Some slime moulds can group together to form a single mass that behaves like a multi-cellular creature. Under stress, they can act like nonvascular plants and create spore pods (sporophytes).
The REALLY COOL thing is that they can spread out into a maze and find the shortest route out. The whole mass communicates without a nervous system to accomplish this feat.
If you get a chance, type "slime mould" in Wikipedia, sit back, clear your mind, think about alien worlds, and then begin reading...