Three-eyed lizards are not uncommon. Four-eyed ones are a novelty

Spot the eye in the back of the head UNLIKE invertebrates, most of which have at least four eyes, vertebrates usually have only two. Yet, there are exceptions. Some fish, amphibians and reptiles have a third, so-called parietal, eye. This organ, a modification of a brain structure called the parapineal gland (itself an associate of the better-known pineal gland, which regulates an animal's body clock), is usually covered with skin and sits on the top of the head, as the picture below shows. It h...