Phylum Porifera (sponges) are multicellular organisms, but are not true metazoans. They have only a few cell types and these are not really organized into tissues. They are usually sessile (non-mobile), benthic (live on the sea floor) animals, and obtain food by filtering it out of the water around them. Most sponges have some sort of skeleton composed of spongin fibers, and/or spicules of calcium carbonate or silica.