79 Cover Your Eyes!

Nature has devised 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 irritants, and tears wash away any particles 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 twigs, dirt, and sand. The third eyelid, however, is a semitransparent tissue that crosses over the eye from the inside corner to the outside corner. Because of this protective membrane, birds seldom have to blink. They close their eyes only when they go to sleep. In 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. In 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 the eye by shading it from glare. They also act like miniature brushes to remove dust. Camels have lashes that are four inches long to protect their eyes from windblown sand in the desert.

79 保护你的眼睛！

大自然设计了多种方式来保护生物的眼睛。最常见的保护装置是眼睑——一片能覆盖眼睛的皮肤褶皱，可保护眼睛免受伤害。睫毛能阻挡灰尘和其他刺激物，眼泪则会冲走突破其他防御的颗粒物。

包括大多数鸟类在内的一些生物有三层眼睑：上下眼睑的作用和人类的眼睑一样，能阻挡细枝、灰尘和沙子。而第三层眼睑是一种半透明的组织，从眼内角延伸到眼外角，覆盖整个眼球。有了这种保护膜，鸟类很少需要眨眼，只有睡觉的时候才会闭上眼睛。对鸭子来说，这层第三眼睑就像水下潜水面罩，能帮助它们寻找食物。

大多数鱼类和蛇类根本没有眼睑，取而代之的是一层坚硬的玻璃状覆盖物来保护眼睛。对鱼类来说，水会不断冲刷这层覆盖物上的污垢；而蛇的视力通常很差，一点遮挡视线的污垢不会对它造成太大影响。

睫毛通过遮挡强光来保护眼睛，还能像微型刷子一样清除灰尘。骆驼有四英寸长的睫毛，用来保护眼睛免受沙漠中风沙的侵袭。