Choanoflagellate

The choanoflagellates are a group of flagellate protozoa. They are considered to be the closest living relatives of the animals, and the last unicellular ancestors of animals are thought to have resembled modern choanoflagellates.

Each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). The flagellum pulls water through the collar, and small food particles are captured by the microvilli and ingested. The flagellum also pushes free-swimming cells along, as in animal sperm, whereas most other flagellates are pulled by their flagella. Many choanoflagellates build complex basket-shaped "houses" called lorica, from several silica strips cemented together.

Most choanoflagellates are sessile, with a stalk opposite the flagellum. A number of species such as those in the genus Proterospongia are colonial, 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. There exist historical accounts of an extracellular matrix and non-flagellated cells in some colonies, but these accounts are unsubstantiated.

The choanocytes (also known as "collared cells") of sponges have the same basic structure as choanoflagellates. Collared cells are occasionally found in a few other animal groups, such as flatworms. These relationships make colonial choanoflagellates a plausible candidate as representative of the ancestors of the animal kingdom.