Birds have this third eyelash called the nictitating membrane.

鸟有第三层睫毛，叫做瞬膜。

That's the nictitating membrane and this little fold is the remnant.

那是瞬膜，这个小褶皱是残余的。

To see underwater they have a nictitating membrane.

为了在水下观察，它们有瞬膜。