The nictitating membrane present in frogs and birds serves as a covering membrane for the eyes. but this membrane is vestigial in humans. why
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The nictitating memrane is a translucent (or transparent) protective membrane that can cover part or all of the conjunctiva of the eye of vertebrates. It is for protection of the eye and still allowing to distinguish light and dark and large objects.
In some sharks, fish, amphibians, some reptiles, some birds and some mammals (camels, polar bears, seals, aardvarks) it can cover all the eye as a further eye lid:
In some reptiles as most snakes it is the only lid.
In humans and other mammalians (mostly primates) it is left as a rudiment called plica semilunaris of conjunctiva:
In some sharks, fish, amphibians, some reptiles, some birds and some mammals (camels, polar bears, seals, aardvarks) it can cover all the eye as a further eye lid:
In some reptiles as most snakes it is the only lid.
In humans and other mammalians (mostly primates) it is left as a rudiment called plica semilunaris of conjunctiva:
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