Sponges are suspension-feeders (also known as filterfeeders), which are animals that collect food particles from water passed through some type of food-trapping equipment. Sponges trap food from the water circulated through the porous body. Lining the inside of the body are flagellated choanocytes, or collar cells (for the membranous collar around the base of the flagellum). The flagella generate a water current, and the collars trap food particles and ingest them by phagocytosis. The similarity between choanocytes and choanoflagellates (the protistan cells in FIGURE 26.27) supports the hypothesis that sponges evolved from colonial choanoflagellates