Sharks are the most ferocious fish in the sea. There are over 250 kinds, some quite small, but nearly all sharks are hunters, with a mouth full of rows of razor-sharp teeth. Fortunately, most prey on fish. Only 12 species are dangerous to humans. Most fish have skeletons of bone, but a shark's skeleton is made of a rubbery substance called cartilage, like the cartilage in your nose.
Sharks also have gill slits on their heads rather than gill covers - typically five on each side.
Like sharks, rays and skates are cartilaginous, but while sharks are long and thin, most rays are broad and flat, with giant pectoral fins extending either side like aeroplane wings. They have eyes on the top of their head and often a sting in their long, slender tail. Most sharks have many rows of large, sharp teeth and powerful jaws that enable them to bite through almost anything. As the outer rows of teeth wear out or break, the inner rows move forward to replace them. The great white shark is the most dangerous of all sharks.