Almost three-forths of the world's surface is covered in water. This water is home to over 20,000 different species of fish. The earliest fossils of fish date back over 400 million years.
There are a wide variety of fish — from the goby which is less than one half an inch long, to the whale shark which can be over 60 feet long.
Most fish breathe through gills. Gills perform the gas exchange between the water and the fish's blood. They allow the fish to breathe oxygen in the water.
Fishes are vertebrates that have a skeleton made of either bone or cartilage. About 95% of fishes have skeletons made of bone. These bony fishes have a swim bladder, a gas-filled sac, that they can inflate or deflate allowing them to float in the water even when not swimming. Fishes with a cartilage skeleton tend to be heavier than water and sink. They must swim to keep afloat. Cartilaginous (cartilage) fish include the ray and the shark.
Most fish swim using a tail fin. Muscles in the tail fin move it from side to side, forcing water backward, and propeling the fish forward. Other fins help the fish change direction and stop. Pectoral fins on their side help them swim up and down. Dorsal and anal fins on the top and bottom keep the fish upright. Pelvic fins on the underside help steer left and right.
Many fish eat plants, while others such as the shark, eat other fish.
From: http://www.kidport.com/RefLIB/Science/Animals/Fish.htm
Fish
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