ARTHROBRANCHI^ AND PODOBRANCHI^. 77 



second maxillipede. The first maxillipede and the last *' 

 ambuhitory hmb have none. Moreover, where there are 

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

 external to the other. 



I'hese 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 

 b.y a longitudinal partition. The stem is beset with a great 

 number of delicate branchial filaments, so that it looks i 

 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. I 

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

 15, av) is eventually poured into the inner canal {ev), which 1 

 again communicates with the channels (hcv) which lead to 

 the pericardial sinus {i)). 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 b}^ a 

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

 Hence, an exchange of gaseous constituents readily takes 

 place, and as much ox3^gen is taken in as carbonic acid is 

 given out. 



The sixjKKlobranchiae, 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 inter articuL'ir membranes. Each con- 

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

 5 



