120 



MOLLUSCA 



63 projecting from the branchial 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 over the shell, though it is withdrawn 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 

 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 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 



[G. 61. Pnlycrra cristatd, 

 one of the Pygobranchi- 

 ate Opisthobranchs (dor- 

 sal view), o, anus ; br, 

 the ctenidium peculiarly 

 modified so as to encircle 

 the anus; *, cephalic ten- 

 tacles. External to the 

 branchial ctenidium are 

 seen ten club-like pro- 

 cesses of the dorsal wall, 

 these are the "cerata" 



sub - order of Opistho- 

 branchs, the Ceratonota 

 (see fig. 62, A). (From 

 Gegenbaur, after Alder 

 and Hancock.) 



Fin. 62. 



A. Eolis papillosa (Lin.), dorsal view. (/, b, posterior and :nil. ii,,r cephalic 



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



B. Ti'tli;/* !>/>>'ii/<i, dorsal virw. a, the cephalic hood ; b, cephalic tentacles ; 



c, neck ; </, genital pore ; e, anus ; /, large cerata ; p, smaller cerata ; 

 ft, margin of the fi>ot. 



C. Z>on'.s (At'lhi/ifii'lii.*) tubercutohu (Ouv.), seen from the pedal surface, m, 



mouth ; '', margin of the head ; / sole of the fout ; .y, tin. 1 mantle-like 



r|H J" "lilUIL. 



D. E. Dorsal and lateral view of Elysia (Jc/jron) I'iridis. ep, epipodial out- 



growths. (After KeferstiMii ) 



(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 FIG. 63. Aplysi(thporina(camehts, Cuv.), with epipodia 

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

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

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

 dium ; e, hinder part of visceral hump ; ff, posterior 

 extremity of the foot ; fa, anterior part of the foot 

 f ,, underlying the head; 17, the ctenidinm (branchial 



space IS ttUly ex- plume) ; A, 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 



mantle is pushed 

 well to the left, 

 the sub-pallial 



POSed as in fi cf 



^' , 



OO, and the van- (male and female) ; /, orifice of the grape-shaped (sup- 

 P us e d P oiso nous) gland ; ?n, the osphradium (olfac- 



OUS apertures of lory organ of Spengel) ; w, outline ofpart of the" renal 



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

 i ture of the uephridium ; p. anus. (Original.) 



Posteriorly we 



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 J~ 

 be poisonous. On the under side h 

 of the free edge of the mantle are 

 situated the numerous small cu- 

 taneous glands which, in the large 

 A]i! i/sin ciiu/eh/s (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 



,., Fin. 64. Gonad, and accessory 



of the common or hermaphrodite ^lamis and duets of Aplysia. 



duct. From this point there \> ovo-testis ; A, hermaphrodite 



stretches forward to the right 

 side of the head a groove the 



duct; (7, albuniiniparous gland ; 

 /, vesicula seminalis ; A-, open- 

 ingof the alliuminiparous uland 

 into the hermaphrodite duct; 



Spermatic groove down Which <, hermaphrodite diiH (ill, , ine 



. . T portion); 6. vaginal port 



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. 



, 



the uterine duct; c, sperni.'L- 

 tln-ra; <l, its duct; a, gmitul 

 pi 'iv. (original.) 



