264 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



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

 of the same numhers of podohranchise, arthrobran- 

 chise, and pleurobranchise attached to corresponding 

 somites. In PaUnurus and in the genus Astacopsis 

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

 six podohranchiee attached to the thoracic limbs from 

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

 branchife 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 pleurobranchiae are fixed to the 

 epimera of the four hindmost thoracic somites. More- 

 over, in Astacopsis, 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 : — 



TJic branchial formula of Astacopsis. 





