  We find three distinct phases; a tubular phase, a planar phase, and the
spherical phase, in a triangulated fluid surface model. It is also found that
these phases are separated by discontinuous transitions. The fluid surface
model is investigated within the framework of the conventional curvature model
by using the canonical Monte Carlo simulations with dynamical triangulations.
The mechanical strength of the surface is given only by skeletons, and no
two-dimensional bending energy is assumed in the Hamiltonian. The skeletons are
composed of elastic linear-chains and rigid junctions and form a
compartmentalized structure on the surface, and for this reason the vertices of
triangles can diffuse freely only inside the compartments. As a consequence, an
inhomogeneous structure is introduced in the model; the surface strength inside
the compartments is different from the surface strength on the compartments.
However, the rotational symmetry is not influenced by the elastic skeletons;
there is no specific direction on the surface. In addition to the three phases
mentioned above, a collapsed phase is expected to exist in the low bending
rigidity regime that was not studied here. The inhomogeneous structure and the
fluidity of vertices are considered to be the origin of such variety of phases.
