| These spicules are up to 0.03 mm in diameter and taper to sharp tips. |
| Recently spicules from sponges of the class Hexactinellida have been identified in Ediacaran age rocks. |
| The spicules are interlocked or joined to form the shell wall. |
| A general overview on the structural organization of the proteinaceous filament inside spicules is presented. |
| Test walls appear to lack compositional zonation, consisting mostly of small quartz grains with some incorporated sponge spicules. |
| If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. |
