So - I know we have examples of the intermediate stages, both light sensitive cells as well as cups, and so on. Do some of them have the nerves sticking up? Or, are there other ideas about how this arrangement evolved, and from what precursors on the skin?
Either way up the eye develops the top layer(s) need to be transluscent, and thus it is possible to have skin cells, nerves, retina cells for the initial light sensitive spot.
As taz pointed out the octopuses, squids, nautilus all have the retina light cells on the light side and the nerves behind.
It could be a flip of the coin for one or the other, or there could be some slight protection from UV for those venturing on land (or near the surface of the water).
Enjoy.
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