264 THE COMPARATIVE MORPHOLOGY OF THE CRAyFISH. 



and when this numher is present, the total is made up 

 of the same numbers of podobranchiie, arthrobran- 

 chise, and pleurobranchise attached to corresponding 

 somites. In PaUniiru^ and in the genus Astacopsis 

 (which is one of the Parastacidce), for example, there are 

 six podobranchiaB attached to the thoracic limbs from 

 the second to the seventh inclusivel}^ ; five pairs of arthro- 

 branchiaG are attached to the interarticular membranes 

 of the thoracic limbs from the third to the seventh 

 inclusively, and one to that of the second, making eleven 

 in all ; while four pleurobranchise are fixed to the 

 epimera of the four hindmost thoracic somites. More- 

 over, in Astacopsis f the epipodite of the first thoracic 

 appendage (the first maxillipede) bears branchial fila- 

 ments, and is a sort of reduced gill. 



These facts may be stated in a tabular form as 

 follows : — 



The bran cliial formula of Astacopi 



