Ultrastructure of Spines in Heart Urchins of the Genus Echinocardium

We studied and compared the ultrastructure of the skeleton of spines in the sea urchins Echinocardium cordatum (from the Sea of Japan and the Mediterranean Sea) and E. mediterraneum. The spines were identified depending on their position on the test: anterior, anal, latero-dorsal, latero-ventral, subanal, apical spines, and plastron spines. In addition to the high individual plasticity of spines, which is now used as a specific character of heart urchins, we propose a more stable morphological pattern including the features of several types of spines that helps more clearly characterize heart urchins from the Sea of Japan and the Mediterranean Sea.