268 BIOLOGY FOR BEGINNERS 



on the left side. This has an internal connection to the right gills 

 so that both are supplied with water. 



These latter changes may have occupied nearly two months, 

 and the tadpole is now a fish-like animal, with gills, lateral line, 

 fins, two-chambered heart, and one-circuit circulation, but soon 

 other changes follow, gradually adapting the aquatic animal for 

 land life. 



A sac-like chamber develops backward from the throat like the 

 fish's air bladder, but soon separates into two lobes with cellular 

 walls which we recognize as lungs. To correspond with this, the 

 circulation is gradually modified; the gill arteries are changed to 

 carotids, pulmonaries, and aortic arches; the heart becomes three 

 chambered, and the circulation flows in two circuits. At this 

 stage the tadpole may be seen coming to the surface for air to fill 

 his new lungs as his gills no longer are used for breathing but are 

 being modified into mouth parts and other organs. 



While these notable changes are occurring to the respiratory 

 and circulatory systems, others no less remarkable are taking 

 place elsewhere. The mouth widens, teeth develop, and the 

 intestine becomes shorter and larger to adapt it for animal diet 

 which the young frog now begins to use. 



The external changes, which have accompanied these last 

 mentioned, have been more conspicuous, though less important, 

 and are as follows. The tail is gradually absorbed (not shed), 

 limbs develop at the place where it joined the body, and the body 

 itself changes shape. The front legs begin growth about the same 

 time but do not show so soon since they start beneath the operculum 

 in the gill chamber and are smaller even when full grown. 



By this time, the tadpole is a well-developed frog which comes 

 on land, breathes air, eats animal food and gradually grows in size 

 till he reaches the full stature of an adult. These latter changes 

 have occupied usually another month, making a total of about 

 three months for an average frog metamorphosis, though growth 

 in size may continue much longer. 



Representatives. Let us now briefly take up a few of the common 

 representatives of the amphibia, which includes, besides the frog, 

 the toads, salamanders, newts, etc. 



