BOUGHT WITH THE INCOME ^ FROM THE SAGE ENDOWNENT FUND THE GIET OP Henrg W. Sage 189X .Admjo MlWlpA Cornell University Library QL 47.S5S Zoology of the Invertebrata : 3 1924 024 761 201 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/cu31924024761201 ZOOLOGY OF THE INVERTEBKATA ZOOLOGY / OF THE PVERTEBEATA A TEXT-BOOK FOR STUDENTS BY AETHUE E. SHIPLEY, M.A. FELLOW AND ASSISTANT TUTOR OF OHE-IST'S COLLEGE AND DEMONSTRATOR OF COMPARATIVE ANATOMY IN THE UNIVERSITY OF CAMBEIDOE LONDON ADAM AND CHAELES BLACK 1893 A. \'^i^'io PKEFACE In this book I have tried to give such an account of the Invertebrata as might be useful to students ia the upper forms of Schools and at the Universities, who are already acquainted with the elementary facts of Animal Biology. The volume is in no sense a work for advanced students, and hence no references to original sources have been given, and the names of the various investigators who have promoted our knowledge have been mentioned as sparingly as possible. In order to keep the book within reasonable limits, I have not described fully certain types which are dealt with in the admirable elementary text-books of Huxley and Martin, and Marshall and Hurst ; but, with this reservation, I have en- deavoured to describe some one example of each of the larger groups, and then to give a short account of the most interesting modifications presented by other members of the group. The last few years have witnessed a great extension in our knowledge of the structure and relationship of the Inver- tebrata. The earth has been ransacked for new forms, and improvements in microscopes and in technique have facilitated a more minute and thorough examination of these forms in the laboratory. This increase in our knowledge has neces- sarily been accompanied by a rearrangement of material ; many intermediate forms have been discovered, and unexpected relationships have been revealed, and these have entailed a revised classification. ZOOLOGY These facts have led me to treat the subject largely from a morphological standpoint, touching but lightly on the Histology, Embryology, and Natural History of the forms described. More space has been, as a rule, devoted to those animals which are regarded as intermediate between the larger groups than to the more specialised members of the groups. Any system of classification is to some extent a matter of personal judgment. I do not suppose that adopted here has any finality, but I hope the tables given will be of use to the student as expressing the results of the most recent research. In preparing the volume I have been much helped by numerous friends, to whom my best thanks are due. Dr. D. Sharp, Dr. Hickson, Mr. Beddard, Mr. J. J. Lister, Mr. F. G. Sinclair, Mr. C. Warburton, and Mr. MacBride, have aU given me the most generous assistance, and, above aU, I am most deeply indebted to my friend Mr. S. F. Harmer, who has in the most kind way read through the proof-sheets, and whose careful revision has saved me from many errors. To the Delegates of the Clarendon Press I owe thanks for permission to use Fig. 133, taken from EoUeston and Jackson's Forms of Animal Life. Herr Fischer of Cassel has kindly given me leave to use some reductions from the admir- able diagrams of Professor Leuckart ; these occur in the groups Echinodermata and Arthropoda, and are acknowledged under each cut; similarly the firm of Wieweg and Son have been good enough to allow me to use four figures taken from Vogt and Yung's Lehrlmch der Prahtischen Vergleichenden Anato^nie. I am also indebted to Messrs. Macmillan and Co. for their kindness in allowing me to use Figs. 37, 89, and 90, all of them taken from Professor Parker's Elementary Biology. AETHUE E. SHIPLEY. Christ's College, Cambridge, March 1893. CONTENTS CHAPTER I Introduction . PAGE 1 Pbotozoa CHAPTER n CHAPTER m MbTAZOA PORIFEHA 36 CHAPTER IV COELENTBEATA 47 COELOMATA CHAPTER V CHAPTER VI Platyhelminthes 80 CHAPTER VII Nemertba 115 CHAPTER VIII Nbmatoda . 124 CHAPTER IX HiRUDINEA 131 ZOOLOGY Chabtopoda CHAPTER X PAGE 138 Gbphyrea CHAPTER XI 159 Brachiopoda CHAPTER XII . 169 POLYZOA CHAPTER Xin 179 MOLLUSCA CHAPTER XIV . 189 ECHINODEBMATA CHAPTER XV 224 CHAPTER XVI Arthropoda — Crustacea . 253 CHAPTER XVII Prototracheata . 300 CHAPTER XVIII Mtriapoda . 310 CHAPTER XIX Insecta 321 CHAPTER XX Arachnida . 388 CHAPTER XXI Chordata . 423 CHAPTEE I INTEODUCTION Protoplasm is the name given to that colloidal, jelly-Uke substance which forms the basis of all life on this globe. Every living organism consists of protoplasm and the products of protoplasm. Whilst life lasts it is continually renewed from food which passes into the organism, and which, by the action of the protoplasm already there, is built up into new protoplasm. At the same time other portions of the protoplasmic body of the organism are being broken down, and the products thus formed are either thrown out from the body as excreta, or remain in the body, either stored away as useless, or in most cases performing some useful function, such as that of protecting the organism by forming a cyst or shell or internal skeleton. The protoplasm of living beings is arranged in a series of units or elements, termed cells, and with very few exceptions each cell contains one or Hciore speciaUsed portions of proto- plasm which take up staining material more readily than the body of the cell, and which are termed nvdei. An organism may consist of but one ceU with its nucleus or nuclei, but more commonly it is composed of an enormous number of cells, connected together, and each dominated by a single nucleus. In either case, whether the organism is unicellular or multicellular, the cell is capable of an extraordinary degree of differentiation, and may assume the most diverse forms. In the multicellular beings similar cells are massed together into aggregates which form the various tissues composing the body of the higher organisms. In unicellular forms the cells 1 ZOOLOGY composing the body sometimes remain in connection with one another, but they never form definite tissues, and the cells of such an aggregate are physiologically distinct and independent of one another, the whole forming a colony of unicellular beings. The organic world has developed in two diverging direc- tions, one corresponding to the animal the other to the vegetable kingdom, and though there is no difficulty in distinguishing the higher forms of these two kingdoms, it is . often by no means an easy matter to determine whether some of the lower forms should be grouped with the plants or with the animals ; hence any scheme of classification is largely dependent on individual opinion. There are a number of characters which if met with in an organism would justify us in claiming it as an animal, but in many cases one or more of these animal features are absent, and again other features may be present which, as a rule, are only found in plants, so that it becomes at once evident that the line between animals and plants, at any rate in their lowest forms, represents no scientific frontier, but is an arbitrary boundary which is apt to be shifted, now forward now backward, according to the opinion of the various investigators. The most important morphological difference between plants and animals is perhaps the presence of a cellulose coat which encloses, at any rate during some part of its life, the vegetable cell. Cellulose is a substance which has a definite chemical composition, and which, though practically universal in plants, is very rarely met with in animals. Another constituent found in all green plants, but rare in animals, is chlorophyll ; the presence of this enables the plant in sunlight to take in carbon dioxide, which serves as part of its food ; chlorophyll is, however, not found in all plants, the Tungi, an important section of the vegetable kingdom, being devoid of it. The physiological differences between plants and animals are more striking than the morphological. Plants can live upon much simpler compounds than animals ; they can absorb their nitrogen in the form of nitrates or simple compounds of ammonia, and their carbon in the form of carbonic acid, or some other soluble compound; thus they can live on liquid inorganic food INTRODUCTION which may enter the organism at any point, and consequently plants require no mouth or digestive cavity, or organs for the pre- hension of food. Animals, on the other hand, require more com- plex compounds; their nitrogen, with scarcely an exception, must be supplied in the form of proteids, and their carbon in the form of starch, sugar, or fat. Some of these compounds are not soluble, and hence an animal must ingest its food in a more or less solid state ; and to that end it is usually provided with a mouth and digestive tract, with organs for the prehension of food, and with locomotor organs so that it may find its food. Since the food of animals does not exist in nature except as the products of living beings, it is obvious that animals are ultimately depend- ent on the plant world for their means of subsistence. The broken - down products of the protoplasm are usually excreted by special organs set apart for this purpose in animals, but in plants the waste products are either diffused from the surface of the organism, or are stored away in the plant. There are no special excretory organs. In both plants and animals the most lowly organised beiags consist of one cell, and the unicellular organisms are termed the Protophyta and Protozoa respectively. The Metaphyta and Metazoa, or the multicellular plants and animals, consist of a number of cells arranged in more or less definite tissues, but even these multicellular beings pass through a unicellular stage, that of the ovum, whose repeated divisions after fertilisation give rise to the cells composing the body of the animal or plant. The Protozoa are therefore the simplest and most primitive animals, and it is natural to place them at the bottom of the animal kingdom. SCHEME OF CLASSIFICATION OF THE ANIMAL KINGDOM I a M I r Gymnomtxa PROTOZOA i COKTICATA ACOELOMATA METAZOA i rProteomyxa Myxomyoetes Lobosa Lahyrinlhviidea Reticularia ^. COELOMATA ' Heliozoa Radiplaria . Flagellata . Dinofiagellata Rhynchoflagellata Ciliata Acinetaria . Sporozoa / Porifera. \ Coelenterata. Platyhelminthes. Nemertea. Nematoda. Hirudin ea. Chaetopoda. Gephyrea. Braoniopoda. Polyzoa. Mollusca. Echinodei-mata. Arthropoda. .Chordata. Protomyxa, Vampyrella. Ohondrioderma, Fuligo. ( Nuda — Amoeba, Pelomyxa. \ Testacea — Arcella, Difflugia. I Perforata — Sotalia, Globi- J gerina. j Imperforata — Gromia, Orln- l tolites. Actinophrys, Actinosphaerium. Acanthometra, Collozowm. t LissoflageUata —Eugleim. A Choanoflagellata — Codosiga, \ Proterospmigia. Noctiluca. Peritricha — Trichodina, Vor- ticel.la. Heterotricha — Stmtor. Holotrioha — Parainoecium, ^Hypotrioha — Euplotes. Acineta, Dendrocameles. C Monooystidae Gregarinidea p^^^f^ i. Clepsidrina. Coocidiidea— Coccidium. Note. — In the tables of classification those groups which are not described in the text are printed in italics. After the title of the minor sub-divisions of the group, the name of one or more typical genera belonging to that sub-division is added in italics. CHAPTEE 11 PROTOZOA Chaeacteeistics. — Unicellular, or if composed of more than one cell, such elements not arranged in tissues. Food ingested hy a special mouth or ty any part of the cell substance. Reproduction never takes place hy ova and spermatozoa. Some forms are colonial. Group A. Gymnomyxa. The Protozoa have been divided into two groups, the Gymnomyxa, corresponding with the old group Ehizopoda ; and the Corticata, which comprise the Infusoria and Gregarin- idea. The former group includes all those forms which, like Amoeba, have, during the dominant phase of their hfe-history, no limiting membrane. Their protoplasm is consequently exposed, at any rate at one portion of their surface, and tends to run into processes or pseudopodia, which vary in appearance in the different species. Food may generally be ingested at any point of the naked protoplasm. Although the amoeboid condition is the one in which these organisms most frequently occur, they may pass through other phases, such as rounded spores enclosed in a membrane {chlamydospore), naked spores with a lash-shaped pseudopodium {flagellula), etc. Not infrequently two or more individuals fuse together, and this fusion may be the precursor of repro- duction. When the bodies of numerous amoebiform indi- viduals run together to form a large mass of protoplasm, the result is known as a Plasmodium. ZOOLOGY The classification here adopted is taken from Lankester's article on Protozoa. Class I. Proteomyxa. The simplest forms of Gymnomyxa are grouped together in the class Proteomyxa. As an example of this class the life- history of Protomyxa aurantiaca, a minute organism found in 1867 by Professor Haeekel, living on the coiled shells of the MoUusc Spirula, in the Canary Isles, may be described. Many of these shells were found bearing on their white surface a minute globular mass of an orange - brown colour. Each globule or cyst consisted of a central mass of protoplasm, surrounded by a structureless membrane ; in the older cysts the central protopksm appeared to be segmented into a num- ber of parts, each of which, on the bursting of the membrane, escaped in the form of a flagellula or pear-shaped swarm-spore. These moved actively about by the lashing of their whip-like pseudopodium, and soon underwent a change in form ; instead of one pseudopodium which acted as a flagellum, they devel- oped several, and then moved about like so many amoebae. After creeping about for some time, these amoeboid organisms fused together and formed a plasmodium, which in some cases attained such a size as to be visible to the naked eye. The Plasmodium gave rise to many branching ragged pseudopodia, by whose aid it ingested great numbers of diatoms and other food particles. It was much vacuolated, although none of the vacuoles were contractile. After crawhng over the Spirula shell for a time the plasmodium retracted its pseudopodia and became spherical ; it then surrounded itself with a cell waU, and the contents of the cyst thus formed broke up into flageUulae in the way indicated above. No nucleus has yet been observed in any phase of the life-history of this organism. Other genera have been described which live parasitically upon Spirogyra ( Vampyrella spirogyrae) or Diatoms (Archerina Boltoni, described by Lankester). In the latter chlorophyll corpuscles are present, and seem to dominate the cell body in a manner suggestive of a nucleus, which is otherwise absent. PROTOZOA Class II. Myxomycetes. The Myxomycetes differ from the Proteomyxa in their spores being always coated (chlamydospores), and in the fact that these are formed in definite cysts (Fig. 1, B), sometimes supported on columns, or in naked groups called sori. They Pw. 1. A. Plasmodium of Didymium leucopus (after Cienkowski). B. Spore cyst of Arcyria incamata (after De Bary). X 350. differ from, all other Protozoa in being rarely aquatic ; they usually live in the air, in damp places. Their plasmodia may attain a great size, several square inches in area, and form the largest masses of undifferentiated protoplasm to be met with. They hve on organic particles ; they are often of brilliant colour. The life-history of most of the Myxomycetes is a repeti- tion of that of Protomyxa : in some the flagellula phase is omitted, the chlamydospore giving rise directly to an amoeboid ZOOLOGY organism, provided however with a nucleus. These amoebulae may multiply by fusion, but ultimately they run together and form the plasmodia (Fig. 1, A), which form the dominant and characteristic phase in the life-history of the Myxomycetes. Fig. 2. A portion of the Plasmodium of Bad- hamia, x 3J, showing a pseudo- podium (1) commencing to enclose a piece of mushroom stem. After Lister. Spore of Chondrioderma. Spore of Chondrioderma dehiscing. FlageUulae which have emerged from the spores. B. ATnoebulae formed by meta- morphosis of flagellulae. F. Two amoehulae fusing to form F'. 6 and Gt'. Stages in the formation of a three-celled Plasmodium. H. A small Plasmodium. (B-H., after Cienkowski.) CJiondriodernia' difforme, the species illustrated in Fig. 2, has a smaU plasmodium, easily visible to the naked eye. This species occurs commonly on old bean-stalks. The plasmodia PROTOZOA 9 may be easily obtained by soaking some dried bean-stalks in water for twenty-four hours, and then keeping them in a moist chamber for ten days or so ; at the end of this time plasmodia may be observed crawling over the stems, etc. The sulphur-coloured Fuligo {Aethalium) is a genus which is met with in considerable masses creeping over the tan in tan- yards ; others occur in rotten wood, decaying bean-stalks, and dung. The spore cysts may or may not be stalked, and the pro- toplasm enclosed within them does not all become spores, but the remnant forms a meshwork of fibres differing in details in the various genera. This network, termed a capillitium, serves to support the spores, and possibly helps in their escape when the surrounding wall gives way. The walls of the cysts may be strengthened by the deposition of calcium carbonate. The coating of the spores is of a cellulose nature : a substance usually associated with the vegetable kingdom, but not unknown among animals, especially amongst the Protozoa. At times the plasmodia contract and surround themselves by a cyst, and pass through a quiescent period. This condition is known as the sclerotium. Myxomycetes are capable of retaining their vitality for long periods of time in a dried-up condition; they resume their active life again when supplied with moisture. About 300 species of Myxomycetes have been described, chiefly by botanists, who regard these organisms as being aUied to the Fungi. Class III. Lobosa. The individuals of this group are those Protozoa in whose life -history the amoeboid phase predominates. The pseudo- podia are lobose, thick, blunt processes of protoplasm, which are never filamentous and never anastomose. One or more contractile vacuoles are found, and it is stated that urates have recently been demonstrated in connection with these vacuoles in some Amoebae. The amoeboid individuals may conjugate from time to time, but do not form plasmodia. They some- times encyst, and the cyst is a resting one (hyprwci/st) and not a reproductive one (sporoeyst). The usual form of repro- duction is fission, which may pass into gemmation. The dis- ZOOLOGY tinction between the endoplasm and ectoplasm found in these and other Gymnomyxa is more apparent than real, and depends only on the presence or absence of food and other granules, the actual protoplasm of the organism being of one consistency. Some of the Lobosa have acquired the power of forming shells, and this affords a convenient character by which we can divide the class into two orders : (1) the N"uda, and (2) the Testacea. Order 1. Nuda. — The most famihar example of the former order is the Amoeba, of which there are many species quite distinct from the amoeboid spores of the Myxomycetes, which are often taken for Amoebae. The various species differ one from another in the nature of their pseudopodia and in the character of their nuclei. In some species the former are little more than low eminences, standing out from the general surface, in others they are long finger-shaped processes which stream rapidly hither and thither. Some members of this order, as the Amoeba princeps and Pelomyxa, have numerous nuclei scattered through the body : in the first-mentioned form these arise by the gradual " fragmentation " of the original nucleus. Such a multinucleated condition is constant in some species. In some cases the soft protoplasmic body has been observed to contract away from, and to lie within, a very thin cuticular membrane, which maintains the outline that the Amoeba possessed the moment before contracting ; this cuticle is not usually visible, except in Zithamoeba, when it exists it must be very attenuated and elastic. Pelomyxa is one of the largest of the Lobosa, the species P. palustris having a diameter of more than 2 mm. The external protoplasm is clear and produced into pseudopodia (Fig. 3). The inner mass is crowded with vacuoles, and contains in addi- tion to the numerous nuclei (5, Pig. 3) a number of refringent bodies of unknown function (6, Fig. 3), and many food particles. It has been observed to set free minute amoeboid spores, which probably grow into new Pelomyxas. Order 2. Testacea. — The shell which encloses the proto- plasmic body of these Lobosa may be soft and cuticular, and may then be strengthened by grains of sand adhering to it, or it may be hard. In either case the protoplasm can be extruded from an aperture in the shell. Arcella (Fig. 4) is a common PROTOZOA genus found in the soft debris ^at the bottom of clean ponds and ditches, and on the surface of aquatic plants. The shell is chitinoid, and arched on the upper surface, flat on the lower, so Fig. 3. — Portion of a Pelomyxa, highly magnified. 1. Clear external layer of protoplasm, ectoplasm. 2. Central protoplasm, crowded with granules, etc. — endoplasm. 3. Pseudopodia. 4. Refractive bodies. 5. Nuclei. 6. Cylindrical crystals scattered through the protoplasm. that it is somewhat dome -shaped or hemispherical in form. In the centre of the flat surface is a circular opening through Fig. 4. — Arcella vulgaris, Ehr. 1. Shell. 2. Protoplasm within the shell. 3. Protoplasm without the shell— pseudo- podia. 4. Nucleus ; there is more than one. 5. Contractile vacuole. 6. Aperture of shell. 7. Space where the protoplasm has with- drawn from shell. 8. Gas vacuoles. which the protoplasm protrudes in the form of blunt pseudopodia. The protoplasm within the shell encloses more than one nucleus, several contractile vacuoles, arranged round the circular ZOOLOGY border, and numerous food vacuoles. In addition we find one or more contractile vacuoles which enclose no liquid, but a gas, possibly CO^. This gas vacuole serves as a hydrostatic balance ; when it disappears the ArceUa sinks. Two individuals are sometimes found lying with their flat surfaces applied to one another in the process of conjugating. This has been in some cases observed to precede reproduction, which takes place by the constriction of small portions of protoplasm, either from Fig. 5. — Arenaceous Foraminifera. a. Exterior of Saccammina. i. The same laid open. c. A portion of the test more highly magnified. d. FUulina. e. Portion of test more highly magnified. the protruding pseudopodia or from the protoplasm enclosed in the shell. In the latter case the abstricted portions escape through the shell mouth and grow up into new ArceRae. Diffiugia is a genus with a soft shell strengthened by the presence of sand particles and diatoms ; the various species have various shapes, some being flask or urn shaped, and one is slightly coiled. Class IV. Reticularia (Foraminifera). In these Protozoa, the pseudopodia are filiform, and anas- tomose into a fine reticulum, along the strands of which gran- PROTOZOA 13 ules may be seen streaming, evidence of the active movement of the protoplasm. They are never entirely naked, but are enclosed in a shell, which may be chitinous, calcareous, or com- posed of agglutinated sand grains (Fig. 5). There may be one or many nuclei, and a contractile vacuole has not been observed in most cases. Their method of reproduction is not very weU known ; it may take place by fission, or by the formation of buds. There are both marine and freshwater representatives, of this class. The enormous variety of forms under which the shells of the Eeticularia present themselves, and their importance in building up large masses of chalk, limestone, etc., have always attracted the attention of naturalists. The class was formerly divided into two groups : the Perforata, those whose shell is pierced by numerous fine pores all over its surface, through which the filiform pseudopodia find exit ; and the Imperforata, Fig. 6. ^Globigerina, as cap- tured in the tow -net near the surface. without the minute pores, but with one or more larger openings, for the exit of the protoplasm. This division is, however, tending to be obliterated. Many of the shells consist of one chamber only (monothalamia. Fig. 5), others, as they grow in size, accom- modate their increased bulk by the addition of more chambers (polythalamia. Fig. 7), and it is chiefly the'marveUous variety of ways by which the new chambers are added which 14 ZOOLOGY produces the great divergency of forms. In some cases the protruding pseudopodia deposit a secondary shell, which obliterates the outline of the primary shell, and usually masks Fig. 7. — Olobigerina buUoides, as seen in three positions. its form. The mud at the bottom of the Atlantic and other seas is composed to such an extent of the calcareous shells of Globigerina buUoides (Fig. 7), which, when the protozoan dies, sink to the bottom, that it is usually known as Globigerina ooze. Fig. 8. — GloMgerina ooze from 1900 fathoms. The living Globigerina (Fig. 6) floats at the surface of the sea, the protoplasm extending round the shell and forming a much vacuo- lated envelope to it. Some slight idea of the enormous number of these organisms which must have lived to build up the foram- iniferous rocks which extend from the Palaeozoic times onward may be formed from the fact that D'Orbigny estimated there were 160,000 shells in a gramme of sand from the West Indies, and Schultze gives 1,500,000 in 15 grammes of sand from PROTOZOA 15 the coast of Sicily. The nummuUtic hmestone of the Medi- terranean basin is composed of the calcareous shells of Fig. 9. 1. A piece of Nummulitic limestone from 2. Vertical section of Nummulites. the Pyrenees, showing Nwmmulites 3. Oriiioides. laid open by fractures through the median plane. Nummulites, a Foraminiferan, which sometimes acquired the diameter of a shilling. Other species, such as Fumlina and PiQ, xO. Rotalia with pseudopodia extended through the pores of the shell. Botalia (Fig. 10), also took a large share in building up the imestones of the Old World. i6 ZOOLOGY Gromia is a form found in both fresh and salt water ; it has a membranous shell of the imperforate type, with an opening at Fig. 11. — Gromia miformis, Duj. 1. The shell. 2. Protoplasm inside the shell. 3. Protoplasm outside the shell. 4. Numerous nuclei. 5. A Diatom surroimded hy pseudopodia. 6. Pseudopodia anastomos- ing. 7. Ingested diatoms. one or both ends, from these the protoplasm passes out and forms a layer round the outside of the shell, from which the fine reticulating pseudopodia arise. The shell is thus completely imbedded in protoplasm, both inside and outside. In Lieber- r PROTOZOA 17 kuhnia (Fig. 12), an aUied form, the pseudopodia anastomose to a great extent and form a close reticulum. Fig. 12. — LieberkiAnia, with reticulate pseudopodia. Class V. Heliozoa. Mostly spherical in form, sometimes supported by a siliceous skeleton, and with radiating stiff pseudopodia. The protoplasm of the body is very vacuolated, and contains one or more nuclei. Near the surface of the body one or more contractile vacuoles may be observed. With few exceptions, they inhabit fresh water. Actinophri/s sol, the sun animalcule (Fig. 13), is one of the common microscopic objects found in still fresh water. It may be met with floating amongst the leaves of submerged plants, and presents a globular body which undergoes slight changes of outline, and is usually very vacuolated. The single nucleus occupies a central position, the contractile vacuole is on the surface, and food vacuoles containing portions of algae, infusoria, etc., may be seen throughout the body. The pseudopodia are stiff and hair-like, and are supported by an axial fibre ; they can be withdrawn into the body. When they come in con- tact with a particle of food they bend slowly over it, and bring it near to the surface of the protoplasmic body, when it is swallowed with the surrounding drop of water, and thus a food 2 i8 ZOOLOGY vacuole is formed. Encystment rarely takes place. Actinophrys and another genus, Ehaphidiophrys^ have been observed to form Fig. 13. — Actinophrya sol, Ehrb. From Bronn. 1. Nucleus in centre of body. 2. Axis of pseudopodium extending into cell as far as nucleus. 3. Contractile vacuole. 4. A mass of food in a food vacuole. 5. Superficial vacuolated protoplasm. 6. Deep, finely granular protoplasm. colonies by incomplete fission. Eeproduction commonly takes place by fission, but in some cases spores have been observed ; those of Aetinosphaerium being provided with a siliceous shell. This last-named genus (Fig. 14) is much larger than Actino- phrys; it contains numerous nuclei, situated in the deeper protoplasm. The pseudopodia are supported by an axial ray. Bhaphidiqphrys is usually found in colonies ; it has a skeleton of siliceous spicules, matted together round the body, each spicule lying tangentiaUy to the surface. Acanthocystis has sihceous rays arranged radially ; they are of two kinds : short ones, which are forked at their outer end, and long stout ones. They are attached to the body by a small disk. Finally, leading on to the condition found in the Eadiolaria, Clathrulina (Fig. 15), PROTOZOA 19 a stalked genus, has a spherical sUiceous shell perforated by numerous openings. Fig. 14. — A portion oiActino- aphaerium Eichhomii, Elirl)., highly magnified, seen in optical section. From Bronn. 1. Nuclei. 2. Ectoplasm. 3. Endoplasm. 4. Pseudopodia with axis. 5. Food mass in food vacuole. Fig. 15. — Glathmlma elegans, Cienk. X 150-200. From Bronn. 1. Stalk. 2. SheU. 3. Pseudopodia protruding through apertures in shell. ZOOLOGY Class VI. Radiolaria. Organisms which are either spherical or with one principal axis whose body is divided into a central mass containing one or more nuclei and a peripheral portion, by the presence of a membrane known as the " central capsule." This is perforated so that the intracapsular protoplasm is continuous with the extracapsular protoplasm. A well-developed skeleton, in most cases siliceous, is present. This consists either of loose siliceous spicules or of a continuous skeleton which may take the form of lattice -work spheres, arranged con- centrically, and united to one another by radial spicules, which project beyond the sur- face of the body. The skele- tons of Eadiolarians occur ia vast numbers on the floor of some seas, forming a layer of sOiceous ooze (Fig. 16). The skeleton may be wholly outside the central capsule, or it may be partially within it. Numerous fine pseudo- podia radiate around the body ; these unite to some extent, nodes of protoplasm being found at the point of union. A streaming of the protoplasm along the pseudopodia, as in Heliozoa and Eeticularia, takes place, and granules have been seen to circulate between the intra- and extra -capsular protoplasm. No contractile vacuole has ever been observed. The protoplasm is much vacuolated : a condition commonly met with iu those Protozoa which, like the Eadiolaria, swim near the surface of the sea. Some very remarkable bodies, known as yellow cells, are found widely distributed amongst the Eadiolaria. These are small oval yeUow bodies, only found in the extracapsular pro- toplasm. They were formerly regarded as part of the body of Fig. 16. — Radiolarian ooze from 4475 fathoms in Central Pacific. PROTOZOA 21 the Eadiolarian ; more recent research has, however, shown that they continue to live after the death of the animal, that they multiply in more than one way, occasionally forming mobile swarm-spores, that similar cells occur in the tissues of many Coelenterates, and that they contain chlorophyll, although this colouring matter is masked by a yellow pigment. A nucleus and a cellulose cell-way are also present. These features have caused these yellow cells to be regarded as unicellular algae, living in a state of commensalism with the. Eadiolarian, and they have received the name of Zooxanthella nutricola. They are not found in all species, and are usually absent in Acanthometra, and in the other species with a horny skeleton. The protoplasm contains, in addition to the yeUow cells, numerous oil or fat globules, and crystals and concretions of unknown use. No conjugation has ever been observed in this class ; repro- duction is sometimes by simple fission, which commences first in the central capsules. Spore formation in the central capsules also takes place, and results in the formation of mobile spores ; but the details are complex, and the exact sequence of events not thoroughly understood. Many form colonies by the fusion of their extracapsular protoplasm. That of Gollosoum, the individuals forming which are devoid of skeleton, may be an inch or more long. The various members of the colony are held together by a gelatin- ous matrix. Group B. COETICATA. The animals which are grouped together in this second division of the Protozoa have as a common feature a differen- tiation of the protoplasm into a more fluid central portion and a firmer cortical layer usually associated with a limiting mem- brane, which surrounds their body and gives it a definite shape. As a consequence of the presence of this cortical layer, these forms which take solid food have acquired one or more channels through which the nutriment is ingested, and usually a definite area whence the undigested remnants are extruded. The parasitic forms which live in the nutritive fluids of their hosts, are usually devoid of any such cell mouth or anus. The presence ZOOLOGY of the cortical layer has also rendered locomotion by pseudo- podia rare, and those Corticata which move about actively do so as a rule by means of rows of cHia or by a single or paired flageUum. Class I. Flagellata. The bodies of the members of this class are usually very minute, and always contain a nucleus ; they are moved by the lashing of one, sometimes by two or three flageUa. A movith Fio. 17. — I. Typical form of Euglena viridis, Ebih., after Sav. Kent. 1. Contractile vacuole. 3. Gullet and origin of flageUum. 