Concept
A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation. The hole in the head which contains the eye is known as a pineal foramen or parietal foramen, since it is often enclosed by the parietal bones. The parietal eye was discovered by Franz Leydig in 1872 from lizards. Franz Leydig, professor of zoology at the University of Tübingen, dissected four species of European lizards, consisting of the slow worm (Anguis fragilis) and three species of Lacerta, in 1872. He found the cup-like protrusions under the middle portion of the brains. He believed them to be glandular in nature and gave the name frontal organ (German stirnorgan). In 1886, University of Oxford anatomist Walter Baldwin Spencer reported his dissection results of 29 species of lizards and noted the presence of the same structure Leydig had described. He called them pineal eye or parietal eye, as they were associated with the parietal foramen and the pineal stalk. In 1918, Swedish zoologist Nils Holmgren found the pineal eye in frogs and dogfish. He noted that the structure contained sensory cells that looked like the cone cells of the retina. He hypothesised that this pineal eye could be a primitive light-sensor organ (photoreceptor), and became popularly known as the third eye. The parietal eye is found in the tuatara, most lizards, frogs, salamanders, certain bony fish, sharks, and lampreys. It is absent in mammals, but was present in their closest extinct relatives, the therapsids, suggesting it was lost during the course of the mammalian evolution due to it being useless in endothermic animals. It is also absent in the ancestrally endothermic ("warm-blooded") archosaurs such as birds. The parietal eye is also lost in ectothermic ("cold-blooded") archosaurs like crocodilians, and in turtles, which may be grouped with archosaurs in Archelosauria.
