Trill (Star Trek) - Depicted Biology

Depicted Biology

The hosts, as they are called, are humanoids that appear much like humans, except with distinctive black or brown spots visible from the top of the head, down each side of the face and neck, and down across the shoulders and the sides of the chest, thighs, legs to their feet. Their hands are noted for being unusually cold to the touch ("A Man Alone", DS9 Season 1). Trill are strongly allergic to insect bites, because the toxins interfere with the biochemical connections between host and symbiont ("The Siege", DS9 Season 2).

The symbionts are helpless, worm-shaped lifeforms who contain the memories of their previous hosts, and who inhabit the abdomens of the humanoid hosts. When a host and a symbiont are joined, the resulting individual is considered a new being. When a host dies, the symbiont is transplanted into a new host. Ninety-three hours after the joining, the host and symbiont are completely interdependent, but up to then, the joining may be reversed without killing the host. A symbiont who is neither implanted into a new host nor returned to their habitat (pools of nutrient-rich liquid on the Trill homeworld) will quickly die, as will a joined Trill host within one to two days of the symbiont being removed.

In rare cases, Trill symbionts can be joined with non-Trill humanoids, but the differences in biology means the results are often unstable ("The Host", TNG). Commander William Riker was briefly joined to the Odan symbiont so that Odan could complete peace negotiations, and to keep Odan alive until a new Trill host could arrive. While this effort saved Odan's life, it nearly caused Riker's death.