Trill are a symbiotic species consisting of a humanoid host and a worm-like symbiont which lives in the abdomen of the host. The worm-like symbiont is extremely long-lived. An example of this is Dax, who is on his eighth host with Jadzia. The host does not lose any of their memories, and they are capable of accessing memories of the symbiont. If the host body is dying, the symbiont is transferred to another host. After four days, the merger between the host and the symbiont is permanent. Should the host body die without removing the symbiont within 96 hours, the symbiont will also die.

Trill humanoids are very interested in the welfare of their symbiont. They even enter into contests from which the winners become the next class of people honored to become hosts. Trill humanoids are very honored by their selection to become hosts. Some potential hosts who do not make the cut consider suicide.

Occasionally, Trill symbionts may be damaged by Transporter use, but most Trill do not seem to be bothered by beaming.

Trill symbionts remember all the memories of all of their previous hosts. Trill appear very human, except for a slight spotty marking along the sides of the face. These markings are unique to each Trill, and are just as unique as a Terran human's fingerprints. They do not detract from appearance.

A Trill symbiont can access memories from all of its previous hosts. The symbiont can survive for 96 hours following the death of the host. If it is transferred to a new host within that time, it will continue to survive. It cannot survive absolute destruction of the host body. All skills are transferred to the new host, but are based off of the new host's physical and mental abilities.

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