CHAPTER VIII 



THE DAPHNIA AND ITS ALLIES 



Relationships. Daphnia 1 belongs to the division of 

 Crustacea called Entomostraca. 2 The Entomostraca 3 are 

 distinguished from the higher Crustacea the Malacos- 

 traca by the negative character that the number of 

 segments and appendages in the body is variable, instead 

 of there being constantly 19 pairs of appendages. 



The group of Entomostraca to which Daphnia belongs 

 comprises the more primitive of living Crustacea. They 

 occur in both fresh and salt water and, excepting Protozoa, 

 are the most abundant aquatic animals. They are of great 

 economic importance, since they constitute the main food 

 supply of fish. On the coast of Norway and Scotland the 

 fishermen prepare for a catch of herring or mackerel when 

 the sea becomes red with Entomostraca. Whalers like- 

 wise seek their booty where these Crustacea are abundant 

 on the open seas, for even the whalebone whales devour 

 the small animals in great quantity, straining them out of 

 the sea-water by means of their whalebone strainers. De- 

 spite all their enemies, the numbers of Entomostraca are 

 maintained by virtue of an enormous fertility. The 



1 From Aa0j/r;, daughter of the river-god Peneus ; she was transformed 

 into a laurel tree. 



2 evTojjwv, cut into, segmented ; 6ffrpaKov, shell. 



8 The five orders of Entomostraca may be distinguished by means of 

 the key given at the end of this Chapter, page 131. 



125 



