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Originally appearing in Volume V11, Page 526 of the 1911 Encyclopedia Britannica.
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ICI. 17FIG. 24.—Animal and shell of Phorus exutus. a, Snout (not introversible). d, Pro- and meso-podium; to the right b, Cephalic tentacles. of this is seen the metapodium c, Right eye. bearing the sculptured operculum. there is, strictly speaking, no invagination (emboly), but an over-growth (epiboly) of the smaller cells to enclose the larger. The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally. In such cases the blastopore may entirely close, and both mouth and anus develop as new Ingrowths (stomodaeum and proctodaeum), whilst, according to the observations of N. Bobretzky, the closed blastopore may coincide in position with the mouth in some instances (Nassa, &c.), instead of with the anus. But in these epibolic forms, just as in the embolic Paludina, the embryo proceeds to develop its ciliated band and shell-gland, passing through the earlier condition of a trochosphere to that of the veliger. In the veliger stage many Pectinibranchia (Purpura, Nassa, &c.) exhibit, in the dorsal region behind the head, a contractile area of the body-wall. This acts as a larval heart, but ceases to pulsate after a time. Similar rhythmically contractile a, Snout or rostrum. f, Operculum. b, Cephalic tentacle. h',Prolonged siphonal notch of the c, Eye. shell occupied by the siphon, d, Propodium and mesopodium. or trough-like process of the e, Metapodium. mantle-skirt. areas are found on the foot of the embryo Pulmonate Limax and on the yolk-sac (distended foot-surface) of the Cephalopod Loligo. The preconchylian invagination or shell-gland is formed in the embryo behind the velum, on the surface opposite the blastopore. It is surrounded by a ridge of cells which gradually extends over the visceral sac and secretes the shell. In forms which are naked in the adult state, the shell falls off soon after the reduction of the velum, but in Cenia, Runcina and Vaginula the shell-gland and shell are not developed, and the young animal when hatched has already the naked form of the adult. One further feature of the development of the Pectinibranchia deserves special mention. Many Gastropoda deposit their eggs, after fertilization, enclosed in capsules; others, as Paludina, are viviparous; others, again, as. the Zygobranchia, agree with the Lamellibranch h Conchifera (the bivalves) in having simple exits for the ova without glandular walls, and therefore discharge their eggs unenclosed in capsules freely into the sea-water; such unencapsuled eggs are merely enclosed each in its own delicate chorion. When (After Lankester, 17.) dc, Directive corpuscle (outcast cell). f, Foot. ae, Arch-enteron or cavity lined by mes, Rudiments of the the enteric cell-layer or endo- skeleto-trophic tissues. derm. pi, The pedicle of invagina- bl, Blastopore. tion, the future rectum. vr, Velum or circlet of ciliated cells. shgl, The primitive shell-sac dv, Velar area or cephalic dome. or shell-gland. sm, Site of the as yet unformed m, Mouth. mouth. an, Anus. A, Diblastula phase (optical section). B, The diblastula has become a trochosphere by the development of the ciliated ring vr (optical section). C, Side view of the trochosphere with commencing formation of the foot. D, Further advanced trochosphere (optical section). E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac. F, Side view of the same, showing foot, shell-sac (shgl), velum (vr), mouth and anus. N.B.—In this development the blastopore is not elongated; it persists as the anus. The mouth and stomodaeum form independently of the blastopore. egg-capsules are formed they are often of large size, have tough walls, and in each capsule are several eggs floating in a viscid fluid. In some cases all the eggs in a capsule develop; in other cases one egg only in a capsule (Neritina), or a small proportion (Purpura, Buccinum), advance in development; the rest are arrested either after the first process of cell-division (cleavage) or before that process. The arrested embryos or eggs are then swallowed and digested by those in the same capsule which have advanced in development. This is clearly the same process in essence as that of the formation of a vitellogenous gland from part of the primitive ovary, or of the feeding of an ovarian egg by the absorption of neighbouring potential eggs; but here the period at which the sacrifice of one egg to another takes place is somewhat late. What it is that determines the arrest of some eggs and the progressive development of others in the same capsule is at present unknown. In the tribe of Pectinibranchia called Heteropoda the foot takes the form of a swimming organ. The nervous system and sense organs are highly developed. The odontophore also is remarkably developed, its lateral teeth being mobile, and it serves as an efficient organ for attacking the other pelagic forms on which the Heteropoda prey. The sexes are distinct, as in all Streptoneura; and genital ducts and accessory glands and pouches are present, as in all Pectinibranchia. The Heteropoda exhibit a series of modifications in the form and proportions of the visceral mass and foot, leading from a condition readily comparable with that of a typical Pectinibranch such as Rostellaria, with the three regions of the foot strongly marked and a coiled visceral hump of the usual proportions, up to a condition in which the whole body is of a tapering cylindrical shape, the foot a plate-like vertical fin, and the visceral hump almost completely atrophied. Three steps of this modification may be a, Mouth and odontophore. n, Dorsal surface overhung by b, Cephalic tentacles. the mantle-skirt; the letter c, Eye. is close to the salivary gland. d, Propodium (B) and meso- o, Rectum and anus. podium. p, Liver. e, Metapodium. q, Renal organ (nephridium). f, Operculum. s, Ventricle. h, Mantle-chamber. u, The otocyst attached to the i, Ctenidium (gill-plume). cerebral ganglion. k, Retractor muscle of foot. w, Testis. 1, Optic tentacle. x, Auricle of the heart. m, Stomach. y, Vesicle on genital duct. z, Penis. distinguished as three families:—Atlantidae, Carinariidae and Pterotrachaeidae. They are true Pectinibranchia which have taken to a pelagic life, and the peculiarities of structure which they exhibit are strictly adaptations consequent upon their changed mode of life. Such adaptations are the transparency and colourlessness of the tissues, and the modifications of the foot, which still shows in Atlanta the form common in Pectinibranchia (compare fig. 27 and fig. 24). The cylindrical body of Pterotrachaea is paralleled by the slug-like forms of Euthyneura. J. W. Spengel has shown that the visceral loop of the Heteropoda is streptoneurous. Special to the Heteropoda is the high elaboration of the lingual ribbon, and, as an agreement with some of the opisthobranchiate Euthyneura, but as a difference from the Pectinibranchia, we find the otocysts closely attached to the cerebral ganglia. This is, however, less of a difference than it was at one time supposed to be, for it has been shown by H. Lacaze-Duthiers, and also by F. Leydig, that the otocysts of Pectinibranchia even when lying close upon the pedal ganglion (as in fig. 21) yet receive their special nerve (which can sometimes be readily isolated) from the cerebral ganglion (see fig. I I). Accordingly the difference is one of position of the otocyst and not of its nerve-supply. The Heteropoda are further remarkable for the high development of their cephalic eyes, and for the typical character of their osphradium (Spengel's olfactory organ). This is a groove, the edges of which are raised and ciliated, lying near the branchial plume in the genera which possess that organ, whilst in Firoloida, which has no branchial plume, the osphradium occupies a corresponding position. Beneath the ciliated groove is placed an elongated ganglion (olfactory ganglion) connected by a nerve to the supra-intestinal (therefore the primitively dextral) ganglion of the long visceral nerve-loop, the strands of which cross one another—this being characteristic of Streptoneura (Spengel). The Heteropoda belong to the " pelagic fauna " occurring near the surface in the Mediterranean and great oceans in company with the Pteropoda, the Siphonophorous Hydrozoa, Salpae, Leptocephali, and other specially-modified transparent swimming representatives a, Mouth and odontophore. b, Cephalic tentacles. c, Eye. d, The fin-like metopodium. d', Its sucker. e, Metapodium. f, Salivary glands. of various groups of the animal kingdom. In development they pass through the typical trochosphere and veliger stages provided with boat-like shell. Sub-order I.—TAENIOGLOSSA. Radula with a median tooth and three teeth on each side of it. Formula 3 : i : 3. Tribe I.—PLATYPODA. Normal Taenioglossa of creeping habit. The foot is flattened ventrally, at all events in its anterior part (Strombidae). Otocysts situated close to the pedal nerve-centres. Accessory organs are rarely found on the genital ducts, but occur in Paludina, Cyclostoma, Naticidae, Calyptraeidae, &c. Mandibles usually present. This is the largest group of Mollusca, including nearly sixty families, some of which are insufficiently known from the anatomical point of view. Fain. i.—Peludinidae. Pedal centres in the form of ganglionated cords; kidney provided with a ureter; viviparous; fluviatile. Paludina. Neothauma, from Lake Tanganyika. Tylopoma, extinct, Tertiary. Fain. 2.—Cyclophoridae. No ctenidium, pallial cavity trans-formed into a lung; aperture of shell circular; terrestrial. snout when retracted. i, Intestine. c, Pericardium. n, So-called nucleus. ph, Pharynx. br, Branchial plume (ctenidium) oc, Cephalic eye. w, Osphradium. g, Cerebral ganglion. mt, Foot (metapodium). g', Pleuro-pedal ganglion. , Caudal appendage. pr, Foot (metopodium). Pomatias, shell turriculated. Diplommatina. Hybocystis. Cyelobhorus, shell umbilicated, with a short spire and horny operculum. Cyclosurus, shell uncoiled. Dermatocera, foot with a horn-shaped protuberance at its posterior end. Sj-iraculum. Fain. 3.—Ampullariidae. To the left of the ctenidium a pulmonary sac, separated from it by an incomplete septum, am- phibious. Ampullaria, shell dextral, coiled. sinistral, spire short or obsolete. Meladomus. Fain. 4.—Littorinidae. Oesophageal pouches present; pedal nerve-centres concentrated; a pedal penis near the right tentacle. Littorina, shell not umbilicated, littoral habit. Lacuna, foot with two posterior appendages, marine, entirely aquatic. Cremnoconchus, entirely aerial, Indian. Risella. Tectarius. Fam. 5.—Fossaridae. Head with two lobes in some Rhipidoglossa. Fossaria. Fain. 6.—Purpurinidae, extinct. Fain. 7. Planaxidae. Shell with pointed spire; a short pallial siphon. Planaxis. Fam. 8.