2. Pigment spot. 4. Nucleus. II. III. IV. V. Pour views of Euglena viridis, showing the change of shape consequent upon the euglenoid movement. may be present, but in those forms which live in nutritive fluids the nourishment is usually imbibed by the whole surface of the body. One or more contractile vacuoles occur, and sometimes a pigment spot is situated at one end of the body. Conjugation sometimes occurs, and is followed by the breaking up of the body into spores, reproduction also takes place by PROTOZOA 23 simple fission. Many Flagellata form colonies, the individuals of which are imbedded in a gelatinous matrix. EugUna viridis (Fig. 17) is a minute oval Flagellate found in puddles by the wayside, or on roofs, etc. It has a thin cuticle, and undergoes curious rhythmical changes of outline. The elon- gated spindle-shaped body shortens, and becomes correspond- ingly thicker. The thickening appears then to travel to the posterior end of the body and die out. The animal has at this moment its elongated spindle-shaped form; it then shortens again, and the whole movement is repeated. - At the anterior end is a single long flagellum, which by its lashing drags the body swiftly through the water. Lankester has dwelt on the difference between the action of such a flagellum (tractellum) and of one that propels an organism in front of it, as the tail of a spermatozoa or the flagellum of Bacteria (pulsellum). Eound the base of the flagellum is a depression, the mouth, which leads into the central protoplasm ; and close to this, and apparently opening into it, is a reservoir communicating with a contractile vacuole. A pigment spot also is found in the same region, but the reticulate nucleus occupies the centre of the body. The whole body is coloured green, by chlorophyll granules. Grains of paramylum, a body with the same com- position as starch, are also found in the protoplasm. Hypnocysts, or resting encysted forms, are frequently formed amongst the Flagellata, when they find themselves in un- favourable circumstances. The encysted Eioglena may emerge after a certain period of rest from the Hypnoeyst, or it may whilst in the cyst divide into 2 or 4 spores each of which emerges as a young Euglena. Eeproduction by multiple fission has also been described in this species, a vast number of spores being formed, each of which grows into a new individual. Con- jugation also takes place in the Flagellata, and is usually followed by encystment and the division of the contents of the cyst (sporocyst). At other times fission may occur in the free state. Sometimes macrogonidia and microgonidia are produced, and the latter fuse with one another or with adult individuals (Frotococciis). The Flagellata are divided into two groups : the Idsso- 24 ZOOLOGY flagellata, with no collar round the base of the flagellum; and the Choanojlagellaia, in which the protoplasm is produced into a collar which surrounds the anterior end, from the middle of this the single large flagellum takes its origin. Godosiga(Evg.lSi) is a colonial form of this kind, composed of long branching stalks, the end of each branch bearing an individual. These collared flagellates have a striking resemblance to the collar cells lining the flagellate chambers in a sponge ; and a genus, Pro- terospongia (Fig. 19), dis- covered by Saville Kent, in which the individuals of the colony are sunk in a jelly, lends some support to the view that Sponges may have originated from colonies of Choanoflagel- lata. In this genus the individuals near the sur- face are of the typical form ; but certain wander- ing amoeboid cells have sunk into the central jelly, and some of these have become spherical, and then divided up into micro- gonidia, in a manner recall- ing the formation of sper- matozoa in a Sponge. Most Flagellata live in fresh water ; some are marine, and some parasitic, living in the alimentary canal or blood of FlQ. 18. — A branch of Co(io- siga cymosa, Sav. Kent. 1. The stalk. 2. Protoplasmic cell body, showing nucleus and granular protoplasm. Collar. 4. Single flagellum. PROTOZOA 25 Vertebrates and Arthropods. Euglena viridis often exists in such numbers as to turn the water green, and a species of Haematococcus is responsible for the red snow of the Arctic regions. Many of them, such as Polytoma, thrive in putrid Fig. 19. — Proterospongia Haeokeli, Sav. Kent, x 800. 1. Nucleus. 2. Contractile vacuole. 3. Collar. 4. Flagellum. 5. Amoetoid individual sunk in sup- porting jelly. 6. Other individuals undergoing fission. 7. Individual with collar contracted. 8. Individual divided up into numter of spores (miorogonidia). 9. Jelly-lilce supporting matrix. liquids, such as the water of macerating tubs, and these forms are mostly saprophytic. Some of the lower forms are apt to withdraw their flagellum and become amoeboid {Oilwphrys) ; in others {Gercomonas), the posterior end of the body is very apt to throw out pseudopodia, and the young of many species exhibit amoeboid movements. These facts support the view that the Flagellata are derived from the Gymnomyxa; and indeed there are certain forms which might equally well find a place in the class Proteo- myxa. Class II. RhynchoflageUata. This class contains but one or two genera. One of them, Nodiluca, attains a large size : ^ of an inch in diameter. It is 26 ZOOLOGY of a spherical form, and grooved on one side like a peach. From the bottom of this groove a very large transversely- striated fiagellum takes its origin. Near the base of the flageUum is the mouth, opening into a sort of pharynx ; a second smaller flageUum has its origin in the latter. The protoplasm of the globular body is very reticulate. No contractile vacuole has been observed. At times NoctiLuca withdraws its flagella and passes into a resting condition, but it does not form a cyst. This animal is iateresting, as it is phosphorescent, and gives rise to a large part of the phosphorescence of temperate seas. The seat of the light is said to be the superficial protoplasm. Eeproduction is by fission, but motile swarm-spores are also formed ia large numbers. Conjugation has been observed. Class III. Ciliata. This class is characterised by the possession of ciHa as locomotor organs, arranged either in a perioral ring, or forming a more or less complete covering. A nucleus is always present ; this may be single, and is then accompanied by a paranucleus, or it may be distributed ia small fragments throughout the body. One or more contractile vacuoles are present. The shape of the body is very various. Some Ciliata are united into colonies, and some form gelatinous tubes in which they live ; the majority, however, are free-swimming. Conjugation is common, but usually does not end in permanent fusion. Fission, usually simple, but sometimes multiple, is the usual method of reproduction. This class has been divided into four Orders, characterised by the arrangement and nature of the ciha. Order I. Peritricha. — Cilia a/rranged in an anterior ring or spiral, to which a posterior ring may he added. The rest of the hody unciliated. Torquatella has its cilia fused to one another, and so a vibratUe membrane is formed which surrounds the anterior end. Trichodina is very common, crawling on the tentacles and body- wall of Hydra ; it is pyramidal in shape, and has two circlets of cilia. On its sucker-like base a curious horny toothed ring is situated. PROTOZOA 27 Vorticella is attached by a stalk to submerged water-weeds, etc. Up the centre of this stalk runs a muscle iibre, a differ- entiation of the protoplasm, attached at iatervals to the cuticular sheath of the stalk. The differentiation of cortical and medullary protoplasm is well marked. The nucleus is a coiled loop. The animal sometimes encysts, but this is prob- -^ ^^%fe. Fig. 20.- —Trichodina pediculus, Ehrb. x 300. f^ » I. View of the base. 1. Mouth. 2. Contractile vacuole. II 3. Corneous collar. IT View from the «iidp «^ .LXt T .Lu TV 1.L\JILL ull\> OXlA^a ^^M ^m^^^ 1. Corneous collar. mk. ^p|j/..4 2 and 3. Ciliated rings. W^ ^^^-3 4. Nucleus. ably only the formation of a hypnocyst, and has no repro- ductive significance. Binary longitudinal fission is the com- monest form of reproduction, one half remaiaing on the stalk, the other (macrozooid) acquiring a ring of cilia and swimming away to settle elsewhere. At other times the Vorticella divides into eight microzooids, which conjugate permanently with the sessile iudividuals. These also occasionally produce microzooids by budding, and the colonies are also increased by the formation of buds. Order 11. Heterotricha. — The body is covered uniformly with short cilia, and a circlet or spiral of long cilia is developed in relation to the mouth. Stentor has a moniliform or beaded nucleus, and a consider- able number of paranuclei. In some cases these latter correspond in number with the beads of the nucleus. >S^. folymorphus is one of the largest CUiata, reaching a length of Jjr in. In this genus conjugation takes place by the oral face, 28 ZOOLOGY as in Paramoecium, and fission is oblique. Eecent observation on this and other forms shows that as long as the products of artificial division contain part of the nucleus with its chromatin, they are capable of regenerating the lost parts ; those portions of the body which are without any portions of the nucleus die. BalaTdidiwm is a genus which lives parasiticaUy in the human colon, and with Nyetotherus, is found in the rectum of Anura, etc. The latter is interesting, since it is provided with a permanent anus with a cuticular lining. In most Cihata the situation of the cell anus is constant, but there is nothing to indicate its position, except when waste matter is being expelled. Order III. Holotricha. — In this order the body is uni- formly clothed with short cilia, arranged in regular rows. Some- times those on the adoral surface are slightly longer than the others. Paramoecium is one of the commonest genera of this order. Close underneath its cuticle in the ectosarc, is a layer of oval bodies, the Trichocysts; these, when the animal is irritated, discharge threads, which have probably the same functions as the stinging threads in the nematocysts of Coelenterata. Maupas has recently described the conjugation of Para- moecium aurelia, which takes place as follows. As soon as two individuals come together, the paranuclei, of which there are two (Fig. 21, 1), separate themselves from the nucleus, increase in size (Fig. 22, A), become spindle-shaped, and ultimately divide into two (2 and B). The two halves of each divide again (C), so that a stage is found with eight similar portions of the original paranucleus in each individual (3 and 4). Of these eight corpuscles, seven are absorbed and disappear (5) ; the eighth alone, and this is always that one which lies nearest to the mouth, undergoes further change. This corpuscle increases in size and divides into two (6 and D), thus giving rise to the male and the female pronucleus. The former of these passes out of each conjugating individual into the other, and there fuses with the female pronucleus (7 and E). The conjugating animals now separate. The " fertilised " paranucleus now divides into PROTOZOA 29 7 8 9 10 Fig. 21. — Paramoecium aurdia in process of conjugating. Maupas. 1. The two paranuclei in each individual becoming spindle-shaped and com- mencing to divide. 2. Stage with four paranuclei in each individual. 3. The four paranuclei again divide. The division is almost complete in the left Paramoecium. 4. Stage with eight paranuclei. 5. Seven paranuclei being absorbed, and the one nearest the mouth remaining and dividing in two, the male and female pronuclei. 6. Exchange of male pronuclei. 7. The male has fused with the female pronucleus to form the fertilised paranucleus. 8. The fertilised paranucleus divides. 9. The resulting halves divide again. 10. The two halves at the posterior end form the new paranucleus ; the two at the anterior end, the new nucleus. The disintegration and dissolution of the nucleus are shown in the same series of figures. ZOOLOGY two (8 and 9, and F), and each half divides again (G), so that each individual contains four fragments of paranucleus : two at one end of the hody and two at the other (9). The two at 12 3 4 Fig. 22. — Diagram of changes undergone by paranuclei of Paramoecium aurdia during conjugation. Maupas. A. Increase in size of paranuclei B. 1st division of paranuclei. C. 2nd division of paranuclei, and disappearance of seven-eighths. D. Division of remaining portion into male and female pronuclei. E. Fusion of male and female pronuclei. F. Division of fertilised paranucleus into two halves, G. Division of these halves. H. Of the four quarters shown in G, one forms the new nucleus of young Paramoecium which is formed by fission, the other divides into two and forms two paranuclei, one in each of the new indi- viduals. the posterior end undergo no change, and form the paranuclei of the new individuals which result from the subsequent fission ; the two at the anterior end increase, and are destined to form the nuclei of the new individuals (10). Before this is completed, however, the two paranuclei have again divided Fig. 23. — Opalind rananim, Ehrb. 1. Nuclei. 2. Ectoplasm. From Bronn. (H), SO that after fission each new individual contains one nucleus and two paranuclei. Whilst the paranucleus of the original Paramoecium has been undergoing these changes, the nucleus has first become PROTOZOA 31 mammillated and then band -like, and ultimately undergoes fragmentation. The fragments persist some time, and in other species, P. cavdatum, possibly take part in forming the new nucleus. In P. aurelia they ultimately disappear, the majority of them being in all probability extruded. Opalina ranarum (Pig. 23) lives in the rectum of a Frog. As it grows, its nucleus divides, until a great number of nuclei are found. The animal then slowly segments, until each portion contains only one or two nuclei; these form a cyst, and in this condition leave the body of the frog. When eaten by a Tadpole, they emerge, grow, and the nucleus again begins to divide. In Opalinopsis this division of the nucleus has been carried further, and it exists in the form of a fine powder scattered through the endoplasm, the particles of which at times coalesce and form a single nucleus again. Order IV. Hypotricha. — These Ciliata have a flat ventral surface, completely ciliated, or provided with enlarged muscular cilia. The dorsal surface is convex, unciliated, hut sometimes hears retractile setae. Both mouth and anus are well developed. In this group the cuticle is sometimes strongly developed, and forms a protective plate of some thickness; in the Euplo- FlG. 24. — Huplotes patella. After Btitsohli. 1. Mouth. 2. Hypotriohous processes. 3. Nucleus. 4. Cilia of oral groove. 5. Contractile vacuole. tidae this exists on the dorsal surface only. The distinction between endoplasm and ectoplasm is ahnost lost. Encyst- ment is not uncommon, and one species (Gastrostyla vorax) has been kept aHve in a hypnocyst for the space of two years. 32 ZOOLOGY Class IV. Acinetaria. No cilia or flagella present, but a number of tentacular processes, which may be adhesive or may be tubular and suctorial. The Acinetaria are usually fixed, and most com- monly stalked (Fig. 26). The nucleus is single, and often branched. One or more contractile vacuoles occur. Eepro- duction takes place by binary fission and gemmation ; the latter is often internal. The Acinetaria are either marine or freshwater, and they are all carnivorous, living chiefly on the soft parts of Infusoria. Fig. 25. — Sphaerophrya magna, Maupas. x 300. 1. IxLiWiinsls oi Colpoda panifrona, a ciliated Infus- orian whose soft parts are being sucked up by the Spfiaerophrya. 2. The nucleus. 3. One of the hollow tentacles ending in a knob. The cuticle may be thin, but in some cases a definite mem- branous capsule is formed. Two kinds of tentacles may be found : one long and adhesive, whose function is to catch and hold the prey ; the other is shorter, tubular, and ends in a sucker, — these latter are sometimes provided with a spiral thickening. The soft protoplasm of Infusoria, on which they prey, is sucked up through these hollow tentacles (Fig. 25). Dendrocometes has a round body, from which four to six many- branched stout arms project. Each branch ends in a point, which is said to be hollow, and by means of which food is sucked into the body. In this genus, and in one or two others, the contractile vacuole has an excretory duct. Acineta (Fig. 26) is a stalked form with a membranous cup ; the tentacles are arranged in two clusters at each side of the body. Eeproduction may be by fission, or by external gemmation, or by internal gemmation, in which case a brood - pouch is formed, which may be open, but is more commonly closed. PROTOZOA 33 -1 The bud, or buds, are formed in the floor of this cavity. When they leave the parent they are ciliated ; a fact which lends support to the vievy that the Acine- taria are descended from the Oihata. They at first lead a wandering Kfe, but after a time settle down, lose their cilia, and acquire tentacles. Class V. Sporozoa. The members of this class are all parasitic ; and correlated with this condi- tion of life is the absence of locomotor organs, the absence of any mouth, — the nutriment being ab- sorbed all over the body,- — and a reproductive process which results in the forma- tion of a very large number of young. The nucleus is always single even when the ceU is divided into two chambers. No contractile vacuole is present. The cortical layer of protoplasm may show traces of fibrilla- tion. Of the four sub-classes which compose this class, that of the Gregarinidea is the most important. The true Gregarines, with very few exceptions, pass their early life as cell parasites, afterwards Fig. 26. — Acineta grandis, Sav. Kent. 1. Stalk. 5. An example of 2. Knobbed tentacles. Acineta lemnarum, 3. Membranous shell. stowing relative size 4. Nucleus. of the two species. 34 ZOOLOGY emerging and living parasitically in the various cavities of their host. The Coccidiidea pass their whole life as, cell parasites. The true Gregarines are divided into the Mono- cystidae, whose body is not divided into two chambers, and which inhabit Annelids, Gephyrea, Platyhelminthes, and Tuni- cata; and the Polycystidae, in which a transverse partition divides the cell into an anterior and posterior chamber, the latter invariably containing the nucleus. An anterior outgrowth, the epimerite, which serves to attach the Gregarine to the tissues of its host, is often present, but this is shed sooner or later. The Polycystidae have hitherto only been found in the alimentary canal of Arthropods. Monocystis magna (Pig. 27) is frequently to be found with its anterior end embedded in one of the epithelial cells of the en- ¥m. 27. — Five individuals of Monocystis irmgna (Schmidt), with their anterior ends embedded in the cells of the rosette-shaped inner end of the vas deferens of Lu/mJbricus Urrestris. 1. Part of the vas deferens of L. 3. The nucleus. terrestris. i. The ectoplasm. 2. The endoplasm of the Gregarine. 5. The cuticle. larged inner rosette-shaped openings of the vas deferens of the common earthworm. It is visible to the , naked eye, and sometimes attains a length of 5 mm. The body is limited by PROTOZOA IS a thin cuticle ; within this lies the cortical protoplasm, which, though full of granules, is transparent. The medullary proto- plasm is dark brown and opaque. BiitschK has shown that some of the granules are composed of a starch-like material. The body exhibits movements of a euglenoid character ; waves of contraction passing down the elongated cell. The flowing about of the protoplasm is rendered visible by the granules. The nucleus is clear and vesicular, with few granules, and it lies in the centre of the cell. At times two individuals come together and surround them- selves by a spherical capsule ; apparently no true fusion takes place, but the bodies of the Gregarines commence to form spores. This spore formation proceeds from without inwards in each cell, but the whole protoplasm is not always used up for this purpose. The spores are shuttle-shaped, they acquire a capsule (chlamydospores), and are often spoken of as pseudonavicdlae. The pseudonavicellae escape from the cyst by its bursting, or in Clepsidrina, a Polycystid, by special sporoducts. Their contents divides into eight elongated bodies, known from their shape as falciform bodies ; these leave the pseudonavicella, and probably grow up directly into the adult form. A recent observer has, however, stated that the contents of the pseudonavicella does not break up into falciform bodies, but the protoplasm becomes grooved, and thus the appearance of segmentation is produced. According to him, the whole contents of the pseudonavicella escapes and grows in a new Gregarine. Many of the Polycystidae are more highly differentiated than the species described above ; their cuticle may be ridged or tuberculated, and is frequently produced into hooks in the epimerite, and the cortical layer of protoplasm may show traces of fibrillation. When these septate forms conjugate, they usually lie side by side. Oregarina gigantea, which inhabits the alimentary canal of lobsters, attains the astonishing length of |- of an inch. Coccidiidea are minute spherical cells which infest the epithelium of the intestine, the liver cells, etc., of Vertebrates, MoUusca, and Insects. Whilst still in their cell host, they give rise to chlamydospores and falciform young. CHAPTEE III METAZOA Chaeacteeistics. — Multicellular animals, which pass through a unicellular stage, the ovum or egg. This multiplies by division, and the cells thus formed, instead of remaining equivalent to one another, become differentiated and are arranged in tissues. Reproduction takes place by means of ova and spermatozoa. Acoelomata. Metazoa in which a two-layered condition is the predomi- nant one. The ectoderm and the endoderm may constitute the whole animal, but in many cases an intermediate layer (the mesoderm or mesogloea) lies between them. This middle layer may be homogeneous, but is more usually invaded by cells from one of the two layers, or from both. The cavities of the Acoelomata, except certain ectodermal pits, are in all cases con- tiQuations of the primary central cavity lined by endoderm, and no cavities exist lined by mesoderm comparable to a coelom. Eadial symmetry about an axis passing through the mouth is a primitive and common feature of this subdivision. The animals which constitute it are exclusively aquatic, and almost entirely marine. Q, / Homocoela — Ascetta. f \ Heterocoela — Orantia, Leucandra. PORIFERA-! rHyalospongiae — Byalonema, Euplectella. \ NoN-OALCAREA-j Spiculispongiae — Halisarca, Oscarella, Geodia. [_ Cornacuspongiae — Euspongia, Velinea, Spongilla. METAZOA 37 PORIFERA. (the Sponges). Chaeactbeistics. — Animals of very varied size and sha^pe. Nurnerous minute pores allow the passage of water into the interior of the sponge, and the water is discharged through larger openings known as Oscula. The current of water is maintained hj certain flagellate cells, which are usually aggregated in what are known as flagellate charnbers. The mesoderm is ivell developed, and usually gives rise to a skeleton of calcareous, siliceous, or horny material ; it also gives origin to the reproductive cells. Sponges may be unisexual or hermaphrodite. They are aquatic, and, ivith the deception of the Spongillidae, they are marine. They are devoid of tentacles and of nematocysts. The simplest type of Sponge is that of Ascetta primordialis, described by Haeckel. It is a hollow vase-hke structure borne on a stalk with its free end open. This opening is the osculum. The walls are per- forated by a series of small circular apertures, the " pores," and its cavity is luied by a layer of flagellate collared cells, whose activity keeps a current of water entering the pores and finding an exit through the osculum. The flagellate cells are endodermal. The outside of the vase-hke body is covered with ectoderm, and between these two layers is a mesodermic tissue which produces triradiate calcareous spicules. The flageUate endoderm cells are said to possess contractile vacuoles. In the more highly organised Sponges the endodermic Hning of the central cavity has lost its flageUate character and become a flat epithelium. Grantia compressa is a sponge with a calcareous skeleton, which is frequ.ently met with attached to rocks and stones round our coast. It is of a whitish colour, seldom more than an inch long, and rather variable in outline : Fig. 28. — Ascetta pri- mordialis, Haeckel. After Haeckel. 38 ZOOLOGY common form is somewhat flask shaped. At its free end are situated one or more slit-like openings, the oscuta. The body is compressed from side to side, and its wall is pierced by numerous minute inhalent pores, which lead by a system of braiiching tubes into the central cavity. The substance of the sponge is composed of three layers the ectoderm, the endoderm, and between them the mesoderm. The ectoderm consists of flat- tened cells covering the outside of the sponge, and lining certain pits or depressions which are pushed into the substance of the sponge, and are termed in- tercanal spaces. The openings from the exterior into the inter- canal spaces are termed "pores" ';5 (1, Fig. 29). Several pores are usually grouped together, and form the pore area. The inter- canal spaces open on their inside 7 by numerous apertures, called Fig. 29. -Part of a section through by SoHas " prosopyles," into the Grantia lahynnthica, vertical to flagellate chambers (4, Fig. 29). the margin and to the two surfaces ° , of the wall of the cup. After ihese iiagellate chambers are Dendy. 1. Inhalent pore. 2. Exhalent canal. 3. Inhalent canal. 4. Cavity of flagellate chamber. 5. Pore area. 6. Gastral skeleton. 7. Dermal skeleton. 8. Tubar skeleton. 9. Embryos. the most characteristic feature of the Sponges. They are lined by collared flagellate cells similar to those of the Choanoflagel- lata. Their flagella keep up a constant current of water, which passes in at the pores through the intercanal spaces and flagellate chambers. METAZOA 39 and into the central cavity, thence it leaves through the oscula. Embryological research shows that vs^e must regard the collar cells of the flagellate chambers, the cells lining the tubes which lead from them to the central cavity, and the cells lining the latter cavity, as endoderm. The mesoderm is a gelatinous tissue in which certain cells are found embedded ; some of these form ova or break up into spermatozoa, whilst others give rise to the skeleton of calcareous spicules. In sections of Grantia the intercanal spaces (3, Fig. 29) may be seen lying between the flagellate chambers, but quite distinct from these. They are lined by flat epithelial cells, and they ultimately open by a more or less wide mouth on to the exterior on the one hand, and by a series of circular pores, the prosopyles, into the flagellate chambers. These intercanal spaces are formed by the pushing in of the outer coatiUg of the sponge, and are lined by ectodermal cells similar to those covering the outside of the sponge. In some sponges they reach a great degree of complexity. The ectoderm of sponges is, as a rule, composed of flat cells 3- Fig. 30. — Stelospongus fldbeUi/ormis. Diagram of a portion of a flagel- late chamber, showing the various parts of the collared cells and their relation to Sollas's membrane, which con- nects together the mar- gins of the collars. After Dendy. 1. Body of flagellate cell. 2. Its nucleus. 3. Its collar fusing at its edges to form 5. 4. The flagellum. 5. Sollas's membrane. in a single layer ; rarely these cells become columnar, and bear flagella (Oscarella lobularis). The endoderm consists, in all but the Homocoela, of two kinds of cells : (i.) flat pavement 40 ZOOLOGY cells, which line the central cavity and the ducts opening into it ; and (ii.) collared cells, which line the flagellate chambers, and in the Homocoela the general cavity. In Grantia the flagellate chambers do not open directly into the central cavity, but into a short exhalent canal (2, Fig. 29), the entrance to which is guarded by a sphincter diaphragm. The endoderm lines the whole canal system from the prosopyles to the osculum. In several genera distributed among several orders the collars which surround the base of the flagella are at their outer ends fused to form a membrane, which was first described and figured by Sollas (Fig. 30, 5). Bidder has shown that if a Leucandra be placed in water with carmine suspended in it, the water which comes from the oscula is always free from carmine granules, thus showing the presence of a very efficient filter, presumably the fused collars of the flagellate cells. It is still an open question whether the space within this membrane, between the body of the flagellate cells, is empty, or occupied by a transparent gelatinous substance. The mesoderm is a gelatinous layer, with branched or stellate cells scattered through it. Amoe- boid cells (Fig. 31) wander through its substance, and convey nutriment from one part to another, and when occasion arises assist in_ removing irritant foreign matter from the body of the sponge {phagocytes). The branching mesoderm cells have been traced into direct proto- plasmic continuity with both ectoderm and endoderm. The reproductive cells are also meso- dermal, and the fertilised ovum develops in a space in the gelatinous mesoderm (9, Fig. 29), which is lined by a layer of flat endothelial cells, also meso- dermal in origin. Certain mesodermal cells in the neighbourhood of the Fig. 31. — Branchiug connective tissue cells from the mesoderm of Thenea muricata. After Sollas. METAZOA 41 diaphragms in the exhalent canals of the flagel- late chambers are believed to be muscular, and a nervous nature has been attributed to others situated near the inhalent pores. The skeleton of Grantia consists of cal- careous spicules, most of which are of a triradiate shape (6, 7, and 8, Kg. 29). Each spicule commences to appear in one of the mesodermal cells, but as it increases in size it may protrude from this. The spicules are said to gradually work towards the exterior of the sponge, and to be discharged as waste matter. The character of the skeleton is made use of in classification. Only a very few sponges — Halisarca, Oscar ella, and Chondrosia — are devoid of any kind of supporting structure. Those which possess calcareous spicules have been grouped together as the Calcarea, and opposed to all the other sponges, or Non-calcarea. The skeletons of this latter group may consist of siliceous spicules, or of a fibrous substance — spongin. The siliceous, like the calcareous spicules, originate in a single mesodermic cell (Fig. 32) ; both assume a great variety of size and shape, and the former may be articulated or fused to one another. The organic skeleton, found in the Order Cornacuspongiae, consists of spongin, a substance chemically allied to silk. It is secreted by a number of mesodermic cells termed spongoblasts, which form a layer all round the fibre, and a multicellular cap covering the ends (Fig. 34). The fibrous skeleton of Bu- spongia, devoid of spicules, and characterised by the regular arrangement of the network and the smallness of the meshes, forms the bath Fig. 32. — Spicules « originating in sponge of Commerce. single cells. Jq Qrantia some of the amoeboid mesodermic After SoUas. I. From Stelletta. II. From embryo of Craniella cranium. III. The same from adult Craniella cranium. IV. A four-rayed spicule from Theonella Simnhoei. II III 42 ZOOLOGY cells may be noticed withdrawing their pseudopodia, and passing into a spherical resting condition. These form the ova. Others Fig. 33. — Various forms of Sponge spicul^. divide up into an immense number of spermatozoa (Fig. 35), each with a head and long vibratile tail. In Grantia lahyrinthica Fro. 34. — Sections tlu-ougli horny fibre of Dendrilla. A, Longitudinal section; layers of spongin, surrounded by a layer and surmounted by a cap of spongo- blasts ; a, fibrous sheath. B, The same in transverse section. After Von Lendenfeld. Dendy has described the ripe ova making their way through the mesoderm to the epithelium of the inhalent canals, and passing METAZOA 43 through the epithdial lining of these ducts. They then hang freely, supported by a short peduncle, into the lumen of the canal (Fig. 36), and are doubtless fertilised in this position by Fig, 35. — 1-7, Development of spermatozoa in Symndra raphanus ; 8, mature spermatozoa, after Polejaeff ( x 792) ; 9, a sperm morula in Oscarella lobularis ( X 500); 10, an isolated mature spermatozoon, after Schulze ( x 500). spermatozoa carried in by the inhalant current from another sponge. After fertilisation they must return back into the mesoderm, where the larva develops. A similar migration of ova has been described by Weismann in many Hydroids. Fig. 36. 1. Ovum [Orantia lahyrinthica) hanging in lumen of inhalent canal. 2. Epithelium lining inhalent canal. After Dendy. The early stages of development take place whilst the ovum is in the mesoderm ; and in the Non-calcarea a special invest- ment of epithelial cells lines the cavity in which the embryo lies. Many sponges are hermaphrodite, and then the spermatozoa usually mature before the ova ; others are unisexual. Eepro- duction by gemmation occurs in some species, buds being formed which, as a rule, include portions of all the three layers ; these separate from the parent form, and grow into new sponges. In Spongillidae, the only freshwater family, resting buds or gemmules are produced, and their production involves the death of the sponge. The gemmules consist of certain yolk -bearing cells enclosed in a complicated capsule, which serves to protect the cells until the external circum- stances permit them to emerge and grow up into a new sponge. 