  We report the results of self-gravitating simulations of spiral galaxies,
modeled by stellar and gaseous components, developed to investigate in
particular the role of dissipation in the evolution of galaxy disks. The gas
disk is simulated by the Beam-Scheme method, where it is considered as a
self-gravitating fluid. The results suggest that the gravitational coupling
between the stars and gas plays a fundamental role in the formation and
dissolution of stellar bars, depending on the gaseous mass concentration and on
the degree of dissipation. In addition we remark that initially concentrated
gas disks can be unstable to the one-armed (m=1) spiral perturbations, which
may explain the lopsided features observed in the gas distribution of the
late-type isolated galaxies. The development of the m=1 feature slows down the
radial gas flows towards the center, since the large-scale gravity torques are
then much weaker.
