120 



MOLLUSCA 



63 projecting from the brancliial sub-pallial space. 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. 

 56, C, but may be better understood 

 by a glance at the figures of the allied 

 genus Umbrella (figs. 54, 55), in which 

 the margin of the mantle-skirt coin- 

 cides, 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 cen- 

 tral area s where the naked shell is 

 exposed. This enclosure of the shell 

 is a permanent development of the 

 arrangement seen in many Strepto- 

 neura (e.g., Pyrula, Ovulum, see figs. 

 38 and 41), 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 over the shell, though it is withdrawn 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 F ~ 61 ._ Po ,,, CCT . a 

 are irritated. From the fact that one of the pygobranciu- 

 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 con- 

 founded 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 tran- 

 sitory "shell-sac" or "shell-gland" of Molluscan embryos 



ate Opisthobranchs (dor- 

 sal view), a, anus ; 6r, 

 the ctenidium peculiarly 

 modified so as to encircle 

 theanus; t, cephalic ten- 

 tacles. External to the 

 branchial ctenidium are 

 seen ten club-like pro- 

 cesses of the dorsal wall, 

 these are the "cerata" 

 which are characteristic- 

 ally developed in another 

 sub - order of Opistho- 

 branchs, the Ceratonota 

 (see fig. 62, A). (From 

 Gegenbaur, after Alder 

 and Hancock.) 



FIG. 62. 



A. Rolls papillosa (Lin.), dorsal view, a, 1>, posterior and anterior cephalic 



tentacles; c, the dorsal "cerata" (hence Ceratobranchia). 



B. Tethys leporina, dorsal view, a, the cephalic hood ; 6, cephalic tentacles ; 



c, neck ; rf, genital pore ; e t anus ; /, large cerata ; g, smaller cerata ; 

 ft, margin of the foot. 



C. Doris (Actinocydus) tubercitlatus (Cuv.), seen from the pedal surface. TO, 



inouth ; b, margin of the head ; /, sole of the foot ; sp, the mantle-like 

 epipodium. 



D. E. Dorsal and lateral view of Elysia (Actteon) viridis. ep, epipodial out- 



growths. (After Keferstein.) 



(see figs. 7, 68, and 72***). Aplysia, like other Mollusca, 



develops a primitive shell-sac in its trochosphere stage of 

 development (fig. 68), which disappears and is succeeded 

 by a nautiloid shell (fig. 60). This forms the nucleus of 

 the adult shell, 

 and, as the ani- 

 mal grows, be- 

 comes enclosed 

 by a reflexion of 

 the mantle-skirt. 

 In reference to 

 the possible com- 

 parison of the 

 enclosed shell of 

 Aplysia and its 

 allies with those 

 of some Slugs and 

 of Cuttle-fishes, 

 the reader is re- 

 ferred to the para- 

 graphs dealing 

 especially with 

 those Molluscs. 

 When the shell 

 of an Aplysia 



enclosed in its 



mantle is pushed 

 well to the left, 

 the sub-pallial 



io. 63. Aplysia leporina (mmelus, Cuv.), with epipodia 

 and mantle reflected away from the mid-line, a, an- 

 terior cephalic tentacle; 6, posterior do.; between a 

 and 6, the eyes ; c, right epipodium ; d, left epipo- 

 dium ; 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 



space IS Hilly ex- plume) ; ft, the mantle-skirt tightly spread over the 

 ho: 



posed as in fig. 

 63, and the vari- 



ous apertures OI tory organ of Spe'ngel) ;n, outline of "part of the" renal 

 the body are Seen. sac (nephridium) below the surface ; o, external aper 



rny shell and pushed with it towards the left side ; 



the spermatic groove ; t, the common genital pore 

 (male and female) ; I, orifice of the grape-shaped (sup- 

 posed poisonous) gland ; m, the osphradium (olfac- 



Posteriorly we 



ture of the nephridium ; p, anus. (Original.) 



have the anus, in front of this the lobate gill-plume, be- 

 tween the two (hence corresponding in position to that of 

 the Azygobranchia) 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 

 Spengel, yellowish in colour, in 

 the typical position, and overly- 

 ing an olfactory ganglion with 

 typical nerve-connexion (see fig. 

 20). 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 cu- 

 taneous glands which, in the large 

 Aplysia camelus (not in other 

 species), form the purple secretion 

 which was known to the ancients. 

 In front of the osphradium is the 

 single genital pore, the aperture 



. -. IT, Fio. 64. Gonad, ami accessory 



ot the common or hermaphrodite glands and ducts of Aplysia. 

 duct From this point there ^Snl^g^ 

 stretches forward to the right 



this point there 



f, vesicula seminalis ; , open- 

 side of the head a groove the into f tiie e heni"irmT di^ 



spermatic groove down which ', hermaphrodite duct (uterine 



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 (not 



shown in any of the diagrams) is the opening of a large 



mucous-forming gland very often found in the Molluscan 



foot. 



portion) ; ft, vaginal portion of 

 the uterine duct ; c, spenna- 

 theea; ri, its duct; a, genital 

 pore. (Original.) 



