A HARD-SHELL CLAM 103 



PELECYPODA 



A HARD-SHELL CLAM {Venus mercenaria) 



This is a very common marine mollusk which inhabits the 

 sandy bottoms of the ocean along our shores. The soft-shell 

 clam (My a arenaria), which lives in mud flats between tides, 

 resembles it very much in structure and may be used for this 

 dissection. 



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

 mented and is 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 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 present proportions by addi- 

 tion to its ventral edge. Note the parallel lines of growth. 

 The ventral edges of the shell are thus the youngest portions 

 of them. 



Exercise l. 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 



