IT has long been recognized that the magnesian calcite skeleton of echinoderms is laid down as a crystal network by a syncytial protoplasmic mass that permeates the cavities within it. It is usually stated that all components except echinoid teeth1, be they spines, thecal plates, pedicellaria valves, or any other skeletal feature, behave crystallographically as a single crystal of calcite2–4. In other metazoan tissues that have been investigated the arrangement of the skeleton is that of a two phase system, with a softer and a harder phase intimately mingled. This is true, for example, of bone5, dentine, enamel6, arthropod cuticle7,8, brachiopod shell9,10, mollusc skeleton11 and coral12. Two phase construction is also seen in wood13. This double component nature of hard skeletal elements appears to impart considerable mechanical strength14. The almost universal occurrence of two phase construction in skeletal materials elsewhere and the considerable advantages such a structure clearly bestows make its lack in echinoderms surprising.