Evolutionary attempts at 4 eyes in vertebrates.

PURPOSE: To understand and compare the optical, histological, and ecological differences among 4 vertebrate species that have had evolutionary attempts toward 4 eyes. METHODS: An evolutionary attempt at 4 eyes in defined as the duplication or one or more structures integral to the refraction or interpretation of the visible spectrum for that animal. We reviewed and compared the known optics, histology, and ecology of each of these vertebrate …