Parietal eye - AbsoluteAstronomy.com

A parietal eye, also known as a parietal organ or third-eye or (often but less correctly) pineal eye, is a part of the epithalamus

Epithalamus

The epithalamus is a dorsal posterior segment of the diencephalon which includes the habenula, the stria medullaris and the pineal body...

present in some animal species. The eye may be photoreceptive and is usually associated with the pineal gland

Pineal gland

The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...

, regulating circadian rhythm

Circadian rhythm

A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

icity and hormone production for thermoregulation

Thermoregulation

Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

Function

The lizard-like reptile tuatara

Tuatara

The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...

has a "well-developed parietal eye, with small lens and retina". Parietal eyes are also found in lizard

Lizard

Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s, frog

Frog

Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

s and lamprey

Lamprey

Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, as well as some species of fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, such as tuna

Tuna

Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

and pelagic shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, 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 salamander

Salamander

Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s. In bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s and mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s the parietal organ (but not the pineal gland) is absent.

Anatomy

The parietal eye is a part of the epithalamus

Epithalamus

The epithalamus is a dorsal posterior segment of the diencephalon which includes the habenula, the stria medullaris and the pineal body...

, which can be divided into two major parts; the epiphysis

Epiphysis

The epiphysis is the rounded end of a long bone, at its joint with adjacent bone. Between the epiphysis and diaphysis lies the metaphysis, including the epiphyseal plate...

(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

Diencephalon

The diencephalon is the region of the vertebrate neural tube which gives rise to posterior forebrain structures. In development, the forebrain develops from the prosencephalon, the most anterior vesicle of the neural tube which later forms both the diencephalon and the...

. In some species, it protrudes through the skull

Skull

The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

. The parietal eye uses a different biochemical method of detecting light than rod cell

Rod cell

Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

s or cone cell

Cone cell

Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

s in a normal vertebrate eye.

Many of the oldest fossil vertebrates, including ostracoderm

Ostracoderm

Ostracoderms are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe...

s, placoderms, crossopterygians, and even early tetrapod

Tetrapod

Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

s, had a socket in the skull that appears to have held a functional third eye. This socket remains as a foramen between the parietal bone

Parietal bone

The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....

s even in many living amphibians and reptiles, although it has vanished in birds and mammals. The third eye, where present, is always much smaller than the main paired eyes, and, in living species, it is always covered by skin, and is usually not readily visible externally.

Among fish, lamprey

Lamprey

Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s retain two functional "third" eyes, one developed from the parietal gland, and the other from the pineal gland. These are located one behind the other in the centre of the upper surface of the braincase. Because lampreys are amongst the most primitive of all living vertebrates, it is possible that this was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above.

Comparative anatomy

As shown in the accompanying figures, the parietal eye of amphibians and reptiles appears relatively far forward in the skull; thus it may be surprising that the human pineal gland

Pineal gland

appears far away from this position, tucked away between the corpus callosum

Corpus callosum

The corpus callosum , also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication...

and cerebellum

Cerebellum

The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

. Also the parietal bone

Parietal bone

s, in humans, make up a portion of the rear of the skull, far from the eyes. To understand this, 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. Likewise, in the brain of the frog, the diencephalon

Diencephalon

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

Cerebral hemisphere

A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...

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

Mesencephalon

The midbrain or mesencephalon is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal , and temperature regulation....

, which is part of the brain stem

Brain stem

In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

. In humans the optic tract

Optic tract

The optic tract is a part of the visual system in the brain.It is a continuation of the optic nerve and runs from the optic chiasm to the lateral geniculate nucleus....

, commissure, and optic nerve

Optic nerve

The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

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 form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s have a single eye atop of the head as a nauplius (first stage larva). This eye has a lens and senses direction of light but cannot see images. Later, more sophisticated segmented eyes develop on sides of the head but the initial eye stays for some time. Hence it is possible to say that at some stage of development crustaceans have the "third eye" as well. Some species like Artemia salina
Artemia salina
Artemia salina is a species of brine shrimp – aquatic crustaceans that are more closely related to Triops and cladocerans than to true shrimp. It is a very old species that does not appear to have changed in .-Description:...

, the brine shrimp, retain the primary eye, being three-eyed in the adult stage.

Many arthropod

Arthropod

An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s have simple eyes, frequently between the main eyes, called ocelli.

Function

Anatomy

Comparative anatomy

Analogs in other species

See also