The six podobranchiæ, or gills which are attached to the basal joints of the legs, play the same part, but differ a good deal in the details of their structure from those which are fixed to the interarticular membranes. Each consists of a broad _base_ (fig. 17, A and B; _b_) beset with many {78} fine straight hairs, or _setæ_ (F), whence a narrow _stem_ (_st_) proceeds. At its upper end this stem divides into two parts, that in front, the _plume_ (_pl_), resembling the free end of one of the gills just described, while that behind, the _lamina_ (_l_), is a broad thin plate, bent upon itself longitudinally in such a manner that its folded edge lies forwards, and covered with minute hooked setæ (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 branchiæ 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 branchiæ 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 × 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 _coxopoditic setæ_, 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 branchiæ it is obvious that they must share in the movements of the basal joints of the {79} legs; and that, when the crayfish walks, they must be more or less agitated in the branchial chamber. The eighteenth branchia resembles one of the eleven arthrobranchiæ in structure; but it is larger, and it is attached neither to the basal joint of the hindermost ambulatory limb, nor to its interarticular membrane, but to the sides of the thorax, above the joint. From this mode of attachment it is distinguished from the others as _pleurobranchia_ (fig. 4, _plb. 14_). Finally, in front of this, fixed also to the walls of the thorax, above each of the two preceding pairs of ambulatory limbs, there is a delicate filament, about a sixteenth of an inch long, which has the structure of a branchial filament, and is, in fact, a rudimentary pleurobranchia (fig. 4, _plb. 12_, _plb. 13_).