scale (zoology)

scale in zoology, an outgrowth, either bony or horny, of the skin of an animal. The major component of the scales of fishes is bone, and they are formed directly in the skin membrane as the fish grows. The number of rows of scales, as well as the kind, figures in the identification of a species. The growth of the scales is marked by rings, which aid in determining the age of the fish. The placoid scales of sharks, which have a dentine base with a pulp cavity, are thought to be similar to the forms from which the teeth of the higher vertebrates evolved. Ganoid scales, found in primitive fishes such as the gar pike and the sturgeon, are heavy and platelike. Other fishes have either rough scales (ctenoid) with comblike edges or smooth scales (cycloid). The horny scales, or scutes, of most reptiles develop embryologically as outpushings of the epidermis. In some lizards the scales are modified to form tubercles or granules. Other lizards and snakes have overlapping scales, highly developed in the snakes as aids to locomotion. The crocodile has both horny and bony scales. Among turtles and their relatives scales are usually found on the head, neck, limbs, and tail; in most of the group horny scales also form a pattern of flat plates overlying the bony dermal skeleton of the back and belly. Birds have horny scales on the feet and sometimes on the legs. Some mammals, e.g., the mouse and the rat, have scales on the tail; the pangolin and the armadillo have a body covering of large horny scales.

scales The small bony or horny plates forming the body covering of fish and reptiles. The wings of some insects, notably the Lepidoptera (butterflies and moths), are covered with tiny scales that are modified cuticular hairs.

In fish there are three types of scales. Placoid scales (denticles), characteristic of cartilaginous fish, are small and toothlike, with a projecting spine and a flattened base embedded in the skin. They are made of dentine, have a pulp cavity, and the spine is covered with a layer of enamel. Teeth are probably modified placoid scales. Cosmoid scales, characteristic of lungfish and coelacanths, have an outer layer of hard cosmin (similar to dentine) covered by modified enamel (ganoine) and inner layers of bone. The scale grows by adding to the inner layer only. In modern lungfish the scales are reduced to large bony plates. Ganoid scales are characteristic of primitive ray-finned fishes, such as sturgeons. They are similar to cosmoid scales but have a much thicker layer of ganoine and grow by the addition of material all round. The scales of modern teleost fish are reduced to thin bony plates.

In reptiles there are two types of scales: horny epidermal corneoscutes sometimes fused with underlying bony dermal osteoscutes.

scale In biology, small hard plate that forms part of the external skin of an animal. It is usually a development of the skin layers. In most fish, scales consist of bone in the dermal skin layer. The scales of reptiles and those on the legs of birds are horny growths of the epidermal skin layer and consist mostly of the fibrous protein keratin.