CLASS OF BATBACHIANS. 



367 



the tail wastes away and disappears (Figs. 373, 374). About 

 the same time the lungs appear, and begin to perform their, 

 functions, so that at this period the tadpole is strictly an 

 amphibious animal ; but although this strictly amphibious 

 state continues in some, in general it does not; the gills 

 disappear, and in the adult there remain no traces of such an 

 apparatus. 



3 a ap av c ab 2 vb 



Fig. 375. Bloodvessels of the Tadpole of the Frog.* 



The circulatory apparatus also undergoes important meta- 

 morphoses. The heart of the adult batrachian is composed 

 of two auricles and a ventricle, whence springs a large artery, 

 bulbous (like that of fishes) at its commencement, and which 

 soon divides to form the two arches of the aorta; but when 

 the young animal breathes by branchiae only, the blood ex- 

 pelled from the ventricle is distributed to the gills (as in 

 fishes), whence it proceeds, for the greater part, to the dorsal 



* o, artery arising from the single ventricle, and dividing into six 

 branches (ab), which go to the three pairs of branchiae, and ramify there 

 they are called branchial arteries ; br, the branchiae, showing the distribu- 

 tion of the branchial arteries upon them, and carrying the blood to them and 

 the branchial veins (vb) , bringing back the blood from them after it has been 

 exposed to the oxygen of the atmosphere contained in the water sent across 

 the gills : those of the two last pairs of branchiae unite to form on each side a 

 vessel (c) which, anastomosing in its turn with that of the opposite side, forms 

 the ventral aorta or dorsal artery (av), which, proceeding backwards, dis- 

 tributes the blood to the greater part of the body. The branchial veins of 

 the first pair of branchiae curve forwards, and carry the blood towards the 

 head (t, t) ; 1, a very fine and anastomotic branch unites the branchial artery 

 and vein together at the base of the first pair of branchiae, and this enlarging 

 afterwards, allows the blood to pass between those two vessels without pass- 

 ing across the gill ; 2, a small anastomotic branch, uniting in the same way 

 the artery and vein of the second pair of branchiae ; 3, a vessel which, by 

 uniting with a small branch situated more inwards, connects the artery and 

 vein of the posterior branchiae ; o, orbitar artery; ap, rudimentary pulmonary 

 arteries. 



