Ophiuroidea are characterized by

• A. Sea urchins with spherical tests
• B. Sea cucumbers with elongated bodies
• C. Sea stars with five arms and tube feet
• D. Brittle stars with a central disk and slender, highly flexible arms

Answer: (D)

The key idea is recognizing the distinctive body plan of brittle stars within echinoderms. Ophiuroidea have a central disk from which long, slender arms extend, and those arms are highly flexible and can bend in many directions. This setup lets them move with quick, graceful arm motions rather than relying on a broad, spreading body shape. In contrast, sea urchins have a rigid, spherical body with spines, sea cucumbers are elongated and leathery, and sea stars typically have broader arms radiating from a central disk and use their tube feet on the underside for movement. So, a central disk with slender, highly flexible arms best describes brittle stars.