What defining characteristic separates Annelida from Nematoda and Platyhelminthes?

Being internally and externally segmented.

The segmentation of the body structure is the key anatomical feature that places an organism within the phylum Annelida, distinguishing it clearly from the other major groups often called worms. Annelids, such as earthworms, exhibit clear internal and external segmentation, meaning their bodies are divided into repeating units or segments. In contrast, Nematodes, or roundworms, are unsegmented, and Platyhelminthes, or flatworms, are characterized by their simple, flattened, unsegmented body organization. This segmental arrangement in Annelids is fundamental to understanding their specialized internal organization and locomotion capabilities, setting them apart taxonomically from the other two large groups mentioned.