The Atlantic horseshoe crab (Limulus polyphemus) is an important model for the neurobiology of vision, but the molecular mechanisms that control eye development in this species remain largely unknown. I propose to use spatial quantification of gene expression, single-cell transcriptomics, and genome editing in L. polyphemus embryos to characterize the gene networks that orchestrate the formation of their median and lateral visual systems. This approach will enable revisiting the observation that the eyes of L. polyphemus develop without the input of the conserved transcription factor Pax6. This study will serve as a valuable comparative data point with respect to the compound eye of the fruit fly Drosophila melanogaster, toward a better understanding of the evolution of developmental controls in arthropod visual systems.