688 



ZOOLOGY 



SECT, 



part or mesopodium, which is the most important, with a smaller 

 anterior propodium and posterior metapodium. In many burrowing 

 forms (Fig. 588) the propodium is well developed and sharply marked 

 off to act as a burrowing organ. In a few cases a pair of tentacles 



FIG. 588. Sigaretus Isevigatus, exemplifying great development of propodium (/v.), and 

 metapodium (met.), in a burrowing Gastropod. The shell has been removed. 1. liver ; s. ap. 

 aperture of proboscis ; t. t. tentacles. (From the C'amlrid'je Natural Jli^to,-//, after Quoy and 

 Gaimard.) 



the pedal tentacles are situated at the anterior end of the foot : 

 still rarer is a pair of similar appendages at the posterior end. 

 The whole foot becomes reduced in the few Gastropods that remain 

 fixed. The metapodium very usually in the Streptoneura bears 

 a disc or stopper the operculum usually horny, rar,ely completely 

 calcined, more commonly horny 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. 589), the foot develops a 

 pair of lateral lobes the epipodia which 

 act as fins ; and in the Pteropods (Fig. 590), 

 which are specially modified for a pelagic 

 existence, these constitute the largest part 

 of the foot. In the Heteropoda (Figs. 

 591, 592) which are also pelagic, the foot 

 is also modified to act as a swimming 

 organ. In one family of this sub-order 

 (Fig. 591) 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 

 developed and forms a laterally-compressed, 

 vertically-elongated fin. 

 A pedal gland is present in the majority: it is a simple or 

 branched invagination of the integument,4ined 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. 



Fio. 589. Aplysia, dorsal 

 view, r, epipodia. (After 

 Keferstein.) 



