THE CRAYFISH 



231 



living of animals in a mutually helpful relation has been referred 

 to as sytnbiosis. Of this we have already had some examples in 

 plants as well as among animals. (See page 187.) 



Habitat. Most crustaceans are adapted to live in the water ; 

 a few forms, however, are found living on land. Such are the 

 wood lice, the pill bugs, which have the habit of rolling up into a 

 ball to escape attack of enemies, the beach fleas, and others. The 

 coconut crab of the tropics climbs trees in search of food, return- 

 ing to the water at intervals to moisten the gills. 



Characters of Crayfish and its Allies. Our study of crayfish 

 shows us that animals belonging to the same group as itself have 

 several well-marked characteristics. The most important are the 

 presence of a segmented limy exoskeleton, gills, jointed appendages, 

 usually a pair to each segment of the body (except the last), stalked 

 compound eyes, and the fact that they pass through a metamor- 

 phosis or change of form before they reach the adult state. 



We find that the Crustacea, fall naturally into two classes, 

 those in which the number of pairs of appendages varies, and 

 those in which the number is fixed at nineteen. In this latter class 

 are placed the crayfish, lobster, blue crab, shrimp, and most of 

 our common crustaceans. 



Entomostraca. Another sub- 

 class of crustaceans, in which the 

 number of appendages varies, is 

 the group Entomostraca. They are 

 mostly small animals, some species 

 existing in countless numbers. One 

 of the largest Entomostracans in- 

 habiting fresh water is the fairy 

 shrimp (branchippus) found ap- 

 pearing in early spring in fresh- 

 water ponds, a little translucent 

 swimming animal from one half to 

 three fourths of an inch in length. 

 Another fresh-water form often 

 seen in aquaria is the water flea 

 (daphnia). From the economic 

 standpoint, probably the most im- Cyclops (note the single eye-spot). This 

 portant crustaceans that we shall is a very common copepod and is mag- 

 study are the copepods. These tiny nified about forty times, e, 



