  We study the collective motion of confined spherical microswimmers such as
active colloids which we model by so-called squirmers. To simulate hydrodynamic
flow fields including thermal noise, we use the method of multi-particle
collision dynamics. We demonstrate that hydrodynamic near fields acting between
squirmers as well as squirmers and bounding walls crucially determine their
collective motion. In particular, with increasing density we observe a clear
phase separation into a gas-like and cluster phase for neutral squirmers
whereas strong pushers and pullers more gradually approach the hexagonal
cluster state.