—Cyclostomatidae. Pallial cavity transformed into a lung; pedal centres concentrated; a deep pedal groove. Cyclostoma, shell turbinated, operculum calcareous, British. Omphalotropis. Fain. 9. Aciculidae. Pallial cavity transformed into a lung; operculum horny; shell narrow and elongated. Acicula. Fam. io.—Valvatidae. Ctenidium bipectinate, free; hermaphrodite; fluviatile. Valvata, British. Fain. i i.—Rissoidae. Epipodial filaments present; one or two pallial tentacles. Rissoa. Rissoina. Stiva. Fain. 12. Litiopidae. An epipodium bearing three pairs of tentacles and an operculigerous lobe with two appendages; inhabitants of the Sargasso weed. Litiopa. Fam. 13. Adeorbiidae. Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity. A deorbis, British. Fam. 14.—Jeffreysiidae. Head with two long labial palps; shell ovoid; operculum horny, semicircular, carinated. Jeffreysia. Fain. t5. Homalogyridae. Shell flattened; no cephalic tentacles. Homalogyra, British. Ammoniceras. Fam. i6.—Skeneidae. Shell depressed, with rounded aperture; cephalic tentacles long. Skenea, British. Fain. 17.—Choristidae. Shell spiral; four cephalic tentacles; eyes absent ; two pedal appendages, Choristes. Fam. 18. Assimineidae. Eyes at free extremities of tentacles. Assiminea, estuarine, British. Fam. 19.—Truncatellidae. Snout very long, bilobed; foot short. Truncatella. Fam. 2o.—Hydrobiidae. Shell with distant from right tentacle, generally appendiculated; brackish water or fluviatile. Hydrobia, British. Baikalia, from Lake Baikal. Pomatiopsis. Bithynella. Lithoglyphus. Spekia, viviparous, from Lake Tanganyika. Tanganyicia. Limnotrochus, from Lake Tanganyika. Chytra. Littorinida. Bithynia, British, fluviatile. Stenothyra. Fam. 21.—Melaniidae. Spire of shell somewhat elongated; mantle-border fringed; viviparous; fluviatile. Melania. Faunus. Paludomus. Melanopsis. Nassopsis. Bythoceras, from Lake Tanganyika. Fam. 22.—Typhobiidae. Foot wide ; shell turriculated, with carinated whorls, the carinae tuberculated or spiny. Typhobia. Bathanalia, from Lake Tanganyika. Fain. 23. Pleuroceridae. Like Melaniidae, but mantle-border not fringed and reproduction oviparous. Pleurocera. Anculotus. Fain. 24.—Pseudomelaniidae. All extinct. Fam. 25.—Subulitidae. All extinct. Fam. 26.—Nerineidae. All extinct. Fam. 27.—Cerithiidae. Shell with numerous tuberculated whorls ; aperture canaliculated anteriorly; short pallial siphon. Ceri- thium. Bittium. Potamides. Triforis. Laeocochlis. Ceri- thiopsis. Fam. 28.—Modulidae. Shell with short spire; no siphon. Modulus. Lanistes, shell a A, The animal. B, The shell removed. C, D, Two views of the shell of Cardiopoda. h, Border of the mantle-flap. u, Cerebral ganglion. i, Ctenidium (gill-plume). v, Pleural and pedal ganglion m, Stomach. w, Testis. n, Intestine. x, Visceral ganglion. o, Anus. y, Vesicula seminalis. p, Liver. [ventricle. z, Penis. t, Aorta, springing from the prominent spire ; penis 0 o, Mouth. op, Operculum. br, Ctenidium (branchial plume). x, Filiform appendage (? rudimentary ctenidium). The freely projecting ctenidium of typical form not having its axis fused to the roof of the branchial chamber is the notable character of this genus. Fam. 29.—Vermetidae. Animal fixed by the shell, the last whorls of which are not in contact with each other; foot small; two anterior pedal tentacles. Vermetus. Siliquaria. Fam. 3o.—Caecidae. Shell almost completely uncoiled, in one plane, with internal septa. Caecum, British. Fain. 31.—Turritellidae. Shell very long; head large; foot broad. Turritella, British. Mesalia. Mathilda. Fam. 32.—Struthiolariidae. Shell conical ; aperture slightly canaliculated ; siphon slightly developed. Struthiolaria. Fam. 33.—Chenopodidae. Shell elongated; aperture expanded; siphon very short. Chenopus, British. A l o r i a, Spinigera, Diartema, extinct. Fam. 34. Slrombidae. Foot narrow, compressed, without sole. Strombus. Pteroceras. Rostellaria. Terebellum. Fam. 35.—Xenophoridae. Foot transversely divided into two parts. Xenophorus. Eotrochus, Silurian. Fam. 36.—Capulidae. Shell conical, not FIG. 31.—Shell of Crucibulum, seen coiled,butslightlyin- from below so as to show the inner whorl curved posteriorly; b, concealed by the cap-like outer whorl a. atongue-shaped projection between snout and foot. Capulus. Thyca, parasitic on asterids. Platyceras, extinct. Fam. 37. Hipponycidae. Shell conical; foot secreting a ventral calcareous plate; animal fixed. Hipponyx. Mitrularia. Fam. 38.—Calyptraeidae. Shell with short spire; lateral cervical lobes present; accessory genital glands. Calyptraea, British. Crepidula. Crucibulum. Fam. 9.—Naricidae. Foot divided into two, posterior half bearing the operculum; a wide epipodial velum; shell turbinated. Narica. Fain. 4o.—Naricidae. Foot large, with aquiferous system; propodium reflected over head; eyes degenerate; burrowing habit. Natica, British. Amaura. Sigaretus. Fam. 41.—Lamellariidae. Shell thin, more or less covered by the mantle; no operculum. Lamellaria. Velutina. Marsenina, Oncidiopsis, hermaphrodite. Fain. 4z.—Trichotropidae. Shell with short spire, carinate and pointed. Trichotropis. Fam. 43.—Seguenziidae. Shell trochiform, with canaliculated aperture and twisted columella. Seguenzia, abyssal. Fam. 44.—Janthinidae. Shell thin; operculum absent; tentacles bifid; foot secretes a float; pelagic. Janthina. Recluzia. Fam. 45.—Cypraeidae. Shell inrolled, solid, polished, aperture very narrow in adult; short siphon; anus posterior ; osphradium with three lobes; mantle reflected over shell. Cypraea. Pustularia. Ovula. Pedicularia, attached to corals. Erato. Fam. 46.—Tritonidae. Shell turriculated and siphonated, thick, each whorl with varices; foot broad and truncated anteriorly; pallial siphon well developed; proboscis present. Triton. Per- sona. Ranella. Fain. 47.—Columbel- linidae. All extinct. Fam. 48.—Cassididae. Shell ventricose,with elongated aperture, J and short spire; proboscis and siphon long ;operculum with marginal nucleus. Cassis. Cassidaria. Oniscia. Fam.49—Oocorythidae. Shell globular and ventricose; aperture oval and canaliculated; operculum spiral. Oocorys, abyssal. Fam. 50. Doliidae. Shell ventricose, with short spire, and wide aperture; no varices and no operculum; foot very broad, with projecting anterior angles; siphon long. Dolium. Pyrula. Fam. 51.—Solariidae. Solarium. Torinia. Fluxina. Fam. 52.—Scalariidae. Shell turriculated, with elongated spire; proboscis short; siphon rudimentary. Scalaria. Eglisia. Crossea. Aclis. The three following families have neither radula nor jaws, and are therefore called Aglossa. They have a well-developed proboscis which is used as a suctorial organ; some are abyssal, but the majority are either commensals or parasites of Echinoderms. Fam. 53.—Pyramidellidae. Summit of spire heterostrophic; a projection, the mentum, between head and foot; operculum present. Pyramidella. Turbonilla. Odostomia, British. Myxa. Fam. 54. Eulimidae. Visceral mass still coiled spirally; shell thin and shining. Eulima, foot well developed, with an operculum, animal usually free, but some live in the digestive cavity of Holothurians. Mucronalia, 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 pseudopallium. Styl fer, the operculum is lost, animal fixed by a large proboscis which forms a pseudopallium covering the whole shell except the extremity of the spire, parasitic on all groups of Echinoderms. Entosiphon, visceral mass still coiled; shell much reduced, proboscis very long forming a pseudopallium which covers the whole body and projects beyond in the form of a siphon, foot and nervous system present, eyes, branchia and anus absent, parasite in the Holothurian Deima blakei in the Indian Ocean. Fam. 55.—Entoconchidae. No shell ; visceral mass not coiled ; no sensory organs, nervous system, branchia or anus; body reduced to a more or less tubular sac; hermaphrodite and viviparous; parasitic in Holothurians; larvae are veligers, with shell and operculum. Entocolax, mouth at free extremity, animal fixed by aboral orifice of pseudopallium, Pacific. Entoconcha, body elongated and tubular, animal fixed by the oral extremity, protandric hermaphrodite, parasitic in testes of Holothurians causing their abortion. Enteroxenos, no pseudo- pallium and no intestine, hermaphrodite, larvae with operculum. Tribe 2.—HETEROPODA. Pelagic Taenioglossa with foot large and laterally compressed to form a fin. Fam. 1. Atlantidae. Visceral sac and shell coiled in one plane; foot divided transversely into two parts, posterior part bearing an operculum, anterior part forming a fin provided with a sucker. Atlanta. Oxygyrus. Fain. 2.—Carinariidae. Visceral sac and shell small in proportion to the rest of the body, which cannot be withdrawn into the shell; foot elongated, fin-shaped, with sucker, but without operculum. Carinaria. Cardiopoda. Fam. 3.—Pterotrachaeidae. Visceral sac very much reduced; without shell or mantle; anus posterior; foot provided with sucker in male only. Pterotrachaea. Firoloida. Pterosoma. Sub-order 2.—STENOGLOSSA. Radula narrow with one lateral tooth on each side, and one median tooth or none. Tribe I.—RACHIGLOSSA. Radula with a median tooth and a single b, egg-capsules; c, ctenidium (gill-plume); d, cephalic tentacles. tooth on each side of it. Formula 1 : 1 : 1. Rudimentary jaws present. Fain. I.—Turbinellidae. Shell solid, piriform, with thick folded columella; lateral teeth of radula bicuspidate. Turrinella. Cynodonta. Fulgur. Hemifusus. Tudicla. Strepsidura. Fain. 2.—Fasciolariidae. Shell elongated, with long siphon; lateral teeth of radula multicuspidate. Fasciolaria. Fusus. Clavella. Latirus. Fam. 3.—Mitridae. Shell fusiform and solid, aperture elongated, columella folded; no operculum; eyes on sides of tentacles. Mitra. Turricula. Cylindromitra. Imbricaria. Fam. ,4.—Buccinidae. Foot large and broad ; eyes at base of d, Foot. h, Mantle-skirt, which is naturally carried in a reflected condition so as to cover the sides of the shell. a, Apex. ac, Siphonal notch of the mouth of the shell. ac to pc, Mouth of the shell. w, w, Whorls of the shell. s, s, Sutures. Occupying the axis, and exposed by the section, is seen the columella " or spiral pillar. The upper whorls of the shell are seen to be divided into separate chambers by the formation of successively formed " septa.” ,;V'o 00.004i!44 444ioniusl144,uthp tentacles ; operculum horny. Buccinum. Chrysodomus. Liomesus. Cominella. Tritonidea. Pisania. Euthria. Phos. D~spsacus. Fam. 5.—Nassidae. Foot broad, with two slender posterior appendages; operculum unguiculate. Nassa, marine, British. Canidia, fluviatile. Bullia. Fain. 6.—Muricidae. Shell with moderately long spire and canal, ornamented with ribs, often spiny; foot truncated anteriorly. Murex, British. Trophon, British. Typhis. Urosalpinx. Lachesis. Fain. 7. Purpurulae. Shell thick, with short spire, last whorl large and canal short; aperture wide; operculum horny. Purpura, British. Rapana. Monoceros. Sistrum. Concholepas. Fain. 8.—Haliidae. Shell ventricose, thin and smooth, with wide aperture; foot large and thick, without operculum. Halia. Fam. q.—Cancellarizdaa. Shell ovoid, with short spire and folded columella; foot small, no operculum; siphon short. Can- cellaria. Fam. io.