Due to their habitation of both terrestrial and freshwater locales at various stages in their life cycle, amphibians have a unique kind of permeable skin. Both oxygen and water are allowed to pass in and out of the delicate integument in order that homeostasis and a sufficient level of respiration may be maintained at all times, even when the animal is underwater. The skin of amphibians is also unusual because it must be kept in a relatively moist to help draw oxygen through the skin. In order to sustain the necessary moisture level, amphibians secrete mucus via glands contained in the skin. This mucous chiefly serves as a protective layer around the body when the animal is on land, but also facilitates a proper salt and water balance within the internal organs when the amphibian is submerged in water.