264 AQUATIC MICROSCOPY FOR BEGINNERS. 



The two Entomostracans, Didptomus , and Canthocdmp- 

 tus, are separated in the Key by the number of the 

 joints in their long antennae. This, seems to be a very 

 minute character to use in so artificial a table, but it 

 need not be an annoyance to the reader, since the 

 antennae of these two common little crustaceans differ 

 so conspicuously in size and length that the joints 

 need not be actually counted; a glance will show 

 which is Canthocamptus, with its short, rather incon- 

 spicuous antennse, and the single egg-sack, and which 

 is Cyclops, with the long antenfiee and two external 

 egg-sacks. 



The beak ,referred to is the front part of the shell 

 extended in a long, usually curved and pointed pro- 

 longation, containing the eye and portions of the ani- 

 mal's head. 



Key to Genera of Entomdstraca and Phylldpoda. 



1. Legs with flat plates near the body; animal swim- 



ming on the back {K). 



2. 'Legs without flat plates («). 



a. Body enclosed in a bivalve shell (V). 



a. Body not enclosed in a shell iyg). 



b. Shell with a sharp posterior spine, or a tooth on or 



near the upper posterior angle (c). 



b. Shell without a posterior spine, or with from one 



to four small teeth on the lower posterior 

 angle id'). 



c. Smooth; spine on the upper angle, or near the 



middle of the border. . Ddphnia, i. ^ 

 c. Smooth, brown; spine on the lower angle. Sca- 



pkoUberis, 2. 

 c. Reticulated; spine on the lower angle; antennae 



large, cylindrical. Bdsmina, 3. 



