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Joseph Ruskin (Suliban Doctor) appeared in every Star Trek television series except Star Trek: The Next Generation (1987) (although he did appear in Star Trek: Insurrection (1998) starring the TNG crew). With the exception of Majel Barrett, who has appeared in every Star Trek series, he was the only actor to appear in all four of the series in question. Furthermore, given that Barrett only provided the computer voice in Star Trek: Voyager (1995) and Star Trek: Enterprise (2001), Ruskin was the only actor to appear on screen in all four series mentioned above. Along with Majel Barrett, Clint Howard, Jack Donner, and Vince Deadrick, he was one of only five actors to appear in both Star Trek (1966) and Star Trek: Enterprise (2001). He, Barrett and Howard also appeared in Star Trek: Deep Space Nine (1993). Jonathan Frakes also appears in four series (all but the original series). Ruskin also worked on two Star Trek video games, lending his voice to Master Si'tann in Star Trek: Hidden Evil (1999) and to Admiral Nolotai and Vulcan Master N'Kal in Star Trek: Away Team (2001).
Hoshi's insult to T'Pol, "Ponfo mirann", loosely translates to "Go to hell".
Three of the prominent Starfleet officers are called Admiral Forrest (Vaughn Armstrong), Admiral Leonard (Jim Beaver) and Commander Williams (Jim Fitzpatrick). These names are an homage to the three lead actors from Star Trek (1966): DeForest Kelley, Leonard Nimoy and William Shatner. One of the Vulcans from the High Command is called "Tos", which is a reference to TOS, as The Original Series is often called.
Ensign Mayweather, when discussing the transporters, says "from what I'm told, [the Captain] wouldn't even put his dog through this thing." The dog in question is Captain Archer's beagle, Porthos. Years later, in the film reboot Star Trek (2009), a surly Montgomery Scott describes his ongoing punishment by Starfleet as a consequence of attempting to prove a transporter theory correct by transporting "Admiral Archer's prize beagle," who was transported and never rematerialized.
Up to this series, the pilot of each Star Trek series after the first included a cameo of a character from the previous one. However, because "Enterprise" takes place 100 years before Star Trek (1966), the cameo was not of a series character, but of Zefram Cochrane (James Cromwell), who appeared in Star Trek: First Contact (1996) and in Metamorphosis (1967) portrayed by Glenn Corbett.
Vaughn Armstrong, a longtime Trek actor, appears here as a human for the first time, bringing his total Trek appearances to eleven - as 9 different characters, and 7 different species (human, Cardassian, Klingon, Romulan, Borg, Vidiian, Hirogen).