SEA-FISHERIES LABORATORY. 223 



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 into 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. 23). 



S e t se . 

 Each notopodial seta is a slender capillary chitinous 

 structure inserted at its proximal end, along with many 

 other similar seta?, 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, pointed 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 in the large setse of the Laminarian variety, 

 in which the lamina may be traced for about a millimetre 

 along the seta, and attains a width of 20/x (fig. 13). In 

 some of the seta? 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 

 laminee. It seems probable that the lamina at first 



