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A Centaurian slug was a small parasitic animal which entered a humanoid host through the mouth, eventually traveling through the body to latch onto the brain stem, where it released a toxin forcing the host to answer any and all questions posed.
In 2258, the Romulan Nero used a Centaurian slug in his interrogation of Captain Christopher Pike in a successful attempt to learn the subspace frequencies of Starfleet's border protection grids surrounding Earth. (Star Trek)
Screenwriters Roberto Orci and Alex Kurtzman invented the Centaurian slug as a homage to the Ceti eel from Star Trek II: The Wrath of Khan. They considered having Nero actually use a Ceti eel, but "wanted 'homage' instead of a direct lift. [1] Deleted scenes from the film establish that Nero got the slugs from the Klingons on Rura Penthe, who used them to interrogate prisoners, and include dialogue stating a different mechanism for their effects: when placed into dark spaces, they furiously attempt to burrow their way out.