The skin of animals serves as a barrier between the exterior environment and the internal environment of the animal's body. It is, however, a barrier that is in many cases permeable and vulnerable to damage and so scales have evolved in most vertebrate groups as a protective element for the skin. Scales have the advantage of being considerably lighter than armor plating of the body with dermal bone and confer much more flexibility upon the body. Scales have evolved a number of different times in vertebrates, and variations in their form and structure reflect these separate developments of scale armor.
Principal Terms ctenoid scales: scales with comblike serrations on rear edge, found on many bony fishes dentine: the ivory portion of a tooth or scale; dentine or dentinelike substances such as cosmine are found in the scales of most fishes ganoid scales: heavy, dense scales containing ganoine found in primitive bony fishes keratin: a dense and rigid protein that makes up the scales of reptiles, birds and some mammals Placodermi: extinct class of fishes characterized by dense armor plating made of dermal bone placoid denticles: toothlike scales found in sharks and rays
Scales have evolved in most vertebrate groups to provide a layer of protection for the integument, the outer layers of tissue that protect the internal environment of the animal from the external world. Scales are hardened plates that are made either of bonelike substance or the protein collagen, both of which are formed in the dermis (the dense, connective tissue layer of the skin) or from other rigid proteins such as keratin, which are secreted by epithelial cells in the outer layer of the skin. The protection afforded by scales does not come at the expense of flexibility, however, for most scales are attached only at one edge to folds in the skin and thus form overlapping plates which can slide over each other as the animal moves. As such, scales provide a protective armor that is lighter and more flexible than the armor formed by large plates of dermal bone such as those seen in extinct fishes such as the Placodermi.
Sharks and rays possess a distinctive type of scales known as placoid denticles. These scales bear a strong resemblance to teeth; indeed, in sharks the teeth are enlarged and exaggerated placoid denticles that are replaced when lost. Each placoid denticle consists of a flat basal plate that is embedded in the dermis and a prominent, curved midline spine with a hollow interior pulp cavity. Both the basal plate and the spine are composed of dentine, the dense ivory material of teeth. The outer layer of the spine consists of a layer of vitrodentine, which is secreted by special cells (ameloblasts) in the epidermis. Within the central pulp cavity are blood vessels and nerve fibers, and fine canals (canaliculi) extend from the cavity to all parts of the dentine.
Some primitive bony fishes, such as gars and the African bichirs, possess ganoid scales. These heavy and dense scales are roughly diamond-
shaped and consist principally of a dentinelike substance called cosmine. The outer surface of the cosmine layer is coated with many layers of a dense, silvery material called ganoine, which provides a lustrous, metallic sheen.
Modern bony fish, which comprise the vast majority of familiar fish species, have abandoned the heavy ganoid scales in favor of greater mobility and other protective devices. Their thinner and lighter scales contain neither cosmine nor ganoine, but rather consist of an osteoid layer (bonelike, but containing no bone cells) and an inner fibrous layer. There are two principal forms of these scales, cycloid scales, which are smoothly rounded in shape, and ctenoid scales, in which the free rear surface has comblike teeth (ctenii). Both types are fully embedded in the dermis and arranged in an overlapping fashion similar to roof shingles. The scales are covered with a thin layer of epidermis that contains single-celled mucous glands. Like the trunks of trees, these scales also exhibit "growth rings" (lamellae), the shape of which are uniquely characteristic to an individual species.
When amphibians became the first vertebrates to move from an aquatic to a terrestrial habitat, the weight of their scales may have become a liability in their less physically supportive environment. The earliest amphibians possessed dense and bony scales of cosmine, but these have been lost in most amphibian groups. Some of the legless modern amphibians (the caecilians) have very small dermal scales, but modern frogs, toads and salamanders lack them completely.
Scales have reappeared in reptiles; however, these scales lack any of the enamel-like dentine or cosmine components observed in aquatic vertebrates. Reptilian scales are composed primarily of a hard structural protein called keratin, which has two molecular forms, a-keratin and P-keratin. Many modifications of these scales are seen in reptiles. In turtles, the scales rest upon a layer of dermal bones (bones that arise from the layer of dermis). In snakes and lizards, the entire epidermis, including the scales, is shed periodically during growth and a new epidermis, with scales, replaces it.
Scales offer protection for an animal's skin while still allowing a great deal of flexibility. (Corbis)
Birds and mammals evolved independently from separate reptilian lineages, and birds typically retain scales on their feet, lower legs, and the base of the beak. These scales are very similar to those observed in reptiles and are also made of keratin. Most mammals have no scales whatever. However, some mammals, particularly rodents, possess scales of keratin on the epidermis of the tail. More dramatic are the epidermal scales of mammals like the armadillo, which are greatly enlarged and form an effective armor plating for these animals.
—John G. New See also: Anatomy; Beaks and bills; Bone and cartilage; Brain;
Circulatory systems of invertebrates; Circulatory systems of vertebrates; Claws, nails, and hooves; Digestive tract; Ears; Endoskeletons; Exoskel-etons; Eyes; Fins and flippers; Immune system; Kidneys and other excretory structures; Lungs, gills, and tracheas; Muscles in invertebrates; Mus cles in vertebrates; Nervous systems of vertebrates; Noses; Physiology; Reproductive system of female mammals; Reproductive system of male mammals; Respiratory system; Sense organs; Skin; Tails; Teeth, fangs, and tusks; Tentacles; Wings.
Bailey, J., and L. Bailey. Animal Life: Form and Function in the Animal Kingdom. New York: Oxford University Press, 1994. A useful book on animal form and function targeted primarily at the young adult reader.
Moyle, P. B., and J. J. Cech. Fishes: An Introduction to Ichthyology. Upper Saddle River, N.J.: Prentice Hall, 1999. A thorough guide to the biology of fishes, the largest group of scaled vertebrates.
Walker, W. F., Jr., and T. S. Walker. Functional Anatomy of the Vertebrates: An Evolutionary Perspective. 3d ed. Fort Worth, Tex.: Saunders College Publishing, 1997. A collegelevel, readable text on the comparative anatomy of the vertebrate body.
Zug, G. R. Herpetology: An Introductory Biology of Amphibians and Reptiles. San Diego, Calif.: Academic Press, 1997. Perhaps the leading text on the biology of reptiles, generally aimed at college-level students.
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