Sharks existed even before dinosaurs. Today there are 368 shark species. Fully-grown sharks can range in size from 18 cm long to 15 m long. Most sharks have streamlined bodies, shaped like a torpedo and are about the same size as people, 1.5 – 2.1 m long. Their shape allows them to glide easily through water. They cannot stop swimming, otherwise they would sink to the bottom. Some bottom-dwelling sharks have flattened bodies, which allows them to hide in the sand of the ocean bed.