Trill

The Trill are a symbiotic species in the Star Trek universe, first introduced in the ST:TNG episode "The Host", and most elaborated in ST:DS9 when a Trill (Dax) was a regular feature.

The homeworld (also named Trill) holds both a humanoid race (the hosts) and the symbiont species, which looks something like a worm. When joined, the symbiont rests in the humanoid abdomen. It carries the memories of its previous hosts, and the personalities of the previous hosts are not wholly obliterated, but incorporated into the new symbiont.

The Trill use the host's name as the first name, and the symbiont's name as the last name; hence, for Jadzia Dax, Jadzia is the host's name, and Dax is the symbiont's name. When Dax was joined with a different humanoid host, its name became Ezri Dax. (See naming conventions.)

For another work dealing with symbiotic/parasitic relationships, see Octavia Butler's "Bloodchild".

In ST:DS9 and ST:TNG, the show's creators used the Trill's body-switching to raise issues of same-sex sexuality.