126 CRAYFISH, LOBSTERS, SPIDERS, AND INSECTS 



Heart 



one is composed of several segments which permits freedom 

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

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

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

 These cover the mouth and aid in crushing the food. The 

 next three pairs of appendages constitute the jaws. The 

 first pair, the mandibles, are short, hard, and toothed and 

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



and weak, but each 

 member of the second 

 pair, carries a curved, 

 paddlehke plate, the 

 gill scoop, that per- 

 forms an important 

 function in respiration 

 to be explained later. 

 On the front of the 

 head ^re the two long 

 antennae. Above each 

 antenna is a forked 

 appendage, the anten- 

 nule. 



The situation, form, 

 and attachment of the 

 gills. — That part of 

 the carapace covering 

 the thorax is plainly divided into three pieces, a narrow 

 middle piece and two wide side pieces that arch downward 

 and cover the sides of the thorax. Beneath these two 

 large side pieces are two long chambers containing the 

 gills. The gills are borne in two sets, an upper set and a 

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



Fia. 69. ^ Cross section of crayfish. 



