228 VERTEBRATE ANIMALS. 



inhabit various of the lakes of the American Continent, the best- 

 known species being the Siredon pisciforme of the Mexican 

 lakes (Fig. 112). It attains a length of a foot or more, and 



FIG. 112. The Axolotl (Siredon pisciforme). (After Tegetmeier.) 



possesses both pairs of limbs, the fore-feet having four toes, 

 the hind-feet five toes. The branchiae 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 Axolotl 

 is certainly perennibranchiate, and it breeds freely in this 

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

 New Haven, that some species, when kept in confinement, lose 

 their gills, and undergo certain other changes, becoming ul- 

 timately converted into a Salamandroid, apparently belonging 

 to the genus Amblystoma. The Proteus is an extraordinary 

 Amphibian which is found inhabiting the waters of caves in 

 Illyria and Dalmatia. It attains a length of about a foot, and 

 is of a pale flesh-color or nearly white. The gills, which are 

 retained throughout life, are of a bright scarlet. Both pairs 

 of limbs are developed, but they are only short and weak, the 

 fore-limbs having three toes each, and the hind-limbs only 

 two. The eyes are extremely small, the animal spending its 

 existence in darkness; and swimming is effected mainly by 

 means of the tail. The Siren, or Mud-eel, is a large lizard-like 

 Amphibian, which is found abundantly in the swamps of South 

 Carolina, and attains the great length of three feet. The ex- 

 ternal branchiae are retained throughout life, and they are the 

 main organs of respiration. The fore-limbs are present, but 

 the hinder pair of limbs is never developed. 



