AKTHROPODA. 



283 



pressed into a very small space, yet have the following 

 members : the eye-stalks ; the short and the long anten- 

 nae; the mandibles, or jaws, between which the mouth 

 opens ; the two pairs of maxillae ; and a pair of modified 

 limbs, called " foot-jaws." The thorax carries two more 

 pairs of foot-jaw r s and five pairs of legs. The foremost 

 legs, "the great claws," 

 are extraordinarily de- 

 veloped, and terminat- 

 ed by strong pincers 

 (chelce). Of the four 

 slender pairs succeed- 

 ing, two are furnished 

 with claws, and two 

 are pointed. The last 

 pair of swimmerets, to- 

 gether with the telson, 

 form the caudal fin — 

 the main instrument of 

 locomotion ; the others 

 (called "false feet") 

 are used by the female 

 for carrying her eggs. 

 The eyes are raised on 

 stalks so as to be mov- 

 able (since the head is 

 fixed to the thorax), 

 and are compound, 

 made up of about two 

 thousand five hundred square facets. At the base of each 

 small antenna is a minute sac, whose mouth is guarded by 

 hairs : this is the organ of hearing. The gills, twenty on 

 a side, are situated at the bases of the legs and enclosed in 

 two chambers, into which water is freely admitted, in fact, 

 draw r n, by means of a curious attachment to one of the 



Fig. 250.— Under-side of the Cray-fish, or Fresh- 

 water Lobster (Astacus fluviatilis) : a, first pair 

 of antennae; b, second pair 4 c, eyes ; <7, open- 

 ing of kidney ; e, foot-jaws ; /, g, first and fifth 

 pair of thoracic legs; h, abdominal feet; i, 

 anus ; Jc, caudal fin. 



