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In this article we will discuss about:- 1. Habitat of Liver Fluke 2. External Feature of Liver Fluke 3. Body Wall 4. Alimentary System 5. Respiratory System 6. Excretory System 7. Nervous System 8. Reproductive System 9. Life-History.
Contents:
- Habitat of Liver Fluke
- External Feature of Liver Fluke
- Body Wall of Liver Fluke
- Alimentary System of Liver Fluke
- Respiratory System of Liver Fluke
- Excretory System of Liver Fluke
- Nervous System of Liver Fluke
- Reproductive System in Liver Fluke
- Life-History of Liver Fluke
1. Habitat of Liver Fluke:
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The liver fluke is a flat worm belonging to the phylum platyhelminthes. It is a parasite living within the bile ducts of the sheep, or of other cattle, and rarely of man. It may invade other organs and causes a disease known as ‘liver rot’.
Heavy infection often ends in the death of the sheep. The parasite requires two hosts to complete its life cycle. One stage of its life-history is spent within the tissues of a snail (Limnaea truncatula). Consequently liver fluke thrives in marshy meadows where the appropriate snails are abundant.
2. External Feature of Liver Fluke:
The liver fluke is a flat leaf-shaped worm about 2.5 cms long and 15 cms broad, with a blunt triangular projection, the head lobe, at its anterior end. A cup-shaped anterior sucker is situated at the apex of the head lobe.
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The mouth is an oval opening in the middle of the anterior sucker. A short distance behind the mouth there is an imperforate posterior sucker on the ventral surface of the body. The small genital aperture or gonopore lies ventrally between the two suckers. The single excretory aperture is situated at the narrow and bluntly pointed posterior end of the body.
3. Body Wall of Liver Fluke:
The animal is covered with a cuticle in which are embedded small backwardly directed spinules. Beneath the cuticle there are successive layers of circular, longitudinal and diagonal muscle fibres. The epidermal cells lie among the longitudinal muscle fibres. There is no Coelom.
The space between the body wall and the gut is filled up by a tissue called parenchyma which is composed of a mesh work of protoplasm with scattered nuclei and a few oval cells. Muscle fibres are found to pass across the parenchyma from the dorsal to the ventral side.
4. Alimentary System of Liver Fluke:
The mouth leads into an oval muscular pharynx with a short oesophagus which at once divides into left and right branches. These are the intestines, which extend backwards along the two sides of the middle line to the posterior end of the body giving off a number of Caeca on either side which in turn may be much branched.
The intestines end blindly and there is no anus. The liver fluke feeds mainly on the blood of its host, which is sucked from the wall of the bile duct with the help of its sucker. It can also absorb predigested sugars such as glucose sctprozoically through its cuticle.
5. Respiratory System of Liver Fluke:
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There is no respiratory organs in the liver fluke. The bile of its host is saturated with oxygen which is absorbed by the general body surface. CO2 is released by exosmosis through the cuticle. There is no evidence to believe that the liver fluke is anaerobic because a reduced oxygen tension leads to the death of the parasite.
6. Excretory System of Liver Fluke:
The excretory system consists of a number of much-branched capillary tubules ending in flame cells. These are the protonephridia which ultimately drain into a single median tube, the excretory vessel, opening into the excretory aperture at the posterior end of the body. The flame cells extract fluid waste products and drive them out via the excretory channels.
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7. Nervous System of Liver Fluke:
There is a ganglionated nerve ring surrounding the oesophagus at the anterior end of the body. From theses arise a number of slender nerves which innervate the different parts of the body, thereby maintaining nervous co-ordination two large lateral nerves, one on either side, run backwards to the posterior end giving off slender branches to the parts nearby.
8. Reproductive System in Liver Fluke:
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The liver fluke is hermaphrodite, which means that the male and female reproductive organs are found simultaneously in the same individual. The male reproductive organ consists of: Two much branched tube-like testes, one lying behind the other, in the middle part of the body.
A vas deferens arises from each testis, and oviduct the vasa deferentia run side by side towards the anterior end. Close to the posterior sucker, the vasa deferentia join with one another to produce a large pear- shaped seminal vesicle. A twisted narrow tube, the ejaculatory duct, connects the seminal vesicle with a hollow muscular structure, the penis or Cirrus, which opens at the gonopore.
Normally the Cirrus is enclosed in a Cirrus-sac but it can be everted and thrust out through the genital pore. The female reproductive organ consists of: A single much- branched ovary lying on the right side in front of the testes.
A narrow oviduct comes out from the ovary and passes towards the middle line to join the median vitelline duct from the yolk glands which lie laterally along the sides of the body. The junction of the oviduct and the vitelline duct is surrounded by a sac-like ootype, and the rounded shell-gland which secretes the egg-shell.
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A wide twisted tube, the uterus, runs forwards from the ootype to open at the gonopore. Arising from the ootype, there is a narrow channel, Laurer’s canal, which is connected to a small aperture on the dorsal surface of the fluke near the anterior end. The animal is usually self-fertilised but cross-fertilisation may occur through Laurer’s canal.
9. Life-History of Liver Fluke:
Liver fluke’s life-history is an extremely interesting process. Numerous eggs are laid in the bile ducts of the sheep: They are carried to the intestine and passed outside with the faeces. They develop only when they are placed in damp marshy places and if the temperature of the environment is above 50°F. If placed suitably a microscopic larva, called miracidium, hatches out of each egg in about ten to twelve days.
The miracidium is multicellular and ciliated. It is provided with a pointed knob at its anterior end, a nerve ganglion, two eyespots, two flame cells and a small gut. If it is hatched in water it swims actively in search of the intermediate host, that is the snail Limnaea truncatula. It dies within about eight hours unless it can bore its way into the pulmonary chamber of the snail.
In the soft tissues of the new host the miracidium loses its cilia, increases in size and is converted into a sac-like sporocyst. The sporocyst may divide transversely and produces other sporocysts. There are germ cells within the sporocyst each of which can develop by parthenogenesis, that is without fertilisation, into another kind of larva called redia. A sporocyst usually produces four to eight rediae.
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A redia is a hollow elongated structure with a mouth and a short gut, a pair of blunt projections near the posterior end and a minute birth pore. Within a short time the rediae burst out of the sporocyst and migrate into the liver of the snail.
A redia has germ cells within its cavity which now develop parthenogenetically to produce other rediae. The daughter rediae escape from the parent through the birth pore. Several generations of rediae are produced in this way during the summer months.
Eventually, the redia produces the fourth type of larva, called cercaria. It has a flat heart-shaped body, with a slender tail which is twice as long, with two suckers and a forked gut.
The cercariae emerge from the redia by the birth pore, work their way out of the snail and swim through water by the lashing of the tail. Very soon it settles upon the blade of a grass, near the water surface, loses its tail, secretes a cyst around itself and becomes a meta- cercaria.
The encysted metacercariae remain alive for several months in damp places, if not subjected to very high temperature or drying. When infected vegetation is eaten by sheep or other suitable final host, the cyst wall dissolves and the larvae burrow through the intestinal wall into the body cavity. The bore their way into the liver in a day or two and are converted gradually into the adult fluke.
