Others have razorlike cutting teeth, and still others have pointed teeth. Some people believe that sharks must turn over on their back to bite, but this is not true. Small, toothlike scales cover a shark's body. These placoid scales make the skin of a shark very rough. Dried sharkskin, called shagreen, was once used as sandpaper.
Sharks get oxygen from the water through gills, as do other fish.
But sharks have no gill cover, a bony plate that shields the gills of most fish. Instead, sharks have from five to seven slits in the skin on each side of the head. Water passes out of these slits after the shark's gills remove the oxygen. Many sharks cannot pump water over their gills, as do the majority of fish. These sharks rely on their constant swimming to force water through their mouth and over their gills.