Class HEXACTINELLIDA Schmidt, 1870

Hexactinellida

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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. [186] ((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.

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. [1201]

Diagnosis

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.

ID Keys

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