396 ESSENTIALS OF ZOOLOGY 



Habitat and Behavior of Crayfish 



For the most part crayfishes (crawfishes, crawdads, fresh-water 

 lobsters) are inhabitants of fresh-water streams and ponds where 

 there is sufficient calcium carbonate in solution for purposes of 

 skeleton formation. . These animals may be found moving about on 

 the bottom, or they may be in hiding under some stone or log, or 

 they may be in the mouth of a burrow beneath the water's edge. 

 Some species carry air tunnels vertically from the original hori- 

 zontal burrow to the surface of the earth and deposit mud around 

 the opening of a tunnel. They are much more active at night than 

 during the day. It is possible for them to walk about on the bottom 

 of the stream or pond, moving the body in almost any direction. 

 Their swimming habits are rather peculiar in that they dart back- 

 ward through the water, as a result of the strong downward stroke of 

 the tail. One stroke of the tail will carry the animal a yard and 

 this is commonly sufficient to avoid the enemy. The daytime is 

 usually spent in hiding under objects or in the mouth of the bur- 

 row. Crayfishes may at times desert their aquatic habitat and go 

 foraging out over swampy land. In some localities certain species 

 build their burrows down to the subterranean water table right 

 out in the fields and become important pests. Sight, touch, and 

 chemoreception are important senses in this animal. 



The crayfish captures other animals, such as tadpoles, small fish, 

 and aquatic insects, by waiting in hiding and suddenly seizing them. 

 The crayfish is quite well protected, due to its protective color which 

 matches the background, its chitinous skeletal covering, and its 

 pinchers. In spite of this, they are captured by water snakes, alli- 

 gators, turtles, fish (such as bass and gars), frogs, salamanders, 

 herons, and raccoons in particular. Many have been exterminated 

 by the drainage of swamps, and by their use as food for man. 



External Structure 



The chitin-covered body is divided into cephalothorax, abdomen, 

 and appendages. The cephalothorax is a compound division of the 

 body including the thirteen most anterior segments and is divisible 

 into head and thorax. The boundary between these is marked by 

 the oblique cervical groove on each side of the region. The shell- 

 like covering, whose lateral edges are free, is known as the carapace. 

 The portion anterior to the cervical groove is the head or cephalic 



