Z S9Z6S9E0 LOLI € OLNOYOL JO ALISYSAINN AMVYA] ADO1O0Z Digitized by the Internet Archive in 2010 with funding from University of Toronto http://www.archive.org/details/treatiseonzoolo05lank + A TREATISE ON ZOOLOGY VOLUMES ALREADY ISSUED PART I. (SECOND FASCICLE) INTRODUCTION AND PROTOZOA PART II. PORIFERA AND COELENTERA PART III. THE ECHINODERMA PART IV. MESOZOA, PLATYHELMIA, AND NEMERTINI Demy Svo, bound in cloth, price 15s. each net. On thin paper, in paper covers, price 12s. Gd. each net. AGENTS America . THE MAcMILLAN CoMPANY 64 & 66 FrrrH AvENUE, NEw YORK Canada . THE MAcMILLAN Company OF CANADA, Lip. 27 RicamMonpD Street West, Toronto India . . MacmiLian & Company, Lrp. MACMILLAN BuILpiInG, BomBay 309 Bow Bazaar STREET, CALCUTTA A TREATISE ON ZOOLOGY EDITED BY HK. RAY LANKESTER M.A., LL.D., F.R.S. ae : gs LIBp ; i Ment of : x "Versi, ). In the former case the genital products fall into the peri- cardium and are carried to the exterior by the renal ducts, which thus act as gonaducts. In other cases the genital products may be dis- charged into the reno-pericardial duct (Zvochus, Fig. 55 ; Solenomya), and are thence expelled through the kidneys, or they may be discharged directly into the kidneys, more or less close to the external renal orifice (various archaic Lamellibranchs, the majority of Rhipidoglossa, Scaphopods). Otherwise, in all groups, the gonads open to the exterior by their proper pores, which are nearly always adjacent to the renal openings ; they may, however, be more or less removed to a distance from the latter, and in certain hermaphrodite forms (Pulmonates and Nudibranchs) the renal orifice is near the anus and the genital aperture is secondarily separated from it and shifted towards the penis. Accessory glands are often developed on the course of the genital duct, especially in the female. The male and female elements are formed from the epithelium of the gonad: each oogonium gives rise to a single ovum with its polar bodies, while £ ™ each spermatogonium gives rise to several sperma- tozoa. The eggs of Cephalopods, of the majority Lacuna paliduin, OL olyplacophora, and of the Lamellibranch Laue pallidula. ‘ E The male in situ on Pseudokellya (Fig. 220) are invested by a continuous the shell-aperture of Cellular follicle. In hermaphrodite Molluses the te alcr male. ” Spermatozoa ripen before the ova; the herma- phroditism is therefore protandric. The herma- phroditism also is not self-sufficient, and the ova of one individual must normally be fertilised by the spermatozoa of another individual. The “parthenogenesis” observed in hermaphrodite Pulmonata (Arion, Limnaea), which have been isolated from the time of their birth, is possibly due to an abnormal autofertilisation. “Progenesis” has only been observed in one Gymnosome (Clione), in which the larval characters are preserved for a long time. Copulation only takes place in such Gastropods, whether monoecious or dioecious, as are provided with a penis, and in the Cephalopoda. In several members of the latter class the copulatory organ, or Fia. 7. THE MOLLUSCA 21 hectocotylus, is caducous and travels independently in search of the female. In those Molluscs which do not copulate the eggs are fertilised after oviposition. The eggs are laid separately in the Amphineura, in the more archaic Gastropoda, in the Scaphopoda, and in almost all the Lamellibranchia (that is to say, generally, in the forms that do not copulate), but in the majority of aquatic Gastro- pods and in the Cephalopods the eggs when laid are united into a gelatinous or coriaceous nidus, which may be attached (benthos) or floating (plankton). As a rule, Molluscs do not nurture their progeny, and when once the eggs are laid they take no further heed of them. Some of them, however, retain their eggs till the time of hatching, and are therefore called incubatory forms (Fig. 8). Examples of incubatory forms occur among the Lamellibranchs, especially the specialised eulamellibranchiate Submytilacea ; among marine Gastropods (Vermetus, etc.), among freshwater Gastropods (Melania, etc.), and even among the octopodous Cephalopods (Argo- nauta), but the number of truly viviparous forms is very small. Callistochiton among the Amphi- neura and several genera of aquatic and pulmonate Gastro- . pods are the only instances. The number of eggs laid is very variable. It is always greater in the case of those marine Molluses which abandon their eggs to the mercy of the waves than in those which deposit them in a nidus, agglomerated together in ribands or in shells in which the embryos are naturally protected. Thus Ostraca may lay as many as 60,000,000 eggs, Chiton 200,000. On the other hand, numerous eggs are found in the nidus of certain Nudibranchs (50,000 in the case of Doris), Cephalopods (Loligo, 40,000), and pelagic Gastropods (Cymbulia, 1200). In all cases in which numerous eggs are laid free larval forms are developed, but when the whole of the develop- mental stages are passed through within the egg-membranes, and when the young individual is hatched with the characters of its parents and undergoes no metamorphoses, the number of eggs is generally small (Cenia, 4-12), or the greater number of the eggs laid is absorbed and furnishes nutriment for a few embryos (Buccinum, Purpura, ete.). In the following cases also eggs are laid in small numbers :—(1) In ineubatory forms, such as Vermetus, where from 120-240 eggs are incubated in the pallial cavity, under the protection of the shell; (2) in terrestrial and fluviatile species, in which the number of eggs is always smaller than in marine forms. In this case caenogenesis or embryonic condensation is the rule ; the young animal quits the egg in the adult form, and there is no need for a large number of embryos. Instances in point are— Stenogyra mamillata, left side view, with four embryos in the oviduct. em, embryo. 22 THE MOLLUSCA Limnaea, 20-100 eggs; Helix, 40-100; Ancylus, 5 or 6; Valvata, 17; Neritina, 50-60. Lastly, the number of eggs is small in viviparous forms: in Paludina there are about 15; in Subulina (Fig. 8) there are 4. Ill. Empryouoey. In the Mollusca the segmentation of the egg is unequal. In some primitive forms, it is true, the first two or three divisions are regular (Patella, Fig. 9, A; Chiton), and the two first blastomeres are often equal in size, but it is frequently the case that even the first division is unequal (many Lamellibranchs, Rachiglossa, ete.). From the first phases of segmentation up to the blastula stage one may distinguish two kinds of cells or blastomeres: the ectodermic cells, called ectomeres or micromeres; and the endodermic cells, called endomeres or macromeres. In the more primitive forms, however, such as Patella (Fig. 9, A), and the Protobranchia, there is eS eae ae C ae BO EC O ma Fig. 9. Eggs of various Mollusca, at the same stage, with 4 micromeres. A, Patella (Patten); B, Lacuna; C, l'eredo (Hatschek); D, Sepia (Kolliker). mia, macromeres ; ii, micromeres. scarcely any difference in size between the two kinds of blastomeres, but in proportion as the endodermic cells are charged with food-yolk so does the difference in size become accentuated, as may be seen in the Taenioglossa (Fig. 9, B), and particularly in the Lamellibranchs (Fig. 9, C) and Rachiglossa. In the last-named group the irregu- larity of segmentation reaches its maximum, leading to the stage of incomplete or meroblastic segmentation characteristic of the Cephalopods (Fig. 9, D). It is quite exceptional for the segmenta- tion to become secondarily regular, as in Paludima, where this condition is due to the diminution of the yolk (Fig. 110). The separation of the ectodermic from the endodermic elements of the embryo follows a constant rule, similar to that which obtains among the Annelida. After the formation of the four primary micromeres two new series or quartettes of micromeres are formed at the expense of the large endodermice cells, and all the ectodermie organs are formed from these three quartettes. The micromeres multiply more rapidly than the macromeres, and there are even cases, such as Dentulium (Fig. 184), certain Rachiglossa, and Lamellibranchs (7eredo, Fig. 9, C, Cyclas, Unionidae), in which there THE MOLLUSCA Ww Lo) is only a single macromere during the early stages of segmentation: The segmentation sphere or blastula is thus formed of two more or less unequal moieties, respectively known as the formative (ectodermic) and nutritive (endodermic) moieties. Internally there is a segmentation cavity or blastocoele, generally much reduced (Fig. 10, A), except in certain Lamellibranchs (Cyclas, Dreissensia, etc.) and stylommatophorous Pulmonates. The animal or formative pole of the egg is indicated by the presence of the polar bodies ; the vegetative or nutritive pole is opposite, and at this point the blastopore or orifice of the digestive cavity will be formed (Fig. 110, A, /). Formation of the Digestive Cavity and of the Diblastula or Gastrula. —The final result of the segmentation is that the micromeres form a more or less complete envelope to the segmented ovum, covering over the macromeres which remain within to form the endoderm. But the formation of this double-walled sphere, the diblastula or Fic. 10. Two types of gastrulae. A, invaginate or embolic (Chiton: after Kowalewsky) ; DL, epibolic (Crepidula: after Conklin). /, blastopore ; ec, ectoderm ; en, entoderm. gastrula, may be effected in one or the other of two apparently different methods, invagination or epiboly. Invagination or emboly is certainly the more primitive method, and is realised in ova witli a relatively small amount of food-yolk. In this case the nutritive moiety of the blastula is doubled back within the formative moiety, muchas one half of a deflated hollow indiarubber ball may be pushed by the finger within the other half. These two halves are separated by a remnant of the original segmentation cavity, which is invariably much reduced by the process. The invagination thus produced gives rise to the digestive cavity or archenteron, lined by the endoderm and communicating with the exterior by the blastopore. As examples of this mode of formation one may cite Chiton (Fig. 10, A), Paludina, the Pulmonata, the Nudibranchia, the Pteropoda, the Gymnosomata and Limacinidae, the Seaphopoda, Nucula, Ostraea, Pisidium, and the Unionidae. The gastrula is formed by epiboly when the nutritive cells or macromeres have become so much distended with food-yolk as to be too large to be invaginated into the layer of micromeres. In such case the micromeres as they multiply grow round the 24 THE MOLLUSCA endoderm and gradually surround it, leaving at the nutritive pole an orifice, which is the blastopore. Examples of this mode of formation are—Many streptoneurous Gastropoda (Trochus, Vermetus, Crepidula, Fig. 11, Janthina), the majority of the Rachiglossa (Columbella, Fusus, Nassa, Purpura, Urosalpina), the Tectibranchs (Acera, Philine, Aplysia, Thecosomata), and many Lamellibranchs (Pecten, Modiolaria, Cardiwm, Teredo, etc.). The two processes, however, differ only in appearance, and there are intermediate stages which form an insensible passage from one method to the other. In fact, complete invagination only occurs when the segmentation is quite or very nearly regular (Paludina, Chiton, ete., Figs. 10, A, and 110, A), but in consequence of the progressive increase of the amount of food-yolk contained in them, the macro- meres become larger and larger and are only able to be invaginated at a late stage of development. That is to say, in certain embolic gastrulae there is ~ commencement of epiboly, followed eventually by an invagination of the macromeres (/iroloida, Clione, Nucula). In the various cases enumerated above the segmentation of the ovum is complete or holoblastic. In the Cephalopods the case is different, for the segmentation is incomplete or meroblastic (Fig. 289), a large part of the egg being formed of food-yolk which takes no part in the division. But it must be remarked that in various types, such as the specialised Gastropods (Rachiglossa: Nassa, Purpura, Fusus, etc. ; Tectibranchia: Accra, Aplysia, Cavolinia, ete.), there is a sort of quasi-distinct yolk, formed by the granular portion of the macromeres. Hence the meroblastic or “ discoidal ” segmentation of the Cephalopods is not absolutely distinct from the total segmentation observed in other Molluscs: it is only an exaggeration of epiboly. In fact, as the yolk forms the principal part of the ovum and the protoplasm is concentrated at the formative pole, the ectoderm is formed over a limited region of the yolk (the “ germinal disc” or “ embryonic area”), and is unable to envelop it entirely, so that development proceeds as if the process of epiboly had been left incomplete, the blastopore remaining very large and leaving all that part of the yolk which could not be covered by the ectoderm outside the embryo. Under these circumstances the endoderm is essentially an embryonic tissue, exclusively employed in the constitution of the vitelline mass, and degenerates in the adult, a great part of the digestive tract of the latter, a long stomodaeum and a long proctodaeum, being formed by the ectoderm. The passage to this condition is presented by some Gastropods with an abundant yolk: in Massa a part of the primitive endoderm degenerates in the adult, and in Fusus the four macromeres of the primitive endoderm seem to form a provisional embryonic organ, and it is the ectoderm that forms nearly the whole digestive tube. In the different groups of Molluses the liver THE MOLLUSCA 25 is formed from the wall of the enteron, generally as a pair of diverticula given off from its middle region (Fig. 116), and com- posed exclusively of cells of a fatty nature, while nutrition is effected by the absorption of the yolk. Formation of the Orifices of the Digestive Tube.—The gastrula mouth or blastopore, at a given period of the development at any rate, has very often the form of an elongated slit. This condition is shown in Patella (Fig. 113, III), Bithynia, various Opisthobranchs, such as Aplysia and Nudibranchs, in basommatophorous Pulmonates, Cyclas, ete. This slit gradually closes up from behind forwards, its two margins forming the ventral pedal prominence. In other cases this aperture may be more or less elongated, oval in shape, with an Fic. 11. Eggs of Crepidula, showing the origin of the first mesodermic cell. ima, macromeres ; mes, first mesodermic cell; mi, micromeres. (After Conklin.) anterior groove running forward as far as the velum, as is seen in Paludina. Or again, the blastopore may be circular, and be gradually displaced from behind forwards, suggesting a specialised condition of the slit-like blastopore which closes in the same direction. The linear or circular blastopore is totally closed in a large number of cases; such are, Aspidobranchs: Patella, Trochus, and Neritina; Pectinibranchs: Bithynia, Nassa, Purpura, Nautica, Lamellaria, and Crepidula; Opisthobranchs: Aplysia, various Pteropods and Nudibranchs ; Lamellibranchs: Cyrenidae, Unionidae, Dreissensia, Teredo. In other forms the blastopore, though it may contract so much as to be scarcely visible, remains open ; if it is a linear blastopore it is the anterior end that persists. Examples are, Chiton; several marine Streptoneura, including Vermetus, Fusus, and Heteropods ; Pulmonata ; Dentalium; Nucula ; Ostraea. 26 THE MOLLUSCA An invagination of the ectoderm at the place where the blastopore closed, or surrounding the blastopore if it remains open, places the digestive cavity (enteron) of the gastrula in communica- tion with the exterior. This invagination constitutes the stomodaeum, from which the pharynx and _ oesophagus with all their accessory ap- paratus, the salivary glands, radula, ete., are formed. Thus the blastopore, if it remains open, does not become the mouth of the adult. Paludina, however, is an exception, in that the whole of the blasto- pore remains open and _ be- sees comes the anus (Ray Lan- Trochosphere of Patella, sagittal section. bl, blasto- kester), whilst the stomo- pore ; me, inesoderm ; si.g, shell-gland; the twodorsal, GQaeum is formed at the ve, and the two ventral ciliated cells are the velar : cells. (After Patten.) anterior end of the embryo (Fig. 110, F, m). Later, in Molluses in general, the proctodaeum is formed as a very short anal invagination, at the posterior end of the original blastoporic groove. Its position is generally indicated by the presence of two prominent ectodermic cells, and it perforates the posterior part of the archen- teron, establishing a com- munication between the in- testine and the exterior. Ketodermic Organs. —The embryo, then, has an endo- dermic digestive cavity and a general ectodermic en- velope from which the oesophagus and the anal invagination are derived. But the ectoderm is prin- cipally concerned in the production of the permanent tegumentary So properly Trochosphere of Dreissensia, median sagittal section. sO called, such as the foot, bl, blastopore ; fil, flagellum; in, intestine ; p.G.c, the mantle, and the ctenidia, heen Cilia; of, shell; vw, -velomn. (2 and those organs which, al- though deeply placed, originate from the surface, such as the nervous system and the organs of sensation. The ectoderm further gives rise to the embryonic locomotory organs which result from a special THE MOLLUSCA 27 adaptation to larval life. The most important of these embryonic organs is the velum; primitively a preoral ciliated ring, characteristic of the Trochosphere (Ray Lankester), which delimits an area known as the prostomium or “velar area.” Towards the centre of this area there is often a ciliated tuft or a flagellum, as in the larvae of Amphineura (Figs. 14, 17), Patella (Figs. 14, 113), Dentalium Fic. 14. Three Trochospheres of Mollusca. 4, Ischnochiton(Heath) ; B, Patella (Patten); C, Dreissensia (Meisenheimer). a, anus; /, foot; jl, Hagelluin; m, mouth; p.a.c, post-anal cilia ; sh, shell; ve, veluin. (Fig. 15), and various Lamellibranchs (Figs. 14, C; 16). It is on the buccal side of this tuft, when it exists, that the apical plate is situated, probably the remains of a sensory organ from which the cerebral nerve-centres take their origin. The velum may be differ- entiated in one of two ways :—(1) The preoral ciliated ring may extend itself by growing outwards at all parts of its circumference : the trochosphere larva is thus transformed into a “ veliger ” (Ray Lankester), a larval form highly characteristic of Molluses (Fig. 18). The velum may be divided into two lateral lobes (Fig. 18, v), which in their turn may be divided into two or three secondary lobes (Fig. 121). (2) The velum may retreat for a greater or less distance Fic, 15. towards the posterior end Trochosphere of Dentaliwm,sagittal median section. of the embryo, becoming ™,hastopors:s, agellum: in intestine: a, mantle attached to it in such a manner as to form a more or less extensive superficial investment furnished with multiple ciliated rings, as in Dentalium (Figs. 15, 28 THE MOLLUSCA 185), Neomeniomorpha (Fig. 17), Nuculidae (Figs. 16, 225). The embryo may in this manner be nearly completely invested by a “test” extending posteriorly to an orifice which, however, is only a false blastopore. In both cases the velum eventually atrophies when the animal assumes the definitive habits of the adult. When the whole course of the development is undergone within the egg-membranes, there is either no veliger stage (Cephalopods, Figs. 119, D; 257, 290, 291; Cyclas, Entovalva, ete.) or there is no free veliger, but a more or less rudimentary velum, postero-dorsally atrophied, may be observed, which persists nearly to the time of hatching (viviparous Gastropods, Purpura, Fig. 1, Pulmonata). A post-anal ciliated area is not uncommonly found, as, for example, in Purpura (Fig. 1), among Gastropods, and Dreissensia Fie. 16. Trochosphere of Yoldia, median sagittal section. o.¢, anterior adductor muscle; ap, apical plate ; b/, blastopore ; e.g, cerebral ganglion ; jl, flagellum ; h.a, posterior adductor ; in, intes- tine ; li, liver ; sd, stomodaeum ; ¢, ‘‘ test” or reflected velum, with 3 circlets of cilia. (After Drew.) (Figs. 13, 14, p.a.c) among Lamellibranchs. Finally, post-oral ciliated rings, secondarily acquired, are found in the larvae of certain Opis- thobranchs, Gymnosomata (Fig. 120). The foot is nothing more than a projection of the integument between the mouth and the anus. Its earliest rudiment is evidently paired, since it is formed by the union of the lips of the (commonly) elongated blastopore (Fig. 113). It is only at a late stage of development that it attains its complete development ; during the early stages it is very small and functionless, the velum serving as the sole organ of locomotion (Fig. 112). An ectodermic invagination, bounded by a ridge, makes its appearance at an early period on the dorsal face of the embryo, near the formative pole. This invagination, known as the “shell- gland” (Ray Lankester) or preconchylian invagination (Fig. 110, K, IF, sh.gl), is also the precursor of the mantle, since the edge of the latter structure is formed by the ridge. The shell-gland THE MOLLUSCA 29 spreads from its point of origin in the form of a pallial thickening, which may be only slightly concave, or it may be deeply invagin- ated and afterwards evaginated. The invagination is caused by the very rapid proliferation of the epithelial tissue in the neighbour- hood of the pallial ridge. When evaginated these epithelial cells, having again become external, begin to secrete the shell. The branchiae or ctenidia originate on the inner surface of the mantle as tegumentary projections in the form of papillae or of filaments arranged in series (Fig. 229). The Nervous System and Organs of Sensation.—The various pairs of nerve-centres arise separately, and usually as thickenings of the ectoderm at the points where they are formed. In certain cases, however, the nerve-centres are developed by the more primitive process of invagina- tion, as may be seen in the case of the cerebral ganglia of Den- 2) Fic. 18. Fic. 17. Veliger of Littorina, ventral view, x 80. Trochosphere of Myzomenia banyulensis. e, eye; f, foot; m, mouth; ma, mantle; A, after 36 hours; B, after 100 hours. ji, pa.e, pallial cavity; 7.1, right liver lobe; s, flagellum ; v, velum. (After Pruvot.) stomach ; ¢, tentacle; v, velum. talium, Vermetus, Cavolinia (paired invaginations), Yoldia (Fig. 16, c.g), and Dreissensia (an unpaired invagination), a portion of the cerebral centres in Pulmonates, and the cerebral, pedal, and visceral ganglia of the Unionidae. When the cerebral ganglia are formed by invagination, a single pit, or a pair of pits, is formed on the oral side of the apical plate, from the bottom of which ganglion cells are budded off; the remiainder of the invagination goes to form the labial palps, ete., of Lamellibranchs, and probably the rhinophores of Gastropods. The eyes also, including the pallial eyes of the Pectinidae, and the otocysts are sometimes developed from ectodermic thickenings ; but in many cases these organs are formed by invagination, for ex- ample, in various Cephalopoda (Fig. 119, D); in Gastropoda: in the Aspidobranchs, Paludina, Bithynia, Calyptraca, Crepidula, Nassa, the Heteropoda, and the Pulmonata. The otocysts only are formed 30 THE MOLLUSCA by invagination in some other Gastropods (Fusus), and in Dentalium and various Lamellibranchs. Mesoderm and Mesodermic Organs.—A third intermediate cellular layer is formed, generally at an early stage, between the external ectodermic envelope and the endodermic wall of the digestive tube. This is the mesoderm, from which all the organs situated between the digestive tube and the integuments are produced. The origin | of this layer is often difficult to determine, especially in highly specialised forms, but in all cases in which the origin is distinct there is no doubt about the matter, the mesoderm is derived from the endoderm. This derivation is shown in the Polyplacophora, the Aspidobranchs (Patella, Fig. 12, me; Trochus, Neritina), the Pectini- branchs (Paludina, Bithynia, Crepidula, Fulgur, etc., and seemingly the Heteropoda), the Opisthobranchs (Philine, Umbrella, Aplysia, Clione, Chromodoris, ete.), the basommatophorous and stylommatophor- ous Pulmonates, the Scaphopods, the Lamellibranchs (Pisidium, Unionidae, Dreissensia, Teredo, etc.). Nevertheless we find scattered mesodermie cells, giving rise to unicellular muscular fibres of the integument (Unio, Crepidula), which are derived from the ectoderm. The principal result of the development of the mesoderm is the formation of another cavity in the embryo, the coelom. In the Mollusca the coelom does not originate by the invagination of enterocoelic pouches (Tonniges has shown the inaccuracy of Erlanger’s description of enterocoelic coelomic pouches in Paludina), but, as a result of specialisation, this primitive method is supplanted by solid mesoblastic masses, generally paired, which may be con- sidered as the cardio-genito-renal rudiments. These mesoblastic masses take their origin from the macromeres. As a rule, at the stage when four macromeres are present, it is the most posterior of the four that gives rise, by successive divisions, to the two first mesomeres or primary mesodermice cells (Fig. 11). From these the two mesodermic bands, which constitute the third layer, are produced as solid, or in some cases discontinuous masses. The coelomic cavity or series of cavities are formed by more or less regular fission or delamination of the mesoblastic bands,—evidently a secondarily acquired mode of development. The coelom is therefore physio- logically a schizocoele. Eventually it is placed in communication with the exterior by ectodermic invaginations. The order in which the different parts of the primitive coelomic cavity make their appearance is not constant. The pericardium, in particular, may originate as two symmetrical cavities, which unite more or less rapidly (Paludina, Cyclas, Cephalopoda), or directly, as a single azygos cavity (Dreissensia, Pulmonata). The extension of the mesodermic elements evidently narrows the primitive segmentation cavity or blastocoele, which becomes the cavity of the circulatory system. These elements spread between the ectoderm and endo- THE MOLLUSCA 31 derm, and become specialised to form the internal lining of the circulatory cavity in particular, and may even fill almost entirely the remainder of the blastocoele in the form of a false mesenchyme (caenogenetic or secondary mesenchyme), which constitutes the connective tissue. This naturally restricts the extension of the coelom properly so called, so that it is commonly reduced to the pericardial cavity. Since the mesodermic tissue gives rise, in this manner, to the epithelial wall of the coelomic cavity, to the lining of the circulatory cavity, and to the conjunctive tissue filling up the spaces between the organs, one must recognise it as sharing in the evolution, firstly, of the coelom and the excretory and repro- ductive organs derived from the coelom ; secondly, of the circulatory apparatus—the heart, etc. The coelom, of which the formation has been described above, is essentially a cavity communicating with the exterior, and its epithelial wall may be differentiated in two special ways—into excretory or renal elements, and into reproductive, and therefore caducous elements. In the most primitive process the kidneys are formed in connection with a portion of the coelom, with which they remain in complete continuity (Paludina). In other cases they are formed by a hollowing out of a portion of the mesoderm in contact with the pericardium (Bithynia, Limax, Cyclas, Dreissensia, etc.), or they may be formed independently in their definitive position (Cephalopoda). Eventually each kidney acquires a com- munication with the pericardium, and in all cases makes a connection with the exterior by an ectodermic invagination. The genital organs or gonads originate either from the wall of the coelom or pericardium (Paludinu, Dreissensia), or in contact with the coelomic wall (Cyclas), or from a rudiment common to themselves, the pericardium and the kidney, or, finally, from distinct mesodermic elements. The continuity of the pericardium and gonads is well preserved only in the Aplacophora (Fig. 30, C) and adult Cephalopoda (Fig. 252, coe) ; in all other Molluses the genital organs are separated from the pericardial cavity and acquire communica- tions either with the kidneys or directly with the exterior. In the latter case the terminal portions of the gonaducts, together with the accessory genital glands, are ectodermic in origin. The heart may arise from a portion of the wall of the peri- cardium itself (Paludina), or a common rudiment may give rise to the wall of the pericardium and the heart (Pulmonata, Cyclas, Dreissensia, etc.), and in the latter case the origin of the heart may be paired (Cyclas, Cephalopoda) like that of the pericardium itself. The larvae of such Molluscs as lay their eggs singly and free in the sea are hatched out very rapidly ; a few hours suffice in the case Of Dentalium among the Scaphopoda ; twenty hours in 7'rochus among the Aspidobranchs ; fourteen hours in Yoldia among the 32 THE MOLLUSCA Lamellibranchs ; twenty-four hours in Pholas, ete. The eggs of Gleba, aggregated into a nidus, are hatched after three or four days, those of Jschnochiton after seven days ; but in the majority of Gastropods and the Cephalopods the time required is much longer. The marine larvae of temperate seas are intolerant of a rise of temperature, and generally perish when it approaches 30° C. IV. DEFINITION OF THE MOLLUSCA. From what precedes, it results that in each of the five classes the same lines of specialisation may be observed—viz. the loss of the shell, of the foot, of the ctenidia, of the radula, ete. Thus the general morphological characters are obviously those of the most primitive of the different classes (Fig. 19), and the following diagnosis may be proposed for the phylum :— 1. The Mollusca are originally bilateral organisms, in which signs of primitive segmentation are no longer evident. rd 3 a Fic. 19. Scheme of a primitive Mollusc, viewed from the left side. a, anus ; e.g, cerebral ganglion ; f, foot ; g, gill, in the pallial cavity ; go, gonad; h, heart; k, kidney; la.c, labial commissure ; m, mouth; pa, mantle; pa.n, pallial nerve; pe, pericardium; p.g, pedal ganglion; pl.g, pleural ganglion ; ra, radula; 7.p.0, reno-pericardial orifice ; st, stomach ; st.g, stomato-gastrie ganglion ; v.g, visceral ganglion. 2. They possess a well-developed coelom (gonad and pericar- dium), enteron, and haemocoel, quite distinct from one another. 3. The alimentary tract exhibits (or has lost) a radular sac in its anterior part. 4. The nervous system consists of a peri-oesophageal ring, whose supra-oesophageal (or dorsal) moiety is the cerebral com- missure, and the infra-oesophageal (or ventral) moiety is the labial commissure. ‘The former gives off chiefly sensorial nerves, the latter nerves to the digestive tract. From their union two nervous cords arise on each side, a dorsal or pallial and a ventral or pedal ; from the former arise the visceral nerves, whose main trunks are frequently joined together under the digestive canal to form the infra-intestinal visceral commissure. 5. The general body-wall is differentiated into three regions : THE MOLLUSCA 33 (1) the antero-dorsal or cephalic, on which are borne most of the special sense-organs ; (2) the postero-dorsal or pallial, which forms a projecting fold around the body, and secretes on its external face a calcified cuticle or shell, and on its lower surface develops respiratory organs or ctenidia; (3) the ventral or pedal, which is the organ of locomotion. 6. A so-called “veliger” or free trochosphere larva is nearly always present in embryonic development ; its preoral ciliated ring grows out to form a natatory velum, and at its formative pole there is a “ preconchylian invagination ” or shell-gland. V. Bronomics AND DISTRIBUTION. Molluses are essentially aquatic animals, but the most varied modes of existence may occur, even among members of the same class. The majority are inhabitants of the sea; a few live in fresh water ; a single order of Gastropods and a few isolated members of the same group are adapted to a terrestrial life. They are repre- sented in the three great groups of aquatic organisms, namely, in the Benthos, comprising creeping or fixed animals inhabiting the depth of the sea; the Necton, comprising animals that swim actively and can make headway against the currents ; the Plankton, comprising animals that float passively and cannot contend with the currents. The first group includes the littoral and abyssal Molluscs, among which the Necton is also represented. The two other groups include the pelagic Mollusca, the Cephalopods belonging exclusively to the Necton, while the free-swimming Gastropods, and those which inhabit pelagic Algae, some isolated Lamelli- branchs (Planktomya), and the larvae of various groups belong to the Plankton. The Mollusca are distributed over the whole surface of the earth and in all latitudes. Terrestrial forms are found on the highest mountains—some Stylommatophora at a height of 15,000 feet ; lacustrine forms (Limnaca) are found at a depth of 350 fathoms. The pelagic forms are not only distributed over the surface of the sea, but may descend to a depth of 2600 fathoms without reaching the bottom. Abyssal Molluscs are found in all oceans extending to a depth of 2800 fathoms from the surface. As a rule, Molluses are free-living animals, and crawl, swim, or burrow, but some are sedentary in adult life. Only a few Gastropods and Lamellibranchs are fixed to their habitat. Their modes of alimentation are various. Some are commensal with Ascidians, e.g. Modiolaria; some with Echinoderms, as Montacuta, Lepton, and Scioberetia ; some with Crustacea, Lepton squamosum and Ephippodonta, or with Sponges (Vulsella), or Annelids (Cochliolepis). Others again are ectoparasitic on Echinoderms, such are Zhyca and 3 34 THE MOLLUSCA Stylifer (Fig. 20) ; or, like Eulima and Entovalva, are endopavrasitic, also in Echinoderms. In the latter case the animal may become so degenerate in the adult state as to consist of little more than a sac containing the genital products, as for example Lntosiphon (Fig. 21), Entocolax (Fig. 138), Entoconcha (Fig. 139), and Enteroxenos (Fig. 140). There are many cases of protective adaptation and mimicry in the various groups of Mollusca. The pelagic species are generally transparent and colourless or tinged with blue, but the most remarkable examples of mimicry are found among the forms un- protected by a shell (Nudibranchs, Lamellaria, ete.), in which the Fic. 20. Stylifer celebensis, X12. pr, pro- Entosiphon deimatis, x 2. 0, orifice boscis ; ps, pseudopallium ; sp, spire of the proboscis; ov, ova; pr, pro- of the shell not covered by the pseudo- boscis ; ps, pseudopallium ; s, fixative pallium. (After Kiikenthal.) siphon. (After Koehler and Vaney.) individual assumes the colour and aspect of its habitat, as for instance Hermaca dendritica on green algae, and Hermaea bifida on Griffithsia, a red alga, etc. The abyssal Molluscs are colourless, and are characterised by the thinness of the shell, the atrophy of the visual organs, and the development of tactile organs. Examples of convergence of form may also be noted, such as Marginella and Pseudomarginella among Gastropods, Pholas candida and Petricola pholadiformis among Lamellibranchs. The duration of life in individual Molluses is ordinarily rather - short. Marine Streptoneura may live for several years, and Littorina littorea, When in captivity, has attained an age of nearly twenty years. Freshwater Molluscs may live for eight years (Paludina). THE MOLLUSCA 35 _ The Pulmonata are generally biannual, but Helix pomatia may attain ‘an age of six years. The majority of Nudibranchs and Tecti- branchs appear to live for one year only. Many Lamellibranchs (Mytilus, Teredo) ave adult at the end of one year; Avicula is adult at the end of two years; Ostraca edulis is sexual at two years, becomes adult in five years, but may live for ten years in oyster- beds. The huge Zridacna lives for about eight years, the Cyrenidae only two years, but the Anodontidae are remarkable for their longevity ; they do not become sexually mature till they are five years old, and they continue to grow to the age of twenty or thirty years. In the Cephalopoda it seems that Lossia does not live for more than a year, and Octopus not more than four years. Many Molluses are able to fast for a long time. Cold affects them less than heat. Helix has been known to survive a tempera- ture of — 120° C., and small Gastropods live in thermal springs at a temperature of 42°C. Molluscan embryos and larvae generally perish, in temperate climates, at temperatures of + 31° C. and — 3°C. Descriptive zoologists have enumerated more than 28,000 species of living Molluscs, of which more than half are Gastropods. Fossil representatives of Molluscs are found in all deposits from the. Palaeozoic onwards. 1. Distribution in Space. I. Marine Molluscs — A. Littoral Fauna.—The shores of con- tinents are divided into three provinces by great thermal variations. a. The North Polar province, with characteristic genera such as Cryptochiton, Molleria, Lacuna, Velutina, Onchidiopsis, Solariella, Machaeroplax, Volutharpa, Torellia, Cyprina, Mya. fp. The South Polar province, with the characteristic genera Photinula, Struthiolaria, Cominella, Eatoniella, Cyamium, Lissarca, Philippiella, Modiolarea. y. The tropical province, or region of coral reefs, in which one may distinguish four sub-regions :—(1) The Indo-Pacific, the home of Nautilus and the chief forms of the Toxiglossa. The Mediterranean should be included in this sub-region, and the Australo-Zealandic division of it, just as is the case with the terrestrial fauna, presents special characters, as shown in T’rigonia, Amphibola, etc. (2) The West African and (3) the East American sub-regions, which have several forms in common. (4) The West American, characterised by such genera as Monoceros, Concholepas, etc. Occasionally species may be naturally or artificially acclimatised in various parts of these regions, but exchanges are only definitively effected between similar latitudes, as for example Littorina littorea between Europe and North America, and reciprocally Venus mercenaria and Petricola pholadiformis. B. The Abyssal Fauna, is not divisible into distinct provinces, and many of its species are universally distributed either in the 36 THE MOLLUSCA northern and southern parts of the same ocean or in several different oceans. Thus Limopsis aurita, Semele profundorum, Verticordia deshayesiana, Arca pteroessa are found in the Atlantic and Pacific ; Hyalopecten pudicum and Silenia sarsii are common to the Atlantic and Indian oceans. Characteristic abyssal genera are Leptochiton, Scissurella, Margarita, Cyclostrema, Setia, Leda, Bathyarca, Limopsis, Hyalopecten, Dacrydium, Callocardia, and Septibranchs in general. C. The Pelagic Fauna.—a. The superficial forms are very widely distributed, but may nevertheless be described as belonging to polar and tropical provinces. The North and South Polar fauna are different : to the former belong Clione limacina, Limacina helicina ; to the latter Spongiobranchaca australis, Limacina antarctica. fp. The zonary or deep forms are probably more universally distributed, but are still imperfectly known; they include the luminous Cephalopoda. Il. Land and Freshwater Molluscs.—It is only a small number of groups that have quitted the sea to assume a freshwater or terrestrial existence, and among these no Amphineura nor Scapho- poda nor Cephalopoda are found. Among the Lamellibranchs only a few families are found in fresh water, viz. Cyrenidae, Dreissensiidae, Unionidae, Aetheriidae (none of them primitive in organisation), and a few isolated types. Among the Gastropods we find very few Rhipidoglossa, Neritina, Hydrocaena, Titiscania ; chiefly Taenioglossa, e.g.the Valvatidae, Paludinidae, Ampullariidae, Hydrobiidae, Melanidae ; some isolated types of Rachiglossa, and practically no Opistho- branchs. As for the terrestrial Mollusca, there are only a few families of streptoneurous Gastropods (Helicinidae, Cyclophoridae, etc.), and the whole order of Pulmonata. Of the last-named, one sub-order, the Basommatophora, has returned to an aquatic life, chiefly in fresh water, but retains for the most part a pulmonary respiration. It is mostly in warm regions, and particularly in those in which the sea is inclined to be brackish, that marine forms have penetrated into fresh waters. Certain inland seas also have become separated from the ocean, and have preserved a fauna which is partly of marine origin. Lakes Baikal and Tanganyika appear to belong to this category. The tropical regions in general are characterised by terrestrial forms, such as Vaginula, Helicina, Ampullaria, ete. Chilina and Bulimus belong to the Neotropical region. Clausilia is not found in North America, and, generally speaking, the Pulmonates with folded branchiae are absent from the New World. The Ethiopian province is the home of Achatina ; the Australo-Zelandic of Janella, Rhytida, Vanganella, Latia ; the Oriental region of Cyclophorus and the Rathouisiidae. The distribution of terrestrial and fluviatile THE MOLLUSCA 37 Molluses shows that the Asiatic and Australian regions are separated not by Wallace’s line, but by another line farther east. 2. Distribution in Time. The five classes of Molluscs were already differentiated at a remote epoch of the Palaeozoic era. The Polyplacophora, the Cephalopoda; and Dentaliwm were represented in the Ordovician ; Diagrams of the five classes of Mollusca, from the left side. A, Amphinewa; B, Scapho- poda; C, Gastropoda ; D, Lamellibranchia; £, Cephalopoda. a, anus; a.a, anterior adductor ; ¢.g, cerebral ganglion; /, foot; fu, funnel; g, ctenidium; h, heart in the pericardium ; h.a, posterior adductor; m, mouth; pa, pallium or mantle; p.g, pedal ganglion; pl.g, pleural ganglion ; ra, radula ; st, stomach ; st.g, stomato-gastric ganglion ; v.g, visceral ganglion. the Rhipidoglossa and the Palaeoconchs (Lamellibranchia allied to Solenomya), in the Cambrian. “ On the other hand, many ancient stocks have left no surviving descendants. But the appearance of existing genera or families supports: the phylogenetic conclusions drawn from the study of comparative anatomy. Among the Cephalopoda it is the Tetra- branchs that appear in the Ordovician; the Ammonites appear 38 LITERATURE OF THE MOLLUSCA only in the Devonian, and the Dibranchs (Belemnites) at the beginning of the Secondary. But at the present day all the species of Tetrabranchs, some 7500 in number, are only represented by a few species of the single genus Nautilus; and as for the group of Belemnites, it has entirely disappeared since the end of the Cretaceous, its sole surviving and more or less distant represent- ative at the present day being the genus Spirula. Among the Gastropods the Aspidobranchs (Rhipidoglossa) are the first to appear; several families, such as the Capulidae and Pyramidellidae, date back to the Upper Silurian. On the other hand, there are but few Ctenobranchs in the Palaeozoic ; most of the families of this order make their appearance in the Secondary, and the same may be said of the Opisthobranchs, with the exception of the Actaeonidae, which are found, as also are the Pulmonata, in the Carboniferous. Of Lamellibranchs various Protobranchs and Filibranchs are found in the Silurian, but the other groups, as a rule, do not appear till the Secondary. The Palaeoconcha of the Primary, and the Rudistae, Diceratidae, Mono- pleuridae, and Caprinidae of the Secondary, have died out without leaving descendants. LITERATURE OF THE MOLLUSCA GENERALLY. I. Conchological. 1. Cooke. Molluscs. The Cambridge Natural History, vol. iii. 1895. 2. Fischer. Manuel de Conchyliologie, 1887. 3. Jeffreys. British Conchology, 1862-1869. 4, Simroth. Mollusca. Bronn’s Klassen und Ordnungen des_ Thierreichs, Bd. iii. 1895 till now. 5. Taylor. Monograph of the Land and Freshwater Mollusca of the British Isles, 1900 till now. 6. Tryon. Manual of Conchology, 1878 till now. 7. Woodward. A Manual of the Mollusca, 1880. II. Morphological. 7 bis. Cuénot. L’excrétion chez les Mollusques. Arch. d. Biol. t. xvi. 1899. 8. Geddes. On the Mechanism of the Odontophore in certain Mollusca. Trans. Zool. Soe. London, vol. x. 1879. 9. Huxley. On the Morphology of the Cephalous Mollusca. Phil. Trans. 1853. 10. von Jhering. Vergleichende Anatomie des Nervensystemes und Phylogenie der Mollusken, Leipzig, 1877. 11. —— Die Gehorwerkzeuge der Mollusken, Erlangen, 1876. Zur Morphologie der Niere der sog. ‘‘ Mollusken.” Zeitschr. f. wiss. Zool. Bd. xxix. 1877. 13. Lankester. Mollusca. Encyclopaedia Britannica, 9th edit. vol. xvi. 1883. 14. —— Contributions to the Developmental History of the Mollusca._ Phil. Trans. 1875. 15. —— Note on the Coelom and Vascular System of Mollusca and Arthropoda. Quart. Journ. Micr. Sci. vol..xxxiv. 1893. LITERATURE OF THE MOLLUSCA 39 . Milne-Edwards. Observations sur la circulation chez les Mollusques. Ann. d. Sci. Nat. Zool. sér. 3, t. viii. 1847. . Moynier de Villepoix. Recherches sur la formation et l’accroissement de la coquille des Mollusques. Journ. Anat. et Phys. 1892. . Pelseneer. Introduction d l’Etude des Mollusques, Bruxelles, 1894. . — Recherches morphologiques et phylogénétiques sur les Mollusques archaiques. Mém. Cour Acad. Belg. t. lvii. 1899. — Hermaphroditism in Mollusca. Quart. Journ. Mier. Sci. vol. xxxvii. 1895. . Plate. Bemerkungen iiber die Phylogenie und die Kntstehung der Asym- metrie der Mollusken. Zool. Jahrb. (Anat. u. Ontog.), Bd. ix. 18965. . Schiemenz. Ueber die Wasseraufnahme bei Lamellibranchiaten und Gastro- poden. Mitth. Zool. Stat. Neapel, Bd. v. 1884, and vii. 1887. . Spengel. Die Geruchsorgane und das Nervensystem der Mollusken. Zeitschr. f. wiss. Zool. Bd. xxxv. 1881. . Thiele. Die Systematische Stellung der Solenogastren und die Phylogenie der Mollusken. Zeitschr. f. wiss. Zool. Bd. Ixxii. 1902. CHAPTER II THE AMPHINEURA CLASS I.—AMPHINEURA, von JueriIne (1876) (=IsopLruRA, Ray Lankester ; ACULIFERA, Hatschek). Order 1. Polyplacophora (Chitones). Sub-Order 1. Eoplacophora. Family 1. Lepidopleuridae. Sub-Order 2. Mesoplacophora. Family 2. Ischnochitonidae. 3. Mopaliidae. 4. Acanthochitonidae. 5. Cryptoplacidae. 9 PP] 9 Sub-Order 3. Teleoplacophora. Family 6. Chitonidae. Order 2. Aplacophora. Sub-Order 1. Neomeniomorpha. Family 1. Lepidomeniidae. 2. Neomeniidae. 3. Proneomeniidae. 4. Parameniidae. 9 bP) Sub-Order. 2. Chaetodermomorpha. Family 5. Chaetodermatidae. Historical_—The Chitones were formerly the only known forms of this group, and for a long time they were placed in the Gastropoda, near the genus Patella, the two forming the Order Cyclobranchia of Cuvier. When Chaetoderma and Neomenia were investigated from an anatomical point of view, von Jhering united them to the Chitones (1876), and placed the two in a division of “ Worms,” which he called Amphineura. But the molluscan nature of these Amphineura is so clear, that they were reintroduced into the 40 THE AMPHINEURA 41 phylum Mollusca by Spengel, Hubrecht, Lankester, etc., and even von Jhering has since admitted this interpretation. Although Gegenbaur and Claus have again separated Chaetoderma and Neomenia from the Chitones, and placed the former in a distinct class (“Solenogastres ”), and the latter once more in the Gastro- poda, the unity of the Amphineura, as well as their molluscan nature, is now very generally accepted. Definition. —The Amphineura are a_ group of Mollusca characterised, firstly, by their more or less elongated and quite symmetrical body, with the mouth and the anus situated at its two ends; and secondly, by their mantle, which is always provided with numerous spicules embedded in a cuticle. General Description.—The mantle is very large, and always covers at least the dorsal surface and the sides of the body. The whole external symmetry reappears in the various internal organs. In the nervous system there are, on each side, two longitudinal cords (one pedal, one pallial) with ganglionic cells along their whole extent. They are united with one another in front, where there is a supra-oesophageal cerebral commissure. The two pedal cords are also united by anastomoses, and in addition each of them also exhibits anastomoses with the corresponding pallial cord. The two pallial cords are united by a thick posterior com- missure on the dorsal side of the rectum. There are no otocysts. The buccal cavity is very generally provided with a radula (but mandibles are only present in a species of Chaetoderma). The anus and renal orifices are posterior. The heart is dorsally situated in the hind part of the body, and its ventricle is more or less intimately united to the dorsal wall of the pericardium. All the Amphineura are marine in habit. They are found in all oceans and at nearly all depths. They existed in very ancient geological ages, for they are already present in the Ordovician (Lower Silurian). There are two very distinct Orders of Amphineura: (1) the Polyplacophora, (2) the Aplacophora. ORDER 1. Polyplacophora, Blainville. Dejfinition.