entom6straca and PHyLL6P0DA. 271 



terior margin is nearly straight, and has no spine nor 

 tooth. The antennae are large and somewhat resem- 

 ble those of Daphnia, although in Sida they are rather 

 stouter, and are divided into two unequal branches. 

 There is but one species, Sida crystdllina. It is com- 

 mon in some localities. 



10. DiAPTOMUS (Fig. 184). 



Diaptomus may be recognized by the very long an- 

 tennae, which are often equal to the body in length. 

 The stout body, including the head, is composed of 

 six joints or segments, while the posterior, narrowed 

 or tapering region, the abdomen, of five, although in 

 the female, two of the latter may be united, thus giv- 

 ing it a three-jointed appearance. 



The animal is among the largest of the Entomos- 

 traca, frequently measuring one-tenth of an inch in 

 length. The color is often brilliant, varying in the 

 different species, and even in the different parts of 

 the body of the same specimen. It may be deep red, 

 brilliant purple, bluish with purple-tipped antennae, 

 whitish or colorless. The animals may found in 

 shallow pools in the fall and early spring, and occa- 

 sionally in ■ slowly flowing streams. The external ovary 

 is single. 



Fig. 184. — Diaptomus. ' Fig. 185.— Canthocamptus with a young form. 



