CORNELL UNIVERSITY LIBRARY FROM RETURN TO ALBERT R. MANN LIBRARY ITHACA, N. Y. Cornell University Library QP 34.C29P 1854 The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003139924 PRINCIPLES OP COMPARATIVE PHYSIOLOGY PRINCIPLES COMPARATIVE PHYSIOLOGY, WILLIAM B, iAEPENTEE, M.D., F.K.S., F.G.S. IIXAMINER IN PHYSrOLOQT AND COHPAHATITE ANATOMY IN THE UNrVBRSITT OF LONDON J PnOFESBOB OF MEDICAL JURISPRUDENCE IN UNITERSITT COLLEGE; PRESIDENT OF THE HICRO3C0FICAL SOCIETY OF LONDON, ETC. ETC. Mit^ i^m liankeir Maaii (Kjtgra&ittgs. Jpourtf) ^Ottton, LONDON : JOHN CHURCHILL, NEW BURLINGTON STREET. (Established in Princes Street, Soho, 1784.) MDCCCLIV. SIR JOHN F. W. HERSCHEL, BART. K.H. P.K.8. L. AND K. ETC. THIS VOLUME IS MOST BESPECTFULLY DEDICATED, AS A TBIBUTE DUE ALIKE TO HIS HIGH SCIENTIFIC ATTAINMENTS, AND MOEAL WOETH, AUD AS AN EXPRESSION OF GRATITUDE FOE THE BENEFIT DERIVED FROM HIS " DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY," BY THE AUTHOR. PEEFACE. SWENOE IS THE KNOWIBDOB 01' MAST, OKDEELY AND MBTHODIOALLY DIGESTED AHD AKRANOED, SO AS TO BECOME ATTAINABLE BY oHB." — Sir John F. W. Herschel. The success of the Third Edition of the " Principles of Physiology, General and Comparative," — as evinced, not merely by its rapid sale, but by the numerous expressions of high appreciation which it drew forth from those most competent to judge of its merits, — ^has encouraged the Author to carry into effect a change of plan which had suggested itself to him during its preparation. For having been led-on by the desire of rendering his work aa complete as possible, to enlarge it to the utmost admissible dimensions of a single volume, he felt that it would be impossible to do justice to any subsequent extensions which its subject might receive, without making some alteration in its form. And this conclusion acquired a greatly-increased force, when the demand for a new Edition led him to survey the deficiencies, which, notwithstanding all his care, had been left in the former one ; and to estimate the amount of new matter, not only deserving but requiring notice, which the diligence of observers in various departments of this comprehensive Science had accumulated in the short interval. Instead of dividing the entire Treatise into two Volumes, however, as suggested to him by many of his friends, the Author has preferred to divide its subjects, and to treat of them separately though connectedly. The present Volume, therefore, consists of the " Comparative Phy- siology" of the last Edition, extended from 530 pages to 744, and with 300 illustrations instead of 130. The First Chapter (the Eighth of the former Edition) has been entirdy re-written, with the view of bringing into greater prominence the general doctrine of Progress from the General to the Special; the enunciation of which by Von Baer (nearly thirty years ago) appears to the Author to mark a most important era in the Philosophy of Physiology, although it is far from having received the notice which it deserved from various subsequent writers who have followed iu the same track. The priDcipal additions and alterations in the succeeding Chapters (as to the general arrangement of which no change has been found necessary) occur in the part of Chapter VI. which relates to the Water- Vascular System, and in the part of Chapter XI. which treats of the SexuaKty of the Cryptogamia; in regard to which latter point the Author would observe, that the course of recent discovery has fully borne-out the anticipations he expressed in the former edition. The whole work, however, has been most care- fully revised; and the Author ventures to think that the present Edition more completely represents the state of the Science at the period of its publication, than any of its predecessors have done. He can honestly say that he has spared no time or labour ia its preparation, which it has been in his power to bestow. And he looks with hope, therefore, to a continuance of that friendly indulgence with regard to errors and short- • comings, which has been so liberally afforded on previous occasions. As to certain points on which his opinions have undergone modification, he can again refer with satisfaction to the following passage in the Preface to his former editions: — "Truth is his only object; and, even if his own doctrines should be overthrown by more extended researches, he will rejoice in their demolition, as he would in that of any other error. The character of the true philosopher as described by Schiller, — one wlw has always loved trvth better than his system, — will ever, he trusts, be the goal of his intellectual ambition." In attempting to embody in a Systematic Treatise the general aspect of Physiology or any other Science of like comprehensiveness, it will be obvious that an Author, however extensive his own range of acquire- ment, must largely avail himself of the labours of others; and that the scientific character of such a treatise must depend, not so much on the amount of original matter it may contain, as on the degree in which " the knowledge of many" has been " orderly and methodically digested and arranged, so as to become attaiuable by one." It is by this standard that the Author desires his work to be tried; and he cheerfully leaves the verdict to the judgment of those, who are qualified by their own knowledge of the subject to pronounce it. He feels it due to himself however, to state that he has devoted considerable time and attention to the verification of the statements of other observers, especially on points under dispute,— a kind of labour which is but little appreciated by those, who contemptuously designate works like the present as " mere compilations;" and that a large amount of materials, di-awn from his PREFACE. ix own original enquiries, is scattered through the work. It would have been easy for him to bring these last into greater prominence, had he been so disposed; but as his constant aim has been, to work-out his general plan harmoniously and methodically, rather than to force any one portion of it into undue prominence, he has generally preferred to allow his own contributions to pass undistinguished, rather than to be continually obtruding his personal claims upon the attention of his readers. He would remark, moreover, that originality may be as much shown in the development of new relations between facts and phenomena observed by others, as in the first discovery of such facts ; and he believes that by the mode in which he has combined and arranged his materials, he has fsequently been enabled to impart a new and unexpected valu.e to statements, which, in their previously isolated condition, were of comparatively insignificant import. Although, in the selection of these materials, the Author has endea- voured to avail himself of the best and most recent information he could procure upon each department of the subject, it is scarcely to be expected that he shoidd be equally well-informed upon every point; and those who have followed particular departments into detail, will doubtless find scope for criticism in what they may regard as deficiencies, or even as errors. Here, again, the Author must beg that his work may be estimated by its general merits; and rather by what it does, than by what it does not contain. It would have been far easier to expand it by mere compilation to twice its present dimensions, than it has been found to compress the accumidated mass within the space which it even now occupies. It has been the Author's endeavour, wherever practicable, to draw the materials, both for his text and for its illustrations, direct from original Treatises and Monographs; and thus to avoid the errors which too frequently arise from second-hand transmission. To have attempted, however, to assign each individual fact to its original discoverer, each doctrine to its first enunciator, would have augmented the bulk of the volume, far beyond the dimensions appropriate to a Text-Book; and while most desirous to avoid taking credit for what is not his own, the Author has felt himself compelled to limit his references, for the most part, to those rtew facts and doctrines, which cannot be yet said to have become part of the common stock of Physiological Science. The number of such references has been largely increased in the present edition ; and the " Index of Authors" which has been added, will, it is hoped, be found nseful in enabling the reader at once to turn to the notice of any original observation that he may desire to retrace. The Illustrations not his own, which likewise have received numerous important additions, are referred to their originals in the list at the commencement of the Volume; and this list will also afford useful assistance to those, who may desire to carry-out their enquiries in any particular direction. The Author cannot bring his task to a conclusion, without expressing the great obligations under which he lies to his friend Mr. T. H. Huxley, not only for many valuable suggestions, but also for the readiness which he has on all occasions evinced, to impart to him whatever he might seek from his own extensive stores of original and acquired information; nor without paying his tribute of regard to the memory of his lamented friend Mr. G. Newport, whose premature death has deprived British Science of one of its most ardent and disinterested votaries, at a time when he was beginning to reap, in the appreciation of Ms discoveries on the Impregnation of the Amphibia,* the credit so justly due to his laborious, accurate, and sagacious researches, in the new field to the cultivation of which he had latterly applied himself. It is the Author's intention to reproduce the " General Physiology" of his former Edition, as a companion-volume to the present, so soon as the numerous demands upon his time may permit him to bestow upon that part of his revision the careful attention which it requires. University Hall, London, June 1, 1854. In a Postscript to the work referred-to in the note to p. 536, written ahnost contem- poraneously with Mr. Newport's decease, Prof. Bischoff states that he has himself con- firmed Mr. N.'s observation of the penetration of the Spermatozoon into the ovum of the Frog, and gives him full credit for the determination of this important fact TABLE OF CONTENTS. CHAPTER I. ON THE GENERAL PLAN OF ORGANIC STEUCTUBE AND DEVELOPMENT. 1. Analysis and Comparison of Phenomena afforded by Organic Structure :— Homology and Analogy . . . . . 1 2. Conformity of Structure of each group to General Design or Archetype :— Progress from General to Special in its Tarious modifications . 10 3. Diversities in Grade of Development . . . 17 4. General Survey of Vegetable Kingdom. Protophyta . 22 Thallogens (Algae, Lichens, Fungi) , 23 Acrogens (Hepaticse, Mosses, Perns) . . . ,28 Phanerogamia . , 33 5. General Survey of Animal Kingdom. Protozoa (Porifera, Khizopoda, Infusoria) . 39 Radiata (Polypifera, Acalephse, Echinodermata) . . 41 Molluaca (Bryozoa, Tunicata, Braohiopoda, Lamellibranchiata, Gasteropoda, Pteropoda, Cephalopoda) . . .50 Articulata (Entozoa, Annelida, Myriapoda, Insects, Crustacea, Arachnida) . . 59 Yertebrata (Fishes, EeptUes, Birds, Mammals) . 71 6. Progress from General to Special in Development , 95 Rudimentary Organs . . . 101 Monstrosities . . . .105 7. Geological Succession of Organic Life . 107 CHAPTER II. GENERAL VIEW OP THE VITAL OPERATIONS OP LIVING BEINGS, AND OF THEIR MUTUAL RELATIONS. 1. Analysis and Classification of Phenomena presented by Vital Action 121 2. Mutual Relations of Organic and Animal Functions . . . 123 3. Organic Functions separately considered . .... 125 4. Animal Functions separately considered ... ,128 5. Progress from General to Special in Function 129 XU CONTENTS. CHAPTER III. OF AlilMENT, ITS INGESTION, AND PEBPAHATION . PAGE 1. Sources of Demand for Aliment . .... .132 2. Nature of the Alimentary Materials 139 3. Ingestion and Preparation of Aliment in Plants . . . . 151 4. Ingestion and Preparation of Aliment in Animals . 153 163 153 157 168 162 164 Agastric Animals Unicellular Animals . Polystome Animals . Oral Apparatus Prehensile Appendages Reducing Apparatus Digestive Apparatus . . 169 Nature of Digestive Process 183 CHAPTER IV. OP ABSORPTION AND IMBIBITION. 1. General Considerations ... . ... 186 2. Absorption in Vegetables . . .193 3. Absorption in Animals . . . . .199 CHAPTER V. OP THE OIRCUIATION OP NUTRITIYE FLUID. 1. General Considerations ..... ... 212 213 221 225 227 229 2. Circulation in Vegetables .... 3. Circulation in Animals Absence of Special Circulation in Certain Classes Circulation in Radiata: — Ecbinodermata Circulation in ArticvZata: — Annelida . Myriapoda . 236 Insects . . . . 237 Arachnida . 239 Crustacea . . 242 Circulation in Mollusca : — Bryozoa . . . 245 Tunicata . 246 BracMopoda 249 LameUibrauchiata 250 Gasteropoda 251 Cephalopoda 253 Cu'cnlation in Vertehrata: — Pishes . 266 Reptiles . . 258 Birds Mammals . ... Forces which move the Blood . . 264 Development of Circulating Apparatus . . , 268 Malformations of Circulating Apparatus 27' 263 263 CONTENTS. CHAPTER VI. OP EESPIEATIOK. 1. General Considerations PAGE . 280 2. Respiration in Plants 283 3. Bespiration in Animals . . . . . 290 Aquatic Respiration ; — Protozoa, Zoophytes, and Acalephse . 294 Eoliinodermata 296 Water- Vascular System ; — Rotifera . 297 Eutozoa 298 Branchial Respiration ; — Bryozoa 300 Tunicata 301 Brachiopoda . 304 Lamellibranohiata . 305 Gasteropoda . 306 Cephalopoda . . 307 Annelida 308 Crustacea 310 Pishes . 312 Batraohia 316 Atmospheric Respiration : — Myriapoda . 317 Insects 318 Arachnida 322 Pishes (air-bladder) . 323 Perennibranchiata 325 Reptiles 325 Birds 327 Mammals 330 Development of Respiratory Apparatus 332 Alterations effected by Respiratory Process . 333 CHAPTER VII. or THE EXHALATION OF AQUEOUS VAPOUR. 1. General Considerations . . 339 2. Exhalation in Plants . 339 3. Exhalation in Animals . . 346 CHAPTER VIII. OP NUTEITION. 1. General Considerations . . . . . 361 Term of Duration of Indiyidual Parts . 352 Assimilation and Formation ... ... 356 2. Nutrition in Vegetables . . . ■ • 360 Growth and Multiplication of Cells . . . 360 Assimilating Process in Vascular Plants . . . 369 Production of Vegetable Organic Compounds . . . . 372 CONTENTS. Nutrition in Animals ...... Asaimilation of Nutritive Materials .... Chyle and Lymph . ... Vascular Glands ...... Composition and Properties of Blood of Vertebrata NutritiTe Fluid of Zoophytes Echinodermata . Articulata MoUusca . Growth and Multiplication of Cells Production of Animal Organic Compounds Conditions of Nutritive Activity in Animals PAGE 379 379 381 384 392 393 395 396 401 405 CHAPTER IX. OP SECRETION AND EXCRETION. 1. General Considerations .... 2. Secretion in Vegetables 3. Secretion in Animals Structure of Glands in general The Liver, and the Secretion of Bile Biliary Apparatus of Invertebrata Vertebrata . Development of Liver Properties and uses of Bile Of the Kidneys and the Urinary Excretion Urinary Apparatus of Invertebrata Vertebrata Development of Kidney Composition and Properties of Urine Cutaneous and Intestinal Secretions Special Secretions Metastasis of Secretion 407 409 410 411 417 417 421 424 425 429 429 429 432 434 438 438 439 CHAPTER X. EVOLUTION OP LIGHT, HEAT, AND ELECTRICITY. 1. General Considerations . 441 2. Evolution of Light ... . 442 Evolution of Light in Vegetables .... . 442 Evolution of Light in Animals . 443 Luminosity of the Sea . . . . 443 Luminous Insects . 446 CONTENTS. 3. E solution of Heat Evolution of Heat in Vegetables . Evolution of Heat in Animals Cold-blooded Animals Insects Warm-blooded Animals Conditions of Evolution of Heat . i. Evolution of Electricity Evolution of Electricity in Vegetables Evolution of Electricity in Animals Electric Fishes . PAGE 449 450 452 452 454 457 460 461 462 463 467 CHAPTER XI. OP aENEBATION AND DEVELOPMENT. 1. 0«neral Considerations Developmental and Eegenerating Power Multiplication by Gemmation True Generative Process Alternation (so-called) of Generations 2. Generation and Development in Plants Multiplication of Phytoids . Generation and Development of Protophyta Characese Lichens Fungi . Hepaticse and Mosses Ferns . Lyoopodiacese MarsUeaceffi. Gymnospermeffi Angiospermous Phanerogamia Generation and Development in Animals Multiplication of Zooids Development and Actions of Spermatozoa Development and Structure of Ova Fecundation of Ova, and subsequent Changes Generation and Development of Protozoa Infusoria Porifera ; Generation and Development of Radiata Polypifera Compound Hydroida Acalephffi Echinodermata 473 476 480 481 482 483 485 486 491 495 497 499 602 505 611 612 513 614 615 628 628 529 634 636 639 639 543 544 544 649 553 661 CONTENTS. (Tpneration and Develcipment oi Molliisca Bryozoa Tunicata Braobiopoda . Lamellibranchiata . Gasteropoda Cephalopoda C-teneration and Development of Artieiilata Entozoa Rotifera Annelida Myriapoda Insects . Crustacea Cirrhipeda Arachnida Generative Apparatus of Vertebrata Fishes Reptiles Birds . Mammals Embryonic Development of Vertebrata Area Germinativa Formation of Amnion Development of AUantois Formation of Placenta Conditions determining Sex . Lactation : — Composition of Milk On the Laws of the Exercise of the Reproductive Function Species and Varieties . Hybridity Modifying influence of External Conditions Origination of New Varieties Transmission of Acquired Peculiarities . PAGE 668 568 570 574 574 576 580 585 685 690 591 595 596 602 605 607 609 610 611 612 615 619 ~ 620 622 625 626 629 630 632 632 634 634 637 CHAPTER XII. OP THE SENSIBLE MOTIONS OP LIVING BEIN&S. 1. General Considerations 2. Motions of Plants 3. Motions of Animals . 640 642 645 CHAPTBU XIII. OP THE PUNOTIONS OP THE NEKVOUS SYSTEM. 1. General Considerations 2. Comparative View of the Nervous System in the Animal Series No Evidence of Nervous System in Zoophytes 648 651 651 CONTENTS. xvii PAGE Nervous System of Radiata .... . 653 Acalephse . 653 Ecliinoderinata . 653 Nervous System of Mollusca . 654 Bryozoa . . 655 Tunicata . 655 BracMopoda 656 Lamellibrancliiata . 656 Grasteropoda . . 658 Cephalopoda . 660 Nervous System of Articulata . 663 Eutozoa . . 670 Amielida . 670 Myriapoda 670 Insects . 670 Crustacea . 672 Arachnida . 673 Nervous System of Yertebrata 675 Fishes . 677 Reptiles . . 681 Birds . 681 Mammalia 682 History of Development of Brain . 679—684 Functions of the Cranio-Spinal Axis . . 685 Spinal Cord . 687 Medulla Oblongata . 689 Sensory Ganglia . . 690 Functions of the Cerebellum 696 Functions of the Cerebrum . 698 Functions of the Sympathetic System . 703 General Summary . 704 CHAPTER XIV. op SENSATION AND THE ORGANS OP THE SENSES. 1. Of Sensation in General ... ... 2. Of the Sense of Touch, and its Instruments 3. Of the Sense of Taste, and its Instruments . 