266 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



The branchial formula of Astacus, 



In Camharus, the number of the branchiae is reduced 

 to seventeen by the disappearance of the last pleuro- 

 branchia ; while, in Astacoides, the process of reduction 

 is carried so far, that only twelve complete branchiae are 

 left, the rest being either represented by mere rudiments, 

 or disappearing altogether. 



The branchial formula of A stacoides, 



Arthrobran chise. 



(ep. r.) 



1 + r 

 2 



2 + r 

 2 + r 

 2 + r 

 2 + r 

 1 



6 + ejJ.r 5 + 7* + + 4 r + 1 = 12 f ep. r. + 5 f . 



