THE GILLS. 153 



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. (See Section E, p. 155.) 



2. The gills may be described in three groups according 

 to their position of attachment. 



a. Podobranchiae are gills attached to the coxopodites 



of appendages VII. to XII. The coxopodite of 

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



b. Arthrobranchiae are gills arising from the arthrodial 



membranes at the bases of the thoracic appen- 

 dages. At the base of each of the limbs VIII. to 

 XII. are two arthrobranchiae, an anterior and a 

 posterior one ; and attached to the arthrodial 

 membrane of VII. is a single one. 



c. Pleurobranchiae 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-developed pleuro- 

 branchia on each side, on the thirteenth segment, 

 but abortive ones are found on the next two or 

 three segments in front. 



Primitively all the gills arose as outgrowths 

 of the basal joint of the appendages, three to 

 each. Movements of these appendages induced 

 shaking of the gills. With the increased size 

 and strength of the animal, improved respi- 

 ratory mechanism arose. The branchiostegites 

 covered over the gills, and the latter shifted 

 their points of attachment proximally, so that 

 now only the podobranchiae retain their original 

 position 



