Sponges belong to the phylum known as porifera, which includes three groups known as hexactinellida, demospongia and calcarea, according to the University of California, Berkeley. There are more than 5,000 sponges in the phylum porifera.
To determine what is considered a sponge and a member of the porifera phylum, researchers look at the feeding system of the animal. Members of the porifera phylum do not use their mouths to eat and instead use pores connected to many different inner channels to draw in water and nutrients. The members of the porifera phylum are also the oldest fossils ever found as of 2015, and they date back as far as the Late Precambrian era.