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.
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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. Three subclasses can be distinguished: Homoscleromorpha, Tetractinomorpha, and Ceractinomorpha. Several genera lack a skeleton, and it is through a study of these seemingly primitive forms that the complicated structure of most adult Demospongiae may be understood. The Demospongiae constitute the most abundant 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 an estimated 6000 accepted species (85% of all sponges) in about 13 well-defined orders, 88 families, and 500 genera. Three families have invaded freshwater. The species vary in size from thin encrustations that are several centimeters in diameter to huge cake-shaped forms that may measure up to as much as 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: Porifera
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