Comparison of lamprey and gnathostome retinas. Schematics of the organization of the retina of the lamprey lateral eye. A, Silver lamprey (Ichthyomyzon unicuspis, post-metamorphic juveniles). B, Gnathostome. The main difference is that in the lamprey the inner plexiform layer is vitreal to the majority of ganglion cells, so that most of the ganglion cells are located in the inner nuclear layer. C, D, Southern hemisphere lamprey, Geotria australis: in two of the animal's adult forms: (C) downstream migrant phase (post-metamorphic juvenile), and (D) upstream migrant phase (ready to spawn). The retina of the downstream migrant is densely packed with cells, lying ~10 layers deep in the INL (including at least two layers of horizontal cells); the retina of the upstream migrant is much larger, and is dominated by large photoreceptors and reduced layering of cells in the INL. Abbreviations in upper case are standard, with the addition of: IGCL, OGCL, inner and outer ganglion cell layers; IHC, OHC, inner and outer horizontal cells; OFL, optic fiber layer; SPL, superficial plexiform layer. a, axon; ap, ascending process of unknown origin; bg, biplexiform ganglion cell; e, efferent fiber; g, ganglion cell. A, B, from Fritzsch & Collin (75). C, D from Nivison-Smith et al (80).