720 



ZOOLOGY 



SECT. 



developed and sharply marked off to act as a burrowing organ. In 

 a few cases a pair of tentacles the pedal tentacles are situated at 

 the anterior end of the foot ; still rarer is a pair of similar appen- 

 dages at the posterior end. The whole foot becomes reduced in the 

 few Gastropods that remain fixed. The metapodium very usually 



ineb 



SMJJ 



FIG. 636. Sigaretus laevigatus, exemplifying great development of propodium (pr.) and 

 metapodhim (met.), in a burrowing Gastropod. The shell has been removed. /. meso- 

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

 Natural History, after Quoy and Gaimard.) 



in the Streptoneura bears a disc or stopper the operculnm already 

 referred to usually horn-like, rarely completely calcified, more 

 commonly horn-like with a thin calcareous investment by means of 

 which the aperture of the shell is closed when the animal is retracted. 



In some forms, such as the Sea-hares 

 (Aplysia, Fig. 637), the foot develops a pair 

 of lateral lobes the parapodia which act as 

 fins ; and in the Pteropods (Fig. 638), which 

 are specially modified for a pelagic existence, 

 these constitute the largest part of the foot. 

 In the Heteropoda (Figs. 639, 640), which are 

 also pelagic, the foot is also modified to act 

 as a swimming organ. In one family of this 

 sub-order (Fig. 639) all three parts of the 

 foot are well developed, the mesopodium bears 

 a sucker, and the metapodium an operculum ; 

 in the rest the mesopodium is alone well de- 

 veloped and forms a laterally-compressed, 

 vertically-elongated fin. The term epipodium 

 is applied to a ridge or fold, which, when 



FK,637.-A P iysia, dorsal best developed, runs around the entire side 

 view. r, parapodia. ot the foot, and may be beset with papillae 



(After Keferstein.) , \., 



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 constriction or neck. The mouth, situated at the anterior 