—Amphineura, whose chief characteristics are (1) the _ foot, occupies the whole ventral face of the body; (2) the mantle, bears eight transverse calcareous plates; (3) between mantle and foot there is on each side a more or less complete row of branchiae. I. GENERAL DEscRIPTION AND EXTERNAL CHARACTERS. The mantle covers the whole body on the dorsal side; its ventral extension is inversely proportional to that of the foot, and 42 THE AMPHINEURA all around the latter there is a pallial groove. ‘The mantle secretes a shell consisting of eight plates or valves articulated with one another and arranged in longitudinal series. Each of these valves partially overlaps the following, except in some species of Cryptoplax ( = Chitonellus), in which the three hindermost are isolated. This articulation of the valves allows the animal to roll up. The two terminal (first and eighth) valves are semicircular, the six inter- mediate are quadrangular. They may be partially (Cryptoplax and some species of Acanthochiton) or even wholly (in adult Cryptochiton, but not in young ones) concealed by a redupli- cation of the mantle. Each valve is made up of two quite dis- similar calcareous layers: («) the uppermost or tegmentum, which alone is visible externally ; (b) the deeper layer or articulamentum, which is porcellaneous, quite compact, and invisible in the living animal. In most of the lower Polyplacophora these layers are coextensive and have smooth edges, but in the higher forms the articulamentum projects beyond the : -*% outer layer into the substance of the mantle, enh ele a er Ge lpted to which it is firmly attached. These pro- Hee che inten at tz jJections of the outer or peripheral margins first eyes on the second of the valves are termed ‘insertion plates”; shell-plate; III, third . shell-plate. they are generally slit or notched to form the so-called “teeth,” which may be either smooth and sharp along the edge or crenulated. The anterior margin of each valve, except the first, is invariably provided with two pro- jections called ‘ sutural laminae,” which underlie the hind margin of the valve next in front. The tegmentum has no representative in the shells of other Mollusca. It is formed by the fold of the mantle covering the edge of the articulamentum, and, as it grows in width, it extends over the latter. It is much reduced in Acanthochiton and aborted in the adult Cryptochiton. The stratified layers of the tegmentum are traversed by a system of numerous, nearly parallel, ramified canals through which special sense-organs pass to the surface (Fig. 24). Nearly the whole of the peripheral part of the mantle or “‘oirdle,” as it is called, is covered with chitinous or calcareous spicules of various shape, acicular or squamose. Each spicule rests on an epidermic papilla and is formed by a single matrix cell. The head is more or less cylindrical, consisting of a short down- wardly curved snout with the mouth at its extremity. On either side of the mouth is a somewhat angular labial palp. A narrow furrow separates the head from the foot. The latter forms a ventral creeping surface, extending the whole length of the body Fic. 23. THE AMPHINEURA 43 from head to anus. The breadth of the foot is inversely: pro- portional to the width of the lower edge of the mantle: it is broad in most of the Polyplacophora, but narrow in the Cryptoplacidae. Extending all round the foot, between it and the mantle on the ventral side, is the pallial groove, in which lie the gills. In the more primitive Chitons mucous thickenings extend into the groove, Fic. 24. Transverse section of the lateral portion of the pallial teguments of Chiton. I, tegmentum ; II, articnlamentum ; III, pallial epithelium under the shell; IV, epithelium of the margin of the mantle; V, spicula; VI, cuticle of the mantle-margin; VIa, periostracum; VII, megal- aesthetes ; VIII, micraesthetes. (After Blumrich.) reaching from the anus to its upper corner, or to the foot, or even to the inner wall of the mantle. II. ANATOMY. 1. The Alimentary Canal extends from one extremity of the body to the other. The mouth leads into the buccal cavity, on the ventral wall of which opens the radular caecum. The radula is long and reaches nearly as far back as the stomach. Each radular row includes seventeen teeth of various shape (Figs. 2, A; 74, E). The three central teeth are simple: on each side of them is a large 44 THE AMPHINEURA recurved thick and dark lateral tooth. Externally are six polygonal marginal teeth: of these the third differs from the rest, being more or less narrow, elongated, and curved, and sometimes its concave edge is ciliated (Zrachydermon). The fore part of the radula rests upon a cartilaginous mass, moved by a great many mus- cular bundles. Two pairs of glands open into the buccal cavity. The true salivary glands lie at the sides, well forward, but behind the cerebral commissure ; they are slightly branched, but rather short, and have a very short duct. On the ventral wall, under the subradular organ, le two very small mucous glands close to one another and to the middle line. On either side, at Boreochiton cinereus, dorsal view of a female, the point where the pharynx without the shell-plates. 1, first “‘intersegmen- passes into the short oesophagus, tum”; II, ovary; III, oviduct; IV, ventricle ~ 3 of heart ;’V, dorsal right muscle; VI, dorsal 18 the opening of the sugar ppeeee Pel mieelss VII, retractor muscle of gland—a large glandular pouch with a papillose internal surface. The large and thin-walled stomach is surrounded by the liver mass. The two liver lobes are symmetrical in young Chitons, but become asymmetrical in the adult, the right lobe being the smaller and Fic. 25. Fic. 26. Cryptoplax larvaeformis, left-side view, the posterior end partially opened. I, gonad; II, genital duct; III, genital pore ; IV, ventricle of heart; V, anus; VI, renal pore; VII, gills; VIII, foot ; 1X, mantle ; 6, 7, 8, position of the sixth, seventh, and eighth shell-plates. anterior. They open into the stomach either by two distinct orifices (Chiton aculeatus), or by two orifices in a single duct (Lepidopleurus), or by a single aperture (Hanleya). The intestine is very long, as is usual in phytophagous animals, and is thrown into numerous THE AMPHINEURA 45 coils. The anus opens at the posterior extremity of the body, between the mantle and the foot (Fig. 28, @). 2. Circulation and Respiration—The heart, enclosed in a large pericardium, occupies the postero-dorsal region of the body. It consists of a median elongated ventricle and two elongated symmetrical auricles. The openings of the auricles into the ventricle are subject to some variation. In the Lepidopleuridae, the Mopalidae, and Tonicella, Trachydermon, Boreochiton, etc., among the Ischnochitonidae, the auriculo-ventricular aperture on either side is single and anterior (Fig. 27); in the Acanthochitonidae, the Cryptoplacidae, and the Chitonidae generally there are two apertures on either side (Figs. 4 and 26); in Chiton squamosus there are three, and in Chiton goodalli four. The auricles are united to one another posteriorly, but there is never a posterior median auriculo-ventricular opening. The posterior auriculo-ventricular orifices are frequently asymmetrical. A single anterior aorta rises from the ventricle anteriorly and carries the blood to the various organs and inter- visceral blood-spaces. The venous blood from the different parts of the body is conducted back to a large sinus on either side near to the line of union of the mantle with the body. Closely connected with this same line of union on either side is a row of gills, situated between the foot and the mantle. The number of pairs of gills varies from four in Lepidopleurus pagenstecheri to eighty in Acantho- pleura spiniger, but a careful comparison shows that the number of gills in the right hand row does not always correspond with that in the left. The gill-rows may be either of the holobranchial type, in which case they ex- tend over the whole length of the body, or of the mero- satis - eI : oy - Heart of Mopalia, dorsal aspect. I, auriculo- branchial ty pe, mW hich they ventricular cominunication ; II, ventricle; III, right are confined to a more or auricle; IV and V, afferent vessels ; VI, mantle ; 6, 7, ae 8, indicate the position of the sixth, seventh, and less limited space at the pos- eighth shell-plates. terior end of the body (Fig. 28). But these two types are connected by transitional forms, and they are not, generally speaking, characteristic of natural groups, nor are they determined by the greater or less size of the species. The genera with few gills are naturally merobranchial, and in the Lepido- pleuridae these organs are confined to the region covered by the two last shell-valves. The gills are inserted at the bottom of the pallial groove. The largest gill, which is also the last one in those forms in which no adanal gills are present, is always the first 46 THE AMPHINEURA behind the renal opening: it is the first to be formed and is the starting-point from which the rest of the gills are added either forwards or both forwards and backwards. Occasionally individual gills may be bifurcated or trifurcated. _ Each gill has the typical ctenidial structure, consisting of an axis bearing an anterior and a posterior row of gill-lamellae or filaments. The blood from the above-mentioned longitudinal vessel is distributed to each gill by an afferent vessel running along the internal or pedal margin of the axis, and, after being oxygenated in the lamellae, is carried back by an efferent vessel running along the external or pallial edge of the axis to another longitudinal vessel which conducts it back to the corresponding auricle. ‘3. Excretory Organs.—There are two symmetrical kidneys, whose relations were first discovered by Sedgwick. Each of them con- Fic. 28. Ventral aspect of three species of Polyplacophora, showing the various sorts of gill-rows. A, Lepidopleurus bentlius ; B, Boreochiton cinereus; C, Schizochiton incisus. a, anus; f, foot; g, gills; m, mouth; pa, mantle; pa’, anal lobe of the mantle; p.s, pallial slit; te, pallial tentacles. d sists of an elongated renal canal, situated on the lateral side of the visceral mass, and once folded on itself, so that its two ends are posterior. The internal or dorsal end opens into the pericardial cavity, through a ciliated aperture or funnel. The external or ventral end opens to the exterior, between two of the gills at the hinder part of the body. The renal canal is dilated immediately behind its external opening. It is excretory throughout its length, and the excretory surface is increased by numerous small much- branched caeca which lie close to the body-wall laterally and ventrally and open into the canal (Fig. 29). Various kinds of kidneys are to be found (Plate). They generally extend more or less forwards, and their extension is THE AMPHINEURA 47 generally correlated with that of the branchial row. The two branches of the renal canal may be fused together from before backwards until, as in Callistochiton and Nuttalochiton, the kidney has the form of a simple sac with more or less numerous arborescent appendages, and the pericardial and external apertures adjacent to one another at its hinder end. A similar form of specialisation may be seen in the kidneys of Lamellibranchs. The renal canal may be complicated by the addition of two accessory Fia. 29. Renal organs of Boreochiton cinereus, dorsal aspect; on the right-hand side, the exterior ramifications of the antero-posterior renal part are alone drawn ; on the left-hand side, all the other ramifications are drawn. I, pedal cord ; II, mantle; III, external ramifications of the antero-posterior (or terminal) renal part ; IV, reno-pericardial orifice ; V, posterior point of the urinary chamber; VI, external renal pore; VII, urinary chamber; VIII, postero-anterior (or initial) portion of the kidney ; IX, antero-posterior (or terminal) portion of the kidney; X, internal ramifications of the antero-posterior portion of the kidney ; X1, left outer limit of IIL; XII, rainifications of the initial portion of the kidney ; 1-8, the eight ‘‘segments” of the body. longitudinal branches, an anterior larger and a posterior smaller, which lie in the foot near the middle of the body. 4. Nervous System and Sense-organs.—There is no concentration of nerve-ganglion cells to form distinct ganglia, but the larger nerve- cords are ganglionic throughout their extent. There are two pairs of longitudinal nerve-cords, united in front of the buccal mass by a supra-oesophageal or cerebral commissure. Ganglionic enlargements on this commissure are found only in Callochiton doriae. 48 THE AMPHINEURA The two ventral or pedal cords are united beneath the digestive tract by numerous transverse anastomoses. The two lateral’ or Fic. 30. Diagrams of the excretory and reproductive organs of Amphineura. C, Proneomenia; D, Chiton. Br, ctenidia; Cl, cloacal or pallial chamber of Proneomenia ; g, external aperture of the genital duct of Chiton; N, renal organ; 0, gonad; P, pericardium ; r, rectum; u, external aperture of renal organ of Chiton. (From Lankester, after Hubrecht.) Fic. 30vis, Ventral aspect of Acanthopleura incana. I, mantle; II, mouth; III, foot; IV, gills; V, anus; VI, right renal pore; VII, right genital pore (these two pores are better seen on the left-hand side of the body). pallial cords are united posteriorly, dorsad of the anus, by a thick supra- rectal commissure (Fig. 31, VIII). The cerebral commissure inner- vates the palps, the lips, and the muscles of the buccal bulb. Below the buccal bulb it is prolonged into an anterior labial commissure, which in turn gives rise to a stomato-gastric commissure: the last-named is to some extent ganglionic and has two branches, which unite with those of the opposite side on the upper and under side of the pharynx, and also with the infra-oesophageal subrad- ular commissure. The subradular commissure supplies a pair of ganglia’ which are in close connection with a peculiar sense-organ lying on the floor of the mouth, in front of the radula. The labial and sub- radular commissures, together with the subradular organ, correspond to the homonomous parts in the Scaphoda and Cephalopoda. <7} ’ THE AMPHINEURA 49 The two great ventral or pedal cords give rise to the pedal nerves. The two great lateral or pallial cords chiefly send nerves to the mantle and the gills, and thus correspond to the whole of the pleural ganglia and the pallial nerves of the other Mollusca ; a great part of the viscera genital glands, kidneys, and heart) also receives nerves from these pallial cords. - The little differentiated head region bears no special sense -organ, except that the outer edges of the snout taper to form the labial palps. The lower wall of the buccal cavity is furnished with cyathiform gustatory bodies, whose nerves arise from the cerebral commis- vill Fic. 31. Nervous systein of Par ge § menia, Pruvot. Paramenia eryo- 3 e phila, ventral as- Faminy 4. PARAMENIUDAE, Pruvot. Short, and ect. mouth ; si er ° * . . Pl hs 4 truncated in front ; thick cuticle (often without papillae) ; cills and radula present. Genera—Paramenia, Pruvot (Fig. 41). Macellomenia, Simroth. Pararhopalia, Simroth. Dino- menia, Nierstrasz. Cyclomenia, Nierstrasz. Proparamenia, Nierstrasz, Uncimenia, Nierstrasz. Kruppomenia, Nierstrasz. THE AMPHINEURA 61 SuB-ORDER 2. CHAETODERMOMORPHA. Aplacophora without distinct longitudinal ventral (or pedal) groove, with unpaired unisexual gonad, with differentiated liver, and with posterior cloacal chamber provided with two bipectinate gills. Anatomy.—The mantle covers the whole surface of the body, which is therefore cylindrical and vermiform in appearance. The hinder half of the body is a little stouter than the anterior; the posterior extremity swollen and bell-shaped, forming the widely cloacal chamber. The whole body has a uniform covering of short, compressed, calcareous spicules implanted in the cuticula. The mouth is anterior, terminal, and crescentic, owing to the presence of a rounded ventral shield. Chaetoderma radulifera alone is provided with mandibles. The buccal cavity, whose anterior part is partially protrusible, bears on its floor a very peculiar radula, which may consist of (a) a single large tooth (Fig. 43, C), upon which two small teeth are placed (C. nitidulum and C. productum); (b) a single large tooth, upon which is a row of teeth (C. guttu- rosum; (c) no large tooth, several rows of three teeth one behind the other (C. raduli- fera) ; (2) several distichous Chaetoderma nitidulum, Loven. The cephalic rows of two teeth each (C. enlargement is to the left, the cloacal or pallial Z é . chamber (containing the concealed pair of ctenidia) challengeri). Two pairs of sali- to the right. (From Lankester, atter Graff.) vary glands, similar to those in the Neomeniomorpha, open into the buccal cavity. The diges- tive tract is quite straight, and narrows towards the middle of its course to form the intestine. Just before it narrows it receives the duct of a more or less extensive hepatic caecum, which extends backwards on the ventral side of the intestine. The hepatic caecum, large in most species, is feebly developed in C. challengeri. The anus opens in the median line in the cloacal chamber (Fig. 43, B). The heart is posterior and dorsal, and lies nearly free in the pericardial cavity. It is traversed by the retractor muscles of the gills. In its main features the circulatory system resembles that of the Neomeniomorpha. The posterior extremity of the body is hollowed to form a bell-shaped cloacal cavity, which has a con- tractile aperture and contains a pair of large branchiae placed symmetrically right and left of the anus. Each branchia bears a double row of branchial plates, as is the case in the Polyplacophora (Fig 43, B). The two renal ducts are more evidently true excretory organs than in the Neomeniomorpha. They originate from the posterior Fic. 42. 62 THE AMPHINEURA corners of the pericardial cavity, run forward under the floor of the pericardium, and then turn outwards and backwards to run back to their respective apertures right and left of the anus. Their thin walls are lined by a ciliated epithelium, and. there are no accessory generative organs. In the nervous system there are two intimately fused cerebral Fia. 43. Chaetoderma nitidulum. A, median sagittal section; B, sagittal section of the posterior extremity ; C, sagittal section of the anterior extremity. a, anus; br, retractor muscle of the branchiae ; ¢.g, cerebral ganglion ; d.t, digestive tract ; g, gill; go, gonad ; h, heart; 7, intestine ; k, kidney ; J, liver; m, mouth; me, “‘mesothorax”’; p.c, pallial suprarectal commissure ; p.d, pericardial duct ; pe, pericardium ; pe.c, pedal commissures ; pr, “‘ prothorax”; r, radula; s.c, sublingual commissure. (After Wiren.) ganglia bearing accessory lobes. Each ganglion gives rise to two longitudinal nerve-cords, the ventral or pedal cord being more slender than the dorsal or pallial cord. In the anterior part of their course the pedal and pallial cords of either side run parallel and adjacent to one another, but in the posterior region of the body they are fused together, as in Paramenia, and the two pallio- pedal cords thus formed are united dorsad of the rectum by a ? LITERATURE OF THE AMPHINEURA 63 ganglionic swelling. A small perirectal commissure originates from this swelling. The pedal cords are united with one another and with the pallial cords of the same side by anastomoses in the anterior region of the body. A small stomato-gastric commissure, bearing two small stomato-gastric ganglia on the middle of its course, arises from the cerebral ganglia and surrounds the oesophagus. ‘There are no organs of special sense except a dorsal posterior and median pit, corresponding to the precloacal fossa of the Neomeniomorpha. The sexes are separate. The azygos gonad occupies the same position as the paired gonads of the Neomeniomorpha, and com- municates by a median aperture with the pericardial cavity. The generative products are conducted from the pericardium to the exterior by the renal ducts. The embryology of the group is quite unknown. The Chaetodermomorpha are marine animals feeding on lowly- organised forms of life, such as Protozoa, etc. They are found in oozy bottoms from a depth of 15 fathoms to abyssal regions. The nine recorded species of the single genus Chaetoderma come from the North Atlantic, North Pacific, and Arctic Oceans, the Sea of Marmora, the Eastern Archipelago, and the Philippine Islands. Family CHAETODERMATIDAE, von Jhering. Genus—Chaetoderma, Loven. The characters are those of the sub-order. Limifossor, Heath (Alaska). PHYLOGENY OF THE AMPHINEURA, The Polyplacophora present the most archaic characters among the Amphineura. The Aplacophora, on the other hand, are specialised in the following respects: (1) in the great reduction of the foot ; (2) the disappearance of the shell (Cryptoplaz, among the Polyplacophora, shows how these two reductions may take place simultaneously) ; (3) the absence of the radula in several forms. The Chaetodermomorpha seem to be more specialised in these points than the Neomeniomorpha. LITERATURE OF THE AMPHINEURA. I. Polyplacophora. 1, van Bemmelen. Zur Anatomie der Chitonen. Zool. Anzeiger, 1883, p. 340. 2. Blumrich. Das Integument der Chitonen. Zeitschr. f. wiss. Zool. lii. 1891, p. 404, 8. Burne. Notes on the Anatomy of Hanleya Hanleyi, M. Sars. Proc. Malacol. Soe. ii. 1896, p. 4. 4. Garnault. Sur la structure et le développement de l’ceuf et de son follicule chez les Chitonides. Arch. Zool. Expér. (2), vi. 1888. - Haddon. On the Generative and Urinary Ducts in Chitons. Proc. R. Dublin Soc. new ser. iv. 1885. or 64 LITERATURE OF THE AMPHINEURA 6. Haddon. Report on the Polyplacophora. Challenger Reports, Zoology, Part xliii. 1886. 7. Haller. Die Organisation der Chitonen der Adria. Arb. Zool. Inst. Wien, iv. v. 1882, 1883. 8. Beitriige zur Kenntniss der Placophoren, Morph. Jahrb. xxi. 1894. 9. Heath. The Development of Ischnochiton. Zool. Jahrb. Anat. u. Ontog. xii, 1899: 10. von Jhering. Beitrige zur Kenntniss der Anatomie von Chiton. Morph. Jahrb. iv. 1878. 11. Kowalewsky. Embryogénie du Chiton Polii. Ann. Mus. Marseille, Zool. i. 1883. 12. Loven. Ueber die Entwickelung von Chiton. Arch. f. Naturgesch, 1856. 13. Metcalf. Contributions to the Embryology of Chiton. Stud. Biol. Labor. Johns Hopkins Univ. v. 1893. 14. Middendorf. Beitrige zur einer Malacozoologia Rossica—I. Beschreibung und Anatomie neuer Chitonen. Mém. Acad. Pétersbourg (6), vi. 1849. 15. Moseley. On the Presence of Eyes in the Shells of certain Chitonidae, and onthe Structure of these Organs. Quart. Journ. Micr. Sci. new ser. xxv. 1885. 16. Pelseneer. Recherches morphologiques et phylogénétiques sur les Mollusques archaiques. Mém. cour. Acad. Belg. lvii. 1899. 17. Plate. Die Anatomie und Phylogenie der Chitonen. Zool. Jahrb. Suppl. iv. v. 1897, 1899, 1901. 18. Reincke. Beitriige zur Bildungsgeschichte der Stacheln im Mantelrande der Chitonen. Zeitschr. wiss. Zool. xviii. 1868. 19. Sampson. The Musculature of Chiton. Journ. of Morphol. xi. 1895. 20. Schiff. Beitriige zur Anatomie von Chiton piceus. Zeitschr. wiss. Zool. ix. 1858. 21. Sedgwick. On certain Points in the Anatomy of Chiton. Proc. R. Soe. London xxxiil. 1881. 21%s, Wetistein. Zur Anatomie von Cryptoplax larvaeformis Burrow. Jenaische Zeitschr. xxxvili. 1903. Il. Aplacophora. 22. von Graff. Anatomie des Chaetoderma nitidulum. Zeitschr. wiss. Zool. xxvi. 1875. 23. Hansen. Anatomiske Beskrivelse af Chaetoderma nitidulum. Nyt. Magaz. Naturvid. xxii. 1877. 938 Heath. The Nervous System and Subradular Organ in two genera of Solenogastres. Zool. Jahrb. (Anat. und Ontog.) xx. 1904. 24, Heuscher. Zur Anatomie und Histologie der Proneomenia Sluiteri. Jenaische Zeitschr. Naturw. xxvii. 1893. 25. Hubrecht. Proneomenia Sluiteri. Nied. Arch. f. Zool. Suppl. i. 1881. 26. —— Dondersia festiva gen. et spec. nov. Donders Feestbundel. Neder]. Tijdschr. van Geneesk. 1888. 27. Kowalewsky et Marion. Contribution a histoire des Solenogastres ou Aplacophores. Aun. Mus. Marseille, Zool. iii. 1887. 28. Kowalewsky. Sur le genre Chaetoderma. Arch. Zool. Expér. (3), ix. 1901. 29. Nierstrasz. The Solenogastres of the ‘‘Siboga” Expedition. Résultats des Explorations . . . & bord du Siboga, xlvii, 1902. “sor POrganisation de quelques Néoméniens des Cétes de France. Zool. Expér. (2), ix. 1891. Bu deux Néoméniens nouveaux de la Méditerranée. Arch. Zool. (8) vii. 1901. le développement d’un Solénogastre. Comptes rendus Acad, Sci. exi. 1890. Baitrige zur vergleichenden Anatomie der Amphineuren. Zeitschr. s. Zool. lviii. 1894, . Neomenia, a new Genus of Invertebrate Animals. Bihang K. sk. Vet. Akad. Handl. iii. 1875. en. Studien iiber die Solenogastres. K,. Svensk. Vet. Akad. Handl. iii, and xxiv. 1892, 1893, CHAPTER III THE GASTROPODA CLASS IIL—GASTROPODA, CUvVIER (= PARACEPHALOPHORA, Blainville ; ANISOPLEURA, Lankester). Sup-CLass J. STREPTONEURA. Order 1. Aspidobranchia, Sub-Order 1. Docoglossa. : 2. Rhipidoglossa. : | : Order 2. Pectinibranchia. Sub-Order 1. Taenioglossa. 2. Stenoglossa. Susp-Ciass II. EuUTHYNEURA. Order 1. Opisthobranchia. Sub-Order 1. Tectibranchia. * 2. Nudibranchia. Order 2. Pulmonata. Sub-Order 1. Basommatophora. mp 2. Stylommatophora. Definition. —The Gastropoda, together with the Scaphopoda and the Lamellibranchia, form the branch Prorhipidoglossomorpha of the Mollusca, that is to say, a group in which the gonads are no longer in direct communication with the pericardium, the foot is wholly posterior to the head, and a visceral commissure is present. The Gastropods are a class of the Prorhipidoglossomorpha specially characterised, firstly, by their asymmetrical organisation ; secondly, by their well-developed head ; and thirdly, by their shell, which is formed of one piece and coiled in a spiral, at least in the larval stage. 66 THE GASTROPODA 67 I. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. The three external divisions of the body, head, foot, and mantle are well defined. The head is well developed, and forms a more or less cylindrical mass, but is sometimes flattened. At its anterior extremity is the mouth, and dorsally it bears one or two pairs of tentacles. There is one pair of tentacles in the Streptoneura (Pig. 44), the “ Thecosomata” (Fig. 63), in Phyllirhoé (Fig. 161), Thecucera, the Proctonotidae, the Elysiidae (Fig. 170), the basom- matophorous Pulmonates, and Janella (Fig. 178). There are two pairs in the majority of Opisthobranchs (Fig. 154) and in the stylommatophorous Pulmonates (Fig. 172). The tentacles either are or bear sensory organs; they are contractile, and in the Stylom- matophora invaginable. The right tentacle of both sexes bears an appendage in Buthysciadium (Fig. 126), certain Trochidae (Fig. 130), and Calyptraca. The form of the tentacles varies greatly in different groups. Sometimes they are atrophied, and they may even dis- appear without leaving a trace, as in Olivella, Homalogyra, certain species of Terebra, Pterotrachea (Fig. 143), Limapontia, and Pseudo- vermis (Fig. 169). In the majority of the Bullidae the two pairs of tentacles are enlarged and transformed into a quadrangular shield (Fig. 148), the four corners of which correspond to the tips of the four tentacles. The single pair, much reduced in certain Basom- matophora (Amphibolidae, Otinidae, Fig. 173, Siphonariidae), similarly gives rise to the appearance of a flattened disc on the top of the head. The anterior pair of tentacles in the Pleurobranchidae (Fig. 157) and in various Nudibranchs (Tritoniidae, Fig. 83, Den- dronotidae, Tethyidae, etc.) is transformed into a more or less well-developed frontal veil. Finally, the tentacles are flattened (Narica) ; split (Pyramidellidae, Fig. 137, Solarium, the posterior pair in many Opisthobranchs) ; bifurcate (Janthina, certain Elysio- morpha); or multifid (the posterior pair in many Nudibranchs, Dendronotus, Ancula, Fig. 163). In some species, on either side of the buccal orifice, there is another pair of appendages of greater or less length, known as the labial palps. These are found in Zrochus infundibulum, Ampullaria, Jeffreysia, Choristes, among the Strepto- neura ; and among the Euthyneura in sundry Pulmonates (Glandina,, Limnaea, in which they form a sort of buccal veil, Fig. 107), and in Tectibranchs. In addition to the above-mentioned cephalic appendages of the adult, the following structures should be noted : the cephalic or frontal lobes, situated between the two tentacles and consisting of projections of various shape, in many Rhipido- glossa and in Fossarus; the dorsal median crest in Olivella and Janus; and finally, the pseudopallium, an expansion of the cephalic integument surrounding the whole shell, with the excep- 68 THE GASTROPODA ; tion of the summit of the spire, in Stylifer and allied parasitic forms (Fig. 20). 1. The Foot.—This is primitively and normally formed by a powerful mass of ventral muscles with a more or less elongated Fic. 44, A, Triton variegatum, animal and shell, seen from the right side. a, siphonal notch of the shell occupied by the siphenal fold of the mantle-skirt ; b, edge of the mantle-skirt resting on the shell; c, cephalic eye; d, cephalic tentacle ; e, proboscis or buccal introvert in a state of eversion ; f, foot; g, operculum; h, penis; i, under-surface of the mantle-skirt, forming the roof of the sub-pallial chamber. (From Lankester, after Poli.) B, sole of the foot of Hemifusus tuba, to show, a, the pore of a pedal gland ; b, median line of the toot. (From Lankester, after Souleyet.) ventral creeping surface (Fig. 44, B). But this primitive condition may be modified in relation to different conditions of existence. Thus, among sedentary Gastropods the foot is reduced to a simple discoidal prominence in such fixed forms as Vermetus (Fig. 45) and Fie. 45. Vermetus triqueter, with broken shell; left-side view. co, columellar muscle; f, foot; m, mouth ; 0, aperture of the shell ; op, operculum ; ov, eggs ; pa.s, mantle slit ; p.t, pedal tentacles ; sh, shell. (After Lacaze-Duthiers.) Magilus. In Bathysciadium (Fig. 126) the ventral surface of the foot assumes the form of a sucker, the central portion of which is covered by a thick cuticle, and the circumference is ciliated. In the parasitic forms Stylifer and Thyca the foot is atrophied and is THE GASTROPODA 69 represented only by a small ventral appendage. Among the free- swimming Gastropods we find that in the Heteropods the foot is laterally compressed to form a vertical natatory lobe held upper- most in swimming (Fig. 142), but in Phyllirhoé it no longer exists as a differentiated organ (Fig. 161). In leaping Gastropods, such as Rostellaria among the Strombidae (Fig. 46), the foot is also laterally compressed, and its ventral surface, if not displaced anteriorly, is not flat. In Harpa the posterior part of the foot may be cast off by a process of autotomy. The creeping sole is often divided by a median longitudinal furrow; this may be seen in sundry Rhipidoglossa, e.g. Trochus, Stomatella, Phasianella; and in Taenioglossa such as Littorina and Cyclostoma: in the last-named genus the two halves of the foot contract alternately during progression. A transverse furrow, crossing the anterior half of the foot, is found in the Olividae, Pomatiopsis, many Auriculidae, Otina, and Cyerce. Certain parts of the foot may exhibit special differentiations. (1) Its two anterior angles are prolonged into tentacles in Cyclostrema, Valvata (Fig. 132), Choristes, Olivella, Eolis, etc. (2) The anterior margin of the foot may be furnished with a number of small tactile papillae as in Tvochus, etc., or there may be a small fleshy projection, called the mentum, between it and the mouth, below the aperture of the supra-pedal gland, as in the Pyramidellidae (Fig. 137), Siliquaria, Aclis, Vermetus. In Capulus there is a little projecting tongue-shaped structure above the anterior margin of the foot and below the snout, and in Vermetus two symmetrical tentacles are present in the same position, on either side of the aperture of the supra-pedal gland. (3) In various fossorial Gastropods the whole anterior region of the foot is somewhat elevated above the head, to form the propodium. This region is distinctly separated from the rest of the foot by a constriction in the Harpidae and by a transverse furrow in the Olividae. The propodium is particularly well developed in the Naticidae, in which it is reflected over the whole cephalic region to form a powerful digging organ (Fig. 47). (4) The lateral margins of the foot are expanded to form fins or parapodia in certain Olividae, and particularly in a number of Opisthobranchs, as, for example, in Gastropteron, Acera, etc. ; among the Bullidae, the Pteropods, Aplysia, etc. In Notarchus these two lobes are united above the body in such a manner as to form a muscular sac open in front, but closed behind and at the sides. By forcibly expelling water through the anterior aperture, the animal makes use of the sac as an organ of locomotion. (5) The posterior region of the foot is often separated off as a distinct operculigerous lobe, as may be seen in the Strombidae (Fig. 46), Xenophorus (Fig. 134), and the Atlantidae (Fig. 141). In some Marginellidae there is a posterior dorsal discoid lobe. In Nassa and in allied 70 THE GASTROPODA forms the posterior extremity of the foot bears a pair of tentacles which are sometimes bifurcated, and in Phos there is a single filament in this position. In Pterotrachea the foot terminates posteriorly in along filiform contractile appendage, bearing several annular vari- cosities. In Cymbulia the posterior lobe of the foot also ends in a long whip-like appendage. (6) On the sides of the foot, at about the middle of its height, there is often a ridge, the epipodium, extending from the head to the posterior end of the foot. This ridge is specially well developed in various Rhipidoglossa (Fig. 130, VIII), and may bear appendages of greater or less length, sensory organs, and pigment spots, the last-named, however, showing no trace of the structure of eyes. The anterior part of the epipodium Fic. 46, Rostellaria rectirostris, animal and shell, right-side view. a, snout or rostrum ; }, cephalic tentacle ; c, eye; d, anterior part of the foot ; e, posterior (operculigerous) part of the foot; / operculum ; h’, anterior canal of the shell, occupied by the pallial siphon. (From Lankester, after Adams.) generally forms a cervical lobe, which exhibits a characteristic asymmetry in certain Trochidae. The epipodium is found also in Litiopa, the Rissoidae, Narica,_Janthina, ete., and a portion of it is represented by the cervical lobes in Paludina, Ampullaria, and Calyptraea. The surface of the foot is normally furnished with a large number of unicellular mucous glands, and very often these cells are specially accumulated in invaginations of the integument, distinguished as pedal glands. The most important of these invaginations are—(1) The anterior groove of the foot (Fig. 144, IV), into which open the so-called labial glands: it is often continued into a fairly long canal. This anterior pedal gland is very generally present in the aquatic creeping species of Streptoneura and Opisthobranchs ; it secretes the mucus which lubricates the surface of the foot, and is auxiliary to creeping whether it be on the bottom of the sea or on the Oo —ro THE GASTROPODA 71 surface in a reversed position. (2) The supra-pedal gland opens in the middle line between the snout and the anterior border of the foot. It is most commonly found in sessile Streptoneura (Vermetus, Hipponyx) and in terrestrial forms such as Cyclostoma and the Pulmonata. It is often very deep, and extends for nearly the whole length of the foot: its walls are thrown into folds and are ciliated ventrally in the majority of the Pulmonates. (3) The ventral pedal pore, situated in the middle line in the anterior moiety of the foot, is the aperture of a more or less extensive and often ramified cavity into which the glands of the sole or the pedal glands properly so called pour their secretion (Figs. 44, B; 144, I). This organ is comparable with the byssogenous gland of Lamelli- branchs (Fig. 197), and is found in the following genera: in Cyclostoma, in which it is composed of multiple tubules ; in Cypraea, Hemifusus (Fig. 44), Cassis, and a large number of Rachiglossa and Toxiglossa, viz. in the Fasciolariidae, Turbinellidae, Massa, Murez, the Olividae, Marginellidae, and Conidae (Fig. 144). Its opening was formerly mistaken for an aquiferous pore. (4) The posterior mucous glands may be either dorsal or ventral in position. The former are characteristic of terrestrial Gastropods, such as the Pulmonates and certain Cyclostomatidae, in which they are often surmounted by a simple or multiple horn-shaped protuberance (Orpiella, Plectrophorus, Dermatocera). The ventral posterior glands are simple localisations of the dermic glands, and occur in various Opisthobranchs ; they are not sensibly invaginated in the Pleurobranchidae and Pleuro- phyllidae, but are invaginated and form a long canal in Gastropteron. The product of secretion of the pedal glands in many cases solidifies on contact with the air or water and serves for the suspension of the animal. In some species of Limaz, Litiopa, Cerithidea, etc., it assumes a filamentous form ; and in both sexes of Janthina, whether viviparous or not, it is filled with air-bubbles and forms a float, covering the ventral surface of the foot, beneath which the animal is suspended (Fig. 135). The ventral border of the flattened and fin-like mesopodium of the Heteropoda exhibits, in the male at any rate, an invagination in the form of a sucker (Figs. 141, 142, d’). A similar sucker exists on the ventral or pedal area of some species of Phyllirhoé, but in no Gastropod does the foot exhibit an aquiferous pore, in the sense formerly attached to this term. In some forms, however, and at all events in the Naticidae, there is a system of aquiferous spaces in the foot ; these spaces are completely separated from the circulatory apparatus and serve to distend the foot (Fig. 47, VIII) in the action of burrowing in the sand or mud. The foot often bears on its posterior dorsal aspect a solid sclerite, known as the operculum, which, on the retraction of the animal, serves to close the aperture of the shell. The operculum 72 ITHE GASTROPODA is sometimes borne on a differentiated portion of the foot as in Xenophorus (Fig. 134), Pteroceras (Fig. 75), Atlanta (Fig. 141), or on a distinct expansion, which in Naticaw is reflected over a portion of the shell. An operculum is present in almost all adult Streptoneura, the exceptions being the Docoglossa, the Fissurellidae, the Haliotidae, Fic. 47. Natica josephina, fully expanded ; right-side view. I, exhalant orifice ; II, propodium ; ITT, part of propodium reflected on the shell; IV, tentacles; V, shell; VI, posterior part of foot reflected on the shell; VIII, hind-part of the foot. (After Schiemenz.) Gena, Stomatia, the Proserpinidae, Calyptraeidae, Capulidae, Hip- ponycidae, Cypraeidae, Doliidae, Marginellidae, Harpidae, the majority of the Mitridae, many Cancellariidae and Conidae, Oliva, the Janthinidae, etc. But in all these cases, with the exception of the larva of Stylifer, an operculum is present during development, Fic. 48. Opereculum of Oxygyrus, x 30, external aspect. a.s, attachment surface ; , nucleus of the opercular spire. as may be seen, for example, in the Patellidae, Fisswrella, Calyptraea, Janthina, Carinaria, ete. The naked Streptoneura, Hntoconcha, Enteroxenos, Pterotrachea, Firoloida also have an operculated shell in the larval stage of development. Among the Euthyneura, on the other hand, only SSS <> LACE KZ KE CLTLLL zz SSS ZZ SSE Ulli h ZZ et Z - NS 4 D &z NK LLL G. Sl. Dorsal view of a specimen of Fissurella from which the shell has been removed, and the anterior area of the mantle-skirt has been longi- tudinally slit and its sides reflected. a, cephalic tentacle ; b, foot; d, left (archaic right) gill-plume; e, reflected mantle-flap ; ji, the fissure or hole in the mantle-flap traversed by the longitudinal incision; f, right (ar- chaic left) renal aperture; g, anus ; h, left (archaic right) renal aperture ; p, snout. (After Lankester.) lie parallel to one another and are disposed perpendicularly along one or two faces of a branchial axis. Such a ctenidium is called “pectinate.” In the Opisthobranchs—the only Euthyneura that possess ctenidia—-the ctenidium is a simple flat and projecting tegumentary lamina, transversely folded from its base to its ex- tremity in such a manner that the ridges of one face correspond to the furrows of the other face: such a branchia is called “ plicate.” Among the dibranchiate Aspidobranchs, Plewrotomaria, the Fissurellidae, and the Haliotidae have two rows of pectinations to each ctenidium, one on either face of the branchial axis (Fig. 81). Each ctendium is therefore formed like that of Chiton, Nautilus, or 102 THE GASTROPODA Nucula, and is similarly free to a greater or less extent at its distal extremity. But in Scisswrella (Fig. 54, 1V) the right ctenidium is already degenerate in so far that it has only a single row of filaments inserted directly on the wall of the pallial cavity. The other ctenidiate Aspidobranchs have only a single and equally bipectinate ctenidium, as has also Vulvata (Fig. 132, gi). The two rows of respiratory filaments are equal in the dibranchiate Rhipidoglossa, in the Acmaeidae and Valvatidae, but in the mono- branchiate Rhipidoglossa the dorsal row—that is to say, the row between the mantle and the branchial axis-—is already much reduced, and in the remainder of the Streptoneura this row of filaments has disappeared, as in the right ctenidium of Scissurella, and the single ctenidium is attached to the mantle for the whole of its length (Fig. 99, XVII). The individual branchial filaments are Fic. 82. Anterior part of the body of Acmaea, showing nervous and circulatory systems, dorsal aspect. «.g, abdominal ganglion; ao, aorta; aw, auricle; br.n, branchial (etenidial) nerve ; br.v, branchial vein; ce.g, cerebral ganglion; gi, gill; 7.i.g, infra-intestinal ganglion; mu, columellar muscle ; 0s’, os’, left aud right osphradia ; pa, mantle ; pa.v, pallial vein ; pe.c, pedal cord ; pe.g, pedal ganglion ; p/.q, pleural ganglion ; s.i.g, supra-intestinal ganglion ; te, tentacle ; ve, ventricle. usually simple, but sometimes their surfaces are folded, and again each filament may be in its turn leafy or beset with plications as in the Cephalopoda: this condition is found in Janthina. Each filament is a simple tegumentary projection without any internal endothelial lining. The wall of the blood-space contained in it is formed of connective tissue, thickened and compacted along the borders of the filament, where it forms a supporting structure, specially well developed on the ventral side. By these means the rigidity of the filaments, which are often very long as in Calyptraea, etc., is ensured. The cavities of the filaments are traversed by muscular trabeculae, by whose agency the whole filament may be contracted. In spite of the presence of ctenidial branchiae, there is a certain number of Gastropods in which the oxygenated blood returned to the auricle is not derived from these organs alone. A considerable quantity may come from various other parts of the mantle, or, in = ese Oe ee THE GASTROPODA 103 forms in which the mantle has disappeared as a shell-forming organ, from the dorsal envelope of the body, which in this case serves as an accessory respiratory organ. ‘This phenomenon is to be seen in the Acmaeidae (Fig. 82, pa.v), the Heteropoda, the Pleuro- branchidae, and the Pneumo- dermatidae, these last-named families being naked. In the Pleurobranchidae, the Hetero- poda, and certain Acmaeidae the mantle is no longer fur- nished with accessory respira- tory structures, but in other Acmaeidae, such as Scewrvia, etc., and certain Pneumoder- matidae (Fig. 84, VI, VII), a Fia. 84. Preumonoderma, right-side view, with the head above. I, the expanded proboscis; II, anterior tentacle; III, posterior tentacle; IV, genital (herma- phroditic) opening; V, right fin; VI, Fig. 88. ctenidium ; VII, posterior pallial gill ; Tritonia lineata, dorsal view. VIII, posterior lobe of the foot; IX, I, rhinophore or posterior reno-anal cloaca; X, lateral margin of tentacle; II, dorsal appendage the foot; XI, penial orifice ; XII, (pallial gill); III, right eye; sucker-bearing appendage; XIII, ven- IV, frontal veil; o, genital tral median papilla of the proboscis ; (hermaphroditic) orifice. (After XIV, seat of the mandibles; XV, ex- Hancock.) panded right hook-sack. etenidium, or branchia properly so called, coexists with secondary respiratory organs or pallial branchiae: these lie below the mantle edge in Scurria and on the free surface of the posterior part of the body in the Pneumodermatidae. If the ctenidium is atrophied and disappears altogether, the mantle itself resumes the respiratory function which was previously localised in the ctenidium. ‘This 104 THE GASTROPODA phenomenon may be found both in aquatic species and in forms adapted to terrestrial life, the different modifications of the mantle being as follows :— (1) There may be branchial structures varying in form and position, but not homologous to a ctenidium. In the Docoglossa these pallial branchiae are situated on the internal face of the mantle, as in Patella (Fig. 125, f). In various Gymnosomata (Clionopsis, Notobranchaea, ete.) they are situated on the posterior surface of the body as in the Pneumodermatidae. In the majority of the Nudibranchs they are on the dorsal surface of the body, sometimes localised round the anus as in the Doridomorpha (Fig. 79), sometimes concealed below a fold of the dorsal integu- ment as in Plewrophyllidia and certain porostomatous Doridomorpha, Phyllidia, and Corambe (Fig. 164, g). Or all kinds of accessory branchial formations may have disappeared, and the function of respiration is distributed over the whole free surface of the pallial integuments, as may be seen in various Docoglossa such as the Lepetidae and Bathysciadium ; in Firoloida among the Heteropoda ; in Dermatobranchus, Heterodoris, the Elysiomorpha (with the ex- ception of the Hermaeidae), and Phyllirhoé (Fig. 161) among the Nudibranchs ; in the Clionidae and Halopsychidae (Fig. 156) among the Gymnosomata. (2) An adaptation to a terrestrial life and the pulmonary respiration resulting therefrom is found in very different groups of Gastropods, but the different stages of evolution are best studied in the Streptoneura. In this group certain aquatic and littoral forms, though they possess ctenidia, have acquired the habit of living for a longer or shorter time beyond the reach of the water. This is the case with various species of Littorinw (L. rudis, L. neritoides, ete.), Cremnoconchus, Neritodryas, several Cerithiidae, ete. Consequently certain modifications of the internal surface of the mantle are induced, in the interior of the pallial or respiratory cavity. The filaments of the ctenidium—bipectinate in Neritodryas, but monopectinate in other forms—are often reduced in height and are prolonged more or less indefinitely on the right side of the internal pallial surface to form vascular arborisations, as may be seen in the semi-aerial species .of Lattorina (Fig. 85, ) and in Cremnoconchus. Finally, the ctenidium disappears altogether, and with it the hypobranchial gland and the efferent branchial sinus, and the venous blood of the rectal sinus is con- ducted to the afferent cardiac vein (corresponding to the efferent branchial vein) by the system of arborisations that extends over the whole roof of the pallial chamber. Such is the case in Cerithidea obtusa, which retains only the vestiges of the anterior extremity of the ctenidium. In many aerial Gastropods the ctenidium has totally disappeared and the roof of the pallial cavity THE GASTROPODA 105 is permeated by a rich vascular network (Fig. 86) in which the blood is oxygenated. In this manner the respiratory pallial chamber is transformed into a pulmonary cavity or lung, whose vascularised surface is irrigated by the blood derived from various parts of the body. The “lung” of Gastropods, then, is not a spongy organ, but a cavity x i strictly homologous to the pallial cavity. The pulmonate Gastropods exhibiting this structure are polyphyletic, that is to say, they belong to several different groups. Among the Strepto- neura we find three families of Si! ut qu Fic. 85. Littorina littorea, male, removed from its shell; dorsal aspect ; the mantle-skirt cut along its right line of attachment and thrown over to the left side of the animal so as to expose the Fic. 86. organs of its inner surface. a, anus; br, ctenidium ; ¢, heart; h, liver; i, intestine; m.c, Roof of the pallial cavity (lung) of columellar muscle (muscular process grasping Limax. Ventral aspect. I, cloacal (reno- the shell); p, penis; p.br, osphradium; 7, anal) orifice ; II, pneumostome ; III, reno- kidney ; 7’, aperture of the kidney ; t, testis; v, pericardial orifice; IV, rectum; V, renal stomach ; v.d, vas deferens ; v.d’, the groove-like duct; VI, kidney; VII, heart - ventricle ; part of latter; «, vascular prolongations of the VIII, pericardium (eut open); IX, heart- etenidial leaflets; y, hypobranchial gland. auricle ; X, ramifications of the pulmonary (From Lankester, after Souleyet.) vein. (After Leidy.) Rhipidoglossa, viz. the Helicinidae, Proserpinidae, and Hydrocenidae ; and three sub-groups of Taenioglossa without probosces, viz. the Cyclophoridae, Cyclostomatidae, and Aciculidae; and among the Euthyneura all the Pulmonates proper, including the aquatic as well as the terrestrial forms. In one family only of the Strepto- neura, the Ampullariidae, is the ctenidium preserved at the same time that a pulmonary cavity is present. In this family the pallial cavity is divided by an incomplete septum into a lung and a branchial cavity, the former being situated to the left of the ctenidium. The animal is therefore able to breathe by its gill in the water, and by its lung when out of the water, the air being 106 THE GASTROPODA admitted by a very extensible pallial siphon. In the pulmonate Streptoneura the pulmonary chamber retains the whole of the primitive opening of the pallial cavity ; in the Euthyneura, on the contrary, the opening of the lung or pneumostome is much reduced by the fusion of a large extent of the mantle border with the neck of the animal, a fusion that leaves only a minimal but extensible posterior aperture (Fig. 177, V) in the neighbourhood of the anus. ‘This disposition allows of the blood, on its arrival at the lung, being carried round a more or less annular circumpulmonary venous sinus. In the Oncidiidae the lung is somewhat rudimentary, being reduced to arborisations ramifying among the lobes of the kidney. In other Pulmonates such as Ancylus and the Vaginulidae (Fig. 87) the reduction of the lung is carried to the point of complete disappearance. Finally, there is a family of Pulmonates in which, instead of a vascularised lung, there is a pulmonary | p i °. ca j oun pe Fic. 87. Vaginula occidentalis, right-side view, with the mantle partially removed on this side. an, anus ; aw, auricle ; 0.f, female orifice ; 0.7, renal opening in the rectum ; 0.7.p, reno-pericardial pore ; 0.7.u, Orifice of the kidney in'the ureter ; p, foot ; pa, mantle; pe, pericardium ; 7, kidney ; re, rectum (the dotted line shows the direction of the intestine); ten, tentacles; ur’, ur’, primary and secondary ureters ; ven, ventricle. chamber continued into numerous tubules which penetrate into the surrounding blood sinuses: these tracheate Pulmonates are the Janellidae (Fig. 90, tr). A large number of Pulmonate Gastro- pods, while preserving their aerial respiration, have returned to an aquatic life; such are the Basommatophora (Limnaeidae, ete.). Among these the marine genera Amphibola, Siphonaria, and Gadinia ; Limnaea abyssicola, an inhabitant of deep lakes; and Planorbis noutilus, have a pallial pulmonary cavity which, instead of being filled with air, may temporarily or continuously be filled with water, as in the larvae of aquatic Pulmonates. Here we see a return and readaptation to aquatic respiration, but for all that the ctenidium does not reappear, a fact which illustrates the irreversibility of evolution. But in these cases respiratory pallial outgrowths or secondary branchiae may be formed near the opening of the pulmonary cavity or even in its interior. Such is the contractile extrapulmonary tegumentary appendage at the base of which the anus opens in Planorbis (as this is a sinistral ——— Ss Pym tf & THE GASTROPODA 107 genus the appendage is to the left of the pallial aperture). In Planorbis corneus (Fig. 89, g) there is a single respiratory lobe, with a richly vascularised surface, and in Ancylus there is a_ similar Fic, 88. Diagram of a sagittal section of Patella vulgata. br.a, branchial afferent vessel (artery) ; br.v, branchial efferent vessel (vein); b.v, blood-vessel ; c, muscular substance forming the root of the foot ; cor, heart within the pericardium ; e, mantle-skirt ; f, papilla of the larger kidney ; g, anus ; 7, smaller kidney ; k, larger kidney ; 1, pericardium ; 7, liver; 0, mouth ; od.m, muscles and cartilage of the odontophore; p, snout; y, intestine in transverse section; 7, radular or lingual sac ; rd, radula; s, lamellated stomach ; ¢, salivary gland; uv, duct of same; v, buccal cavity; w, gonad. (After Lankester.) structure, but the lung has disappeared. In Pulinus, including the sub-genera Jsidora, Pulmobranchia, ete., and in Miratesta there is a folded branchia. In Siphonariu the long plicated branchia which extends across the interior of the roof of the pulmonary Fic. 89. Planorbis corneus, removed from the shell; anterior view. «@, anus; jf, foot; g, gill; k.o, renal aperture; m, mouth; os, osphradium; pu, mantle; pn, pneumostome; si, pulmonary siphon. cavity, between the kidney and the rectum, is of the same character. This branchia is situated more posteriorly and to the right than the ctenidium of monobranchiate Gastropods (Fig. 174). The diverse characters of the respiratory apparatus of Gastropoda may be advantageously summed up in the following table :— THE GASTROPODA 108 ‘snayynou siquounjig “njoossliqn nanu \ unt ‘muy ‘onupoy “npoquydup i “‘punjauorgy ‘aepl{noloy “ovprzeutoys | -opoXg ‘avpitoydopaAg ‘aeprutdiosoag ‘geptumooo1pAP] OVpluLorpey] “eyeuow[ud | ‘sasdouoyg ‘empourrqrpuy ‘eepr{[oyed J ‘ayohsdoymyT ‘aworg “nprojoug — \ ‘snphoup : , . ‘ . * Suny jNoyyIAa pue T]1s Atossaoov YALA ‘ppsomnuy ‘snuyng ‘snoutoo siqsounjg [ts Laossaoov Arvuowypndesyxe | , ’ ? . Spagetti ae a, ‘muinuoydgs [tS aossooow Areuowynderzat f P Te “pynurbDn 4 : : : : ; : ‘ : : * (Suny ou) Arequournsey : : Suny oenbe yyLM - AILYZRAIG [VIC - | : : * Suny ev Aq ATeatsnpoxe Suny [eee yy menyaWegneNT ' - g]]13 Arosseo0e a1 | - ; : c ; : * onvube ony * p18 Arosseoov qnoyytar | ‘soyurplyg ‘eydaowuorsétg ‘oepryeday f prsojyndupr ‘aUplpRULlepouoMNeug puv ovplovmMoy ‘auplovlmuloy olos pUR sepItToUBAqoINs] gy *AT[RIOMOS VITOURIGT}OIT, ‘AT[RIOUAS BITPOWeIqrUl4oeg “essoLnoprdiyy “yan A “D)/OINSsv9y ‘OVPIJOIV]Y PUB 12.0W07}0.N9)T ‘ORPI[[OANSST . . . . . . . . . . . . . Suny 2 Yat | — , : : ; : , : : : A Suryyveiq ails Arossao0v YIM ered pur perpitsi9 : . 