Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and ...

Cataracts can develop in one or both eyes and interrupt the flow of light through the lens. The lens is usually transparent, allowing light to pass though to the retina at the back of the eye.

It's hard to miss the huge eye of a squid. But now it appears that certain squids can detect light through an organ other than their eyes as well. It's hard to miss the huge eye of a squid.