CHAPTER V 
MOLLUSCA 
PELECYPODA 
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 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. 
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