Scale (zoology)
In biology, a scale is a small rigid plate that grows out of an animal's skin to provide protection. Scales are quite common and have evolved multiple times with varying structure and function.
Fish scales are bony and covered with a smooth transparent tegument to improve the flow of water over them. Reptile scales are more like fingernail. Birds also have scales, commonly on their feet, and their feathers are thought to have been derived from modified scales. A few mammals also have scales, such as the pangolin, and these are originally derived from hair.
Sharks do not have scales, instead being covered with small denticles which are similar in structure to teeth. Some other fish are also scaleless or have incomplete scale coverage.
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Types of scales
There are various types of scales according to shape and class. The scales of all bony fishes are laid head to tail. This reduces drag. Fish scale types include:
Cosmoid
True cosmoid scales can only be found on the extinct Crossopterygians. The inner layer of the scale is made of compact bone. On top of this lies a spongy layer and then a layer of dentinelike material called cosmine. The upper surface is enamel. The coelacanth has modified cosmoid scales, that are thinner than true cosmoid scales.
Ganoid
Ganoid scales can be found on bichirs and reedfishes (family Polypteridae) and gars (family Lepisosteidae). Ganoid scales are similar to cosmoid scales, but a layer of ganoin lies over the cosmine layer and under the enamel. Ganoid scales are diamond-shaped, shiny, and hard.
Placoid
Placoid found on cartilaginous fish
Ctenoid and cycloid
As they grow, cycloid and ctenoid scales add concentric layers.
Ctenoid which have a toothed outer edge, and are usually found on fishes with spiny fin rays. Cycloid which have a smooth outer edge, and are most common on fish with soft fin rays.
Others
Reptile scale types include: Cycloid, Granular which appear bumpy, and Keeled which have a center ridge.