of some days-possibly weeks-the ova of the second Aplysia | digestive tract, but we find very numerous hepatic diverticula on a commence to descend the hermaphrodite duct; they become en- 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 FIG. 42.-Follicles of the hermaphrodite gonads of Euthyneurous Gastropods. A, of Helix; B, of Eolis; a, ova; b, developing spermatozoa; c, common efferent duct. ce closed in a viscid secretion at the point where the albuminiparous ab sp -pl shell. FIG. 44.-Bulla vexillum (Chemnitz), as seen crawling. 6, oral the epipodia; b, b', cephalic tentacles. (From Owen.) hood (compare with Tethys, fig. 46, B), possibly a continuation of 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 communicating nephridium or renal organ in all Opisthobranchs. The typical character is retained by the heart, pericardium, and An interesting example of this is furnished by the fish-like transparent Phyllirhoë (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 nervesupply 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. In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct. The cuthyneurous 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 development 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 The development of many Opisthobranchia has been examineddoes not give the small pair of buccale.g. Aplysia, Pleurobranchidium, Elysia, Polycera, Doris, Tergipes. ganglia which are, as in all glossophorous All pass through trochosphere and veliger stages, and in all a nautiFIG. 43-Nervous Molluscs, present upon the nerves passing loid or boat-like shell is developed, preceded by a well-marked system of Aplysia, as from the cerebral region to the odontophore. "shell-gland" (see fig. 36). The transition from the free-swimming a type of the long- For a comparison of various Opistho- veliger larva with its nautiloid shell looped Euthyneurous branchs. Aplysia will be found to present (fig. 36) to the adult form has not condition. The un- a convenient starting-point. It is one of been properly observed, and many twisted visceral loop the more typical Opisthobranchs, that is interesting points as to the true nature is lightly shaded. to say, it belongs to the section Tecti- of folds (whether parapodia or mantle (After Spengel.) branchia, but other members of the sub- or velum) have yet to be cleared up ce, Cerebral ganglion. order, namely, Bulla and Actaeon (figs. 44 by a knowledge of such development pl, Pleural ganglion. and 45), are less abnormal than Aplysia in forms like Tethys, Doris, Phyllidia, pe, Pedal ganglion. in regard to their shells and the form of the &c. As in other Molluscan groups, ab. sp, Abdominal gan- visceral hump. They have naked spirally we find even in closely-allied genera glion which re- twisted shells which may be concealed from (for instance, in Aplysia and Pleuro- shell; b, oral hood; d, foot; FIG. 45. Actaeon. h. presents also the view in the living animal by the expansion branchidium, and other genera), the supra-intestinal and reflection of the parapodia, but are not greatest differences as to the amount f, operculum. ganglion of Strep- enclosed by the mantle, whilst Actaeon is of food-material by which the egg-shell is encumbered. Some toneura and gives remarkable for possessing an operculum form their diblastula by emboly, others by epiboly; and in the off the nerve to like that of so many Streptoneura. later history of the further development of the enclosed cells (archthe osphradium The great development of the parapodia enteron) very marked variations occur in closely-allied forms, due (olfactory organ) seen in Aplysia is usual in Tectibranchiate to the influence of a greater or less abundance of food-materia! o, and another to Opisthobranchs. The whole surface of the mixed with the protoplasm of the egg. an unlettered so- body becomes greatly modified in those called "genital" Nudibranchiate forms which have lost, not ganglion. The only the shell, but also the ctenidium. Many buccal nerves of these have peculiar processes developed and ganglia are on the dorsal surface (fig. 46, A, B), or omitted. 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 Sub-order 1.-TECTIBRANCHIA. Opisthobranchs provided in the adult state with a shell and a mantle, except Runcina, Pleurobranchaea, Cymbuliidae, and some Aplysiomorpha. There is a ctenidium, except in some Thecosomata and Gymnosomata, and an osphradium. Tribe 1.