 Amphibians are ectothermic , tetrapod vertebrates of the class Amphibia . Modern amphibians are all Lissamphibia . They inhabit a wide variety of habitats , with most species living within terrestrial , fossorial , arboreal or freshwater aquatic ecosystems . Amphibians typically start out as larvae living in water , but some species have developed behavioural adaptations to bypass this . The young generally undergo metamorphosis from larva with gills to an adult air @-@ breathing form with lungs . Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin . They are superficially similar to reptiles but , along with mammals and birds , reptiles are amniotes and do not require water bodies in which to breed . With their complex reproductive needs and permeable skins , amphibians are often ecological indicators ; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe . 