4. Of the Sense of Smell, and its Instruments 5. Of the Sense of Hearing, and its Instruments 6. Of the Sense of Sight, and its Instruments .... 709 712 716 716 718 725 CHAPTER XV. OF THE PBOBTJCTION OT SOUNDS BY ANIMALS 738 LIST OF ILLUSTEATIONS. FIG. 1. 2. 3. 4. 5. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. Pterodactyl'm crassvrostris (xxti.) Different forms of Anterior Member . Diagram illustrating the Nature of Limbs (lxit.) Gboup of J»aS8 .—a, posterior view of the oeoipital vertebra of ZetnSsirm mineetens .— B, posterior view of the occipital vertebra of Av^hmma didaxti/lum, : —0 ooaterior view of the pelvic vertebra of Zepidoairen .— d, postenor view of the pelvic ver- tebra of P»-o«»s OMommis. In the several diagrams, the foUowing references indicate crare- snonding parts; c, oentram; m, nenrapophyses ; «, nenral spine ; p«, 61, pleurapophysiB of the occipitaWertebra, or scapula; ft, 52, hsmapophysiB of the occipital vertebra, or coracoid bone • a. 53-67, diverging appendages of the occipital vertebra, or antenor hmbs j i)i, 62, pleu- rapophyiis of tie pel™ lertebra, or iliac bone ; \, 63, htemapophysis of the pelvic vertebra, or iscUic bone ; a, 65-69, diverging appendages of the pelvic vertSira, or postenor hmbs. composition between the jaws and the true legs of the Crustacea, is shown by the transitional gradations presented by the feet-jaws. And turmng to the Vegetable kingdom, we find the mutual relations of the parts of the flower, and their homology with the leaves, to be indicated by those cases in which' there is a gradational passage from the leaf to the bract, from the bract to the sepal, from the sepal to the petal, from the petal to the stamen, and from the foliaceous type to the carpel (§ 30).— But it is most certain that, of aU the means of discovering the structural relations of organs the study of t^eii development is most important; smce this, if carefully pursued, will probably never faU to clear up whatever doubts may be left by other modes of investigation. It is in this manner that the true solution has been at last attained, of many of the most difficult and most controverted questions in the science;— questions which have reference, not merely to the nature of particular organs, but to the relations subsisting between different groups of living beings. And it is in this path, therefore, that the Philosophic Naturalist can press forward with the most assured prospect of success, in the search for that germ-alplmi of Organisation, which it is his highest object to discover 11 Thus we are led by the study of Morphology (that is, by the recomition of ' homologous ' organs, under whatever forms they may pre- sent), to the perception of that great general truth, which is, perhaps, the highest yet attained in the science of Organisation, and which is even yet 10 GENERAL PLAN OP ORGANIC STRUCTUKE AND DEVELOPMENT. fai- from being fully developed; that in the sevei-al tribes of organised beiQgs, we have TWt a mere aggregation of individuals, each formed upon an independent model, and presenting a type of structure peculiar to itself; but that we may trace throughout each assemblage a conformUy to a general plcm, which may be expressed in an 'archetype' or ideal model,* and of which every modification has reference either to the peculiar conditions under which the race is destined to exist, or to its relations to other beings. Of these special modifications, again, the most important themselves present a conformity to a plan of less generality; those next in order to a plan of stUl more limited extent : and so on, until we reach those which are peculiar to the individual itself This, in fact, is the philosophic expression of the whole science of Classification. For, to take the Vertebrate series as our illustration, we find that Fishes, ReptUes, Birds, and Mammals agree in certain leading features of their structure, which constitute them vertebrated animals ; but this structure is displayed under diversified aspects in these classes respectively, which constitute their distinctive attributes. Thus, of the general vertebrated type, the Fish presents one set of special modifications, adapting it to its peculiar mode of Ufe; the ReptUe, another; the Bird, a third; the Mammal, a fourth. So again, in each of these classes, we find its general type pre- senting subordinate modifications in the respective orders; thus, for example, the EeptUian type exhibits itself under the diverse aspects of the Frog, the Snake, the Lizard, and the Turtle; the Mammalian, under those of the Whale and the Bat, the Sloth and the Deer, the Elephant and the Tiger, the Kangaroo and the Monkey, the Ornithorhyncus and Man. Each order, again, is subdivided into families, in accordance with the subordinate or more special modifications which the type of the order presents; every one of these families displaying the type of the class and order, with distinctive variations of its own. Each family consists of genera, in every one of which the fa mil y type is presented under a some- what diversified aspect. Eaeh genus is made up of an aggregation of species, which exhibit the generic character under a variety of modifica- tions ; these, however slight, being Tiniformly repeated through successive generations. Lastly, each species is composed of an assemblage of indi- viduals, every one of which repeats the type of its kingdom, sub- kingdom, class, order, family, genus, and species, through its whole line of descent. 12. Thus, in assigning to any particular being its place in the Organised Creation, we have to proceed from the general to the special. — ^We will suppose an unknown body to be brought for our determination ; the first business is to ascertain whether it be an Organised fabric, or a mass of Inorganic matter. This is soon discriminated, in the majority of cases, by an appeal to those most general characters which distinguish aU oroan- ised structures from inorganic masses ; and the next question is to deter- mine its Animal or Vegetable nature, by the aid of those characters which are not common to both, but are distinctive of each respectively. We will suppose the Animal nature of our unknown body to have been ascertained; the question next arises, to which of the four sub-kingdoms * For an admirable exposition of this doctrine, as it respects the osseous system of Vertebrated Animals, see Prof. Owen's treatise on "The Archetype and Homologies of the Vertebrate Skeleton." DIFFICULTIES IN DETERMINATION OF ARCHETYPES. 11 it stall be referred ; and this, agaia, has to be ascertained by an appeal to characters which are less general than the preceding, not being common to all organised structures, nor yet to all animals, but being restricted to each of the four sub-kingdoms respectively. Then having ascertained that it is a Vertebrated, Molluscous, Articulated, or Radiated animal, as the case might be, the Naturalist would determine its prder by characters of still less generality, which are peculiar to that order ; its family, by features which are still more limited; its genus, by those modifications of family character which are presented by the several genera it includes; and lastly, its species, by characters which are the most special of all, that is, which are limited to that race alone. 13. Now if our classification were perfect, it would be comparatively easy to determine the ' archetype' or ideal model of each group ; because we should have all the forms before us, by the comparison of which the Philosophical Zoologist seeks to educe what is common to the whole. But in practice it has often been found extremely difficult to determine what shall be considered as characters of classes, what of orders, and so on ; since their respective values are very commonly mistaken by those who are imperfectly acquainted with the true principles of classification, and some- times even by the instructed Naturalist. Thus in popular ideas, a Bat ranks as a Bird, because it flies by wings through the air; whilst the Whale ranks as a Fish, on account of its fish-like form, habitation, and mode of progression. But the scientific Zoologist has no hesitation in placing the Bat amongst the Mammalia, because it presents all the characters which are essential to that class, and which distinguish it from that of Birds ; namely its viviparous and placental generation, its subsequent nurture of its young by lactation, its covering of hair, the dental armature of its mouth, its diaphragmatic respiration, its highly-developed cerebrum, and many other peculiarities of conformation ; whilst its apparent resemblance to the class of Birds merely results from the adaptation of the Mammalian type to an aerial life. So, again, notwithstanding its fish-Uke habits, and the peculiarities of structure which adapt it to these, the Whale is a Mammal in all which is essentially characteristic of the class, and which distinguishes it from that of Fishes ; namely, its atmospheric respiration, its complete double circulation, its warm blood, its viviparous generation and subsequent lactation, its well-developed cerebrum, its osteological and many other peculiarities. — Here, then, the determination is easy to those who possess but a smattering of Zoological knowledge. But we will take another case, in which a fundamental error was committed even by a great Master in modern science, owing to his misapprehension of the value of characters. Following too closely the indications afforded by the teeth (which are valuable in so far only as they serve as a key to the general plan of conformation), Ouvier placed the Mwrswpial MammaUa in the first instance as a subdivision of his order Garncma; and even when he subsequently raised them to the rank of a distinct order, he gave them a position intermediate between the Carnaria and the Rodentia. Likewise, on account of the absence of teeth, he associated the Monotre- mata with the Sloths and Ant-eaters, in his order Edentata; satisfying himself with indicating that they presented a certain degree of affinity to the Marsupiata. Now the mutual resemblance of these two orders is extremely close ; and their unlikeness to all other Mammalia, in the 12 GENEEAL PLAN OF ORGANIC STKUCTUEE AND DEVELOPMENT. structure of their cerebrum, and in the mode in which the genital function is performed (two characters of fundamental importance), is such as unquestionably to require their detachment as a distinct sub- class. — So, again, it is now coming to be perceived, that the adaptation of the Mammalian structure to a fish-like habit of life, is not of itself suffiqient to assemble all the animals which present it into a distinct order; for whilst the greater part of those which agree in possessing the Cetacean form, agree also in structure and carnivorous habit, there are certain genera (the Dugong, Manatee, and Stelhrme) which have been until recently ranked with them, but which are found rather to corre- spond in the more essential peculiarities of their organisation with the great series of herbivorous Mammals, and to be connected with that series by forms now extinct. 14. Many other examples might be cited, illustrative of this difficulty, which is one that especially presents itself among the lower classes of animals, with whose structure and physiology the acquaintance of the Naturalist is as yet very imperfect. It is one, however, which is continu- ally lessening with the progress of research ; and whilst, therefore, we should avoid placiag too much confidence in existing systems of classifi- cation, and in existing ideas of what really constitute- natural groups, we may look forwaa-ds with hope, if not with absolute confidence, to the gradual accumulation of those materials, which shall enable the Philo- sophic Naturalist to do that for each group, which has been already Fia. i. A Fig. 5. Anatifa l interior tenor structure, enlargea, as shown by the romovaTof the valves of nnn »M„ ^ ^'"J* =""> mantle; o, cephalic portion of the body ; rf, mouth; o.artioiilnted i.,™!, 'AP^*™"'" ; jpendoges '(or Wnclfiffi) ; g. abdominal appendage. *""""'"«'l members j /, flabelliform IMPOETANCE OP THE STUDY OF DEVELOPMENT. 13 effected, in great measure, for the Vertebrated series. In the determina- tion of the relative importance of characters, it is certain that great assistance may be expected from the study of development ; although we may not perhaps go the full length -with those who maintain, that it should constitute the sole basis of all classification (§ 76). — It would be scarcely possible to adduce a more apposite illustration of the essential importance of this knowledge, to the determination of the true position and relations of a group of Animals distinguished by characters that seem to isolate it from all others, than is afforded by the case of the Cirrhipeds, or Barnacle tribe (Fig. 4). By all the earlier Naturalists, this group was unhesi- tatingly referred to the Molluscous sub-kingdom; being allied to the classes of which that division is composed, in the softness of its body and appendages, — in the inclusion of these within a hard casing, that is not fitted upon them, like the ' test' of a Crab or Lobster, but loosely enve- lopes the whole, like the ' shell' of a Mussel or Limpet, — and in the fixity of the animal to one spot during (apparently) the whole of its existence, the Ba/rnacle being anchored by a long flexible tubular peduncle, as a Pinna is anchored by its byssus, whilst the Balanus is attached, like the Oyster, by the adhesion of the shell itself to some solid basis of support. Even Cuvier left them in this position; although he had been led by the study of the anatomy of the animal inhabitants of the shells, to recognize their strong afiinities to the Articulated series. For he perceived that their bodies are quite symmetrical, and present indications of division into a longitudinal succession of segments, each of them furnished (Fig. 5.) with a pair of articulated appendages; their mouth he observed to be furnished with lateral jaws; he found their heart to lie in the dorsal region; whilst along their ventral region he detected the dou.ble ganglionic nervous chain, so characteristic of Articulated animals. Those Naturalists who considered this last assemblage of characters to possess a higher value than the preceding, — as being more indicative of the essential nature of these animals, whilst their relations to the Molluscous series are rather such as adapt them to a particular mode of Ufe, — trans- ferred the Cirrhipeds to the Articulated series ; and the propriety of this transference was made manifest by the discovery (first announced by Mr. J. V. Thompson in 1830*), that the Cirrhipeds in their early state are free-moving animals, conformable in all essential particulars to the Crustaceam, type ; and that they only attain their adult form and cha- racter after a series of metamorphoses, which progressively remove them to a greater and greater distance from it, and which, while they constantly tend to evolve the peculiar conformation that distinguishes the Cirrhiped group, adapt the animal, in each of its stages, to maintain its own exist- ence. The researches of Mr. Thompson, with the extensions which they have subsequently received from others, show that there is no essential difference between the early forms of the sessile and of the pedwneidated Cirrhipeds ; but that both are active little animals (Fig. 5, a), possessing three pairs of legs and a pair of compound eyes, and having the body covered with an expanded shield, like that of many Entomostracous Crustaceans, so as in no essential particular to differ from the larva of Cyclops (Fig. 60). After going through a series of metamorphoses, one * "Zoological Kesearches," No. III. 14 GENEEAL PLAN OF ORGANIC STKUCTUBE AND DEVELOPMENT. Stage of wliioh is represented in Fig. 6, b, these larvse come to present a form D, which reminds ns of that of Dwphnia, another Entomostracous Fig. 6. Development oiBalanus baltmoides; — A, earliest form ; — B, larva after eecond moult ;— c, side view of me same ; — v, stage immediately preceding the loss of activity ; a, stomach (?) ; 6, nucleus of future attachment (?) . Crustacean ; the body being enclosed in a shell, composed of two valves, which are united along the back, whilst they are free along their lower margin, where they separate for the protrusion of a large and strong anterior pair of prehensile limbs provided with an adhesive sucker and hooks, and of six pairs of posterior legs adapted for swimming. This bivalve shell, with the prehensile and natatory legs, is subsequently thrown off; the animal then attaches itself to its head, a portion of which Fis. 7. Comparison of Leucyer, a Stomapod CmBtaceg.n, with lepas ; — in the former a the abdo men, which becomes rudimentary in Cirrhipeds, is represented in outline ;— in the latter B the antenna! and eyes, which really exist in the larva, are represented as if they had been retained and had contmued to grow ; m marks the position of the mouth in both DIVERSITY OF PRIMARY TYPES OP ORGANISATION. ] -5 becomes excessively elongated into tte peduncle of the Barnacle (Fig. 7), whUst in the Balanus it expands into a broad base or disk of adhesion ; the first thoracic segment sends backwards a prolongation which arches over the rest of the body so as completely to enclose it (no uncommon occurrence among the Crustacea), and the exterior layer of this is conso- lidated into the 'multivalve' shell; whilst from the qther thoracic segments are evolved the six pairs of cirrhi which are characteristic of these animals in their adult state. — Hence, whether we consider the peculiarities of the group, in the fully-developed condition, as sufficient to entitle it to take rank as a distinct class, or whether we regard it as con- stituting merely a section of the great Crustacean class, there can be no longer any question that the Cirrhipeds bear any extremely close affinity to the latter, and that they must be placed near its borders (whether within or beyond them), as an aberrant form of the higher Articulate type, adapted to a life essentially Molluscous.* 15. Much of the controversy which has taken place among Physiologists, in regard to the general doctrines which may be deduced from the com- parison of different plans of organisation, has been due to a neglect of this difference between functional and structwral correspondence, that is, between a/nalogy and homology ; whereby phenomena which are essen- tially dissimilar, have been brought under the same category. But by some who have clearly recognized organic identity as the basis of their reasoning, it has been attempted to show that the law of Unity of Com- position has an unlimited application ; it having been maintained that the same elementary parts exist alike throughout the Vegetable and Animal Kingdoms, and that the difference between the several classes of each lies solely in the respective development of these parts. Such a doctrine, however, can only be supported by assertion, since Nature affords no sanction to it ; as the most cursory survey of these two of her kingdoms will at once make obvious. 