The spirotrichs are a large and diverse 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 flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone 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, beginning anterior to the oral cavity and running down to the left side of the mouth. There may also be one or two paroral membranes on its right side. The body cilia are fused to form polykinetids called cirri in some, and are sparse to absent in others. Forms with cirri are common throughout soil, freshwater, and marine environments. Individuals tend to be flattened, with cirri confined to the ventral surface. These are variously used for crawling over objects, acting as feet, swimming, or assisting in food capture. They are generally divided into

hypotrich The hypotrichs are a group of ciliated 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 ...

s 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 ...

s, but were originally all considered hypotrichs. Forms with sparse or absent body cilia tend to be smaller and are mostly marine, but a few are common in freshwater. Again, they are generally divided into

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 and

choreotrich The choreotrichs are a group of small marine 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 sh ...

s, but were originally all considered oligotrichs. The latter group includes 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, which produce loricae or shells and are the predominant fossil ciliates. As first defined by Bütschli in 1889 the spirotrichs were one of two orders, together with the now-abandoned holotrichs, and included all ciliates with prominent oral cilia:

heterotrich The heterotrichs are a class of ciliates. They typically have a prominent adoral zone of membranelles circling the mouth, used in locomotion and feeding, and shorter cilia on the rest of the body. Many species are highly contractile, and are ...

s, hypotrichs, oligotrichs, and

peritrich The peritrichs (Latin: Peritrichia) are a large and distinctive group of ciliates. The peritrichs were first defined by Friedrich von Stein in 1859. Initially they were considered spirotrichs, then treated as a separate category, before receivi ...

s, although the last were soon separated. The heterotrichs have an adoral zone of membranelles, but molecular and ultrastructure studies have shown they are a separate group that diverged from most other ciliates early on. A few of the smaller groups included with them may be genuine spirotrichs, however, such as the Protocruziida. The remaining spirotrichs form a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

group, but their relationships are uncertain. For the most part the oligotrichs and choreotrichs appear to form closely related, natural groups. However ''Halteria'' and its close relatives, originally considered oligotrichs, form a separate group and may even be modified stichotrichs. Studies also suggest the hypotrichs are

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

to the stichotrichs, and possibly to the oligotrichs and choreotrichs as well. This stands in contrast to the earlier belief that they were the most advanced of all protozoa.

References

{{Taxonbar, from=Q117735

See also

References