Hello Isshika,

The plica semilunaris is a fold of the conjunctiva at the inner corner of the human eye.

The similarity to the nictitating membrane, or third eyelid, of other animals led to the conjecture that it might be the vestige of such a structure, which is still a part of the eye in some primates, including gorillas.

However in chimpanzees, one of the human species’ closest relatives, the plica semilunaris appears to be vestigial.

The purpose of the nictitating membrane in many animals is that of protection. For instance,  it helps in keeping the eye clean and moist and concealing the iris from predators. The membrane, in certain species, is sufficiently transparent to enable vision when underground or underwater.

Though the reason for the loss of a nictitating membrane in humans in rather ambiguous, changes in our habitat and the physiology of the eye may have put the tissue out unnecessary.

Hope this helps you. We wish you the best for your future endeavours!

Regards