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In this article we will discuss about:- 1. Habitat of Monocystis 2. Morphology of Monocystis 3. Nutrition Required 4. Respiration Process 5. Excretion Process 6. Reproduction 7. Life-History 8. Life-Cycle.
Contents:
- Habitat of Monocystis
- Morphology of Monocystis
- Nutrition Required by Monocystis
- Respiration Process in Monocystis
- Excretion and Locomotion Process of Monocystis
- Reproduction in Monocystis
- Life-History of Monocystis
- Life-Cycle of Monocystis
1. Habitat of Monocystis:
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Monocystis is a parasite and belongs to the class sporozoa. It lives within the seminal vesicles of the common earthworm, which, therefore, is its host. As it lives within the body of the host, it is an endoparasite.
Monocystis causes injury by destroying the sperms which are formed and nourished in the seminal vesicles of its host. But the amount of injury seems to be very slight, since the earthworm thrives quite well in spite of heavy infection. Almost very earthworm dissected in the class-room is found to be infected with Monocystis.
2. Morphology of Monocystis:
In the full-grown stage the parasite is known as the trophozoite. Its body is elongated and more or less fusiform in shape. It is composed of a nucleus lying within a mass of cytoplasm, the whole thing surrounded by a thick cuticle.
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The nucleus is spherical in shape. It is surrounded by a delicate nuclear membrane and is filled with nuclear sap. The chromatin is aggregated into a solid mass called endosome which is either attached to a side of the nuclear membrane or suspended in the nuclear sap.
The cytoplasm is divided into a clear stiff outer portion or ectoplasm, and a granular soft inner portion or endoplasm. In the deeper layers of the ectoplasm there are a number of fine contractile threads called myonemes.
The endoplasm include reserve food materials, chiefly glycogen, para-glycogen and droplets of fat. Vacuoles of any kind are totally absent. The cuticle surrounding the cell body is thick, porous and permeable. The animal is now in its feeding or trophic stage and hence the name trophozoite.
3. Nutrition Required by Monocystis:
The adult trophozoite is suspended in the nutrient fluid meant for the nourishment of the earthworm’s spermatozoa. It is surrounded by soluble food which is absorbed into the endoplasm by slow diffusion through its porous cuticle. The young trophozoite lives within the sperm mother cell of the earthworm.
It secretes digestive enzymes from its outer surface which dissolve the protoplasm of the sperm mother cell. The digested product is then absorbed by diffusion through the cuticle. The parasite requires no food vacuole, because it either gets digested food or digests food outside its own body. As the parasite absorbs soluble food through the outer surface of its body, the mode of nutrition is saprozoic.
4. Respiration Process in Monocystis:
The parasite gets a poor supply of oxygen within the seminal vesicles of its host. Respiration, therefore, is mainly anaerobic. Release of energy occurs by breaking down of glycogen and para-glycogen molecules into lactic acid with the help of enzymes. Carbon dioxide, formed as by-product, passes out by diffusion through the porous cuticle.
5. Excretion Process and Locomotion of Monocystis:
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Being slow and inactive, the animal produces very little nitrogenous waste products. This is excreted in solution through its cuticle. This explains the absence of contractile vacuole in Monocystis.
Movement is sluggish because the animal has no necessity to run after its food. It is capable of slow wriggling movements by bending the cell body to one side or the other. This is due to rhythmical contractions of the myonemes in its ectoplasm.
It can also glide along by producing-series of wave-like contractions of the myonemes, which consist of both, longitudinal and circular fibrils. Expansion and contraction of the cell body is effected by using the two sets alternately.
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6. Reproduction in Monocystis:
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Monocystis reproduces sexually and the process of reproduction is complicated. Two mature trophozoites associate with each other for this purpose. They are called gametocytes or gamonts. They grow shorter and secrete round them a two-layered cyst called gametocyst.
The individuals, thus enclosed, are identical and they never fuse with one another. Within the gametocyst, each gametocyte undergoes multiple fission. Its nucleus divides rapidly to produce a number of daughter nuclei which migrate towards the periphery. Each bit of the nucleus is surrounded by a fragment of cytoplasm, and each of these nucleated fragment is a gamete.
