Systematics

CALCAREA

Marine Porifera in which the mineral skeleton is composed entirely of calcium carbonate. The skeleton is composed of free diactine, triactine, tetractine and/or polyactine spicules, to which can be added a solid basal calcitic skeleton with basal spicules either cemented together or completely embedded in an enveloping calcareous cement. The aquiferous system can be asconoid, syconoid, sylleibid or leuconoid. Members of the Calcarea are viviparous, and their larvae are blastulae (Manuel et al. 2002).

DEMOSPONGIAE

Demospongiae (Porifera) have discrete cellular elements, parenchymella or blastula larvae, with either viviparous or oviparous reproductive strategies. The skeleton is composed of monaxonic or tetraxonic siliceous spicules (never triaxonic) bound together with spongin in discrete fibres or loosely aggregated, and ubiquitous collagenous filaments forming the ground substance of the intercellular matrix. Spicules and/or fibres, or both, may be absent in some taxa, and several other groups with solid calcitic or siliceous skeletons are also included (‘sclerosponges’, ‘sphinctozoans’, ‘lithistids’) rendering the class morphologically heterogeneous.

HEXACTINELLIDA

Hexactinellida Schmidt (Porifera) are deep-water marine sponges defined by their production of siliceous spicules of hexactinic, triaxonic (cubic) symmetry, or shapes clearly derived from such forms by reduction of primary rays or terminal branches added to the ends of primary rays. They lack calcareous minerals and sclerified organic spongin as skeletal components. Siliceous spicules may be entirely loose, or partially fused to form a rigid basal and choanosomal framework. Their living tissues are mainly syncytial, with distinctive porous plugs joining differentiated regions of the syncytium to each other or to discrete cellular components. Flagellated-collar units are anucleate (Reiswig, 2002).

HOMOSCLEROMORPHA

Homoscleromorphs are a small group of exclusively marine sponges. They are encrusting or lumpy with a smooth surface, usually occurring at shallow depths. If present, it consists of small calthrops (peculiar type of tetractines spicules) and/or their derivatives (lophose calthrops, diodes and triodes). These spicules are evenly distributed in the sponge body and do not form a well-organized skeleton. Spicules are secreted by sclerocytes, pinacocytes of the external epithelium and also, to a lesser degree, by pinacocytes of the internal epithelia (Maldonado & Riesgo, 2007). In contrast to the axial filament of siliceous spicules in Demospongiae and Hexactinellida, the organic core of the spicules of Homoscleromorpha is amorphous, indicating a possible lack of a tertiary structure of the protein contained (Uriz, 2006). see also Gazave et al. 2012.

Sources

Manuel M, Borojevic R, Boury-Esnault N, Vecelet J. 2002. Class Calcarea Bowerbank, 1864. dalam Hooper JNA, van Soest RWM [editor], Systema Porifera: A Guide to the Classification of Sponges. New York: Kluwer Academic/Plenum Publisher. pp 1103-1116. doi: 10.1007/978-1-4615-0747-5_115

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