50 



GENERAL ZOOLOGY 



The relationships of the classes of arthropods are shown 

 in Fig. 28. Let us now consider each of them in more detail. 

 The remainder of this chapter will be devoted to the 

 Crustacea. 



CLASS 1. CRUSTACEA 



This class may be defined as including arthropods which 

 live for the most part in water, and usually breathe through 

 gills. We can perhaps best get an idea of one of these 

 animals by considering the activities of an example which is 

 familiar to everyone the common crayfish, or crawfish, 



FIG. 29. Crayfishes. 



found in fresh water the world over. The daily life of this, 

 or any other, animal is shaped primarily for three ends 

 self-maintenance, self-protection, and race preservation. 

 To use the words of Alcock "the three great exigencies: 

 to find something to eat, to avoid being oneself eaten, and 

 to disseminate one's species, give rise to a perpetual struggle 

 in which the fittest is successful. " 



THE CRAYFISH, Cambarus virilis Hagen 



Self -maintenance. Crayfishes (Fig. 29) eat nearly any- 

 thing that is available. They prefer animal food, and lurk 



