Ouroboros

The Ouroboros was the symbol of a snake or worm (PROSE: Falls the Shadow) eating its own tail, representing all of time, i.e. eternity.

Demesne Furze, the Great White Worm/Wyrm, was worshipped by the Romans and served as the origin of the symbol on Earth. (AUDIO: Trail of the White Worm)

The signature emblem of the Corsair was an Ouroboros. According to the Eleventh Doctor, he had the image tattooed on his body after each regeneration, even when he regenerated as a female. Without it, the Corsair didn't feel like him/herself. (TV: The Doctor's Wife)

The Oroborus entity (TV: Oroborus) and the Oroborous Worm (PROSE: The Mary-Sue Extrusion) were named after the symbol.