Amphibians are cold-blooded vertebrates. They have a backbone, and they maintain their body temperature through external means, such as laying in the sun. Amphibians spend part of their lives on water and part of their lives on land. They have permeable skin, which gases and some molecules penetrate; they also have gills for some portion of their lives.
Most amphibians go through a period of metamorphosis. For example, frog eggs develop into tadpoles, which are small aquatic larvae with external gills. As they grow, tadpoles develop back legs and form internal gills. When they mature into frogs, they undergo numerous physical and biochemical changes, such as growing a large mouth and a tongue; losing gills; developing lungs; growing front legs; synthesizing new visual pigment in the eyes; and developing oxygen-binding hemoglobin protein in the blood.
Amphibians are the most threatened class of animals due to extensive water pollution and the appropriation of their aquatic habitat for human needs. About 6,340 amphibians have been identified, 32 percent of which are either threatened or extinct and over 42 percent of which are declining in number. In the United States, there are over 230 species of amphibians, many of which are on the endangered species list. The most common U.S. amphibians are bullfrogs, American toads, mole salamanders and hellbenders.