100 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



two pairs of maxillae. The thorax carries three pairs 

 of modified limbs, called "foot jaws," and five pairs 

 of legs. The foremost legs, "the great claws," are 

 extraordinarily developed, and terminated by strong 

 pincers (chelce). Of the four slender pairs succeeding, 

 two are furnished with claws, and two are pointed. The 

 last pair of swimmerets, together 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 movable (since the head is fixed to 

 the thorax), and are compound, made up of about two 



FIG. 55. Internal anatomy of the Crayfish: em, extensor muscle of abdomen; Jm, 

 flexor muscle of abdomen; m, mouth; cs, cardiac portion of stomach; ps, pyloric 

 portion of stomach; dg, digestive gland; k t heart; r, reproductive gland; i, intes- 

 tine; g, " brain " ; vc, ventral nerve cord. 



thousand five hundred square facets. On 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 inclosed in two chambers, into which water is 

 freely admitted, in fact, drawn, by means of a curious 

 attachment to one of the maxillae, which works like 

 a paddle or scoop. The heart is a single oval cavity, 

 and drives arterial blood a milky fluid full of corpus- 

 cles. The alimentary canal consists of a short gullet, 



