PHYLUM MOLLUSCA 



291 



quently bears a disc or stopper, the operculum, usually 

 horny, or partly calcified, by means of which the aperture 

 of the shell is closed when the animal is retracted. 



In some forms, such as the sea-hares (Aplysia), the foot 

 develops a pair of lateral lobes, the epipodia, which act as 

 fins; and in the Pteropods (Fig. 178), which are specially 

 modified for a pelagic existence, these constitute the largest 

 part of the foot. 



The organs of respiration in the majority of the aquatic 

 Gastropoda are in the form of gills or ctenidia, usually plume- 

 shaped appendages consisting of a central stem bearing two 



Fig. 178. — Shell-bearing Pteropoda. y,/, fins; /.liver; o, ovary; sk, shell. 

 (From Cooke, after Souleyet.) 



rows of compressed filaments or lamellae, or a single row. 

 Two ctenidia may be present or only one may be developed; 

 they are enclosed in the mantle-cavity. 



In the Nudibranchs two ctenidia are absent, but their 

 place as breathing organs is taken by a number of secondary 

 branchice, sometimes simple, sometimes branched or pin- 

 nate processes, which are distributed over the dorsal surface, 

 as in Eolis, or as in Doris (Fig. 177), forming a circlet 

 surrounding the anus, or, as in PleurophyUidia, a row on 

 each side beneath the mantle-flap. 



