CRYSTALLINE AXES OF THE SPINE AND TEST OF THE SEA URCHINSTRONGYLOCENTROTUS PURPURATUS:DETERMINATION BY CRYSTAL ETCHING AND DECORATION

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Abstract
By means of crystal etching and scanning electron microscopy the c- and a- axes of skeletal units of Strongylocentrotus purpuratus have been determined. Crystal etching and crystal decoration were found to be equally useful in demonstrating that the spines are single crystals of magnesian calcite. All regions of spine cross sections had common c- and a-axes. Comparisons of etched figures to decorated crystals on the same cross section of a spine showed that the grooves of the etch figures were parallel to the edges of the upper faces of the decorated crystals. The a-axes of the primary spines differed from those of the interambulacral plates to which they were attached. Certain regions of the tubercle were shown by SEM examination of decorated tests to be polycrystalline aggregates lacking uniform crystal orientation, whereas the remaining portions of the interambulacral plates shared the same a-axes. The primary plates and demiplates of the ambulacral plates were each single crystals whose a-axis, and often c-axis, orientations were independent of adjacent compartments. Reflective properties of decorated plates coated with gold-palladium have been shown to be useful in demonstrating differences in crystal orientation between adjacent interambulacral plates and between compartments of individual ambulacral plates.

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