60 



CLASS CONCHIFERA.. 



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 suflicient for respiration, and that they keep 

 the valves closed till the return of the tide ; whereas, 

 those Oysters which are taken from greater depths, 

 Avhere 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 expe- 

 rience, improvidently open their shells after the sea has 

 left them ; and, by allowing the water to escape, soon 

 perish. 



The CanUioH, or Cockle. 



Many bivalve Mollusca are provided with an instru- 

 ment shaped like a leg and foot, which they employ 

 extensively for progressive motion. In the Cardiumy or 

 cockle, this organ is composed of a mass of muscular 

 fibres, interwoven together in a very complex manner, 

 and which may be compared to the muscular structure 

 of the human tongue ; the effect in both is the same, 

 namely, the conferring a power of motion in all possible 

 Avays ; thus it may be readily protruded, retracted, or 

 intlected at every point. 



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



