The ciliates (Class Ciliata)
These protozoans have many cilia and nuclei of two sizes. Because the mastigophoran Opalina also has many flagella (or cilia), it is the nuclei that act as the criterion and establish the true ciliates as a distinct group. Other features include a kinety system of fibrils and granules for co-ordinating the cilia, characteristic cross-wise fission and a characteristic form of sexual reproduction called conjuga-don.
The range of form exhibited in this class is so great that it suggests a very long independent evolutionary history. While it is relatively easy to place a protozoan as a ciliate it is often quite difficult to determine its order unless the genus is already known, because elaborations and fusion of some cilia, often accompanied by loss of others, has occurred in each order. This type of parallel evolution produces genera that are superficially similar.
The only sure method of identification is the application of one of the silver line staining techniques, in which it is possible to show portions of the kinety system and so obtain the necessary characterization upon which their taxonomy is based. But this is a method not readily or easily performed.
The habitats are various: parasitic, commensal and free-living species are found in many orders. The degree of morphological specialization in some of the spirotrichs is noteworthy as an example of elaboration in a single cell: adaptation to unusual habitats such as the interstitial spaces in sand has lead to great elongation of up to 5 mm. in some species, while some free-living freshwater species are only 15μ long.
Holotrichs (Subclass Holotricha)
This group is regarded as more primitive than the other subclasses for there is lack of development in the peristomial membranelles. A feature that cuts across all other complications and simplifications of the structure is the way the cilia are fused to form accessory organelles in the funnel-shaped cyto-pharynx, or cell mouth. It is characteristic of this subclass that no fused structures are present and the mouth is plainly open. Typically the body (somatic) cilia are of about the same length and present all over the cell.
Gymnostomes (Order Gymnostomatida) As the name suggests the mouth is without ciliary organelles. This is a most diverse group of structurally simple ciliates. But diversity has led to considerable changes in the classification in recent years. A comparison of current classifications shows that no general agreement has yet been reached. Most species in the order contain one meganucleus and one or more micronuclei. The mouth may be anterior and terminal or lateral. If the mouth is lateral then the body is produced into a thin extension to form the new anterior end.
This is the basis for the classification of the suborder Rhabdo-phorina, in which the cytopharynx (the space immediately within the cell opening, the cytostome) is extensible. The other suborder is the Cyrtophorina which has a rigid inextensible cytopharynx.
Suborder Rhabdophorina In this group are found some remarkably voracious feeders, carnivorous in every sense of the word despite their smallish size. A simple form like Holophrya is oval and about 160μ long. It is found on the surface of freshwater where it feeds on dino-flagellates.
The remains of past meals are clearly visible in several food vacuoles in the cytoplasm. A contractile vacuole partly maintains the water balance in the cell. Didinium is similar in shape but has much reduced ciliary fields. Only two bands are left, one round the mouth and one near the equator. This organism can ingest a whole Paramecium although the prey is several times larger than itself.
The vigour with which the prey is attacked, apparently paralysed and then drawn into a mouth which seems far too small is fascinating to watch. Other genera include Dileptus and Lionotus which have extended trichocyst and cilia-bearing anterior processes giving a comb-like appearance to the region in front of the mouth. This shape is an adaptation to living between sand grains and environments where free swimming is impossible.
The higher ciliates have a food-gathering ring of cirri, known as the adoral wreath, around the gullet, and in this photograph it can be clearly seen, together with the nucleus and food vacuoles ( x 1,680).
The ventrally placed mouth is believed to be of evolutionary significance in view of the fact that the majority of ciliates are like this but have additional elaborations. This is a small group. A typical genus is Chilodonella. The ventral mouth is visible from the dorsal side through the cell. The anterior process is considerably thinner than the main posterior part. A line of cilia, the dorsal brush, runs from the edge of the cell to a region over the mouth, and should not be confused with the feeding organelles in other ciliates which sweep food into the mouth.
Suctorians (Order Suctorida)
The suctorians are a compact group only recently considered as holotrichs. The adult form is sessile and without cilia. Food is captured by long knob-ended tentacles which are capable of paralysing prey.
After capture the prey is not drawn to the body, for there is no mouth, but is sucked dry by the tentacles. Recent film studies show the steady flow of cytoplasm from an organism such as Tetrahymena into a suctorian. Reproduction is by several methods.
The commonest is internal budding. After division of the nucleus the anterior pellicle sinks into the cytoplasm in a circle and a young ciliated suctorian develops around the new nucleus. When mature the daughter cell leaves the parent and swims freely. Later it settles down on some object, secretes a non-contractile stalk and loses its cilia to become a small adult. Stained preparations of suctorians show that all stages of development
the kinety system is present even when cilia are not. Typical genera are Tokophyra, Podophyra and Adult:.
Trichostomes (Order Trichostomatida) This group has many features in common with the gymnostomes but is distinguished from them by the presence of a vestibulum. This organ is a depression of the pellicle complete with the cilia which forms a tube leading to the mouth. The cilia of the vesti-bulum urge food into the mouth.
The cytoplasm adjoining the mouth appears to be highly vacuo-lated, and algae or bacteria upon which a free-living genus, Coelosomides, feeds, are digested here. The structure of the gut parasite of amphibians and mammals, Balantidium colt, superficially resembles some hymenostomes or heterotrichs.
