The Crayfish. 41 



certain exotic crayfishes this epipodite has a few rudimentary 

 branchial filaments. 



When the podobranchs have been removed, other gills 

 come into view. These are termed arthrobranchiae, from the 

 fact that they are attached to the articular membrane, to which 

 is also attached the limbs. These gills correspond to the same 

 appendages that bear the podobranchs; there are, however, 

 a pair to each limb, except No. 8, which has but one — that 

 is to say, there are eleven in all. The structure of these 

 arthrobranchiae differs from that of the podobranchiae in that 

 there is no lamina; the stem which bears the gill filaments 

 is not expanded into the folded plate characteristic of the 

 podobranchs. 



Finally, the last thoracic limb (No. 13 of the entire series) 

 bears a single gill, which is attached at a level above that 

 of the podobranchs and arthrobranchs — to the epimeron, in 

 fact. This is the pleurobranch. In front of it, and occupying 

 a similar position with regard to the two limbs in front, is 

 a tiny unbranched filament which is believed to represent a 

 rudimentary pleurobranch ; from the fact that it occupies 

 a similar position, and — more important — from the fact that in 

 various exotic crayfish there are fully developed pleurobranchs 

 corresponding to the limbs in question. The rudiment, 

 such as it is, may be compared to the stem of the otherwise 

 missing gill. 



It is convenient for the purposes of a ready comparison 

 with the gills of other crayfishes to express the arrangement of 

 the gills as a formula : — 



18 + ep. + 2.1- 

 When the body of the crayfish is opened by dissecting off 



