XII 



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 



r\ 



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 



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

 h. heart; k, nephridium ; 1. "liver" ; m. mouth ; o. ovary ; p, operculum ; t. testis 



siphon which lies 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 



