256 



ZOOLOGY 



other hand, it is among the more active types the cephalo- 

 pods that the ancient geological forms have least success- 

 fully adapted themselves to modern conditions. The cephalo- 

 pods appear much less numerous and varied now than in earlier 

 geological time, although the types which have persisted are 

 the most perfect and active ones that have appeared. 



301, Relation to Human Interests. This is the first phylum 

 that we have studied that furnishes any food, worthy of mention, 



FIG. 1 1 6. 



PIG. 116. 



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

 quickly retracted when the animal is disturbed. (After Kingsley.) 



This is 



Questions on the figure. What is the function of the much elongated siphons? 

 Which is the anterior end of the animal? Which the dorsal side? What would 

 seem to be the chief function of the foot in this case? 



to man. It is estimated that $65,000,000 annually are derived 

 from our oyster fisheries. There are several species of clams 



