136 TWELFTH ANNUAL KEPORT. 



is five-jointed, though in the female these joints are so united as to 

 cause the abdomen to appear three-jointed. The antennae are 

 twenty-five-jointed, and the right male limb is modified by a coal- 

 escing of some of the terminal joints, a thickening of others and 

 the development of certain spines, hooks, and knife-like ridges to 

 form a prehensile organ. The first pair of feet has two-jointed 

 inner rami. The remainder have both rami three-jointed, save the 

 last. This fifth foot is differently formed in the sexes, the inner 

 branch of the fifth foot being one or two-jointed. Terminal joint of 

 the outer ramus of this limb in the female very small or apparently 

 absent, second joint produced to form a stout curved claw. The left 

 foot is reduced in the male, serving, in some species, to affix the sper- 

 matophore to the body of the female, while the abdomen is held by 

 the right foot. The last segment of the thorax has one or two sharp 

 spines below. The spermatophore, or sperm case, is a long tube 

 with coagulating expansive lining, which forces out its contents on 

 exposure to the water. The colors are frequently brilliant. 



Three or four species of this genus are known in Europe, the 

 first being Diaptomus castor which seems universally distributed. 

 It can hardly be doubted that the six forms mentioned below belong 

 among the varieties of this species; yet these forms can be disting- 

 uished very well, and are deserving of distinct names. Two other 

 forms are nearest D. gracilis of Sars, but sufiicieutly distinct. These 

 stand related as do the pairs of the other section, and can not be 

 readily distinguished. 



The following is the most convenient arrangement of the genus 

 I have been able to devise. 



Key to the Genus Diaptomus. 



I. Form robust; right antenna of the male with a hook, much 

 swollen anterior to the geniculatiug joint. 



A. Head not greatly dilated. 

 * Last segment of thorax prolonged into a sharp-spined angle or tooth. 

 t With but one tooth (?). 



1. D. c(xsior, Jurine. 

 t+ With two teeth. 



t Length under 3 rum. 



§ Inner rami of fifth feet in the female 1 -jointed. 



2. D. sanguineus, Forbes. 

 4. D. armatus, Herrick. 



§§ Inner rami 2-jointed. 



3. D. minnetonka, Herrick. 

 ti Length over 3 mm. 



