CHAPTER XXII 

 THE WATER FLEAS (CLADOCERA) 



BY EDWARD A. BIRGE 



Dean, University of Wisconsin 



WHEN men began to study nature by the aid of the microscope 

 in the seventeenth century the " insects" were among the first ob- 

 jects to be examined. In 1669, the Dutch physician, Swammerdam, 

 described in his history of insects the " pulex aquaticus arborescens" 

 the water-flea with branching arms. This was one of the 

 Cladocera, still called Daphnia pulex, the commonest species in 

 shallow pools. These creatures he described and figured, giving an 

 account of their structure and habits and speaking of their sudden 

 appearance in enormous numbers, and their equally sudden dis- 

 appearance. So the Cladocera made their debut into science along 

 with the microscope. 



For nearly a century little was added to the knowledge of the 

 group. In 1755, the German, Schaeffer, gave the first really good 

 account of their structure. In 1785, 0. F. Mueller, the Danish 

 naturalist, issued the first general systematic work upon Entomo- 

 straca. This described many of the species as we now know them 

 and gave a firm scientific basis for further knowledge of the Clado- 

 cera. In the rapid advance of science during the latter half of the 

 nineteenth century the systematic work of the group was substan- 

 tially done, the Norwegian, G. 0. Sars, having contributed more 

 than any other one man. This work showed that the Cladocera 

 constitute the largest group of fresh-water Crustacea in number of 

 species; the most diversified in size, in structure, and in habits. 



During the opening years of the present century the scientific 

 study of fresh-water life has advanced rapidly and the biology of 

 the Cladocera is receiving much attention. The conditions of 

 variation and the nature of the variants are examined, as well as 

 the conditions of sexual reproduction, the centers of origin and 

 dispersal of species, and other similar matters. 



The Cladocera are particularly well suited for study by those 

 persons who are interested in observing animals with the micro- 



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