Poriferans, the sponges, are named for their pores. Most sponges have thousands of pores into which water flows; thus their body forms contribute in large part to current-induced water flow through their bodies. Because poriferans have no tissues or organs, like the placozoans (Phylum Placozoa), they belong to the Parazoa subkingdom. Also like placozoans, most sponges lack anterior-posterior and left-right symmetry. Between 5000 and 10,000 species of sponges are known; all are aquatic and only 150 species live in fresh water. Their fantastic forms - fans, cups, crusts, and tubes - range from a few millimeters wide to the 2 m tall Scolymastra joubini, a barrel-like glass sponge of the Antarctic. Most classes encompass various body forms.