First of, they are in two different phylum. Flat worms are in the planarian phylum while, Porifera is the sponges phylum. Sponge has no organs, and can reproduce asexually. Many flat worms are parasites while no sponges are. Another thing is sponges have no symmetry while flat worms have bilateral symmetry. Hmmm what else... on yes flat worms have one body opening while, i not 100% sure sponges have two.
