The peritrichs are a large and distinctive group of ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

. They are usually bell or disc shaped, with a prominent paroral membrane arising from the oral cavity and circling counter-clockwise around the anterior of the cell, accompanied by a smaller series of membranelles
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

. The oral cavity is apical and funnel shaped, with a contractile vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...

 discharging directly into it. When disturbed, the anterior of the cell can contract. The rest of the body is unciliated, except for a telotroch band circling the posterior in mobile species and stages.

The larger order of peritrichia are the Sessilida. Most of these have modified posterior kinetosomes which secrete a contractile stalk. The unattached stage, called a telotroch
Telotroch is the free-swimming stage of members of the order Sessilida. Sessilida are ciliate protozoa of the subclass Peritrichia.A sessile individual can turn into a motile one to migrate to a better place....

, is mouthless. These are common in both freshwater and marine environments, and many live attached to aquatic plants and animals. They are either solitary or produce branched colonies. A few secrete a lorica
Lorica (biology)
In biology, a lorica is a shell-like protective outer covering, often reinforced with sand grains and other particles that some protozoans and loricifera metazoans secrete. Usually it is tubular or conical in shape, with a loose case that is closed at one end. An example is the protozoan genus...

. Vorticella
Vorticella is a genus of protozoa, with over 16 known species. They are stalked inverted bell-shaped ciliates, placed among the peritrichs. Each cell has a separate stalk anchored onto the substrate, which contains a contractile fibril called a myoneme. When stimulated this shortens, causing the...

is one of the best-known genera. Stalks may be as long as 2 mm, and in some cases where they are highly contractile can be extended up to 3 mm.

The other peritrichia make up the order Mobilida. In these the posterior of the cell is enlarged and modified to form a complex holdfast, allowing the cell to temporarily attach to some host organism. Most live on the integument or gills of freshwater and marine invertebrates, but other hosts occur, including fish and even other ciliates, and other locations as well. Some can be pathenogenic in high populations.

The peritrichs were first defined by Friedrich von Stein in 1859. Initially they were considered 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, then treated as a separate category, before receiving their modern placement.
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