cells can coordinate by diffusion of signal molecules larger multicellular organisms can use circulatory transport to reduce latency of signals. larger complex organisms coordinate systemic functions with neurons and the electrical signaling between distant parts of the body.

latency is not a good thing to have when you are a motile animal, it limits your overall size and complexity. animal phyla with nervous systems were able to evolve larger bodies avoid being consumed by being too big to eat and became consumers of smaller organisms.