Pax-6 usually causes the production of a type of light-receptor pigment. in vertebrate eyes, though, a different gene (the rh gene family) is responsible for the light-receptor pigments of the retina. the rh gene, like pax-6, is ancient. in the marine ragworm, for example, the rh gene causes production of c-opsin, which to regulate the worm’s biological clock. what most likely accounts for vertebrate vision?