The ultrastructure of the echinoid skeleton

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Abstract
The ambulacral and interambulacral plates of S. droebachiensis, S. pallidus, and S. sachalinicusconsist of four types of calcareous layers: a thin compact layer on the outer surface of the plate, a layer of skeletal trabeculae in the interior, another thin compact layer, and a spongy layer on the inner surface of the plate. The resorption, reorganization, and new growth of the skeleton takes place on the outer surface of the plates. Primary tubercles increase their size through the resorption of the surrounding secondary and miliary tubercles, and thorns. These primary tubercles then develop a new skeleton on the resorbed area. Skeletal processes which develop the mammelon and areole bear the spine, while other skeletal processes, which remain simple, bear the pedicellariae. The plates increase in thickness and area by increasing the number of skeletal trabeculae and by the growth of these trabeculae at their free ends. The pore pair develops from a notch in the adorai side of the primary- and demiplates. The adradial side of the plate grows around the adradial tube of the bipartite proximal section of the tube foot, and thus subdivides the pore hole. A maximum of one ambulacral plate per series is resorbed in old S. droebachiensis, test diameter 39.5-43.5 mm.

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