The second species is known as the Symbionts. These are relatively small, slug like 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 that often causes the death of the Host. Only one in every few thousand Hosts is suitable to be joined, and these must go through an exhaustive series of tests and training in order to qualify.
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. 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.
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.
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.
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.
Trills are beings which are a triumph of artificial selection and forced evolution. Many years ago the Trill home world was ravaged by awful ecological disasters. The two intelligent species found that the only way to survive was to work together and ultimately to become one combined life form
