-Believed to have been formed by the uniting of protozoa to a colonial organism.
-Benthonic, with an internal skeleton of spicules of calcite or silica.
- Classified on the nature of these spicules and way in which they unite.
-13 genera, of which 4 survived into Triassic.
-Surviving Palaeozoic forms dominated sponge faunas through Triassic. Didn't radiate quickly however.
-Doesn't have abundant fossil record, not being found at many locations.
