THE GILLS. 



145 



movement, driving the water forwards out of the 

 gill-chamber, and discharging it in front just below 

 the renal aperture. (See Section E, p. 146.) 



2. The gills may be described in three groups according 

 to their position of attachment. 



a. Fodobraucliise are gills attached to the eoxopodites 



of appendages VII. to XII. The coxopodite of 

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



b. Arthrobranchise 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 arthrobranchise, an anterior and a 

 posterior one ; and attached to the arthrodial 

 membrane of VII. is a single one. 



c. FleurobrancMse 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. 



The following table shows the number and 

 arrangement of the gills in the crayfish. 



Rud.=Abortive rudiment. Bp.=Epipodite. 



Verify the above table by carefully counting the gills, and 

 turning them down as they are checked off in the table. 



3. The structure of the gills is different in the different 

 groups. 



Bemove and examine in turn a plenrobranchia, an arthro- 

 branchia, and a podobranchia. 



