102 ESSENTIALS OF ZOOLOGY 



the internal gills. The skeleton is cartilaginous; lateral line sense 

 organs are present on the sides ; the pineal organ is evident ; and the 

 animal is similar to a fish. 



The tadpoles feed primarily on plant substances. In the mouth of 

 the bullfrog tadpole is one row of teeth above and three rows below, 

 plus a border of projections, known as papillae, for testing food sub- 

 stances. Between the lips is a horny heak somewhat like that of a 

 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 horny 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 dis- 

 cussed in the chapter on Meiazoan Organization. 



Economic Importance of Amphibia 



The entire group of Amphibia is of considerable economic value 

 because they feed to such a large extent on insects, thus becoming 

 valuable aids to the farmer in controlling noxious insects. In the 

 flooded rice fields of Louisiana, bullfrogs grow fat eating insects, 

 crayfish, and other small animals. 



Frogs are used throughout the world as an article of food by 

 man as well as by other animals. In the eastern United States, 

 large quantities of the leopard frog and wood frog are consumed. 



