The Trill are a humonoid species who are visibly similar to humans with the exception of spotted markings that go from from their temples, down their bodies, to their feet. Some of the species, usually selected by a rigorous approval process overseen by the Symbiosis Commission, are 'joined' to a symbiont whose life span far exceeds the unjoined Trill's and who will retain the memories of previous hosts, melding them into the memories and personality of the Trill they join with.
Trills resemble humans, with the exception of the markings that go down both sides of their body from the temples to their feet. Below the surface however there are significant differences. The Trill have a pouch in their abdomen into which the Trill symbiont, should the Trill be joined, is placed and lives. Their neural chemistry is also slightly different to most other humanoids due to the ability to join with the symbiont and given the need, with joined Trills, to avoid disruption of the neural link between the two neural clusters of the Trill and symbiont brain. As a result of this, there exists a separate class of anesthetics and sedatives for use on Trills in more serious cases. This delicate neural chemistry of a joined Trill is also vulnerable for insect bites and other allergic reactions, meaning a minor irritation for others might be seriously life threatening without treatment for a joined Trill. Trill hands are also noticeably colder than other species.