CHAPTER V 



MOLLUSCA 

 LAMELLIBRANCHIATA 



AN ASJPHONATE LAMELLIBRANCH. A FRESHWATER 

 MUSSEL (Anodonta or Unio) 



These animals are common in most parts of the country; 

 they inhabit the sandy bottoms of freshwater streams and 

 lakes. 



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

 mented and entirely enclosed in a bilateral, bivalve shell, 

 which is the cuticula of the animal richly charged with cal- 

 careous 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 towards 

 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 parallel lines of 

 growth. The ventral edges of the shell are thus the youngest 

 portions of them. 



Exercise i. 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. 



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