A parietal eye (third eye, pineal eye) is a part of the

epithalamus The epithalamus (: epithalami) is a posterior (dorsal) segment of the diencephalon. The epithalamus includes the habenular nuclei, the stria medullaris, the anterior and posterior paraventricular nuclei, the posterior commissure, and the pine ...

in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the

pineal gland The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep, sleep patterns following the diurnal c ...

, which regulates

circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...

icity and hormone production for

thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...

. The hole that contains the eye is known as the pineal foramen or parietal foramen, because it is often enclosed by the

parietal bone The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...

s. The parietal eye was discovered by Franz Leydig, in 1872, from work with

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

Discovery

Franz Leydig, a professor of zoology at the

University of Tübingen The University of Tübingen, officially the Eberhard Karl University of Tübingen (; ), is a public research university located in the city of Tübingen, Baden-Württemberg, Germany. The University of Tübingen is one of eleven German Excellenc ...

, dissected four species of European lizards—the slow worm ('' Anguis fragilis'') and three species of '' Lacerta''. in 1872; He found cup-like protrusions under the middles of their brains. He believed the protrusions to be glandular and called them frontal organs (German ''Stirnorgan''). In 1886,

Walter Baldwin Spencer Sir Walter Baldwin Spencer (23 June 1860 – 14 July 1929), commonly referred to as Sir Baldwin Spencer, was a British-Australian Evolutionary biology, evolutionary biologist, anthropology, anthropologist and Ethnology, ethnologist. He is k ...

, an anatomist at the

University of Oxford The University of Oxford is a collegiate university, collegiate research university in Oxford, England. There is evidence of teaching as early as 1096, making it the oldest university in the English-speaking world and the List of oldest un ...

, reported the results of his dissection of 29 species of lizards; he noted the presence of the same structure that Leydig had described. Spencer called it the pineal eye or parietal eye and noticed that it was associated with the parietal foramen and the pineal stalk. In 1918,

Nils Holmgren Nils Frithiof Holmgren (September 28, 1877 – September 7, 1954) was a Swedish zoologist and comparative anatomist. He was professor of zoology at Stockholm University from 1921 to 1944. He took a special interest in the taxonomy of termites. ...

, a Swedish zoologist, found the pineal eye in

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

s and dogfish. He noted that the structure contained sensory cells that looked like the cone cells of the retina, and hypothesised that the pineal eye could be a primitive light-sensing organ ( photoreceptor). The organ has become popularly known as the "third eye".

Presence in various animals

The parietal eye is found in the

tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...

, most lizards, frogs,

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...

s, certain

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s, and

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

s. It is absent in

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s but was present in their closest

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

relatives, the therapsids, suggesting that it was lost during the course of the mammalian evolution due to it being useless in

endotherm An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inst ...

ic animals. It is also absent in the ancestrally endothermic ("warm-blooded")

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

s such as

birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

. The parietal eye is also lost in

ectotherm An ectotherm (), more commonly referred to as a "cold-blooded animal", is an animal in which internal physiological sources of heat, such as blood, are of relatively small or of quite negligible importance in controlling body temperature.Dav ...

ic ("cold-blooded") archosaurs like

crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...

ns, and in

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

s, which may be grouped with archosaurs in

Archelosauria Archelosauria is a clade grouping turtles and archosaurs (birds and crocodilians) and their fossil relatives, to the exclusion of lepidosaurs (the clade containing lizards, snakes and the tuatara). The majority of phylogenetic analyses based on ...

. Despite being lepidosaurs, as lizards and tuatara are,

snake Snakes are elongated limbless reptiles of the suborder Serpentes (). Cladistically squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales much like other members of the group. Many species of snakes have s ...

s lack a parietal eye.

Anatomy

The third eye is much smaller than the main paired eyes; in living species, it is always covered by skin, and is usually not readily visible externally. The parietal eye is a part of the

, which can be divided into two major parts—the

epiphysis An epiphysis (; : epiphyses) is one of the rounded ends or tips of a long bone that ossify from one or more secondary centers of ossification. Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, inc ...

(the pineal organ; or the pineal gland, if it is mostly endocrine) and the parapineal organ (often called the ''parietal eye'' or, if it is photoreceptive, the ''third eye''). The structures arise as a single anterior evagination of the pineal organ or as a separate outgrowth of the roof of the

diencephalon In the human brain, the diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as t ...

; during development, it divides into two bilaterally somewhat symmetric organs, which rotate their location to become a caudal pineal organ and a parapineal organ. In some species, the parietal eye protrudes through the

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

. The parietal eye's way of detecting light differs from the use of

rod cell Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in ...

s and

cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...

s in a normal vertebrate eye. Many of the oldest fossil vertebrates, including ostracoderms,

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

s, crossopterygians, and early

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

s, have in their skulls sockets that appear to have held functional third eyes. The socket remains as a

foramen In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...

between the

s in many living amphibians and reptiles, although it has vanished in birds and mammals.

Lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

s have two parietal eyes, one that developed from the parapineal organ and the other from the pineal organ. These are one behind the other in the centre of the upper surface of the braincase. Because lampreys are among the most primitive of all living vertebrates, it is possible that was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above. '' Saniwa'', an extinct varanid lizard, probably had two parietal eyes, one that developed from the pineal organ and the other from the parapineal organ. ''Saniwa'' is the only known

jawed vertebrate Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned ...

to have both a pineal and a parapineal eye. In most vertebrates, the pineal organ forms the parietal eye, however, in lepidosaurs it is formed from the parapineal organ, which suggests that ''Saniwa'' re-evolved the pineal eye.

Comparative anatomy

The parietal eye of amphibians and reptiles appears relatively far forward in the skull; thus it may be surprising that the human

appears far away from this position, tucked away between the

corpus callosum The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental ...

and

cerebellum The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or eve ...

. Also the

s, in humans, make up a portion of the rear of the skull, far from the eyes. To understand further, note that the parietal bones formed a part of the skull lying between the eyes in

sarcopterygian Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

s and basal amphibians, but have moved further back in higher vertebrates. Likewise in the brain of the frog, the

, from which the pineal stalk arises, appears relatively further forward, as the

cerebral hemisphere The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...

s are smaller but the optic lobes are far more prominent than the human

mesencephalon The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

, which is part of the

brain stem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...

. In humans the optic tract, commissure, and

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...

bridge the substantial distance between eyes and diencephalon. Likewise the pineal stalk of '' Petromyzon'' elongates very considerably during metamorphosis.

Analogs in other species

Crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s at the nauplius stage (first-stage larva) have a single eye atop the head. The eye has a lens and senses the direction of light but can not resolve details. More sophisticated segmented eyes develop later on the sides of their heads, but the initial eye also stays for some time. Thus it is possible to say that, at some stage of development, crustaceans also have a "third eye". Some species, like the

brine shrimp ''Artemia'' is a genus of aquatic crustaceans also known as brine shrimp or ''Sea-Monkeys, sea monkeys''. It is the only genus in the Family (biology), family Artemiidae. The first historical record of the existence of ''Artemia'' dates back to t ...

, retain the primary eye throughout all stages of their life. Most

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s have one or more simple eyes, called ''ocelli'', between their main,

compound eye A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of a cornea, lens (anatomy), lens, and p ...

References