152 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



FIG. 129. Head and gills of Necturus. Cayuga 

 Lake. Copyright, 1901, by N. Y. Zoological 

 Society. 



between the two extremes of the vertebrate series; no 

 fundamental part is excessively developed. 



There are about seven hundred living species, grouped 

 in three orders : 



1 . Urodela, characterized by retaining the tail through- 

 out life, and in usually having two pairs of limbs approx- 

 imately equal in size. 

 In this group are 

 the Proteus of Aus- 

 tria and Necturus of 

 the Eastern United 

 States, both of which 

 retain their gills ; 

 Amphiuma of North 

 America, and the 

 salamanders and 



newts, in which the gills are lost in the adult, though 

 the former retains a gill slit as an evidence of their 

 presence in the larval stage. 35 



2. Anura include all the well-known amphibians 

 which are tailless in the adult stage, as frogs and toads. 

 They have a moist, 



naked skin, ten ver- 

 tebrae, and no ribs. 

 They breath by 

 swallowing the air. 

 They have four 

 limbs the hinder 

 the longer, and the first developed. They have four 

 fingers and five toes. The tongue is long, and, fixed 

 by its anterior end, it can be rapidly thrown out as an 

 organ of prehension. 36 The eggs are laid in the water 

 enveloped in a glairy mass ; and the tadpoles resemble 

 the urodelans till both gills and tail are absorbed, no 

 gill slit persisting. Frogs (Rand) have teeth in the 



FIG. 130. Red Salamander ( Spelerpes ruber). 

 United States. 



