ARTHEOrODA. 



283 



ages. The five segments of the head are compressed into 

 a very small space, yet have the following members:"" 

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

 between which the mouth opens ; and the two pairs of 

 maxillae. The thorax carries tliree pairs of modified limbs, 

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

 legs, " the great claws," 

 are extraordinarily de- 

 veloped, and terminat- 

 ed by strong pincers 

 {chelm). 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 

 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 b}' 

 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 



Pig. 260.— Under-side of the Cray-flsh, or Fresh- 

 water Lobster (Aataats fiuviatilu) : a, first pair 

 of antennae; 6, second pair, c, eyes; d, open- 

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

 pair of thoracic legs; h, swimmerets; i, anus; 

 kf caudal fin. 



