COPEPODA 



775 



The main points to be noted in the specific determination of the genus are: 

 length and number of segments in the antenna of the female; 

 armature of the antennal segments, especially of the terminal segments; 

 form of the abdomen, especially the form and armature of the furcal rami; 

 form and armature of the rudimentary fifth feet; 



structure of the second antennae, of the maxillipedes, and of the swimming feet. 

 These last structures are of less importance. 



77 (98) Antennae composed of twelve or more segments. ...... 78 



78 (92, 93) Antennae composed of seventeen segments 79 



79 (80) Fifth feet composed of one segment armed with one spine and 



two long setae Cyclops ater Herrick 1882. 



It is a large dark-colored species, rather rare, probably distributed very 

 widely, and growing in shallow water. In spite of its wide distribution, 

 however, it is a rare form. 



Length of female, 1.77 to 2.88 mm. 



FIG. 1213. Fifth foot of Cyclops ater. X 296. (Original.) 



80 (79) Fifth feet composed of two segments. . '." ''."". .'.'.'. . . . 81 



8 1 (84, 89) Second segment of fifth feet armed with seta and short spine. 82 



82 (83) Spine of second segment of fifth feet small and near end of seg- 



ment; last three segments of female antenna without hya- 

 line membrane Cyclops viridis Jurine 1820. 



FIG. 1214. Abdomen of fe- 

 male Cyclops viridis, var. 

 americanus. X 77- (Origi- 

 nal.) 



FIG. 1215. Abdomen of 

 female Cyclops viridis, 

 var. brevispinosus. X 

 66. (Original.) 



FIG. 1216. Fifth foot of 

 Cyclops viridis. X 218. 

 (Original.) 



A widely distributed species, being found both in pools and lakes. It varies greatly in its 

 form and general appearance, so that it has received a number of different specific names, 

 which are now reduced to varieties, since it has been found that there are intermediate forms 

 showing all the stages between the extremes. When living in pools it is apt to be deeply 

 colored, while its relatives living in the open waters of our lakes are colorless and almost trans- 

 parent. Especially noticeable is the difference in the form of the furcal rami, as shown in 

 Figs. 1214 and 1215. The forms found in pools generally have comparatively short and stout 

 furcal rami; on the other hand, the forms in deep waters have long and slender furcal rami. 

 Even in the limnetic forms there is wide variation. In typical viridis there is a short seta on 

 the outer angle of the furcal ramus. This is replaced in the form which Herrick called brevi- 

 spinosus by a short broad spine. This variety is a common limnetic form in some classes of 

 lakes; a form with the furca armed at its outer angle with a seta like typical viridis, but differing 

 from viridis in the structure of the swimming feet and of the fifth feet, called americanus, is 

 common in shallow waters, and is the variety that is most frequently seen in the waters of the 

 United States. Wherever a collection is made one is likely to get some form of viridis, and 

 generally it will be americanus. 



Length of female, 1.25 to 1.5 mm. 



