  We discuss the properties of spiral arms in a N-body simulation of a barred
galaxy and present evidence that these are manifold-driven. The strongest
evidence comes from following the trajectories of individual particles. Indeed,
these move along the arms while spreading out a little. In the neighbourhood of
the Lagrangian points they follow a variety of paths, as expected by
manifold-driven trajectories. Further evidence comes from the properties of the
arms themselves, such as their shape and growth pattern. The shape of the
manifold arms changes considerably with time, as expected from the changes in
the bar strength and pattern speed. In particular, the radial extent of the
arms increases with time, thus bringing about a considerable increase of the
disc size, by as much as ~50% in about a Gyr.
