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The Demospongiae are the largest class in the phylum Porifera. In fact, the demosponges include 81% of all species of sponges with nearly 7,000 species worldwide (World Porifera Database〔(【引用サイトリンク】title=World Porifera Database )〕). They are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges. The many diverse orders in this class include all of the large sponges. Most are marine dwellers, but one order (Spongillida) live in freshwater environments. Some species are brightly colored, with great variety in body shape; the largest species are over across.〔 They reproduce both sexually and asexually. == Classification and Systematics == The Demospongiae have an ancient history, the first demosponges may have appeared during the Precambrian deposits at the end of the Cryogenian "Snowball Earth" period, where their presence has been indirectly detected by fossilized steroids, called steranes, hydrocarbon markers characteristic of the cell membranes of the sponges, rather than from direct fossils of the sponges themselves. They represent a continuous 100-million-year-long chemical fossil record of demosponges through the end of the Neoproterozoic.〔Gordon D, Love ''et al.'', "Fossil steroids record the appearance of Demospongiae during the Cryogenian period", ''Nature'', 2009〕 The earliest Demospongiae fossil was discovered in the lower Cambrian (Series 2, Stage 3; approximately 515 Ma) of the Sirius Passet Biota of North Greenland:〔Botting J.P., Cárdenas P., Peel J.S. (2015) "A crown-group demosponge from the early Cambrian Sirius Passet Biota, North Greenland". ''Palaeontology''. 58(1):35–43 〕 this single specimen had a spicule assemblage similar to that found in the subclass Heteroscleromorpha. The earliest sponge-bearing reefs date to the Early Cambrian (they are the earliest known reef structure built by animals, exemplified by a small bioherm constructed by archaeocyathids and calcified microbes at the start of the Tommotian stage about 540–535 million years ago (Mya), found in southeast Siberia.〔Robert Riding and Andrey Yu. Zhuravlev, "Structure and diversity of oldest sponge-microbe reefs: Lower Cambrian, Aldan River, Siberia", ''Geology'' 23.7 (July 1995:649-52) 〕 A major radiation occurred in the Lower Cambrian and further major radiations in the Ordovician possibly from the middle Cambrian. (Finks, 1970 The Systema Porifera (2002) book (2 volumes) was the result of a collaboration of 45 researchers from 17 countries led by editors J. N. A. Hooper and R. W. M. van Soest. This milestone publication provided an updated comprehensive overview of sponge systematics, the largest revision of this group (from genera, subfamilies, families, suborders, orders and class) since the start of spongiology in the mid-19th century. In this large revision, the extant Demospongiae were organized into 14 orders that encompassed 88 families and 500 genera. Hooper and van Soest (2002) gave the following classification of demosponges into orders: * Subclass Homoscleromorpha Bergquist 1978 * * Homosclerophorida Dendy 1905 * Subclass Tetractinomorpha * * Astrophorida Sollas 1888 * * Chondrosida Boury-Esnault & Lopès 1985 * * Hadromerida Topsent 1894 * * Lithistida Sollas 1888 * * Spirophorida Bergquist & Hogg 1969 * Subclass Ceractinomorpha Lévi 1953 * * Agelasida Verrill 1907 * * Dendroceratida Minchin 1900 * * Dictyoceratida Minchin 1900 * * Halichondrida Gray 1867 * * Halisarcida Bergquist 1996 * * Haplosclerida Topsent 1928 * * Poecilosclerida Topsent 1928 * * Verongida Bergquist 1978 * * Verticillitida Termier & Termier 1977 However, molecular and morphological evidence show that the Homoscleromorpha do not belong in this class. The Homoscleromorpha was therefore officially taken out of the Demospongiae in 2012, and became the fourth class of phylum Porifera.〔Gazave E., Lapébie P., Ereskovsky A., Vacelet J., Renard E., Cárdenas P. and Borchiellini C, "No longer Demospongiae: Homoscleromorpha formal nomination as a fourth class of Porifera", ''Hydrobiologia'' 687.1 (April 2012:3-10) 〕 Morrow & Cárdenas (2015)〔Morrow, Christine and Cárdenas, Paco, "Proposal for a revised classification of the Demospongiae (Porifera)", ''Frontiers in Zoology'' 12.7 (April 2015:1-27) 〕 propose a revision of the Demospongiae higher taxa classification, essentially based on molecular data of the last ten years. Some demosponge subclasses and orders are actually polyphyletic or should be included in other orders, so that Morrow and Cárdenas (2015) officially propose to abandon certain names: these are the Ceractinomorpha, Tetractinomorpha, Halisarcida, Verticillitida, Lithistida, Halichondrida and Hadromerida. Instead, they recommend the use of three subclasses: Verongimorpha, Keratosa and Heteroscleromorpha. They retain seven (Agelasida, Chondrosiida, Dendroceratida, Dictyoceratida, Haplosclerida, Poecilosclerida, Verongiida) of the 13 orders from Systema Porifera. They recommend to resurrect or upgrade six order names (Axinellida, Merliida, Spongillida, Sphaerocladina, Suberitida, Tetractinellida). Finally, they create seven new orders (Bubarida, Desmacellida, Polymastiida, Scopalinida, Clionaida, Tethyida, Trachycladida). These added to the recently created orders (Biemnida and Chondrillida) make a total of 22 orders in the revised classification. These changes are now implemented in the World Porifera Database (http://www.marinespecies.org/porifera/), part of the World Register of Marine Species. * Subclass Heteroscleromorpha Cárdenas, Pérez, Boury-Esnault, 2012 * * order Agelasida Verrill, 1907 * * order Axinellida Lévi, 1953 * * order Biemnida Morrow et al., 2013 * * order Bubarida Morrow & Cárdenas, 2015 * * order Clionaida Morrow & Cárdenas, 2015 * * order Desmacellida Morrow & Cárdenas, 2015 * * order Haplosclerida Topsent, 1928 * * order Merliida Vacelet, 1979 * * order Poecilosclerida Topsent, 1928 * * order Polymastiida Morrow & Cárdenas, 2015 * * order Scopalinida Morrow & Cárdenas, 2015 * * order Sphaerocladina Schrammen, 1924 * * order Spongillida Manconi & Pronzato, 2002 * * order Suberitida Chombard & Boury-Esnault, 1999 * * order Tethyida Morrow & Cárdenas, 2015 * * order Tetractinellida Marshall, 1876 * * order Trachycladida Morrow & Cárdenas, 2015 * * Heteroscleromorpha incertae sedis * Subclass Verongimorpha Erpenbeck et al., 2012 * * order Chondrillida Redmond et al., 2013 * * order Chondrosiida Boury-Esnault et Lopès, 1985 * * order Verongiida Bergquist, 1978 * Sublclass Keratosa Grant, 1861 * * order Dendroceratida Minchin, 1900 * * order Dictyoceratida Minchin, 1900 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Demosponge」の詳細全文を読む スポンサード リンク
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