The term Metazoa was coined to distinguish multicellular animals from single-celled organisms

In 1874, the German biologist Ernst Haeckel coined the term Metazoa to differentiate multicellular animals from Protozoa, the single-celled organisms that were previously grouped with them. Haeckel’s classification initially identified five major phyla, ranging from coelenterates (like jellyfish) to vertebrates, later adding sponges as a sixth. This distinction was a crucial step in defining the biological boundaries of the animal kingdom as we understand it today.