The visual systems of most invertebrate groups arise following the outgrowth of axons from photoreceptors which develop from the ectodermal epithelium independently of the nerve centers which they innervate; peripherally derived visual interneurons are exceptional. This is in contrast to vertebrates where the retina forms from an embryological outgrowth of the forebrain, with several classes of interneurons. Most studies on invertebrates are descriptive and preferentially treat eye development. Few studies are analytical and fewer treat the visual centers. Yet the topographic ordering, the small numbers of their identifiable neurons, and the many other model characteristics of invertebrate neuropils especially recommend these centers for studies in neural development and regeneration, notably the compound eyes of arthropods and the highly developed single-lens eyes of certain molluscs. Modern studies have tended to canonize selected species (especially the flies Drosophila and Musca, and the water-flea Daphnia) to the unwarranted neglect of other equally deserving groups—cephalopods, pectinid scallops, spiders, and salps to name but a few. A comprehensive review by Meinertzhagen and Macagno is forthcoming.