380 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



In the Mollusca the characteristic organ of loco- 

 motion is the foot, which is made up of muscular 

 fibres, which are transversely striated, but are 

 spindle-shaped cells, and so have the general form of 

 unstriated muscular tissue. This foot, which can be 

 withdrawn into the shell of such forms as Anodon by 

 retractor muscles attached anteriorly and posteriorly, 

 seems to be protruded or put into a state of erection 

 by an increased flow of blood into its substance, and 

 not, as has sometimes been supposed, by the intaking 

 of water from without. While it has a somewhat 

 conical or hatchet- shaped form in the fresh-water 

 mussel, and in those Lamellibranchs which move about 

 with some activity, it is very strong in boring forms 

 such as Solen, and long and curved in Trigonia, where 

 it is used as a leaping organ ; on the other hand, it is 

 very small in the scallop (Pecten), and quite incon- 

 spicuous in the still more sedentary oyster. 



Among the Gastropoda the foot has often, as in 

 the common snail, a broad disc-like lower surface, and 

 is adapted for creeping or crawling. When the snail 

 is in movement waves of contraction may be seen 

 passing over the lower surface of the foot from behind 

 forwards, and it has been found that smaller have 

 greater locomotor power than larger forms. Within 

 limits, snails are able to carry weights, and it follows, 

 therefore, that unloaded snails do not make use of all 

 the activity of which they are capable. 



The foot may become modified in a most remark- 

 able manner, as, for example, in the Heteropoda, 

 which are forms found only on the surface of the 

 ocean ; the animal swims with its shell downwards, 

 and its foot (Fig. 161 ; f) is converted into a high 

 crest-like fin, which is no doubt aided functionally by 

 the fin-like prolongation of the hinder end of the 

 body. 



In the Pteropoda the sides of the foot become 



