Of the republic of kazakhstan state university

The body shape is usually constant and in general is asymmetrical; however, radial symmetry with an anterior mouth is probably the primitive condition (Fig. 2-26). Although the majority of ciliates are solitary and free swimming, there are both sessile and colonial forms. The bodies of tintinnids and some heterotrichs, peritrichs, and suctorians are housed within a lorica, a girdle-like encasement, which is either secreted or composed of foreign material cemented together. In the peritrichs the lorica is attached to the substratum, but in many others the lorica is carried about by the organism (Fig. 2-27).

Figure 2-26 Prorodon, a primitive ciliate. (After Faure-Fremict from Corliss.)

Figure 2-27 Tintinnopsis, a marine ciliate with lorica, or test, composed of foreign particles. Note conspicuous membranelles and tentacle-like organelles interspersed between them. (After Faure-Fremiet from Corliss.)

Figure 2-28 Section through cilium and pellicle of Colpidium. Note that alveoli are greatly flattened and their inner and outer membranes fused at base of cilium. At top right is an enlarged view of surface and alveolar membranes. At lower right is a cross section of a cilium and surrounding pellicle taken at the level indicated by the dashed line. Note the circle of nine doubled peripheral ciliary fibrils. (After Pitelka.)

Figure 2-29 Pellicular system in Paramecium. (After Ehret and Powers from Corliss.)

The trichocyst is a peculiar rodlike organelle characteristic of many ciliates that can be explo­sively discharged as a filament. In the undischarged state they are oriented at right angles to the body surface The discharged trichocyst consists of a long, striated, threadlike shaft surmounted by a barb, which is shaped somewhat like a golf tee (Fig. 2-30). The shaft is not evident in the undischarged state and is probably polymerized in the process of discharge. The function of trichocysts is uncertain, but they may be used in anchoring the ciliate when feeding.

Toxicysts are vesicular organelles found in the pellicle of gymnostomes (Dileptus and Didinium), which on discharge consist of bulbous bases that taper into long threads (Fig. 2-31A and B). Toxi­cysts are used for defense or for capturing prey by paralysis and cytolysis. They are commonly re­stricted to the parts of the ciliate body that contact prey, such as around the smooth region in Didinium.

Figure 2-30 Electron micrograph of discharged tricho­cysts of Paramecium. Note golf-tee-shaped barb and long, striated shaft. (By Jacus and Hall, 1946: Biol. Bull., 91:141.)

outer ciliary membrane
Mucigenic bodies (mucocysts) are another group of pellicular organelles found in some cil­iates. They are arranged in rows like trichocysts and discharge a mucoid material that may function in the formation of cysts or protective coverings (Fig. 2-28).

Cilia have the same structure as flagella; they differ from flagella chiefly in that they are generally more numerous and shorter. Compound ciliary or­ganelles, evolved from the adhesion of varying numbers of individual cilia, are of common occur­rence and will be described later.

The ciliature can be conveniently divided into the body (or somatic) ciliature, which occurs over the general body surface, and the oral ciliature, which is associated with the mouth region. Distri­bution of body cilia is quite variable. In the primi­tive groups, cilia cover the entire animal and are ar­ranged in longitudinal rows (Fig. 2-26), but in many of the more specialized groups they have be­come limited to certain regions of the body.

As mentioned earlier, each cilium arises from a basal body or kinctosome, located in the alveolar layer (Figs. 2-29). The kinetosomcs that form a particular longitudinal row arc connected by means of fine, striated fibrils, called kineto-desma. The cilia, kinetosomcs, and fibrils of a row make up a kinety. The longitudinal bundle of fi­brils runs to the right side of the row of kineto-somes, and each kinctosome gives rise to one ki-netodesmos (fibril), which joins the longitudinal bundle and extends anteriorly. Single kinctodes-mata arc tapered and extend for varying distances as parts of the bundle. At the kinctosome, the ki-nctodesmata are connected to certain of the kinc­tosome triplets.

