Class HEXACTINELLIDA Schmidt, 1870
Compiler and date details
April 2012 - John N.A. Hooper, Queensland Museum, Brisbane, Queensland, Australia(1994, updated 1999, 2004, 2011); Felix Wiedenmayer Naturhistorisches Museum Basel, Basel, Switzerland (1994)
- Hexactinellida Schmidt, E.O. 1870. Grundzüge einer Spongien-Fauna des Atlantischen Gebietes. Leipzig : Engelmann 88 pp. 6 pls. (as Hexactinellidae Schmidt, 1870).
- Coralliospongia Gray, J.E. 1867. Notes on the arrangement of sponges, with the description of some new genera. Proceedings of the Zoological Society of London 1867: 492-558 pls 27-28 (in part).
- Vitrea Thomson, C.W. 1868. On the ‘Vitreous’ Sponges. Annals and Magazine of Natural History 4 1(2): 114–132, pl. IV.
- Hyalospongiae Claus, C.F.W. 1872. Spongiae (Porifera). pp. 186–197 in Claus, C.F.W. Grundzüge der Zoologie. Marburg und Leipzig : Elwert.  ((in part)).
- Sexradiatospongia Gray, J.E. 1874. On the Arrangement of Sponges. Annals and Magazine of Natural History 4 13(76): 284–290.
- Triaxonia Schulze, F.E. 1886. Ueber den Bau und des System der Hexactinelliden. Abhandlungen der Königlich-Preussischen Akademie der Wissenschaften. Berlin 1886: 1-97.
2012 - INTRODUCTION
The Hexactinellida was revised by Reiswig (2002, and additional papers therein), and most recently the systematics was corroborated by independent molecular datasets (e.g. Dohrmann et al. 2008, 2012), supporting the monophyly of the two subclasses and six orders.
In January 2012 there were 613 valid species of Hexactinellida recognised (from 1,019 nominal species) (Van Soest et al. 2011), all exclusively marine and predominantly from deeper waters, with the list of higher taxa and their current global diversity summarised below (from Hooper, Van Soest & Pisera 2011).
Class Hexactinellida Schmidt, 1870 (2 subclasses)
Subclass Amphidiscophora Schulze, 1886 (1 order)
Order Amphidiscosida Schrammen, 1924 (3 families)
Family Hyalonematidae Gray, 1857 (5 genera, 129 species)
Family Monorhaphididae Ijima, 1927 (1 genus, 1 species)
Family Pheronematidae Gray, 1870 (6 genera, 45 species)
Subclass Hexasterophora Schulze, 1886 (5 orders)
Order Hexactinosida Schrammen, 1903 (8 families, + 1 incertae sedis)
Family Aphrocallistidae Gray, 1867 (2 genera, 12 species)
Family Auloplacidae Schrammen, 1912 (1 genus, 4 species)
Family Craticulariidae Rauff, 1893 (1 genus, 1 species)
Family Cribrospongiidae Roemer, 1864 (1 genus, 1 species)
Family Dactylocalycidae Gray, 1867 (2 genera, 7 species)
Family Euretidae Zittel, 1877 (17 genera, 67 species)
Family Farreidae Gray, 1872 (6 genera, 61 species)
Family Tretodictyidae Schulze, 1886 (8 genera, 30 species)
Hexactinosida incertae sedis (4 genera, 4 species)
Order Aulocalycoida Tabachnick & Reiswig, 2000 (2 families)
Family Aulocalycidae Ijima, 1927 (6 genera, 8 species)
Family Uncinateridae Reiswig, 2002 (2 genera, 3 species)
Order Fieldingida Tabachnick & Janussen, 2004 (1 family)
Family Fieldingiidae Tabachnick & Janussen, 2004 10 (1 genus, 4 species)
Order Lychniscosida Schrammen, 1903 (2 families)
Family Aulocystidae Sollas, 1887 (2 genera, 6 species)
Family Diapleuridae Ijima, 1927 (1 genus, 3 species)
Order Lyssacinosida Zittel, 1877 (3 families)
Family Euplectellidae Gray, 1867 (27 genera, 99 species)
Family Leucopsacidae Ijima, 1903 (5 genera, 12 species)
Family Rossellidae Schulze, 1885 (22 genera, 192 species)
1994 - INTRODUCTION
In the glass sponges (class Hexactinellida) the skeleton is made up of six-rayed siliceous spicules (hexacts), occurring 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. The ‘tissue’ is syncytial in both the dermal region (pinacoderm) and the choanosome: the multinucleate protoplasm is only partially divided into cells. Uniflagellated choanocytes are absent from this class of sponges, and the choanocytes are really only collar-flagellum units (or ‘collar bodies’) lining cylindical to spherical chambers. The chambers are referred to as ‘flagellated chambers’ or ‘choanochambers’ rather than ‘choanocyte chambers’ as in the classes Calcarea and Demospongiae. The unusual ‘collar bodies’ are embedded in the membranous protoplasm stretched between spicules (by ‘plugged bridges’). Some authors have suggested that these unusual features support differentiation of the glass sponges from other sponges at the subphylum (Reiswig & Mackie 1983) or even phylum level (Bergquist 1985). Spicules occur in three different regions, and 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); the 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 and, unlike the other classes, axial canals of spicules are always square in cross-section. Larvae, known for only one species, are incubated and parenchymellae-like (termed trichimella (Boury-Esnault & Vacelet 1994)).
