_Hair._—The external surface of the greater number of members of the class is thickly clothed with a peculiarly modified form of epidermis, commonly called hair. This consists of hard, elongated, slender, cylindrical or tapering, filiform, unbranched masses of epidermic material, growing from a short papilla sunk at the bottom of a follicle in the derm or true skin. Such hairs upon different parts of the same animal, or upon different animals, assume various forms, and are of various sizes and degrees of rigidity,—as seen in the delicate soft velvety fur of the Mole, the stiff bristles of the Pig, and the spines of the Hedgehog and Porcupine, all modifications of the same structures. Each hair is composed usually of a cellular pithy internal portion, containing much air, and a denser or more horny cortical part. In some animals, as Deer, the substance of the hair is almost entirely composed of the medullary or cellular substance, and it is consequently very easily broken; in others the horny part prevails almost exclusively, as in the bristles of the Wild Boar. In the Three-toed Sloth (_Bradypus_) the hairs have a central horny axis and a pithy exterior. Though generally nearly smooth, or but slightly scaly, the surface of some hairs is strongly imbricated, notably so in some Bats; while in the Two-toed Sloth (_Cholœpus_) the hairs are longitudinally grooved or fluted. Though usually more or less cylindrical or circular in section, hairs are often elliptical or flattened, as in the curly-haired races of men, the terminal portion of the hair of Moles and Shrews, and conspicuously in the spines of the Rodents _Xerus_ and _Platacanthomys_. Hair having a property of mutual cohesion or “felting,” which depends upon a roughened scaly surface and a tendency to curl, as in domestic Sheep (in which animal this property has been especially cultivated by selective breeding), is called “wool.”