  In this article,we investigate some features of the perturbation theory in
spatially closed universe. We will show that the perturbative field equations
in a spatially closed universe always have two independent adiabatic solutions
provided that the wavelengths of perturbation modes are very longer than the
Hubble horizon. It will be revealed that these adiabatic solutions do not
depend on the curvature directly. We also propound a new interpretation for the
curvature perturbation in terms of the unperturbed geometry.
