Of all the groups of unicellular or paucicellular (excuse the neologism) eukaryotes generally lumped under the heading of 'protozoa' or 'protists', ciliates are one of the most noteworthy. Together with sporozoans, they were one of the very few groups to be recognised as distinctive* before the microbial classificatory revolution that was permitted by the appearance of SEM and molecular phylogeny. Through the example of Paramecium, they are also one of the few protist groups whose existence is widely known by the general public. While other prostists such as many flagellate** groups tend to be morphologically fairly plain, ciliates attain a diversity of form and complexity that seems incredible for unicellular organisms.
*Except for an unfortunate tendency for the non-ciliate Stephanopogon to keep trying to mooch its way into the ciliate party. Researchers still have pretty much no idea what to do with Stephanopogon, but the ciliates are adamant that they want nothing to do with it.
**Pre-revolution classifications generally divided protozoans on the basis of locomotory structures between flagellates (with flagella), amoebae (pseudopods), ciliates (cilia) and sporozoans (parasitic taxa without locomotory structures). While it is well-recognised by now that these divisions are largely artificial*** (as is the term 'protozoa' itself), they retain a certain degree of utility as descriptive conveniences (as does 'protozoa'), though 'amoeba' should probably be passed over for 'amoeboid' so as not to cause confusion with the actual genus Amoeba. Also, while light microscopists distinguished flagella (relatively long and few) and cilia (relatively short and usually arranged in tracts), there is no real difference between the two. Some researchers would prefer to refer to all such structures in eukaryotes as 'cilia', reserving the term 'flagella' for bacterial locomotory structures, which are very different.
***Especially as many protists have both amoeboid and flagellate stages in their life cycles.
While the new technologies allowed ciliates as a whole to retain their integrity, they did incite a bit of reshuffling within the clade. Earlier classifications emphasised features of the oral apparatus, but from the 1980s the importance of ultrastructural characters such as arrangement of cilia was recognised (Lynn, 2003). With the addition of molecular data, the ciliates settled (a little uneasily) down into eleven or so classes, some of them well-supported by both molecular and morphological data, some by only one or the other. It is with one of these classes, the Spirotricha, that we concern ourselves today.
The Spirotricha are a diverse bunch, and support for them as a total group is, admittedly, fairly low (though support increases if the divergent Protocruzia is left out of the mix). The classic feature of the spirotrichs are the cirri - bunches of cilia fused into tendril-like structures, which can be seen fairly well in the photo at the top of the post of Euplotes (from A Micronaturalist's Notebook. Not all taxa united molecularly with spirotrichs possess cirri, but features of macronuclear* division also support the grouping.
*An individual ciliate possesses multiple nuclei - one small micronucleus and one or more larger macronuclei. The macronuclei are involved in the day-to-day production of enzymes and such, while the micronucleus is involved in reproduction. When conjugation (sexual reproduction) occurs, the macronuclei break down and only the micronucleus is propagated. The macronuclei are then regenerated from the daughter micronuclei (see here for a more detailed and accurate description - like many so-called 'simple' organisms, ciliates make up for simplicity of structure by indulging in obscenely complicated life cycles).
Euplotes is one of the best-known of the spirotrichs. The photo above well illustrates how Euplotes uses its cirri to walk along the substrate, though they can also be used for swimming. Euplotes is a predator of other ciliates, and as such has a rather large oral cavity. Its voracity in feeding can be remarkable - Kloetzel noted in 1974 that "In extreme cases (with small Tetrahymena, which are eaten much more rapidly than large ones) a Euplotes cell can ingest 17 Tetrahymena within 5 min, representing an area of food vacuole membrane approximately twice that of the entire Euplotes surface". Trust me, I'm fighting the urge to add exclamation marks after that one.
Spirotrichs also include the only ciliate group to have a significant fossil records, the tintinnids. Unlike other ciliates, tintinnids form a lorica (a vase-shaped shell) that may be preserved after the death of the organism (shown above in an SEM by Fiona Scott from Australian Antarctic Division). A detailed taxonomy exists of tintinnids based mainly on lorica structure and composition, and it has been suggested that tintinnids with agglutinated loricas are basal to those with hyaline loricas. However, studies based on living tintinnids show that different lorica types may be possessed by species with the same or similar ciliary arrangements, and there does not appear to be a close correlation between lorica structure and ultrastructure of the living organism (Agatha & Strüder-Kypke, 2007).
Agatha, S., & M. C. Strüder-Kypke. 2007. Phylogeny of the order Choreotrichida (Ciliophora, Spirotricha, Oligotrichea) as inferred from morphology, ultrastructure, ontogenesis, and SSrRNA gene sequences . European Journal of Protistology 43 (1): 37-63.
Kloetzel, J. A. 1974. Feeding in ciliated protozoa. I. Pharyngeal disks in Euplotes: a source of membrane for food vacuole formation? Journal of Cell Science 15: 379-401.
Lynn, D. H. 2003. Morphology or molecules: How do we identify the major lineages of ciliates (phylum Ciliophora)? European Journal of Protistology 39 (4): 356-364.