eye © Sam23/Fotolia

The plica semilunaris is a fold of conjunctiva at the inner corner of the human eye. Its likeness to the nictitating membrane, or third eyelid, of other animals led to the idea that it might be the vestige of such a structure, which is still part of the eye in some primates, including gorillas. In the chimpanzee, however—one of the human species’ closest relatives—the plica semilunaris also appears to be vestigial. The function of the nictitating membrane in many animals is protective—for example, keeping the eye clean and moist or concealing the iris from predators. In some species, the membrane is sufficiently transparent so as to enable vision when underground or underwater. Though the reason for the loss of a nictitating membrane in humans in unclear, changes in habitat and eye physiology may have rendered the tissue unnecessary.