Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.

Ciliates are an important group of protists, common almost anywhere there is water—in lakes, ponds, oceans, rivers, and soils, including anoxic and oxygen-depleted habitats.

Ciliate species range in size from as little as 10 μm in some colpodeans to as much as 4 mm in length in some geleiids, and include some of the most morphologically complex protozoans.

[9] In some older systems of classification, such as the influential taxonomic works of Alfred Kahl, ciliated protozoa are placed within the class "Ciliata"[10][11] (a term which can also refer to a genus of fish).

In the taxonomic scheme endorsed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata.

[12][13] Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus (the "generative nucleus", which carries the germline of the cell), and a large, ampliploid macronucleus (the "vegetative nucleus", which takes care of general cell regulation, expressing the phenotype of the organism).

[16][15] Division of the macronucleus occurs in most ciliate species, apart from those in class Karyorelictea, whose macronuclei are replaced every time the cell divides.

[15] After a certain number of generations (200–350, in Paramecium aurelia, and as many as 1,500 in Tetrahymena[17]) the cell shows signs of aging, and the macronuclei must be regenerated from the micronuclei.

The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia.

The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates, Apicomplexa, and dinoflagellates.

This usually includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures.

Alternatively, it may proceed as a result of self-fertilization (autogamy),[23] or it may follow conjugation, a sexual phenomenon in which ciliates of compatible mating types exchange genetic material.

[27] ln clonal populations of Paramecium, aging occurs over successive generations leading to a gradual loss of vitality, unless the cell line is revitalized by conjugation or autogamy.

In 2007, Li et al. published a description of fossil ciliates from the Doushantuo Formation, about 580 million years ago, in the Ediacaran period.

[32] A fossil Vorticella has been discovered inside a leech cocoon from the Triassic period, about 200 million years ago.

Additionally, two big sub-groups are distinguished inside subphylum Intramacronucleata: SAL (Spirotrichea+Armophorea+Litostomatea) and CONthreeP or Ventrata (Colpodea+Oligohymenophorea+Nassophorea+Phyllopharyngea+Plagiopylea+Prostomatea).

Mesodiniea Karyorelictea Heterotrichea Odontostomatea Armophorea Litostomatea Spirotrichea Cariacotrichea Protocruziea Discotrichida Colpodea Nassophorea Phyllopharyngea Oligohymenophorea Prostomatea Plagiopylea Several different classification schemes have been proposed for the ciliates.

The following scheme is based on a molecular phylogenetic analysis of up to four genes from 152 species representing 110 families:[1] Some old classifications included Opalinidae in the ciliates.