500 ZOOLOGY SECT. 



middle of its posterior border. The cephalic region of the cara- 

 pace is produced in front into a large median spine, the rostrum 

 (Fig. 396, r) : immediately below it is a plate from which spring- 

 two movably articulated cylindrical bodies, the eye-stalks, bearing 

 the eyes at their ends. 



The appendages are seen at a glance to differ from those of 

 Apus in their vastly greater degree of differentiation : obvious at 

 a glance are the long feelers (Fig. 396, a, 1, a. 2) attached to the 

 head, the five pairs of legs (9-13) 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. 397, 10), the resemblance of which to the biramous limbs 

 of the larval Apus is obvious. There is an axis or protopoditc 

 consisting of a very short proximal (pr. 1) 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 

 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 seg- 

 ment, while that of the first is more or less rudimentary. In the 

 male the first and second pleopods (9) are modified into incom- 

 plete tubes which act as copulatory organs, serving to transfer 

 the spermatophores to the body of the female. The sixth pair of 

 abdominal limbs (11) are alike in the two sexes : they are very 

 large, both endo- and exopodite having the form of broad fiat 

 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), upon which the animal 

 walks : in front of these is a pair of very large legs terminating 

 in huge claws or chelce-, and hence called chelipeds (Fig. 396, 9). 

 The three anterior 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 

 (Fig. 397, 5-7). 



The structure of these appendages is best understood by a con- 

 sideration of the tliird nm.i //ll/>cd (7). The main portion of the 

 limb is formed of seven podomeres arranged in a single series, 

 strongly calcified, and, with the exception of the second and third, 

 which are fused, movably articulated with one another. The second 

 podomere, counting from the proximal end, bears a many-jointed 

 feeler-like organ (ex), and from the first springs a thin folded 



