MOLLUSCA 



257 



On the other hand, it is among the more active types — the 

 cephalopods — that the ancient geological forms have least suc- 

 cessfully adapted themselves to modern conditions. The cepha- 

 lopods 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, 



Pig. 116. 



Fig. 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 about $15,000,000 annually are 

 derived from our oyster fisheries. There are several species of 

 17 



