Alternative hypothesis on the evolution of photoreceptor deployment in cerebral eyes. Schematic representation of three hypotheses accounting for the deployment of ciliary photoreceptors in the cerebral eyes of Terebratalia and vertebrates, versus rhabdomeric photoreceptors in Platynereis and other protostomes. (A) Deployment of rhabdomeric photoreceptors as the ancestral state in cerebral eyes, with the larval eyes of Terebratalia, containing ciliary photoreceptors, representing an evolutionary novelty. The deployment of ciliary photoreceptors is the result of a substitution (with ciliary photoreceptors having replaced rhabdomeric photoreceptors in the cerebral eyes) early in the chordate lineage. (B) Larval eyes in Terebratalia are homologous to the cerebral eyes in other protostomes, but ciliary photoreceptors have been substituted for rhabdomeric photoreceptors, as in the vertebrates. (C) Ciliary photoreceptors in cerebral eyes represent the ancestral condition, inherited by Terebratalia and vertebrates. Deployment of rhabdomeric photoreceptors in the cerebral eyes of Platynereis and other protostomes are the result of substitution events.