This begs the question of how this happened. Once is near impossible considering eyes are not the only thing need to make sight possible, because you need to a supply of blood to both keep it cool and for sources a repair mechanisms so the eye doesn't get damaged from the light it takes in. Also there needs to be facial muscles that control the movement of the eye. There needs to proper wiring so that the signal can sent and lastly there needs to be the brain that is capable of interpreting the signals from the eye. Eyesight is a vastly complex thing.
Something like this:
http://en.wikipedia.org/wiki/Evolution_of_the_Eye
TL;DR The eye is probably the worst thing you can possibly throw irreducible complexity at. For every stage of the development of the eye it makes perfect sense how the development would represent an improvement over the previous iteration.
-Cells that can detect light simply (light/no light) aid in orienting an organism towards circadian rhythms.
-Further improvements help detect light intensity and direction
-Development of an "eye cup" helps refine direction
-Eventually deepening the cup becomes more of a hindrance and you get pinhole camera-esque design to help refine resolution
-Layer of cells added to help protect the photoreceptor cells.
-Specialization of eye components further helps improve vision
-Lens helps direct, focus, and discriminate against light.
There's even some inefficiencies in eyes that are really only explainable due to evolutionary arguments. For example we are only able to see along the "visible spectrum" because only blue and green visible light are capable of penetrating water.
You can even track the development:
Planarians still have that simple "eye spot" of a cup of photosensitive cells that can distinguish brightness and direction but not much else
The Nautilus has the "pinhole camera" type of eye that has no cornea or lens but is able to distinguish objects at very very low resolution