The word "amphibian" comes from the Greek for "two lives," because most of these animals live both in the water and on land at various stages of their lives. There are three orders of amphibians. The order Anura includes frogs and toads, tailless animals that are distinguished by their powerful hind limbs, which have been adapted for leaping.
The order Caudata includes salamanders and newts, which have tails and two pairs of limbs of approximately the same size. The order Apoda or Gymnophonia are all caecilians, wormlike creatures with no limbs.
Amphibians are similar to fish in that their eggs are anamniotic; that is, they don't have the covering sac that protects the eggs of birds, mammals, and reptiles.
Other distinguishing features are mucus-secreting glands that keep the skin moist; a three-chambered heart; and two lungs.
Amphibians live almost everywhere in the world except the extreme latitudes of the Arctic and Antarctic, some ocean islands, and some very dry desert areas.
The smallest amphibian is a Brazilian brachycephalid that's less than 10 millimeters (about four-tenths of an inch) long. The largest is the giant aquatic salamander of China, which may reach a length of 5 feet or more.