Nature has created many ways to protect creatures' eyes. The most common protection is the eyelid—a fold of skin that closes over the eye, protecting it from damage. Eyelashes are useful for keeping out dust and other things, and tears wash away any small pieces that get through the other defenses.
Some creatures, including most birds, have three eyelids. The upper and lower lids act like human lids and keep out little tree branches, dirt, and sand. The third eyelid, however, is a semitransparent tissue(半透明组织) that crosses over the eye from the inside corner to the outside corner. Because of this protective membrane(隔膜), birds seldom have to blink. They close their eyes only when they go to sleep. For ducks, this third eyelid serves as an underwater diving mask that helps the ducks find food.
Most fish and snakes have no eyelids at all. Instead, a hard glassy covering protects their eyes. For fish, water constantly sweeps away dirt from the covering. And a snake's eyesight is usually so bad that a little dirt obscuring its vision does not disturb it greatly.
Eyelashes defend(保护) the eye by shading it from strong light. They also act like mini brushes to remove dust. Camels have eyelashes that are four inches long to protect their eyes from wind-blown sand in the desert.
