ACEPHALA. 255 



sight suppose it possible that such a creature could 

 exercise. The changeable organ, the foot, which, 

 (absent in the Oyster, and but slightly developed in 

 the Scallop,) we have seen to be the mould which 

 forms the cable in the Pinna, now assumes the 

 function more in conformity with its name, and 

 becomes the organ of locomotion. In the genus 

 before us, it is developed to a great size ; in structure 

 it much resembles the human tongue, as well as in its 

 power of assuming various shapes. It is used for 

 burrowing in the sand, for which it is admirably 

 adapted. The animal lengthens the foot, and thrusts 

 it deep into the sand, then turning the end into the 

 shape of a hook, and thus acquiring a hold, it forci- 

 bly contracts the foot, by which the whole animal is 

 gradually brought into the hole and buried, till 

 only a projecting tube can be seen, through which 

 it breathes. 



By an opposite process, bending the end of the 

 foot, and pushing against the sand at the bottom of 

 the hole, the shell is again protruded in a succession 

 of efforts. At the bottom of the water, the Cockle 

 can also move with considerable speed, by pushing 

 with its foot against the ground, as a ferryman poles 

 his boat across a river. Nor is this all. For, by 

 stiffly bending the same wonderful organ in a hook, 

 and pressing it forcibly against the ground, a sudden 

 spring-like extension of the hook throws the Cockle 

 into the air, and, by a repetition of these jerks, it 

 skips along with marvellous agility. 



The foot attains a very great size in the genus 



