THE VASCULAR SYSTEM 133 



AF 8 , from the afferent vessel of the second branchial arch, 

 which as yet ends blindly. There is also a well-developed 

 efferent vessel, E F 3 , which opens into the dorsal aorta. 



In the fourth branchial arch there is no afferent vessel, but 

 an efferent vessel, EF 4 , is present, opening into the dorsal 

 aorta. From this efferent vessel, just before it reaches the 

 aorta, a backwardly directed branch arises, which will become 

 later the pulmonary artery, A P. 



In front of the first branchial arch, vessels are present in the 

 hyoid and mandibular arches, which clearly belong to the same 

 category as the branchial vessels, but which never attain full 

 development, probably owing to the fact that no gills are 

 formed on these arches. Efferent branches, EH, EM, open- 

 ing into the dorsal aorta, are present in both hyoid and 

 mandibular arches ; but these have no connection with the 

 heart, as there are no afferent vessels corresponding to them. 



The condition of the bloodvessels, while the tadpole is 

 breathing by external gills, may be summarised thus : Com- 

 plete systems of afferent and efferent vessels, connecting the 

 heart with the aorta through the gill capillaries, are present 

 in the first and second branchial arches, and at a stage slightly 

 later than that shown in Fig. 32 in the third branchial arch as 

 well. A similar set of vessels, but incomplete, is present in the 

 fourth branchial arch : and vessels formed on the same plan, 

 but still less complete, and showing signs of degenerative 

 changes, are present in the hyoid and mandibular arches. 



There are thus six sets of branchial vessels on each side of 

 the pharynx : of these, three, in the first, second, and third 

 branchial arches, are complete ; one, in the fourth branchial 

 arch, is incomplete ; and two, in the hyoid and mandibular 

 arches, are rudimentary. 



2. The Circulation during the time the tadpole is breathing 

 by its opercular gills. 



On the formation of the gill-slits, additional loops of com- 

 munication are formed in the gill tufts between the afferent 

 and efferent vessels of the first, second, and third branchial 

 arches, and also a series of similar loops between the afferent 

 and efferent vessels of the fourth branchial arch. The vessels 

 in the hyoid and mandibular arches undergo further retrograde 

 changes, and need not be described in detail. 



