290 MANUAL OF ZOOLOGY SECT. 



The mantle is usually developed into a fold the mantle 

 flap originally posterior, but subsequently becoming 

 shifted round to the right side. This covers over a cavity 

 the mantle cavity situated anteriorly, in which are found 

 the anal and nephridial apertures and the ctenidia. The 

 edges of the mantle-flap may become united together in 

 such a way as to form a chamber opening on the exterior 

 by a comparatively narrow opening. In many Gastropods 

 the edges of this aperture are drawn out into a spout-like 

 prolongation open ventrally the siphon which lies in the 

 corresponding prolongation of the peristome of the shell 

 (p. 287) and serves as a channel for the ingress and egress 

 of water. In some Gastropods, however, there is no definite 

 mantle-cavity, the anus, nephridial apertures, and ctenidia 

 merely lying under cover of a comparatively slightly- 

 developed lateral mantle-flap. 



The foot varies in the extent of its development in the 

 different families of the class. It usually presents an 

 elongated flat ventral surface on which the animal creeps 

 by .wave-like contractions of the muscular tissue. In the 

 typical Gastropods the foot is usually distinguishable into 

 three portions, a middle part or mesopodium which is the 

 most important, with a smaller anterior propodium and 

 posterior metapodium. The whole foot becomes reduced 

 in the few Gastropods that remain fixed. The metapodium 

 very frequently bears the operculum, usually horny, or partly 

 calcined, by means of which the aperture of the shell is 

 closed when the animal is retracted. 



In some forms, such as the Sea-hares (Apfysia), the foot 

 develops a pair of lateral lobes the epipodia which act 

 as fins ; and in the Pteropods (Fig. 1 66), which are specially 

 modified for a pelagic existence, these constitute the largest 

 part of the foot. 



