INTEGUMENT

in biology, network of features that forms the covering of an organism. The main purpose of an integument is to protect the organism from the outside world and against drying out of internal parts. The integuments of unicellular animals consist of an external cell wall that maintains the shape of the cell and affords protection. Multicellular invertebrates have an outer layer of epithelial cells, and these may secrete a variety of surface coatings, ranging from the mucous coat of cnidarians to the tough cuticle of insects. In arthropods (insects, crustaceans, and spiders), rigid, noncellular materials deposited by the epithelial layer provide the animal with a protective armour or form an exoskeleton to which muscles are attached. The integument of all vertebrate animals is made up of two main layers: a relatively thin outer layer, the epidermis; and a thicker, tougher inner layer, the dermis. The closely packed cells of the epidermis provide a barrier against desiccation, microbes, radiation, and chemicals. The underlying dermis is composed of connective tissue in which are nerve endings, muscle cells, and blood and lymph vessels. It provides nourishment for the epidermis and is most highly developed in mammals. Vertebrates have various skin types. The smooth, flexible skin of fishes facilitates swimming and is covered with dermal scales. Amphibians have a thick, horny layer of surface epidermis whose thickness depends on the amount of time that the animal typically spends out of water. Reptiles have a similarly thick horny epidermis, which in their case is protected from desiccation by waxes and is covered with scales formed by overlapping folds of skin. Birds have a thin epidermis covered with feathers except at the legs and feet, where the skin is scaly. Mammals are the only vertebrates with hair or fur, which insulates them against cold. See also skin. in biology, network of features that forms the covering of an organism. The integument delimits the body of the organism, separating it from the environment and protecting it from foreign matter. At the same time it gives communication with the outside, enabling an organism to live in a particular environment. Among unicellular organisms, such as bacteria and protozoans, the integument corresponds to the cell membrane and any secreted coating that the organism produces. In most invertebrate animals a layer (or layers) of surface (epithelial) cellsoften with additional secreted coatingsconstitutes the integument. Among the vertebrates the boundary coveringwith a variety of derived elements such as scales, feathers, and hairhas assumed the complexity of an organ system, the integumentary system. The integument is composed of layers that may be of single cell thickness, as in many invertebrates, or multiple cell thickness, as in some invertebrates and all vertebrates. In every case the cells that give rise to the integuments belong to that class of tissue called epithelium, which in most animals is called epidermis. Underlying the epidermis and supplying it with nourishment is the dermis. In addition to the cellular layers, the integument often includes a noncellular coating, or cuticle, that is secreted by the epidermis. Such coatings are found in most invertebrates. The vertebrate skin has generated many kinds of glands and a variety of horny structures, but it lacks coatings. The wide diversity of integuments among vertebrates further exemplifies the adaptive character of the body covering: from the almost impenetrable shield of an armadillo and the dense furry coat of an Arctic bear to the slimy, scaled covering of a cod and the exceptionally smooth skin of a porpoise. Amphibians and fishes often have mucous glands that lubricate their skins and prevent waterlogging and deterioration. Reptiles have thick, leathery skins that help reduce water loss and serve as an armour against enemies. Birds use their feathersskin derivativesto fly and to insulate their bodies. The hairy or furry coats of many terrestrial mammals insulate them, shed water, and provide a dense guard against injury. Additional reading J. Bereiter-Hahn, A.G. Matoltsy, and K. Sylvia Richards (eds.), Biology of the Integument, 2 vol. (198486), is a comprehensive account of all aspects of the integument of animals and a valuable reference source. Information on integumentary systems of different animals can be found in the appropriate sections of such comprehensive texts as Robert D. Barnes, Invertebrate Zoology, 5th ed. (1987); William E. Duellman and Linda Trueb,Biology of Amphibians (1986); and Joel Carl Welty, The Life of Birds, 3rd ed. (1982). William Montagna, Cutaneous Comparative Biology, Archives of Dermatology, 104(6):577591 (December 1971), is an advanced but readable article on the biologic properties and adaptive structure and function of mammalian skin. See also R.I.C. Spearman and P.A. Riley (eds.), The Skin of Vertebrates (1980). The vertebrate integumentary system Skin layers In all vertebrates the skin has two major layers. The outer, relatively thin epidermis is composed of closely packed cells with little intercellular material; it provides the barrier against attack by chemicals, radiation, or microbes. The underlying dermis (cutis, corium) is thicker and tougher, and its bulk is formed by extracellular materials manufactured by scattered cells. One of its major functions is physical protection. The sensory functions of skin are shared by both epidermis and dermis. The various structures or appendages such as scales, feathers, claws, glands, and hair follicles are derived from either layer or from both. There are considerable differences between the skins of different vertebrate classes, and they are closely related to the environments of the various groups. The epidermis is the product of the deepest layer of its cells, those that lie immediately over the dermis. From this generative layer, known as the stratum germinativum, cells move outward and become progressively flattened. The surface cells of terrestrial vertebrates, mere remnants of once living cells, are scaly and compressed; they constitute the horny layer, or stratum corneum. The cell fragments of the stratum corneum are composed largely of keratin, a tough insoluble protein. In most land vertebrates the stratum corneum is shed or molted, either periodically and in large fragments or sheets, as in reptiles, or continuously in small patches or scales, as in mammals. The dermis, which is best developed in mammals, consists largely of fibrous connective tissue (composed of collagen fibres), blood and lymph vessels, smooth muscle cells, and nerve endings. It gives rise to so-called membrane bonesthe bony scales of fishes, the bony plates in certain reptiles and mammals, and the membrane bones of the vertebrate skull. Through its blood network the dermis supplies nourishment to the overlying epidermis. Skin derivatives and appendages Among the notable changes that have taken place during the course of evolution is the development in vertebrates of a variety of glands, pigmentary structures, scales, claws, nails, horns, feathers, and hairs as adaptations to their changing environments.