Amphibian populations are undergoing dramatic declines, which has been attributed to ultraviolet radiation, environmental toxins, etc. Parasites have been identified as the cause of deformities in frogs. Most species occur in North America. The common North America salamanders are less than 15 cm long.
Some aquatic species are rather large.
Respiration all salamanders hatch with external gills, but during development, they are lost in all except aquatic forms or in some species which do not undergo complete metamorphosis. The skin contains an extensive vascular network of capillaries which allow for gas exchange to take place just below the epidermis. Cutaneous respiration is facilitated by pumping air in and out of the mouth where further respiratory gas exchange can take place. They can exist for short periods out of the water axolotls are perennibranchiate forms of Ambystoma tigrinum that can be transformed using thyroxin. Larvae have external gills and a fin-like tail Terrestrial species lay masses of eggs under logs or in holes of soft dirt.