2GG THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



The hrancliial formula of Astacus. 



6 + ep. + 6 +5 + 1 +2or3/-=^ 18 + ep. + 2or3r. 



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 onl}- twelve complete branchiie are 

 left, the rest being either represented by mere rudiments, 

 or disappearing altogether. 



The hrancliial formula of Astacoides. 



XIV. 



1 



Q + cjj.r 5 + 7" + + 4: ?• + 1 = 12 + ep. r. + 5 r. 



