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The ciliates (Ciliophora, Infusoria) are one of the most important groups of Protista, common almost everywhere there is water--lakes, ponds, oceans, soils--with a few parasitic members. Ciliates tend to be large protozoa, a few reaching 2 mm in length, and are some of the most complex in structure. They belong to a group called the Alveolates, along with the Dinoflagellates and Apicomplexa.

The name "ciliate" comes from numerous, short flagella called cilia that are present at some point in every group. Each cilium arises from a structure called a kinetosome, which are grouped into distinct rows caled kineties that coordinate their motion. The outer layer of the cell containing these is separated off by microfilaments and may also contain extrusomes of various kinds, and of course the alveoli, which are packed together to form a pellicle.

Each ciliate cell has a cytostome (mouth), leading down into a cytopharynx at the end of which food vacuoles form. These circulate the cell on a definite path as their contents are digested and absorbed, and finally are discharged at a point called the cytoproct. Usually contractile vacuoles are often present, and have a unique star shape. Most ciliates graze on smaller organisms which are swept into the mouth by nearby cilia, which may be modified to form an undulating membrane or membranelles.

In motile ciliates, the cilia are used either for swimming or crawling along the substrate, in which case they tend to be fused into tufts called cirri. Many ciliates lived attached for part or most of their life, however. One group of these, the Suctoria, actually lack cilia in the adult stage and are unusual for having multiple cytostomes on the end of tentacles, through which they suck in the cytoplasm of prey.

Unlike other eukaryotes, ciliates have two different kinds of nuclei: a large, polyploid macronucleus responsible for protein synthesis, and one or more small diploid micronuclei. In asexual reproduction, which is by fission, the micronuclei undergo closed mitosis but the macronucleus simply pinches in half. Sexual reproduction is necessary from time to time and occurs through conjugation--two cells line up, the micronuclei undergo meiosis and some are exchanged, and the macronuclei disintegrate. The micronuclei then undergo fusion and are used to reform the macronucleus.

The currently accepted classification of ciliates is as follows: