“…Direct cell type homology (in the sense of cell types in modern cephalopod and vertebrate eyes descending from specific ancestral cell types in their common ancestor (Arendt et al, 2016)) is therefore unlikely, since the last common ancestor of molluscs and chordates did not have an image forming eye (Nilsson, 2009;Salvini-Plawen et al, 1977). Instead, we must consider that convergent structures could arise by some combination of (1) parallel reuse of shared ancestral genes or networks (Shubin et al, 2009), perhaps following common organizational principles (Sanes and Zipursky, 2010) and (2) the parallel acquisition of new genes and pathways, such as novel adhesion molecules or transcriptional activators (Albertin et al, 2015a(Albertin et al, , 2022b. From this perspective, the a priori convergent cephalopod and vertebrate visual systems provide an ideal opportunity to begin to compare the genes and cell types underlying convergently evolved complex systems.…”