1. Ambulation in Nereis involves two phenomena: (a) the spread, at a rapid rate, of an ambulatory pattern over the segments of the body, the pattern being propagated (during forward progression) from the anterior end of the animal towards the tail; (b) the transmission of this pattern, at a relatively slow rate, in an anterior direction.
2. During rapid ambulation, the activity of the parapodia is co-ordinated with that of the longitudinal muscles and progression is, largely, attributable to these muscles. Since one side of each segment is fixed to the ground when the underlying longitudinal muscles are fully relaxed, it follows that the animal must progress in the direction in which the muscular waves travel over the body, and not, as in the case of the earthworm, in the opposite direction.
3. The mechanism of swimming is described.