MOLLUSCA ACEPHALA. 199 



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 respiration, 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 after- 

 wards 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 molUisca are provided with an 

 instrument shaped like a leg and foot, which they 



employ extensively for pro- 

 104 ^^^^^^^Hjj^ gressive motion. Its form 



in the Cardium, or cockle, is 

 seen in Fig. 104. This organ 

 is composed of a mass of 

 muscular fibres, interwoven 

 together in a very complex 

 manner, and which may be compared to the mus- 

 cular structure of the human tongue : the effect in 

 both is the same, namely, the conferring a power 

 of motion in all possible ways ; thus it may be 

 readily protruded, retracted, or inflected at every 

 point. The Solen, or razor-shell fish, has a foot of 

 a cylindrical shape, tapering 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 



* Journal de Physique, xxviii. 244. 



