sentence: Movement is done by the action of the spines.
sentences_before: All sand dollars have a rigid skeleton known as a 'test'. This is the typical white disk found washed up on beaches. The living animals have a skin of movable spines on the test.
sentences_after: Like other sea urchins, sand dollars have five paired rows of pores. The pores are arranged in a petal-like pattern. These pores are perforations in the endoskeleton through which the tube feet, used in gas exchange, project from the body.
concept_name: movement
headings: Sand dollar
categories: Echinoderms