THE WATER FLEAS (CLADOCERA) 685 



Little is known regarding the length of life of the individual 

 Cladocera. It doubtless varies from a few weeks to several months. 

 Limnetic forms probably have a longer life than the littoral, as the 

 food supply and other conditions of life are more constant. Individ- 

 uals from the broods of Daphnia longispina (hyalina) which are born 

 late in October and in November may survive through the winter 

 and produce one or more broods of young in the spring. The last 

 survivors die in June, weakened by old age and attacked by par- 

 asitic fungi. This is probably about the maximum length of 

 life. 



The Cladocera are found in all sorts of fresh waters. Lakes and 

 ponds contain a much larger number of forms than do rivers. 

 The shallow, weedy backwaters of a lake whose level is fairly 

 permanent harbor a greater variety of species than does any other 

 kind of locality. Here are found almost all of the Chydoridae and 

 Macrothricidae, as well as most of the representatives of the other 

 families. In such localities are found the best conditions for the 

 life of these animals: warmth, shelter from enemies, and abundant 

 food. It must not be supposed, however, that each square rod of 

 such waters harbors a like population. On the contrary, anyone 

 who collects frequently in one lake will come to know certain places 

 as especially favorable to these creatures, which are present in 

 greater number and variety than in places apparently quite simi- 

 lar and closely adjacent. While by far the greater number of 

 species belong to the littoral region, living among the weeds and 

 feeding on algae and similar organisms, a few species live near the 

 bottom. Several species are commonly found in or near the mud, 

 although not specially adapted to a life in the mud; such are 

 Alona quadrangularis and Drepanothrix. The genera Ilyocryptus and 

 Monospilus live regularly on the bottom; their structure is adjusted 

 to a life in the mud and their shells are often overgrown by algae. 

 These forms may and do swim, but more often scramble about on 

 the bottom, pulling with their antennae and pushing with the post- 

 abdomen. In both forms the old shell is not cast off in molting, 

 the new and larger shell appearing beneath it (Figs, mo, 1168). 



The species of Moina are found most commonly in muddy pools, 

 such as those in brick-yards, though not confined to such waters. 



