  We theoretically describe the dynamics of swimmer populations confined in
thin liquid films. We first demonstrate that hydrodynamic interactions between
confined swimmers only depend on their shape and are independent of their
specific swimming mechanism. We also show that due to friction with the walls,
confined swimmers do not reorient due to flow gradients but the flow field
itself. We then quantify the consequences of these microscopic interaction
rules on the large-scale hydrodynamics of isotropic populations. We investigate
in details their stability and the resulting phase behavior, highlighting the
differences with conventional active, three-dimensional suspensions. Two
classes of polar swimmers are distinguished depending on their geometrical
polarity. The first class gives rise to coherent directed motion at all scales
whereas for the second class we predict the spontaneous formation of coherent
clusters (swarms).
