What is the name of the tuatara's light-sensitive 'third eye'?

The tuatara’s light-sensitive “third eye” is called the parietal eye. It sits on the top of the head and is part of the animal’s pineal complex, alongside the pineal gland.

The parietal eye has a lens-like structure, a retina containing rod-like photoreceptors, and a nerve connection to the brain. It does not produce ordinary detailed images; instead, it detects light levels and may help regulate circadian rhythms, seasonal cycles, and body temperature.

The structure is most visible in young tuatara, whose translucent head scales reveal a small patch. After roughly four to six months, scales and pigment cover it. “Pineal eye” and “third eye” are legitimate alternative names, but “parietal eye” is the standard anatomical term and the clearest canonical quiz answer.

The tuatara is especially notable because its parietal eye is unusually well developed. Related structures occur in some lizards, frogs, salamanders, fish, and lampreys, but they are absent in modern mammals and birds.