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Science 28 November 1969:
Vol. 166. no. 3909, pp. 1150 - 1152
DOI: 10.1126/science.166.3909.1150
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Articles

Crystal Orientation and Plate Structure in Echinoid Skeletal Units

Hans-Ude Nissen 1

1 Labor für Elektronenmikroskopie, Institute für Kristallographie und Petrographie, ETH-Aussenstation Hönggerberg, CH-8049 Zurich, Switzerland

The submicroscopic morphology of magnesian calcite skeletal units of echinoids, revealed by scanning electron microscopy, was compared with crystal orientation data obtained by x-ray methods and with macroscopic morphology. The Perischoechinoidea and the Euechinoidea differ with regard to the shapes of their trabeculae. Nearly all plates and spines are single crystals. A variety of diJfferent directional relations of c- and a-axes to the main morphological directions are found for different species; adjacent plates with identical c-axis orientation differ strongly in orientation of their a-axes. Fracture surfaces of single trabeculae show cleavage planes and zonal layers attributed to changes in secretion conditions.


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Science. ISSN 0036-8075 (print), 1095-9203 (online)