438 THE ANATOMY OF IN VERTEBRATED ANIMALS. 



is no pallial cavity, and the branchiae are numerous lamellae, 

 placed on each side of the body, between the free edge of the 

 mantle and the foot. In Aplasia, the mantle is relatively 

 small, and possesses an internal shell ; the branchiae, the 

 anus, and the reproductive apertures, are placed on the right 

 side of the body. In this genus, and in G aster opter on, there 

 are very large epipodial lobes, by the aid of which some 

 species propel themselves like Pteropods. 



The Nudibranchiata have no mantle, and the anus is 

 usually situated on the right side of the body ; sometimes, 

 however, as in Doris, it is terminal. In the pelagic Phylli- 

 rhde, the foot aborts, as well as the mantle, and the body has 

 the form of an elongated sac. 



The gastric portion of the alimentary canal becomes com- 

 plicated by division into several portions, some of which are 

 provided with chitinous or calcareous plates, or teeth, in 

 Aplysia, Bulla, and other genera. In many Nudibranchs, 

 as Eolis, the liver is represented by a much-branched tubular 

 organ, the caecal ultimate ramifications of which end in the 

 elongated dorsal papillae. The apices of these papillae contain 

 thread-cells. 



In the series of the Prosobranchiata, the great majority 

 are not only chlamydate, but there is a spacious branchial 

 chamber, and the pallial wall of the body is produced into a 

 conical visceral sac, which contains the stomach, liver, and 

 genital organs. It is usually asymmetrically coiled, and is 

 protected by the shell. No Opisthobranch possesses a large 

 visceral sac of this kind. On the other hand, no Prosobranch 

 is, like Phyllidia, symmetrical, with the anus at the posterior 

 end of the body. Patella and Fissurella are nearly sym- 

 metrical, but the anus is anterior. 



The Prosobranchiata have, at most, rudiments of epipodia, 

 bat the rest of the foot often acquires a much greater develop- 

 ment than in the Opisthobranchiata, and a chitinous or shelly 

 plate — the operculum — is frequently developed from the dor- 

 sal or haemal aspect of the metapodium. The differentiation 

 of the foot attains its highest degree in the so-called Hetero- 

 poda, in which the propodium, mesopodium, and metapodium 

 differ widely in form ; the propodium being broad and fin-like, 

 and constituting the chief organ of locomotion in these free- 

 swimming; oceanic animals. 



In the Limpets (Patellidce), the visceral sac forms merely 



