ULTRASTRUCTURAL ORGANIZATION OF CHOANOCYTE CHAMBERS IN THE HAPLOSCLERID PELLINA-SEMITUBULOSA (PORIFERA, DEMOSPONGIAE) - A CUE FOR WATER-FLOW INTO THE SPONGE BODY

The choanocyte chambers of the haplosclerid Pellina semitubulosa were investigated with regard to their ultrastructural architecture. They a re separated from the mesohyl and contain central cells arranged to fo rm a uniform layer. Their cytoplasmic extensions enter the distal part s 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 an d related filter feeding activity is proposed, which supports a water passageway through the small microvillar slits into the interior of th e collar. A slight detachment of the central cells from the microvilla r ends originates spaces for a direct water flow into the chamber. Suc h a plastic organization might enhance the filtration efficiency and p revent chamber clogging as well.