16. If, for example, we commence by comparing the various tribes of Flowering-Plants with each other, we find that they may all be referred to a certain ' archetype' or ideal form, consisting of an ascending axis or stem with its foliaceous and fioral appendages, and of a descending axis or root with its absorbent fibres. Their most obvious diversities are generally attributable to the deficiency or excess of some or other of these component parts ; thus many of the trees most remarkable for the massive perfection of their stems, have the less essential parts of their flowers undeveloped; whilst many of the plants most remarkable for the beauty and luxuriance of their blossoms, never form a true woody stem. But amid this general conformity, the Botanist recognizes two very dis- tinct though subordinate types, each of them including a long series of gradational forms, from the lofty tree to the humble plant; the difference between which consists rather in the diversity of the plan on which the very same elementary parts are combined and arranged, than in any superior elevation possessed by one over the other. Thus if we compare the Palm and the Oak, which may be considered as presenting typical * For the most complete aoooimt of these metamorphoses, as well as of the Anatomy, Physiology, and Classification of the pedunculated division of the group, see Mr. C. Darwin's "Monograph on the suh-class Cirripedia," published by the Eay Society, 1851. — It is hoped that this admirable work will be soon completed, by a like description of the sessile division. 16 GENERAL PLAN OF OKGANIO STRUOTUHE AND DEVELOPMENT. examples of the Endogenous and JExogmous stems, we find that the same materials— cellular tissue, woody fibre, and ducts of various kinds— are worked-up, as it were, on two difierent patterns; and as a like difierence of plan extends itself also to the arrangement of the elementary parts of the leaf and to the number of the components of the flower, and even shows itself iji almost the earliest stage of the life of the embryo, it becomes aj)parent that the diversity is one which belongs to the funda- mental nature of the two groups. There are instances, it is true, in which there is such a general conformity in external appearance between certain of their members (between Oi/cads and Pahns for example), as might deceive a mere superficial observer; yet there is no assumption of the essential characters of the latter of these groups by the former, the stem being exogenous and the embryo polycotyledonous. So, again, although there are certain Flowering-plants (such as Lenma, duckweed, and Zost&ra, sea-wrack) which, alike in habit and in general simplicity of structure, correspond with aquatic Oryptogamia, these are at once reco- gnized as degraded forms of the Phanerogamic type, when their generative apparatus is examined ; reduced, though this is, to a condition of extreme simplicity. — The Cryptogamic series cannot be referred with equal pro- priety to a single ' archetype,' so diversified are the types of structure, as well as of grades of development which its principal groups ' present. Still the plan on which their generative apparatus is constructed, though not so dissimilar to that of Phanerogamia as was formerly supposed (since no reasonable doubt can now remain of their true seoc/UMlity) present a certain fundamental uniformity ; whilst its several modifications serve to distinguish the subordinate groups of Ferns, Mosses, Liverworts, &c. Although the Oryptogamia as a whole rank below Flowering- plants, yet no one can help recognizing in a Tree-Fem a far more elaborate structure than that oi Lem/na or Zoster a; so that the essential distinction between the two series lies, not in grade of development, but in type of conforma- tion. So among the Oryptogamia themselves, we find paraMd series, such as those of Algce, Lichens, and Fungi, through each of which a cer- tain distinctive type is preserved, notwithstanding that between their several varieties of grade there is a close correspondence. — Hence we see that although, from the comparatively small number of distinct organs which the Plant possesses, and from the less complete separation even of these, there is not by any means the same scope for varieties in plan of organisation as we shall find in the Animal Kingdom, it is not the less certain that a considerable number of distinct types of structv/re exists, which cannot be reconciled to any other theory of fundamental unity, than that which refers them aU to their common starting-point ^the single cell. 17. Turning, now, to the Animal Kingdom, we find that even a slight general survey affords ground for. the recognition of those foitr very dis- tinct pkvns of structwre, which Ouvier was the first to mark-out clearly namely, the Radiated, the Molluscous, the Articulated, and the Verte- brated ; and these are found to be more and more clearly distinguishable from each other, the more profoundly we examine into the fundamental peculiarities of each, and the more fully we become acquainted with the history of its development. For by acciirately studying and comparino- the various modifications under which these respectively present them- DIVEKSITY OP TYPES OF ORGANISATION. 17 selves, we see that beneath the apparent mixture of characters which occasionally presents itself (as, for example, in the case of the Cirrhipeds, 14), there is an essential conformity to one type, and that the departure from the ordinary aspect is merely superficial, being such as adapts the animal or group of animals to a particular mode of existence. Now siace modifications of a similar kind may take place in groups of animals belonging to different types, they may come to present very striking resemblances to each other in their adaptive characters (as is the case between Birds and Insects), although there is no conformity whatever in their general plan of structure. Taken as a whole, no animal belonging to any one of these types can be likened to any animal belonging to another; although comparisons may be legitimately made between their individual organs. Thus, as Von Baer justly observes, " metamorphose a Cephalopod as you will, there is no making a Fish out of it, save by building-up all the parts afresh ;" yet in many portions of their organisa- tion, Cephalopods are unquestionably intermediate between the lower MoUusks and the typical Fishes. Again, although the higher Cepha- lopods indubitably take a more elevated rank as Animals than the lowest Fishes, and in this respect might be considered as approximating more closely to Man, yet in the conformity of its organisation to the Verte- brated type, the lowest Fish bears far more resemblance to Man, than does the highest Cephalopod. — Moreover it is to be observed, that the general type of construction manifests itself not merely ia the mode in which the organs are grouped together, but also in the conformation of the organs themselves. Thus we shall hereafter see, that whilst there is a remarkable correspondence between the condition of the Circulating apparatus ia the two series of Articulated and Molluscous animals, — especially as regards the imperfection of its vascular system, and its com- munication with the visceral cavity, — there is a type which is peculiar to each, and which shows itself in the structure of the heart, as well as in the general distribution of the blood-vessels. For the type of the heart, in the Articulated animal, is the elongated dorsal vessel, which, if divided at all, has a repetition of similar chambers for the several segments of the body; whilst in the MoUusk it is a concentrated organ, with much thicker walls, usually having the auricle or receiving cavity separated from the ven- tricle or impelling cavity, and presenting no other repetition of similar parts than the occasional doubling of the auricle, where the two sets of gills (whence the blood returns to the heart) are placed wide asunder. So, again, in the various Glandular organs of the Articulata, the required extent of surface is usually afibrded by the elongation of a small nuniber of narrow tubes ; whilst in the MoUusca, the same extension is provided for by the mviltipUcation of short and wide follicles. Yet we find that iu certain Crustacea, which are adapted in many respects to the conditions of the MoUusk, both the heart and the glandular apparatus present a very striking approximation to the Molluscous type; whilst no such approxi- mation is seen in the general-plan of the fabric, which is as obviously arti- culated in the Crustacea, as it is in the Insect. 18. But although it is in type, or plan of organisation, that the most essential difierences lie, among the several forms of Plants and Animals, it is not the less true that they are distinguished by very marked diver- sities in grade of devehpment ; by which is to be understood, the degree m 18 GENEBAL PLAN OP OEGANTIC STRUCTUBB AND DEVELOPMENT. which the several parts that make up the entire fabric are cha,racterised by specialities of confonnation, so that each becomes a distinct organ, adapted to perform a fwnction more or less different from that which other parts can discharge. The lower we descend in the scale of being, whether in the Animal or in the Vegetable series, the nearer approach do we make to that homogeneousness which is the typical attribute of inorganic bodies, wherein every particle has all the characters of indivi- duality, so that there is no distinction either of tissues or of organs. Thus in Sponges and Sea-weeds, even when of considerable size, every part resembles other parts in intimate structure, and differs but slightly from them even in external configuration ; so that the whole mass is little else than a repetition of the same organic components. On the other hand, as we ascend the scale of being, we find the fabric — whether of the Plant or the Animal — becoming more and more heterogeneous; that is, to use Von Baer's language, " a differentiation of the body into organic systems, and of these again into separate more individualized sections," presents itSelf. Thus, as we ascend from the lowest towards the highest forms of Vege- table hfe, we find that out of the homogeneous aggregation of cells which forms the simple frond of the humble Algae (§ 22), a differentiation gradually arises between the ' axis' and the ' appendages to the axis / that in the axis, there is a gradual separation established between the ascend- ing portion, or stem, and the descending portion, or root; and that among its appendages, the foKaceous organs become more and more completely separated from the generative apparatus. Even in the highest Plants, however, we find an extensive repetition of similar pa/rts; and there is always, too, a close correspondence in the intimate structure of even the iQost antagonistic organs, such as the roots and leaves. — The differentia- tion, both as regards external conformation and intimate structure, pro- ceeds to a far wider extent in the Animal kingdom, in virtue of the much greater variety of purposes to be attained in its existence; and we see this carried to its highest degree in Man, in whose organism the principle of speoiaUsation everywhere manifests itself, no part being a precise repe- tition of any other, except of the corresponding part on the opposite side of the body.* 19. It is only, however, by a very gradual succession of steps, that this elevation is attamed. The simplest Animals are precisely upon a level with the simplest Plants, as regards their homogeneity of character • and no sooner does a differentiation of organs show itself, than these are in the first instance almost indefinitely repeated, so that, however numerous may be the parts of which the entire organism is composed, they are (so to speak) thefac-smiiles of one another. Thus not only in Zoophvtes bu^ also m the lower MoUusca and Articulata, we find this repetition extendmg to those entire grovps of organs, which, when detached from the rest, can mamtam an independent existence, and are therefore com monly accounted distinct individuals. But we find the same to hold good as * This fact is most curiously exempUfied in the speciality of the ?^«#, «/ „?.,„*• ^ those disorders of nutrition, which obviously depend upon the creslcl "■? f *"» ■" morU in the blood, rather than upon any primary local disturban^q T, w" f?"''' Budd's Memoir on ''Symmetrical Diseases," in the "McS^^CtTI^T ' ^?^^ Vol. XXV.; M.-. Paget'B "Lectures on Surgical PathZy " Vdl ^ ^^r™';''"*'""''^ the Author's " Principles of Human Physiololy," 4th Ed p 192 ^' *'*•' ""^ REPETITION OP SIMILAE PARTS IN LOWER ORGANISMS. 19 OpMura. regards mdimdual organs, in the highest members of each of the Inverte- brated sub-kingdoms, and even (though to a less extent) among Verte- brated animals. Thus among the Echmodermata, there is a precise repetition of similar parts around a common centre ; and although this repetition is limited to fioe in the highest forms ^^'^- 8. of the class, yet it extends to a much greater num- ber in those of inferior organisation, — as we see in comparing the Ophiura with its five simple arms (Pig. 8) and the Pmta- crinus (Fig. 9), whose ten arms all subdivide into such numerous branches, that the aggregate num- ber of pieces in the whole is estimated at above a hundred thousand. So, again, in the Cephalopo- da, which constitute the highest division of the Molluscous series, we find the tentacula sur- rounding the mouth to be almost indefinitely multiplied in the lower or tetrahromcMate division {NautiMs and its allies); whilst they are reduced to eight or ten in the dibramcMate order (CiMtU-fish, Fig. 47), at the same time acquiring a much higher individual development, and often hav- ing one pair difierentiated from the rest, for some special purpose. So in the Annelida and other inferior groups of the Articulate series, we find the loco- motive, respiratory, and other important organs almost indefinitely multiplied in the longitudinally- repeated segments; but as we ascend towards the higher Articulata, the number of segments becomes strictly limited and greatly reduced, even where these divisions are stUl little else than repetitions of one another, — being only twenty-two in the Centipede, and thirteen in the Insect- Larva ; whilst in the perfect Insect, the difierentiation is carried to its highest extent, the locomotive apparatus being restricted to the three thoracic segments, and all the other organs, even when re- peated throughout, being unequally de- veloped in the several parts. The same priaciple of gradual differentiation c2 Fia. 9. ■ !*,-■ ^^ I..' .■■ ■-! II': Pentacriniiee bria/reus. 20 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. stows itself most remarkably in the conformation of the members of Verte- brata: for, taking the many-jointed but [single rod-like appendage oi the Lepidosvren (Fig. 3, a, and Fig. 1 50) as their lowest type, we find this simply repeated even to the extent of a hundred-fold or more, m the digital rays supporting each of the pectoral and ventral fins of Fishes; as we ascend thence, through the extinct Enaliosauria (Ichthyosaurus, Plesiosaurus &c.) to the typical Reptiles, we find the number of these multiplied digits dimi- nishing, until it settles down at five, and the number of joints in each also reduced, until it becomes restricted to the six rows (two carpal, one metacarpal, and three phalangeal) which characterise the hand (or foot) of Man ; in Birds and Flying Mammals there is a most marked difieren- tiation between the anterior and posterior extremities, as there is also (though in a less degree) in Man; and in the Quad/runiana, we begin to see that specialisation of the first digit (this being usually common to all their members) which is carried to its highest point in the hand of Mem, whose other digits, also, have their distinctive capabilities, whereby this member as a whole becomes the most highly-organised of all instruments, in virtue of the unequalled variety of actions which it is adapted to perform. 20. Thus we see that, whether we trace the ' Archet3rpe' of each great subdivision of the Animal kingdom into those modifications which it presents in the more restricted groups, — or whether we follow any organ or system, from the form under which it at first presents itself, to that which it assumes in its state of most complete development, — ^we recognise one and the same plan of progression, namely, from the general to the special; and, as Von Baer justly remarks, the relations of any organised fabric to any other, must be expressed by the product of its type with its grade of devehpnient. Neither alone suffices to characterise it; for under the same type, difierent grades of development may present themselves; whilst conversely, a like grade of development may be attained under different tjrpes. And this general fact needs to be constantly borne in mind, not merely when a Plant or Animal is being considered as a whole, but also when we are studying the evolution of any individual organ or system in the ascending series ; since it is no more possible to follow this through one unbroken progression, than it is to arrange the entire assem- blage of beings composing either kingdom in a single linear series. — It may in some degree assist the reader in his perusal of the subsequent pages, if we here pause to take a general survey of the principal types of Vegetable and Animal conformation, and of the chief diversities in grade of development which present themselves under each. 21. Vegetable Kingdom. — If we commence by examining any Plant of high organization, we observe, in the first place, that there is a complete differentiation between its organs of Nutrition and its organs of Repro- duction; and further, that its principal organs of Nutrition, the root and the leaf, are separated from each other by the interposition of the stem or axis, around which the various appendages are arranged with a con- siderable degree of regularity. Further, we notice that a corresponding differentiation presents itself, as to the intimate structv/re of these several organs; for whilst the parts most directly concerned in the vital onera- GENEBAL VIEW OF VEGETABLE KINGDOM. — PEOTOPHYTA. 