INTRAMACRONUCLEATA

Most of the ciliates are found in this subphylum, and they vary enormously. The group has 8 classes, which are distinguished on the basis of ciliation. That is, the cilia may be uniform over the whole cell (holotrichous). They may form membranelles as in the heterotrichs like Stentor. They may be fused into composite structures called cirri. The cell may also be mostly cilia-free with ciliation on restricted areas or relegated parts of the life cycle.

Members of this group can also be distinguished on the basis of the cytostome and the structure of the cytostomal apparatus. The combination of the cytostome and ciliary structures also determine how the taxa feed. Some feed on whole organisms or large pieces of tissue, and others are filter feeders. They range from free-swimming, attached, to parasites.

PHALLOPHYRINGIA

Members of this class have mostly monokinetids with a distinctive infraciliature. The oral region is surrounded and supported by radiating microtubular ribbons called phyllae. Some members are free-swimming, but many are sessile and some of those are commensals.

Spirochona (Figure 5) is a commensal on the outside of crustaceans. There, it is a sessile cell with a thin winding row of cilia on the edge of membranous flange of an otherwise naked cell body. The anterior end opens as a funnel with the membranelle along its edge carrying food particles to the cytostome. They reproduce by means of budding to form motile swarmer cells.

Suctorians (Figure 6) although ciliates, do not display cilia when attached. Rather, they elaborate tentacles that have feeding disks at their ends (suctorial tentacles). When a prey item like a small ciliate contacts the disk, it stops moving and its cytoplasm is taken into the cell body of the suctorian through the tentacle. Because they have no flagella in the attached form, suctorians divide and release ciliated swarmer cells for dispersal and as gametes. Some species are stalked and some are loricate.

NASSOPHOREA

The cilia on these cells vary and the alveoli are well-developed. The cells do, however, have a distinctive tubular cytostome called a cyrtos. They have no toxicysts. Nassula (Figure 7) is a typical example of this class with a densely-ciliated holotrichous cell and an obvious cyrtos. Though it is not apparent from Figure 4, both Vd’ačný et al. (2010) and Gong et al. (2009) determined that Nassophorea was polyphyletic and form three separate clades generally associated with the monophyletic Phyllopharyngea.

OLIGOHYMENOPHORA

This large and diverse class has 6 subclasses. The cytostome tends to be ventrally located and is in the bottom of a deep funnel called an infundibulum. They have distinctive cytostomal ciliature that often includes small (or large) membranelles.

Paramecium (Figure 8) is a common holotrichous cell. The cytostome is found in the base of a long groove in the slipper-shaped cell.

Urozona (Figure 9) is unusual in its order in that its somatic ciliature is reduced to a central girdle. It has a trailing thigmotactic (text with tooltip) Thigmotactic cilia are ciliary structures that are sensitive to touch and thus function as holdfasts, etc. tail-like cilium.

Ichthiophtherius (Figure 10), which literally means fish louse, lives its life as a parasite under the skin of certain fish. During that time, the lesion caused by the ciliate swells and looks like a grain of salt. Fish show discomfort and distress when infested and try to rub themselves against the bottom rocks, plants, etc. When mature, the ciliate emerges as a trophozoite and swims to the bottom where it covers itself with mucilage and divides into hundreds of offspring. The emergent tomites can swim for up to 3 days in searching for another host fish. It is during this brief free-swimming stage that they are susceptible to treatment.

Trichodina (Figure 11) is in a group called the peritrichs, which have broad disk-shaped peristomes surrounded by two rows of ciliary membranelles that wind into the cytostome in a counter-clockwise direction. Trichodina is a surface symbiont/parasite of fish. It has a typical peritrich oral disk and an adhesive base. Vorticella (Figure 12) is an attached peritrich with an oral region like that of Trichodina; however, this species has a stalk with a contractile myoneme (text with tooltip) A myoneme is a structure that operates as a muscle fiber. It can be found in the stalk of certain sessile ciliates like <em>Vorticella</em>. .

PLAGIOPYLEA

Taxa in this class are restricted to anaerobic habitats. They have a somatic ciliature that is monokinetid but have a row of dikinetids surrounding the cytostome. Plagiopyla (Figure 13) is an example of this small group.

PROTOSTOMATEA

This class is characterized by having a ciliature made of monokinetids. The cytostome is apical or subapical and is tubular with toxicysts inside (called a rhabdo). Prorodon (Figure 14) is a typical member of this class and has an apical cytostome.

COLPODEA

The cilia of the cell body is dikinetid (ciliary pairs) with a regular network for the infraciliature. Typically members of this class make resting cysts.

Colpoda (Figure 15) is an oval cell with an obvious small cytostome about midway down the cell. The cytostome has rows of small membranelles. Colpoda is one of the most common cells to appear in a hay infusion.

Bursaria (Figure 16) is a large holotrichous cell with a deep cytostomal cleft, along its left side is a long oral membranelle (text with tooltip) Membranelles are structures that resemble undulating membranes, however, they are formed by the lateral fusion of many flagella-cilia. . This species feeds on other ciliates like Paramecium.

