Skin is a layer of usually soft, flexible exterior tissue that covers a vertebrate animal's body and has three primary functions: protection, regulation, and sensation. The exoskeleton of arthropods, for example, has a different developmental origin, structure, and chemical constitution than other animal coverings. The adjective cutaneous (from Latin cutis ‘skin') implies "of the skin." The origin, structure, and function of extra skeletal apparatus are all influenced by the skin (including cutaneous and subcutaneous tissues). The term "cutaneous" refers to the outer layer of the skin. Subcutaneous refers to the area beneath or beneath all of the skin's layers. A subcutaneous cyst, for example, is located beneath the skin. Any medical problem that affects the integumentary system—the organ system that encloses the body and includes skin, hair, nails, and associated muscle and glands—is referred to as a skin condition. This system's primary function is to act as a barrier against the outside world.