There are approximately 3,140 existing species of amphibians. They are divided into three orders: Anura, or Salientia (about 2,660 species); Urodela, or Caudata (about 320 species); and Apoda, or Gymnophiona (about 160 species). Amphibians are distributed throughout the world, except in regions covered with snow all year long.
The anurans include true frogs, tree frogs, and toads. True frogs have long hind legs and well-developed swimming and leaping powers. Tree frogs have suction pads on their fingers and toes so that they can hold on to smooth surfaces. Toads have shorter legs than frogs, and their skin has a warty appearance. The skin of the toad is usually dry to the touch, while the frog's skin is moist and smooth. The toad is brownish olive, usually with some darker spots, and with a yellowish streak down the middle of the back. Most frogs are drab green, brown, gray, and yellow; but some are brightly colored and able to change their appearance somewhat so as to blend in with their environment as a form of camouflage.
Anurans live as far north as the swamplands of northwestern Canada. They are found on all the major landmasses of the world except Antarctica, New Zealand, and Greenland.
All anurans begin life as tadpoles, tiny fishlike larvae with tails and gills. As they become adults, they lose their tails and gills and develop hind legs suitable for jumping. There are more frogs than any other kind of amphibian. (See also Frog, Anatomy of.)
The urodeles are the tailed amphibians. All of them resemble the most numerous members of this order--the salamanders. Basically animals of the Northern Hemisphere, urodeles live in or near streams, and are sometimes found under rocks and logs. They have long tails, poorly developed legs, and smooth, moist skin. They were probably the first vertebrates (animals with backbones) to exist for any length of time on land. The giant salamander of Japan is the largest of all amphibians. It grows to a length of about 5 feet (1.5 meters).
Aquatic urodeles that have tiny forelegs, no hind legs or pelvis, and feathery gills are called sirens, or trachystomes. Their long, slender bodies, which resemble those of eels, are usually brown, dark gray, or greenish. The lidless eyes are buried in the smooth, scaleless skin. Sirens usually burrow in mud at the bottom of marshes, but they sometimes venture briefly onto dry land.
The gymnophions, or caecilians, are the least understood and most rarely seen amphibians. They are blind and limbless, with long, slender bodies, like worms or snakes. Buried in a pit near each useless eye is a protruding tentacle. The animal uses this organ to feel its way about. Gymnophions range in size from 4 inches (10 centimeters) to 53 inches (135 centimeters). They burrow in the moist soil, feeding mostly on earthworms. Some gymnophions eat dead animals as well. They are found throughout the tropics, mostly in South America, Africa, and islands of the Indian Ocean. Their scales are buried in their skin.
Main Page - Kinds of Amphibians - Physical Characteristics - Behavior
The Evolutionary Record - Anatomy of the Frog