million years ago. The teeth are not replaced. As their teeth wear down, older tuatara have to switch to softer prey such as earthworms, larvae, and slugs, and eventually have to chew their food between smooth jaw bones. It is a common misconception that tuatara lack teeth and instead have sharp projections on the jaw bone, though histology shows that they have enamel and dentine with pulp cavities. The brain of Sphenodon fills only half of the volume of its endocranium. This proportion has actually been used by paleontologists trying to estimate the volume of dinosaur brains based on fossils. However, the proportion of the tuatara endocranium occupied by its brain may not be a very good guide to the same proportion in Mesozoic dinosaurs since modern birds are surviving dinosaurs but have brains which occupy a much greater relative volume in the endocranium. Sensory organs Eyes The eyes can focus independently, and are specialised with three types of photoreceptive cells, all with fine structural characteristics of retinal cone cells used for both day and night vision, and a tapetum lucidum which reflects onto the retina to enhance vision in the dark. There is also a third eyelid on each eye, the nictitating membrane. Five visual opsin genes are present, suggesting good colour vision, possibly even at low light levels. Parietal eye (third eye) The tuatara has a third eye on the top of its head called the parietal eye. It has its own lens, a parietal plug which resembles a cornea, retina with rod-like structures, and degenerated nerve connection to the brain. The parietal eye is visible only in hatchlings, which have a translucent patch at the top centre of the skull. After four to six months, it becomes covered with opaque scales and pigment. Its use is unknown, but it may be useful in absorbing ultraviolet rays to produce vitamin D, as well as to determine light/dark cycles, and help with thermoregulation. Of all extant tetrapods, the parietal eye is most pronounced in the tuatara. It is part of the pineal complex, another part of which is the pineal gland, which in tuatara secretes melatonin at night. Some salamanders have been shown to use their pineal bodies to perceive polarised light, and thus determine the position of the sun, even under cloud cover, aiding navigation. Hearing Together with turtles, the tuatara has the most primitive hearing organs among