Spicules

Spicules, small and typically needle-shaped structures which form the elements of the skeleton in many invertebrates. They are composed of different materials, such as carbonate of lime in the Gorgonias, silica in the siliceous sponges, or chitinoid or fibrous material, as in freshwater sponges. They occur in many different groups, including the sponges, echinoderms, alcyonarians, corals, tunicates, and bryozoa. They may be united together to form regular lattice-like skeletons, as in the Hexactinellid Sponges, or into dense masses with the spicular structure obliterated, as in Corallium, the Red Coral, or may be loosely scattered through the soft fibres, as in the outer crust of Gorgonias and in horny sponges.