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.

Body

Nearly all fish 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.

Fins

The fins are the most distinctive features of a fish.

• The dorsal fin is located on the back.
• The caudal fin corresponds to the fish tail.
• The anal fin is located on the bottom, behind the anus.
• The pectoral fins are located on each side, usually at a middle height on the body just behind the head.
• The pelvic fins are on the belly.
• Some types of fish have a small fleshy adipose fin on the back just forward of the caudal fin.

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

Scales

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)

• Found in: only sharks
  

2. Ganoid: flat, basal-looking scales that cover a fish body without much overlapping.

• Found in: gar
  

3. Cycloid: Small oval-shaped scales with growth rings.

• Found in: bowfin, remora
  

4.Ctenoid: Similar to the Cycloid scales, with growth rings. Distinguishable by the spines that cover one edge.

• Found in: halibut

(internal anatomy next)