The Ceti eel was a burrowing desert animal native to the planet Ceti Alpha V. Capable of surviving extremes in its environment, the Ceti eel was the only known indigenous creature that survived the orbital shift of Ceti Alpha V following the explosion of Ceti Alpha VI.
Ceti eels incubated their larvae within the plates of their jointed carapace. Once emerged, an eel larva could enter the ear of a larger animal, where it wrapped itself around the cerebral cortex. The effect caused the host extreme pain and rendered them extremely susceptible to outside suggestion. Over time, as the larva matured, the subject suffered from madness and eventual death.
In 2285, Khan used Ceti eels to gain the cooperation of Captain Clark Terrell and Commander Pavel Chekov of the USS Reliant. However, both men fought against the eels' control of them; Terrell even committed suicide (vaporizing himself with his own phaser), rather than execute Admiral James T. Kirk as Khan had ordered him to do. After Terrell's death, Chekov collapsed in agony, whereupon the eel that had infected him – possibly sensing the death of its "brother" – crawled out of Chekov's ear and fell to the ground, where it was quickly vaporized by Kirk. (Star Trek II: The Wrath of Khan)
Gene Roddenberry took exception to Kirk killing the eel. Roddenberry commented: "Remember when the eel came out of Chekov's ear? What did Kirk do? He had a look of disgust on his face and grabbed his phaser and went 'zap'. Now, how dare he destroy a lifeform that had never been seen before. It needs studying. They had [Kirk] act like a old woman trampling on a tarantula. Now that's not the Kirk we built up for three years". (Captains' Logs: The Unauthorized Complete Trek Voyages, page 118)
As part of its campaign to protect its exclusive right to market Star Trek-related items, Paramount sought and obtained a design patent from the US Patent and Trademark Office for the Ceti eel. Called an "ornamental design for a toy animal", it was issued in 1984.
The name "Ceti eel" is never referenced during the movie Star Trek II: The Wrath of Khan. Khan describes the animal without naming it, and McCoy doesn't recognize it when it exits Chekov's ear, though Chekov had told Kirk and McCoy of it when they were on Regula I.
In Evan Carlos Somers' original draft for DS9: "Melora", Fallit Kot (then named Megzie Del) – who had served an eight-year prison sentence due to Quark – threatened to kill Quark by using a type of Ceti eel, planning to insert it into Quark's ears. According to Somers, these creatures would have been "the Ceti eels introduced in Star Trek II: The Wrath of Khan, but this variety didn't cause mental vulnerability for mind control [....] I thought that would be a fitting, torturous end for someone who had caused eight years of suffering." (The Official Star Trek: Deep Space Nine Magazine issue 6, p. 62) In the first draft script of "Melora", the creatures were described as "tiny lizards" and were kept in "amber liquid" which was in "a transparent tube". They were described as "a cousin of the Ceti Alphan Cortical Parasite" and were said to "enter through the ears causing instant madness", though the time they took to destroy the victim's mind was also referred to as potentially variable (from a couple of hours to a week), depending on the wishes of their owner. The idea of using the creatures in "Melora" was later altogether abandoned, in rewrites.