'And "nictitating membranes" are a fancy way of saying "eye lids"' Not exactly. A nictating membrane is a part or the eye on a reptile and some birds and amphibians. There are a few mammals with them, too, if I'm not mistaken, but very few. It's an eyelid, yes, but not a proper one as we think of them. Instead, it's clear so the animal can see through it, but it's protective against things like sand and water, allowing the animal to see, if not as well, under adverse envrinmental conditions. Most animals with nictating membranes also have eyelids, a notable exception being about half the geckoes.