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 (Mya 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 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 
