THE CRAYFISH AND OTHER CRUSTACEA 85 



legs are called chelipeds because they end in a pair of 

 chela or pincers. There are small chelae on the two follow- 

 ing pairs, but the two posterior pairs of legs end in a simple 

 claw. The large chelipeds are used as organs of defense 

 and in the capture of prey. The other legs are all employed 

 in walking, but the small chelipeds are used also in clean- 

 ing the body, and in picking up small bits of food and 

 passing them forward to the mouth parts. The abdomen 

 is capable of being curved downward and extended, the 

 various segments being articulated by hinge joints at 



/5 



14 



FIG. 72. Crayfish with the side of the carapace cut away to show gills. 

 6, branchiae or gills; r, rostrum; I, eye; 2, first antenna; 3, second antenna; 

 4, mandible; 6, second maxilla; 7, 8, 9, maxillipeds; 10-14, bases of legs. 

 (After Huxley.) 



the sides. On each abdominal segment except the telson 

 there is a pair of appendages (swimmerets) . In the male 

 the first two pairs are modified into organs for the transfer 

 of the sperm cells; the other appendages are nearly alike 

 in both sexes. The posterior pair is furnished with two 

 expanded branches which, with the telson, form a tail 

 fin used in swimming. When the crayfish is disturbed 

 in the water it suddenly bends the abdomen downward 

 and forward, thus causing the animal to dart quickly 

 backward. In the female the small abdominal append- 

 age? are used for carrying the eggs which become attached 

 to the hairs of these organs by a sticky secretion. 