Glass sponges are found most commonly in deep water, typically below 200 m, although in boreal and austral seas many species may extend into shallower waters. There are about 600 named species described for the class, although probably fewer than this are valid, with an estimated 400 living species in two subclasses and four orders.
The glass sponges of the subclass Amphidiscophora have birotulate microscleres (Hooper & Wiedenmayer 1994: figs 218–221) but lack hexaster microscleres. They are not attached to the substratum but are embedded in soft sediments by means of single or tufts of basal monactine spicules (Hooper & Wiedenmayer 1994: fig. 193). Flagellated chambers are continuous at their openings, not discrete as in other classes of sponges. One order of living species is recognised, the Amphidiscosida, comprising three families. Records are published for two of these for the Australian fauna. The third family is the Monorhaphididae from the coast of east Africa, Timor, and the Philippines (Hartman 1982). This family has been collected recently on the continental slope off the Great Barrier Reef (P. Arnold, pers. comm.) and New Caledonia (J. Vacelet, pers. comm.) but these records are not yet published.
Sponges in the second subclass, Hexasterophora, have hexasters as their microscleres (Hooper & Wiedenmayer 1994: figs 208–217, 222–224), but lack birotulates. Growth forms are diverse and the sponge usually has a fixed basal attachment. When present, basal spicules occur in tufts and consist of pentactines (e.g. Figs 198, 200, 204) or anisodiactines (e.g. Fig. 194). Three extant orders and twelve families are recognised.
In members of the order Hexactinosida, the fusion of hexactines (Hooper & Wiedenmayer 1994: fig. 201) produces a rigid parenchymal skeleton; dermal and gastral spicules are usually pentactines (Hooper & Wiedenmayer 1994: figs 198, 200, 204), with the unpaired ray directed inwards, or sometimes stauractines (Hooper & Wiedenmayer 1994: fig. 202), and these spicules are usually connected by ‘tissue’ only. Of the six families included, published records are available for only three for Australian waters. Two of the three extra-limital families, the Aphrocallistidae and the Tretodictyidae, are widely distributed in the world’s oceans and the third is known from the Atlantic Ocean and Antarctic region.
In the Lychniscosida parenchymal megascleres are lychniscs (Hooper & Wiedenmayer 1994: fig. 226) or derivatives, united in a rigid framework (Hooper & Wiedenmayer 1994: fig. 225), and the central part of each spicule is surrounded by twelve struts arranged like the edge of an octahedron; sponges are firmly attached to substrata. Three extant families are recognised in this order, one with published records for the Australian fauna. Of the others, the Dactylocalycidae, is known from Bermuda, the Caribbean Sea and off the coast of Portugal (Hartman 1982), and the Diapleuridae is known from depths of 90–204 m off the western tip of New Guinea (Ijima 1927; Hartman 1982).
Members of the order Lyssacinosida have parenchymal megascleres varying from hexactines (Hooper & Wiedenmayer 1994: fig. 201) to rhabdodiactines (similar to Figs 194, 195, but curved), usually occurring free in tissues, sometimes secondarily fused to form a rigid framework; dermal spicules consist of a single layer of large pentactines (Figs 198, 200, 204) or hexactines (Figs 201, 206), with the single, long, proximal ray directed inwards, or with a layer of small dermal spicules overlying the larger hypodermal pentactines, with the unpaired ray extending inwards.
