350 THE CRAYFISH chap, vi 



five pairs of legs springing from the thorax, and the little 

 fin-like bodies arising from the sterna of the abdomen. It 

 will be convenient to begin with the last-named region. 



The third, fourth, and fifth segments of the abdomen 

 bear each a pair of small appendages, the swimming feet or 

 pkopods (Fig. 83, P, N, X). A pleopod (Fig. 84, 10) consists 

 of an axis or/w/('/^fl'//'e having a very short proximal (Fig. 84 

 pr. i) and a long distal {pr. 2) podomere, and bearing at its 

 free end two jointed plates, fringed with setse, the endopodite 

 {en) and exopodite (ex). These appendages act as fins, 

 moving backwards and forwards with a regular swing, and 

 probably aiding in the animal's forward movements. 



In the female a similar appendage is borne on the 

 second abdominal segment, while that of the first is 

 more or less vestigial (p. 159). In the male the first 

 and second pleopods (9) are modified to form incomplete 

 tubes which serve to transfer the spermatophores (p. 368) 

 to the body of the female. The sixth pair of pleopods (11) 

 are alike in the two sexes : they are very large, both 

 endopodite and exopodite ' having the form of broad, 

 flat plates : in the natural position of the parts they lie 

 one on each side of the telson, forming with it a large 

 five-lobed tail fin : they are therefore conveniently called 

 uropods or tail-feet. The telson itself bears no appendages. 

 The thoracic appendages are very different. The four 

 posterior segments bear long, slender, jointed legs (8), 

 with which the animal walks : in front of these is a pair of 

 very large legs terminating in huge claws or chela:, and 

 hence called chelipeds (Fig. 86, hf. 4). The three anterior 

 thoracic segments bear much smaller appendages, more 

 or less leg-like in form, but having their bases toothed to 

 serve as jaws : they are distinguished as maxillipeds or foot- 

 jaws. 



