Answer: rotifers

AnswerCrossword Clue
ROTIFERSMicroscopic aquatic animals
rotifersany of a class (Rotifera of the phylum Aschelminthes) of minute usually microscopic but many-celled chiefly freshwater aquatic invertebrates having the anterior end modified into a retractile disk bearing circles of strong cilia that often give the appearance of rapidly revolving wheels
rotifersROTIFER, a microscopic aquatic organism