Not just the forts but amphibians in general have a very thin layer of skin which is also moist. This is for the very important reason that the frog has poorly developed lungs. The skin is so thin that both gases and liquids can move in and out of the frog without taking up much energy. The reason that the skin is moist is because it is very easy for the frogs to lose moisture to the air through the thin skin and become dehydrated easily. That is why amphibians in general like to stay in habitats where water is nearby and they can keep their skin wet.
