Plate construction of the spatangoid test

摘要

Twelve spatangoid species belonging to seven families were examined. All test plates observed have the common basic three-layer construction composed of outer perforate stereom, middle galleried stereom, and inner laminar stereom. The thickness of the dorsal and the latero-ambital plates is proportional to the test size and not related to taxonomic difference. In sand-dwelling spatangoids, the anterior plates are thicker than other plates, while in mud-dwelling spatangoids, the posterior plates are thicker. The difference of the plate thickness is attributable to different methods of burrowing. In an epifaunal spatangoid, which crawls on muddy bottoms of deep sea, has obviously thinner plates than burrowing urchins.