722 ZOOLOGY 



end of the Lead on its ventral aspect, is in many instances provided 

 with a protrusible proboscis or introvert, sometimes of considerable 

 length. In the dorsal surface of -the head are a pair of tentacles 

 which vary a good deal in shape, but are usually cylindrical or club- 

 shaped. In most cases the eyes are situated on tubercles at the 

 bases of the tentacles, or elevated towards the middle ; but in the 

 snails and slugs (Pulmonata, Fig. 641) the eyes are elevated on the 

 extremities of a second, longer, pair of tentacles (oc. tent) placed 

 behind the first. 



The mantle is usually developed into a fold the mantle-flap 

 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 renal 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 



oc.lenc 



FIG. 641. Helix nemoralis. an. anus ; gen. ap. genital aperture ; oc. tent, posterior eye- 

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

 Pelseneer.) 



by a comparatively narrow opening. In many of the Prosobranchia 

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

 tion open ventrally the siphon which lies in the corresponding 

 prolongation of the peristome 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 lateral mantle-flap. Usually there is on the inner surface 

 of the mantle a glandular area the pallial mucus-gland. 



Respiratory Organs. There are typically two ctenidia, one 

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

 cavity ; but in the great majority of the Streptoneura and branchiate 

 Euthyneura the primitively right (actually left) ctenidium alone is 

 retained. In those Gastropoda that possess two ctenidia, and in 

 many forms with only one, the axis of the ctenidium bears two 

 rows of compressed filaments, and is attached only towards its 

 base. But in the majority of those with one ctenidium there 



