Fig. 11

The two photoreceptor types found in animals. A Rhabdomeric photoreceptors, which use extensions of the membrane (apical microvilli) to increase the amount of photopigment (r-opsin) that they can contain. Rhabdomeric photoreceptors are the predominant type found in protostomes (most invertebrates). B Ciliary photoreceptors, which make use of a modified ciliary membrane to increase the surface area available for storing photopigment (c-opsin). Ciliary photoreceptors are the main type observed in deuterostomes (vertebrates, echinoderms, and relatives). The two cell types differ in their phototransduction pathways and in the electrical responses that result. It is thought that both cell types were already present in the ancestor of all bilaterian animals, having been derived from one ancestral photoreceptor type (Fig. 14). From Arendt and Wittbrodt (2001), reproduced by permission of The Royal Society and D. Arendt