Litostomatea

The Litostomatea are a class of ciliate protozoa, divided into two groups, ranked as subclasses. The Haptoria includes mostly carnivorous forms, for instance Didinium, a species of which preys exclusively on the ciliate Paramecium. The Trichostomatia (trichostomes) are mostly endosymbionts in the digestive tracts of vertebrates. These include the species Balantidium coli, which is the only ciliate parasitic in humans.

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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 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 extrusomes 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.