132 



The branchial ridge, however, will act as a formidable 

 barrier, and although some of the water will undoubtedly 

 pass directly over into the pre-branchial chamber, it is 

 reasonable to suppose that some of it will have its course 

 changed by the branchial ridge and will pass backward, 

 following the direction of the latter. 



The Gills (Figs. 63, 64). 



The gills arise from each side of the thorax and lie 

 upon the inner wall of each branchial chamber, i.e., on 

 the thoracic epimera. According to the terminology 

 introduced by Huxley, the gills may be placed in three 

 categories — the podob ranch arising from an appendage, 

 the arihrobranch arising between an appendage and the 

 epimeron, and the pleurobranch arising from the epimeron. 



In Cancer there are only nine gills at each side. The 

 following is the branchial formula. 



First gill. Podobranch of the second thoracic somite 

 (tigs. 63, 64, g. 1). It arises from the coxopodite of the 

 second maxillipede between the exopodite and the flabellum. 

 It lies with its outer face in contact with the posterior 

 side of the branchial groove, and its inner face is closely 

 applied to the basal portions of the gills 2 to 6. Its apex 

 points backward and outward. Length 22 mm.* 



Second gill. Arthrobranch of the second thoracic 



somite (g. 2). Arises from the arthrodial membrane of 



* The measurements of the gills are taken from a crab having a 

 carapace breadth of 12 cm. 