21 tions of the organism are chiefly made-up of aggregations of ceUs, which resemble in all essential particulars those of which the simpler forms of vegetation entirely consist, these are supported upon a framework of woody fibre, an extension of that which gives strength and solidity to the stem and roots; and further, in order that air and liquids may the more readily find their way from one part of the structure to another, than they could do by transmission from cell to cell, a set of ducts is inter- posed, which establish a ready communication through the stem between the roots and the leaves. These organs are all mutually dependent and connected ; and contribute, each in its own special manner, to the life of the Plant as a whole. But since all the most essential organs are many times repeated, the loss of some of these does not involve the destruction of the entire organism ; and even the -separated parts may develope the organs in which they are deficient, and may thus evolve themselves into entire plants, and maintain an independent existence.* In this way a multi- plication of the products of the original germ may be effected ; but these, . as will be shown hereafter (chap, xi.), are not distinct individuals in the highest sense of that term ; and the process by which they are evolved is simply a modification of the ordinary Nutritive operation, and is so far from being a form of true Generation, as to be essentially antagonistic to it. This distinction is one of much importance ; since on it depends the recognition of the organs inCryptogamia, which are homologous with those of Flowering-Plants. 22. Having thus determined, by the analysis of one of the highest Plants, what it is that constitutes the most complete type of Vegetable organisation, we shall commence with the lowest division of the series, and endeavour to trace-out the principal lines of ascent by which that type is attained. This can only be accomplished, at present, in a very imperfect manner; since it is only within a very recent period, that the homologies of the reproductive apparatus of Phanerogamia have been discovered among Cryptogamia ; and little more than a guess can be as yet made, as to the conditions which these present in some of the humbler forms of Crjrptogamic Ufe. — The lowest type of vegetable existence is afibrded by those organisms, which either consist of single cells, or of aggregations of si/milar cells, each of which can maintain an independent existence, living for and by itself, and not needing the co-operation of other cells, save for the purpose of generation, of which the re-union of the contents of two cells, by an act of ' conjugation,' is an essential condi- tion. Any one of these cells may multiply itself indefinitely by sub- division, the results of which process are seen in the accompanying ex- ample (Fig. 10) ; but these products are all mere repetitions one of another, and often detach themselves spontaneously, so that the descendants of a single cell may cover a very extended area, as is the case, for example, * This is usually the case under favourable conditions with regard to leaf-b-uds, which can put forth rootlets, and then evolre a stem, from which other leaf-buds and their flower-buds are developed. But there are some plants, as Bryophyllum, which have the same power in every leaf, or even in every fragment of a leaf; a small portion, laid upon damp earth, or suspended in a humid atmosphere, gradually evolving itself into the entire organ, and at the same time developing the other parts most essential to the performance of its nutritive operations, from which the reproductive apparatus is subsequently put forth. 22 GENERAL PIAN OP ORGANIC STRUCTURE AND DEVELOPMENT. Witt the Protococms nivalis, or ' red snow.' There is here, therefore; not the least show of differentiation; no special cells bemg set aparteven for the performance of the generative act. Where Via. 10. the multiplied cells remain in continuous connexion with each other, being imbedded in a common sub- stratum of gelatinous substance, so as to form but a single mass (Fig. 10), this may be perfectly homoge- neous throughout; no definite form being presented by it as a whole, and no trace of 'organs' being dis- tinguishable in any part of it. The first indication of progress towards a higher grade, is given by the limitation of the direction in which the increase takes place : so that, instead of an amorphous aggre- gation of cells, we find a linear series (Fig. 11, a) which is formed by successive trans- FiG. 11. verse subdivision; and this filament may increase in breadth by longitu- dinal subdivision(B), so as at last to pro- duce a laminar ex- pansion, such as that of the common JJhioB, which is termed a thallus. In the simplest forms of this thallus, we do not meet with the Bangia velutina. slightest traCe of differentiation; and every one of its component cells appears to live as much for and by itself, as if it were completely detached from the rest. Every one of them, moreover, seems able to multiply itself, not merely by sub- division, but also by the emission of a portion of its contents enclosed in a cell-wall, in the condition of a 'spore' or detached gem/ma; and this in the tribe now under consideration, being usually furnished with cilia, and endowed with the power of spontaneously moving for a time, is termed a 'zoospore.' When the zoospore has been thus carried to a distance from the organism from which it proceeded, it begins to develope itself into a similar organism by the process of duplicative subdivision ; and in arriving at the highest of these stages of development, it passes through the simpler forms which remain permanent in yet humbler grades of vegetation. The true Generation of the plants of this grouiJ, to which the term Protophytes may perhaps be advantageously restricted, seems to be always accomplished by the process of ' conjuo'ation ' in which any or all of the component cells may alike participate • but we see, in its higher forms, a tendency to the distinction between the ' sperm- cell ' and the ' germ-cell,' that is, to the differentiation of sexes into male and female, — ^the only mark of heterogeneousness which yet presents itself The product of this act is a new cell, from which a new plant originates GENERAL VIEW OF VEGETABLE KINGDOM. ALGjB. 23 by duplicative subdivision, as in the case of the zoospore. Here, then, we find that each individual (understanding by this term the aggregate result of a generative act) is made up of an indefinite number of cells, which, being precisely similar to each other, have no relation of mutual dependence; so that the Life of the whole is merely the swn of the lives of the component parts, and not, as in higher organisms, the product of it. 23. In the next stage of development, the difierentiation of parts begins to manifest itself more decidedly ; but this not so much in a distinction of organs adapted to separate oifices in the act of Nutrition, as in the limitation of the Reproductive act to particular portions of the organism, and in the setting-apart of special organs for its performance. For we have as yet no real distiaction between stem, roots, and leaves ; although some semblance of such a distinction may present itself. The primordial cell, by repeated subdivision, extends itself into a ' thallus,' whose form has but little definiteness, and whose tissue is nearly homogeneous throughout, being entirely composed of cells of various forms, without either woody fibres or vessels of any kind; and it is chiefly by its appa- ratus of fructification, which presents itself under many different aspects, that this group, which may be 'designated by the term Thallogbns, is distinguished from the preceding. Nearly the same degree of general development is presented by three tribes of these humble Cryptogamia, — namely, Algce* Lichens, and Fungi, — which, nevertheless, are fitted to exist under very diverse conditions, and which present corresponding diversities of structural type ; and all of them seem to agree (according to the most recent investigations, of which an account will be given here- after, CHAP. XI.) in the possession of a special generative apparatus, in which the distinction of sexes is clearly marked. This consists of a set of ' sperm-cells' developed in certain parts of the organism, and of a set of 'germ-cells' evolved elsewhere, usually (but not always) in the same indi- vidual ; the product of the former is a ' spermatoid' body, which comes into contact with the latter and fertilizes its contents; and the result is the formation of a germ, which must be considered as the commencement of a new generation. This germ, however, frequently remains for some time in connection with the parent, and multiplies itself by duplicative subdivision at the expense of the nutriment which it draws from it, so as at last to evolve itself into a collection of ' spores' contained within a special envelope, every one of which, when liberated from the parent, may develope itself into a new plant in which the same processes are repeated. It is by the general relation of this apparatus of fructification to that of nutrition, that the three groups already named are most dis- tinctively characterized. 24. Thus the Algce vegetate exclusively in water or in damp situa^ tions; they require no nutriment but such as is supplied by water and by the air and inorganic substances dissolved in it; they absorb this nutriment equally by every part of their surface; and they show a great tendency to the extension of the ' thallus' by the multiplication of cells in continuity with the existing fabric, so that it frequently attains most * The group of Algse, as here limited, does not include the Protophytes described in the preceding paragraph; for although these, being mostly aquatic plants, are usually ranked in it, yet their type of reproductive apparatus is so distinct from that of the higher Algis, as to require that they should be separately considered. 24 GENEEAL PLAN OP ORGANIC STEUCTUBE AND DEVELOPMENT. extraordinary (iimensions. In some of the simpler forms of the group, we find but a slight advance upon those aggregations of similarly-shaped cells, of which the fabrics of the Protophyta are made up. Thus in Mesogloia (Fig. 12), although we have a distract axis with radiating appendages, the former is composed of elongated cells very loosely adhe- rent, while the latter consist of single rows, bearing the generative cells at their extremities; and in Zonaria (Fig. 13), it is only the character of the fiiictification that raises it above the type of an Ulva. In the highest Fig. 13. Meaogloia vermicula/ris. Zonaria plantaginetL, Algse, however, we find some differentiation in the texture of their inte- rior and exterior substance; and there is also a certain foreshadowing of Fig. 14. Fig. 15, I>aaya Imetzvngitma. Marginaria gigm. the separation between the stem, the roots, and the leafv exnan^i-nr, frond; but there is nowhere a departure from the simple cellular f "'" nor IS there any real specialization of function, save that the fruotific t^^' GENERAL VIEW OF VEGETABLE KINGDOM. LICHENS. 25 IS evolved from the frondose portion, and not from the stem-Hke or root- like axis. Most Algse are provided with a special apparatus (such as the siickidmm of Basya, Fig. 14, a) for the evolution of free gemm», which are sometimes ciliated like the zoospores of Protophyta, and which mul- tiply the original fabric iadependently of any true generative act. The proper generative organs are frequently very obscure, and are often buried m the general substance of the frond; occasionally, however, they form conceptacles, which are prominent externaUy (Fig. 15), or are developed on particular branches only. The embryo-cells, which are the products o± the fertilization of the germ-cells by the contents of the sperm-ceUs, do not usually undergo any great amount of subdivision into 'spores,'* before each spore that has originated from it begins to develope itself into a new plant. Hence it is obvious that the whole nisus of vital activity in the Algae, is towards Nutrition rather than Generation,— the multiplication of independent organisms of the existing generation, rather than the origination of new series by the proper generative act. 25. On the other hand. Lichens grow upon living Plants, upon rocks and stones, upon hard earth, or other situations in which they are sparingly supplied with moisture, but are freely exposed to light and air. They derive their food from the atmosphere, and from the water which this conveys to them ; but this they do not seem to absorb equally over the whole surface, the least exposed side being the softer, and being pro- bably the one through which most liquid is imbibed, whilst it is rather through the other that carbon is drawn-in from the air. The ' nisus' or tendency of development is here to form a hard crust-like thallus, of slow growth, and of rather limited dimensions, but of great durability (Pigs. 16, 17); and in the several layers of this thallus, there is consi- Fio. 16. Pia. 17. I^armelia acetabulum. Sphaerophoron coralkrides. * The term ' spore' has been used to designate many things homologioally different. The Author belicTes that it will be most accordant mth existing usage, to continue to apply it to the bodies contained in the capsules of Mosses, Ferns, &c., which are imme- diate or remote products of the subdivision of the embryo-cell, and to those bodies in Algse, Lichens, &c., which are homologous with them. On the other hand, the germ- cells which themselves take part in the generative act, and from which the embryo-cells originate, should never be designated by the term spore. 26 GKlfERAL PLAN OF ORGANIC STRUCTUKE AND DEVELOPMENT. derable diversity of texture, although (as in the Alg£e) there is no departure from the simple cellular type. As in the Algse, moreover, we usually jSnd a special arrangement for the production of free gemmre {soredia), by which the number of independent organisms of the same generation may be multiplied; and the evolution of these has been frequently considered as the true reproductive process. It is now almost certain, however, that in this ill-understood group, both ' sperm-cells' and ' germ-cells' exist, although usually buried in the substance of the thallus; and that of the clusters of 'spores' which make their appearance within special conceptacles, each, as in the AlgsB, is the result of the subdivision (to a limited extent) of a single embryo-cell produced by the generative act. These conceptacles are sometimes buried in the substance of the thallus, although their presence usually makes itself known by the prominence which it causes (Figs. 16, 17). Some tribes of Lichens very closely approximate to Algse, both in their conditions of growth, and in their general character; whilst others present an equally close approxi- mation to Fungi; so that, as some botanists have ranked this group with the former, and others with the latter, it seems reasonable to regard it as an intermediate section, the types of which are equally far removed from both. 26. The group of Fungi differs from both the preceding, in reqiuring as the most favourable, if not as the absolute condition, for the develop- ment of the Plants belonging to it, the presence of dead or decaying organic matter, which shall afford by its decomposition a larger' supply of carbonic acid and ammonia than the atmosphere and its moisture would alone furnish ; their growth is favoured by darkness rather than by light; and, like higher plants when not acted-on by light, they absorb oxygen and set-free carbonic acid. Their simpler forms (Fig. 18) strongly remind us of the lower Algse (compare Fig. 12) in their grade of development, the nutritive and reproductive portions not being differentiated ; but in the higher we find a very marked separation between these, the reproductive apparatus being here as predo- minant, as is the nutritive apparatus in the Algee. The vegetative thallus of these plants, which extends itseK indefinitely in situations favourable to its development, has a very loose flocculent texture, and is composed of elongated branchiag cells interlacing amongst each other, but having no intimate connec- tion (Fig. 19, a); and this mycelmm, as it is termed, has such a want of definiteness of form, and varies so little in the different tribes of Fun- tinue to vegetate when separated from the rest, it does not appear that there is any provision for the spontaneous detachment of free gemince for the multiplication of the individual. The whole nisus of vital activitv in the Fungi seems to be concentrated upon the Generative apparatus Fig. 18. GENERAL VIEW OF THE VEGETABLE KINGDOM. — FUNGI. 27 which, when fully developed, separates itself completely from the nutri- tive, and constitutes all that commonly attracts notice as the Plant Fia, 19. CUmcma mspula; — a, portion of the mycelium magnified. (Fig. 20). Late observations render it probable that Fungi possess a true sexual apparatus, certain cells of the mycelium being developed into sperm-cells, and others into germ-cells; and that what is known as the Via. 20. -n ^cidimn iuBBilagmig ; — A, portion of the plant magnified ; — b, section of one of the concep- ta«]£8 with its sporecles. ' fructification' is the product of an act of conjugation, the immediate result of which is the formation oi an embryo-cell, which afterwards sub- divides almost indefinitely, so as to produce an immense mass of 'spores.' These become detached from each other; and, being usually of extreme minuteness, are carried about in the atmosphere, so as to become deposited in remote soils, and to give rise to vast numbers of separate beings consti- tuting a new generation.* * It is interesting to otserve that the mode of evolution of many of these Thallogens is greatly influenced by the conditions under which it takes place. Thus, if Lichens be removed from the influence of light, and be over-supplied with moisture, they show a ten- dency to the extension of the vegetative or foKaceous portion of the thallus, with a non- development of the fructification ; and the thaUus often assumes the lyssoid form of the myeeUum of Fungi, so that it might be readily mistaken for this. So, again, if the simpler forms of Fungi develope themselves in liquids, they show an unusual tendency to the extension of the mycelium ; and may even take-on so much of the characteristic appear- ance and mode of growth of Alg%, that their true nature becomes apparent only when the fructification is evolved. — See the description of ' a Confervoid state of Mucor olavatus,' by the Rev. M. J. Berkeley, in the "Magazine of Zoology and Botany," Vol. II. p. 340. