268 TEXTBOOK OF ZOOLOGY 



them. The crayfish is quite well protected, due to its protective 

 color which matches the background, its chitinous skeletal cover- 

 ing, and its pinchers. In spite of this, they are captured by water 

 snakes, alligators, turtles, fish (such as bass and gars), frogs, sala- 

 manders, herons, and raccoons in particular. Many have been ex- 

 terminated 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 

 portion, while the portion posterior to the grooves is the thorax. The 

 anterior end of the cephalothorax is drawn out to almost a point, and 

 this portion is called the rostrum. The mouth is located on the 

 ventral side of the head portion and not at the tip of the rostrum 

 where most people look for it. The lateral portions of the carapace 

 are known as hranchial areas or hrancliiostegites, and they cover the 

 gills. Their ventral edges are free. On the ventral side of the thorax 

 between the twelfth and thirteenth segments (about the level of the 

 fourth walking leg) of the female is a cuplike pouch called the 

 annulus or seminal receptacle. It serves in reproduction for the 

 receipt and storage of spermatozoa. 



The portion posterior to the thorax, which is frequently called 

 "tail" by fishermen, is really the aldomen, and the tail proper is at 

 the posterior end of this. The abdomen is divided into six typical 

 segments and the terminal telson, which has no appendages but is 

 often called the seventh abdominal segment. The anus is found on 

 the ventral side of this part. The skeletal part of the abdominal seg- 

 ment consists of: the dorsally arched tergum; a thin, overhanging 

 lateral plate, the pleuron; and the slender ventral sternum in the 

 form of a narrow bar extending from side to side. A thin arthro- 

 podial membrane extends between successive sterna and allows for 

 movement of the segments upon one another. 



