The Spline are a race of huge sentient space-faring creatures. They are described as fleshy spheres, encrusted with a variety of pods and orifices. The various pods contain sensory organs, weapons emplacements and other equipment, while the orifices allow for the ingress and egress of smaller ships.
The Spline evolved on a watery planet as giant, whale-like creatures with articulated limbs. The Spline the made a decision to rebuild themselves, coating their hides with armor and allowing them to survive in the vacuum of space. They then hired themselves out to other intelligent species, becoming organic spaceships, capable of faster than light travel (using hyper-drives embedded in themselves), although they still had to return to their watery home-world to breed.
Humanity first encountered the Spline during the Qax occupation of the solar system, as Spline vessels were the primary mode of Qax transport. Following the collapse of the Qax occupation, humans entered into the same sort of arrangement as the Qax had with the Spline. When the third expansion reached the Spline home-world, humanity seized control of them, preventing other species from becoming Spline clients, and gaining a monopoly on them and suppressing the conscious control previously exercised by the creatures.