CHAPTER XIV 
THE MOLLUSKS 
171. The Mollusks. — This group includes such animals as 
clams, oysters, snails, slugs, squids (skwids), and octopi 
(ok'to-pi). These forms differ from the crustaceans in 
having a soft, unsegmented body and, in most cases, a 
shell as their exoskeleton. The squids have a shell that 
is internal, and in some 
of the snails the shell is 
absent. 
172. Clams.— The fresh¬ 
water clam is a convenient 
type of mollusk to study. 
It is found in canals and 
in many streams and lakes. 
This clam has two shells Water enters throu £ h ls -> inhaIent 
. . . siphon, and leaves the body of the clam 
or valves and, when mov- through e ^ exhaIent siphon . 
ing naturally, the hinge is 
uppermost, while the opened valves allow the foot to be 
extended into the mud. The foot is a thick, muscular mass, 
not at all foot-like in appearance, but it enables the clam to 
move, although slowly and at an uneven rate. 
Structure. — The structure of the fresh-water clam shows 
how it has adapted itself to its peculiar method of life. 
The shell is lined with a membrane called the mantle. 
The mantle secretes the shell-material and adds to its size 
year by year. At the back, the edges of the mantle are 
united at three points, thus forming two openings known as 
siphons (sl'fons). Through one of these siphons water 
Figure 170. — Clam Showing Foot. 
