Although difficult to see through all the spines, sea urchins also have a hard outer body like that of its relatives. Their outer skeleton--called a test--is made up of ten fused plates that encircle the the sea urchin like the slices of an orange.

What type of skeleton does a sea urchin have?

The skeleton of sea urchin is an endoskeleton: the elements of test and spines are covered with a thin epidermis layer.

What is the skeleton of a sea urchin made of?

Echinodermata: Morphology Echinoderm skeletons are made up of interlocking calcium carbonate plates and spines. This skeleton is enclosed by the epidermis and is thus an endoskeleton. In some, such as sea urchins, the plates fit together tightly.