258 AQUATIC MICROSCOPY FOR BEGINNERS. 



of the Entomdstraca, meaning little animals in a bhell, 

 but the translation of the word has no distinctive sig- 

 nification, since members of both groups are little ani- 

 mals and both have shells. 



The Entomostraca are more abundant in fresh water 

 than the Phyllopoda, and are remarkably active. 

 They are usually visible to the unaided eye as little 

 whitish specks, skipping, flirting, or jerking them- 

 selves through the water, although probably few. will 

 measure more than one-twelfth of an inch in length. 

 Under the microscope some are, as already stated, 

 seen to be enclosed in a bivalve shell, while others are 

 entirely free from so distinct a covering. 



The feet are arranged in pairs, and may be numer- 

 ous. They serve in the- shell-bearing forms, not only 

 as swimming organs, but as gills o'r similar contri- 

 vances for the absorption of air. from the water for the 

 aeration of the little animals' blood. This is probably 

 one reason why they are. kept in such incessant 

 motion. Even when the shell-bearing Entomostraca 

 come to rest, to feed, or for some other purpose, certain 

 of the feet keep up a ceaseless beating of. the water, 

 as can be readily seen through their transparent case. 



The mouth-parts are complicated, much patience 

 and microscopical skill being needed to investigate 

 and understand them. On each side of the head, 

 however, and usually near the mouth, are two thread- 

 like'but jointed organs called the antennse, and these 

 the beginner must recognize, as they often become 

 important aids in learning the animal's place and 

 name. They vary in length, one on each side often 

 being short and difficult to see distinctly, while the 

 other two are usually long and conspicuous. They are 



