what is the difference between flatworms nematodes and annelids?
Flatworms (phylum Platyhelminthes) are simple animals that are slightly more complex than a cnidarian. Roundworms (phylum Nematoda) have a slightly more complex body plan. Segmented worms (phylum Annelida) are the most complex animals with worm-like body plans. ... Worms are invertebrate animals with bilateral symmetry.