The phylum of marine and freshwater invertebrates that comprise the sponges, which live permanently attached to rocks or other surfaces. The body of a sponge is hollow and consists basically of an aggregation of cells between which there is little nervous coordination. The body is supported by an internal skeleton of spicules, whose composition defines the three groups of sponges. Flagellated cells (choanocytes) cause water to flow in through openings (ostia) in the body wall and out through openings (oscula) at the top; food particles are filtered from the water by the choanocytes. Glass sponges and demosponges have spicules made of silica, whereas calcareous sponges have spicules made of calcium carbonate. Demosponges are the most abundant group and have a skeleton consisting chiefly of a horny protein called spongin, as typified by the bath sponges. Various antibiotics and other potentially therapeutic compounds are produced by sponges, including possible anticancer drugs.

http://animaldiversity.org/site/accounts/information/Porifera.html Overview of the sponges, including their biology and classification, from the University of Michigan’s Museum of Zoology Diversity Web