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This is a very interesting question. The general answer comes from understanding how amphibians get oxygen. In general, when amphibians evolved onto land, they became air-breathing organisms. Most evolved lungs for this purpose, but the lungs and pumping mechanisms are primitive compared to those of reptiles, birds and mammals. Not unexpectedly, many amphibians supplement their oxygen needs by absorbing oxygen through the relatively thin skin of their mouth, head and body. For many species of terrestrial salamanders like the locally common red-backed salamander, transport through the skin is the only means of getting oxygen. Adults of this and related species don't have lungs or gills. Since oxygen must first be dissolved in water before it can be taken into the body of an organism, the skin must be moist before the animal can respire. And since quite a few amphibian species get some of their oxygen through their skin despite having lungs or gills, most amphibians have moist skin. 

Among the amphibians that are totally aquatic, like our local hellbender salamander and the mudpuppy, the skin is moist and slimy, and some gas exchange occurs through their skin as well. The slime of the hellbender skin may also help to keep too much water from entering the hellbender's body from the streams and rivers, which is a common problem for freshwater organisms. 

From the above generality, one might expect drier skin in amphibians that depend almost entirely on lungs for getting oxygen. Toads usually have drier, warty, almost reptile-like skin, and certainly one's sensation is that the skin of a common toad is not very moist. It turns out that gas exchange across the skin in toads is minimal among amphibians.

Remaining in our brief inventory of amphibians are those common frogs and salamanders with moist skin and lungs that spend most of their lives out of water. Most of these

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