Giant barrel sponge (Xestospongia testudinaria) on the seabed. The sponge is the most primitive of the multicellular animals, lacking organs and true body tissues. The giant barrel sponge seen here is one of the largest sponges, reaching 1.5 meters in height and diameter. The barrel sponge is host to a variety of other organisms, including crabs, shrimps and fish. These use the sponge to access the plankton-rich water currents which it generates. Photographed at the Lighthouse Reef, Belize.