METAMORPHOSES OF BATRACHIA. 



579 



a large artery, which swells at its base into a contractile bulb, 

 and which is sometimes bifurcated to form the two arches of the 



6r3 



vb 3 a ap av c ab 2 



FIG. 338 BLOOD-VESSELS OF THE TADPOLE IN ITS FIRST STATE. 



a, artery originating from the single ventricle, and dividing into six branchial arteries, 

 06 ; br. I , br. 2, br. 3, the three pairs of gills, from which return the branchial veins vb. The 

 second and third branchial veins on each side form a trunk, c, which unites with the op- 

 posite one to form the great dorsal artery, av ; the first pair of branchial veins send off the 

 trunks, t, t, to the head. I, 2, 3, communicating branches, connecting the branchial ar- 

 teries with the branchial veins, in this stage very small ; ap, pulmonary arteries, as yet 

 but little developed. 



aorta. But when the young animal respires by the gills only, 

 the blood forced out of the ventricle is distributed to these organs ; 

 and thence the greater part is returned into a dorsal artery, 



FIG. 339. The same, in a more advanced state; the communicating branches are'non- 

 enlarged, so that much of the blood transmitted to ihe branchial arteries finds its way at 

 once into the branchial veins, without passing through the gills ; the pulmonary arteries, 

 ap, are also much increased in size. 



whose branches convey it to the rest of the body (Fig. 338). 

 We have elsewhere seen (Axor. PHYSIOL. 289), that amon? 



