Sea urchin spine. Light micrograph of a transverse section through the concentric and radial structures of an Echinus sp. sea urchin spine. Sea urchins are marine invertebrates. Their spines consist of calcareous radial plates 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. The spines of a sea urchin are designed to protect it from predators, and some of the spines carry poison glands. Magnification: 20x when printed at 10 centimetres wide.