HOW ARE TUATARA DIFFERENT FROM LIZARDS?
The tuatara is commonly mistaken for a lizard, due to its superficial reptile-like appearance. The tuatara has a number of structural features which clearly distinguish it from an ordinary lizard: the structure of its skull having two pairs of arches like a crocodilian, the vertebrae, which are hollow at each end; a small projection on the back of each rib; a bony skeleton located in the abdominal wall and jaw teeth, which are not separate structures but instead, serrations of the jaw bone.
Aside from this, the tuatara does not have an ear drum, middle ear and male copulatory organ. Various features of the tuatara's brain and of its locomotion, resemble conditions in amphibians and its heart is far more primitive than that of any other reptile group.
TUATARA HAVE SPECIAL EYES
Some of the tuatara's behavioural features seem to be associated with an altering environment or habitat since the days of its early ancestors.
Now a nocturnal burrower, the tuatara has a duplex retina containing two types of visual cell, presumably enabling the animal alike crocodiles and turtles,
to see at night and during the day. Accompanying this retina is a lave which reflects light back through the sense cells a second time.
THE MYSTERIOUS THIRD EYE
The tuatara has a "third eye" on the top of its head. This special eye contains all the usual features of a regular eye including a lens, retina and nervous connection the brain, but it has been discovered that although it is sensitive to light, it does not contribute to the formation of visual images. Despite this, the tuatara has a structure in its pineal body which would permit light sensitivity even if it is not known to be sensitive to light. The "third" (parietal) eye is only visible in hatchlings, and is assumed to be useful in absorbing ultraviolet rays to produce vitamin D, as well as to determine circadian rhythm (light/dark cycle). Aside from this, it also assists the tuatara with thermoregulation- the regulation of core body temperature.