270 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



long and strong (fig. 71, B) ; the remaining thoracic 

 limbs are terminated by simple claws. The five anterior 

 abdominal somites are all provided with large swimmerets, 

 which are used like paddles, when the animal swims 

 quietly ; and, in the males, the first pair is only slightly 

 different from the rest. The rostrum is very large, and 

 strongly serrated. 



None of these differences from the crayfish, however, 

 is so great, as to prepare us for the remarkable change 

 observable in the respiratory organs. The total number 

 of the gills is only eight. Of these, five are large pleuro- 

 branchiae, attached to the epimera of the five hinder 

 thoracic somites ; two are arthrobrandhise, fixed to the 

 interarticular membrane of the external maxillipede ; and 

 one, which is the only complete podobranchia, belongs 

 to the second maxillipede. The podobrancliise of the 

 first and third maxillipedes are represented only b}^ small 

 epipodites. The branchial formula therefore is : — 



