Fig. 7.

The classes of photoreceptive tasks plotted on a metazoan phylogeny, using the arguments from the functional discussion in this paper together with information on receptor morphology (mainly from Salvini-Plawen & Mayr, 1977), and the occurrence of opsins (Plachetzki et al., 2007; Porter et al., 2011; Schnitzler et al., 2012). For clarity, minor bilaterian taxa were omitted, and because these only have class II photoreception, the most parsimonious transitions to class III are the ones indicated by color. The exact pattern of transitions should be interpreted with caution because it is sensitive to the choice of phylogenetic tree, which is here based on Philippe et al. (2009), with the placement of the Acoela according to Philippe et al. (2007).