138 



THE CEAYFISH 



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 gills in the crayfish : 



Verify the above table by carefully counting the gills, a/iid 

 twrning them down as they are cheched off in the table. 



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



groups. 

 Remove and examine in turn a pleurobrancMa, an arthro- 

 branchia, and a podobranchia. 



a. The pleurobrancMse and arthrobrancMse resemble. 



bottle-brushes, each consisting of a series of delicate 

 branchial filaments arranged upon a central stem, 

 which is traversed by afferent and efferent blood- 

 vessels. It is in the branchial filaments that- 

 respiration is effected, diffusion taking place very 

 readily through the thin cuticle covering them. 



b. A podobraneMa is more complicated, and consists of,. 



(i.) a basal plate arising from the outer surface, 

 of the coxopodite, and covered with finely plumose 

 setsB ; (ii.) a stem arising from the dorsal border- 

 of the basal plate close to its apex ; (iii.) a lamina, 

 which is a corrugated plate borne on the distal 

 end of the stem, doubled longitudinally upon itself, 

 and beset with small hooked sets ; and (iv.) a. 

 plume, which arises from the apex of the stem, 

 and resembles an arthrobranchia. The plume, 

 and the outer face of the stem are covered with. 

 ■M' branchial filaments. 



