SPECIES: TRILL

Trill is a member of the Federation and is a Alpha quadrant world.  The Trill are humanoid in appearance, with a series of brown spots that start at the temples and work downward along the side of the Trill's body.
Some Trill have a slug-like creature called a "symbiont" implanted in their abdomen.  The Trill consider it a great honor to be joined with these creatures but very few Trill are considered "compatible" for such joining.

When a humanoid Trill (called a "host") is joind with a symbiont, the two beings function as one in all respects. Humanoid trills without a symbiont are basically identical to humans except for the spots.  The symbiont lives for about 500 years and during these years is joined with many hosts as the symbiont far outlives a humanoid Trill.  The symbiont retains the personality, memories, and skills of all its previous hosts and these traits are passed on to any future host it
may join with.

A symbiont will die within twelve hours if seperated from its host, unless it is returned to its natural environment on the Trill homeworld (a silty, subterranean pool).  The host usually dies within a few days and welcomes death as the emotional trauma of being seperated is usually too much for the humanoid host to bear.

Today, the joining of a symbiont with a host is managed by a group of sages called "The Symbiosis Commission," a branch of the Trill government.  Many trill never receive a symbiont as they are not considered "compatible", and many symbionts spend their lives in the silt pools underground.  The joining of a host and symbiont usually occurs during the early adulthood of the humanoid host, though it could occur at any time after the host reaches physical maturity.

Trills are one of the very few joined species known in the alpha quadrant. Two separate intelligent species have evolved on the Trill homeworld - the hosts are a humanoid species who have two distinct races. The first, or alpha race, comprises approximately 10% of the Trill population; they are distinguishable by their pronounced foreheads, but in most respects their physiology is similar to that of the beta race who comprise the remaining 90% of the population. These are easily identifiable by a pattern of skin markings which extend from the forehead down the side of the face and all the way down the body. These spots move and change gradually over time, although they remain in the same region of the body. Both of these races are known as the Trill Hosts.
The second species is known as the Symbionts. These are relatively small, sluglike creatures who live underwater for most of their life. The Symbiots are extremely long lived and very intelligent, but lack any kind of limbs or tool-using ability.

Some thousands of years ago the Trill began living as a joined species - the Hosts developed the ability to physically join with the Symbionts, linking their nervous systems together. The effects of this are different between the two races of Host; with the alphas the Symbiot totally submerges the Hosts personality upon joining, essentially using the body as a home for its own mind. Alphas tend to be less physically robust than betas; for example, many alphas are extremely sensitive to the energy processes involved in transporters, and using this technology would cause fatal damage to the Symbiont. For this reason betas are the preferred recipients in the joinings.
Betas also have some difficulties due to the joining process. Betas become physically dependant on the Host within 93 hours of the joining; after this period, removing the Symbiont is fatal to the Host. After a joining the mind of a beta is not submerged by the Host; the two separate minds merge, forming a new personality who is a combination of both the Host and Symbiont. This can cause severe psychological problems if the two minds are incompatible, resulting in rejection of the Host after only a few days - a rejection which often causes the death of the Host. Only one in every few thousand Hosts are suitable to be joined, and these must go through an exhaustive series of tests and training in order to qualify.
After the death of the Hosts the Symbiont is removed and placed into a new Host, carrying into the new joining all the personality and memories of the old. Many Symbionts have lived in this manner for centuries, accumulating experiences from many different lifetimes.