Amphibians are small vertebrate animals that have four legs and glandular skin that lacks feather, scales or hair. Internally, the structure of living amphibians is intermediate between that of fishes and amniotes. Amphibians include as many species as there are species of mammals and exhibit more variation in reproduction than any group but fishes. Amphibians have undergone a remarkable adaptation and great diversity. The termp amphibian can be interpreted in tow ways - either as an animal spending part of its life in water and then changing to an aquatic adult, or as an animal that alternates life in and out of water. Actually both interpretations are valid in part but neither applies to all amphibians, some of which are aquatic throughout of their lives, but others of which neither enter water nor have aquatic stages in their life histories.
The three major groups of amphibians, frogs & toads, salamanders & newts, and caecilians, are very different. Frogs and toads are specilized for jumping, with greatly enlarged hind legs, shortened bodies, no tail, and large heads and eyes. Salamanders and newts are much more elongated, with front and back legs of approximately equal size and a long tail. Despite these differences, all amphibians share certain characteristics that set them apart from other terrestrial animals. One such feature is that their highly permeable skin allows for rapid passage of both water and respiratory gasses. This feature prevents them from living in salt water or being active in very dry environments. However, it allows them to make use of the skin for respiration to a much greater degree than any other vertebrate. Amphibians are incapable of generating their own body heat and consequently have much lower metabolic rates than birds and mammals.
Caecilians have no legs and look like worms. Most live underground. They live in tropical areas of south America and Africa. These amphibians have small sense organs and no external ear openings. Their main sense is probably touch. short tentacles between the eys and nostrils may be used to feel their way aronund and finding food. Massive numbers of caecilians appear on the surface when their burrows are flooded after heavy rain, even in places where they seem rare and are seldom seen.