74 INTRODUCTION TO ZOOLOGY. 



be removed, and fresh water must take its place. 

 There is a necessity for the production of a current of 

 water over the gills. This is brought about by the 

 action of the gill scoop, situated in front of the gills. 

 The gill scoop, when working rapidly, moves about 

 three times in a second, and produces a current from 

 backward to forward. 



Since this is the direction of the current of water 

 over the gills, it is an advantage in breathing for the 

 crawfish to swim backward. The movement back- 

 ward assists tlie current flowing over the gills. So, 

 also, when crawfishes lie in a running stream, they 

 will usually lie with their heads down stream. 



The blood in crawfiishes is white, and it exists in 

 considerable quantity. When a leg of a live crawfish 

 is cut off, the blood flows freely from the wound. It 

 contains an abundance of white corpuscles, but no red 

 ones such as we shall find in vertebrates. 



Some kinds of crawfishes dig holes from the sur- 

 face of the land downward several feet, usually piling 

 the excavated dirt around the mouth of the buiTow, 

 forming a chimney. These holes generally reach 

 downward to the water, and are most numerous in 

 low places, from the surface of which the water has 

 almost or quite disappeared. 



Crawfishes molt as insects do. They molt about 

 five or six times the first year of their lives, and 

 usually molt once each year thereafter. A crawfish 

 may live to be twelve years old. 



All crawfishes living in the eastern and central 

 part of the United States belong to the genus Cam- 

 barus, and have seventeen gills on each side of their 

 body. English crawfishes, and those of California, 

 have eighteen gills on each side and belong to the 

 genus Astacus. 



The crawfish is the fresh-water lobster. In general 

 appearance, and in everything except size, the crawfish 



