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Choanoflagellates are solitary or colonial heterotrophic flagellates. They are distributed widely in the fresh water and seawater habitats of the world. The name “Choanoflagellate” is derived from the morphological characteristics, i.e. a flagellate with a collar (choano-) surrounding the flagellum. They are a large and highly diverse group, and nearly 250 spp. have been described. Traditionally, three families are recognized in this group of organisms, i.e. Codonosigidae (apparently naked organisms), Salpingoecidae (organisms with cell holders of organic membranes, thecae) and Acanthoecidae (organisms with siliceous basket-like cell holders, loricae). The cell is oval, ovoidal or obovoidal, and possesses a flagellum at one end, that is surrounded with a funnel shaped collar composed of 30–40 microvilli. The flagellum beats to produce a water current flowing from the cell body to the tip of the flagellum. Therefore, cells of free swimming forms swim with the flagellum (posterior propulsion), like the swimming of a sperm. The cell traps micro-particles, mainly bacteria, moving wit the water current produced by the flagellum and these attach to the outside of the collar. This is a well-adapted cell apparatus for filtering suspended particles. These organisms are typical bacterio-feeders in the aquatic ecosystem. They play a role connecting dissolved organic matter and bacteria to bigger animals in the microbial loop. The morphological characteristics, i.e. possessing a flagellum surrounded with collar of microvilli, are highly similar to those of the choanocyte in sponges, and the systematically close relationship between the choanoflagellate and the sponge has been pointed out early in the study of choanoflagellates. This inference has been supported by other similarities in the microstructures of their cells. Moreover, the close relationship, and also the isolation of choanoflagellates from the Protista group, has been confirmed by an increasing body of genetic information, which indicates that choanoflagellates are a sister clade to the clade of metazoans and the closest unicellular group to multicellular animals. [ABSTRACT FROM AUTHOR]