Speaking of frogs – this is what a red eyed tree frog looks like with its eyes closed. They actually have three eyelids. The third is called the nictitating membrane, and it allows the frog to protect their eye from danger and to keep it moist while still allowing some visibility.
This membrane is found in many different groups of animals, but most of them it simply looks like an opaque membrane. The tree frog’s is a bit more striking!