VERTEBRATES — CLARK 289 



others are wholly subterranean, A few salamanders are confined to 

 caves or to underground waters. Nearly all terrestrial amphibians 

 are strictly aquatic in their early stages. 



In marked contrast to the other vertebrates, the amphibians are 

 strictly confined to fresh water, only a single Indian frog frequent- 

 ing somewhat brackish water. 



As adults most amphibians feed on insects, some adding other in- 

 vertebrates, but the large frogs and toads often eat small mammals 

 or birds, some of them other frogs. Some amphibians feed largely 

 or wholly on vegetable detritus, and nearly all feed on algae and 

 detritus in the larval stages, reversing the cycle of such birds as spar- 

 rows which are insectivorous when young, seed eaters as adults. 



All amphibians are naked, with a moist skin, least moist in some of 

 the toads. They breathe by means of external or covered gills or by 

 internal lungs, many by gills in the early stages and by lungs later, 

 and also through the skin. Some may, as adults, retain the larval form, 

 breathing by gills, or acquire an alternative form, breathing by lungs. 



Most amphibians have four functional limbs terminated by sep- 

 arate digits, which may be subequal, or the hinder pair much enlarged 

 and used for jumping or swimming, or absent. In some the limbs 

 may be greatly reduced, and in one group they are wholly absent. 



The body temperature is uncontrolled, and is approximately that 

 of the surroundings. In temperate regions the frogs, toads, and 

 salamanders hibernate, usually in water. In very dry regions some 

 frogs and toads estivate deep in the soil, encasing themselves in a 

 hard cocoon of earth mixed with mucus. 



Amphibians are not dependent on bright illumination, and most of 

 them are nocturnal, or at least avoid the sunlight. A few (as Eyla 

 micans) secrete luminous mucus. 



Except for touch, the senses are poorly developed, although some 

 frogs and toads have good eyesight, and the fact that many are highly 

 vocal indicates good hearing. 



Most amphibians lay eggs which are fertilized either outside the 

 body, in some cases long after deposition, or internally. The eggs 

 are soft with a gelatinous covering or in a gelatinous matrix and 

 are deposited individually or in masses or strings, usually in water 

 or in situations where the young will fall or be washed into water, 

 rarely in moist situations on land. A few species make more or less 

 elaborate nests. In other species the eggs are carried about on the 

 back of the female or in the mouth or wrapped about the body of the 

 male until they hatch. The young are not tended after hatching. A 

 few salamanders and two African frogs are viviparous. 



Except for the viviparous species all amphibians pass through a 

 superficially fishlike limbless larval stage, with vertical fins but no 

 fin rays, in water. In some wholly terrestrial frogs the eggs are laid 



