The choanocyte chambers of the haploscleridPellina semitubulosawere investigated with regard to their ultrastructural architecture. They are separated from the mesohyl and contain central cells arranged to form a uniform layer. Their cytoplasmic extensions enter the distal parts of the choanocyte collars, thus enveloping the end of the microvilli. Central cells seal up all the spaces between choanocyte collars. On the basis of choanocyte chamber architecture, a water flow dynamics and related filter feeding activity is proposed, which supports a water passageway through the small microvillar slits into the interior of the collar. A slight detachment of the central cells from the microvillar ends originates spaces for a direct water flow into the chamber. Such a plastic organization might enhance the filtration efficiency and prevent chamber clogging as well.