Bdelloid rotifer, light microscopy. Rotifers are microscopic aquatic animals that are related to roundworms. A ring of cilia surrounds the mouth at the head end (left), which is used to ingest bacteria, algae and organic detritus. The pharynx, or mastax, pulsates as the rotifer feeds. The majority of rotifers are found in fresh water. They are very common and widespread. Bdelloid rotifers are unusual in that they are all female and reproduce parthenogenetically. Filmed with differential interference contrast illumination.