As animals grew larger, simple diffusion was no longer enough, so internal fluid movement evolved.

As multicellular animals grew larger, simple diffusion became insufficient to move nutrients and remove waste. Early animals evolved internal fluid movement within body cavities, allowing nutrients to circulate beyond direct cell-to-cell contact.

In the earliest multicellular animals, every cell could no longer rely on direct contact with the outside world. Internal fluid transport through body cavities was a major step forward, helping larger bodies move nutrients and remove waste before the evolution of fully developed blood vessels and hearts.