The internal skeleton of a sea urchin, or test, is made up of many hexagonally shaped calcareous plates known as ossicles. Each ossicle is a single crystal of calcium carbonate in a form known as calcite. The ossicles are fenestrated (have holes) which, remarkably, increases their relative strength. The adjoining plates or ossicles in a sea-urchin test are not fused; rather, they are separated by tissue sutures within which crystalline calcium carbonate is secreted for growth. A newly metamorphosed sea urchin Strongylocentrotus purpuratus has 10 interambulacral plates (5 in each column). The number of interambulacral plates increases wtih age/size from 10 to over 40 in 70-80mm diameter individuals.