Why do zoologists classify sponges as animals rather than plants?
Organisms are classified, not by their behavior, but by their phylogeny (evolutionary relationships). Sponges have unique and complex molecules in their intracellular matrix that developed in a common ancestor of sponges, and are shared with sponges and all other animals. These molecules include collagen, proteoglycans, integrin, and adhesive glycoproteins. These molecules are made by the cells, exist outside the cells, and fill the space between them.