Parietal eye

The eye is at the top of the head; is photoreceptive; and is associated with the pineal gland, which regulates circadian rhythmicity and hormone production for thermoregulation.

[2] In 1886, Walter Baldwin Spencer, an anatomist at the University of Oxford, reported the results of his dissection of 29 species of lizards; he noted the presence of the same structure that Leydig had described.

[5] The parietal eye is found in the tuatara, most lizards, frogs, salamanders, certain bony fish, sharks, and lampreys.

[7][8][9] It is absent in mammals but was present in their closest extinct relatives, the therapsids, suggesting that it was lost during the course of the mammalian evolution due to it being useless in endothermic animals.

[17] Many of the oldest fossil vertebrates, including ostracoderms, placoderms, crossopterygians, and early tetrapods, have in their skulls sockets that appear to have held functional third eyes.

The socket remains as a foramen between the parietal bones in many living amphibians and reptiles, although it has vanished in birds and mammals.

To understand further, note that the parietal bones formed a part of the skull lying between the eyes in sarcopterygians and basal amphibians, but have moved further back in higher vertebrates.

[21] Crustaceans at the nauplius stage (first-stage larva) have a single eye atop the head.