Parietal eye
The apex eye, and parietal or organ, is an upturned, central third eye ( parietal bone ) on the parietal bone of the skull that serves as a light sensory organ for the perception of brightness differences in the original vertebrates.
Description
The apex eye is like an ordinary paired eye from the lens, retina and optic nerve, only the iris is missing. The institution is not free, but is hidden under the skin. The connection to the diencephalon is established via the hole in the apex ( Parietalforamen ), which is located between the paired bones of the parietal bone.
The apex eye was in the vertebrates of the Paleozoic, the placoderms ( armored fish), the " jawless " and the tetrapods, amphibians and reptiles, in general formed. In today living ( extant ) animals, for example, several iguana species, the parietal eye in the form of a translucent scale can be seen in the center of the head, the light gets involved in the cranial cavity. Also in the New Zealand tuatara ( Sphenodon punctatus ), a living fossil, and the primitive lampreys the eye is still present. Behavioral observations suggest that the parietal eye at least used to light-dark distinction, may possibly be used for the detection of movement and so to protect them from predators such as birds of prey.
Evolution of the peak eye
Originally, the apex of the eye in accordance with the paired system parietal also appears to have been formed in pairs; its two parts were subsequently converted independently of one another are each homologous to light sensory organs ( Parietalorgan and pineal gland ). Where the left eye was moved forward, it became the Parietalorgan ( Parapinealorgan ) and - eye of the higher vertebrates such as lizards and tuatara, where it moved to the rear, which was originally right eye to the pineal or - eye of anurans was and lampreys. The other eye can be maintained even under loss of function in exceptional cases, as the sea lamprey, or, as in higher vertebrates, the function under exchange - yet often light-sensitive - convert pineal gland. This assures the release of the hormone melatonin for the control of circadian rhythms of day and night activity and regulates specifically in reptiles and some birds body temperature ( thermoregulation ) and food intake. In evolutionary terms, it is the oldest institution to control the daily rhythm.