—Columbellidae. Spire of shell prominent, aperture narrow, canal very short, columella crenelated; foot large. Columbella. Fain. it.—Coralliophilidae. Shell irregular; radula absent; foot and siphon short; sedentary animals, living in corals. Coralliophila. Rhizochilus. Lepioconchus. Magilus. Rapa. Fam. t2.—Volutidae. Head much flattened and wide, with eyes on sides; foot broad; siphon with internal appendages. Voluta. Guivillea. Cymba. Fain. 13.- Olividae. Foot with anterior transverse groove; a posterior pallial tentacle; generally burrowing. Olivia. Olivella. Ancillaria. Agaronia. Fam. t4.—Marginellidae. Foot very large; mantle reflected over shell. Marginella. Pseudomarginella. Fain. 15.—Harpidae. Foot very large; without operculum; shell with short spire and longitudinal ribs; siphon long. Har pa. Tribe z.—TOXIGLOSSA. No jaws. No median tooth in radula. Formula : t : o : t. Poison-gland present whose duct traverses the nerve-collar. Fain. t.--Pleurotomatidae. Shell fusiform, with elongated spire; margin of shell and mantle notched. Pleurotoma. Clavatula. Mangilia. Bela. Pusionella. Pontiothauma. Fain. z.—Terebridae. Shell turriculated, with numerous whorls; aperture and operculum oval; eyes at summits of tentacles; siphon long. Terebra. Fain. 3.—Conidae. Shell conical, with very short spire, and narrow aperture with parallel borders; operculum unguiform. Cones. Sub-Class II.—EuTHYNEURA The most important general character of the Euthyneura is the absence of torsion in the visceral commissure, and the more posterior position of the anus and pallial organs. Comparative anatomy and embryology prove that this condition is due, not as formerly supposed to a difference in the relations of the visceral commissure which prevented it from being included in the torsion of the visceral hump, but to an actual detorsion which has taken place in evolution and is repeated to a great extent in individual development. In several of the more primitive forms the same torsion occurs as in Streptoneura, viz. in Actaeon and Limacina among Opisthobranchia, and Chilina among Pulmonata. Actaeon is prosobranchiate, the visceral commissure is twisted in Actaeon and Chilina, and even slightly still in Bulla and Scaphander; in Actaeon and Limacina the osphradium is to the left, innervated by the supra-intestinal ganglion. But in the other members of the sub-class the detorsion of the visceral mass has carried back the anus and circumanal complex from the anterior dorsal region to the right side, as in Bulla and Aplysia, or even to the posterior end of the body, as in Philine, Oncidium, Doris, &c. Different degrees of the same process of detorsion are, as we have seen, exhibited by the Heteropoda among the Streptoneura, and both in them and in the Euthyneura the detorsion is associated with degeneration of the shell. Where the modification is carried to its extreme degree, not only the shell but the pallfal cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in Pterolrachaea and in Doris, Eolis, and other Nudibranchia. These facts afford strong support to the hypo-thesis that the weight of the shell is the original cause of the torsion of the dorsal visceral mass in Gastropods. But this hypothesis leaves the elevation of the visceral mass and the exogastric coiling of the shell in the ancestral form unexplained. (Formula, x.l.x.) radula has a number of uniform teeth on each side of the median tooth in each transverse row. The head in most cases bears two pairs of tentacles. All the Euthyneura are hermaphrodite. In the most primitive condition the genital duct is single throughout its length and has a single external aperture; it is therefore said to be monaulic. The hermaphrodite aperture is on the right side near the opening of the pallial cavity, and a ciliated groove conducts the spermatozoa to the penis, which is situated more anteriorly. This is the condition in the Bullomorpha, the Aplysiomorpha, and in one Pulmonate, Pythia. In some cases while the original aperture remains undivided, the seminal groove is closed and so converted into a canal. This is the modification found in Cavolinia longirostris among the Bullomorpha, and in all the Auriculidae except Pythia. A further degree of modification occurs when the male duct takes its origin from the hermaphrodite duct above the external opening, so that there are two distinct apertures, one male and one female, the latter being the original opening. The genital duct is now said to be diaulic, as in Valvata, Oncidiopsis, Actaeon, and Lobiger among the Bullomorpha, in the Pleurobranchidae, in the Nudibranchia, except the Doridomorpha and most of the Elysiomorpha, and in the Pulmonata. Originally in this condition the female aperture is at some distance from the male, as in the Basommatophora and in other cases; but in some forms the female aperture itself has shifted and come to be contiguous with the male opening and penis as in the Stylommatophora. In all these cases the female duct bears a bursa copulatrix or receptaculum seminis. In some forms this receptacle acquires a separate external opening remaining connected with the oviduct internally. There are thus two female openings, one for copulation, the other for oviposition, as well as a male opening. The genital duct is now trifurcated or triaulic, a condition which is confined to certain Nudibranchs, viz. the Doridomorpha and most of the Elysiomorpha. The Pteropoda, formerly regarded as a distinct class of the Mollusca, were interpreted by E. R. Lankester as a branch of the Cephalopoda, chiefly on account of the protrusible sucker-bearing processes at the anterior end of Pneumonoderma. These he considered to be homologous with the arms of Cephalopods. He fully recognized, however, the similarity of Pteropods to Gastropods in their general asymmetry and in the torsion of the visceral mass in Limacinidae. It is now understood that they are Euthyneurous Gastropods adapted to natatory locomotion and pelagic life. The sucker-bearing processes of Pneumonos derma are outgrowths of the proboscis. The fins of Pteropods are now interpreted as the expanded lateral margins of the foot, termed parapodia, not homologous with the siphonof Cephalopods which is formed from epipodia. The Thecosomatous Pteropoda are allied to Bulla, the Gymnosomatous forms to Aplysia. The Euthyneura comprises two orders, Opisthobranchia and Pulmonata. In those Euthyneura in which the shell is entirely absent in the adult, it is, except in the three genera Cenia, Runcina and Vaginula, developed in the larva and then falls off. In other cases (Tectibranchs) the reduced shell is enclosed by upgrowths of the edge of the mantle and becomes internal, as in many Cephalopods. A few Euthyneura in which the shell is not much reduced retain an operculum in the adult state, e.g. Actaeon, Limacina, and the marine Pulmonate, A mphibola. The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the. whole nervous system is aggregated in the cephalic region, either dorsally or ventrally. The Order I.—OPISTHOBRANCHIA. Marine Euthyneura, the more archaic forms of which have a relatively large foot and a small visceral hump, from the base of which projects on the right side a short mantle-skirt. The anus is placed in such forms far back beyond the mantle-skirt. In front of the anus, and only partially covered Y A, Veliger-larva of an Opisthobranch (Polycera). f, Foot; op. operculum; mn, anal papilla; ry, dry, two portions of unabsorbed nutritive yolk on either side of the intestine. The right otocyst is seen at the root of the foot. B, Trochosphere of an Opisthobranch (Pleurobranchidium) showing—shgr, the shell-gland or primitive shell-sac; v, the cilia of the velum; ph, the commencing stomodaeum or oral invagination; ot, the left otocyst; pg, red-coloured pigment spot. C, Diblastula of an Opisthobranch (Polycera) with elongated blastopore oi. (All from Lankester.) by the mantle-skirt, is the ctenidium with its free end turned back-wards. The heart lies in front of, instead of to the side of, the attachment of the ctenidium—hence Opisthobranchia as opposed to " Prosobranchia," which correspond to the Streptoneura. A shell is possessed in the adult state by but few Opisthobranchia, but all pass through a veliger larval stage with a nautiloid shell (fig. 36). Many Opisthobranchia have by a process of atrophy lost the typical ctenidium and the mantle-skirt, and have developed other organs in their place. As in some Pectinibranchia, the free margin of the mantle-skirt is frequently reflected over • the shell when a shell exists; and, as in some Pectinibranchia, broad lateral outgrowths of the foot (parapodia) are often developed which may be thrown over the shell or naked dorsal surface of the body. The variety of special developments of structure accompanying the atrophy of typical organs in the Opisthobranchia and general degeneration of organization is very great. The members of the order present the same wide range of superficial appear- ance as do the Pectini- branchiate Streptoneura, forms carrying well-de- veloped spiral shells and large mantle-skirts being included in the group, together with flattened or cylindrical slug-like forms. But in respect of the sub- stitution of other parts for the mantle-skirt and for. the gill which the more degenerate Opisthobranchia exhibit, this order stands alone. Some Opisthobranchia are striking examples of de- generation (some Nudibranchia), having none of those regions or processes of the body developed which distinguish the archaic Mollusca from such flat-worms as the Dendrocoel Planarians. In- deed, were it not for their retention of the characteristic odontophore we should have little or no indication that such forms as Phyllirhoe and Limapontia really belong to the Mollusca at all. The interesting little Rhodope veranyii, which has no odontophore, has been associated by systematists both with these simplified Opisthobranchs and with Rhabdocoel Planarians. In many respects the sea-hare (Aplysia), of which several species are known (some occurring on the English coast), serves as a convenient example of the fullest development of the organization characteristic of Opisthobranchia. The woodcut (fig. 38) gives a faithful representation of the great mobility of the various parts of the body. The head is well marked and joined to the body by a somewhat constricted neck. It carries two pairs of cephalic tentacles and a pair of sessile eyes. The visceral hump is low and not drawn out into a spire. The foot is long, carrying the oblong visceral mass upon it, and projecting (as metapodium) a little beyond it(f). Later-ally the foot gives rise to a pair of mobile fleshy lobes, the parapodia (ep), which can be thrown up so as to cover in the dorsal surface of the animal. Such parapodia are common, though by no means universal, among Opisthobranchia. The torsion of the visceral hump is not carried out very fully, the consequence being that the anus