ganoid n : primitive fishes having thick bony scales with a shiny covering [syn: ganoid fish]

In most biological nomenclature, a scale (Greek lepid, Latin squama) is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran (butterfly and moth) species, scales are plates on the surface of the insect wing, 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.

Fish scales

Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically.

Cosmoid scales

True cosmoid scales can only be found on the extinct Crossopterygians. The inner layer of the scale is made of lamellar bone. 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 coelacanth has modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales.

Ganoid 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 ganoin lies over the cosmine layer and under the enamel. They are diamond-shaped, shiny, and hard.

Placoid scales

Placoid scales are found on cartilaginous fish and sharks. These scales, also called denticles, are similar in structure to teeth.

Leptoid scales

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

Cycloid scales have a smooth outer edge, and are most common on more primitive fish with soft fin rays, such as salmon and carp.

Ctenoid scales

Ctenoid scales have a toothed outer edge, and are usually found on more derived fishes with spiny fin rays, such as bass and crappie. |}

Reptilian scales

Reptile scale 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 osteoderms underlying the epidermal scale, as do crocodilians and turtles. Snakes, tuataras and 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.

Arthropod scales

Butterflies and 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 wavelength of visible light the plates lead to structural coloration and iridescence through 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.

Bibliography

• Vertebrates: Comparative Anatomy, Function, Evolution

References