Sea urchin spine. Polarised light micrograph of a cross-section through a spine from an Echinus sea urchin. Sea urchin spines consist of supporting calcareous radial plates, which run the length of the spine, embedded in a reticulate calcareous and organic matrix. The concentric layers indicate the age of the spine, in a manner similar to that of the growth rings of trees. Muscles attached to the base of the spine allow the spine to be moved. Such spines are typically a few millimetres across.