Pineal bodies, and other associated parietal structures, are remarkably varied among vertebrates These organs apparently assumed, in the primitive vertebrates, a principal role in integration of photic information. The pineal photoreceptor cell seems to have evolved into the secretory pinealocyte that is found in most of the higher vertebrates. Along with the evolution of the photoreceptor element into the pinealocyte, there is a concomitant shift in the neural connection of the pineal organ. The pinealo-fugal, sensory innervation gives way to an autonomic, pinealo-petal motor innervation. Thus, direct photosensitivity was superceded by indirect, optically-mediated control of the now secretory pineal gland. Even though pineal organs display such unusual plasticity and diversity across groups, responsivenses to light remains a constant feature. Photoperiod may modify the diurnallyrhythmic patterns of melatonin secretion across seasons and invoke appropriate “programs” which permit an animal to anticipate seasonal changes. Thus, melatonin may be a key molecule, attuned to photoperiodicity, which has been selected through evolution to effect adaptation to annual events.