Answer to Question #49368 in Molecular Biology for Kevin Ko
Why do animals with millions of cells "need" a circulatory system while animals with few cells can do without one?
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.