Ciliophora

The ciliates (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.

Ciliates are mainly classified according to the structure of the kinities. The various classes are as follows:

Class Prostomatea
Class Litostomatea

Subclass Haptoria (Didinium, etc)
Subclass Trichostomatia (Balantidium, etc)

Class Karyorelictea
Class Spirotrichea

Subclass Heterotrichia (Blepharisma, Stentor, etc)
Subclass Choreotrichia (Tintinnidium, etc)
Subclass Stichotrichia (Stylonychia, etc)

Class Phyllopharyngea

Subclass Phyllopharyngia
Subclass Chonotrichia
Subclass Suctoria

Class Nassophorea

Subclass Nassophoria (Paramecium, etc)
Subclass Hypotricha (Euplotes, etc)

Class Colpodea (Colpoda, etc)
Class Oligohymenophorea

Subclass Hymenostomatia (Tetrahymena, Ichthyophthirius, Colpidium, etc)
Subclass Peritricha (Vorticella, etc)
Subclass Astomatia
Subclass Apostomatia
Subclass Plagiopylia