CLASS CONCHIFERA. 61 



sliape, taperinof at the end, and much more resembling 

 in its form a tongue than a foot. In some bivalves, the 

 dilatation of the foot is effected by a curious hydraulic 

 mechanism ; the interior of 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 Mytilus eduli's, or common mussel, 

 can be advanced 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 towards it, and by re- 

 peating the operation successively 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 struc- 

 ture is then peculiarly adapted for scoopmg out. The 

 cockle 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 movements 

 are repeated two or three times in a minute. The ap- 

 parent progress is at first but small, the shell, which 

 was rfiised on its edge at the middle of the stroke, fall- 

 ing 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 ad- 

 vances more rapidly, sinking visibly at every stroke, till 



