ARTHEOBRANCHI^ AND PODOBRANCSHIiE. 77 



second maxillipede. The first maxillipede and the last 

 ambulatory limb have none. Moreover, where there are 

 two arthrobranchiae, one is more or less in front of and 

 external to the other. 



These eleven arthrobranchise are all very similar in 

 structure [fig. 17, C). Each consists of a stem which con- 

 tains two canals, one external and one internal, separated 

 by a longitudinal partition. The stem is beset with a great 

 number, of delicate branchial filaments, so that it looks 

 like a plume tapering from its base to its summit. Each 

 filament is traversed by large vascular channels, which 

 break up into a net-work immediately beneath the surface. 

 The blood driven into the external canals of the stem (fig. 

 15, av) is eventually poured into the inner canal (ev), which 

 again communicates with the channels {bcv) which lead to 

 the pericardial sinus (p). In its course, the blood traverses 

 tlie branchial filaments, the outer investment of each of 

 which is an excessively thin chitinous membrane, so that 

 the blood contained in them is practically separated by a 

 mere film from the aerated water in which the gills float. 

 Hence, an exchange of gaseous constituents readily takes 

 place, and as much oxygen is taken in as carbonic acid is 

 given out. 



The six podobranchiee, or gills which are attached to 

 the basal joints of the legs, play the same part, but differ 

 a good deal in the details of their structure from those 

 which are fixed to the interarticular membranes. Each con- 

 sists of a broad base (fig. 17, A and B ; b) beset with many 

 7 



