Absence of Organic Phase in Echinoderm Calcite

Abstract

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 calcite^2–4. 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 cuticle^7,8, brachiopod shell^9,10, 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.