Porifera
. Sponge, any of several thousand species constituting a phylum of simple invertebrate animals. Sponges are mainly marine, with a few freshwater species. They are abundant throughout the world and especially in tropical waters, where they and other invertebrates such as corals are important in the formation of calcareous deposits. The structural components of a sponge include the outer, protective layer of cells and the spiny spicules, which form the skeleton. Sponges feed off microorganisms in the water that flow in through small openings known as ostia. The flagella on the inner layer of cells move the water through the sponge, absorbing food particles as the water flows past. Eventually the water exits through the osculum, the cavity at the top of the sponge. Scientific classification: Sponges make up the phylum Porifera. Four major groups exist. All the Calcarea are marine, with skeletal spicules composed of calcium carbonate. The Hexactinellida are found in the deep sea; because