It is a question often posed by those who doubt Darwin. From “Genetic Link Between Visual Pathways of Hydras and Humans Discovered” (ScienceDaily, Mar. 5, 2012), we learn,
The research found that the light-sensitive protein opsin in sensory cells regulates the firing of the hydra’s harpoon-like cnidocytes. These same cells are found in the mechanisms hydra use to grasp prey, and to summersault through the water.
The linking of opsin to the stinging cells helps explain how hydra can respond to light despite the absence of eyes, the scientists said, because the sensory neurons also contain the ion channels and additional proteins required for phototransduction — the process by which light is converted to electric signals. Phototransduction in humans occurs in the retina.
“I wouldn’t call this vision, because as far as we know the hydra are not processing information beyond what’s light and what’s dark, and vision is much more complicated than that. But these genes that we’re studying are the keystones of vision,” Oakley said. “For us, as evolutionists, the message is that photoreception can do other things besides just facilitate vision. It can do unexpected things. What good is half an eye? Even without eyes there are other functions for light sensitivity that we may not be thinking of.”
The problem is that opsin isn’t anything like half of an eye, nor on the way to becoming one, so far as we can see. It can help trigger stinging cells, and that is very interesting, but doesn’t answer questions about eyes.
Hunting strategy of the freshwater hydra: