Sharks have lived in the oceans for more than four hundred million years, long before the first dinosaurs appeared. Unlike mo fish, th do n have bo. Instead, th skeletons a made o cartilage, t same li and flex material that is found in human ears and noses. Many sharks must keep swimming in order to breathe, because water has to flow over their gills. Although sharks are often feared, most species are harmless to people.

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