264 THE COMPABATIVE MORPHOLOGY OF THE CKAyFISH. 



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

 of the same numbers of podobranchise, arthrobran- 

 chise, and pleurobraachise attached to corresponding 

 somites. In Polinurus and in the genus Astacopsis 

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

 six podobranchiae attached to the thoracic limbs from 

 the second to the seventh inclusively; five pairs of arthro- 

 branchiae 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, 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 branchial formula of Astacopsis. 



+ ep. 1-. + C 



4 = 21 + ep. r. 



