  The sponge-like biomineralised calcite materials found in echinoderm
skeletons are of interest in terms of both structure formation and biological
function. Despite their crystalline atomic structure, they exhibit curved
interfaces that have been related to known triply-periodic minimal surfaces.
Here, we investigate the endoskeleton of the sea urchin Cidaris rugosa that has
long been known to form a microstructure related to the Primitive surface.
Using X-ray tomography, we find that the endoskeleton is organised as a
composite material consisting of domains of bicontinuous microstructures with
different structural properties. We describe, for the first time, the
co-occurrence of ordered Primitive and Diamond structures and of a disordered
structure within a single skeletal plate. We show that these structures can be
distinguished by structural properties including solid volume fraction,
trabeculae width, and to a lesser extent, interface area and mean curvature. In
doing so, we present a robust method that extracts interface areas and
curvature integrals from voxelized datasets using the Steiner polynomial for
parallel body volumes. We discuss these very large scale bicontinuous
structures in the context of their function, formation, and evolution.
