part of part of in taxon Arctocebus calabarensis Haplorrhini Amphibia Serpentes Aves Ursidae Phocidae Orycteropodidae Camelidae eyelid conjunctiva nictitating membrane nictitans membrana nictitans NCIT:C77657 UBERON:0010207 Wikipedia:Nictitating_membrane A fold of the mucous membrane of the conjunctiva in many animals. At rest, it is hidden in the medial canthus. It can extend to cover part or all of the cornea to help clean the cornea. We exclude the shark structure for this via a taxon constraint. The Calabar angwantibo (Arctocebus calabarensis, a lorisoid) is the only primate to have a functioning nictitating membrane[Wikipedia:Calabar_angwantibo] In most primate species, a plica semilunaris is present, although fully developed nictitating membranes can be found in lemurs and lorisoid primates[Wikipedia:Nictitating_membrane] SCTID:113326005 haw third eyelid in mice, the nictitating membrane is very small and composed of a very thin core of cartilage covered by loose connective tissue Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as camels, polar bears, seals, and aardvarks, have full nictitating membranes. fully developed nictitating membranes can be found in lemurs and lorisoid primates. UMLS:C0028058 MESH:D009541 palperbra tertia Wikipedia:File:Chickenblinking.jpg