136 THE CRAYFISH 



Verify the above table by carefully counting the gills, and 

 turning them downwards 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 pleurobranchia, an arthro- 

 branchia, and a podobranchia. 



a. The pleurobranchiaB 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 very 

 readily through the thin cuticle covering them. 



b. A podobrancMa is more compHcated and consists of, 



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

 of the coxopodite, and covered with finely plumose 

 setae ; (ii.j 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 seta ; 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. 



E. Demonstration of the Respiratory Current of water 

 through the gill-chamber. 



Place a Uvvng crayfish in a shallow dish of water ; and, 

 when the animal is at rest, run into the dish close to the base 

 of the hinder legs, by means of a pipette, a few drops of water 

 coloured with suspended carmine or other pigment. Watch the 

 currents entering under the edges of the branchiostegites behind, 

 and issuing in front from the mouths of the cervical canals. 



Open the cervical canal of one side by making two cuts, 

 one immediately behind the cervical groove, the other parallel 

 to it and a quarter of an inch further back ; and removing the 

 strip between the two cuts. Lay the animal on its side in the 



