The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates

Abstract

In the course of evolution the median eyes, in addition to paired lateral eyes, have independently developed in several classes of animals including crustaceans, insects and vertebrates. Within the order of vertebrates some lower classes including fishes, amphibians and reptiles possess photosensitive structures within the epiphyseal complex deriving ontogenetically from the diencephalon. While the gross anatomy of the median eyes has been well examined for over 100 years, the fine structure and knowledge of the physiological properties of median photoreceptors has remained remarkably incomplete. Until recently their functional role was based mainly on indirect evidence and speculation, except for the classes of crustaceans and insects in which both the functional operation and the sensory significance of the median eyes (ocelli) are well known.