Research into the activity of the epiphysial complex and its hormone, melatonin, has been part of the growing general interest in chronobiology and in the potential therapeutic applications of melatonin. However, the studies have been almost exclusively oriented to humans and, by extension, to mammals, although humans are diurnal animals, while most other mammals are either nocturnal or crepuscular. The aim of this review is to direct attention to non-mammalian vertebrates to show that their epiphysial complexes have a set of structures and adaptations that are extremely complicated and versatile and which in themselves merit careful study. In addition, most poikilotherms and birds are diurnal like man, which extends the practical side of interest in such studies. Furthermore, many fish and bird species have immense commercial interest and the other groups, amphibians and reptiles, include many cases of endangered animals which need to be protected. Finally, it is widely recognized that an evolutionary perspective is required for a true understanding of biology. Hence, this review will stress the comparative approach to the anatomy and physiology of the epiphysial complex and the function of melatonin, a truly exceptional substance which is perhaps the only biomolecule which has undergone no structural change in all living beings and has probably always been dedicated to biological time measurement and control. As a general conclusion, the epiphyseal system of non mammalian vertebrates has an extremely wide pattern of adaptations, being involved in alimentation. This was probably due to their dependence on environmental conditions. On the contrary, the high constancy of the internal milieu of mammals has caused a reduction in both the complexity and the functions and secretions of the epiphyseal complex.
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