198 



THE FOOT 



The Foot 



One of the most characteristic organs of the Mollusca is the 

 foot, which, under one form or another, occurs throughout the 

 whole phylum. The foot is a thickening, on the ventral side, 

 of a portion of the integument of the animal, modified to serve 

 different forms of motion. It attains its maximum relative area 

 in tlie Chitonidae, many Nudiljranclis, and the slugs generally, 

 in nearly all of which there is no portion of the body which is 

 not sul)tended by the foot. Here too it presents the form of a 

 regular disc or ellipse, which is uiore or less produced. In many 

 cases, however, the foot becomes modified in such a way that 

 we are enabled to recognise well-marked anterior and posterior 

 portions, which have received the name of ^>v'()2?of?i?«/i and mcta- 



s.a-ji 



Fig. 97. — St'f/aretics laevir/atus Lam., showing excessive developmeut of tlie propodhim 

 (^j/-) and metapodium (met) in a mollusc living in sand (the shell, which covers 

 only the liver and adjacent parts, has been removed) ; /, liver ; s.a2), apertiire 

 of proboscis, here dellected from the median line ; f, t, tentacles. (After Quoy and 

 Gaimard. ) 



jwdium respectively, while the intervening central portion is 

 termed the mesoiiodium. 



The propodium is most strongly developed in genera which 

 crawl about in wet sand, e.g. Natica, Sigmrtus, Oliva, Harpa, 

 Scaplyinder (Figs. 9 7 and 98, and compare Fig. 91). In such cases 

 it seems to serve as a sort of fender or snow-plough, to push the 

 sand away on both sides of the path the aninud is traversing. 

 In some species of Sigarctas the propodium becomes as it were 

 Ijanked up against the head and proboscis, which are thus 

 unnaturally elevated, or tend to disappear altogether. Bidlia 

 (Fig. 62), which crawls about rapidly on wet sand, appears 

 to attain its object by a wide extension of the foot on all sides, 

 and so slides over the sand instead of plougliing through it ; 



