52 GENERAL ZOOLOGY 



crayfish is colorless, but carries food, oxygen, and waste 

 products. 



A crayfish labors under one great disadvantage the 

 exoskeleton is so rigid that it cannot grow from day to day, 

 as do animals with pliable skins. This shell is an admir- 

 able protection but prohibits growth. This difficulty is 

 surmounted by a periodical casting of the entire exo- 

 skeleton including the lining of the esophagus and stom- 

 ach. After such a moult the crayfish is a "soft shell" for a 

 day. During this period the body absorbs a great deal of 

 water, and there is very rapid increase in size. The shell 

 then hardens and no more growth can take place until the 

 next moult. Just before the shell is shed a little stony 

 secretion, the gastrolith, is formed on either side of the 

 stomach. This is a reserve supply of lime which is rapidly 

 transferred by the blood to the new shell after moulting. 



The crayfish has a special respiratory system for securing 

 oxygen, which consists of a number of plume-like gills along 

 the sides of the thorax. These delicate breathing organs 

 are protected by the hard shell, or carapace, which covers 

 the cephalothorax. One of the mouth parts has a little 

 paddle which projects into the respiratory chamber and by 

 its movements keeps a current of water passing forward over 

 the gills and out under the head. The gills are full of 

 moving blood which receives oxygen from the passing 

 water. The carapace fits over the thorax much as a man's 

 coat covers his body, and the gill chamber is kept from 

 getting dirty by little bristles along its edge which strain 

 the incoming water. 



The excretions resulting from general metabolism in the 

 crayfishes' body are eliminated through the " green gland" 

 a form of kidney consisting of a long tube which is ex- 

 panded into a bladder near the discharg ; ng end. Excre- 

 tions are absorbed from the blood by the green gland and 

 passed out through a pore which opens through the basal 

 segment of the second antenna. There is only one pair of 

 green glands and these are wholly within the head. 



