364 THE CRAYFISH CHAP, vn 



head, the 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 pleopods (Fig. 90, P, N, X). A pleopod (Fig. 91, 

 10) consists of an axis or protopodite having a very short 

 proximal (pr. i) and a long distal (pr. 2) podomere, and 

 bearing at its free end two jointed plates, fringed with 

 setae, the endopodite (en) and exopodite (ex). These appen- 

 dages 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 (Fig. 91, 9) are modified to form in- 

 complete tubes which serve to transfer the spermato- 

 phores (p. 382) to the body of the female. The sixth 

 pair of pleopods (n) 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 either side of the telson, form- 

 ing 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. 93, bf. 4). The three 

 anterior thoracic segments bear much smaller appen- 

 dages, more or less leg-like in form, but serving as jaws: 

 they are distinguished as maxillipeds or foot- jaws. 



