PHYLUM MOLLUSCA 



Class : Pelecypoda 

 A FRESH-WATER CLAM {ANODONTA OR UNTO) 



These animals are common in most parts of the country ; they 

 inhabit the sandy or gravelly bottoms of fresh-water streams and 

 lakes. There are many species of fresh-water clams, two com- 

 mon genera being Anodonta and Unio, the former of which is 

 characterized by the thinness of the shell and the latter by its 

 thickness. 



Study first the live animal, if possible. Its body is unsegmented 

 and entirely inclosed in a bilateral, bivalve shell, which is the 

 cuticula of the animal richly charged with calcareous salts. The 

 two valves of the shell cover the right and left sides of the animal 

 and are joined together on its dorsal side by the dark-colored 

 hinge ligament, while their ventral edges are open ; the animal is 

 thus very much compressed laterally. The anterior end of the 

 animal is more rounded and less elongated than the posterior end. 

 Which is the right-hand valve ? The elevation on each valve near 

 the hinge toward the forward end is called the umbo. It is the 

 oldest portion of the shell ; from it as a beginning point the shell 

 has grown in size by additions to its ventral edge. Note the par- 

 allel lines of growth. The ventral edges of the shell are thus the 

 youngest portions of them. 



Exercise 1. Make a drawing of the right-hand valve, indicating the 

 anterior, posterior, dorsal, and ventral aspects, and showing the 

 lines of growth. 



Exercise 2. Make a drawing of the dorsal aspect of the animal. 



Kill the animal by immersing it for a few minutes in hot water 

 (70° C). As the shell is kept closed by the contraction of the two 

 muscles which pass between its valves, it will gape open as soon as 