4 : : [Ls Atossa00e qnoyyras } + “Es * papyULi : : : , oyeurqoodououl h [ES yu dadAvL WOLFVULZoEd [LAZUOA THIN gavurqoadiq poqeurqoad | ‘eIptueqzoou0Ul * — suorjeurjood yenba z yyt\ J ee : : : ; : ; : oqwurjood } Suryyeorq [erpius}) -OUOUL WNIPItoyo USI YT | rerpruoyorq * Jaywals WNTptteyd qe] 18 | exprayo aieneloeiea nie | prae}o} * vIprmoyo TeotajoummAs Z TAIN THE GASTROPODA 109 3. Excretory Organs.—In the Gastropoda the kidneys are the essential organs of excretion, but the pericardial glands serve as accessory excretory organs, as also certain parts of the body in which the products of excretion are collected, forming veritable accumulative kidneys. (1) The kidneys are originally paired, as in all other Mollusca, and a single pair is found (Figs. 55, II, XIII; 81, f, 2; 91, 127) in all the Aspidobranchia, except the Neritacea, including the Neritidae and allied families. These two kidneys open one on each side of the anus, but they do not retain their primitive symmetry in any Gastropod, and although they are independent of one another, the topographically left kidney is rudimentary, and that of the right side alone is functional in almost every case. Fic. 90. Transverse section of the lung of Janella. k, ureter; pa.c, pallial or pulmonary cavity ; po, oe si, blood sinus ; tr, ‘‘ tracheae” or diverticula of the pulmonary cavity. (After late.) In the Neritacea (Neritidae, Titiscaniidae, Helicinidae, Hydro- cenidae, and Proserpinidae) and in all the Pectinibranchia and Euthyneura the topographically right kidney no longer exists. In Paludina the two kidneys coexist during development, but in the adult that of the topographical right side has disappeared. As regards the position of these organs, their primitive situation is wholly within the visceral mass (Docoglossa, Fig. 88, /), and their migration outside the visceral mass is a specialisation which begins to show itself in the Rhipidoglossa—at any rate, in the case of the left kidney (Fig. 127)—and is completely realised in the case of the single kidney in other Streptoneura and Tectibranchia, in which the excretory organ is more and more localised in the mantle (Figs. 75,k; 63,7). The kidney is always a dorsal organ, situated in the neighbourhood of the pericardium, with which it communicates by a ciliated aperture. In the detorted Aspidobranchs (Fissurellidae), however, the very rudimentary left kidney has lost this pericardial 110 THE GASTROPODA communication. Hlysia is exceptional in that the kidney is placed below and partly surrounds the pericardium, and the reno-pericardial orifices are multiple, some ten being present (Fig. 92). As a rule the external opening of the kidney is situated near the anus (Figs. 81 and 88), and sometimes the two open together into a sort of common cloaca, as may be seen in the Gymnosomata (Fig. 84, IX) and in certain Pulmonates, such as Limaxz (Fig. 86, I), the Onci- diidae (Fig. 59), and Vaginula (Fig. 87), but not in /. willeyi. In rare cases, however, such as the Nudibranch Janus, the excretory aperture is distant from the anus. The external renal orifice is borne on a papilla in various Aspidobranchs with two kidneys (Fig. 88, /), but is a simple slit, shaped like a button-hole, in the majority of Pectinibranchia (Fig. 99, IV) and Tectibranchia (Fig. 154, 0). Among the Pec- tinibranchs, however, Paludina kesv Fig, 91. Diagram of the two renal organs of Patella, to show their relations to the rectum and to the pericardium. /f, papilla of the larger kidney; g, anal papilla, with rectum leading froin it; h, papilla of the smaller kidney, which is only represented by dotted outlines ; J, peri- cardium, indicated by a dotted outline (at its right side are seen the two reno- pericardial pores) ; ff, the sub-anal tract of the large kidney given off near its papilla and seen through the unshaded smaller kidney ; ks.a, anterior superior lobe of the large kidney ; ks./, left lobe of same; ks.i, inferior sub-visceral lobe of same; ks.p, posterior lobe of the Fig, 92. Elysia viridis, heart and kidney, dorsal aspect. (somewhat schematic). I, ventricle of heart; II, external renal pore; III, auricle; IV, kidney; V, the various reno-pericardial pores on the left side (there are five such pores on the right-hand side); VI, the ventral right kidney. (After Lankester.) reno-pericardial pore ; VII, pericardium. and Valvata are exceptional in possessing an ureter which opens at the edge of the mantle. The same arrangement is found in many Pulmonata, especially in the Stylommatophora, in which an elongated ureter opens alongside of the anus at the margin of the pneumo- stome (Fig. 86, V). As regards its structure, the kidney in its simplest form is a sac lined by a secretory epithelium. By the infolding of its walls, the cavity of the sac is subdivided and the organ acquires an alveolar structure of spongy appearance, but in various pelagic forms it again becomes more or less tubular and transparent, é.g. in the Heteropoda (Fig. 141, q), in certain “ Pteropoda” (Fig. 60, &), THE GASTROPODA III in Phyllirhoé (Fig. 161, 1). Asa rule the kidney is a compact mass, without external projections, but it is divided into two lobes in Stenoglossa in general and also in some Taenioglossa, viz. Paludina and Cypraca. In a fairly large number of Nudibranchs (Dorido- morpha, Janus, etc.) the kidney is divided into ramifications which extend between the visceral organs of the greater part of the body (Fig. 79, XIII). In sundry Pectinibranchs—e.g. Littorina—there is a “‘nephric gland” which opens into the kidney, and consists of ciliated canals surrounded by conjunctive tissue. In addition to its excretory function the kidney may also serve for the conduction of the genital products. Thus in all Gastropoda with two kidneys, that is to say, in all the Aspidobranchia (Pleurotomaria, Trochus, Fig. 55, Fissurellidae, etc.) except the Neritacea, the gonad opens into the right kidney by a papilla situated near the external renal aperture. (2) The pericardial glands in the Aspidobranchs and Valvata are placed on the external walls of the auricles. In other forms they are localised on the internal wall of the pericardium, as in Littorma and Cyclostoma among the Pectinibranchs and in the Pleurobranchidae and Nudibranchia among the Opisthobranchs, or they are situated within the pericardium on the origin of the aorta, as in Aplysiidae. (3) Various excretory products may be accumulated in plasmatic cells (known as the “cells of Leydig”) in the conjunctive tissue of different parts of the body. This phenomenon is particularly common on the walls of arterial trunks, and may be seen in the caudal artery of Carinaria, and on the wall of the arterial trunks of certain Streptoneura and many terrestrial Pulmonates, in which calcareous concretions are found in the perivascular conjunctive tissue. The different forms of excretory apparatus and the special function of each can be revealed by the method of physiological injections. 4. Nervous System.—With the exception of the endoparasitic Entoconchidae, all Gastropods possess a well-developed nervous system in which the same cerebral, pedal, pleural, visceral, and stomato-gastric nerve-centres, and the same connectives and com- missures, are to be found as in other Molluscs. But the special character of the Gastropod nervous system is the asymmetry of the visceral centres and of the nerves arising from them, an asymmetry resulting from that of the visceral organs themselves. The most primitive form of nervous system is characterised, as in the Poly- placophora, by the absence of concentration in the ganglia. The cerebral centres in the Rhipidoglossa are situated at the sides of the oesophagus and are united by a long commissure which is itself ganglionated (Fig. 94). The pedal centres in Aspidobranchs (Fig. 94, pe.c), Paludina, and some other taenioglossate Pectinibranchs 112 THE GASTROPODA such as Cyclophorus and Cypraea, have the form of long ganglionated cords with multiple commissures or anastomoses. The pleural ganglia are but slightly differentiated in Pleurotomaria, in which genus they are placed on the dorsal pedal connective, at a nearly equal distance from the cerebral and pedal centres (Fig. 94, pl.c). In forms in which they are better developed, the pleural ganglia are still in intimate contact with the anterior part of the pedal centres, and there are two long connectives, the cerebro-pleural and the AMIN xv vin Fic. 93. Central nervous system of Patella vulgata, dorsalaspect. I, tentacular nerve ; II, left cerebral ganglion ; III, cerebro-pedal connective ; IV, cerebro-pleural connective ; V, left otocyst; VI, left osphradium ; VII, pallial nerve ; VIII, anterior part of the pedal cord ; IX, supra-intestinal ganglion ; X, pedal cords (their posterior endings are not drawn); XI, abdominal ganglion ; XII, pleural ganglion ; XIII, otocystic nerve ; XIV, stomato-gastric ganglion ; XV, optic nerve ; XVI, labial commissure ; XVII, cerebral commissure, cerebro-pedal, on either side of the digestive tube, the pleuro-pedal connective being, on the contrary, very short. This arrangement is known as the ‘‘ hypoathroid,” and is found in Aspidobranchia (Fig. 93) and several Taenioglossa, viz. Ampullaria, Cyclophorus, and Nassopsis. As a result of specialisation we get the ‘ dystenoid ” condition, in which the cerebral centres are approximated and the pleural ganglia are shifted nearer to the cerebrals, so that the pleuro-pedal connectives are elongated (Fig. 123, A). Finally, in the “ epiathroid ” condition, the pleural centres are either in contact or are fused with the cerebrals (Fig. 123, B), as is the case in the THE GASTROPODA 113 majority of the Pectinibranchia, including the Heteropoda, and in various Bullomorpha (¢.g. Actaeon, Fig. 57) and the thecosomatous “ Pteropods” among the Opisthobranchia. At the same time the pedal ganglia are concentrated anteriorly to form more or less globular masses (Fig. 123, C, pe.g). Primitively the visceral commissure is somewhat extensive, and its ganglionic centres are tolerably far removed from one another, as may be seen in all the Streptoneura and the less specialised Euthyneura (Figs. 94, 57, etc.). These ganglionic centres are normally three in number: one is median, and is called the abdominal or the visceral ganglion proper (Fig. 93, XI); two are lateral, placed right and left on correspond- ing sides of the visceral commissure. The ganglion on the morphologically left side may be but slightly developed or may not be differentiated at all, as, for example, in monobranchiate KRhipidoglossa. In con- sequence of the torsion of the visceral mass of Gastropoda, the visceral commissure is normally twisted into a figure of eight ; that is to say, the right moiety with the visceral ganglion is situated above the alimentary tract and is displaced to the left, the left moiety remains below the alimentary tract, but is inclined to the right (Fig. 57). Hence the names supra-intestinal and infra-intestinal are respectively given to the two moieties and to the ganglia borne on them (Fig. 123). This disposition of the visceral com- missure is common to all the Streptoneura (as the name of the group signifies), in- cluding the Heteropoda and all the forms formerly called “ Orthoneura,” i.e. forms in which the visceral loop was believed to have never been twisted ; it may also be clearly Pleurotomaria, nervous system, dorsal aspect. br.g, branchial ganglion ; ¢.¢c, cerebral commissure; ¢.p.c, cerebro- pedal commissure; c.pl.ec, cerebro - pleural commissure : i.i, infra-intestinal portion of the visceral commissure; la.c labial commissure ; of, otocyst ; pa.n, pallial nerve; pe.c, pedal cord; pl.c, pleural centre ; pl.p.c, pleuro-pedal connec- tive; s.i, supra-intestinal part of the visceral commissure ; st.g, stomato-gastric ganglion. (After F. M. Woodward.) seen in the more archaic Euthyneura (which, as has been explained above, are detorted Gastropods), for instance, in various Bullomorpha (Actacon, Fig. 57, Scaphander, Bulla, ete.), and in Chilina. But in the three last-named genera the detorsion of the visceral commissure is already manifest, that is to say, its supra-intestinal moiety shows a tendency to return to the lower side of the alimentary tract, and its sub-intestinal moiety tends to return to the left side. This detorsion of the visceral commissure is complete in the rest of the Euthyneura, as may be 8 114 THE GASTROPODA seen in the Opisthobranchia (Figs. 95 and 159) and the Pulmonata (Figs. 96 and 97). Further, in all the Euthyneura but those which are the most primitive from this point of view, such as the Bullomorpha and Aplysia among the Opisthobranchs, the Auriculidae, Chilina, and Latia among the Pulmonata, there is a tendency to the approximation of ‘the ganglionic centres and at the same time a shortening of the visceral commissure. This is carried so far that the ganglia come into contact and form a chain of several united nerve- centres between the pleural ganglia (Fig. 97). When it has reached this stage of evolution the whole nervous system is Fia. 95. Nervous system of Aplysia (dorsal aspect), as a type of the long-looped Euthyneurous con- dition. The untwisted visceral loop is lightly shaded. ab.sp, visceral ganglion which repre- sents the abdominal + the supra- intestinal ganglia of Streptoneura, and gives off the nerve to the Fic. 96. osphradium and another to an Latia neritoides, central nervous system, dorsal unlettered so-called ‘‘ genital ” ganglion; ce, cerebral ganglion ; 0, osphradium ; pe, pedal ganglion and double pedal commissure ; pl, pleural ganglion (the stomato- gastric commissure and ganglia are omitted). (From Lankester, after Spengel.) view (the buccal mass is indicated by a dotted line). bu, buccal mass; co.vi, visceral commissure; g.a, abdominal ganglion; g.bu, buccal ganglion; g.ce, cerebral ganglion; g.i.i, infra-intestinal ganglion ; g.pa, parietal ganglion; g.pe, pedal ganglion and double pedal commissure; g.p/, pleural ganglion ; g.s.i, supra-intestinal ganglion; n.p.i, nerve of the inferior pallial lobe ; osp, osphradium ; rad, radula. concentrated in the cephalic region round the anterior part of the oesophagus, and finally all the ganglia—cerebral, pleural, pedal, and visceral—are intimately united and localised on the dorsal surface of the oesophagus, a condition which may be seen in Pleurobranchus and sw) = Zi _ | a THE GASTROPODA 115 the majority of the Nudibranchs (Fig. 159), and is pushed to an extreme in Z¢thys. In this case the pedal and visceral commissures are destitute of ganglia or nearly so on the ventral part of their course. It is only in the thecosomatous “ Pteropods” that the concentration of the ganglia takes place ventrad of the oesophagus, the dorsal part of the nervous system being formed only by a long cerebral commissure (Fig. 60, 2.s). In all Gastropoda there is an infra-oesophageal stomato-gastric commissure. In the more primitive forms (Aspidobranchia, Fig. 94) the stomatogastric system originates from a sub-oesophageal labial commissure, but in other Gastropods from the cerebral ganglia. Normally this stomato-gastric commissure bears a pair of Pe bs 0 Fic. 97. Nervous system of Limnaea stagnalis (dorsal aspect), as a type of the short-looped Euthy- neurous condition. The short visceral loop, with its three ganglia, is lightly shaded. ab, abdominal ganglion (+infra-intestinal); ce, cerebral ganglion; 0, osphradium; pe, pedal ganglion ; pl, pleural ganglion ; sp, parietal ganglion or left visceral ganglion ; opposite to it is the visceral ganglion of the right side, or supra-intestinal ganglion, which gives off the long nerve to the osphradium (in Planorbis, the osphradium and supra-intestinal ganglion are on the left side). (From Lankester, after Spengel.) ganglia situated below the radular caecum at the point of origin of the oesophagus from the buccal bulb (Fig. 146, g.bu). As regards the innervation of the different organs, the cerebral centres supply the head and buccal lips, the tentacles and other cephalic appendages, the eyes and the otocysts. The pedal ganglia send out nerves to the whole mass of the foot, including the epi- podium, and to a portion of the cervical region. The mantle and the organs connected with it are primitively—as in the Streptoneura —almost entirely innervated from the pleural centres, but they are also partially innervated by nerves which issue from the visceral commissure and the supra- and infra-intestinal ganglia, and pass to the ctenidia and osphradia. The last-named nerve-centres take a preponderant share in the innervation of the mantle in Euthyneura, 116 THE GASTROPODA especially in the Pulmonata, in which group the pleural ganglia scarcely ever give off any nerves. In all the Euthyneura except Actacon (Fig. 57), Chilina, and Latia (Fig. 96) the infra-intestinal ganglion is fused with the abdominal (Fig. 97, ab) in such a manner that the latter appears to participate in the innervation of the mantle—for instance, in the innervation of the inferior pallial lobe and the pallial branchia of the Planorbidae—and the pallial nerves issuing from the left moiety of the visceral commissure originate from a special and newly-developed ganglion, viz. the parietal ganglion of the Pulmonata (Figs. 96, g.pa; 97, sp). As regards the viscera, the heart, the kidneys, and the gonad receive their essential nerve supply from the abdominal ganglion, the alimentary canal receives its nerves from the stomato-gastric centres, and these latter nerves sometimes exhibit accessory ganglia on specialised parts of the alimentary tract, such as the gizzard of Tectibranchia, ete. 5. Sense Organs.—In addition to sensory cells scattered over the whole surface of the body, Gastropods possess special sensory organs usually designated by the names rhinophore, osphradium, otocyst, and eye. While the whole surface of the integument is sensitive, tactile sensibility is more particularly localised in the anterior regions of the body: in the head, in the margin of the foot (in marine Streptoneura the whole ventral surface of the foot exhibits a fine nervous network), and in the regions of the body specialised to form tactile appendages of various shape and situa- tion. Such are the cephalic tentacles—especially the anterior pair in quadritentaculate Euthyneura; the labial palps (Fig. 117, #”), which bear a row of tubercles in some Pulmonates; the pedal tentacles of Vermetus (Fig. 45, p.t); the epipodial tentacles of the Rhipidoglossa (Fig. 130, XI), which have ciliated sensory organs at their bases; and the pallial appendages, such as the dorsal papillae of the Nudibranchs. The Rhinophores, or olfac- tory organs, as they are called, Fic. 98. are likewise constituted by cnppaa (damineo) navies, sahtside view. et, the cephalié tentacles, espect- orifice; il, inferior pallial lobe; m, mouth; m.o, ally by the posterior pair in male one; hy hinophore (Haneork’s 0); <2 the quadritentaculate Buthy- neura. The whole surface of these tentacles is covered by little ciliated papillae, giving them a silky appearance, in many [Khipidoglossa, ¢.g. Scisswrella, Haliotis, Trochus, Gena, Molleria, Cyclostrema, Neritina, in Caecum (Fig. 68, te), ete. The olfactory nerve divides into many ramifications which end on the surface of the tentacle in olfactory sensorial cells. In many a cael oe bs . =e THE GASTROPODA 117 forms, such as the terrestrial Pulmonata, the majority of the naked Opisthobranchia, Cyclostrema, Xenophorus, and all the Gastropods without an osphradium, these ramifications issue from a rhinophoric ganglion situated at the extremity of the olfactory nerve. The olfactory end-cells are frequently localised in a tract of higher epithelium at the extremity of the tentacle, or in a furrow ex- cavated in the surface of the tentacle (Pyramidellidae, Fig. 137, fe, Solarium), and in many Opisthobranchia the sensitive surface of this olfactory prominence or cavity is increased by the development of numerous parallel pleats or foliations (Fig. 163, ¢). In terrestrial Pulmonates—e.g. Helix—the sense of smell does not extend for a greater distance than half a metre, and then only in the case of exceptional odours ; the most usual distance at which odours are recognised is from one to three centimetres, but certain carnivorous marine Streptoneura—for example, Nassa—are able to recognise odours at a distance of more than two metres. The Osphradia are the sensory organs of the pallial or respiratory cavity, and exist in diverse forms. ‘There is a pair of osphradia in all the bictenidiate Aspidobranchia and in the Docoglossa : in all other Gastropods the osphradium is unpaired. It disappears only in some terrestrial Streptoneura (Helicinidae and Cyclophoridae), in the Pleurobranchidae, the Nudibranchia, and all the Stylommato- phorous or terrestrial Pulmonates ; nevertheless in all the stylom- matophora (Limaz, Helix, etc.) the osphradium is present during development and during the first few days after hatching. To sum up, then, the osphradium is absent in aerial species or in aquatic forms devoid of a respiratory cavity, and when it is absent a rhino- phoric ganglion is present. An osphradium consists of a specialised and usually elevated and ciliated region of the epithelium, in which there is an accumulation of sensory cells. In the ctenidiate Gastro- pods the organ is situated on the outer side of the ctenidium (Fig. 99, XVI). The most simple form of osphradium is seen in the Strepto- neura, in which it is not differentiated into a definite organ, but is merely a localisation of neuro-epithelial cells on the course of the branchial nerve along the two supporting margins of the ctenidia, as in the Fissurellidae, or on an osphradial nerve running along the support and formed by a differentiation of the branchial nerve, as in other Rhipidoglossa, or again on a ganglion placed on the extremity of this special nerve at the base of the ctenidium. In other Gastropods the osphradium becomes a distinct terminal organ at the base or at the left (external) side of the single ctenidium, athwart the current of water which supplies the latter organ. The osphradium may persist in this place after the disappearance of the ctenidium, but only in aquatic forms such as the Patellidae, Gymnosomata, and basommatopherous Pulmonates. In the most archaic Taenio- glossa, viz. Paludina, Littorina (Fig. 85, p.br), Cyclostoma, Vermetus, 118 THE GASTROPODA the osphradium is a filiform epithelial ridge, overlying a nerve or ganglionated cord. Then, as a result of specialisation and multi- plication of its surface, the two sides of the ridge become garnished with pectinations, so that the organ acquires a deceptive resemblance to a branchia. This condition may be seen in the more specialised Taenioglossa, such as Natica, Cerithium, and the Strombidae, in which the pectinations themselves are arborescent ; in Cypraca, in which the xin vil Fig. 99. A male Hemifusus tuba, removed from its shell and with the pallial cavity opened. I anus ; II, hypobranchial gland; III, spermiduct; IV, renal pore; V, heart, in the opened pericardium ; VI, testis; VII, liver; VIII, oesophagus ; IX, columellar muscle; X, the spermi- duct cut through, with the mantle; XI, penis; XII, foot; XIII, proboscis; XIV, head; XV, pallial siphon ; XVI, osphradium; XVII, ctenidium. (After Souleyet.) organ is trifid; and in the Rachiglossa (Fig. 99, XVI) and Toxiglossa. In the Euthyneura the osphradium is a simple epithelial projection of circular or elongated shape, lying above an osphradial ganglion, into which it is sometimes invaginated—viz. in certain basommatophorous Pulmonates—and the invagination is bifurcated in Limnaea. In the last-named genus the osphradium is situated in the pallial cavity, to the left of the ctenidium ; in the Basommatophora it is close to the pneumostome ; in Siphonaria (in which the lung is filled with water) Piick? & el i i i ee De. THE GASTROPODA 119 it is inside the pulmonary cavity (Fig. 174); in other forms with an aerial lung it is external (Fig. 89, 0s). Cyathiform bodies or gustatory bulbs, formed of taste-cells, are found on the lateral and ventral surfaces of the buccal cavity in sundry Rhipidoglossa, and at the sides of the buccal aperture in some Heteropoda. Analogous bodies have been found on the epipodial tentacles of Rhipidoglossa. The Otocysts, or statocysts, are hollow spherical vesicles, whose internal walls are lined by a ciliated epithelium containing sense- cells. These vesicles contain a liquid secreted by their epithelial walls, and in this liquid are calcareous auditory concretions of -erystalline structure. There is a single large and spherical con- cretion or otolith in the more specialised Tectinibranchs and in a few adult Opisthobranchs (Lobiger, the Elysiomorpha, Hedylidae, Pseudovermis, Fiona, and sundry Eolidomorpha), (viz. the Tergipedi- nidae, Capellinia, Eolidiella, Eolis aurantiaca and E. olivacea, Galvina piecta and G. cxigua). There are numerous and usually ovoid and elongated concretions, called Otoconia, in the Aspidobranchia (except Bathysciadium), in some of the less specialised Taenioglossa, such as Paludina, Ampullaria, Cyclophorus, Valvata, Nassopsis, and the majority of the Melaniidae, and in the Euthyneura in general, with the exception of the Opisthobranchs mentioned above. Oto- conia sometimes coexist with an otolith in certain Cerithiidae, Twrritella, Doto, and Oncidium, but in all larvae there is only a single otolith (Fig. 116, A, III). Otocysts are absent in the adult Vermetus and in some Janthina. In creeping Gastropods the otoliths are situated in the foot, in the neighbourhood of the pedal ganglia (Fig. 93, V), and are often adherent to these nerve-centres. In swimming Gastropods, such as Heteropoda, Phyllirhoé, and Glawcus, they show a tendency to approach the cerebral centres, and the same tendency may be seen in the majority of Nudibranchs. In all cases the otocysts are innervated from the cerebral ganglion, as may be most clearly seen when they are at some distance from the pedal ganglia (Figs. 123, C, of; 142, uw; 146, of). The neuro-epithelial elements are concentrated in a macula acustica, placed opposite the expansion of the otocystic nerve, in the otocyst of Heteropoda. Cephalic eyes exist in almost all Gastropods, and there are, in addition, pallial eyes in certain Oncidiidae. The two cephalic eyes are situated on the tentacles, in the Euthyneura on the posterior pair of tentacles. In the Streptoneura these eyes are placed at the outer side of the base of each tentacle, and are borne on tubercles (Fig. 44, A, c) which may fuse with the tentacles, and thus, in a number of instances, give the eyes the appearance of being placed half-way up the tentacles, as may be seen in the Potamidae among the Ceri- thiidae, in Cypraea, many Rachiglossa, certain species of Conus (Fig. 144, V), and Plewrotoma: in the last named they are very near the 120 THE GASTROPODA extremities of the tentacles in the sub-genera Drillia and Clavatula. The ocular tubercle is better developed than the tentacle in the Strombidae (Fig. 75 #), and finally the tentacle may be aborted and the eye appear to be placed on its summit (Terebellum). It is really placed on its summit in A ssiminea and in the adult ter- restrial Pulmonates or Stylom- matophora (Figs. 172, 177), but during the development of these forms it is some distance removed from it. In the basom- matophorous Pulmonates, and ‘ = in the Opisthobranchia the eye 46. 160, is at the base of the tentacle, Axial section of the eye of T’rochus umbilicaris and in the latter group some- 1 crrstaline ne; Hi, retinas TH, eptie nerVe; times at some distance from it and often buried beneath the integuments, especially in the Nudibranchia. As regards its struc- ture, the Gastropod eye typically consists of a retina or invagination of the tegumentary epithelium, in which sensory and pigment cells may be distinguished. The former are known as retinophora and are colourless ; their free extremities are much contracted, and their opposite extremities are continuous with prolongations of nerve- fibres. The latter, or retinulae, have expanded free extremities, and surround the retinophora. As these two kinds of cells arise by the differentiation of normal epithelial cells, they may not in all cases possess sharply defined characters, and may pass insensibly into one another: the colourless cells actually appear to be absent in the eyes of certain Opisthobranchia that are buried beneath the integuments. The visual organ is completed by accessory structures, of cuticular nature, secreted by the epithelium, and are more dis- tinct from one another in proportion as the eye is more highly specialised. These cuticular structures comprise the layer of rods and the refracting bodies properly so called. The layer of rods, or retinidia, caps the epithelial cells of the retina. These rods, little developed in the Aspidobranchia (Fig. 100, IV), attain their highest degree of specialisation in certain Rachiglossa (Strombidae) and in the Heteropoda (Fig. 101, B, VII). In the last named they are disposed in furrows perpendicular to the optic axis, an arrangement analogous to that found in another pelagic Gastropod, Gastropteron. The refractive elements are the crystalline lens—a spheroidal body formed of concentric layers, which does not as a rule fill the cavity of the eye—and a less dense cuticular substance, known as the vitreous body, which surrounds the crystalline lens. In its most primitive condition the visual organ consists simply of an entirely THE GASTROPODA I2! retinal or pigmented invagination, still widely open to the exterior, whose epithelial cells are covered by a layer of rods, but the crystalline lens and vitreous body are altogether absent: this con- dition is realised in the Docoglossa. In a more advanced stage of specialisation the margins of the invagination become approximated, so that the ocular cavity, whose walls are pigmented throughout their extent, retains a small external aperture, through which water is admitted to bathe the crystalline lens: this condition may be seen in certain Rhipidoglossa, viz. Pleurotomaria, the Haliotidae, the Trochidae (Fig. 100), the Stomatellidae, and the Delphinulidae. Finally, the aperture of the ocular cavity is closed, and the crystal- line lens is covered in by two superimposed transparent epithelial layers, separated by a transparent layer of connective tissue. These two layers are (1) the internal cornea or “ pellucida,” a layer of small extent which is simply the anterior continuation of the retina, the two together forming the internal wall of the ocular sphere ; and (2) the cornea proper, which is external, and continu- ous with the tegumentary epithelium. This form of eye is found in all the Rhipidoglossa, with the exceptions mentioned above. In most other Gastropods the structure of the eye is practically the same as in the Rhipidoglossa with a closed cornea, with this differ- ence, that the pellucida is more and the pigmented retina propor- tionally less extensive. The retinal area becomes more and more restricted in proportion as the eye becomes more specialised—e.g. the Heteropoda—or ceases to be functional, ¢.g. Guivillea. There is often a blood space above the pellucida, as may be seen in Dolium, the Heteropoda, the Elysiomorpha, and the Basommato- phora. As regards the function of the eyes, it has been experiment- ally demonstrated that aquatic Gastropods are incapable of dis- tinguishing the form of objects, while the terrestrial species are able to distinguish them at a distance of one or two millimetres. The eye becomes rudimentary when it is buried in the integuments ; and further, it diminishes in size, though retaining its pigmenta- tion, in the following burrowing Gastropods: several Naticidae (Natica alderi, Amawra, ete.), various Bullidae (Scaphander, Philine, Doridium, Gastropteron, ete.), the Pleurobranchidae, many Nudi- branchs, and some Pulmonates, viz. Siphonaria, Auricula midae, and A. judae. ‘The eye may also become rudimentary through loss of its retinal pigment, while still retaining its superficial position : this is the case in species living in situations beyond the reach of the light, whether they be abyssal species (Guivillea) or inhabitants of subterranean waters (Bithynia pellucida). Finally, regression may be carried so far that the eye, as a consequence of functional disuse, is wholly wanting in the adult state. This phenomenon may be seen in burrowing forms, such as various Naticidae, sundry species of Terebra, the Olividae (Olivella, Agaronia, Ancillaria), certain 122 THE GASTROPODA Marginellidae and Sullia; in subterranean Pulmonates, such as Caecilianella and Helix hauffeni; in abyssal Gastropods, such as Lepeta, Propilidium, Bathysciadium, Puncturella, Cocculina, a species of Eulima, Choristes, Oocorys, some species of Fossarus, Addisonia, a species of Chrysodomus, Plewrotoma nivalis, Bathydoris, and Gonieolis ; Entoconchidae and Ento- in internal parasites, such as the siphon ; among pelagic Gastropoda in Janthina and the “ Pteropoda.” j vu Fic. 101. I, retractor muscle; II, optic nerve; IIT, Eye of Pterotrachea. A, the whole left eye. carina ; IV, the pellucida, or inner cornea, lacerated in order to show the lens; V, lens; VI, outline of the pellucida. 3B, sagittal section of the deeper part of the eye. I, lens; Il, vitreous body; III, limiting membrane; IV, retina; V, carina; VI, optic nerve; VII, retinal rods on their stands; VIII, retractor muscle ; IX, pigmented epithelium. (After Grenacher.) In addition to the cephalic eyes, certain species of Oncidiidae (Peronia) possess a large number of pallial eyes, situated on tubercles Structurally these organs are character- on ends are directed towards the interior of the body, the optic nerve The traversing the retina, just as is the case in the vertebrate eye. optic cavity is filled by a crystalline lens formed of a few large Another example of a pallial eye is found in the dorsal surface. ised by the fact that the retinal cells are reversed and their free transparent cells. Cerithidea obtusa ; in this case it is single, situated in the interior of aa THE GASTROPODA 12 Los) the respiratory cavity at the anterior extremity of the osphradium, and in it also the optic nerve traverses the retina. 6. Reproductive Organs.—The Streptoneura are dioecious, with the exception of the genera Bathysciadium, Cocculina, Valvata, Mar- senina, Oncidiopsis, Odostomia, Entosiphon, Entoconcha, and Enteroxenos. All the Euthyneura are monoecious. In the dioecious Gastropoda sexual dimorphism is generally very slightly marked. The males are externally recognisable only by the penis, when this organ exists; their shape, however, is more as, for instance, in Rhipidoglossa, Paludina, Littorina, various species of Crepidula, Plewrotoma, ete.: the most typical case in this respect is that of Lacuna pallidula (Fig. 7), in which the females are on the average ten times as heavy as the males. In addition, sexual differences are sometimes found in the aperture of the shell (Littorina obtusata), in the operculum (some species of Cerithiwm, Quoy and Gaimard), in the radular teeth (certain Buccinidae, Troschel), in the absence of the pedal sucker in the female Pterotrachea, of ten- tacles in the female of some Firoloida, and of the slit in the mantle in the male Vermetus. The gonad is always unpaired, even in the most archaic Aspido- branchia. It is generally placed on the dorsal side and at the summit of the visceral mass. It has the form of a racemose gland, made up of a great number of acini, and it may be compact or arborescent, with ramifications extending over and into the liver mass. In Aspidobranchia such as Plewrotomaria, the Trochidae, and Fissurellidae the gonad opens into the reno-pericardial duct, in the same manner as in some protobranchiate Lamellibranchs, viz. Solenomya. In all other Aspidobranchs, except the Neritacea, the gonad discharges into the kidney. In the Neritacea and Pectini- branchia the reproductive apparatus always possesses its own proper orifice, and there is a genital duct of greater or less length, which, however, is incompletely closed in various Melaniidae, Cerithiidae, Turritellidae, and Vermetidae. This duct opens into the pallial cavity to the right of the intestine in both sexes in the Ampul- lariidae, and in such forms as have not acquired a penis, that is to say, besides the four families mentioned above, in the Capulidae, Hipponycidae, and Solariidae. In all forms the male duct or spermiduct differs from the female duct or oviduct in the fact that it terminates in a copulatory organ (Fig. 99, XI). In its less special- ised form the spermiduct is continued into a seminal furrow or groove (Fig. 85, v.d’) which extends from the primitive genital orifice to the extremity of the penis, and is capable of being closed for part of its course, remaining open only in the neighbourhood of or on the penis. This condition is found in a Jarge number of Taenio- glossa, viz. Ampullaria, the Littorinidae, Modulidae, Struthiolariidae, 124 THE GASTROPODA Chenopodidae, Cassididae, Doliidae, Trotonidae, Naticidae, Cyp- raeidae, Calyptraeidae, Xenophoridae, Strombidae (Fig. 75, s.gr) ; and in some Stenoglossa, viz. the Muricidae, Magilus, Voluta, Lyria, the Harpidae, Terebra, and in all the Heteropoda. In all other forms, that is to say, in a certain number of Taenioglossa'and almost all the Stenoglossa, the spermiduct is closed in for the whole of its extent and the penis is hollow (Fig. 44, /). Thus the male orifice is secondarily removed to the extremity of the penis, and consequently is at a considerable distance from the primitive position of the genital aperture, a position which is retained by the female aper- ture. A penis exists in the Neritacea among the Rhipidoglossa, and in all the Pectinibranchia, with the exception of those families of the Taenioglossa enumerated above. When it does not exist copulation cannot take place, and the ova are fertilised in the sea by contact with the spermatozoa emitted by the male. The penis exists only in a rudimentary form in sedentary species, but in all others it is a well-developed, non-invaginable excrescence, situated on the right side of the anterior part of the body, except in cases of situs inversus, when it is on the left. All the aerial Streptoneura are necessarily provided with a penis, since in them copulation is indispensable. But the penis is not homologous throughout the group; it is developed at different parts of the body, at the point where the spermiduct ends. Thus the Neritacea have a cephalic penis, as has also Paludina, whose penis is a part of the right tentacle. In the Ampullariidae and Cyclostomatidae the penis is developed from the mantle, but in all other forms it is exclusively of pedal origin. Sometimes it is provided with an external whip-hke appendage or flagellum: such is the case in many Taenioglossa, viz. nearly all the Littorinidae except Cremnoconchus, in Dolium, and especially in Hydrobia, Bithynia, the Naticidae, the Lamellariidae, and the Heteropoda. The genital ducts are rarely provided with well-differentiated accessory organs in the dioecious Gastropoda. In certain cases there is a glandular tract in the oviduct, which is sometimes specialised to form an albuminiparous gland (Ampullaria, Paludina, the Naticidae, Lamellariidae and Calyptraeidae, 7riton and Cassidaria). There is a copulatory pouch or receptaculum seminis in the Neritacea, Paludinidae, Cyclostomatidae, and Heteropoda. In some Neritacea—viz. Neritina, Nerita, Navicella, ete.—the receptaculum has its own external opening distinct from the oviducal aperture ; this probably corresponds with the right kidney of other Rhipido- glossa, which in this case has been lost by the male sex (Thiele). In some freshwater Taenioglossa, e.g. Tanganyicia (Fig. 78) and Melania episcopalis (Fig. 109), the oviduct is continued into a ciliated groove which leads to an incubatory pouch situated in the head ; this pouch has been homologised with the penis by Moore. The THE GASTROPODA 12 at males of Ampullaria and the Heteropoda also possess a vesicula seminalis, and the penis frequently is furnished with well-marked superficial glands (Littorinidae, Cassis, Terebra, and the Heteropoda). In some Taenioglossa, such as Paludina and Pteroceras, and in several Stenoglossa, such as Murex, Nassu, Purpura, ete., there are two kinds of spermatozoa, the one normal and filiform, the other vermiform ; the function of the latter kind is not yet explained. In Paludina, for example, these two kinds of spermatozoa exist in equal quantities, but the filiform kind, with a single cilium, originate from spermatids which have increased but little in size during the growth period, and contain the normal quantity of nuclear substance: these are the eupyrenic spermatozoa of Meves. The vermiform spermatozoa, on the other hand, have about six cilia apiece, originate from spermatids which have increased largely Fic. 102. Follicles of the hermaphrodite gonads of Euthyneura. 