The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eye lid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and shark have full nictitating membranes
If attack is imminent, defend yourself with whatever weapons you can, advises the ReefQuest Centre for Shark Research. “Avoid using your [bare] hands or feet if youcan avoid it; if not, concentrate your blows against the shark‘s delicate eyes or gills.” A shark‘s snout is also said to be sensitive.
Source: Wikipedia, Google