AMPHIBIA. 251 



horny scales embedded in it. The eyes are concealed by the skin, 

 and are rudimentary. There is no tail, and the anal aperture is 

 placed almost at the extreme end of the body. When adult, respira- 

 tion is carried on by means of lungs, but gills are jjresent in the 

 young, and there can thei'efore be no doubt as to their being genuine 

 Amphibians. 



The CcecilicB are found burrowing in marshy ground, and they are 

 not unlike large earth-worms in appearance, but they sometimes 

 attain a length of several feet. 



Order II. Urodela. 



In this order are a number of fi.sh-like Amphibians, of which the 

 Newts and Land-salamanders are the most familiar examples. In 

 all the members of this section the skin is naked, and never develops 

 any hard structures, and in all there is a well-developed fish-like 

 tail, which is retained throughout life. The vertebras are sometimes 

 hollow at both ends {amphicoslous), sometimes hollow behind and 

 convex or rounded in front {opisthocoeloics). The ribs are rudiment- 

 ary, and the bones of the fore-arm {radius and nbui), and of the 

 shank (tibia B.nd Jibula), are separate, and are not combined so as to 

 form single bones. 



The Urodela are not unfrequently spoken of as the "Tailed" Am- 

 phibians, and they fall into two natural sections, according as the 

 gills are permanently retained throughout life, or are cast off before 

 maturity is attained. The animals belonging to the first section are 

 often called " perennibranchiate," whilst those belonging to the 

 second are said to be " caducibranchiate." 



Among the Perennibranchiate forms, in which the gills are )]er- 

 manently retained after the lungs make their appearance, the best- 

 known examples are the Axolotl, the curious Proteus anguinus, the 

 Menohranchus, and the Mud-eel (Siren). Axolotls inhabit various of 

 the lakes of the Americaif continent, the best-known species being 

 the Siredon pisciforme of the Mexican lakes. It attains the length 

 of a foot or more, and possesses both pairs of limbs, the fore-feet 

 having four toes, the hind-feet five toes. The branchise are in the 

 form of three long ramified processes on each side of the head ; and 

 the tail is compressed, and fringed by a fin which is prolonged on 

 the back between the shoulders. In a state of nature, the Mexican 

 Axolotl is certainly perennibranchiate, and it breeds freely in this 

 condition. It has been shown, however, by Prof. Marsh, of New 

 Haven, and others, that some species, when kept in confinement, 

 lose their gills, and undergo certain other changes, becoming ulti- 

 mately converted into Salamandroids, belonging to the genus Amhly- 



