Flatworms and roundworms are two types of worms with many differences in their body. Flatworms belong to the phylum Platyhelminthes while roundworms belong to the phylum Nematoda. The main differencebetween flatworms and roundworms is that flatworms consist of a dorso-ventrally flattened body whereas roundworms consist of a cylindrical body tapered to a fine point at each end. Both roundworms and tapeworms are triploblastic animals with bilateral symmetry. They are protostomes, exhibiting the spiral, determinate cleavage.