PHYLUM MOLLUSCA 



289 



situated at the anterior end of the head on its ventral 

 aspect, is in many instances provided with a protrusible 

 probosis or introvert, sometimes of considerable length. 

 On 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 situ- 

 ated on tubercules at the bases of the ten- 

 tacles, or elevated towards the middle; 

 but in the snails and slugs (Pulmonata) 

 (Fig. 171) 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 

 to the right-hand side. This covers over 

 a cavity, the mantle-cavity, situated ante- 

 riorly, 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 cham- 

 ber opening on the exterior by a compara- 

 tively narrow opening. In many 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, 

 and serves as a channel for the ingress 

 and egress of water. In some Gastro- 

 pods, 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. 



IV 



Fig. 175.— Shell of 

 Terebra oculata. 



