Fish are water animals that evolved about 500 million years ago. They were the first animals to have an internal skeleton. Most fish have scale-covered bodies with fins and a tail for swimming. They breathe using gills to absorb oxygen from the water, although a few, such as the lungfish, can survive in air. The four classes of fish – jawless fish, sharks, lungfish, and bony fish – have common characteristics, but are only distantly related.
Bony fish are good swimmers. Muscles, called myotomes, contract in sequence as the fish moves. The tail fin provides thrust, while other fins help the fish to change position or direction. The lateral line, nerve endings along the side of the fish, detects movement in the water. The swim bladder contains the right amount of air, so that the fish neither floats nor sinks.
The manta ray looks strange, but is a harmless filter feeder. Large flaps on each side of its head channel water into its mouth, and the gills filter out animal plankton and small fish for the ray to swallow. It moves through the water by beating its powerful, triangular pectoral fins. It can accelerate suddenly and leap out of the water, if threatened.
The African lungfish does not have gills. It lives in stagnant water and breathes air with its lungs. The African lungfish survives droughts by burrowing into mud and making a cocoon where it lies dormant until the rains fill the pool again.
The sea lamprey has a circular sucker instead of a mouth. Around the edge of the sucker are rows of small teeth, while larger teeth surround the opening. The lamprey sucks blood with its rough tongue. Lampreys do not have jaws or scales, and their internal skeleton is a stiffened rod, or notochord.
The term fish is an informal grouping of chordate animals (with backbones), living in water. They are ectothermic (cold-blooded) and move using fins. There are about 25,000 species.
(cartilaginous fish, including sharks, skates, rays)
Features: skeleton of cartilage, tooth-like scales
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