  We show that an effective particle Lagrangian yields the
Mathisson-Papapetrou-Dixon (MPD) equations. The spin of the effective particle
is defined without any reference to a fixed body frame or angular velocity
variable. We then demonstrate that a continuous body, defined by a congruence
of world lines and described by a general action, can be rewritten as an
effective particle. We analyze the gauge freedom of the body and show that a
natural center of mass condition is related to a spin supplementary condition.