Figure 2-33 Reconstruction of section of the pellicle of Tetrahymena. Right side is on viewer's left. Abbreviations: kinetodesmos |k); transverse microtubules (tm); postciliary microtubules (pm); longitudinal microtubules (lm); basal microtubules (bm); alveolus (a); cilium (c); epiplasm (c); mitochondrion |m); mucigenic body (mb). (From Allen, R. D, 1967: Fine structure, reconstruction and possible function of components of the cortex of Tetrahymena pyriformis. J. Protozool., 14:553-565.)

A kinety system is characteristic of all ciliates, although there are variations in details of the pat­tern. Even groups such as the Suctorida, which pos­sess cilia only during developmental stages, retain part of the kinety system in the adult.

The ciliates are the fastest moving of the protozoa. In its beat each cilium performs an effective and a recovery stroke. During the effective stroke the cil­ium is outstretched and moves from a forward to a backward position (Fig. 2-34A and B). In the re­covery stroke the cilium is bent over to the right against the body (when viewed from above and looking anteriorly) and is brought back to the for­ward position in a counterclockwise movement. The recovery position offers less water resistance and is somewhat analogous to feathering an oar. A cilium moves in three different planes in the course of a complete cycle of beat, and the positions have been captured and recorded in scanning electron micrographs of freeze-dried Paramecium (Tamm, 1972).

Figure 2-33 Reconstruction of section of the pellicle of Tetrahymena. Right side is on viewer's left. Abbreviations: kinetodesmos |k); transverse microtubules (tm); postciliary microtubules (pm); longitudinal microtubules (lm); basal microtubules (bm); alveolus (a); cilium (c); epiplasm (c); mitochondrion |m); mucigenic body (mb). (From Allen, R. D, 1967: Fine structure, reconstruction and possible function of components of the cortex of Tetrahymena pyriformis. J. Protozool., 14:553-565.)

The Protozoa

In addition to kinetodesmata, there are also rib­bons of microtubules that extend posteriorly and transversally from the kinetosome and apparently function as part of the cilium anchorage system.

A kincty system is characteristic of all ciliates, although there are variations in details of the pat­tern. Even groups such as the Suctorida, which pos­sess cilia only during developmental stages, retain part of the kinety system in the adult.

Locomotion

The ciliates are the fastest moving of the protozoa. In its beat each cilium performs an effective and a recovery stroke. During the effective stroke the cil­ium is outstretched and moves from a forward to a backward position (Fig. 2-34A and B). In the re­covery stroke the cilium is bent over to the right against the body (when viewed from above and looking anteriorly) and is brought back to the for­ward position in a counterclockwise movement. The recovery position offers less water resistance and is somewhat analogous to feathering an oar. A cilium moves in three different planes in the course of a complete cycle of beat, and the positions have been captured and recorded in scanning electron micrographs of freeze-dried Paramecium (Tamm, 1972).

Figure 2-34 Ciliary beating and locomotion. A, Cycle of a ciliary beat seen from the side. In the effective stroke the Cilium is outstretched and moves from left to right. B, Path described by tip of cilium during beat cycle as seen from surface. E is effective stroke and R is the recovery stroke. (From Sleigh, M. A., 1973: The Biology of Protozoa. Edward Arnold Publishers, London, p. 38.) C, Series of adjacent cilia in a metachronal wave in various stages of the beat cycle. Direction of effective stroke (solid arrow) is same as metachronal wave (dashed arrow). Letter с indicates wave crest. D, As in В except metachronal wave is moving in opposite direction to effective stroke. |C and D from Jones, A. R., 1974: The Ciliates. St. Martin's Press, Hutchinson in London, p. 70.) E, Metachronal waves in Paramecium during forward swimming. Wave crests are shown by lines and their direction by arrows (dotted on opposite side). Movement of cihate is indicated by large arrow. (From Machemer, H., 1974: Ciliary activity and metachronism in Protozoa. In Sleigh, M. A. (Ed): Cilia and Flagella. Academic Press, London, p. 224.) F, The avoiding reaction of Paramecium. (After Hyman, L. H., 1940: The Invertebrates, McGraw-Hill Book Co., N.Y. Vol. I.)

