Notice the imperfections in the eye of this green heron. After striking for a fish the nictitating membrane cleans the eye readying for another attempt.

A nictitating membrane, or “third eyelid,” is a movable protective fold found in many vertebrates, including the Okefenokee’s alligators, frogs, and birds. It typically sweeps across the eye from the inner corner and consists of a connective-tissue core covered by epithelial cells, with cartilage providing support in some species. Its transparency varies: some membranes allow considerable vision, while others are opaque. Like a windshield wiper and shield combined, it spreads tears, clears debris, and protects the cornea from injury.

Evolution has shaped different forms of this protective system across vertebrate lineages. Keeping eyes moist, clean, and protected likely helped drive the evolution of blinking as vertebrates moved onto land. Humans retain a reduced third-eyelid remnant—the plica semilunaris, a small fold at the eye’s inner corner. Biologically, translucent membranes help animals balance eye protection with continued visual awareness. In some species, mucus-producing cells and associated immune tissue also support lubrication and defense against infection, making this structure part of both the eye’s mechanical protection and its immune defenses.