has a posterior position a little to the right of the median line above the metapodium, whilst the branchial chamber formed by the overhanging mantle-skirt faces the right side of the body instead of lying well to the front as in Streptoneura and as in Pulmonate Euthyneura. The gill-plume,which in A plysia is the typicalMolluscan ctenidium, is seen in fig. 39 projecting from the branchial sub-pallial space. C 1, Anterior cephalic tentacles. m, Mantle-flap reflected over the t2, Posterior cephalic tentacles. thin oval shell. e, Eyes. os, s, Orifice formed by the un- f, Metapodium. closed border of the reflected ep, Epipodium. mantle-skirt, allowing the g, Gill-plume (ctenidium). shell to show. pe, The spermatic groove. The relation of the delicate shell to the mantle is peculiar, since it occupies an oval area upon the visceral hump, the extent of which is indicated in fig. 38, C, but may be better understood by a glance at the figures of the allied genus Umbrella (fig. 40), in which the margin of the mantle-skirt coincides, just as it does in the limpet, with the margin of the shell. But in Aplysia the mantle is reflected over the edge of the shell, and grows over its upper surface so as to completely enclose it, excepting at the small central area s where the naked shell is exposed. This enclosure of the shell is a permanent development of the arrangement seen in many Streptoneura (e.g. Pyrula, Ovula, see figs. 18 and 32), where the border of the mantle can be, and usually is, drawn over the shell, though it is withdrawn (as it cannot be in Aplysia) when they are irritated. From the fact that Aplysia commences its life as a free-swimming veliger with a nautiloid shell not enclosed in any way by the border of the mantle, it is clear that the enclosure of the shell in the adult is a secondary process. Accordingly, the shell of Aplysia must not be confounded with a primitive shell in its shell-sac, such as we find realized in the shells of Chiton and in the plugs which form in the remarkable transitory " shell-sac " or " shell-gland " of Molluscan embryos (see figs. 26, 6o). Aplysia, like other Mollusca, develops a primitive shell-sac in its trochosphere stage of development, which disappears and is succeeded by a nautiloid shell (fig. 36). This forms the nucleus of the adult shell, and, as the animal grows, becomes enclosed by a reflection of the mantle-skirt. When the shell of an A plysia enclosed in its mantle is pushed well to the left, the sub-pallial space is fully exposed as in fig. 39, and the various apertures of the body are seen. A 9' a, Mouth. b, Radular sac. c, Oesophagus. d, Stomach. c', Intestine. f', Anus. g, g', g'', gm, The four lobes of the liver. h, The heart (auricle and ventricle). 1, The renal sac (nephridium). 1', The ciliated communication of the renal sac with the pericardium. m, The external opening of the renal sac. n, The cerebral ganglion. o, The cephalic tentacles. f, The genital pore. y, The ovo-testes. w, The parasitic hydromedusa Mnestra, usually found attached in this position by the aboral pole of its umbrella. Posteriorly we have the anus, in front of this the lobate gill-plume, between the two (hence corresponding in position to that of the Pectinibranchia) we have the aperture of the renal organ. In front, near the anterior attachment of the gill-plume, is the osphradium (olfactory organ) discovered by J. W. Spengel, yellowish in colour, in the typical position, and overlying an olfactory ganglion with typical nerve-connexion (see fig. 43). To the right of Spengel's osphradium is the opening of a peculiar gland which has, when dissected out, the form of ,,, a bunch of grapes; its secretion is said to be poisonous. On the under side of the free edge of the mantle are situated the numerous small cutaneous glands which, in the large A plysia camelus (not in other species), form the purple secretion which was known to the ancients. In front away from the mid-line. (Lankester.) the aperture of the com- a, Anterior cephalic tentacle. mon or hermaphrodite b, Posterior cephalic tentacle; between a duct. From this point and b, the eyes. there passes forward to , Right epipodium. the right side of the c head a groove—the spermatic groove—down which the spermatic fluid passes. In other Euthyneura this groove may close up and form a canal. At its termination by the side of the head is the muscular introverted penis. In the hinder part of the foot (riot shown in any of the diagrams) is the opening of a large mucus-forming gland very often found in the Molluscan foot. With regard to internal organization we may commence with the disposition of the renal organ (nephridium), the external opening of which has already been noted. The position of this opening and other features of the renal organ were determined by J. T. Cunningham. There is considerable uncertainty with respect to the names of the species of Aplysia. There are two forms which are very common in the Gulf of Naples. One is quite black in colour, and measures when outstretched 8 or 9 in. in length. The other is light brown and some-what smaller, its length usually not exceeding 7 in. The first is flaccid and sluggish in its movements, and has not much power of contraction; its epipodial lobes are enormously developed and extend far forward along the body; it gives out when handled an abundance of purple liquid, which is derived from cutaneous glands situated on the under side of the free edge of the mantle. According to F. Blochmann it is identical with A. camelus of Cuvier. The other species is A. depilans; it is firm to the touch, and contracts forcibly when irritated; the secretion of the mantle-glands is not abundant, and is milky white in appearance. The kidney has similar relations in both species, and is identical with the organ spoken of by many authors as the triangular gland. Its superficial extent is seen when the folds covering the shell are cut away and the shell removed; the external surface forms a triangle with its base bordering the pericardium, and its apex directed posteriorly and reaching t6 the Iefthand posterior corner of the shell-chamber. The dorsal surface of the kidney extends to the left beyond the shell-chamber beneath the skin in the space between the shell-chamber and the left parapodium. When the animal is turned on its left-hand side and the mantle-chamber widely opened, the gill being turned over to the left, a part of the kidney is seen beneath the skin between' the attachment of the gill and the right parapodium (fig. 39). On examination this is found to be the under surface of the posterior limb of the gland, the upper surface of which has just been described as lying beneath the shell. In the posterior third of this portion, close tc that edge which is adjacent to the base of the gill, is the external opening (fig. 39, o). When the pericardium is cut open from above in an animal otherwise entire, the anterior face of the kidney is seen forming. the posterior wall of the pericardial chamber; on the deep edge of this face, a little to the left of the attachment of the auricle to the floor of the pericardium, is seen a depression ; this depression contains the opening from the pericardium into the kidney. To complete the account of the relations of the organ: the right anterior corner can be seen superficially in the wall of the mantle-chamber above the gill. Thus the base of the gill passes in a slanting direction across the right-hand side of the kidney, the posterior end being dorsal to the apex of the gland, and the anterior end duct, which very soon becomes miniparous gland into phrodite the hermaphrodite entwined in the spire of a gland—the duct. albuminiparous gland. The latter opens e, Hermaphrodite duct into the common duct at the point k, (uterine portion). and here also is a small diverticulum of b, Vaginal portion of the the duct f. Passing on, we find not uterine duct. far from the genital pore a glandular c, Spermatheca. spherical body (the spermatheca c) open- d, Its duct, ing by means of a longish duct into a, Genital pore. the common duct, and then we reach the pore (fig. 39, k). Here the female apparatus terminates. But when the male secretion of the ovo-testis is active, the seminal fluid passes from the genital pore along the spermatic groove (fig. 39) to the penis, and is by the aid of that eversible muscular organ introduced into the genital pore of a second Aplysia, whence it passes into the spermatheca, there to await the activity of the female element of the ovo-testis of this second Aplysia. After an interval d, Left epipodium. e, Hinder part of visceral hump. fp, Posterior extremity of the foot. fa, Anterior part of the foot underlying the head. g, The ctenidium (branchial plume). h, The mantle-skirt tightly spread over the horny shell and pushed with it towards the left side. i, The spermatic groove. k, The common genital pore (male and female). 1, Orifice of the grape-shaped (supposed poisonous) gland. m, The osphradium (olfactory organ of Spengel). n, Outline of part of the renal sac (nephridium) below the surface. o, External aperture of the nephridium. p, Anus. ventral to the right-hand corner. As so great a part of the whole surface of the kidney lies adjacent to external surfaces of the body, the remaining part which faces the internal organs is small; it consists of the left part of the under surface; it is level with the floor of the pericardium, and lies over the globular mass formed by the liver and convoluted intestine. Thus the renal organ of Aplysia is shown to conform to the Molluscan type. The heart lying within the adjacent pericardium has the usual form, a single auricle and ventricle. The vascular system is not ° rl extensive, the arteries soon ending in the well-marked spongy tissue which builds up the muscular foot, parapodia, and dorsal body-wall. The alimentary canal commences with the usual buccal mass; the lips are cartilaginous, but not armed with horny jaws, though these are common in other Opisthobranchs; the lingual ribbon is multidenticulate, and a pair of salivary glands pour in their secretion. The oesophagus expands into a curious gizzard, which is armed internally with large horny processes, some broad and thick, others spinous, fitted to act as crushing instruments. From this we pass to a stomach and a coil of intestine embedded in the lobes of a voluminous liver; a caecum of large size is given off near the commencement of the intestine. The liver opens by two ducts into the digestive FIG. 4i—Gonad, and tract. accessory glands and The generative organs lie close to the ducts of Aplysia. (Lan-coil of intestine and liver, a little to the kester.) left side. When dissected out they ap- Ovo-testis, pear as represented in fig. 41. The h, Hermaphrodite duct. essential reproductive organ or gonad g, Albuminiparous gland. consists of both ovarian and testicular f Vesicula seminalis. cells (see fig. 42). It is an ovo-testis. k, Opening of the albu-From it passes a common or herma- of some days—possibly