AMPHIBIA. 251 



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

 and are I'udimentary. There is ncj tail, and tiie anal aperture is 

 placed almost at the extreme end of the bod)'. When adult, respira- 

 tion is carried on by means of lungs, but gills are ])resent in the 

 young, and there can therefore be no doubt as to their being genuine 

 Amphibians. 



The CcecUia; are found burrowing in marshy ground, and they ai'e 

 not unlike large eaith-woinis in appearance, but tliey sometimes 

 attain a lengtii of several feet. 



Order II. Urodela. 



In this order are a number of tish-like Amphibians, of which the 

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

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

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

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

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

 convex or rounded in front (npisthoccelous). The ribs are I'udiment- 

 ary, and the bones of the fore-arm {radins and vlmi), and of the 

 shank {tibia and fibula), are separate, and are not combined so as to 

 form single bones. 



The Urodela are juit unfrequently sjioken of as the "Tailed" Am- 

 phibians, and they fall into two natural sections, accoiding 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 per- 

 manently retained after the lungs make their a|ipeai-ance, the best- 

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

 Menohrinc/ius, and the Mud-eel {Siren). Axolotls iidiabit various of 

 the lakes of the American continent, the be.st-known species being 

 the Siredoii piseiforme of the Mexican lakes. It attains the length 

 of a foot or more, and possesses both jiairs (jf limlis, the fore-feet 

 having four toes, the hind-feet five toes. The branchi.'e are in the 

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

 the tail is compiesseil, 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 Prcjf. 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 geims Ambhj- 



