THE VASCULAR SYSTEM 139 



an efierent vessel, EF^ 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, opening 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 blood-vessels, 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. 31 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 internal gills. 



On the formation of the internal gills, additional loops of 

 communication 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. 



In tadpoles of 12 mm. length, in which the internal gills are 

 fully established, and the external gills are shrivelling up, the 

 condition of the blood-vessels is shown in Figs. 33 and 34. 



The truncus arteriosus divides at once into right and left 



