CRUSTACEANS 



size, but to their enormous reproductive capacity. One water- 

 flea {Daphnia pulex) produces a brood of eggs every two or 

 three days and with its descendants it is said to produce 

 13,000,000,000 in sixty days. Like many other entomos- 

 tracans, this httle water-flea has two kinds of eggs — thin- 

 shelled eggs produced all through the summer which de- 

 velop without fertilization, and thick-shelled eggs produced 

 in autumn. These autumn eggs are regularly fertilized by 

 male cells. Their shells are thickened and sometimes pro- 

 vided with air cells like the statoblasts of the bryozoan, Pec- 

 tinatella (Fig. 107), and they are finally enclosed in a saddle- 

 like cover, the ephippium, which forms around them. 



Fig. 125. — A water-flea, Ceratodaphnia reticulata, 

 carr>'ing resting egg in the ephippium. 



Ceratodaphnia carries the ephippium about on her back for 

 some time, but eventually it falls off and drops to the bottom or 

 the animal dies. Such resting eggs will live through the win- 

 ter or long droughts, and then develop into daphnids which go 

 on with routine life again. 



There is a large cladoceran population in the open water, 

 which feeds on floating diatoms and algae of the plankton 

 (p. 51) and constitutes a large part of plankton society. They 

 are abundant through the summer, at night feeding close to the 

 surface of the water and in the daytime dropping to a little 

 lower level. Cladocerans can easily be seen swarming near 

 shore and thousands of them can be dipped up in one cup of 

 water. 



165 



