 Amphibians are ectothermic ( cold @-@ blooded ) vertebrates that do not maintain their body temperature through internal physiological processes . Their metabolic rate is low and as a result , their food and energy requirements are limited . In the adult state , they have tear ducts and movable eyelids , and most species have ears that can detect airborne or ground vibrations . They have muscular tongues , which in many species can be protruded . Modern amphibians have fully ossified vertebrae with articular processes . Their ribs are usually short and may be fused to the vertebrae . Their skulls are mostly broad and short , and are often incompletely ossified . Their skin contains little keratin and lacks scales , apart from a few fish @-@ like scales in certain caecilians . The skin contains many mucous glands and in some species , poison glands . The hearts of amphibians have three chambers , two atria and one ventricle . They have a urinary bladder and nitrogenous waste products are excreted primarily as urea . Most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis to become terrestrial adults . Amphibians breathe by means of a pump action in which air is first drawn into the <unk> region through the nostrils . These are then closed and the air is forced into the lungs by contraction of the throat . They supplement this with gas exchange through the skin . 
