Integument

integument, 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) cells—often with additional secreted coatings—constitutes the integument. Among the vertebrates the boundary covering—with a variety of derived elements such as scales, feathers, and hair—has 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 feathers—skin derivatives—to 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.