though sea urchins don’t have eyes, they are covered in photoreceptors which collectively act as a retina, effectively making their entire bodies one big compound eye. sea urchins, one of the few marine organisms to have their genome sequenced, have about 23,000 genes (like a human), several of which are associated with sight, including those that govern the development of animal eyes.
"comparing all the genes of the sea urchin, it’s actually quite similar to humans," said george weinstock, who led the sequencing project. they are one of the few invertebrates on the human branch of the evolutionary tree. yet interestingly, they seem to be the only example of a deuterostome to have the rhabdomeric light sensors associated with protostomes, suggesting that rhabdomeric light detectors have been the norm for eyes throughout much of the animal kingdom’s history.
"we think of animals that have a head with centralized nervous systems and all their sense organs on top as being the ones capable of sophisticated behavior, but we’re finding more and more some animals can do pretty complex behaviors using a completely different style," notes sönke johnsen, a marine biologist at duke university who conducted the study on sea urchin vision.
the way that urchins apparently carry out eyesight - with a diffuse nerve net, where no region looks like a central processing unit - reflects how scientists are now often designing robots. “they’re finding it might be a lot better with a distributed system with many little processors and simpler sensors and simple rules, which end up creating fairly complicated behaviors as emergent properties.” [see: starlings post]”
photos of sea urchin tests up close by paul richman. when alive, tube feet would be seen coming from the holes, which the sea urchin uses primarily for sight, with the smaller dents seen in the tests, also containing photoreceptors, used for shading and blocking light. text sources.