ARTHROPODA. 



283 



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ages. The five segments of the head are compressed into 

 a very small space, yet have the following members: 147 * 

 the short and the long antennae ; the mandibles, or jaws, 

 between which the mouth opens; and the two pairs of 

 maxilla?. The thorax carries three pairs of modified limbs, 

 e.-illed " foot-jaws," and five pairs of legs. The foremost 

 legs, " the great claws," 

 are extraordinarily de- 

 veloped, and terminat- 

 ed by strong pincers 

 (choice). 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 "swimmerets") 

 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 Fl6 - 250.-Under-side of the Cray-flah, or Fresh- 



water Lobster (Astacm fluviatilis) : a, first pair 



fixed tO the thorax), f antennae; b, second pair, c, eyes; d, open- 

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



and are Compound, pair of thoracic legs ; h, swimmerets ; i, anus ; 

 i / v k, caudal fin. 



made up ot 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, 

 drawn, by means of a curious attachment to one of the 



