AMPHIBIA 



291 



animal, like many others, is sensitive primarily to moving objects. 

 The tongue is supplied v^\t\\ papillae, bearing taste organs. In order 

 to detect a substance it must be liquefied. 



Hearing is quite well developed. Near the center of the tym- 

 panic membrane is a little protuberance, the tip of the columella, 

 the small bone at its inner end connected with an opening in the 

 skull which communicates with the inner ear. When the tympanic 

 membrane vibrates in response to sound waves, the vibrations com- 

 municated to the internal ear produce the sensation of hearing. 



Amphibian tadpoles have functional lateral line organs sensitive 

 to vibrations of low frequency, one step below the sensation of 

 hearing. In adult amphibia they disappear. In the skin there are 

 thermal and tactile organs. 



General Consideration of the Amphibia 



Distribution. — Amphibia live in or near swamps, ponds and 

 streams and are never found in salt water. They are on this ac- 

 count absent from most oceanic islands. Although many of them 

 have lost their gills and taken up terrestrial life, they return to 

 water to spawn. The salamanders are for the most part limited to 

 the temperate zone, while the Anura are found widely distributed 

 and hibernate or aestivate as temperature demands. 



External Anatomy. — There is a great variation in the matter of 

 retention of gills in the salamander, some having persistent gills and 

 lungs while others lose the gills entirely. 



In the toads and most tree frogs, webs are lacking from the hind 

 toes, while certain species of frogs have their toes ending in rounded 

 sucking discs. 



Amphibia respire largely by the skin, which is full of capillaries. 

 The stratum corneum of the skin is shed periodically. Warts and 

 granules are cornifications of the epidermis. In the males of the 

 African " hairy frog " {Astylosternus), the granules of the skin are 

 developed at the breeding season into hairlike structures, which, 

 according to Kingsley, are supplied with nerves and are probably 

 sensory in function, although certain writers have described them 

 as accessory breathing organs. 



The skin of amphibia is rich in mucus glands, Cryptobranchus 

 being especially slimy. Some species of amphibia have well-de- 

 veloped poison glands. (See page 300.) 