weeks—the ova of the second Aplysia commence to descend the hermaphrodite duct; they become en- A 1 closed in a viscid secretion at the point where the albuminiparous gland opens into the duct intertwined with it; and on reaching the point where the spermathecal duct debouches they are impregnated by the spermatozoa which escape now from the spermatheca and meet the ova. The development of Aplysia from the egg presents many points of interest from the point of view of comparative embryology, but in relation to the morphology of the Opisthobranchia it is sufficient to point to the occurrence of a trochosphere and a veliger stage (fig. 36), and of a shell-gland or primitive shell-sac (fig. 36, shgr), which is succeeded by a nautiloid shell. In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct. The euthyneurous visceral loop is long, and presents only one ganglion (in Aplysia camelus, but two distinct ganglia joined to one another in Aplysia hybrida' of the English coast), placed at its extreme limit, representing both the right and left visceral ganglia and the third or abdominal ganglion, which are so often separately present. The diagram (fig. 43) shows the nerve connecting this abdomino-visceral ganglion with the olfactory ganglion of Spengel. It is also seen to be connected with a more remote ganglion—the genital. Such special irregularities in the develop- ment of ganglia upon the visceral loop, and on one or more of the main nerves connected with it, are very frequent. Our figure of the nervous system of Aplysia does not give the small pair of buccal ganglia which are, as in all glossophorous Molluscs, present upon the nerves passing from the cerebral region to the odontophore. For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point. It is one of the more typical Opisthobranchs, that is to say, it belongs to the section Tectibranchia, but other members of the sub-order, namely, Bulla and Actaeon (figs. 44 and 45), are less abnormal than Aplysia in regard to their shells and the form of the visceral hump. They have naked spirally twisted shells which may be concealed from view in the living animal by the expansion and reflection of the parapodia, but are not enclosed by the mantle, whilst Actaeon is remarkable for possessing an operculum like that of so many Streptoneura. The great development of the parapodia seen in Aplysia is usual in Tectibranchiate Opisthobranchs. The whole surface of the body becomes greatly modified in those Nudibranchiate forms which have lost, not' only the shell, but also the ctenidium. Many of these have peculiar processes developed on the dorsal surface (fig. 46, A, B), or retain purely negative characters (fig. 46, D). The chief modification of internal organization presented by these forms, as compared with Aplysia, is found in the condition of the alimentary canal. The liver Is no longer a compact organ opening by a pair of ducts into the median digestive tract, but we find very numerous hepatic diverticula on a shortened axial tract (fig. 47). These diverticula extend usually one into each of the dorsal papillae or " cerata " when these are present. They are not merely digestive glands, but are sufficiently wide to act as receptacles of food, and in them the digestion of food proceeds just as in the axial portion of the canal. A precisely similar modification of the liver or great digestive gland is found in the scorpions, where the axial portion of the digestive canal is short and straight, and the lateral ducts sufficiently wide to admit food into the ramifications of the gland there to be digested; whilst in the spiders the gland is reduced to a series of simple caeca. The typical character is retained by the heart, pericardium, and the communicating nephridium or renal organ in all Opisthobranchs. An interesting example of this is furnished by the fish-like trans-parent Phyllirhoe (fig. 37), in which it is possible most satisfactorily to study in the living animal, by means of the microscope, the course of the blood-stream, and also the reno-pericardial communication. In many of the Nudibranchiate Opisthobranchs the nervous system presents a concentration of the ganglia (fig. 48), contrasting greatly with what we have seen in Aplysia. Not only are the pleural ganglia fused to the cerebral, but also the visceral to these (see in further illustration the condition attained by the Pulmonate Limnaeus, fig. 59), and the visceral loop is astonishingly short and insignificant (fig. 48, e'). That the parts are rightly thus identified is probable from J. W. Spengel's observation of the osphradium and its nerve-supply in these forms; the nerve to that organ, which is placed somewhat anteriorly—on the dorsal surface—being given off from the hinder part (visceral) of the right compound ganglion—the fellow to that marked A in fig. 48. The Eolid-like Nudibranchs, amongst other specialities of structure, possess (in some cases at any rate) apertures at the apices of the " cerata " or dorsal papillae, which lead from the exterior into the hepatic caeca. Some amongst them (Tergipes, Eolis) are also remarkable for possessing peculiarly modified cells placed in sacs (cnidosacs) at the apices of these same papillae, which resemble the " thread-cells " of the Coelentera. According to T. S. Wright and J. H. Grosvenor these nematocysts are derived from the hydroids on which the animals feed. The development of many Opisthobranchia has been examined—e.g. Aplysia, Pleurobranchidium, Elysia, Polycera, Doris, Tergipes. All pass through trochosphere and veliger stages, and in all a nautiloid or boat-like shell is developed, preceded by a well-marked " shell-gland " (see fig. 36). The transition from the free-swimming veliger larva with its nautiloid shell (fig. 36) to the adult form has not been properly observed, and many interesting points as to the true nature of folds (whether parapodia or mantle or velum) have yet to be cleared up by a knowledge of such development in forms like Tethys, Doris, Phyllidia, &c. As in other Molluscan groups, we find even in closely-allied genera (for instance, in Aplysia and Pleurobranchidium, and other genera), the greatest differences as to the amount of food-material by 'which the egg-shell is encumbered. Some form their diblastula by emboly, others by epiboly; and in the later history of the further development of the enclosed cells (archenteron) very marked variations occur in closely-allied forms, due to the influence of a greater or less abundance of food-material mixed with the protoplasm of the egg. Sub-order I. —TECTIBRANcHIA. Opisthobranchs provided in the adult state with a shell and a mantle, except Runcina, Pleuro• branchaea, Cymbuliidae, and some Aplysiomorpha. There is a ctenidium, except in some Thecosomata and Gymnosomata, and an osphradium. Tribe I.—BULLOMORPHA. The shell is usually well developed, except in Runcina and Cymbuliidae, and may be external or internaL No operculum, except in Actaeonidae and Limacinidae. The pallial cavity is always well developed, and contains the ctenidium, at least in part; ctemdium, except in Lophocercidae, of folded type. With 2 ce, Cerebral ganglion. pl, Pleural ganglion. pe, Pedal ganglion. ab. sp, Abdominal ganglion which re-presents also the supra-intestinal ganglion of Streptoneura and gives off the nerve to the osphradium (olfactory organ) o, and another to an unlettered so-called " genital " ganglion. T h e buccal nerves and ganglia are omitted. d b the exception of the A plustridae, Lophocercidae and Thecosomata, the head is devoid of tentacles, and its dorsal surface forms a digging A, Eolis papillosa (Lin.), dorsal view. a, b, Posterior and anterior cephalic tentacles. c, The dorsal"cerata." B, Tethys leporina, dorsal view. a, The cephalic hood. e, Anus. b, Cephalic tentacles. f, Large cerata. c, Neck. g, Smaller cerata. d, Genital pore. h, Margin of the foot. C, Doris (Actinocyclus) tuberculatus (Cuv.), seen from the pedal surface. m, Mouth. f, Sole of the foot. b, Margin of the head. •sp, The mantle-like epipodium. D, E, Dorsal and lateral view of Elyria (Actaeon) viridis. ep, epipodial outgrowths. (After Keferstein.) disk or shield. The edges of the foot form parapodia, often trans-formed into fins. Posteriorly the mantle forms a large pallial lobe A, Cerebral, pleural and visceral ganglia united. B, Pedal ganglion. C, Buccal ganglion. D, Oesophageal ganglion con- nected with. the buccal. a, Nerve to superior cephalic tentacle. b, Nerves to inferior cephalic tentacles. c, Nerve to generative organs. d, Pedal nerve. e, Pedal commissure. e', Visceral loop or commis-sure (?). under the pallial aperture. Stomach generally provided with chitinous or calcified masticatory plates. Visceral commissure fairly long, except in Runcina, Lobiger and Theeosomata. Hermaphrodite genital aperture, connected with the penis by a ciliated groove, except in Actaeon, Lobiger and Cavolinia longirostris, in which the spermiduct is a closed tube. Animals either swim or burrow. Fam. i.—Actaeonidae. Cephalic shield bifid posteriorly ; margins of foot slightly developed; genital duct diaulic; visceral commissure streptoneur- ous; shell thick, with prominent spire and elongated aperture; a horny operculum. Actaeon, British. Solidula. Tornatellaea, extinct. Adelactaeon. Bullina. Bullinula. Fam. 2. — Ringiculidae. Cephalic disk enlarged anteriorly, forming an open tube posteriorly ; shell external, thick, with prominent spire; no operculum. Ringicula. Pugnus. Fam. 3. — Tornatinidae. Margins of foot not prominent; no radula; shell external, with inconspicuous spire. Tornatina, British. Retusa. Volvula. Fain. 4.— Scaphandridae. Cephalic' shield short, truncated posteriorly; eyes deeply embedded; three calcareous stom- g, achal plates; shell external, with reduced spire. Seaphander, British. Atys. Smaragdinella. Cylichna, British. Amphisphyra, British. Fam. g.—Bullidae. Margins of foot well developed; eyes superficial; three chitinous stomachal plates; shell external, with reduced spire. Bulls, British. Haminea, British. Fam. 6.—Aceratidae. Cephalic shield continuous with neck; twelve to fourteen stomachal plates; a posterior pallial filament passing through a notch in shell. Acera, British. Cylindrobulla. Volutella. Fam. 7.—A plustridae. Foot very broad ; cephalic shield with four tentacles; shell external, thin, without prominent spire. A plustrum. Hydatina. Micromelo. Fain. 8.—Philinidae. Cephalic shield broad, thick and simple; shell wholly internal, thin, spire much reduced, aperture very large. Philine, British. Cryptophthaltnus. Chelinodura. Phanerophthalmus. Colpodaspis, British. Colobocephalus. Fam. 9.—Doridiidae. Cephalic shield ending posteriorly in a median point; shell internal, largely membranous; no radula or stomachal plates. Doridium. Navarchus. Fam. to.--Gastropteridae. Cephalic shield pointed behind; shell internal, chiefly membranous, with calcified nucleus, nautiloid; parapodia forming fins. Gastropteron. Fan'. II.