and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This "irregular" body form has evolved to allow the animals to burrow through sand or other soft materials.

Systems

Musculoskeletal

The internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate covered by a thin dermis

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