Hexoflagella is a phyllum of very small, predatory single celled hexoideans which can be solitary or colonial and which mostly live in saltwater. Some species are motile, but some, especially colonial species, are sessile filter feeders that pump water through their bodies to filter out particles of food matter. They lack true tissues (parazoa), they lack muscles, nerves, and internal organs.
Hexoflagellates have a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). The flagellum draws water through the collar, and bacteria and detrital particles are captured by the microvilli and ingested The flagellum also pushes free-swimming cells along, as in animal sperm — most other flagellates are pulled by their flagella. Some are sessile, with a stalk opposite the flagellum.
A number of species such as those in the genus Proterospongia may form simple colonies, usually taking the form of a cluster of cells on a single stalk, but often forming planktonic clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated. Many build complex basket-shaped "bodies", from several silica strips cemented together.