- Scale (zoology)
In most biological nomenclature, a scale (Greek "lepid", Latin "squama") is a small rigid plate that grows out of an
animal's skinto provide protection. In lepidopteran ( butterflyand moth) species, scales are plates on the surface of the insectwing, and provide coloration. Scales are quite common and have evolved multiple times with varying structure and function.
Scales are generally classified as part of an organism's
integumentary system. There are various types of scales according to shape and to class of animal. Although the meat and organs of some species of fish are edible by humans, the scales are usually not eaten.
Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically.
True cosmoid scales can only be found on the extinct
Crossopterygians. The inner layer of the scale is made of lamellarbone. On top of this lies a layer of spongy or vascular bone and then a layer of dentinelike material called cosmine. The upper surface is keratin. The coelacanthhas modified cosmoid scales that lack cosmineand are thinner than true cosmoid scales.
Ganoid scales can be found on
gars (family Lepisosteidae) and bichirs and reedfishes (family Polypteridae). Ganoid scales are similar to cosmoid scales, but a layer of ganoinlies over the cosmine layer and under the enamel. They are diamond-shaped, shiny, and hard.
Placoid scales are found on cartilaginous fish including
sharks. These scales, also called denticles, are similar in structure to teeth.
Leptoid scales are found on higher order bony fish and come in two forms, ctenoid and cycloid scales.
As they grow, cycloid and ctenoid scales add concentric layers. The scales of bony fishes are laid so as to overlap in a head-to-tail direction, a little like roof tiles, allowing a smoother flow of water over the body and therefore reducing drag.
Cycloid scales have a smooth outer edge, and are most common on more primitive fish with soft fin rays, such as
Ctenoid scales have a toothed outer edge, and are usually found on more derived fishes with spiny fin rays, such as bass and
Reptilescale types include: cycloid, granular (which appear bumpy), and keeled (which have a center ridge).
The scales of all reptiles have an epidermal component (what one sees on the surface), but many lizards have
osteodermsunderlying the epidermal scale, as do crocodilians and turtles. Snakes, tuatarasand many lizards lack osteoderms. All reptilian scales have a dermal papilla underlying the epidermal part, and it is there that the osteoderms, if present, would be formed.
Butterfliesand moths - the order Lepidoptera(Greek "scale-winged") - have membranous wings covered in delicate, powdery scales, which are modified setae. Each scale consists of a series of tiny stacked platelets of organic material, and butterflies tend to have the scales broad and flattened, while moths tend to have the scales narrower and more hair-like. Scales are frequently pigmented, but some types of scales are metallic, or iridescent, without pigments; because the thickness of the platelets is on the same order as the wavelengthof visible lightthe plates lead to structural coloration and iridescencethrough the physical phenomenon described as thin-film optics. The most common color produced in this fashion is blue, such as in the " Morpho" butterflies. Other colors can be seen on the Sunset moth.
* cite book | last =Kardong | first =Kenneth V. | authorlink =Kenneth Kardong | title =Vertebrates: Comparative Anatomy, Function, Evolution | edition = second edition | publisher =
McGraw-Hill| date =1998 | location =USA | pages =747 pp. | url =http://www.amazon.com/Vertebrates-Comparative-Anatomy-Function-Evolution/dp/0072909560 | doi = | id =
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