ARTHROBRANCHIZ AND PODOBRANCHIZ. 77 
second maxillipede. The first maxillipede and the last 
ambulatory limb have none. Moreover, where there are 
two arthrobranchiz, one is more or less in front of and 
external to the other. 
These eleven arthrobranchie 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 alongitudinal 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 
the 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 aérated 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 podobranchie, 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, Aand B; b) beset with many 
