Sharks have upper and lower eyelids, but these lids don't move. Some sharks have a membrane, the nictitating eyelid, that can be closed over the eye to protect it from damage. As the shark grows, the denticles are shed and replaced by slightly larger ones. The layer of dermal denticles creates a rough surface that is sometimes compared to sandpaper. Untanned shark skin, called shagreen , is used to sand and polish furniture.
Most sharks swim by thrusting their tail fins from side to side.
The unpaired first dorsal fin, along with the smaller second dorsal and anal fins, keeps the shark's body stable as it swims through the water. Many sharks, especially those that live near the surface, are a a dark color on their backs and a lighter color on their bellies. This network is sensitive to external motion, and helps the shark detect prey and potential predators. The unpaired second dorsal fin assists in thrust as the shark swims, and helps stabilize it, preventing it from rolling from side to side as it swims. Not all sharks has a second dorsal fin.