Final answer:
Option C is the correct option. Polychaetes use leg-like structures called parapodia for movement and respiration, which also bear setae. These appendages enable polychaetes to crawl along the seafloor and participate in gas exchange.
Step-by-step explanation:
Polychaetes, a group within the annelids, exhibit locomotion and respiration through specialized structures known as parapodia. The question asks which leg-like structures polychaetes use for these functions. The correct option is parapodia, which are fleshy, flat appendages located on each segment of the polychaete's body. These structures not only assist in movement but also play a role in gas exchange. Polychaetes may use parapodia to crawl along the seafloor or swim in some cases, depending on their adaptation. Furthermore, bristles, also known as setae, are arranged on these parapodia. The biology of polychaetes is quite complex, with different species having evolved to inhabit various environmental niches, including hydrothermal vents.
To clarify, while setae (option b) are bristle-like structures associated with parapodia, they are not the primary means of locomotion and respiration. Cirri (option c) are elongated, slender appendages often found in marine arthropods and are not characteristic of polychaetes. Podia (option d), more correctly referred to as tube feet, are associated with echinoderms like sea stars and are not part of polychaetes' anatomy.
Therefore, the leg-like structures that polychaetes use for crawling and respiration are indeed parapodia, which also bear the setae. Please, take note of this correct option in your final answer.