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The Barreleye fish has a see-through head, but that's not the weirdest part.  Its eyes are in fact the two green domes sitting towards the back of its head.  Each retina is packed with cells that are super-sensitive to light.  The fish uses them to look upwards, searching for the faint silhouettes of prey or predators far above.

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Join Nat Geo WILD as we showcase some of the strangest, most shocking animal appendages on the planet, and uncover their evolutionary advantages.