PLEUROBRANCHIZ, COMPLETE AND RUDIMENTARY. 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 
arthrobranchie in structure; but it is larger, and it is 
attached neither to the basal joint of the hindermost ambu- 
latory limb, nor to its interarticular membrane, but to the 
sides of the thorax, above the joint. From this mode ot 
attachment it is distinguished from the others as a pleuro- 
branchia (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. 18). 
The quantity of water which occupies the space left in 
the branchial chamber by the gills is but small, and as 
the respiratory surface offered by the gills is relatively 
very large, the air contained in this water must be 
rapidly exhausted, even when the crayfish is quiescent ; 
while, when any muscular exertion takes place, the quan- 
tity of carbonic acid formed, and the demand for fresh 
oxygen, is at once greatly increased. For the efficient 
performance of the function of respiration, therefore, the 
water in the branchial chamber must be rapidly renewed, 
and there must be some arrangement by which the 
supply of fresh water may be proportioned to the 
demand. In many animals, the respiratory surface is 
