Fishes are aquatic animals with a streamlined shape and a functional tail that allow them to move rapidly through water. Fishes exchange gases with their environment through gills, although a few species have lungs that supplement the gills for gas exchange.

One class of fishes called Agnatha is made up of species that have no jaws. Lampreys and hagfishes are species within this class. These fishes feed by sucking blood and other body fluids from their prey, usually other fishes.

Another class of fishes is Placodermi. The placoderms are now extinct and are recognized as the earliest known fishes with jaws. Placoderms were freshwater bottom-dwellers with a partially bony endoskeleton and bony, armored plates. Another class of fishes is Chondrichthyes. These fishes have endoskeletons composed entirely of cartilage. This class includes the sharks, skates, and rays. Gas exchanges are made at several prominent, vertical gill openings on either side of the throat.

Bony fishes belong to the class Osteichthyes. Most of the familiar fishes living today are bony fishes. They live in the oceans (for example, tuna, mackerel, and herring) and in freshwater (for example, striped bass, trout, and goldfish). Bony fishes prospered during the Devonian Period, which is also called the Age of Fishes. Bony fishes have gills, as well as fleshy pectoral and pelvic fins. The ray-finned fishes are the predominant type of bony fishes living today. These fishes have a swim bladder, a gas-filled sac near the gut that permits a fish to change its buoyancy.