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 27. The next important mode of elevation consists in the differen- tiation of the parts of the Nutritive apparatus, and in their stUl more complete separation from the Generative. In ascending through the series formed by HepaticcB, Mosses, and Ferns, we observe a progressive approximation to that distinction between the ' axis' and its ' appendages,' which is characteristic of the highest forms of Vegetable lifej but the growth of th^e axis is limited to one or both of its extremities, the part already formed being subject to very little, if any, increase; and from this character it has been proposed (by Dr. Lindley) to distinguish this higher division of the Cryptogamio series by the title Acrogens, signi- ficant of growth at their points alone. — The lower foijms of the Hepaticce (such as the Ricoiacem) closely abut upon the Lichens, and differ from them but little as regards the or- Fie- 21. ganisation of their nutritive apparatus, although their fructification evolves itself after a different type. In the common MarcJicmtia (Fig. 21), however, the soft green thallus now assumes more of the structure and aspect of a leaf, having an upper and under cuticle (the former perforated with stomata), and an interven- ing soft, loose parenchyma; and distinct radical fibres are thrown out from the lower surface, for the imbibition of moisture. In the Jvmgermarmia there is a distinct axis of growth, on which the foUa- ceous appendages are symmetrically arranged; these are not completely differentiated from it in some species, but in others they are quite separated, and have an indica- tion of a central mid-rib; the stem, however, stm trails on the ground, and radical fibres are developed from every part of it.— A slight eleva- tion in this type brings us to that of the Mosses, which always have a distinct axis of growth com- monly more or less erect, with the foliaceous ap- pendages symmetrically arranged upon it (Fig 22) A transverse section of this axis shows an indi- cation of a separation between its cortical and its medullwry portions, by the intervention of a layer of elongated cells, that seems to prefigure the wood of higher plants; and from this layer pro- longations pass into the leaves, in which' thev form a kind of mid-rib. The leaves, however, do not themselves present any considerable ad vance towards the more perfect type, being merely solid homogeneous aggregations of cells .. ,. , , . ^^^ "1° P'^°P®'' ^°°^ is yet evolved as a descending continuation of the axis, radical-fibres being put forth from every mrt f Frond of Marckamiia polymorpha. Fig. 22. FiBaidens bryoides. GENERAL VIEW OP THE VEGETABLE KINGDOM. MOSSES. 29 the lower portion of the axis (Fig. 25), and even from the under-surfaces of the leaves. Both in Hepaticse and Mosses, we find a special arrangement for the multiplication of the plant by the formation of detached gemmce; and Fia. 23. Pia. 24. MareJtwntia polymorjilui, witii peltate receptacles bearing antlieridia. Marcka/ntia polymorpha, with lobed recep- tacles bearing pisti]]i( some species owe their dispersion and perpetuation much more to this mode of propagation, than to the regular generative operation. There is no longer any doubt that both these tribes of plants possess true sexual organs; namely, antheridia conta inin g ' sperm-ceUs,' saidpistillidia or archegonia containing ' germ-cells.' In Marchantia, these are borne upon distinct plants, and both are sufiiciently conspicuous (Figs. 23, 24) ; in Mosses, on the other hand, they are usually very obscure, and are gene- rally combined in the same individual. The pro- duct of the fertilization of one of the germ-cells by the spermatoid bodies set free from the sperm- cells, is an embryo-cell which develops itself into a capsule containing a mass of ' spores;' and this, in the Mosses, is raised by the elongation of its foot- stalk, far above the original situation of the pistiUi- dium, and becomes the only ostensible fructification of the plant (Fig. 25). In any one of the spores thus formed by the duplicative subdivision of the embryo-cell, a new plant may originate. — It is chiefly by specialities in the structure of their gene- rative apparatus, that the preceding groups are distinguished from each other; each having its own peculiar type of fructification, whilst presenting (as we have just seen) a tolerably regular gradation Foij/tHchum commune. in the development of the organs of nutrition. 28. Passing from these to the Ferns, we find such a rapid elevation in the character of the apparatus of nutrition, as causes the group to approximate closely in this respect to the Phanerogamic division; indeed its members may be said to be more highly organized in most respects than the inferior Phanerogamia, although, the type of their generative apparatus being essentially Cryptogamic, they must be considered as 30 GENEEAL PLAN OP ORGANIC STRUCTURE AND DEVELOPMENT. belonging to the lower rank in the Vegetable scale. It is in the Tree-Fems that we have the most perfect evolution of the characters of Fia. 26. Fig. 27. Trichomanei. Fcoaioi Smlojiendrum. the group ; and here we find, not only an ascending axis or stem, around which the foliaceous appendages are symmetrically arranged in a spiral, Fig. 28. Fig. 29. b Frond of Ommmda regaUs; — a, sterijo or folia- ceous portion ; h, fertile portion ; — a, part of the latter enlarged, to show the thecce. Jiqukclnm arvcn. GENEBAL VIEW OP THE VEGETABLE KINGDOM. FERNS. 31 but a proper descending axis or true root, from which alone the radical fibres are given off. In the stem, the cortical portion is separated from the medullary by the interposition of bundles composed of woody fibre and vascular tissue; and the principal difference which exists between these and the woody layers of Exogenous stems, lies in the absence of any tendency to regular iacrease, except in length. Prom the fibro- vascular bundles in the stem, prolongations are given off, which pass into the leaf-stalks, and thence iato the mid-rib and lateral branches of the foUaceous appendages, to which they form a kind of skeleton, as in the leaves of Phanerogamia. These organs, which are distinguished as ' fronds,' on account of their combining the character of a leaf with that of an apparatus of fructification, are constructed upon the same type with the leaves of Flowering-Plants; being composed of a cellular parench3rma, enclosed between two layers of epidermis, and having air-chambers to which access is given by stomata; and they can scarcely be less com- plete as organs of nutrition, although still made to bear a share in the function of reproduction. Even in this respect, however, a differentiation exhibits itself in certain Ferns, as the Osnivmda regalia (Fig. 528) ; whose fructification is restricted to particular fronds, or parts of fronds, hence designated 'fertile,' which lose their foliaceous character; whilst the remainder bear no fructification, and are hence designated as ' sterile,' performing the functions of leaves alone. The ostensible organs of fructi- fication are far from constituting (as they were until lately supposed to do) the real generative apparatus; for this is evolved at a, period in the life of the plant, at which its appearance was totally unexpected. Each of the ' spore-cells' which are set free from conceptacles on the under siu&ce of the fronds (Pig. 27), when received upon a damp soil, extends itself, by duplicative subdivision, into a frondose body closely resembling the thallus of the Marchantia; it is in this that the 'sperm- FiG. 30. Iti/copodium cernu/iim. 32 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. cells' and ' germ-cells' are evolved, and that the fertilization of the latter, by self-moving spermatoid filaments set free from Fig. 31. the former, takes plii'ce ; and from the embryo- ceU, which is the product of this operation, there arises — not, as in the Mosses and Liverworts, a conceptacle filled with spores, each of which may give origin to a separate plant, — but a single young Fern, which, having attained its full deve- lopment by duplicative subdivision, detaches cer- tain of its cells, as ' spores,' to continue the race by the same process. In this departure from the plan which prevails among the inferior Crypto- gamia, we have an obvious tendency towards that of the Flowering-Plants : the entire product of each generative act being worked-up (so to speak) in the Fern, as in the Flowering Plant, into the diversified parts of a single organism ; instead of being subdivided, as in the inferior Cryptogamia, amongst an indefinite number of independent fabrics, which are mere repetitions one of another. Still the type of the generative apparatus in the Ferns is essentially Cryptogamic. — That of the MarsUeaquadrifoiia. EquisetacecB (Fig. 29) appears to be essentially the same; but in Lycopodiacece (Fig. 30), Isoe- tacece, and HMzocarpece (Fig. 31), there is a stUl closer approximation to the Phanerogamic type, the ' sperm-cells' (' small spores') being directly produced by the parent-structure, and the ' germ-cells' alone being evolved, after the detachment of the ' large spores,' upon the ' prothaUium' into which each of these developes itself. 29. The distinctive character of the Phanerogamia or ' Flowering- Plants' is not the possession of what are commonly designated as ' flowers,' since these may be reduced to a condition in which they are scarcely distinguishable from the fructification of the Cryptogamia. In fact, the group of Ehizocarpese, in which the concurrent action of the small and large spores had been ascertained to be necessary for the production of an embryo, was referred by many Botanists to this division, at a period when the existence of distinct sexes had not been recognized among the Cryptogamia generally, and when it was, in fact, not merely doubted, but usually denied. Still, it is in the peculiar type of their Generative apparatus, that the essential distinction lies ; for the fertilizing process is performed among them in a manner not elsewhere seen, namely, by the emission of a long tube from the ' germ-cell' (pollen-grain), which finds its way (often through a distance of some inches) to the ' sperm-cell' buried in the ovule ; and it is among them alone that a true seed is pro- duced, in which, with the embryo, a store of ready-prepared nutriment is laid-up for its early development. This sub-division of the Vegetable kingdom includes a vast range of species that difier very greatly ia the degree of development, both of their nutritive and their generative appa- ratus; but for our present purpose, it will be sufficient to sketch the typical plan, which is more or less obviously manifested in the conforma- tion of the entire group. — If we analyse the fabric of any common Phanero- gamous Plant, we find that it consists essentially of an axis and appendages ■ GENERAL VIEW OF VEGETABLE KINGDOM. — PHANEROGAMIA. 33 Fia. 32. tie former being made up of an ascending portion or stem, and of a descending portion or root, with, their respective ramifications ; and the latter being distinguishable into foliaceoua and floral organs, which will be pre- sently shown to be modifi- cations of the same funda- mental parts. The axis (Fig. 32, A, a a) is composed of cellular parenchyma, ■with a larger or smaller proportion of fibro-vascular tissue ; and it is upon the mode in which these com- ponents are arranged rela- tively to each other, and in which progressive additions are made to the diameter of the axis, that the dis- tinction is founded between the Endogenous and Exo- genous tjrpes, which, toge- ther with corresponding distinctions in the struc- ture of the leaves, flowers, and seeds, afibrds a basis for the sub-division of the Phanerogamia into two pri- mary classes. From the central axis, bundles of fibro-vascular tissue pass down into the root-fibres which form the ultimate ramifications of its descend- ing portion; these are en- veloped in firm tissue, that limits their absorbent power to their extremities, which, being still soft and succulent, are known as ' spongioles.' On the other hand, the fibro-vascular bundles of the ascending portion of the axis pass rato the footstalks of the leaves ; and their ultimate ramifications form the skeletons of these organs, the interstices being filled up with cellular parenchyma, and the whole being clothed with an epidermis, quite distinct in texture from the parenchyma it covers, and perforated by -the peculiar apertures termed 'stomata' (Fig. 155). Various modifications present themselves in the form of the leaves, and in the arrangement of their component parts; but none of these afiect the essential character of the organs. The modes, too, in which they are arranged on the stem, present a great apparent variety; but they seem all reducible to one fundamental type, namely, a spiral, which is the result of the radiation of the appendages, not from a single A, Ideal Plant, after Sehleiden ; a to avi, the axiSj a being the root, ai, aii, ai'i, aiv, and av the successive intemodes of the stem, and a" the terminal development of the asis into an ovule ; b, rootlets ; c to cvii the successive fohaceous ap- pendages to the axis, c being the cotyledons, ei, eii, and ciii the ordinary leaves, civ the outer floral leaves or sepals, cv the inner floral leaves or petals, evi the stamens, and cvii the carpeUary leaves ; d, leaf-buds : — b, carpel enclosing an ovule, seen externally and in section, showing a the stig ' b the style, c the ovary : — 0, leaf-buda, as seen externa d'f and in section at d". ly at 34 GENERAL PLAN OP ORGANIC STEUCTtfBE AND DEVELOPMENT. point, but from a longitudinal axis. When this plan is characteristically exhibited, the leaves come off at regular intervals along the axis, but not in a vertical line one -with another, — the second not being above the first, but a little to one side of it, — the third holding the same relation to the second, — and so on ; in such a manner that a line carried through the points of origin of the successive leaves, -which are termed ' nodes,'_ -will not only ascend the stem, but -wdll gradually turn round it, and -will at last pass through a point directly above the origin of the first leaf. The leaves -whose origiQ has been intersected by this line, -whilst it makes one turn round the stem, are said to form a cycle; and the number of leaves -which this cycle contains, is subject to great variations. Thus ia Dicoty- ledonous plants generally it maybe said to he five; that is, the sixth leaf ■will be directly above the first, the eleventh directly above the sixth, and so on. In Monocotyledoiw, ho-wever, the typical number is three; the fourth leaf being above the first, the seventh above the fourth, and so on. There are oases ia -which the cycle seems to consist of only two leaves; each leaf springing from the side of the stem precisely opposite to that from -which the leaf belo-w it, as -well as the one above it, arises. The most common departures from the spiral type, sho-wn in the disposition of the leaves, are those -which are kno-wn as the opposite and the vertidllate (or radiate) arrangements. These may be reconciled with it in. three modes, each of -which has some evidence to recommend it ; and perhaps the deviation does not al-ways take place in the same vray.* 30. The complete _;?ora^ apparatus of Phanerogamia consists externally of a 'perianth,' composed of a series of verticils of foliaceous organs, -which do not depart -widely, except in colour, from the ordinary type of the leaf, and are arranged according to the la-w of spiral development round the axis. For the first or outermost layer of the ' perianth,' in a perfectly regular flo-wer, is formed of a -whorl of bracts; the calyx is com- posed of a -whorl of sepals (Fig. 32, c'^) alternating -with the preceding; and the corolla, in like manner, consists of a -whorl of petals (c^, -which alternates -with that of the sepals, but corresponds -with that of the bracts. These -whorls, in many flo-wers, are considerably multiplied, and the spiral arrangement of their component parts is often very ob-vious ; and, -when * Thus, ' opposite' leaves -would be produced in a plant whose ' cycle' consisted only of t"WO, by the non-development of every alternate segment, or ' intemode' of the stem, so that each leaf and its successor on the opposite side come to be developed from the same part of the stem, -whilst separated by an interval from the next pair. But this explanation does not suit those cases, in which the successive pairs of leaves are arranged on the stem at right angles to each other ; and this arrangement may either be attributed to the development of two opposite leaves from each node, the successive pairs being then arranged in a cycle of four ; or to the existence of two spirals proceeding up the stem simultaneously. — In like manner, a ' verticil' of five leaves originating from the same point of the stem, may be conceived to result from the non-development of the intemodea between five successive nodes ; and it sometimes happens that leaves which have a ver- tioillate arrangement at one part of the stem, are spiral at another, being separated by the development of the intermediate intemodes. But this does not account for the fact that the successive whorls themselves usually alternate "with each other ; each leaf of the verticU being over the spaces between the leaves of the verticil beneath it. And here again it would seem necessary, either to imagine that all the leaves of one verticil mav originate from a single intemode, or to suppose several spirals to be passing round the stem. In either way, however, this very common arrangement is reconcilable with the general theory of spu-al development, which is thus readily carried into application as regards the disposition of the parts of the Flower. GENERAL VIEW OP VEGETABLE KINGDOM. PHANEBOGAMIA. 