The Eye of the Hydra
Here's one of the most recent, and most interesting. Researchers using a non-symbiotic species of hydra have discovered that its stinging cells exhibited a sensitivity to light - they are more likely to fire at low levels of light or in darkness, while bright light actually inhibits their firing. That's interesting in its own right - hydra prey on water fleas and other small swimming animals, whose activity may correlate with the activity of the hydras' stinging cells. But there's more. That activity is regulated by a species of light-sensitive chemical, opsin, which is also found in the visual systems of higher animals, including mammals. So although hydras don't have physical structures analogous to eyes, they are photosensitive, and that photosensitivity is regulated by a chemical that has a very similar function in the human eye. The 'eye' of the common ancestor of hydras and the eyes of higher animals (fish, fowl, mammals, us) share a common pathway.
Parenthetically, it would be fun to examine to role of opsin in the behaviour of green hydra, which will migrate towards a bright light shone in one corner of their culture dish, presumably to maximise the photosynthetic output of their symbiotic algae. It might also be interesting to discover if green hydra's feeding behaviour is diurnal, or if it is just as active feeding by day as at dusk, or night (the polyps of reef-forming corals seems to photosynthesise by day and feed by night, getting the best of both being a plant and a predator).
There's also an important evolutionary angle, as one of the researchers, Professor Todd Oakley, points out: "What good is half an eye? Even without eyes there are other functions for light sensitivity that we may not be thinking of."
This is precisely the problem that Charles Darwin raised in On The Origin of the Species, in a sentence that's often quoted by opponents of evolutionary theory:
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.
When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.On The Origin of the Species, 6th Edition, Chapter 6