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Trill is a class-M planet located in the Alpha Quadrant that is hope to the joined species known as Trill. Trill is similar to Terra in respect to weather and general atmospheric conditions and is about the same size. One distinct difference is that the oceans on Trill are a majestic purple hue rather than the typical blue found on Terra. Trill is also home to several interesting geological formations including the Tenaran ice cliffs and the Caves of Mak'ala.

The Caves of Mak'ala contain interconnecting pools that stretch for kilometers under the surface of the planet. These pools are not only visually stunning, but also play a critical part in the life of Trills as they serve as the breeding environment for Trill symbionts.


The Trill are comprised of two species which can be surgically joined to form a joined species. They consist of a humanoid host and a small veriform symbiont that resides in an internal abdominal pocket within the host body. While most of the personality and memories of the Trill reside in the symbiont, which can leave for many decades, the host does contribute to the personality of the joined lifeform. 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. Upon the death of the host, the symbiont can be transplanted into another host. When the host dies, although the symbiont lives, the new host is not obligated to uphold the commitments and responsibilities of the previous hosts.


At the center of Trill society rests the Trill Symbiosis Commission. The Trill Symbiosis Commission is a powerful Trill governmental body that oversees the testing, selection, and joining of hosts with symbionts. The Symbiosis Commission maintains strict guidelines for host candidates as it is believed that only one in every thousand hosts are suitable for joining. (To become a joined trill, you must complete the joined trill class.) The Symbiosis Commission uses these stringent guidelines for host selection because if a symbiont were joined to an unsuitable host, it would be rejected in a matter of days and possibly lead to the death of either the host or the symbiont itself. Recent research and rumors have come to light, however, that would suggest nearly half the Trill population is capable of being joined. This information has not been confirmed by the Symbiosis Commission, however, due to the fear that the symbionts, which are very few in number, would become commodities to be bought and old.

The decision of who is allowed to be joined is the decision of the Symbiosis Evaluation Board. One Trill in ten is chosen to be joined with a symbiont. Those Trills that do get accepted are placed in what is known as the Initiate Corps. Some joined Trills on the Symbiosis Commission serve as field docents who show unjoined Trills what it is like to be joined and then, based on the experience, evaluate whether or not that Trill should stay in the program.

Trill hosts enter voluntarily into their association with the symbiont and in fact there is rigorous competition among potential hosts to determine who will be accorded this honor. Children are directed by their parents towards becoming a host. For a joined Trill, NOTHING is more important than protecting the life of the symbiont. Sometimes friendships with other species do not survive when the Trill moves on to the next host. Trill society disapproves of reassociation of subsequent hosts in cases where previous hosts have been married. It is strictly taboo and is punishable by banishment from the Trill homeworld which would result in the death of the symbiont when the current host dies.

The symbionts residing in the Caves of Mak'ala are protected by unjoined Trills called Guardians who devote themselves to the care of the symbionts. They communicate with the symbionts telepathically and are often regarded as odd by other Trills.

Joined Trills sometimes engage in what is known as a zhian'tara ritual. A zhian'tara ritual, also called the Trill Rite of Closure, is when all of the personalities of former hosts leave the symbiont and are taken on by friends of the current host so that they can talk with them as separate people and by doing so learn more about themselves. The temporary host is a volunteer and can resume control at their leisure. These rituals are conducted by Guardians who use their telepathy to conduct the zhian'tara ritual.


Trills are a joined species comprised of a humanoid host and a small invertebrate symbiont that resides in an internal abdominal pocket of the host body when joined. The symbiont is a small, sightless, intelligent lifeform. They are bred in underground chambers on Trill known as the Caves of Mak'ala. While they remain in the pools there, they symbionts communicate with each other by means of a visible electric discharge. The symbiont lives within the humanoid host, gaining sustenance and mobility from the host. Symbionts can have enormously long life span and, upon death of their current host, can be transplanted into a new host. The resulting new joined life form is considered to be another person. Once a symbiont is removed, the host usually dies within hours. Improper joining of host and symbiont can also cause psychological damage to both.

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 newly joined lifeform is considered to be another person, although it retains the memories of previous joinings. Trill hosts sometimes have residual sexual feelings, but they do their best to rise above them.

Trills are highly allergic to insect bites as the toxins released often interfere with the biochemical connections between host and symbiont. Trill hands are naturally cold due to the body's desire to keep most of the blood flowing in the torso to keep the symbiont nourished and protected. While it is not a dominant trait, many Trill symbionts are unable to use transporters for travel due to adverse reactions which can damage or kill the symbiont and host. Although a major area of research, Trill scientists have yet to find a reason as to why some are affected and others are not.


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