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Ciliates

The ciliates (Ciliata, Ciliophora) 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]].

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 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 distinctive rows called kineties that coordinate their motion.  Also associated with the cell surface is a flexible [[pellicle]] composed of alveoli, a series of microfilaments that separate these outermost structures from the interior, and various accessories like defensive [[trichocysts]].

Many ciliates feed on smaller organisms like bacteria, but a few are predatory.  There is at least one distinct cytostome (mouth), usually surrounded by cilia that draw in food, at the end of which bud off a series of digestive [[vacuoles]] that circulate through the cell on a definite course before being discharged at a point called the cytoproct.  Usually contractile vacuoles are also present; these have a distinctive star-shape.

Among the [[Suctoria]] adults live attached to the substrate and lack cilia altogether.  Aside from them, cilia are always present on at least part of the body, especially around the mouth, where they may be fused together to form membranelles or undulating membranes.  Elsewhere cilia may be clumped into tufts called ''cirri''.  Ciliates move either by swimming or crawling, although many adults are sessile.