MOLLUSCA ACEPHALA. 163 



taching the point to such hody, and retracting the foot, this 

 animal drags its shell towards it; and by repeating the ope- 

 ration successively on other points of the fixed object, con- 

 tinues 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 conti- 

 nually employs its foot for this purpose: first elongating it 

 and 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 re- 

 tract it are thrown into action, and the whole shell is alter- 

 nately raised and depressed, moving on the foot as on a ful- 

 crum. The effect of these exertions is to drag the shell 

 downwards. When the animal is moderately active these 

 movements are repeated two or three times in a minute. 

 The apparent progress is at first 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 bu- 

 ried 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 perceived above the sand. JNIr. 

 Osier, who has given us this account,* observes that the in- 

 stinct, which directs the animal thus to procure a shelter, 

 operates at the earliest period of its existence. Tlie Mija 

 iruncata, when fully grown, will not attempt to burrow; 

 but on placing two young ones, which were scarcely more 

 than a line in length, and apparently but just excluded, on 

 sand, in a glass of sea-water, he found that they buried them- 

 selves immediately. 



By a process exactly the inverse of this, that is, by dou- 

 bling up the foot, and pushing with it downwards against the 

 sand below, the shell may be again made to rise by the same 

 kind of efforts which before protruded the foot. By this 

 process of burrowing the animal is enabled quickly to retreat 



* Phllos. Trans, for 1826, p. 349. 



