MOLLUSC A ACEPHALA. 221 



contained water, with a degree of force, which 

 by the reaction of the fluid in the opposite direc- 

 tion, gives a sensible impulse to the heavy mass. 

 He notices the singular fact that oysters, which 

 are attached to rocks occasionally left dry by the 

 retreat of the tide, always retain within their 

 shells a quantity of water sufficient for respira- 

 tion, and that they keep the valves closed till the 

 return of the tide : whereas those oysters which 

 are taken from greater depths, where the water 

 never leaves them, and are afterwards removed 

 to situations where they are exposed to these 

 vicissitudes, of which they have had no previous 

 experience, improvidently open their shells after 

 the sea has left them, and by allowing the water 

 to escape, soon perish.* 



Many bivalve mollusca are provided with an 

 instrument shaped like a leg and foot, which 

 ^^^^^^ they employ extensively 



for progressive motion. 

 Its form in the Cardium, 

 or cockle, is seen in Fig. 

 104, This organ is com- 

 posed of a mass of mus- 

 cular fibres, interwoven together in a very com- 

 plex manner, and which may be compared to 

 the muscular structure of the human tongue : 

 the effect in both is the same, namely, the con- 



* Journal de Physique, xxviii. 244. 



