The Jolibrii Worldships are a subluminal species that has largely integrated into superluminal civilizations. This unusual state of affairs is due to the extremely slow life cycle of the Jolibrii.
Evolved from something more similar to a plant than an animal, the Jolibrii never gained the efficiency, speed, or aggressiveness of other evolved sapient species. Instead, they formed into coalitions of unmoving life forms, with new growth and partial die-off being their primary method of change.
The expedition to their homeworld did not realize the Jolibrii were sentient for quite a while, but the Jolibrii quickly realized what these visitors were. The expedition only realized what the Jolibrii were when they noticed the Jolibrii terraforming regions to be more hospitable, building new homes for them, and leaving gifts for their newborn.
It wasn't too long (in Jolibrii terms) before both sides began communicating and understanding each other.
The Jolibrii learned that their new friends had come via space ships, and this led to an effort by the Jolibrii to "ascend" - in a more literal sense than we normally use the word. They piggybacked their way into space, and with much effort and testing, discovered how to turn asteroids and small moons into "Worldships".
These Worldships create a contained space bound by the Jolibrii's various vegetative elements, with an interior containing whatever layout the Worldships prefer. They are not strictly self-sufficient, but the largest ones can survive for hundreds of years with no additional matter intake, simply absorbing light and using solar sails for propulsion.
The "vigorously gregarious" (their words) Worldships seem to enjoy containing many kinds of life, and they are quite flexible as to the kinds of atmospheres they can contain. It has been pointed out that what the Worldships really like is having a species aboard that can build in things like sensor packages, comms devices, computers, video screens, and warp engines, since the Worldships cannot make such things on their own. It is a common sight to enter an empty room on a Worldship and find the video screen playing a popular show... one frame an hour.
Most Worldships serve as space stations, since they are quite heavy and bulky and not terribly structurally rigid.
The most famous of these is probably the Ponderous Vision, which was involved in the Imputative Transfer.
While most Worldships are space stations, the most famous Worldships are the ones used as ships. From the Solace and Contemplation of the Child's Crusade to the Brevity is Paramount, which was a renowned sociologist in its own right.
To this day, Jolibrii Worldships remain active in interstellar affairs... despite rarely traveling even a fraction of the speed of light. Comparing them to other living ships such as the Cerdian Generation Fleet and The Starlifters clearly shows just how fortunate we are that the Jolibrii are a friendly, well-adjusted species that would never edit their own wiki articles to downplay disasters they definitely weren't involved with.