveying the sperm from the spermary to the exterior, 292 Spermatogen'esiS {