A parietal eye, also known as a parietal organ or third-eye is a part of the epithalamus present in some animal species. The eye may be photoreceptive and is usually associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation.
The lizard-like reptile tuatara
has a "well-developed parietal eye, with small lens and retina". Parietal eyes are also found in lizards
, as well as some species of fish
, such as tuna
and pelagic sharks
, where it is visible as a light-sensitive spot on top of their head. A poorly developed version, often called the parapineal gland, occurs in salamanders
. In birds
the parietal organ (but not the pineal gland) is absent.
The parietal eye is a part of the epithalamus
, which can be divided into two major parts; the epiphysis
(the pineal organ, or pineal gland
if mostly endocrine) and the parietal organ (often called the parietal eye, or third eye if it is photoreceptive). It arises as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon
. In some species, it protrudes through the skull
. The parietal eye uses a different biochemical method of detecting light than rod cells
or cone cells
in a normal vertebrate eye.