Fig. (4) .

Homology (deep homology) and non-homology in eye evolution. Camera eyes of vertebrates and squid are not homologous; that is, they have evolved independently. However, Pax6, which is homologous for the more inclusive (deeper level of) Bilateria, has been co-opted independently in eye development of both squid and vertebrates. Rhabdomeric photoreceptors and r-opsins are also homologous for Bilateria. Vertebrate ganglion cells have been hypothesized to be a transformational homology of rhabdomeric photoreceptors. Camera eyes of box jellies are thought to be derived independently from those of vertebrates. The figure is modified from Shubin et al. [10].