44 ZOOLOGY Artificial fission has been successfully adopted in the cultiva- tion of sponges for commercial purposes. All sponges are marine, with the exception of the family Spongillidae, the commonest species of which is Sfongilla flimatilis, an incrusting mass with crater -like oscula, commonly found on the woodwork of locks, weirs, etc., in our rivers. It may be coloured green by the presence of chlorophyll. Many other sponges are brightly coloured, and they may assume a very great variety of shapes. Some emit a powerful and un- pleasant odour, which may, like the presence of the spicules, tend to prevent their being devoured. The classification of the Porifera cannot be regarded as settled. That at the head of this chapter is suggested by Vosmaer. A. The Calcarea. includes those forms which possess a skeleton of calcareous spicules, generally triradiate in shape. The collar cells are large. This class comprises two orders : (i.) Homocoela, in which the endoderm consists wholly of collared cells, which line the central cavity. Ex. Leuco- solenia. This group includes all those forms which were formerly known as Ascones. (ii.) Heterocoela, in vjhich the endoderm is differentiated into (a) fat epithelial cells, lining the central cavity and excurrent canals, and (b) collar cells, confined to the flagellate chambers. Ex. Grantia, Sycon, Leucandra. B. The NoN-CALCAEEA possesses a skeleton of siliceous spicules or spongin ; very rarely none at all. The spicules may he isolated, articidated, or fused. Tlie collar cells are ma^-kedly smaller than those of the Calcarea. The group includes three classes : (i.) Hyalospongiae (Hexactinellidae), (ii.) Spiculispongiae, and (iii.) Cornacu- spongiae. The first of these three classes is much more clearly defined than the other two ; indeed the latter tend to run into one another. METAZOA 45 (i.) Hyalospongiae. — Skeleton of siliceous spicules, which are usually sexradiate, isolated, or fused into a trellis-work. Usually deep-sea forms. Many fossils. Ex. Hyalonema, whose spicules may reach a length of two feet ; Euplec- tella, or Venus's flower-basket. (ii.) Spiculispongiae. — Skeleton absent in a few forms ; in- the great majority consisting of siliceous spicules, usually independent, hut sometimes articulated together, or united hy organic material. One of the sub-orders, the Myxospongiae, is devoid of skeleton. Ex. Hali- sarca, Oscarella. In the Tetractinellidae, one of the largest subdivisions, the spicules are to a great extent four-rayed. Ex. Geodia, Tetilla. (iii.) Cornacuspongiae. — Skeleton of uniaxile spicules united hy spongin, or of spongin. Inhabit the sea, brackish or fresh water. Ex. Euspongia, the sponge of commerce ; Velinea, Spongilla (the freshwater sponge), etc. li! ill I , E-HO I Sh r~) El rf, O l-H 03 o o Q o o 5 R? o W Ph o H O CHAPTEE IV COELENTEEATA Chaeacteeistics. — Acoelomata with a definite shape. The body is usually radially symmetrical about an axis which passes through the mouth. The ectoderm is separated from the en- doderm by a middle layer — the mesogloea — which may be structureless and devoid of cells, or may contain numerous cells. The tissues are not pierced by a series of pores, and there are no collar cells. Protective organs known as nematocysts are characteristic of the group, with the exception of almost all the Ctenophora. Alternation of generations is common, and also the formation of colonies by budding. All the members of the group are aquatic, and most of them marine. This group is divided into four classes : A. Hydeomedusae ) ,, j T. o r =Hydrozoa. B. Scyphomedusae j "' C. ACTINOZOA. D. Ctenophoka. Class A. HYDEOMEDUSAE. Chaeacteeistics. — The Hydroidform may be free or sessile, single or colonial. It is rarely without tentacles, which are nearly always solid. A horny perisarc or a calcareous skeleton may be developed. Asexual reproduction by gemmation usually taJees place, the hydroid form budding off a medusiform sexual individual. The 'Medusa has a velum-, and a double nerve ring. The sensory organs are ocelli and otocysts or modified tentacles. The Hydromedusae are bisexual, and the sexual cells are typically ectodermic, sometimes endodermic. iVb gastral filaments are present. The medusa may arise directly from the egg, but this is rare. ZOOLOGY Fig. 37. A. Vertical section of ByO/ra. 1. Ectoderm. 2. Endoderm. 3. Mesogloea. 4. Mouth. 5. Coelenteron. 6. Hypostonie or oral cone, 9 and 9'. Large and small nemato- cysts. 10 and 10'. Two tuds in different stages of development. 11. Ovary. 12. Single ovum. 13. Testis. 7. Pseudopodia of endoderm B. Portion of transverse section througli cells. body wall of Hydra, more highly 8. Flagella of endoderm cells. magnified. COELENTERATA 49 The simplest form of Hydromedusa is represented by Hydra, which exists only in the hydroid form. Its reproductive organs show no trace of a medusoid nature ; and although the medusae can be traced in other and more specialised species through stages of degeneration till they become Little more than pro- tuberances on the body-wall full of sexual cells, still there is nothing in the ovary and testis of Hydra to warrant the view that they are n(^t simply sexual organs. In some Hydromedusae a distinct alternation of generations is present ; that is, the hydroid person produces asexually, by budding, a medusoid person which produces sexually, by means of ova and spermatozoa, the hydroid person again. Thus asexual and sexual modes of reproduction alternate in the life-history of these animals, and each mode is associated with a distinct kind of animal : the asexual with the hydroid, usually a fixed form; the sexual with the medusoid, a free-swimming form. This kind of alternation of generation — budding alternating with the sexual method — has been termed metagenesis ; it occurs in many of the lower animals. Although the fixed hydroid differs a good deal in appear- ance from the free-swimming medusa, they can both be reduced to a common type. In both forms of person very considerable complexity of form is often combined with great simplicity of ultimate structure. Many forms are colonial, and the in- dividuals composing the colonies are commonly modified to subserve various functions, and thus may become degraded to the level of organs. 1. Ectoderm. 12. FlageUa. 2. Endoderm. 13. Vacuole. 3. Mesogloea. C. Two ectoderm cells. 4. Layer of muscular processes of 1. Nucleus. ectoderm cells cut across just 2. Muscular tails. outside mesogloea. D. An endoderm cell of S. viridis. 5. Interstitial cells. 1. Nucleus. 6. Cnidoblast containing nemato- 2. Chromatophores. cysts. 3. Ingested nematocyst. 7. Neniatocyst. E. A large nematocyst with extended 8. Cnidocil. barb. 9. Nucleus of endoderm cell. F. A small nematocyst. 10. Ingested diatom. G. A spermatozoon. H. Pseudopodium. A, B, and C, after Jeffery Parker ; D, after Lankester ; F and G, after Howes. 4 50 ZOOLOGY Tubularia indivisa is a marine colonial Hydromedusan borne on a branching anastomosing stolon attached to sub- Fia. 38. — A colony of Tubularia indivisa. Alter Allman (natural size). 1. Ciroumoral tentacles. 2. Aboral row of tentacles. 3. Mouth. 4. Blastostyle. 5. Hydrocaulus. 6. Hydrorhiza. merged objects. At intervals the stolon sends up straight unbranched stems, each of which terminates in a hydroid person, generally termed a hydranth. The branching stolon may be COELENTERATA 51 termed the hydrorhiza, and the single stem the hydrocaulus. Both the hydrorhiza and the hydrocaulus are protected by a thin chitinoid investmont, the joerisarc. This tubular covering, however, stops short at the base of each hydranth. The living tissue within the perisarc is known as the coenosarc ; it consists externally of a single layer of ectoderm, which secretes the perisarc, and internally of an axis of endoderm traversed by ciliated canals ; these serve to place the cavities of the various hydranths in communication with one another. Each hydranth is a somewhat flask - shaped structure bearing two rings of tentacles ; one composed of short tentacles situated round the mouth, the other, of much larger tentacles, arises from the middle of the body, where the diameter is greatest. The bases of the circumoral tentacles are visible as slight projections on the body- wall for some little distance below their point of emergence. Their ectoderm is crowded with nematocysts, and contains muscle cells, glandular cells, ganglion cells, usually bipolar, and four kinds of nematocysts. Their endoderm consists of several rows of cartilaginoid cells, which serve as a stiffening skeleton. The digestive cavity of the hydranth is spacious, it is lined by endoderm cells, which in many species digest iutracellularly. There is a thickened layer of endoderm forming a ring for the support of the larger tentacles. The middle layer or mesogloea is structureless and thin. The relationship of the hydroid to the medusoid person is best explained by means of diagrams (Fig. 39). The oral axis of the hydroid is shortened, and the circumference of the middle part of the body is correspondingly increased. The ring of large tentacles is thus carried out to the edge of what is known as the umbrella ; the oral cone, with its ring of tentacles, remaining in the position of the umbrella handle. This change of external shape is accompanied by the obliteration of certain parts of the coelenteron or central cavity. Along certain areas in the umbrella the endoderm cells come in contact and fuse, forming the endodermal lamella (Fig. 39, II. 12). This fusion takes place in such a way as to leave certain tracts open, the most important of which are the circular vessel running round the 52 ZOOLOGY edge of the umbrella, and the radial canals, which communicate with the cavity of the oral cone on the one hand and the circular canal on the other. On the aboral surface of the endodermal lamella the middle layer usually becomes much Fro. 39, thickened, its gelatinous substance may remain structureless, or cells may invade it. Tubularia is one of those Rydromedusae which do not give rise to free medusae. The medusa is produced, but does not break away, it remains connected with the hydranth by the aboral pole. In a mature individual a racemose extension of the body-waU is formed between the two rows of tentacles. COELENTERATA 53 This is the UastostyU (Fig. 3 8, 4), and it bears a series of buds called gonophores. These buds increase in maturity as they approach the end of the blastostyle, so that a single blastostyle may illustrate the various stages in the development of the buds. The gonophores or rudimentary medusoid persons arise as elevations of the ectoderm, into which the endoderm pro- jects. At the free end of the bud the ectoderm then thickens, Fig. 39. — Diagrams to show the relations of (I.) a Hydroid polype to (II.) a Hydro- medusa and to (III.) a Soyphomedusa. 1. Ectoderm, shaded. 2. Endoderm, dotted. 3. Structureless lamella, black line. 4. Coelenteron. 5. Extension of coelenteron into tentacles in I., and forming radial canals in II. and III. 6. In I., Mouth. In II., Velum. 7. Gelatinous tissue, cross-hatched. 8. Circular canal. 9. Tentacle into which the endoderm extends. 10. Generative organs. In II. they occupy Anthomedusan position. 11. In II., Generative organs in Lepto- medusan position. In III., Sub-genital pit. 12. In II., Endodermal lamella. Inlll., Eoldofumbrellaovertentacle. 13. The gastric filaments, remains of the taeniolae. and in this thickened portion a split appears in such a way as to separate a central portion, the manubrium, from a circular por- tion which surrounds the former and forms the umbrella. The space thus constituted, which is lined everywhere by ectoderm, is the sub-umbrella cavity (Fig. 40). The endoderm projects both into the manubrium and the umbrella ; in those forms which break off as free medusae, the manubrium acquires a mouth, which leads into a cavity — the coelenteron — lined by endoderm. The endoderm of the umbrella in Tuhularia is a ilat plate of tissue enclosing no cavity except round the rim, where slight remnants of a circular canal can be found, and from this rudiments of radial vessels lead to the cavity of the manubrium. Eudi- 54 ZOOLOGY mentary tentacles may occur on the gonophore. As stated above, the medusoid persons in Tubularia do not break away or develop further. The gener- y ative organs are found on the walls of the manubrium (Fig. 39, II.), the medusoid persons bearing either male or female cells, but not both. In those species in which Pig. 40. — Diagrams of the forma- tion of Medusae by tudding. The ectoderm and endoderm are ^^ medusoid person is completed, represented by the same shadmg ^ , -, as in Fig. 39. The Coeienteron other Structures develop ; is black. After Korschelt and ^^^^^^ ^^^ ^^^^^ ^^^^^^ Heider. I. — Commencement of bud- II. — Thickening of ectoderm and cupping of endoderm. ten- arise round the edge of the umbrella, a muscular fold, the velum, is formed inside the rim of the um- brella (Fig. 40, IV.) and functions as a swimming organ, and the medusa breaks away and swims freely through the water. The existence of these two phases in the life-history of one organism has rendered necessary a double classification. The order to which Tubularia belongs is the Gymno- blastea-Anthomedusae : the first word referring to the nature of the hydroid form, the second to that of the medusoid. Order 1. Gymnoblastea-Anthomedusae. "^TtyTn'^^Sotom™^"'"* '^'" CHARACTERISTICS.— rA«S6 Hydro- zoa have probably always a hydroid stage, which is very various in appearance, sometimes non-colonial, but more usually the hydroids are united into colonies. The ectoderm may form a perisarc, but this is never continued into a cup or hydrotheca surrounding COELENTERA TA 55 tli& hydranth, nor are there capsules surrounding the gono- phores. The medusae may he vjell developed and free, or they may he permanently attached to the hydranth (gonophore), or they may not develope hey ond rudimentary huds (sporosacs), and finally, in Hydra they are not formed at all. When free, they may he provided with ocelli at the hose of their marginal tentacles; usually on their outer surface. The sexual cells are arranged round the manuhrium, either uniformly or in hands. Most Anthomedusae are small. The tentacles of the hy- droid are nearly always solid. Hydra, however, is an exception to this rule; they are usually arranged in one circle as in Hydra, or in two as in Tuhularia; in the family Clavidae, however, they are irregularly scattered over the surface of the hydranth. There is no special aggregation of nerve cells in the hydroid stage, but in the medusa a nerve ring is present round the edge of the umbrella ; it is split into an upper and lower nerve ring by the insertion of the velum. The ocelli consist of a collection of pigment spots, and a cuticular lens is present in lAzzia. The sexes as a rule are separate, the genus Hydra again forming an exception. Eeproduction by fission is rare, but sexual reproduction by budding is common in both the hydroid and the medusoid stage. When it takes place in the IV. Fig. 40. -Sub-umtrella cavity opened, manubrium arising, and the de- velopment of gelatinous tissue. 1. Ectodei-m. 2. Endoderm. 3. Structureless lamella. 4. Manubrium. 5. Radial canal. 6. Velum. 7. Gelatinous tissue. 8. Sub-umbrella cavity. latter, a medusa is invariably the result. Order 2. Calyptoblastea-Leptomedusae. Characteristics. — The hydroid forms of this genus never have more than one row of tentacles round the mouth. The S6 ZOOLOGY tentacles are solid. The ectoderm secretes a perisarc which is continued into a cup, the Hydrotheca, surrounding the Hydranth. The groups of nudusiform buds are similarly enclosed in a capsule, the Gonotheca. The medusae of this group are all Zeptomedusae, and are characterised hy having four, eight, or more radial canals, on which the generative glands are always situated. The tentacles on the rim may be few or very numerous. Ocelli are found in two families ; more commonly the sense organs take the form of hollow vesicles, the otocysts, in which the otolith cells are formed from the ectoderm. The hydroids belonging to this order are colonial, the colonies being arranged in branching filaments, which have a superficial resemblance to some of the branching colonies of Polyzoa. Certain families are distinguished by the way in which the hydranths are placed on the branch. In the Cam- pa nulakidab each hydranth is stalked; in the Plumulaeidae the hydrothecae are sessile on one side of the branch only ; in the Sertulaeidae they are sessile, and on both sides of the branch. Certain tentaculoid structures, termed nematophores, occur in relation to the hydrothecae of the Plumulaeidae. They are sohd, with an endodermic axis, and knobbed at the end, and the knob contains nematocysts and sense cells. Their ectoderm has been seen to ingest carmine granules, and they have been observed to bend into the hydrotheca and eat up the remains, of dead hydranths of the same colony. ' The medusae are often rudimentary, and remain attached to the blastostyle, as the hydroid individual which gives rise to them is called. The free medusae arise from the Cam- panularian hydroids ; the gonophores of the Plumularians and Sertularians do not become detached. Order 3. Hydrocorallinae. This is a very well-marked order, in which the hydroid stage only has hitherto been found.^ The hydrorhiza in these animals deposits a copious secretion of carbonate of lime instead of a chitinoid perisarc. By this means considerable masses 1 A very primitive form of medusa bearing male organs only (spermospores) has been recently described in MiUepora Murrayi. COELENTERA TA 57 of lime are built up, simulating some of the true corals. Certain tubular spaces are left for the accommodation of the hydranths, which can be withdrawn into them. The hydranths are colonial, and of two kinds : gastrozoids, provided with mouths, which are the nutritive persons of the colony ; and dactylozoids, elongated mouthless persons well armed with nematocysts. The order is divided into two famihes : (i.) the Mille- PORIDAE and (ii.) the Stylasteeidae. (i.) The Millepoeidae. — This family may have an arborescent or branching skeleton, consisting of an outer living part which surrounds an inner and older dead 'part. The younger parts of the fores, in which the living hydranths are found, are separated from the older ones, wherein their predecessors lived, by horizontal partitions known as tabulae. Hie colonies are hermaphrodite. (ii.) The Stylasteeidae. — These are always arborescent, never encrusting, and their colonies are unisexual. The hydranths may be scattered irregularly, but more usually the gastrozoids are surrounded by a regular circle of Fig. 41. — Longitudinal median section througli a Stylasterid. After Hiclcson. 1. Gastrozoid. 2. Dactylozoid. 3. Style. i. Coenosarcal canals. 5. Coenosarc, calcareous. 6. Mouth. 7. Tentacle. dactylozoids. The gastrozoid (Fig. 41) is usually provided with tentacles, but in some of the Stylasteeidae these are wanting. The dactylozoids are long tentacular structures with 58 ZOOLOGY no mouth, they bear short knobbed branches, or tentacles, in the Milleporidae. The generative cells of Millepora, which is hermaphrodite, arise in the ectoderm of the coenosarcal tubes, which connect the gastrozoids and the dactylozoids. The spermospores or cells which are destined to break up into spermatozoa, migrate through the mesogloea into the endoderm, and then travel along the tubular coenosarc to a hydranth, usually a dactylozoid. They pass into the cavity of the hydranth, and then re-enter the endoderm, and push out the mesogloea into a diverticulum between the tentacles. The number of the diverticula or sporosacs on each dactylozoid varies from one to five. The spermatozoa mature in the sporosacs. The ova make their way in a similar manner through the mesogloea into the endoderm ; here they attach themselves by a stalk to the inside of the mesogloea and grow considerably. The ovum may wander about to seek a more favourable position for fertilisation, which probably takes place in the lumen of the canals. Similar wanderings of the sexual cells take place among the Hydro- medusae. Order 4. Siphonophora. Chaeacteeistics. — These are Hydkomedusae whicli live together in colonies floating at or near the surface of the sea (PELAGic). The members of the colonies are hydriform individuals alone (VeIiELLa), or hydriform and medusiform; theyhave undergone great modifications, so that in each colony a great number of persons are present performing various fionctions, and exhibit- ing a great diversity of form. The chief modifications are as follows : (i.) the Gastrozoid, whose function is to absorb nutriment. It has a mouth, and communicates at its aboral end with the tubular coenosarc connecting the various persons, (ii.) Hydrophyllia : these are protective covering -pieces, which have a central endodermic canal, and are usually borne on the coenosarc. (iii.) Tentacles: these may be short and tubular, or may attain the length of many feet (Physalia). They are usually branched, and each branch is provided with a battery of nematocysts. (iv.) Dac- tylozoids : hydroid structures without a mouth, and usually COELENTERATA 59 well armed with nematocysts. (v.) Nectocalyces : swimming bells. They consist of the umbrella of a medusoid, with four radial canals, but without a manubrium or mouth, (vi.) Pneu- 'mato'phons : these form floats, and the air secreted within them serves to keep the whole colony in a vertical position, (vii.) The sexual persons : these may become free medusae of the craspedote type — that is, provided with a velum ; or they may remain undeveloped as sporosacs. The medusae have their generative organs on the radial canals (Velella), or on the manubrium (Physalia). The colonies are usually herma- phrodite. The seven modifications described above are not all to be met with in every Siphonophoran, but usually four or five of them coexist in each colony, thus giving rise to a form of extreme complexity ; various combinations of these persons also permit great variety amongst the different species. Many of them are brilliantly coloured in parts, and are amongst the most beautiful marine objects which float along the surface of the sea. They are common in the Mediterranean and open seas, Velella and Diphyes being occasionally found on our shores. The two remaining orders of the . HydromeduSae — ithe Tkachomedusae and the Naecomedusae — have no hydroid stage, but the ova in most cases give rise immediately to the medusae. They are characterised by the possession of sense organs formed by modifications of the tentacles on the rim of the umbrella. They are termed tentaculocysts, and into them alone of the sense organs of the Hydromedusae does the endo- derm enter. One or more otoliths formed from the endoderm cells, which correspond with the axis of the ordinary tentacle, contain crystalline concretions, and form the auditory organ. Order 5. Trachomedusae. Characteristics. — The Trachomedusae Juive their tentaculocysts free or enclosed in capsules. Coecal radial cancels may he present, which open into the circular canal, but never reach the central stomach. 6o ZOOLOGY The generative glands are on the sub-umbrella surface of the radial canals. Zimnocodium, the only freshwater medusa with whose anatomy we are acquainted, is placed by Lankester in this order. It forms an exception to the rule of the absence of a hydroid form. A small hydranth has been described as budding off the medusae, which are found at intervals in tanks of the Victoria regia in the Botanical Society's Garden, Eegent's Park. Order 6. Narcomedusae. Chaeacteeistics. — The Narcomedusae always have their ten- taculocysts free. The tentacles arise some way up the aboral face of the umbrella, not from its edge. Their bases are con- nected with the edge of the umbrella by stiffening rods of tissue termed Peronia. These may also he found in the Tracho- medusae. The generative organs arise from the sub-umbrella surface of the stomach, not on the radial canals. Class B. SCYPHOMEDUSAE. Chaeaoteristics. — The hydroid stage of this sub -class is a polype, which may give rise to the medusa, not by budding but by transverse fission. It is termed the Scyphistoma. The mouth of the Scyphistoma is squarish, and the coelenteron is sub-divided by four ridges, termed taeniolae, which project from the sides into its lumen. The 'medusa has also a square mouth, whose angles may be produced into four oral processes. The edge of the umbrella is lobed. The sense organs are modified tentacles, into the base of which the coelenteron is produced. No continuous nerve ring exists, but scattered nerve centres are found round the margin, and no true velum is present. Chaeybdaea forms, however, an exception to this statement. The generative cells are endodermic in origin, and the medusae are unisexual. In the Scyphomedusae a primary series of four tentacles corresponding to the four angles of the mouth are the first to be formed. This affords a convenient method for mapping out the regions of the body. The radii on which they are COELENTERATA 6i formed are termed jperradii, the next four are situated half- way between the perradii, and are known as inferradial tentacles ; and eight more may be added, one between each perradius and interradius, and these are called adradial. The same convention is made use of in describing the position of the various organs in medusae of other classes. The most striking features in the Scyphistoma are the taeniolae, which project into the enteric cavity and divide its outer portion into four chambers (Fig. 39, 13). The taeniolae are four in number, and are inter- radial in position. They are comparable with the mesenteries in the Actinozoa. Towards the end of spring the Scyphistoma passes into the Stro- bila stage, it then becomes con- stricted on the aboral side of its tentacles, and thus a disk is partly cut off, behind this a series of furrows arise, cutting the Scyphi- stoma into a series of segments (Fig. 42). These separate off and form the immature medusae or the Ephyrae. Eight bifid lobes have grown out, which give to the Ephyrae their characteristic eight- rayed appearance. Each lobe encloses in its notch either a perradial or interradial tentacle destined to become a tentaoulocyst. The gastric cavity extends into each lobe, and the taeniolae be- ,.,..,. -, , 1. J j^ ™ +1,0 alinral PiQ. 42. — Strobila of ^«reZia attnto. come detached from the aboral After Haeckei. 1. Base of StroHla. 2. Sense organ. 3. Marginal tentacle. 4. Lappet on side of sense organ. side of the disk and give rise to gastric filaments. The same pro- cesses of growth go on in each of the segments of the Scyphistoma. The Ephyra becomes a medusae chiefly by the filling out of the adradial spaces, and it thus acquires a circular outline. 62 ZOOLOGY The position of the tentaculocysts, however, always marks the position of the eight original lobes. Aurelia is a Scyphomedusan very commonly met with round our coasts swimming at the surface of the sea. If the adult medusa be examined, it will be seen that the original four-lobed chamber of its Scyphistoma has given off a number of branch- ing enteric canals which ultimately open into a circular vessel. Between these canals the aboral and oral endoderm has given Fio. 43. — Awelia aurita. 1. Mouth. 6. An adradial canal. 2. Circumoral perradial processes. 8. The circular canal. 3. Tentacles on the edge of the umhrella. 9. Marginal lappets hiding tentaculo- 4. One of the tranching perradial canals. cysts. There are four of these, and four 11. Gastral filaments, similar interradial canals. rise by concrescence to the endodermal lamella (Fig. 39). Eight adradial canals are given off from the central chamber between these branching canals, and these pass directly to the circular canal without giving off any branches. The mouth of Awelia is surrounded by four perradial COELENTERA TA 63 processes, and the manubrmm is short. The coelenteron is ciliated. In BMzostoma the four oral processes are divided into eight, and have fused together in such a way as to occlude the mouth. This is compensated for by the presence of fine canals which open at one end into the gastric cavity, and at the other on the frilled edges of these processes. The food is absorbed through these canals. The gelatinous layer of the mesogloea in Aurelia is traversed in all directions by fibrous cells and by wandering amoeboid cells. The muscles are in some cases formed of transversely-striated processes of epithelial cells ; in the oral processes of some medusae, however, distinct nucleated muscle cells occur. The sense organs of Aurelia are modified tentacles, which bear endodermal otocysts and ectodermal pigment spots or eyes. An aboral and an oral pit, both lined by specialised epithehum on the surface of the disk, are regarded as olfactory. The coelenteron is prolonged into the modified tentacles. No regular nerve ring exists in Scyphomedusae, with the exception of one genus, Charybdaea ; but nerve fibres occur in the region of the sense organs, and are connected with scattered ganglion cells. The ova and spermatozoa arise from endodermal cells, and escape through the mouth. Four large pits may be excavated in the sub-umbrella ectoderm, and these form sub-genital pouches, the skin of which is thin, and forms only a slight mem- brane between the sea- water and the genital cells (Fig. 39, III.). This arrangement may promote the respiration of these parts. In some Scyphomedusae, e.g. Pelagia, the medusa rises directly from the ovum without the intervention of a Scyphi- stoma. Class C. ACTINOZOA. Chaeacteeistics. — Single or colonial Coehnterata. The mouth leads into an eetodermic invagination, the stomodaeum or oeso- phagus ; this is attached to the walls of the body hy a series of radial mesenteries, so that the coelenteron is divided into a central portion and a series of radiating intermesenteric chambers. The generative cells are endodermic, and lie in 64 ZOOLOGY the mesentery. There are no special sense organs. Nemato- eysts occur, and are more complicated than those usually found in the Hydrozoa. The most fundamental distinction between the Actiuozoa and the Hydrozoa lies in the presence of the mesenteries and the consequent division of the coelenteron into a central chamber and several radiating lateral chambers, which all open below iato the central chamber. In the Hexactinia these chambers are put into direct communication with each other by one or two pores, which pierce the mesenteries near their upper end. The existence of these mesenteries is to some extent foreshadowed by the taeniolae of the Scyphomedusae. In two forms recently described by Danielssen, Fenja and Aeytr, the oesophagus is continued to the aboral disk, where it opens to the exterior by an anus ; thus a body cavity is shut off from the digestive cavity. In Aegir, however, these two cavities communicate by a series of pores which lead from the inter- mesenteric chambers into the alimentary canal near the anus. In Fenja each intermesenteric chamber opens to the exterior by a pore close to the anus. Thus a ring of genital pores js formed. In both genera the various intermesenteric cavities communicate with one another by pores at the oral end of the mesentery. The Actinozoa have an apparent radial symmetry which does not hold true for all parts ; but a genuine bilateral symmetry exists. The Actinozoa are sub-divided into two orders : (i.) the Hexactinia (Zoanthaeia) and (ii.) the Octactinia (Alcy- onaeia). Order 1. Hexactinia. Charactbkistics. — Tentacles sirnple ; they and the mesenteries are very generally some multiple of six. Single or colonial ; when colonial a continuous organic or calcareous skeleton is usually deposited hy the ectoderm. Actinia mesembryanthemum is a beautiful red sea-anemone common in rock pools round our coasts. When in the expanded condition it may be seen that the animal is in the form of a short cylinder, with one end firmly attached to a stone COELENTERATA 65 or the rocks, the other end free. The mouth is an elongated slit in the middle of the peristome or free end, it is surrounded by several rows of tentacles (Fig. 44). At the ends of the elongated mouth are special grooves which are continued down the oesophagus and are lined with especially long cilia (Fig. 46). Fig. 44. Colony of sea-anemones {Sagartia parasitica) on shell of hermit crab. "When the mouth is closed the central parts are in apposition, but the grooves, called Siphonoglyphs, remain always open, and through them a current of water may be kept circulating in and out of the animal even when it is in its most contracted condition. Gerianthus has only one siphonoglyph. The oeso- phagus ends with a free edge, and never reaches the base of the sea-anemone. The mesenteries are vertical radial partitions which extend from the peristome to the base. The outer edge is continuous vnth the inner side of the body-wall. The inner edge in the primary mesenteries is divided into two parts (Fig. 45). The part nearest the peristome is continuous with the outside of the oesophagus, but below the lower edge of the oesophagus the inner 66 ZOOLOGY edge of the mesentery is free. The secondary mesenteries have not reached the oesophagus in Fig. 46, so that the whole extent of their inner edge is free. The mesenteries are grouped in pairs, and the members of each pair are separated by an Fig. 45, — A mesentery of Tealta crassieomis. After the Hertwigs. 1. Edge of mouth. 2. External and internal pores in the mesentery. 3. Mesenterial filament. i. Testis follicles. 5. Circular muscle. 6. Radial muscle fibres. 7. Longitudinal muscle fibres. 8. Parietal longitudinal muscle fibres. 9. Lower edge of the stomodaeum. intramesenteric space, while the various pairs have an inter-septal chamber between them. The secondary mesenteries always occur between two primary ones. Each mesentery is pierced by a round hole, and in some species by two, so that the various intermesenteric chambers open into one another above, as well as all communicating with the central chamber below. Along one side of each mesentery runs a longitudinal bundle of muscle fibres from the peristome to the base. These assist the sea-anemone to contract. They are as a rule facing one another in each pair. But on each side of each siphono- glyph is a mesentery known as the directive mesentery (Fig. 46), and on these the longitudinal muscles turn away from each other. Transverse muscle fibres are found on the mesenteries on the side where the longitudinal muscles do not exist. There are also sphincter muscles running round the peristome, and parietal muscles running obliquely from the walls to the base COELENTERATA 67 of the body. The nervous system consists of scattered ganglion cells, chiefly in the peristome and tentacles. The free edge of the mesentery below the level of the oesophagus is divided into three lobes; the middle one is crowded with nematocysts and glandular cells, the outer lobes Pig. 46. — Transverse sec- tion through the body of Adamsia dia~ pTuma, in the region of the stomodaeum. After the Hertwigs. 1. The directive mesen- teries. 2. Longitudinal muscle fibres in the mesen- teries, cut across. 