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 eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...
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 ciliate
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to e ...
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
hypotrichs and
stichotrichs, 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
oligotrichs and
choreotrichs, but were originally all considered oligotrichs. The latter group includes the
tintinnids, 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
holotrich
Holotricha is an order of ciliates. The classification has fallen from use as a formal taxon, but the terms "holotrich" and "holotrichous" are still applied descriptively to organisms with cilia of uniform length distributed evenly over the surfa ...
s, and included all ciliates with prominent oral cilia:
heterotrichs, hypotrichs, oligotrichs, and
peritrichs, 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 ...
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.
See also
*
Ciliate MDS/IES database
References
{{Taxonbar, from=Q117735