126 



CRAYFISH, LUBSTEKtS, Sl'IIJEHS, AND INSECTS 



Heart 



one is composed of severul segments which permits freedom 

 of movement. Each of tlie first pair of legs terminates in 

 a large pair of furceji.s, or pincers for grasping. Just an- 

 terior to this pair of legs are the three pairs of foot jaws. 

 Tliese cover the mouth and aid iir crusliing the food. The 

 next three pairs of appendages constitute the jaws. The 

 fiirst pair, the mandibles, are short, hai'd, and toothed and 

 grind the food. The two following pairs are small, soft, 



and weak, but each 

 member of the second 

 pair carries a cui-ved, 

 paddlelike plate, the 

 gill scoop, that per- 

 forms an important 

 function in respiration 

 to be explained later. 

 On the front of the 

 head are the two long 

 antenme. Above each 

 antenna is a forked 

 appendage, the anten- 

 nule. 



The situation, form, 

 and attachment of the 

 gills. — That pai't of 

 the carapace covering 

 the thorax is plainly divided into three pieces, a narrow 

 middle piece and two wide side pieces tluit arch downward 

 and cover the .sides of the tliorax. Beneath these two 

 lajge side pieces are two long chambers containing the 

 gills. The gills ai-e liorne in two sets, an upper set and a 

 lower set. The lower set is attached to the basal seg- 



FiG. 69. — Cross section of crayfis 



