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 branchiae 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 branchiae subserve the respiratory function. 



The crayfish has eighteen perfect and two rudimentary 

 branchiae in each branchial chamber, the boundaries of 

 which have been already described. 



Of the eighteen perfect branchiae, six (podobranchice) 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- 

 branchiai) 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 branchiae, 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 