—Runcinidae. Cephalic shield continuous with dorsal integument; no shell; ctenidium projecting from mantle cavity. Runcina. Fam. 12.—Lophocercidae. Shell external, globular or ovoid; foot elongated, parapodia separate from ventral surface; genital duct diaulic. Lobiger. Lophocercus. The next three families form the group formerly known as Thecosomatous Pteropods. They are all pelagic, the foot being entirely transformed into a pair of anterior fins; eyes are absent, and the nerve FIG. go.—Shell of Cavolinia centres are concentrated on the ven- tridentata, seen from the side. tral side of the cesophagus. f, Postero-dorsal surface. Fam. 13.—Limacinidae. Dextral g, Antero-ventral surface. animals, with shell coiled h, Median dorsal spine. pseudo-sinistrally : operculum i, Mouth of the shell. with sinistral spiral; pallial cavity dorsal. Limacina, British. Peraclis, ctenidium present. Fam. 14.—Cymbuliidae. Adult without shell; a sub-epithelial pseudoconch formed by connective tissue; pallial cavity ventral. Cymbulia. Cymbuliopsis. Gleba. Desmopterus. Fam. i5.—Cavoliniidae. Shell not coiled, symmetrical; pallial cavity ventral. Cavolinia. Clio. Cuvierina. Tribe 2.—APLYSIOMORPHA. Shell more or less internal, much reduced or absent. Head bears two pairs of tentacles. Parapodia separate from ventral surface, and generally transformed into ph, Pharynx. Midgut, with its hepatic appendages h, all of which are not figured. e, Hind gut. an, Anus. C C a, Mouth. b, Pair of cephalic tentacles. C, C, Pteropodial lobes of the foot. d, Median web connecting these. e, e, Processes of the mantle-skirt reflected over the surface of the shell. The shell enclosing the visceral hump. The median spine of the shell. h, 522 swimming lobes. Visceral commissure much shortened, except in Aplysia. Genital duct monaulic; hermaphrodite duct connected with penis by a ciliated groove. Animals either swim or crawl. Fain. i.—Aplysiidae. Shell partly or wholly internal, or absent; foot long, with well-developed ventral surface. Aplysia. Dolabella. Dolabrifer. Aplysiella. Phyllaplysia. Notarchus. The next six families include the animals formerly known as Gymnosomatous Pteropods, characterized by the absence of mantle and shell, the reduction of the ventral surface of the foot, and the parapodial fins at the anterior end of the body. They are all pelagic. Fam. z. Pneumonodermatidae. Pharynx evaginable, with suckers. Pneumonoderma. Dexiobranchaea. Spongiobranchaea. Schizobrachium. Fam. 3.—ClionoIsidae. No buccal appendages or suckers; a very long evaginable proboscis; a quadriradiate terminal branchia. Clionopsis: Fam.4. Notobranchaeidae. Posterior branchia triradiate. Notobranchaea. Fam. 5.—Thliptodontidae. Head very large, not marked off from the body; neither branchia nor suckers; fins situated near the middle of the body. Thliptodon. Fam. 6.—Clionidae. No branchia[ETJTHYNEURA anterior tentacles form a frontal veil; mantle contains spicules. Pleurobranchus. Beathella. Haliotinella. Oscanius, British. Oscaniella. Oscaniopsis. Pleurobranchaea. Sub-order 2.—NUDIBRANCHIA. Shell absent in the adult; no ctenidium or osphradium. Body generally slug-like, and externally symmetrical. Visceral mass not marked off from the foot, except in Hedylidae. Dorsal respiratory appendages frequently present. Visceral commissure reduced; nervous system concentrated on dorsal side of oesophagus. Marine; generally carnivorous, and brightly coloured, affording many instances of protective resemblance. Tribe I.—TRITONIOMORPHA. Liver wholly or partially contained in the visceral mass. Anus lateral, on the right side. Usually two rows of ramified dorsal appendages. Genital duct diaulic; male and female apertures contiguous. Fam. i.—Tritoniidae. Anterior tentacles form a frontal veil; foot rather broad. Tritonia, British. Marionia. Fam. 2.—Scyllaeidae. No anterior tentacles; dorsal appendages broad and foliaceous; foot very narrow; stomach with horny plates. Scyllaea, pelagic. Fam. 3.—Phyllirhoidae. No anterior tentacles, and no dorsal appendages; body laterally compressed, transparent; pelagic. Phyllirhoe. Fam. 4.—Tethyidae. Head broad, surrounded by a funnel-shaped velum or hood; no radula; dorsal appendages foliaceous. Tethys. Melibe. Fam. 5.—Dendfonotidae. Anterior tentacles forming a scalloped frontal veil; dorsal appendages and tentacles similarly ramified. Dendronotus. Campaspe. Fam. 6.—Bornellidae. Dorsum furnished on either side with papillae, at the base of which are ramified appendages. Bornella. Fam. 7.—Lomanotidae. Body flattened, the two dorsal borders prominent and foliaceous. Lomanotus, British. Tribe 2.—DORIDOMORPHA. Body externally symmetrical; anus median, posterior, and generally dorsal, surrounded by ramified pallial appendages, constituting a secondary branchia. Liver not ramified in the integuments. Genital duct triaulic. Spicules present in the mantle. Fam. I. Polyceratidae. A more or less prominent frontal f, Median portion of the foot. pn, Pteropodial lobe of the foot. h, Heart. i, Intestine. m, Mouth. ot, Otocyst. q, Shell. r, Nephridium. a, Anus. br, Secondary branchia surrounding the anus. t, Cephalic tentacles. External to the branchia are seen ten club-like processes of the dorsal wall, these are the " cerata " which are characteristically developed in another sub-order of Opisthobranchs. Rang. sp. enlarged. (From Owen.) C, C, The wing-like lobes of the foot. d, Median fold of same. e, Copulatory organ. h, Pointed extremity of the s, Oesophagus. shell. a, Sac containing nutritive yolk. i, Anterior margin of the mb, Mantle-skirt. n, Stomach. [shell. mc; Sub-pallial chamber. o, Liver. Kn, Contractile sinus. u, Hermaphrodite gonad. of any kind ; a short evaginable pharynx, bearing paired conical buccal appendages or " cephalocones." Clione. Paraclione. Fowlerina. Fain. 7.—Halopsychidae. No branchia; two long and branched buccal appendages. Halopsyche. Tribe 3: PLEUROBRANCHOMORPHA. Two pairs of tentacles. Foot without parapodia; no pallial cavity, but always a single ctenidium situated on the right side between mantle and foot. Genital duct diaulic, without open seminal groove; male and female apertures contiguous. Visceral commissure short, tendency to concentration of all ganglia in dorsal side of oesophagus. Fain. I.—Tylodinidae. Shell external and conical; anterior tentacles form a frontal veil; ctenidium extending only over right side; a distinct osphradium. Tylodina. Fam. 2.—Umbrellidae. Shell external, conical, much flattened; anterior tentacles very small, and situated with the mouth in a notch of the foot below the head; ctenidium very large. Umbrella. Fam. 3.—Pleurobranchidae. Shell covered by mantle, or absent; a, The mouth. c, The pteropodial lobes of the foot. f, The centrally :placed hind-foot. d, 1, e, Three pairs of tentacle-like processes placed at the sides of the mouth, and developed (in all probability) from the fore-foot. o', Anus. y, Genital pore. k, Retractor muscles. o and p, The liver. u, v, w, Genitalia. veil; branchiae non-retractile. Euplocamus. Polycera, British. Thecacera, British. Aegirus, British. Plocamopherus. Palio. Crimora. Triopa, British. Triopeila. Fam. 2.—Goniodorididae. Mantle-border projecting; frontal veil reduced, and often covered by the anterior border of the mantle. Goniodoris, British. Acanthodoris, British. Idalia, British. Ancula, British. Doridunculus. Lamellidoris. Ancylodoris, the only fresh-water Nudibranch, from Lake Baikal. Fam. 3.—Heterodorididae. No branchia. Heterodoris. Fam. 4.—Dorididae. Mantle oval, covering the head and the greater part of the body; anterior tentacles, ill-developed; branchiae generally retractile. Doris, British. Hexabranchus. Chromodoris. Fam. 5•—Doridopsidae. Pharynx suctorial; no radula; branchial rosette on the dorsal surface, above the mantle-border. Doridopsis. Fam. 6.—Corambidae. Anus and branchia posterior, below the mantle-border. Corambe. Fam. 7.—Phyllidiidae. Pharynx suctorial; branchiae surrounding the body, between the mantle and foot. Phyllidia. Fryeria. The last three families constitute the sub-tribe Porostomata, characterized by the reduction of the buccal mass, which is modified into a suctorial apparatus. Tribe 3.—EOLIDOMORPHA (Cladohepatica). The whole of the liver contained in the integuments and tegumentary papillae. Genital duct diaulic; male and female apertures contiguous. The anus is antero-lateral, except in the Proctonotidae, in which it is median. Tegumentary papillae not ramified, and containing cnidosacs with nematocysts. Fam. I.-Eolididae. Dorsal papillae spindle-shaped or club-shaped. Eolis, British. Facelina, British. Tergipes, British. Gonieolis. Cuthona. Embletonia. Galvina. Calma. Hero. Fam. 2.—Glaucidae. Body furnished with three pairs of lateral lobes, bearing the tegumentary papillae; foot very narrow; pelagic. Glaucus. Fam. 3.—Hedylidae. Body elongated ; visceral mass marked off from foot posteriorly; dorsal appendages absent, or reduced to a single pair; spicules in the integument. Hedyle. Fam. 4. Pseudovermidae. Head without tentacles; body elongated; anus on right side. Pseudoverm;s. Fam. 5. Proctonotidae. Anus posterior, median; anterior tentacles, atrophied; foot broad. Janus, British. Proctonotus, British. Fam. 6.—Dotonidae. Bases of the rhinophores surrounded by a sheath; dorsal papillae tuberculated and club-shaped, in a single row on either side of the dorsum; no cnidosacs. Dote', British. Gellina. Heromorpha. Fam. 7.—Fionidae. Dorsal papillae with a membranous expansion; male and female apertures at some distance from each other; pelagic. Fiona. Fam. 8.—Pleurophyllidae. Anterior tentacles in the form of a digging shield; mantle without appendages, but respiratory papillae beneath the mantle-border. Pleurophyllidia. Fam. 9.—Dermatobranchidae. Like the last, but wholly without branchiae. Dermatobronchus. Tribe 4.—ELYSIOMORPIIA. Liver ramifies in integuments and ex-tends into dorsal papillae, but there are no cnidosacs. Genital duct always triaulic, and male and female apertures distant from each other. No mandibles, and radula uniserial. Never more than one pair of tentacles, and these are absent in Alderia and some species of Limapontia. Fam. I.—Hermaeidae. Foot narrow; dorsal papillae linear or an. fusiform, in several series. Hermaea, British. Stiliger. Alderia, British. Fam. 2.—Phyllobranchidae. Foot broad; dorsal papillae flattened and foliaceous. Phyllobranchus. Cyerce. F a m. 3. — Plakobranchidae. Body de-pressed, without dorsal papillae, but with two very large lateral ex= pansions, with dorsal plications. Plako- A B branchus. Fam. 4.—Elysiidae. Pleurophyllidia lineata (Otto), one of lateral expansions; the Eolidomorph Nudibranchs. (After tentacles large; foot Keferstein.) narrow. Ely s i•a, b, The mouth. British. Tridachia. 1, The lamelliform sub-pallial gills, Fam. 5.—Limaponiiidae. which (as in Patella) replace the No lateral expansions, typical Molluscan ctenidium. and no dorsal papillae; body planariform; anus dorsal, median and posterior. Limapontia, British. Actaeonia, British. Cenia. Order 2 (of the Euthyneura).