The Trill Problem

TNG: "The Host" showed us a symbiosis of two species, one humanoid host and one Trill symbiont implanted into the host. I didn't like the episode for several reasons. First of all, it is a recurring theme in TNG that the Enterprise has guests who don't reveal their true nature. It is just not credible that friendly guests always remain silent about their (potentially hazardous) peculiarities, and that no scans indicate their nature either. Secondly, no humanoid would be keen on being subdued to the Trill symbiont the way it was shown in "The Host". Thirdly, Beverly, who has fallen in love with Odan and is suddenly confronted with Odan in Riker's body and finally a female host, seems to come to the conclusion that heterosexuality is something that needs to be overcome in favor of something more universal or more inclusive. I firmly disagree, because sexual reproduction has obviously been the foundation of all human life (and of most other species likewise) for millions of years. Tolerance for gay and lesbian relationships is one thing, but Beverly would essentially deny her own identity if it was her goal that it doesn't have to matter who or what she is attracted to.

Anyway, my criticism of the episode itself is not what this brief article is about. The issue is that the producers of DS9 liked the TNG Trill episode obviously more than I did, and they decided to include a Trill character, namely Jadzia Dax, in the new series. It is usually pleasant to see such a kind of inter-series continuity, but in the case of the Trills it turned out a disaster. DS9 Trills are completely different than TNG Trills in almost all respects - although the people who created Dax probably thought they would make only a few insignificant changes to the species' outer appearance. The real-world reason for the decision for the change was probably that the make-up with the forehead ridges didn't look good on Terry Farrell, according to the DS9 Companion.

Dax with TNG look (DS9 test shot)

Dax with different spots (DS9 test shot)

The Star Trek Encyclopedia almost completely dismisses the problem and only comments on the fact that Odan doesn't want to use the transporter: "Certain Trill symbionts can be severely damaged by beaming, which is why Odan insisted on using the shuttlecraft in 'The Host' (TNG). Other Trills, including Jadzia Dax, don't seem to have the same problem."

Evidence

If we compare the biological and other characteristics of DS9 Trills (Jadzia and Ezri Dax plus several more of her species) and TNG Trills (at least the two that were in "The Host"), numerous significant differences become evident:

In TNG, Odan and the female host shown at the end of the episode have a quite different outer appearance than all Trills shown in DS9, including Jadzia and Ezri Dax and dozens more. Neither was any TNG Trill with forehead bones shown in DS9, nor was there any mention that there are two types of Trill hosts in any of the two shows. It is very unlikely that there two different humanoid species have evolved on the same planet, and both are able and willing to share a symbiosis with even a third intelligent species on that planet. The DS9 Trills, moreover, with their characteristic stain pattern look much the same as the Kriosians (TNG: "The Perfect Mate"), which remains an unexplained coincidence.

Odan in TNG: "The Host"

Kareel in TNG: "The Host"

Jadzia Dax in DS9: "A Man Alone"

Ezri Dax in DS9: "Afterimage"

Kriosian make-up in TNG: "The Perfect Mate"

Kriosian make-up in ENT: "Precious Cargo"

Note It appears that the Kriosian make-up for Kaitaama in ENT: "Precious Cargo" was modified to smaller spots, in order to be not quite as evocative of the DS9 Trill look.

The symbiont in the TNG episode looks only remotely similar to the one in DS9: "Invasive Procedures". I wouldn't expect a lifeform that can only survive with a host to exist in a variety of shapes and sub-types. There doesn't seem to exist a difference between male and female symbionts either.

TNG symbiont in TNG: "The Host"

DS9 symbiont in DS9: "Invasive Procedures"

What is the nature of the Trill species? In TNG we learn that the symbiont totally controls the host's body and mind. It is a key message of the episode that the prominent Ambassador Odan (who falls in love with Beverly) consists of the symbiont only, while his hosts merely serve as containers. The host bodies change more than once. This is impressively demonstrated when Odan takes over Riker's body after the death of his preceding host, and a female body some time later. It is no surprise that Bev is embarrassed about that. Fortunately, Will's memory is not wiped out by the symbiont. However, as I already criticized above, it is anything but credible that humanoids sacrifice themselves by becoming a slave for the all-dominating TNG Trill. The DS9 Trills couldn't be more different. Once the symbiont is implanted into the host's body, the two individuals form a joined species and therefore a new person. This person does not only take over the memories of the symbiont, the current and all of the previous hosts, but also parts of their personalities. Thus the new humanoid host does not really lose their personality, but even feels as if it were extended by the symbiosis (which is why many DS9 Trills are eager to get one of the rare symbionts). This fact plays a decisive role in the DS9 episode "Dax" when Jadzia is accused of murder. Sisko, who is in charge of defending her, points out that not Jadzia, the young woman, would have to be accused, but Curzon, the old man who was holding the symbiont the time when the murder occurred. So if Curzon should really be guilty, the court would not have the right to punish his successor who is in fact a different person. (Curzon) Dax eventually turns out to be innocent anyway.

