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- 
branchie, attached to the epimera of the five hinder 
thoracic somites; two are arthrobranchiz, fixed to the 
interarticular membrane of the external maxillipede; and 
one, which is the only complete podobranchia, belongs 
to the second maxillipede. The podobranchie of the 
first and third maxillipedes are represented only by small 
epipodites. The branchial formula therefore is :— 
Somites and Arthrobranchie. 
their Podo- —_—"_“— Pleuro- 
Appendages. branchie. Anterior. Posterior. branchia. 
VIL... 0 (ep.) 0 0 0 =  0(ep.) 
VIN. ax 1 on 0 0 0 = 1 
Ix, ... 0 (ep.) 1 1 0 = = 2(ep.) 
x sa 0 0 0 1 = 1 
XI. 0 0 0 | a | 
x1... «0 0 0 1 = 1 
XII. ... 0 0 0 1 = 1 
xIV. 0 0 0 1 = 1 
14+2ep + 1 + 1 + 5 = 8 + 2ep 
