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How do earthworm setae prevent the body from sliding backward?
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The structure of polychaetes. (Left) Free-moving polychaetes. (A) Neanthes, (B) Nereis. (Right) Tube-dwelling (sedentary) polychaetes. (C) Amphitrite, (D) Sabella.
Earthworm setae are small, bristle-like structures embedded in the body wall that play a critical role in locomotion.
To prevent the body from sliding backward during movement, specific body muscles extend the setae of a segment outward. By anchoring these bristles against the surface, the earthworm creates a point of resistance that holds the segment in place while longitudinal muscles contract to pull the rest of the body forward. Once this phase of movement is complete, the setae are retracted during the circular contraction period to allow the segment to move freely again.1