THE GILLS 



135 



branohioBtegite. 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, drawing the water forwards out of the 

 gill-chamber, and discharging it in front just below 

 the renal aperture.- {Gf. Section E, p. 136.) 



2. The gills may be arranged in three groups according to 

 their position of attachment. 



a. PodobrancMsB are giUs attached to the coxopodites 



of appendages VII. to XII. The coxopodite of 

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



b. ArthrobrancMse are giUs 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 arthrobranchias, an anterior and- 

 a posterior one ; and attached to the arthrodial 

 membrane of VII. is a single one. 



c. PleurobranoMsB 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 rudiments are found on the next two or three 

 segments in front. 



The following table shows the number and ar- 

 rangement of the gUls in the crayfish : — 



