SEM micrographs of a Sea Urchin spine.

A: Cross-section of the spine showing its hollow center and porous wall architecture (scale bar = 1.0 mm). B: Outer surface of the spine. Barbs point toward the spine’s tip, shown here on left (scale bar = 200 micron). C: Fracture surface of a wedge of the spine (scale bar = 100 micron). The appearance is reminiscent of fracture morphology of glass. Top arrow points to the root of crack initiation. Bottom arrow points to a feature on the external surface of the wedge, also seen in B, which identifies the external surface, and confirms that crack initiation started on the outer surface.