The three main classes of sponges include glass sponges (Hexactinellida), calcareous sponges (Calcarea), and demosponges (Demospongiae). Parazoa from the phylum Placozoa include the single species Trichoplax adhaerens. These tiny aquatic animals are flat, round, and transparent. They are composed of only four types of cells and have a simple body plan with just three cell layers.