Cajal’s fascination with the retina is well documented – his publications on retinal neurons and organization in various species span 45 years, the length of his career. Shown above are schematics of insect (left), cephalopod (middle), and vertebrate (right) retinas, highlighting their similarities. All of these retinas are composed of photoreceptors (A) which are activated by light and transmit signals through synapses of intermediary cell types (B, C, F) onwards toward deeper brain processing (G). Cajal has oriented all these diagrams with the photoreceptors at the top for ease of comparison, but in fact the diagram of the vertebrate retina is more correctly shown inverted compared to the other two, with its photoreceptors located at the bottom of the diagram. This is because the photoreceptors of invertebrates, such as insects and cephalopods, are exposed to light directly as the top layer of the retina, while the photoreceptors of vertebrates are actually at the bottom of the retina. Light must pass through the transparent layers of nerves above them before reaching them. 3D print by Jeremy Swan based on an original illustration by Santiago Ramon y Cajal. Courtesy of the Cajal Institute, Spanish National Research Council or CSIC©