What are parapodia?
Parapodia are primarily the organs of locomotion used both in creeping and in swimming. Since they are highly vascularised, they also serve the function of respiration.
Nereis is a genus of Polychaeta worms in the family Nereidae. They possess setae and parapodia for locomotion. Each segment of the body, except the peristomium and the anal segment, bears on either side a flat, fleshy, hollow and vertical flap like out growth called parapodia.
They may have two types of setae and parapodia for locomotion. Acicular setae provide support. Locomotor setae are for crawling and are the bristles that are visible on the exterior of the Polychaeta.
Slow creeping movements of Nereis virens are carried out by the action of parapodia only. During locomotion each parapodium performs two strokes - an effective or back stroke and recovery or forward stroke. The combined effective and recovery strokes of numerous parapodia propel the worm forward. The parapodia of the two sides work alternatively causing successive waves along each side of the worm.