In particular, how did the close cooperation of its many component species evolve?
My hypothesis is that it began with a few, probably no more than two zooids cooperating in symbiosis, and overtime, either entirely new species of zooid adapted to first parasitize and then cooperate with the existing company, or that some of the existing zooids mutated to acquire different functions. As the new zooids join the cooperation, the other re-adapt to form the "species" we see today.
Is that a somewhat accurate description of the evolutionary history of this colonial organism?
Addendum: Would the relative simplicity of the polyps forming the colony be a crucial prerequisite in their cooperation and specialization?