Phylum: Porifera (por-i'-fer-uh): sponges, which are aggregations of specialized cells around a skeleton of spicules composed of spongin, silica or glass and calcium carbonate. These organisms are filter feeders. There are three groups of sponges based on skeletal material. Sponges are often asymmetrical. Feeding is accomplished by flagellated cells drawing water in through innumerable tiny openings, from which the sponge extracts detritus and bacteria as food. The water leaves by the large excurrent holes called oscula. (sing.: osculum). Sponge with oscula; sponge spicules;