86 THE MUSSEL. 



ing the foot, this creature drags its shell towards it ; 

 and, by repeating the operation, it continues slowly to 

 advance. 



This organ is of great use to such shell-fish as con- 

 ceal themselves in the mud or sand, which its structure 

 is admirably adapted for scooping out. The cardium 

 continually employs its foot for this purpose. It first 

 lengthens and directs its point downwards, and insinu- 

 ates it deep in the sand ; it next turns up the end, and 

 forms 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. Thus the shell is dragged 

 onwards ; the animal is moderately active, and these 

 movements are made two or three times in a minute. 



The apparent progress is at first 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 more rapidly, sinking visibly at every stroke, 

 till nothing but the extremity of the tube can be per- 

 ceived above the sand. The instinct which thus secures 

 a shelter for the animal, is said to operate at the earliest 

 period of its existence. 



