nictitating membrane(redirected from Semilunar fold)
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a third eyelid, located in the inner corner of the eye in some Euselachiae (fishes) and the majority of terrestrial vertebrates. The nictitating membrane is formed from a fold of the conjunctiva and is located behind the upper and lower eyelids. In terrestrial vertebrates, a transparent nictitating membrane periodically covers the cornea to clean and moisten it. Nictitating membranes are most highly developed in reptiles and birds. Among mammals, a third eyelid is absent in Echidna and Cetacea; apes and humans have rudimentary nictitating membranes in the form of semilunar folds.