Rotifer, a microscopic animal. There are approximately 2,000 species. Most species live in freshwater; some live in the sea; and a few live as parasites in or on other organisms. A rotifer's body consists of a head, a long middle section that holds the internal organs, and a tailpiece bearing one to four pointed projections called toes. An adhesive substance, with which rotifers adhere to surfaces, is secreted by glands in the toes. The rotifer has a two-lobed "brain" (a small mass of nerve tissue), and most species have eye spots (cells that are sensitive to light).

The mouth is surrounded by hairlike cilia that sweep in food, which typically consists of bacteria, protozoans, and other small organisms. The cilia are also used for swimming. The motions of the cilia, which suggest the turning of a wheel, are responsible for the name of the rotifer, which means wheel bearer. Rotifers can move by creeping over submerged objects or by swimming.

Female rotifers produce both summer eggs and winter eggs. Summer eggs are thin-shelled and develop without fertilization. They are not all the same size; the larger ones produce females, the smaller ones males. Winter eggs are thick-shelled and must be fertilized. They produce only females. Winter eggs can survive internal water loss and freezing.

Rotifers make up the phylum Rotifera.