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. 2013 Mar;30(1-2):5–20. doi: 10.1017/S0952523813000035

Fig. 8.

Fig. 8.

Schematic view of the head region of bilateria indicating the evolutionary shift of position and function of photoreceptor organs argued for in the text. The photoreceptor/opsin types are denoted by (r) for rhabdomeric/r-opsin and (c) for ciliary/c-opsin. Note that the vertebrate lateral eyes are composite structures formed by fusion of components from different photoreceptor organs in Urbilateria. It is also possible that vertebrate ancestors went through a stage with only median eyes (Vopalensky et al., 2012). Polychaetes have only made functional shifts and duplicated their lateral rhabdomeric eyes, whereas crustaceans/insects have formed a new median eye from rhabdomeric photoreceptors.