4A, of Helix; B, of Kolis. a, ova; b, developing spermatozoa ; c, common efferent duct. (Irom Lankester, after Gegenbaur.) in size during the growth period, and contain only a small quantity of nuclear substance: they are known as oligopyrenic spermatozoa. In the monoecious Gastropods the gonad ordinarily occupies the same position and has the same relations as in the dioecious Streptoneura, but it may be much more subdivided, especially in certain Nudibranchs, viz. Phyllirhoé (Fig. 161, y) and Elysiomorpha. It has always a duct with its proper external orifice and a penis which is invaginable in most Euthyneura, but this latter organ is absent in hermaphrodite parasitic Streptoneura. The gonad differs from that of the dioecious Streptoneura in producing ova and spermatozoa in the same individual. In the most simple arrange- ment the two kinds of genital products are developed side by side, as may be seen in Valvata and in the majority of the Tectibranchia -and Pulmonata (Fig. 102, A). In the more specialised condition there are male and female acini, the latter opening into the spermatogenous sacs in Oncidiopsis, the Pleurobranchidae, the majority of the Nudibranchia (Fig. 102, B), with the exception of 126 THE GASTROPODA the Elysiomorpha. Lntoconcha, Enteroxenos, and bathysciadium are the only genera in which the male and female acini are quite distinct. In its most primitive condition the genital duct is hermaphrodite, that is to say, it is a spermoviduct throughout its length, and is therefore called monaulic. It generally is provided with an internal double longitudinal fold. The hermaphrodite aperture is situated on the right side, near the opening of the pallial cavity, and is connected by a ciliated seminal groove with the more anteriorly situated penis. This condition is found in the Bullomorpha (Fig. 98, s.g) in general, including the Thecosomata ; in the Aplysiomorpha (Fig. 154, 2), including the Gynmosomata (Fig. 84, IV, XI); and in the Pulmonata Pythia (Fig. 171). The edges of this seminal groove unite to form a complete tube in Cavolinia longi- rostris among the Bullomorpha, and among the Pulmonata in all the Auriculidae except Pythia, and as a consequence the primitive genital aperture serves only for the emission of the female pro- ducts, the male products passing through a spermiduct closed throughout its extent. In subsequent stages of evolution ot the genital duct the spermiduct takes its origin from the hermaphro- dite duct above the external opening: this latter duct, therefore, bifureates or becomes “diaulic,” the female branch of the duct opening by the primitive hermaphrodite orifice. This condition is characteristic of Valvata and Oncidiopsis (Fig. 103), of Actaeon and Lobiger among the Bullomorpha, of the Pleurobranchidae and the Nudibranchia except the Doridomorpha and most of the Elysio- morpha, and of the Pulmonata. At the point of bifurcation the male and female sections of the duct are separated by a narrow slit, which only allows the spermatozoa to pass. In this case therefore, as in the dioecious Gastropoda, the female orifice remains in the same place as the primitive genital aperture, and the male orifice is carried far forward, to the extremity of the penis. The two external orifices, male and female, are thus at some distance from one another, as may be seen in Valvata, Oncidiopsis (Fig. 103, f.o, pe), the Basommatophora in general, the Oncidiidae (Fig. 59, of, om), and Vaginula (Fig. 87, of), But the female aperture itself may be secondarily shifted from its original position, and come so near to the penial aperture as to be contiguous to it, a condition found in the Pleurobranchidae and the Nudibranchs in general ; or the two apertures may reunite in a common cloaca, as in the Stylommatophora (Fig. 177, Il), Siphonaria, and Amphibola. In these various cases the female duct, like the hermaphrodite duct of the monaulic forms, bears a bursa copulatrix or receptaculum seminis, which in certain stylommatophorous Pulmonates, such as Helia aspersa, Clausilia, etc., is provided with an accessory branch (Fig. 104, Jt.s). A third differentiation of the genital ducts is brought about THE GASTROPODA 127 when the female duct becomes bifurcated through the separation of the bursa copulatrix, the latter acquiring a separate external aperture but remaining in connection with the oviduct by its deeper extremity. In this manner two female orifices are formed; the one is the copulatory orifice, the other is the oviducal orifice serving for the passage of the ova. The genital duct is thus trifurcated or “ tri- aulic,” a condition which is not found in any Pulmonate, but is confined to certain Nudibranchs, viz. the Doridomorpha and the majority of the Elysiomorpha (Fig. 105). Fia. 104. Hermaphrodite reproductive appara- tus of Helix hortensis. d, digitate acces- sory glands on the female duct; E.d, Fic. 103. albuminiparous gland; jl, flagellum; p, penis; p.s, calciferous gland or dart-sac Oncidiopsis, hermaphrodite genital on the female duct; 2.s, receptaculum apparatus, dorsal view. a.g, albumini- seminis or spermatheca, opening into the parous gland; /f.o, female orifice; g.g, female duct ; uv, uterine dilatation of the hermaphrodite gonad; pe, penis; pr, hermaphroditic duct; v.d, spermiduct or prostate; 7.s, receptaculum seminis; vas deferens; v.e, hermaphroditic duct ; so, spermoviduct; sp, spermiduct; s.v, z, ovo-testis. (From Ray Lankester, seminal vesicle. after Gegenbaur.) The penis is invaginable in all the Euthyneura with the exception of Actacon (Fig. 148, VI) and Umbrella. It is a pedal structure in nearly all Opisthobranchia, but in Umbrella it is cephalic. In the majority of Pulmonates the penial nerve arises from the cerebral ganglion, but the fibres of the nerve originate from the pedal and only traverse the cerebral ganglion. In monaulic species the penis often bears an appendage, and occasionally chitinous accessory structures: there is a_ single stylet in several species of Planorbis, in Glaucus, and many other 128 THE GASTROPODA Nudibranchs, the stylet being contained in a special pouch in certain species of Doris: in some other Nudibranchs there are multiple stylets. In addition to the bursa copulatrix, there are numerous and various accessory genital organs in the hermaphrodite Gastropoda. An albuminiparous and a mucous gland are found on monaulic ducts, generally near their distal end (Fig. 171, muc). In the diaulic Pulmonata there is a large albuminiparous gland on the hermaphrodite section of the duct (Fig. 104, #.d), and on the female part of the duct of Basommatophora there is an albumen gland corresponding to the uterine glands of Stylommatophora (Fig. 104, w). The diaulic and triaulic Opis- thobranchs have also contiguous albumen and mucous glands on the oviducal part of the genital duct. The terminal portion of the oviduct of Stylommatophora is further provided with a glandular zone (Zonites), or with two multifid vesicles with a variable number of ramifications Limapontia, reproductive appara- (Fig. 104, d). Opening between the tus, dorsal aspect. a.d, albumini- two is a special pouch—seemingly a parous duct; a.g, albuminiparous So > : : gland; h.d, hermaphroditie duct; Specialised multifid vesicle, which secretes Kgs Ovo. testis; mg, mucous ovi' 4» sharp calcareous dart (Fig. 104, p.s). ducal gland; 0.0, oviducal orifice ; ; ; 4 ov, oviduct; p, penis; pr, pros- Before copulation the dart-sac is evagin- peta atin tad together with all the terminal part (vestibule) common to the repro- ductive organs, and the dart, which is caducous, pierces the skin of the conjugate. The spermiduct is sometimes furnished with amore or less elongate “prostate” gland, as in Valvata, Oncidiopsis (Fig. 103, pr), various Bullomorpha and Elysiomorpha (Fig. 105, pr). The penis of certain Stylommatophora is provided with a long hollow caecum, the “ flagellum” (Fig. 104, //), in the interior of which is secreted the spermatophore or capreolus. This is a thin-walled chitinous tube closed at one end and cleft at the other and filled with a quantity of sperm. When there is no flagellum the spermatophore is formed by the deeper portion of the penis. Sometimes the spermatophores are furnished with denticulations and even with arborisations (Fig. 106). The hermaphrodite gland (gonad) does not liberate ova and spermatozoa simultaneously, the discharge of the former occupying a very short time after copulation. Generally speaking, the hermaph- roditism is protandric, the spermatozoa being the first of the genital products to come to maturity. This hermaphroditism is THE GASTROPODA 129 not self-sufficient, and the union of two individuals is necessary for fertilisation. There are, however, instances of Pulmonates which have been insolated from the time of birth and have nevertheless laid eggs which have developed (Zonites cellarius, Limnaea). In all species possessing a penis, fertilisation is effected by copulation ; in species without a penis—e.g. Patella—artificial fertilisation is possible. During the venereal act the penis is thrust into the bursa copulatrix, when this latter structure is present, and discharges into it a quantity of sperm which subsequently fertilises the ova during their passage down the oviduct. Copulation and the act of oviposition that follows it take place at various seasons. In temperate regions they continue from early spring onwards, extending even into the winter, as is the case in Patella in the Fic. 107. Fic. 106. Two Limnaea stagnalis in copulation, Spermatophore of Nanina the left one acting as male. I, tentacle wallacet, magnified. (After and eye; II, penis; III, foot; 1V, buccal Pfeffer. ) veil. (After Stiebel.) Atlantic and TZ'rochus striatus in the Mediterranean. In _ the stylommatophorous Pulmonates with a common genital orifice, the two copulating individuals mutually fertilise one another, each one acting as male and female, and the same is the case in the majority of Nudibranchs. In the hermaphrodites with distant genital apertures the same animal may act as male and female, but as a rule not simultaneously, unless, indeed, it unites with two or more individuals to form a chain, as may be seen in Limnaea, the Aplysiomorpha, etc. The copulation of two individuals is effected in the same manner as in the dioecious Gastropoda (Fig. 107). Ill. Empryouoey. The eggs may be laid or may develop within the maternal organism. In the oviparous species that do not copulate, the a 130 | THE GASTROPODA unfertilised ova are generally laid one by one and are not united by an accessory envelope (Patellidae, Haliotis, certain Trochidae of the sub-genera Gibbula and Trochocochlea), but in Fissurella and in Trochidae of the sub-genus Zizyphinus they are united by a gelatinous investment. In the species that copulate the ova are deposited within a few days after the act of copulation, the time varying from one day in sundry Nudibranchs to as many as fifteen days in some species of Heliz. The nidus may assume very various shapes. In aquatic species the shells surrounding the eggs may be embedded and united in a single gelatinous mass, which may be ribbon-shaped, more or less coriaceous, attached in littoral species, floating in pelagic species. This is more especially the case in the Euthyneura—viz. in the Basommatophora and Opisthobranchia, in which the ribbon is often coiled into a spiral—and also in many Taenioglossa (Littorinidae, Rissoidae, Hydrobiidae, etc.). In such cases each egg-shell contains a single ovum. Again, the egg-shells may be coriaceous (Rachiglossa), more or less independent, simply attached to one another (Buccinum, Fusus, Pyrula), or fixed side by side on a common support (Purpura, Murex, Nassa, Fig. 108, Trophon, Voluta, etc.). In this case each shell contains a consider- able number of ova, but all of them do not complete their develop- ment. As special forms of nidus may be mentioned that of Natica, in which the eggs are united by agglutinated sand into a horny-looking ribbon coiled in a ring; that of Lamellaria, whose eggs are deposited in a sort of nest excavated in colonies of Synascidians. Finally, various forms of Streptoneura attach their eggs to various parts of their bodies, and thus appear to be more or less incubatory, as, for instance, Hipponycidae and Capu- lidae (in Calyptraca the eggs are attached below the neck); or they may attach them to the external surface of the shell, e.g. Neritina, Hydrobia ulvae, and in excep- tional cases /tssoa ; or to the internal face of the shell, e.g. Vermetus (Fig. 45, ov). In the oviparous Janthinae the eggs are attached to the float (Fig. 135, 0). The stylommatophorous or terrestrial Pulmonates generally lay in the earth 7 ; __-_____- isolated ova enclosed either in a gelatinous Neg-capsule of Nassa reticulata, ; . x 18. 0, aperture; ov, eggs. envelope (Limaz, etc.) or in a calcified shell, e.g. certain species of Helix, Testacella, ete. In Bulimus these eggs may attain a length of three centi- metres, thus exceeding in size the eggs of many birds. The eggs of Ampullaria have also a calcified envelope; they are laid in the water and are agglomerated together. When Succinea lays its o Fic. 108. THE GASTROPODA 131 eggs in the water it surrounds them with a gelatinous mass, like the basommatophorous Pulmonates. In the ovoviviparous Gastropoda the progeny are born living after undergoing their development within the parent. They develop in the terminal portion of the oviduct in the following Streptoneura: Paludina, Typhobia; various species of Melania (Fig. 109), Littorina, Cymba, Janthina, Nassopsis, and the Entocon- chidae ; in the Opisthobranch Halopsyche; and in numerous Pul- monates—viz. Glandina algira, Rhytida aequalis, Selenites voyanus, Helix rupestris, H. inversicolor, H. inaequalis, H. wunidentata, H. erronea, H. studeriana, Patula cooperi, Acanthinula harpa, the genera Partula, Balea, Coeliaxis, Pupa muscorum, P. wnbilicata, P. cylindracea, Clausilia ventricosa, C. similis, Achatina alabaster, and A. zebra, Stenogyra mammillata (Fig. 8), S. octona, S. terebraster, S. domini- ciensis, S. decollata, S. lamellata, Ferussacia folliculus, F. lamellifera, Fia. 109. Melania episcopalis, out of its shell, showing the female genital apparatus, right-side view. a, anus; %.0, brood-pouch opening; b.p, brood-pouch; /, foot; g.gr, genital ciliated groove ; g.0, genital orifice ; m, mouth ; oc, eye ; od, oviduct; op, operculum ; ov, ovary. (After Moore.): F. procerula, F. debilis, Vaginula vivipara. The embryos develop in a special incubatory pouch excavated in the interior of the foot and connected with the extremity of the ciliated canal which passes from the female orifice to the head in Melania episcopalis (Fig. 109, bp.) and Tanganyicia rufofilosa (Fig. 78, b.p). In viviparous Gastropods the ovum contains but little yolk, but in other forms the quantity of deutoplasm is greater, and is especially large in a number of Rachiglossa, such as Nassa, ete. The segmentation of the ovum is always total, and, except in cases in which the deutoplasm is scanty (Paludina), it soon becomes irregular. As a rule in Aspidobranchia, Taenioglossa, and Pul- monata, the two first cleavage planes are meridional, the first separating the right and left halves of the future animal. The third cleavage is equatorial and» cuts off the micromeres at the animal pole from the macromeres at the opposite pole, so that, as a rule, there are four macromeres from the beginning (Fig. 9, A, B). These macromeres give rise to two more generations of micro- or 132 THE GASTROPODA ecto-meres, and the latter proliferate very rapidly. In such forms as Patella, Planorbis, and Limaz, the blastula formed in this manner P19, O40) 40K Fic. 110. Development of the river-snail (Paludina vivipara), in which the mouth and stomodaeum are formed independently of the blastopore, the latter persisting as the anus. ae, archenteron, or endodermic cavity; an, anus; bl, blastopore; d.c, directive or polar corpuscle; d.v, velar area or cephalic dome; f, foot; m, mouth ; mes, rudiments of the mesoderm; p.i, pedicle of invagination, the future rectum ; sh.gl, the shell-gland ; s.m, site of the as yet unformed mouth ; vr, velum. A, Gastrula phase (optical section). B, the Gastrula has become a Trochosphere by the development of the ciliated velar ring (optical section). C, side view of the Trochosphere with commencing formation of the foot. D, further advanced Trochosphere (optical section). E, the Trochosphere passing into the Veliger stage ; dorsal view showing the formation of the shell-gland. J’, side view of the same, showing foot, shell-gland, velwnm, mouth, and anus, (After Lankester.) contains a large blastocoel between the micromeres and macromeres, but in other Gastropods this cavity is much reduced. In some —" ‘ _—_. THE GASTROPODA 133 types such as Paludina, Planorbis, etc., the endoderm formed by the macromeres is invaginated into the ectodermic layer formed by the micromeres, but in many cases, in consequence of the far more rapid multiplication of the micromeres and the much larger size of the macromeres, gastrulation is effected by epiboly, and the endoderm is invaginated at a later period; in this case the endodermic cavity or enteron is of small size. In most cases the blastopore closes, and the definitive mouth is formed by a new invagination at the point of closure: it is only in Paludina that a portion of the blasto- pore remains open and becomes the anus (Fig. 110, C, 01; F, an). The mesoderm is formed as two primary mesomeres from the more posterior of the two primitive macromeres (Fig. 11, mes). The mesodermic organs (definitive kidney, heart, etc.) do not arise until a late period, their place being taken, during the development, by provisional larval organs, such as superficial contractile sinuses and larval kidneys (Figs. 114, 118). The ciliated trochosphere larva is sometimes consti- tuted at a very early period, before the formation of the mesoderm — e.g. in Patella, Acmaea, and Trochus—and in such cases the embryo be- comes free at once. But in most cases the larva is not Fic. 112. Larva of Cavolinia tridentata, ventral aspect. Fic. 111. a, anus; f, median portion of the foot; h, heart; i, intestine; An, contractile sinus; Young veliger of Trochus, ven- m, mouth; mb, mantle-skirt ; me, subpallial tral aspect. /f, foot ; m, mouth; chamber ; ot, otocyst; pn, lateral lobe of the pa, mantle; pa.c, pallial cavity ; foot (the future left fin); g, shell; r, kidney ; sh, shell; ve, velum. (After Ss, oesophagus; o, sae containing nutritive Robert.) yolk. (From Lankester, after Fol.) hatched out until a much later period, and a large part of the development is effected within the egg envelope. The larva—whose shell is often different from that of the adult—is characterised by its velum and by some other external or superficial larval organs. 134 THE GASTROPODA The velum is the locomotory ciliated ring, which arises antero- dorsally (Fig. 110, C, vr) and circumscribes the apical area. As in the Amphineura, it is only slightly prominent in the most archaic marine Aspidobranchia (Patella, Fig. 14, Acmaea, Trochus, Fig. 111, ve), but in other Gastropods it becomes more and more prominent, and eventually gives rise to a natatory velum formed of two lateral lobes with ciliated margins (Fig. 112). These lobes may in turn be subdivided into two (Fig. 121, ve) or three secondary lobes (for example, in Atlanta and in “ Ethella,” a larval form attributed to one of the Strombidae). In viviparous species, or in forms like Cenia and the Pulmonata, in which the young are Fic. 113. Trochosphere of Patella, in the 34th hour. I, flagellum in the apical area ; II, left lip of the blastopore ; III, blastopore; 1V, velum. (After Patten.) hatched in the adult condition, the velum becomes rudimentary or disappears altogether; in the Basommatophora (Fig. 119, A) it is developed to a slight degree, but only on the sides, and is not con- tinuous ; in the Auriculidae, Siphonariidae, and Oncidiidae, however, all of which are marine, a normal velum is present. The foot is always very short at first (Fig. 1, f), being repre- sented by a mere papilla, which in sundry instances is formed from two paired rudiments (Fig. 113, II) situated between the two ex- tremities of the primitive blastopore when this latter structure is elongated. The operculum is formed early, and exists in all testaceous larvae, even in cases in which the adult possesses neither shell nor foot: the only exceptions to this statement are the Pulmonata (excepting the Auriculidae, the Amphibolidae, and the THE GASTROPODA 13 at Siphonariidae), the Cavoliniidae, and the Gymnosomata. The pedal glands are formed by ectodermic invaginations, and in some species exist in the larva though they are absent in the adult (Purpura). The preconchylian invagination or shell-gland (Fig. 110, sh.gl) appears at the beginning of development in the centro-dorsal area behind the velum, on the side of the body opposite to the blastopore. It is surrounded by a ridge which gradually extends over the visceral sac and secretes the shell (Fig. 117, si). In some Stylommatophora —viz. Clausilia and Succinea, a pallial sac is formed which covers up the shell, but eventually opens again. ‘The shell grows in thickness internally, fresh matter being added to it from the external surface of the mantle, but its increase in extent is dependent on the activity of the border of the mantle, where there are special glands which degenerate when the animal reaches the adult state. It is only at this period that the aperture of the shell acquires a lip, Fic. 114. Embryo of Vermetus, ventral Fic. 115. aspect. I, velum; II, contractile Larval shell of Nassa reticulata, sinus (“embryonic heart”); III, ventral aspect, x 30. h, hook of opening of the pallial cavity; IV, the dorsal edge of the aperture ; si, shell ; V, foot; VI, lefteye. (After future canal or shell-siphon; sp, Salensky.) spire. or is contracted in various ways to form, for example, the linear aperture of Cypraea, Cavolinia, ete. In Gastropoda that are naked in the adult state, the shell falls off soon after the reduction of the velum (Fig. 116), but in Cenia, Runcina, and Vaginula the shell- gland and shell are not developed, and the young animal, at the time of escaping from the egg, has already the naked form of the adult. The superficial contractile sinuses are portions of the wall of the body, temporarily modified to ensure the circulation of the nutritive fluid in the system of cavities destined to become the circulatory apparatus of the adult. In the walls of these sinuses are muscular elements, whose fibres are sometimes disposed in regular meshes, as, for example, in the nuchal sinuses of the rachiglossa. These organs are acquired in the course of ontogeny, and are developed in different regions: they are frequently found 136 THE GASTROPODA between the foot and anus, in front of the pallial cavity, C9 Helix, Bithynia, Vermetus, the Rachiglossa, and nearly all the marine Gastropoda, including the “Pteropoda,” Heteropoda, and Nudi- Fic. 116. Larvae of Holis (Galvina) exigua. A, on the second day, left-side view. I, radula; II, foot; III, otocyst ; IV, operculum ; V, shell. #£, on the third day, after the loss of the shell, dorsal aspect. I, eye; II, left liver lobe; III, foot; IV, anus; V, octocyst; V1, radula. (After Schultze.) B branchia. This sinus is displaced, together with the pallial aper- ture, along the right side towards the neck (Fig. 114, I), and finally is partly contained in the pallial cavity. In the Basom- matophora there is a velar dorsal sinus, and in the Stylommatophora, Fia. 117. Helix aspersa, embryo of the tenth day, right-side view. cog, shell; o.ca.c, external opening of the cerebral cavity ; p, foot; pa, mantle; po, contractile vesicle or podocyst; te’, the true tentacles ; te’, labial palp ; vit, vitellus. Arion, Limax, Clausilia, Helix, etc., but not in Succinea, there is a caudal vesicle, known as the pedal sinus or podocyst (Fig. 117, po). The larval kidneys are paired and generally symmetrical organs situated at the anterior end of the body, immediately behind the velum or apical area. In the marine Streptoneura they are caducous ectodermic projections, in which the products of excretion are THE GASTROPODA 137 accumulated. In the Opisthobranchs they are closed pouches ; in Paludina, Bithynia, and the Pulmonates they are canals opening to the exterior. In the last-named group these organs consist of tubes of ectodermic origin, each of which bears an ampulla on the middle of its course, and its internal portion is formed of perforated cells and ends internally in a closed flame-cell. In the Basommatophora (Fig. 118, ve) these cells of the larval kidney are four in number, three being perforated and one a flame-cell. These organs are absent in Vaginula. The nerve-centres and organs of special sense originate from the ectoderm, almost always from an ectodermic thickening on the buccal side of the apical organ of the larva (Crepidula), but in Vermetus and the Cavoliniidae an ectodermic invagination has been described as taking part in the formation of each cerebral centre. Again, in the Fic. 118, Embryo of Limnaea stagnalis, viewed from the right side. a, anus; ¢.g, cerebral ganglion ; J, foot; in, intestine; m, mouth; o.7, aperture of the embryonic kidney; pa, mantle; ra, radula ; re, embryonic kidney ; sh, shell; st, stomach; vi, vitellus. (After Erlanger.) Pulmonates, even if the principal part of the cerebral centres is formed from an epithelial thickening, the posterior accessory lobe origin- ates from a subsequent ectodermic invagination (Fig. 117, 0.ca.c) —the ‘“‘cerebral tube ”—the cavity of which generally disappears after the animal is hatched out, but persists in the adult Planorbis and Limnaea. The eyes arise in the velar field, near the cerebral centres, and are formed by invagination in the Aspidobranchia, Paludina, the Stylommatophora, etc., but from thickenings in Vermetus, and frequently after the veliger larva is hatched, as, for example, in many Nudibranchia, with the exception of the Tergi- pedinae (Fig. 61, ¢). The otocysts are always formed at an early stage, on the antero-lateral aspects of the foot, by invagination in the majority of marine Gastropods, but from ectodermic thickenings in many Pulmonata. At first they invariably contain a single otolith, even in the species which in the adult state possess multiple otoconia. In pelagic larvae the otocysts may often be seen to be asymmetrical, and sometimes the tentacles share this peculiarity 138 THE GASTROPODA As regards the formation of the internal organs of the adult, the stomach, the liver, and nearly the whole of the intestine arise from the endoderm. The liver lobes are formed before the absorption of the nutritive sacs borne on the posterior part of the larval stomach. The proctodaeal invagination, placing the intestine in communication with the exterior, is always of small importance, but, on the other hand, the buccal bulb and oesophagus, with their numerous accessory organs, are formed from an im- portant stomodaeal ectodermic invagination, which always corre- sponds in position with the extreme anterior end of the blastopore, whether the latter is closed or remains open. The remaining organs are formed in the same manner as in other Mollusca, as has been de- scribed in the first chapter, and the ontogeny of the Gastropoda does Fic. 119. Embryos and larvae of various Molluscs, after the ventral flexure and before the torsion (for A), ventralaspect. A, Gastropod (Limnaea, after Fol) ; B, Dentaliwm (after Lacaze) ; C, Lamelli- branch (Dreissensia, after Meisenheimer); D, Cephalopod (Oigopsid, after Grenacher). an, anus ; ar, arms ; e, eye; f, foot; fu, funnel; g, gill; m, mouth ; ot, otocyst; pa, mantle ; pa.o, posterior pallial orifice ; pe.g, pedal ganglion ; sh, shell ; ve, velum ; v7, vitellus. not exhibit any special features other than certain post-larval meta- morphoses and the torsion produced during development (Fig. 51). Up to the trochosphere stage the larva is strictly symmetrical (Fig. 14, B), but afterwards the torsion sets in, as a result of which the asymmetry characteristic of adult Gastropoda is established. At first the aperture of the pallial cavity and the anus are always posterior (Fig. 118), as is the case in all symmetrical Molluses (Fig. 22, A, B, D, E); then they are carried forward by a ventral flexure (Fig. 119) in the same manner as in the Cephalopoda, Seaphopoda, and many Lamellibranchia. But in the Gastropods a lateral torsion is superadded to this primitive flexure, causing the pallial aperture to pass from the postero-ventral surface (Fig. 51, A) over to the right side (Fig. 51, B), and thence to the antero-dorsal surface (Fig. 51, C). If the animal be supposed to have the mouth turned towards the observer, this torsion may be seen to follow the movements of the hands of a watch (Fig. 52). THE GASTROPODA 139 Post-larval metamorphoses occur in various cases. The velum, as seen above, disappears, being absorbed by a process of phago- cytosis. In various genera the operculum falls off, and so also does the shell in naked forms (Fig. 116, B) and in Lamellaria, in which a new shell is formed replacing the larval shell or Echinospira. It is only in rare cases that a second larval form exists after the disappearance of the velum and before the adult state is reached ; this is the case, however, in the gymnosomatous ‘‘ Pteropods,” in which three transverse and parallel ciliated rings are formed before the fins are completely developed (Fig. 120). The most anterior of these rings is made up of inter- rupted portions; the two others, on the contrary, are continuous, and are situated respectively at the middle of the body and near the aboral extremity. These two continuous ciliated circles, and especially the more posterior, are preserved till a very late period, and sometimes persist in the adult, whose habits do not differ from those of the larva (Fig. 155). Sooner or later after their ex- pulsion from the oviduct, or after the nidus is laid, the eggs are hatched out: after a period of some twenty hours in 7rochus, after ten days in certain Nudibranchs (Zer- gipes), at the end of eighteen days in others (Cenia), after three or four weeks in Limnaea, after more than a month in Valvata and certain i meemee of Arion and Limar. Itis tata of Sponpiobranchaes aushans ‘ fi al aspect. , mouth; c.e.a, anterior only In exceptional cases that the ciliated ring; ¢.c.m, middle ciliated ring ; s c.c.p, posterior ciliated ring; na, fin; p, young are hatched out with the foot? (After Racovitza.) characters of the adult, but this is the case in all the Pulmonates—vwith the exception of the Siphonariidae which have a marine- veliger larva—in the Opisthobranchs Cenia and Runcina, and in sundry Streptoneura such as Littorina and Lacuna among the Taenioglossa, and Purpura and Buccinum among the Rachiglossa. In normal cases the young Gastropods are hatched out as free-swimming or pelagic veliger larvae (Fig. 61). This veliger has a very small foot and a more or less voluminous velum, the latter organ being smallest in the least specialised forms, such as Z'’rochus (Fig. 111), Patella, Fissurella, ete., and is the characteristic larval form in most opisthobranchiate 140 , THE GASTROPODA Euthyneura and in the majority of Streptoneura. Even in those Gastropods that are hatched out in the adult form, the veliger stage can generally be recognised, in a more or less reduced condition, within the egg membranes, e.g. in Buccinwm, Cenia, the basommato- phorous Pulmonates. In all Gastropods the velum is reduced in proportion as the foot develops; nevertheless, in a considerable number of pelagic larvae the veliger stage is preserved for a long time, and the velum persists, and often develops excessively long lobes even after the creeping foot is fully and normally developed : such is the case in “ Macgillivraya” (Fig. 121), “ Agadina,” ‘ Chele- tropis,” “ Sinusigera,” “‘Echinospira,” etc., all of which are special pelagic larval forms of Streptoneura which were long considered to Fic. 122. Shell of a young Fie. 121. Purpura —haemastoma, “ Macgillivraya,” pelagic Jarva of a_ siphonate enlarged, dorsal aspect. Streptoneurous Gastropod (Doliwm), ventral aspect, ca, canal of the adult ; x 12. f, foot; m, mouth ; sh, shell; si, siphon ; fe, e.s, limit of the embry- tentacle and eye; ve, lobes of the velum. (After onic shell; sp, spire. MacDonald.) (After Dautzenberg.) be distinct genera. The velar lobes may even produce lobate expansions of the margin of the aperture of the shell, but these dis- appear when the velum is absorbed and the shell assumes the adult form (Fig. 122). IV. DEFINITION. The asymmetry of some of the principal organs of the body is the chief characteristic of the Gastropoda. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane; that the ctenidium, the osphradium, the hypo- branchial gland, and the auricle of the heart are azygos, or at least are more developed on one side of the body than the other; and that there is only one genital orifice, which lies on the same side of the body as the anus. In other words, one-half—generally the morphologically left but topographically right half—of the anal complex is either atrophied or has disappeared altogether. This asymmetry, expressed by the transfer of the morphologically right THE GASTROPODA 141 organs to the left side, is the result of a torsional movement, which has carried the anus and pallial cavity from an originally posterior to an anterior position and at the same time has twisted the visceral commissure. V. BIONOMICS, The Gastropoda are essentially aquatic animals, and the more archaic species are marine. Some species are specially adapted to brackish waters. In fresh waters there are found sundry Strepto- neura, viz. certain Neritidae, the Ampullariidae, Paludinidae, Valva- tidae, Bithyniidae, Hydrobiidae, several Cerithiidae, the Melaniidae, Cremnoconchus, and Canidia; nearly the whole pulmonate group of Basommatophora ; and a single Opisthobranchiate, Ancylodoris. Finally, the stylommatophorous Pulmonates and Halicinidae, Cyclo- phoridae, Cyclostomatidae, and Aciculidae among the Streptoneura are terrestrial. In some forms that live in torrential streams, or are subject to being dried up periodically, the respiration is alternately aquatic and aerial, and the Amphibolidae, Siphonariidae, and Onci- diidae are examples of Pulmonates that have returned to a marine existence. The Gastropoda crawl at the bottom of the water, or on the land, or in a reversed position, on the film of mucus secreted on the surface of the water by the glands of the anterior groove of the foot (Basommatophora, Nudibranchia). The Strombidae are jumpers, and a considerable number of Gastropods are swimmers, e.g. the Heteropoda—which swim in a reversed position with the foot upwards—Janthina (Fig. 135), the “ Pteropoda,” Phyllirhoé, Acera (Fig. 147), etc. Some families both of Streptoneura and Opistho- branchia burrow in mud or sand, e.g. the Naticidae, Bullidae, ete. Some genera are more or less sedentary, though able to move from place to place—such are Patella and Bathysciadiwm—but others are completely sedentary when adult, and may be fixed either by the substance of their shells—such are Vermetus and Magilus (the latter inhabits corals)—or by a calcareous plate secreted by the foot, as is the case in Hipponyx. The diet of Gastropoda varies according to the group under consideration. Generally speaking, the carnivorous habit is due to specialisation, often accompanied by the development of a_pro- boseis. Various forms of Gastropods live and feed on colonial invertebrates such as Synascidians, Hydrozoa, Anthozoa, and the like, and to a certaindegree mimic these forms. Thus Ovula lives on Gorgonia, Pedicularia on Corallium, Lamellaria on Leptoclinum, various Nudibranchs on sponges or Hydroids. Some Gastropoda are parasitic, generally in or upon Echinoderms, and belong either to the sub-group Capulidae, in which case they are ectoparasites, and had already acquired this habit in Palaeozoic times (Platyceras), or to the “ Aglossa,” that is to say, to the little group formed by 142 THE GASTROPODA the families Eulimidae (including Stylifer, parasitic on Asterids, Echinids, and Crinoids) and Entoconchidae, including Lntosiphon, Entocolax, Entoconcha, and Enterorenos, all parasitic in Holothurids. Some thirty thousand species of Gastropoda have been enu- merated, of which twenty thousand belong to the present epoch and are distributed in every region of the globe. Of existing species more than twelve thousand are branchiate forms. Some marine species are found at a depth of over 2500 fathoms, and some Pulmonata live in the Himalayas at a height of nearly 17,000 feet above the level of the sea. Some freshwater Gastropoda (Hydro- biidae, Basommatophora) exist at a depth of 180 fathoms below the surface of certain lakes, e.g. Lake Baikal; others live in subterranean waters, and some Pulmonates are found in caverns into which the daylight does not penetrate. Palaeontology shows that these animals were already in existence in the Cambrian period, at the commencement of the Palaeozoic epoch. The size of Gastropods varies from a fraction of a millimetre to more than fifty centimetres. The largest forms are found not only among the testaceous species, such as Fusus, Tritonium, Ancistromesus, Strombus, ete., but also among the naked forms: Tethys, for example, is more than thirty centimetres in length, and some species of Den- dronotus as much as twenty-five centimetres. VI. SysTemMATIC REVIEW OF THE SUB-CLASSES, ORDERS, AND FAMILIES OF GASTROPODA. The class Gastropoda includes two well-defined sub-classes, Streptoneura and Euthyneura. Sus-Ciass I. STREPTONEURA, Spengel (= Prosobranchia, Milne-Edwards = Cochlides, von Jhering). These are dioecious Gastropoda, with the exception of a few aberrant genera, and are characterised by the maximum torsion exhibited by the visceral mass and visceral commissure, the latter being always twisted into a figure of eight (Fig. 124, VII, IX). The right moiety of this commissure is situated above the digestive tube, and is known as the supra-intestinal ; the left moiety is situated below the digestive tube, and is known as the infra-intestinal. The pleural ganglia are often united to the opposite branch of the visceral nerve by an anastomosis of the pallial nerve, this condition constituting “dialyneury ” (Fig. 123, A, di’, di”): or there may be a direct connection by means of a longer or shorter connective pass- ing from the pleural ganglion to the ganglion borne on the visceral branch of the opposite side; this constitutes “zygoneury” (Fig. 123, B, C, zy’, zy). Zygoneury is more frequently found on the ——_ Fia. 123. Nervous system of 3 Streptoneurous Gastropods, showing the dialyneury and zygoneury, dorsal aspect. A, Paludina (after Bouvier, somewhat modified); B, Triton (after Haller); C, Lanveliaria (after Bouvier). ab.g, abdominal ganglion ; br.n, branchial nerve; ce.g, cerebral ganglion ; ¢.pe, cerebro-pedal connective ; ¢.pl, cerebro-pleural connective ; di’, di’, left and right dialyneury ; i.i.g, infra-intestinal ganglion ; ot, otocyst ; pa.n, pallial herve ; pe.g, pedal ganglion ; pl.g, pleural ganglion ; pl.pe, pleuro-pedal connective ; s.i.g, Supra-intestinal ganglion ; st.q, stomato-gastric ganglion ; vi.c, visceral commissure ; vic’, vi.c’, supra-intestinal and infra- intestinal part of the visceral commissure ; zy’, zy", left and right zygoneury. 143 144 THE GASTROPODA right side; the connective passes from the right pleural to the infra-intestinal ganglion, and may have the effect of bringing XX! \ Fie. 124. Trochus cinerarius, central nervous system, dorsal aspect, with the anterior part of the digestive tract. I, salivary gland; Il, cerebral ganglion; III, cerebro-pleural connective ; IV, pleural ganglion; V, right pallial nerve ; VI, pedal ganglion; VII, supra - intestinal part of the visceral commissure ; VIII, posterior part of the glandular oeso- phagus; IX, infra-intestinal part of the visceral commissure ; X, abdominal ganglion; XI, oesophagus ; XI, radula ; XIII, supra-intestinal ganglion 5, XIV, osphradial ganglion and branchial nerve; XV, left pallial anastomosis or dialyneury ; XVI, glandular oesopha- gus; XVII, left pallial nerve; XVIII, buceal mass ; XIX, cerebro-pedal con- nective ; XX, stomato-gastric ganglion ; XXI, snout. the latter ganglion between the two pleural centres (Fig. 123, C, 7.1.9). The head of Streptoneura bears only a single pair of tentacles (Fig. 125, @). The radular teeth, when there is more than one on either side of the median tooth, are of several different kinds in each transverse row (Fig. 74, C, F). The heart is almost always posterior to the branchia. The sub-class in- cludes two orders, Aspidobranchia and Pectinibranchia. OrpDER 1. Aspidobranchia. These are Streptoneura in which the nervous system is still but little concentrated (Fig. 124). The pedal centres have the form of long gan- glionated cords, to the anterior end of which the pleural centres are attached: the cerebral ganglia are widely separated from one another, and are united by a long commissure lying in front of the buccal mass and the salivary glands (Fig. 127, c.c). An infra-oesophageal or “labial ” cerebral commissure is present. The osphradium is but little specialised, and is situated on the branchial nerve. The otocyst contains numerous oto- conia. The eye is open (Fig. 100), or if closed has a very small pellucida. The central teeth of the radula are multiplied. Ctenidia are almost always present; they are bipectinate and free at their distal ends (Fig. 81, d). Asa rule, the Aspidobranchs exhibit well-marked traces of the original bilateral symmetry, having two auricles to the heart and two kidneys (Fig. 127), the last named opening to the exterior at the end of short papillae (Fig. 88, /). The gonad has no accessory organs and discharges its products into THE GASTROPODA 145 the right kidney. In the Neritacea, however, there is only one kidney, namely, the left kidney, with a slit-shaped external aperture, and there is a distinct genital orifice, the oviduct being diaulic in the Neritidae. The order Aspidobranchia includes the most archaic Gastropods : it includes two sub-orders, the Docoglossa and Rhipidoglossa. Svusp-ORDER 1. DocoGuLossa. In these Aspidobranchs the nervous system (Fig. 93) is without dialy- neury, that is to say, there is no anastomosis between the pleural ganglia _aud the visceral nerve of the opposite side through the intermediary of the pallial nerve. The eyes are open and devoid of a crystalline lens. There are two osphradia, but neither hypobranchial glands nor operculum. The mandible is unpaired and dorsal. The radula generally has trabeculi- form teeth (Fig. 74, F), and there are at most three marginal teeth on either side. The heart has only a single auricle (Fig. 82, au), and neither it nor the pericardium are traversed by the rectum. The visceral mass is cone-shaped, without a spire. The sub-order includes about 1400 species. Faminy 1. AcMAEIDAE, Philippi. A single bipectinate ctenidium, free for the greater part of its extent, is present on the left side (Fig. 82). Genera—Acmaea, Eschsholtz ; without pallial branchiae ; Great Britain. Scurria, Gray ; with pallial branchiae arranged in a circle beneath the mantle. Pectinodonta, Dall. Scenella, Billings ; from the Cambrian. Palaeacma, Hall ; from the Silurian. Famity 2. Trypiipupag, Pilsbry. Muscle sear divided into numerous separate impressions. Genus—Tryblidium, Lindstrém ; Silurian. Faminy 3. Pareniipa®, Guilding. No ctenidia, but only pallial branchiae disposed in a circle between the mantle and the foot (Fig. 125). Genera—Patella, Linnaeus ; the pallial branchiae forming a complete circle ; no epipodial tubercles; British seas. Ancistromesus, Dall; radula with an unpaired central tooth, which is wanting in Patella. Nacella, Schumacher; branchial circle complete; epipodial tentacles present. Helcion, Montfort; cirelet of pallial branchiae interrupted anteriorly, beneath the head ; British seas. Helcioniscus, Dall. FAmIty 4. Lereripar, Gray. Dioecious, with otoconia; the head symmetrical, the foot elongated ; neither ctenidia nor pallial branchiae present ; a central tooth in the radula. Genera—Lepeta, Gray; without eyes. Lepetella, Verrill ; with eyes. Pilidiwm, Forbes. Propilidiwm, Forbes and Hanley. Famity 5. Baruysctapipar, Dautzenberg and Fischer. Monoecious, with otoliths ; head provided with an appendage on the right side; radula without a central tooth. Genus— Bathysciadium, Dautzenberg and Fischer ; abyssal (Fig. 126). Susp-OrpDER 2. RHIPIDOGLOSSA. Aspidobranchia with a dialyneurous nerve-system, that is to say, with a pallio-visceral anastomosis (Fig. 124, XV); eyes with a crystalline lens (Fig. 100, I) ; a single osphradium, except in genera with two ctenidia ; one or two hypobranchial glands. Mandibles paired, lateral. Radula 10 146 THE GASTROPODA with very numerous marginal teeth, arranged like the sticks of a fan. Fic. 125. Patella vulgata, in its shell, seen from the pedal surface ; 2, y, the median antero-posterior axis. a, cephalic tentacle; b, plantar surface of the foot; c, free edge of the shell; d, the branchial efferent vessel carrying aerated blood to the auricle, and here interrupting the cirelet of gill lamellae; e, margin of the mantle-skirt ; /, gill lamellae—special pallial outgrowths (uot ctenidia) ; g, the branchial efferent vessel ; h, factor of the branchial advehent vessel ; 7, inter- spaces between the muscular bundles of the root of the foot. (After Lankester.) Oesophagus with a frill, oesophageal glands (Fig. 124, XVD, and a stomachal caecum, often coiled in a spiral (Fig. 127, sp.c). Heart with two auricles ; ventricle traversed by the rectum (Fig. 55) except in the Helicinidae, in which there is only a single auricle and the rectum only passes through the pericardium. An epipodial ridge on each side of the foot (Fig. 130, VIID, and cephalic expansions between the tentacles often present. FamMILy 1. PLEUROTOMARIIDAE. Visceral mass and shell spiral; mantle and shell with an anterior fissure (Fig. eet ian: 54, IIT) near the median line. ‘Two Bathysciadium, ventral aspect, magni- ctenidia ; a horny operculum. Genera— fied. ap, cephalie appendage ; f, foot; Plewrotomaria, Defrance ; epipodium with- epee. pee, pallial out tentacles; two bipectinate ctenidia (Fig. 127). Five living species from the Antilles, Japan, and the Moluccas. The first recent species (P. guoyana, Crosse and Fischer) was discovered in 1856; the animal was first THE GASTROPODA 147 obtained in 1871, in a collection made off the Barbadoes by the *‘ Hassler” (A. Agassiz). The Moluccan species is nineteen centimetres in height. The genus includes several hundred extinct species, ranging from the Silurian to the Tertiary, but is rare in the last named. Scissurella, d’Orbigny ; epipodium with tentacles ; right ctenidium mono- pectinate (Fig. 128). Schismope, Jeffreys; the slit at the margin of the shell is closed in the adult and transformed into an orifice (Fig. 62). The following genera are exclusively fossil: Porcellia, Léveillé ; Devonian and Carboniferous. Kokenella, Kittl; Trias. Ditremaria, d’Orbigny ; Jurassic. Polytremuria, de Koninck; Carboniferous. Trochotoma, Deslongehamps; ‘Trias and Jurassic. Cantantostoma, Sandberger ; Fig. 127. Pleurotomaria, with the pallial cavity laid open, right-side view. a, anus; aa, aorta ; a.v, afferent branchial vessel; br.g, branchial ganglion; c.c, cerebral commissure ; co, columellar muscle ; cr, crop ; e.v, efferent branchial vessel; f, foot; g, right or minor gill; g.d, genital duct; gg, gonad ; h, heart in the pericardium; hy.g, hypobranchial gland; in, intestine ; li, liver ; /.k.0, opening of the left kidney; m, mouth; m.s, mantle slit; od, odontophore ; oe, oesophagus ; op, operculum ; os, osphradium ; pe.c, pedal cord ; 7, rectum ; 7./k, right kidney ; r.k’, anterior part of ditto (dotted line) ; r.k.o, opening of the right kidney ; 7.s, radular sac ; s.