|Habitat||Need a high-pressure atmosphere|
|Height||Same as Humans (approximately)|
|Diet||Either herbivorous or omnivorous|
|Behind the Scenes|
|Universe||Star Trek Universe|
The Xyrillians are a species of space-faring humanoid being whose skin is almost entirely covered by smooth brown scales. Xyrillian metabolism is highly different from that of Humans and most other races, as it isn't based on water. They are a peaceful, reclusive and technologically-advanced culture.
Xyrillian biology doesn't require either water or oxygen for its metabolism. They live on a very dense, high-pressured atmosphere. Curiously, despite the fact that Xyrillians are not oxygen-breathers, Humans can still survive in their atmosphere without the need for a space-suit, but will suffer from hallucinogic effects if they don't spend some time in a "weaning chamber" before entering it. Also oddly, due to biological differences, Humans seem to be able to eat Xyrillian food and even become pregnant with Xyrillian offspring.
Xyrillian copulation can occur with minimal physical contact between male and female. After it, it is the male who becomes pregnant, carrying the offspring in a pouch in the upper chest region. According to Dr. Phlox's analysis, males of this species serve only as hosts, and the offspring only contains genetic material from its mother. Human Starfleet officer Charles Tucker was once impregnated by a female Xyrillian, and the embryo started developing inside his body. Even more oddly is that Tucker developed a nipple on his wrist. Whether this was a weird reaction of the Human body or a natural one which occurs in Xyrillian males as well is unclear, but the former seems more likely since Xyrillian females have breasts, indicating that they nourish the young.
Xyrillian starships are of rather interesting design, and their internal walls are used to grow edible vegetation, which gives them a peculiarly organic look. There is also an aquarium filled with several eel-like creatures swimming freely. Whether these are also used for food, are simply decorative or have yet other functions is still unclear. Xyrillians have amazing stealth technology and have also developed holodeck technology long before Humans, Vulcans or Klingons, although in 2151 they did give this technology to Klingons in payment for not being destroyed after the Xyrillian starship was discovered secretly hiding within the warp plasma field of a Klingon ship and trying to use the Klingons' plasma wake to generate power, since the Xyrillians were having trouble with their own engines.
- Star Trek: Enterprise - "Unexpected" (First appearance)