THE BRANCHIiE OF THE CKABS. 277 



The branchial formula of Cancer pagiirus. 



2 + ep. + 3 + 2 + 2 = 9 + ep. 



It will be observed that the suppression of branclii^ 

 lias 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 pleurobranchise alone, as in the case of the prawns, 

 but of the arthrobranchiae 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 possibihty of closing the 

 inhalent 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 



