Which lineage of echinoderms is made up of about 950 species of sea urchins and sand dollars, which have their ossicles fused into solid tests that restrict flexibility but provide excellent protection?

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Echinoidea is the lineage that includes sea urchins and sand dollars, characterized by their ossicles fused into a solid structure known as a test, which offers protection but limits flexibility.

Which Lineage of Echinoderms Is Made Up of About 950