35 such, is the case (as in the Garden Pseony), we may observe such a gradual passage from the type of the ordinary leaf, through the succession of bracts and sepals, to the most characteristic petal, that the essential con- formity of this last to the same general type with the preceding cannot be for a moment doubted. In the flowers of Dicotyledons, the typical number of components of each whorl, as of that of the cycle of ordinary leaves, is^e, whilst in the Monocotyledons it is three. The regularity of a flower may be interfered-with by the suppression or by the multiplica- tion of whorls; but the greatest departures from archetypal simplicity are those which result from the unequal development of different parts of the same whorl, some being very imperfectly evolved or even entirely suppressed, whilst others are extraordinarily augmented in size, and strangely altered in figure and character. The scientific Botanist, how- ever, can seldom be at a loss in the investigation of their real nature, if he proceed on the morphological principles already explained; and he continually finds his determinations justi£ed by the occurrence of ' mon- strosities,' which exhibit a more or less complete reversion to the arche- typal form (§ 82). The non-essential character of the perianth is indi- cated by the deficiency of one or more of its whorls in many tribes of Plants, which are nevertheless truly Phanerogamic. It is interesting to remark, however, that the group of GymmosperincB, in which the deficiency is most complete, reaUy form a transition-step to the higher Cryptogamia, in virtue of certain peculiarities in their proper generative apparatus, which will be explained hereafter (chap. xi). — It is within the protection of the perianth, that the true generative organs are developed; and these consist of the cmiher-s, (Pig. 32, c") from which the ' sperm-cells' (here termed pollen-grains) are evolved, and the carpels (c™), whose aggrega- tion forms the ^»si*Z, containing the ovules (a^), each of which includes a ' germ-cell' imbedded in a mass of nutritious matter, the whole invested by two or more seed-coats. Now the anthers, which with their support- iug ' filaments' constitute the stcmi&ns, depart more widely than do the sepals and petals from the ordinary condition of the leaf; but it is quite certain, aJike from the history of their development, from the series of intermediate forms which some flowers (as the Nynvphcea alba, or white water-lily) present, and from their occasional reversion in monstrous flowers to the form of petal or sepal, or' even to that of the ordinary leaf, that they too belong to the same type of structure. The carpels (b), again, may be regarded as leaves folded together at the edges ; as is indicated by their frequent retention of much of the leafy character, even in the normally-developed flower, and by their occasional more or less complete reversion to the type of the leaf in monstrous blossoms, sometimes when (as in the common ' double cherry') the stamens have undergone a less complete transformation. In the Gymtiosperms, indeed, the carpellary leaves are not folded together so as to enclose the ovules, which are deve- loped upon their internal surfaces ; and merely protect them during their immaturity, by their own mutual adhesion. — It is the general rule for the two kinds of sexual organs to be developed in the sanie organism ; and where, as is most commonly the case, every flower contains both stamens and carpels, it is said to be hermaphrodite. There are certain cases, however, in which, by the suppression of one or other of these whorls, the flowers become vmisexwd; when the staminiferous or male d2 36 GENEEAL PLAN OF OEGAiflO STEUCTUEE AND DEVELOPMENT. flowers are borne on the same plant or tree witt the pistilline, it is said to be monceciotis ; whilst if the two sets of flowers are developed by dif- ferent individuals, the species is said to be dioecious. This last arrange- ment, in which the generative apparatus attains its highest degree of difierentiation, is comparatively infrequent ; but we find examples of it in several groups of Cryptogamia, as well as among Phanerogamia. 31. The ' embryo-cell,' which is formed within the germ-cell, after the admixture of the contents of the sperm-cell with its own by the means already adverted-to, developes itself by duplicative subdivision, just as among the lowest Cryptogamia ; but the nourishment which it requires for the continuance of this operation is furnished by the store previously laid-up in the ovule ; and the entire mass of cells thus formed, instead of subdividing to constitute a multitude of independent organisms, remains connected so as to form biit a single fabric ; and this exhibits at a very early period a tendency to become heterogeneous, by the development of distinct organs, every kind of organ, however, being very numerously repeated. For, at the time that the seed is detached, as a self-sustaining struc- ture, from the parent, the embryonic rudiments of the stem and root are already formed, and a temporary leaf-like expansion, the single or double cotyledon (Fig. 32, A, c), is prepared to evolve itself; whilst a supply of nutriment for its further development is stored-up within it, either form- ing a separate albumen external to the embryo, or being contained within its cotyledons, which are in that case thick and fleshy. The subsequent evolution of the plant, of which ' germination' is the first stage, consists in the progressive development of the ascending and descending axes and of their respective ramifications, these remaining permanent j and in the evolution, from the ascending axis, of a succession of mutually-simUar appendages, foUaceous and floral, which have only a temporary existence, each set being in its turn replaced by another. Thus the individuality of the whole fabric is maintained, whilst a continual change is taking place in certain of its component parts. 32. It is with the performance of the true generative act, and the consequent production of a new embryo- cell, that each " new generation" originates. But it is not in this mode alone, that Phanerogamic Plants (for the most part at least) are multiplied. Por each leaf-bud usually possesses within itself the capacity of putting forth roots, when separated from the parent-stock and placed in circumstances favourable to its growth, so that it thus becomes capable of maintaining an independent existence, and of developing itself into a perfect Plant; and there are some Phanerogamia which spontaneously detach leaf-buds or ' bulbels,' and which thus multiply themselves after a manner analogous to that which prevails so remarkably among the lower Cryptogamia. This is pre- eminently the case, for example, with the common Lemna (duck-weed) each plant of which consists of but a single foliaceous body, with a root- fibre hanging from its under surface; this puts forth buds from its margin; and these buds, early detaching themselves from their stocks henceforth maintain an independent existence, so that the plant thus becomes rapidly multipHed by gemmation, large surfaces of water being covered by the growth proceeding from a single individual, without the intervention of any process of generation.— It is interesting to remark that this little plant seems to hold almost the same relation to GENEEAL VIEW OP ANIMAL KINGDOM. 37 Phanerogamia, that the lowest Protophyta do to Oryptogamia. For it scarcely presents any distinction of parts, the leaf and stem being fused together into a single flattened lobe, whilst the organs of reproduction are reduced to their very simplest form, being developed in a sUt in its edge. Its texture, too, is of the simplest kind, being composed of scarcely any- thing but ordinary cellular tissue. And the developmental process here, as m the Protococci, consists in the multiplication of organs which repeat each other in every particular, and which, having no relation of mutual dependence, can exist as well detached as coherent; instead of tending, as in the higher forms of Vegetable life, to the evolution of a single fabric,' whose several parts present a marked differentiation of external form and of internal structure, and have such a functional dependence on one another, that they can only exist as living bodies so long as they remain mutually connected. 33. Animal Kingdom. — Turning, now, to the other great division of the Organised Creation, we shall in the first place examine, as in the previous case, what is the highest form under which its life expresses itself The whole nism of Vegetative existence consists in the activity of the organs of Nutrition and Keproduction; but, on the other hand, the nisus oi Animal life tends towards the evolution of the faculties of Sensation and of Self-determined motion, and, in its highest manifestation, to that of the Intelligence and Will. The instruments of these faculties, how- ever, are in the first place developed, and are afterwards sustained, by the Organic apparatus with which they are connected; whilst, in their turn, they become subservient to its operations : so that, in those forms of Ajoimal existence, in which there is the greatest differentiation of organs, there is at the same time the closest relation of mutual depen- dence in their actions; and every thing tends to render the entire pro- duct of each generative act a single individual, in the most restricted sense of that term, no multiplication by the subdivision of that product ever taking place (save as a monstrosity), but the whole of it evolving itself into a congeries of different but mutually-related organs. It is only in the higher forms of Animal existence, however, that we meet with this complete individualisation, and this marked predominance of the anvmal over the vegetative. In a large proportion of the beings composing this kingdom, the apparatus which is subservient to the strictly animal functions is scarcely differentiated from that which ministers to organic life; in many of the cases in which the former is separately distinguished, it seems but a mere appendage to the latter; and it is only in the highest or Vertebrate type, that we find the general plan of the fabric distinctly arranged with special reference to the mani- festations of Animal power, which involve the exercise of its highest attri- bute, — Intelligence. The nearest approach to this is made in the higher forms of the Articulated series; in which a very remarkable degree of development is given to the instruments of the lower animal powers, especially the locomotive apparatus ; and in which the general plan of structure, and the arrangement of the nutritive apparatus, have evident reference to this. But in the MoUusoa, we find a marked predominance of the Vegetative apparatus ; it being in only a small proportion of the group, that there is any considerable power of movement. And in the Radiata, it becomes obvious that the general plan has reference rather 38 GENERAL PLAN OF OBGANIC STEUOTUEE AND DEVELOPMENT. to the 'vegetative repetition' of the organs of Nutrition and Reproduction, than to any manifestation of the higher Animal powers ; the apparatus for which, in so far as it is developed, exhibits a like repetition of similar parts. — Notwithstanding the diversity of these types of structure, how- ever, and the marked differences which they present in regard to the relative development of their several organs, we observe in the higher forms (at least) of each of them, a differentiation of all the most important parts by which the Animal is especially characterised. For we find in each type a digestive cavity for the reception and preparation of aliment ; chylij&rous channels or vessels, into which the liquid prepared by the diges- tive process transudes from this cavity, to be conveyed to the remoter parts of the organism ; a circulating system, by which the distribution of the nutritive fluid is effected, the surplus materials brought back, and the waste or refuse matter removed from the tissues and conveyed for elimi- nation to appropriate organs; a respiratory surface, through which the circulating fluid is exposed to the influence of atmospheric air; seoreti/ng glands for the separation of certain products from the blood, either for its purification, or for special uses in the economy, or for both purposes combined; generative organs, in which 'sperm-cells,' or 'germ-cells,' or both, are developed, the latter being enclosed (as in Phanerogamous Plants) in a store of nutriment prepared for the nutrition of the germ, so as to constitute an ovwm; organs of support and protection, forming a 'skeleton' of some kind, either external or internal; organs oi sensation; organs of consciovsness and self-direction; and organs of locomotion. 34. It is true that in the least-developed forms of each type, we may find some or other of these organs but little distinguished from the general structure, or even entirely absent ; but the proportion of such forms is smaller, the higher we ascend in the scale. Thus, in a large part of the Radiated series, there is but Kttle differentiation of the several parts of the nutritive apparatus; and although the reproductive is nearly always very distinct from it, yet even this is scarcely segregated in the lowest examples of the type : wMlst even the very slight develop- ment which the organs of animal Life attain in the higher Radiata is altogether wanting in the lower, among which they are not distinguish- able by any structural mark. — But in the Molluscous series, it is only among the very lowest that we have a difiiculty in distinguishing all the essential parts of the apparatus of nutrition and reproduction, the absorbent and circulating apparatus being usually that which is most imperfectly developed; and although the organs of sense and locomotion axe not evolved in the same proportion, we never fail to find a nervous gangKon, which must be considered as marking the existence of some degree of consciousness. — On the other hand, in the lowest forms of the Articu- lated series, it is the imperfection of the nutritive apparatus which most strikes us; and although distinct sensori-motor organs are there also very- deficient, yet they present themselves very prominently in higher parts of this series, in which the type of nutritive system is still com- paratively low. In both these sub-kingdoms, however, it is only in a small proportion of each series respectively, that we fail to discern all the essential parts of the assemblage of organs just now enumerated • those higher forms of each, in which the differentiation is complete, constitut- ing the great bulk of its entire series, instead of being, as among the GENEBAL VIEW OP ANIMAL KINGDOM. PBOTOZOA. 39 Eadiata, exceptional as to number, and probably to be so considered in regard to tjrpe likewise.* — ^No-w, in the Yertehrated series, the complete differentiation of all these structures is nearly the invariable rulej it being only in one of the very lowest Fishes (the Amphioxus), that we meet with such an imperfect development of any of the systems above enumerated, as reminds us of those simpler organisms in which they are absolutely deficient. — There is another point of interest nearly related to the preceding, in regard to which thpse primary types of Animal confor- mation present a marked contrast ; and this is the degree in which they are severally capable of being multiplied by gemmation. This power exists among Zoophytes in exactly the same degree as among the higher Plants ; for, whilst the gemm», in the former as in the latter, usually remain connected with the parent-stock, they are capable of maintaining their existence if detached, and are regularly thrown-off in some species, so as to become independent organisms, possessing all the capabilities of that from which they have separated themselves; and in the very simplest Zoophytes (as the Hydra), we even find a capacity for reproducing the entire fabric to lie in every fragment of the body, just as a fragment of the leaf of Bryophyllum will give origia to an entire plant (§ 21, note). A like capacity exists in the lowest group of the Mollusca, which, in this and in many other particulars, closely borders upon Zoophytes. It is only among a very small number of the lowest Articulated animals, however, that this method of multiplication presents itself. And among Vertebrata it seems entirely wanting as a regular habit, although there is reason to think that it may occasionally occur as an abnormality, at that early period of the evolution of the germ when its grade of develop ment has not advanced beyond the Zoophytic stage (chap, xi.) 35. Underlying these well-marked types of Animal organisation, how- ever, there is a group of beings which cannot be regarded as presenting even a rudiment of the plan of conformation that is characteristic of any one of them, and in which scarcely any differentiation of organs is to be discerned, — a group, in fact, which holds a rank in the Animal kingdom, that is precisely parallel to that of the Protophyta in the Vegetable (§ 22), and which may therefore be appropriately designated Protozoa. Between these two groups, indeed, no definite line of demarcation can be drawn; and the same beings have been reckoned as Plants or as Animals, accord- ing to the particular views of the classifier in regard to the mode in which they should be distinguished. A large proportion of the Protozoa consist of single cells, or of aggregations of cells ia which there is no differentiation of character; and in the lowest forms of them, there is not even that distinctness of the cell-wall from the cell-contents which exists in every completely-developed cell, but the whole forms one mass of living jelly (Fig. 33). The animal character of this, however, is marked in its mode of nutrition; for it does not draw its aliment. Eke the Proto- phytes, from the surrounding air and moisture, but is dependent for its support upon organic substances previously elaborated by other bemgs, which it envelopes with its own jelly-like substance, and of which it gradually dissolves and appropriates that which is fitted for its own * In the Author's opinion, the Zoophytes, not the Eehinodermata, are the typea of the Radiated aeriea i—Gasteropods of the Mollmscous /—Insects of the Articulated ;—md Mammals of the Verteirated. 40 GENEEAL PLAN OP ORGANIC STBUCTUEE AND DEVEWPMENT. increase. The animal character of this body is also indicated by its movements: for although the ' zoospores' of the Protophyta and lower Fio. 33. AmiBbi frincepi, in'diiferent forms, a, B, c. Algse are rapidly propelled through the water by ciliary action, yet they do not exhibit that motion of one part upon another, which is often seen in the simplest Protozoa. But there are as yet no special instruments either for sensation or for motion. As every part of the body is equally adapted for digestion, for absorption, for circulation, for respira- tion, and for secretion, so does every part appear equally capable of receiving impressions made upon it, and of responding to them by a con- tractile movement. From this starting-point we may proceed in either of two directions; for we find in the Infasory Animalcules a tendency to the individualisation of the single cell, which seems to attain in them its highest development as a separate entity ; whilst in the Rhizopoda (Fora- minifera) and Pon/era (Sponges) we find aggregations of gelatinous bodies (which present more or less distinctly the characters of true cells) assum- ing certain definite types of form, and approaching the individuality of higher organisms. — In the true Animalcules (excluding the Rhizopods and the Protoph3rta which have been confounded with them) we find an obvious distinction between cell-wall and cell-cavity ; there is a definite opening into the latter, through which food is introduced, instead of its being received into any part of the mass ; and there is frequently, also, a second orifice, through which iadigestible particles are expelled. More- over, the locomotion of these beings is performed, as in the Protophyta, by the agency of cilia; these being prolongations of the cell itself, to which the contractile power is especially delegated. Their midtiplication is ordinarily accomplished, like that of the Protophyta, by duplicative sub- division ; and in this way a vast number of similar beings may be pro- duced, each of which is a repetition of the rest, and lives altogether inde- pendently of them. But it seems probable that, like the Protophyta, they have a proper generative process, consisting in the ' conjugation' of two similar cells ; no sexual distinction as yet manifesting itself between these, and both of them apparently contributing in the same manner and GENERAL VIEW OF ANIMAL KINGDOM. PROTOZOA. 