Trill

Trills are humanoids capable of hosting a small vermiform symbiont within their abdominal cavity. The process of joining links the cerebral cortex of both host and symbiont, thus intermingling the previous experiences from other hosts with the new host to form a new unique identity. Trills themselves are easily recognized by the twin columns of spots that run from the forehead all the way down to the feet. Otherwise, they look completely Terran. That and their tendency to overlap the memory of past lives with the present set Trills apart. The symbiont roughly resembles a snail without the shell and is bred on the Trill homeworld in the vast interconnected breeding pools underneath the surface. They are tended, even pampered, by the Guardians, unjoined Trills who devote their lives to tending the pools and tracking the symbionts after joining.
 Symbionts live for thousands of years, but Trills are considerably shorter-lived. So symbionts are often transferred from one host to another when the host dies. The knowledge and experiences of each host is retained by the symbiont, who share it with a new host. Trill hosts voluntarily join with a symbiont, but once joined, both Trill and symbiont become biologically interdependent. After some 83 hours, neither Trill nor symbiont can survive without the other. The preservation of the symbiont's health is of paramount importance to the Trill, and if one must be sacrificed to save another, the Trill will invariably save the symbiont's life. The most important gauge of the health of the joined Trill is the isoboromine level. If it falls below 50, the symbiont will be removed from the Trill. Certain death awaits the host, but this is the standard policy for the Commission and is followed without question by all joined Trills. TNG "The Host" 4-97
SocietyTrill society regards a joining with a symbiont as a very high honor. The Trill Symbiosis Commission has been the only Trill government agency mentioned frequently, an indication of its powerful influence. The commission was set up to determine which Trill could receive the honor of joining, and tended to the medical needs of the symbionts. Trill hosts are selected only after rigorous training as an initiate under the close scrutiny of the Symbiosis trainers. Everyone was led to believe that only a few Trills are capable of hosting a symbiont, but Jadzia Dax and Benjamin Sisko realized that this was not the case. A doctor within the Committee admitted that while over half of the Trill population is eligible for joining, this truth could never be made public. If the truth got out, the symbionts would become a marketable item instead of the priceless preserver of experience it is.
The memories and lifetime of a previous host should not interfere with the duties and obligations of the new host. This is known as reassociation, a retrograde link strictly forbidden by the Symbiosis Commission. For once every personal obligation is carried on to the next host, there'd be no end to the debts and pledged from previous lifetimes. If such a disregard of Trill society occurs, both hosts will be banished from Trill for life, and the symbiont will die. Since nothing is as paramount to a Trill as the safety of the symbiont, the risks involved in reassociation make occurrences few and far between.
Trill symbionts (second image) Only 300 symbionts are available for hosting each year on average. If a host is weak, the personality of the symbiont will overwhelm it. Unless its current Joined HOST objects, a prospective Host may request a specific symbiont. Since the war, and the fleet departure from the system, only 12 are with us.