From Star Trek II: The Wrath of Khan
[Khan, about to put Ceti Eels in Terrell and Chekov's ears
You see, their young enter through the ears and wrap themselves around the cerebral cortex. This has the effect of rendering the victim extremely susceptible to suggestion. Later, as they grow, follows madness and death.
Khan, listen to me...
These are pets, of course. Not quite domesticated.