What animals possess a median eye?

Vision is the dominant sense in most reptiles, and their eyes are similar to those of amphibians. Upper and lower  eyelids,  a  nictitating membrane, and a blood sinus protect and cleanse the surface of the eye. In snakes and some lizards, the upper and lower eyelids fuse in the embryo to form a protective window of clear skin called the spectacle. Some reptiles possess a median (parietal) eye that develops from outgrowths of the roof of the optic tectum (midbrain). In the tuatara, the median eye is complete with a lens, nerve, and retina. In other reptiles, the median eye is less developed. Skin covers median eyes, which probably cannot form images. They can, however, differentiate light and dark periods and are used in orientation to the sun. Fig. 2.28


Figure 2.28



Median Eye of Reptiles. (a) Median eye in a reptile and its relationship to the lateral eyes (dorsal view). (b) Sagittarius section of the median eye.


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