The flagellated chambers of the marine sponge Reniera sp. (Haplosclerida) are found at the junctions of the incurrent and excurrent canal systems, in the lumens of the incurrent canals. Each is inserted into the pinacocyte lining of the incurrent and excurrent canal systems by means of a cell ring in the apopyle region. The cohesion of the choanocytes is brought about by processes of the cells that form a lattice bridging the intercellular spaces. The water in the incurrent canals is sucked into the flagellated chambers through these cavities between the choanocytes, driven through the apopyle into the excurrent parts of the canal system by the beating of the choanocyte flagella, and eventually expelled from the sponge. The flagellated chambers are thus pumps to keep the water flowing and simultancously devices to trap the food particles carried in by the current. The described arrangement of the flagellated chambers in the canal system of Reniera sp. differs from all known Leucon-types and should be regarded as a new basic type of leuconoid organisation.