Parietal eye

A parietal eye, also known as a third eye or pineal eye, is an eye located at the top of the head. Reptiles, amphibians have this eye. The third eye is always smaller than the main paired eyes.[1] In living species, it is always covered by skin and is not visible externally. [1]

The parietal eye of a juvenile bullfrog

References
  1. Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 471–473
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