OPISTHOBRANCHIA.] MOLLUSCA 659 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. 20) shows the nerve connecting this abdomino- visceral ganglion with the olfactory ganglion of Spen- A gel. It is also seen to be connected with a more re mote ganglion the genital, Such special irregularities in the development of gan glia upon the visceral loop, and on one or more of the main nerves connected with it, are, as the figures Of Fia. 67. Central nervous system of Fiona (one of the Ceratonotous Opistho branchs), showing a tendency to fusion of the great ganglia. A, cerebral, pleu- ral, and visceral ganglia united ; II, pe dal ganglion ; C, buccal ganglion ; D, wsophageal ganglion connected with the buccal ; a, nerve to superior cephalic tentacle ; 6, nerves to inferior cephalic tentacles ; c, nerve to generative organs; d, pedal nerve ; e, pedal commissure ; e , visceral loop or commissure (?). (From Gegenbaur, after Bergh.) Molluscan nervous systems given in this article show, very frequent. Our figure of the nervous system of Aplysia does not give the small pair of buccal ganglia which are, as in all Glosso- phorous 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 Palliata, but other members of the Palliata, namely, Bulla and Tornatella (figs. 52 and 53), 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 reflexion of the epipodia, FIG. 68. Young veliger larva of an Opisthobranch (Pleuro-branchiclium). m, mouth ; r, ci! : ".ted band marking off the velum ; ng, cerebral ganglion de veloping from epiblast, within the velar area ; of, otocyst also developing from epiblast ; /, foot ; i, intestine ; ry, residual nutritive yelk ; shs, primi tive shell-sac or shell-gland. (From Lankester.) but are not enclosed by the mantle, whilst Tornatella is remarkable amongst all Euthyneura for possessing an oper- ctilum like that of so many Streptoneura. The great development of the epipodia seen in Aplysia is usual in Palliate Opisthobranchs ; it occurs also in Elysia (fig. 62, D) among Non-Falliata ; in Doris it seems prob able that the mantle-like fold overhanging the foot is to be interpreted as epipodium, the mantle-skirt being alto gether absent, as shown by the naked position of the gills and anus on the dorsal surface (figs. 61 and 62, C). The whole surface of the body becomes greatly modified in those Non-Palliate forms which have lost, not only the mantle-skirt and the shell, but also the ctenidium. Many of these (Ceratonota) have peculiar processes developed on the dorsal surface (fig. 62, A, B), or retain purely negative characters (fig. 62, 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. 66). These diverticula extend usually one into each of the dorsal papilke 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 ceeca. The typical character is retained by the heart, peri cardium, and the communicating nephridium or renal organ in all Opisthobranchs. An interesting example of this is furnished by the fish-like transparent Phyllirhoe (fig. 58), 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 communi cation. With reference to the existence of pores placing the vascular system in open communication with the surrounding water, see the paragraph as to Mollusca gener ally. In a form closely allied to Aplysia (Pleurobranchus) such a pore leading outwards from the branchial vein has been precisely described by Lacaze Duthiers. No such pore has been detected in Aplysia. In many of the Non-Palliate Opisthobranchs the nervous system presents a concentra tion of the ganglia (fig. 67), 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. 22), and the visceral loop is astonishingly shcrt and insignificant (fig. 67, e ). That the parts are rightly thus identified is probable from Spengel s observation of the os- phradium and its nerve-supply in these forms ; the nerve to that organ, which is placed somewhat anteriorly on the dor sal surface being given off from the hinder part (visceral) of the right compound ganglion the fellow to that marked A in fig. 67. The Ceratonotous Opisthobranchs, amongst other specialities of structure, are stated to 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. This requires confirmation. Some amongst them (Tergipes, Eolis) are also remarkable for possessing peculiarly modified epidermic cells placed in sacs at the apices of these same papillae, which resemble the "thread- cells " of the Planarian Flatworms and of the Ccelentera. The existence of these thread-cells is sufficiently remark able, seeing that the Non-Palliate Opisthobranchs resemble in general form and habit the Planarian worms, many of which also possess thread-cells. But it is not conceivable that theirpresence is an indication of genetic affinity between the two groups, rather they are instances of homoplasy. The development of many Opisthobranchia has been examined e.g., Aplysia, Pleurobranchidium, Elysia, Poly- cera, 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 figs. 60 and 68). The transition from the free-swimming veliger larva with its nautiloid shell (fig. 60) to the adult form has not been properly observed, and many interesting points as to the true nature of folds (whether epipodia or mantle or velum) have yet to be cleared up by a knowledge of such
development in forms like Tethys, Doris, Phyllidia, itc.