AMPHIBIANS 145 



Number and Distribution. — About i ,400 species of amphibians are 

 known, of which 150 species are Caudata, or salamanders, and 1,200 

 species are Salientia, or frogs and toads. The burrowing, hmbless 

 Apoda, which are confined to tropical America, Africa and India, 

 number about 50 species. The Caudata occupy principally the temper- 

 ate and subtropical portions of Europe, Asia and America, penetrating 

 only into the extreme northern parts of Africa, and not occurring at 

 all in Australia and the east Indian archipelago. The Salientia inhabit 

 the entire world, except the polar regions and the smaller oceanic islands. 



The amphibian fauna of the United States is very rich, a feature in 

 which it parallels the distribution of other animal groups with a similar 

 habitat, namely, fresh water fish and fresh water mollusks. A com- 

 parison of the number of species of amphibians inhabiting New York 

 State and Germany or Great Britain will illustrate the statement: in 

 Germany the Caudata are represented by 4 species and in New York by 

 15; Great Britain possesses 3 species of Salientia, while New York 

 has 13. 



Key to the Orders of Amphibia in the United States 



ai Body elongate, with a tail which persists throughout life; salaman- 

 ders I. Caudata. 



32 Body of adult short and taUless; young with a long tail which 



disappears in the metamorphosis; frogs and toads 2. Salientia. 



Order i. Caudata (Urodela). — Salamanders. Amphibians with 

 an elongate body and usually two pairs of weak limbs. The vertebrae 

 are numerous and are accompanied by ribs. The skin is without scales 

 and is very glandular, the secretion in some species being poisonous. 

 The eyes are small and the eye-lids are not present in the lower forms. 

 The ear is entirely internal, as in fishes, there being no tympanum or 

 tympanic cavity. The body muscles have a conspicuously segmental 

 arrangement, like those of fishes, the vertical grooves which separate 

 the muscle segments of the trunk being usually easily seen on the 

 outside of the body, and called the costal grooves (Fig. 72). Teeth may 

 be present on the maxillaries, premaxillaries, vomers, pterygoids, 

 parasphenoid and mandibles. The eggs are usually deposited in the 

 water and the larvae, when born, are provided with three pairs of 

 external gills, which are expansions of the outer integument and not 

 homologous to the gills of fishes. These gills disappear during the 

 metamorphosis, except in Necturus, Typhlomolge and Siren. In some of 

 the Plethodontid(B the greater part of the larval stage is spent in the egg 

 and the young are terrestrial from the time of birth and without gills. 



