Croc-Eye-Dile
Click here to watch this crocodile (a sub-adult Australian saltwater crocodile or 'salty', if you must know) open its eye. You'll notice the nictitating membrane, or third eyelid, which is stored in the lower corner of the eye nearest the nose, sweep across the eye as it opens. The nictitating membrane is transparent except for the leading edge, which is pigmented and contains some cartilage. This third eyelid cleans and lubricates the eye with the help of fluid secreted from the lacrymal (tear) duct. Although nobody is quite sure how well the crocodile can see underwater, the nictitating ensures that light can reach the eye during diving.

All crocodilians have this "third eyelid," but other land animals have them as well. Birds, such as this owl use the nictitating membrane constantly because their eyes tend try out quickly while flying. And if birds had to blink all the time, they'd be flying blind. A polar bear's membranes prevent snow-blindness, and the aardvark's nictitating membrane protects its eyes against termite bites. Horses and dogs have them, too.

Most aquatic animals, like fish, have no eyelids at all, because bright light and dust aren't a concern underwater. Eyelids evolved in land-dwelling animals to protect the delicate eye from incoming debris and to keep the corneal surface clean by sweeping tear fluid across the eye.

Crocodile image and movie courtesy Dr. Adam Britton.