The members of this class may be supported by siliceous spicules; these spicules are never six-rayed. In some, the spicules are scattered and may or may not be linked by a horny material called spongin. Other members of the class rely on spongin alone for support. Found at all depths, this class constitutes 90% of the Phylum Porifera including the freshwater sponges, the bath sponges, the boring sponges and Oscarella, the only genus of sponge which lacks any supporting material at all.
Top right: Chalina oculata, from the German deadnought battle-ship 'Kaiserine' scuttled at Scapa Flow, Scotland. Colonisation of ships, buoys and other structures by the brief, motile larval stages of sponges contributes to fouling of ships and boats by the undesirable growth of sea-weeds and animals that damages vessels and slows them down.