41 degree to the production of the germ. — In the Rhizopoda, we find the simple jelly-like mass extending itself by gemmation, and at the same time very commonly forming a calcareous envelope upon its extei-ior; whilst through apertures in this are put forth extensions (pseudopodia) of the soft substance in its interior, through which the introduction of nutriment into the body seems to be chiefly effected. Notwithstanding the small amount of differentiation which appears to exist among the several products of gemmation, yet a strong tendency to individuahsa- tion in the entire aggregate is shown in the very definite plan of growth which each species exhibits, as is most obviously seen in Nurmnulites and other higher forms of Foraminifera. Of the mode of multiplication of these animals, nothing is yet known. — In the Porifera, or Sponges, there is, with less definiteness of configuration in the aggregate mass pro- duced by gemmation from the single primordial cell, a much higher degree of mutual interdependence; for we now find the component particles so arranged as to form the rudiments of differentiated organs, whilst the general plan of structure approaches that which we meet-with among the lower Zoophytes, in whose fabrics the individuality of the components is stUl more completely merged in that of the organism as a whole. For, in the first place, we have a marked distinction between the internal fibrous skeleton and the soft flesh which clothes it; and these components have a very definite and characteristic arrangement, which varies in different parts of the mass ; being dissimilar, near the external surface, and around the internal canals, to that which prevails in the intervening substance. Again, in the system of absorbent pores for the entrance of liquid, and of ramifying canals for its discharge, we have the first rudiment of a digestive and circTilatory apparatus, not yet marked-off, however, from the general cavity of the body. And although the organs of nutrition do not present any further specialisation, yet those of reproduction are differentiated from them, and are limited to particular parts of the mass. Even in this lowest form of an aggregate Animal, there is reason to believe that a true ovmn is produced; so that we here already advance to the same essential type of generation, as that which prevails in the highest plants. 36. Among the four definite types of structure under which all the higher forms of Animal organization may be ranked, the Radiated, as already remarked, unquestionably holds the lowest rank: in virtue alike of the close conformity of its general plan to that which prevails in the higher Plants ; of that predominance of its Vegetative or Nutritive apparatus over that of Animal Ufe, which is conspicuous even in its higher types; and of that very imperfect differentiation of 'the organs of the former, which prevails through the larger part of the group. Each of these points will now be noticed in some detail. — The radial synv- mei/ry must he regarded as in itself a vegetative character, for it cor- responds with that which is seen in the disposition of the appendages around the axis in the leaf-buds and flower-buds of plants; and it is inti- mately connected with another vegetative character, the repetition of smdla/r paHs. Thus, in the animals in which it prevails, we find the central mouth to be surrounded externally by a circular series of pre- hensile appendages; which may be mere oral tentacles, as in the Polypes (Figs. 34, 35), the Medusae (Fig. 93), and the Holothwria (Fig. 40), true 42 GENEBAL PLAN OP OEGANIC STEUCTUEE AND DEVELOPMENT. arms, as in the Ophiwra and Comatvla (Figs, 8, 38), or divisions of the body itself, as in tlie Star-fish (Fig. 37). In the arrangement of the internal organs, a similar character is exhibited; that is, a circular disposition of parts -which precisely repeat each other. There are, it is true, modifi- cations of the radial type in certain aberrant forms of the group, which tend towards a bi-lateral symmetry; but these are comparatively rare exceptions, which it is only necessary here to mention. It is not only in their radial symmetry, however, that the animals of this division are conformable to the type of the higher portion of the Vegetable kingdom; for this conformity is equally shown by a large proportion of the group, in the development of composite structures by gemmation. From a single polype, as from a single leaf-bud, an arborescent structure may be evolved, bearing hxmdreds or even thousands of polype-bodies, all origi- nating from the first, and maintaining an intimate organic connection with each other; thus bearing a close physiological resemblance to a tree, and requiring to be considered (like it) as a single individual, although its several members have no relation of mutual interdependence, and can maintain a separate existence if detached. It is not to be won- dered at, then, that the older Naturalists, who were only acquainted with the skeletons of Zoophytes, should have considered them as vege- table structures, and that many of them should even now be popularly regarded in that light; whilst even the movements exhibited by the living polypes, not being apparently very different in nature from those per- formed by the Sensitive-Plant, or the Venus's Fly-trap, did not seem sufficient to establish their animal nature. This extension of the original fabric by gemmation may take place among Zoophytes to an indefinite extent ; and the mode in which it occurs is the chief determining cause of the particular type or plan of growth which is traceable in each species, but which is liable to great variation from the influence of external conditions. In nearly all the members of the class of Acelephce, it seems to take place at some period of life or other; for although we find few traces of it in the fully developed Medusae, yet (as will be shown hereafter, chap, xl) multiphcation by gemmation takes place to an extraordinary extent during the early stages of their existence; and in some of the lower forms of the group, especially those which closely approximate to the Zoophytic type, it continues during the whole of life, and gives rise to those composite fabrics of the Cirrhigrade and Physograde orders, which, until the recent discovery of their true cha- racter, have been a source of so much perplexity to Naturalists. In the class Echinodermata, multiplication by gemmation very seldom takes place; but its' members retain throughout their lives an extraordinary measure of that power of reproducing lost parts, of which the production of an entire organism by gemmation is only a higher manifestation. 37. The low development of the proper Animal powers in Radiated animals, as compared with their Vegetative activity, is one of the most remarkable features of the group taken as a whole; nor are there are any exceptions to this general character. In none of the true Zoophytes is the nervous system differentiated from that general fibro-gelatinous tissue of which the entire bodies are composed; every part seems more or less impressionable and contractile, although these attributes are most strongly displayed in the oral tentacula; and there is no evidence that GENERAL VIEW OP ANIMAL KINGDOM. ZOOPHYTES. 43 Fio. 34. the respondence to external impressions whicli is probably the source of all their movements, proceeds from any distinct consciousness of these impressions. It is in the Aoalephce, that the first traces present them- selves of a nervous system, and of organs peculiarly fitted to receive sen- sory impressions; but it is probable that a large part of the movements executed by even these animals, are not dependent upon any influence transmitted through this apparatus. In the Echinodermata, whose organs and tissues attain a far higher grade of development, the nervous system is more clearly marked out; and the distinction between nerve-cords and ganglionic centres, which has not yet been clearly established in the Acalephse, may be unmistakeably affirmed to exist. There are also rudi- ments of eyes in certain members of this class; and there is some evidence that their movements are directed by visual impressions received through these organs. 38. Between the lowest and the highest members*' of the Radiated series, there is a very marked contrast in regard to the differentiation of the principal organs of Vegetative life; but a number of intermediate grada- tions present themselves, which establish a tolerably complete transition from the one condi- tion to the other. — Commencing with the Hydra (Fig. 34), we find the digestive apparatus re- duced to a state of the greatest simplicity, the whole body seem- ing to be nothing else than a stomach, with a circle of pre- hensile tentacula around its ori- fice, which, being single, and serving alike for the reception of food and for the ejection of its indigestible portions, must be considered as representing in itself the cardiac and pyloric orifices of . the stomachs of higher animals. The wall of this cavity and the general in- tegument of the body are so closely connected together, as to seem like two layers of one and the same membrane; there are, however, some lacunar spaces between them, constituting the first indication of that ' general cavity of the body' which exists in almost every other animal. A, Syd/ra fasca, or Brown Fresh-water Polype, attached to a piece of stick, with its anna extended, aa in search of prey; a, the mouth surrounded by tentacula; 6, foot or base, with its suctorial disk : at b ia seen a portion of one of the arms near ite origin, and at another portion near its termination, more highly magnified. and which performs, as we shaU see. very important functions; and these lacunar spaces commtmicate 44 GENERAL PLAN OF OBOANIC STEUCTUEE AND DEVELOPMENT. Fia. 35. ■with similar cavities in the interior of the tentacula. There does not yet appear to be amy decided structural or functional differentiation between the layer which lines the stomach and that .which clothes the body; since each can perform all the offices of the other, as is shown by the result of Trembley's well-known experiment. No circulating apparatus is yet distinguishable, the nutritive liquid, which is the pro- duct of the digestive operation, being at once absorbed from the parietes of the stomach into the general substance of the body and arms ; nor is there any special respiratory or secretory apparatus. Even the gene- rative organs, which are usually the first to be differentiated from the rest of the fabric, cannot here be distinguished; for ovules and sperm-cells are evolved in the substance of the ordinary tissue; and the only indication of their specialization is afforded by the restriction of their production to particular situations, the sperm-cells usually making their appearance just beneath the arms, whilst the ovules protrude nearer the foot. The homogeneousness of the entire body, however, is Most remarkably evinced in the facts, that gemmce which develope themselves into new Hydrae sprout almost indifferently from any part of it, and that a minute frag- ment from any region will (under favourable circum- stances) regenerate the whole. In the composite fabrics which are formed after the Hydra- form type (Fig. 99), the conso- lidation of the external in- tegument necessitates several other changes; amongst the rest, the evolution of a special reproductive apparatus, and the separation (within the polype-cells) of the wall of the stomach from the external integument, so as to com- mence the formation of the 'general cavity of the body.' This, however, is carried much further in the Actinia (Fig. 35), and in all the Polypes Diagrammatic section of Actinia, shoTring its interna formed UDOn its tvnp ■ fni- in structure;— a, a, base or foot; 6, 6, oral disk; c, e.tenta- j-i, "^ j, * "Jr^' ^"^ ^" cnla; i, mouth; e, stomach; p,^, i, i, vertical partitions ^'ISSe We tmd the Stomach out across in different directions;/,/, apertures in these; susnPTirlprl tao ;+ „■ \ ■ *, passages opening into the t4ntMula ; I, I, testes or ^^P^Haea (^as it were) in a OTaria; m, m, filiferous filaments. large Space, which is Subdi- J -^ • • 4.1, 1, 1, ^t, r 1 / , • , "^^"^ ^y radiating partitions; and it IS m the chambers thus formed (which are prolonged into the interior of the tentacula) that the generative apparatus is situated. Very distinct organs for the production of sperm-cells or of ova are here evolved • these organs (according to late researches, chap, xi.,) not being combined' in the same individuals. There is still a direct connection between the interio f the digestive sac and the general cavity of the body, by an &r,erh-,J «l the bottom of the former; and through this, the nutritive products of digestion find their way into the surrounding cavity, mingled with the GENERAL VIEW OF ANIMAL KINGDOM. AOALEPHiE. 45 ■water which, is introduced through the mouth. This is the only mode in which the tissues are nourished, as there is not yet any special circulating apparatus; and, in like manner, it is only by the expulsion of the fluid that has remained for some time in the general cavity, that the excre- tory products which have found their way into it from the tissues, can be carried out of the body, in those species which have no orifices at the extremities of the tentacula. Thus the very same liquid answers aU the purposes, in these simply-formed animals, which are seized in Vertebrata by chyme, chyle, arterial blood, and venous blood; and it also serves as a medium for respiration, the external integument being usually so thickened and hardened, that the amount of aeration of the interstitial fluids which takes place through it must be extremely limited, in com- parison with that which will be carried on through the delicate mem- branes clothing the internal surfaces. Thus, with some very important points of difierentiation, the general type of these animals remains extremely low; and their power of multiplying by gemination, and of reproduoiag lost parts, in which they are only inferior to the Hydra, is what we might anticipate from their general homogeneousness. In the composite Aotiniform Zoophytes, a certain degree of connection remains between the general cavities of the Polypes which have budded-off one from another; but this connection is more intimate in the Aloyonian Zoophytes (Fig. 100), among which the 'general cavity' extends through- out the polypidom, forming a branching system of canals which strongly resembles that of Sponges. In fact, when we compare the two organisms, we can scarcely fail to perceive that the Alcyonium is essentially a Sponge of which certain parts have been difierentiated from the rest, and evolved into special organs. And this view is confirmed by the circumstance, that when a bud is put forth from one of those polypidoms, it has all the ordi- nary characters of a Sponge, except that its canals do not open upon the external surface (Fig. 9 1) ; the formation of a polype-mouth and stomach not taking place until a later period. 39. The lower forms of the class of Acakphce carry us back to the grade of development proper to the composite Hydraform Zoophytes. But in the higher, such as the ordinary Medusa (Fig. 36), there is a far less amount of repetition of similar parts, the gemmse detaching themselves from each other at an early stage of development, and subsequently maintaining an entirely independent existence. There cannot be here said, any more than in the Hydra, to be any 'general cavity;' for the space between the walls of the digestive sac and of the ovarial chambers which surround it, and the external integument, is occupied by homo- geneous solid tissue. But a series of gastro-vascular canals, commencing from the stomach, radiates towards the margin of the disc ; and these serve the double purpose of conveying the nutritive product of the diges- tive operation to the remoter parts of the body for the supply of their wants, and of subjecting it to the aerating influence of the surrounding medium. In its return to the centre, the fluid will of course carry back with it whatever excretory products it may have received from the tissues through which it has passed; and thus, like fluid of the stomach and general cavity of the Actinia, it answers to the chyme, chyle, arterial blood, and venous blood, of Vertebrated animals. In the .Beroe (Fig. 102), and certain allied forms, the digestive cavity has an anal as well as an oral 46 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. orifice; and there also appears reason to think, that in its system of gastro- vascular canals a difierence already exists between the afferent and efierent tubes, the fluid passing forth from the stomach by one set, and returning to it by the other. The generative apparatus in this class always exhibits Fio. 36. Stmcture of Cyamsa omnia. — Disk seen from above, showing the quadrilateral mouth a, the four OTarieB&&&&, thefonr orifices of the ovarian chambers cocc, the stomach dddd,«ai its radiating prolongations, the eight anal [?] orifices e e, &c., and the eight ocelli \y'\ff, &c. a very well-marked differentiation; its type being in many respects higher than that of the true Zoophytes. For in the Medusa, the four ovaries or testes (6, 6) are lodged in cavities roimd the mouth, each of which has its own proper outlet (c, c), so that the mouth is no longer (as it is in those species of Actinia the extremities of whose tentacula are closed) the only channel for the escape of the fertilized ova or of the rudimentary young. The sexes are here distinct, the ova and testes not being combined in the same bodies : and this is true also of many of the composite forms which develope medusa-Uke buds containing sexual organs, each indi- vidual producing buds of only one sex, as in dioscious plants; in others however, male and female medusa-buds are developed on the same stock as in monoecious plants, although in no case are the two sets of genera- tive organs combined in the same medusoid body. 40. In the class Echinodermata, the Asterias (Fig. 37) holds bv no means an elevated rank; yet we find in it a very marked advance upon either of the types previously described. The stomach with its single orifice, suspended in the midst of the ' general cavity of the body ' reminds us of that of Actinia; but it is entirely cut ofi" fi-om that cavitv which consequently remains closed. The nutritive products of digestion probably find their way into it, however, by transudation through the GENERAL VIEW OP ANIMAL KINGDOM. EOHINODERMATA. 