The symbiont was a sentient vermiform lifeform from the planet Trill. It was transferred from one humanoid Trill host to another, upon the death of the prior host. Humans could also be used as a host to a symbiont; however, the symbiont could not remain in their bodies for long without physically harming the Human host. (TNG: "The Host") A symbiont could survive normally in a partly Trill host. (DS9: "Children of Time") The symbiont took with it all of its memories of the previous Trill hosts. Upon joining, the new Trill host gained those memories. The symbionts lived in the Caves of Mak'ala on Trill and were cared for by the Guardians. They communicated with each other via electrical impulses transmitted through the milky water that they inhabited. They were long lived compared to most humanoid species, and could easily live beyond 550 years.
Once a symbiont was joined to a Trill for over ninety-three hours, the two beings became permanently dependant on each other; the Trill would die within hours without a symbiont, and the symbiont could not return to the Caves of Mak'ala (if joined for the first time) nor survive a significant period of time outside of a host.
The Symbiosis Commission oversaw the joining of the humanoid Trills and vermiform symbionts. In the 24th century, the general belief was that one in every 1,000 Trills was suitable for joining; however, the number was nearer to 50% of the population. This was a closely guarded secret because every year about 5,000 Trills became initiates while only about 500 symbionts became available. (DS9: "Equilibrium")
Rites and ceremonies Edit
Background Information Edit
The series bible of Star Trek: Deep Space Nine mentioned that "Many centuries ago, the symbionts lived underground, while the humanoids were on the surface ... and due to an environmental disaster, they were forced to 'join' to survive." 
For TNG: "The Host", the design of the symbiont was based upon a caterpillar with an octopus' head, but when next seen in DS9: "Invasive Procedures", the design had changed significantly, having what Michael Westmore refers to as a more "streamlined" design (Michael Westmore's Aliens: Season Two, DS9 Season 2 DVD, special feature).