The mouth, however, is fed by vestibular cilia which are derived from and develop in a way similar to the normal somatic cilia covering the cell. A common freshwater genus is Colpoda, characteristically kidney-shaped and about 75μ long. The mouth is on the indented side, and the vestibular cilia, clearly visible in stained preparations, are a little shorter than the somatic cilia.
Hymenostomes (Order Hymenostomatida)
This order marks the starting point for the elaborations found in the remaining ciliates. The feeding apparatus is described in some detail here and the names of structures are applicable later. The first of these structures is the buccal cavity between the cytostome and the buccal overture, which is a line of demarcation between the somatic cilia and the buccal cilia.
The buccal cavity has two types of ciliary organelles: the adoral zone of membranellae (AZM) on the left of the cytostome and the undulating membrane (UM) on the right. (Left and right are used for the organism whose mouth is considered to be ventral when the observer is looking from the dorsal surface to the ventral, as left and right are on our own bodies.
In order to get clear pictures and because of the standard way microscopical preparations are made the organisms are always viewed from the ventral surface. So, in drawings and photographs, left and right are trans-posed, as when two people look attach other face to face.)
The UM is a row of cilia that are fused together and beat in co-ordination. The AZM is variable and may be a row of fused cilia as in the spirotrichs or may be divided into several sheets of fused cilia as in Tetrahymena. The buccal cavity has no cilia except for those contained in the UM and the AZM.
The oral ciliature is developed into two fields of cilia in the buccil cavity which opens into a vesti-bulum lying in a shallow oral groove. Paramecium feeds largely on bacteria which are gathered by a feeding current created by cilia in the oral groove. The particles are swept down this groove into the shallow vestibulum and, if accepted into the buccal cavity, to the cytostome. Particles can be rejected, in which case they are swept across the buccal cavity at right angles to the normal ciliary beat. Movement in Paramecium is, as in most ciliates, by means of the somatic cilia covering the body. The cilia beat in a co-ordinated pattern to give waves of propulsion running down the surface.
The cilia lie in rows called meridians, running in the antero-posterior direction. Each row or kinety has cilia connected by fibrils running under the pellicle. The kinetics are cross-connected by transverse fibrils at a level deeper than the longitudinal fibrils. The whole of these interconnections beneath the pellicle is known as the infra-ciliature.
Paramecium is well known for its avoidance reaction. The normal course of swimming is forwards and rotating about its own long axis. But when an unpleasant stimulus is reached the organism stops, reverses at a slight angle for two to four lengths and then moves forward again at a slight angle to the reverse direc-tion. The net result is that the organism turns from its original course and heads away from the unpleasant stimulus. It appears not to feed when moving quickly.
while the macronucleus divides by splitting without the characteristic mitotic apparatus. The methods of sexual reproduction in the ciliates are conjuga-tion and autogamy. In conjugation two individuals come together and join at their oral surfaces. Both macronuclei partially degenerate by enlarging and becoming diffuse and then drawing out into strands which later break down to give numerous weakly staining remnants of the original nuclei. The micronuclei, of which Paramecium aurelia has two, divide meiotically, giving eight daughter nuclei. Seven of the eight degenerate and the remaining one divides again to produce two homozygous gametic nuclei.
One gametic nucleus from each partner migrates into the other partner and fuses with the stationary gametic nucleus. The zygotic nucleus divides twice and while these divisions are occurring the con-jugants separate. Of the four resultant micronuclei two become macronuclei and two micronuclei.
The latter divide again and the exconjugant divides, one macronucleus going to one daughter while the other goes to the .second. Each daughter thus receives two mitotically derived micronuclei and one whole macronucleus.
The second form of sexual reproduction is autogamy. Here a single organism goes through the motions of conjugation without another conjugant. Two homozygous gametic nuclei are formed and these fuse to form a zygote. Thereafter divisions and a new macronucleus are formed as in conjugation. Genetically autogamy brings about recombination of genetic material in a wasteful manner as the genes contained in the daughter micronuclei are lost for ever. The process is similar to the endo-meiosis recently reported to occur in aphids. Recent genetical studies have shown that Paramecium species have classical Mendelian inheritance for many characters.
Although cytological studies . have suggested the above description of sexual reproduction for some eighty years it was only from genetical analysis that certain divisions could be more surely determined as meiotic. Further, some characters were found to be inconstant, in particular the properties of the surface protein of cilia. These proteins developed in response to the temperature of the environment. Whether the change in protein structure is accidental or has a function is not known, for each variation is a newly discovered phenomenon even in parasitic protozoans.
Peritrichs (Order Peritrichida)
These include the sessile ciliates (Sessilina), which live on contractile stalks and are common near the surface of overgrown aquaria, and motile forms (Mobilina) which are believed to be derived from the sessile forms. The characteristics of the order are three specialised rows of cilia around the mouth and the general absence of somatic cilia. Viewed from the anterior the oral cilia are seen to wind counter-clockwise into the mouth. Vorticella is a typical and common genus. The body is bell shaped and mounted with the pointed end on a contractile stalk. During feeding the stalk is elongated and the crown of peristomial cilia expanded.