The heterotrichs are a class of

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

s. They typically have a prominent adoral zone of membranelles circling the mouth, used in locomotion and feeding, and shorter

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

on the rest of the body. Many species are highly contractile, and are typically compressed or conical in form. These include some of the largest

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

, such as '' Stentor'' and '' Spirostomum'', as well as many brightly pigmented forms, such as certain '' Blepharisma''.

Etymology

The term ''heterotrich'' derives from the

ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

(), meaning "another, different", and , (), meaning 'hair', because of the contrast between the regular somatic ciliation and that of the oral zone.

Ultrastructure

A number of ultrastructural details characterize the group. The cilia on the body are in dikinetids, in which either the anterior one or both kinetosomes may be ciliated, and which are associated with fibers composed of overlapping postciliary

microtubule 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 ...

s, called ''postciliodesmata'' and found only in this group and the closely related

Karyorelictea Karyorelictea is a class (biology), class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''Lox ...

. A series of oral polykinetids, each containing two or three rows of kinetosomes, support the membranelles. These run from the left to the anterior of the mouth, and often spiral out of the oral cavity. The macronucleus is divided by external microtubules, whereas in the

it forms by differentiation of micronuclei, and in all other ciliates it is divided by internal microtubules.

Pigmentation

Many species are highly pigmented, e.g. the blue '' Stentor coeruleus''. In '' Blepharisma'', the red pigment is associated with light sensitivity. Species of blue-pigmented '' Eufolliculina'' form extensive mats in the deep sea that have been called "blue mats".

Systematics

Early classification schemes by Otto Bütschli, Alfred Kahl, Emmanuel Fauré-Fremiet, and John O. Corliss classified the heterotrichs as a subgroup of spirotrichs. They included groups such as Armophorida, Odontostomatida, Licnophorida, Clevelandellida, and Plagiotomida within the Heterotrichea. However, more recent classification systems, which have incorporated information from molecular phylogenetics, place the above groups within the Intramacronucleata, because they are not closely related to the "core" heterotrichs at all. Almost all of the "true" heterotrichs belong to a single order, Heterotrichida. In the classification scheme of Lynn (2008), the order Heterotrichida contains the following nine families: * Blepharismidae Jankowski in Small & Lynn, 1985 * Chattonidiidae Villeneuve-Brachon 1940 * Climacostomidae Repak, 1972 * Condylostomatidae Kahl in Doflein & Reichenow, 1929 * Folliculinidae Dons, 1914 * Maristentoridae Miao, Simpson, Fu & Lobban 2005 * Peritromidae Stein, 1867 * Spirostomidae Stein, 1867 * Stentoridae Carus, 1863 Two new families were established in 2014 on the basis of molecular phylogenies: * Gruberiidae * Fabreidae

References

{{Taxonbar, from=Q3078759 Ciliate classes