Feeding on Small Particles
Microscopic algae and bacteria can be taken in directly into the cell by the digestive vacuoles. But one of the most successful methods of feeding on small particulate matter is filter feeding or suspension feeding. Particulate matter includes detritus, living and dead plankton. Most filter feeders use ciliated surfaces to produce currents - that draw drifting food particles into the mouth.
Figure: Sponges obtain their food by filtering seawater. Food particles pass down the collar cells or choanocytes and enter them through phagocytosis
The animal extracts the suspended food particles by means of structures that act as filters often aided by secretion of mucous which traps the food particles. In sponges, the flagella of the choanocytes, the cells that line the body cavity, create internal water currents. The body wall has numerous pores called ostia and the water is drawn in across the flagellated chamber into the body cavity. Food particles are trapped by the flagella and directed to the surface of choanocyte, which ingest the particles by phagocytosis.