MOLLUSCA ACEPHALA. 223 



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 down- 

 wards. 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 

 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 perceived above the sand. Mr. Osier, 

 who has given us this account,* observes that 

 the instinct, which directs the animal thus to 

 procure a shelter, operates at the earliest period 

 of its existence. The Mya tnmcata, 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 themselves imme- 

 diately. 



By a process exactly the inverse of this, that 

 is, by doubling up the foot, and pushing with 

 it downwards against the sand below, the shell 



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



