176 ANIMAL FORMS 
In the meantime large internal changes are also taking 
place. The wall of the esophagus has gradually pouched 
out to form the lungs. They are richly supplied with blood- 
vessels, closely resembling in their general features the 
lungs of the lung-fishes. The animal now rises to the sur- 
face occasionally to gulp in air, and it also continues to 
breathe by means of gills. At this stage of its existence, 
therefore, the larva is amphibious (two-living), and we have 
the interesting example of an animal extracting oxygen 
from both the water and the air. The diet of the tadpole 
at this time changes from vegetable to animal substances, 
and horny teeth give way to the small teeth of the frog, 
and the digestive system undergoes an entire remodeling 
to adapt it to its new duties. The young amphibian— 
whether frog, toad, or salamander—is now a four-legged 
creature, with well-developed head and tail, with lungs and 
gills, though the latter are usually fast disappearing, and is 
rapidly assuming those characters which will fit it for a 
terrestrial or semiaquatic existence. 
168. The salamanders.—The changes which now ensue in 
such a larva in reaching the adult condition are relatively 
slight in the lower salamanders. The external gills often 
persist (Fig. 110), the lungs are also functional, and the 
changes are largely those of increase of size. In the larger 
number of species the gills disappear more or less com- 
pletely (Fig. 108), such species often abandoning the water 
for homes in damp soil or under stones and logs, returning 
to it only when the time comes for their eggs to be laid. 
The limbs are always relatively weak, never supporting the 
body from the ground, but serving in a clumsy way to push 
it from place to place. In the aquatic forms the tail con- 
tinues to serve as a Swimming organ. In some species the 
hind legs become rudimentary, or even entirely lacking. 
A still further modification occurs in a few burrowing spe- 
cies, which move by wrigglings of the body, and are with- 
out either pairs of legs. 
