Domain Eukaryota
Choanomonada

The Choanomonada (or "collared-flagellates") are a kingdom of single-celled organisms with flagella.

They are genetically the closest relatives of Animals, and may be the ancestors of the Sponges.

The Choanomonada have a single flagellum, surrounded by a ring of hair-like protrusions (microvilli), The microvilli form a conical "collar" which gives the group its name (Choanomonada - "collared cell" in Greek). The flagellum pulls in water and the microvilli collar absorbs and digests food particles.

The flagellum pushes the cells forward, like animal sperm, instead of pulling it forward as in other eukaryotic groups. They share this feature with the other members of the super-kingdom Opisthokonta.

Some Choanomonada are attached to surfaces by a stalk opposite the flagellum. Others form colonies, some of which show cellular differentiation ('division of labor') with the outside cells retaining collared flagella and the inside cells without. In this they resemble primitive sponges.

There are three groups based on the compositon of their shell: the Monosigidae (no shell), the Salpingoecidae (cellulose shell) and the Acanthoecidae silica shell).

Some Animal groups have collared cells at some point in their life cycle, further demonstrating their relationship to the Choanomonada.