Starseeds are lifeforms found through interstellar space. They seem to migrate from the galactic core toward the rim, ejecting their eggs along the way, and then back to the core. Sometimes the eggs are ejected towards other galaxies, but most are left to hatch and migrate themselves to the core.
When folded on themselves, they look like a spherical mass roughly a mile in diameter, but when in need of changing direction, they unfold into a large squarish solar sail with its corners connected via strands to a small knob. Most of the internal organs are located in a cross-shaped swelling in the sail.
Source: Larry Niven, A gift from Earth