
About five hundred million years ago, fish were the first vertebrates to appear on earth. Since that time, they have evolved into one of the most diverse and successful of animal groups. The "lobe-finned" fish, such as the lungfish, can live for brief periods on land. Sharks and rays have no bones, only cartilage, while the bony fish range from sturgeon to trout to seahorses. Eels, the snakes of the water, can slip in and out of small spaces.
The other animals, who live in water, are the anemones, cephalopods, clams, crustaceans, echinoderms (starfish), and jellyfish. Crustaceans live in freshwater, deep oceans, and tidal pools. Their claws and hard shells serve to protect them from predators. The cephalopods, with their tentacles, are known for their inky defenses. Jellyfish float from North Pole to South Pole, seeking food. Anemones have tentacles traps to prey on unsuspecting shrimp. Clams will quickly burrow in the tidal flats with their tube feet. Sea cucumbers put out sticky tentacles to catch food particles that drift by.
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