The Class Amphibia ("double life") contains over 4,000 species of animals that are somewhat transitional between fishes and reptiles. Amphibians make their first appearance in the fossil record almost 400 million years ago, and the surviving species represent a small fraction of the total number of amphibians that once existed on Earth. General characteristics of amphibians include a bony skeleton and usually four limbs. The skin of amphibians is moist and thin with no scales.  Since it is too thin to provide much protection against dehydration or predators, they must employ other means of defense. 

For example, amphibian skin contains many mucous glands that keep them moist and make them slippery, which helps them escape from predators.  The skin of all amphibians also contains poison (serous) glands that produce toxins that range from mildly noxious to deadly poison (some are among the most potent toxins produced by any vertebrates).  In keeping with their toxic nature, many amphibians are brightly marked with aposematic (warning) colors that advertise their toxicity.  These colors are due to pigment-containing cells call chromatophores that come in several varieties.

Living amphibians include the: leg-less amphibians (caecilians), tailed-amphibians (salamanders and newts) and tail-less amphibians, or anurans, (frogs and toads).