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
membranelles, 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
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
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 biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
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
Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
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