The integumentary system is made up of three layers: the outermost epidermis, the dermis, and the hypodermis.
The epidermis is typically ten to thirty cells thick, its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.
Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair. It is a definitive characteristic of the class. Some mammals have very little, but nonetheless, careful examination reveals the characteristic, often in obscure parts of their bodies. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth and the polar bear may seem to have green fur, but this color is caused by algae growths.
The skin is a soft outer covering of an animal, in particular a vertebrate. Other animal coverings such the arthropod exoskeleton or the seashell have different developmental origin, structure and chemical composition. The adjective cutaneous literally 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 B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.
Hair with sufficient density is called fur. The fur mainly serves to augment the insulation the skin provides, but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. A frog sitting in an anesthetic solution could quickly go to sleep.