The hypotrichs are a group 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 eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

d protozoa, common in fresh water, salt water, soil and moss. Hypotrichs possess compound ciliary organelles called " cirri," which are made up of thick tufts of cilia, sparsely distributed on the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

surface of the cell. The multiple fused cilia which form a cirrus function together as a unit, enabling the organism to crawl along solid substrates such as submerged debris or sediments. Hypotrichs typically possess a large oral aperture, bordered on one side by a wreath or collar of

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 (small membranous structures made up of fused cilia), forming an "adoral zone of membranelles," or AZM.

Classification

Stylonychia mytilus ventral view with cirri labeled

In older systems of classification, the term hypotrich comprised all ciliates possessing a relatively flattened body shape, strong cirri restricted to the ventral surface, and a large oral region (peristome) partially surrounded by an "adoral zone of membranelles". From a

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

point of view, this historic grouping--which included both euplotid ciliates (such as '' Euplotes'' and '' Aspidisca''), and

stichotrich The stichotrichs were a proposed group of ciliates, in the class Spirotrich, Spirotrichea. In a classification system proposed by Eugene Small and Denis Lynn in 1985, Stichotrichia formed a subclass containing four orders: Stichotrichida, Urostyl ...

ian ciliates (such as '' Oxytricha'' and ''Urostyla'')--is paraphyletic. Any natural group, or clade, that contains both '' Euplotes'' and '' Oxytricha'' would also have to include many morphologically dissimilar

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 ...

ciliates, such as the

tintinnid Tintinnids are ciliates of the choreotrich order Tintinnida, distinguished by vase-shaped shells, the name deriving from a Latin source meaning a small tinkling bell, that are called'' loricae'', which are mostly protein but may incorporate min ...

s and the

oligotrich The oligotrichs are a group of ciliates, included among the spirotrichs. They have prominent oral cilia, which are arranged as a collar and lapel, in contrast to the choreotrichs where they form a complete circle. The body cilia are reduced to ...

s. In the classification system developed by Denis Lynn in collaboration with Eugene B. Small, the subclass Hypotrichia was restricted to euplotids and one small order of marine ciliates called Kiitrichida, while most of the traditional hypotrichs were placed in the subclass Stichotrichia. However, some prominent researchers—notably, the Austrian ciliatologist Helmut Berger—rejected Lynn's nomenclature, and continued to assign non-euplotid "hypotrich" ciliates, such as oxytrichids and urostyloids, to some variant of

Friedrich von Stein Samuel Friedrich Nathaniel Ritter von Stein (November 3, 1818 – January 9, 1885) was a German entomologist. He was Professor at the Royal Saxon Academy of Forestry in Tharandt from 1850–55; and Professor, and later Rector, at the Charles Unive ...

's original order Hypotricha. In their revised classification of the phylum Ciliophora, published in 2016, Gao et al. place the "stichotrichs" of Lynn & Small in the synonymous subclass Hypotrichia, and place all euplotid and discocephalid ciliates in the subclass Euploti

The same high-level taxa are used, without ranks, by Adl et al. in their "Revised Classification of Eukaryotes" of 201

File:Oxytricha hymenostoma - 160x (8999935961).jpg, ''Oxytricha'' sp. File:Oxytricha chlorelligera - 400x (10403483023).jpg, A hypotrich, ventral view File:Stichotricha secunda - 400x - Macronuclei (14994677881).jpg,

References

` {{Taxonbar, from=Q130515 Spirotrichea