The ability to resist water loss through the skin and the development of a land egg are perhaps the differences between reptiles and amphibians that are of the greatest evolutionary significance. The amnion of the reptile egg makes it more able to resist desiccation, and the eggs can be laid in relatively dry places. Lacking this membrane, amphibian eggs must be laid in water or in very moist places. A most important difference between the two groups is the absence of the amnion in the Amphibia, and its presence in the Reptilia.
Young (larval) amphibians have gills, but there is no comparable gill-breathing, larval stage in the life history of a reptile. Reptiles usually have a dry, scaly skin that is relatively impervious to water loss and very different from the amphibians with their moist skin that permits much evaporation. There are many exceptions to these generalizations, however. In general, modern amphibians as adults differ from fishes in lacking scales, breathing by means of the skin and lungs instead of gills, and having limbs in place of fins. Present-day amphibians, however, are highly specialized animals, rather different from the primitive forms that probably first arose from crossopterygian fishes and far removed from those that gave rise to the earliest reptiles. The closest relatives of the amphibians are the fishes, from which they evolved, and the reptiles, to which they gave rise. A classification scheme for the Amphibia follows:Ī typical amphibian is characterized by a moist, glandular skin, the possession of gills at some point in its life history, four limbs, and an egg lacking the embryonic membrane called the amnion. The orders in the subclasses Labyrinthodontia and Lepospondyli existed in the geologic past and are now extinct. The living amphibians number approximately 2460 species, and are classified in three orders: the Anura or Salientia (frogs and toads, slightly less than 2000 species) Urodela or Caudata (salamanders, 300 species) and Apoda or Gymnophiona (caecilians, about 160 species). One of the four classes composing the superclass Tetrapoda of the subphylum Vertebrata, the other classes being Reptilia, Aves, and Mammalia.