PHYLUM ARTHROPODA — CLASS CRUSTACEA 283 



reason they must live in moist plac6s. They are a garden pest in 

 that they eat leaves of delicate plajits. There are a number of 

 aequatic forms which are parasitic on fish and others, such as the 

 gribble (Limnoria), which tunnel in submerged wood. 



The amphipods are sand and beach dwellers which may be found 

 burrowing or jumping around on the seashore or walking on the bot- 

 tom in fresh water. Gammanis is the best known fresh-water form. 

 The legs of representatives of this order are divided into two groups, 

 with the legs of each group pointing toward each other. These are 

 of particular value as fresh-water fish food. 



Entomostraca as a group is composed of many smaller crustaceans 

 occurring in great numbers in both marine and fresh waters. The 

 fairy shrimps (EuhrancJiipus) are delicate, transparent and feathery 

 appearing. They are about three-fourths of an inch in length. They 



Fig. 159. — Asellus, a common fresh-water crustacean. (Courtesy of General Bio- 

 logical Supply House.) 



swim with the ventral side up and their long, leaf-like appendages 

 hang from the body; these appendages serve also as respiratory 

 organs. The^^ live in cool streams during the spring ajid fall. The 

 summer is passed in the egg, which can withstand complete dryness. 

 Many of them are parthenogenetic, hence, males are rare. The 

 common marine form is Artemia, often called brine shrimp. 



The water fleas including Daphnia of order Branchiopoda, Cyclops 

 and Diaptomus of order Copepoda and other small Crustacea con- 

 stitute an important common group. Daphnia is one that is en- 

 closed in a delicate bivalve shell. The second pair of antennae are 

 very large and are used in swimming. The shell is beautifully 

 marked and terminates in a caudal spine. They are only about 

 one-tenth of an inch in length. Cyclops is another common fresh- 

 water form with the antennae shorter than the cephalothorax whose 



