Chap. XII.] THE CRAYFISH. 239 



or gills will be exposed to view. They lie in the branchial 

 cavity, which is bounded on the inner side by the delicate 

 calcified outer wall of the thorax (epimeron), and opens in front 

 by a canal in which a curved plate, the scaphognathite, in con- 

 nection with the second maxilla, is constantly working in such a 

 way as to produce a continuous current from behind forwards, 

 over the gills and through the canal. 



The branchial cavity is completely filled with the gills and 

 with certain hair-like processes (setae), developed on the coxo- 

 podites of the neighbouring limbs. Of branchiae there are 

 eighteen pairs : 



Six podobranchice ; these are the gill-bearing epipodites of the 

 limbs from the second maxilliped to the third leg inclusive. 



Eleven arthrobranchice ; attached to the inter-articular mem- 

 branes between the basal joints of the appendages and the 

 thoracic wall ; one to that of each second maxilliped, and two 

 to those of the third maxilliped to the third leg inclusive. 



One pleurobranchia ; attached to the wall of the thorax above 

 the fourth leg. There are two or three minute processes 

 representing rudimentary pleurobranchiae anterior to this. 



The arthrobranchiae consist of a stem beset with branchial 

 filaments forming a plume. In the stem are two canals, an 

 afferent and an efferent. The pleurobranchiae are similar but 

 larger. The podobranchiae have a base beset with fine straight 

 hair-like setae. From this arises a stem which, above, carries a 

 plume in front, and behind a V-shaped lamina, with the point 

 of the V directed forwards. The lamina is plaited longi- 

 tudinally, and is beset with short hooked setae. 



In examining the branchiae practically, lay the crayfish on its 

 side, and, after removing the branchiostegite, remove carefully, 

 one by one, the six podobranchiae. Then count the arthro- 

 branchiae, turning them carefully forwards as you do so. Note 

 that they are attached to the soft inter-articular membranes. 

 The last branchia is the pleurobranchia. Note that it is 

 attached to the calcified epimeron above the inter-articular 

 membrane. 



The gills are highly vascular, the blood being separated from 



