Excretory system of Leech
Excretory system consists of 17 pairs of small coiled tubes, the nephridia, arranged segmentally, one pair in each segment from 6th to22nd.
Nephridia are of two types
i ) testicular
ii) pre-testicular
i) Testicular nephridia
Posterior 11 pairs of nephridia lying one pair in each segment from 12 to 22nd, are termed testicular nephridia.
They are called testicular nephridia, due to the presence of a pair of testis sacs in each of these segments.
A typical testicular nephridium is a horseshoe-shaped structure traversed by a complicated system of canals
It consists of 6 parts:
1) main lobe
2) vesicle and vesicle duct
3) apical lobe
4) inner lobe
5) initial lobe
6) ciliated organ
1) Main lobe:
Forms the horseshoe proper
Lies in a ventro-lateral position between two adjacent caeca of crop
Forms the major part of nephridium
Consists of two unequal limbs
One limb is longer and anterior in position and the other shorter and posterior
Cells of main lobe are big and polyhedral in shape
2) Vesicle and the Vesicle duct.
A narrow vesicle duct arises from the inner ventral end of anterior limb of main lobe
Runs posteriorly to open into a large bladder or terminal vesicle, situated ventro-laterally behind the rest of nephridium
Vesicle is a large oval sac, with a non-contractile thin wall, internally lined by a ciliated epithelium
A short and narrow excretory duct, lined with a non-ciliated epithelium, leads from vesicle to open to the exterior through a rounded aperture, the nephridiopore
At its origin from vesicle, the excretory duct is provided with a sphincter muscle that controls the flow of excretory substances out of the vesicle.
M.L. Bhatia (1940)- bladder is lined by cilia
B. Dev- the so called cilia are in fact non-motile bacteria, the nephridial microflora, 2.8 to 7 microns in length
3) Apical lobe
Inner free end of posterior limb of main lobe is continued to form a stout apical lobe
Present antero-posteriorly beneath the crop
Its anterior end is slightly swollen and bent on itself like the handle of a walking stick
Its cells are big and traversed by regular intracellular canals.
4) Inner lobe or incurrent lobe
seen extending between the anterior and posterior limbs of main lobe
also runs forward along the outer side of apical lobe for about half of its length
5) Initial lobe
Long, narrow, transparent and cord-like structure
Formed of a single row of elongated tubular cells and closely coiled around the apical lobe.
Its posterior end joins the main lobe, while its anterior end runs inwards and reaches over the testis sac of its own side, where it ends blindly close to the perinephrostomial ampullae.
The intracellular canal of initial lobe gives off many diverticula in each cell.
6) Ciliated organ
Present inside peristomial ampullae
Suspended from the inner walls of ampullae by 4 to 5 strands or trabeculae.
It corresponds to the funnel or nephrostome of a typical annelid nephridium, but is a greatly modified and compound structure
Ciliated organ consists of a spongy central reservoir and ciliated funnels
The reservoir contains the central mass of connective tissue cells which manufacture the coelomic corpuscles
Outer wall of central mass, made of a single layer of cells, and bears numerous minute pores.
A ciliated funnel fits into each pore on the outside
Each funnel is like an ear lobe, with about one-fourth of its margin incomplete
Funnel covered with outwardly directed cilia on its outer margin and inner surface
In the embryo, ciliated organ has a distinct cellular connection with the nephridium.
But, in adult Hirudinaria, it loses the connection as well as excretory function and becomes a part of the haemocoelomic system.
It manufactures coelomic corpuscles for the haemocoelomic system.
II Pre-testicular nephridia
First six pairs of nephridia are termed pre-testicular nephridia because of their location in segments 6 to 11 without testis sacs themselves, but in front of those containing testis sacs.
These nephridia resemble testicular nephridia in all respects except that their initial lobes end loosely in general connective tissue on their side of ventral nerve cord.
There are no testis sacs, peri-nephrostomial ampullae and ciliated organs in their segments.
