I’ll settle your bet, but did you know sharks have eyelids?

Today’s question:

I’m trying really hard to imagine the circumstances under which this matter led to a bet. I wonder if there was alcohol involved. It sort of sounds like a family dinner-table discussion that went tragically awry.

And I wonder what’s involved in the bet. If it’s money, I think I am due a share of the proceeds for taking the time and trouble to settle it. Sort of a finder’s fee.

In the course of researching this matter, I learned that some sharks have eyelids, while almost all other fish don’t. (They don’t need them because they don’t need to worry about their eyeballs drying out.) Some sharks have two eyelids that don’t close all the way but also have a third eyelid called a nictitat- ing membrane that protects the eye when they are feeding or fighting. Great white sharks and a few other types don’t have eyelids but roll their eyes way back in their heads when they need protection. Anyway, back to the numbers. Worldwide, there are about 10,000 species of birds and about 32,000 species of fish.

Why this should be so, I don’t know. Perhaps it’s because there are a lot more bodies of salt water and freshwater than there is solid ground. This would give fish more diverse environments in which to flourish and evolve new species than would be available to birds.

Since that was the first explanation I thought of, I suspect it is shallow if not plain wrong. But why should today’s answer be any different from any of the others?