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Paramecia are often described as slipper shaped. Paramecium caudatum is 150 to 300 µm in length and is blunt anteriorly and somewhat pointed posteriorly. The organism has an asymmetrical appearance because of the oral groove, a depression that runs obliquely backward on the ventral side.
The pellicle is a clear, elastic membrane that may be ornamented by ridges or papilla-like projections, and its entire surface is covered with cilia arranged in lengthwise rows. Just below the pellicle is the thin clear ectoplasm that surrounds the larger mass of granular endoplasm. Embedded in ectoplasm just below the surface are spindle-shaped trichocysts, which alternate with the bases of cilia. The infraciliature can be seen only with special fixing and staining methods.
A cytostome at the end of the oral groove leads into a tubular cytopharynx, or gullet. Along the gullet an undulating membrane of modified cilia keeps food moving. Fecal material is discharged through a cytoproct posterior to the oral groove. Within the endoplasm are food vacuoles containing food in various stages of digestion. There are two contractile vacuoles, each consisting of a central space surrounded by several radiating canals that collect fluid and empty it into the central vacuole.
Paramecium caudatum has two nuclei: a large kidney-shaped macronucleus and a smaller micronucleus fitted into the depression of the former. These can usually be seen only in stained specimens. The number of micronuclei varies in different species; for example, P. multimicronucleatum may have as many as seven.
Paramecia are holozoic, living on bacteria, algae, and other small organisms. Cilia in the oral groove sweep food particles in the water into the cytostome, from which they are carried into the cytopharynx by the undulating membrane. From the cytopharynx food is collected into a food vacuole that is constricted into the endoplasm. Food vacuoles circulate in a definite course through the cytoplasm while the food is being digested by enzymes from the endoplasm. Indigestible parts of the food are ejected through the cytoproct.
The body is elastic, allowing it to bend and to squeeze through narrow places. Its cilia can beat either forward or backward, so that the organism can swim in either direction. The cilia beat obliquely, causing the organism to rotate on its long axis. In the oral groove the cilia are longer and beat more vigorously than the others so that the anterior end swerves aborally. As a result of these factors, the organism moves forward in a spiral path.
When a ciliate, such as a paramecium, contacts a barrier or a disturbing chemical stimulus, it reverses its cilia, backs up a short distance, and swerves the anterior end as it pivots on its posterior end. This behavior is called an avoiding reaction.
A paramecium may continue to change its direction to keep itself away from a noxious stimulus, and it may react in a similar fashion to keep itself within the zone of an attractant. A paramecium may also change its swimming speed. How does a paramecium “know” when to change directions or swimming speed? Interestingly, reactions of the organism depend on effects of the stimulus on the electrical potential difference across its cell membrane.
Paramecia slightly hyperpolarize in attractants and depolarize in repellents that produce the avoiding reaction. Hyperpolarization increases the rate of the forward ciliary beat, and depolarization results in ciliary reversal and backward swimming.
Reproduction in Paramecium Paramecia reproduce only by binary fission across kineties (ciliary rows) but have certain forms of sexual phenomena called conjugation and autogamy. In binary fission the micronucleus divides mitotically into two daughter micronuclei, which move to opposite ends of the cell. The macronucleus elongates and divides amitotically.
Conjugation occurs at intervals in ciliates. Conjugation is the temporary union of two individuals to exchange chromosomal material. During the union the macronucleus disintegrates and the micronucleus of each individual undergoes meiosis, giving rise to four haploid micronuclei, three of which degenerate. The remaining micronucleus then divides into two haploid pronuclei, one of which is exchanged with the other conjugant. The pronuclei fuse to restore the diploid number of chromosomes, followed by several more nuclear events detailed in. Following this complicated process, the organisms may continue to reproduce by binary fission without conjugation.
The result of conjugation is similar to that of zygote formation, for each exconjugant contains hereditary material from two individuals. The advantage of sexual reproduction is that it permits gene recombinations, thus increasing genetic variation in the population. Although ciliates in clone cultures can apparently reproduce repeatedly and indefinitely without conjugation, the stock seems eventually to lose vigor. Conjugation restores vitality to a stock. Seasonal changes or a deteriorating environment usually stimulate sexual reproduction.
Autogamy is a process of self-fertilization similar to conjugation except that there is no exchange of nuclei. After the disintegration of the macronucleus and the meiotic divisions of the micronucleus, two haploid pronuclei fuse to form a synkaryon that is completely homozygous.
Useful External Links
- Paramecium ciliate genus by Britannica
- Paramecium: Characteristics, biology and reproduction by LIVESCIENCE
- Integrative Neuroscience of Paramecium, a “Swimming Neuron” by NIH
