Photosensitive membrane structures in the retinular cells of the Limulus lateral eye are broken down and renewed daily. The first light onset causes a rapid, synchronous disassembly and buildup of the rhabdom in each photoreceptor cell. The entire process is complete within 30 minutes. Blocking the efferent input to the retina from the brain blocks the turnover of the rhabdom, and mimicking the efferent input by optic nerve stimulation restores it.