3. The siphonoglyphs. A. Primary mesenteries, 12. B. Secondary mesenteries, 12. C. Tertiary mesenteries, 24. D. Quaternary mesenter- ies, 48. are ciliated. When a piece of soUd food is swallowed, the edges of several of the mesenteries come together and surround it, and the secretion of the glandular cells helps to digest it. The sexes are usually separate, the generative cells arising from the endoderm of the mesenteries (Fig. 45) ; the young escape through the mouth, A. mesembri/anthemum being vivi- parous. In some species the free edges of the mesenteries are pro- duced into long whip-Kke processes, called acontia, armed with nematocysts. These are protruded through the mouth or through special pores in the body-wall called cinclides. They are found in Sagartia. Occasionally there is a central pore in the base of the body, and as a rule the tentacles are perforated at their ends ; and in some of the deep-sea forms the tentacular pores are large, in others the tentacles are reduced, the pores only remaining as a circlet of holes surrounding the mouth. 68 ZOOLOGY The Hexactinia are divided into three groups : (i.) The AcTiNiAEiA, which are devoid of any kind of skeleton, are usually single, and are mostly adherent to some foreign tody ; occasionally they live lialf embedded in mud or sand. Cerianthus, Actinia. (ii.) The Antipathaeia : they possess a horny axial skeleton secreted from their ectoderm ; they are colonial, and form large branching structures. Antipathes. (iii.) TJie Madeepoeaeia are solitary or colonial ; their most remarkable characteristic is their power of secreting a calcareous skeleton. The skeleton is often very massive, and recent research has shown that it is entirely formed from the ectoderm. Oculina, Astraea, Madrepora, Fungia, Caryophyllia. The Madeepoeaeia are of the greatest importance in the history of the earth. They are the true corals, and their skeletons form hy far the greater part of the coral rock which has huilt up a considerable portion of the earth's crust. Eeef- forming corals do not as a rule grow below the forty-fathom line, and are not usually found north or south of a belt ex- tending 30° each side of the equator. As the coral grows, large masses of the coenenchyma or common skeleton become covered over by the younger forma- tions. This skeleton may be quite solid and dead, or it may be pierced by canals which shelter coenosarcal tubes of living matter, connecting one individual with another. This enables us to divide the Madreporaria into two divisions : (i.) Perforata, with the skeleton perforated by the cavities which lodge the coeno- sarcal tubes, and (ii.) the Aporosa, in which no such perforations exist. The form and shape of the skeleton is extremely varied, and often complicated by the colonial habits of the actinozoan. But whatever its shape, and however deeply it may have pene- trated into the body of the soft gelatinous-looking animal, it is always formed by ectoderm, and is consequently always outside the animal, whose tissues are, as it were, moulded over it. COELENTERATA 69 The skeleton commences to appear by the ectoderm of the base secreting a flat plate between it and the substance on which the young actinozoan is fixed. From this plate a number of radially-arranged vertical ridges grow up (Fig. 47, 2). These are, Fig. 47.— Diagram to illustrate anatomy of Galaxea esperi. 1. The theca. 7. Mesentery. After Hickson. 2. The septa. 3. The basal plate. 4. Flesh covering the theca. 5. Tentacles. 6. Mouth. 8. Lower edge of stomodaeum. 9. Free edges of mesenteries beneath the stomodaeum. 10. The mesenteric filaments. like the rest of the skeleton, secreted by ectoderm ; and as they grow they push the ectoderm and the skin of the base up into the coelenteron. These vertical plates are termed septa. Partly by the fusion of the external edges of the septa, and partly by the upgrowth of a circular rim from the basal plate, a circular ridge is formed. This ridge forms the wall of the cup or theca, and like the septa it projects into the coelenteron, pushing the body -wall before it; occasionally a second circular rim, external to but concentric with and close to the first, is formed, the epitheca, and this forms a ridge round 70 ZOOLOGY and outside the base of the animal. Some of the septa may- coalesce in the centre of the body, and thus the columella is Fig. 48. — Caryopliyllia borealis, Fleming, a simple coral, twice the natural size. After Sir WyTille Thomson. produced ; this is usually continued as a little, pillar pointing towards the mouth. The outer wall of the theca frequently FiQ. 49. — Astraea pallida, Dana, a compound coral in its living condition. After Dana. bears ridges, the costae, which do not necessarily correspond with the septa ; the latter may be connected with their neigh- bours by horizontal shelves, termed dissepiments. COELENTERATA 71 Neither the, septa nor the eostae correspond with the mesenteries, but are situated in intermesenteric spaces. Like the mesenteries, they may increase in number, secondary septa arising between the primary. Most of the corals which form massive skeletons increase the number of individuals in the colony by budding ; in some the theca of each member of the colony remains distinct {Madr&pora, Oculina, Astraea) (Fig. 49), in others complications arise by the fusion and obliteration of the walls of the theca, etc. {Meandrina). Order 2. Octaotinia. Chaeacteristics. — Colonial AdiTiozoa with eight pinnate ten- tacles and eight mesenteries, which bear the longitudinal muscles on their ventral surface, that is, on the face which looks towards the single siphonoglyph. , The well-known skeleton of the organ-pipe coral, Tubipora, consists of a stolon or encrusting lamina which attaches the colonies to some foreign body, and of a series of tubes in which the polyps live, termed corallites, which arise from the stolon. The corallites are externally connected together by horizontal plates, forming the platforms or exothecal tabulae (Fig. 50) ; and within each coralHte is a series of tabulae, the top one of which Fig. 50. — A portion of the oorallum of Tviipora musica of the natural size, showing the tuhular corallites and the exothecal tabulae or plat- forms. cuts off the polyp from the dead skeleton below. New coral- lites constantly arise from the platforms, so that the whole coral increases in circumference as well as in height as it gets older. The platforms are formed by outgrowths of the lips of the hving coral, and are at first very thin; they are traversed by many branching canals. 72 ZOOLOGY Within, the cavity of the corallites is divided into a series of chambers by the presence of partitions termed tabulae ; these may be simple flat plates, but more often they are cup- shaped, and in some cases are drawn down in the centre into funnel-shaped structures. The tabulae often give off tubes, which run out into the platforms. The whole skeleton is so compact that it appears to be formed of a continuous homogeneous deposit of calcium car- bonate. In reality, however, it consists of a number of spicules, each with minute serrations which fit into other serrations on the neighbouring spicules. The polyp wall is built up of the ectoderm, mesogloea, and endoderm. The mesogloea contains a few scattered cells and fibres, as well as the skeletal spicules, which, however, in those cases where the embryology is known, originate in certain ecto- dermal cells which wander afterwards into the mesogloea. The tentacles, eight in number, have about fifteen pinnae on each side in a single series. They are hollow, their cavities Fig. 51. — Transverse section of a polyp of TuHpora purpurea. After Hickson. 1. Stomodaeum. 2. Cilia of siplionoglyph. 3 . Ectoderm of stomodaeum and of outer surface of body- wall. 4. Endoderm lining mesenteric chambers. 5. Longitudinal muscle fibres in mesentery, cut across. 6. Calcareous spicules. 7. Parasitic diatoms on the ectoderm. 8. Mesogloea. D. Dorsal surface. V. Ventral surface. opening into the intermesenteric chambers, over which they are placed. Their ectoderm is ciliated. The whole body of the polyp, including the tentacles, can be withdrawn into the corallite. The stomodaeum is ciliated throughout, and on one COELENTERA TA 73 side, the ventral, there is a special groove (Fig. 51) lined with long cilia, the siphonoglyph. The stomodaeum is supported by eight mesenteries ; on the ventral side of each of these are placed the powerful retractor muscles, which draw the polyp swiftly into the corallite at the approach, of danger. The two dorsal mesenteries alone are continued far below the lower edge of the stomodaeum in the form of mesenteric filaments, their edge being much thickened, bilobed, and covered with cilia. The endodermal cells are probably some of them amoeboid, and digestion may be intra- cellular. The ova are found attached to the dorsal and dorsolateral mesenteries, immediately below the stomodaeum. The tabulae are formed by the mesogloea splitting into two layers : the outer remains attached to the ectoderm, the inner layer with the endoderm ; the latter shrinks away from the outer layer, and then in this contracted condition begins to secrete a fresh layer of spicules, which ultimately stretches across the corallite. Hence the space below each tabula is morphologically a space in the mesogloea. The polyps of the TuUpora remain free and distinct one from another ; in other groups of the Ootactinia, however, they may be sunk in a well-developed coenosarc, as in Alcyonium, commonly known as " dead men's fingers," or they may be arranged side by side, their lateral surfaces fusing in the form of a leaf-like plate, as in the Pennatulidae (Fig. 52). In Alcyonium the skeleton is not continuous, but consists of spicules scattered loosely through the coenosarc. The leaf-like plates of Penna- tula are borne on each side of a rachis, this is continued into a stalk free of polyps. Both stalk and rachis are traversed and supported by a long calcified horny rod, secreted by an epithelium whose origin is uncertain. Among the Alcyonidae and Pennatulidae the individual zooids are often of two kinds. In Fennatula, for instance, the leaf-like expansions are composed of a single layer of polyps (aufozoids) fused side by side, whilst the zooids (siphonozoids) cover that surface of the rachis on to which the bases of the leaves do not extend, and pass up between the leaves. The zooids differ from the polyps, having no tentacles or 74 ZOOLOGY retractor muscles, only two of ^their mesenteries are continued below the level of the oesophagus and bear mesenteric fila- FiG. 52. — A longitudinal section through a polyp of Pennatvia phosphorea. After Marshall. 1. Mouth. 2. Walls of stomodaeum. 3. Free edges of mesenteries below the level of the stomodaeum, showing short mesen- terial filaments. 4. Long mesenterial filaments. 5. Feathered tentacles. 6. Calyx. 7. Spicules in body-wall. 8. Spermatospheres. 9. Mesentery showing protractor and retractor muscle fibres. 10. Coenenchyme. ments, they have a well-developed siphonoglyph, but no repro- ductive system. The cavities of all the individuals are put into communi- cation by means of a number of ramifying coenosarcal tubes lined by endoderm. The Penhatulidae are phosphorescent, and emit a bright glowing light when disturbed, this is said to originate from COELENTERA TA 75 eight cords of fatty cells attached to the oesophagus in both kinds of polyps. Class C. CTENOPHORA. Chaeacteeistics. — Pelagic Coelenterata, usually spheroidal, more rarely hand-shaped in form. Eight meridional rows of vibrating plates composed of fused cilia form the locomotor apparatus. A large pair of retractile tentacles are usually present, which can he withdrawn into pouches. At the ahoral pole is a special sense organ. Tliey are all herma- phrodite, and pelagic. This group of animals possesses considerable importance from a phylogenetic point of view. Haeckel has described a Fig. 53. — Hormip/wra plumosa. After Chun. Side view. 1. Mouth leading into stomach. 2. Aboral pole with sense organ. 3. Funnel. 4. Recurrent canals running back to- wards oral pole. 5. One of the eight bands of fused cilia. 6. One of the eight canals running to- wards 5. 7. A tentacular pouch. 8. A tentacle. 9. Gelatinous tissue. remarkable Anthomedusan named Ctenaria ctenophora which has many of the characteristic features of a Ctenophor ; whilst 76 ZOOLOGY on the other hand two curious organisms, the CoeloplaTia of Kowalevsky and the Ctenoplana of Korotneff, have lately been discovered which unite some characters of the Ctenophora with others of the Turbellaria. In a typical Ctenophor, one of the Cydippidae, such as ITormiphora, the mouth leads into a flattened stomach hned with ectoderm, which in its turn opens iato the funnel, Uned with endoderm (Fig. 53). The funnel gives off two gastric canals, which pass out towards the base of the long tentacles. These two primary canals give off a secondary canal on each side, which forks, and forms tertiary canals (Fig. 54), of which there- fore there are eight. Each of these eight canals opens into one of eight meridional canals which lie under the rows of vibratile plates, and which end blindly both at the oral and aboral pole. The two long tentacles can be completely retracted into their pouches ; they bear peculiar adhesive cells. The characteristic vibratile plates are formed by a number of very large cUia fused together side by side ; an arrangement also met with in some of the Hypotricha. The central nervous system consists of an area of ciliated Fig. 54. — Hormiphora plumosa. After Chun. Aboral view. 2. Aboral pole with sense organ. 5. One of tlie eight bands of fused cilia. 6. One of the eight canals running towards 5. 8. A tentacle. 9. Gelatinous tissue. SS. sagittal plane. cells at the aboral pole. This area is sunk into a depression which contains certain otoliths, and from which nerve cells pass to the meridional rows of paddles. This specialised sensory COELENTERA TA 77 apparatus possibly serves as a balancing organ, and helps to keep the floating animal the right way up. The reproductive organs are arranged along the meridional canals, the male cells on one side and the female cells on the other of each canal. The ova and spermatozoa escape through the canals, and eventually leave the body through the mouth. The main features of the anatomy of the Ctenophora have been indicated in order to render intelligible their possible relationship on the one hand to the Anthomedusae and on the other to the Turbellarians. There is a mass of interesting detail with reference to these animals which cannot be referred to here. The Anthomedusan Gtenaria has the mouth of its umbrella very much contracted, and the edges have grown round and over the manubrium, which is small. The opening into this sub-umbrella cavity corresponds with the opening into the stomach of the Ctenophor ; and the lumen of the latter, lined as it is with ectoderm, corresponds with the sub-umbrella cavity of Gtenaria. The shape of the medusa is very like that of Cydippe, and its surface is provided with eight rows of modified ectodermal cells, which correspond in position with the eight rows of vibratile plates in Ctenophors. The arrange- ments of the enteric canals also approaches that of Cydippe, and the resemblance between the two animals is further in- creased by the presence in both of two long fringed tentacles which project from pouches as in the Ctenophora. The Gestus veneris, or Venus's girdle, is a Ctenophor in which the spherical form has been replaced by a flattened band-like shape. It is found swimming at the surface of warm seas, and moves through the water by a series of graceful undulations. Beroe, in which the stomach attains a very great size, has no tentacles. The group is a carnivorous one, the chief food being pelagic organisms. Many of them are phosphorescent. CHAPTEE V COELOMATA In the Acoelomata there is a common cavity, the Coelenteron, which is lined by endoderm cells, and which pervades various parts of the body. Whatever cavities exist in these animals, with the exception of certain ectodermic pits, are all diverticula of this one primitive cavity, and remain in connection with it. The Coelomata, on the other hand, start with an Arch- enteron, probably the equivalent of the Coelenteron ; but this is replaced by two distinct and separate cavities — that of the alimentary canal, and that of the body. The latter is termed the Coelom, and is entirely shut off from the cavity of the digestive system. The cavity of the alimentary canal is lined by endoderm ; between the endoderm and ectoderm a new layer of cells has appeared, the mesoderm, and it is in this layer, and lined by it, that the coelom appears. The Coelomata have typically a bilaterally symmetrical form, which is however in many cases lost. In front of the mouth there is a prostomium, or preoral lobe. The mouth and anus, when present, are lined by invaginations of the ecto- derm, termed the sfomodaeum and prododaeum respectively. The very important cavity, the coelom, which distinguishes the Coelomates from the lower animals, is characterised by the following peculiarities. (i.) It develops as one or more diverticula from the primitive archenteron (enterocoel), or it arises as a space or spaces in the mesoblast (schizocoel), or it is a remnant of the segmentation cavity, into which a lining of meso- blast grows (archicoel) ; it is consequently always lined by meso- blast. (ii.) Its walls give rise to the reproductive cells, which are set free into a portion of the coelom, and leave the body COELOMATA 79 either through the nephridia or by special ducts or apertures, (iu.) It communicates with the exterior through the nephridia, or the organs which excrete waste nitrogenous matter. It is prohable that the body-cavities or coeloms which originate in different manners may not be homologous through- out the Coelomata; on the other hand they all possess the characteristics enumerated above. There are other cavities which arise in the mesoblast to which the above characteristics do not apply. These are the vascular and lymphatic systems ; they contain blood and lymph. In some Coelomata these systems are composed of vessels with certain muscular differentiations to propel the contained fluid ; in others, they form large spaces which simulate the appearance of the body-cavity of other animals. Such spaces are termed pseudocoels or haemocoels ; they occur in Arthropods and Molluscs, and when they are present the true coelom is very much reduced. It is not impossible that the vascular system is but a part of the general coelom ; in many cases the two systems are distinct, but in others — as the Leeches, where they communicate through the botryoidal tissue, and in Vertebrates through the thoracic duct — there is an indirect connection, whilst in Nemertines the coelom and vascular system appear to be one. The mesoderm, which is such an important feature in the Coelomata, occupies the same position between the ectoderm and endoderm as the mesogloea of the Coelenterata ; it is not, however, always regarded as homologous with the mesogloea, partly because the latter appears originally as a clear gelatinous layer which is devoid of cells, and may remain so throughout life ; but also because cells, when they do wander into the meso- gloea, do not arrange themselves in definite tissues. CHAPTEE VI PLATYHELMINTHES ( Acoela — Convoluta, Proporus. )r" " " " "" Rhabdocoelida Rhabdocoela-Jfeoiiioma, Mi- I crostoma, Vortex. TUKBELLAE1Ay an incision a little to the left of the median dorsal line, so as to show the internal organs. The introvert is retracted. 1. Opening of introvert. 2. Position of brain, the two eye- spots are shown. This marks the level of the head. 3. Dorsal vessel. 4. Left dorsal retractor muscle. 5. Left ventral retractor muscle. 6. Generative ridge. 7. Ventral nerve cord. 8. Nephridia. 9. Coiled intestine. 10. Rectum. 11. Spindle muscle. and forming a continuous sheath in the trunk ; internal to this are the longitudinal muscles, continuous in the introvert, but arranged in about twenty anastomosing bundles in the trunk. Within this layer, the coelom is lined by flat epithelial GEPHYREA 163 cells. From the longitudinal bundles four stout muscles arise, two dorsal and two ventral. These pass to form a muscular ring ensheathing the oesophagus, just behind the head. They are termed the four retractors, and their function is to draw in the introvert. The alimentary canal consists of a straight oesophagus, into which the mouth passes without any armature, and which in its turn passes into a coiled intestine. Both these parts are ciliated, the ciha of the oesophagus being continuous with those of the lower lip and tentacles. The intestine is coiled round a special " spindle " muscle, which arises from the extreme posterior end of the body, passes up the axis of the coil, and joins the longitudinal muscles of the body-wall near the anus (Fig. 104). A short rectum passes to the anus which terminates the alimentary canal, the anus pierces the body-waU just behind the line of division between the introvert and the trunk. The vascular system is closed and is confined to the anterior end of the animal. Its most conspicuous part is a vessel which lies on the dorsal side of the oesophagus between the retractor muscles. The vessel is closed behind, and gives off no capillaries. At the anterior end it opens into a large sinus into which the brain protrudes ; from this sinus a circular vessel is given off which runs round the lower lip, and when fuU of blood, it serves to distend the latter. Another part of the vessel runs along the base of the lophophore, giving off branches into each tentacle. It is possible that the blood may become oxygenated in the tentacles, but the chief function of the whole system is to distend the tentacular crown and lower lip. The fluid in this system is corpusculated. The coelom is very spacious, and contains a corpusculated fluid which bathes aU the internal organs. The corpuscles are larger than those of the vascular system. The contraction of the circular muscles of the skin forces this fluid forward, and in this way the introvert is everted. The nephridia, or excretory organs, of the Gephyrea are often termed " brown tubes." In Phymosoma they are two in number, one on each side of the ventral nerve cord (Fig. 104). They have the form of elongated sacs, which hang down 1 64 ZOOLOGY into the body-cavity. At their upper ends the sacs are attached to the body-wall, and open to the exterior a little in front of the level of the anus. Each sac consists of two portions : a posterior glandular part Uned by large glandular cells, which give off vesicles containing their excretion, and a muscular non-glandular anterior half, which opens both on to the Pig. 105. — Diagram showing relation of nervous system, vas- cular system, and oesophagus in Phymosoma varians. Partly after Selenka. 1. The brain, represented rela- tively too small. 2. Nerves to skin of preoral lohe. 3. Lophophore ; each tentacle is represented by its blood sinuses and its nerve. 4. Blood sinuses of lower lip. 6. Oesophagus. 6. Dorsal blood-vessel. 7. Ventral nerve cord. exterior and into the coelom. The opening iato the latter space is situated close to the external opening, and is guarded by a frilled, funnel-shaped Hp, thickly cihated. The wall of the organ contains many muscle fibres, and it is capable of considerable change of form. The nervous system consists of a bilobed brain in con- tinuity with the epidermis of the concavity of the lophophore (Fig. 103). It gives off a pair of lophophoral nerves, which run along the base of the tentacles, sending off a nerve into GEPHYREA 165 each. Laterally the two lobes are continued into stout nerves which embrace the oesophagus, and fuse to form a ventral cord. On each side the ventral cord is supported by two longitudinal muscles, and the whole is loosely attached to the ventral surface of the body-wall by muscular strands. The cord shows but slight traces of double origin, it bears no ganglia, but ganglion cells are uniformly distributed on its ventral surface. It gives off a series of lateral nerves, which form complete rings round the body, situated in the skin (Figs. 103 and 105). Two pits of large ectodermal cells, crowded with dense black pigment, have sunk on each side into the brain. They are hollow, and contain a coagulum in dead specimens. They are usually spoken of as eyes. Phymosoma is dioecious. Both the ovary and testis are formed of a ridge of the peritoneal epithelium which runs across the body at the base of the ventral retractor muscles. Certain of the cells of this ridge break off and float in the coelomic fluid. In the female they become ova, in the male they are the mother cells of the spermatozoa. The ova grow a good deal whilst in the body-cavity, and secrete a thick egg shell ; ultimately they leave the body through the nephridia. The spermatozoa derived from one mother cell always remain connected as long as they are in the body-cavity, and in this condition are taken up by the funnel-shaped internal openings of the nephridia. The ova are fertilised externally in the water. Certain of the Gephyrea achaeta differ in many points from Phymosoma. Sipvnmdus has no lophophore, and the mouth is surrounded by a frayed fringe, which, like the tentacles of other forms, is well supplied with nerves and blood-vessels. Many species are without the hooks on the introvert. A layer of obhque muscles lies very commonly be- tween the circular and longitudinal fibres. The capacity of the dorsal vessel, which acts as a reservoir for receiving the blood when the tentacles and head are retracted, is in- creased in some species of Phymosoma by a number of lateral diverticula, and in some Sipunculids by the addition of a i66 ZOOLOGY ventral vessel. Sipunculus and Phascolosoma, have remarkable bodies known as " urns " floating in their coelomic fluid. They are bell-shaped structures, with a ring of cilia round the mouth, and a nucleus. These remarkable corpuscles are formed by the division of certain large cells on the wall of the dorsal blood- vessel, they were formerly thought to be parasitic Infusoria. The Achaeta have no special organs of locomotion, and probably do not move about much. Sipunculus and Phas- colosoma usually live half embedded in the sand, which they swallow in large quantities. Phascolion lives in empty worms' tubes or iu mollusc shells, and its body is often permanently twisted, accommodatiag its shape to that of its home. Pliymo- soma lives in holes or passages in coral rock, or in holes between stones. As a rule the members of this subdivision occur only in comparatively shallow water. The Gephyrea chaetifera are provided with a pro- stomium, which may acquire enormous proportions. In Ponellia it may, when fully extended, attain a length of 2 or 3 feet, whilst the body is only 1^ to 2 inches long. In this genus it is bifid at the end. In Echiurus, BoTielUa, and Thalassema there are a pair of large chitinoid hooks placed anteriorly on the ventral side of the body, and in some species of JEchiurus there is one, sometimes two, posterior circlets of setae, each seta originating from a single cell, like those of the Chaetopods. Bonellia viridis is coloured a bright green by a pigment termed " bonellein/' which is not identical with chlorophyll. The mouth in the Chaetifera lies at the base of the pro- stomium, which is ciliated and grooved, and is doubtless used to catch minute organisms for food; the iatestiae is looped and the anus terminal. In Bonellia, Echiurus, and Thalassema a " siphon " or collateral intestine, such as is found in the Capitellidae and Echinids, is present. Branched organs open into the rectum in most of the Chaetifera. At the end of each branch is a small funnel- shaped ciliated opening leading into the coelom. The ceUs lining the tubes of these branches have been seen crowded with excretory granules, and they may possibly function as nephridia as well as serve to regulate the amount of fluid in the coelom. GEPHYR^A 167 The vascular system is more complex in the Chaetifera than in the Achaeta. The dorsal vessel in Echiurus opens behind into a circular blood-vessel which surrounds the oeso- phagus. At its anterior end it enters the prostomium and runs to the tip of this organ, here it splits, and the two branches return, one down each side of the prostomium, tiU they have passed the mouth, when they unite to form a median supra- neural blood - vessel. This is connected with the peri- oesophageal circular ring by a- transverse vessel. Haemo- globin has been detected in the coelomic corpuscles of TJialassema. The last-named genus may have from one to four pairs of nephridia, according to the species, Echiurus has usually two pairs. The nervous system, like the vascular system, is continued into the prostomium, running all round the edge, and finally uniting below the oesophagus, thus forming a circum-oeso- phageal ring, which gives off the ventral cord. In no place is the nerve riag or cord thickened to form anything like a ganglion. In Echiurus and the female Bonellia the coloemic epi- thelial cells which surround the ventral vessel enlarge and form the reproductive cells, which are thus favourably situated for receiving nourishment. There is a .very remarkable dimorphism in the genus Bonellia. The female is a fair-sized animal, with a body 2 inches long, but the male is a microscopic planarian-like organism which Hves in a recess of the nephridium of the female. It is from 1 to 5 mm. long, and is ciliated all over. Its intestine is not functional, and it ends bKndly both in front and behind. The spermatozoa arise from the coelomic epi- thelium, and escape by a modified nephridium. A nervous system, but no vascular system, is present. The male larva is said to chng to the prostomium of the female, and thence to pass into the mouth, where it undergoes its final changes, then it creeps out from the mouth and into the nephridium, where it spends the rest of its life. Another genus, Hamingia, has a similarly degenerate male, which also lives in the nephridia of the female. i68 ZOOLOGY Those members of the armed Gephyrea whose developement has been investigated show unmistakable affinities to the Chaetopods. Their larvae exhibit a metameric segmentation, but the somites disappear early. Traces of segmentation are retained in the adult in a few cases, such as the four pairs of nephridia in one species of Thalassema, the double ring of setae in UcMurus Pallasii, and possibly in the rings of hooks and circular nerves of many forms. A connecting link between the Gephyrea armata and the Chaetopoda may exist in the curious worm SterTiaspis. This animal, usually classed with the Chaetopoda, retains a well-marked segmentation ; and its blood-vessels, whUst resembling in their disposition the more important vessels of the Gephyrea, open into a well- developed system of capillaries. On the other hand the looped intestine, one pair of brown tubes, retractile anterior end of its body, and — in Sternaspis spinosa — a long bifid prostomium, de- scribed by Sluiter, are all features shared in common with the Gephyrea. The unarmed Gephyrea have an abbreviated developement which shows no traces of metameric segmentation, but this hardly seems a sufficiently important difference to warrant the breaking up of the group. CHAPTEE XII BEACHIOPODA Brachioi)oda-f ^•'^'''iiii^s — lAngula, Crania, Discina. ^ \ Testicardines — Argiope, Terebratula, Waldkeimia. Chaeacteristics. — Coelomata devoid of organs of locomotion, and usually fixed in the sand on to some foreign iody, hy a peduncle. A Mvalved shell encloses the body. The valves are dorsal and ventral, and in one subdivision are hinged to one another. They are lined hy dorsal and ventral extensions of the body-wall, termed the mantles ; these often hear chitin- oid setae round their edges. A lophophore surrounds the mouth, bearing ciliated tentacles. The alimsntary canal is ciliated, and receives the secretion of two branched glands, the liver ; it is in one sub-division aproctous. One, rarely two, pair of nephridia exist. Uxclusively marine. The existing BracHopoda are interesting as the survival of what in early geological time was a very widely distributed and very numer- ous group of animals. The two genera Lingula and Discina extend from the Cambrian, the oldest group of the Silurian rocks, to the present day ; and, judging by their shells, they appear to have undergone but httle change during the vast period of time which must have elapsed since they lived. They are found in great numbers, both of indi- viduals and of species, in these older Paleozoic formations ; but the group seems to have been most flourishing in the Devonian seas, for upwards of 60 genera and Fig. 106. — Waldheimia cranium. A. Ventral, B. Dorsal valve. I70 ZOOLOGY 1100 species have been described from Devonian rocks. Since this epoch they have dwindled, and at the present day not more than about 100 species exist. Argiope (Cistella) neapolitana is a small Brachiopod found attached by a peduncle to pieces of rock at a depth of about 70 metres in the Mediterranean. The dorsal and ventral shells entirely cover the body except the peduncle, which projects through a " beak " formed by the ventral or larger shell. The Fig. 107. — A longitudinal vertical median section through Argiope neapoUtana. 1. Ventral shell. 2. Canal containing blood-vessel. 3. Sub-oesophageal nerve ganglion. 4. Mouth. 5. Stomach. 6. Peduncle. 7. Plexus of blood-vessels. 8. Median crest on dorsal shell. 