154 



THE CRAYFISH. 



The following table shows the number and 

 arrangement of the gills in the crayfish : 



Rud.=Abortive rudiment. Ep.= Epipodite. 



1 See p. 149. 



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. 



Remove and examine in turn a pleurobranchia, an arthro- 

 branchia, and a podobranchia. 



a. The pleurobranchise and arthrobranchise 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 through the thin cuticle covering 

 them. 



b. A podobranchia is more complicated, and consists of 



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

 of the coxopodite, and covered with finely 

 plumose setae ; (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 setse ; 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 

 branchial filaments. 



