  The kinematic degrees of freedom of spinning particles are analyzed and an
explicit construction of the phase space and the simplectic structure that
accomodates them is presented. A Poincare invariant theory of classical
spinning particles that generalizes the work of Proca and Barut to arbitrary
spin is given using spinor variables. Second quantization is naturally
connected to the unphysical nature of zitterbewegung. Position variables can
not be disentangled from spin in a canonical way, nor can the phase space be
reduced to the usual description $(x,p)$ and a vector spin.
  Pacs: 03.20.+i, 03.65.Sq, 03.30.+p, 11.30.Cp