—PuLmoxATA. Euthyneurous Gastropoda, probably derived from ancestral forms similar to the Tectibranchiate Opisthobranchia by adaptation to a terrestrial life. The ctenidium is atrophied, and the edge of the mantle-skirt is fused to the dorsal integument by concrescence, except at one point which forms the aperture of the mantle-chamber, thus converted into a nearly closed sac. Air is admitted to this sac for respiratory and hydrostatic purposes, and it thus becomes a lung. An operculum is present only in Amphibola; a contrast being thus afforded with the operculate pulmonate Streptoneura (Cyclostoma, &c.), which differ in other essential features of structure from the Pulmonata. The Pulmonata are, like the other Euthyneura, hermaphrodite, with elaborately developed copulatory organs and accessory glands. Like other Euthyneura, they have very numerous small denticles on the lingual ribbon. In aquatic Pulmonata the osphradiuei is retained. In some Pulmonata (snails) the foot is extended at right angles to the visceral hump, which rises from it in the form of a coil as in Streptoneura; in others the visceral hump is not elevated, but is extended with the foot, and the shell is small or absent (slugs). Pulmonata are widely distinguished from a small number of Streptoneura at one time associated with them on account of their mantle-chamber being converted, as in Pulmonata, into a lung, and the ctenidium or branchial plume aborted. The terrestrial Streptoneura (represented in England by the common genus Cyclostoma) A, Helix pomatia. (From Keferstein.) B, Helicophanta brevipes. (From Keferstein, after Pfeiffer.) C, Testacella haliotidea. (From Keferstein.) D, Arian ater, the great black slug. (From Keferstein.) a, Shell in A, B, C, shell-sac (closed) in D; b, orifice leading into the subpallial chamber (lung). have a twisted visceral nerve-loop, an operculum on the foot, a complex rhipidoglossate or taenio-glossate radula, and are of distinct sexes. The Pulmonata have a straight visceral nerve-loop, usually no operculum even in the embryo, and a multidenticulate radula, the teeth being equi-formal; and they are hermaphrodite. Some Pulmonata (Limnaea, &c.) live in fresh waters although breathing air. The remarkable discovery has been made that in deep lakes such Limnaei do not breathe air, but admit water to the lung-sac and live at the bottom. The lung-sac serves undoubtedly as a hydrostatic apparatus in the aquatic Pulmonata, as well as assisting respiration. The same general range of body-form is FIG.57.—Ancylus shown in Pulmonata as in the Heteropoda fluviatilis, a patelliand in the Opisthobranchia; at one extreme form aquatic Put-we have snails with coiled visceral hump, at monate. the other cylindrical or flattened slugs (see fig. 56). Limpet-like forms are also found (fig. 57, Ancylus). The foot is always simple, with its flat crawling surface extending from end to end, but in the embryo Limnaea it shows a bilobed character, which leads on to the condition characteristic of Pteropoda. The adaptation of the Pulmonata to terrestrial life has entailed might be expected, what is found to be the case in all " reversed " little modification of the internal organization. In one genus Gastropods. - (Planorbis) the plasma of the blood is coloured red by haemoglobin, The shell of the Pulmonata, though always light and delicate, is in this being the only instance of the presence of this body in the blood many cases a well-developed spiral ' house," into which the creature of Glossophorous Mollusca, though it occurs in corpuscles in the blood can withdraw itself ; and, although the foot possesses no operculum, of the bivalves Area and Solen (Lankester). yet in Helix the aperture of the shell is closed in the winter by a The generative apparatus of the snail (Helix) may serve as an complete lid, the " hybernaculum," more or less calcareous in nature, example of the hermaphrodite apparatus common to the Pulmonata which is secreted by the foot. In Clausilia a peculiar modification of and Opisthobranchia (fig. 58). From this lid exists permanently in the adult, attached by an elastic stalk $ the ovo-testis, which lies near the apex to the mouth of the shell, and known as the " clausilium." In of the visceral coil, a common herma- Limnaeus the permanent shell is preceded in the embryo by a wellphrodite duct ve proceeds, which marked shell-gland or primitive shell-sac (fig. 6o), at one time sup-receives the duct of the compact white posed to be the developing anus, but shown by Lankester to be albuminiparous gland, Ed, and then identical with the " shell-gland " discovered by him in other Molbecomes much enlarged, the additional lusca (Pisidium, Pleurobranchidium, Neritina, &c.). As in other width being due to the development of Gastropoda Anisopleura, this shell-sac may abnormally develop glandular folds, which are regarded as a plug of chitinous matter, but normally it flattens out and disforming a uterus u. Where these folds appears, whilst the cap-like rudiment of the permanent shell is shed cease the common duct splits into two out from the dome-like surface of the visceral hump, in the centre of portions, a male and a female. The which the shell-sac existed for a brief period. male duct vd becomes fleshy and In Clausilia, according to the observations of C. Gegenbaur, the muscular near its termination at the primitive shell-sac does not flatten out and disappear, but takes the genital pore, forming the penis p. form of a flattened closed sac. Within this closed sac a plate of cal- Attached to it is a diverticulum fl, in careous matter is developed, and after a time the upper wall of the which the spermatozoa which have sac disappears, and the calcareous plate continues to grow as the descended from the ovo-testis are nucleus of the permanent shell. In the slug Testacella (fig. 56, C) stored and modelled into sperm ropes the shell-plate never attains a large size, though naked. In other A or spermatophores. The female por- slugs, namely, Limax and Arion, the shell-sac remains permanently tion of the duct is more complex. Soon closed over the shell-plate, which in the latter genus consists of a after quitting the uterus it is joined by granular mass of carbonate of lime. The permanence of the primi- a long duct leading from a glandular five shell-sac in these slugs is a point of considerable interest. It is sac, the spermatheca (Rf). In this duct clear enough that the sac is of a different origin from that of Aplysia and sac the spermatophores received (described in-the section treating of Opisthobranchia), being primi- in copulation from another snail are tive instead of secondary. It seems probable that it is identical lodged. In Helix hortensis the sperma- with one of the open sacs in which each shell-plate of a Chiton is theca is simple. In other species of formed, and the series of plate-like imbrications which are placed Helix a second duct (as large in Helix behind the single shell-sac on the dorsum of the curious slug, Plectro- Reproductive Apparatus of from the spermathecal duct, and in the sacs on the back of that animal similar to those which we find in the Garden Snail tHelixhor- natural state is closely adherent to the Chiton. Whether the closed primitive shell-sac of the slugs (and tentis). wall of the uterus. This second duct with it the transient embryonic shell-gland of all other Mollusca) is has normally no spermathecal gland at precisely the same thing as the closed sac in which the calcareous r, Ovo-testis.. its termination, which is simple and pen or shell of the Cephalopod- Sepia and its allies is formed, ve, Hermaphrodite duct. blunt. But in rare cases in Helix is a further question Ed, Albumimparous gland. aspersa a second spermatheca is found which we shall con- P a, Uterine dilatation of at the end of this second duct. Tracing eider when dealing the hermaphrodite the widening female duct onwards we with the Cephalopoda. duct. now come to the openings of the It is important here d, Digitate accessory digitate accessory glands d, d, which to note that Clausilia glands on the female probably assist in the formation of the furnishes us with an duct. egg-capsule. Close to them is the re- exceptional instance ps, Calciferous gland or markable dart-sac ps, a thick-walled of the continuity of the dart-sac on the female sac, in the lumen of which a crystalline shell or secreted pro-duct. four-fluted rod or dart consisting of duct of the primitive Rf, Spermatheca or recep- carbonate of lime is found. It is sup- shell - sac with the tacle of the sperm in posed to act in some way as a stimulant adult shell. In most copulation, opening in copulation, but possibly has to do other Mollusca (Anisointo the female duct. with the calcareous covering of the pleurous Gastropods, vd, Male duct (vas de- egg-capsule. Other Pulmonata exhibit Pteropods and Con- ferens). variations of secondary importance in chifera) there is a want p, Penis. the details of this hermaphrodite ap- of such continuity; fl, Flagellum. paratus. the primitive shell-sac The nervous system of Helix is not contributes no factor favourable as an example on account of the fusion of the ganglia to the permanent shell, to form an almost uniform ring of nervous matter around the or only a very minute FIG. 59.—Nervous System of the Pond, oesophagus. The pond-snail (Limnaeus) furnishes, on the other knob - like particle Snail, Limnaeus stagnalis, as a type of the hand, a very beautiful case of distinct ganglia and connecting (Neritina and Palle- short-looped euthyneurous condition. The cords (fig. 59). The demonstration which it affords of the ex- dins). It flattens out short visceral " loop " with its three ganglia treme shortening of the Euthyneurous visceral nerve-loop is most and disappears before is lightly-shaded. instructive and valuable for comparison with and explanation of the work of forming ce, Cerebral ganglion. the condition of the nervous centres in Cephalopoda, as also of the permanent shell pe, Pedal ganglion. some Opisthobranchia. The figure (fig..59) is sufficiently described commences. And just pl, Pleural ganglion. in the letterpress attached to it; the pair of buccal ganglia joined as there is a break ab, Abdominal ganglion. by the connectives to the cerebrals are, as in most of our figures, at this stage, so (as sp, Visceral ganglion of the left side; op- omitted. Here we need only further draw attention to the osphra- observed by A. Krohn posite to it is the visceral ganglion of dium, discovered by Lacaze-Duthiers, and shown by Spengel to in Marsenia 'Echino- the right side, which gives off the long agree in its innervation with that organ in all .other Gastropoda. spire) there may be a nerve to the olfactory ganglion and On account of the shortness of the visceral loop and the proximity break at a later stage, osphradium o. of the right visceral ganglion to the oesophageal nerve-ring, the nerve the nautiloid shell In Planorbis and in Auricula (Pulmonata, to the osphradium and olfactory