740 



ZOOLOGY 



SECT. 



pelagic existence, these constitute the largest part of the foot. 

 In the Heteropoda (Figs. 629, 630) which are also pelagic, 

 the foot is also modified to act as a swimming organ. In one 

 family of this sub- order (Fig. 629) all three parts of the foot 



js.aji 



Fir. 626.— Sigaretus laevigatas, exemplifying great development of propodium (pr.) and 

 metapodium (met.), in a burrowing Gastropod. The shell has been removed, f. mesopodium ; 

 I. "liver" ; s. ap. aperture of siphon ; t. t. tentacles. (From the Cambridge Natural History, 

 after Quoy and Gaimard.) 



are well-developed, the mesopodium bears a sucker, and the 

 metapodium an operculum ; in the rest the mesopodium is alone 

 well developed and forms a laterally-compressed, vertically- 

 elongated fin. The term epipodium is applied to a ridge or 

 fold, which, when best developed, runs around the entire edge 

 of the creeping sole of the foot, and is beset 

 with papillae or tentacle-like processes. 



A pedal gland is present in the majority : 

 it is a simple or branched invagination of 

 the integument, lined by mucus-secreting 

 cells. Very commonly, as in Triton, it opens 

 on the exterior in the middle line of the 

 ventral surface of the foot. 



The Gastropoda have a well-marked 

 head, separated from the body by a con- 

 striction or neck. The mouth, situated at 

 the anterior end of the head on its ventral 

 aspect, is in many instances provided with 

 a protrusible proboscis or introvert, some- 

 times 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 situated on tubercles at the 

 bases of the tentacles, or elevated towards 

 the middle ; but in the snails and slugs (Pulmonata, Fig. 631) 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 — 



Fig. 627.— Aplysia, dorsal 

 view, r, parapodia. (After 

 Keferstein.) 



