Starbase 565

A Class M planet that is home to the joined species known as the Trill. Interesting geological formations on the Trill home world include the Tenaran ice cliffs and the caves of Mak'ala. The oceans on planet Trill are purple.

The People: A joined species comprised of a humanoid host and a small vermiform symbiont that lives in an internal abdominal pocket of the host body. Most of the personality and memories of the Trill reside in the symbionts, which are extremely long-lived, although the host also contributes personality traits to the joined lift-form. Upon the death of a host body, a Trill symbiont is usually transplanted into another host. For a joined Trill, nothing was more important than protecting the life of the symbiont.

Trill hosts enter voluntarily into their association with the symbiont, and in fact there is intense competition among potential hosts to determine who will be accorded this high honour. Only one Trill in ten is chosen to be joined with a symbiont. The decision of who is to be allowed to be joined is the responsibility of the Symbiosis Evaluation Board. Improper joining is believed to cause damage to both host and symbiont.

Children are directed by their parents towards becoming a host. While the influence of the symbiont is very strong, good host candidates can balance the influence of the symbiont with their own interests, and hence produce the best possible merge of the two personalities.

Hosts are accepted into their mid-twenties. Once joined, the host and symbiont become biologically interdependent, and after 93 hours, neither can survive without the other. The resulting new joined life form is considered to be another person, although it retains memories of previous joinings.

Despite the fact that a symbiont retains memories in subsequent joinings, Trill society very strongly disapproves of re-association of subsequent hosts, in cases where previous hosts had been married. Trills' hands are naturally cold. Sometimes friendships with other species don't survive when the Trill moves to the next host. Trills are highly allergic to insect bites, as the toxins released interfere with the biochemical connections between the host and the symbiont. Trill hosts sometimes have sexual feelings, but they do their best to rise above them, at least according to Jadzia Dax. They do not have to uphold commitments of the previous hosts. Certain Trill symbionts can be severely damaged by beamings.

Trill Symbiosis Commission: Powerful Trill government body that oversees the testing, selection, and joining of Trill humanoid hosts with symbionts. The Symbiosis Commission maintained strict guidelines for those host candidates, as it was commonly believed that only one in a thousand Trill humanoids was capable of being joined, and to join an unsuitable host would result in rejection of the symbiont within a matter of days.