200 THE MECHANICAL FUNCTIONS. 



the organ is formed of a spongy texture, capable of 

 receiving 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 Mytilusedulis, or common muscle, 

 can be protruded 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 to- 

 wards it; and by repeating the operation succes- 

 sively 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 

 strncture is then peculiarly adapted for scooping 

 out. The Cardiuni continually employs its foot for 

 this purpose : first elongating it, directing its point 

 downwards, and insinuating 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 fixed in its position, and then the muscles which 

 usually retract it are thrown into action, and the 

 whole shell is alternately raised and depressed, 

 moving on the foot as on a fulcrum. The effect of 

 these exertions is to drag the shell downwards. 

 When the animal is moderately active, these move- 

 ments are repeated two or three times in a minute. 

 The apparent progress at first is but small ; the 

 shell, which was raised on its edge at the middle of 

 the stroke, falling back on its side at the end of 

 it ; but when the shell is buried so far as to be 

 supported on its edge, it advances moi'e rapidly, 

 sinking visibly at every stroke, till nothing but the 

 extremity of the tube can be perceived above the 



