Choanocytes (collar cells) form the lining of spongocoel in poriferans (sponges). Flagella in collar cells provide circulation to water in the water canal system. These distinctive cells line the interior body walls of sponges. These cells have a central flagellum that is surrounded by a collar of microvilli. Choanocytes are versatile cells. Their flagella beat to create the active pumping of water through the sponge, while the collars of the choanocytes are the primary areas where nutrients are absorbed into the sponge. Furthermore, in some sponges, the choanocytes develop into gametes.