The skeleton is  composed of celestite (strontium sulphate). When they die the skeleton rapidly dissolves. The skeletal elements usually consist of 20 evenly spaced, radiating, solid spicules, not always of the same length, which characteristically meet at the centre Where they meet, the spicules may cross loosely or fuse. The spicules produce lateral plates, forming a spherical perforated shell  but in the latter the spicules do not extend to the centre. Generally spherical, but some have their greatest diameter around the equator