:: Star Trek Media Online

Planet: Trill. A unique "joined race" whose humanoid members have provided host bodies for small soft-bodied entities in a symbiotic relationship; Among the humanoid Trill joined species, apparently at least two races of hosts are used to house a symbiont: those of the ridged forehead, like Odan, and those like Jadzia, Curzon and Selin, with a narrow speckled band running along the hairline to the sides of the neck, then down along the body.

The symbiont body is a foot-long purplish lump of cranial and exoskeletal tissue, some are susceptible to damage by transporter beam. At Peliar Zel, the Trill can have a replacement body on hand for Odan in 40 hours; he will last only 1-2 hours in stasis, and a human body makes a temporary but limited host.

Since the symbiont integrates each new host's personality with its own after joining, Trill relationships are often hard for single-unit species to deal with and sometimes don't survive the change. Aside from the exceptions like Curzon, Trills are usually very responsible and moderate or reserved in personality; the symbionts don't seek romance and try to adhere to a "higher plane," feeling it a nuisance and a weakness of the young — although the host body occasionally indulges, as Curzon and Odan did. The symbiont does carry memories of time shared with each of its hosts, and each new combination — aside from gender, height and weight — carries a different blood type, metabolism, nervous system, and brain wave pattern. The complex brain carries two cerebral nuclei — one in each being, linked together — whose brain wave patterns can each be scanned separately as well as together. The two are biologically interdependent; at 93 hours after their joining, neither can survive without the other. Legally, the joined race is a tangle for Science Officer Dax's extradition arbiter Renora, since records of any legal and medical decisions regarding antecedent host culpability are almost nonexistent. A Trill host's hands feel cold-blooded to the touch; their more overtly developed senses include identifying a known individual only by the audible rhythm of footsteps.

Trill physiology may be the reason why Science Officer Dax is the first of the Deep Space Nine senior staff to show symptoms of the alien telepathic archival matrix. The humanoid host species' unruly children are not above throwing rocks at windows, just like their Human and Bajoran counterparts, at least. To others, Trill may seem arrogant side. Only one Trill in 10 is chosen to be joined to the "thousands" of symbionts, but those not joined still lead normal, productive lives. The brains of hosts produce endorphins as in the human brain. Thousands of symbionts live on Trill. Joined Trills are allergic to insect bites, since the biochemical connections of the symbiont and host can't tolerate the toxins. As seen with Jadzia, they are more resistant to a noxious paralyzing volcanic gas found on planet LS VI than other humanoids such as Bajorans. Among outworlders, they have been in contact with Klingons at least since 2289.

Judging by the Belar brothers, a Trill's lineal or family name follows the given name, as is done on Earth, but a host drops his lineal name in favor of the symbiont's. Renhol and others at the Symbiosis Commission worry that the symbionts would be bartered as commodities if the truth were known that half the population could be hosts, rather than the 1-in-1,000 ratio officially quoted and the reason behind the rigorous selection and training program. Social role equality seems apparent among the population, at least in medicine, with doctors and nurses of both genders.