Spirotrichs (Subclass Spirotricha)
Members of this class are mostly free-living and have a more highly developed AZM than the primitive hymenostomes. The AZM winds clockwise to the cytostome. Many of the more advanced spirotrichs have no somatic cilia but this loss is secondary. There is a wide variety of structure which prevents generalizations on morphology; most of the orders, however, are relatively compact.
Heterotrichs (Order Heterotrichida)
The somatic cilia are more or less well represented and of uniform size. The oral cilia are several times larger than the somatic cilia and may be fused into cirri. Various habitats are used: Clevelandella and Melanesia are both invertebrate gut symbionts, Spirostomum is found in freshwater and is highly motile, and Stentor is found in freshwater but is sessile. Spirostomum and Stentor are both large and maybe three mm. long. Both are highly contractile and can shorten to less than one-quarter of the original length. Spirostomum has its cytostome situated about two-thirds of the way down the body and is fed by particles passing down the oral groove.
The latter is wound spirally around the animal for between half and one turn. A contractile vacuole is in the posterior and from it a single tubule for drainage of the cytoplasm extends nearly the whole length of the organism. Spirostomum feeds on bacteria and other protozoans. Food vacuoles take a regular course, first passing forward to the anterior tip and then journeying to the posteriorly situated permanent cytoproct.
Stentor is sessile when feeding, being attached by a ‘foot’ at the thin end of the conical body. The base of the cone carries the oral cilia (AZM) as a single row on the rim. Only at the buccal region is there an indentation and the food particles are led to the cytostome. The macronucleus is in the form of a string of beads with up to twenty segments. There are many micronuclei.
Hypotrichs (Order Hypotrichida)
Examples of this order are among the most active and commonly found ciliates. They are generally of medium size (70-l50µ long) and occur in mud and rotting vegetation. There are no generally distributed somatic cilia. These are replaced by bristle-like cirri composed of fused cilia on the dorsal surface. The distribution of cirri is a factor used in the separation of several families and genera. The AZM is the main feature of the oral ciliature.
Euplotes and Stylonichia are good examples of the order. In Euplotes the macronucleus is horseshoe-shaped. Adjacent to the center of the macronucleus and on its outer surface is a single micronucleus. The dorsal cirri are reduced to only eighteen bristles. Some of the posterior cirri are connected by a system of subpellicular fibrils to a centre, the motorium, at the anterior of the cell. The other cirri have several fibrils running from their bases at different angles. Experiments in which some fibrils are cut suggest that they serve to co-ordinate the posterior cirri.
Entodiniomorphs (Order Entodiniomorphida)
Structurally these are the most complex of the ciliates and therefore of the whole phylum. Somatic cilia are absent, and tufts of cirri are confined to a maximum of three regions.
The posterior is sometimes produced into spines. They are found in the rumen and reticulum portions of the stomach of ruminants, where they always occur in great num-ben. Being extremely specialised in this bizarre environment with its continuous comings and goings of food materials, low pH, and the near absence of oxygen, the order is, not surprisingly, most difficult to culture. In consequence little is known about them except their morphology.
The question of importance is how does the presence of perhaps 101° 100g long ciliates affect the nutrition of a cow. It was once thought that the ciliates converted cellulose into digestible carbohydrates, which would be of considerable benefit to the host. Later work has shown that cattle thrive every bit as well when their rumens have been sterilized of ciliates.
More recently it has been claimed that the frequently fatal disease, bloat, is caused by cattle feeding on too rich pasture, which in turn provided food for and produced many bacteria which were ingested too readily by the ciliates. The overfed ciliates bunt in the rumen, liberating their cell proteins which stabilize the foam of normally produced gases. The foam cannot escape and the rumen swells.