ganglion is very long. The position formed on the larva allied to Limnaeus) the olfactory organ is of the osphradium corresponds more or less closely with that of the being cast, and a new on the left side and receives its nerve from vanished right ctenidium, with which it is normally associated. In shell of a different form the left visceral,ganglion. (After Spengel.) Helix and Limax the osphradium has not been described, and being formed afresh on possibly its discovery might clear up the doubts which have the surface of the visceral hump. It is, then, in this sense that w' been raised as to the nature of the mantle-chamber of those may speak of primary, secondary and tertiary shells in Mollusca genera. In Planorbis, which is sinistral (as are a few other genera recognizing the fact that they may be merely phases fused by con-or exceptional varieties of various Anisopleurous Gastropods). tinuity of growth so as to form but one shell, or that in other cases instead of being dextral, the osphradium is on the left side, they may be presented to us as separate individual things, in virtue and receives its nerve from the left visceral ganglion, the of the non-development of the later phases, or in virtue of sudden whole series of unilateral organs being reversed. This is, as changes in the activity of the mantle-surface causing the shedding or disappearance of one phase of shell-formation before a later one is entered upon. The development of the aquatic Pulmonata from the egg offers considerable facilities for study, and that of Limnaeus has been elucidated by E. R. Lankester, whilst H. Rabl has with remarkable skill applied the method of sections to the study of the minute embryos of Planorbis. The chief features in the development of Limnaeus are exhibited in fig. 6o. There is not a very large amount of food-material present in the egg of this snail, and accordingly the cells resulting from division are not so unequal as in many other cases. The four cells first formed are of equal size, and then four smaller cells are formed by division of these four so as to lie at one end' of the first four (the pole corresponding to that at which the " directive corpuscles " are extruded and remain). The smaller cells now divide and spread over the four larger cells; at the same time a space—the cleavage cavity or blastocoel—forms in the centre of the mulberry-like mass. Then the large cells recommence the process of division and sink into the hollow of the sphere, leaving an elongated groove, the blastopore, on the surface. The invaginated cells (derived from the division of the four big cells) form the endoderm or arch-enteron; the outer cells are the ectoderm. The blastopore now closes along the middle part of its course, which coincides Pi' sx r ph, Pharynx (stomodaeal in- attachment to the ecto- vagmation). derm is coincident with the v, v, The ciliated band marking hindmost extremity of the out the velum. elongated blastopore of fig. ng, Cerebral nerve-ganglion. 3 C. re, Stiebel's canal (left side), tge, Mesoblastic (skeletotrophic probably an evanescent and muscular) cells invest- embryonic nephridium. ing gs, the bilobed arch- sh, The primitive shell-sac or enteron or lateral vesicles shell-gland. of inva inated endoderm, pi, The rectal peduncle or whichwill develop Into liver. pedicle of invagination; its f, The foot. in position with the future " foot." One end of the blastopore becomes nearly closed, and an ingrowth of ectoderm takes place around it to form the stomodaeum or fore-gut and mouth. The other extreme end closes, but the invaginated endoderm cells remain in continuity with this extremity of the blastopore, and form the " rectal peduncle " or " pedicle of invagination " of Lankester, although the endoderm cells retain no contact with the middle region of the now closed-up blastopore. The anal opening forms at a late period by a very short ingrowth or proctodaeum coinciding with the blind termination of the rectal peduncle (fig. 6o, pi). The body-cavity and the muscular, fibrous and vascular tissues are traced partly to two symmetrically disposed " mesoblasts," which bud off from the invaginated arch-enteron, partly to cells derived from the ectoderm, which at a very early stage is connected by long processes with the invaginated endoderm. The external form of the embryo goes through the same changes as in other Gastropods, and is not, as was held previously to Lankester's observations, exceptional. When the middle and hinder regions of the blastopore are closing in, an equatorial ridge of ciliated cells is formed, converting the embryo into a typical trochosphere. The foot now protrudes below the mouth, and the post-oral hemisphere of the trochosphere grows more rapidly then the anterior or velar area. The young foot shows a bilobed form. Within the velar area the eyes and the cephalic tentacles commence to rise up, and on the surface of the post-oral region is formed a cap-like shell and an encircling ridge, which gradually increases in prominence and becomes the freely depending mantle-skirt. The outline of the velararea becomes strongly emarginated and can be traced through the more stature embryos to the cephalic lobes or labial processes of the adult Limnaeus (fig. 6i). The increase of the visceral dome, its spiral twisting, and the gradual closure of the space overhung by the mantle-skirt so as to f convert it into a lung-sac with a small contractile aperture, belong to stages in the development later than any represented in our figures. We may now revert briefly to the internal organization at a period when the trochosphere is beginning to show a prominent foot growing out from the area where the mid-region of the elongated blastopore was situated, and having therefore at one end of it the mouth and at the other the anus. Fig. 6o represents such an embryo under slight compression as seen by transmitted light. The ciliated band of the left side of the velar area is indicated by a line extending from v to v; the foot f is seen between the pharynx ph and the pedicle of invagination pi. The mass of the arch-enteron or invaginated endodermal sac has taken on a bilobed form, and its cells are swollen (gs and tge). This bilobed sac becomes entirely the liver in the adult; the intestine and stomach are formed from the pedicle of invagination, whilst the pharynx, oesophagus and crop form from the stomodaeal invagination ph. To the right (in the figure) of the rectal peduncle is seen the deeply invaginated shell-gland ss, with a secretion sh protruding from it. The shell-gland is destined in Limnaeus to become very rapidly stretched out, and to disappear. Farther up, within the velar area, the rudiments of the cerebral nerve-ganglion ng are seen separating from the ectoderm. A remark-able cord of cells having a position just below the integument occurs on each side of the head. In the figure the cord of the left side is seen, marked re. This paired organ consists of a string of cells which are perforated by a duct opening to the exterior and ending internally in a flame-cell. Such cannulated cells are characteristic of the nephridia of many worms, and the organs thus formed in the embryo Limnaeus are embryonic nephridia. The most important fact about them is that they disappear, and are in no way connected with the typical nephridium of the adult. In reference to their first observer they were formerly called " Stiebel's canals." Other Pulmonata possess, when embryos, Stiebel's canals in a more fully developed state, for instance, the common slug Limax. Here too they disappear during embryonic life. Similar larval nephridia occur in other Gastropoda. In the marine Streptoneura they are ectodermic projections which ultimately fall off; in the Opisthobranchs they are closed pouches; in Paludina and Bithynia they are canals as in Pulmonata. Marine Pulmonata.—Whilst the Pulmonata are essentially a terrestrial and fresh-water group, there is one genus of slug-like Pulmonates which frequent the sea-coast (Oncidium, fig. 62). Karl Semper has shown that these slugs have, in addition to the usual pair of cephalic eyes, a number of eyes developed upon the dorsal integument. These dorsal eyes are very perfect in elaboration, possessing lens, retinal nerve-end cells, retinal pigment and optic nerve. Curiously enough, however, they differ from the cephalic Molluscan eye in the fact that, as in the vertebrate eye, the filaments of the optic nerve penetrate the retina, and are connected with the re organs. Endodonta. Punctum. Sphyradium. Laoma. Pyramidula. Fain. 9.—Orthalicidae. Shell external, ovoid, the last whorl swollen, aperture oval with a simple border; radular teeth in oblique rows. Orthalicus. Fain. Lo.—Bulimulidae. Jaw formed of folds imbricated externally and meeting at an acute angle near the base. Bulimulus. Peltella. Amphibulimus. Fain. It.—Cylindrellidae. Shell turriculated, with numerous whorls, the last more or less detached. Cylindrella. Fam. 12.—Pupidae. Shell external, with elongated spire and numerous whorls, aperture generally narrow; male. genital duct without multifid vesicles. Pupa, British. Eucalodium. Vertigo, British. Bnliminus, British. Clausilia, British. Balea. Zospeum. Megaspira. Strophia. Anostoma. Fain. 13.—Stenogyridae. Shell elongated, with a more or less obtuse summit; aperture with a simple border. Achatina. Stenogyra. Ferussacia, British. Cionella. Caecilianelia. Azeca. Opeas. Fam. 14.—Helicteridae. Shell bulimoid, dextral or sinistral ; radular teeth, expanded at their extremities and multicuspidate. Helicter. Tornatellina Tribe 2.—AGNATHA. No jaws; teeth narrow and pointed; carnivorous. Fam. i.—Oleacinidae. Shell oval, elongated, with narrow aperture; neck very long; labial palps prominent. Oleacina (Glandina). Streptostyla. Fam. 2.—Testacellidae. Shell globular or auriform, external or 4artly covered by the mantle. Streptaxis. Gibbulina. Aerope. ,~hytida. Daudebardia. Testacella. Chlamydophorus. Schizo-glossa. Fam. 3.—Rathouisiidae. No shell, a carinated mantle covering the whole body; male and female apertures distant, the female near the anus. Rathouisia. Atopos. Tribe 3.—ELASMOGNATHA. Jaw with a well-developed dorsal appendage. - Fam. 1.—Succineidae. Anterior tentacles much reduced ; male and female apertures contiguous but distinct; shell thin, spiral, with short°spire. Succinea, British. Homalonyx. Hya- limax. Neohyalimax. Fam. 2. Janellidae. Limaciform, with internal 'rounded shell; mantle very small and triangular; pulmonary chamber with tracheae; no anterior tentacles. Janella. Aneitella. Aneitea. Triboniophorus. Tribe 4.—DITREMATA. Male and female apertures distant. Fam. 1.—Vaginulidae.. No shell; limaciform; terrestrial; female aperture on right side in middle of body; anus posterior. Veginula. Fam. 2.—Oncidiidae. No shell; limaciform; littoral; female aperture posterior, near anus; a reduced pulmonary cavity with a distinct aperture. Oncidium. Oncidiella, British. Peronia.
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