Ceti eel 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 orbit al shift of Ceti Alpha V following the explosion of Ceti Alpha VI . Ceti eels incubated their larva e within the plates of their jointed carapace . Upon emergence , the eel larvae 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 larvae matured , the subject suffered from madness and eventual death . Twenty of Khan Noonien Singh ' s followers were killed by Ceti eels in this fashion , including his " beloved wife . " Ceti eels could also be eaten , and were being sold at an Orion outpost on Qo ' noS in the 2250s . ( ) In 2285 , Khan used Ceti eels to gain the cooperation of Captain Clark Terrell and Commander Pavel Chekov of the . 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 . ( ) Background information . 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 " " . ( " " , 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 . The name " Ceti eel " is never referenced during the movie . 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 , 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 . " " ( ) 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 . Roberto Orci and Alex Kurtzman , screenwriters of the film , created the Centaurian slug as an homage to the Ceti eel .