Locomotory Structures
Annelids possess three kinds of locomotory structures, namely parapodia, setae and suckers. Parapodia are segmentally arranged, lateral, hollow extensions of the body into that also extends the coelomic cavity. Each parapodium basically consists of two lobes, a dorsal notopodium and a ventral neuropodium and every lobe bears a bundle of bristles or setae supported by an aciculum. Related with each parapodium are dorsal and ventral sets of oblique muscles, and as well the intrinsic protractor and retractor muscles. During movement, two parapodia of a segment remain in opposite phases of motion and so cause a sort of paddling activity by water. The bristles and acicula are protruded and withdrawn through the activity of the intrinsic muscles. Parapodia are the major locomotory organs of polychaetes. In accordance with the different functions which they perform, parapodia exhibit variations of form among different polychaetes.