146 ELEMENTARY ZOOLOGY 



kinds among them have led to much modification and to 

 great specialization of structure. The course of develop- 

 ment, too, is made very complicated by the elaborate 

 metamorphosis undergone by many of the members of the 

 branch. 



We shall study the Arthropoda by getting acquainted 

 with a few examples of each class and thus learning the 

 special class characteristics. 



CLASS CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS, 



ETC. 



THE CRAYFISH (Cambarus sp.) 



Structure. The structure of the crayfish has been 

 already studied (see Chapter IV and figs. 3 and 4). 



Life- history and habifs. Crayfish frequent fresh-water 

 lakes, rivers, and springs in most parts of the United 

 States. Many of them perish whenever the small prairie 

 ponds dry up. But some burrow into the earth when the 

 dry season comes. There may be noticed in meadows 

 where water stands for certain seasons of the year many 

 scattered holes with slight elevations of mud about them. 

 These are mostly the burrows of crayfish. During the 

 dry season the crayfish digs down until it reaches water, 

 or at least a damp place, where it rests until wet weather 

 brings it to the surface once more. One of these burrows, 

 followed in digging a mining shaft, extended vertically 

 down to a distance of twenty-six feet, where the crayfish 

 was found tucked snugly away. 



The eggs are carried by the female on her abdominal 

 appendages. Previous to the laying of the eggs the 

 female rubs off all foreign matter from the appendages, 

 thus preparing them for the reception of the eggs. This 

 cleaning is done with the fifth pair of legs. When the 



