688 



ZOOLOGY 



SECT. 



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

 anterior propodium and posterior iinla^ctdiinn. 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 



s.a]J 



FIG. 5S8. Sigaretus laevigatus, exemplifying great development of propodium (pi:), and 

 metapodium (,</. '.). in a burrowing Gastropod. The shell has been removed. /. liver : s. ap. 

 aperture of proboscis ; t. t. tentacles. (From the Cambridgt Natural Ji;.-*tn,-/'. after Quoy and 

 Gaimard.) 



the pedal fen fades 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 opcrculum- usually horny, rarely completely 



calcified, 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 cpipodia 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 Heter.opoda (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, 

 \ ertically-elongated fin. 

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

 branched invagination of the iiitegument,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. 



l-'n:. :.s;i. Aplysia, dnr.-al 



view, r, epipodia. (After 

 Keferstein.) 



