Amphibians are the oldest land-dwelling vertebrates and first appeared about 400 million years ago during the Devonian period. The class Amphibia includes frogs, toads, newts, salamanders, efts and caecilians. The name "amphibian" describes the creatures' ability to thrive in water and on land. Although many species are land dwellers, amphibians require a moist environment to reproduce; their eggs have no shells and dry out in more arid environments.
All animals must have oxygen to live. They need a means to gather oxygen from the surrounding water or air and release respiratory byproducts, including carbon dioxide. Mammals and reptiles do this with lungs, but amphibians rely on a combination of gas exchange via lungs, gills and skin. Not all amphibians have all three means of gas exchange throughout their life cycles. Species that rarely leave the water may keep their gills, while arboreal species typically lose theirs as they mature.
Most amphibians start life with gills -- heavily vascularised structures that facilitate gas exchange. Frogs and toads hatch in water as tadpoles -- limbless tailed creatures that need an aqueous environment to live. Their primary gas exchange is via primitive internal gill structures. As they sprout limbs, lose their tails and grow into their adult forms, their gills seal as their lungs develop. Aquatic amphibians like the axolotl and certain species of salamander retain their gills into adulthood. Some species have dramatic feathery "headdresses" that are actually external gills for taking in oxygen from the surrounding water.
Amphibians can also respire through their soft, thin skin. While this ability helps the animals thrive in water and on land, it also puts them at risk of suffocating if their moist environment becomes too dry. Desiccated tissues don't permit oxygen and carbon dioxide transfer. Amphibians' moist and permeable skins also leave them vulnerable to toxins in their environment. Researchers study amphibian populations as bioindicators of environmental disruption because these creatures are so susceptible to change.
Adult amphibians typically rely on lungs for gas exchange, although they never lose their skin permeability. Unlike mammals who use a diaphragm to pull air into the lungs via negative pressure, amphibians force air into their lungs from their mouths. Amphibian lungs contain fewer alveoli, or air sacs, than do the lungs of creatures that evolved later such as reptiles and mammals.