Burrowing creatures of Ceti Alpha V whose young enter a victim's ear and wrap around the cerebral cortex, leaving the host open to suggestion.
Ceti eels were among the few creatures to survive on Ceti Alpha V after the planet was devastated. Adults carried their young within their bodies. The young crawled into a victim's ear and wrapped themselves around the brain, causing great pain and making the host highly suggestible, which eventually led to madness and death.
In Star Trek II: The Wrath of Khan, Khan blamed the eels for the deaths of many of his followers, including his wife. He placed them in Chekov and Captain Terrell to control them. The creature later left Chekov's ear, freeing him.