The Porifera, sponges, are sessile benthic invertebrates that represent one of the oldest, possibly even the oldest metazoan lineage. Sponges comprise approximately 8,500 described species divided among 4 extant classes: Calcarea, Homoscleromorpha, Hexactinellida, and Demospongiae, with the Demospongiae being the most diverse group. Sponges maintain numerous roles in the marine ecosystem such as nutrient cycling, habitat forming, and symbiosis with several different organisms.