  We simulate a two dimensional model of self-propelled particles confined by a
deformable boundary. The particles tend to accumulate near the boundary and the
shape of the boundary deforms upon the collisions. We find that there are two
typical stages in the variation of the morphology with the increase of active
force. One is at small force characterized by radially inhomogeneous
redistribution of particles and suppression of local fluctuations of the
boundary. The other is at large force featured by angularly redistribution of
particles and global shape deformation of the boundary. The last two processes
are strongly cooperative. We also find different mechanisms in the particle
redistribution and opposite force-dependences of the rate of the shape
variation at low and high particle concentrations.
