The opheliids Amniotrypane and Armundiu have very small parapodia bearing few chaetae, and these are pressed back against the body when the animals swim; the archiannelid Polygordius lacks parapodia and chaetae. All three worms have smooth bodies and swim by retrograde sinusoidal movements in the same way as other long, narrow, smooth-bodied animals, unlike nereidiform polychaetes in which the parapodial beat provides the driving force and the body undulations travelin the same direction as that of locomotion.
The wavelength of the undulations is comparable to the body length of these smooth polychaetes and this generally results in pronounced yaw. The kinetics of swimming are similar to those observed in nematodes and the amphioxus, and these polychaetes have comparable structural and mechanical features. A thick cuticle containing a spiral fibre system and lacking circular body-wall muscles is comparable to the situation in some nematodes, but in these polychaetes transverse muscles antagonize the longitudinal muscles and may allow adjustment of the internal hydrostatic pressure-and hence the stiffness of the body-as in the notochord of amphioxus which has similar swimming characteristics.
Like amphioxus, these polychaetes leave and re-enter the substratum in which they live. The small archiannelid Protodrilus has a similar muscular anatomy to that in Polygordius, but has a thin cuticle without spiral fibres. Prorodrilus has not with certainty been observed to swim.
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