Fish have scales as a protective coating for the skin. In fact, not all fish have them. But we usually think of a fish as a cold-blooded, aquatic animal that swims by means of fins, breathes by means of gills, and is covered with scales. Scales may be of four different kinds-placoid, ganoid, cycloid and ctenoid.
Placoid scales are long, spiny and tooth like, and are made of enamel and dentine. These are found on fishes which have a back-bone made of gristle, such as sharks and rays.
Ganoid scales are rather like placoid scales but are mainly bony and covered with a kind of enamel called ganoin. These thick scales are found especially in garfish.
Cycloid scales are thin, large round or oval scales arranged in an overlapping pattern. They are found in carps and similar fishes.
Ctenoid scales are similar to the cycloid ones, but have spines or comb like teeth along their free edges. These are found in the higher bony fishes, such as perches and sunfishes.
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