THE COCKLE. 89 



and hence the lengthening of the tube of the mantle, 

 which is protruded to the mouth of their burrow, for 

 the purpose of aquatic respiration ; in other words, for 

 the transmission of water to the branchiae ; water, full of 

 animalcules, being driven to the mouth by means of the 

 minute vibratory cilia. It has been observed, that one 

 creature, my a truncata, when fully grown, will not 

 attempt to burrow ; but on placing two young ones, 

 which were scarcely more than a line (the tenth of 

 an inch in length) on sand, in a glass of sea-water, 

 they buried themselves immediately. 



Another process is also remarkable. By doubling 

 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. 

 Thus the animal is enabled quickly to retreat when 

 danger presses ; and when this is past, it can, with 

 equal ease, come forth from its refuge. 



We have hitherto only spoken of shells, of which 

 the valves when closed, fit each other so as to leave 

 no aperture, and which therefore require to be opened 

 for the protrusion of the tube ; but there is a group, 

 called les infermes by Cuvier, because the mollusks 

 are so completely enveloped in the mantle, in which the 

 closed valves always remain open, from their shape at 



