Skin is the soft outer covering of vertebrates. Other animal coverings such as the arthropod exoskeleton or the seashell have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis, skin). In mammals, the skin is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds. All mammals have some hair on their skin, even marine mammals which appear to be hairless. Because it interfaces with the environment, skin plays a key role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the protection of vitamin D folates. Severely damaged skin may heal by forming scar tissue. This is often discoloured and depigmented.