Analogs in Other Species
Crustaceans have a single eye atop of the head as a nauplius (first stage larva). This eye has a lens and senses direction of light but cannot see images. Later, more sophisticated segmented eyes develop on sides of the head but the initial eye stays for some time. Hence it is possible to say that at some stage of development crustaceans have the "third eye" as well. Some species like Artemia salina, the brine shrimp, retain the primary eye, being three-eyed in the adult stage.
Many arthropods have simple eyes, frequently between the main eyes, called ocelli.
Read more about this topic: Parietal Eye
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