  We analyze evolution of live disk-halo systems in the presence of various gas
fractions, f_gas less than 8% in the disk. We addressed the issue of angular
momentum (J) transfer from the gas to the bar and its effect on the bar
evolution. We find that the weakening of the bar, reported in the literature,
is not related to the J-exchange with the gas, but is caused by the vertical
buckling instability in the gas-poor disks and by a steep heating of a stellar
velocity dispersion by the central mass concentration (CMC) in the gas-rich
disks. The gas has a profound effect on the onset of the buckling -- larger
f_gas brings it forth due to the more massive CMCs. The former process leads to
the well-known formation of the peanut-shaped bulges, while the latter results
in the formation of progressively more elliptical bulges, for larger f_gas. The
subsequent (secular) evolution of the bar differs -- the gas-poor models
exhibit a growing bar while gas-rich models show a declining bar whose vertical
swelling is driven by a secular resonance heating. The border line between the
gas-poor and -rich models lies at f_gas ~ 3% in our models, but is
model-dependent and will be affected by additional processes, like star
formation and feedback from stellar evolution. The overall effect of the gas on
the evolution of the bar is not in a direct J transfer to the stars, but in the
loss of J by the gas and its influx to the center that increases the CMC. The
more massive CMC damps the vertical buckling instability and depopulates orbits
responsible for the appearance of peanut-shaped bulges. The action of resonant
and non-resonant processes in gas-poor and gas-rich disks leads to a converging
evolution in the vertical extent of the bar and its stellar dispersion
velocities, and to a diverging evolution in the bulge properties.
