Short answers, pulled from the story.
Skin has three main functions: protection, regulation, and sensation. It interfaces with the environment as the first line of defense against external factors, protecting the body against pathogens and excessive water loss while aiding insulation, temperature regulation, and the production of vitamin D folates.
Mammalian skin is composed of two primary layers: the epidermis and the dermis. The epidermis provides waterproofing and acts as a barrier to infection, while the dermis is a layer of connective tissue that holds the skin's appendages and cushions the body from stress and strain.
Human skin is thinnest under the eyes and around the eyelids at 0.5 mm, making it one of the first areas to show aging such as crows feet and wrinkles. The thickest skin is on the palms and the soles of the feet at 4 mm thick.
On the 11th of January 2024, biologists reported the discovery of the oldest known skin, fossilized about 289 million years ago. It possibly belonged to an ancient reptile.
Amphibian skin is not a strong barrier, especially regarding the passage of chemicals, and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated quickly as the chemical diffuses through its skin.
Granular glands are venomous and produce toxins that can be fatal to most vertebrates, and they are larger but fewer in number. Mucous glands are non-venomous, cover the entire body surface, and keep it lubricated while controlling pH, aiding thermoregulation, and fighting bacteria and viruses.
The word skin is a borrowing from Old Norse skinn, meaning animal hide or fur, ultimately from the Proto-Indo-European root sek-, meaning to cut. It originally referred only to dressed and tanned animal hide, while the usual word for human skin was hide.