Ciliates comprise the phylum Ciliophora. They are protozoans that, during at least part of their life cycle, use many short, hairlike cilia for locomotion and feeding. On the outside of the cell, ciliates have a flexible pellicle; the cilia are arranged in rows, or kineties, on this covering. In some places on the pellicle, the cilia can form clumped tufts called cirri, which act as limbs and can support a considerable deal of pressure. Ciliates have tubular mitochondrial cristae, and almost all have contractile vacuoles and toxicysts or other trichotrysts. These are small organelles, similar to the cnidocytes of cnidarians but on a smaller scale, with thorn-like parts that can be discharged in defense, for capture of prey, or to provide anchorage. Many ciliates have distinctive "mouthparts," oral apparatuses made of cilia fused near the cytostome. These can form membranelles or undulating membranes, and serve to sweep food into the cell.

Ciliates are unique in possessing two types of nuclei at the same time: a large, polyploid macronucleus and many smaller micronuclei. The macronucleus controls cell metabolism and development, while as many as 80 diploid micronuclei function in sexual reproduction. Predominant among the several modes of sexual reproduction used by ciliates are conjugation, the transfer of genetic material from one cell to another, and autogamy, reorganization of a nucleus within an individual. Despite this capability, ciliates reproduce primarily asexually by budding or by transverse binary fission.

Most ciliates are free-living, inhabiting aquatic areas across the world. Some, however, live harmlessly inside invertebrate gills or integument, and a few species are parasitic, including the parasite that causes dysentery. The ciliates are considered the most evolved and complex group of protozoans.

Phylum Ciliophora

• Class Karyorelictea
• Class Spirotrichea (Polyhymenophorea)
• Class Litostomatea
• Class Prostomatea
• Class Phyllopharyngea
• Class Nassophorea
• Class Oligohymenophorea
• Class Colpodea