Frontonia leucas from Gary Calkins drawn by Schewiakoff

Holotricha is an order 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. The classification has fallen from use as a formal

taxon In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

, but the terms "holotrich" and "holotrichous" are still applied descriptively to organisms with cilia of uniform length distributed evenly over the surface of the body.

Etymology

The term ''holotrich'' derives from the

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

(), meaning "whole, entire", and , (), meaning 'hair', because of the even distribution of cilia over the surface of the cell.

Classification

The order Holotricha was erected in 1859 by

Friedrich Stein Friedrich may refer to: Names *Friedrich (surname), people with the surname ''Friedrich'' *Friedrich (given name), people with the given name ''Friedrich'' Other *Friedrich (board game), a board game about Frederick the Great and the Seven Years' ...

, who developed a system of classification in which ciliates were categorized according to the type and distribution of their cilia. The four major subdivisions of Stein's Ciliata were: Heterotricha, Peritricha, Hypotricha and Holotricha. The latter was reserved for organisms with cilia covering the entire surface of the body. The order included five families, within which a very diverse collection of

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

were grouped. Among the better-known ciliates in Stein's Holotricha were ''

Coleps ''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate. Description Species of ''Coleps'' can grow up to 250 µm in length, but are usua ...

'', ''

Nassula ''Nassula'' is a genus of unicellular ciliates, belonging to the class Nassophorea. Like other members of the class, ''Nassula'' possesses a basket-like feeding apparatus (''nasse'', or '' cyrtos'') made up of cytopharyngeal rods (''nematodesmat ...

'', '' Lacrymaria'', ''

Loxodes ''Loxodes'' is a genus of karyorelictean ciliates, belonging to family Loxodidae. It is the only known karyorelictean ciliate that lives in freshwater habitats. The term ''Loxodes'' derives from the ancient greek (), meaning "oblique, tilted". ...

'', ''

Colpoda Colpoda is a genus of ciliates in the class Colpodea, order Colpodida, and family Colpodidae. Description ''Colpoda'' are distinctly reniform (kidney-shaped) and are strongly convex on one side, concave on the other. The concave side often ...

'', ''

Glaucoma Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...

'' and ''

Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...

''. Stein's scheme was amended in 1889 by

Otto Bütschli Johann Adam Otto Bütschli (3 May 1848 – 2 February 1920) was a German zoologist and professor at the University of Heidelberg. He specialized in invertebrates and insect development. Many of the groups of protists were first recognized by him. ...

, who divided the Ciliata into two orders: Holotricha and

Spirotrich The spirotrichs are a large and diverse 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 t ...

a. Bütschli's Holotricha were described much as Stein's had been, as ciliates with relatively uniform somatic cilia. The other three orders of Stein's Ciliata were placed in a lower rank under the new order Spirotricha: ciliates possessing a well-developed structure of spiralling

membranelle Membranelles (also membranellae) are structures found around the mouth, or cytostome, in ciliates. They are typically arranged in series, to form an "adoral zone of membranelles," or AZM, on the left side of the buccal cavity (peristome). The membra ...

s around the oral cavity. Versions of Bütschli's two-order scheme were used by taxonomists throughout the first half of the twentieth century, although many adjustments were made to the arrangement of families and genera within those groups. Meanwhile, improved laboratory methods and the advent of the electron microscope began to reveal inadequacies in the old scheme; and, as inconsistencies accumulated, dissatisfaction with the system grew. Nonetheless, Holotricha remained in use as a high-level taxon until 1974, when John Corliss proposed a complete revision of the phylum Ciliophora. In his system, Spirotricha is retained as an order, under the class Polyhymenophora, but Holotricha does not appear at any rank. Recent systems of classification, including an influential scheme proposed by Pierre de Puytorac in 1994, and Denis Lynn's revision of 2002, make no mention of Holotricha.

References

{{Reflist Obsolete eukaryote taxa Ciliate orders