Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/684

Rh 656 MOLLUSCA [oriSTHOBRANCHI. Sub-order 2. Ccratonota. Characters. The typical Molluscan ctenidium is not developed ; upon the dorsal area is developed a more or less numerous series of cylindrical or branched processes (the cerata) into each of which the intestine usually sends a process ; anus dorsal, median, or right-sided. Family 7. Tritoniadae. Genera: Tritonia, Cuvier; Scyllasa, L. ; Tetliys, L. (fig. 62, B) ; Dendronotus, A. and H. ; Doto, Oken. Family ^.Eolidse. Genera : Eolis, Cuvier (fig. 62, A) ; Glaucus, Forster ; Fiona, A. and H. (fig. 67) ; Embletonia, A. and H. ; Prodonotus, A. and H. ; Antiopa, A. and H. ; Hermsea, Loven ; Alderia, Allman. Sub-order 3. Haplomorpha. Characters. No ctenidia, cerata, mantle-skirt, or other processes of the body- wall ; degenerate forms of small size. Family 9. Phyllirhoidae. Genera : Phyllirhoe, Peron and Lesueur (fig. 58) ; Acura, Adams. Family 10. Elysiadse. Genera : Elysia, Risso (fig. 62, D, E) ; Actconia, Quatref. ; Ccnia, A. and H. ; Limapontia, Johnston ; Rhodope, Koll. Further Remarks on the Opisthobranchia. - The Opis- thobranchia present the same wide range of superficial appearance as do the Azygobranchiate Streptoneura, forms rp Fio. 56. Three views of Aplysia sp., in various conditions of expansion and retraction, t, anterior cephalic tentacles ; &amp;lt; 2, posterior cephalic tentacles ; , eyes ; /, metapodium ; ep, epipodium ; g, gill-plume (ctenidium) ; m, mantle- flap reflected over the thin oval shell ; os, s, orifice formed by the unclosed border of the reflected mantle-skirt, allowing the shell to show ; pe, the sper matic groove. (After Cuvier.) carrying well-developed spiral shells and large mantle- skirts being included in the group, together with flattened or cylindrical slug- like forms. But in respect of the substi tution of other parts for the mantle -skirt and for the gill which the more degenerate Opisthobranchia ex hibit, this Order stands alone. Some Opisthobranchia are striking examples of degeneration (some Haplomorpha), hav ing none of those re gions Or processes of Flo 57 _ I)orsal and ven t ra i view of Plevrophylli- the body developed &amp;lt;lw lineata (Otto), one of the Phyllidiobranchiate L-.U j- 4 -i Palliate Opisthobranchs. b, the mouth; I, the Anguisn lamelliform sub-pallial gills, which (as in Patella) the archaic Mollusca replace the typical Molluscan ctenidium. (After , Keferstein.) from s^^cll flat-worms as the Dendrocoel Planarians. Indeed, were it not for their retention of the characteristic odontophore we should have little or no indication that such forms as Phyllirhoe and Limapontia really belong to the Mollusca at all. The inter esting little Rhodope Veranyii, which has no odontophore, has been associated by systematists both with these simpli fied Opisthobranchs and with Rhabdocoel Planarians (29). In many respects the Sea-Hare (Aply sia) of which several species are known (some occurring on the English coast), serves as a conven ient example of the of the organization characteristic of Opisthobranchia. The woodcut (fig. 56) gives a faithful repre sentation of the great mobility of the vari- j. ,, Fio. 5S.PhyUirhoe lucephala, twice the natural lUllest development size, a transparent pisciform pelagic Opistho- branch. The internal organs are shown as seen by transmitted light, a, mouth ; 6, radular sac ; c, oesophagus ; d, stomach ; c, intestine ; / , anus ; !7i 9 9&quot;t ff&quot; &amp;gt; the four lobes of the liver ; h, the heart (auricle and ventricle) ; /, the renal sac (ne- phridium) ; I, the ciliated communication of the, renal sac with the pericardium ; m, the external opening of the renal sac ; n, the cerebral ganglion ; o, the cephalic tentacles ; /, the genital pore ; y, the ovo-testes ; w, the parasitic hydromedusa Mnestra, usually found attached in this position by OUSpartS of thebody. theaboral Ple of its umbrella. (After Keferstein.) The head is well marked and joined to the body by a some what constricted neck. It carries two pairs of cephalic tentacles and a pair of sessile eyes. The visceral hump is low and not drawn out into a spire. The foot is long, carrying the oblong visceral mass upon it, and projecting (as metapodium) a little beyond it (/). Laterally the foot gives rise to a pair of mobile fleshy lobes, the epipodia (ep), which can be thrown up so as to cover in the dorsal Fm. 59.--Acera btdlata. A single row of teeth of the radula. (Formula, x.l.x. ) surface of the animal. Such epipodia are common, though by no means universal, among Opisthobranchia. The torsion of the visceral hump is not carried out very fully, F IO. 60. A. Veliger-larva of an Opisthobranch (Folycera). /, foot ; op, oper- culuin ; inn, anal papilla; ry, dry, two portions of unabsorbed nutritive yelk on either side the intestine. The right otocyst is seen at the root of the foot. B. Trochosphere of an Opisthobranch (Pleurobranchidium) show ing : shgr, the shell-gland or primitive shell-sac ; v, the cilia of the velum ; ph, the commencing stomodseum or oral invagination ; ot, the left otocyst ; pg, red-coloured pigment spot. C. Diblastula of an Opisthobranch (1 oly- cera) with elongated blastopore oi. (All from Lankester.) the consequence being that the anus has a posterior posi tion a little to the right of the median line above the metapodium, whilst the branchial chamber formed by the overhanging mantle-skirt faces the right side of the body instead of lying well to the front as in Streptoneura and as in Pulmonate Euthyneura. The gill-plume which in Aplysia is the typical Molluscan ctenidium is seen in fig.