If they lose an arm it will grow back. It takes a long time though. Maybe as long as a year. Sea Star Cousins Sea urchins are related to sea stars. They look like a pin cushion crawling around in the tide pool.
Urchins have a round body without arms.
Their body is protected by a shell called a test. The test is covered by many movable spines and tube feet. The spines and tube feet help in walking. The sea urchins' mouth is on the bottom of the test. Five large chewing teeth scrape little plants off rocks and chew seaweed.