Litostomatea
Encyclopedia
The Litostomatea are a class of ciliate protist
s. Until recently, the class Litostomatea was divided into two groups, the Haptoria and the Trichostomatia. However, new genetic information has resulted in the resurrection of a third subclass, Rhynchostomatia, first proposed by A. W. Jankowski in 1980. The Haptoria includes mostly carnivorous forms, for instance Didinium
, a species of which preys exclusively on the ciliate Paramecium
. The Trichostomatia (trichostomes) are mostly endosymbiont
s in the digestive tracts of vertebrates. These include the species Balantidium coli, which is the only ciliate parasitic in humans. The Rhynchostomatia include two free-living genera previously included among the Haptoria, but now discovered to be genetically distinct from them, the Dileptida and the Tracheliida.
The body cilia arise from monokinetids, which have an ultrastructural arrangement characteristic to the group. The mouth 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 membranelle
s and cirri of spirotrich
s (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
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.
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s. Until recently, the class Litostomatea was divided into two groups, the Haptoria and the Trichostomatia. However, new genetic information has resulted in the resurrection of a third subclass, Rhynchostomatia, first proposed by A. W. Jankowski in 1980. The Haptoria includes mostly carnivorous forms, for instance Didinium
Didinium
Didinium is a genus of ciliophora. They are free-living carnivores, mostly found in fresh and brackish water environments, although at least three marine species are known. Their diet consists primarily of Paramecium, but some species such as D...
, a species of which preys exclusively on the ciliate Paramecium
Paramecium
Paramecium is a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion at speeds of approximately 12 body...
. The Trichostomatia (trichostomes) are mostly endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s in the digestive tracts of vertebrates. These include the species Balantidium coli, which is the only ciliate parasitic in humans. The Rhynchostomatia include two free-living genera previously included among the Haptoria, but now discovered to be genetically distinct from them, the Dileptida and the Tracheliida.
The body cilia arise from monokinetids, which have an ultrastructural arrangement characteristic to the group. The mouth 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 membranelle
Membranelle
Membranelles are structures found around the mouth, or cytostome, in ciliate protists. They are typically arranged in series, to form an "adoral zone of membranelles," or AZM, on the left side of the buccal cavity . The membranelles are made up of kinetosomes arranged in groups to make up...
s and cirri of spirotrich
Spirotrich
The spirotrichs are a large and distinctive group of ciliate protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelles, beginning anterior to the oral cavity and running down to the left side of the mouth. There may also be...
s (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
Extrusome
Extrusomes are membrane-bound structures in some eukaryotes which, under certain conditions, discharge their contents outside the cell. There are a variety of different types, probably not homologous, and serving various functions....
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.