  We describe simulations of the response of a gaseous disc to an active spiral
potential. The potential is derived from an N-body calculation and leads to a
multi-armed time-evolving pattern. The gas forms long spiral arms typical of
grand design galaxies, although the spiral pattern is asymmetric. The primary
difference from a grand-design spiral galaxy, which has a consistent 2/4-armed
pattern, is that instead of passing through the spiral arms, gas generally
falls into a developing potential minimum and is released only when the local
minimum dissolves. In this case, the densest gas is coincident with the spiral
potential, rather than offset as in the grand-design spirals. We would there
fore expect no offset between the spiral shock and star formation, and no
obvious co-rotation radius. Spurs which occur in grand-design spirals when
large clumps are sheared off leaving the spiral arms, are rare in the active,
time-evolving spiral reported here. Instead, large branches are formed from
spiral arms when the underlying spiral potential is dissolving due to the
N-body dynamics. We find that the molecular cloud mass spectrum for the active
potential is similar to that for clouds in grand design calculations, depending
primarily on the ambient pressure rather than the nature of the potential. The
largest molecular clouds occur when spiral arms collide, rather than by
agglomeration within a spiral arm.
