CHAPTER VIII 



BRANCH MOLLUSCA: THE OYSTER, CLAMS, AND THEIR 



ALLIES 



THE FRESH-WATER CLAM 



EXTERNAL ANATOMY 



I. — Place a clam in a glass of tepid water and observe 



the gradual opening of the shell, and the protrusion 

 from one end of a white, blunt, muscular projection, 

 the so-called foot. This marks the anterior end of 

 the animal, and by its protrusion and retraction the 

 clam steadily moves along the sand, leaving grooves 

 made by the sharp edges of the shell. In removing 

 the creature from the water in preparation for the 

 study of its structure, notice how the two halves of the 

 shell are quickly brought together and the foot with- 

 drawn. 



II. — The lateral surfaces are easily distinguished. They 

 are the broad oval surfaces upon which the clam rests 

 when placed upon the table. The sharp edges of the 

 two parts of the shell form the ventral surface. It is 

 this surface that makes the groove markings in the 

 sand, mentioned above. The opposite thicker edge is 

 the dorsal surface. 



III. — With a clear idea of the lateral, dorsal, and ventral 

 surfaces in mind, determine the anterior and posterior 

 ends by the aid of the following directions. On the 



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