An amphibian must spend part of its life cycle in
the water where its eggs are laid and fertilized. The eggs develop into a larval
stage, or tadpole, that has fish-like characteristics. In tadpoles breathing is
by means of gills, blood is pumped by a two-chambered heart, and swimming is by
means of tail and body movements made possible by muscles in the body wall. Most
amphibians undergo metamorphosis into a lung-breathing adult with a 3-chambered
heart.
