ThirstyFish

Nictitating membrane is a protective and moisturizing third eyelid commonly found in birds, reptiles, amphibians, and fish and less commonly in mammals. The membrane is either transparent or translucent, allowing the animal to see while blinking.

The Nictitating membrane exists in cats and dogs but is usually not visible because it lacks a sufficient amount of muscle fibers. If visible this could be a sign of poor health for the cat or dog. In humans it is believed that a pink fold of tissue on the inside corner of the eye is a Nictitating membrane that has become vestigial. Vestigiality is the evolutionary loss of function of organs or behaviors in organisms due to lack of utilization. Although almost all animals have Vestigial characters, the wings of ostriches, emus, and other flightless birds are given as the obvious examples; They are the remnants of their flying ancestors' wings.