  We propose a new dynamical picture of galactic stellar and gas spirals, based
on hydrodynamic simulations in a `live' stellar disk. We focus especially on
spiral structures excited in a isolated galactic disk without a stellar bar.
Using high-resolution, 3-dimensional N-body/SPH simulations, we found that the
spiral features of the gas in galactic disks are formed by essentially
different mechanisms from the galactic shock in stellar density waves. The
stellar spiral arms and the interstellar matter on average corotate in a
galactic potential at any radii. Unlike the stream motions in the galactic
shock, the interstellar matter flows into the local potential minima with
irregular motions. The flows converge to form dense gas clouds/filaments near
the bottom of the stellar spirals, whose global structures resemble dust-lanes
seen in late-type spiral galaxies. The stellar arms are non-steady; they are
wound and stretched by the galactic shear, and thus local densities of the arm
change on a time scale of ~ 100 Myrs, due to bifurcating or merging with other
arms. This makes the gas spirals associated with the stellar arms non-steady.
The association of dense gas clouds are eventually dissolved into inter-arm
regions with non-cirucular motions. Star clusters are formed from the cold,
dense gases, whose ages are less than ~30 Myrs, and they are roughly associated
with the background stellar arms without a clear spatial offset between gas
spiral arms and distribution of young stars.
