690 



ZOOLOGY 



SECT. 



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 



clcn 



sh 



FIG. 592. Pterotrachea scutata. alt. alimentary canal ; cten. gills ; eye, eye ; fl. float ; 

 mo. mouth ; prob. proboscis ; repr. reproductive gland ; sh. shield covering a portion of the 

 dorsal surface ; su. sucker. 



situated anteriorly, in which are situated 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 of the Prosobranchia 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 peri- 

 stome of the shell, and serves as a channel for the ingress and egress 



oc.tenl 



lent 



pulrrt 



FIG. 593. Helix nemoralis. an. anus ; gen. ap. genital aperture ; oc. tent, posterior eye-bearii 

 tentacles ; pulm. opening of pulmonary sac ; tent, anterior tentacles. (After Pelseneer.) 



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

 mantle-cavity, the anus, nephridial apertures, and ctenidi* 

 merely lying under cover of a comparatively slight ly-developec 

 lateral mantle-flap. Usually there is on the inner surface of th< 

 mantle a glandular area the pallial mucus gland. 



Respiratory Organs. There are normally two ctenidia, one 01 

 the right side and the other on the left, contained in the mantle- 



