A large class of marine echinoderms that includes sea urchins, sand dollars, and heart urchins, characterized by round or flattened bodies covered with spines.
A taxonomic class name formed from echino- + -oidea (resembling), formally classified by paleontologists studying fossil and living sea urchins in the 19th century.
Fossil sea urchins are found on every continent and helped scientists prove that Earth's landmasses move—the same extinct species on Africa and South America proved continental drift.
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