In forms such as Paramecium the direction of the effective stroke is oblique to the long axis of the body (Fig. 2-34E). This causes the ciliate to swim in a spiral course and at the same time to rotate on its longitudinal axis. The ciliary beat can be re­versed, and the animal can move backward. This backward movement is associated with the so-called avoiding reaction. In Paramecium, tor ex­ample, when the animal comes in contact with some undesirable substance or object, the ciliary beat is reversed (Fig. 2-34F). The animal moves backward a short distance, turns slightly clockwise or counterclockwise, and moves forward again. If unfavorable conditions are still encountered, the avoiding reaction is repeated. External stimuli are probably detected through certain long, stiff cilia that play no role in movement and are probably en­tirely sensory. The direction and intensity of the beat are controlled by levels of Ca++ and К/ ions (Eckert, 1972).

The highly specialized hypotrichs, such as Urostyla, Stylonychia, and Euplotes (Fig. 2-32Л), have greatly modified body cilia. The body has be­come differentiated into distinct dorsal and ventral surfaces, and cilia have largely disappeared except on certain areas of the ventral surface. Here the cilia occur as a number of tufts, called cirri. The cilia of a cirrus beat together, and coordination here is believed to result from some sort of struc­tural coupling as a result of the close proximity of their bases.

Some ciliates, especially sessile forms, can un­dergo contractile movements, either shortening the stalk by which the body is attached, as in Vorticella (Fig. 2-32C), or shortening the entire body, as in Stentor (Fig. 2-32D). Contraction is brought about by bundles of contractile filaments, or myonemes, that lie in the pellicle. In Vorticella and the colo­nial Carchesium, both of which have bell-shaped bodies attached by a long slender stalk, the myonemes extend into the stalk as a single, large, spiral fiber. The contractions of this spiral my-oneme, which functions very much like a coiled spring, produce the familiar popping movements that are so characteristic of Vorticella and related genera.

Feeding in ciliates parallels, on a microscopic level, feeding in multicellular animals. Typically a dis­tinct mouth, or cytostome, is present, although it has been secondarily lost in some groups. In prim­itive groups the mouth is located anteriorly (Fig. 2-26), but in most ciliates it has been displaced posteriorly to varying degrees. The mouth opens into a canal or passageway called the cytopharynx, which is separated from the endoplasm by a mem­brane. It is this membrane that enlarges and pinches off as a food vacuole. The wall of the cy­topharynx is strengthened with rods (nemades-mata) arranged like the staves of a barrel. Primi­tively, the ingestive organelles consist only of the cytostome and cytopharynx (Figs. 2-26 and 2-35Л), but in the majority of ciliates the cytos­tome is preceded by a preoral chamber. The preoral chamber may take the form of a vestibule, which varies from a slight depression to a deep funnel, with the cytostome at its base (Fig. 2-35B). The vestibule is clothed with simple cilia derived from the somatic ciliature.

In the higher ciliates the preoral chamber is typ­ically a buccal cavity, which differs from a vesti­bule by containing compound ciliary organelles in­stead of simple cilia (Fig. 2-35C to F). There are two basic types of such ciliary organelles: the un­dulating membrane and the membranelle. An un­dulating membrane is a row of adhering cilia form­ing a sheet (Fig. 2-36Л and B). A membranelle is derived from two or three short rows of cilia, all of which adhere to form a more or less triangular or fan-shaped plate and typically occur in a series (Figs. 2-27, 2-32D, and 2-36B). Although there is no actual fusion of adjacent cilia in these com­pound organelles, their kinetosomes and bases are sufficiently close together to produce some sort of structural coupling that causes all of the cilia of a membranelle to beat together.

