short notes on scales of fishes and reptiles
Answers
Answer: Scales vary in size and shape in different species. The body of all fishes except members of family Siluridae and a few bottom dwellers is covered by scales. Scales are usually found covering entire body surface, but in some fishes such as Chimaeras, Polyodon and Acipenser the scales are present in localised regions or areas of the body.
Primitive fossil fishes of Silurian and Devonian periods possess exoskeleton in the form of plates and scales which consist of three distinct layers. The innermost layer consisted of a compact bone, isopedine, the intermediate or middle layer of spongy vascular bone and the outer layer is of dentine.
According to the mode of the their origin, there are two types of scales:
(i) Those which are formed due to the secretory activity of both epidermis and dermis, as the placoid scales of elasmobranchs and
ii) Non-placoid scales that are derived from the dermis only as the scales of teleosts.
Structurally, the scales are classified as cosmoid, ganoid or rhomboid, placoid, cycloid and ctenoid, the last two are also called the bony ridge scales
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