  We use numerical simulations to examine the structure of merger remnants
resulting from collisions of gas-rich spiral galaxies. When the gas fraction of
the progenitors is small, the remnants structurally and kinematically resemble
elliptical galaxies, in agreement with earlier work. However, if the progenitor
disks are gas-dominated, new types of outcomes are possible. In fact, we show
that a prominent disk may survive in certain cases. To illustrate this
scenario, we analyze an extreme example with progenitor galaxies consisting of
dark matter halos, pure gas disks, and no bulges, as might be appropriate for
mergers at high redshifts. While rapid star formation triggered by tidal
torques during the merger forms a central, rotating bulge in the remnant, not
all the gas is consumed in the burst. The remaining gas cools very quickly and
settles into an extended star-forming disk, yielding an object similar to a
spiral galaxy, and not an early type galaxy. This is contrary to the usual view
that major mergers invariably destroy disks. The morphological evolution of
galaxies can therefore be more complicated than often assumed, and in
particular, theoretical constraints based on the fragility of spiral disks need
to be reevaluated.
