PLEUROBRANCHIiE, 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 

 arthr.obranchise 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 of 

 attachment it is distinguished from the others as apleuro- 

 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. 13). 



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 



