122 



MOLLUSCA 



Aplysia kybrida 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- 

 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 Fl<J . 67 *_ C entral nervous system of Fiona 



Molluscan nervous systems ( ne f tne Ceratonotous Opistho- 



.,. i- i i_ branchs), showing a tendency to fusion 



given in this article show, of the great ganglia. A, cerebral, pleu- 



very frequent. Our figure !j an ^ Yion e ^c' 8 Tuccai U gar?giLn'- */>" 



of the nervous System of ccsophageal ganglion connected with the 



Ai>lvifl rlnpq not rrivp trip buccal; a, nerve to superior cephalic 



Aplysia Q tentacle ; 6, nerves to inferior cephalic 



small pair of buccal ganglia tentacles; c, nerve to generative organs; 



* . i . ,, 7^. d^ pedal nerve ; e, pedal commissure ; e', 



Which are, as in all (jrlOSSO- visceral loop or commissure (?). (From 



phoTOUS Molluscs, present Gegenbaur, after Bergh.) 



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, 



Fin. 68. Young veliger larva of an Opisthobranch (Pleuro-branchiilium). m, 

 mouth ; v, ciliated band marking otf the velum ; ng, cerebral ganglion de- 

 veloping from epiblast, within the velar area ; at, 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- 

 culum 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-Palliata ; 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 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, 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 short 

 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 papillje, 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 C'nelentera. 

 The existence of these thread-cells is sufficiently remark- 

 able, seeing that the Non-Palliate Opisthobranchs resemble 

 in general form and habit the Planarian w r orms, 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" (seefigs. 

 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, Ac. 



