538 TEXTBOOK OF ZOOLOGY 



bird's with which the tadpole can scrape thin pieces from leaves of 

 aquatic plants, or algae and other plant material from sticks and 

 stones. 



Near the end of the larval period (about two years in the bull- 

 frog), which varies considerably with species and environment, the 

 tadpole prepares to metamorphose into a frog. First the hind legs 

 push through the skin at the base of the tail; then the forelegs 

 appear, forcing their way through the operculum on the right and 

 the spiracle on the left side. As the lungs develop, the tadpole has 

 to come to the surface of the water frequently to give out a bubble 

 of impure air and take in a purer one. The tail is gradually ab- 

 sorbed, the intestines shorten, the homy beak disappears, the mouth 

 widens, the gills are resorbed, the legs develop, and the tadpole 

 becomes a frog. 



In general, it may be stated that the ectoderm gives rise to the 

 nervous structures, the epidermis and its outgrowths. The endo- 

 derm forms the epithelial lining of the intestine, and outgrowths 

 of the intestine, such as the epithelial lining of gills, lungs, liver, 

 pancreas, gall bladder, urinary bladder, etc. From the mesoderm 

 are formed the muscular, vascular, and skeletal systems. Most of 

 the organs are formed not from a single germ layer but from a 

 combination of these tissues. The elementary tissues have been 

 discussed in the chapter on Metazoan Organization. 



THE TOAD 



One of the common toads is the American toad, Bufo americamis. 

 It resembles closely its relatives, the Woodhouse's toad {Bufo wood- 

 housii Girard) and Fowler's toad (Bufo fowleri), and only by close 

 scrutiny can they be distinguished. Bufo woodJiousii ranges, in gen- 

 eral, from Texas to Kansas and Nebraska and westward to Arizona 

 and southeastern California. 



Habitat 



In contrast to the bullfrog, the toad is entirely terrestrial except 

 when it goes to the water during the breeding season to lay its 

 eggs. Although the toad's skin is tougher and better protected 

 from drying out, water evaporates through it rapidly, and it can- 

 not endure dry heat. Most of the toad 's supply of water is obtained 



