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Fish anatomy
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Fish anatomy

Fish anatomy is primarily governed by the physical characteristics of water, which is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs light more than does air.

Table of contents
1 Body
2 Fins
3 Scales


Nearly all fishes have a streamlined body, which is divided into head, trunk, and tail, although the dividing points are not always externally visible.

The head includes the snout, from the eye to the forwardmost point of the upper jaw, the operculum or gill cover, and the cheek, which extends from eye to preopercle. The lower jaw defines a chin. The head may have several fleshy structures known as barbels, which may be very long and resemble whiskers. Many fish species also have a variety of protrusions or spines on the head. The nostrils or nares of almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth. The outer body of the fish is covered with scales.


The fins are the most distinctive features of a fish.

For every fin, there are a number of fish species in which this particular fin has disappeared during evolution.


There are four types of fish scales.
1. Placoid: these scales are "composed of body dentin covered by a layer of enamel." (Gilbertson, 7.4)

2. Ganoid: flat, basal-looking scales that cover a fish body without much overlapping. 3. Cycloid: Small oval-shaped scales with growth rings. 4.Ctenoid: Similar to the Cycloid scales, with growth rings. Distinguishable by the spines that cover one edge. (internal anatomy next)