-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; ctenidium, except in Lophocercidae, of folded type. With the exception of the Aplustridae, Lophocercidae and Thecosomata, the head is devoid of tentacles, and its dorsal surface forms a digging long, except in Runcina, Lobiger and Thecosomata. Herma- 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. b, Cephalic tentacles. c, Neck. d, Genital pore. C, Doris (Actinocyclus) surface. m, Mouth. b, Margin of the head. sp, The mantle-like epipodium. D, E, Dorsal and lateral view of Elysia (Actaeon) viridis. epipodial outgrowths. (After Keferstein.) disk or shield. The edges of the foot form parapodia, often transformed into fins. Posteriorly the mantle forms a large pallial lobe FIG. 47.-Enteric Canal of Eolis papillosa. (From Gegenbaur, after Alder and Hancock.) ph, Pharynx. m, Midgut, with its hepatic appendages h, all of which are not figured. e, Hind gut. an, Anus. under the pallial aperture. Stomach generally provided with chitinous or calcified masticatory plates. Visceral commissure fairly Fam. 5.-Bullidae. Margins of foot well developed; eyes super- Fam. 6.-Aceratidae. Cephalic shield continuous with neck; Fam. 7.-A plustridae. Foot very broad; cephalic shield with Fam. 8.-Philinidae. Cephalic shield broad, thick and simple; Fam. 10.-Gastropteridae. Cephalic shield pointed behind; shell Fam. 11-Runcinidae. Cephalic shield continuous with dorsal Fam. 12.-Lophocercidae. Shell external, globular or ovoid; foot from ventral surface; genitalistun gniteroge The next three families form the 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 archus. 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. Fam. 1.-Aplysiidae. Shell partly or wholly internal, or absent; foot long, with well-developed ventral surface. Aplysia. Dolabella. Dolabrifer. Aplysiella. Phylla plysia. NotThe 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. 2.-Pneumonodermatidae. Pharynx evaginable, with suckers. Pneumonoderma. Dexiobranchaea. Spongiobranchaea. Schizobrachium. Fam. 3-Clionopsidae. No buccal appendages or suckers; a very long evaginable proboscis; a quadriradiate terminal bran chia. Clionopsis. Fam. 4.-Notobranchaeidae. Posterior branchia triradiate. Nolobranchaca. 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 anterior tentacles form a frontal veil; mantle contains spicules. Pleurobranchus. Berthella. 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 1.-TRITONIOMORPHA. Liver wholly or partially contained in the visceral mass. Anus lateral, on the right side. Úsually two rows of ramified dorsal appendages. Genital duct diaulic; male and female apertures contiguous. Fam. 1.-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. Phyllirhoë. Fam. 4.-Tethyidae. Head broad, surrounded by a funnel-shaped velum or hood; no radula; dorsal appendages foliaceous. Tethys. Melibe. Fam. 5.-Dendronotidae. 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:-Lomanolidae. 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 Fam. 1.-Polyceratidae. A more or less prominent frontal in the mantle. of any kind; a short evaginable pharynx, bearing paired conical buccal appendages or cephalocones.' Clione. Paraclione. Fowlerina. Fam. 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. Fam. 1.-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; c, The pteropodial lobes of the foot. The centrally-placed hind-foot. d, l, 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. and p, The liver u, v, w, Genitalia. veil; branchiae non-retractile. Euplocamus. Polycera, British. Thecacera, British. Aegirus, British. Plocamopherus. Palio. Crimora. Triopa, British. Triopella. Fam. 2.-Goniodorididae. Mantle-border projecting; frontal veil reduced, and often covered by the anterior border of the Fam. 1.-Eolididae. Dorsal papillae spindle-shaped or clubcshaped. Eolis, British. Facelina, British. Tergipes, British.namas Visa Gonicolis. Cuthona. Embletonia. Galvina, Calma. Hero. alamat adT Fam. 2.-Glaucidae. Body furnished with three pairs of lateral wood golago a im lobes, bearing the tegumentary papillae; foot very narrow; all pelagic. Glaucus. Fam. 3.-Hedylidae. Body elongated; visceral mass marked min off from foot posteriorly; dorsal appendages absent, or reduced Baxipoja Lod to a single pair; spicules in the integument. Hedyle. Delongated; anus on right side. Pseudovermis. ha Fam. 6.-Dotonidae. Bases of the rhinophores surrounded by of Ca sheath; dorsal papillae tuberculated and club-shaped, in au yuver single row on either side of the dorsum; no cnidosacs. Doto, tom British. Gellina. Heromorpha.. Fam. 7.-Fionidae. Dorsal papillae with a membranous ex pansion; male and female apertures at some distance from toda each other; pelagic. Fiona. Fam. 8.-Pleurophyllidae. Anterior tentacles in the form of anal digging shield; mantle without appendages, but respiratory papillae beneath the mantle-border. Pleurophyllidia. Fam. 9.-Dermatobranchidae. Like the last, but wholly withouts branchiae. Dermatobranchus. Tribe 4.-ELYSIOMORPHA. 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 musco b, The mouth. 