sentences:  Most species have two series of spines, primary (long) and secondary (short), distributed over the surface of the body, with the shortest at the poles and the longest at the equator.The spines are usually hollow and cylindrical.Contraction of the muscular sheath that covers the test causes the spines to lean in one direction or another, while an inner sheath of collagen fibres can reversibly change from soft to rigid which can lock the spine in one position.