are among the least complex members of the animal kingdom;
have no organs or tissues, though they do have many different kinds of cells;
have no distinct front, back, dorsal or ventral surface (though a few are radially symmetrical);
are built around a system of canals and chambers that allows them to filter large volumes of water;
have unique collar cells with a flagellum that drives water through the canals and chambers, and can move up to ten times the sponge’s body volume of water in an hour;
feed chiefly on particles the size of bacteria or smaller;
digest their food within cells, not inside a body cavity such as a stomach;
almost all produce a supportive mineral (silica or calcium carbonate) or organic (protein fiber) skeleton, or both;
are individuals, not colonies;
have separate sexes, or change sexes as they grow;
can reproduce asexually through buds or fragments that can give rise to another individual;
grow into a distinctive shape or vary widely depending upon factors such as currents, seafloor configuration or turbidity;
mostly live in the ocean (a few live in freshwater), attached to the seafloor for most of their lives (although their larvae can crawl or swim);
range in size from almost microscopic to over 3.5 meters across;
harbor diverse communities of bacteria with which they maintain unique relationships that may involve mediating bacterial metabolisms, and carrying out processes such as photosynthesis or nitrification;
probably number over 15,000 species (7,000 have been described scientifically);
often require microscopic examination of skeletal structures for identification.
