o Metazoa `--o PORIFERA |--Hexactinellida `--+--+--Demospongiae | `?-Archaeocyatha |==Sclerospongiae (polyphyletic)? |==Stromatoporoidea (polyphyletic)? `--+--Calcarea `--Radiata / Eumetazoa
Demosponges are the most widespread and advanced class of sponges, as well as the largest and most diverse class. Some 90 to 95% of all sponge species belong here.
Demosponges construct their skeletons from one-rayed to four-rayed (monaxon or tetraxon) siliceous spicules, or of spongin fibres, or both. Spicules, when present, usually differentiated into megascleres (larger) and microscleres (smaller). Most living demosponges have skeletons of unfused spicules, although due to preservational effects, the fossil record of demosponges is mostly of fused forms.
Demosponges have a leuconoid construction (above), with many small, round, flagellated chambers.
While this class does contain freshwater groups, most are marine. Some demosponges are more than a cubic meter in size. There are three subclasses, the Homoscleromorpha, Tetractinomorpha, and Ceractinomorpha.
Fossil RecordLithistida, a taxon that ranges from the Cambrian to the present, and are common from the Jurassic onward.
Lithistids are distinguished by irregular knobby spicules known as "desmas" that interlock to hold the skeleton together in a reticulate wall of great thickness. They fossilize readily and in recognizable form because the interlocking desmas retain the original form of the sponge. This group, which includes many fossil demosponge taxa, is now considered polyphyletic with members in both the Tetractinomorpha and the Ceractinomorpha. However, most workers have refrained from allocating the various families, owing to limited knowledge of the group and the fact that the numerous fossil genera would be difficult or impossible to place since the free spicules are almost always lost upon death of the sponge. [Moore et al, p.93, Clarkson, p.53, Smecher iNet ]