In what direction was the evolution of body integuments in animals?

We can observe changes in the integument of the body on the example of vertebrates. The fish had scales. She disappeared in the course of evolution. Amphibians, which descended from fish, received mucus-covered skin. Its task is not only to protect the body, but also in gas exchange. Scales appeared in some reptiles, and the possibility of gas exchange was lost. In mammals, the skin performs a protective function, carries out heat exchange, the release of various toxins, and also contains various receptors. Therefore, the skin of mammals is more developed than that of other vertebrates.