1, The lamelliform sub-pallial gills, Fam. 5.-Lima pontiidae. The adaptation of the Pulmonata to terrestrial life has entailed | little modification of the internal organization. In one genus (Planorbis) the plasma of the blood is coloured red by haemoglobin, this being the only instance of the presence of this body in the blood of Glossophorous Mollusca, though it occurs in corpuscles in the blood of the bivalves Arca and Solen (Lankester). EL The generative apparatus of the snail (Helix) may serve as an example of the hermaphrodite apparatus common to the Pulmonata and Opisthobranchia (fig. 58). From the ovo-testis, which lies near the apex of the visceral coil, a common hermaphrodite duct ve proceeds, which receives the duct of the compact white albuminiparous gland, Ed, and then becomes much enlarged, the additional width being due to the development of glandular folds, which are regarded as forming a uterus u. Where these folds cease the common duct splits into two portions, a male and a female. The male duct vd becomes fleshy and muscular near its termination at the genital pore, forming the penis p. Attached to it is a diverticulum fl, in which the spermatozoa which have descended from the ovo-testis are stored and modelled into sperm ropes or spermatophores. The female portion of the duct is more complex. Soon after quitting the uterus it is joined by a long duct leading from a glandular sac, the spermatheca (Rf). In this duct and sac the spermatophores received in copulation from another snail are lodged. In Helix hortensis the spermatheca is simple. In other species of Helix a second duct (as large in Helix aspersa as the chief one) is given off from the spermathecal duct, and in the natural state is closely adherent to the wall of the uterus. This second duct has normally no spermathecal gland at 2, Ovo-testis. its termination, which is simple and ve, Hermaphrodite duct. blunt. But in rare cases in Helix Ed, Albuminiparous gland. aspersa a second spermatheca is found Uterine dilatation of at the end of this second duct. Tracing the hermaphrodite the widening female duct onwards we now come to the openings of the d, Digitate accessory digitate accessory glands d, d, which glands on the female probably assist in the formation of the egg-capsule. Close to them is the remarkable dart-sac ps, a thick-walled sac, in the lumen of which a crystalline four-fluted rod or dart consisting of carbonate of lime is found. It is supposed to act in some way as a stimulant in copulation, but possibly has to do with the calcareous covering of the egg-capsule. Other Pulmonata exhibit variations of secondary importance in the details of this hermaphrodite apparatus. FIG. 58.-Hermaphrodite Reproductive Apparatus of the Garden Snail (Helix hortensis). 16, duct. duct. ps, Calciferous gland or dart-sac on the female duct. Rf, Spermatheca or receptacle of the sperm in copulation, opening into the female duct. vd, Male duct (vas deferens). Penis. f, Flagellum. The nervous system of Helix is not favourable, as, an example on account of the fusion of the ganglia to form an almost uniform ring of nervous matter around the ocsophagus. The pond-snail (Limnaeus) furnishes, on the other hand, a very beautiful case of distinct ganglia and connecting cords (fig. 59). The demonstration which it affords of the extreme shortening of the Euthyneurous visceral nerve-loop is most instructive and valuable for comparison with and explanation of the condition of the nervous centres in Cephalopoda, as also of some Opisthobranchia. The figure (fig. 59) is sufficiently described in the letterpress attached to it; the pair of buccal ganglia joined by the connectives to the cerebrals are, as in most of our figures, omitted. Here we need only further draw attention to the osphradium, discovered by Lacaze-Duthiers, and shown by Spengel to agree in its innervation with that organ in all other Gastropoda. On account of the shortness of the visceral loop and the proximity of the right visceral ganglion to the oesophageal nerve-ring, the nerve to the osphradium and olfactory ganglion is very long. The position of the osphradium corresponds more or less closely with that of the vanished right ctenidium, with which it is normally associated. In Helix and Limax the osphradium has not been described, and possibly its discovery might clear up the doubts which have been raised as to the nature of the mantle-chamber of those genera. In Planorbis, which is sinistral (as are a few other genera or exceptional varieties of various Anisopleurous Gastropods). instead of being dextral, the osphradium is on the left side, and receives its nerve from the left visceral ganglion, the whole series of unilateral organs being reversed. This