The phylum Rotifera consists largely of free-living animals less than 1 mm long that are common in aquatic ecosystems throughout the world. Although most rotifers have fewer than 1000 cells, they have muscles, ganglia, sensory organs, structures for feeding and swimming, and digestive and secretory organs. Rotifers are characterized by a ciliated head structure, the corona, used for locomotion and food gathering, and a muscular pharynx, the mastax, used to process food. All rotifers have a syncytial epidermis with an intracytoplasmic lamina that is unique among metazoans. Rotifers employ a wide variety of reproductive modes: some species reproduce only sexually, some alternate between sexual and asexual reproduction, and others reproduce only asexually. The successful evolution of asexuality in rotifers renders them important model organisms in studying why sexual reproduction is the dominant form of reproduction in animals.