The term peristome, which is commonly en­countered in the literature, is synonymous with buccal cavity. In members of a number of orders the buccal organelles project from the buccal cav­ity, or, as in the Hypotrichida (Fig. 2-32A), the buccal cavity is somewhat shallow so that the or­ganelles occupy a flattened area around the oral region. Such an area is called the peristomial field. In forms like Paramecium there is a vestibule in front of the buccal cavity (Figs. 2-35D and 2-36D).

Figure 2-35 Oral areas of various ciliates. A, In rhabdophorine gymnostomes, such as Coleps, Prorodon, and Didi-nium. B, In a trichostome such as Colpoda, with a vestibule that is displaced from anterior end. C, In a tetrahymenine hymenostome, such as Tetrahymena. D, In a peniculine hymenostome, such as Paramecium. E, In a peritrich, such as Vorticella. F, In a hypotrich, such as Euplotes. (Modified after Corliss, J. O., 1961: The Ciliated Protozoa. Pergamon Press, N.Y.)

Didinium has perhaps been most studied of all the raptorial feeders. This little barrel-shaped ciliate feeds on other ciliates, particularly Parame­cium (Fig. 2-37A). When Didinium attacks a Par­amecium, it discharges toxicysts into the Paramecium and the proboscis-like anterior end attaches to the prey through the terminal mouth, which can open almost as wide as the diameter of the body.

An interesting group of raptorial ciliates is the aberrant subclass Suctoria, formerly considered a separate class. Free-living suctorians are all sessile and are attached to the substratum directly or by means of a stalk (Fig. 2-37). Cilia are present only in the immature stages. The body bears tentacles, which may be knobbed at the tip or shaped like long spines (Fig. 2-41B). Each tentacle is supported by a cylinder of microtubules and carries special or­ganelles, called haptocysts (Fig. 2-37D). Suctorians feed on other ciliates, and when prey strikes the tentacles, the haptocysts are discharged into the prey body, anchoring it to the tentacles (Figs. 2-37D to F and 2-38). The contents of the prey are then sucked through the tubular tentacle into the suctorian, where they are collected into food vacuoles.

Typically characteristic of suspension feeders is the buccal cavity. Food for suspension feeders con­sists of any small organic particles, dead or living, particularly bacteria that are suspended in water. Food is brought to the body and into the buccal cavity by the compound ciliary organelles. From the buccal cavity the food particles are driven through the cytostome and into the cytopharynx. When the particles reach the cytopharynx, they col­lect within a food vacuole.

The order Hymenostomatida—"membrane-mouthed"—contains some of the most primitive suspension feeders. Tetrahymena is a good exam­ple of such a primitive type (Fig. 2-36B). The cy tostome is located a little behind the leading edge of the body. Just within the broad opening to the buccal cavity are four ciliary organelles—an un­dulating membrane on the right side of the cham­ber and three membranellcs on the left. The three membranelles constitute an adoral zone of mcm-branelles, which in many higher groups of ciliates is much more developed and extensive.

Figure 2-36 A, Pleuronema. (After Noland from Corliss.) B, Tetrahymena. (After Corliss, J. O., 1961: The Ciliated Protozoa. Pergamon Press, N.Y.) C, Scanning electron photomicrograph of Uronychia, a marine ciliate, showing the highly developed membranelles. (By Small, E. В., and Marszalek, D. S., 1969: Science, 163: 1064-1065. Copyright 1969, American Association for the Advancement of Science.) D, Buccal organelles of Paramecium. (After Yusa from Man-well.) £, Lacrymaria. (After Conn from Hyman.)

