Fig. 14

The current distribution and origin of rhabdomeric (dark gray) and ciliary (white) photoreceptor cells among animals (see also Fig. 11). These are thought to have descended from cells already present in the last common ancestor of all bilaterally symmetrical animals (the Urbilaterian). It is thought that ciliary and rhabdomeric photoreceptor cells are descended from a single ancestral cell type, though there is some ongoing debate as to whether the Urbilaterian possessed (a) a ciliary precursor type from which both are derived, (b) a dual ciliary or rhabdomeric precursor that later diverged into two types, or (c) both ciliary and rhabdomeric cells that had already diverged earlier in animal evolution. From Arendt and Wittbrodt (2001), reproduced by permission of The Royal Society and D. Arendt