Starting from a microscopic definition of an alignment vector proportional to the polarization, we discuss the hydrodynamics of polar liquid crystals with local $C_{infty v}$-symmetry. The free energy for polar liquid crystals differs from that of nematic liquid crystals ($D_{infty h}$) in that it contains terms violating the ${bf{n}}to -{bf{n}}$ symmetry. First we show that these $mathcal{Z}_2$-odd terms induce a general splay instability of a uniform polarized state in a range of parameters. Next we use the general Poisson-bracket formalism to derive the hydrodynamic equations of the system in the polarized state. The structure of the linear hydrodynamic modes confirms the existence of the splay instability.
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