PROTEID.E; AXOLOTL; PROTEUS. SIRENID^. 585 



the branchial arches with small laminae. They evidently form 

 a transition to the next order, in which the external gills are 

 persistent. The species, some of which reach a length of three 

 feet, are for the most part inhabitants of North America. Their 

 limbs are very small, and they live principally in the mud. 



ORDER III. AMPHIPNEUSTA. 



532. The species of this order in some respects resemble the 

 Urodela ; they have a naked skin and a permanent tail. They 

 are distinguished, however, by the existence of persistent ex- 

 ternal branchiae, projecting from behind the head. They form 

 only two families, the PROTEIDJE and the SIRENID^E. In the 

 former the tail is compressed, as in the Newts ; the four limbs 

 are developed, and the branchiae are of large size. The Axolotl 

 (Fig. 19) is an example of this family. It is a native of the 

 Lake of Mexico, where it attains a length of from ten to fifteen 

 inches, and is considered as an article of luxury by the inhabit- 

 ants of the city of Mexico. The Proteus, a curious blind animal, 

 found in the subterranean waters of the Carinthian caves, also be- 

 longs here. The SIRENID^E, or /Sirens, have only the two an- 

 terior legs developed, and these are small ; the body is nearly 

 cylindrical, and the branchiae small. They are peculiar to the 

 Southern States of North America, where they live principally 

 in the marshy rice grounds. The Siren lacertina, the best 

 known species, sometimes measures three feet in length. It 

 feeds upon worms and insects. 



ORDER IV. APODA. 



533. The apodal Batrachia. or those in which the feet are 

 entirely wanting, and the body cylindrical and worm-like, form 

 only a single family, that of the CJECILIAD;E, so called from the 



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