Practice question

Sea urchins display classic echinoderm traits that place them firmly within phylum Echinodermata alongside starfish, brittle stars, and sea cucumbers. Adults show pentaradial symmetry, endoskeletal ossicles with spines, and a water vascular system operating tube feet for locomotion and feeding. Embryologically they are deuterostomes with radial cleavage, enterocoelic coelom formation, and blastopore becoming anus, aligning them phylogenetically closer to chordates than molluscs or arthropods. Bilateral larvae metamorphose into radial adults, confirming echinoderm affiliation rather than chordate, molluscan, or arthropod body plans.