148 
The afferent branchial vessels arise from the ventral 
vessel. They enter the gills, and at once divide to supply 
a trunk to each of the main stems. ‘These trunks in turn 
give off branches into each pinna, and these again to each 
gill filament. There is a corresponding series of efferent 
vessels. The afferent and efferent vessels merge imto one 
another in each gill filament, where one limb of the 
vascular loop is derived from the afferent, while the other 
is in connection with the efferent vessel. The first six 
gills return blood to the sub-intestinal vessels, while the 
efferent vessels of the remaining gills open into the dorsal 
vessel (fig. 25). 
Setzx. 
Kach notopodial seta is a slender capillary chitinous 
structure inserted at its proximal end, along with many 
other similar sete, in a setal sac, which is moved by 
special retractor and protractor muscles (figs. 24 and 36). 
The seta has an almost uniform diameter for a consider- 
able portion of its length, but it tapers at its distal end to 
a fine point (fig. 12). On the distal fourth or fifth of its 
length the _seta bears numerous minute, regularly 
arranged, pomted processes, which are usually present on 
both sides of the seta. They are moderately obvious on 
one side, but on the other they are very minute and borne 
on the edge of a thin border or lamina (fig. 14). This is 
well marked im the large sete of the Laminarian variety, 
in which the lamina may be traced for about a millimetre 
along the seta, and attains a width of 20u (fig. 15). In 
some of the sete the lamina is not denticulate at its 
margin, and in others is only very faintly so; but it is 
crossed by fine oblique lines the intervals between which 
correspond roughly to the size of the teeth on dentigerous 
laminze. It seems probable that the lamina at first 
