The setae vary much in form and are often longer and stronger than in the Oligochaetes.
In these forms the bundles of setae are either capilliform or uncinate, and the dorsal setae of the thorax are like the ventral setae of the abdomen.
They possess (save for certain Archiannelida, most Hirudinea, and other very rare exceptions) setae or chaetae implanted in epidermal pits.
The setae are organs of locomotion, though their large size and occasionally jagged edges in some of the Polychaeta suggest an aggressive function.
The setae are implanted metamerically in accordance with the metamerism of the body, which consists of a prostomium followed by a number of segments.