740 



ZOOLOGY 



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 Isevigatus, exemplifying great development of propodium 0?r.) and 

 metapodlum {met.\ in a bin-rowing Gastropod. Tlie shell has been removed. /. mesopodium ; 

 I. "liver" ; s. ap. apertin-e of siphon ; t, t. tentacles. (From the CamhHdgc Natural HUtoi^j, 

 after Quoy and Gaimard.) 



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

 raetapodinm an operculum ; in the rest the mesopodium is alone 

 well developed and forms a laterally-compressed, vertically- 

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

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

 of the creeping sole af 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 {Fulmonata, 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. 027.— Aplysia, dorsal 

 view. V, parapodia. (After 

 Keferstein.) 



