The New Student's Reference Work/Amphibia

Amphibia (ăm-fĭb′ e-a), a natural class of vertebrate animals including frogs, toads, salamanders, newts, etc.  The peculiarity about the amphibia is that they pass through a tadpole state.  They are at first adapted to living in the water and breathe by gills and have no legs; but they gradually change; usually the gills disappear, and lungs and legs are developed.  They are in a way intermediate between fishes and land animals.  Sometimes the gills are retained throughout life, as in the mud-puppy (Necturus) and Mexican axolott.  These forms have both lungs and gills.  The salamanders are like lizards in form but are different in structure and development.  Fossil amphibia are found imbedded in the rocks with a body as large as an ox.  See Frog, Toad, Salamander, etc.