The Science of Biology
Animals: Vertebrates
The amphibians are animals that live both on land and in the water. The members of the class Amphibia are believed to have evolved from the lobe-finned fishes about 20 million years ago, taking advantage of the higher concentration of oxygen in air than water. They are represented by the frogs, toads, and salamanders.
Amphibians live on land and breathe air to meet their oxygen demands. Amphibians are also able to exchange gases through their lungs, skin, and the inner lining of their mouth. Gas exchange is enhanced by a very efficient circulatory system.
Amphibians remain in moist environments or water to avoid dehydration. Also, amphibians lay their eggs in water because the eggs would quickly dry out on land. Sperm cells are released into the water where they fertilize the egg mass. The early-stage tadpoles lead an aquatic existence and later emerge onto land as adult amphibians.
