Organisms under the class Echinoidea can cause major damage to kelp, but they also help keep algal populations under control. The skeleton is almost always made up of tightly interlocking plates that form a rigid structure or test -- in contrast with the more flexible skeletal arrangements of starfish, brittle stars, and sea cucumbers. Test shapes range from nearly globular, as in some sea urchins, to highly flattened, as in sand dollars. Living echinoids are covered with spines, which are movable and anchored in sockets in the test. Echinoideas are also classified by the symmetry of the test, the number and arrangement of plate rows making up the test, and the number and arrangement of respiratory pore rows called petals. Traditionally, echinoids have been divided into two subgroups: regular echinoids, with nearly perfect pentameral symmetry; and irregular echinoids with altered symmetry.