Fig. 6.

Possible evolutionary transitions between classes of photoreceptive sensory tasks and related sensory structures. For functional reasons, the process cannot skip classes except for reductions (dashed line) when functions are lost through behavioral modifications. The color coding agrees with Figs. 2, 6, and 7. Note that the classes concern only the spatial modality of photoreception, which is fundamental for eye evolution. The other photoreceptive modalities, color and polarization, are very important for the visual ecology of many species but not so much for the evolution of eye design. Class II organs are appropriately called eyespots because the screening pigment makes them visible, but they do not provide vision (image information) and are thus not true “eyes.”