  Three-dimensional simulations with fully resolved hydrodynamics are performed
to study the collective motion of model swimmers in confinement. We show that
certain swimming mechanisms can lead to traveling wave-like collective motion
even without any direct alignment mechanism. It is also shown that by varying
the swimming mechanism, this collective motion can be suppressed, contrary to
the perception that hydrodynamic effects are completely screened at high volume
fraction. From an analysis of bulk systems, it is shown that this traveling
wave-like motion, which can be characterized as a pseudo-acoustic mode, is
mainly due to the intrinsic swimming property of the particles.
