266 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



The branchial formula of Astacus. 



Somites and Arthrobranchise. 



their Podo- ^— ' Plenro- 



Appendages. branchial. Anterior. Posterior, branchiae. 



In Cambarus, 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 Astacoides. 



