Among the protists, choanoflagellates are the closest known relatives to the animals, so they have attracted the attention of many scientists who are curious about the origins of multicellularity among the animals.
The name means “collar-flagellate”, and refers to the collar of microvilli (minute finger-like projections of the cytoplasm) that surrounds the single central flagellum. Beating of the flagellum allows the free-living choanoflagellates to move, and also aids in the capture of bacteria and other food particles, which are engulfed by the cell.
This example was found attached to a green alga. Beside the original image is a copy where the image contrast has been increased to highlight the undulating flagellum and the collar, which are labeled with red arrows.
Stalked choanoflagellate. The individual cells are grouped in a bunch at the end of a long stalk.