126 ZOOLOGY 



Entomostraca are rich in species also, partly because they 

 occur in such diverse environments. Thus they live in 

 fresh water, in the sea, and even in the Great Salt Lake 

 and in vats where salt is crystallized out. They live in 

 little pools, such as dry up in summer. They are found 

 also as parasites on the gills or in the skin of fishes. 



Habitat and Food. - - Daphnia 1 lives in ponds, lakes, and 

 slow-running streams over all the globe. During the fall 

 in northern latitudes the Daphnias, of at least certain 

 species, lay fertilized eggs, called winter eggs, which may 

 lie dormant, however, not merely for the winter but 

 throughout an entire year. During most of the year 

 females alone occur and unfertilized "summer eggs" are 

 alone produced. 



The abundance of Daphnia in any pond is determined 

 by a number of causes. One of the most important of 

 these is food. The food of Daphnia consists chiefly of 

 fresh-water algre, such as nostocs and diatoms ; and it has 

 been shown that the abundance of Daphnia in a pond is 

 closely determined by the abundance of the kind of alga 



1 Key to the principal genera of the family Daphnidse : 



a i. Head rounded, not beaked. 



&i. Antennules long ; abdomen not wholly cov- 

 ered by shell Moina 



b-2. Antennules short ; whole body enclosed in 



shell Ceriodaphnia 



a-2- Head beaked below. 



bi. Beak slight ; shell angled below or extending 

 in long spines from lower angle ; pigment 

 spot roundish Scapholeberis 



ft 2 . Shell rounded below, with a blunt spine above ; 



pigment spot elongate .... Simocephalus 



6 3 . Shell extending in sharp spine at upper pos- 

 terior angle ; pigment spot small . . Daphnia 



