THE ORGANS OF RESPIRATION. 75 
activity of all the organs, and especially of the muscles, 
is inseparably connected with the absorption of oxygen 
and the evolution of carbonic acid; and the only source 
from which the one can be derived, and the only recep- 
tacle into which the other can be poured, is the blood 
which bathes and permeates the whole fabric to which it 
is distributed by the arteries. 
The blood, therefore, which reaches the branchie has 
lost oxygen and gained carbonic acid; and these organs 
constitute the apparatus for the elimination of the inju- 
rious gas from the economy on the one hand, and, on the 
other, for the taking in of a new supply of the needful 
‘vital air,” as the old chemists called it. It is thus that 
the branchiz subserve the respiratory function. 
The crayfish has eighteen perfect and two rudimentary 
branchie in each branchial chamber, the boundaries ot 
which have been already described. 
Of the eighteen perfect branchiz, six (podobranchie) are 
attached to the basal joints of the thoracic limbs, from the 
last but one to the second (second maxillipede) inclusively 
(fig. 4, p. 26, pdb, and fig. 17, A, B); and eleven (arthro- 
branchiz) are fixed to the flexible interarticular mem- 
branes, which connect these basal joints with the parts 
of the thorax to which they are articulated (fig. 4, arb, arb’, 
fig. 17, C). Of these eleven branchie, two are attached 
to the interarticular membranes of all the ambulatory 
legs but the last, (=6) and to those of the pincers and of 
the external maxillipedes, (=4) and one to that of the 
