The skin surface of teleost fish dififers from that of higher vertebrates in that it is a living vital substrate at the margin between the water and the tissues. Its surface is continually sloughing into the water, to be replaced from the mucoid and formed elements of the epidermis. The surface, or cuticle, is altered by cyclical physiological changes in the animal, and also by pathological conditions. These, and the nature of the external medium, affect the range and the ecology of the biota of the surface. The secretions on the skin surface may also have a protective capacity. Fish epidermis is delicate and easily traumatized and there is great mobility of the Malpighian cells which allows rapid sealing of any breaches.