ABSTRACT
Among 16 spatangoid species examined, 5 species showing morphological extremes in test profile are recognized. They fall into three morphological types. Each of them corresponds to a distinctive habitat; the urchins with globular profiles burrow deeply in sand, the urchin with a flat profile burrows to a shallow depth in sand and the urchins with wedge-shaped profiles burrow in mud. The three morphological types adopt different burrowing mechanisms derived from differences in test shape and resultant function of spines. Each burrowing mechanism corresponds to specific conditions of habitat (i.e. burrowing depth and grain size of sediment).