47 walls of the stomach; and it is thence taken up by a regular system of vessels, the distribution of which, however, is very limited, so that the Fio. 37. Asteriaa wwramUcuiay with the upper aide of the hard envelope removed : — a, central stomach j 6, cseca npon its upper surface (salivary glands ?) ; e e, ceecalprolongations of the stomach into rays ;c', the same empty ; d, the same laid open ; e, the und^r surmce, seen &om vrithin after the removal of the ceeca, showing the vesicles of the tubular cirrbi ; jT, the same in a contracted state, showing the skeleton between them. fluid of the general cavity seems still to take the largest share in the nutritive operation. It is interesting to remark, that in this class we already meet with a differentiation, however imperfect, not only between the fluid of the gastric cavity, or chyme, and that of the surrounding visceral cavity, or chylaqueous Jlmd* but also between the latter and the * Theterm chylaqueous fimd, introduced by Dr. T. WiUiams, appears _ to the_ Author to be well adapted to designate the fluid of the ' general cavity, ' when (as ia Echiuodermata and Annelida) this is distinct alike from that of the digestive sac, and from that of the proper circulating system. It is far more extensively employed, however, by Dr. Williams in his ingenious Memoir ' On the Blood proper and Chylaqueous Fluid of Invertebrated Animals,' in the " Philos. Transactions," 1852; being there applied to the immediate product of gastric digestion which passes directly into the ' general cavity' of the Aotini- form and Alcyonian Zoophytes, and even to that which is confined within the stomach and gastro-vaacular canals of Medusse. 48 GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. hlood contaiQed within the proper circulating system. A special pro- vision appears to be made for respiration in these animals, by the trans- mission of the ' fluid of the general cavity' into a multitude of short delicate cjecal tubes, -which pass between the pieces of the calcareous Fig. 38. Comatula rosacea. framework and project externally in little tufts, and which are lined with cUia that keep up a constant movement ia their contents. And there are various secretory or- ^^'^- ^^- gans possessing a dis- tinct glandular charac- ter, whose special uses are not yet certainly known. The genera- tive apparatus here attains a high develop- ment, the ovaries and testes (as in the higher Acalephse) being no longer combined in the same individuals, and having separate orifices for the discharge of ,. , , , ^t, ..• ^ , ,^ their products; it is in- teresting to remark, however, that m Comatula (Fig, 38), whose digestive apparatus is framed upon a higher type than that of Asterias, the ovaries are dispersed in isolated spots through the integument of the arms. The Star-fish exhibits a series of elaborate provisions for locomotion in the beautiful articulation of the plates of the calcareous skeleton, in the con- tractility of the general integument of the body, by which its lobes (misnamed 'arms') are moved in various directions, and in the multipli- lEchinua vmmmUlattis. GENERAL VIEW OF ANIMAL KINGDOM. EOHNINODEEMATA. 49 cation of tubular drrhi fumislied witli suckers, by the contraction of wbicb, wlipn tbe suckers (forced out by the injec- tion of fluid into the cirrhi from the 'general ca- vity') have taken an attachment, the body is drawn towards the points to which they have adhered. — The chief feature of advance in the Eclmms (Fig. 39) is the conversion of the digestive sac with a single orifice, into an alimentary canal with a separate mouth and anus ; and around the mouth we find a very elaborate dental apparatus, furnished with distinct muscles, such as do not make their ap- pearance in any lower forms of or- ganisation. The locomotive appa- ratus, too, is still more highly de- veloped; for the body being now enclosed in an im- movable case, so that its parts are not themselves capable of flex- ure, a new set of instruments is evolved, namely, — ^the calcareous Fig. 40. Anatomy o{ Soloihuriatubulosa i—a, anus; i, mouth, snrroundedbyZO tentacula; c, cloaca, surrounded by muscular dilators c'; i, intestinal tube; m, mesentery ; tjiI, ml, longitudinal muscles ; mt, transverse muscles lining the entire inner surface of the integument; o, ovary; ap, caecal appen- dages, probably seminiferous ; p, contractile vesicle, probably a heart ; r,r, respiratory apparatus, originating in the cloaca; ^, oral tentacula; t\cmodl reservoirB ; va, annularvessel surrounding the mouth and supplying the ten- tacula; ve, external intestinal vessel, giving off a large anastomotic branch va' which enters another part of the same trunk ; m, internal intestinal vessel, with contractile dilatations ; dI, longitudinal tegumentary vessel, giving off transverse branches vV, seen by removing the longitudinal muscles ; om, piesenteric vessels, connecting the branches of the external intestinal vessel with those of the respiratory system of vessels, w. 50 GENERAL PLAN OF OEGANIC STBrOTUBE AND DEVELOPMENT. spines, which project from the surface, and are put in motion by the contractile integument, upon the ball-and-socket joints at their base. — The Holothwna presents us with certain interesting features of more complete differentiation, without, however, any very decided advance upon the type of the Echinus. The absence of a solid 'test' enables its movements to be performed by the flexure of the body generally; and for this a regular series of longitudinal and transverse muscular bands (Fig. 40, m I, m t,) is provided, remindiag us of those of the Worm-tribe. The alimentary canal (i) does not yet present any distinc- tion of parts into oesophagus, stomach, or intestine, but remains of nearly the same diameter throughout its length; it is held in its place in the midst of the general cavity of the body, however, by a regular mesentery, upon which the blood-vessels are minutely distributed. The circulating system is more complete than among other members of this class, especially in its peripheral portion; and it is furnished with a pulsatile vesicle (jp), whose contractions assist the onward movements of their fluid. For respiration there are two special provisions ; the fluid of the circulatory vessels being aerated by transmission to the branching oral tentacula (t) ; whilst that of the ' general cavity' receives the same in- fluence from the water introduced through the respiratory tree (r, r). The restriction of the outlet of the genital apparatus (o) to a single aperture (the five equal and separate portions of this apparatus in the Echinus and Asterias having each its own outlet) is a very decided character of elevation ; which seems to have been presented also by the extinct group of CysUdea (Fig. 81), notwithstanding that in the attachment of these animals by a stalk to a fixed basis, they (in common with the Crinoidea) showed a decidedly zoophytic tendency. 41. The Molluscous sub-kingdom, like the Eadiated, is remarkable for the high development of its apparatus of vegetative life in comparison with that of anvmal Ufe; but its type of conformation is altogether dif- ferent. It is true that, in the lowest group of this series, there is such a close apparent conformity to the Zoophytic type^ that the animals belong- ing to it were, untU recently, unhesitatingly ranked xinder that designa- tion. But it is now perceived that the resemblance is only superficial ; being dependent, in part upon the mode in which these animals extend themselves by gemmation, so as to form arborescent structures very ana- logous to those of true Zoophytes; and being partly caused by the state of degradation to which various organs are reduced, whereby their true type is obscured. — Taking it as a whole, the Molluscous series is charac- terized rather by the absence, than by the presence, of any definite or symmetrical form. In the Zoophytoid Mollusks, it is true, we are reminded of the radiated type by the circular arrangement of organs around the mouth (Fig. 49, a) ; whilst in the family of Ghitonidce, we meet with a division of the external skeleton into segments (Fig. 41) which reminds us of the articulated type. But these are peculiar excep- tions ; and a Molluscous animal is essentially a bag of viscera, enveloped in a skin which is thickened in parts by muscular fibres that are not arranged after any constant plan. In the ' archetype' MoUusk, the mouth and anus are situated at the two extremities of the sac ; and the various organs are disposed symmetrically on the two sides of a lono-itudinal GENEEAL VIEW OF ANIMAL KINGDOM. MOLLUSCA. 51 Fig. 41. A r median plane, just as in a Vertebrate or Articulate embryo ; the centres or principal trunks of the circulating apparatus being on the dorsal aspect (which may hence be termed the ' hsemal'), whilst the prin- cipal centres and trunks of the ner- vous system are on the ventral aspect (which may hence be termed ' neural'.) But this simple and symmetrical arrangement is very commonly obs- cured by subsequent inequalities in the development of particular regions, so that an entire change takes place in the relative position of the dif- ferent organs, and the types of con- forma,tion thus evolved seem to have little or no affinity to one another. * — The nearest approach to the archetype is presented on the whole by those of the Twnicata, in which the two orifices retain their original positions at the poles of the body (Fig. 42) ; and their chief peculiarities consist, in the first place, in the enormous development of their pharynx to form the J? \^ jL, Chiionellus. — B, Chiton. Fia. 42. Salpa maamna ; a, oral orifice ; i, vent j c, nucleus, composed of the stomach, hver, &o. ; d, brancliial lamina,; e, the heart, from which proceeds the longitudmal trunk/, sending trans- Terse branches across the body ; g, g, projecting parts of the external tunic, servmg to umte the different individuals into a chain. branchial sac, and secondly, in the inversion of their integument around the anal orifice, so as to form an immense cloacal cavity, the wall of which extends so far into the interior, and so completely envelopes the general mass of the body, as to constitute what is known as their ' inner tunic.'t— In the Bwalve MoUusks, on the other hand, the principal extension of the integument takes place externally; a duplica- ture of the thickened glandular skin of the ' dorsal' or ' hsemal region * See Mr. Huxley's admirable Memoir ' On the Morphology of the Cephalous MoUusoa,' in the " Philos. Transact. 1852." j ^i.- * • „„ „-™„ t Such is Mr. Huxley's very ingenious account of the production of this tump, as given in his " Eeport on the Tunicata" to the British Association, 1852. See also his Memoirs ' On the Anatomy of Salpa and Pyrosoma,' and his ' Remarks upon Appendiculana and Doliolum,' in the " Philos. Transact." for 1851. 52 GENERAL PLAN OF OKGANIO BTEUCTURE AND DEVELOPMENT. (here termed the mantle) being prolonged on either side into two lobes, which enwrap the body like a cloak, and form the valves of the shell upon their outer surface (Fig. 43). Again, a special development of muscular 3. Ventral Margin. Anatomy otMacira; — a, anus; &, posterior muscle; c, branchial ganglion; d, ovary; e, i, intestine ; /, shell ; p, nervous cord, connecting cesophageal and branchial ganglia ; h, stoina<:h ; if heart ; ft, liver ; I, anterior or oesophageal ganglia ; m, anterior adductor muscle ; n, nervous filaments ; o, mouth ; p, one of the oral tentacula ; q a;, mantle ; r, margin of the sheU ; s, foot ; 10, branchial lamellae ; if, oral siphon ; e, anal siphon. tissue in the integument of the ' ventral' or ' neural' region constitutes the ' foot' of those Lamellibranchiata which possess such an organ, In the Gasteropoda this foot assumes the form of an expanded disk (Fig. 44, a), upon which the animal can crawl ; the two extensions of the upper part of the integument are wanting; but the form of the body itself is entirely altered by the extraordinary and commonly unsymme- trical development of the hindmost portion of the haemal region into a ' post-abdomen,' which contains the heart and a considerable part of the alimentary canaJ, and from the mantle of which a shell (Fig. 45) is very frequently produced. In the ^jM^morcaJec? Gasteropods (Fig. 124) however the development takes place before instead of behind the anus • so that an ' abdomen' is formed instead of a post-abdomen. — This is also the case in the Pteropoda, in which the ' foot' proper is but little developed, whilst two lateral expansions {epipodia) sent off from it constitute the win„sacl.9fl ■aSc g'p CO 5 I'^SejI s§i =g n " 'S m- b a ■o"* ,a -1 5 ^■sll -1-1111- II ■§ si ^.^0.1^ 'gall IlillPlHii ¥ ° o 3 ^ ea m {8*09 R d ^ t^ ^ ft ^ m Woo ax) bp " _ ^ 'l||-|.|ii*si|li .|llill'|glli.i cao 74 GENEEAL PLAN OF OEGANIC STRUCTUBE AND DEVELOPMENT. 56. The Vertebral skeleton, as a whole, is very differently developed in various parts of the series. Thus in the ' apodal' Fishes, locomotion is per- formed by flexion of the body itself; and here, as in the Worm tribe, we find the skeleton extremely flexible, and its divisions indistinct, the spinal column being a continuous cylinder of cartilage with scarcely a trace of segmentation. In the Serpent tribe, again, which is destitute of loco- motive appendages, the vertebrse are extraordinarily multiplied, and are so connected by ball-and-socket joints, that the entire column possesses a most extraordinary degree of flexibility. In proportion, however, as distinct members are developed, and the power of locomotion is committed to them, we find the firmness of the spinal column increasing, and its flexi- bility diminishing : and in Birds — in which, as in Insects, the movements of the body through the air are effected by muscles that must have very fixm points of support — the vertebral column is much consolidated by the union of its different parts, so as to form a compact framework. As a general rule, then, the mobility of the extremities, and the firmness of the vertebral column, vary in a converse proportion. ' The number of these extremities in Vertebrata, never exceeds /biwy and two of them are not unfrequently absent. The power of locomotion is not developed to nearly the same proportional extent as in the Articulata ; the swiftest Bird, for example, not passing through nearly so many times its own length in the same period, as a large proportion of the Insect tribes : but it is far greater than that which is characteristic of the MoUusca; and there is no species that is fixed to one spot, without the power of changing its place. On the other hand, the highest Mollusca approach them very nearly in the development of organs of special sense, of which Vertebrata almost invariably possess all four kinds — sight, hearing, smell, and taste. 57. A Hke union of the characters of the Articulated and Molluscous sub -kingdoms, may be noticed in the general relations which the organs of Nutritive and those of Animal life bear to one another. The former, contained in the cavities of the trunk, are highly developed ; but, as in the MoUusca, they are for the most part unsyinmetrically disposed. Of the latter, the nervous system and organs of the senses occupy the head, whilst the muscles of locomotion are principally connected with the extremities; both are symmetrical, as in the Articulata; but, whilst that part of the nervous centres, which is the instrument of reason, is very largely developed, the portion which is specially destined to locomotion, together with the muscular system itself, bears much the same proportion to the whole bulk of the body, as it does in the Articulated series. Hence we observe tha,t the Vertebrata unite the imsymmetrical apparatus of nutrition, characteristic of the Mollusca, with the symmetrical system of nerves and muscles of locomotion, which is the prominent characteristic of the Articulata; both, however, being rendered subordinate to the great purpose to be attained in their fabric — ^the development of an organiza- tion through which intelligence may peculiarly manifest itself. Tor the operation of this, a degree of general perfection is required, which is not manifested elsewhere; and, in order that the body may be always in a state of readiness for active exertion, it is requisite, not merely that it should be adequately nourished, but that it should be constantly mamtained at a high temperature. This is fully the case, however, with only the two higher classes of Vertebrata; which, in their power of gene- GENERAL VIEW OP ANIMAL KINGDOM. VBKTEBEATA. 75 rating sufficient heat to counteract the effect of external vicissitudes, and in the uniformity of their rate of vital activity, contrast strikingly with the Invertebrated classes. Indications of a like power, however, though less energetic in its operation, are presented in Reptiles and Fishes. The maintenance of a constantly-high temperature, and the support of the system under the demands created by its unceasing activity, involve an energetic performance of the functions of respiration and circulation ; and these again require a constant supply of alimentary material, and great activity iu the process of digestion. The Digestive apparatus usually presents a high degree of specialization ; its simplest forms (as among the Invertebrata) being found in those tribes which obtain their food by the suction of animal juices, and its most complicated in the vegetable- feeders. Not only do we almost invariably find the stomach anatomically separated by valvular constrictions from the cBsophagus and iatestinal tube, but it is completely differentiated from them in function also ; the intestine itself generally exhibits varieties of conformation and of endow- ments in different parts of its course ; and the number and complexity of its accessory glands is greatly augmented. The anterior part (considering the animal as placed horizontally) of the visceral cavity is usually occu- pied by the heart and respiratory organs, and the posterior by the diges- tive and excretory apparatus ; it is only in Mammals, however, that the separation between these portions is completely made, by the iaterposition Fig. 62. . Ideal Section otB.Mamiml, iUustrating the Vertebrate type of structure :-p, pectoral ex- tremities • T Tentral extrenuties; as, anus; a», organ oflearmg; 4, brain; ch ch svuial cK • I diaptopn ; ep, epiglottis ; g, stomach ; A>art j i, intestine ; ^, ^, jaws ; i kidney ; fCr; Clmgi^». mu^aea;