Structure and function of the fresh-water sponge Ephydatia fluviatilis (Porifera). 10. The demonstration of the open mesenchyme by feeding yeast cells (Saccharomyces cerevisiae )

Baker's yeast (S. cerevisiae ) suspended in the water that enters the incurrent canal system of the fresh-water sponge E. fluviatilis by way of the dermal pores is carried into the mesenchyme, where it is deposited against the surface of the flagellated chambers that generate the current. In the first phase of ingestion, as soon as the imported yeast cells have touched the choanocytes, the latter send extensive pseudopoda out into the mesenchymatic space to phagocytise the yeast cells there. Similar responses are elicited in the mesenchymatic cells during the second phase of ingestion and later in the pinacocytes of the excurrent canal system. All the cells of E. fluviatilis can phagocytise yeast cells. However, the yeast cells cannot be digested by the sponge cells and in the course of a few hours to 2 days they are expelled from the cells into the mesenchyme, whence they move out transcytotically.