allee] small crustaceans 109 



line of fish food that may be traced to its beginning. Thus fish 

 eat crayfish, crayfish eat isopods and amphipods, which in turn 

 eat entomostraca. The entomostraca live on smaller organisms, as, 

 for example, the protozoa, and are reported to be able to absorb, 

 through their external covering, food substances dissolved in the 

 water. One other example of the importance of entomostraca 

 as animal food is that they are almost the sole food of the fresh 

 water clam. In fact they are the basis of the food supply of all 

 the animals living in our fresh waters, and in addition are im- 

 portant as water scavengers. Although many other representa- 

 tives of each group are parasitic, yet, so far as I know, none of 

 these fresh water forms are harmful to man. 



Summary of Characteristics. 



1. Isopods have a plainly segmented body. The thorax bears 

 seven pairs of similar legs. They are strongly flattened dorso- 

 ventrally. They move by crawling and seldom if ever swim. 

 The adult size is from two-fifths to two-thirds of an inch in 

 length. 



2. Amphipods also have a plainly segmented body, the legs 

 are less prominent than in the isopods and they are flattened 

 laterally rather than dorso-ventrally. They move mainly by swim- 

 ming and often swim on their side. They may reach an inch in 

 length but are more often taken about a quarter of an inch long. 



3. Palaemonetes, the common shrimp, closely resembles a 

 small crayfish except that it is a semi-transparent light green 

 color and that it lacks the large pinchers of the crayfish. The 

 adult is from one to one and a half inches long. 



4. Eubranchipus, or the fairy shrimp, is about the same size 

 as the common shrimp, but it has no carapace, the thorax is 

 plainly segmented, the appendages are leaf-like, and the animal 

 swims on its back. 



5. Copepods are very small, about one-sixteenth of an inch 

 long. They have cylindrical bodies that are plainly segmented, 

 and carry their eggs in sacs that hang from the under side of the 

 animal near the posterior end. 



6. O'Stracods vary in size from microscopic animals up to 

 those a quarter of an inch long. The body is entirely enclosed in 

 a chitonous bivalve shell which may be closed as in the fresh 

 water clam. 



7. Cladocerans are usually less than one-eighth of an inch in 

 diameter. The body is not completely enclosed in a shell and the 

 shell may be reduced to a small plate completely covered by the 

 body wall. 



