THE GILLS 137 



by diffusion through the thin cuticle, of the gases dissolved 

 in the water and in the blood. The gills are enclosed and 

 protected by downward growths of the carapace at the sides 

 of the thorax, the brancMostegites. 



1. The brancMal chambers are situated one at each side of 

 the thorax, between the body and the branchiostegite. 



Expose the left brarichial chamber by cutting away the 

 branchiostegite of that side ; and fix the animal on its right 

 ^ide under water. 



The chamber is bounded on its inner side by the 

 thoracic epimera, and on its outer side by the 

 branchiostegite. It is open below and behind, but 

 the former opening is partially stopped by the basal 

 joints of the legs. The greater portion of the 

 chamber is occupied by the gills ; but in front of 

 them and just behind the line of the cervical groove, 

 is a channel, the cervical canal. In this canal the 

 scaphognathite works to and fro with a sculling 

 movement, driving the water forwards out of the 

 gill-chamber, and discharging it in front just below 

 the renal aperture. {Cf. Section E, p. 139.) 



,2. The gills may^be described in three groups according to 

 their position of attachment. 



a. Fodobranchise are gills attached to the coxopodites 



of appendages VII. to XII. The coxopodite of 

 VI. bears a large epipodite in place of a giU. 



b. Arthrobranchise are gills arising from the arthrodial 



membranes at the bases of the thoracic appen- 

 dages. At the base of each of the hmbs VIII. 

 to XII. are two arthrobranchise, an anterior and 

 a posterior one ; and attached to the arthrodial 

 membrane of VII. is a single one. 



c. Pleurobraachise are gills arising from the side-wall 



of the thorax, above the ridges which form the 

 dorsal boundaries of the arthrodial membranes. 

 The crayfish has only one well-developad pleuro- 