The gametes adhere to a central lump of undivided cytoplasm called residual body. Usually a large number of gametes are produced by each gametocyte and these are nourished by the residual body which is full of reserve food. Fully formed gametes are finally set free within the gametocyst.
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They are more or less rounded or oval bodies which now fuse in pairs. The gametes produced by the two gametocytes are exactly similar and are, therefore, isogametes. But during fusion, the two in each couple are derived from different parents. The fusion of the gametes is complete, involving both nucleus and cytoplasm.
The product is a larger cell called zygote or sporoblast which clothes itself with a resistant boat-shaped covering called sporocyst. The spoiocyst is also known as the pseudonavicella, because it appears to resemble a unicellular plant called Navicella.
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Enclosed within the pseudonavicella the zygote undergoes three successive divisions, producing eight sickle-shaped falciform youngs called sporozoites. At this stage the original gametocyst is loaded with a large number of pseudonavicellae, each containing eight sporozoites.
Further development cannot take place until the sporozoites are transferred to a new host. The actual method of transmission is not known. The earthworm may die and disintegrate, liberating the sporocysts in the soil; or the worm may be devoured and digested by a bird, and in this case the sporocysts, being resistant, escape digestion and pass with the bird’s faeces into the soil.
By chance, a sporocyst may be eaten along with soil by another earthworm. In the alimentary canal of the new host the sporocyst dissolves, liberating eight falciform sporozoites. The sporozoites bore through the wall of the gut and ultimately find their way into the seminal vesicles. Each spoiozoite now enters a sperm-mother cell and becomes an intracellular parasite.
It grows rapidly by digesting and absorbing the cytoplasm of the sperm-mother cell and is converted into a young trophozoite. The young trophozoite digests the heads of the spermatozoa which the sperm-mother cell is producing.
Consequently the slender wavy tails of the sperms remain attached to its outer surface and the young animal appears to be covered by cilia. In course of time it is converted into a full-grown adult trophozoite and the cycle, just described, is repeated.
7. Life-History of Monocystis:
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There are two important features in the life-history of Monocystis which require special consideration.
Firstly, the remarkable simplicity of its structure and the easy-going life of the adult trophozoite; and secondly, the complex nature of its mode of reproduction. Both are associated with the parasitic mode of life. Compared with a free-living Amoeba, the trophozoite of Monocystis appears to be a degenerated individual.
The Amoeba, possesses several organelles such as food vacuoles for digestion, contractile vacuole for excretion and pseudopodia for active locomotion. None of these are found in Monocystis; because, in the seminal vesicle of its host, ready-made food is abundant and competition is absent. Consequently structures to carry on normal activities are unnecessary.
This comfortable life has at least one drawback. The parasite must be transferred from host to host for perpetuation of its race. This involves extra risks, for the animal has to pass through the outside world which is full of danger.
Further, actual transmission depends upon mere chance, because a sporocyst may never be eaten by a new host. This explains the complexity of its reproductive method. The parasite, reproducing sexually, releases a large number of resistant sporocyst. Most of the sporocysts may perish but the few that survive will perpetuate the race.
8. Life-Cycle of Monocystis:
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There are two generations in the life-cycle of Monocystis. The zygote or sporoblast represents the diploid generation because it possesses a double set of chromosomes represented by number as ‘2n’. The zygote divides thrice to produce eight sporozoites. As one of these divisions is reduction division, each sporozoite contains a reduced or haploid number of chromosomes represented by number as ‘n’.
Sporozoites are converted into trophozoites and the latter into gametocytes. The gametocytes produce gametes by multiple fission. All these forms represent the haploid generation.
With the fusion of gametes, that is, sexual reproduction, the diploid or double number of chromosomes (represented by ‘2n’) is restored. Thus, in Monocystis the zygote alone is diploid and all the other stages of the life-cycle are haploid as in higher plants.
