Parietal eye

A parietal eye is 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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