Note DS9: "Facets" establishes that the single Trill individualities and memories can be isolated in the zhian'tara ceremony and transferred to other individuals, even aliens. This telepathic dimension is in contrast to everything else we know about the DS9 Trills and their nature of being inseparably joined. Also, their telepathic abilities would even surpass those of the Vulcans by far, if it were that simple. And if it were true, it would be no evidence either that the TNG Trills may be the same as DS9 Trills. On the contrary, telepathic transfer as in DS9: "Facets" may have been an option in TNG: "The Host", but was not taken into consideration.

Side note In a deleted scene in TNG: "The Host", Odan, in Riker's body, comes to the officers' poker game. He reaffirms that the symbiont totally controls the host's body and that he has no idea of poker, but he concedes that he somehow felt compelled to go there.

In TNG, the symbiont's name is Odan. Unlike the DS9 Trills he does not seem to have a first name that denotes the host. Otherwise Beverly, who was in love with him, would definitely have used it. In addition, a first name would have stressed Odan's claim that he is the son of the Odan who led the negotiations with Peliar Zel years ago (just like Jadzia Dax may have claimed she was the daughter of Curzon Dax, had their been a need to keep her true nature a secret). If there had been one, he would definitely have used it himself. Therefore we can assume for sure that TNG Trills don't have a first name at all. The missing first name matches with the fact that the host is totally suppressed by the TNG symbiont.
We do not know for sure why Odan of TNG cannot be beamed, but the impression is created that this is a general peculiarity of Trills. Beverly surmises: "It would have damaged the symbiont." Dax, on the other hand, never has any problems using the transporter, and not any other DS9 Trill either.
In TNG, it does not appear to be a problem to implant the Trill symbiont into another host, although Beverly has never performed this procedure before and it is not sure whether a human host (Riker) would be compatible with the symbiont. Moreover, it is possible to insert and remove the symbiont without damage to host. In DS9 there is a 93 hours time limit of the symbiosis. If the Trill is removed after this time has expired, the host will probably die, since his physiology has become dependent on the symbiont. Dax is really lucky to have Julian to save her in the DS9 episode "Invasive Procedures". He is not only affected to her, but obviously an excellent surgeon too.
Beverly is believed to be one of the most qualified medical officers of Starfleet. Maybe we have to revise our opinion on her after the TNG episode "The Host". Here, she does not have any knowledge about the Trills. In fact, she is surprised when she becomes aware that there is a symbiont in Odan's body in the first place or, to be more precise, the symbiont is Odan (according to the TNG Trill physiology). Odan himself lies to her at the beginning of the episode when he claims to be the former Odan's son, to keep the true nature of his species secret. When Picard talks with Deanna about the Trills, they agree that the Federation knows very little about them. At that time, Curzon Dax has been in charge of diplomatic and other official missions for the Federation for decades, and it is ridiculous that Sisko should be the only human being to know about his real nature. Emony Dax was on Earth around 2245 where she acted as a judge in a gymnastics competition, which almost definitely requires that humans had at least some basic knowledge about Trills already then. On the other hand, in DS9: "Emissary" Bashir says he has not seen a symbiotic species yet, but we may interpret this statement in a way that the young doctor has not met one in person (or as a patient). Finally, it is obvious that DS9 Trill is a member of the Federation and most likely even a long-time member, whereas this definitely isn't the case with TNG Trill.

Note Jadzia's spots were removed for the away mission on the Enterprise NCC-1701 in DS9: "Trials and Tribble-ations". It was apparently a precaution because, given the composition of the Enterprise crew, a human looking crew member would raise far less suspicion, not because Trills were entirely unknown at that time.

Odan, having been implanted into a new female host, does not mind continuing his relationship with Beverly, whereas exactly this is strictly prohibited for DS9 Trills (DS9: "Rejoined"). Well, the Odan symbiont may be a renegade or may be willing to become one, but why always assume the unlikely?

Allowed relationship in DS9: "Rejoined"

Forbidden relationship in DS9: "Rejoined"

Note It has been objected that the Trill law against reassociation may not apply to relationships with aliens. However, in DS9: "Afterimage" Sisko mentions to Ezri that Worf may avoid her because "joined Trill aren't supposed to get involved with people who were married to their previous hosts."

Conclusion

Given all this evidence, there is absolutely no way for the TNG Trills and the DS9 Trills to be the same species. The only thing they have in common, besides the name, is that they are both living in symbiotic relationships. But otherwise the two Trills are quite obviously not closer related to one another than humans to Klingons, for instance. On the other hand, why should two different species, who only have in common that they consist of a host and a symbiont, coincidentally have the same name? I don't have a clue. This is just awfully bad continuity.

When TPTB insisted on DS9 Trills looking different, they had better given Jadzia's species a different name too. This would have solved most of the above problems, except for Beverly's astonishment when she obviously encounters a joined species for the very first time.