Reiswig and Mackie, 1983
Definition: Porifera with syncytial organization of soft tissues, including the choanocytes (here dubbed choanomeres or collar units, to distinguish them from the discrete choanocytes) and the lining of the surface and the canals. These syncytial units are separated from each other by "plug-junctions". The collar units are supported by a secondary organic reticulum at the level of the microvilli. Mineral skeleton of siliceous spicules of triaxone symmetry with a quadrangular axial filament. Larvae are parenchymella types with stauractinic larval spicules.
A single class: Class Hexactinellida with characters of the subphylum.
N.B. The existence of the subphylum remains in debate. Possibly, the possession of siliceous spicules is an indication of a subphylum Silicea excluding the Calcarea.