THE BRANCHIAL BLOOD-VESSELS. 177 



arrangement of the vessels is practically the same in all. In 

 each arch (Fig. 80) there are two main vessels, afferent and 

 efferent, which lie side by side close to each other. Of these, 

 the afferent vessel is a branch of the truncus arteriosus, and lies 

 in the posterior part of the arch ; while the efferent vessel 

 lies immediately in front of the afferent, and opens at its 

 dorsal end into the aorta. The afferent vessel is confined to 

 the ventral half of the arch ; while the efferent extends along 

 its whole length, its ventral termination lying in the floor of 

 the mouth, close to the origin of the afferent vessel from the 

 truncus arteriosus. 



The afferent and efferent vessels of each arch are connected 

 together in two ways : (i) by the capillary loops of the gills, 

 of which the most dorsally placed belong to the external gill, 

 and the ventral series to the internal gills ; (ii) by each afferent 

 vessel opening directly into the corresponding efferent vessel, 

 the communication (Fig. 80) being ventral to the gills, and of 

 very small size. This direct connection between the afferent 

 and efferent vessels is present in all four branchial arches, though 

 its position and relations are not easy to determine. The blood 

 entering an afferent vessel from the heart has thus two courses 

 open to it : it may either continue along the afferent vessel, 

 and pass through the gill capillaries to the efferent vessel, and 

 so reach the aorta ; or it may pass across at once, through the 

 aperture of communication, to the efferent vessel, and reach 

 the aorta without having passed through the gills. 



At the commencement of the metamorphosis these direct 

 communications enlarge, so that an increasing quantity of blood 

 passes from the heart to the aorta without going through the 

 gills. The gills thus receive less and less blood, and gradually 

 diminish in size and in efficiency. Increased work is thereby 

 thrown on the lungs ; and an increasing supply of blood is 

 sent to the lungs and skin by enlargement of the pulmonary 

 and cutaneous arteries, which are branches of the efferent vessel 

 of the fourth branchial arch (Fig. 80), close to its dorsal end. 



By further enlargement of the direct communications 

 between the afferent and efferent vessels, the definite aortic 

 arches are formed, leading directly from the heart to the aorta ; 

 each aortic arch (cf. Figs. 80 and 81) consisting of the basal or 

 proximal end of the afferent branchial vessel, and the whole 



