Trill

The Trill are a dual species, consisting of an internal slug/brain like organism or "symbiont" and a humanoid host. The symbiont is dependent on the host for survival and will die within hourse of removal if not transplanted into a new host. The hosts are born without symbionts and can survive without one. However, once a symbiont is implanted, the host will quickly die if it is removed.

The species was first introduced 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 is ever given.