1 86 



Agnatic Organisms 



herbivores of the open water. They are themselves 

 important food for fishes. 



The importance of water fleas in the economy of 

 water is largely due to their very rapid rate of reproduc- 

 tion. During the summer season broods of eggs suc- 



Fig. 92. A water-flea (Simocephalus vetulus) in its ordinary 



swimming position. Note the striated shell, and the ali- 

 mentary canal, blackish where packed with food-residue in 

 the abdomen. 



cessively appear in the chamber enclosed by the shell 

 on the back of the animal (see figure 93) at intervals 

 of only a few days. The young develop rapidly and 

 are themselves soon producing eggs. In Daphne pulex, 

 for example, it has been calculated that the possible 



