252 



ZOOLOGY. 



fact that there is scarcely a nook into which they have not 

 penetrated, except where continuous drouth prevails. On the 

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

 opods that the ancient geological forms have least success- 

 fully adapted themselves to modern conditions. The cephal- 

 opods appear much less numerous and varied now than in 

 earlier geological time. 



295. Classification. The following are the principal classes : 

 Class I. Pelecypoda or Lamellibranchiata (Mussels, Oysters, etc.). 

 Lamellibranchs are mollusks in which the fundamental bilateral sym- 



FIG. 114. 





FIG. 114. Mya arenaria, a burrowing clam. The siphon is represented as fully ex- 

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



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? 



