OPISTHOBRANCHIA..] MOLLUSCA 657 G3 projecting from the branchial sub-pallia! 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Ovulum, see figs. 38 and 41), where the border of the mantle can be, and usually is, drawn over the shell, though it is withdrawn (as it cannot be in Aplysia) when they F ~ ei __ rolycera cris(atai are irritated. From the fact that one of the Pygobrancin- A i , i-r r ate Opisthobranchs(dor- Aplysia commences its hie as a tree- sa i view). Wj anus; br, swimming Veliger with a nautiloid the cteudium peculiarly ... i i 11 modified ao M to encircle shell not enclosed in anyway by the theanus; *, cephalic ten- nf f!ir> mintlp it i flpir tVmt tacles. External to the 01 me mantle, it is clear tnat the enclosure of the shell in the adult branchial c tenidium are seen ten club-like pro- , A j i cesses of the dorsal wall, IS a Secondary process. Accordingly, these are the "cerata" the shell of Aplysia must not be con- ** ic j> ar ? characteristic- ., il/ ... in- ally developed in another founded with a primitive shell in its sub -order of Opistho- shell-sac, such as we find realized in g^S 1 S lJToSSS the shells Of Chiton and in the plugs Gegenbaur, after Alder i . , f ,-, iii and Hancock.) which form in the remarkable tran sitory " shell-sac " or " shell-gland " of Molluscan embryos Fio. 62. A. Eolls )xipillosa (Lin.), dorsal view, a, b, posterior and anterior cephalic tentacles; c, the dorsal "cerata" (hence Ceratobranchia). B. Tet/njs leporina, dorsal view, a, the cephalic hood ; b, cephalic tentacles ; c, neck ; d, genital pore ; e, anus ; /, large cerata ; g, smaller cerata ; h, margin of the foot. C. Doris (Actinncyclus) tuberculatits (Cuv.), seen from the pedal surface. TO, mouth ; b, margin of the head ; /, sole of the foot ; sp, the mantle-like epipodium. D. E. Dorsal and lateral view of Elysia (Actieon) viridis. ep, epipodial out growths. (After Kefersteiu ) (see figs. 7, 68, and 72***). Aplysia, like other Mollusca, develops a primitive shell-sac in its trochosphere stage development (fig. 68), which disappears and is succeec by a nautiloid shell (fig. 60). This forms the nucleus of of succeeded 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 space is fully ex posed as in fig. 63, and the var- ous apertures of the body are seen. Posteriorly we IQ. 63. Aplysia leporina(camehts, Cuv.), with epipodia and mantle reflected away from the mid-line, a, an terior cephalic tentacle ; b, 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 ; fir, the ctenidium (branchial plume) ; h, the mantle-skirt tightly spread over the horny shell and pushed with it towards the left side ; i, the spermatic groove ; A , the common genital pore (male and female) ; J, orifice of the grape-shaped (sup posed poisonous) gland ; m, the osphradium (olfac tory organ of Spengel) ; n, outline of part of the renal sac (nephridium) below the surface ; o, external aper 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 ,.. Fio. 64. Gonad, and accessory of the common or hermaphrodite glands and ducts of duct. From this point there stretches forward to the right side of the head a groove the spermatic groove down which 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. XVI. 85 ? , ovo-testis ; h, hermaphrodite duct; g, albuminiparous gland ; /, vesicula seminalis ; k, open ing of the albuminiparous gland into the hermaphrodite duct ; e, hermaphrodite duct (uterine portion) ; 6, vaginal portion of the uterine duct ; c, sperma- theca ; d, its duct ; a, genital
pore. (Original.)