The tuatara’s famous third eye is also known as a parietal eye. It is located on the top of the head, and it is very small and difficult to notice unless you are looking for it. It has a lens, a retina, and its own nerve connection to the brain.  Scientists aren’t exactly sure why tuataras have this third eye, but they suspect that one use is to help them determine cycles of light and dark. It could also be present to help them absorb the UV light necessary to create vitamin D or to aid in regulating their body temperatures.