 Sublensar sensory structures have been found in the eyes of some phacopid trilobites.The structures consist of what appear to be several sensory cells surrounding a rhadomeric structure, resembling closely the sublensar structures found in the eyes of many modern arthropod apposition eyes, especially "Limulus", a genus of horseshoe crabs.Secondary blindness is not uncommon, particularly in long lived groups such as the Agnostida and Trinucleioidea.