THE BRANCHIAZ OF THE CRABS. 277 
The branchial formula of Cancer pagurus. 
Somites and Arthrobranchiz, 
their Podo- —. Pleuro- 
Appendages. branchie. Anterior. Posterior. branchiz. 
VII. ... 0 (ep.) 0 0 o = 0 
VIII. 1 Bhs 1 0 0 = 2 
IX, 1 1 1 0 = 8 
x. 0 1 1 0 = 2 
XI... 60 0 0 1 — 1 
XI, ay 0 0 0 1 = 1 
XIII 0 0 0 0 = 0 
XIV. 0 0 0 0 = 0 
2+ep +3 +4 2 +4 2 = 9+ ep. 
It will be observed that the suppression of branchize 
has here taken place in all the series, and at both the 
anterior and the posterior ends of each. But the defect 
in total number is made up by the increase of size, not of 
the pleurobranchie alone, as in the case of the prawns, 
but of the arthrobranchie as well. At the same time 
the whole apparatus has become more specialized and 
perfected as a breathing organ. The close fitting of the 
edges of the carapace, and the possibility of closing the 
inbalent and exhalent apertures, render the crabs much 
more independent of actual-immersion in water than most 
of their congeners; and some of them habitually live on 
dry land and breathe by means of the atmospheric air 
which they take into and expel from their branchial cavities. 
Notwithstanding all these wide departures from the 
structure and habits of the crayfishes, however, attentive 
examination shows that the plan of construction of the 
