Sponges are aquatic and, as adults, sessile and nearly immobile multicellular animals. Their body is called amorphous because no precise body axes can be recognized. Sponges lack distinct organs for circulatory, respiratory, digestive, excretory, and reproductive functions. Instead, the water flow system and the individual cells provide all of these essential physiological tasks. Sponges filter food particles out of the water flowing through them. The water flow is generated by specific flagellated cells called choanocytes. Almost all of the 10,000 known sponge species are marine, and only a few, about 150 species, live in freshwater. However, most people would be surprised to learn that the freshwater spongeSpongilla lacustrislives in almost all of our local rivers, ponds, or creeks.