What is the main evolutionary novelty presented by annelids?
The main evolutionary novelty shown by the beings of the phylum Annelida is the coelom, the internal body cavity totally covered by mesoderm, a feature also there in arthropods, molluscs, chordates and echinoderms. Platyhelminthes are acoelomate and nematodes are pseudocoelomate (their internal cavity is partly covered by mesoderm).