Class GASTEROPODA. 



Sub-class OPISTHOBRANCHIATA. 



Gills exposed, or only slightly covered by a fold of the 

 mantle, situated behind the heart, and never lodged in a 

 distinct, cervical cavity. Sexes united in the same indi- 

 vidual. Abdomen rudimentary, not spirally developed in 

 the adult, or protected by a shell. Larva shell-bearing, 

 and furnished with deciduous cephalic fins. 



The Mollusks which comprise the Opisthobranchiate 

 division of the Branchiferous Gasteropods are charac- 

 terised, in the larval state, by the same peculiarities 

 of organization which are found in the Prosobranchiate 

 tribes, the young animal being provided with a turbinate 

 or spiral shell the aperture of which is closed by an 

 operculum, and the head bearing on the fore part a large 

 membranous veil, more or less bi-lobed, and fringed at the 

 margin with vibratile cilia which serve as organs of loco- 

 motion ; in fact, as Milne Edwards has remarked, it 

 is difficult to distinguish the larva; of Eolis and Aplysia 

 from those of Buccinum and Vermetus; in the perfect 

 state, also, they are provided with gills, and their re- 

 spiration is aquatic, as in the former sub-class, but in 



VOL. II. b 



