Throughout all of the movies, TV shows, books and more, hundreds of species have been introduced throughout the Star Trek universe. Some of them only appear once in a single episode, if at all sometimes as a species occasionally only gets mentioned in conversation. Then there are species that are integral to the ongoing story, like the Klingons or the Romulans.
Deep Space Nine brought in a variety of new species such as the Changelings and the Jem'Hadar. But DS9 also made good use out of a few secondary species from other series, giving them a new life and more backstory. The Ferengi are a great example of this as they were introduced on The Next Generation but came into their own on Deep Space Nine.
Trills are another such species. First appearing in a fourth season episode of The Next Generation, the species went on to become key to Deep Space Nine. Lieutenant Commander Jadzia Dax served as the station's Science Officer under Captain Benjamin Sisko.
The species is unique because they are, in some cases, actually two beings joined together. The process and it's impacts make Trills one of the most interesting species in the galaxy.
Ian Goodwillie is a freelance writer based out of Saskatoon, SK, Canada. He has a BA in English Literature from the University of Saskatchewan and completed the Writing Program at Vancouver Film School. More importantly, Ian is also a wrestling fan, comic book reader, video game player and photographer. He loves to write and writes about what he loves. Ian's also an unrepentant, unapologetic Cougar Town fan, a show he will defend until the day he dies.