Spirotrich
Encyclopedia
The spirotrichs are a large and distinctive group of ciliate
protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelle
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
s and stichotrich
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
s and choreotrich
s, but were originally all considered oligotrichs. The latter group includes the tintinnid
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 holotrich
s, and included all ciliates with prominent oral cilia: heterotrich
s, hypotrichs, oligotrichs, and peritrich
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 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 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.
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...
protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelle
Membranelle
Membranelles are structures found around the mouth, or cytostome, in ciliate protists. They are typically arranged in series, to form an "adoral zone of membranelles," or AZM, on the left side of the buccal cavity . The membranelles are made up of kinetosomes arranged in groups to make up...
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
Hypotrich
The hypotrichs are a group of ciliate protozoa, included among the spirotrichs. Most are oval in shape, with a rigid pellicle, and have cirri distributed in isolated tufts on the ventral surface of the cell. Some also have dorsal cilia, which function as sensory bristles. Euplotes and Aspidisca...
s and stichotrich
Stichotrich
The stichotrichs are a group of ciliate protozoa, included among the spirotrichs. Like the hypotrichs, with which they were originally classified, they have body cilia fused into cirri, but these are mostly arranged into rows, running along the ventral surface or edges of the cell. Most...
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
Oligotrich
The oligotrichs are a group of ciliate protozoa, 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 a girdle and ventral cilia...
s and choreotrich
Choreotrich
The choreotrichs are a group of small, marine ciliates. Their name reflects the impression that they appear to dance . The group includes the tintinnids, which produce species-specific loricae , and are important because these may be preserved as microfossils...
s, but were originally all considered oligotrichs. The latter group includes the tintinnid
Tintinnid
Tintinnids are ciliates of the choreotrich taxon Tintinnida, distinguished by vase-shaped shells called loricae, which are mostly protein but may incorporate minute pieces of minerals. Although appearing as early as the Ordovician period, tintinnids became abundant in the fossil record during the...
s, which produce loricae or shells and are the predominant fossil ciliates.
As first defined by Bütschli
Otto Bütschli
Johann Adam Otto Bütschli was a German zoologist and professor at the University of Heidelberg. He specialized in invertebrates and insect development...
in 1889 the spirotrichs were one of two orders, together with the now-abandoned holotrich
Holotrich
Holotricha is an order of ciliate protists. 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 surface of the body.The order Holotricha was...
s, and included all ciliates with prominent oral cilia: heterotrich
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 typically compressed or conical in form...
s, hypotrichs, oligotrichs, and peritrich
Peritrich
The peritrichs are a large and distinctive group of ciliate protozoa. They are usually bell or disc shaped, with a prominent paroral membrane arising from the oral cavity and circling counter-clockwise around the anterior of the cell, accompanied by a smaller series of membranelles...
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 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 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.