^w^ Sharks' eyes more closely resemble those of land vertebrates than other fish, and they see quite well.
^w^ The eyeball has a transparent cornea that allows light to enter the eye, an iris that opens and closes, an elaborate pupil with pinhole, vertical, or horizontal slits (depending on the species) and a lens that focuses light.
^w^ Numerous retinal rods provide the sensitivity necessary for vision in low light conditions, allowing sharks to adjust their eyesight for night time hunting, while the retinal cones provide clear, colored images for daytime vision.
^w^ A series of mirror-like plates behind the retina reflect back the light entering the eye and increase the shark's sensitivity to low light. Unlike any other animal, the shark has a moveable pigment that descends like a curtain in daylight and reduces the glare, thus protecting the eye from excessive light.
^w^ In a shark, the transparent cornea is mostly protective, and the ellipsoidal lens does all the work of focusing the image. The result is that underwater, the shark sees better than we do. To be on an equal footing, we need a swim mask to protect our eyes from the water.
^w^ The eyes of some sharks are rimmed by two tiny eyelids with lids that open and close. In some species, like Lemon Sharks, a third eyelid called the `nictitating membrane` closes during attack to protect the eye from damage.
^w^ The Great White keeps its eyes open all the time, but it can rotate its eye back into the head for protection.
