9 Sea Urchin Facts

Test

The skeleton of a sea urchin is referred to as its test. It is composed of plates of calcium carbonate fused together to impart hardness to the skeleton. The test is covered by a thin layer of epidermis.

Shell

The shell of a sea urchin is found within the rigid test. The shells are also composed of plates fitted together tightly in order to provide protection to the organism from physical damage. Spines outline the shells and help the animal camouflage or fight predators. Each spine ranges from 1 to 3 centimeters in length with a width of 1 to 2 millimeters on average.

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