Formation of Compound Plates in the Skeleton of the Sea Urchin Tripneustes gratilla
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The sea urchin’s body, or test, is divided into 20 columns of calcareous skeletal plates. These columns are divided into paired, alternating regions. The ambulacral region is distinguished by the presence of tube feet. The interambulacral region separates the amulacral columns. The ambulacral region is composed of compound plates. Compound plates are three skeletal plates that have compounded (fused). Compound plates are distinctive because they typically bear one spine. Singular skeletal plates within the ambulacral region are added at the top of the organism under a specialized plate called the ocular plate. The present study explores how compound plates form from singular compound plates.