title for sandworms & relatives section of A SNAIL'S ODYSSEY
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Free-living polychaetes locomote using moveable segmental parapodia bearing clusters of bristles or setae.  When walking, the worms use their parapodia as stilts, photograph of sandworm Nereis vexillosa walking on its parapodiaaround 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.
photograph of an unidentified west-coast polychaete crawling on the sea floor

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

It seems that this unidentified west-coast syllid polychaete is walking on its elongated
dorsal cirri and raising them alternately to minimise inter-cirral interference. But
looks can be deceiving. The actual parapodia on which the worm is moving are oriented downwards and are out of view. For whatever reason, the dorsal cirri in this species and
othes in the Family Syllidae alternate in length, but are not involved in locomotion

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Research study 1
  photograph of early larva of nereid Nereis vexillosa showing unusual jointed setae, courtesy Gustus & Cloney 1973 J Morph 140: 355drawing of unusual seta of a larval nereid worm Nereis vexillosaAn early study of larvae of the sandworm Nereis vexillosa at Friday Harbor Laboratories, Washington describes the presence of jointed setae (see photograph of larva on Right).  The authors list 4 types of setae in 4-10d-old larvae, including the jointed homogomph falcigers shown in the drawing.  The functions of the different setal types are not well understood, but the authors suggest that the bladed teeth of the falcigers may increase friction between the setae and substratum during the crawling stages of settlement.  Gustus & Cloney 1973 J Morph 140: 355.
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Research study 2

drawings of 3 stages in the formation of a jointed seta in the sandworm Nereis vexillosaThe same University of Washington research group later describes morphogenesis of jointed setae in larval sandworms Nereis vexillosa. Several setae are produced within a single sac containing special epidermal cells termed chaetoblasts ("chaet"= seta + "bud"; no drawings of these sacs are provided). The process takes 3d at 10oC and involves secretion of filaments of protein/chitin in plastic form to create the main shaft of the seta, with the ultimate complex shape being configured by microvilli on the surface of the chaetoblast (see drawings). The microvilli not only shape the seta, but the initial fluid state of the protein/chitin material is thought by the authors to be polymerised into chitin chains and cross-linked with protein to form a solid glycoprotein (chitin + protein) by enzymes on the microvilli. Throughout the process the microvilli shift in position, sometimes fusing temporarily, and extending and withdrawing to mold the material into its final complex shape. It is not clear how many microvilli are involved in production of a single seta or whether this number changes over time, but the authors list 30 as being present during later formation of the cortex of the shaft and 6-14 as being involved in early tooth formation. The external surface of the distal part of each seta is eventually covered with an enamel-like material. As they reach their final size the setae break through the cuticle that covers the epidermis of the larva. The authors provide a number of photomicrographs of the developing setae. O’Clair & Cloney 1974 Cell Tiss Res 151: 141.

NOTE as one reads the authors’ description the process seems vaguely reminiscent of a combination of injection molding with 3-D printing, obviously pre-dating these contemporary manufacturing methods by many hundreds of thousands of years

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Research study 3

drawings showing different gaits used during locomotion of the polychaete worm Ophiodromus pugettensisFree-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 photographs of jointed setae of larval polychaete worms Ophiodromus pugettensis, courtesy Merz & Edwards 1998 J Exp Mar Biol Ecol 228: 273length, 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 other data on swimming and walking, not included here

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