774 FRESH-WATER BIOLOGY 



74 (75) Endopodites of all swimming feet composed of three segments, 

 antennae of 23 segments (according to Herrick 24), furca 

 short. Only one species. 



Osphranticum labronectum Forbes 1882. 



Found widely distributed in the United States, more frequently 

 in the Mississippi Valley but never in large numbers, so that it is 

 comparatively rare in collections. 



Length of female, 1.7 mm. 

 Length of male, 1.36 mm. 



FIG. 121 1. Abdomen of female Osphranticum labronectum. X Si. 

 (Original.) 



7S (74) 



L 



Endopodites of all swimming feet composed of three segments, 

 antennae of 25 segments, furca long. 



Limnocalanus macrurus Sars 1862. 



Found only in deep lakes. It is especially interesting, as it is the only 

 species of the Centropagidae found in both Europe and America. It is 

 widely distributed in northern Europe and Asia and is found in salt water 

 as well as in fresh. It is considered a representative of the "fauna re- 

 licta," that is, it is a salt water form which has become adapted to the 

 environment of fresh water. 



Length of female, 2.4 mm. 



Length of male, 2.2 mm. 



FIG. 1212. Abdomen of female Limnocalanus macrurus. X 37. (Original.) 



76 (2) Antennae short, never longer than cephalothorax, generally much 

 shorter, and composed of from six to seventeen segments; 

 antennae of male symmetrically geniculate; fifth feet rudi- 

 mentary, composed of from one to three segments. 



Family CYCLOPIDAE. 

 Only one genus Cyclops . . 77 



