Roundworms, or nematodes, are unsegmented, cylindrical worms with a tough outer cuticle and a simple body structure, lacking specialized respiratory and circulatory systems. In contrast, annelids, such as earthworms, are segmented and have a more complex body structure with a true coelom, allowing for a more advanced circulatory system and segmentation that facilitates movement. Additionally, annelids possess setae (bristle-like structures) for locomotion, while roundworms do not. These differences highlight the distinct evolutionary adaptations of each group within the animal kingdom.
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