I'm not sure but if specifically the third eyelid is showing it is a sign of illness or possibly injury and you should get the cat or dog to the vet.
Eye lids protect the eyes. Eyelids also provide moisture for the eyes. The moisture is provided when a person blinks. When a person blinks, the eyes are lubricated with tears.
they have no eye lids
a fish
yes just like cats and humans
Their eyes do not dry out.
Wasps have compound eyes and no eye lids
They are right above your eyes.
When its eyes are bloodshot and his eye lids are droopy?
Their eye lids protects their eyes. Beavers eyes let them to see both underwater and on land.
yes it can ...they scratch the inside of your eye lids
Eye lids
In order to make your eyes stand out better it would be appropriate to apply eyeliner to the top lids. Unless you are trying to make your eyes smaller looking. Then just apply eyeliner to the bottom lids instead.