CRUSTACEA 



139 



crab. (Pinnotheres ostreum) the female of which lives in 

 oysters. 



The water fleas just visible to the naked eye are present 

 in nearly all waters and 

 become very abundant 

 in stagnant water. The 

 common fresh water gen- 

 era are Cypris, Daph- 

 nia, and Cyclops. They 

 make up a large part 

 of the food for young fish. Many species of small crus- 

 taceans are parasites on fish or other aquatic animals, 

 and because of their habits become so changed in ap- 



Fm. 161. Daphnia enlarged forty times. 



FIG 1G2. Acorn-shell barnacles on a sand dollar. 