Four extant lyssacinosid families are recognised, for three of which records are published for Australian waters. The fourth, the Leucopsacadidae is found in the Indian Ocean, in the vicinity of Japan, and in the South Atlantic Ocean (Hartman 1982). So far no records have been published for this group in the Australian fauna.
Further taxonomic information on this class is given by Ijima (1927), Hartman (1982), Reiswig & Mackie (1983) and Reiswig (1990).
Taxonomic Decision for Synonymy
- Reiswig, H. 2002. Class Hexactinellida Schmidt, 1870. pp. 1201-1202 in Hooper, J.N.A. & Soest, R.W.M. Van (eds). Systema Porifera. A guide to the classification of sponges. New York : Kluwer Academic/Plenum Publishers Vol. 2. 
Body form variable, including monomeric tube-, cup-, funnel-, cap-, club-, lobate- and blade-forms, with or without lateral diverticula, and branching tubule or solid cylinders, with or without anastomoses; encrusting forms unknown. Entirely marine on hard or soft substrate; attached directly to hard substrate either firmly by basal disc cementation (basiphytous) or loosely by grappling anchor spicules (lophophytous); attached to soft substrate by rooting spicules (lophophytous) or rarely by solid roots (rhizophytous). Skeletons of (mineral) siliceous spicules, as separate elements or joined by silica deposition and (organic) thin lattice of collagen; dense spongin, calcareous deposition and aspicular forms unknown. Siliceous spicules generally divisible into megascleres and microscleres on basis of form, size, and function; intermediatesize surficial spicules sometimes distinguished as mesoscleres. Megascleres basically hexactins but reduction of one or more rays results in pentactins, tetractins (usually stauractins), triactins (usually tauactins), diactins and monactins (usually basal anchors, sceptrules, or sceptres); megasclere ray branching is rare. Rigid skeletal frames of hexactine megascleres often fused at or soon after spicule formation along parallel rays or at ray crossing points are known as dictyonal frameworks; rigid frames formed by fusion of non-hexactine megascleres (usually diactins) long after spicule formation in more basal body regions of sponges with otherwise separate spicules are not considered dictyonal frameworks. Living tissues are mainly syncytial, including dermal and atrial membranes, internal trabeculae and flagellated chamber walls with anucleate collar bodies. Discrete nucleate cellular components embedded in pockets or capsules of the syncytium may be joined together or to syncytium by distinctive porous plugs or may be entirely separate. Cellular differentiation is slight. Translocation of intrasyncytial materials is by symplastic flow transport across plugs and cytoplasmic streaming within open syncytia; stimuli are disseminated throughout entire individuals by membrane conduction. The flagellated chambers are large and eurypylous, arranged between very thin-walled inhalant and exhalant water conducting canals in syconoid, sylleibid or leuconoid pattern. All known members are active water pumpers and particle filterers; food particle acquisition by passive flow and sedimentation appears minor or non-existent. Larval form, the trichimella, is well-known for only one species; among many distinctive features it has an equatorial, subepidermal sheath of multiflagellate cells, the flagella of which project through pores of the overlying syncytial epidermis to provide motility. Reproductive strategy is viviparous in the few forms where known.
KEY TO ORDERS OF RECENT HEXACTINELLIDA
(1) Amphidisc microscleres present; hexaster microscleres absent ------------------------------------ Amphidiscosida
Astral (hexasters) microscleres present; amphidisc microscleres rare --------------------------------------------------- 2
(2) Dictyonal framework formed of hexactins fused by secondary silicification ----------------------------------------- 3
Dictyonal framework of fused hexactins absent; fusion of non-hexactine megascleres (diactins, triactins, tetractins) may occur in older parts --------------------------------------------------------------------------------------- Lyssacinosida
(3) Dictyonal nodes mainly lychniscid, but some may be simple ----------------------------------------- Lychniscosida
Dictyonal nodes simple ---------------------------------------------------------------------------------------------------------- 4
(4) Dictyonal meshes consistent in size and shape; dictyonal rays one mesh in length; dictyonal strands as serially aligned beam pairs -------------------------------------------------------------------------------------------- Hexactinosida
Dictyonal rays irregular in size and shape; dictyonal rays exceed mesh length; dictyonal strands as long single rays or multiple (more than 2) overlapping rays ----------------------------------------------------------------- Aulocalycoida
Reiswig, H. 2002. Class Hexactinellida Schmidt, 1870. pp. 1201-1202 in Hooper, J.N.A. & Soest, R.W.M. Van (eds). Systema Porifera. A guide to the classification of sponges. New York : Kluwer Academic/Plenum Publishers Vol. 2. 
