Star Trek's alien race the Trill were physically altered after their first appearance in Star Trek: The Next Generation for a surprisingly shallow reason. The Trill were introduced in the TNG episode "The Host," where it was revealed they are a symbiotic species; they consist of both a humanoid host and a small, slug-like life form that resides in the host body's abdomen. Most of the personality and consciousness is controlled by the symbiont being, and when the host body dies the symbiont is transferred to a new body, retaining its former hosts' memories.
It was decided that a Trill would be a main character on the spinoff Star Trek: Deep Space Nine, this time in the form of Lieutenant Jadzia Dax, played by former model Terry Farrell. While the Trills' initial appearance on Star Trek: The Next Generation featured pronounced foreheads ridges on the host, that look was given a massive overhaul before Deep Space Nine began production. Executive Producer Rick Berman deemed Farrell too attractive to cover her face in heavy prosthetics, so a completely new look for the Trill was conceived, one that has endured ever since.
How DS9 Casting Terry Farrell As Dax Changed The Trill
While Terry Farrell had a successful modeling career and some small acting roles prior to Star Trek: Deep Space Nine, her venture into the final frontier was her first regular leading role on television. Admittedly cast for her conventional beauty, Farrell's Jadzia Dax would go on to become a much-beloved member of the Star Trek family and the defining member of the Trill race. And while Rick Berman's reasoning for altering the look of the Trill may be suspect, it's hard to argue with the eventual results.
During the development of Deep Space Nine, the Trill head ridges were ditched in favor of two rows of dark spots running down both sides of their bodies. This design was actually borrowed from the Star Trek: The Next Generation episode "The Perfect Mate," where it was featured on the Kriosian metamorph known as Kamala, played by future X-Men star Famke Janssen. This design was the best of both worlds; while it still clearly indicated these were aliens, it allowed the actors greater freedom to emote without the hindrance of heavy facial makeup.
Every Trill In Star Trek Besides Jadzia Dax
While the Trill are largely associated with Star Trek: Deep Space Nine, they've had a fairly consistent presence across the franchise. Ambassador Odan was the original Trill character on Star Trek: The Next Generation, though he has little in common with the Trill characters that would follow. Apart from Jadzia, the most prominent Trill character in the franchise is Ezri Dax, who took over the Dax symbiont after Jadzia Dax's death in DS9, murdered by the crazed Cardassian Gul Dukat.
More modern Star Trek has included the Trill in both conventional and innovative ways. Star Trek: Lower Decks and Star Trek Prodigy feature the fairly traditional Trill characters Barnes and Asencia, respectively, both young female Starfleet officers much like Dax. Star Trek: Discovery's third season introduced Gray Tal, a young male Trill who passed on his symbiont to his partner, the human Adira, after a seemingly fatal accident. The story of Gray and Adira has opened up the mythology of the Trill in some surprising ways, and the race's future in Star Trek could go virtually anywhere from here.