The body of a typical fish tapers from the middle toward both ends and is slightly compressed, or flattened, from side to side. Eels, however, look like snakes. Sunfish are considerably compressed from side to side, while flounders are greatly compressed from top to bottom. There are many other variations from the shape of the typical fish.

Most fish are covered with horny scales that overlap like shingles and help to protect the body from injury. The placoid scales of sharks and related fish do not overlap. They are embedded in the skin, and each bears a tiny toothlike projection, or denticle. Bowfins and others of their group have hard, bony ganoid scales that meet edge to edge. Some catfishes and some other species have no scales.

As a fish ages, its scales grow in size but not in number. The scales of many fish bear annual rings similar to those of a tree trunk. From these rings the age of the fish can be determined. A thin layer of skin covers the scales. This skin secretes mucus, a slimy substance that helps protect the fish from bacterial and fungal infections.

Most fish have both paired and unpaired fins. The paired fins are the pectoral fins, which grow on each side just behind the head; and the pelvic fins, which grow on each side farther back on the body. The paired fins are usually used in steering and braking, but in some fish, such as the seahorses, they are important swimming organs.

The unpaired fins consist of one or more dorsal fins, which grow along the back; an anal fin, at the rear of the anus; and a caudal fin, which grows at the end of the tail. Dorsal and anal fins serve as keels, to keep the fish upright. The caudal fin is used as an aid in swimming.

Some fish, such as eels, swim by making a series of muscular contractions which alternate on each side of the body. Most other fish swim by lashing their tails from side to side. When the tail is whipped in one direction, the front part of the body tends to turn the opposite way. Water pressure resists the turning movement and changes it into a forward motion.

Fish breathe through four or more pairs of gills, organs that are situated on each side of the body just behind the head.

Almost all fish focus their eyes by using their eye muscles to change the position of the lens rather than by changing the shape of the lens or cornea, as do the higher vertebrates. However, some sharks and rays are capable of slight changes in the shape of the lens.

Most fish are nearsighted. Sharks are farsighted. Some fish have eyes mounted on tubes that can be moved to and fro. Some fish can see colors.

The eyes of the four-eyed fish, which habitually swims on the surface with its eyes half submerged, have divided pupils. The upper half of each eye is adapted to seeing in air; the lower half can see in water. Archerfish and some other species can see well in air. Many fish that live in caves and in the depths of the ocean have no eyes.

The mouth is usually large in flesh-eating fish and small in plant eaters. In skates and most sharks and rays, the mouth is located under the body. In suckers and sturgeons the mouth is also under the body, and can be protruded to pick up substances from the bottom.

Various fish species have teeth adapted to piercing, cutting, crushing, and chopping. Some fish have no mouth teeth. Several, however, have grinding teeth at the top of the gullet, the tube through which food passes to the stomach.

Fish have inner ears but lack external and middle ears. They can hear sounds made in water, but they hear only faintly sounds made in air. In some fish, the gas bladder serves as a resonator that aids in hearing.

A fish's sense of hearing is supplemented by an organ called the lateral line, which senses vibrations in the water. The position of this organ is indicated in many species by a line extending along each side of the body. The lateral line also branches over the head, though the branches are usually not visible.

The lateral line is a canal just under the surface of the skin. It is connected to the surface by numerous pores, each of which is located over a group of cells sensitive to pressure changes. The cells are connected to a nerve, deeper in the body, that leads to the acoustico-lateralis area of the brain.

Within the belly of many fish is a gas bladder (also called an air, swim, or sound bladder) containing gases absorbed from the blood. In most fish the gas bladder absorbs or gives off gases to enable the fish to float at any depth of water without effort. In some fish, such as the lungfish, the gas bladder has evolved into a lung.

It is difficult to distinguish between the senses of smell and taste in fish, and they may be identical. Certainly fish have keen smell-taste senses, for a large area of a fish's brain is given over to these senses. Fish have sensory nerves within their nostrils, which in most species consist of a pair of double sacs that open only outward. Other sensory nerves are found within the mouth and around the snout. Still other sensory nerves associated with smell and taste are distributed over the body of a fish. Catfish and some other species have additional taste organs in their barbels (fleshy whiskers that trail from their chins or snouts).