On the bizarre and wonderful complexity of the fanworm vision system. I bet none of you woke up thinking you'd read about that today.
The butt eyes – pygidial eyes, to be less crude and more precise – are quite simple, and their job seems to be to find the darkest spot to build a new tube if a worm has been unhoused. The thorax eyes seem to alert worms who’ve embarrassingly exposed themselves to clamber back down into the tube.
Together with the cerebral eyes – little more than simple pigment cups now, buried inside fanworm brains – all these eyes do little more than detect light. And they are structurally and biochemically similar to the eyes of other invertebrates.
The tentacular eyes, on the other hand, are cut from very different cloth. Photoreceptors can be constructed from two types of cellular protrusions: microvilli, and cilia. A microvillus is a little finger-like cell membrane projection. Our small intestines are lined with these (they help us absorb nutrients). A cilium is a beating hair.
What seems to have happened in fanworms is that a few simple, ciliated photoreceptors employed only in sensing ambient light happened to be hanging out in the vicinity of the radiolar tentacles, minding their own business, when the fanworm nabbed them and gave them a new job as lookouts... It is possible these eyes provide only shadow-detection. But it is also possible the compound eye versions may be capable of rudimentary motion detection and image formation, and that fanworms have something like “vision” as a result.
https://blogs.scientificamerican.com/artful-amoeba/fanworms-natures-eye-factories-stick-them-pretty-much-anywhere/
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