¢, salivary gland ; sp.c, spiral caecuin ; st, stomach. (After F. M. Woodward.) Devonian. Murchisonia, d@’Archiac and Verneuil ; Cambrian to Trias. Odontomaria, Roemer ; Devonian. Famity 2. BELLEROPHONTIDAE, Mac- Coy. An exclusively fossil family, comprising more than 300 species extending from the Cambrian to the Trias. The shell is coiled in one plane and has an incision in the mid-dorsal margin of the aperture. Genera—Bellerophen, Montfort. Euphemus, MacCoy. Salpingo- stoma, Roemer. Trematonotus, Hall. Cyrtolites, Conrad. Famity 3. EvoMPHALIDAE, de Koninck. Also an extinct family, extending from the Cambrian to the Cretaceous. Spire slightly prominent ; umbilicus deep ; operculum calcareous. Genera—Zuomphalus, Sowerby. Strapa- rollina, Billings. Ophileta, Vanuxem. Maclwrea, Lesueur. Platychisma, MacCoy. Straparollus, Montfort. Phanerotinus, Sowerby. Discoheliz, 148 THE GASTROPODA Dunker. Famity 4. Hauiotmpan, Fleming. Spire of the visceral mass and shell much reduced; two bipectinate ctenidia, the right being mate Fic. 128. Scissurella euglypta, removed from its shell, veutral aspect, magnified. br.d, right gill ; br.s, left gill; m.vi, visceral mass ; oc, right eye ; op, operculum ; p, foot; pa, mantle; ¢, snout; fe, left cephalic tentacle; te.ep, epipodial tentacles; te.pa, pallial tentacle; te.p.o, post-ocular tentacle. the smaller; no operculum. Genus—Halvotis, Linnaeus (Fig. 129), FamiIty 5, VELAINIELLIDAE, Vasseur. An extinct family from the Eocene. Shell elongate, with numerous whorls ; columella and partitions Fic. 129. Haliotis tuberculata, right-side view. d, foot; i, tentacular process of the mantle, passing through the shell-foramina, (From Lankester, after Cuvier.) between the whorls absent ; internal cavity open from base to summit. Genus, Velainiella, Vasseur. FAmity 6. FissurELLIDAR, Risso. Visceral mass and shell conical ; the anterior part of the mantle exhibits a slit or a hole in the median line; two symmetrical ctenidia ; no operenlum. THE GASTROPODA 149 Genera — Emarginula, Lamarck; anterior border of the mantle and shell with a slit; British seas. Rimula, Defrance. Subemarginula, Blainville. Scutwm, Montfort ; mantle split anteriorly and partially reflected over the shell, which has no anterior slit. Zeidora, Adams. Puncturella, Lowe; mantle and shell with a foramen in front of the summit of the visceral cone; British. Fisswrella, Bruguiere; mantle and shell perforated by a hole at the summit of the visceral cone, the hole leading into the branchial chamber; British. Glyphis, Carpenter. Fissurellidea, VOrbigny. Pupillia, Gray; mantle completely covering the shell. Lwucapina, Gray. Megatebennus, Pilsbry. Macrochisma, Swainson. Lucapinella, Pilsbry. Faminy 7. CoccuLinipar, Dall. Shell conical, symmetrical, without slit or perforation; the summit inclined backwards. Genus— Cocculina, Dall; dioecious; abyssal. Famity 8. Trocuipar, d’Orbigny. Visceral mass and shell spirally \X vin Vil Fic. 130. Trochus (Gibbula) cinerarius, right-side view. I, shell; II, frontal lobes ; III, right eye and peduncle ; LV, right tentacle; V, appendage of the right ocular peduncle ; VI, snout; VII, right epipodial lobe; VIII, epipodium ; 1X, claviform appendage under the epipodial tentacle; X, posterior (operculigerous) part of the foot ; XI, epipodial tentacle. coiled ; a single ctenidium; eyes open (Fig. 100); a horny operculum ; flattened lobes between the tentacles (Fig. 130, II). Genera—T'rochus, Linnaeus ; no jaws; shell umbilicated ; spire pointed and prominent. Monodonta, Lamarck ; no jaws; spire not prominent; no umbilicus ; columella toothed. Clanculus, Montfort. Elenchus, Swainson. Photinula, Adams. Gaza, Watson, (Gibbula, Risso; with jaws; three pairs of epipodial cirrhi without pigment spots at their bases (Fig. 130); British. Margarita, Leach ; from five to seven pairs of epipodial tentacles with a pigment spot at the base of each. Livona, Gray. Basilissa, Watson. FAMILY 9. STOMATELLIDAE, Gray. Spire of the visceral mass and shell much reduced; a single ctenidium, Genera — Stomatella, Lamarck ; foot truncated posteriorly ; an operculum present; no _ epipodial tentacles. Gena, Gray; foot elongated posteriorly; no operculum ; epipodial tentacles present. Stomatia, Helbling; foot not truncated ; operculum and epipodial tentacles absent. Famity 10. DELPHINULIDAE, Fischer. Visceral mass and shell spirally coiled ; operculum horny ; in- 150 THE GASTROPODA tertentacular lobes absent. Genus—Delphinula, Lamarck ; with five pairs of epipodial tentacles. Famity 11. Liotimpan, Gray. Shell globular ; margin of aperture thickened; operculum horny, with a calcareous layer. Genus—Liotia, Gray. Famity 12.’ CycLostREMATIDAE, Fischer. Shell flattened, umbilicated, not nacreous; foot truncated anteriorly and with the two angles prolonged into tentacles. Genera—Cyclostrema, Marryat (Fam. 50), Teinostoma, Adams. Famity 13. TROCHONE- MATIDAE, Zittel. Exclusively fossil, from Cambrian to Cretaceous ; shell spiral and nacreous internally ; whorls without keels; aperture rounded. Genera—Trochonema, Salter ; from the Cambrian and Silurian. Eunema, Salter ; from the Ordovician to the Devonian. Amberleya, Morris and Lycett; from the Trias to the Cretaceous. Oncospira, Zittel ; Jurassic. Famity 14, Turpinipar, Gray. Visceral mass and shell spirally coiled ; epipodial tentacles present; eyes closed; operculum calcareous and thick. Genera—Turbo, Linnaeus; shell globular, thick, with short spire. Astralium, Link. Molleria, Jeffreys; shell thin, umbilicated, with very short spire. Cyclonema, Hall. Faminy 15. PHASIANELLIDAE, Troschel. Shell not nacreous, without umbilicus, with prominent spire and polished surface. Genus— Phasianellu, Lamarck. Faminy 16. Umponrpar, Adams. Shell flattened, not umbilicated, generally smooth, without a nacreous layer ; operculum horny. Genera—Umboniwm, Link. Isanda, Adams. Famity 17. NERITopsIDAR, Fischer. Shell semiglobular, with short spire ; operculum calcareous, not spiral. Genera—Neritopsis, Grateloup. Naticopsis, MacCoy ; from the Devonian to the Trias. FAMILY 18. Macturitipat, Fischer. Shell discoid, deeply umbilicated, with few whorls ; operculum spiral, thick. Genus—Muclurites, Lesueur ; from Cambrian and Silurian. Famity 19. NERI?IDAR, Lamarck. Shell with very low spire; without um- bilicus and without a nacreous layer ; internal partitions frequently absorbed ; operculum calcareous, provided with an apophysis; epipodium slightly developed, without tentacles; a single ctenidium; a cephalic penis present. Genera— Nerita, Adanson ; marine. Nerttina, Lamarck ; freshwater ; British. Neritodomus, Morris and Lycett; Jurassic. Deianira, Stoliezka ; Cretaceous. Septaria, Férussac ; shell boat - shaped, with a large aperture and symmetrical muscular im- pressions. Pleolus, Sowerby ; from the Jurassic and Cretaceous. Famity 20. TiriscaAnrpak, Bergh. With- out shell and operculum, but with a pallial cavity and a etenidium. Genus—Tivtiscania, Bergh (Fig. 131) ; iwahiaclinubind, from the Pacifie Ocean. Faminty 21. Henictnipar, dorsalaspect. I,eye; Pfeiffer. No ctenidium, but a pulmonary cavity peice ae aie allial present; epipodium without tentacles; heart with a Bhar eiegh.) gill. single auricle, and not traversed by the rectum; no inandible ; operculum without apophyses. Genera— Helicina, Lamarck. Hutrochatella, Fischer. Stoastoma, Adams. Bourcierta, Pfeiffer. Famimy 22. Hyprocentpar, Fischer. No ctenidium, but a pulmonary cavity present; foot obtuse ; operculum calcareous, with an Fig. -131. THE GASTROPODA 151 apophysis. Genus—Hydrocena, Parreys; from Dalmatia. Faminy 23. PROSERPINIDAE, Fischer. Differs from the two last families in not having an operculum, Genus—Proserpina, Gray ; from Central America. ORDER 2. Pectinibranchia. These are Streptoneura with a somewhat concentrated nervous system ; without a labial commissure, except in Paludina and Ampul- lavria. The nerve-collar is situated behind the buccal bulb, except in Ampullaria. ‘There is a single well-differentiated, independent, and often pectinated osphradium. The eye is always closed, and the internal cornea (pellucida) is extensive. Each otocyst contains a single otolith, except in some forms of Taenioglossa devoid of a pro- boscis, e.g. Paludina, Valvata, Ampullaria, Cyclophorus, Typhobia, Bythoceras, Nassopsis, certain Cerithiidae, etc. The central tooth of the radula is single or absent. There is no longer any trace of bilateral symmetry in the circulatory, respiratory, and excretory organs, the topographically right half of the pallial complex having completely disappeared. The heart has only a single auricle—that of the morphologically right side—and is not traversed by the rec- tum. The ctenidium is monopectinate and attached to the mantle throughout its length, except in Adeorbis and Valvatu, the latter genus being the only Pectinibranch with a bipectinate ctenidium. The single kidney usually opens directly by a slit-shaped aperture (but exceptionally by a ureter in Paludina, Cyclophorus, and Valvata), and never serves for the passage of the sexual products. The gonad always has a separate orifice of its own. The male generally has a penis (Fig. 44, A, 2). The Pectinibranchia are divided into two sub-orders—Taenio- glossa and Stenoglossa. Sus-OrpDER 1. TAENIOGLOSSA. In these Pectinibranchs the radula has normally three teeth on each side of the median tooth, viz. one lateral and two marginals (Fig. 2, B; 74, B). The stomatogastrie ganglia are situated behind the buccal mass, and are united to the cerebral centres by long connectives which are in part recurrent and deeply situated. The salivary ducts, when sufficiently long, traverse the nerve-collar. The oesophagus is nearly always devoid of an unpaired gland. Usually there is neither a probgscis##for a siphon. The sub-order includes two distinct groups or tribes, which are respectively creeping and swimming forms, namely, the Platypoda and Heteropoda. Trise 1. PLatypopa. Normal Taenioglossa, but slightly modified, and of creeping habit. The foot is flattened ventrally, at all events in its anterior part (Strom- bidae). The otocysts are situated close to the pedal nerve-centres. Acces- 152 THE GASTROPODA sory organs are rarely found on the genital ducts, but are present in Palu- dina, Cyclostoma, the Naticidae, Calyptraeidae, etc. Mandibles are usually present. The intestine is long. The Platypoda form the largest group of the Mollusca, comprising nearly sixty families of unequal value, some of which are not thoroughly well known from an anatomical point of view. Faminy 1. Patupinipag, d’Orbigny, Ctenidium monopectinate ; pedal centres in the form of ganglionated cords ; the kidney is provided with a ureter ; viviparous; fluviatile. | Genera—Paludina, Lamarck. Neo- thawma, Smith ; from Lake Tanganyika. Tylopoma, Brusina; from the Tertiary. Famity 2. CycLopHoripar, Gray. Pallial cavity devoid of a ctenidium and transformed into a lung; pedal centres in the form of ganglionated cords ; otocysts with otoconia; aperture of the shell and the operculum circular ; terrestrial. Genera—Pomatias, Hartmann ; shell turriculated. Diplommatina, Benson. Hybocystis, Benson. Cyclophorus, Montfort ; shell umbilicated, with a short spire and horny operculum. Cyclosurus, Morelet ; shell uncoiled. Dermatocera, Adams; foot provided with a horn-shaped protuberance at its posterior end. Spiraculum, Pear- son ; aperture provided with a sutural tube at its superior angle. FAMILY 3. AMPULLARIIDAE, Gray. A monopectinate ctenidium present, and to the left of it a pulmonary sac, separated from the ctenidium by an incom- plete septum ; oesophageal nerve-collar in front of the buccal bulb ; penis pallial; amphibious. Genera — Ampullaria, Lamarck ; visceral sac and shell coiled dextrally. Lanistes, Montfort ; shell sinistral ; spire short or obsolete. Meladomus, Swainson; shell elongated, sinistral. Famity 4, Lirrortnipak, Gray. Ctenidium monopectinate ; oesophageal pouches present ; pedal nerve-centres concentrated ; a pedal penis near the right tentacle. Genera—Littorina, Férussac ; shell not umbilicated ; foot devoid of appendages ; marine forms of semi-aerial habit (Fig. 85). Lacuna, Turton ; foot with two posterior appendages ; marine forms of exclusively aquatic habit (Fig. 7). Cremnoconchus, Blandford ; shell umbili- cated ; of exclusively aerial habit; Indian. Risella, Gray. Tectarius, Valen- ciennes, Famity 5. FossartDA®, Fischer. Shell turbinated and umbili- cated ; head with two cephalic lobes, as in some Rhipidoglossa. Genus— Fossarus, Philippi. Famity 6. Purpurtnipas, Zittel. An exclusively fossil family ; shell thick, with prominent spire, angular whorls, and oval aperture. Genera— Purpurina, VOrbigny ; Jurassic. Brachytrema, Morris and Lycett ; Jurassic. Scalites, Conrad ; Ordovician. Famity 7. PLANAXIDAE, Adams. Shell not umbilicated, with pointed spire; the external border of the shell sharp; a short pallial siphon. Genus — Planaxis, Lamarck, Famity 8. Cycrosromatspar, Pfeiffer. Pallial cavity transformed into a lung; pedal centres concentrated ; otocysts with otoliths ; no mandibles; a deep longitudinal pedal groove present ; terrestrial. Genera—Cyclostoma, Draparnaud ; shell turbinated ; opereu- lum caleareous; British. Omphalotropis, Pfeiffer; shell turriculated ; operculum horny. Famity 9. AcicuLipan, Gray. Shell narrow and elongated ; operculum horny ; pallial cavity a pulmonary chamber ; otocysts with otoconia. xenus — Acicula, Hartmann. Famiry 10. VALVATIDAE, Gray. Ctenidium bipectinate, free throughout its length ; a pallial filament on the right side ; hermaphrodite ; fluviatile. Genus— THE GASTROPODA 153 Valvata, Miiller (Fig. 132); British, Famity 11. Rissoripar, Gray. A monopectinate ctenidium ; epipodial filaments present ; one or two pallial tentacles; snout elongated. Genera — Rissoa, Fréminville ; operculum simple. Rissoina, d’Orbigny ; operculum with an apophysis. Stiva, Hedley. Famity 12. Lirioprpas, Fischer. Foot with an epipodium bearing three pairs of tentacles and an operculigerous lobe with two appendages ; in- habitants of the Sargasso weed. Genus—Litiopa, Rang. Faminy 13. ADEORBIIDAE, Fischer. Mantle with two posterior appendages ; ctenidium large and capable of being protruded from the pallial cavity ; shell depressed and umbilicated. Genus—Adeorbis, Wood ; British (Fig. 133). Famity 14. JErrreystpag, Fischer. Head with two long labial palps ; shell ovoid, umbilicated ; operculum horny, semicircular, concentric, and earinated. Genus—Jeffreysia, Alder. Faminry 15. HoMALOGYRIDAE, Sars. Shell flattened ; operculum horny, circular; no cephalic tentacles. & Fig. 132. Valvata piscinalis, dorsal aspect, Fig. 158. magnified. jf, foot; gi, gill; m, Adeorbis subcarinatus, ventral aspect. f, mouth ; op, operculum ; pa.t, pallial foot; g, gill; m, mouth; op, operculum ; tentacle ; sh, shell; te, cephalic ptt.c, pallial cavity ; pa.t, pallial tentacle ; sh, tentacle. (After Bernard.) shell; t, tentacle. (After F. M. Woodward.) Genera—Homalogyra, Jeffreys ; British. Ammoniceras, Vayssiere. FAMILY 16. SkeNEIDAE, Fischer. Shell depressed and umbilicated, with a rounded aperture ; cephalic tentacles long. Genus—Skenea, Fleming ; British. Famiiy 17. Cuortistipa®, Fischer. Shell spiral ; four cephalic tentacles; eyes absent; two pedal appendages behind the operculum. Genus—Choristes, Verrill. Faminty 18. ASsIMINEIDAE, Fischer. Shell conical, with a short spire; operculum horny, spiral ; eyes situated at the free extremities of the tentacles. Genus—Assiminea, Leach ; estuarine ; British. Famity 19. Truncatrertipan, Gray. Ctenidium mono- pectinate ; snout very long, bilobed ; foot very short; spire elongated and truncated ; marine and littoral. Genus—Truncatella, Risso. FAMILY 20. Hyprosipar, Fischer. Shell with prominent spire; ctenidium monopectinate ; sexes separate ; penis distant from the right tentacle and generally appendiculated ; otocysts with otoliths; brackish water or fluviatile. Genera—Hydrobia, Hartmann; shell conical, smooth with searcely convex whorls; operculum horny; brackish water; British. Baikalia, von Martens; from Lake Baikal. Pomatiopsis, Tryon ; foot 154 THE GASTROPODA divided into two sections by a transverse furrow; penis without an appendage. ithynella, Moquin-Tandon. Lithoglyphus, Miihlfeldt ; shell globular with short spire. Spekia, Crosse ; viviparous; from Lake Tanganyika. Tanganyicia, Crosse (Fig. 78). Limnotrochus, Smith ; from Lake Tanganyika. Chytra, Moore. Littorinida, Eydoux and Souleyet. Bithynia, Gray ; shell conical with an oval aperture; oper- culum calcareous, concentric ; habitat fluviatile; British. Stenothyra, Benson ; aperture rounded and contracted ; operculum calcareous, spiral. Famity 21. MELANiIDAE, Gray. Shell spiral, the spire somewhat elon- gated ; operculum horny ; foot and snout short ; mantle border fringed ; viviparous (Fig. 109); fluviatile. Genera—Melania, Lamarck ; shell turri- culated ; aperture rounded and enlarged anteriorly. Hauwnus, Montfort ; spire very long; aperture of shell notched anteriorly. Paludomus, Swainson; shell short, thick, with rounded aperture. Melanopsis, Férussac. Nuassopsis, Sinith. bythoceras, Moore ; from Lake Tanganyika. Famity 22. TypHopripar, Moore. Foot wide; tentacles elongate ; shell turriculated, with carinated whorls, the carinae tuberculated or spiny. Genera—Typhobia, Smith. Bathanalia, Moore; from Lake Tanganyika. Famity 23. PLeuRocERIDAE, Fischer. Like the Melaniidae, but the border of the mantle is not fringed and the reproduction is oviparous. Genera—Pleurocera, Rafinesque; shell elongated ; the aperture canaliculated anteriorly. Anculotus, Say ; shell short, globular ; the aperture rounded anteriorly. Famity 24. PSEUDOMELANIIDAE, Fischer. An exclusively fossil family ; shell turriculated, with prominent spire and elongated oval aperture. Genera—Pseudomelania, Pictet and Campiche ; Secondary and Tertiary. Loxonema, Phillips; Palaeozoic. Macrochilus, Phillips ; Devonian to Trias. Famity 25. Supuniripasr, Fischer. An exclusively fossil family ; shell turriculated with a narrow aperture, elongated and canaliculated anteriorly. Genera—Subulites, Conrad ; Cambrian to Car- boniferous. Fusispira, Hall; Ordovician. Huchrysalis, Laube ; Trias. Faminy 26. NERINEIDAE, Zittel. An exclusively fossil family ; shell with numerous whorls, with multiple folds in the lumen of the whorls. Genera— Nerinea, Defrance ; Jurassic and Cretaceous, Aptyxiella, Fischer ; Trias and Jurassic. Ptygmatis, Sharpe ; Jurassic and Cretaceous. FAMILY 27. CeritHupAr, Fleming. Shell with elongated spire and numerous tuberculated whorls; aperture canaliculated anteriorly; snout long; pallial siphon short. |Genera—Cerithiwm, Adanson ; aperture oval ; operculum oval, with submarginal nucleus. Sittiwm, Gray ; operculum circular, with central nucleus ; siphon rudimentary. Potamides, Brong- niart ; eyes situated above the bases of the tentacles; ctenidium rudi- mentary ; brackish water. Triforts, Deshayes ; shell sinistral. Laeocochlis, Dunker and Metzger. Cerithiopsis, Forbes and Hanley. Faminy 28. Mopuripag, Fischer. This family differs from the Cerithiidae in having a shell with a short spire, without a siphon ; the eyes are placed midway up the tentacles. Genus—Modulus, Gray. FAmity 29. VERMETIDAB, d’Orbigny. The animal is fixed by the shell, the last whorls of which are not in contact with one another ; foot small, discoidal, with two anterior pedal tentacles, one on each side of the supra-pedal gland. Genera—Vermetus, Adanson ; shell without a notch on the exterior border of the aperture ; 7. THE GASTROPODA 155 mantle slit in the female only (Fig. 45); pedal tentacles elongate. Siliquaria, Bruguitre ; mantle and shell slit in both sexes for the whole length of the branchial cavity ; pedal tentacles rudimentary. Famity 30. CAECIDAE, Gray. Shell almost completely uncoiled in one plane, and furnished with internal septa; aperture circular. Genus—Caecum, Fleming (Fig. 68) ; British. Faminy 31. Turrire.iipar, Clark. Shell very long with numerous whorls ; head large and prominent ; mantle border fringed ; no siphon; foot broad and truncated. Genera—Tuwrritella, Lamarck ; British. Jfesalia, Gray. Mathilda, Semper; the summit of the shell hyperstrophic. Fairy 32. SrruTHIOLARMDAL, Fischer. . Spire of shell conical; aperture pointed and subcanaliculated anteriorly ; foot oval, rather small; head elongate with short tentacles ; siphon very slightly developed. Genus—Struthiolaria, Lamarck. FAMILY 33. CHENOPODIDAE, Fischer. Spire of shell elongated ; margin of aperture expanded ; foot elongated and narrow ; snout short ; tentacles long; siphon very short. Genera — Chenopus, Philippi; British. Alaria, Morris and Lycett ; Jurassic and Cretaceous. Spinigera, d’Orbigny ; Jurassic. Dtartema, Piette ; Jurassic. Faminy 34. Stromprpak, Gray. Foot narrow, arcuate, compressed laterally, without ventral sole (Fig. 75, f) ; snout long ; ocular peduncles longer and stouter than the tentacles. Genera—sStrombus, Lin- naeus ; shell ovoid, with elongated aperture ; mantle border and aperture of shell not digitate. Pteroceras, Linnaeus ; mantle border and aperture of shell digitate. Rostellaria, Lamarck ; spire of shell elongate ; aperture prolonged anteriorly into a canal and laterally into an aliform expansion (Fig. 46). Terebellum, Klein ; shell elongated with a short spire ; tentacles aborted. Famity 35. XeNopHoRIDAE, Philippi. Snout elongated ; foot divided transversely into two parts, the posterior part bearing the oper- eulum ; shell conical, carinated. _Genera—Xenophorus, Fischer (Fig. 134) ; with foreign substances agglutinated on the shell. otrochus, Whitfield ; from the Silurian. Faminy 36. Caputtpan, Fleming. Visceral sac and shell conical, but slightly incurved posteriorly ; a tongue-shaped projection between snout and foot; columellar muscle horseshoe-shaped. | Genera— Capulus, Montfort. Thyca, Adams ; parasitic on Asterids; without a radula ; foot rudimentary. Platyceras, Conrad; from the Silurian onwards. Famity 37. Hipponycrpar, Fischer. Visceral mass and shell conical ; foot feebly muscular, capable of secreting a ventral calcareous plate ; animal fixed. Genera—Hipponyx, Defrance. Mitrularia, Schumacher ; the shell with an internal appendage shaped like a half-horn. FamILy 38. CALyPTRAEIDAE, Broderip. Visceral mass spiral; shell flattened, with a short spire; lateral cervical lobes present ; foot short and circular ; accessory genital glands present. Genera—Cualyptraea, Lamarck ; shell spiral, with central summit and circular aperture; British. Crepidula, Lamarck ; shell oval, with nearly obsolete spire and marginal summit, furnished with an internal horizontal posterior septum. Crucibulwm, Schumacher ; shell conical, with an internal corniform appendage (Fig. 69). Famity 39. Naricipar, Recluz. Foot divided into two, the posterior half bearing the operculum ; a wide epipodial velum ; tentacles flattened ; snout elongate; shell turbinated. Genus—Nurica, Recluz. FamiLy 40. Naticipak, Swainson. Foot highly developed and provided 156 THE GASTROPODA with an aquiferous system; propodium reflected over the head; eyes deeply seated or absent ; operculum spiral ; burrowing animals. Genera —wWNatica, Adanson; shell globular, thick and polished, umbilicated, with a semi-lunar aperture (Fig. 47); British. Amaura, Moller ; shell not umbilicated, thin, with an oblong aperture. Stgaretus, Lamarck ; shell auriform, with a very short spire and large aperture ; operculum small and rostrate. Faminty 41. LAMELLARIIDAE, d’Orbigny. Shell thin, more or less covered by the mantle, and with a small spire ; no operculum or propodium ; mandibles fused dorsally. Genera—Velutina, Fleming ; shell only partially covered by mantle; British. Lamellaria, Montagu ; shell internal, spiral, transparent ; British. Marsenina, Gray ; shell not completely covered by the mantle; hermaphrodite. Oncidiopsis, Beck ; Fic. 134. Xenophorus exutus, animal and shell, left-side view. a, snout; b, cephalic tentacles ; ¢, left eye; d, anterior part of the foot (to the right of this is seen the posterior lobe of the foot bear- ing the sculptured operculum f). (From Lankester, after Owen.) shell internal, membranous, without spiral; hermaphrodite. FAMILY 42, TrICHOTROPIDAE, Gray. Shell with short spire, umbilicated, carinate and pointed. Genus—Tvrichotropis, Broderip and Sowerby. Faminy 43. Secvenziwar, Verrill. Shell trochiform, with canaliculated aperture and twisted columella ; operculum spiral. Genus—Seguenzia, Jeffreys ; abyssal. Faminy 44. Janrarnipar. Shell thin; operculum absent ; tentacles bifid ; eyes absent ; foot short, provided with an epipodium and secretes a float ; radula with similar pointed teeth (Fig. 74, D); pelagic. Genera—Janthina, Lamarck ; shell blue, with a short spire ; ctenidium with long pointed filaments, capable of being protruded from the pallial cavity (Fig. 135). Recluzxia, Petit; shell white with elongated spire. Faminy 45, CyprarIpak, Fleming. Shell inrolled, solid, polished, the spire nearly hidden, the aperture very narrow in the adult; pallial aperture provided with a short anterior siphon ; a short proboscis ; anus posterior ; foot broad ; osphradium with three lobes; mantle reflected THE GASTROPODA 157 over the shell (Fig. 70). Genera—Cypraea, Linnaeus ; shell ventricose with a crenelated columella. Pustularia, Swainson ; differs from Cypraea in having an internal shell. Ovula, Bruguiére ; columella smooth, both ends of the aperture canaliculated (Fig. 136). Pedicwlaria, Swainson ; attached to corals ; foot small; shell irregular with an expanded aperture. Erato, Risso; shell piriform, with a prominent spire. Faminy 46. Trironipar, Adams. Shell turriculated and siphonated, thick, each Fic. 135. Female Janthina, with egg-float (a) attached to the foot. 0, egg-capsules ; c, ctenidium ; (7, cephalic tentacles. (From Lankester, after Owen.) whorl of the spire provided with varices; foot broad and truncated anteriorly ; pallial siphon well developed ; a proboscis. _Genera—Triton, Montfort ; varices not continuous from one whorl to another ; eyes at the bases of the tentacles (Fig. 44, A). Persona, Montfort ; whorls irregular ; eyes half-way up the tentacles. Ranella, Lamarck ; varices continuous from one whorl to another. Famity 47. CoLUMBELLINIDAE, Fischer. An exclusively fossil family ; shell with prominent spire, narrow aperture, and callous columella. Genera—Oolumbellina, VOrbigny ; cretaceous. Coluwm- bellaria, Rolle; Jurassic. Zittelia, Gemellaro; Jurassic. Petersia,Gemellaro ; Jurassic. Famity 48. Casstpipar, Adams. Shell ventricose, with elongated aperture and short spire ; foot broad and rounded anteriorly ; proboscis and siphon long ; oper- eculum with marginal nucleus. Genera—Cassis, Lamarck ; shell varicose, with narrow aperture. Cassidaria, Lamarck ; shell with- out varices, aperture oval and canaliculated. Oniscia, Sowerby ; shell oval, with a linear aperture. FaMiIty 49. OocoRYTHIDAE, Fischer. Shell globular and ws ops ventricose ; aperture oval and ,qj(ztltauinal and shell, right-side view. canaliculated : operculum spiral. is naturally carried in a reflected condition so as Genus—Oocorys, Fischer ; abyssal. Ba’ le of the shell. (From Lan- Famiry 50. Donupar, Adams. Shell ventricose, with short spire and wide aperture; no varices and no operculum ; foot very broad with projecting anterior angles ; siphon long. Genera—Doliwm, Lamarck; shell with a short canal; ocular tubercles distinct from the tentacles; mantle not reflected over the shell. Pyrula, Lamarck ; canal long ; spire very short ; mantle reflected over the shell; eyes sessile (Fig. 71). Famitry 51. Sovarpag, 158 THE GASTROPODA Chenu. Shell spiral, conical, with flattened spire, umbilicated ; head short; tentacles split throughout their length; foot short. Genera— Solarium, Lamarck. Torinia, Gray. Flusvina, Dall. Faminy 52. SCALARIIDAR, Broderip. Shell turriculated with numerous whorls and an elongated spire; head short, with a short proboscis; foot small, truncated anteriorly ; siphon rudimentary. Genera—Scalaria, Lamarck ; shell elongate with a circular aperture, whorls very convex, ornamented with longitudinal projecting lamellae ; British. Lyglisia, Gray. Crossea, Adams. Aclis, Loven. The three following families of Taenioglossa Platypoda have neither radula nor jaws, and are therefore called Aglossa. ‘They are suctorial animals with a well-developed proboscis, and are often commensal or parasitic on Echinoderms; some are abyssal. The series affords a remarkable example of the regressive evolution of various organs as a result of parasitism. Famity 53, PyrAMIDELLIDAE, Gray, Summit of spire heterostrophic (Fig. 65, B); tentacles deeply grooved externally or split at their extremities ; foot truncated anteriorly ; a projection, the “mentum,” between the head and foot; an operculum present. het Moke Oh ‘f Fic. 137. Turbonilla scalaris, right-side view. jf, foot; m, mouth; me, mentum ; op, operculum ; pa, mantle; sh, shell; te, tentacle. (After Loven.) Genera—Pyramidella, Lamarck ; columella folded, tentacles corniform. Turbonilla, Leach; columella not folded (Fig. 137). Odostomia, Fleming ; columella provided with a tooth; hermaphrodite ; British. Myxa, Hedley. Famity 54. Evuuimmpar, Adams. Visceral mass still coiled spirally ; shell thin and shining, generally with a pointed summit; tentacles without a groove. Genera—LEulima, Risso; foot well developed, and with an operculum ; animal usually free, but some live in the digestive canal of Holothuriae in the Fiji Islands, in the Philippines, and in Europe, e.g. Lulima distorta in Holothuria intestinalis. Niso, Risso. Scalenostoma, Deshayes. Hoplopteron, Fischer. Mucronalia, Adams (=Stylina, Fleming); foot reduced, but still operculate ; eyes present ; animal fixed by its very long proboscis, which is deeply buried in the tissues of an Echinoderm ; no ‘paeudop: illium. Stylifer, Broderip ; the operculum is lost, but a sadinient of the foot remains ; tentacles very small or absent ; eyes, otocysts, and a branchia present ; animal fixed by a large proboscis forming a pseudopallium which surrounds the whole of the shell except the more or less projecting extremity of the spire (Fig. 20); sexes separate ; parasitic on all groups of Echinoderms in different seas. | Entosiphon, Koehler and Vaney ; visceral mass still coiled ; shell much reduced ; proboscis very long, forming a pseudo- _ Sac ; THE GASTROPODA 159 pallium, which covers the whole body and projects beyond in the form of a siphon, and serves to put the animal in communica- tion with the external world and for the passage of the ova (Fig. 21); a foot is retained, and also a nervous system and oto- cysts; neither eyes, branchia, anus, nor rectum; the stomach is a sac with ramifying caeca ; hermaphrodite ; parasitic in the Holothurian Deima blakei, in the Indian Ocean. ntosiphon forms the transition to the next family. Famity 55. Enrto- CONCHIDAE, Fischer (= Cochlo- syringia, Voigt). Neither shell nor spirally coiled visceral mass ; no sensory organs, nervous system, branchia, or anus ; body reduced to a more or less tubular endoparasitic in Holo- thurians ; probably all herma- phrodite, with separate male and female gonads ; incubatory (“viviparous”); with conchi- ferous and operculiferous veliger larvae, without a retractor veli muscle. Genera — Entocolax, Voigt ; visceral mass essentially genital and forming a swelling surrounded by the pseudo- pallium ; digestive orifice or proboscis at the free extremity ; orifice of the pseudopallium at the opposite extremity by which the animal is fixed; a second accessory aperture of the pseudo- pallium serves for the passage of the genital products. Two species parasitic in Holothurians in the Pacific: EF. ludwigi, in Myriotrochus rinkiit from the Behring Sea (Fig. 138); and £. schiemenzt in Chirodota pisanii from Chili. Entoconcha, J. Miiller Fia. 138. Entocolux ludwigi, in situ, x 30. I, fixative apparatus; II, ovary; III, uterus; IV, buccal orifice ; V, oviduct; VI, genital orifice ; VII, ova separated from the ovary, by dehiscence; VIII, cavity around the ovary, formed by the pseudo- pallium ; [X, orifice of this cavity ; X, integument of the Holothuria. (After Voigt.) (Fig. 139) ; body elongated and tubular ; the aperture of the digestive tract rudimentary and situated at the fixed extremity of the body ; protandric 160 THE GASTROPODA hermophrodite with separate male and female gonads ; parasitic in the testis of Holothurians, causing their abortion. Three species are known: one in Synapta digitata (Mediterranean), one in Holothwria edulis (Philippines), and one in a Holothuria from Puget Sound in the North-East Pacific. Hnteroxenos, Bonnevie ; no pseudopallium and no alimentary tract; male and female gonads separate, with a single common genital orifice; larvae operculiferous. #. ostergreni (Fig. 140); parasitic in the intestine of Stichopus (Norway). Fic. 139. Entoconcha mirabilis, in_situ, mag- nified. I, oral extremity ; I1, remains of the digestive tract ; III, testis; IV, Fic. 140. ovary; V, antimesenteric vessel of : , the Synapta in which Entoconcha is Enteroxenos ostergreni, Bonnevie. parasite. (After J. Miiller.) ov, eggs. (After Bonnevie.) TRIBE 2. HETEROPODA. These are free-swimming Taenioglossa, with the foot flattened laterally and the otocysts situated near the cerebral ganglia. There are no mandibles and the intestine is short. All the Heteropoda are pelagic, and are much modified in adaptation to this mode of existence. The foot is very large, and has the form of a fin compressed bilaterally ; it bears, in the male at least, a sucker on its ventral aspect (Fig. 142, d’). The visceral sac or “nucleus” and mantle form a progressively smaller and smaller part of the mass of the body (compare Figs. 142 and°143), but the head always remains large and forms a cylindrical snout. The cerebral nerve-centres are in juxtaposition; the pleural ganglia, still visible in the Atlantidae and Pterotracheidae, are attached to them, and there are thus two pedal connectives on either side, namely, the cerebro- pedal and the pleuro-pedal ; these are separate proximally in Atlanta, but THE GASTROPODA 161 fused together for their whole length in other forms, The pedal centres are situated at the base of the fin (Fig. 142, v). The visceral commissure is fairly long, is crossed, and bears several ganglia, but there is neither dialyneury nor zygoneury. In the Carinariidae, however, there are secondary uncrossed viscero-pedal anastomoses, and in the Pterotracheidae the pedal connectives are fused with the anterior part of the visceral commissure, and behind the pedal ganglia the two branches of this com- missure are fused together for the greater part of their length. The osphradium is a more or less elongated ciliated organ, situated in the pallial cavity to the left of the branchia. The otocysts are situated near Fic. 141. Oxygyrus keraudreni, male, right-side view. A, head ; a, mouth and odontophore ; B, anterior part of the foot; bh, cephalic tentacles ; c, eye; d, natatory foot and its sucker ; e, posterior lobe of the foot; f, operculum; h, mantle and pallial cavity ; 7, ctenidium ; /:, retractor muscle of the foot(columellar muscle) ; 7, optic tubercle ; m, oesophagus ; n, salivary gland ; 0, rectum and anus ; 7, liver ; 4, kidney ; s, ventricle ; wu, the otocyst attached to the cerebro-pleural ganglion ; w, testis; xz, auricle of the heart; y, vesicula seminalis; z, penis. (From Lankester, after Souleyet.) the cerebral ganglia (Fig. 141, uv). The eyes are very large and highly differentiated in structure ; they are placed at the sides of the cerebral ganglia and at the bases of the tentacles (Fig. 141, c) when the latter organs exist (Pterotrachea and the female in some Firoloida are devoid of tentacles). The alimentary canal is furnished with a protractile pharynx containing a characteristic Taenioglossate radula with very powerful lateral and marginal teeth. The oesophagus is very long and slightly dilated in the middle of its length. The stomach and liver are situated posteriorly (Fig. 142, n) , the intestine is always very short, and in the Pterotracheidae it is no longer bent forward (Fig. 143). The heart is situated near the stomach, and in the less specialised Heteropoda (Atlantidae, Carinariidae) is clearly disposed in the same manner as in other Streptoneura, but in the Pterotracheidae, which have undergone detorsion, it has clearly become an opisthobranch II 162 THE GASTROPODA heart. In the Atlantidae there is an aortic bulb; the arterial vessels always end abruptly in sinuses. The ctenidium is monopectinate and completely enclosed in the pallial cavity in Atlanta (Fig. 141, 2), but it pro- jects in Carinaria (Fig. 142, 2), is no longer covered by the mantle in Ptero- trachea (Fig. 143, br), and finally has completely disappeared in Firoloida. The kidney is a transparent and sometimes contractile sac, which has the same relations as in other Taenioglossa and opens not far from the anus (Fig. 141, q). The gonad is situated beside the liver (Fig. 141, w). The genital duct is always rather short, and opens alongside of the anus; in the male it exhibits a dilatation, the vesicula seminalis (Figs. 141 and 142, y), and its aperture communicates with the penis by means ofa seminal groove. The penis is situated at the base of the foot, and is provided with a glandular Fia. 142. Carinaria mediterranea, male, right-side view. A, the animal; B, the shell removed; C, D, two views of the shell of Cardiopoda. a, mouth and odontophore ; b, cephalic tentacles ; ¢, eye ; d, the fin-like anterior lobe of the foot ; @’, its sucker ; e, posterior part of the foot; f, salivary glands ; h, margin of the mantle ; 7, ctenidium ; m, oesophagus ; , stomach ; 0, anus ; p, liver ; t, aorta, springing from the ventricle ; w, cerebro-pleural ganglion ; v, pedal ganglion ; w, testis ; x, visceral ganglion ; y, vesicula seminalis ; z, penis. (From Lankester, atter Souleyet.) appendage or flagellum. In the female the genital duct is furnished with a copulatory bursa and an albuminiparous gland. The Heteropoda lay floating eggs imbedded in a gelatinous matrix ; the larvae are charac- terised by the velum, which is divided into four or six lobes. All the Heteropoda are pelagic and transparent, and are generally found in dense bands in warm and temperate zones, swimming slowly in a reversed, position, that is to say, with the foot uppermost. They are all car- nivorous. The tribe includes three families which afford a good example of regressive evolution accompanying a process of detorsion and a return to bilateral symmetry, as in the Opisthobranchs, The specialisation of the group is marked by a progressive reduction, and finally by the disappearance first of the operculum, afterwards of the mantle, and finally of the ctenidium and tentacles) The genus Atlanta is still provided with a well-developed coiled shell and an operculum, and is THE GASTROPODA 163. characteristically prosobranchiate. In Carinaria the shell is uncoiled and rudimentary, and there is no operculum. Pterotrachea has neither shell nor tentacles and is opisthobranchiate. Finally, Firoloida has lost the ctenidium. Faminty 1. ATLANTIDAE, Rang. Visceral sac and shell spirally coiled in one plane ; foot divided transversely into two parts, the posterior part bearing an operculum with a sinistral coil (Fig. 48), while the anterior art forms a fin provided with a sucker. Genera—Oxygyrus, Benson ; shell capable of containing the entire animal, carinated only on the last whorl and near the aperture. Atlanta, Lesueur; shell capable of con- taining the whole animal, carinated throughout ; aperture with fissures (Fig. 141). Famrny 2. CartNaran, Grasset. Visceral sac and shell conical and small in proportion to the rest of the body, which cannot be withdrawn into the shell ; foot elongated, fin-shaped, with a sucker but without an operculum. Genera—Carinaria, Lamarck (Fig. 142). Cardio- Fic. 143. Pterotrachea mutica, seen from the right side. a, pouch for the reception of the snout when retracted ; br, ctenidiuin; c, pericardium ; g, cerebral ganglion ; g’, pedal ganglion ; i, intestine ;: mt, posterior part of the foot ; n, so-called visceral nucleus ; oc, cephalic eye ; ph, pharynx ; pr, fin-like anterior part of the foot ; v, oesophagus ; w, osphradium ; z, caudal appendage. (From Lankester, after Keferstein.) poda, VOrbigny (Fig. 142, C, D). Faminy 3. PrerorracnErmpar, Gray. Visceral sac very much reduced, without shell and mantle ; anus on the posterior part of the body ; foot provided with a sucker in the male only. Genera—Pterotrachea, Forskil ; no tentacles ; a ctenidium present ; a filiform appendage at the posterior extremity of the foot (Fig. 148). Firoloida, Lesueur ; tentacles present, but no ctenidium and no posterior appendage to the foot. Pterosoma, Lesson. SuB-ORDER 2. STENOGLOSSA. Pectinibranchs in which the nervous system is much concentrated and always zygoneurous. The perioesophageal nerve-collar is always posterior to and is not traversed by the salivary glands. The stomato- ‘gastric ganglia are situated close to the cerebral nerve-centres and far behind the buccal mass, the last-named organ being greatly reduced. A well-developed proboscis, an unpaired oesophageal gland (the gland of Leiblein or poison-gland), a pallial siphon, and a penis are always present. The osphradium is bipectinate, The radula is narrow, and in the majority of genera (Rachiglossa) has a single lateral on each sidé of the median or rachidian tooth: in the remainder of the group (Toxiglossa) there is no median tooth, and the radular formula is therefore 1.0.1. The sub-order is accordingly divided into two tribes. 164 THE GASTROPODA TRIBE 1. RACHIGLOSSA. These are Stenoglossa with a highly-developed proboscis, a pallial siphon, and rudimentary jaws: the radular formula is 1.1.1 (Fig. 74, H). Famity 1. TURBINELLIDAE, Sowerby. Shell solid, piriform, with a thick folded columella ; foot broad ; proboscis long; tentacles conver- gent; lateral teeth of the radula bicuspidate. Genera—Turbinella, Lamarck ; shell with short spire and long canal. Cynodonta, Schumacher ; spire and canal short; shell tuberculated. Fulgur, Montfort; shell piriform ; tentacles short. Hemifusus, Swainson; shell fusiform with carinated whorls ; tentacles short (Fig. 99). Tudicla, Link. Strepsidura, Swainson. Faminy 2. FascroLarimpar, Adams. Shell elongated, with a long siphon ; head small and narrow, with short tenacles ; foot rather broad and short; lateral teeth of the radula multicuspidate. Genera— Fasciolaria, Lamarck. Fusus, Lamarck. Clavella, Swainson. Latirus, Montfort. Famity 3. Mirripar, Adams. Shell fusiform and solid, the spire pointed, the aperture elongated and the columella folded; no operculum ; tentacles elongated, bearing the eyes at their sides; foot narrow ; proboscis very long; siphon moderately long. Genera—WMitra, Lamarck. Turricula, Klein. Cylindromitra, Fischer. Imbricaria, Schumacher. Faminy 4. Buccinipan, Fleming. Foot large and broad ; eyes at the bases of the tenacles; shell ovoid, with oval aperture; a horny operculum. Genera—Chrysodomus, Swainson; shell fusiform, solid, with an unguiculate operculum; British. Liomesus, Stimpson ; shell ovoid, with a very short canal ; tentacles short ; lateral teeth of the radula unicuspidate. Buccinwm, Linnaeus ; shell ventricose with a wide aperture ; operculum oval with sub-central nucleus ; tentacles moderately long ; lateral teeth with three or four cusps; British. Cominella, Gray ; shell fusiform ; the operculum oval to piriform, with an apical nucleus. Tritonidea, Swainson ; shell ventricose ; operculum like that of Cominella. Pisania, Bivona; shell with a short canal; operculum unguiculate ; lateral teeth tricuspidate. Huthria, Gray ; shell fusiform, with elongate spire and canal. Phos, Montfort ; foot broad with two lateral projections anteriorly and a slender posterior filament. Dipsacus, Klein; foot elongated ; tentacles long ; shell ovoid, solid, with a short canal; lateral teeth bicuspidate. Faminy 5. Nassiparn, Swainson. Foot broad, with two slender posterior appendages ; siphon long; shell ovoid, with a short canal; operculum unguiculate. Genera— Nassa, Lamarck ; external border of the aperture of the shell thickened ; marine ; British. Candia, Adams; exterior border of the aperture simple; fluviatile. Bullia, Gray; shell polished; tentacles without eyes; foot very broad; a burrowing form. Famitry 6. Muricipan, Fleming. Foot truncated anteriorly ; tentacles elongated, bearing the eyes on their sides, more or less high up; shell with moderately long spire and canal, ornamented with ribs, often spiny. Genera—Murex, Linnaeus ; eyes half-way up the tentacles ; canal almost closed ; British. Trophon, Montfort ; eyes at the bases of the tentacles; shell lamellar; canal open; British. Typhis, Montfort ; sheli with closed canal and tubular spines. Urosalpina, Stimpson. Lachesis, Risso. Faminy 7. PurpurmIDAE, Broderip. Foot THE GASTROPODA 165 short, obtuse posteriorly ; shell thick with a short spire, the last whorl large and the canal short ; aperture wide ; columella flattened ; operculum horny. Genera—Purpura, Bruguiére; shell not umbilicated, aperture smooth ; British. Rapana, Schumacher ; shell ventricose, umbilicated. Monoceros, Lamarck ; shell like that of Purpura, but the aperture shifted backward and bearing a conical tooth on its external border. Sistrwm, Montfort ; shell thick, spiny, the aperture contracted by the thickening of the margins of the aperture. Concholepas, Lamarck ; shell ovoid, the spire short and the aperture widely dilated. Faminy 8. Ha.ipag, Fischer. Foot large and thick ; without an operculum ; tentacles thick and flattened ; shell ventricose, thin, and smooth, with a wide aperture. Genus—Halia, Risso; from Cadiz and Morocco, Faminty 9. CANcEL- LARIIDAE, Adams. Snout short ; tentacles long, with the eyes at their bases and external; foot small; no operculum; siphon short; shell ovoid with short spire and folded columella. Genus — Cancellaria, Lamarck. Famity 10. CoLUMBELLIDAE, Adams. Foot large, tentacles long and convergent; spire of shell prominent, aperture narrow, the canal very short and the columella crenelated. Genus — Columbella, Lamarck. Famity 11. CORALLIOPHILIDAE, Chenu. Foot short; ten- tacles slender and convergent ; siphon short ; radula absent; shell irregular; sedentary animals living in corals. Genera— Corallio- phila, Adams; shell deformed, with a wide aperture and a short canal ; operculum present. Rhizochilus, Steenstrup; no operculum; the aperture of the shell irregular, with the canal prolonged into a tube. Leptoconchus, Riippel ; no operculum ; the shell globular with a wide aperture. Magilus, Montfort; an operculum present ; the last whorl of the shell uncoiled and very thick. Rapa, Klein; an operculum present ; shell globular and umbilicated, the aperture provided with a canal. Famiry 12. Vouuripar, Gray. Head very flattened, and transversally widened, with the eyes on the — Conus tineatus, ventral aspect. sides ; snout short ; foot broad; siphon with aa Sree et ate pegs amnd raat internal appendages. Genera—Voluta, Lin- cavity ; III, peeling ‘WW, nn naeus; head with eyes; Australian seas. ee Ynare and bokoke vw Guivillea, Watson ; no eyes ; abyssal. Cymba, siphon; VII, mouth. (After Broderip and Sowerby ; viviparous. Famiiy cesta 13. Oxivipak, d’Orbigny. Eyes, when present, on the middle of the tentacles; fore part of the foot with a transverse groove; a posterior pallial tentacle ; generally burrowing. Genera—Oliva, Bruguitre ; eyes ; no operculum. Olivella, Swainson ; tentacles without eyes ; an operculum. Ancillaria, Lamarck. Agaronia, Gray. Famity 14. MARGINELLIDAE, Adams. Foot very large; mantle reflected over the shell. Genera— Marginella, Lamarck ; foot without operculum; a central gland-pore. Pseudomarginella, Carrivre ; foot with an operculum and an anterior gland- Fic. 144. 