PHYLUM MOLLUSCA 



741 



originally posterior, but subsequently becoming shifted round, in 

 the course of the displacement already referred to, to the right- 

 hand side. This covers over a cavity— the mantle-cavity— 

 situated anteriorly, in which are situated the anal and nephridial 



Fig. 628.— Shell-bearing Pteropoda. /. /. fins; I. liver; o. ovary; sh. shell. (From Cooke 



after Souleyet.) 



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 of the Prosobranchia the edges- of this aperture are 

 drawn out into a- spout-like prolongation - open ventrally — the 



Fig. 629.— Atlanta peronii. a, cerebral ganglia ; b, pedal ganglia ; e. eye ; g, ctenidia ; 

 h. heart ; k, nephridinm ; I. " liver" ; in. mouth ; o. ovary ; p, operculum ; t. testis 



siphon — which liss in the corresponding prolongation of the peri- 

 stome of the shell 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 



