Fish can be divided into two groups according to their skeleton. Some fish have a skeleton made of gristle or cartilage. Other fish have a skeleton made of bone. The fish which have cartilage skeletons are the sharks and rays. All the other fish such as salmon, herring and eel have bony skeletons.

Fish have special organs that let them breathe the oxygen in the water. These organs are called gills. They are located just behind the head. In bony fish they are covered with a body plate and may be seen through a flap that the plate makes with the rest of the body. The fish breathes by gulping in water and pushing it over its gills. The flap opens to let the water out so more water can be taken in at the mouth. By opening and closing its mouth and gill flap in a regular sequence the fish can keep a constant flow of water over its gills. Shark's gills are not covered by a bony plate. There is a slit in the side of the shark's body next to every gill so the water can flow out after oxygen has been removed from it.