The tuatara has a third eye on the top of its head called the parietal eye. It has its own lens, cornea, retina with rod-like structures, and degenerated nerve connection to the brain, suggesting it evolved from a real eye. The parietal eye is only visible in hatchlings, which have a translucent patch at the top centre of the skull.
In most iguanas, the parietal eye shows up as either a dark spot or an opalescent spot. In these photos of Bailey and Mary, you can see that Bailey has a dark parietal eye while Mary has the opalescent kind.
【中 文 維 基1】
【英 文 維 基1】
【英 文 維 基2】
+ 扇貝迷人的眼睛 +
For one, about sixty primitive tiny bright blue eyes eyes reside in rows along a scallop’s mantle edge to detect motion, light and dark. A scallop can easily regrow any lost or injured eyes. Although these eyes may or may not produce clear images, the ability to sense an object moving with the speed of one of the scallop’s predators allows the scallop to save its skin (or to be scientifically correct, its shells) by either shutting immediately or swimming away.
As far as I am aware, no other living vertebrate animal has three eyes. Some invertebrates have more than two eyes - scallops, for instance.