166 THE GASTROPODA pore. Famity 15. Harpipar, Chenu. Foot very great ; without oper- culum; shell with a short spire and longitudinal ribs; siphon long. Genus—Harpa, Lamarck. TRIBE 2, TOXIGLOSSA. Stenoglossa without jaws, and with a radular formula of 1.0.1; a **poison gland” present, whose duct traverses the nerve-collar. Famity 1. PLevrotomatipak, Loven. Shell fusiform with an elongated spire ; the margin of the shell and mantle notched ; siphon rather long ; eyes situated on the sides of the tentacles. Genera—Clavatula, Lamarck ; operculum piriform with a lateral nucleus ; eyes near the extremities of the tentacles. Plewrotoma, Lamarck ; operculum oval with nucleus near the summit ; eyes near the bases of the tentacles. Mangilia, Risso ; no operculum ; eyes half-way up the tentacles. Bela, Gray. Pusionella, Gray. Pontiothawma, Smith. Famity 2. TEREBRIDAE, Adams. Shell turriculated, with numerous whorls ; aperture and operculum oval ; foot small ; eyes at the summits of the tentacles; siphon long. Genus— Terebra, Adanson. Famity 3. Contpar, Gray. Shell conical, with a very short spire and a narrow aperture with parallel borders; eyes borne near the middle of the external sides of the tentacles ; an ungui- form operculum. Genus—OConus, Linnaeus (Fig. 144). Sus-Ciass II. EurHyNEuRA, Spengel (= Platymalakia, von Jhering = Androgyna, Mérch). These are hermaphrodite Gastropoda, whose radula is generally composed of uniform teeth on each side of the median tooth (Fig. 145). The head in most cases bears two pairs of tentacles ; it is only in Lophocercus, the | Elysiomorpha, Hedyle milaschewitchi, \\\ and the Janellidae that iN \)\ there is a single pair. © The Euthyneura are My oe ee specially characterised ao fart lain estes rowetieetofthemauln; forms: “hy the detox organisation whenadult; this detorsion is particularly well manifested in the visceral com- missure, which is no longer twisted, except in some archaic forms of Tectibranchs (Actaeon, Fig. 57) and Pulmonates (Chilina), and shows a tendency to the concentration of all its elements round the oesophagus (Fig. 146). To such a degree is this concentration carried that, with the exception of the majority of the Bullomorpha and of Aplysia (Fig. 95), the whole central nervous system is aggregated in the cephalic region (Fig. 97), sometimes on the dorsal side, as in the Pleurobranchidae and Nudibranchia (Fig. 159), sometimes on the ventral side as in the Thecosomata (Fig. 60, 7.8). The pedal centres are frequently united by a second “ parapedal ” THE GASTROPODA 167 (Figs. 95, pe, and 96, g.pe) commissure. ‘The sub-class includes the two orders, Opisthobranchia and Pulmonata. ORDER 1. Opisthobranchia, Milne-Edwards. Marine Euthyneura with aquatic respiration ; the ventricle of the heart is generally anterior, and the pallial cavity, when present, is widely open. There is a marked tendency to a reduction of the shell, which may become internal or disappear. In the naked forms spicules are sometimes developed (Pleurobranchidae, Dorido- morpha, Hedylidae, Fig. 168, sp.). The order comprises two sub- orders, Tectibranchia and Nudibranchia. Fie. 146. Central nervous system of Limnaew stagnalis, right-side view. bu, buccal mass; g.a, ab- dominal ganglion ; g.vu, buccal or stomato-gastric ganglion; ¢.ce, cerebral ganglion ; g.p, pedal ganglion ; g.pl, pleural ganglion; g.s.i, supra-intestinal ganglion ; lo.d, dorsal lobe of cerebral ganglion ; /o.1, lateral lobe of cerebral ganglion ; ”.co, columellar nerve ; n.la, labial nerve ; 7.0. optic nerve; n.pa, right pallial nerve; “.par, parietal nerves; n.pe, penial nerve; 1.te, tentacular nerve ; oe, oesophagus ; of, otocyst and nerve. Sus-OrpER 1. TECTIBRANCHIA, Cuvier. Opisthobranchs provided in the adult state with a mantle and a shell, with the exceptions Runcina, Plewrobranchaea, the Cymbuliidae, and some Aplysiomorpha. There is a ctenidium, except in some “Thecosomata ” and “ Gymnosomata,” and an osphradium. The sub-order includes three tribes, the Bullomorpha, the Aplysiomorpha, and the Pleurobranchomorpha. TRIBE 1. BULLOMORPHA. In these Tectibranchs the shell is usually well developed (it is want- ing in Ituncina and the Cymbuliidae), and may be external or internal. There is no operculum except in the Actaeonidae and Limacinidae (Fig. 49, op). The pallial cavity is always well developed, and contains the ctenidium, in part at least: this ctenidium, except in the Lophocercidae, is of the “folded” type. With the exception of the Aplustridae, Lopho- cercidae, and Thecosomata, the head is devoid of apparent tentacles, and its dorsal surface forms a digging dise or shield usually separate from the neck, and with more or less scolloped margins. The edges of the foot (parapodia) are continuous with the ventral face of that organ (Fig. 147, f), and are often transformed into highly-developed fins (Fig. 151, VI). 168 THE GASTROPODA Posteriorly the mantle forms a large “pallial lobe” under the pallial aperture (Figs. 98, 7.1; 148, I). The stomach is generally provided with chitinous, or even calcified, masticatory plates (Fig. 76, m.p). The visceral commissure is fairly long, except in such specialised forms as Runcina, Lolnger, and the Thecosomata (Fig. 60, ”.s). The hermaphrodite genital aperture is connected with the penis by a ciliated groove, except in Actaeon (Fig. 148), Lobiger, and Cavolinia longirostris, in which the spermiduct is a closed tube. The Bullomorpha are swimmers or burrowers, Faminy 1. ActazontpAr, Adams. Cephalic shield bifid posteriorly ; the margins of the foot slightly developed ; the genital ducts diaulic ; the visceral commissure streptoneurous ; the shell thick, with a prominent spire and elongated aperture; columella generally folded ; a corneous paucispiral operculum. Genera—Actacon, Montfort ; British (Fig. 148). Solidula, Fischer von Waldheim. Tornatellwa, Conrad ; extinct. Triplaca, Tate ; from the Eocene. Adelactacon, Cossmann. Actaeonina, dOrbigny ; Carboniferous to recent. Bullina, Férussac. Bullinula, Beck, Actaeo- Fig. 147. Acera bullata, swimming, left-side view. /, fin (foot) ; m, mouth 3 sh, shell. (After Guiart.) nella, VOrbigny ; from Fic. 148. the Cretaceous. Vol- Actaeon tornatilis, removed from its shell, right-side view. yee " I, inferior lobe of the mantle; II, glandular hollow pallial Carra,, Lamarck ’ Eocene. appendage (extending to the first whorls) ; III, pallial (hypo- Odostomiopsis, Thiele, branchial) gland ; IV, opening of the pallial cavity ; V, eye and Fawiy 2. Rinaicu- cephalic hood ; VI, penis. LIDAE, Fischer. Cephalic disc enlarged anteriorly and forming an open tube posteriorly ; shell external, thick, with a prominent spire ; no operculum. Genera—Ringicula, Deshayes. Pugnus, Hedley. Cinulia, Gray; from the Cretaceous. in oll. otber Lamellaaaa kines; Ay, wisceral commsire:,V_ntesting; and especially in the siphonate liver; X, afferent sinus ; XI, retractor muscle of forms, there are always two the labial palps; XII, auricle; XIII, ovary; XIV, * Piha. ev. ventricle. aortae, an anterior and a pos- terior, clearly separated from one another and of more or less equal importance. The anterior aorta is dorsad and the posterior ventrad of the intestine, except, of course, in Nucula and other forms in which the heart is dorsal. The pedal branch of the anterior aorta passes between the cerebral and pedal ganglion-pairs. In Ostraea, Vulsella, Tridacna, and Teredo the two aortae are secondarily fused to form one ; again an instance of convergence in unrelated species, due to the shortening of the antero-posterior axis of the body. vill Fic. 204. _—_ —— - ae . ae “a THE LAMELLIBRANCHIA 225 The foot, the mantle, and the siphons derived from the latter are gorged with blood when their muscles are relaxed, and their sudden contraction often produces a reflux of arterial blood towards the heart. In Lamellibranchs with a well-developed foot and siphons, the return of blood into the ventricle is prevented by valves situated at the origins of the aortae, and a sphincter is also often to be found at the root of the posterior aorta, and sometimes a valve in the siphonal artery. In addition, highly developed aortic bulbs, separated from the ventricle by one of the above- mentioned valves, are frequently present, generally on the posterior aorta, where a large bulb may be seen, within the pericardium, in many Siphonates, particularly in the Veneridae (Zapes, Fig. 202, a.0), Petricolidae, Tridacnidae, Mactridae, etc. A bulb or aortic dilata- tion also occurs on the anterior aorta, inside the pericardium in Pecten and the Mytilidae, outside the pericardium in Anodonta. The arterial blood forced back towards the heart by the contraction of the foot or mantle or siphons enters and fills these various bulbs. The blood carried to the different parts of the organism by the ultimate ramifications of the arterial trunks finally enters the venous sinuses, of which the most important are the pallial sinuses, the pedal sinus, and the great median ventral sinus. The last named is situated between the pericardium and the foot, and is separated from the pedal sinus by the valve of Keber, which prevents the foot from emptying itself of blood when in movement. It is from this great unpaired median sinus that the greater part of the blood is derived that passes through the kidneys and thence goes on to the gills. But a certain quantity of blood is carried to the auricles without having passed through the gills: this blood is brought from Bits 20h the mantle, for example, in Pecfen, _izansvere section through an Andon, The essential respiratory organ.) vba gil plat nui el pat of the Lamellibranchs is a pair of i, intestine; m, mantle-flap; p, 7’, peri- ctenidia. Each ctenidium is a pa Renta (From Lankester, after lateral pallial offset, occupying a longer or shorter space between the mantle and the posterior part of the visceral mass.. It may extend as far forward as the labial palps (Fig. 241, dr’), but in the most archaic forms the gills still 92 occupy a relatively posterior position (Figs. 230, 231, g), while in e 15 226 THE LAMELLIBRANCHIA specialised forms they reach to the anterior extremity of the body (Fig. 221). Each ctenidium consists of a hollow vascular axis bearing on each face a row of more or less flattened hollow filaments, which are nothing more than simple expansions of the axis. In the Protobranchia the filaments are broad, simple, and free (Fig. 230, q), and the two rows are situated on opposite sides of the axis (Fig. 206, A, B). In all other Lamellibranchia the filaments are more or less narrow, and the two rows are normally parallel to one another C D E F (he 4e 4D J H K : ui ¥ a a x t € d. 2 a Fic. 206. Diagrammatic sections taken transversely to the axes of the gills of various Lamellibranchs. A, Nucula; B, Solenomya ; C, Dimya; D, the majority of the Lamellibranchs; 2, Donax faba ; F, Donax variabilis, Tapes, Venus; G, Lasaea; H, Tellina; J, Lyonsia; K, Lucina, Montacuta. a, axis; b, direct (usually descending) lamella of the outer gill-plate; c, reflected (usually ascending) lamella of the outer gill-plate ; d, direct or descending lamella of the inner gill-plate ; e, reflected or ascending lamella of the inner gill-plate; f, leaflets of the outer gill-plate ; g, leaflets of the inner gill-plate. (After Ridewood.) and directed towards the ventral surface (Fig. 206, C-K). The distal moieties of the filaments are, however, reflected ectaxially and dorsalwards, in such a manner that each row forms a double lamina, that is to say, consists of two leaves or lamellae (Fig. 205, br, br’) between which there is an interlamellar space or cavity, serving, in some species, for the incubation of the ova. In the Filibranchia the successive filaments of each ctenidial row are locked together by ciliary junctions, sometimes specialised to form ciliated discs whose cilia interdigitate closely with one another (Fig. 210, A, ¢j). The direct and reflected limbs of each THE LAMELLIBRANCHIA 227 individual filament—and consequently the two lamellae of each gill- plate—are joined together by bridges or interlamellar junctions, which are formed of connective tissue only in the Pectinidae, but are vascular in the Aviculidae. Finally, the different elements of the branchial apparatus are much more intimately connected in the various groups of Eulamellibranchs, in which there are always vascular interfilamentar and interlamellar junctions (Fig. 237). Thus the blood brought to the gill by the afferent vessel is conducted by vessels which run between the lamellae and communicate with Fic. 207. Adacnarca nitens, Pels., transverse section. br, right internal gill-plate ; 67”, left internal gill-plate (without reflected lamina): br’, external gill-plates (with reflected lamina); ca.b, byssus cavity ; com.v, visceral commissure ; hep, liver; in, intestine; pa, mantle ; per, peri- eardium ; 7, kidney ; st, stomach ; tes, testis. the filaments on either hand, forming in this manner the inter- lamellar junctions. Each gill-plate may be thrown into a very regular series of transverse folds, each fold involving a fixed number of filaments ; this is the case in the Pectinacea, the Ostraeacea, and the more specialised forms of Eulamellibranchia. In the last-named the folding is still but slightly marked in the Veneridae, but becomes much more so in the Cardiacea (in 7ridacnu a single fold may contain as many as seventy filaments), the Myacea, etc. In the Pectinacea and Ostraeacea the filament forming the junction between two successive folds becomes thicker and more important than the 228 THE LAMELLIBRANCHIA others, constituting a principal filament lying at the bottom of the furrow between two successive folds. Not all the Lamellibranchia possess complete gills consisting of two gill-plates each formed of two reflected lamellae ; in Anomia aculeata and Dimya none of the gill- plates have reflected lamellae; in 1 the interlamellar space without undergoing any appreciable structural modification. In the following spring they are hatched out, and escape through the dorsal or anal pallial aperture in the form of a peculiar larva THE LAMELLIBRANCHIA 2 vw _ called a “ glochidium” (Fig. 228). This larva is characterised by the possession of hooks in the middle of the lateral borders of its valves and by its larval byssus—which is not homologous with that of other Lamellibranchs. This byssus appears to issue from, but in reality winds round, the adductor muscle, and originates from a single glandular epithelial cell, deeply embedded in the tissues on Fig. 228. Two stages in the development of Anodonta ; both figures represent the ‘‘ glochidium” stage. A, when free swimming, shows the two dentigerous valves widely open. JB, a later stage, after fixation to the fin ofa fish. a.ad and ad, anterior adductor muscle ; al, alimentary canal ; aw.v, otocyst ; br, branchial filaments ; by, byssus ; f, foot; mt, mantle-flap ; p.ad, posterior adductor ; 4, teeth of the shell; sh, shell. (From Lankester, after Balfour.) the dorsal and anterior side of the muscle. The glochidia swim actively by clapping together the valves of the shell, and eventually attach themselves to the gills or fins of a fish, and become encysted in consequence of a pathological development of the epithelium of their host. This parasitic existence lasts for a period varying from two to six weeks, during which the glochidia are nourished by the epidermic elements of their host, absorbing them by means of the ectodermic cells of the embryonic mantle. During this time most of the definite organs of the adult, the foot, otocysts, gills, ete., which were not required in larval life, are de- veloped, largely as the result of the proliferation of the cells of two sym- metrical cavities situated behind the adductor muscle. In a general way the development of the organs follows the normal course, but some—the borders of the mantle, for instance— are formed anew. The glochidium bes : . : Parasitic larva of Anodonta on the shell is not cast off but persists, though eighth day of parasitic life ; ventral view. it undergoes a considerable change {) 0th eee eta entes of shape. The posterior ciliated shield and the byssus disappear. During the early part of the parasitic life the mouth acquires an opening into the previously closed endodermic cavity or archenteron, but the anus is not formed till Fic. 229. 252 THE LAMELLIBRANCHIA the close of parasitic life, and its formation is not accompanied by a sensible ectodermic invagination. When the young Unio quits its host its evolution is not complete. The gills continue to grow slowly, their external plates not being developed until the third year, and sexual maturity is not attained until the fifth year, but growth continues for some time after. IV. BronoMiIcs AND DISTRIBUTION. All the Lamellibranchia are aquatic. The great majority are marine, but some few families have penetrated into fresh waters. All the members of the class feed upon microscopic organisms, chiefly Diatomaceae and other low forms of plant life. Only the Septibranchia and some other abyssal forms are truly carnivorous. In general, the Lamellibranchs are burrowing forms, living half-buried in muddy or sandy bottoms, and in this case their plane of symmetry is vertical. But many forms are completely sedentary and are fixed by the byssus, or in a more definitive manner, by the shell itself, as is the case in Spondylus, Ostraea, Aetheria, Myochama, etc. In these genera the plane of symmetry becomes horizontal, and the animal usually lies on the right side, e.g. Pinna, Hinnites, Spondylus, Plicatula, Anomia, and the Rudistae ; more rarely on the left side as in Ostraca, Requienia, and Chama generally. Some Lamellibranchs live in holes which they excavate either in wood, as in the case of Zeredo, or in stone, as Lithodomus, Saxicava, Pholas, Clavagella, etc., or even in the shells of other Molluses. Lithodomus is only found in calcareous rocks, and bores its hole by the aid of the acid secretion of glands situated in the antero-dorsal and postero-dorsal regions of the mantle. Some Lamellibranchs, such as Lima, are nidamentous, and construct a nest by means of the byssus. Lima hians builds its nest in the space of three weeks, and may afterwards return and reconstruct another from it. Modiolaria marmorata and Entodesma cuncatum pass their existence deeply buried in the tests of Ascidians, and Vulsella lives in a similar manner in sponges ; but the few commensalistic or parasitic forms generally live on or in Echinoderms: thus JJontacuta lives on Spatangids, Seioberetia in the incubatory pouch of an Asterid, Hntovalva in the oesophagus of a Synapta. On the other hand, Lphippodonta is commensal with a prawn, and certain species of Lepton with Gebia. Only a few species are very active: Tellina, Yoldia, ete., execute leaping movements by forcibly contracting the foot ; Lasaea, Cyclas, etc., crawl on immersed bodies or on the surface of the water ; other forms, notably the Pectinidae and Limidae, swim by rapidly opening and closing the valves of the shell; and some elongated forms in which the mantle edges are fused for a considerable a THE LAMELLIBRANCHIA 253 extent swim by forcibly expelling water from the posterior aperture of the mantle (Solen, Solenomya). In point of size the Lamellibranchs vary from a length of a few millimetres to more than seventy centimetres (Pinna and Tridacna, some specimens of the latter genus weighing as much as 310 lbs.). The fossil Hippurites attained to the length of a metre. There are more than 5000 living species of Lamellibranchia, of which 1000 are Unionidae. They are distributed all over the world, and some marine forms extend to a depth of 2700 fathoms. Fossil forms appear in the Cambrian, and become very numerous in species from the Silurian onwards. Some large groups, such as the Palaeoconcha of the primary and the Rudistae of the secondary deposits, are quite extinct. VY. REVIEW OF THE ORDERS AND FAMILIES OF LAMELLIBRANCHIA. The classification of this homogeneous group has long presented great difficulties, for the different organs or apparatus, such as shell, muscles, siphons, etc., that have successively been employed as bases of classification, have not given satisfactory results. Ray Lankester was the first to suggest (in 1884) that the structure of the gills might furnish characters of classificatory value, and the present writer has constructed on this basis a phylogenetic classification in which the class is divided into five groups. This classification has put various families, such as the Anomiidae, Trigoniidae, Dreissensiidae, ete., into their proper places, and has been largely adopted. Objections to it have, however, been raised, notably by Dall, who has urged that the genera LEuciroa (Anatinacea) and Callocardia (or Vesicomya, Cyprinidae) have protobranchiate gills, and that the system of classification according to branchial characters is consequently without foundation. But the recent investigations of Ridewood, undertaken at the instance of Ray Lankester, have shown that it was the objections of Dall that had no foundation: Luciroa and Callocardia have typical eulamellibranchiate gills. As the result of the advancement of our knowledge, the classification of the Lamellibranchia founded on the structure of the gills has been ameliorated by the suppression of the order “ Pseudolamellibranchia,” and the two diphyletic sub-orders which it included, the Pectinacea and the Ostraeacea, may be respectively located in the Filibranchia and the Eulamellibranchia, thus making these two old-established orders correspond to the new orders proposed by Ridewood under the names Eleutherorhabda and Synaptorhabda. On the other hand, the shell (and particularly its hinge) is the only other organ that has been retained as a basis of the general 254 THE LAMELLIBRANCHIA classification of the Lamellibranchia, especially by palaeontologists, and the subdivisions adopted in this system correspond more or less with those based on the structure of the respiratory organs. Thus the following are very nearly synonymous terms :— Prionodesmacea = Protobranchia + Filibranchia. Teleodesmacea = Eulamellibranchia — Anatinacea. Anomalodesmacea = Septibranchia + Anatinacea. As regards the value of the last order, Septibranchia, in it the characteristic organs that have given the name to the whole class Lamellibranchia are so profoundly modified, that they differ much more from all the other different kinds of gills than the latter differ from one another, and therefore, even if the Septibranchia should not be placed in contrast to all other Lamellibranchia, they at least constitute a group equivalent to the three other groups, Protobranchia, Filibranchia, and Eulamellibranchia. Thus the Lamellibranchs are divisible into these four orders. It will be remarked that the numerous studies on the organisation of Lamellibranchia made since 1891, have shown that there has been a progressive evolution in each of these four orders, and that consequently such important organs as the heart, kidneys, and otocysts may exhibit marked differences in relatively nearly related types, and that no strictly pure primitive types have been retained. From the point of view of phylogeny the most archaic Lamellibranchia are those in which the foot has a “ plantar” ventral surface like that of Gastropoda and Pulsellwm among the Scaphopoda. These archaic forms constitute the Protobranchia (Solenomya, Fig. 230, Yoldia, Fig. 231, etc.), in which the gonads still retain openings into the initial #r pericardial portion of the kidneys, and the branchial filaments are free and not reflected. From these Proto- branchia are derived the Filibranchia, whose branchial filaments are reflected, but are still devoid of vascular junctions: these in turn have given rise to the Eulamellibranchia, which are more specialised in respect of the complication of the ctenidia. Finally, eulamellibranchiate forms analogous to the Anatinacea represent the source from which the Septibranchia have been derived. ORDER 1. Protobranchia. These are Lamellibranchia whose distinctive character is the possession of gills with flat and non-reflected filaments disposed in two rows on opposite sides of the branchial axis (Fig. 206, A, B). The mantle is provided with a hypobranchial gland lying on the outer side of each gill. The foot has a plantar ventral surface (Fig. 230, f) and the byssogenous apparatus is but slightly developed. The nervous system generally presents a distinct pair of pleural - ; F Jj “i THE LAMELLIBRANCHIA 255 ganglia, and the otocysts are generally open. The gut may be pro- vided with a relic of the pharyngeal cavity, which in some cases is furnished with two lateral glandular sacs. The auricles of the heart are muscular ; the kidneys are rather simple in structure and glandular throughout their extent. The sexes are separate: the gonads have retained their primitive communications with the initial or internal extremities of the kidneys, but as the two branches of each kidney have acquired a secondary communication at their anterior ends, the genital products pass direct to the external orifice of the kidney by this passage (Fig. 213). Fic. 230. Adult specimen of Yoldia limatula, represented as seen from the right side, and showing the interna] organs. a.a, anterior adductor muscle; a.f.m, anterior foot muscle ; 6.9, byssal gland ; c.g, cerebral ganglion ; ¢.s, exhalant siphon; f, foot; g, gill; h, heart; int, intestine; i.s, in- halant siphon; l.p, labial palp; ot, otocyst; p.a, posterior adductor muscle; pap, palp appendage ; p.e, posterior expansion of the margin of the mantle; p.f.m, posterior foot muscle ; p-g, pedal ganglion ; s.t, siphonal tentacle ; sto, stomach ; v.g, visceral ganglion. (After Drew.) Famity 1. SoteNoMympaz, Gray. In the gills one row of branchial filaments is directed dorsally and the other ventrally (Fig. 231, g). The mantle has a long postero-ventral suture, and a single posterior orifice. The labial palps of each side are fused together. The shell is elongate ; the hinge has no teeth ; the periostracum is thick. Genus—Solenomya, Lamarck. Famity 2. Nucuniparn, Gray. The labial palps. free, very broad and provided with a posterior appendage; all the branchial filaments are oriented transversely ; the shell has an angular dorsal border and the hinge is pliodont; the mantle is open throughout its extent. Genera—Nucula, Lamarck ; the heart situated on the dorsal side of the rectum. Acila, Adams (Cretaceous, Tertiary, and Recent). Pronucula, Hedley. Famity 3. Leprpar, Adams. The same characters as the Nuculidae, but the mantle has two posterior sutures and two united siphons; the heart traversed by the rectum. Genera—Leda, Schumacher; the mantle borders produced posteriorly into two lobes which simulate a third siphon. Yoldia, Moller; siphons elongate ; 256 THE LAMELLIBRANCHIA ligament internal (Figs. 200, 230). Malletia, Des Moulins ; the ligament external. Nuculina, d’Orbigny. Famity 4. CrENopontTipAr, Wohr- mann. An extinct family from the Silurian; the shell is nuculiform and the hinge presents an uninterrupted arcuate row of teeth. Genera— Ctenodonta, Salter. Cuculella, Fischer. Cardiolaria, Meunier-Chalmas. The fossil group Palaeoconcha, Neumayr, is connected with the Protobranchia through the Solenomyidae. It contains the following families, all of which are extinct :—Famity 1. PRarcaRDIDAE, Neumayr Shell equivalve, with the hinge dentition of Arca. Genus—Praecardium, Barrande ; from the Silurian and Devonian. Faminy 2. ANTIPLEURIDAE, Neumayr. Shell inequivalve ; the hinge with an obscure resemblance to that of Arca. Genus— Antipleura, Barrande; from the Silurian. FaMILy 3. CARDIOLIDAE, Neumayr. Shell equivalve and ventricose ; the hinge without teeth. Genus—Cardiola, Broderip ; from the Silurian and Devonian. Famity 4. GRamMMysSIIDAE, Fischer. Shell thin, equivalve, Fic. 231. : Solenomya togata, Poli, left-side view (the left pallial lobe and gillcut away). a, anus; ad’, ad’, anterior and posterior adductor; wo, aorta; a.or, anal orifice of the mantle; au, heart- auricle ; c.g, cerebral ganglion ; f, foot; /.e, foot elevator ; /.p, foot protractor ; fr’, fr’, anterior and posterior foot retractor ; g, right gill; hy.g, hypobranchial gland; k, kidney ; m, mouth ; pa, mantle; pe, pericardium ; p.g, pedal ganglion; p./, palp appendage ; pl.g, pleural ganglion ; 7.0, renal opening ; st, stomach ; v, heart-ventricle ; vi.c, visceral commissure ; vi.g, visceral ganglion. oval, or elongate, the cardinal border thickened but without teeth. Genera — Grammysia, Verneuil; from the Silurian and Devonian. Protomya, Hall, from the Devonian. Cardiomorpha, de Koninck ; from the Silurian to the Carboniferous. Faminy 5, Vuastipan, Neumayr. Shell thin, very inequivalve, the hinge without teeth. Genus—Vdasta, Barrande; from the Silurian. FAMILY 6. SoLENOPsIDAE, Neumayr. Shell equivalve, thin, greatly elongated, the umbones very far forward. Genus—Solenopsis, MacCoy, from the Devonian to the Trias. ORDER 2. Filibranchia. These are Lamellibranchs whose main character is the possession of gills formed of parallel, ventrally directed, and reflected filaments. The successive filaments are joined together by cilia disposed in ‘ciliated discs” (Figs. 210, A; 232, 7/7). The foot is generally “7, THE LAMELLIBRANCHIA 257. provided with a highly developed byssogenous apparatus. The order comprises five sub-orders—the Anomiacea, Arcacea, Mytilacea, Pectinacea, Dimyacea. Sus-OrpErR 1. ANOMIACEA. Very asymmetrical animals with a single large posterior adductor muscle. The heart is not contained in the pericardium, lies dorsad of the Fic. 232, A portion of the gill of Mytilus, showing the filaments ; the median part is cut out. a.b.v, afferent branchial vessel ; e.).v, efferent branchial vessel ; i.fj, interfilamentar cilifted junction ; i.L.j, interlamellar connective tissue junction. (After Bonnet.) rectum, projects into the pallial cavity, and gives off a single and anterior aorta. ‘The reflected borders of the inner gill-plates of either side are fused together in the middle line. The gonads open into the kidneys, and the right gonad extends into the mantle. The shell is thin and the animal fixed. Family Anomipag, Adams. Foot small. The inferior (right) valve of the adult is perforated to admit of the passage of the byssus. Genera —Anomia, Linnaeus; byssus large and calcified; British. Placuna, Bruguitre; byssus atrophied in the adult. Hypotrema, d’Orbigny. Carolia, Cantraine. Ephippium, Bolten. Placunanomia, Broderip. 17 258 THE LAMELLIBRANCHIA Sus-OrDER 2. ARCACEA. Symmetrical animals, with the mantle open throughout its extent, and with generally well-developed anterior and posterior adductor muscles. The heart lies in the pericardium and gives off two aortae. The gills are free and without interlamellar junctions, The renal and genital orifices are separate. Famity 1. Arciparn, Gray. The borders of the mantle bear com- pound pallial eyes. The labial palps are direct continuations of the lips (Fig. 199). The hinge is ‘pliodont,” that is to say, it has numerous teeth on either side of the umbones, and the teeth are perpendicular to the edge. Genera—Arca, Linnaeus ; foot byssiferous ; heart above the rectum ; hinge straight (Figs. 188,199); British. Pectunculus, Lamarck ; foot without byssus, but with a plantar surface ; the heart traversed by the rectum ; the hinge curved ; British (Fig. 193, A). Scaphula, Benson ; from fresh water; India. Argina, Gray. Bathyarca, Kobelt. Barbatia, Gray. Senilia, Gray. Anadara, Gray. Adacnarca, Pelseneer. Faminy 2. PARALLELODONTIDAE, Dall. The shell of Arca, but with the posterior hinge teeth elongated and parallel to the cardinal border. Genera— Cucullaea, Lamarck ; recent and fossil from the Jurassic. All the other genera are fossil, e.g. Parallelodon, Meek and Worthen ; from the Devonian to the Tertiary. Carbonaria, Meek and Worthen ; from the Carboniferous, etc. Famity 3. Limopsrpag, Dall. Shell sub-orbicular, the hinge curved, the ligament simple with the trans- verse axis longer than the longitudinal; foot elongate, pointed anteriorly and posteriorly. Genera—Limopsis, Sassi ; shell covered with a hairy epidermis ; the anterior adductor frequently much reduced (Fig. 233). Trinacria, Mayer ; from the Tertiary. Faminy 4. Parto- BRYIDAE, Bernard. The animal, like that of Limopsis, without an anterior adductor muscle ; the shell thin, very inequilateral, the anterior part atro- phied, the umbones projecting and Fic. 233. formed by the prodissoconch. Genera Limopsis longipilosa, Pels., interior aspect —Philobrya, Carpenter (Figs. 196, of the right valve. «.a, anterior adductor 234). FAMILy 5. CYRTODONTIDAE, impression ; /, ligamentar fossa; p.a, pos- e ' é . terior adductor impression ; t, hinge-tooth. Wohrmann. An extinct family with an equivalve short, convex and inequilateral shell, the anterior side of which is short; the hinge teeth oblique or horizontal. Genera — Cyrtodonta, Billings; Silurian and Devonian. Cypricardites, Conrad ; Silurian. Vanuxemia, Billings ; Silurian. Famity 6. Triconimpan, Fleming. Foot elongated, pointed in front and behind, the ventral border sharp. The byssogenous apparatus atrophied and devoid of a byssus. The labial palps distinet from the lips. Shell thick. Hinge with striated teeth. Genera— Trigonia, Bruguitre ; shell sub-triangular, the umbones directed back- THE LAMELLIBRANCHIA 259. wards. This genus was very abundant in the Secondary epoch, particularly in Jurassic seas. There are six living species, all of which live in Australian seas. The animal of Jrigonia was first found by Quoy and Gaimard in 1827. Schizodus, King ; from the Permian. Myo- phoria, Bronn ; from the Trias. Famity 7. LYRODESMIDAE, Ulrich. Shell inequilateral, the posterior side being the shorter. The hinge short, bearing teeth disposed in the shape of a fan. An extinct family, from the Silurian. Genus — Lyrodesma, Conrad. Sup-OrperR 3. MyrTInacra. Symmetrical Lamellibranchia in which the anterior adductor Philobrya sublaevis, Pels., viewed from the left muscle is alway = less developed side ; the left pallial lobe removed. ad”, posterior than the posterior (the “amiso- adductor muscle; an, anus; au, auricle of the mee 35 “4° : heart; br, gill; br.s, branchial axis; by, byssus; myarian condition) or is absent gl.ge, gonad; n.pa, pallial nerve; p, foot; pa, Gig, 192, B,C, D, B. The mane, mt able ml sep spe an heart gives off a single vessel stomach; ven, ventricle of the heart. only, the anterior aorta. The gills are smooth, the gill-filaments all alike and provided with inter- lamellar junctions. The gonads generally extend into the mantle and open at the sides of the kidneys. The foot is linguiform and byssiferous. Famity 1. Myvrinipar, d’Orbigny. Shell inequilateral, the anterior side being short ; the hinge without teeth ; the ligament external. The mantle has a posterior suture. Cephalic eyes present. Genera—Mytilus, Linnaeus ; the shell with terminal umbones; British. Modiola, Lamarck ; the umbones behind the anterior extremity ; British. L7thodomus, Cuvier ; the shell sub-cylindrical, adapted to boring. Modiolaria, Loven ; posterior pallial orifice provided with an elongated siphon; anterior adductor fairly high; British. Crenella, Brown. Stavelia, Gray. Dacrydiwm, Torell. Myrina, Adams. TIdas, Jeffreys. Septifer, Recluz. Famity 2. Mopto.opsipak, Fischer. Shell elongate, thin, inequilateral, enlarged posteriorly ; the ligament external ; the adductor muscles subequal. An extinct family from the Silurian to the Cretaceous. Genera—Modiolopsis, Hall; from the Silurian. Modiomorpha, Hall; from the Devonian. Myoconcha, Sowerby ; from the Carboniferous to the Cretaceous. FamIny 3. PERNIDAE, Fleming. Mantle open throughout. No anterior adductor muscle. Shell very inequilateral ; the ligament multiple and lodged in a series of vertical fossae. Genera—Perna, Bruguiere. Shell sub-quad- rangular, the right valve notched for the passage of the byssus ; gills free posteriorly. Crenatula, Lamarck ; shell thin, flattened, irregular, without a byssal notch ; inhabits sponges. Bakewellia, King ; fossil from the Permian. Gervilleia, Defrance ; fossil from the Trias to the Eocene. Fic. 234, Hpber 260 THE LAMELLIBRANCHIA Odontoperna, Frech ; fossil from the Trias. Inoceramus, Sowerby ; fossil - from the Jurassic to the Cretaceous. Sus-ORDER 4. PECTINACEA. Lamellibranchia with an open mantle and devoid of an anterior adductor muscle. The gills are folded, and the filaments at the summits and bottoms of the folds are different from the others. The gonads are contained in the visceral mass and generally open into the kidneys. Foot usually rudimentary. y= Nig ah . eae pe \oh q pa. % Fic. 235. Pecten jacobaeus, ventral aspect. a, anus; e, pallial eyes; f, foot; g, gill; h.a, posterior adductor; i, intestine ; /, lips; l.p, labial palps; m, mouth; ov, ovary; pa, mantle (reflected edge); p.c, pallial cavity; sh, shell; ¢, testis. (After Poli.) \ Faminy 1. VULSELLIDAE, Adams. Mantle open ; foot without byssus ; the shell high and the hinge without teeth. Genus—Vulsella, Lamarck. Famuy 2. AvicuULIDAE, Swainson. Foot provided with a very stout byssus (Fig. 236). The gills fused to the mantle ; shell very inequilateral ; the cardinal border straight, provided with two auriculae, of which the posterior is the longer. Genera—Avicula, Bruguitre ; the auriculae of the shell very prominent; heart attached to the ventral face of the rectum ; British ; fossil from the Devonian to the present day. Meleagrina, Lamarck ; shell sub-quadrangular, the auriculae not very prominent. A species of this genus, Meleagrinw margaritifera, from the Indian Ocean, Persian Gulf, ete., forms precious pearls, Malleus, Lamarck ; shell irregular, high and narrow, with broad subequal auriculae. The following genera are exclusively fossil :—Limopteria, THE LAMELLIBRANCHIA 261 Hall ; Devonian and Carboniferous. Pseudomomnotis, Beyrick ; Devonian and Cretaceous. Cassianella, Beyrich ; Trias. Monotis, Bronn; Trias. Daonella, Mojsisovics; Trias. Posidonomya, Bronn ; Silurian to Jurassic. Famity 3. PrastnipAx, Stoliczka. Shell inequilateral with anterior umbones, and a prominent anterior auricula ; the dorsal border arched ; the hinge with a single fossa and a single tubércule on each valve. Genus—Prasina, Deshayes. Famity 4. PrerInerpar, Goldfuss. Shell thick, very inequilateral ; the cardinal border straight, with two auriculae and a notch for the byssus under the right anterior auricula ; an extinct family from the Palaeozoic. Genera— Pterinea, Goldfuss ; Silurian to Carboniferous. Rhombopteria, Jackson; Silurian. Actinodesma, Sand- Fia 2386. Avicula tarentina, Lamarck, from below. a, anus ; ad, adductor muscle ; b.gr, byssal groove of the foot ; by, byssus; e, eye; f, foot; g, gill; l.p, labial palp; m, mouth; pa, mantle; sh, shell ; vi.c, visceral commissure ; vi.g, visceral ganglion. (After Poli.) berger ; Devonian.’ ! Famiry 5. LUNULICARDIIDAE, Fischer. Shell thin, triangular, very inequilateral, the anterior end truncated ; the umbones terminal ; the cardinal border straight; without hinge teeth. An extinct family from the Silurian and Devonian. Genera—Lwnuli- cardium, Miinster; Silurian and Devonian. Patrocardiwm, Fischer ; Silurian. Balinka, Barrande; Silurian. Famity 6. ConocarpIIDAg, Neumayr. Shell thick, subtriangular, the anterior side truncated and gaping ; cardinal border straight and prolonged into two auriculae of which the anterior is very long and narrow ; hinge with a lateral tooth and a reduced cardinal tooth. Dimyarian. An extinct family from the Palaeozoic. Genus—Conocardium, Bronn ; Silurian to Carboniferous. Famity 7. AMBONYCHIIDAE, Miller. Shell inequilateral, without an anterior auricula, the umbones anterior and terminal ; hinge with two 262 THE LAMELLIBRANCHIA cardinal teeth and two posterior oblique lateral teeth. Dimyarian, the anterior adductor being very small. An extinct family from the Silurian and Devonian. Genera—Ambonychia, Hall; Silurian. Byssonychia, Ulrich ; Silurian. Gosseletia, Barrois; Devonian. Clionychia, Ulrich ; Silurian. Famity 8. Myarrnipas, Frech. Shell very inequilateral, the posterior part greatly enlarged ; the umbones anterior or terminal ; the hinge straight, without teeth; adductors subequal. An extinct family from the Silurian to the Cretaceous. Genera—Myalina, de Koninck ; Silurian and Devonian. Hoplomytilus, Sandberger ; Devonian. Ptycho- desma, Hall; Devonian. Anthracoptera, Salter; Carboniferous. Per- gamidea, Bittner ; Trias. Mysidea, Bittner;. Trias. Aucella, Kyser ; Jurassic and Cretaceous. Famity 9. AmusstiDAE, Ridewood. Gills with- out interlamellar junctions. Shell orbicular, smooth externally, with radiating costae internally. Genus—Amwussiwm, Klein. Famity 10. SPoNDYLIDAE, Fleming. Shell very inequivalve, fixed by the right valve, which is larger than the left. The ligament elongated in a transverse direction. No byssus. Genera—Spondylus, Linnaeus ; shell with spiny ribs, and adherent by the spines. Plicatula, Lamarck ; shell folded, adherent by the umbo of the right valve. Faminy 11. PECTINIDAE, Lamarck. Shell ornamented with radiating ribs; the dorsal border provided with two auriculae. Foot byssiferous. Mantle borders pro- vided with eyes (Fig. 235). Genera—Pecten, Lamarck ; shell orbicular, with equal auriculae ; without a byssal sinus; British. Chlamys, Bolten ; shell higher than it is long; the anterior auricula the larger, and pro- vided with a byssal sinus; British. Pedum, Bruguiere. Hinnites, Defrance. Pseudamussium, Adams. Camptonectes, Agassiz. Hyalopecten, Verrill ; abyssal. Sus-ORpDER 5. DIMYACEA. Dimyarian Lamellibranchia with an orbicular and almost equilateral shell; adherent; the hinge without teeth and the ligament internal. Gills with free non-reflected filaments. Family Druyipan, Dall; with the characters of the sub-order. Genus—Dimya, Ronault ; recent, in abyssal depths, and fossil since the Jurassic. ORDER 3. Eulamellibranchia. = Lamellibranchia in which the edges of the mantle are generally united by one or two sutures (Figs. 221, 241, ete.). Two adductor muscles are usually present (Figs. 238, 241, 242, ete.). In the gills the branchial filaments are united at regular intervals by vascular junctions which transform the linear interfilamentar spaces into a series of fenestrae (Fig. 237). Similarly the lamellae of each gill- plate have vascular junctions which form afferent vessels in the interior of the plates. The gonads always have their own proper external orifices. The order comprises the following nine sub-orders : —Ostraeacea, Submytilacea, Tellinacea, Veneracea, Cardiacea, Chamacea, Myacea, Adesmacea, Anatinacea. THE LAMELLIBRANCHIA 263 Sus-OrpDER 1. OSTRAEACEA. Monomyarian Eulamellibranchia, or with a very small anterior adductor muscle. The mantle is open; the foot rather small; the branchiae folded ; the shell inequivalve. Famity 1. Lyuwmar, D’Orbigny. Foot digitiform, with a bysso- genous apparatus. Borders of the mantle provided with long and numerous tentacles. Gills not united with the mantle. Shell pro- vided with auriculae. Genera—Lima, Bruguitre ; the individuals of this genus form a sort of nest by means of the byssus, or swim by (31 BS —— of SG g eby Fic. 237. A portion of the gill of Venus. e.b.v, efferent branchial vessel ; g.f, gill filaments ; 9.7, afferent branchial vessels. (After Bonnet.) clapping the valves of the shell together. Limaea, Broun. Famity 2. OsrReEIDAE, Gray. Foot much reduced and devoid of a byssus. Heart generally on the ventral side of the rectum. The gills fused to the mantle. Shell irregular, fixed by the left and larger valve. Genera— Ostraea, Linnaeus ; foot absent in the adult ; eatable and cultivated for commerce ; some species, such as the British O. edulis, are hermaphrodite. Faminy 3. Exiemrpar, Gill. Shell thick, inequilateral, the anterior side being the shorter. Monomyarian, with the muscular impression on a prominent myophorous apophysis. Genus—Eligmus, Deslongchamps ; an extinct genus from the Jurassic. Famity 4. Prnnipar, Meek. Shell elongated. Dimyarian, with a very small anterior adductor 264 THE LAMELLIBRANCHIA muscle. Shell truncated and gaping posteriorly. Foot byssiferous. Genera—Pinna, Linnaeus ; heart traversed by the intestine; anus pro- jecting and appendiculated. Cyrtopinna, Morch. Aviculopinna, Meek ; fossil from the Carboniferous and Permian. Pinnigena, de Saussure ; fossil from the Jurassic and Cretaceous. Atrina, Gray ; from the Car- boniferous to the present day. Sus-OrRDER 2. SUBMYTILACEA. Eulamellibranchia in which the mantle is only slightly closed ; generally there is only a single suture. Siphons absent or very short. Gills smooth. Nearly always dimyarian. Shell equivalve, with an external ligament. Famity 1. DREISSENSIIDAE, Gray. Two pallial sutures and two short siphons ; pedal orifice short. Foot cylindrical with a stout byssus. Shell elongated ; the hinge without teeth ; the summits of the valves with an internal septum. Genus—Dreissensia, van Beneden; an in- habitant of fresh water, but originated from the Caspian Sea ; acclimatised in England about 1824. Famity 2. MopioLarcipAE, Gray. Mantle with two sutures. The foot byssiferous, with a plantar surface and a glandular cavity in front of the byssogenous cavity. The two branchial plates serve as incubatory pouches, Genus—Modiolarca, Gray ; sub- antarctic (Fig. 241). Famity 3. AsSTARTIDAE, d’Orbigny. A single pallial suture. Foot elongate, without a byssus. Shell concentrically striated; the ligament external. Genera—Astarte. Sowerby ; British. Woodia, Deshayes. Opis, Defrance; fossil from the Secondary. Pro- socoelus, Keferstein ; fossil from the Devonian. Famity 4. CRASSATELLIDAE, Gray. Mantle with a single suture; foot short. Shell thick with concentric striae ; the ligament external. Genera—Crassatella, Lamarck. Cuna, Hedley. Faminy 5. Carpitipax, Férussac. Mantle with a single pallial suture; foot carinated, often byssiferous; palps short. Shell thick with radiating costae; the ligament external. Genera—Cardita, Bruguiere. Thecalia, Adams. Milneria, Dall; incubatory, California. Venericardia, Lamarck. Famity 6. ConDYLOCARDIIDAE, Bernard. Dis- tinguished from the family Carditidae by the presence of an external ligament. Genera—Condylocardia, Bernard. Carditella, Smith. Cardi- topsis, Smith. Faminy 7. Cyprinipar, d’Orbigny. Mantle open in front, and with two pallial sutures. The branchial and anal orifices papillose, the latter projecting. External gill-plates smaller than the internal. Genera—Cyprina, Lamarck; British. Cypricardia, Lamarck. Coralliophaga, de Blainville. Plewrophorus, King; fossil from the Devonian to the Trias. Anisocardia, Munier-Chalmas; fossil from the Jurassic to the Tertiary. Veniella, Stoliczka; fossil from the Cre- taceous to the Tertiary. Famitry 8. Isocarpimpar, Gray. Mantle largely closed, the pedal orifice generally small; the anal and branchial orifices sessile ; gill-plates of equal size ; foot short. Shell globular with prominent and coiled umbones. Genus—lIsocardia, Lamarck ; British. Faminy 9. CaLLocARDUDAE, Dall. The anal and branchial orifices of the mantle provided with siphons. The external gill-plate smaller than the internal. Shell ventricose, but elongated ; the umbones not promi- THE LAMELLIBRANCHIA 265 nent. Genus—Callocardia, Adams; abyssal. Famity 10. Lucinrpag, d’Orbigny. The anal orifice of the mantle sometimes produced into a siphon. Anterior adductor muscle within the pallial line. Labial palps very small. Gills without an external plate. Shell rather thin. Genera —Lucina, Bruguiere ; mantle with two sutures; visceral mass smooth ; foot vermiform ; British. Montacuta, Turton ; shell with a single suture, foot short, byssiferous; visceral mass with arborescent projections ; British. Cryptodon, Turton ; mantle with a single aperture ; foot short ; visceral mass smooth. Famity 1]. Corprpag, Dall. Shell thick with denticulated borders. The anal orifice provided with a valve, but not with a siphon. Foot elongated and pointed. Genera — Corbis, Cuvier. Gonodon, Schafhiutl ; fossil from the Trias and Juras- sic. Mutiella, Stoliczka; fossil from the superior Cretaceous. FaMILy 12. UNGULINIDAE, Adams. Mantle without siphons ; the pedal orifice long. Foot greatly elongated, vermiform, ending in a glandular enlarge- ment (Fig. 238, III). Anterior adductor muscle in contact with the pallial line. Gills with two plates; labial palps small. Marine. Genera — Ungulina, Daudin ; Fic. 238. mantle with a single suture ; vis- Axvinus flexuosus, Montagu, viewed from the left ceral mass smooth. Diplodonta, side. I, anterior adductor muscle ; II, glandular Broan ; mantle with two sutures ; Pow pf, the manu IL foot, IY, gonad pro Sit, ; : otews Mella - Ys ¥> ee ee omer ings mantle: vant; 1x, pontacor netrastar of the ois with a single suture ; visceral mass anterior retractor of the foot. with arborescent excrescences (Fig. 238); British, Faminy 13. CyreENELLIDAE, Fischer. Mantle provided with two elongated, united, non-retractile siphons. Two gill-plates to each gill; labial palps elongated. Inhabitants of fresh water. Genera— Cyrenella, Deshayes. Joanisiella, Dall. Famity 14. TancrEpIIDAE, Fischer. Shell elongate, sub-triangular ; the ligament external. Hinge with two cardinal teeth on the right and one or two on the left valve. Posterior lateral teeth stout. An extinct family ranging from the Trias to the Cretaceous. Genera—Tuancredia, Lycett; Trias to Cretaceous. Meekia, Gabb ; Cretaceous. Famity 15. Unicarprpag, Fischer. Shell sub-orbicular, more or less ventricose, nearly equilateral, with concentric striae ; pallial line simple; hinge with a single cardinal tooth on each valve, An extinct family ranging from the Carboniferous to the Cre- taceous. Genera—Unicardium, d’Orbigny ; Trias to Cretaceous. Scaldia, de Ryckholt; Carboniferous. Pseudedmondia, Fischer ; Carboniferous. Famity 16. Lepronrpar, Gray. Shell thin, not covered by the mantle and not gaping. Mantle without siphons; gills with two gill-plates ; foot long and byssiferous. Marine, hermaphrodite and incubatory animals. 