is, as might be expected, what is found to be the case in all "reversed" Gastropods. The shell of the Pulmonata, though always light and delicate, is in many cases a well-developed spiral" house," into which the creature can withdraw itself; and, although the foot possesses no operculum, yet in Helix the aperture of the shell is closed in the winter by a complete lid, the "hybernaculum," more or less calcareous in nature, which is secreted by the foot. In Clausilia a peculiar modification of this lid exists permanently in the adult, attached by an elastic stalk to the mouth of the shell, and known as the "clausilium." In Limnaeus the permanent shell is preceded in the embryo by a wellmarked shell-gland or primitive shell-sac (fig. 60), at one time supposed to be the developing anus, but shown by Lankester to be identical with the " shell-gland" discovered by him in other Mollusca (Pisidium, Pleurobranchidium, Neritina, &c.). As in other Gastropoda Anisopleura, this shell-sac may abnormally develop a plug of chitinous matter, but normally it flattens out and disappears, whilst the cap-like rudiment of the permanent shell is shed out from the dome-like surface of the visceral hump, in the centre of which the shell-sac existed for a brief period. In Clausila, according to the observations of C. Gegenbaur, the primitive shell-sac does not flatten out and disappear, but takes the form of a flattened closed sac. Within this closed sac a plate of calcareous matter is developed, and after a time the upper wall of the sac disappears, and the calcareous plate continues to grow as the nucleus of the permanent shell. In the slug Testacella (fig. 56, C) the shell-plate never attains a large size, though naked. In other slugs, namely, Limax and Arion, the shell-sac remains permanently closed over the shell-plate, which in the latter genus consists of a granular mass of carbonate of lime. The permanence of the primitive shell-sac in these slugs is a point of considerable interest. It is clear enough that the sac is of a different origin from that of Aplysia (described in the section treating of Opisthobranchia), being primitive instead of secondary. It seems probable that it is identical with one of the open sacs in which each shell-plate of a Chiton is formed, and the series of plate-like imbrications which are placed behind the single shell-sac on the dorsum of the curious slug, Plectrophorus, suggest the possibility of the formation of a series of shellsacs on the back of that animal similar to those which we find in Chiton. Whether the closed primitive shell-sac of the slugs (and with it the transient embryonic shell-gland of all other Mollusca) is precisely the same thing as the closed sac in which the calcareous pen or shell of the Cephalopod Sepia and its allies is formed, is a further question which we shall consider when dealing with the Cephalopoda. It is important here to note that Clausilia furnishes us with an exceptional instance of the continuity of the shell or secreted product of the primitive shell-sac with the adult shell. In most other Mollusca (Anisopleurous Gastropods, Pteropods and Conchifera) there is a want of sp -pl ab ce such continuity, the primitive shell-sac contributes no factor to the permanent shell, or only a very minute FIG. 59.-Nervous System of the Pondknob-like particle Snail, Limnaeus stagnalis, as a type of the (Neriting and Palu- short-looped euthyneurous condition. The ding). It flattens out short visceral "loop" with its three ganglia and disappears before is lightly-shaded. the work of forming ce, Cerebral ganglion. the permanent shell pe, Pedal ganglion. commences. And just pl, Pleural ganglion. as there is a break ab, Abdominal ganglion. at this stage, so (as sp, observed by A. Krohn in Marsenia - Echinospira) there may be a break at a later stage, the nautiloid shell In Planorbis and in Auricula (Pulmonata, formed on the larva allied to Lamnaeus) the olfactory organ is being cast, and a new on the left side and receives its nerve from shell of a different form the left visceral ganglion. (After Spengel.) being formed afresh on the surface of the visceral hump. It is, then, in this sense that we may speak of primary, secondary and tertiary shells in Mollusca, recognizing the fact that they may be merely phases fused by continuity of growth so as to form but one shell, or that in other cases they may be presented to us as separate individual things, in virtue of the non-development of the later phases, or in virtue of sudden changes in the activity of the mantle-surface causing the shedding Visceral ganglion of the left side; opposite to it is the visceral ganglion of the right side, which gives off the long nerve to the olfactory ganglion and osphradium o. |