230 FROGS, TCIADS, AND SALAMANDERS 



body of the frog is covered with a smooth, moist, clammy 

 skin devoid of scales. The color of the skin usually accords 

 with the surroimdings and may change to suit new environ- 

 ments. The color is (lue to cells of pigment in the skin. 



Eyes. — The two eyes are very pi-ominent and protrude 

 from the head. Each one is furnishetl with an upper and 

 an under litl. The upper lid is thick and not capable of 

 much movement, but when the eye is withdrawn, this lid 

 drops down over it in a sort of mechanical wa}'. The under 

 hd is thin and can be drawn over the eye. It is called the 

 nictitating membrane. 



The hearing organs. — Back of each eye is a conspicuous, 

 round area, the eardrum or tympamim. An elastic rod, 

 the columella, one end of which is attachetl to the inner 

 surface of the tympanum, extends to the inner ear situated 

 within the skull. When sound waves strike the tympanum 

 and cause it to vil)rate, the vibrations ai'C transmitted by 

 the rod to the inner ear, which is the true organ of hearing. 

 Each ear communicates with the mouth through the Eusta- 

 chian tube. 



The nostrils and manner of breathing. — In front of the 

 eyes and above the mouth are the two nostrils. Each 

 opening is furnished with a valve so that it can lie tightly 

 closefl. Both nostrils communicate directly with the mouth. 



Within the body cavity of the frog are two pink-colored 

 lungs of considerable size that communicate with the mouth 

 through tlie windpipe. They are filled with air spaces, 

 surrovmded with walls jjcrmeated with capillaries containing 

 blood. It must be noted, however, that the lungs of the 

 frog are not ])erfect organs of respiration and ai-e aided in 

 this work by the skin and the mucous membrane of the 

 mouth antl ijharvnx. 



