CHAPTER IX 



THE MOLLUSKS 



89. The Mollusks. — This group includes such animals as 

 clams, oysters, snails, slugs, squids (skwids), and octopi 

 (6k't5-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 the shell is absent in some of the snails. 



90. 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 or 

 valves and, when moving naturally, the hinge is upper- 

 most, while the opened valves allow the foot to be ex- 

 tended into the mud. 

 The foot is a thick, mus- 

 cular mass, not at all 

 foot-like in appearance, 

 but it enables the clam 

 to move, although slowly 

 and at an uneven rate. 



91. 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, 



94 



Figure 93. — Clam Showing Foot. 



Water enters through i.s., inhalent siphon, 

 and leaves the body of the clam 

 through e.s., exhalent siphon. 



