Free-living polychaetes locomote using moveable segmental parapodia bearing clusters of bristles or setae.  When walking, the worms use their parapodia as stilts, around which the body pivots. The slower the gait, the more a worm appears to walk on its parapodia. As speed increases, the body tends more to undulate. When swimming, the body undulates maximally and the parapodia function as paddles.

Nereis vexillosa walks on
the tips of its parapodia.
Legs in polychaetes move
in metachronal waves,
from back to front 1.5X

This unidentified west-coast polychaete seems to be alternating
use of its parapodia to walk, or perhaps it is raising every alternate
pair to minimise inter-parapodial interference 2X

Free-living polychaetes locomote using moveable segmental parapodia bearing clusters of bristles or setae.  Different gaits are used for slow and fast speeds when walking.  During slow walking the body is more or less straight and the parapodia act like little legs moving step-by-step. A step involves the parapodium moving forward, extending to contact the substratum, exerting the power stroke, and recovering for the next step.  At faster speeds of walking, the body undulates, and a different mechanism is used. In this gait the parapodia and setae, when thrust into the sediment, act as pivots around which the body segments move.  The parapodia are extended only on the outside curves of an undulatory wave, and withdrawn on the inside curves (see drawings upper Left).

A feature of many species is that the setae, rather than being a single structure, are jointed or compound. These joints are external to the body and are not directly controlled by muscles or nerves.  However, since jointed setae are associated with motile, but not tube-dwelling polychaetes, it seems that they may play a role in locomotion.  This role is investigated in Ophiodromus pugettensis at Friday Harbor Laboratories, Washington by videotaping worms with and without setal joints as they crawl over sand substratum.  Individuals with setae shortened but with joints left intact show no consistent difference in speed, step length, stride distance or frequency, or swimming speed.  In comparison, individuals with joints removed use different gaits than they would otherwise, such as using undulatory walking instead of slow walking, and swimming speed is lessened.  Worms without joints employ shorter stride distance, but compensate with increased stride frequency.  So, how do the joints actually function in locomotion?  The authors propose that the joints allow the setae to bend in such a way as to increase their contact with the substratum during the power stroke when walking, and similarly against the fluid medium when swimming. The distal part of each seta is serrated, which may also increase frictional resistance with the substratum.  Merz & Edwards 1998 J Exp Mar Biol Ecol 228: 273.

NOTE  the same individuals are observed before and after their compound setae are ablated, either distally or proximally to the setal joints.  The 2 treatments provide the researchers with a kind of sham-ablation control; otherwise, the effects of absence of a joint may not have been separable from the effects of the operation itself

NOTE  the authors provide comparative data on swimming as well as on the 2 walking gaits, but the data are not included here