How do free-moving polychaetes use parapodia to crawl?

Free-moving polychaetes utilize their parapodia—fleshy, lateral outgrowths found on most body segments—to crawl through a coordinated cycle of strokes.123

When moving slowly, the primary contractual force is generated by the sweeping motion of these appendages. The crawling process involves two distinct phases:1

• Effective (backward) stroke: The needlelike aciculae are projected beyond the parapodium to make contact with the surface, providing the necessary leverage to push the worm forward.1
• Recovery (forward) stroke: The aciculae retract, allowing the parapodium to lift free from the surface and reposition for the next cycle.1

This movement is highly organized, as the next parapodium initiates its backward stroke just as the previous one finishes its own. For more rapid movement, these parapodial actions are supplemented by body undulations, which are produced by the contraction of longitudinal muscles stimulated by the backward stroke of the parapodia in specific segments.1