The Litostomatea are a class of

ciliates The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as '' Didinium'', a species of which preys primarily on the ciliate ''

Paramecium ''Paramecium'' ( , , plural "paramecia" only when used as a Common name, vernacular name) is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and Ocean, marine environments. Paramecia are often abundant in stagna ...

''. Trichostomatia (trichostomes) are mostly

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s in the digestive tracts of vertebrates. These include the species '' Balantidium coli'', which is the only ciliate parasitic in humans. The group Rhynchostomatia includes two free-living orders previously included among the Haptoria, but now known to be genetically distinct from them, the Dileptida and the Tracheliida.

Morphology

In litostomes, the body cilia arise from structures in the

cell cortex The cell cortex, also known as the actin cortex, cortical cytoskeleton or actomyosin cortex, is a specialized layer of cytoplasmic proteins on the inner face of the cell membrane. It functions as a modulator of membrane behavior and cell surface p ...

called monokinetids, which are made up of a single cilium and its associated structures, such as

basal bodies A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor ...

and

microtubular Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

fibres. These have an ultrastructural arrangement characteristic to the group. The cell "mouth" (cytostome) is apical or subapical. In trichostomes it lies in a depression, or vestibule, containing modified somatic cilia. In one order, the Entodiniomorphida, the cilia are arranged into tufts or bands, and may be packed together to form syncilia, resembling the membranelles and cirri of spirotrichs (with which they were originally classified) and other ciliates. However, no true compound cilia occur. In haptorians the mouth is typically surrounded by a ring of coronal cilia, arising from dikinetids derived from the anterior of the body kineties, and a ring of characteristic

extrusome Extrusomes are membrane-bound organelles found in eukaryotic cells that are capable of discharging material contained within to the exterior of the cell. Due to the diversity in structure and function, it is unlikely that different types of extruso ...

s called ''toxicysts''. These discharge on contact with prey, penetrating and immobilizing them, and beginning digestion. In some forms the mouth is formed only during feeding, and everts to assist in capture. The cytopharynx takes the form of a straight tube, supported by rods or ''nematodesmata'', which dilates greatly during ingestion. This structure is called a ''rhabdos'', and is functionally and structurally distinct from the '' cyrtos'' found in several other classes.

References

{{Taxonbar, from=Q7663959 Ciliate classes