MOLLUSCA 



119 



Sub-order 2. — Oeratonofa. 



Characters. — The typical MoUuscan 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. — Tritoniadas. 



Genera : Tritonia, Cuvier ; Scyllsea, L. ; Tethys, L. (fig. 62, B) ; 

 Bendronotris, A. and H. ; Doto, Oken. 

 Family 8. — Eolidse. 



Genera : Eolis, Cuvier (fig. 62, A) ; Glaueus, Forster ; Fiona, A. 

 and H. (fig. 67); Erribletonia, A. and H. ; Prodonotus, A. and 

 H. ; Aniiopa, A. and H. ; Sermesa, Loven ; Alderia, AUman. 



Sub-order 3. — Haplomorpha. 



Characters. — No ctenidia, cerata, mantle-skirt, or other processes 

 of the body-wall ; degenerate forms of small size. 

 Family 9. — Phyllirhoidas. 



Genera : Phyllirhoe, Peron and Lesueur (fig. 58) ; Acura, Adams. 

 Family 10. — Mysiadie. 



Genera: Mysia, Risso (fig. 62, D, E) ; Acteonia, Quatref ; Cenia, 

 A. and H. ; Limapontia, Johnston ; Bhodope, Kbll. 



Further Remarks on the Opisthohranchia. — The Opis- 

 tiobranchia present the same -wide range of superficial 

 appearance as do the Azygobranchiate Streptoneura, forms 



FiQ. 56. Three views of Aplysia sp., in various conditions of expansion and 



retraction. (, anterior cephalic tentacles ; P, posterior cephalic tentacles ; 

 e, 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 giU which 

 the more degenerate 

 Opisthohranchia ex- 

 hibit, this Order 

 stands alone. Some 

 Opisthohranchia are 

 striking examples of 

 degeneration (some 

 Haplomorpha), hav- 

 ing none of those re- 

 gions or processes of p^^ jy —Dorsal and ventral view of PUwraphytti- 

 the body developed dta!Mieo!to(Otto), oneofthePhyllidiobranchiate 

 , . , '',.,. .I. Palliate Opisthobranchs. i, the mouth; Z, the 

 which distinguish lamelliformsub-pallialgills, which (as in Patella) 

 the archaic MoUuSCa ^placethe typical Molluscan ctenidium. (After 



from such 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 Ehabdoccel 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 

 fullest development 

 of the organization 

 characteristic of 

 Opisthohranchia. 

 The woodcut (fig. 56) 

 gives a faithful repre- 

 sentation of the great 

 mobility of the vari- 

 ous parts of the body. 

 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 



Fig. 58. — Phyllirhoe i-ucephala, twice the natural 

 size, a transparent piscifonn pelagic Opistho- 

 ■ branch. The internal organs are shown as seen 

 by transmitted light, a, mouth ; &, radular sac ; 

 c, oesophagus ; d, stomach ; c', intestine ; /, anus ; 

 ft S', ff", a", the four lobes of the liver ; h, the 

 heart (auricle and ventricle) ; I, the renal sac (ne- 

 phridium) ; l\ the ciliated communication of the 

 renal sac with the pericardium ; m, the external 

 opening of the renal sac ; n, the cerebral ganglion ; 

 0, the cephalic tentacles ; /, the genital pore ; 

 y, the ovo-testes ; w, the parasitic hydromedusa 

 ilfmsfm, usually found attached in this position by 

 the aboral pole of its umbrella. (After Keferstein.) 



Fig. 59. — Acera hullaia. 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 Opisthohranchia. The 

 torsion of the visceral hump is not carried out very fully, 



Fig 60.— a. Veliger-larva of an Opisthobranch (Polycera). /, foot ; op, oper- 

 culum ; m», 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 ; 

 Vh the commencing stomodseum or oral invagination ; ot, the left otocyst ; 

 W red-coloured pigment spot. C. Diblastula of an Opisthobranch (Poly- 

 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. 



