252 MICROSCOPY FOR BEGINNERS. 



short hairs on the edges, and often reaching to the pos- 

 terior margin of the shell. The surface is usually 

 marked by a few conspicuous diagonal lines. The ani- 

 mal's movements are slow. 



9. SIDA. 



The shell is long and narrow, with the head separated 

 from the body by a depression. The posterior margin 

 is nearly straight, and has no spine or tooth. The 

 antennae are large, and somewhat resemble those of 

 Daphnia, although in Sida they are rather stouter, and 

 are divided into two unequal branches. There is but 

 one species Sida crystallma. It is quite common in 

 some localities. 



10. DIAPTOMUS (Fig. 165). 



Diaptomus may be recognized by the very long an- 

 tennse, which are often as long as the body. The latter, 

 including the head, is formed of six joints, and the pos- 

 terior narrower part or abdomen of five, although in 

 the female two of the latter 

 may be united, thus giving 

 it a three-jointed appearance. 

 The animal is among the 

 largest of the Entomostraca, 



Fig. 1C5 Di&ptomus. 



often 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 individual. It may be deep 