SPIROTRICHEA

The spirotrichs are a large and diverse class within the Intramacronucleata. Taxa in this group have hese have a band of membranelles and cirri (both made of multiple cilia); however, the membranelle band winds in a clockwise direction. The diversity in the spirotrichs is quite high and they vary from being covered with flagella (holotrich) to nearly naked. Some are loricate.

Euplotes (Figure 17) has a dorsoventrally flattened cell with cirri at the posterior end and the ventral side. The cirri are articulated and work almost like legs as they move swiftly over surfaces in search for food. Although much of the cell is naked, the membranelle does allow them to swim.

Tintinnopsis (Figure 18) is a sessile ciliate that constructs a lorica out of sediment and sand grains. The lorica is open at the top and resembles a vase. The active cell resides inside the lorica and exends its membranelle while feeding.

Urostyla (Figure 19) is associated with a substrate and resembles Euplotes in the way it moves and feeds. However, the cirri are more anteriorly located in Urostyla. The cirri are attached to the ventral part of the cell in a zig-zag pattern.

Halteria (Figure 20) is a planktonic cell that is mostly naked but moves slowly by its oral cirri and membranelle. The few somatic cirri are very long (>10µm) and when the contract, the cell can move very rapidly. They have no lorica.

LITOSTOMATEA

Taxa in this class have single cilia emerging from the alveolar structure (monokinetids) rather than the typical pairs of cilia (dikinetids). The underlying kineties are associated with ribbon-like lateral roots and one posterior root.

The two subclasses in this class are quite different from each other. The subclass Haptoria has a thin alveolar layer that is underlain by a thick cortical layer. This gives the cell a refractile appearance.

Dileptus (Figure 21) is a swan-shaped cell, the body of which is holotrichous with monokinetid cilia. The cell creeps over the substrate and waves its proboscis, which is lined with toxicysts that lyse cells. After the pieces are sliced up, they are engulfed by a cytostome at the base of the proboscis. They can go after prey much larger than themselves. We have watched Dileptus surround small planarians and slice them to oblivion.

Balantidium (Figure 22) is a relatively large spherical cell which is holotrichous with the cytostome is at the apex of the cell. It is of some economic importance because Balantidium coli is an intestinal parasite of humans and other vertebrates. The ciliates inhabit the large intestine where they feed on the microbial flora. However, sometimes they will create ulcers in the large bowel wall and feed on the cells of the mucosa. They encyst and go out with the feces. The disease is spread by the host taking up the cysts in food or water.

ARMOPHOREA

Members of this class occur in anaerobic environments, and seem to require a relationship with methanogenic endosymbiotic bacteria. They are bacterivores with complex oral membranellar structures (Figure 23). The analyses of Gong et al. (2009) and Vd’ačný et al. (2010) confirm that they are a sister group to the Litostomatea.

POSTCILIODESMATOPHORA: Classes Heterotrichea and Karyorelictea

The name comes from the structure of the ciliary root system, which is an ordered array of ciliary (flagellar) roots, called kineties) that allow for the coordination of ciliary motion. Associated with the flagellar roots are ribbon-like microtubular structures called postciliodesmata, which overlap and, among other things, support the cytopharynx (text with tooltip) A cytopharynx is an internal structure that is attached to the cytostome. . The ciliary root system underlies the alveolar system, which in these taxa, tends to be poorly developed.

Taxa in this group are found in two classes, Heterotrichea and Karyorelictea, but all tend to be long and worm-like. Stentor, a long trumpet-shaped cell, can swim (Figure 24) or attach to a substrate extending itself and feeding with an adoral disc made of a long winding membranelle (Figure 25) that superficially resembles the ciliary structures of the bdelloid rotifers. Indeed, Stentor is about the size of a rotifer and feeds on the same things: suspended particles, bacteria, etc. Although other ciliate taxa have long winding membranelles that surround the cytostome, those of the heterotrich Polydesmatophora wind in a counter clockwise direction.

ORIGINS AND SYSTEMATICS

The origin of the ciliates has been equally shrouded in mystery. Corliss (1979) proposes that the gymnostome ciliates evolved from an unknown group of flagellates. Taylor (1976) suggests a strong relationship between the dinoflagellates and the ciliates. More recent evidence (Gajadhar et al. 1991; Cavalier-Smith 1993; Patterson 1999; and Taylor 1999) supports a common origin of the ciliates, Dinoflagellata, and Apicomplexata in the “supergroup” called the alveolates. The tree produced by Baldauf (2003a) suggests that the ciliates are among the most primitive of the alveolates.

The systematics of the ciliates has been based on gross morphology (Margulis and Schwartz 1988 and 1998; Kudo 1966; and Grell 1976) for a very long time. More recently, some taxonomic schemes like those of Corliss (1979), Small and Lynn (1985), Sleigh et al. (1984), and Lynn and Small (1990 and 2000) differ significantly as they try to incorporate ultrastructural evidence, mainly the organization of the infraciliature and the cytostomal apparatus. The ultrastructure-based systems have grown to become almost unwieldy. The one that we follow is taken from Lynn and Small (2000) and Lynn (2008) which contains two subphyla and 10 classes. A comprehensive check on the ultrastructure is needed in this group because, although ultrastructure does give important fundamental similarities at present, it is unclear whether those similarities are synapomorphies or symplesiomorphies.