CLASS OF BATBACHIANS. 



331 



reaches them by the cavity of the mouth, and escapes by one 

 or two orifices situated under the neck. In the tadpole of the 

 frog, the hind feet appear first, and they become of some 

 length before the fore feet are visible ; these appear later 

 (Fig. 297). In the salamanders it is the opposite; finally, 

 in the siren the hind legs never appear. The tail of the 

 tadpole continues to grow in the salamander and proteus 

 with the rest of the body ; but in frogs and in many others 

 the tail wastes away and disappears (Figs. 298, 299). 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. 



Fig. 295. 



Fig. 296. 



Fig. 297. Fig. 298. Fig. 299. 



Figs. 295 299. Metamorphoses of the Tadpole of the Frog. 



The circulatory apparatus also undergoes important metamor- 

 phoses. The heart of the adult batrachian is composed of two 

 auricles and a ventricle, whence springs a large artery, bul- 

 bous (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 

 artery, whose branches ramify in the different organs (Fig. 

 300). We have already seen, that in fishes the blood follows 

 the same course ( 109), but when the lungs are developed, 

 the disposition of the vascular apparatus changes; a direct 



