third eyelid

1. A protective fold of skin in the eyes of reptiles and birds and some mammals (synset 105322189)
   is a type of: protective fold - a flap of tissue that protects what it covers
   subtypes: haw - the nictitating membrane of a horse
   is a part of: eye, oculus, optic - the organ of sight
   same as: nictitating membrane

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