Sharks posses the basic eye structure that is found in all vertebrates, but with some modification.  The shark eye has a reflecting layer called a tapetum lucidum located behind the retina.  Essentially the structure consists of a layer of parallel, plate-like cells filled with silver guanine crystals.  The crystals reflect light that has already passed through the retina and redirects it back to restimulate the retina as it passes out through the eye.  This effectively boosts the visual signal, especially in low light levels giving sharks high visual acuity.

Another modification found in some sharks is the presence of a nictitating membrane.  This structure is a denticle covered membrane that protects the eye.  It closes when the shark passes close to a objects and also during biting or feeding.