Figure 4. Photoreceptor arrangements: simple photoreceptor cells without pigment—(A) phaosomal photoreceptor (adapted from Döring et al., 2013); (B) ciliary photoreceptor cell (recreated from Arendt et al., 2004). Photoreceptor arrangements: simple eyes—(C) sipunculid eye (recreated from Randel & Jékely, 2016); (D) polychaete simple (larval) eye (with permission from Nilsson, 2009). Photoreceptor arrangements: complex eyes—(E) leech complex eye (adapted from Döring et al., 2013): (F) polychaete adult eye (recreated from Arendt et al., 2002). Photoreceptor cells are colored green; pigmented cells (where present) are colored brown.