Annelids differ from nematodes because they possess a metamerically segmented body, which means their body is divided into numerous similar segments. On the other hand, nematodes do not have segmented bodies. Annelids also possess a complete digestive system, which means they have a mouth and an anus, whereas nematodes have a tubular digestive system with only one opening, which serves both as mouth and anus. Although both annelids and nematodes are bilaterally symmetrical and triploblastic, these characteristics are not the primary features that differentiate them from each other.