  We simulated the behavior of interstellar gas clouds in a companion galaxy
during a gas-dynamical interaction with the halo and disk of a spiral galaxy.
By ram pressure, the gas clouds are stripped from the companion, and accreted
to ward the disk of the spiral galaxy. If the companion's orbit is retrograde
with respect to the rotation of the spiral galaxy, infalling clouds hit the
nuclear region. Angular momentum transfer causes disruption of the inner
gaseous disk, and makes a void of interstellar gas in the bulge. If the
companion's orbit is either prograde or polar, infalling clouds are accreted by
the outer disk, and form a rotating gas ring. We show that the ram-pressure
stripping-and-accretion is one way from the companion to a gas-rich larger
galaxy, which causes disposal of interstellar gas from the companion and
effectively changes its galaxy type into earlier (redder). The ram-pressure
process is significant durig merger of galaxies, in which interstellar gas is
stripped and accreted prior to the stellar-body merger. Based on the
simulation, we discuss a possible history of interstellar gas in M31 system,
which comprises M32, NGC 205 and possible merged galaxies. The ram-pressure
stripping explains the disposal of mass lost from evolving stars in the dwarf
elliptical companion M32.