9. Organic membrane which has separated from shell during the process of de- calcification. --6 dorsal shell is rather the smaller; both are of a brownish hue with small white spots. The body of the Argiope lies almost entirely in the dorsal shell (Fig. 107), and is supported by certain ridges which this shell bears on its inner surface. The BRACHIOPODA 171 whole animal is about 2 "5 mm. long, and about the same in breadth. The shells are secreted by the body-wall or by the mantle. Siace the body Ues chiefly in the dorsal shell, the larger part of the latter is secreted by the body-wall, and the dorsal mantle is of small extent; on the other hand, the greater part of the ventral shell is lined by a fold of integu- ment, the ventral mantle. The substance of the shell is composed of minute calcare- ous spicules kept together by a network of organic fibrils (Kg. 107). The shell is pierced by numerous canals, whose outer ends are somewhat enlarged and covered with a cuticle. Since the mantle is formed of a duplicature of the body- wall, it is necessarily double, and the body-cavity extends into it, in some places this space lodges the reproductive organs. The mantle sends a prolongation into each of the canals in the shell, which is continuous with some of its blood-vessels. These prolongations contain blood- corpuscles, and doubtless serve to nourish the organic fibrils which keep together the calcareous spicules of the shell. The lophophore occupies a considerable part of the dorsal shell, and forms a large part of the body- wall. Its shape is oval, its border running parallel to the edge of the shell, except at the anterior median line, where a narrow deep in- dentation almost divides it into two, and thus gives it a somewhat horse-shoe shape. The indentation is occupied by a median ridge in the dorsal shell (Fig. 107). The lophophore carries round its edge, on the dorsal side of the mouth, from 70 to 100 tentacles; at the base of the tentacles is a ciliated PiQ. 108. Wdtdheimia Jlavescens. Interior of dorsal valve, to show the position of the lophophore. A portion of the fringe of cirrhi has been removed to show the brachial membrane, and a portion of the spiral extremities of the arms. A. Position of mouth. 172 ZOOLOGY groove, whose other side is formed by a lip which also mas round the edge of the lophophore (Fig. 109). The tentacles are partially ciliated as well as grooved, and any particles of food they come in contact with are carried down the groove to the mouth, which opens in its posterior median line. In other genera the lophophore stands out from the surface of the body and becomes curiously coiled and rolled up, as in Waldheimia (Fig. Ill), in which animal it is supported by a calcareous loop. The mouth is a transverse slit leading into a short oesophagus; this is attached by mesenteric strands to the Fig. 109. — Transverse section through the middle of Argiope neapolUana. The section includes the posterior limit of the lophophore, but is anterior to the brood pouches. 1. Stomach. 2. Gastroparietal bands. 3. Ovary in dorsal shell. 3'. Ovary in ventral shell. 4. Dorsal adjuster muscle. 5. Occlusor muscle. 6. Left mesentery ; posteriorly this fuses with the right to form a single mesentery. 7. Ventral shell. 8. Vascular canal in shell. 9. Canal at base of lophophore, which sends a branch into each tentacle. 10. Lip forming with the tentacles a groove. 11. Dorsal shell. end of the median projection of the dorsal shell, and it opens directly into the globular stomach. On each side of the aliment- ary canal is the liver, composed of six or seven thick tubules, which unite and open into the stomach by a broad mouth. The lumen of the liver is often full of secretion, it is lined by vacuolated cells. The stomach opens behind into a short intes- tine which has no anus, and which, like the rest of the aliment- BRACHIOPODA 173 ary canal, is ciliated. The alimentary canal is supported by a median sheet of connective tissue, the mesentery, which passes from it to the ventral shell (Fig. 109), and by two lateral sheets, termed the gastroparietal bands, which pass out from the stomach to the sides of the body-wall. Owing to the peculiar relations of the animal to its shell, the body-cavity becomes very complicated, it is partly pro- duced into the mantles which line the shells, and here the reproductive organs partially lie. At the posterior and lateral regions the body-wall is pushed in, in such a way as to form two lateral brood pouches, which lie behind the level of the lophophore, and are enclosed by the shell. The embryos undergo the early stages of their developement in these pouches. The coelom is traversed by four bundles of muscle fibres, two of which open and close the shell, the other two move the shell ■,v 2 FlQ. 110. — Woddheimia flcmescens. Diagram showing the muscular system. After Hancock. 1. Ventral valve. 2. Dorsal valve. 3. Calcareous loop. 4. Mouth, 5. Extremity of intestine. 6. Adductor. 7. Divaricators. 8. Accessory divaricators. 9. Ventral adjusters. 10. Peduncular muscles. 11. Dorsal adjusters. 12. Peduncle. on its peduncle. The latter are termed adjusters, and a pair arise from each valve of the shell and are inserted into the peduncle. By their contraction they raise or depress the shell, and by contractiug alternately they may also serve to rotate it. The occlusor muscles have a double origin from the dorsal sheU, 174 ZOOLOGY but the two parts unite to form a single tendon, whicli is in- serted into the ventral shell. The divaricators are very small. They arise from the ventral shell, and are inserted into the dorsal valve in such a relation to the hinge as to cause the shell to open when they contract. Additional muscles are found in other members of the group, those of Waldheimia and Zin- gula are shown in Figs. 110 and 113. Eunning round the edge of the lophophore, at the base of the tentacles, is a canal which is probably continuous with the general body-cavity. It gives off a branch into each tentacle, and the latter are probably extended by the entrance of the coelomic fluid into them (Fig. 109). There is a closed vascular system containing a corpus- FiG. 111. — View of the inner side of the right half of Waldlieimia australis. Prom a dissection by J. J. Lister. 1. Mouth. 2. Lophophore. 3. Stomach. 4. Liver tubules. 5. Median ridge on dorsal shell. 6. Heart. 7. Intestine ending blindly. 8. Peduncular muscle. 9. Internal funnel-shaped opening of uephridium. 10. Peduncle. 11. Body- wall. 12. Tentacles. 13. Coil of lip. 14. Terminal tentacles. culated fluid. The vessels composing it are irregularly scattered through the tissue of the body. They are especially BRACHIOPODA 175 developed in the mantle and in those parts of the body-wall next to the shell, and send oif numerous caeeal processes into the canals permeating the substance of the latter. The blood is probably aerated as it comes through the vessels of the mantle. The presence of a central heart in Argiope is a matter of dis- pute if it exists, it is a contractile vesicle situated dorsal to the stomach ; a spherical vesicle is found in this position in Waldheimia, but its relation to the blood-vessels is not very definitely known. The nephridia, which also function as genital ducts, open internally, with large funnel-shaped mouths, which are directed towards the posterior, end of the dorsal shell. These openings lead to short tubes which run in that part of the body-wall which is pushed in to form the brood pouch ; into this they PiQ. 112. — The posterior quarter of a Waldheimia austredis has been removed, in order to show the relations of alimentary canal, nephridia, etc. The coelom is represented black. From a dissection by J. J. Lister. 1. Dorsal shell. 7. Intestine. 2. Ventral shell. 8. 3. Stomach. 9. 4. Liver. 10. 5. Gastroparietal bands. 6. Heart. Tendons of adductor muscles. Ovary. Internal funnel-shaped opening of nephridia with three ova drop- ping in. eventually open. The walls of the tube consist of a connect- ive tissue basement membrane lined by glandular cells crowded with brown concretions. Some of these cells are also ciliated. 176 ZOOLOGY The ova are modifications of the cells lining the coelom. There are four ovaries, one on each side in the dorsal and in the ventral shell. The cells are borne on an axis, and all stages from the ripe ovum to the unmodified peritoneal cell may be seen on the same axis. The ripe ova fall into the coelom and leave the body through the nephridia. They undergo the early stages of their developement in the brood pouches. No testes have been described, and it is uncertain whether this species is hermaphrodite or not. The nervous system consists of a circum-oesophageal nerve ring, vrhich is enlarged into a weU- marked sub -oesophageal ganglion lying in the epidermis. This lies in that part of the body-wall which overhangs the mouth, just behind the base of the tentacles. The nerve ring swells into a small supra- oesophageal ganglion, which is not so large or so well marked as the sub-oesophageal. The latter gives off a nerve which runs round the edge of the lophophore, and nerves to the dorsal and ventral mantles. The Brachiopoda are divided into two orders : (i.) The Ecardines, whose shell is chitinous and hut slightly strengthened hy a deposit of calcareous spicules. The shell has no hinge and no internal skeleton to support the arms. The ali- mentary canal terminates in an anus, median and ventral in Crania and lateral in Lingula. Lingula, Glottidia, Crania, and Discina. (ii.) The Testicardines have shells composed of calcareous spicules, the valves are hinged together, and there is usually an internal skeleton supporting the arms of the lophophore. There is no anus. Argiope, Terebratula, TerebratuUna, EhynchoneUa, Theci- dium, Waldheimia. In the Lingulidae the dorsal and ventral valves are about the same size ; in all other Brachiopods the ventral is much the larger, and except in Crania always lies uppermost. In some genera, as Argiope and Lingula, the body occupies most of the space enclosed by the valves of the shell ; in Terebratula and some others the body takes up but a small portion of this space, the remainder being occupied by the arms of the lophophore, which stand out from the surface of the BRACHIOPODA 177 body, and may be coiled in a very complicated manner. Rhynehonella the lophophore is protrusible, but this is excep- tional. The edges of the mantle usually carry a row of setae, which arise from ectodermal pits, as in Chaetopods. The intestine in Zingula is of some length, it takes one or two twists, and terminates in an amis which opens at the right side into the mantle cavity between the shells. In Discina also the anus is lateral, but in Crania it opens in the median line into a cavity which lies between the posterior ends of the valves where the peduncle would normally be found. The food of Brachiopods con- sists chiefly of Diatoms and minute unicellular Algae, which are brought to the mouth by the action of the cilia on the lopho- phore. The only case of serial repeti- tion of parts presented by the Brachiopoda is the two pairs of nephridia found in the genus Bhynchonella. The sexes are separate in Crania, but Zingula, and probably some others, are hermaphrodite. The recent Brachiopods are found in all seas, usually at moderate depths, within 100 fathoms. Idngula and Glottidia sometimes live between tide marks, but may extend to 12 In FlO. 113. — iMigvla anatina. Dia- gram showing the muscular system : after Hancock. 1. Dorsal valve. 2. Ventral valve. 3. Peduncle. 4. Heart. 5. Alimentary canal. 6. Anal aperture. 7. Umbonal muscle. 8. Central muscle. 9. Transmedial muscle or sliding muscle. 10. Anterior muscle. 11. Middle muscle. 12. Adjusters, enabling valves to move forward and backward on each other. a 178 ZOOLOGY depth of 70 metres. They anchor themselves in the mud, and their long, hollow peduncle forms a sand-tube around itself. Some Testicardines have been found at great depths ; they usually attach themselves to the rocks, and, when they are found at all, occur in considerable numbers. Polyzoa. CHAPTEE XIII POLYZOA {Phylactolaemata — Plumatella, Cristatella, Lophopus. fCycloatomata— Crisia, Eomera. Gymnolaemata J Ctenostomata — PaludiceUa,Alcyonidmm. I Cheilostomata — Bugula, Memhranipora, I Flui Entoprocta — Loxosoma, PedicelUna. Characteristics. — Small coelomate animals, invariably possess- ing the faculty of budding. The colonies nearly always fixed. The ectoderm secretes, as a rule, a cuticle, which may be horny or calcareous. The intestine is bent in the form of a \J, the anus and mouth being approximated ; between them is situated the nerve ganglion. The mouth is surrounded by a series of ciliated tentacles. The individuals of the colonies may be hermaphrodite, but the generative cells may ripen at different times. The Polyzoa comprise a very great number of species, which can be grouped into two main subdivisions : (i.) The Ectoprocta, in which the anus lies outside the circlet of tentacles which surrounds the mouth. (ii.) The Entoprocta, or those forms in which the circlet of tentacles or lophophore embraces both ends of the alimentary canal, the anus as well as the mouth. The latter subdivision contains very few forms, but the former includes a great number of species, mostly marine ; a few, however, inhabit fresh water. Plumatella fungosa is a fairly common representative of the freshwater Ectoprocta. It occurs all over Europe, on pieces of submerged trees, etc., living by preference in stagnant or slowly-flowing water. Each individual of the colony lives i8o ZOOLOGY in a thin chitinous tube, from the mouth of which the animal protrudes under favourable circumstances, and into which it withdraws in time of danger. The size of the colony varies a good deal, it may, however, attain a diameter of some inches, and may weigh a pound or more. The chitinous tube is a modi- fication of the cuticle, and is secreted by the epidermis of the body -wall. The .cuticle is usually spoken of as the ectocyst, the body - wall underlying it being called the endocyst. The anterior portion of the body is capable of being extended beyond the mouth of the tube, and bears a series of tentacles arranged in a horseshoe -shaped lophophore. This portion can also be retracted, and in this condition the space in which the tentacles lie is termed the tentacle-sheath. The body-wall, with its cuticle, is horny in Plumatella, gelatinous in Lophopus, and calcareous in most marine Polyzoa. It is often known as the zooecium, whilst the extrusible part of the organism, with the tentacular crown and the alimentary canal, etc., is distinguished as the polypide. The body- wall of Plumatella contains a layer of external circular, and of internal longitudinal muscles, and is lined by a ciliated epithelium ; at its lower end it becomes free from the ectocyst, and by the contraction of the muscle fibres in this unattached region the coelomic fluid is forced forward, and serves to extrude the polypide. Certain muscle fibres stretch from the endocyst to the waU of the extrusible portion of the body. These have been termed the parieto-vaginal muscles, and serve to prevent the full extrusion of the polypide. The zooecia at their basal ends open into one another, and the Pig. 114. — A portion of Plwnatdla fungosa seen in vertical section. Allman. 1. Mouth of tubes. 2. Cavity of tubes. 3. Statoblaats. 4. Piece of wood in which the colony is growing. POLYZOA body-cavities of the various members of the colony are in free communication. The lophophore is in the form of a double horse-shoe, it bears from forty to fifty ciliated tentacles, which, by the current they set up, assist in bringing minute algae, etc., as food to the mouth. The bases of the tentacles are connected together by a fine membrane or web, sometimes called the calyx. On the side of the horse-shoe nearest the mouth is situated a ciliated extension of the body-wall. This lobe, which more or less overhangs the mouth, is the epistome. The alimentary canal is U-shaped, the mouth opens into an oesophagus, which is ciliated in some species. This leads into a stomach. From the stomach a rectum turns forward again and opens to the exterior outside the ring of tentacles, but not very far from their base. The walls of the intestine contain muscle fibres ; the single layer of cells lining the stomach enclose brown granules, which apparently increase in mmiber with the age of the polypide. These are probably excreta, which for some reason or another do not find their way out of the body. A strand of tissue of considerable importance in the life- history of the animal passes from the posterior end of the stomach, and is attached to the body-wall of the animal, near the posterior end. It is termed the funiculus, and doubtless serves to prevent the polypide from being extruded too far. The coelom is spacious, and contains a corpusculated fluid which is kept in motion by the ciliated cells lining the body-wall. It is continued into the lophophore, and into each tentacle, but is partly divided into two by an incomplete septum which stretches across the body below the level of the base of the lophophore. The nervous system consists of a bilobed ganglion lying on the oesophagus, between it and the anus. It is situated just in front of the imperfect septum which stretches across the animal in this region. At the sides the two lobes extend round the oesophagus, forming a complete circum-oesophageal nerve ring, each lobe also gives off a nerve which runs along the base of the lophophore, and which furnishes a small nerve to each tentacle. 1 82 ZOOLOGY No nephridium has been described iu Plumatella; in an allied form, Cristatella, a pair of ciliated tubes are, however, said to lead from the body-cavity and to open by a common pore between the anus and brain. The interpretation of these Fig. 115.— View of right half of Pluma- tella fungosa, slightly diagrammatic. After AUman and Nitsche. 1. Lophophore. 2. Mouth. 3. Epistome. 4. Anus. 5. Nerve ganglion. 7. Stomach. 8. Rectum. 9. Edge of fold of body-wall. 10. Wall of tube. Ectooyst. 11. Parieto-vaginal muscles. 12. Funiculus. 13. Body-wall. Endocyst. 14. Testis. 15. Testis more mature. 16. Statoblast. 17. Ovary. 18. Spermatozoa free in body-cavity. 19. Calyx. 20. Retractor muscle. ducts is a subject of uncertainty. The functions of a renal apparatus may possibly be delegated to those large cells in the stomach in which brown granules accumulate during the life of the polypide. Plumatella is hermaphrodite. The ova develope from the cells which line the body-wall, near the anterior end. The testes are formed from cells covering the upper end of the funiculus ; these cells multiply, and become spermatozoa. The cells of the funiculus at its lower end give rise POLYZOA 183 to some very remarkable structures known as statohlasts These have the morphological significance of buds ; they con- sist of a little heap of cells which secrete around them a chitinous shell. The freshwater Polyzoa usually die down at the approach of winter, and the continuance of the race is provided for by the persistence of the statohlasts. These structures are usually formed during the autumn, and escape after the decay of the parent organism. Their chitinous shell in most cases is a complicated structure ; part of it contains air vesicles, the air being secreted from the protoplasm of the cells which have formed the shell. Thus the latter forms a float; in some cases, however, the statohlasts are attached to submerged stones, and the floating ring is then rudimentary or absent. The formation of these structures, capable of resisting the winter frosts, in freshwater Polyzoa acquires an additional interest when they are compared with similar contrivances found in other members of the freshwater fauna. Spon- gilla Jluviatilis, the freshwater sponge, also dies down at the approach of winter; it also forms remarkable bodies, termed gemmules, which consist of a collection of cells protected by curious spicules termed' amphidiscs, and from which a new sponge arises in the spring. In the same connection the ephippian eggs of Daphnia, which are also supported by an air float, and in other respects have a striking resemblance to some statohlasts, and the winter eggs of Eotifers and Planarians, may be mentioned. The low temperature of winter, which affects the comparatively small bulk of fresh water much more than the ocean, where beyond a depth of a few fathoms the cold hardly affects the temperature of the water, has apparently called forth these modifications. A. ECTOPROCTA. The Polyzoa are divided into two groups, — (a) the Ento- PEOCTA and (&) the Ectopeoota, — according as to whether the anus is included or not in the circlet of tentacles which sur- rounds the mouth. The Ectoprocta are further characterised by the possession of a well -developed coelom. This group 1 84 ZOOLOGY includes an immense number of species, and has been divided into two subdivisions : (i.) the Phylactolaemata, which inhabit fresh water ; and (ii.) the Gymnolaemata, which are almost invariably marine. The Phylactolaemata are further distinguished from the Gymnolaemata by the presence of an epistome and the shape of their lophophore, which is that of a horse-shoe, and by the formation of statoblasts. The structure of a member of this subdivision has been illustrated by the description of Plumatella ; in this genus the body-cavities of the various polypides are in communication, though some of the polypides are partially separated by an imperfect septum. In Loplwpus and Gristatella the coelom in each polypide is in free and open communication with that of all the others. The last-mentioned genus forms colonies, which may attain the length of over two inches. The colony is oval in outline, and the polypides project from its upper convex surface; the lower, surf ace is flat, and on this the whole colony creeps slowly along on submerged stems or stones. This mode of progression of the colony is one of the very few instances of any co-ordination of function which exists between the various individuals which compose a Polyzoan colony. The Phylactolaemata are all hermaphrodite. The Gymnolaemata have a circular lophophore, and are devoid of an epistome. With the exception of a few genera, they are marine. In Faludicella the funicular tissue of the various individuals communicates by means of certain perfor- ations known as rosette plates, but in the marine forms the zooecia are more independent. The ectocyst may be calcareous, horny, or gelatinous, and the various zooecia may be aggregated together in an almost infinite variety of ways. Prom time to time the polypide dies down, the tentacular crown and aliment- ary canal degenerating and forming what is known as the " brown body," which is coloured by the concretions which have accumulated within the wall of the stomach. This brown body lies in the zooecium until the endocyst produces a bud, and then it may become included in the alimentary canal of the bud or young polypide ; here the nutritive matter which it may contain is doubtless absorbed, and the undigestible matter passes out of the intestine of the young polypide. POLYZOA 1 8s In some of the Gymnolaemata there is a pore by means of which the sea-water can be admitted into the coelom when it is desirable to expand the polypide. This pore may be guarded by a circlet of bristles, which tend to prevent the entrance of grains of sand or other foreign bodies. In certain individuals of some species of Alcyonidium, etc., there is a ciliated canal which leads from the coelom to the exterior ; this is known as the " inter-tentacular organ," it probably serves as an exit for the generative cells, at any rate spermatozoa have been observed to leave the body by this channel. In most species, however, the generative products escape only by the dying down of the polypide. The G-ymnolaemata are divided into three classes, according to the character of their zooecia and the nature of their cell mouths when the polypides are retracted. (i.) The Cyclostomata. — These have tubular zooecia, always calcareous. The cell raouth is circular, and with no apparatus for closing it. Many of them are found fossilised, probably because their calcareous skeleton is easily preserved. Crisia, Hornera, etc. (ii.) The Ctenostomata. — The zooecia are soft, and their apertures guarded by a folded frill. JBowerbanJcia, one of this sub-order, has a muscular gizzard armed with teeth, situated between the oesophagus and the digestive stomach. Paludicella, one of the few freshwater members of the Gymnolaemata, belongs here, and Alcyonidium. There is some reason to suppose that the Phylactolaemata are derived from this group of the Gymnolaemata. (iii.) The Cheilostomata. — This is the largest subdivision, and its members are clearly characterised by the posses- sion of a lid or operculum, which closes the mouth of the zooecium when the polypide is retracted. The zooecia are calcareous. This subdivision exhibits a considerable degree of poly- morphism. Some of the individuals of the colonies are modified to form structures known as avicularia, resembling in shape a 1 86 ZOOLOGY parrot's beak. The two halves are constantly opening and closing, and by their action in catching small worms, etc., they probably serve as defensive organs, as well as assist in keeping the colony clean. The smaller beak is believed to be a modified operculum, whilst the larger corresponds with a much modified zooecium. These structures exist in very various degrees of perfection, those of the genus Bugula being amongst the most specialised. The vihracula are long stiff processes which move up and down, and are possibly tactile in function, they are believed to be homologous with the lower beak of an avicularium. In one genus they move in unison, and thus the colony exhibits some degree of co-ordination. B. ENTOPROOTA. This group contains but few genera. The mouth and the anus are both surrounded by the lophophore, which is circular. The tentacles can be bent over the mouth, but the anterior end of the body cannot be retracted into the posterior half The coelom is almost completely obliterated. A pair of nephridia are present. This group is chiefly founded on the structure of two com- paratively well-known marine genera, Loxosoma and Pedicellina. Loxosoma is unique amongst Polyzoa, inasmuch as it is not colonial ; like most Polyzoa, it increases by budding, but the buds separate from the parent organism. Both the genera are stalked, in Pedicellina the stalk arises from a creeping stolon, and the calyces or the bodies of the individuals often drop off, and are replaced by the regeneration of new ones at the end of the stalks, a process apparently analogous to the formation of brown bodies in ectoproctous forms. In Loxosoma the stalk, which carries the calyx, is at least in the young condition pro- vided at its lower end with a foot-gland, by means of which it is usually attached to some marine animal. The paired nephridia of Loxosoma consist of ciliated intra -cellular ducts piercing a few large cells, and probably each beginning with a flame cell. The ducts open to the exterior between the ganglion and the oesophagus. POLYZOA 187 Most species of the Entoprocta are dioecious, and the generative organs have special ducts. Besides the sexual reproduction, they multiply also by the formation of buds. This process amongst the Polyzoa is remarkable, inasmuch as the endoderm is not represented in the tissues of the bud ; in most other animals which reproduce by budding, all the three embryonic layers occur in the bud. The affinities of the Polyzoa are somewhat obscure. The developement of the Entoprocta and their adult structures point to the origin of the group from some ancestor which is represented in the ontogeny of the MoUusca, Chaetopoda, and armed Gephyrea by the Trochosphere larva. On the other hand, the Phylactolaemata have a certain marked resem- blance to the unarmed Gephyrea, and if this resemblance be not homoplastic, we must regard the Gymnolaemata, and still more the Entoprocta, as degenerate forms. !2i o o l-H CQ , 8 it =5 i -E3 02 PL, <1 7 Iz; •< -< §. o Ph O ES Ph o Kl O ►J ^ g S •< 1 S g 1-5 CJ 02 O 1 <1 1^ P^ -<^ o w tn Ph PM H o o m o m PLi O h-f hJ hJ C5 o i o CHAPTEE XIV MOLLUSOA Chaeacteeistics. — Uivsegmented Coelomata, with a primitive bilateral symmetry. Their iody is soft, and is dorsally produced info a fold, the mantle, which usually secretes a shell. The ventral part of the hody forms, as a rule, a muscular process, the foot, which may be modified in various ways, hut whose function is usually to assist in locomotion. Respiration is typically carried on hy a pair of vascular processes, which project from the hody-wall, and are termed the ctenidia. Near the hase of these organs is a modified patch of epithelium, whose function is olfactory, and this has been termed the osphradiAim. The portion of the body-cavity in which the heart lies, the pericardium, communicates directly with the exterior by means of the nephridia. The heart is systemic, and the circulation partly lacunar. The nervous system typically consists of a pair of cerebral ganglia in the head, a pair of pedal ganglia in the foot, and a pair of pleural ganglia in the body. The last pair are united by a long commissure, the visceral nerve cord, which may become twisted. The sense organs comprise the osphradia, otocysts in connection with the pedal ganglia, tactile tentacles on the head, and in many cases eyes. The developement includes a characteristic larva, the Veliger. The phylum Mollusca includes a large number of animals which exhibit the greatest variety of structure and habit. The majority of them are marine, some inhabit fresh water, and many are terrestrial. The group includes the class Cephalopoda, the members of which are the largest, and at the same time the most ferocious of invertebrates. Some members of the I90 ZOOLOGY phylum are pelagic, and consist of the most transparent and delicate tissues, others are sessile, being fixed either by cords secreted by a gland in the foot (Mytilus) or by the surface of the shell (Ostrea), whilst, again, others bore long funnel-shaped passages in the rocks or in submerged pieces of wood, etc. The very various animals which compose this phylum may be separated into two main divisions, according as to whether ' they retain a well - marked prostomium or not. Those which have lost a definite cephalic region have probably done so in correlation with a sessile, inactive life. They form the division Lipocephala. The other division comprises those Mollusca which possess a well-developed head, associated with a toothed lingual ribbon, capable of a biting or rasping action, borne on a cushion and moved by certain muscles, the whole apparatus constituting the odontophore. This organ has given a name to the division, the Crlossophora. Division I. LIPOCEPHALA. Chakactbristics. — Mollusca with rudimentary ^prostomium, no odontopJiore, and no eyes. Hither sessile, or with very feeble powers of locomotion. This division contains but one class, the LameUibranchiata. Class LameUibranchiata. Lipocephala which have retained the primitive moUuscan bilateral symmetry. The body is laterally compressed, and the mantle is bilobed, each lobe secreting one valve of the bivalved shell. The two valves, right and left, are united by a dorsal elastic ligament. The ctenidia or gills are largely de- veloped, and by the currents their cilia create, assist in bringing food to the mouth. The foot is usually plough-shaped, and contains part of the viscera. It may be used for boring in sand or rock, more rarely for crawling. The pericardium, part of the coelom, is in communication with the exterior by means of a pair of nephridia. The generative glands are simple, and have no accessory organs connected with them. In the common freshwater mussel, Anodonta cygnea, the shells are eqtiivalve. Each valve is composed of three layers : MOLLUSC A 191 (i.) the periostracum, or outermost layer — this is thin and homy, and not calcified, and is formed by the thickened free edge of the mantle ; (ii.) the prismatic or middle layer, con- sisting of closely-packed calcareous polygonal prisms — this is also deposited by the edge of the mantle ; (iii.) the nacreous or mother-of-pearl layer, which lines the inside of the shell — it is composed of laminae of calcareous matter, and is de- posited by the whole of the surface of the mantle and body in contact with the shell. It is this last layer which, when deposited in concentric layers round foreign particles, such as grains of sand, etc., produces pearls. The shells of some LameUibranchs are not equivalve, e.g. the oyster, Ostrea, which is attached to rocks by means of its larger valve. In Pholas there are additional calcareous plates inserted dorsaUy between the two valves ; and in Teredo, the mollusc which does so much damage by boring into wood, the valves fail to completely cover the body, which secretes a calcareous lining to the tube in which it lives. The valves of the shell are kept in apposition by adductor Fig. 116. — Section through ^jiof^onto, to show mechanism of opening and of closing the valves. After Lankester — Zoological Articles reprinted from the Uncydopcedia Britannica. 1. Right valve of shell. 2. Left valve of shell. 3. Hinge. 4. Elastic ligament. 