  We study the spectrum of collective excitations in the inhomogeneous phases
in the neutron star inner crust within a superfluid hydrodynamics approach. Our
aim is to describe the whole range of wavelengths, from the long-wavelength
limit which can be described by macroscopic approaches and which is crucial for
the low-energy part of the spectrum, to wavelengths of the order of the
dimensions of the Wigner-Seitz cells, corresponding to the modes usually
described in microscopic calculations. As an application, we will discuss the
contribution of these collective modes to the specific heat of the "lasagna"
phase in comparison with other known contributions.
