The plica semilunaris is a small fold of bulbar conjunctiva on the medial canthus of the eye. It functions during movement of the eye, to help maintain tear drainage via the lacrimal lake, and to permit greater rotation of the globe, for without the plica the conjunctiva would attach directly to the eyeball, restricting movement. It is the vestigial remnant of the nictitating membrane (the "third eyelid") which is drawn across the eye for protection. It is present in other animals such as birds, reptiles, and fish. It is rare in mammals, mainly found in monotremes and marsupials. Its associated muscles are also vestigial. It is loose, thus eye movements are not restricted by it. Only one species of primate, the Calabar Angwantibo, is known to have a functioning nictitating membrane.
^Montagna, W.; Machida, H.; Perkins, E.M. (1966). "The skin of primates XXXIII.: The skin of the angwantibo". American Journal of Physical Anthropology25 (3): 277–290. doi:10.1002/ajpa.1330250307. PMID5971502.