5. Adductor muscles. muscles. These may be two in number, an anterior and a posterior, or the posterior may alone persist {Monomyaria). The edge of the mantle is thickened, and in some genera it bears tentacles and eyes. Posteriorly it is notched in such a way as to form two apertures, which remain open when the 192 ZOOLOGY edges of the remainder of the mantle are in contact. These openings form the dorsal and ventral siphons. In some Lamellibranchs, e.g. Mactra, Cyclas, etc., these notches, by the fusion of their edges, are converted into tubes, which in some genera attain a length of several inches. The ventral siphon serves to admit fresh water, bringing with it oxygen and food, and the dorsal siphon gives exit to a stream of water which carries away the waste products and generative cells. The foot is not developed in the Osteeidae, and is small in Mytilvs, the marine mussel. In the cockle, Cardium, and in Trigonia, it can be suddenly bent, and by this means the animal jumps along. In Solen the foot is suddenly retracted, and in this way water is violently forced out of the siphons, and the animal is propelled forwards. Pecten flies through the water, with its dorsal surface downward, by the flapping of the valves of its shell. The foot often bears a special gland, which secretes a number of horny fllaments known as the lyssus, which serve to anchor the animal to the ground. This structure is well seen in Area and in Mytilus. The mouth, which is median, and ventral to the anterior adductor muscle when the latter is present, lies in a groove formed by the anterior and posterior labial palps. These are ciliated structures, which resemble to some extent the gills, and doubtless serve to convey minute organisms to the mouth as food. The alimentary canal is ciliated. The stomach gives off a caecum, which in many genera lodges a crystalline style. The function of the style is obscure, but it appears to consist of an albuminoid material. The intestine is coiled, and leads to a straight rectum, around which the ventricle of the heart is often folded. A fold of the intestine, or typhlosole, increases its surface. A paired gland, the so-called liver, pours its secretion into the stomach. Two auricles return the arterialised blood to the ventricle, which in Area is double ; the ventricle gives off an anterior and posterior aorta, which distributes the blood all over the body. The. blood from the mantle is in Anodonta returned directly to the auricles ; the rest of the blood is collected into MOLLUSC A 193 a vena cava in the floor of the pericardium, and is thence sent through the nephridia to the gills and returned to the auricles. The circulation is partly lacunar, the blood being contained in irregular splits in the tissues and not in distinct vessels. The blood contains amoeboid corpuscles, and is usually colourless ; two species, however, Solen legumen and Area Noe, contain haemoglobin in their corpuscles. The gills consist primitively of an axis, which is fused to the body for the greater part of its course ; this contains an efferent and an afferent blood-vessel. The axis gives off two series of filaments, which hang down parallel to one another, thus forming two lamellae. The filaments of both series may be bent up, forming V-shaped structures, those of the outer series having their free ends external and next to the mantle, whilst those of the inner series have their free ends internal and next to the foot, so that each series forms a gill with an outer and an inner lamella. In Mytilus and some others the outer and inner limbs of each filament are connected by certain pieces of tissue termed inter! amellar concrescences. Neighbouring filaments are kept parallel to one another by an arrangement unique in the animal kingdom. Each filament bears certain patches of ciliated cells, and the cilia of two opposite patches are interlocked, in the same way as a couple of brushes when put together. In more complex genera these ciliary junctions are replaced by interfilamentous concrescences, and in Anodonta the interlamellar and interfilamentous concres- cences are developed to such an extent as to leave but narrow passages through which the water circulates. The free ends of the filaments of the outer lamella of the external gill, and of the inner lamella of the internal gill, very frequently fuse with the contiguous organs, the mantle, or the foot. Between the lamellae of each gill a certain space is de- veloped which is more or less continuous with that of the other gills. This epibranchial space often serves to lodge the developing ova, it communicates with the dorsal siphon, through which the waste products leave the animal. Each gill filament contains a blood-vessel, and it is often stiffened by two rods of a chitinous material. Its outer epithelium bears cilia, which serve to create a current of 13 194 ZOOLOGY water, which enters the pallial chamber by the ventral siphon. The nephridia of Lamellibranchs are usually known as the Organs of Bojanus. There is a single pair, and each con- sists of a glandular or secretory portion which has an opening Fig. 117. ■ 11 10 -Diagrams of a Schematic Mollusc. After Lankester — Zoological Articles reprinted from the EncydojocBdia BrUannica. A. Dorsal view, showing the heart, 9. Internal opening of nephridium. pericardium, generative organs,' 10. External opening of nephridium. and nephridia. 11. Opening of genital gland. B. Ventral view, showing the nervous 12. Cerebral ganglion. system. 13. Pleural ganglion. C. Lateral view. 14. Pedal ganglion. 1. Mouth. 15. Visceral ganglion. 2. Nerve ring. 16. Foot. 3. Oesophagus. 17. Anterior aorta. 4. Liver. 18. GUI. 5. Anus. 19. Tentacle. 6. Pericardium. 20. Auditory vesicle. 7. Heart. Ventricle. 21. Olfactory ganglion. 8 in A. Auricle. 22. Abdominal ganglion developed on 8 in C. Wall of pericardium. the visceral loop. into the pericardium, and of a ureter which opens to the exterior in the neighbourhood of the orifice of the generative glands. In the oyster the kidney is much less compact, and its secretory part is scattered through the body, even reaching the mantle. MOLLUSC A 195 The nerve ganglia are usually rendered conspicuous by their bright orange colour. The cerebral ganglia, which lie one on each side of the mouth, probably represent the cerebral and pleural ganglia of other molluscs ; they are united both with the pedal ganglia in the foot and with the olfactory (parieto-splanchnic) situated on the ventral face of the posterior adductor muscle. A pair of auditory vesicles, lined with ciliated ceUs and containing a single otolith, are usually present close to the pedal ganglia, and are innervated by a nerve from the cerebro-pedal commissure, which probably comes from the cerebral ganglia. Tactile papillae or tentacles are common round the edge of the mantle. In some cases the tentacles have been modified and form eyes, which attain a great degree of complexity. In Pecten, Spondylus, etc., these eyes have a remarkable resemblance to the vertebrate type of eye, inasmuch as the optic nerve passes in front of the retina, and the retinal elements are thus turned away from the light. The epithelium in the neighbourhood of the olfactory ganglion is modified to form an organ of smell, by means of which the quality of the water flowing in through the ventral siphon may be tested. The LamelUbranchs with few exceptions are dioecious. The generative organs are branched glands usually situated in the foot, though in Mytilus they occur in the mantle. The generative cells are formed in the caecal processes of the gland, and, they leave the body by a right and left simple duct which is continuous with the walls of the gland, and in some cases opens into the duct of the kidney {Spondylus, Lima, and Pecten). Division II. GLOSSOPHORA. CiiARAOTEElSTlos. — Mollusca with a prostomium more or less developed and a buccal cavity armed with a rasping tongue, the radula, which together with its accessory parts constitutes the odontophore. The Glossophora comprise three classes : (i.) Gasteropoda, (ii.) Scaphopoda. (iii.) Cephalopoda. 196 ZOOLOGY Class Gasteropoda. Characteristics. — The Gasteropoda Tmm a foot which is in the main a cravMng organ, it is simple, median, and has a broad flat surface. The foot is often divisible into three divisions, termed the pro-, meso-, and meta-podium. The Gasteropoda are divided into two sub-classes : i. Gasteropoda Isopleura. Characteristics. — The Gasteropoda Isopleura retain the primi- tive bilateral symmetry of the group. The body is elongated, the mouth anterior and the anus posterior. The viscera generally are paired and bilaterally symmetrical. This subclass includes six genera, which are distributed amongst three orders. The best-known genus is Chiton, in which the shell is metamerically divided into eight parts. The gills or ctenidia are also metamerically repeated to the number of sixteen or more, and at the base of each is a patch of olfactory epithelium, the osphradium. Chiton, like Chcoetoderma, another member of the subclass, is dioecious, in the former the generative cells escape by special ducts. In Neomenia and Chaetoderma, however, they leave the body by means of the nephridia. The nerve ganglia are not very markedly developed, but ganglion cells are scattered all along the well-defined nerve- trunks. In some Chitons, eyes furnished with a lens, retina, cornea, etc., have been described as existing on the shell plates. ii. Gasteropoda Anisopleura. Characteristics. — In the members of this subdivision the head and the foot have retained a bilateral symmetry, but the visceral hump with its included organs has undergone a twist which has resulted in rotating the anus and posterior part of the viscera to the right. The angle through which the anus has been twisted varies in different groups ; it may be as much as 180°, and in this case the anus lies above the middle line of the neck. One of the ctenidia is usually atrophied, and- one of the nephridia specialised as a generative duct. The MOLLUSC A 197 mantle developes a shell, which often increases the asymmetry of the animal hy being spirally coiled. This shell is often capacious enough to shelter the whole animal, thus forming a kind of house into which the animal can withdraw. The foot is usually provided tvith a mucous gland. The Gasteropoda Anisopleura are subdivided into two branches : Streptoneura (Prosobranchiata) and Euthyneura. BrancliA. STREPTONEURA. Characteristics. — The first branch comprises those Molluscs in which the torsion has proceeded to such an extent that the emus has become anterior, and the right gill and o^hradium have crossed anteriorly to the left, whilst the left gill and osphradium have come rozcnd posteriorly to the right. As a consequence one limb of the visceral nerve loop is pulled over the other and a figure of Q is produced. This branch includes two orders : Zygobranchiata and Azygobranchiata. Order 1. Zygobranchiata. Characteristics.- — The first order includes all those forms in which, although the torsion is complete, so as to bring the anus near to the anterior median line, the atrophy of the cten- idium of one side has not usually taken place, and the generative cells leave the body through one of the nephridia which still retains its renal function. No accessory generative organs occur, and the visceral hump is coextensive with the foot. This group includes three families. The best-known genera are Haliotis, known as the Ormer in the Channel Islands, where it forms an article of diet, Fissurella, and Patella or the limpet. Patella vulgata, the common limpet, is protected by a conical dome-shaped shell, whose average length is about two inches. The edges of the shell are not quite smooth, and their inequalities generally correspond closely with those of the rock upon which the animal is situated. Limpets are usually found between the tide-marks, and if they wander away from the spot on which they usually occur when covered by the 198 ZOOLOGY tide, they are stated always to return to it before the water has again receded. The visceral hump is covered by the conical shell. The body-wall at its edge is produced into a fold, the mantle. The ventral surface of the animal consists of the muscular oval Fig. 118. Diagram of a vertical median section of a Limpet, Patella mdgata. After Lanlsester — Zoological Articles reprinted from the Mncydopcedia Britannica. 1. Mouth. 10. Heart in pericardium. 2. Odontophore. 11. Nephridium. 3. Radula. 12. Opening of larger nephridium. 4. Radula sac. 13. Branchial efferent vessel (vein). 5. Buccal cavity. 14. Branchial afferent vessel (artery). 6. Laminated stomach. 15. Salivary gland. 7. Intestine cut across. 16. Generative gland. 8. Liver. 17. Edge of the mantle. 9. Anus. foot, between which and the mantle a groove exists which lodges the gills. The foot is attached to the shell by a circular muscle which is incomplete anteriorly. A distinct head exists, and this carries a pair of tentacles with a pair of eyes which appear as black specks near the base of the tentacles. Above the head the groove between the foot and the mantle deepens into a large paUial cavity. Into this, not in the median line, but slightly to the left of it, the anus opens, and on each side of the anus lie the openings of the renal organs (Fig. 119). On the neck are also situated two small bodies representing the ctenidia, which are fully developed in the allied forms Haliotis and Fissurella; in connection with these a patch of olfactory epithelium, the osphradium, has also been discovered. The function of these ctenidia, the original breathing organs, has been assumed by certain folds of the mantle forming the actual gills. MOLLUSC A 199 The mouth leads into the cavity of the buccal mass, this is partially obliterated by the developement of a large ventral Fig. 119. — Side view of anterior end of Limpet, Patella milgata. Part of the mantle is cut away to show the contents of the pallial cham- ber. After Lankester — Zoological Articles re - printed from the Encyclopcedia Britannica. 5. Small nephridium. 6. Large nephridium, 7. External opening of small ne- phridium. 8. External opening of large ne- phridium. 9. Anus. 10. Rudimentary otenidium. Head. 2. Tentacle. 3. Mantle skirt. 4. Muscles forming root of foot, and adherent to the shell. 11. Pericardium.' mass, over which the tooth -ribbon or radula works. The ventral mass contains certain cartilaginous nodules, and is very Fig. 120. — Vertical section through the neck of Patella mtlgaia. After R. J. Harvey Gibson. 1. Mouth. 6. Anterior cartilage. 2. Buccal cavity. 7. Oesophagus. 3. Palatal tooth. 8. Radula sac. i. :^adula. 9. Foot. 5. Odontophore. muscular. The radula, which runs over it, is continued into a sac, from the blind end of which it grows (Fig. 120). The radula ZOOLOGY and its sac attain an extraordinary length in the limpet, often twice the length of the animal; they lie between the viscera and the muscular foot. Two pairs of yellowish salivary glands pour their secretion into the buccal cavity by two ducts on each side, and many mucous glands also open into it. The oesophagus leads from the buccal mass into the stomach. The walls of this organ are much folded, it receives by numerous ducts the secretion of the liver. The latter is a large organ occupying the greater portion of the space in the visceral hump, and enveloping a considerable proportion of the PiQ. 121. — Semi - diagraramatio view of intestinal coils of Patella vulgata. After R. J. Harvey Gibson. 4...///./^^ /^ /,'C^^\\ 1- Buccal mass. 2. Rectum. 3. Crop'. 4. Stomach. 5. Coils of intestine. alimentary tract. The intestine which passes from the true stomach makes a loop and then again enlarges into a second stomach, which is bent upon itself ; after this the intestine coils in a most complicated way and ultimately ends in a rectum, which, opens to the exterior on the anal papilla in the anterior pallial chamber (Fig. 119). The whole alimentary canal is lined throughout by ciliated cells ; the extent of its convolutions are shown by the fact that it may attain a length of over fourteen inches, in an animal a little more than an inch long. The heart consists of a single auricle and ventricle, in the allied forms Haliotis and Fissurella two auricles exist. It is enclosed in a pericardium situated in the posterior angle of the anterior pallial chamber. A large vessel, the branchial vein, runs on each side round the edge of the mantle at the base of the gills ; anteriorly the two vessels unite and empty into the auricle. A muscular valve separates the auricle from the ven- tricle. The cavity of the latter is much broken up by strands MOLLUSC A of muscle fibres ; it opens into the left and right aortae, the former supplying the circular muscle. Both aortae soon termi- nate in lacunar spaces, from whence the blood presumably passes to the gills. The blood is colourless, and contains amoeboid corpuscles. The nephridia are paired, but the right is much larger than the left. They open to the exterior by small renal papillae, situated one on each side of the anal prominence, and also, according to some observers, internally by two minute pores into the pericardium. The existence of the reno-peri- cardial openings has recently been denied, both in Patella and in Fissurella. Haliotis and Trochus possess a left reno-peri- cardial duct only. The left kidney lies between the rectum and the pericardial chamber. The right kidney, which is aborted in other Anisopleura, occupies a large space in the visceral hump. In part of its course it is closely applied to the generative organs, and when the ova and spermatozoa are ripe they are stated to burst into the lumen of the kidney, and so to leave the body through the renal papilla on the right of the anus. The lumen of the kidney is much broken up by ridges which project into it from its walls. The ridges are covered with glandular epithelium, which is partly ciliated ; in the substance of the ridges numerous blood-vessels ramify. The nervous system is very complex, it comprises several pairs of ganglia, the most important of which are the cerebral, the pedal, and the pleural. The cerebral ganglia are situated at the base of the tentacles, they give off nerves to the eyes and to the tentacles. The two ganglia are united by a com- missure above the pharynx ; they also give off a commissure on each side which passes to an anterior superior buccal ganglion. From each buccal ganglion two commissures arise, one uniting it with the similar ganglion of the other side, the other pass- ing posteriorly to a posterior superior buccal ganglion, which is in its turn united with the similar one on the other side. Thus the buccal nervous apparatus consists of a square of commis- sures with a ganglion at each angle. The cerebral ganglia are connected with one another by a commissure which runs underneath the buccal mass ; this bears two small ganglia — the inferior buccal ganglia. ZOOLOGY From the posterior end of each cerebral ganglion two com- missures pass backward, the outer one passing into the pleural ganglion, the inner to the pedal. Each pleural ganglion is connected with the pedal of its own side, and the two pedals are united by a pedal commissure. The pleural gives off two stovit nerves. The outer of these soon splits, one branch going to the gills and mantle, the other to the circular muscle which attaches the animal to its shell. The second nerve given off from the pleural forms the origin of the visceral loop. This is a nervous loop, which, starting at each end from the pleural ganglion, forms a figure of 8 twist. In its course it gives off a nerve to an olfactory ganglion lying at the base Of each of the rudimentary ctenidia. The olfactory nerve going to the left ctenidium arises from the loop near to the right pleural gan- glion, that to the right ctenidium arises near the left ganglion. This twisting of the visceral loop is characteristic of the Streptoneura. The pedal ganglia give off each two large nerves, which supply the muscles of the foot. The tentacles have a tactile function; at their base the eyes are situated — they consist of a pair of pits sunk in the surrounding tissue. The epidermal cells lining these pits become modified and deeply pigmented, and are connected by an optic nerve with the cerebral ganglia. A similar simple eye, consisting of an open pit lined with pigmented cells, is found in Nautilus. Limpets are dioecious ; the position of the generative glands is similar in the two sexes (Fig. 118) between the muscular foot and the digestive organs, rather near the posterior end. Like other members of the Zygobranchiata, the generative glands possess no ducts, and theircontents leave the body through the right nephridium. Order 2. Azygobeanchiata. Characteristics. — The Aeygdbranchiata, which constitute the other order of the StreptoTieura, have lost their original left ctenidium and osphradium. TJie right nephridium does not exist as such, hut is most prohably represented iy the generative MOLLUSC A 203 duct. The left nephridium, which was in Patella smaller than the right, and the left ctenidium and osphradium, are retained. The anus lies on the right of the neck of tJie animal. The Azygdbranehiata are dioecious, and the males are often furnished with a large penis. The great majority of Azygobranchiata are adapted for creeping at the bottom of the sea (Reptantia), and for this purpose have a large muscular foot with a flat sole. They are often spoken of as sea-snails ; the shell which encloses and protects the visceral hump is usually coiled. A large gland is not unfrequently found lying alongside the rectum ; in the genera Murex and Purpura its secretion turns purple when exposed to the light, and it was in ancient times used as a dye. The posterior surface of the foot in some species bears a calcareous or horny plate, the operculum, which serves to close the mouth of the shell when the animal is retracted. The foot itself shows a tendency to break up into three portions : anteriorly the propodium, in the middle the meso- podium, and posteriorly the metapodium, which bears the operculum. This group includes, amongst many others, Buccinum, the whelk, and Littorina, the periwinkle. Paludina, the river-snail, and Valvata are freshwater members of the group, and so is the terrestrial Cyclostoma, which has no gill, the mantle chamber having become a respiratory organ. It lives in damp places. Untoeoncha mirabilis lives parasitically in the body of Synapta digitata ; this is exceptional, as parasitism is very rare amongst the MoUusca. A few Azygobranchiata have become modified in con- nection with a pelagic mode of life. These form the section Natantia, also known as the Heteropoda. As in other pelagic organisms, their tissues have become wonderfully transparent, and of a gelatinous consistency. The foot has become a swimming organ. Its division into pro-, meso-, and meta-podium is well marked. Atlanta has a coiled transparent shell, into which the body and foot may be withdrawn, and the metapodium carries an operculum. In Garinaria the foot is by far the largest part of the animal, and the relatively small visceral hump is covered by a small hyaline shell. In 204 ZOOLOGY Pterotrachea the visceral hump is still more reduced, and is devoid of a shell. The sense organs and nervous system are unusually well developed in this subdivision, the otocysts being usually closely attached to the cerebral ganglia. The lingual ribbon has moveable lateral teeth, which divaricate when the tongue Fig. 122. — Diagrams of a series of Molluscs to show the form of the foot and its regions and the relations of the visceral hump to the antero - posterior and dorso - ventral axes. After Lanlcester — Zoological Articles reprinted from the Enoyclo- pcedia BHtannica, I. A Lamellibranch. II. AuAnisopleurousGasteropod. III. A Cephalopod. A. Anterior surface. P. Posterior surface. D. Dorsal surface. V. Ventral surface. 1. Mouth. 2. Anus. 3. Mantle cavity. 4. Foot. is protruded, but come together when that organ is withdrawn, by means of these the Natantia, which are carnivorous, catch other pelagic organisms to feed upon. Von Erlanger has shown in the developement of Paludina, that the glandular portion of the nephridium arises as an evagination of the pericardium, and the mouth of the evagin- ation remains as the reno-pericardial pore, when such an orifice exists. The genital gland arises as a proliferation from the pericardial wall at a spot where a rudimentary evagination has been formed which represents the missing nephridium. The duct of the nephridium and the duct of the gonad are sym- metrically situated and homologous involutions of the epiblast. MOLLUSC A 20S Thus, in this animal at least, and probably in all MoUuscs, the cavity of the generative organs and of the glandular part of the kidney is part of the coelom, and the generative cells are formed from coelomic epithelial cells. Branch B. EUTHYNEUEA. Chabacteeistics. — The, second great branch into which the Gasteropoda Anisopleura is divided differs from the Strepto- neura in the fact that the torsion of the visceral hump has proceeded to a less extent, and that consequently the peri- cardium is placed obliquely in front of the mantle cavity and gill. For the same reason also the one side of the visceral loop has not become pulled over the other by the forward rotation of the gill and osphradium, and the loop is untwisted. The condition of the gill and hidneys is as in Azygobranchiata. The right auricle is wanting. With one exception, no operculum is found. All Euthyneura are hermaphrodite, a sharp distinction from the Streptoneura. The shell is often but slightly calcified, and it and the mantle tend to disappear. The Euthyneura comprise two orders : (i.) the Opistho- branchiata and (ii.) the Pulmonata. The Opisthobranchiata are characterised, as their name implies, by the position of the ctenidium behind the heart. The branchial vein opens into the auricle, which is situated behind the ventricle. The shell is often absent, and when it is developed it is frequently covered by a fold of the mantle. Both the mantle and the ctenidium may, like the shell, be wanting, the function of the ctenidium may be carried on by processes of the body-wall. In some Opisthobranchs the margin of the foot is produced into two broad flaps, which may stand out at right angles to the axis of the foot, or may in some cases be folded up against the sides of the body. One of the most characteristic features of this order is the presence of processes of the body-wall termed cerata or dorsal papillae. The cerata vary very much in size, number, and arrangement. Processes of the liver are prolonged into some of them, these have been termed hepatoeerata to distingusih 2o6 ZOOLOGY them from those which are simply diverticula of the body-wall. These cerata often coexist with well-developed gills, and their minute structure does not point to any very definite respiratory function ; it seems not improbable that their varied shape and colour may be in some cases protective and in others con- spicuous and warning. In those Opisthobranchs which possess hepatocerata, such as Boto and Eolis, the liver is not a compact gland, but consists of a number of diverticula given off from the alimentary canal, each diverticulum passing into one of the cerata, and being large enough for food particles to pass into it and be there digested. In Eolis the liver diverticula do not end blindly, but are stated to open into an invagination of the ectoderm termed the cnidophorous sac. This opening is guarded by a minute sphincter muscle. The cnidophorous sac, which in its turn opens to the exterior, is lined by a number of large cells, cnidoblasts, which are crowded with nematocysts or thread-cells ; these recall the stinging organs of the Coelenterata. The everted threads of these nematocysts are armed with both large and small spines. In Aplysia, Bulla, and Fleurobranchus the original cten- idium has been retained, but is situated behind the heart, in Boris and its allies the ctenidium appears in a modified form as a circlet of feathered processes which surround the median dorsal anus ; in Eolis, Tethys, etc., it has completely disappeared. In Aplysia there is a large gland, whose secretion is said to be poisonous, which opens just below the osphradium near the anterior end of the ctenidium ; and numerous small cutaneous glands open on the under surface of the mantle, in Aplysia these produce a purple secretion. The Opisthobranchs are, like the Pulmonata, hermaphrodite ; and the generative organs • consist of a hermaphrodite gland or ovo-testis. Some of the cells of this gland form ova, whilst others divide up and become spermatozoa. Prom the ovo- testis a hermaphrodite duct leads to an albuminiparous gland, in the substance of which the duct coils. Just where the duct leaves the gland it gives off a small diverticulum, the vesicula seminalis. The duct then passes on to the external opening, but just before it reaches that it receives the duct of a spherical spermatheca. When eggs leave the body by means MOLLUSC A 207 of this external opening situated in front of the ctenidium, they are enveloped in a gelatinous coating deposited by the albuminiparous gland, and are impregnated by the spermatozoa of another individual, which has been stored up in the sperma- theca. The spermatozoa when they pass out of the genital pore pass along the spermatic groove, which runs along the right side of the head and terminates in a muscular penis, by the aid of which the spermatic fluid is introduced into the genital pore of another individual and finds its way to the spermatheca. The Opisthobranchiata show a considerable tendency to lose some of their organs ; this process of degeneration is carried farthest in one of the sub-orders, the Haplomorpha, in which neither mantle - fold, ctenidia, nor cerata are found. Phyllirhoe has little but its odontophore to show its relationship with the MoUusca. It is a flattened, Planarian-like, transparent pelagic organism, about half an inch long. Its skin contains numerous unicellular glands, which are said to secrete a phosphorescent slime. The ovo-testis in this animal is double. A small Hydromedusa, Mnestra, is frequently found attached by the aboral surface 9f its body to these animals. . In Bhodope the degeneration has gone still farther, and the odontophore has disappeared. Eecent research has shown that the group of animals which formerly ranked as a class, the Pteropoda, are really allied to the Opisthobranch Gasteropoda. The Pteropoda are subdivided into two orders, the Thecosomata and the Gymnosomata. The former consists of three recent families, and is allied to the BuUoidea ; the latter contains five families, and is allied to the Aplysioidea, members of the Opisthobranchiata. The failure to recognise the correct affinities of these animals was to some extent due to their external symmetry, but this is a secondary feature which does not affect their internal organs. The animals com- posing this class are all carnivorous and pelagic. In correspondence with their mode of life, they are delicate organisms with transparent tissues, those amongst them pro- vided with a shell — the Thecosomata — having a hyaline one. In both orders the margin of the foot is prolonged and 2o8 ZOOLOGY modified into fins, which in the Thecosomata embrace the head. The anterior portion of the digestive tract is evaginable in Clio, and bears a number of unicellular glands opening upon cones. The secretion of these glands is adhesive, and they serve as organs for the capture of prey. Pneumodermon has a pair of appendages which bear suckers similar in structure to those found in the Cephalopoda. Order 2. Pulmonata. Characteeistics. — This second order of the Euthyneura has possibly hecome modified from a palliate Opisthobranch ancestor, in correspondence with the altered conditions of life involved in the change from an aquatic to a terrestrial habitat. The ctenidium has atrophied, and the edge of the mantle has fused with the body-wall, leaving only one small opening which leads from the pallial chamber to the exterior — this is the respiratory pore. The walls of the pallial chamber are very vascular, and they function as lungs. An operculum is never present. This order includes the land-snails and slugs. As a rule, its members live on land, and they all breathe air when adult, even those like the pond-snail Limnaea, which lives in water, but whose mantle chamber contains air. The mantle cavity of the young freshwater Pulmonates is stated at first to contain water, this is afterwards replaced by air. The great extent to which the blood-vessels and capillaries are developed in the Pulmonata is possibly connected with this habit. No true operculum is found in any Pulmonate, but many secrete a temporary lid to their shell when they withdraw into it for the winter. This temporary operculum is called a hibernaculum. In the slugs the shell may be small and even quite atrophied. Onchidium and Peronia are members of a family which inhabit the sea -shore and brackish marshes. Onchidium is remarkable for possessing, in addition to the normal pair of ceplialic eyes, a number of dorsal eyes scattered over the integument. These latter, like the eyes in the mantle of Pecten and Spondylus, are not constructed on the usual MOLLUSC A 209 invertebrate type, but, as in the eyes of vertebrates, the nerve pierces the layer of sensory cells, and is distributed to that side of them which is nearest the lens. In Onchidium, as in Pecten, etc., the eyes are probably modifications of certain simple tentacles which are borne on the mantle. Class Scaphopoda. Chaeacteeistics. — Elongated cylindrical Glossofhora. The mantle has extended on to the ventral surface, and has fused in the middle ventral line. It has secreted around it a cylindri- cal shell. The foot can he protruded through the anterior opening of the shell. No heart is present. The visceral loop of the nervous system is untwisted. Dioecious, the generative products escape through the right nephridium. This class includes but three genera, of which the best- known is Dentalium. The cylindrical shape of its body may be correlated with a burrowing habit of life, and it is interesting to note that a similar shape is found amongst Fig. 123. — Diagram of the ana- tomy of DentcUmm. Lateral view of organs, showing as though by transparency. After Lankester — Zoological Articles reprinted from the JSncyclo- pcedia Britannica. 10. Peri-oral part of mantle cavity. 11. Foot. 12. Cerebral ganglion. 13. Pleural ganglion. 14. Olfactory ganglion placed on visceral loop as in the Lamellibranchiata, according to Spengel. 15. Pedal ganglion. 16. Left nephridium. 1. Mouth surrounded by pinnate ten- tacles. 2. Oral process. 3. The ctenidial filaments. 4. Odontophore. 5. Oesophagus. 6. Left lobe of liver. 7. Anus. 8. Peri-anal part of mantle cavity. 