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Figure 2-37 A, Four Didinium, raptorial ciliates, attacking one Paramecium. (After Mast from Dogiel.| B, Acineta, a suctorian. (After Calkins from Hyman.| C-F, Suctorian haptocysts and prey cap­ture. Haptocyst isolated |C| and within tentacle tip (D). Attach­ment of tentacle to prey (£) and en-gulfmcnt through tentacle (PL (From Sleigh, M. A., 1973: The Bi­ology of Protozoa, Edward Arnold Publishers, London, p. 64. Based on micrographs of Rudzinska, Bar D E F dele, and Grell.J

Figure 2-38 A colony of the suctonan Heliophrya feeding on Paramecium. Some individuals of Paramecium have just been captured. Others have been ingested to various degrees. (From Spoon et al., 1976: Observations on the behavior and feeding mechanisms of the suctorian Heliophrya erhardi preying on Paramecium. Trans. Am. Micros Soc 95-443-462.|

In feeding, the cilia of the oral groove produce a current of water that sweeps in an arclike manner down the side of the body and over the oral region. The ciliature of the vestibule and buccal cavity pull in food particles and drive them into the forming food vacuole.

In the subclass Peritricha, whose members pos­sess little or no somatic cilia, the buccal ciliary or­ganelles are highly developed and form a large, disclike, peristomial field at the apical end of the animal. In the much-studied peritrich genus Vor-ticella, a peripheral shelflike projection can close over the disc when the animal is retracted (Figs. 2-32C and 2-35E). The ciliary organelles lie in a peristomial groove between the edge of the disc and the peripheral shelf and consist of two ciliary bands, which wind in a counterclockwise direction around the margin of the disc and then turn down­ward into the funnel-shaped buccal cavity. The inner ciliary band produces the water current and the outer band acts as the filter. Suspended parti­cles, mostly bacteria, are transported along with a stream of water between the two bands into the buccal cavity.

Ciliates of the subclass Spirotricha, which in­cludes such familiar forms as Stentor, Halteria, Spi-rostomum, and Euplotes, arc typically suspension feeders. They usually possess a highly developed adoral zone of many membranelles (Fig. 2-32Л and D).

Within the cytopharynx of all ciliates, food par­ticles enter the food vacuole. When the food vac­uole reaches a certain size, it breaks free from the cytopharynx, and a new vacuole forms in its place. Detached vacuoles then begin a more or less cir­culatory movement through the endoplasm.

Digestion follows the usual pattern, and a pH as low as 1.4 has been reported during the acid phase-in some species (Paramecium). Following diges­tion, the waste-laden food vacuole moves to a def­inite anal opening, or cytopyge (Fig. 2-1), at the body surface and expels its contents. The cytopyge varies in position. In Tetrahymena it is located on the side of the animal, near the posterior end (Fig. 2-1), whereas in the pcritrichs it opens into the buccal cavity.

There arc relatively few parasitic ciliates, al­though there arc many ecto- and endocommensals. Many suctorians are commensals, and a few arc parasites. Hosts include fishes, mammals, various invertebrates, and other ciliates. Sphaerophyra, for example, lives within the endoplasm of Stentor, and Endosphaera is parasitic within the body of the peritrich Telotrochidium.

Other interesting commensal ciliates include Kerona, a little crawling hypotrich, and Tricho-dina. a mobile peritrich, which are cctocommen-sals on the surface of hydras. There arc also some free-swimming pcritrichs that occur on the body surfaces of freshwater planarians, tadpoles, sponges, and other animals.

The genus Ralantidium includes many species that are endocommensals or endoparasites in the guts of insects and many different vertebrates. Bal-antidium coli is an endocommensal in the intes­tines of pigs and is passed by means of cysts in the feces. This ciliatc has occasionally been found in humans, where in conjunction with bacteria it erodes pits in the intestinal mucosa and produces pathogenic symptoms. The related highly special­ized Entodiniomorphida (Fig. 2-39) live as harm­less commensals in the digestive tracts of many different hoofed mammals. Like the flagellate sym­bionts of termites and roaches, some of them in­gest and break down the cellulose of the vegetation eaten by their hosts. The products of digestion are utilized by the host.

A few ciliates display symbiotic relationships with algae. The most notable of these is Parame­cium hursaria, in which the endoplasm is filled with green zoochlorellae.