THE HARD-SHELL CLAM 85 



Class : Pelecypoda 

 THE HARD-SHELL CLAM {VENUS MERCENARIA) 



This is a very common marine mollusk which inhabits the 

 sandy bottoms of the ocean along the Atlantic coast. The soft- 

 shell clam {Mya arenaria), which lives in mud fiats visible be- 

 tween tides, resembles it very much in structure and may be 

 used for this dissection. 



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

 and is 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 truncated ; the posterior end is elongated. Which is 

 the right-hand valve ? The elevation on each valve near the hinge 

 ligament is called the umbo. It is the oldest portion of the shell ; 

 from it as a beginning point the shell has grown in size to its pres- 

 ent proportions by additions to its ventral edge. Note the parallel 

 lines of growth. The ventral edges of the shell are thus the young- 

 est 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). Since the shell is kept closed by the contraction of the 

 two muscles which pass between the valves, it will gape open as 

 soon as the animal is dead and the muscles are relaxed. It is the 

 elasticity of the hinge ligament which causes it to open.^ 



1 The shell may also be opened by inserting a sharp, wedge-shaped instrument 

 between the valves. The valves are thus pressed apart far enough to admit the 

 blade of a scalpel, by means of which the adductor muscles should be cut close to 

 the left valve of the shell. The hinge ligament should then be cut and the left 

 valve removed. 



