222 THE MECHANICAL FUNCTIONS. 



ferring a power of motion in all possible ways ; 

 thus it may be readily protruded, retracted, or 

 inflected at every point. The Solen, or razor- 

 shell fish, has a foot of a cylindrical shape, 

 tapering at the end, and much more resembling 

 in its form a tongue than a foot. In some bivalves 

 the dilatation of the foot is effected by a curious 

 hydraulic mechanism : the interior of the organ 

 is formed of a spongy texture, capable of re- 

 ceiving a considerable quantity of water, which 

 the animal has the power of injecting into it, 

 and of thus increasing its dimensions. 



The foot of the Mytilus edulis, or common 

 muscle, can be advanced to the distance of two 

 inches from the shell, and applied to any fixed 

 body within that range. By attaching the point 

 to such body, and retracting the foot, this animal 

 drags its shell towards it ; and by repeating the 

 operation successively on other points of the 

 fixed object, continues slowly to advance. 



This instrument is of great use to such shell- 

 fish as conceal themselves in the mud or sand, 

 which its structure is then peculiarly adapted 

 for scooping out. The Cardium continually em- 

 ploys its foot for this purpose : first elongating it 

 and directing its point downwards, and insinu- 

 ating it deep into the sand ; and next, turning 

 up the end, and forming it into a hook, by 

 which, from the resistance of the sand, it is 