Bergquist, P.R. 1985. Poriferan relationships. In Morris, S.C., George, J.D., Gibson, R. & Platt, H.M. (eds). The origin and relationships of lower invertebrates. Proc. Systematics Assoc. London, September 1983, Special Vol. 28: 14–27.
Boury-Esnault, N. 1994. Preliminary studies on the organisation and development of a hexactinellid sponge from a Mediterranean cave, Oopsacas minuta. pp. 407-415 in Soest, R.W.M. Van, Kempen, T.M.G. van & Braekman, J.-C. (eds). Sponges in Time and Space. Rotterdam : Balkema.
Dohrmann, M., Collins, A.G. & Wörheide, G. 2009. New insights into the phylogeny of glass sponges (Porifera, Hexactinellida): monophyly of Lyssacinosida and Euplectellinae, and the phylogenetic position of Euretidae. Molecular Phylogenetics and Evolution 52: 257-262
Dohrmann, M., Göcke, C., Janussen, D. Reitner, J., Lüter, C. & Wörheide, G. 2011. Systematics and spicule evolution in dictyonal sponges (Hexactinellida: Sceptrulophora) with description of two new species. Zoological Journal of the Linnean Society 163: 1003–1025
Dohrmann, M., Haen, K.M., Lavrov, D.V. & Wörheide, G. 2012. Molecular phylogeny of glass sponges (Porifera, Hexactinellida): increased taxon sampling and inclusion of the mitochondrial protein-coding gene, cytochrome oxidase subunit I. Hydrobiologia in press
Dohrmann, M., Janussen, D. , Reitner, J., Collins, A. G. & Wörheide, G. 2008. Phylogeny and evolution of glass sponges (Porifera, Hexactinellida). Systematic Biology 57: 388-405 
Erpenbeck, D. 2007. On the molecular phylogeny of sponges (Porifera). Zootaxa 126(1668): 107 - 126
Hartman, W.D. 1982. Porifera. pp. 640-666 in Parker, S.P. (ed.). Synopsis and Classification of Living Organisms. New York : McGraw-Hill Vol. 1.
Ijima, I. 1927. The Hexactinellida of the Siboga Expedition. 1-383 26 pls in Weber, M.W.C. (ed.). Siboga-Expeditie, livr. 106, monogr. 6. Leiden : J.E. Brill.
Reiswig, H. 2002. Class Hexactinellida Schmidt, 1870. pp. 1201-1202 in Hooper, J.N.A. & Soest, R.W.M. Van (eds). Systema Porifera. A guide to the classification of sponges. New York : Kluwer Academic/Plenum Publishers Vol. 2.
Reiswig, H.M. 1990. Correction of Ijima's (1927) list of Recent hexactinellid sponges (Porifera). Proceedings of the Biological Society of Washington 103(3): 731-745
Reiswig, H.M. 2006. Classification and phylogeny of Hexactinellida (Porifera). Canadian Journal of Zoology 84(2): 195-204
Reiswig, H.M. & Mackie, G.O. 1983. Studies on Hexactinellid sponges. III. The taxonomic status of Hexactinellida within the Porifera. Philosophical Transactions of the Royal Society of London B 301: 419-428
van Soest, R. 2011. Hexactinellida. In: Van Soest, R.W.M, Boury-Esnault, N., Hooper, J.N.A., Rützler, K, de Voogd, N.J., Alvarez de Glasby, B., Hajdu, E., Pisera, A.B., Manconi, R., Schoenberg, C., Janussen, D., Tabachnick, K.R., Klautau, M., Picton, B., Kelly, M., Vacelet, J. (2011) World Porifera database. Accessed through: Van Soest, R.W.M, Boury-Esnault, N., Hooper, J.N.A., Rützler, K, de Voogd, N.J., Alvarez de Glasby, B., Hajdu, E., Pisera, A.B., Manconi, R., Schoenberg, C., Janussen, D., Tabachnick, K.R., Klautau, M., Picton, B., Kelly, M., Vacelet, J. (2011) World Porifera database at http://www.marinespecies.org/porifera/porifera.php?p=taxdetails&id=22612 on 2012-01-27. http://www.marinespecies.org/porifera/porifera.php?p=taxdetails&id=22612 (checked on 2012-01-27)