MOLLUSKS 121 
inhabitants of the sea. They differ greatly in size and shape, vary- 
ing from microscopic dimensions to four and a half feet in width, 
as in the giant clam (Tridacna gigas) of the tropical Pacific and 
Indian Ocean. This great clam lives half buried in the coral reefs 
with the sinuous gape of its shell uppermost. It commonly remains 
with its valves slightly open, showing the rich, blue edges of 
the mantle, but if the tide retreats or if it be suddenly disturbed 
the valves close tightly. The animal, exclusive of its shell, often 
weighs twenty pounds and, although coarse, is sometimes eaten by 
the natives. 
Excellent photographs showing these clams partially exposed 
at low tide are given by W. Saville-Kent in “The Great Barrier 
Reef of Australia,” Plate X XIX. 
The American Oyster, (Ostrea virginica, Fig. SO), is found 
from the Gulf of St. Lawrence to Texas, but is most abundant in 
Chesapeake and Delaware Bays and in Long Island and Pamlico 
Sounds. It thrives best in shallow bays and estuaries where the 
water is apt to be brackish. Our oyster fisheries are worth 
$16,600,000 annually. 
Oysters usually lie upon their sides, and the lower valve fas- 
tens itself to a rock or other firm anchorage by the shelly secretion 
of the mantle. This lower shell is almost always upon the left side 
of the body, and is quite deep and convex, while the upper shell 
is much flatter or even concave. At the narrow end of the shell 
we will see a dark brown, elastic hinge, which is so arranged 
that it tends constantly to cause the two valves of the shell to open, 
but this is prevented by a strong muscle that extends across from 
both sides of the oyster’s body and binds the valves together. 
This strong adductor muscle is commonly but erroneously called 
the “heart.” In the American oyster the place of attachment of 
this muscle to the shell is marked by a dark brownish-purple area 
while in the common Ostrea edulis of Europe this area is uncolored. 
When undisturbed, and under normal conditions oysters commonly 
remain with their shells gaping slightly open. ‘The inside of the 
shell is lined with a delicate membrane called the mantle, which 
grows out from both sides of the mid-dorsal line of the body and 
secretes the shell itself. 
There are also a pair of gills which arise from the sides 
