78 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



fine straight hairs, or seta (F), whence a narrow stem (st) 

 proceeds. At its upper end this stem divides into two 

 parts, that in front, the plume (pi), resembling the free 

 end of one of the gills just described, while that behind, 

 the lamina (Q, is a broad thin plate, bent upon itself longi- 

 tudinally in such a manner that its folded edge lies for- 

 wards, and covered with minute hooked setse (G). The 

 gill which follows is received into the space included 

 between the two lobes or halves of the folded lamina 

 (fig. 4, p. 26). Each lobe is longitudinally plaited into 

 about a dozen folds. The whole front and outer face of 

 the stem is beset with branchial filaments ; hence, we may 

 compare one of these branchise to one of the preceding 

 kind, in which the stem has become modified and has 

 given off a large folded lamina from its inner and 

 posterior face. 



The branchia? now described are arranged in sets of 

 three for each of the thoracic limbs, from the third 

 maxillipede to the last but one ambulatory limb, and 

 two for the second maxillipede, thus making seventeen 

 in all (3 X 5 + 2 = 17) ; and, between every two there is 

 found a bundle of long twisted hairs (fig. 17, A, cx.s; D and 

 E), which are attached to a small elevation (t) on the basal 

 joint of each limb. These cozopoditic seta, no doubt, serve 

 to prevent the intrusion of parasites and other foreign 

 matters into the branchial chamber. From the mode 

 of attachment of the six branchiae it is obvious that they 

 must share in the movements of the basal joints of the 



