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. 



