Rützler, Klaus Invertebrate Zoology Department, Smithsonian Institution, National Museum of Natural History, Washington, DC.
Hartman, Willard D. Peabody Museum of Natural History, Yale University, New Haven, Connecticut.
Last reviewed:June 2019
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- Biology and morphology
- Fibrous skeletal elements
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A class of the phylum Porifera, including sponges with a skeleton of one- to four-rayed siliceous spicules or of spongin fibers, or both. Four taxonomic classes comprise the phylum Porifera—Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha. Demospongiae is the most abundant and diverse of these classes and is distinguished by three subclasses—Heteroscleromorpha, Keratosa, and Verongimorpha. Several genera lack a skeleton, and it is through a study of these seemingly primitive forms that the complicated structure of most adult members of the Demospongiae (demosponges) may be understood. In general, demosponges (Fig. 1) constitute the most populous and widely distributed group of sponges, occurring in the sea from the tidal zone down to abyssal depths [at least to 5500 m (18,000 ft)]. They contain at least 7500 accepted species (>80% of all sponges) in more than 20 well-defined orders. Most demosponge families are marine, but a few have invaded freshwater habitats. The species vary in size from thin encrustations that are several centimeters in diameter to huge cake-shaped forms that may measure 2 m (6.6 ft) in diameter. Other common shapes are tubular, columnar, arborescent (treelike), cuplike, flabellate (fan-shaped), and excavating (living in galleries bored in limestone). The shallow-water species tend to be more plastic in form than the deep-water species, which usually exhibit little intraspecific variation in shape. See also: Calcarea; Hexactinellida; Porifera
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