438 THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

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

 mantle and the foot. In Aplysia^ 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 opteron^ there 

 arv^ very large epipodial lobes, b}' the aid of which some 

 sjDecies propel themselves like Pteropods. 



The Nudibranchlata 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 Fhylli- 

 rhoe, 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, somic of which are 

 provided with chitinous or calcareous plates, or teeth, in 

 A2)lysia, BtdUt, and other genera. In many Nudibranchs, 

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

 organ, the ca^cal ultimate ramifications of which end in the 

 elongated dorsal 23apillie. The apices of these papilhe contain 

 thread-cells. 



In the series of the Prosohranchiata, 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 Flssurella are nearly sym- 

 metrical, but the anus is anterior. 



The ProsohrancJiiata have, at most, rudiments oi epii^odia, 

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

 ment than in the OpistJiohranchiata, and a chitinous or shelly 

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

 sal or hasmal 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 cliief organ of locomotion in these free- 

 swimming oceanic animals. 



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



