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What are some examples of annelids?

Earthworms, leeches and lugworms are examples of annelids.

Which are the morphological features that differentiate annelids from nematodes and platyhelminthes?

Platyhelminthes are worms with flat bodies (flatworms), nematodes are worms with cylindric (roundworms) but not segmented bodies. Annelids are cylindric worms with segmented bodies (they are metameric).

What is the main evolutionary novelty presented by annelids?

The main evolutionary novelty presented by annelids is the presence of the coelom, internal body cavity totally covered by mesoderm, a feature also present in arthropods, molluscs, echinoderms and chordates. Platyhelminthes are acoelomate and nematodes are pseudocoelomate (their internal cavity is partially covered by mesoderm).

Another important evolutionary novelty of the annelids is the closed circulatory system.