88 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



KEY TO THE FAMILY CYCLOPID.E. 



I. Mandiblak palp obsolescent, a mere tubercle bearing two filameDts. 



a. Second maxillepeds prehensile Thorellia. 



b. Second maxillepeds not prehensile Cyclops» 



II. Mandiblar palp two-branched. 



a. Secondary branch of palp many-jointed Oithoiia. 



i. Secondary branch one-jointed. 



* Fifth pair of feet foliaceous Lopliopliorus. 



** Fifth pair of feet cylindrical, two- or three-jointed. . . . Cyclopina. 



All these genera except Cyclops are marine. 



GENUS CYCLOPS. 



The sole representative of the genera of the Cijclopidxc here treated 

 is the best known of the Gopepoda. Ev^ery one is familiar with the 

 "common Cyclops,''^ but few realize how many are the species included 

 under this name. An attempt is here made to enable the student to 

 recognize the more obvious distinctions upon which the genus is sub- 

 divided and to identify such of the species as seem valid and at the 

 same time recognizable without recondite study of development. 

 Without attempting a complete elucidation of the synonymy, which 

 is practically an impossibility, a proximate classification of all the 

 species known to me is attempted. 



Diagnosis: Cephalothorax robust, oval, broadest in front, com- 

 posed, in the female, of nine segments. Antennie short, of less than 

 twenty segments, in the male geniculate on both sides. Antennules 

 one-branched, four-jointed. Mandiblar j)alp a small two-setose tu- 

 bercle. First four pairs of feet two branched, all the branches three- 

 jointed. Fifth feet small, one- to three-jointed. In the male a still 

 more rudimentary sixth pair. Eye single but composed of two facets. 

 Ovisac double. 



As stated by Brady, the genus Cyclops is, as regards discrimination 

 of species, one of the most difficult and puzzling of all the Copepoda' 

 Chief among the causes of this difficulty is the polymorphism and het- 

 erogenesis which prevail, as well as the great susceptibility to the 

 effects of the environment. Liviog as they do, in pools and streams, 

 undergoing great seasonal and other variation, these forms offer ex- 

 ceptional opportunities for the study of the plasticity of species. 



KEY TO THE GENUS CYCLOPS. 



I. Antennae 18-jointed. 



II. Antennte IT-jointed. 



a. Fifth foot one-jointed ater, 89. 



6. Fifth foot two-jointed. 



