DEVELOPMENT OF THE BLOOD VESSELS IN THE EROG. 239 



vessel (Figs. 13 and 14, A F.2) is of large size, if anything slightly- 

 larger than that of the first arch ; just before reaching the gills it 

 gives off a small branch to the filters. 



The efferent vessel (Figs. 13 and 14, E F.2) lies just in front of the 

 afferent vessel, receives branches from the gills, both internal and 

 external, and then continues its course to the dorsal aorta. 



The ventral end of the efferent vessel lies very close to the afferent 

 vessel, and in some specimens a direct communication takes place 

 between the two, in the same manner as in the first arch. As 

 regards the afferent vessel, this communication is effected beyond the 

 origin of the artery to the filters, but ventral to the lowermost gill 

 vessels. The communication, when present, is effected, as in some 

 specimens of the first branchial arch, by a narrow capillary passage 

 perforating a thin plate of deeply staining cells wedged in between 

 the afferent and efferent vessels, the thickness of the plate being 

 about half the diameter of the vessels. 



e. The Third Branchial Arch. — The vessels are very similar to 

 those of the second arch, though of smaller size. Their course and 

 relations are shown in Figs. 13 and 14, A F.3 and E F.3. There is a 

 small external gill present, lying dorsal to the internal gills, and 

 having both afferent and efferent vessels. The ventral end of the 

 efferent vessel communicates at its base with the afferent vessel, in 

 the same manner as in the second arch. In our specimens this 

 connection is easier to see, and apparently more constant in the 

 third than in the second arch. 



/. The Fourth Branchial Arch. — The vessels of this arch are much 

 smaller than those of the anterior arches. The afferent vessel (Figs. 

 13 and 14, AF.4) arises as a branch from the afferent vessel of the 

 third arch, some distance from the division of the truncus arteriosus ; 

 it gives afferent branches to the gills of the arch. 



The efferent vessel receives the efferent gill capillaries, and then 

 runs nearly vertically upwards in the walls of the pharynx, its course 

 being much straighter than that of the more anterior vessels, and 

 nearly the whole of its length being shown in a single sagittal section. 

 It opens into the dorsal aorta immediately behind the opening of 

 the third branchial efferent vessel (Fig. 13) ; the two vessels some- 

 times appearing to unite just before reaching the aorta. 



From the dorsal part of the efferent vessel, just before it reaches 



