THE ANIMAL KINGDOM 



Rostruni /"Cotripoundeye 



) wimracrdslf] Tels on 



Uropod 

 Figure 16.4. Lateral view of a crayfish. (After Howes.) 



the body iorming the carapace. The same skeleton extends forward 

 over the head of a rostrum. The tapered abdomen is composed of_seveii 

 segments, of which the first six have appendages. The last, called the 

 telson, is often not counted as a segment. The anus is located on its 

 ventral side but it lacks appendages. The abdomen is flattened and has 

 broad dorsal and ventral surfaces. The rigid portion of the ventral 

 skeleton is reduced to narrow transverse rings joined together with 

 broad areas of flexible chitin. This enables the abdomen to flex sharply 

 beneath the body. 



The appendages are modified in a variety of ways (Fig. 16.5). In 

 many of them a base (protopodite), an inner ramus (endopodite) and 

 an outer ramus (exopodite) can be recognized. 



The last appendages (on the 20th segment) are extremely flattened 

 uropods. \V^hen extended, the exopodites, endopodites and the telson 

 between form a tail fan. The crayfish spreads this fan and flexes the 

 abdomen rapidly, pulling itself backward with startling speed. 



The other abdominal appendages are the much more delicately 

 built swimmerets or pleopods, with bristly endopodites and exopodites. 

 The continual gentle beating of these limbs produces a water current 

 backward beneath the animal, probably of use beneath rocks or in 

 burrows where the water would become devoid of ox^gen if not cir- 

 culated. In the male the first pleopods (15th segment) are modified as 

 copulatory organs (Fig. 16.5). The female deposits her eggs on the 

 pleopods, to which they are glued by secretions from the limbs. Con- 

 stant motion then keeps the eggs ^vell aerated. If the pleopods beat 

 vigorously the current produced helps the crayfish to walk forward, and 

 in small individuals may actually produce a gentle forward swimming. 



The last five pairs of appendages on the cephalothorax (segments 

 10 to 14) are the large walking legs or pereiopods. These are uni- 

 ramous in the adult. Each is formed of seven segments, of -ivhich the 

 first t^\•o represent the protopodite and the last five the endopodite. 

 Each joint (Fig. 16.6) can move in a single plane, but the planes of 

 succeeding joints are rotated so that the limb as a whole can move 

 with considerable flexibility. The first three pairs of pereiopods are 

 chelate, and the first pair have large pincers. The jaws of the pincers 

 are made of the two distal segments of the leg. ^\'hich are hinged one 

 upon the other. The pincers are used for fighting and for occasional 

 food capture, and may assist in walking over rough terrain. They are 



