The class of vertebrate chordates (see chordata) that contains over 7000 known species of frogs, toads, newts, and salamanders. The amphibians evolved in the Devonian period (about 370 million years ago) as the first vertebrates to occupy the land, and many of their characteristics are adaptations to terrestrial life. All adult amphibians have a passage linking the roof of the mouth with the nostrils so they may breathe air and keep the mouth closed. The moist scaleless skin is used to supplement the lungs in gas exchange. They have no diaphragm, and therefore the muscles of the mouth and pharynx provide the pumping action for breathing. Fertilization is usually external and the eggs are soft and prone to desiccation, therefore reproduction commonly occurs in water. Amphibian larvae are aquatic, having gills for respiration; they undergo metamorphosis to the adult form. In recent decades many amphibian populations around the world have suffered sharp declines or even extinction. The causes are often unknown, but pesticide pollution, habitat loss, and climate change are among those implicated.

https://amphibiaweb.org/index.html Hosted by the University of California, Berkeley; gives free access to information about amphibian biology and conservation