What do tuatara use their third eye for?
A tuatara's "third eye" is visible at the top of the head between its other two eyes but only when it is very young.
As the animal gets older the transparent membrane of scales that cover the eye become opaque and the eye
is no longer visible. This small mass of nerve tissue has a glandular role and is sometimes known as the "pineal eye" or pineal gland.
This gland regulates daily body rhythms, most notably the day/night cycle, in all vertebrates including humans. But it is closer to the surface in tuatara and some lizards and birds.
It secretes the ancient hormone melatonin which plays an important role in sleep. One theory has it the "eye" evolved in bottom-dwelling fish to alert them to danger from above.
But this function was progressively lost as better binocular depth perception developed with paired eyes.
What we see in tuatara is just an ancestral remnant like our appendix.
French philosopher Descartes decided that the pineal gland in humans, situated between the two hemispheres of the brain, was the seat of the human soul, the location of what we call the mind.
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