9. Appendix of mantle skirt separated by a valve from 8. the sand -boring Lamellibranchiata, such as Solen. The mantle is ensheathed by a shell, which is at first incom- plete ventraUy. The shell resembles a truncated elephant's tusk, it is open at both ends, the larger opening is the anterior one, and the concave surface is dorsal. The foot is protrusible through the anterior opening of 14 2IO ZOOLOGY the shell, it terminates in a trifid lobe. Dorsal to it is the oral cone or head, at the end of which the mouth opens, surrounded by short pinnate tentacles (Fig. 123). A buccal mass and a radula are present ; two liver lobes symmetrically placed open into the stomach, from which the intestine passes to open by the anus in the ventral middle line. A right and left nephridium are present, and open to the exterior on either side of the anus. There is no heart, but the coelom contains a colourless blood. At the base of the oral cone a number of ctenidial filaments have their origin. These are capable of very considerable extension. The nervous system consists of a pair of cerebral ganglia, close to which lie the pleural ganglia. Long commissures connect the cerebral with the pedal ganglia. The visceral commissure is also long, and bears the olfactory ganglia, situated in front of the anus, in the same position as in Lamellibranchs. The generative gland is alike in both sexes ; it is situated dorsally, and its products make their exit through the right nephridium. Class Cephalopoda. Chaeacteristics. — BUatercdly symmetrical Glossaphora. The visceral hu/nvp is elongated, not twisted ; the sub-pallial chamber is chiefly developed posteriorly, and contains the gills, anus, and excretory pores. The shell may he external or internal, in a few cases it is absent. The foot has grown round the head, and is hrohen up into the characteristic arms of the Cephalopoda, provided with siickers. Part of the foot forms a funnel-like siphon, which guides the water as it is expelled from the pallial cavity. The vascular system is well developed, in addition to the central heart consisting of a ventricle and two auricles, an accessory branchial heart exists at tJte base of each gill in all but Nautilus. Powerful beak- like homy or calcareous jaws guard the mxmth, and the radula is well developed. Chromatophores are present in the integument. The Cephalopoda are dioecious. The Cephalopoda are divided into two orders : (i.) The Tetrabeanchiata or the Tentaculifera and (ii.) the Di- BEANCHIATA or the ACETABULIFERA. MOLLUSC A 211 (i.) Characteristics. — The, TetrahraTwhiata are characterised as follows. The siphon is not complete ; the edges overlap one another, hut are not fused; the lobes of the circumoral foot carry tentacles, not suckers. There are two pairs of ctenidia and two pairs of nephridia. The coelom opens straight on to the exterior, and not into the nephridia. There are two oviducts and two vasa deferentia, hut the left is in hoth cases rudimentary. The shell is large, external, and chamhered. No ink sac, salivary glands, or hranchial hearts exist. This order contains very many extinct forms, but only one living genus — Nautilus. (ii.) Characteristics. — The Bihranohiata have hut one pair of ctenidia and one pair of nephridia. The edges of the siphon have fused so as to form a complete funnel. The arms or processes of the foot which surround the head hear cup-like suckers. Branchial hearts exist cut the hose of the gills. The coelom communicates with the exterior through the nephridia, and not directly ; the oviducts may he paired or single ; the vas deferens, with one exception, is single. An ink sac and salivary glands exist. The sense organs are highly developed. The Dibranchiata comprise two sub-orders : the Decappda and the Octopoda. a. Characteristics. — The Becapoda have ten arms, two of which are very long, and differ in appearance from the others. The suckers are stalked, and provided with a horny ring. The hody is elongated, and hears lateral fins. The shell is enclosed hy an upgrowth of the mantle, and is therefore internal. ^. Characteristics. — The Octopoda have eight similar arms, hearing sessile suckers, which are not strengthened hy a horny ring. The hody is short and glohular. The oviducts are paired. There is no shell in or on the visceral hump. The Cuttle-fish or Sepia officinalis is common in most seas, and in the spring, when it approaches the shore to deposit its eggs amongst the rocks, it is easily caught. In considering the anatomy of this form, it is important to orientate the animal correctly; with this view it should be placed mouth down- wards, then its foot will be ventral, its visceral hump dorsal, and its mantle cavity posterior. For the sake of convenience it is, however, better to twist the animal through a right ZOOLOGY angle, and describe it as it swims, with its foot and mouth anterior, the visceral hump posterior, the mantle cavity ventral. Fig. 124. A. Loligo vulgaris, a. Arms. t. Tentacles. C. Side view of one of the suckers, show- ing the horny hooks surrounding the margin. B. Pen of the same, reduced in size. D. View of the head from in front, show- ing the arms (a), the tentacles (t), the mouth (»!), and the funnel (/). ' and the shell or cuttle-bone, which may be felt through the skin, dorsal. The most dorsally-placed pair of arms, really the most anterior, are termed the first pair. Sepia, being a Decapod, has five pairs of arms, of these, the fourth pair are unlike the others. They are much longer. MOLLUSC A 213 and can be withdrawn into pouches at their base ; they do not bear their suckers scattered uniformly over their inner surface, but the suckers are all aggregated in a swollen pad at the free end of the arm (Fig. 126). The suckers are very remarkable organs^ they are cup-like structures whose rim is strengthened by a toothed horny ring. A retractor muscle can deepen the cavity of the cup, so that when the edge of the cup is applied to any object and the muscle contracts, the sucker adheres to the object by the pressure of the surrounding medium. In Fio. 125. a. Internal skeleton of Sepia ornata. Rang. b. Internal skeleton of Sistioteuthis BoneUiana. D'Orl). c. Internal skeleton of Spirvla fragilis. Lamarck. d. Animal of Spirula Peronii. Sepia, as in other Decapods, the suckers are stalked. In the male the fifth arm on the left side has lost some of its suckers, and this is termed the hectocotylised arm, vide p. 222. That portion of the foot which is modified to form the sucker-bearing arms is homologous with the fore-foot or pro- podium of other Mollusca (Fig. 122). The mid-foot or mesopodium has become converted into the siphon, a funnel- 214 ZOOLOGY like structure open at both ends. The posterior aperture communicates with the mantle cavity. "When the edges of the mantle are in close apposition to the body-wall, to aid which a pair of cartilaginous nodules exist on the mantle edge, which iit into corresponding depressions on the outside of the funnel, and the muscles of the mantle contract, the water in the mantle cavity is forced violently through the siphon. The result of this is, the Sepia darts backwards. In the lumen of the siphon is a small valve which only allows the water to pass one way ; this possibly represents the hind- foot or metapodium. The cuttle-bone or shell of Stipia is entirely internal. It lies along the dorsal surface in a sac formed by the concres- cence of certain folds of the mantle in this region (Fig. 126). It consists of a posterior horny portion, which is continued forward by a series of calcareous plates deposited by the inner wall of the sac ; between these plates air is found. This air must be secreted by the surrounding tissues, it probably assists the Seipia to balance itself. If the mantle be divided and the mantle chamber exposed, the anus will be seen situated in the middle line near the posterior end of the siphon. Close to it the duct of the ink sac opens. A little way behind the anus, and on each side of it, is situated a nephridial opening, and at about the same level on the left side is the aperture of the genital duct. The large ctenidia lie one on either side, and in the female Sepia a pair of large nidamental glands are to be seen through the body-wall. The mouth is surrounded by a circular lip and guarded by two strong horny beaks resembling those of a parrot, except for the fact that the under beak is the larger and more pro- minent (Fig. 126). The buccal mass is large and muscular, it contains a well-developed radula. The rows of teeth consist of five central conical teeth and one hook - shaped tooth on each side. The oesophagus, a narrow tube, passes from the buccal mass straight to the posterior end of the body, and opens into a thick-walled stomach. At this point the intes- tine bends forward ventrally, and gives off a curved caecum of considerable size. From this the rectum passes along the MOLLUSC A 215 ventral body-wall to the anus. A pair of small salivary glands lie on either side of the oesophagus just behind the cartilaginous skeleton of the head ; their two diicts unite into a single channel, which opens into the buccal mass in the neighbourhood of the radula. The liver is of considerable size ; it is bilobed, and the anterior end of each half reaches as far forward as the salivary glands ; the duct of each lobe arises about its middle, and runs back parallel with the oesophagus, to open into that part Fig. 126. — Diagram representing a vertical, approximately median antero-posterior section of Sepia officinalis. From a drawing by A. 6. Bourne. After Lankester — Zoological Articles reprinted from Sncydopmdia Britannica. 1. Mouth. 11. Lumen of siphon. 2. Upper beak. 12. Branchial heart. 3. Lower beak. 13. Appendage of branchial heart. 4. Odontophore. 14. Visoero-pericardial aperture. 5. Nerve ring. 15. Eenal glandular mass. 6. Oesophagus. 16. Left ctenidium. 7. Crop. 17. Subpallial chamber. 8. Gizzard. 18. Valve in siphon. 9. Anus. 19. Afferent branchial vessel. 10. Cuttle bone enclosed by a growth of the mantle. of the intestine just behind the stomach which gives oft the caecum. The bile ducts are enwrapped by the unpaired diverticulum of the nephridia, and this, where in contact with them, developes spongy excretory tissue, the so-called pan- creatic caeca. The ink bag lies to the right of the stomach, its duct runs parallel to the rectum, and opens to the exterior. Its secretion forms a pigment named after . the animal, and by the expulsion of this the surrounding water is made inky, and thus serves as a screen to cover the escape of the squid. The coelom of Sepia contains the stomach, the heart with 2l6 ZOOLOGY its chief vessels, the branchial hearts, and in its posterior half the genital gland. It is partially divided into two by an in- FiQ. 127. — Central organs of the cir- culation,*'gills, and renal organs of Sepia officinalis. After John Hunter. v' V, Visceral veins. a. Aorta. V. Vena cava. c. Ventricle. d. Auricles. e. Branchial hearts. h. Branchiae. r. Renal organs. complete septum. It opens into the nephridia by a minute pore on each side. The heart consists of a ventricle with two auricles opening into it. The ventricle is continued in front into an anterior aorta, which gives off vessels to the mantle and liver and runs forward to the head and arms, and behind into the posterior aorta, which supplies the generative organs and the fins, etc. The blood passes largely by capillaries but partly by lacunar spaces, into the veins. Of these, the largest is the anterior vena cava, which has a ventral position and splits into two branchial veins. These latter are beset with diverticula of the nephridia, and they receive numerous veins which bring back the blood from the generative organs, the ink sac, etc. Just before they enter the branchial hearts they are joined by veins from the mantle and posterior end of the visceral hump. The branchial hearts are pulsating muscular enlargements at the base of the ctenidia, whose contractions force the blood through the gill. A mass of excretory tissue — the pericardial gland — MOLLUSC A 217 is developed on the wall of the coelom, beneath each branchial heart. The blood, after passing through the ctenidia, is returned to the auricles. The blood contains colourless corpuscles : in the oxidised condition it is nearly colourless, but when venous it is bluish. The colour is due to haemoeyanin, a substance containing copper, which is diffused through the serum. The ctenidia are organs of considerable size. In the normal state their long axis is parallel with the longitudinal axis of the body, and they are attached throughout their whole length to the body-wall. The axis bears a double row of plate- like lamellae, which decrease towards the anterior end, thus giving a pyramidal shape to the organ. An afferent vein from the branchial heart traverses the axis and gives off branches to the lamellae ; here the blood is aerated, and is then returned by an efferent vein which runs parallel and close to the former, this leads to the auricle. The nephridia are paired, right and left, but they are con- nected by two transverse portions, an anterior and a posterior. The former of these transverse communications gives off a diverticulum which stretches, as the unpaired nephridial sac, back as far as the genital gland. The ventral wall of the nephridia is smooth, but the dorsal wall, which is wrapped round the branchial veins, and into which numerous veinlets run, is spongy and glandular. At the anterior end of each kidney is a short ureter which opens to the exterior at the side of the anus. Near the inner end of the ureter there is a rosette- shaped opening covered with ciliated epithelium, which leads into the coelom. Cephalopods, which are the largest and most ferocious of all the invertebrates, have developed an internal cartilaginous skeleton, a very unusual arrangement outside the phylum Vertebrata. The cartilage consists of a structureless matrix, through which numerous cells are scattered ; the cells give off branching processes which permeate the substance in all direc- tions. Nodules of this cartilage exist in processes of the mantle edge, and fit into corresponding depressions on the edge of the siphon when the mantle is closed, and also along the base of the lateral fins, but the most considerable developement 2l8 ZOOLOGY of cartilage is in the head. Here there is a cephalic cartilage of complicated form, which is pierced by the oesophagus. It ensheaths the chief nerve ganglia, the ear is embedded in it, and it forms two recesses which lodge the eyes. Another portion affords some support to the bases of the arms, and there is also a iiat piece situated in the neck known as the nuchal plate. The chief nerve ganglia are aggregated round the oeso- phagus, close behind the buccal mass, and are embedded in the Flo. 128. — Lateral view of the nervous centres and nerves of the right side of Octopus vul- garis. From a drawing by A. G. Bourne. After Lankester — Zoological Articles re- printed from the Encyclopcedia Britannica. 9. 10. Cerebral ganglion. The optic nerve. Pedal ganglion giving nerves to arms. Pleuro-visceral ganglion. Right visceral nerve. Riglit stellate ganglion of the mantle, con- nected by a nerve to the pleural portion of 4. Branchial branch of 6. Olfactory branch of 5. Buccal ganglion. Buccal mass. cartilaginous skeleton. The cerebral ganglion on the dorsal side of the oesophagus gives off a pair of nerves which end in the superior buccal ganglion, from which a pair pass to the inferior buccal ganglion, both lying on the surface of the buccal mass. In connection with these ganglia there is a well- developed stomatogastric system. Laterally each cerebral ganglion is continued into two very stout optic nerves ; these expand into the optic ganglia, situated at the back of the eye. The cerebral ganglion gives off two circum-oesophageal com- missures, which pass down to the nervous mass on the ventral surface of the oesophagus. This mass is composed of three ganglia very much fused together. Anteriorly lie the pedal MOLLUSC A 219 ganglia, which give off ten large nerves, one to each arm ; they also supply the siphon. The auditory nerves also arise from the pedal ganglia, although their fibres may be traced to the cerebral. The pedal ganglia are partially marked off from the fused pleural and visceral by the presence of a small foramen through which a blood-vessel passes. From the pleural por- tion of this compound nerve centre a stout nerve passes to the stellate ganglion, situated at the angle between the mantle and the head. It can be seen shining through the integument when the mantle cavity is exposed. From this ganglion nerves radiate to the muscles of the mantle. The visceral half of the fused ganglion gives off a pair of stout visceral nerves, which unite to form a loop. These visceral nerves supply the gener- ative organs, the kidneys, and other viscera, and each sends a stout branch to a ctenidium. The eyes of Se'pia. are of great complexity. They have a striking but superficial resemblance to the Vertebrate eye, and fundamental differences exist between these two types of visual organs. Anteriorly the eye is covered by a transparent cornea, which in Se'pia, is closed. The cornea is protected by certain folds of skin, which can cover it in by the contraction of a sphincter muscle, and there is also a horizontal lower eyelid. Within the cornea is the anterior chamber of the eye, into which the folds of the iris project ; they partially cover the lens, which consists of an outer and an inner part separated by a membrane. The lens is supported by the ciliary body, which with the lens occupies the anterior half of the retinal chamber. The retina, which completes the wall of this chamber, is two- layered, and the nerves which pass to it from the optic ganglion enter the retina posteriorly. The auditory apparatus consists of two otocysts sunk in the cephalic cartilage. Their cavities have an irregular shape, and are lined by an epithelium, which is ciliated in places, they contain an endolymph, in which a single spherical otolith floats. An olfactory function is attributed to two small invagina- tions of the skin, situated one just behind each eye. The sacs open to the exterior by a small slit -like aperture; they are lined by a ciliated columnar epithelium, amongst which are ZOOLOGY certain special sense cells, each provided with a single sense hair. This organ is supplied by a nerve which arises from a special ganglion situated near the base of the optic ganglion. No osphradia corresponding with those of other Molluscs have yet been described in Sepia. Certain large cells crowded with pigment, situated in the subepidermal connective tissue, play an important part in the life of a Cephalopod. Attached to these cells, which are called chromatophores, are a number of radiating muscle fibres ; when these contract, the cavity of the cell enlarges, and the contained colour becomes diffuse ; the chromatophores contract by their own elasticity, and when contracted the colour is concentrated. The whole system is under nervous control, and the colour of the animal may change with startling rapidity In the S&pia and other members of the group this faculty is used as a pro- tection, the colour of the animal tending to assimilate itself to that of the surrounding rocks or sand. In addition to the chromatophores, the subepidermal tissues contain other modified connective tissue cells known as iridocysts ; these cells are so modified as to produce iridescent colours by the diffraction of light. Sepia is a dioecious animal which lays eggs. The male is usually somewhat smaller than the female, and its arms are relatively longer; the fifth arm on the left side is hecto- cotylised, that is, it is modified in connection with the process of depositing the spermatozoa. It is thickened at its base, and almost devoid of suckers. The testis lies at the extreme end of the visceral hump, in a capsule — part of the coelom — into which opens a more or less coiled vas deferens, the walls of which are much folded, and provided with numerous glandular diverticula. Whilst passing down this vas deferens the spermatozoa are divided up into packets, and the glandular walls secrete around each packet a cuticular spermatophore. Finally, the sperm duct opens into a large receptacle known as Needham's sac, in which the spermatophores are stored up ; they pass to the exterior by the genital pore situated to the left of the anus, and they are deposited in the hectocotylised arm, and are possibly introduced by it into the mantle cavity of the female at the time of oviposition. MOLLUSC A The spermatophores are complex structures about 2 cm. long, they have a receptacle in which the minute spermatozoa are stored up, and a long tightly-coiled spiral thread, the ex- pansion of which explodes the capsule, and the spermatozoa rush out. In the female the ovary occupies the same position as the testis in the male ; the cavity of both these generative glands communicates with the pericardial portion of the coelom, though partly shut off from it by a septum. A cushion projects into the lumen of the ovary, which bears ova in various stages of developement ; from the ovary the oviduct, which is ciliated, passes to its external opening to the left of the anus. Accessory glands are present ; of these the most important are a large pair of nidamental glands, which deposit the substance of the egg capsules ; in Sifia there is a second smaller pair of Fig. 129. — a, Male of Argonauta argo, with the hectocotylised arm still contained in its enveloping cyst, four times enlarged (after H. Miiller). h, Hectocotylus of Tremoctopus violaceus (after Kblliker). nidamental glands, as well as the large ones. The egg capsule is prolonged into a stalk, by means of which the eggs are kept together, and the collection of eggs somewhat resembles a cluster of grapes. In Sepia one of the arms in the male is slightly modified, and probably assists, in the deposition of the spermatozoa, but in ZOOLOGY certain Octopods this modification is carried much further. In Argonauta argo the third arm on the left, and in Ocythoe tuber- culata the same arm on the right, becomes detached from the male, and is placed in the mantle cavity of the female. It carries a small sac charged with spermatophores, and was at one time looked upon as a parasite, and the name Hectocotyliis was given it. The male, after losing its arm, always reproduces it again. In the female Argonauta the eggs are carried about in the shell; this is the only member of the Octopoda which has a shell, and it does not cor- respond with the sheU of other Cephalopods, but is formed from the expanded ends of the two dorsal arms. In other Dibranchiata the shell varies from the external coiled chambered shell of Spirula to the horny pen of Loligo. Even in Spirula (Fig. 125) the Fig. ISO. — Argonauta argo, the Paper shell is partially Sur- Nautilus, female. The animal is repre- _„ -i„j i ,, p„i j„ „j; 4.-u„ sented in its shell, but the webbed dorsal funded by _ folds of the arms are separated from the shell which mantle, and in Other formS they ordinarily embrace. ^^^ ^^^^^ ^^^^ ^^^^^ ^_ gether so that the shell comes to lie in a closed sac. In the Tetrabranchiata, Nautilus and the extinct Ammonitidae, the shell is external, and chambered. The animal lies in the last-formed chamber, and closely fits it. The chambers are separated from one another by septa, and the whole is traversed by a membranous tube, the siphuncle, which is a continuation of the integument of the animal. The chambers are full of a gas probably secreted by the dorsal integument, and they doubtless serve as a float. In Nautilus the fore-foot is broken up into certain flattened MOLLUSC A 223 lobes, which differ in their arrangement in the two sexes. The lobes bear at their edges cylindrical tentacles, which can be retracted into muscular sheaths. Probably the tentacles cor- respond to the suckers in the Dibranchiata. In some species of cuttle-fish the suckers are replaced by hooks, or both may coexist ; the arms in the Octopoda are usually connected by a fold of skin forming a web, which is no doubt of use in swim- ming. The arms of Architeuihis, a gigantic form, sometimes attain the length of 40 feet, and the total length of the body and arms may measure 60 feet. The beak which guards the mouth is calcareous in Nautilus, and horny in other Cephalopods. The possession by Nautilus of two pairs of auricles which open into the single ventricle is correlated with the two pairs of ctenidia. There are in this same animal two pairs of nephridia; this repetition of parts is almost unknown in MoUusca, the only other case being the gills and shells of Chiton, and it is therefore par- ticularly interesting. .The chief nerve ganglia in Nautilus are band-like, and hardly to be distinguished from the commissures which con- nect them. The nerves to the mantle are numerous, and are not aggregated into one stout cord as in the Dibranchiata. The same animal is provided with a pair of osphradia, situated at the base of the anterior ctenidia ; these organs have not yet been discovered in other Cephalopods. The eye of Nautilus is one of the most remarkable organs found in the order. It has the shape of a kettledrum ; the tense membrane, which is external, being pierced at its centre by a minute hole, which leads into a dark chamber lined by the retina. The latter is bathed by sea water, which enters through the minute pore. The mechan- ism by which images must be formed on the retina resembles that of a pin-hole camera. CHAPTEE XV ECHINODERMATA Eohinodermata Asteroidea — Asterias, Solaster, Brisinga. Ophiuroidea — Astrophyton, OphiophoUs. Crinoidea — Comatula, Pentacrhms. I Eegulares — JEchinus, Toxopneustes. Echinoidea . -j Clypeastroidea— CTjfpcasier, Jtotula. (_Spataiigoidea — Spatangus, Brissus. tr 1 4.T, -J f Actinopoda — Eolothuria, Cucumaria, Deima. Holothuroidea a _ , . , „ „, . ( Paraotinopoda — Synapta, Omrodota. Chaeacteeistics. — Animals with a primitive bilateral symmetry, which is in the adult replaced by a more or less regular radial symmetry, usually pentamerous. The shin is hardened by calcareous deposits, vjhich may take the form of scattered spicules or of plates which build up an almost complete shell, but in all cases they are mesodermic structures. A well- developed coelom is present, and part of it becomes cut off from the rest to form the tvater-vascular system, which is both locomotor and respiratory in function. The five radial vessels of this system correspond with five areas, the '' ambu- lacra " ; the angles between them form the " interambulacra.'' The alimentary canal usually opens to the exterior at both ends, but an anus may be absent. The sexes are usually distinct, and developement is nearly always associated with a metamorphosis. They are exclusively marine. The Echinodermata are divided into five classes : I. Asteroidea. II. Ophiueoidea. III. Ceinoidea. IV. Echinoidea. V. HOLOTHUEOIDEA. ECHINODERMA TA 225 Class I. ASTEROIDEA (Starfishes). Characteristics. — Echinodermata whose body is flattened dorso- ventrcdly, and is produced into arms or rays, which are usually five or more in number. These arms are longitudinally grooved on the ventral surface, and the tube-feet lie in this groove. The madreporic plate is dorsal and interradial in position. The alimentary canal sends caecal diverticula into the arms. The generative organs are interradial in position at the base of the arms. Pedicellariae usually present. Asterias ruhens is oue of the commonest of starfishes, and is constantly left stranded on our shores by the retreating tide. Its body consists of a central disk, from which five arms or radii project. The surface on which it habitually rests or moves, and on which the mouth opens, may be termed the ventral, the upper and more convex, where the anus is situated, may be called the dorsal. From the mouth five grooves radiate along the arms, these are the arnhulacral grooves, and they lodge the tube-feet j between each two grooves, and consequently interradial in position, are five sets of oral spines, which project over the mouth and perhaps assist in feeding. If the tube-feet be removed from each ray, it will be seen that the ambulacral groove is formed of two rows of ambulacral plates, situated right and left of the middle line of the radius (Fig. 131). Each right plate is so placed as to form an angle, open ventrally, with the corresponding left plate, and between the adjacent plates of each side certain pores exist which give exit to the tube-feet. The groove is covered in by the integument, and lodges two radial canals, of these the most ventral is divided by a vertical septum, and is called, for reasons mentioned below, the " peri-haemal " space. The dorsal canal is the radial trunk of the water-vascular- system. At the outer end of the ambulacral plate a series of adambulacral ossicles are situated, and these support three rows of moveable spines. Those spines which are nearest to the centre of the disk form the oral spines mentioned above ; these are borne by the first adambulacral ossicles, one set on each side of an inter- radius. At the distal end of each arm the ambulacral plates end in 15 226 ZOOLOGY a single ossicle, which supports a terminal tentacle bearing a number of pits of pigmented cells, called collectively the eye- spot. Between this single ossicle and the other ambulacral Fig. 131. — Diagram of a 1. Epidermis. 2. Mesoderm. 3. Perihaemal space in the skin. 4. Peritoneal lining of body-cavity. 5. A branchia. 6. Paired caeca from intestine. 7. Mesentery supporting caeca. 8. Spine. 9. Ossicle in skin. 10. Pedicellaria. 11. Ambulacral ossicle. section of the arm of a Starfish. 12. Adambulacral ossicle. 13. Eadial trunk of water - vascular system. Badial trunk of blood - vascular system of Ludwig. Eadial nerve connected with plexus under epidermis. 16. Ampulla of tube-foot. 17. Tube-foot. 18. Perihaemal space. 19. Coelom. U. 15 plates all the new plates appear. The tentacle at the tip of the arm, together with the eye-spot, is surrounded by a circlet of spines. On the dorsal surface of both disk and arms numerous spines are scattered, and amongst them many pedicellariae (Fig. 131). These must be regarded as modified spines; they consist of a basilar plate and of two blades which snap against one another like the two limbs of a pair of forceps — in some of them the blades cross one another as they do in a pair of scissors. The function of these pedicellariae seems to be to catch hold of foreign bodies, and so keep parasites from settling ECHINODERMA TA 227 on the skin or penetrating through the branchiae into the coelom. On the dorsal surface of the disk, situated interradially, lies the madreporic plate, through which the water- vascular system communicates with the exterior. The two arms which lie right and left of this plate are termed the " bivium," and contrasted with the other three or " trivium " ; in mapping out the various organs of the body these will be found to be convenient terms. The anus lies near the centre of the dorsal surface of the disk. The skin is formed of (i.) an outer cylindrical epithelium with nerve fibrils at the base, (ii.) an intermediate connective tissue layer with some muscle fibres, — this is the matrix for the spines and plates, — and (iii.) an inner coelomic epithelium, which is ciliated ; this last lines the true coelom or enterocoel, a spacious cavity containing the alimentary canal, the generative organs, etc. The coelom contains a fluid in which amoeboid corpuscles float. The angle which the two series of ambulacral plates in each arm make with one another is floored in by the outer layer of the integument, the nerve plexus of which is thickened and forms the radial nerve (Fig. 131). The cavity thus formed is the radial perihaemal vessel or blood-vessel of French authors ; it is divided into a right and a left portion by the presence of a median mesentery. This mesentery in this species, but not in others, has a certain amount of glandular tissue in it, which Ludwig describes as a blood-vessel. On the dorsal surface of the starfish numerous delicate processes of the skin may be seen projecting above the general level of the body-wall. These thin-walled extensions of the integument are known as dermal branchiae ; the coelomic fluid passes freely into them, and they doubtless serve as respiratory organs (Fig. 1 3 1). It has been recently shown that some of the amoeboid cells of the coelomic fluid (phagocytes), when they have eaten any particles which it is desirable should be ejected from the body, make their way to the walls of these dermal branchiae, and force a passage through them to the exterior, whence they are washed away. Besides the enterocoelic ciliated body-cavity, there are a 228 ZOOLOGY number of vessels. They constitute the blood system according to French authors. The radial one has already been mentioned ; the five radials unite with a circum-oesophageal ring, which is stated to open into the body-cavity by five interradial pores. Inside this is another ring-shaped vessel, into which a large sinus surrounding the stone canal — the axial sinus — opens. Besides this there is an aboral pentagon which sends off inter- radially pairs of vessels which dilate and surround the genital organs. The mouth is situated centrally on the ventral surface, surrounded by a ring of nervous matter. The mouth leads by a short oesophagus into a large stomach, the walls of which are folded in many sacculi. When the starfish attempts to devour young molluscs or shellfish which are too large to be taken in at the mouth, these sacculi are protruded and enclose the prey. They are retracted by special muscles. The walls of both the oesophagus and the stomach are ciliated, and the eversible portions contain many glands, the secretions of which possibly exercise a paralysing effect on the prey. The stomach is followed by a pentagonal pyloric portion with its angles situated radially. From each angle a short duct passes to the base of each arm, and here opens into two large hepatic caeca, which occupy a large portion of the space in each arm and extend to its tip. Each caecum is supported by two dorsal mesenteries. From the pyloric portion a short rectum passes to the anus, which is in the next interradius to that bearing the madreporic plate, and is almost central. The rectum gives off two short caeca, which lie in two neighbouring interradii — that between the left and central arm of the trivium, and between the left arm of the trivium and bivium. The water-vascular system consists of a circumoral ring which gives off five radial vessels, one running along each arm, and a single interradial stone canal, which passes from the circumoral ring, and opens to the exterior at the madreporic plate, which is calcified. The madreporic plate is marked externally by a number of radial grooves ; at the bottom of each of these is situated a row of pores ; these open into a series of tubules, which collect into an ampulla, and this in its turn opens into the lumen of the ECHINODERMATA ■ 229 stone canal. The stone canal is lined by a ciliated epithelium, surrounded by calcified connective tissue, a ridge projects into its interior, and the free edge of the ridge may bifurcate, each half then folding back upon itself. The circumoral ring bears nine glandular bodies, composed of branching tubules lined with cubical cells, and opening into the ring. These bodies are known as Tiedemann's bodies. The stone canal opens in the position where the tenth of these bodies should be. It is possible that the corpuscles which float in the fluid of the water-vascular system are formed in these bodies. The radial vessels which pass along the arms lie ventral to the ambulacral plates, between them and a transverse muscle which runs between each pair (Fig. 131). Opposite each tube-foot the radial vessel gives off a transverse branch. Each branch passes between the ambulacral ossicles, and opens into a vesicular expansion, the ampulla, situated in the coelom. From this another vessel passes to the tube-foot. The con- traction of the ampulla forces fluid into the tube-foot, and so extends it. At the tip of the arm the radial tube ends in an unpaired terminal tentacle, at the base of which is a thicken- ing beset with eyes. The tentacle has a very well-developed nervous layer. The blood system described by German authors is founded on misinterpretation. They describe a radial vessel, an oral ring, and an aboral ring, and a connecting heart lying inside the corresponding organs described above. The radial and oral vessels are nothing but the thickened septa of the true vessels, the heart is a solid glandular organ, and the aboral vessel is the genital rhachis, partly degenerate. The rhachis is in connection with the so-called heart. The nervous system is diffused all over the body, but better developed in some parts than in others. The epidermis contains numerous sense cells, prolonged at their bases into nerve fibrils ; these are not very abundant on the dorsal surface, but along the ridge which lies between the tube-feet, and in a ring which surrounds the mouth, both sense cells and nerve fibres exist in great quantities. The triangular ridges which occupy the ventral surface of the arms unite in a ring round the mouth, and constitute the central nervous system 230 ZOOLOGY (Fig. 132). The outer cells of this ridge are mainly sense cells, and ganglion cells and nerve fibres occur at their bases. The nerve layer is also well developed on the tube-feet. Fig. 132. — View of blood- vascular system of a Starfish as described by German writers. Modified from Ludwig. 