The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway. 2010. D.C. Plachetzki, et al.Proc. Roy. Soc. B, published online before print March 10.
"We determined which genetic 'gateway,' or ion channel, in the hydra is involved in light sensitivity," said senior author Todd H. Oakley. "This is the same gateway that is used in human vision."
The gene, called opsin, is present in vision among vertebrate animals, and is responsible for a different way of seeing than that of animals like flies. The vision of insects emerged later than the visual machinery found in hydra and vertebrate animals. link