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 is transposed, as when two people look at each other face to face.)
The UM is a row of cilia that are fused together and beat in coordination. The AZM is variable and maybe 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 hymenostomes are mostly free-living and found in freshwater. A simple member of the group is Tetrahymena. This genus has become well known recently because of its usefulness in laboratory studies on cell division. The pellicle, measuring 50 μ x 30 μ is somewhat flexible and ovoid. The oral apparatus is near the anterior end and is ventral.
The UM is single but the AZM is represented as three fused bands of cilia. A macronucleus is always found but many strains are without micronuclei. The latter cannot reproduce sexually. There is one contractile vacuole and also a cytoproct (cellanus) for the elimination of solid waste. Paramecium is a genus that abounds in nature and is large enough for ready observation. The size of the different species ranges from 80μ to 200 p. The oral ciliature is developed into two fields of cilia in the buccal cavity which opens into a vestibulum 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 coordinated pattern to give waves of propulsion running down the surface. The cilia lie in rows called meridians, running in the anteroposterior direction. Each row or kinety has cilia connected by fibrils running under the pellicle. The kineties 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 direction. 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..
Reproduction can be by fission or by a sexual process. Binary fission may occur two or three times a day and the micronucleus divides mitotically and is large enough for ready observation. The size of the different species ranges from 80μ to 200 p.
The oral ciliature is developed into two fields of cilia in the buccal cavity which opens into a vestibulum 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.
while the macronucleus divides by splitting without the characteristic mitotic apparatus. The methods of sexual reproduction in the ciliates are conjugation 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 conjugants separate. Of the four resultant micro nuclei 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 fuses form a zygote. Thereafter divisions and a new macronucleus are formed as in conjugation. Genetically autogamy brings about the recombination of genetic material in a wasteful manner as the genes contained in the daughter micronuclei are lost forever. 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 the 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 specialized 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 is expanded. The slightest disturbance causes both structures to contract. Vorticellids live in clusters and as they are, in the expanded condition, about 500 μ long, contraction of a whole cluster is conspicuous. Food is drawn in particulate form from the surrounding water in the swirling currents set up by the peristomial cilia. Reproduction is by binary fission and conjugation.