Trill
Trills are a joint species of a humanoid host and a veriform symboint. The symbiont lives for many years, surviving several hosts. A trill who is joined takes into the relationship their own personality and gains the past memories and personality of the symbiont. Most Trills can be recognised by their 'spots'. Many trills are suitable for joining but with limited symbionts the public are told that only 1 in 1000 are suitable. The process of choosing a host is highly intense and the candidates undergoing three years of training to prepare them.
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. 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. 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. Social role equality seems apparent among the population, at least in medicine, with doctors and nurses of both genders.
