INVERTEBRATA. 



315 



pleurobranchs. The following table shows those appendages 

 from or beside which gills are found : 



ep. signifies an epipodite without a podobranch. 

 V. signifies a vestigial pleurobranch. 



Thus there are in all eighteen functional gills and two or 

 three vestigial ones on each side. 



In structure the arthrobranchs and pleurobranchs are 

 similar and simpler than the podobranchs. Each consists 

 of a vertical axis from which a number of filaments come 

 off all round, so that the whole gill looks like a test-tube- 

 brush. Oxygenation of the blood takes place in the filaments. 

 There are separate afferent arid efferent channels in the axis. 



The vestigial pleurobranchs consist of a short nxis only, 

 without filaments.* 



The podobranchs arise from oval plate-like epipodites. 

 Each has an axis like the other gills, and filaments come off 

 from the anterior face of this axis; but posteriorly there 

 comes off a double membrane, the two folds of which diverge 

 to embrace the front of the next podobranch behind (figs. 1 50 

 and 157). 



The whole series of gills, as well as the epipodite of the 

 first maxillipede and the scaphognathite of the second maxilla, 



* In the lobster, which closely resembles the crayfish in all 

 respects, all four pleurobrauchs on each side are functional. 



