Final Answer:
Polychaetes possess parapodia, specialized appendages used for locomotion, which is true. Oligochaetes lack parapodia and are equipped with few setae, which is false.
Step-by-step explanation:
Annelids, characterized by a body structure of a tube within a tube, include two principal classes: Polychaeta and Oligochaeta. Polychaetes, represented by marine worms, exhibit parapodia, specialized fleshy protrusions, aiding in movement and respiration. These structures function as paddles, enabling effective swimming or crawling through sediment, facilitating their identification as possessing parapodia, aligning with statement "a" being true.
In contrast, Oligochaetes, such as earthworms, lack parapodia. Instead, they possess few, if any, visible appendages, relying primarily on their muscular movements for locomotion. They do have setae, small bristle-like structures, which aid in anchoring and burrowing. The statement "b" is false concerning the presence of parapodia in Oligochaetes.
Understanding these distinguishing features is crucial for taxonomy and classification within the annelid phylum. These traits play a significant role in differentiating between the two classes and understanding their ecological roles and behaviors within various ecosystems, highlighting the importance of these distinctive characteristics in the classification of annelids.