In submammalian vertebrates the pineal and accessory pineal organs show the structure of light receptors. During shorter or longer periods of life they may function as such. Structurally, their sensory epithelium is, in principle, similar to that of the lateral eyes. Afferent nerve fibers, originating in this epithelium, reach the brain by way of the habenular and/or the caudal commissure. Practically nothing is known about their further central course. There is some indication that, in anurans, the ependymal and the glial cells of the epiphysis are secretory in function. This epiphysis receives also some efferent, possibly autonomic fibers. The mammalian epiphysis shows an extensive orthosympathetic autonomic innervation by postganglionic fibers originating in both superior cervical ganglia. Part of these fibers is often bundled to form bilaterally symmetrical nervi conarii. In monkey, probably also parasympathetic fibers reach the organ. The epiphysis of primates contains a small numer of autonomic ganglion cells. The terminals of the autonomic pineal fibers, very often found in the pericapillary space, contain noradrenaline as well as serotonin. Fibers reaching the mammalian epiphysis from the brain are merely aberrant commissural fibers dervied from both the habenular and the caudal commissure. Most probably, the functional innervation of the mammalian epiphysis happens exclusively by way of its autonomic fibers.