198 



THE FOOT 



CHAP. 



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 the Chitonidae, many Nudibranchs, and the slugs generally, 

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

 not subtended by the foot. Here too it presents the form of 

 a regular disc or ellipse, which is more 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 pos- 

 terior portions, which have received the name of propodium and 



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Fig. 97. — Sigaretus laevigatus Lam., showing excessive development of the propo- 

 dium (pr) and metapodium (met) in a mollusc living in sand (the shell, which 

 covers only the liver and adjacent parts, has been removed) ; I, liver ; s.ap, aper- 

 ture of proboscis, here deflected from the median line ; t, t, tentacles. (After 

 Quoy and Gaimard.) 



metapodium respectively, while the intervening central portion 

 is termed the mesopodium. 



The propodium is most strongly developed in genera which 

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

 Scaphander (Figs. 97 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 animal is trav- 

 ersing. In some species of Sigaretus the propodium becomes 

 as it were banked up against the head and proboscis, which 

 are thus unnaturally elevated, or tend to disappear altogether. 

 Bullia (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 ploughing 



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