Condensation rhythm of fresh-water sponges (Spongillidae, Porifera).

The mesenchyme continuum of spongillids exhibits rhythmic changes that at first glance appear to be contractions. Actually, however, the process is a condensation initiated by the formation of punctate cell contacts and a rapid swelling of all mesenchymal cells. As the cells come into closer contact and the spaces between them are constricted, the volume of the mesenchyme shrinks, giving the impression of a contraction. It seems likely that rhythmic mesenchyme condensation assists the choanocyte chambers in pumping water through the sponge.