The pineal gland, also known as the pineal body, conarium or epiphysis cerebri, is a small endocrine gland in the vertebrate brain. The shape of the gland resembles a pine cone, hence its name. The pineal gland is located in the epithalamus, near the center of the brain, between the two hemispheres, tucked in a groove where the two halves of the thalamus join. The pineal gland produces melatonin, a serotonin derived hormone which modulates sleep patterns in both circadian and seasonal cycles. Nearly all vertebrate species possess a pineal gland. The most important exception is the hagfish, which is often thought of as the most primitive extant vertebrate. Even in the hagfish, however, there may be a "pineal equivalent" structure in the dorsal diencephalon. The lancelet Branchiostoma lanceolatum, the nearest existing relative to vertebrates, also lacks a recognizable pineal gland. The lamprey (considered almost as primitive as the hagfish), however, does possess one. A few more developed vertebrates lost pineal glands over the course of their evolution. The results of various scientific research in evolutionary biology, comparative neuroanatomy and neurophysiology, have explained the phylogeny of the pineal gland in different vertebrate species. From the point of view of biological evolution, the pineal gland represents a kind of atrophied photoreceptor. In the epithalamus of some species of amphibians and reptiles, it is linked to a vestigial organ, known as the parietal eye which is also called the third eye. René Descartes believed the pineal gland to be the "principal seat of the soul" (a mystical concept). Academic philosophy among his contemporaries considered the pineal gland as a neuroanatomical structure without special metaphysical qualities; science studied it as one endocrine gland among many. However, the pineal gland continues to have an exalted status in the realm of pseudoscience.

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