266 THE LAMELLIBRANCHIA Genera—Kellya, Turton ; mantle with two sutures and three orifices, the pedal orifice being the middle and not the anterior of the three (Fig. 187) ; foot linguiform ; the external gill-plate with a reflected lamella; British. Lepton, Turton; mantle with a single suture ; the mantle edges provided with tentacles ; foot with a plantar ventral surface ; commensal ; British. Lasaea, Leach ; a single pallial suture ; the foot linguiform and elongated ; the external gill-plate not reflected (Fig. 206, G); British. Hrycina, Lamarck; fossil from the Tertiary. Pythina, Hinds. Scacchia, Philippi. Sportella, Deshayes. Cyamium, Philippi. Famity 17. GaLEoMMIDAE, Gray. Mantle more or less completely reflected over the shell. Foot well developed, generally byssiferous. Shell thin, gaping; the adductor muscles much reduced. Genera—Galeomma, Turton ; shell incompletely covered by the mantle ; a single pallial suture; a large azygos anterior pallial tentacle, and a short anal siphon present. A byssal groove in the foot ; British. Scintilla, Deshayes. Hindsiella, Stoliczka. Ephippo- donta, Tate ; shell internal; a single pallial suture; gills with two gill- plates; commensal with the shrimp Agius; Australian. The three following genera with an internal shell probably belong to this family :—Chlamydoconcha, Dall; two gill-plates; a pallial suture ; an anterior orifice leading into a caecum ; no adductor muscles; sexes separate ; from California (Fig. 239). Scioberetia, Bernard ; gills with a single gill-plate ; a single pallial suture ; foot large, elon- gated, with a byssal groove ; hermaphro- dite and commensal with a Spatangid, Triphylus ; from Cape Horn. Entovalva, Voeltzkow (Fig. 240); mantle fairly open, with a single suture; foot large, with a posterior pore; hermaphrodite and incubatory ; endoparasitic in Synapta (= Synapticola, Malard), Madagascar and Atlantic. Faminy 18. K®LLYELLIDAER, Fischer. Mantle with a single pallial suture ; anal orifice with a very short Fic. 239. siphon ; foot elongated ; gills with two Chlamydoconcha orcutti, Dall. A, dor- Unequal plates. Shell ovoid ; the liga- sal aspect ; B, left-side view. @.o, anal ment external ; the anterior lateral rate ener Gcany. Bape be ay hinge tooth below the cardinal tooth. Genera — Kellyella, Sars. Turtona, Forbes and Hanley ; British. Allopagus, Stoliczka ; fossil from the Eocene. Lutetia, Deshayes ; fossil from the Eocene. Famity 19. CYRENIDAE, Gray. Mantle with two siphons, which are more or less intimately united together and have papillose orifices. The sexes separate. Shell with external ligament; the pallial line usually with a sinus, Freshwater forms. Genera—Cyrena, Lamarck. Corlicula, Megerle. SBatissa, Gray. Velorita, Gray. Galatea, Bruguitre. Fischeria, Bernardi, Faminy 20. THE LAMELLIBRANCHIA 267 CycLaDIDAk, Clark. Mantle with one siphon or with two free siphons, which have simple orifices. _Hermaphrodite ; the embryos incubated in the external gill-plate. Shell with a simple pallial line. Freshwater. Genera—Cyclas, Bruguiére (= Sphaerium) ; two siphons ; British (Fig. 218). Pisidiwm, Pfeiffer ; asingleanalsiphon; British. Famity 21. RaNGIIDAE. Mantle with two short siphons united at their bases, and with papillose orifices. Foot linguiform. Shell with prominent umbones and an internal ligament. Genus—Rangia, Desmoulins; from brackish water in Florida. Faminy 22. Carpinipak, Zittel. Shell elongated, inequi- lateral, the posterior side being the longer; the ligament external ; the pallial line simple; dimyarian. An extinct family, ranging from the Fic. 240. Entovalva, left-side view. a, anus; c.g, cerebral ganglion ; f.gl, foot-gland ; g.gl, gonad ; i.c, incubatory chamber; in, intestine; /, liver; m, mouth; pa, mantle; sh, shell. (After Voeltzkow.) Devonian to the Cretaceous. Genera—Cardinia, Agassiz; Trias and Jurassic. Anthracosia, King ; Carboniferousand Permian. Anoplophora, Sandberger ; Trias. Pachycardia, Hauer; Trias. Famity 23. Mrca- LODONTIDAE, Zittel. Shell inequilateral, thick, dimyarian, with pro- minent umbones; the posterior adductor impression borne on a myo- phorous apophysis. An extinct family, ranging from the Devonian to the Cretaceous. Genera—Megalodon, Sowerby ; from the Devonian to the Jurassic. Pachyrisma, Morris and Lycett; Trias and Jurassic. Durga, Bohm ; Jurassic. Dicerocardium, Stoppani; Jurassic. Famiry 24. Untonipar, Fleming. Mantle with a single pallial suture and no siphons. Shell equilateral, with lateral hinge teeth or no _ hinge teeth. Inhabitants of fresh water. Development through a glochidium 268 THE LAMELLIBRANCHIA stage (Fig. 242). Genera—Unio, Retzius ; shell thick, the hinge toothed. This genus includes more than a thousand species, the majority from the northern hemisphere. Anodonta, Lamarck; shell thin; the hinge without teeth ; British. Pseudodon, Gould. Quadrula, Rafinesque. Arconaia, Conrad, Monocondylaca, VOrbigny. Solenaia, Conrad. Mycetopus, d’Orbigny ; foot cylindrical, with a terminal swelling ; South America, Faminy 25. MUTELIDAE, Gray. This family differs from the Unionidae in having two pallial sutures and a distinct branchial orifice; the shell is never fur- nished with lateral hinge teeth. Freshwater. Genera—Mutela, Scopoli. Pliodon, Conrad. Spatha, Lea. Jridina, Lamarck. Hyria, Lamarck. Castalia, Lamarck. Aplodon, Spix. Plagiodon, Spix. Faminy 26. AETHERIIDAE, Adams. Shell irregular, generally fixed in the adult state. Mantle with a single suture; foot absent; anterior adductor muscle Fig, 241. Modiolaria trapezina, Lamarck, viewed from the left side ; the left mantle lobe is removed. ad’, ad”, anterior and posterior adductor muscles ; br’, br’, internal and external gill-plates ; gl.p, foot gland ; 0.a, 0.b, anal and branchial orifices of the mantle ; 0.by, byssal orifice of the foot; o.p, pedal orifice of the mantle ; p, foot; pal, labial palp ; re.p, posterior retractor muscle of the foot. sometimes reduced or absent; from fresh water. Genera— Aetheria, Lamarck ; anterior adductor well developed ; African. Miilleria, Férussac ; no anterior adductor; American. artlettia, Adams. Sus-OrpDER 3. TELLINACEA. Eulamellibranchia in which the mantle is not extensively Closed, with two pallial sutures and two well-developed siphons; the gills smooth. The foot is compressed and elongated. The labial palps very large. Dimyarian; the pallial line has a deep sinus. Famity 1. TeLirrpar, Deshayes. The external branchial plate directed upwards (Fig. 206, H). The siphons separate and elongated. Foot with a byssogenous apparatus. Palps very large. Ligament of shell external, Genera—Tellina, Linnaeus; slightly inequivalve ; foot large ; British (Fig. 190). Gastrana, Schumacher ; equivalve ; the foot slightly developed ; British. Capsa, Bruguitre. Macoma, Leach. Famity 2. ScROBICULARIIDAE, Adams. External gill-plate directed upwards. Siphons separate and excessively long. Foot without a byssus. The ligament partly internal, lodged in a concavity in the hinge. Genera— THE LAMELLIBRANCHIA 269 | | a ahay at ; ay UE “bb au or aU Wp fieastitty DP li AMY Fic. 242. Diagrams of the external form and anatomy of Anodonta cygnaea (2), ventral view, all the other figures seen from the left side. (1) animal removed from its shell; a probe g passed into _ the infra-branchial chamber through the excurrent siphonal notch. (2) view from the ventral surface of Anodonta, with its foot expanded and issuing from between the gaping valves. (3) the left mantle-flap reflected upwards so as to expose the sides of the body. (4) diagrammatic sagittal section of Anodonta to show the course of the alimentary canal. (5) the two gill-plates of the left side reflected upwards, so as to expose the fissure between foot and gill where the probe g passes. (6) diagram to show the positions of the nerve ganglia, heart, and kidney. a, centro-dorsal area ; b, margin of the left mantle-flap; c, margin of the right mantle-flap ; d, ex- current (anal) siphonal notch of the mantle-margin ; e, incurrent (branchial) siphonal notch ; f, foot; g, probe passed into the superior division of the sub-pallial chamber through the anal siphonal notch, and issuing by the side of the foot into the infra-branchial chamber ; h, anterior ductor muscle ; i, anterior retractor muscle of the foot ; k, protractor muscle of the foot ; /, posterior adductor muscle; m, posterior retractor muscle of the foot ; 7, anterior labial palp ; 0, posterior labial palp ; p, base-line of origin of the reflected mantle-flap from the side of the body ; q, left external gill-plate ; 7, left internal gill-plate ; rr, inner lamella of the right inner gill-plate ; r.g, right outer gill-plate ; s, line of concrescence of the outer lamella of the left outer gill-plate with the left mantle-flap ; t, pallial tentacles ; w, the thickened muscular pallial margin which adheres to the shell and forms the pallial line of the left side ; v, that of the left side; w, the mouth ; z, aperture of the left kidney, exposed by cutting the attachment of the inner lamella of the inner gill-plate ; y, aperture of the genital duct; 2, fissure between the free edge of the inner lamella of the inner gill-plate and the side of the foot, through which the probe g passes into the supra-branchial chamber ; aa, line of concrescence of the inner lamella of the right inner gill-plate with the inner lamella of the left inner gill-plate ; ab, ac, ad, three pit-like glandular depressions in the median line of the foot ; ae, left shell-valve ; af, space occupied by the liver; ag, space occupied by the gonad ; ah, muscular substance of the foot ; ai, opening of bile-duct into the stomach; ak, stomach ; al, rectum traversing the ventricle of the heart ; am, pericardium ; an, glandular portion of the left kidney ; ap, ventricle of the heart ; aq, auriculo- ventricular orifice ; ar, non-glandular portion of the left kidney ; as, anus ; at, reno-pericardial orifice ; au, pore joining the two parts of the kidney ; av, internal pore of the kidney leading to the external pore x; aw, left cerebral ganglion; az, left pedal ganglion; ay, left otocyst ; az, left visceral ganglion ; bb, floor of the pericardium, separating that space from the kidney. (After Lankester.) 270 THE LAMELLIBRANCHIA Scrobicularia, Schumacher; estuarine; British. Syndosmya, Recluz ; British. Cumingia, Sowerby. Famity 3. Donacripan, Fleming. Ex- ternal gill- plate directed ventrally. Siphons separate, of moderate length, the anal siphon being the longer. Foot large and compressed. Shell inequilateral, the anterior side being the longer; the ligament external. Genera—Donaz, Linnaeus; British. Iphigeneia, Schumacher. Famiry 4, Massopesmaripar, Deshayes. External branchial plate directed ventrally. Siphons separate and equal. Shell inequilateral, the anterior side being the longer ; ligament internal. Genera—Meso- desma, Deshayes. rvilia, Turton; British, Famity 5. CarpInimDAg, Dall. Shell very high and short, ventricose, dimyarian, the posterior adductor impression borne on a prominent myophorous apophysis. Ligament partly internal. Genus—Cardilia, Deshayes ; from the Pacific Ocean. Famity 6. Macrripar, Gray. External branchial plate directed ventrally. Siphons united, more or less invested by a chitinous sheath. Foot long, stout, bent at an angle and without a byssus. Shell sub- triangular and nearly equilateral ; the ligament partly internal. Genera —Mactra, Linnaeus ; British (Fig. 191). Mulinia,Gray. Harvella, Gray. Raeta, Gray. LHastonia, Gray. Heterocardia, Deshayes. Vanganella, Gray. Sus-OrRDER 4. VENERACEA. Eulamellibranchia with two pallial sutures; the siphons generally somewhat elongated and partially or wholly united. Gills slightly folded. A bulb on the posterior aorta. Ligament external. Famiry 1. VENERIDAE, Gray. Foot well developed. Adductor muscles subequal. Pallial sinus shallow or absent. Genera—Venus, Linnaeus ; siphons rather short, their distal extremities free ; foot without byssus ; British. Dosinia, Scopoli; siphons long and fused together throughout their length; foot truncated without a byssus; British Tapes, Megerle; siphons rather long and incompletely fused; foot byssiferous ; British (Fig. 202). Cyclina, Deshayes. Lucinopsis, Forbes and Hanley ; British. Meretriz, Lamarck (Fig. 189). Circe, Schumacher ; British. Venerupis, Lamarck. Famity 2, Prrricotmar, dOrbigny. Boring Lamellibranchs with a reduced foot. The shell more or less elongated, with a deep pallial sinus. Genera—Petricola, Lamarck ; the British species P. pholadiformis, originally an inhabitant of the United States, has been acclimatised for some years in the North Sea: it has boring habits as and mimics Pholas candida. FamiLy 3, GLAUCOMYIDAE, Chenu. Siphons very long and united. Foot small. Shell elongated, thin, with a deep pallial sinus. Inhabitants of fresh or brackish water, Genera—Glaucomya, Woodward ; from 8.E. Asia. Tanysiphon, Benson ; from India. Susp-OrpDER 5. CARDIACEA. Eulamellibranchia with two pallial sutures. Generally with short siphons. The foot cylindrical, more or less elongated, furnished with a byssogenous apparatus. The gills much folded, Shell equivalve, with radiating costae and an external ligament. = THE LAMELLIBRANCHIA 271 Faminy 1. Carpimar, Gray. The mantle slightly closed; siphons very short and surrounded by a single circle of papillae which are often oculiferous (Fig. 243, 0.t). Foot very long, geniculated. Pallial line of the shell without a sinus; two adductor muscles. Genera—Cardium, Linnaeus ; adductor muscles subequal ; British (Fig. 243). Psewdokellya, Pelseneer (Fig. 221). Both Byssocardiwm, Munier-Chalmas, and Litho- cardium, Woodward, fossils from the Eocene, have a much reduced anterior adductor muscle. Faminy 2. Limnocarpmpag, Stoliczka. Siphons very long, united throughout their extent. Shell gaping; two adductor muscles. Inhabitants of brackish waters. _Genera—Adacna, Eichwald ; from the Caspian Sea. Limnocardium, Stoliczka ; from the Caspian Sea and fossil from the Tertiary. Arcicardiwm, Fischer; fossil from the Tertiary. Famity 3. Trrpacntpas, Broderip. Mantle closed to a con- siderable extent, the orifices distant from one another; no siphons. The foot short, with a more or less well developed byssus, A single adductor Fie. 243. Cardium edule, left-side view. «.s, anal siphon ; br.s, branchial siphon ; ), foot ; li, ligamen of the shell; 0.t, eye-spots on the tentacles ; sh, shell. (After Deshayes.) muscle. The gills narrow. The shell thick. Genera — Tridaena, Bruguiére ; byssus stout; shell gaping anteriorly; from the Indian and Pacific Oceans. Hippopus, Lamarck; byssus reduced; shell not gaping. Sup-OrpER 6. CHAMACEA. Asymmetrical, inequivalve, fixed Eulamellibranchia, with extensive pallial sutures and distant pallial orifices; no siphons. Two adductor muscles present. The foot reduced and without a byssus. Shell thick, without a pallial sinus. Famity 1. Caamipar, Gray. Shell with subequal valves and promi- nent umbones more or less spirally coiled ; ligament external. Genera —Chama, Bruguitre; the free valve only slightly ventricose ; shell lamellated or spiny. J)iceras, Lamarck; shell smooth, the umbones largely divergent and coiled ; the adductor muscles (at least the anterior) attached to myophorous apophyses ; fossil from the Jurassic (Fig. 244, A). Requienia, Matheron ; the fixed valve spirally coiled; the free valve 272 THE LAMELLIBRANCHIA operculiform ; fossil from the Cretaceous (Fig. 244, B). Matheronia, Munier-Chalmas ; fossil from the Cretaceous. Famity 2. CaPRINIDAE, d’Orbigny. Shell inequivalve; the fixed valve spiral or conical; the free valve not operculiform but coiled or spiral ; ligament internal ; sub- stance of the shell generally pierced with large parallel canals. An exclusively fossil family, from the Cretaceous. Genera —Caprina, @Orbigny ; the free valve larger than the fixed and coiled (Fig. 244, C). Caprinula, d’Orbigny ; fixed valve elongated and conical, free valve small and coiled (Fig. 244, D). Caprotina, d’Orbigny. Ichthyosarcolites, Des- marets. Plagioptychus, Matheron. Polyconites, Roulland, Famtzy 3. MONOPLEURIDAE, Munier- Chalmas. Shell very inequi- valve ; the fixed valve conical or spiral, the free valve operculiform and slightly or not at all spiral ; ligament external. No canals in the substance of the shell. An exclusively fossil family, from the Cretaceous. Genera—Mono- pleura, Matheron (Fig. 244, E). Valletia, Munier-Chalmas. Baylea, Munier-Chalmas. The a, Soin geners of font Chamaeos and Bulistes two following families, desig Caprina adversa,; D, Caprinula Baylet; £, Mono- nated by the common name of Oe Oren F, Radiolites angeiodes. (Chiefly Rudistae, are closely allied to tha preceding: they also comprise some extinct marine forms from Secondary deposits. These animals, of littoral and often gregarious habit, were fixed by the conical and more or less elongated right valve ; the adductor muscles were not inserted perpen- dicularly to the surface of separation of the two valves; the free left valve has a sub-central umbo, is not spiral, and is furnished with promi- nent myophorous apophyses to whose external faces the muscles were attached ; this valve was only movable in a vertical direction. FAamity 4. RapDIoLitTipak, Gray. Shell conical or biconvex, without canals in the external layer. Genera—Aadiolites, Lamarck ; valves ornamented with longitudinal costae ; a ligament present ; from the Cretaceous (Fig. 244, F). Biradiolites, @Orbigny ; no ligament; Cretaceous. Famity 5. Hip- PURITIDAE, Gray. Fixed valve long, cylindro-conical, with three longitudinal furrows, corresponding internally to two pillars which serve to support the siphons. Anterior adductor muscle with two separate insertions on the fixed valve. Genera—Hippurites, Lamarck ; Cretaceous, Arnaudia, Bayle ; Cretaceous. The family Diceratidae, the most ancient Fic. 244. x a THE LAMELLIBRANCHIA 273 of the Chamacea, and possibly derived from the Megalodontidae of the Palaeozoic and the Trias, has given rise to a branch that has survived to the present epoch (Chamidae) and to various others that became extinct at the close of the Secondary period. In all cases, the forms in which the umbo of the free valve is coiled have preceded the forms with an operculiform free valve—Requienia being derived from Diceras and Chama from Matheronia; in the same way among the Rudistae Radiolites appears to be derived from Caprina. The Hippuritidae, by the depth of the fixed valve, the reduction of the cavity, and the absence of the ligament, indicate the last stage of the evolutionary series, Sus-OrpDER 7. Myacrka. Eulamellibranchia in which the mantle is closed to a considerable _ extent; the siphons are well developed, the gills much folded and frequently prolonged into the branchial siphon. The foot is compressed and generally byssiferous. The shell gaping, with a pallial sinus. Famity 1. PsamMMosiipAr, Gray. Siphons very long and quite separate. Foot large, flattened from side to side and pointed. Shell Se ee Fic. 245. Psammobia florida, right side, showing expanded foot (e) and g, branchial, and g’, anal siphons. (From Lankester, after Garner.) oval, elongated, with a deep pallial sinus and an external ligament. Genera—Psammobia, Lamarck ; the posterior end of the shell sub- truncated; British (Fig. 245). Sanguinolaria, Lamarck. Asaphis, Modeer. Elizia, Gray. Solenotellina, de Blainville. Faminy 2. Myipak, Gray. Mantle largely closed; siphons united for the greater part of their length and surrounded, near their extremities, by a circlet of tentacles. Foot reduced. Shell gaping, with an internal ligament ; the left valve provided with a spoon-shaped projection for the ligament. Genera—Mya, Linnaeus; siphons elongated, covered by a chitinous sheath, and incompletely retractile ; foot small ; palps elongated ; British. Sphenia, Turton ; British. Tugonia, Gray. Platyodon, Conrad. Crypto- mya, Conrad. Famity 3. Corpuntpar, Fleming. Shell sub-trigonal, inequivalve, the left valve less convex than the right ; the pallial sinus shallow ; the ligament partly external. Siphons short, united, com- pletely retractile. Foot large, pointed, often byssiferous. Palps reduced. Genera—Corbula, Bruguitre ; siphons surrounded by a common circlet of tentacles ; shell short; British. Corbulomya, Nyst; shell elongated ; branchial siphon with a special tentacular crown. Paramya, Conrad. Erodona, Daudin, and Himella, Adams, are fluviatile forms from South America. Famiy 4. lLurraripar, Adams. Mantle extensively 18 274 THE LAMELLIBRANCHIA closed ; siphons long and united throughout their length; a fourth opisthopodial pallial aperture. Foot rather large, compressed. Shell elongated, with a deep pallial sinus and a spoon-shaped projection for the ligament on each valve. Genera—Lutraria, Lamarck ; British. Tresus, Gray. Standella, Gray. Famity 5. Soienrpax, Leach. Elongated, burrowing animals. The foot more or less cylindrical and powerful, without a byssus. Gills narrow. Shell long, truncated, and gaping at each end; the ligament external. Genera—®Solenocurtus, de Blainville ; siphons large, partially united, incompletely retractile ; pallial sinus deep ; foot very large and linguiform (Fig. 194); British. Tagelus, Gray ; posterior extremity short ; pallial sinus very deep ; estuarine. Ceratisolen, Forbes and Hanley; siphons long, separate ; gills rather short and not folded ; British. Cultellus, Schumacher ; siphons rather short ; extremity of foot dilated ; British. Siliqua, Megerle ; siphons of medium length ; foot dilated; shell compressed. Solen, Linnaeus; siphons short; foot elongated ; shell rectilinear, cylindrical; the umbones anterior and terminal ; British. Hnsis, Schumacher ; siphons very short ; a fourth pallial orifice ; shell arcuate, the um- bones anterior and sub-terminal; British. Famity 6. SAXICAVIDAE, Gray. Mantle extensively closed ; with a small pedal orifice (Fig. 246, f); siphons elongate, covered by a chitinous sheath and wholly or largely united ; gills prolonged into the branchial siphon. Foot small. Shell gaping, with an external ligament. oh Genera—Saxicava, Fleuriau; bores holes in rocks ; siphons free at their extremities; foot byssiferous ; British (Fig. 246). Glycimeris, Lamarck; siphons very long and completely united ; a burrowing form. Cyrtoduria, Daudin ; shell inequilateral ; the anterior brow a side the longer ; siphons united, incompletely retractile. Fic. 246, FaMILy 7. GASTROCHAENIDAE, Gray. Shell thin, Savicava arctica, ven- Without teeth, gaping widely at the anterior end. tral aspect. as, anal Anterior adductor much reduced. Foot small and siphon ; br.s, branchial : F siphon; by, byssal without a byssus, Gills narrow. Mantle extensively ante a en 2 closed; with long united siphons. Genera—Gastro- chaena, Spengler ; a boring form, with a cylindrical foot ; the shell regular and rarely enclosed in an adventitious tube ; British. istulana, Bruguitre ; a burrowing form with a very small compressed foot ; shell with a denticulate border, always enclosed in a regular, non-adherent, fragile, club-shaped adventitious tube. Sus-OrDER 8. ADESMACEA. Eulamellibranchia with very long united siphons and a largely closed mantle. The foot short, truncated, discoid, and without a byssus. The gills prolonged into the branchial siphon. The shell gaping and devoid of a ligament, but with a styloid apophysis in the umbonal cavities. Famity 1. Paonapmar, Adams. Shell capable of containing all THE LAMELLIBRANCHIA 275 the organs; the heart traversed by the rectum ; two aortae. The shell with a pallial sinus ; the dorsal region protected by accessory calcareous pieces. Genera—Pholas, Linnaeus ; foot cylindrical ; siphons free near their extremities ; British. In Pholas there are four accessory plates ; in Zirphaea, Leach, two; in Barnea, Leach, one. Pholadidea, Goodall ;. foot rudimentary ; siphons completely united and their extremity surrounded by a fringed disc ; shells elongated, in the adult prolonged posteriorly by a short caleareous tube, which surrounds the siphons ; British. Jowannetia, des Moulins; foot rudimentary ; siphons completely united ; shell globular, and the right valve prolonged posteriorly by a rostriform appendage. Xylophaga, Turton ; siphons separate at their extremities ; foot narrow ; shell globular with two accessory dorsal plates ; British. Martesia, Leach ; siphons long, united ; foot absent in the adult ; shell ovoid with a ventral plate in addition to the dorsal plates Faminy 2. TrERE- DINIDAE, Fleming. Shell globular, covering a small portion only of the vermiform body. Heart on the ventral side of the rectum (Fig. 195, h); a single aorta ; siphons long, united to a large extent and furnished with two posterior calcareous ‘‘ pallets” (Fig. 247, II). Genera —Teredo, Linnaeus; a borer in wood; secretes an adventitious non-adherent tube; British. Xylotrya, Leach ; the pallets articulated. Sus-ORDER 9. ANATINACEA. Hermaphrodite Eulamellibranchia, in which the ovaries and testes are distinct and have separate orifices (Fig. 219, 0, t). The foot generally rather small. The mantle frequently presents a fourth orifice. The ex- ternal gill-plate directed dorsally and devoid of a reflected lamella. Hinge of shell without teeth. Famity 1. THracrpar, Dall. Mantle with a fourth pallial orifice; the pedal orifice elongated; siphons rather long, quite separate, and completely retractile “ t Ww uw Fig. 247. Teredo navalis, Linnaeus, ventral aspect. I, shell; II, pallets; III, anal siphon; IV, branchial siphon ; V, siphonal mass ;. VI, foot. and invertible. Shell with a deep pallial sinus. Genera—Thracia, de Blainville ; shell with a large spoon-shaped tooth ; British. Asthenothaerus, Carpenter ; shell without spoon-shaped teeth. FAMILY 2. PERIPLOMIDAR, Dall. retractile, but not invertible. Genera — Cochlodesma, Couthouy. Adams. by a chitinous sheath, and not completely retractile. Pallial sinus well marked. Genera—Anatina, Lamarck. gaping, with spoon-shaped teeth. Jurassic and Cretaceous. extensively closed, with a fourth orifice. united, naked, and incompletely retractile. appendage. Siphons separate, naked, completely Pallial sinus shallow ; Periploma, Schumacher. Famity 3. ANATINIDAE, Gray. Siphons long, united, covered no ligament. Tyleria, Foot slender. Shell thin and Plectomya, de Loriol ; fossil from the Famity 4, PHoLapomyrpan, Gray. Mantle Siphons very long, completely Foot small, with a posterior Shell thin, with an external ligament and a well-marked 276 THE LAMELLIBRANCHIA pallial sinus. Genera—Pholadomya, Sowerby ; some species living and abyssal ; numerous fossil species from the Trias onwards, the maximum in the Jurassic. Faminy 5. ArcomyipAg, Fischer. Shell finely granular, equivalve, thin; the hinge without teeth ; the ligament external ; pallial sinus. An exclusively fossil family, from the Secondary and Tertiary. Genera—Arcomya, Agassiz ; from the Trias to the Eocene. Goniomya, Agassiz ; Jurassic and Cretaceous. Famity 6. PHOLADELLIDAE, Miller. Shell oval, the posterior extremity attenuated and gaping; cardinal border thin and devoid of teeth; ligament external ; posterior adductor muscle large. An exclusively fossil family, from Primary deposits. Genera—Pholadella, Hall; Devonian. Phytimya, Ulrich ; Silurian. Allorisma, King; Carboniferous and Permian. Faminy 7. PLEUROMYIDAE, Zittel. Shell inequi- lateral, thin ; the pallial line deeply sinuous ; the cardinal border of one valve covering that of the other and hiding the ligament, which is therefore sub-internal. An ex- clusively fossil family from Secondary formations. Genera — Plewromya, Agassiz; from the Trias and inferior Cretaceous. Gresslya, Agassiz ; Jurassic. Ceromya, Agassiz ; Jurassic. Famity 8, PanpoRIpAE, Gray. Shell thin, inequivalve, free ; the ligament internal ; no pallial sinus. Siphons very short; foot elongate. Genera— Pandora, Bruguiere ; British. Coelodon, Carpenter. Clidiophora, Carpenter. Famity 9. Myo- CHAMIDAE, Dall. Shell very inequivalve, solid, with a pallial sinus. Siphons short ; a fourth pallial _ orifice present ; foot small. Genera — Myochama, Stutchbury ; shell irregular ; fixed to other shells by the right valve; Australian. Myodora, Gray ; shell free, trigonal ; the left valve flattened. Famity 10. CHAMOSTREI- DAE, Fischer. Mantle largely closed. A fourth pallial orifice present ; pedal orifice small. Siphons very short and separate. Shell Aspergillum vaginiferum ; to the left side, the whole shell, dorsal view, the anterior part below ; to the right fixed by, the right valve, irre- side, the anterior pa aeene to Bhow ote hie gular, without a pallial sinus ; valves a, now embedded in a continuous calcification of 7. 5 y tubular form, (From Lankester, after Owen.) ligament internal. Genus— Chamostrea, de Roissy; Australian. Famiy 11. CLAVAGELLIDAE, d’Orbigny. Mantle largely closed ; pedal orifice extremely small ; a fourth pallial orifice present ; siphons fairly long, united ; foot very rudimentary and without a byssus. The ligament external ; the valves continued backwards into a calcareous Fic. 248. THE LAMELLIBRANCHIA 277 tube secreted by the siphons ; pallial line sinuous. Genera—Clavagella, Lamarck ; left valve fused to the tube; adductor muscles well developed ; a boring form. JDrechites, Guettard (= Aspergillum, Lamarck); the two valves fused to the tube and external (Fig. 248); no posterior adductor muscle ; the anterior adductor much reduced; the anterior extremity bearing numerous tubular projections serving for adhesion ; Indian and Pacific Oceans. Famity 12. Lyonsipaxr, Fischer. Mantle largely closed, with a fourth pallial orifice ; siphons short, invertible ; foot byssiferous. Shell thin, granular externally ; the pallial sinus feeble ; the ligament internal. Genera—Lyonsia, Turton; shell regular and elongated ; British (Fig. 219). EHntodesma, Philippi; shell irregular, truncated behind; a boring form, sometimes found in the tests of Ascidians. Mytilimeria, Conrad ; shell regular, ventricose, gaping behind. Famity 13. VerticorDimpAE, Wood. Siphons short ; the gills papillose ; Fic. 249. Poromya tornata, left-side view. a.a, anterior adductor; a.p, anterior labial palp; a.s, anal siphon ; f, foot; g./, gill lamellae on the septum; h, heart; ha, posterior adductor; in, intestine ; li, liver; pa, pallial suture; p.p, posterior labial palp; p.t, pallial tentacles; r.p, retractor posterior pedis; r.s, retractor of the septum; s, septum; v.f, valvular fold of the branchial aperture. foot small ; palps well developed. Shell globular, very slightly gaping, without a pallial sinus) Many species abyssal. Genera—Verticordia, Wood ; mantle largely closed ; the pedal orifice small. Luciroa, Dall ; heart situated above the rectum. Lyonsiella, Sars; foot byssiferous. Halicardia, Dall. ORDER 4. Septibranchia. The Septibranchia are dimyarian Lamellibranchs in which the mantle remains fairly open and has two sutures and two siphons. The foot is long and slender; the byssus rudimentary or absent. The pallial line is simple or very slightly sinuous. The essential character of the group is the disappearance of the gills as respiratory organs, a character which is not found in any other Lamellibranch. The gills are transformed into a muscular septum (Fig. 249, s) 278 THE LAMELLIBRANCHIA which extends from the anterior adductor muscle to the point of separation of the two siphons, and surrounds and is continuous with the foot. This septum, therefore, has exactly the situation and the relations of the branchial septum of the majority of the Lamellibranchia, which divides the pallial cavity into two chambers. The group is derivable, more or less directly, from the Anatinacea, through the series Lyonsia, Lyonsiella, Poromya, Cetoconcha, Cuspidaria, in which one may observe a gradual increase in the amount of muscular fibre in the gill filaments or in their reduced equivalents, so that any objection to the branchial origin of the septum, because of its muscularity, cannot hold good. The muscular septum is inserted on the shell, especially in the neighbourhood of the two adductor muscles. The origin of the anterior and posterior exten- sions of the septum and of its muscular attachments to the two extremities of the shell is to be found in the physiological contrac- tions necessary to create a current of water on the respiratory surface of the supra-septal chamber. The septum is, in fact, always pierced by paired orifices, which admit of the passage of water. The Septibranchia are all marine, inhabit considerable depths of the sea, and are carnivorous. The order only comprises one sub-order, the Poromyacea. FAMILY 1. PoromyIDA#, Dall. Siphons short and separate ; the bran- chial siphon provided with a large valve. Foot pointed and not byssi- ferous. The branchial septum bears two groups of transversely elongate orifices on either side; these are formed by a few branchial filaments, with or with- out junctions. The palps are large. All the members of the family are her- maphrodite. Genera—Poromya, Forbes ; no pallial sinus; British (Fig. 249). Fic, 250. Dermatomya, Dall; a pallial sinus A ventral view of Cetoconcha, removed ypyesent. Liopistha, Meek ; fossil from from its shell. «a.o, anterior septal orifices; ap, anterior palp ; J, foot; m, the Cretaceous. Famity 2. CETOcON- mouth ; m.o, median septal orifices ; pa, CHIDAR, Ridewood. Branchial septum mantle; p.o, posterior septal orifices ; p.p, posterior palp ; se, branchial sep- bearing three groups of orifices on each (Attar nieve) branchial siphon. side ; these orifices are separated by rudimentary branchial filaments. Palps large ; siphons short, separate, the branchial siphon with a valve. Genus —Cetoconcha, Dall (=Silenia, Smith) ; abyssal (Fig. 250). Famity 3. CusPIDARUDAE, Fischer. Siphons long and united, their extremities surrounded by tentacles. Foot narrow, with a rudimentary byssus. Palps greatly reduced or absent, Branchial septum pierced by four or , LITERATURE OF THE LAMELLIBRANCHIA 279 five pairs of very narrow symmetrical orifices. The sexes separate. Genus—Cuspidaria, Nardo; British (Fig. 251). Fic. 251. Cuspidaria cuspidata (Olivi), left-side view, after removal of left half of the mantle. a, anus; a.a, anterior adductor ; a.f.r, anterior foot retractor ; «.p, anterior labial palp ; @.s, anal siphon ; br.n, branchial nerve ; br.s, branchial siphon; b.v, branchial valve; ¢.g, cerebral ganglion ; f, foot; g.g, genital gland; g.o, genital orifice; hk, heart; in, intestine ; k, kidney ; k.o, kidney opening ; m, mouth ; p.a, posterior adductor; p.f.r, posterior foot-retractor ; p.g, pedal gan- glion; p.l.p, posterior labial palps; s, septum; s.o, septal orifice; s.r, septal retractor; st, stomach ; vi.g, visceral ganglion. LITERATURE OF THE LAMELLIBRANCHIA. A. General. . Ahting. Untersuchungen iiber die Entwickelung des Bojanus’schen Organ und des Herzens der Lamellibranchier. Jenaische Zeitschr. xxxvi. 1901. . Barrois, Th. Les glandes du pied et les pores aquiféres chez les Lamelli- branches. Lille, 1885. . — Le stylet cristallin des Lamellibranches. Revue biol. Nord France, i. 1890. . Bernard, F. Premiere, deuxiéme, troisiéme, quatriéme et derniére note sur le développement et la morphologie de la coquille chez les Lamellibranches. Bull. Soc. Géol. France (3), xxiii., xxiv., xxv. 1895, 1896, and 1897. 5. —— Recherches ontogéniques et morphologiques sur la coquille des Lamelli- branches. Ann. des Sci. nat. Zool. (8), viii. 1898. 6. Blanchard, E. Observations sur le systéme nerveux des Mollusques Acéphales Testacés ou Lamellibranches. bid. (3), iii. 1845. 7. —— JOrganisation du Régne Animal: Mollusques Acéphales. Paris, 1851. 8. Bonnet, R. Der Bau und die Circulationsverhiiltnisse der Acephalentkieme. it, 12, Morph. Jahrb. iii. 1877. . Boutan, L. Recherches sur le byssus des Lamellibranches, Arch. de Zool. Expér. (3), iii. 1895. . Biitschli. Notiz zur Morphologie des Auges der Muscheln. Festschr. 500 jahr. Bestand Ruperto-Carola nat.-med. Ver. Heidelberg, 1886. Carazzi, D. Contributo all’ istologia e alla fisiologia dei Lamellibranchi. Mittheil d. Zool. Stat. Neapel, xii. 1896 ; Internat. Monatsschr. f. Anat. u. Phys. xiv. and xx. 1897 and 1902. Carriére. Die Driisen im Fusse der Lamellibranchiaten. Arbeiten Zool. Zoot. Instit. Wiirzburg, v. 1879. 280 LITERATURE,OF THE LAMELLIBRANCHIA 13. 14, 15. 16. fe 18. ig) Cattie. Les Lamellibranches recueillis dans les courses du Willem Barents. Bydr. tot de Dierk., 1884. Coutance. De Vénergie et de la structure musculaire chez les Mollusques Acéphales. Paris, 1878. : Dail. Tertiary Mollusks of Florida. Part III. A new Classification of the Pelecypoda. Trans. Wagner Free Instit. of Science, iii. 1895. Deshayes. Histoire naturelle des Mollusques (Exploration scientifique de l’Algérie). Paris, 1844-1848. Duvernoy. Mémoires sur le systeéme nerveux des Mollusques Acéphales. Mém. Acad. Sci. Paris, xxiv. 1853. Ehrenbaum. Untersuchungen iiber die Structur und Bildung der Schale der in der Kieler Bucht haiifig vorkommenden Muscheln. Zeitschr. f. wiss. Zool. xli. 1884. Fleischmann. Die Bewegung des Fusses der Lamellibranchiaten. Zeitschr. f. wiss. Zool. xlii. 1885. . Flemming. Ueber Bindesubstanz und Gefiisswandung im Schwellgewebe der Muscheln. Arch. f. mikr. Anat. xiii. 1877. . Georgévitch. Recherches sur les glandes du pied des Lamellibranches. Geneve, 1895. . Grobben. Die Pericardialriise der Lamellibranchiaten. Arb. Zool. Inst. Wien, vii. 1888. Ueber den Bulbus arteriosus und die Aortenklappen der Lamelli- branchiaten. Jbid. ix. 1891. . Hancock. On the boring of the Mollusca into rocks. Ann. Mag. Nat. Hist. (2), ii. 1848. . Horst. Ist der Byssus ein Cuticularbildung? Tijdschr. Ned. Dierk. Vereen. (2), ii. 1889. . Kellog. A Contribution to our Knowledge of the Morphology of Lamelli- pbranchia. Bull. U.S. Fish Comm. x. 1890. . Jameson. On the Origin of Pearls. Proc. Zool. Soc. London, 1902. . Janssens. Les branchies des Acéphales. La Cellule, ix. 1893. . Lacaze-Duthiers. Recherches sur les organes génitaux des Acéphales Lamelli- branches. Ann. des Sci. nat. Zool. (4), ii. 1854. wy, . — Mémoire sur l’Organe de Bojanus des Acéphales Lamellibranches. Ann. des Sci. nat. Zool. (4), iv. 1855. . —— Mémoire sur le développement des branchies des Mollusques Acéphales Lamellibranches. Jbid. (4), v. 1856. . Letellier. Etude sur la fonction urinaire chez les Mollusques Acéphales. Arch. de Zool. Expér. (2), v. bis, 1887. . Loven, Bidrag till Kinnedomen om Utvecklingen af Mollusca Acephala Lamellibranchia. K. Vet. Akad. Handl. 1848. . Menegauwx. Recherches sur la circulation chez les Lamellibranches marins. Besancon, 1890. . Mitra. The cristalline style of Lamellibranchia. Quart. Journ. Mier. Sci. xliv. 1901. . Mitsukuri. On the structure and significance of some aberrant forms of Lamellibranchiate Gills. Quart. Journ. Micr. Sci. xxi. 1881. . Newmayr. Beitrage zur einer morphologischen Eintheilung der Bivalven. Denkschr. k. Akad. d. wiss. Wien, math.-naturw. Cl. lviii. 1891. . Patten. Eyes of Molluses and Arthropods. Mitth. Zool. Stat. Neapel, vi. 1886. Vv LITERATURE OF THE LAMELLIBRANCHIA 281 39. Peck, R. H. The minute structure of the gills of Lamellibranch Mollusca. Quart. Journ. Micr. Sci. xvii. 1877. 40. Pelseneer. Contribution a l’étude des Lamellibranches. Arch. de Biol. xi. 1891. 41. —— Les yeux céphaliques chez les Lamellibranches. Jbid. xvi. 1899. 42. Rawitz. Der Mantelrand der Acephalen. Jenaische Zeitschr. xxii., xxiv., Xxvii. 1888, 1890, 1892. 43. Rice. Die systematische Verwerthbarkeit der Kiemen bei den Lamelli- branchiaten. Jenaische Zeitschr. xxxi. 1897. 44. Ridewood, W. G. On the structure of the Gills of the Lamellibranchia. Phil. Trans. B. exev. 1903. 45. Roule, L. Recherches histologiques sur les Mollusques Lamellibranches. Journ. Anat. et Physiol. 1887. 46. Schreiner. Die Augen bei Pecten und Lima. Bergens Mus. Aarbog, 1896. 47. Sharp, B. On the Visual Organs in Lamellibranchiata. Mitth. Zool. Stat. Neapel, v. 1884. 48. Soubeiran. Essai sur les ganglions médians ou latéro-supérieurs des Mol- lusques Acéphales. Paris, 1858. 49. Stenta, M. Zur Kenntniss der Strémungen im Mantelraume der Lamelli- branchiaten. Arb. Zool. Inst. Wien, xiv. 1902. 50. Thiele, J. Die Mundlappen der Lamellibranchiaten. Zeitschr. wiss. Zool. xliv. 1886. Die Abdominal Sinnesorgane der Lamellibranchier. Jbid. xlviii. 1889. 52. Yung, E. De innervation du ceeur et de V’action des poisons chez les Mol- lusques Lamellibranches. Arch. de Zool. Expér. (1), ix. 1881. B. Special. 53. Alder and Hancock. On the branchial currents in Pholas and Mya. Ann. Mag. Nat. Hist. (2), viii. 1851. 53’, Anthony. Influence de la fixation pleurothétique sur la morphologie des Moliusques Acéphales Dimyaires. Ann. des Sci. nat. Zool. (9), i. 1905. 54. Babor. Ueber das Centralnervensystem von Dreissenia polymorpha. Pall. Sitzungsber. Bohm. gesellsch. Wiss. math.-natur. Cl. 1895. 55. Barrois, Th. Sur la structure del’ Anomia ephippium. Bull. Sci. Dép. Nord (2), ii. 1879. + — Note sur l’embryogénie de Ja Moule commune, Mytilus edulis. Ibid. (2), ii. 1879. 57. Bernard, F. Scioberetia australis, type nouveau de Lamellibranche. Bull. Sci. France et Belgique, xxvii. 1896. 58. —— Condylocardia, type nouveau de Lamellibranches. Journ. de Conchyl. 1896. 59. —— Les genres Philobrya et Hochstetteria. bid. 1897. 60. Anatomie de Chlamydoconcha Orcutti, Lamellibranche 4 coquille in- terne. Ann. des Sci. nat. Zool. (8), iv. 1897. 61. Beuk. Zur Kenntniss des Baues der Niere und der Morphologie von Teredo. Arb. Zool. Inst. Wien, xi. 1899. 62. Bloomer. The Anatomy of the British Species of the Genus Solen. Journ. of Malacol. viii., ix. 1901, 1902, 63. —— The Anatomy of certain species of Ceratisolen and Solecurtus. Jbid. x. 1905. 282 LITERATURE OF THE LAMELLIBRANCHIA 64. 65. 66. 67. 68. 69. 70. (Ae 72. 73. Brooks, W. K. The development of the Oyster. Studies Biol. Labor. Johns Hopkins Univ. i. 1880. Deshayes. Mémoire anatomique sur l’Iridine du Nil. Mém, Soc. Hist, Nat. Paris, iii. 1897. Douvillé, H. Etudes sur les Rudistes. Mém. Soc. Géol. France, Paléon- tologie, i., iii. 1890, 1893. Drew. Some observations on the Habits, Anatomy, and Embryology of Members of the Protobranchia. Anat. Anzeiger, xv. 1899. — YVoldialimatula. Mem. Biol. Labor. Johns Hopkins Univ. iv. 1899. — The Life-History of Nucula delphinodonta. Quart. Journ. Mier. Sci. xliv. 1901. Drost. Ueber das Nervensystem une die Sinnesepithelien der Herzmuschel (Cardium edule). Morph. Jahrb. xii. 1886. Dubois. Anatomie et Physiologie comparée de la Pholade dactyle : Structure. locomotion, tact, olfaction, gustation, vision dermatoptique, photogénie. Ann. Univ. Lyon, ii. 1892. Logger. Jowannetia Cumingii, Sow. Arb. Zool. Zoot. Inst. Wiirzburg, viii. 1887. Faussek. Ueber die Ablagerung des Pigments bei Mytilus. Zeitschr. wiss. Zool. lxv. 1898. . Fullarton. On the development of the Common Scallop (Pecten opercularis). Eighth Ann. Rep. Fish. Board Scot. iii. 1890. - Gotte. Bemerkungen iiber die Embryonalentwickelung der Anodonta piscinalis. Zeitschr. wiss. Zool. xli. . Grobben. Beitriige sur Kenntniss des Baues von Cuspidaria cuspidata. Arb. Zool. Inst. Wien, x. 1892. Beitrage sur Morphologie und Anatomie der Tridacniden. Denkschr. math.-naturw. Cl. K. Akad. wiss. Wien, lxv. 1898. - — Zur Kenntniss der Morphologie und Anatomie von Meleagrina sowie der Aviculiden im Allgemeinen. Jbid. lxix. 1900. . Hancock. On the Animal of Chamostrea albidu. Ann. Mag. Nat. Hist. (2), ii. 1853. On the Animal of Myochama anomoides. Ibid. (2)ii. 1858. - Hatschek. Ueber Entwicklungsgeschichte von Teredo. Arb. Zool. Inst. Wien, iii. 1880. - Hoeck. Les organes de la génération de l’huitre. Tydschr. Ned. Dierk. Vereen, Suppl. Deel i. 1884. 3. Horst. On the development of the European Oyster (Ostrea edulis). Quart. Journ. Micr. Sci. xxii. 1882. - Huxley. Description of the Animal of Trigonia. Proc. Zool. Soe. London, 1849, . Jackson. The development of the Oyster, with Remarks on allied genera. Proc. Bost. Soc. Nat. Hist. xxiii. 1888. . Jhering, H. von. Phylogeny of the Pelecypoda; the Aviculidae and their Allies. Mem. Boston Soc. Nat. Hist. iv. 1890. - — Die Gehorwerkzeuge der Mollusken in ihrer Bedeutung fiir das Naturliche System derselben. Erlangen, 1876. Johnstone. Cardium. Liverpool Mar. Biol. Com. Mem. ii. 1899. . Keer, Bydrage tot de Kennis van den Paalworm. Leiden, 1903. . Kishinouye. Note on the eyes of Cardiwm muticum, Reeve. Journ. Coll. Sci. Tokyo, vi. 1894. v LITERATURE OF THE LAMELLIBRANCHIA 283 91. Korschelt. Ueber die Entwickelung von Dreissena polymorpha. Sitzungsber. Ges. naturforsch. Fr. Berlin, 1891. 92. Lacaze-Duthiers, F. J. H. Mémoire sur l’organisation de l’Anomie (Anomia ephippium). Ann. des Sci. nat. Zool. (4), ii. 1854. 93. Lacaze-Duthiers, H. de. Morphologie des Acéphales. 1° Mémoire. Anatomie de l’Arrosoir (Aspergillum dichotomum). Arch. de Zool. Expér. (2), 1 1883. 94. —— Morphologie de Tridacna elongata et de Hippopus. Ibid. (3), x. 1903. 95. Langer. Ueber das Gefisssystem der Teichmuschel. Denkschr. k. Akad. wiss. Wien math.-naturw. Cl. viii., xii. 1855, 1856. . Lankester, E. Ray. On Green Oysters. Quart. Jour. Micr. Sci. xxvi. 1886. Lilie. The Embryology of the Unionidae. Jour. of Morphol. x. 1895. oie: “Die Mytiliden.. Fauna und Flora Neapel, xxvii. 1902. 99. Mayoux. L’existence d’un rudiment céphalique, d’un systeme nerveux stomato-gastrique et quelques autres particularités morphologiques de la Pintadine. Bull. Soc. Philom. Paris (7), x. 1886. 100. Meisenheimer. Entwickelungsgeschichte von Dreissensia polymorpha. Zeitschr. wiss. Zool. ]xix. 1900. 101. Morse. Remarks on the relations of Anomia. Proc. Bost. Soc. Nat. Hist. xiv. 1871. 102. Pieri. Recherches physiologiques sur Tapes decussata et quelques Tapidées, Laval, 1895. _ A? Purdie. The Anatomy of the Common Mussels (Mytilus latus, edulis, and magellanicus). Studies in Biology, No. 3, published by the Colonial Mus. and Geol. Survey Dept. New Zealand, 1887. ¥ 104. Rankin. Uber das Bojanus’sche Organ der Teichmuschel (Anodonta cygnea, Vv Lam.). Jenaische Zeitschr. xxiv. 1890. 105. Ryder. The Metamorphosis and Post-larval Stages of Development of the Oyster. Rep. U.S. Fish Comm. for 1882, 1884. 106. Sabatier. Anatomie de la Moule commune. Ann. des Sci. nat. Zool, (6), v 1877. 107. Sassi. Zur Anatomie von Anomia ephippiwm. Arb. Zool. Inst. Wien, xv. 1903. 108. Schierholz. Ueber Entwickelung der Unioniden. Denkschr. k. Akad. wiss. Wien math.-naturw. Cl. lv. 1889. 109. Schmidt, F. Beitrag sur Kenntniss der postembryonalen Entwickelung der Najaden. Arch. f. Naturgesch. li. 1885. 110. Sigerfoos. Note on the Organisation of the Larva, and the Post-larval Development of Shipworms. Johns Hopkins Univ. Circul. xv. 1896. 111. Stempell. Beitriige zur Kenntniss der Nuculiden. Zool. Jahrb. Suppl. iv. 1898. 112. —— Zur Anatomie von Solenomya togata. Ibid. (Anat. u. Ontog.), xiii. 1900. 113. Stauffacher. Fibildung und Furchung bei Cyclas cornea. Jenaische Zeitschr. xxviii. 1898. 114. Towreng. Sur le systéme nerveux du Dreissensia polymorpha. Comptes rendus Acad. Paris, 118, 1894. 115. Tullberg. Ueber die Byssus des Mytilus edulis. Nova Acta Reg. Soc. Scient. Upsala, 1877. 116. Vaillant. Recherches sur la famille des Tridacnidés. Ann. Sci. nat. Zool. (5), iv. 1865. 284 LITERATURE OF THE LAMELLIBRANCHIA Ine 118. 119. => Jae yar. 122. 123. 124, Vaillant. Anatomie de deux Mollusques de la famille des Malléacées. Zbid. (5), ix. 1868. Voeltzkow. Entovalva mirabilis. Zool. Jahrb. (Anat. u. Ontog.) v. 1891. Willem and Minne. Recherches expérimentales sur la circulation sanguine chez l’Anodonte. Mém. Cour. Acad. Belg. lvii. 1899. Wilson. On the development of the Common Mussel (Mytilus edulis). Fifth Ann. Rep. Fish. Board Scot. 1887. Woodward, F. M. On the Anatomy of Ephippodonta MacDougalli, Tate. Proc. Malacol. Soe. i. 1893. Note on the Anatomy of the larva of the European Oyster, (Ostrea edulis, Linn.). Proce. Malacol Soc. i. 1895. — Anatomy of Miilleria Dalyi, Smith. Proc. Malacol. Soe. iii. 1898. Ziegler. Die Entwickelung von Cyclas cornea. Zeitschr. wiss. Zool. xli. 1885. CHAPTER VI THE CEPHALOPODA CLASS V.—THE CEPHALOPODA, Cuvier. Order 1. Tetrabranchia. Sub-Order 1. Nautiloidea. = 2. Ammonitoidea. Order 2. Dibranchia. sits Onder 1. Decapoda. Tribe 5 Oigopsida. ‘4 . Myopsida. Sub- Order 2 Octopoda. Tribe 1. Leiozlossa. ,, 2. Trachyglossa. Definition —The Cephalopoda are perfectly symmetrical Mollusca, in which the edges of the foot are transformed into circumoral appendages completely surrounding the head, and the epipodium is modified to form an exhalant muscular tube or funnel consisting of two free or united lobes, situated behind the head at the opening of the pallial cavity, and serving as a conduit for the water from this cavity. In the nervous system all the typical ganglion-pairs are concentrated in the head, and are applied to or contained in the interior of a cartilaginous skeletal piece. The renal organs are constituted by the glandular covering of the afferent branchial vessels. The coelom communicates with the exterior either directly or by the intermediary of the paired kidneys, and by a second pair of ducts serving as gonaducts. The gonad is situated in the coelom and is not continuous with the gonaducts. A portion of the circumoral pedal crown is “ hectocotylised,” that is to say, modified to form a copulatory organ in the male. The development is characterised by the incomplete segmentation of the ovum. I. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. In comparing the Cephalopoda with other Mollusca, one finds that the ventral surface is much abbreviated and the length of the 285 286 THE CEPHALOPODA body reduced (Fig. 22, E). This is the result of the displacement of the foot, whose lateral borders surround the head and are joined together in front of the mouth. In consequence of this shortening of the antero-posterior axis, the two extremities of the digestive canal are closely approximated, and the pallial cavity opens im- mediately behind the head (Fig. 252, m, a). The head is highly developed, but has hardly any other ap- pendages than those formed by the edges of the foot which Fig. 252. Diagram of the structure of a Cephalopod, as seen from the left side in its proper mor- phological position. a, anus; ar, arms; ce, central nervous system, with eye; coe, coelom ; fu, funnel; g, gill; go, gonad; h, Fic. 253. heart; k, kidney; Ul, liver; m, mouth; pa, mantle; v.o, renal orifice; 7.p, reno- pericardial orifice ; st, stomach. Taonius suhmii, Lankester, ventral aspect. e, pedunculated eye; fi, fin; m, mouth ; ¢, tentaculararm. (After Hoyle.) embrace it. Certain Oigopsida, however, e.g. Taonius suhmii, Ray Lankester (Fig. 253), and the embryos of an allied form known as Grenacher’s embryo (Fig. 119, D) and of Loligo peali, have very prominent pedunculated eyes. On the other hand, Nautilus, whose eyes are also somewhat prominent, has in addition two ciliated tentacles on either side of the head, one in front of and the other behind the eye (Figs. 255, i, k; 293, a.o.t, p.o.t). The foot forms a crown of appendages surrounding the mouth: the edges of this crown are not deeply divided in Nautilus, but are much more so in the Dibranchia. In Nuutilus the cireumoral pedal crown is divided into lobes each of which bears a group of tentacles, the total number of tentacles being about ninety in the THE CEPHALOPODA 287 female, but only sixty in the male. These tentacles have laminated but not ciliated surfaces; they are adhesive and prehensile, and are retractile within special tentacular sheaths. When the animal is extended they radiate outwards from the mouth. In the female there are three tentaculiferous lobes in immediate contact with the buccal aperture (Fig. 255, c, d): these are the right and left and the ventral interior lobes. The last named (which is absent in the male) bears a laminated organ, supposed to be olfactory in function and known as Owen’s organ, in the middle of its free border (Fig. 255, n), and fourteen tentacles on each moiety of the lobe. The \\ aie Sox é g Fia. 254. Tremoctopus velifer, Verany, viewed from the dorsal side, showing the four dorsal arms joined together by amembrane. (After Verany.) right and left interior lobes bear twelve tentacles apiece. The muscular mass of the foot forms a broad ring round the three interior lobes, and is particularly thick and strong in the dorsal region (Fig. 255, g), Where it is modified to form a hood which protects the whole animal when it is retracted within its shell. On the external face of the hood is a concavity in which the spire of the shell is lodged. The tentacles borne on this ring are called “digital,” and are larger than the “labial” tentacles borne on the three interior lobes. The digital tentacles are nineteen in number on each side in the female, and are disposed more or less regularly in three unequal rows. It is only the dorsal pair of tentacles that belongs to that part of the muscular ring which forms the hood, the last-named 288 THE CEPHALOPODA structure being largely composed of an extension of the sheaths of the tentacles in question. On the ventral side an extensive part of the internal surface of the muscular ring is laminated, Fiq. 255. Male (upper) and female (lower) specimens of Nautilus pompilius, as seen in the expanded condition ; oral view, showing the disposition of the tentaculiferous lobes and the differences between the two sexes. a, the shell; b, the outer ring-like expansion or annular lobe of the circumoral mass of the foot, dorsally forming the hood ; ¢, the right and left inner lobes of the foot, each carrying twelve tentacles in the female, in the male divided into p, the “ spadix” (hectocotylus) on the left side, and q, the “‘anti-spadix,” a group of four tentacles, on the right side ; d, the inner inferior lobe of the foot (reduced in the male to a paired group of lamellae) ; e, the buccal cone, fringing the jaws with a series of minute papillae; /, the tentacles of the outer lobe, projecting from their sheaths ; 9, the two most posterior tentacles, belonging to the hood ; i, superior ophthalmic tentacle; k, inferior ophthalmic tentacle; l, eye; m, paired laminated organ on each side of the base of the inner inferior lobe of the female; n, olfacto: lamellae upon the inner inferior lobe, in the female ; 0, the funnel ; p, the spadix (in the en or hectocotylised portion of the left inner lobe of the foot, representing four modified tentacles ; q, the antispadix (in the male), being four of the twelve tentacles of the right inner lobe, isolated from the remaining eight. (After Lankester.) THE CEPHALOPODA 289 forming the so-called “organ of Valenciennes,” peculiar to the female and serving for the attachment of the spermatophores (Fig. 255, m). In the Dibranchia the pedal appendages have the form of four or five pairs of symmetrical and generally elongate arms. In the Octopoda there are eight similar arms, and the whole length of the ventral surface of each is covered by suckers which are often very numerous and highly specialised in structure (Fig. 256). It seems probable that it is the suckers and not the arms that are com- parable with the tentacles and tentacle-sheaths of Nautilus. In the Decapoda, in addition to the eight arms corresponding to those of the Octopoda, there are two additional “tentacular” arms, of which one is situated between the third and fourth sessile arms on either side of the posterior part of the head. These two tentacular arms are longer and more slender than the others (Fig. 298, A), and the suckers are generally confined to their free extremities, which are enlarged and club-shaped ; in some forms, however, they bear suckers along their whole length (Fig. 297, Il). The tentacular arms are further distinguished from the sessile arms by the fact that they are more or less retractile within special pouches: they are com- pletely retractile in Sepia, Sepiola, and Rossia, incompletely retractile in Loligo, very slightly retractile in the majority of the Oigopsida, and finally they are united to form a beak-like appendage in Rhynchoteuthis. In some Oigopsida, such as Leachia, Chawnoteuthis, some species of Cheiroteuthis, and Grimalditeuthis (Fig. 258), the tentacular arms are reduced to mere stumps: in the adult Veranya they are similarly reduced, but the young still retain small tentacular arms. Similarly a notable reduction of the ordinary or sessile arms, particularly on the dorsal side, may be observed in some Cheiroteuthidae and Cranchiidae. Some or all of the eight sessile arms may be united by a more or less complete inter- brachial membrane: the four dorsal arms are united in this manner in /remoctopus (Fig. 254), the six dorsal arms in Histioteuthis, and all eight arms in some species of Eledone, in Alloposus, and in the adult Cirrhoteuthidae and Amphitretidae, the membrane ex- tending in the two last-named families to the tips of the arms, but in the young of Cirrhoteuthis (Fig. 260) the membrane is not fully developed. In the female Argonauta the two dorsal arms are enlarged to form a veil (Fig. 301, IV), which is applied to the mantle and secretes a protective calcareous shell. Finally, in most cases a single arm of the male, or a portion of the cireumoral pedal crown in Nautilus, is modified to form a copulatory organ, which is some- times detachable. This is the hectocotylus, or spadix in Nautilus, which will be described in detail under the head of reproductive apparatus. 19 290 THE CEPHALOPODA The suckers are pedunculated in the Decapoda, the peduncles being axial or lateral, but they are sessile in the Octopoda (Fig. 256). They generally form a double series along the internal, that is to say, the ventral or buccal faces of the arms, but there is a single series in LHledone and Cirrhoteuthis (Fig. 260). In some cases, however, there are more than two rows, eg. Spirula (Fig. 268, ar), Gonatus, Dosidicus, Tritaxeopus, Ctenopteryx (on the three dorsal pairs of arms), and Sepia (Fig. 299, c). In point of structure, each sucker consists of a globular or cylindrical projection, comprising an annular surface of application with a central cavity whose capacity can be augmented by the retraction of its floor. The floor is provided with perpendicular muscular fibres (Fig. 256, I), whose contraction causes the sucker to adhere to the prey or to the substratum. The surface of application of the sucker is Fic. 256. Axial section of a sucker of Argonauta. I, muscular fibres retracting the bottom of the sucker ; II, radiating muscular fibres ; ITI, annular muscular fibres ; LY; floor of the cavity of the sucker; V , sphincter muscle; VI, tooth on the mar, gin of the sucker. (After Niemiec.) —F augmented by the action of radiating muscular fibres (Fig. 256, II), and its adherence is further assured by the cuticular rugosities of this surface. These cuticular structures are simply small projections in the Octopoda, but in the Decapoda there is a complete chitinous ring with denticulated edges which are often very prominent, and in some cases a single denticulation may become very large and preponderant and thus transform the sucker into a hook-bearing organ. In Onychoteuthis true functional suckers coexist with the hooks, but in the adult Veranya the suckers are nothing more than the bases of the hooks. In various species of Chetreteuthis the tentacular arms bear suckers in which the muscular system is feebly developed and there is neither a central piston nor a horny ring, but the bottom of the cupule is covered by a great number of anastomosed epithelial filaments which constitute an organ for fishing. In Cirrhoteuthis, in addition to the row of suckers, there are tentacular filaments on each arm alternating with the suckers. a h THE CEPHALOPODA 291 In addition to the foot proper, whose edges constitute the circum- oral appendages, the Cephalopoda possess an epipodium which is well developed, but highly specialised to form a funnel. The epipodial nature of the funnel may be specially well seen in young embryos (Figs. 257, fu; 119, D, fu), in which this organ is situated laterally and posteriorly, between the mantle and the foot. Primitively the funnel has the form of two symmetrical lateral lobes, which simply incline towards one another and overlap in Nautilus (Fig. 276). In the Dibranchia, however, these two lobes become fused together during development (Fig. 290, (9) g) and form a complete tube projecting beyond the pallial cavity (Figs. 253 and 258, etce., fu). Through this tube the excrements, the secretion of the ink-sac, and the generative products are ejected. The interior of the funnel is generally provided with a larger or smaller valve, attached to its anterior or dorsal face; as, for instance, in the Nautilidae (Fig. 276, 6) and the majority of the Decapoda (Fig. 259, fu); but this structure is absent in Leachia among the Oigopsida and in the Octopoda. In addition, the in- Fic. 257. Young embryo of Sepia officinalis, viewed from the dorsal side. a, anus; Fic. 258. e, eye; fu. funnel; gi, gill; m, mouth; Grimalditeuthis richardi, ventral as- ot, otocyst ; pa, mantle and shell; vi, pect. a, arms; e, left eye; fi’, anterior vitellus; 1, 2, 3, 4, 5, arms. (After fin; fi’, posterior tin; fu, funnel. (After Kolliker. ) Joubin.) ternal wall of the funnel is furnished with an epithelial outgrowth of variable form, constituting a mucous gland called Miiller’s organ. Powerful muscular bundles, originating from the cephalopedal mass and from the sides of the funnel, unite together and are inserted symmetrically on the sides of the shell (Fig. 272, m). In Nautilus they are inserted on the interior of the shell, in the Dibranchia on its external surface, in Spirula on the margins of the last chamber. 292 LHE CEPHALOPODA Other differentiated muscular bundles may be recognised ; they are mostly due to the specialisation of the funnel. In the Tetrabranchia (Nautilus) the mantle is covered by an external shell, which is partly overlapped by a small dorsal pallial lobe (Fig. 270, d): the retractor muscles of the head and foot are inserted symmetrically on either side on the internal surface of this shell. The female Argonauta also bears an external shell which covers the mantle, but has no muscular attachments and is not homologous with the shells of other Cephalopods: it does not originate from a pre-conchylian invagination or shell-gland, but is of pedal origin, and is only formed some ten or twelve days after birth by the palmate extremities of the two dorsal arms. The animal is not attached to this shell. In all other Cephalopoda the shell is covered over by the mantle, or at least is partly covered in Spirula (Fig. 295). The shell therefore is internal, and often is rudimentary, as in the majority of Decapoda, or it may be nearly obsolete, as in the Octopoda. The shell of living and fossil Nautiloidea, of Ammonoidea, Spirula (Fig. 268, sp), and of various fossil Dibranchia, such as the Belemnitidae, Spirulirostra (Fig. 262, C), etc.,is provided with internal septa, disposed perpendicularly to the axis of the coil. It is only the last of the chambers thus formed that is occupied by the body of the animal, but a prolongation of the pallial integument known as the pallial siphuncle (Fig. 270, 7) extends back to the initial chamber of the shell, and is enclosed in a calcareous tube or shell siphuncle which perforates all the septa (Fig. 268, si). This pallial siphuncle does not communicate with the coelomic cavity: in Nautilus and Spirula it is a simple vascular vermiform process of the mantle, whose cavity consists of a venous sinus and whose wall contains a ramification of the pallial artery. It apparently plays a part in the hydrostatic function. At the point where the shell siphuncle traverses each septum it is generally surrounded by a small reduplication of the latter, forming the so-called siphuncular neck. The chambers traversed by the siphuncle do not communicate with one another nor with the shell siphuncle: they are filled with a nitrogenous gas and form a hydrostatic apparatus. The external multilocular shell is straight in some palaeozoic Nautiloidea (Orthoceras), but in the majority of Tetrabranchia it is arcuate or more or less completely coiled in such a manner as to form a discoidal shell whose whorls are all in the same plane. In the majority of Tetrabranchia (Nautilus, Fig. 270) the coil is exo- gastric, that is to say, it is turned towards the dorsal aspect, but in some forms, ¢.g. Phragmoceras, Cyrtoceras, Ptenoceras (Fig. 261, B), it is turned towards the ventral side and is therefore endogastric ; the direction of the coil cannot be determined by the position of the siphuncle, which traverses the septa at various points, but by eo THE CEPHALOPODA 293 the form of the aperture and the position of the “hyponomous ” sinus, which corresponds to the funnel (Fig. 261, 7s). In some Nautiloidea, ¢.g. the dextral or sinistral 77rochoceras, and in sundry Ammonoidea, ¢.g. the sinistral Twurrilites and Cochloceras and the dextral Bostrychoceras, the coil may be produced into a helicoidal or turriculated spire. And in other cases again the last whorls of shell, whether it be discoidal or helicoidal, may be partly un- coiled, as may be seen, for example, in Litwites, which is largely uncoiled, or in Ophidioceras, in which only a small extent of the shell is uncoiled. Finally, the shell may become secondarily rectilinear in the adult, as in Baculites, among the Ammonoidea. In Spirula the shell is coiled in one plane, but it is endogastric, that is to say, coiled in the opposite direction to that of Nautilus (Figs. 268 and 270), and it is largely internal. In certain fossil Dibranchia the multilocular shell, whether it be straight or partially coiled, has become internal (Belemnitidae, Spirulirostra) and forms the phragmocone (Fig. 262, C). In such cases it is surrounded by a caleareous secretion of the reflected portion of the mantle, which is not homologous with the shell of other Molluscs, and forms the pointed rostrum or guard at the end opposite to the head (Fig. 262) and the cephalic plate or pro-ostracum at the anterior or dorsal end. Thus there is, in the shells of these Cephalopoda, an element which is not represented in the shells of other Mollusca. In the living Dibranchia, with the exception of Spirula, the phragmocone and the rostrum of this internal shell have become very rudimentary. In Sepia, for example, the shell is composed of parallel layers: united together by short pillars of calcareous sub- stance, and has a stratified and alveolar structure: at its posterior end a little hollow marks the position of the phragmocone, and a short pointed external projection represents the rostrum, the bulk of the shell being formed by the anterior pro-ostracum, on which the retractor muscles of the cephalopedal mass are inserted. In the Oigopsida the guard is no longer calcified, and the shell has the form of a chitinous plume or gladius, but in Ommatostreples there is a small posterior conical cavity representing the remains of the phragmocone. In the Loliginidae and Sepiolidae the shell is similarly repre- sented by a chitinous gladius (Fig. 263), but in these families it is so much reduced that it only occupies the anterior portion of the body. In Jdiosepius this shell is nearly obsolete, and it is absent altogether in certain Sepiolidae and some allied forms such as Stoloteuthis, Inioteuthis, Sepioloidea, and Sepiadarium. Finally, in the Octopoda there is no longer a true shell, but only some simple chitinous rudiments, on which the retractor muscles of the head and funnel are inserted ; these may be paired, as in the case of the lateral stylets of Octopus ; or unpaired, asin the case of Cirrhoteuthis. 294 THE CEPHALOPODA It follows that in all living Cephalopods except Nautilus the shell is localised on the anterior or physiologically dorsal side of the body, and is enclosed by the mantle, which therefore appears to be a naked, bell-shaped sac (Figs. 252, 254, 287, etc.). The whole circumference of the mantle border is free in the Decapoda (Fig. 259), with the exception of Sepiola, in which it is fused to the head anteriorly in the middle line. Similarly, in the Octopoda, the border of the mantle is fused to the head anteriorly and laterally, whereby the pallial aperture is much reduced, especially in Cirrho- teuthis (Fig. 260) and Opisthoteu- this (Fig. 300). In the Decapoda a more secure union between the mantle and the funnel, during the expulsion of the water used in respiration Fic. 259. Ctenopteryx cyprinoides, ventral aspect. Fic. 260. a, arms; e, eye; ji, fins; fu, funnel with valve; ol, olfactory organ or rhinophore ; Cirrhoteuthis meangensis, Hoyle (young speci- so, socket of the funnel; te, tentacular men), ventral aspect. ji, fin; fu, funnel; m, arms. (After Joubin.) mouth. (After Hoyle.) through the latter structure, is ensured by the following mechanism : the free borders of the mantle bear on each side a cartilaginous pro- jection (Fig. 272, c’) which fits into a corresponding depression in the funnel (Figs. 272, ¢; 259, so), the whole constituting the so- called “ resisting apparatus ” of foreign authors. In certain Oigopsida of the family Cranchiidae (Cranchia, Leachia) and in the Octopoda this apparatus is but feebly developed, and in Cirrhoteuthis, where it would be useless, it has disappeared. Otherwise the funnel is quite independent of the mantle: it is only in Amphitretus among the Cirrhoteuthidae that the mantle border is united to the funnel by a ventral suture, leaving an opening into the pallial cavity on either side. Similarly, in (rimalditeuthis and Symplecto- _—— THE CEPHALOPODA 295 teuthis among the Oigopsida, there are two infundibulo-pallial sutures. In the Dibranchia the mantle is a very muscular organ, which, by its contractions, serves two purposes. By alternately and rhythmically drawing in and forcing out the water that enters the pallial cavity between the funnel and the border of the mantle, it acts as an accessory respiratory organ, and by violently expelling water through the funnel it acts as an efficacious locomotory organ, causing the animal to execute sudden retrograde movements. In the majority of Cephalopods with internal shells (Decapoda) and in the Cirrhoteuthidae, the mantle is produced into lateral symmetrical expansions or fins of various form and position (Figs. 253, 260, and 268, fi). These organs always originate at the aboral ig Fic. 261. Two fossil Nautilids, left-side view. A, Ophidioceras simpler, Barrande; A’, mouth of the shell; B, Ptenoceras alatum, Barrande. /.s, foot (arms) sinus ; i.s, infundibular sinus. (After Barrande.) extremity of the mantle (Fig. 290, (4) a)—even in Octopus, in which genus they eventually disappear—as two triangular or rounded out- growths. They remain localised at the aboral extremity in Spirula, in which genus they are situated close together at the point where the two halves of the mantle reunite behind the shell (Fig. 295). In most other Oigopsida they are still terminal and close together, but they tend to shift further forward on the anterior or dorsal surface, as in Tuonius (Fig. 253) and Ommatostrephes (Fig. 297), and they may be duplicated, the two fins on each side lying close together, as in Grimalditeuthis (Fig. 258, ji’, fi’) and Vampyroteuthis. But in all other cases they diverge to take up positions opposite one another on the right and left sides of the body, and show an increasing tendency to occupy the whole length of the body, as in Thysanoteuthis, where they are triangular (Fig. 298, B), and in Sepioteuthis, where they are rounded. In Sepia the fins extend the whole length of the mantle, 296 THE CEPHALOPODA but are reduced so as to be of the same width throughout (Fig. 272, P). The fins of Ctenopteryx are similar, but are pectinated, that is to say, they consist of a thin membrane supported by muscular fibres (Fig. 259, fi). On the other hand, in species with a short and globular mantle the fins are shifted away from the aboral extremity, and are situated either in the middle of the body, as in Sepiola, or even near the anterior end, as in Cirrho- teuthis (Fig. 260). Except in Nautilus (Fig. 270, f) and Spirula (Fig. 268, pa.c), where it is shallower, the pallial cavity extends from its opening behind the head to the aboral extremity or summit of the body. It contains the branchiae and the anal, renal, and genital orifices (Fig. 272, Br, a, 7, g). In some a om a in Sx WGC ) = 7 —"7 — — — — — — os =— — — Fic. 262. Internal shells of Cephalopoda. ——_ — THE CEPHALOPODA 325 devoid of chromatophores. Eventually the membrane of the eyst bursts and remains attached to the dorsal surface of the arm, forming the spermatophore sac. The uncoiled arm is pedunculated, that is to say, is attenuated towards its base, and it bears at its extremity a little pouch (Fig. 287, x) containing a long filament, which is extended prior to the act of fertilisation (Fig. 287, y). The spermatophore sac communicates with a canal in the interior of the arm, and this canal is continued into the terminal filament and opens by an orifice at its free extremity. The hectocotylus when detached is able to live and move about for a considerable time, until finally it penetrates into the pallial cavity of a female and fixes itself in the neighbourhood of the genital aperture. In those Dibranchia in which the hectocotylus is not autotomous the hectocotylised arm (or arms) is inserted into the pallial cavity of the female (Fig. 288, 3) in such wise as to deposit the spermato- phores in the terminal portion of the oviduct in Octopus, or to fix Fic. 288. Copulation.of Octopus. A, the female; B, the male. fu, funnel of the female; 3, third right hectocotylised arin of the male. (After Racovitza.) them in the neighbourhood of the oviducal orifice in Rossia and Sepiola. In Sepia and Loligo the spermatophores are simply deposited on the ventral lobes of the buccal membrane, and in Nautilus they are deposited on the folded lamellae on the ventral side of the buccal orifice (Fig. 255, m). The eggs are laid shortly after copulation. In Nautilus they are laid singly, each egg being about four centimetres long and surrounded by two thick shells, the outermost of which is partly open (Willey). In the Dibranchia the eggs are aggregated together, but in the Octopoda and in Sepia, Sepiola, and Rossia each egg has a separate envelope, whereas they are united to form longer or shorter gela- tinous strings, which are joined together and fixed by one extremity in Loligo, but are single and floating in the pelagic Oigopsida. In Eledone only about sixty eggs are laid at one time, in Octopus more than a hundred, and some species of Loligo lay more than 40,000 eggs. Some Octopods are incubatory: the female drgonauta, for example, protects the eggs in the shell peculiar to her sex. 326 THE CEPHALOPODA III. Empryouocy. Our knowledge of the embryology of Cephalopoda is confined to the Dibranchia, the development of Nautilus being unfortunately still unknown. The ovum is remarkable, even in the cases of Nautilus and the ovarian ovum of Spirula (Fig. 285), for the enormous quantity of yolk contained in it. In contrast to all other Mollusca, the segmentation is incomplete: at no period does the ectoderm completely cover in the vitelline mass, so that there is no proper blastopore, or rather the blastopore is enormous and is represented by that part of the vitellus that is not covered by ectoderm (Fig. 290, (2), (3), e). This peculiarity in the development of Dibranchia, however, is only an exaggeration of the phenomena observable in the epibolic ova, provided with an abundant yolk, of certain Gastropoda (Fig. 10, B), and it has been shown that in the archaic Dibranchia (the Oigopsid Cephalopod of Grenacher, Fig. 119, D, vi) the quantity of yolk is less than in the other members of the order, and that the ectoderm extends much farther over it. As the formative protoplasm is localised at the narrower end of the egg, the segment- ation is restricted to this end (Fig. 289, 0/), and results in the formation of a germinal disc or embryonic area. In the course of subsequent development the embryo is like- wise restricted to this end, and never covers the whole surface of the vitelline mass, on Fs, Oe: which it appears to be seated (Fig. 291). Egg of Loligo in the first Lhe extent of the embryonic area and of pee ein “etage. nd the free surface of the yolk are in inverse vitellus. (After Watase.) ratid to one another: the external vitelline mass is smaller in Loligo than in Sepia, smaller still in Argonauta, and reduced to a minimum in the Oigopsida (Fig. 119, D). The embryonic area forms the ectoderm: the so-called peri- vitelline or yolk membrane is formed as a proliferation of cells from a limited part of the periphery of the ectoderm, the region of proliferation marking the anal side. The cells thus formed migrate over the whole surface of the yolk and form a layer of scattered nuclei investing it (Fig. 290, (7), h). At a later period the same anal edge of the periphery of the embryonic area gives rise to a second cellular layer, the endoderm : it is at first crescentic in shape, but subsequently becomes ring-shaped, and eventually forms a continuous circular sheet below the ectoderm (Teichmann). At a still later period the ectoderm gives rise to cells constituting the genital rudiment and other mesodermic elements: these cells UL —————————<—<_— CC THE CEPHALOPODA 327 also originate at the anal side of the blastoderm, behind the place where the shell gland is formed externally, and extend in the form of a crescent, right and left, between the ectoderm and endoderm, toward the anterior part of the blastoderm. After the mesoderm has been established in this manner, a thickened specialised portion of the endoderm constitutes the rudiment of the mesenteron and forms a little vesicle lying close upon and widely open to the yolk, and situated in the middle line, below the posterior part of the mantle, between the rudiments of the two branchiae (Fig. 290, (6), 7). This vesicle is the rudiment of the alimentary tract : it ultimately gives rise to the stomach, the two lobes of the liver (which are separate from the first), and the intestine (Fig. 290, 7). The oesophagus and its annexes, viz. the radula, the salivary glands, etc. (Fig. 290, (7), J, s), are formed by a precocious stomodaeal invagination, and the anus is formed later, by an excessively short proctodaeal invagination. Thus the mouth arises relatively near to the nutritive or vegetative pole, as it does in other Mollusca, and the less abundant the yolk, the nearer it is to the vegetative pole, as in the Cephalopod of Grenacher (Fig. 119, D). The mantle arises in the middle of the embryonic area (Fig. 257, pa), and in its centre is the shell gland, but the borders of the latter structure are reflected inwards and approach one another to form the shell sac. In certain highly differentiated Octopoda (Argonauta, Ray Lankester) the shell sac disappears before it is closed up, but in the Decapoda, with the exception of Spirula, it is completely closed, and it grows pari passv with the mantle (Fig. 290, ¢), while the shell develops within it. Posteriorly to the mantle, between it and the epipodium, appear the bud-like rudiments of the branchiae (Fig. 290, (6), 2), and the folds that form the branchial lamellae gradually make their appearance and become in their turn folded. As development advances the pallial cavity becomes deeper, and the branchiae are gradually covered by the mantle. Throughout the earlier part of embryonic life the cephalic mass is excessively large (Fig. 290, (8), (9)), but its preponderance insensibly diminishes in subsequent stages. This cephalic mass is formed by the antero-lateral regions of the embryonic area, and it bears the rudiment of an eye at each posterior corner (Fig. 290, (9), d). During these earlier phases of embryonic development the mouth is not in any sense surrounded by the circumoral append- ages. The foot, in fact, is at first formed by the lateral and posterior borders (Fig. 257, 1, 2, 3, 4, 5) of the embryonic area, and these borders are rapidly divided into ten projections in the Decapoda, or eight projections in the Octopoda and the Cephalopod of Grenacher (Fig. 119, D). But in the course of development these lobes, while they grow in length, also advance gradually THE CEPHALOPODA 328 290. Fic. ] | . a a ee ee THE CEPHALOPODA 329 along the sides until the most anterior of them reach the mouth (Fig. 291), and becoming united in front of it, eventually surround it completely. A paired epipodial outgrowth, the origin of the funnel, is formed early in development: its two posterior lobes become prominent and bend inwards toward one another (Fig. 290, ¢), thus establishing the condition which is permanent in the adult Nautilus (Fig. 276), but finally, in the Dibranchia, they fuse together completely and form a perfect tube. All the nervous centres—the cerebral, optic, visceral, and pedal —are formed separately as proliferations of the ectoderm. The pedal centres give rise, by subdivision, to the ganglia of the arms. The eyes (Fig. 292, A, B) and otocysts originate as invaginations of the ectoderm, which eventually close up. The otocysts arise Fic. 290. Development of Loligo. (1) view of the cleavage of the egg during the first formation of embryonic cells. (2) lateral view of the egg at a little later stage ; a, limit to which the layer of cleavage-cells has spread over the egg; b, portion of the egg as yet uncovered by cleavage- cells; ap, yolk membrane cells; kp, cleavage-pole where first cells were formed. (3) later stage, the limit a now extended so as to leave but little of the egg-surface (b) unenclosed ; d, eyes; e, mouth; u, mantle sac. (4) later stage, anterior surface, the embryo is becoming nipped off from the yolk sac (g). (4) view of an embryo similar to (3) from the cleavage-pole or centro-dorsal area. (6) later stage, posterior surface. (7) section in a median sagittal plane of au embryo of the same age as (4). (8) viewof the anterior face of anolderembryo. (9) view of the posterior face of an embryo of the same age as (8). Letters in (3) to (9) :—a, lateral fins ; 6, mantle-skirt ; c, supra-ocular invagination to forin the ‘‘ white body”; d, the eye; e, the mouth ; ep, outer layer of the embryo; 1, 2, /%, f4, /°, the five paired processes (arms) of the foot; g, rhythmically contractile area of the yolk sac; h, dotted line showing internal area occupied by yolk; xk, first rudiment of the funnel; /, sac of the radula; m, stomach; mes, mesoderm ; 7, rudiments of the gills; 0, the otocysts; p, optic ganglion; q, distal portion of the ridges which form the funnel; 7, vesicle-like rudiment of the intestine formed independ- ently of the parts connected with the mouth; s, rudiment of the salivary gland ; ¢, the closed shell sac ; uv, the open shell sac, formed by an uprising ring-like growth of the central dorsal area ; w, the mantle-skirt commencing to be raised up around the area of the shell sac. (After Lankester.) laterally on the sides of the foot outside the epipodium (Fig. 290, (6), 0); they close up at a relatively late period, often retaining a rudiment of the original external canal, and then approach one another till they come in contact in the median line. When the ocular cavity is closed, the external part of the crystalline lens is formed separately from the internal segment. At the sides of the optic ganglia a pair of cellular masses, formed by ectodermic invaginations, becomes the white bodies of the adult (Fig. 290, c¢) ; they are the relics of a pair of embryonic ganglia (lateral cerebral lobes). The coelomic cavity is hollowed out in the mesoderm as two symmetrical spaces, right and left of the intestine ; it gives rise to the kidneys and the pericardium. The two kidneys are formed independently of one another in their definitive positions. The heart is also formed from the pericardial wall as two paired rudiments. Finally, a portion of the coelomic wall gives rise to the gonad. 330 : THE CEPHALOPODA The vitelline mass diminishes insensibly during the growth of the embryo, and is for the most part absorbed at the time of hatching. The yolk sac is independent of the stomach, and is only in contact with it over a small area in the middle line. Fic. 291. Embryo of Sepia officinalis, on its vitellus, left-side view. an, anus; br, gill ; 7.lo.1, invagina- tion of the lateral cerebral lobe; na, fin; pa, mantle; vit, vitellus. I, II, III, IV, V, the five left arms. IV. Bronomics AND DISTRIBUTION. All the Cephalopoda are marine, and very active animals. They swim rapidly by expelling the water from the pallial cavity through the funnel, sometimes with so much violence that they can spring for some distance out of the water (Ommatostrephes). The fins of the Dibranchia are organs of balance rather than of locomotion. All the Cephalopods are in the highest degree carnivorous: many Fia. 292. Diagrams of sections showing the early stage of development of the eye of Loligo, when it is, like the permanent eye of Nautilus, an open sac. A, first appearance of the eye as a ring- like upgrowth; B, ingrowth of the ring-like wall so as to form a sac, the primitive optic vesicle. (After Lankester.) of them destroy a large number of edible fish and Crustacea, but, on the other hand, many of the pelagic forms fall a prey to the toothed whales. Some Cephalopods attain to a very considerable size: the body of some species of Architeuthis, without the head, may measure two and a half metres in length, and when the head and extended — ————— THE CEPHALOPODA 331 tentacular arms are taken into account, they may be from twelve to eighteen metres long. Hence these Mollusca have given rise to various fabulous tales, and they have been known by man from the remotest periods of antiquity, as is evidenced by their representa- tions on some of the most ancient monuments from Mycenae, Egypt, and Greece. In the present day some four hundred species of Cephalopoda are distributed throughout all the seas of the world. Some species, especially those with a short and rounded pallial sac, such as the Octopodidae and Sepiola, are strictly littoral—indeed Sepiola and also Rossia are fossorial in habit. Other species are inhabitants of the open sea, and among these various forms of Oigopsida dwell in great depths: Spirula is found down to 1000 fathoms; Cranchia and Bathyieuthis down to 1700 fathoms; Histiopsis at a depth of nearly 2000 fathoms; Calliteuthis at 2200; and Cheiroteuthis down to 2600 fathoms. Many of these deep-sea Oigopsida are luminous. The history of the Cephalopoda extends back to the remotest geological times. Orthoceras and other forms allied to Nautilus, but as yet uncoiled, are abundant in the most primitive Palaeozoic formations. The subdivision of the Ammonitoidea, related to the Tetrabranchia, is distributed from the Devonian to the end of the Secondary period. The Dibranchia do not appear till the end of the Secondary epoch, during which they were characteristically represented by the Belemnitidae, a group which, like the Ammoni- toidea, became nearly completely extinct at the end of this period. V. REVIEW OF THE ORDERS, SUB-ORDERS, AND FAMILIES OF THE CEPHALOPODA. The class Cephalopoda comprises two orders, the Tetrabranchia and the Dibranchia. Palaeontology, as well as morphology, shows that the Tetrabranchia (Nautilus, etc.), that is to say, the Cephalopods with multiple branchiae, auricles, and kidneys, and with an external chambered shell, are the most archaic. The Dibranchia are more specialised, inasmuch as they have lost the anterior branchiae, auricles, and kidneys, and their shell has become rudimentary. The earliest Dibranchia were descended from rectilinear forms with a multilocular external shell devoid of a rostrum, and they gave rise in turn to Spirula, the Belemnitidae, and the allied Oigopsida. From the last named were derived, as the result of a yet more profound specialisation, on the one hand the Myopsida, on the other hand the Octopoda, by the loss of the tentacular arms (already so much reduced as to be almost lost in some Oigopsida), and by the more and more complete atrophy of the shell. 332 THE CEPHALOPODA ORDER 1. Tetrabranchia, Owen. In these Cephalopoda the whole of the visceral mass is protected by an external, multilocular, siphunculated shell, which may or may not be coiled; only the last compartment of the*shell is occupied by the body of the animal. The head bears numerous appendages in the form of pedal tentacles, which are retractile within sheaths (Fig. 293, te). The funnel is formed of two separate moieties. There are four branchiae, and four kidneys without reno-pericardial orifices. The pericardium opens directly to the exterior. The cephalic cartilage is wholly situated on the ventral side of the oesophagus (Fig. 270, h) and only supports the ventral part of the nervous centres. The eyes are open and have no crystalline lens (Fig. 6, A). The Tetrabranchia comprise two sub-orders, the Nautiloidea and the Ammonitoidea. Sus-OrRDER 1. NAUTILOIDEA. This group is distinguished from the Ammonitoidea by the initial chamber, which is in the form of an obtuse cone bearing on its summit a “ cicatrix,” elongated dorso-ventrally and situated opposite the extremity of the blind end of the siphuncle: it is probable that the siphuncle passed through this cicatrix on emerging from a true initial chamber or proto- conch, which may have been uncalcified or caducous. The sub-order comprises nearly 2500 fossil species, but only a few living species of the genus Vautilus, In certain fossil forms the aperture of the shell may be contracted to such an extent that the animal was probably able to protrude only the appendages of the circumoral crown, but not its head. These contracted apertures are said to be “composite” when they have lobes of different form, as in Gomphoceras, Phragmoceras, ete. In these apertures the ventral part, corresponding to the funnel, is separated from the rest by a constriction, and constitutes the “ hyponomous sinus” ; the remainder of the aperture is more or less lobate and corresponds to the external parts of the circumoral crown. The shell may attain to a length of two metres (Hndoceras). Faminy 1, OrTHOCERATIDAE. Shell straight or slightly curved, with a simple aperture, a large terminal chamber, and a cylindrical siphuncle. Genera—Orthoceras, Breyn ; from the Silurian to the Trias. Baltoceras, Holm; Silurian. Famiry 2. Acrinoceratipar. Shell straight or slightly curved, with a wide siphuncle contracted at the level of the septa by rings or swellings. Genera—Actinoceras, Bronn; from the Silurian to the Carboniferous. Discosorus, Hall; Silurian. Huronia, Stokes ; Silurian. Lozxoceras, MacCoy ; from the Silurian to the Carbon- iferous, Famity 8. Enpoceratipar, Shell straight, with a wide marginal siphuncle, the siphuncular necks produced into tubes which fit into one another. Genera—lHndoceras, Hall ; shell straight ; from the Silurian. Famiy 4. GompHoceraAtipar. Shell globular, straight or arcuate, the aperture contracted to the shape of a T. Genera—Gompho- ceras, Sowerby ; Silurian. Phragmoceras, Sowerby ; Silurian, Famity 5. THE CEPHALOPODA 330 ASCOCERATIDAE. Shell straight, ampulliform ; the summit truncated ; the terminal chamber occupies nearly the whole length of the shell on the ventral side. Genera—Ascoceras, Barrande ; Silurian. Glossoceras, Barrande ; Silurian. Famity 6. PorertoceratipAr. Shell straight or curved, fusiform, contracted at the two extremities ; the aperture simple ; the siphuncle contracted at the levels of the septa. Genera— Poterioceras, MacCoy ; from the Silurian to the Carboniferous. Streptoceras, Billings ; Silurian. Faminy 7. Cyrroceratmpar. Shell slightly curved; the aperture simple ; the siphuncle wide and the septa approximated. Genus —Cyrtoceras, Goldfuss ; Devonian. Famiry 8. Livorrmpar. Shell coiled in one plane with the ter- ininal part uncoiled ; the aperture contracted. Genera—Lituites, Bar- rande ; Silurian. Ophidioceras, Barrande ; Silurian (Fig. 261, A). Famity 9. TROCHOCERATIDAE. Shell helicoidally coiled; dextral or sinistral ; the last whorl gener- ally uncoiled. Genera — T’rocho- ceras, Barrande; Devonian. Adelphoceras, Barrande ; Devonian. Famity 10. Navrinipar. Shell coiled in one plane ; the aperture wide and simple; the siphuncle central. Genera—Nuutilus, Lin- naeus (Figs. 270 and 293); four living species are known from the Indian and Pacific Oceans: they are gregarious, nocturnal animals, living at some depth. Trocholites, Conrad; Silurian. (yroceras, de Koninck ; from the Silurian to cl ant the Carboniferous. Hercoceras, 4, Neullus macromphalus creeping on a horizon Barrande ; Silurian. Ptenoceras, mic tentacle; e, eye ; ho, hood; in, infundibulum ; Hyatt ; Devonian (Fig. 261, B). Sphthaimic tentacle; sh, shell. (Atter Willey.) Discites, MacCoy ; Carboniferous. Famity 11. Bacrritipar, Shell straight, conical ; the siphuncle narrow and marginal, and the siphuneular necks long and infundibuliform ; septa united to the shell by an undulating line. Genus—Bactrites, Sandberger ; Silurian and Devonian. Sus-Orper 2, AMMONITOIDEA. The Ammonitoidea are distinguished from the Nautiloidea by their initial chamber, which is spheroidal like that of the Belemnitidae and Spirulidae ; by their siphuncle, which is narrow and simple, whereas that of the Nautiloidea is wide, but often reduced in diameter by internal deposits ; by their septa, which are generally convex on the side nearer 334 LHE CEPHALOPODA the aperture, instead of being concave as in the Nautiloidea; finally, by the sutures (intersections of the septa with the shell), which form a more or less complex sinuous line, instead of being simple as in the Nautiloidea. Although the Ammonitoidea have a globular initial chamber which is absent in Nautiloidea (though it may possibly be present but caducous or not calcified in the latter group), their shell has the same structure as that of Nautilus, and was indisputably external. The chamber containing the body of the animal is very deep, more so than in Nautilus. Like the Nautiloidea, the Ammonitoidea did not possess an ink-sac, A consider- able number of Ammonitoidea resemble such Nautiloid forms as Gomphoceras, Phragmoceras, etc., in having a contracted aperture, indicat- ing an analogous structure of the cephalopedal organs: such contracted apertures may be seen in Arcestes, Lobites, Stoliczkaia, and especially in Stephanoceras, but this contraction is carried to an extreme in Morphoceras pseudoanceps (Fig. 294). In this genus the aperture is almost completely closed by the lobes which form its borders and circumscribe five small orifices ; there is a central oblong orifice which probably corresponded to the mouth ; two circular lateral orifices, one on either side of the central, may perhaps have served as windows for the eyes; and the two remaining orifices, which are partly limited by the preceding whorl, probably served for the passage of the pedal appendages, and do not correspond to any part of the contracted apertures of Nautiloidea. A calcified structure, consisting of a single piece (Anaptychus) or of two symmetrical moieties (Aptychus) is sometimes found in the terminal chamber of Ammonitoidea : its constant position shows that it could not have been an operculum, and it is supposed to have been a calcified cartilage situated at the base of the funnel. The Ammonitoidea are, geologically speaking, younger than the rest of the Tetrabranchia. They appeared in the Devonian and became com- pletely extinct at the end of the Secondary period. They were littoral in habit, and lived in troops like Nautilus. Some of the coiled forms are as much as seventy centimetres in diameter. More than 5000 species have been described, and it has been found necessary to divide the originally single genus Ammonites first into genera, then into families, and even into tribes. TRIBE 1. RETROSIPHONATA. The siphuncular necks project behind the septa as in the Nautiloidea, These are the most ancient Ammonitoidea, belonging exclusively to the superior Palaeozoic strata, from the Devonian upwards. The sutures of the septa form simple undulations, those which point backwards being known as “lobes,” and those which point forward towards the aperture as ‘“ saddles.” Faminy 1. Gonrarrripar. Shell nautiloid with simple sutures and a ventral siphuncle. Genera—Goniatites, de Haan ; Devonian and Carbon- iferous. Anarcestes, Mojsisovics; Devonian. Famity 2. CLYMENIIDAE. Shell nautiloid; sutures simple; the siphuncle dorsal, that is to say, internal. Genus—Clymenia, Miinster ; from the Upper Devonian. THE CEPHALOPODA 335 TRIBE 2, PROSIPHONATA. The siphuncular necks project in front of the septa. The septal sutures present deeply indented lobes and saddles. Famity 1. ArcestTipaAr. Globular and smooth or nearly smooth forms, with a reduced umbilicus; the terminal chamber very deep, occupying nearly a whole whorl of the spire; an aptychus present. Genera — Popanoceras, Gemmellaro; Permian. Cyclolobus, Waagen ; Permian. Arcestes, Mojsisovics; Trias. Lobites, Mojsisovics; Trias. Famity 2. Tropitmpar. The shells globular, but differing from those of the Arcestidae in having radiating and tuberculated costae. Genera— Thalassoceras, Gemmellaro; Permian. Tropites, Mojsisovics; Trias. Sibirites, Mojsisovies ; Trias. Faminty 3. Ceratitmpar. Shells coiled, with a large umbilicus; the terminal chamber short; sutures with simple saddles. Genera—Trachyceras, Laube; Upper Trias. Ceratites, de Haan; Trias. Diénarites, Mojsisovics; Trias. Some genera with helicoidal shells are related to these coiled forms, viz. Cochloceras, Hauer; Trias: also some straight forms, e.g. Rhabdoceras, Hauer ; Trias : they have been placed in distinct families by some authors. Famity 4. PINACOCERATIDAE. Shell compressed, smooth ; the terminal chamber short; the suture very complicated, convex. Genus — Pinacoceras, Mojsisovics ; Trias. Faminy 5. PHyLLoceRATIDAE. Shells coiled, the whorls overlapping one another; the suture formed of numerous lobes and saddles. Genera — Phylloceras, Suess; Jurassic. Rhacophyllites, Zittel. Famity 6. Lyroceratipan. Shell discoid, the whorls loosely united or uncoiled ; the sutures deeply indented but with only three saddles and lobes. Genera—Lytoceras, Suess; Jurassic and Cretaceous. Macroscaphites, Meek; uncoiled and recurved; Cretaceous. Hamvites, Parkinson ; flexed three times; Cretaceous. Ptychoceras, d’Orbigny ; Cretaceous. Twrrilites, Lamarck ; coiled in a sinistral helicoidal spire ; Cretaceous. Baculites, Lamarck; the adult straight and elliptic in section; Cretaceous. Faminy 7. AmmMonitipar. Shell coiled, with narrow whorls which do not embrace one another ; aperture simple; a horny anapty- chus present. 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