266 UNIVERSITY OF COLORADO STUDIES 



part of the zooplankton of both fresh and salt waters, are much more 

 abundant in standing water than in streams, and find most favorable 

 conditions in small bodies of water with much plant growth, or in 

 shallow margins of larger bodies that present similar conditions. So 

 common are these forms that every lake and pond has its entomos- 

 tracan fauna. 



Aside from their general scientific interest, the Entomostraca are 

 of considerable economic importance. Many young and small fishes 

 in both salt and fresh water feed almost exclusively upon these forms, 

 and they are an important part of the diet of many of the larger ones. 

 Also they are fed upon largely by certain insect larvae. The small 

 fish and the insect larvae are in turn eaten by larger fish. The 

 Entomostraca themselves feed almost exclusively upon microscopic 

 plant forms which are abundant in all waters and furnish the ultimate 

 source of food for aquatic animals. Thus Entomostraca furnish the 

 first link in the utilization of this fundamental food supply. 



It is by no means possible in a few pages to give an adequate 

 account of the structure of so large and diverse a group and I shall 

 merely attempt to include a few facts necessary for a general under- 

 standing of specimens and identification of species. In the key, at 

 appropriate places, will be found more detailed accounts of certain 

 peculiarities of each group. 



In common with all Crustacea, the Entomostraca have a definitely 

 segmented body in which head and trunk regions may be recognized. 

 In by no means all, however, is the trunk definitely divided into 

 thorax and abdomen. The carapace, or covering of the body, is 

 variously developed, and takes diverse forms, so that the different 

 subdivisions of the group present a wide range of possibilities as to 

 general appearance. 



The appendages of the Entomostraca, especially those of the 

 trunk, are primitive. Those of the head are five in number, as in all 

 Crustacea — in each group modified in a characteristic manner. The 

 two pairs of antennae serve as sensory or as locomotor organs; and 

 either or both pairs may be well developed or much reduced. In 

 the males they may be peculiarly modified to serve as clasping organs. 



