The reptile with a fully functioning third eye
The tuatara, a rare reptile native to New Zealand, boasts a fully formed third eye on the top of its head. Known as the parietal eye, it comes complete with a lens, retina, and nerve endings. While it is covered by scales in adults and cannot form images, it is highly sensitive to light and helps the tuatara regulate its body temperature and track seasonal cycles.
The Anatomy of a Third Eye
Positioned on the crown of the tuatara's skull lies one of the most remarkable sensory structures in the animal kingdom: a fully formed parietal eye. Often referred to as a pineal eye or 'third eye,' this organ is far more sophisticated than a simple cluster of photosensitive cells. Anatomically, it exhibits many of the same essential components found in standard lateral eyes, including a rudimentary cornea, a cellular lens, a retina lined with photoreceptors, and nerve tracts connecting it directly to the brain.
The eye rests within a small opening in the skull called the parietal foramen. From this position, its dedicated parietal nerve connects to the epithalamus and pineal complex in the brain. Unlike the animal's two primary lateral eyes, the parietal eye lacks an iris, eyelids, and the muscular apparatus required for movement or focusing. It cannot dart around or resolve detailed shapes, serving instead as a fixed, upward-facing optical sensor designed specifically to measure ambient light.
Visibility and the Developing Scaled Window
The visibility of the parietal eye changes dramatically over the course of a tuatara's life. When a young tuatara hatches, the third eye is directly visible on the top of its head. At this early life stage, the organ sits beneath a clear, translucent patch of skin that functions as a small skylight, allowing unobstructed light to strike the underlying lens and retina directly.
As the tuatara matures over its first several months, this transparent window gradually thickens and becomes covered with opaque scales and pigment matching the surrounding skin. By adulthood, the third eye is hidden from external view and can no longer be seen with the naked eye. Despite this epidermal cover, the underlying photoreceptive structure remains intact throughout the animal's life, as sufficient wavelengths of light continue to penetrate through the skin and skull opening.