Nature 🦋🌳🦩🌬️ on Instagram: "Sea urchins are spiny, globular marine invertebrates that live on the ocean floor. They are echinoderms, which also includes sand dollars and sea cucumbers. Appearance: • Have a round, spiny shell called a test • Have tube feet that they use to move • Have spines that they use for defense, anchoring, and moving Behavior: • Crawl slowly over the ocean floor • Use their spines as stilts to move • Are non-aggressive, but can injure humans if stepped on or brushed int