DAPHNIA PULEX Miiller 



Material. Specimens of Daphnia pulex may be collected 

 at any time of the year, but are more abundant in spring. It 

 is the common large Daphnia of our fresh-water ponds. The 

 animals may be kept well in large aquaria, provided all hydras 

 have been carefully removed. Every student should receive 

 several live specimens. 



Descriptive Part 



Daphnia pulex is a common representative of the Suborder 

 Cladocera, Order Entomostraca, Class Crustacea. These little 

 crustaceans inhabit the fresh-water ponds of North America and 

 Europe and may occur in millions in small pools. They swim by 

 the aid of their second antennae which are developed into power- 

 ful organs for a jerkwise method of propulsion. Their food con- 

 sists mostly of microscopic algae. Owing to their ability to 

 propagate parthenogenetically, i. e., without fertilization, one 

 meets during the spring and summer with scarcely any males. 



The body of Daphnia pulex, like that of all arthropods, is 

 segmented or composed of somites. But the somites can be 

 recognized only by their appendages of which there is never 

 more than one pair to a somite, and by the internal metamer- 

 ization of the organs. We shall see later that the number "of 

 somites in higher crustaceans, is definite and that the entire 

 body may be readily divided into a head, thorax and abdomen; 

 the head is composed of six somites, the thorax of eight and the 

 abdomen of seven. But the Entomostraca form a group of 

 crustaceans in which near relatives may have a different number 

 of somites. Only the head is always composed of six somites and, 



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