Amphibians are ectothermic, four-limbed vertebrates that belong to the class Amphibia. The phylum Lissamphibia includes every extant amphibian. They inhabit a variety of environments, the majority of which are terrestrial, fossorial, aquatic, or arboreal for freshwater species. Amphibians use their skin as a secondary respiratory surface, and some small terrestrial salamanders and frogs lack lungs and rely only on their skin for breathing. Amphibians use gills for breathing early in life and acquire rudimentary lungs in adulthood. In amphibians, metamorphosis is the process through which a larva develops into a small adult duplicate, typically moving from an aquatic to a terrestrial or semi-terrestrial existence. The beginning of metamorphosis indicates that the larval stage has come to an end. Tadpoles have gills, which eventually vanish as adult frogs grow lungs. However, cutaneous respiration is advantageous during the winter, when frogs commonly hibernate underwater. Frogs can obtain their oxygen without breathing in the surrounding air. To be able to breathe throughout the winter, frogs typically hibernate in water that is high in oxygen. The gills atrophy, the paired lungs form, and the metamorphosing larvae start venturing to the water's surface to breathe air as amphibian larvae grow. With the exception of a few scales that resemble fish on some caecilians, its skin has negligible keratin content and is scale-free. Numerous mucous glands as well as poison glands are found in some species of skin. Amphibians use a pumping mechanism to breathe, first drawing air into their buccopharynx through their nostrils. These are then shut, and by contracting the throat, the air is pumped into the lungs.