Fish are aquatic vertebrates that have vertebral column called spine. A classic fish is a torpedo shaped. The fish contains head containing a brain and sensory organs, a trunk with a muscular wall surrounding a cavity with the internal organs and a muscular post-anal tail. The general characteristic is: ectothermic, lateral line, two chambered hearts, gills used for respiration, no external ears, no eye lids.

A fish’s fins are used for balance and to help propel and steer through the water. Most fish have 2 types of fins: single fins that are found along the centerline (top and bottom) of the fish, and paired fins. On the outside of the skin, most fish have scales. These overlap in rows and help protect the fish against injuries and infection. Some species (for example, pufferfish) do not have scales. The edges of scales are jagged and sharp in some fish and smooth and rounded in others. Fish secrete a covering of mucus over the scales to further protect against infection. The mucus traps and immobilizes bacteria and viruses, keeping them from entering the fish’s body. This covering also helps reduce friction, allowing the fish to move easily through the water.

Fish have a unique internal organ known as the swim bladder or air bladder. It is usually found in the abdomen, and it helps fish move up or down in the water. By adjusting the amount of air in the bladder, fish can adjust the depth at which they float without continuously having to swim. In some fish, the swim bladder is also used to produce sounds. Members of the shark and ray class (elasmobranchs) do not have a swim bladder.

To breathe underwater, fish have developed special organs called gills. The gills, found on the side of the fish just behind the head, contain thousands of capillaries, or tiny blood vessels. Water is constantly pumped over the gills, which filter the oxygen out of the water and directly into the fish’s blood. A gill cover, the operculum, is a flexible bony plate that helps protect the sensitive gills. Gills are also important for excretion of waste products, particularly ammonia, from the fish’s bloodstream.