88 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
KEY TO THE FAMILY CYCLOPIDE. 
I. MANDIBLAR PALP OBSOLESCENT, a mere tubercle bearing two filaments. 
a. Second maxillepeds prehensile. . ..... . . . +. Thoreliia. 
b. Second maxillepeds not prehensile. . . . ... =... =. Cyclops. 
II. MANDIBLAR PALP TWO-BRANCHED. 
a. Secondary branch of palp many-jointed. . . . . . . . . Oithona. 
b. Secondary branch one-jointed. 
* Fifth pair of feet foliaceous. . . . . . . Lophophorus. 
** Fifth pair of feet cylindrical, two- or three jautted: . . . Cyelopina. 
". All these genera except Cyclops are marine. 
GENUS CYCLOPS. 
The sole representative of the genera of the Cyclopidw here treated 
is the best known of the Copepoda.. Every 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. Autenne short, of less than 
twenty segments, in the male geniculate on both sides. Antennules 
one-branched, four-jointed. Mandiblar palp 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. Living 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. Antenne 18-jointed. 
II. Antenne 17-jointed. 
a. Fifth foot one-jointed . Rn mE ry tray trea a CY 15) Os Lele 
b. Fifth foot two-jointed. 
beget 
