The camel has a third eyelid.
The third eyelid on a frog is called a nictitating membrane.
the 3 eyelids work together to create a constant barrier against the saltwater
A cat's third eyelid becomes more visible when they are sleepy because their eye muscles relax, allowing the third eyelid to move across the eye and become more noticeable.
A nictitating membrane The nictitating membrane is the THIRD eyelid, the SECOND eyelid that you asked about is just that, the eyelid, just like in humans
Nictitating Membrane
Cats sometimes have their third eyelid showing when they are sleeping because it helps protect their eyes and keep them moist while they are resting.
Yes because it is to help the frogs see under water
The third eyelid in the cat (and dog) is found in the lower corner of the eye on the side near the nose. It can be visualized on occasion as it may partially elevate and partially cover the eye. In birds, the third eyelid is very functional and sweeps across the entire eye. It too is located in the same spot as the cat and dog. Even humans have a third eyelid, however, it is vestigial (a small non-functional remnant).
Seeing your cat's third eyelid is a common sign of disease in cats. To answer your question, yes, it is very possible it means that your cat is sick.
A cat's third eyelid, called the nictitating membrane, is visible when it is sleeping because it helps protect and moisten the eye while the cat is in a relaxed state.
It is called a nictating membrane.