MOLLUSCA. 



22 3 



records of life. They ruled the early seas. They have 

 always been numerous both in species and individuals. 

 They occupy the ocean at all depths, the fresh waters, 

 and the land. They devour, and in turn furnish food 

 for, many other forms. They have their share of para- 

 sites; there are boring snails that rasp through the shell 



FIG. 72. Mya arenaria, a burrowing clam. The siphon is represented as fully 

 extended. This is quickly retracted when the animal is disturbed. 



Questions on the Figure. Why the much elongated siphon? 

 Find other figures of syphons and compare. Which is the anterior 

 end of the animal? What seems to be the chief function of the 

 foot in this case? 



