Question

Why do animals with millions of cells "need" a circulatory system while animals with few cells can do without one?

Answer 1

Animals which consist of few cells have a capacityto transport the nutrients by cell interactions and diffusion. The maximum
distance of the last one is around 0.1 mm. So it is not enough to supply all
cells quickly in the multicellular organism without specific proteins carriers
and a circulatory system as a result.