Trill
The Trill are a dual species, consisting of an intelligent "symbiont" and a humanoid host. The symbiont is dependent on the host for survival and will die within hours of removal if not transplanted into a new host. The hosts are born without symbionts and can survive without one. However, after a symbiont is implanted, it eventually becomes necessary for the host's survival.
The number of symbionts is small compared to the humanoid host population, making Trill with symbionts rare even on their homeworld.
Attributes
Trill symbionts have extremely long life expectancies, while host life expectancies are typical for Star Trek humanoids. Symbionts retain their memories when transplanted from a dying host to a new host, allowing Trill with symbionts to draw upon many lifetimes worth of knowledge and experience.
Some Trill with symbionts avoid using transporters because they believe that being transported can harm or kill the symbiont[1], but others have no objection to the process[2]. There have also been suggestions that phasers set to stun are more dangerous to symbionts than to normal humanoids[3].
Appearances
The species was first introduced as an alien of the week in TNG "The Host". The Enterprise-D is transporting a Trill diplomat to settle a dispute between two factions of another species. The host is mortally wounded in a shuttle accident and another host will not arrive in time. Riker volunteers to be a temporary host to both keep the symbiont alive and carry forward the negotiations.
Trill are more prominent in Deep Space Nine. Two main characters are trill hosts of the same symbiont; Jadzia and Ezri Dax. However, the general appearance of the host was altered in this series and no canon explanation for the change was ever given.