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pineal body[′pin·ē·əl ‚bäd·ē]
(also called pineal eye), an eyelike organ in some higher fishes, such as dipnoans and some holosteans, and in such reptiles as the tuatara and many lizards. It develops from an outgrowth of the roof of the diencephalon and communicates with the diencephalon by means of an unpaired nerve. The structure of the pineal body is particularly similar to that of the ordinary paired eye in reptiles: their pineal body has a crystalline lens that faces the parietal foramen in the roof of the skull and that has a multilayered retina with photosensitive and pigmentary cells. The pineal body in some lizards perceives differences in light. The pineal body is reduced in higher vertebrates.