1. Circumoral ring. 2. Eadlal vessel with branches to am- pulla. 3. Heart. 4. Circumoral ring. 5. Dorsal end of heart passing into the skin. 6. Vessels to intestine. 7. Paired vessels passing to generative glands. Asterias rubens is dioecious. The generative organs con- sist of five interradial pairs of glands, which are alike in both sexes, and when mature each extends into two neighbouring arms (Fig. 133). Except during the breeding season, the size of the glands is inconsiderable. Each gland opens to the exterior by a single duct, which terminates in a perforated plate situated dorsally and interradially. The various glands are connected together by a genital rhachis, and they are supplied by the above-mentioned genital vessels, which dilate to form a sinus round the glands. Fertilisation takes place externally. The Asteroidea are mainly inhabitants of shallow water, though a considerable number of species from great depths have been described. The arms are usually five in number; one species of Solaster has, however, thirteen, and Brisinga has nine to twelve arms, which are more sharply marked off from the disk than is the case with other Asteroidea. The same genus is devoid of dermal branchiae, of eye-spots, and of ampullae at the base of the tube-feet. The family Asteopectinidae is, with one exception, charac- terised by the anus being absent, and by the tube-feet being ECHINODERMA TA 231 Pig. 133.— The common Starfish {AsUrias ruhens), dissected to show motor, digestive, and reproductive systems. After Eolleston and Jackson. 1. Central radius of trivlum. 2. Right arm of trivium. 3. Left arm of trivium. 4. Left arm of bivium. 5. Eight arm of bivium. 6. Madreporio plate and canal. 7. Arborescent "hepatic" caeca, two in each arm. 8. Generative glands. 9. Ampullae of tube-feet. 10. Ambulacral plates, inner surface. 11. Pyloric portion of stomach. 12. Duct leading from stomach to paired caeca. 13. Cardiac division of stomach bulging into arm. 14. Anus. 232 ZOOLOGY arranged in two rows on the ventral surface of each arm, and not in four, as appears to be the case in Asterias. Their tube-feet have pointed extremities, and not a sucking- disk. Besides the ampullae on the radial vessels, additional Fxo. 134. — Solasier papposus (upper surface). reservoirs for the water-vascular fluid usually occur on the circumorai ring ; these are termed Polian vesicles, and are usually five or ten in number. It is doubtful whether the vesicles which occur near the right position in Asterias ruhens are really Polian vesicles, that is, opening into the ring, or whether they are the first pair of ampullae of each radial vessel. In one species, Crihella oculata, some of the openings in the madreporic plate lead into that section of the body- cavity which surrounds the heart and stone canal, instead of into the latter canal. The Asteroidea have great powers of regenerating lost parts. Arms broken off grow out again from the disk, and even the whole disk may be regenerated from a single separated arm. ECHINODERMA TA 233 Class II. OPHIUROIDEA (Brittle Stars). Chaeactekistics. — Echinodermata with a central disk hearing long slender arms, into the cavity of which no part of the alimentary canal is prolonged. There is no anus. The madreporic plate is ventral, and usually is an oral plate. There is no amhulacral groove, and the tube-feet are lateral in position. This class is allied to the Asteroidea, and is sometimes included with the latter in a single class. The Ophiuroids, however, differ from the Asteroids in the sharp dis- tinction between disk and arms, a condition approached by Brisinga, in the absence of any digestive diverticula in the arms, in the ventral position of the madreporic plate, and in the almost universal absence of pedi- cellariae. In the adult also the ectoderm is absent ex- cept on the tube-feet. The arms are long and slender, in most cases they ^^^_ us,-OpUophoii^ um (upper surface), are protected by four rows of plates, a ventral, a dorsal, and two lateral, the tube-feet protrude between the ventral and lateral; they have no ampullae. The nervous system has sunk under the skin, and is protected by the ventral plates. Dorsal to it is the radial blood-vessel, and dorsal to that the water-vascular vessel. In a transverse section of the arm, the greater part of the space is occupied by the ambulacral ossicles. Originally paired, these have fused and become single; they are grooved dorsally and ventrally. The dorsal groove lodges part of the coelom, the ventral the above-mentioned vessels and nerve cord. The mouth is armed with certain modified ossicles ; it is central in position, and leads into a spacious stomach, which 234 ZOOLOGY is produced into five radial and five short interradial caeca. The walls of the stomach are lined by a ciliated epithelium. Fio. 136. — Diagram of a transverse sectioiL of an Ophiuroid, 1. Eadial nerve, witli lateral branches. 2. So-called radial blood-vessel. 3. Eadial -water-vascular trunk. 4. Tube-foot. 5. Ventral plate. 6. Lateral plate. 7. Ambulacra! ossicles. 8. Dorsal plate. 9. Dorsal portion of coelom. 10. Muscles. 11. Lateral nerve. 12. Origin of lateral nerve. and are supported by connective tissue strands, which traverse the coelom to the body-wall. There is no anus. The water-vascular system consists of a circumoral ring, which bears four Polian vesicles ; in the fifth interradius it gives off the ciliated stone canal, which is simple and un- calcified, this passes to the madreporic plate on the ventral surface. In Astrophyton there are five madreporic plates, one in each interradius, and five stone canals. The radial vessels which arise from the ring bear no ampullae, but give off branches which pass directly to the conical tube-feet. Cor- puscles tinged with haemoglobin occur in the water-vascular fluid of one species. The true vascular system resembles that of Asterids. The aboral ring has, however, an undulatory curve, being ventral in the interradii. MacBride has recently proved that both the axial sinus and the aboral ring are involutions of the coelom. The so-called heart is nothing but a genital stolon, whence the genital rhachis grows out. The genital stolon in the earliest ECHINODERMA TA 235 stage is a mere thickened ridge of peritoneum, so that here, as in other Coelomata, the generative cells are derived from the lining of the coelom. The circumoral nerve ring, like the radial nerves, has lost its connection with the epidermis, and has sunk into the body. V 8* 10 6» Fig. 137. — A diagrammatic vertical section of an Ophiuroid, after Ludwig. The circumoral systems of organs are seen to the left, cut across, their radial pro- longations cut longitudinally, to the right. 1. Body-wall. 2. Month. 3. Body-cavity. 3\ Body-cavity of the arm. 4. Oral ossicles. 5. Torus angularis. 6. Oral plate. 7^. 1st ambulacral ossicle. 1^, V, 7*. 2nd to 4th ambulacral ossicle. 8\ 8^ 8^ 1st to 3rd ventral plate. 9. 1st oral foot. 10. Transverse muscle of the 2nd joint. 10^. External interradial muscle. 10^. Internal inten'adial muscle. (The line should point to the dotted tissue. ) Water - vascular system: to the left the circumoral ring, to the right the radial vessel. Polian vesicle. Nerve ring and radial nerve. So-called blood-vessel. 14 {to the right). Genital rhachis enclosed in aboral sinus. 15. Radial perihaemal canal. 11 12. 13. 14. The radial nerves in the arms are frequently segmented, a ganglionated swelling occurring corresponding with each ossicle. The generative organs consist of numerous caeca which open into a genital bursa. The bursae are ten in number, and lie one on each side of each arm ; they open ventrally by a slit- like aperture at the base of each arm. A genital rhachis con- nects the generative organs, which are surrounded by a blood-sinus, as in Asterids. Aviphiwa squamata is hermaph- rodite, and it is stated that when certain internal parasites, Orthonectidae, infest the coelom, it ceases to produce eggs, but produces a greater number of spermatozoa. Some of the Ophiuroids give off a phosphorescent light from the back of their arms. OpMopholis bellis (Fig. 135) exists in great numbers in the 236 ZOOLOGY northern European seas. Like many other members of the class, it is brilliantly coloured. The different specimens of the same species exhibit a surprising amount of variation both in their colour and markings. Class III. CRINOIDEA (Sea Lilies). Characteristics. — The dorsal or aboral surface usually prolonged into a jointed stalk, hy which the animal is fixed. The calyx, consisting of the dish and arms, in some species breaks off from the stalk and leads a detached existence. The jointed arms hear lateral pinnules. The tube-feet take the form of ten- tacles arranged in groups on the disk, arms, and pinnules. No madreporic plates exist, but certain holes lead from the water-vascular system into a ramifying system of vessels, whence others open to the exterior. The skeleton of the Crinoids is composed of a number of ossicles with a very definite arrangement. The topmost seg- ment of the jointed stalk is termed the centro-dorsal plate ; in the Comatulidae, which lose their stalk when adult, this persists as the central aboral plate, and bears several whorls of cirrhi which have a root-like appearance. The stalked forms, such as Penfacrinus caput Med- usae, also have numerous cirrhi, arranged in whorls on their stalks. From the centro-dorsal piece five radial plates radiate; these are continued by second and third V' radial ossicles, and the last of N these bears two brachials (Fig. Fia. 138.— Pentacrinoid larval fonns 141). These brachials form the of Coinatula. Natural size and fy.^^ „£ g, series which form the magnified. axis of each of the ten arms. The growing point of the arm forks at short intervals, and one ECHINODERMA TA 237 branch of the fork alternately, right and left, remains small and constitutes a pinnule, a method of branching which occurs in plants, and iS termed by botanists scorpioid dichotomy. The arms and their pinnules have a grooved ciliated ventral surface, at the disk the grooves of the two arms of a pair unite, and the five grooves thus formed run to the mouth. The arms are flexible, and the free Crin- oids swim through the sea by the graceful undula- tions of these processes. In a transverse section of the arm the following parts may be distin- guished : dorsally a large brachial ossicle which is traversed by an axial nerve, the contiguous Ossicles being united and moved by a pair of muscles (Fig. 140). Ventral to the ossicle is the body- cavity broken up into four spaces which com- municate atintervals. One of these is dorsal, one ven- tral, and two lateral, the ventral portion is traversed by the sterile generative rhachis. Below these coelomic spaces lies the radial water-vascular vessel which gives off alternating branches to the nonsuctorial tube -feet. At the side of the ambulacral groove some spherical bodies of unknown function are situated, these are termed sacculi, and consist of a membrane enclosing a large group of spherules. Fig. 139. — Pentacrinus caput Medusae. After Gxittard. 238 ZOOLOGY The pinnules resemble the arms, with the exception that the generative rhachis has become functional, producing either ova or spermatozoa. The rhachis, both in the arms and in the Fia. 140. — Transverse section of a Cri- noid arm (partly diagrammatic). After Milnes Marshall. 1. Ambnlaoral groove. 2. Ambulacral nerve. 3. Amtulaoral water- vessel. 4. Tube-feet. 5. Pinnule. 6. Coeliao (dorsal) canal. 7. Subtentacular (lateral) canal. 8. Ventral canal: oontainsgenital rhachis. 9. Muscles connecting the joints of arm. 10. Axial cord. 11. Its branches. 12. Branch to pinnule. pinnules, is surrounded by a blood-plexus, and the whole is enclosed by the ventral division of the body-cavity, which is relatively much larger in the pinnules, corresponding with the enlargement of the rhachis. The generative cells escape through a series of special pores. At the tips of both arms and pinnules all the sections of the body-cavity communicate with each other. The mouth is central, and the anus is interradial in position and on the oral surface of the disk ; the alimentary canal is coiled, and lined by a ciliated epithelium. The coelom in the disk is much broken up by strands of connective tissue which support the viscera. The mouth is surrounded by vascular, water-vascular, and nervous rings, which each give oif extensions into the arms (Fig. 141). The water- vascular ring gives off numerous ciliated canals which open into a series of vessels which communicate with the exterior by a series of ECHINODERMA TA 239 ciliated pores which traverse the integument. This system represents the stone canal of Asterids. In one ■ species of Fia. 141. — A longitudinal section through the plane of the mouth and anus of Pentacrinus decorus, Wyv. Th. After Carpenter. 1. Mouth. 2. Alimentary canal. 3. Anus. 4. On the left, the axial cord of the ray ; on the right, extensions of nerves from the axial cord into the plated perisome of the ventral side. 4i, ambulacral nerve. The central nervous mass is shown at 4, near the basal plate. 5. Basal plate. 6i, 62, 63. First, second, and third radial plates. 7i, Ta. First and second distichal (brachial) plates. 8. The more or less calcified connective tissue in the body-cavity. 9. Central vascular axis of stem. 10. A cirrhus. 11. Genital rhachis. 12. Ligament between the ray joints. 13. Radial water-vessel. The black plexus of blood-vessels in the centre of the figure is the plexiform gland, containing the genital stolon. Ehizocrinus there is one canal and one pore in each interradius, but the number is much increased in other Crinoids. The five genital strands are continuous with a central genital stolon, which here, as in Asterids, has been mistaken for a heart by German authors. Around this stolon are numerous vessels, which in the central capsule of the dorsal nervous 240 ZOOLOGY system dilate into chambers which give off vessels to the cirrhi. Above they communicate with a plexus of vessels around the oesophagus, this plexus communicates with the distal portion of some of the stone canals. The chief nervous system is situated dorsally ; it consists of a mass of nervous matter, lying within the circle of basal ossicles, and giving off a large nerve to the stalk, which supplies branches to all the cirrhi, and five radial nerves, each of which divides into two, and the resulting nerves supply each arm and govern their movements. This system is con- tinued into the pinnules ; it is probably connected here and there with the ambulacral system of nerves, whose function seems to be mainly sensory. This dorsal or anti-ambulacral system may be derived from concentrations of a subepidermal nervous system, such as exists in Asterids, which have sunk into the body. Crinoids are attacked by an order of highly-modified Chaetopods, termed Myzostomidae. These occur only on the Crinoidea, and live parasitically either on the disk or arms ; their presence often causes local abnormalities of growth, pro- ducing swellings sometimes termed galls. The order includes two genera, Myzostoma and Stelechopus. Extinct Crinoids seem to have suffered from the same parasite. Class IV. ECHIIfOIDEA (Sea Urchins). Chaeacteeistics. — Spheroidal or heart-shaped Echinodermata, sometimes flattened dor so-v&ntr ally. The calcareous ossicles take the form of definitely-arranged plates usually immovably united iy their edges, and of moveable spines. The number of radii always five in recerd forms. Mouth and anus present. A ciliated ectoderm covers the body of the Echinoids, beneath this is a nerve plexus. The calcareous plates which constitute the shell of the animal are developed in the con- nective tissue of the integument. The apical series of plates consists of a dorso-central piece surrounded by ten plates ; five of them, the radials or ocular plates, bear sense organs, the alternating five, interradial in position, are pierced by the genital pores. The ambulacral plates abut against the radials, and it is ECHINODERMA TA 241 between the most dorsal ambulacral and the radial plates that new ambulacral pieces are intercalated. One of the ambulacra Fig. 142. — A portion of the shell of Echinus graaUis, Alter Agassiz. It. Ambulacral plates. b. Poriferous zone. c. Interambulacral plates. is regarded as anterior, and an interradius is posterior ; in those forms in which the anus is not central, it lies in this posterior interradius. Adopting this orientation, the madreporic pores are usually found on the right anterior genital plate. Both the ambulacral or radial and the interambulacral or interradial areas are composed of a double row of pentagonal plates, firmly united with all the contiguous plates. Each of the ambulacral plates is formed by the fusion of several small plates, the pore-plates ; these latter are pierced by two holes. Pig. 143. — Spine of an Echinid. After Leuckart. 1. Spine. 2. Basal knob. 3. Circular muscle of spine. 4. Ligament. •:;.:2 ■/••■• through which two processes from the water-vascular system pass and fuse to form one tube-foot. Both the radial and interradial plates bear calcareous knobs, upon which long spines are articulated; these are moved by certain muscles attached to their base, and form important locomotor organs. Pedicellariae, with usually three jaws, are also present. Some 16 242 ZOOLOGY of these are provided with glands which open to the exterior near the tip of the jaws ; the glands are said to secrete a sticky fluid by means of which the Echinoid attaches to itself pieces of seaweed, etc., which screen it from observation. The smaller Fig. 144. — Pedicellariae of Echimus saxatilis. After Gegenbaur. a. Open. b. Closed, pedicellariae serve chiefly to clean the surface of the body, and some of them serve as locomotor organs, and to catch passing worms, etc. They are well supplied with nerves, and some of them have in addition a special tactile organ. The peristomial area immediately surrounding the mouth Pig. 145. — View of Sea Urchin, witli part of the shell removed to show the course of the alimentary canal (from Leuckart). After Cuvier. 1. Mouth, surrounded by five teeth (dis- 5. The siphon. placed). Lantern of Aristotle. Oesophagus, coiled rectum. Ovaries with oviducts. 6. Oral vascular ring. 7. Aboral sinus. intestine, and 8. Blood-vessel accompanying intestine. 9. Ampullae at base of tube-feet. ECHINODERMATA 243 is soft and membranous, with scattered ossicles. The mouth opens into an oesophagus, surrounded in the Eegulares and Clypeastroids by a complicated masticatory apparatus known as Aristotle's Lantern. The oesophagus extends into the inter- radius of the madreporic plate, and opens into an intestine which takes a spiral course, finally opening by the anus, which may be nearly central in position or quite eccentric. The intestine is accompanied by a second tube, the siphon, which may have been pinched off from the intestine, into which it opens at each end. The whole is held in position by numerous strands of connective tissue. The body-cavity is spacious and is filled with a fluid in which amoeboid corpuscles float, similar to those found in the water-vascular system. The circumoral water- vascular ring lies at the dorsal end of Aristotle's Lantern. The ring gives off in each inter- radius a diverticulum or Polian vesicle, and in each radius a radial vessel which runs along the inner surface of the ambu- lacral plates ; it bears a number of ampullae, which open, as a rule, by two ducts into the tube-feet, these vary much in structure; when suctorial the sucker contains calcifications. In the interradius of the madreporic plate a stone canal, which may be membranous or calcified, passes to an ampulla which opens by the madreporic plate. The blood system of Echinoids is still involved in obscurity. There is a circumoral ring adjacent to the water-vascular ring, giving off two vessels which run one on each side of the intestine, and there are probably radial vessels, and one or more vessels accompanying the stone canal. Glandular tissue representing the so-called heart of other forms is developed in the wall of this structure. In the Eegulares ten buccal gills are usually found pro- jecting from the peristomial area around the mouth ; these are hollow arborescent diverticula of the coelom, resembling in essential structure the dermal branchiae of the Asteroids. There is a circumoral nervous ring situated in the angle between the base of Aristotle's Lantern and the peristome ; this gives off five radial nerves, each of which ends in a sensory prominence of the epidermis, which traverses the ocular plate. 244 ZOOLOGY The radial nerves send branches to the tube-feet, from the base of which a nerve passes to the sub-epidermal plexus of nerve 14- FiG. 146.- -Diagrammatio vertical section of a Sea Urchin (from Leuokart). After Hamann. 1. Mouth. 10. Ahoral sinus containing so-c 2. Intestine cut short. hlooJ-vessel. 3. Siphon. 11. Circumoral water-vascular ring 4. Rectum. 12. Oral nerve ring. 5. Anus. 13. Tube-foot with ampulla. 6. Ventral vessel on intestine. 14. Radial nerve. 7. Dorsal vessel on intestine. 15. Radial water-vascular vessel. 8. Stone canal. 16. Polian vesicle. 9. Madreporio plate. 17. Muscles. 18. Ocular plate. fibrils, which ramify all over the body just outside the calci- fications, and govern the movement of the pedicellariae and spines. The generative organs typically consist of five arborescent glands, though the number is often reduced, lying interradially, and opening on the genital plates. In the young they are all connected by a circular genital rhachis ; they become very conspicuous in the breeding season. The pore plates of the paired ambulacral areas are in the female Memiaster phiLvppii extended and depressed so as to form four deep oval cups. In these the eggs are deposited ECHINODERMA TA 245 and the young develope, being kept in position by some of the spines bending over them. Fig. 147. — Spatangus purpureus. The Echinoidea are divided into three subdivisions : (i.) Regulares. — Sphaewidal or flattened circular bodies. Ambulacral and interamtulacral areas equal in length. Central mouth and subcentral amis. Complex masticatory apparatus — Aristotle's Lantern — present. Echinus, Toxopneustes. (ii.) Clypeastroidea. — Shield-shaped, often flattened todies. Central mouth, with Aristotle's Lantern. Very broad ambulacra, with their dorsal ends forming a peta- loid rosette round the apical plate ; small tube -feet. Anus excentric. Clypeaster, Eotula. (iii.) Spatangoidea. — Irregular heart-shaped bodies. Mouth and anus excentric. No Lantern of Aristotle. Ambu- lacra petaloid, and the anterior one unlike the others. Spatangus, Brissus. Class V. THE HOLOTHUROIDEA (Sea Cucumbers). Characteristics. — Bchinodermata with elongated bodies, usually pentagonal in cross section. The integument is leathery. 246 ZOOLOGY and contains small scattered calcareous ossicles. The mouth is surrounded hy a circlet of retractile tentacles, into which the water-vascular ring sends extensions. The madreporic plate usvMlly opens into the hody -cavity. Tlie anus is usually terminal. The body of the Holothurians is elongated along an oral- apical axis. The ambulacra are five in number ; they may be equally developed, or three of them, the trivium, may be flattened and form a creeping sole upon which the animal rests ; the bivium is then con- vex. This occurs in Psolus and in all the Elasipoda. When this specialisa- tion of radii takes place, the tube-feet are modified on the trivium. In other ises the tube-feet are scattered all rer the body, and in others — the ynaptae — they are entirely wanting. I he skin is covered by an ectoderm ith an external cuticle ; within this a layer of connective tissue, in which ills laden with pigment and cal- ireous ossicles are scattered. This I yer also includes a nervous plexus. The connective tissue sheath surrounds a muscular layer whose fibres run in a circular direction, and more inter- nally are five radial bands of longi- tudinal muscles, one running along each ambulacrum, and lying beneath the water-vascular vessel and nerve ; anteriorly these bands are attached to the pharyngeal ossicles, which are radial and interradial in position. The ossicles in the integument are always small in size ; they may be simple spicules, or may assume a number of very elegant forms in the different genera. In the Elasipoda they exist in the mesenteries and in the walls of the alimentary canal, as well as in the integument. The coelom is large, and is lined with ciliated cells ; a special section of it surrounds the pharynx, and in the outer Pig. 148. — Holothuria papulosa. ECHINODERMA TA 247 walls of this the pharyngeal ossicles are formed ; these are notched for the passage of the radial nerve and water- vascular vessel. In some of the Synaptae the alimentary canal runs nearly #1^ ^w^ Pig. 149. — Spicules of Holotlinroidea. After Semper. d a and i. Anchor and anchor plate of Syiwpta mdivisa. Semper. c. Spicule of Chirodota rigida. d. Wheel spicule of Chirodota vitiensis. GrafFe. e. Spicule of Thyone chUenms. Semper. /, g, h. Anchor and anchor plate of Synapta godefroyn. Semper. i. Spicule of Rhopalodima lageni- formis. Gray. straight from the mouth to the anus, but as a rule it forms a coil with three limbs. The mouth is situated in the centre of Fio. 150. — Diagi'am of a transverse section through the body of a Holothurian. From Leuckart. 1. Dorsal surface, dorsal interradius. 2. Ventral surface. 3. Left dorsal radius. 4. Bight dorsal radius. 5. Right ventral radius. 6. Left ventral radius. 7. Radial nerve. 8. So-called radial blood-vessel. 9. Water-vascular vessel. 10. Radial muscles. -11" 11', 11", 11'". The mesenteries of the three limbs of the intestine. 12', 12", 12"'- The three limbs of the intestine. 13. Respiratory trees, the left surrounded by a rete mirabile of blood-vessels. 14. Two tubules of the generative organs lie to the right of the genital duct. 15. Ventral blood-vessels. a peristomial area, and is in the Elasipoda directed ventrally. The mesentery of the first descending limb of the alimentary canal is situated in the interradius, in the middle dorsal line. 248 ZOOLOGY Fio. 151. — View of the internal organs of a Holothuriau which has been cut open along the middle dorsal line. From Leuckart. 1. Badial ossicle of the calcareous ring, into which the longitudinal muscle is inserted. 2. Interradial ossicle of the calcareous ring. 3. Badial water-vascular vessels. 4. Circumoral ambulacra! ring, 5. Polian vesicle. 6. Two stone canals ending in Madre- poric plates ; the upper one is attacaed to the dorsal mesentery, the lower one hangs freely. 7. Circumoral blood-vessel. 8. Ventral blood-vessel. 9. Dorsal blood-vessel. 10. Anastomosing branch between dif- ferent parts of the ventral blood- vessel. ECHINODERMATA . 249 that of the second or ascending hmb in the left dorsal inter- radius, and that of the third or second descending limb in the right ventral interradius. The pharynx is followed by a stomach with muscular walls ; the intestine forms the longest portion of the alimentary canal ; the posterior end of the rectum, or the cloaca, is rhythmically contractile, and takes in and sends out sea water ; special muscles run between it and the body- wall. Certain appendages known as respiratory trees open into the cloaca. These are sometimes regarded as homologous with the two interradial caeca which open into the rectum of Asterids. They are branched structures, usually two in number; each terminal ramification opens by a fine tube into the coelom, and they doubtless serve to introduce sea water into that space. Their function is probably in part respiratory, and they re- semble in structure the similar organs in the armed Gephyrea. In some species some of the basal ramifications of the respiratory trees are modified into the so-called Cuvierian organs. In these the peritoneal covering becomes glandular, and when the animals are irritated they are discharged into the water through holes torn at their base in the cloacal wall, and swell up into long tenacious elastic threads, which serve the purpose of entangling their enemies. At times, when much disturbed, the Holothuroidea wiU throw out their whole intes- tine through the anus ; but it is probably regenerated. The water-vascular system consists of an oral ring, which gives off five radial vessels, these run at first upwards and out- wards, and give off branches to the oral tentacles ; the tentacles may increase in number with age. The tentacles assist in the pro- cess of feeding, either shovelling in mud or sand — in which case 11. Anterior part of alimentary canal. 19. Generative organs. 12', 12", 12'". The three limbs of aliment- 20. Opening of generative duct, ary canal. 21. Circular muscles in body-wall. 13. Cloaca. 22. Bight dorsal muscle. 14. Cloacal opening with five teeth. 23. Eight ventral muscle. 15. Radiating muscles of cloaca. 24. Median ventral muscle. 16. Organs of Cuvier. 25. Left ventral muscle. 17', 17". Bespiratory trees. 26. Left dorsal muscle. 18. Dorsal mesentery with free posterior margin. The tentacular ampullae are omitted ; the mouth is in the centre of the divided tentacles. 250 ZOOLOGY the intestine, like that of the Gephyrea, is full of sand — or en- tangling food particles ; and in the latter case the tentacles are then thrust into the mouth, which removes any nutritive particles. The radial vessels pass through notches in the radial ossicles of the pharyngeal calcareous ring, and run along the ambulacra giving off tube-feet outside the bundles of muscle fibres. They are absent in one group, and devoid of tube-feet in others. The ampullae of the tube-feet are embedded in the circular muscle layer in the Elasipoda, and in many of this group the stone canal opens on the dorsal surface, and in others it lies in the tissue of the integument ; in other subdivisions it is supported by the mesentery, and the madreporic plate opens freely into the coelom. It may or may not have calcareous walls ; the fluid in this system contains numerous corpuscles. The vascular system consists of spaces in the connective tissue not lined by an epithelium. There is a circular space round the pharynx, just behind the water-vascular ring. This Fig. 152. — Sea Cucumber Ciuiumaria crocea (Falkland Islands) bearing its young. After Sir Wyville Thomson and Murray, "Cliallenger" Narrative. communicates with a dorsal and a ventral intestinal vessel, and these two are connected by numerous anastomoses round the walls of the alimentary canal. The dorsal vessel is in connec- tion with a plexus which surrounds the left respiratory tree. There are no radial vessels. The circumoral nerve ring gives off five radial nerves, and nerves to the tentacles. There is a nerve plexus in the skin ECHINODERMA TA 251 and the tube-feet. In Synapta ten auditory vesicles contain- ing numerous otoliths have been described at the base of the radial nerves. The generative organs are either a single gland situated at the left of the dorsal mesentery, or a double gland, one on each side of it ; the glands are continuous with a single duct, which opens in the dorsal middle line close to the base of the ten- tacles. With one or two exceptions, the Holothuroidea are dioecious. The young of one species, Gucumaria erocea, found near the Falkland Isles, are attached in rows on each side of the dorsal ambulacrum. The early stages of developement ap- parently take place rapidly, and the embryos are arranged in position by the tube-feet of the ambulacrum. In the East Indies some species form an article of com- merce under the name of BSche-de-mer. They are dried and sold to the Chinese, who use them in the preparation of soup. The Holothuroidea are classified as follows : I. AcTiNOPODA. — Badial canals present in the water-vascular system. a. Aspidochirotae. — The tentacles are peltate in form, respirator]/ trees are present. Holothuria, Miilleria. h. Elasipoda. — Tentacles as above. The dorsal tube -feet produced often info very long stiff processes. Bespira- tory trees rudimentary or absent. Stone canal sometimes opens to the exterior. Deima, Elpidia. c. Dendrochirotae. — The tentacles have a dendriform shape. Gucumaria, Thyone. d. Molpadiidae. — The tentacles are simple or pinnate. The radial canals bear tenta/iles, but no other tube -feet. Molpadia, Caudina. II. Paeactinopoda. — The tentacles are pinnate. No radial canals, no tube-feet, no respiraiovy trees. Synapta, Chirodota. !2i o I— I o CO ll I". III ^ T3 t. PJ O hi o _ o.a 1^ « g