MOLLUSCA 



119 



Sub-order 2. Ccrntonota. 



Oinractfrs. The typical Mollnsran rtriiiilium is not <lcv<']n|>r,l ; 

 upon the dorsal area is developed a more or less niniii-rous scries <>t 

 e\ lindrieal or liraneheil processes ^Ihe eera(a) into eaeli of which 111*' 

 intestine usually sends a process ; anus dorsal, inudian, or rifjht-sidcd. 

 Family 7. Trilmiitn/ir. 

 Genera: Tritonia, Cuvier; ScyUma, L.; Tcthys, L. (fig. 62, B); 



lii'in/ronotus, A. and H. ; Dolo, Oken. 

 Family 8. Eolidse. 



Genera : En! is, Cuvier (fig. 62, A) ; Olaticus, Forstcr ; Fiona, A. 

 and H. (tig. 67); Embletonia, A. and H. ; Proc/onatits, A. and 

 H. ; Antiopa, A. and H.; Hcrimrn, Lovi-n ; Alilcria, Alhuan. 



Sub-order 3. Haplumorji/ia. 



Characters. No ctenidia, cerafoi, mantle-skirt, or other processes 

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

 Family 9. PhyllirJiuiiltr. 



Genera : Plii/llirhoe, Peron and Lesucnr (fig. 58) ; Acura, Adams. 

 Family 10. Elysiadie. 



Genera: Elysia, Risso (fig. 62, 1), E) ; Action in, Quatref. ; Ccnia, 

 A. and H. ; Limapontin, Johnston ; Ehodopc, Koll. 



Further Remarks on the Opisthobranchia. The Opis- 

 thohranchia present the same wide range of superficial 

 appearance as do the Azygobranchiate Streptoneura, forms 



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

 retraction, t, anterior cephalic tentacles ; t" t posterior cephalic tentacles ; 

 e, eyes ; f, metapodium ; ep, epipodiuiu ; g, gill-plume (ctenidium) ; m, mantle- 

 flap reflected over the thin oval shell ; os, s, orifice formed by the unclosed 

 In mier of the reflected man tie- skirt, allowing the shell to show ; pe t 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 Fm 5T " Dorsal and ventral view of p hvTorln f, H . 

 the body developed dioZinato(Otfco\oneofthePhyllidiobranchiate 

 , . , A' t oli Palliate Opistholjranchs. ft, the mouth ; I. the 



Which distinguish ] amP nifnnn sub-pullial Rills, which (as in Patella) 

 the archaic MollllSCa replace the typical Molluscan ctenidium. (After 

 , , ,, Keferstein.) 



from such flat-worms 



as the Denclroccel Planarians. Indeed, were it not for their 

 retention of the characteristic oclontophore we should have 

 little or no indication that such forms as Phyllirlioe and 



B 



3-! 



Limapontia really belong to the Mollusca at all. The inter- 

 esting little Ji'/Kii/H/ii' Viriini/ii, \vliieh has no odontophore, 

 lias been associated by systematists both with these simpli- 

 fied Opistlicil>niiiehs and with Khabdcieiel I'lanarians (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 



brunch. The intemai organs are shown as SITU 

 by transmitted light, o, mouth ; (/, radular sac ; 

 c, oesophagus ; '/, stomach ; c', inteslinf ; ;'. .inn , ; 

 .". :' ''". if", the turn loljes of tl liver; /;, tin- 

 heart, (auricle and vi-ntricle) ; t, the renal sac (ne- 

 phridinm) ; /', the ciliated communication of the 

 renal sae \\ilh Hie prrieanlkiiii ; in, the --xlnn;il 

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

 o, the cephalic ti-nlaeles ; / the genital pore; 

 y, the ovo-testes ; , the parasitic l!y<lr"jm'<lnsa 

 V'c'.^r'f.usual^ 1 found attached in this position by 



Fio. 08. rJujllirhoi' }ir,}>}tftl(f. f twice the nntnrnl 

 size, a transparent pisdfnrm pelagic 



thcaboral jxileofits umbrella. (After Keferstein.) 



of the organization 

 characteristic of 

 Opisthobranchia. 

 The woodcut (fig. "'i!) 

 gives a faithful repre- 

 sentation of the great 

 mobility of the vari- 

 ous parts of thebody. 

 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 



Fie. M.AceTalullata. 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, 



Fin. GO. A. Veliger-iarva of an Opisthobranch (Pnlycera). /, foot ; op, oper- 



pft, the commencing bi^nnjv^,*... ~. D 



pg, r.-d-1-oloureil pij-'inetit sput. C. liilil.istula of an Opisthobranch (Poly- 

 cera) with elongated blastopore oi. (All from LankeBter.) 



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. 



