Definition: Skeleton is composed of six-rayed siliceous spicules (triaxones or hexacts), ooccurring individually or fused together, usually forming rigid lattice-like skeletons. The body wall has a cavernous structure, with living tissue stretching across a framework around the cavities like a membrane. This tissue is syncytial, on both the dermal region (pinacoderm) and in the choanosome, in which the multnucleolate protoplasm is not divided into cells. Uniflagellated choanocytes are absent from this class of sponges, and the choanocytes are really only collar-flagellum units lining cylindrical chambers; hence they are referred to as 'flagellated chambers' or 'choanochambers' rather than 'choanocyte chambers" found in the classes Calcarea and Demospongiae. These unusual choanocytes are embedded in the membraneous protoplasm stretched between spicules (called 'plugged bridges' or trabeculae). For these reasons it has been suggested that glass sponges may in fact diverge from other sponges at the subphylum or even phylum level. Spicules occur three different regions, amd the localization of particular spicule types to particular areas is very
precise. Three zones are differentiated: the first lying on or just below the dermal membrane (dermal); tbe second lying in the interior of the trabeculae (parenchymal), and the third lying below the membrane around the atrial cavity (gastral). The geometry of megascleres and microscleres in Hexactinellida is very diverse (Hexactinellida spicules) and unlike the other classes axial canals of spicules are always square in cross-section. Larvae are incubated parenchymellas.
Remarks: Glass sponges are most commonly found in deep water, typically below 200 m in depth, although in boreal and austral seas many species may extend into shallower waters. There are about 600 described living species in two subclasses and four orders. Example: Regadrella
Revisions: Ijima (1927).
Taxa included in this file: none.
Species not treated here:
Caulophacus arcticus (Hansen, 1885) (Hexasterophora: Caulophacidae), Norway, deep water, cf. Arndt, 1935: 22.
Pheronema grayi Kent, (1869), Ireland, France, Portugal, deep water
Hyalonema thomsoni (Marshall, 1875), Ireland, France, Portugal, deep water
Hyalonema infundibulum Topsent (1896), France, Portugal, deep water
Aphrocallistes beatrix Gray (1858), Ireland, France, Portugal, deep water