ENTOMOSTRACA AND PHYLLOPODA. 239 



in a shell, but the translation of the word has no dis- 

 tinctive signification, since members of both groups 

 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 specks, 

 skipping, flirting, or jerking themselves through the 

 water, although probably few will measure more than 

 one-tenth of an inch in length. Under the microscope 

 some are, as already stated, seen to be enclosed in a 

 bivalve shell, and others are entirely free from so dis- 

 tinct a covering. The feet are arranged in pairs, and 

 may be very numerous. They serve, in the shell-bear- 

 ing forms, not only as swimming organs, but as gills or 

 similar contrivances 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 inces- 

 sant motion. Even when the shell - bearing Entomos- 

 traca 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 un- 

 derstand them. On each side of the head, however, and 

 usually near the mouth, are two thread-like but jointed 

 organs called the antennae, and these the beginner must 

 recognize, as they often become important aids in learn- 

 ing the animal's name. They vary in length, one on 



