Amphibians were the first true land vertebrates. They emerged from the oceans in the early Devonian Period, 410-345 million years ago. Altogether they comprise the smallest class of vertebrates. The class is separated into three main orders: Apoda (the tailless, legless and blind caecillians), Caudata (salamanders and newts) and Anura (frogs and toads).
Amphibians inhabit all ecological niches. They are not found in high mountain regions permanently snow covered, the polar regions or in completely marine environments. This class is most diversely represented in the tropics.
The skin of amphibians is smooth and moist, except for the caecillians. Numerous skin glands secret a fluid that is often irritating and poisonous. The inner skin contains color cells which may allow the skin to change color by expansion or contraction of the cells. The outer skin is frequently shed either whole or in large patches and is often eaten by the animal.
The skeletal, muscular, digestive and nervous systems are similar to those in higher animals. The number of vertebrae ranges from 6 in some species of salamanders to over 300 for certain species of caecillians. The teeth and tongue vary in forms and are sometimes absent. In many frogs, the tongue is fixed in the front and free behind. In the males of the order Anura, the lining of the mouth is often pushed outward into resonating vocal sacs. Breathing takes place by means of gills, lungs or through the skin.
The eggs of most amphibians have a gelatin like covering and are usually laid in water. Larvae hatch from the eggs and later undergo metamorphosis from egg to larva to adult. In some species, the eggs are laid on land, but the larvae must enter the water to complete their development. Other amphibian species never undergo metamorphosis, but instead remain larva-like and reproduce as larva. Few species of amphibians are live-bearers. The number of eggs in a clutch varies from a few to well over 25,000. The time required to hatch varies from 36-52 hours to as long as 3-4 months. While a great number of species lay their eggs and desert them, there are many species which do provide parental care.
Amphibians are poikilothermic vertebrates, which means they are unable to maintain a constant body temperature, and therefore have a body temperature similar to that of the enviroment. Modern amphibians have naked skin and lose water through pores in the skin. They must have access to moisture or they will dehydrate and die. Although they depend on water, they cannot drink so they must get much of the water they need from the food they eat.