Zoology in the Classroom - is a blog for teachers and students of zoology. I have been teaching as Zoology teacher for the last 30 years. I post the notes or handouts that I supply to my students in my classroom. Hope this will benefit Zoology fraternity
Wednesday, January 20, 2016
Wednesday, January 6, 2016
ECOLOGICAL SUCCESSION
3. Explain the ecological succession with suitable examples.
Ecological succession is defined as an orderly process of changes in the community structure and function with time mediated through modifications in the physical environment and ultimately culminating in a stabilized ecosystem known as climax. The whole sequence of communities which are transitory are known as Seral stages or seres whereas the community establishing first of all in the area is called a pioneer community. Ecological successions starting on different types of areas or substrata are named differently as follows:
(i) Hydrarch or Hydrosere: Starting in watery area like pond, swamp, bog (ii) Mesarch: starting in an area of adequate moisture.
(iii) Xerarch or Xerosere: Starting in a dry area with little moisture. They can be of the following types:
Lithosere : starting on a bare rock
Psammosere : starting on sand
Halosere : starting on saline soil
Process of Succession
The process of succession takes place in a systematic order of sequential steps as follows:
(i) Nudation: It is the development of a bare area without any life form. The bare area may be caused due to landslides, volcanic eruption etc. (topographic factor), or due to drought, glaciers, frost etc. (Climatic factor), or due to overgrazing, disease outbreak, agricultural/ industrial activities (biotic factors).
(ii) Invasion: It is the successful establishment of one or more species on a bare area through dispersal or migration, followed by ecesis or establishment. Dispersal of the seeds, spores etc. is brought about by wind, water, insects or birds. Then the seeds germinate and grow on the land. As growth and reproduction start, these pioneer species increase in number and form groups or aggregations.
(iii) Competition and coaction: As the number of individuals grows there is competition, both inter-specific (between different species) and intra-specific (within the same species), for space, water and nutrition. They influence each other in a number of ways, known as coaction.
(iv) Reaction: The living organisms grow, use water and nutrients from the substratum, and in turn, they have a strong influence on the environment which is modified to a large extent and this is known as reaction. The modifications are very often such that they become unsuitable for the existing species and favour some new species, which replace them. Thus, reaction leads to several seral communities.
(v) Stabilization: The succession ultimately culminates in a more or less stable community called climax which is in equilibrium with the environment. The climax community is characterized by maximum biomass and symbiotic (mutually beneficial) linkages between organisms and are maintained quite efficiently per unit of available energy. Let us consider very briefly two types of succession. A. Hydrosere (Hydrarch): This type of succession starts in a water body like pond. A number of intermediate stages come and ultimately it culminates in a climax community which is a forest. The pioneer community consists of phytoplanktons, which are free floating algae, diatoms etc. Gradually these are replaced by rootedsubmerged plants followed by rooted-floating plants. Growth of these plants keep on adding organic matter to the substratum by death and decay and thus a layer of soil builds up and shallowing of water takes
place. Then Reed swamp (marshy) stage follows in which the plants are partly in water and partly on land. This is followed by a sedgemeadow stage of grasses then by a woodland consisting of shrubs and trees and finally by a forest acting as climax. (Fig. 3.15)
B. Xerosere (Xerarch): This type of succession originates on a bare rock, which lacks water and organic matter. Interestingly, here also the climax community is a forest, although the intermediate stages are very different.
The pioneer community here consists of crustose and foliose lichens. These lichens produce some weak acids and help in disintegrating the rock, a process known as weathering. Their growth helps in building up gradually some organic matter, humus and soil.
Then comes the community of mosses, followed by herbs, shrubs and finally the forest trees. Throughout this gradual process there is a slow build up of organic matter and water in the substratum.
Thus, succession tends to move towards mesic conditions (moderate condition), irrespective of the fact, whether it started from a dry (Xeric) condition or a moist (hydric) condition and it culminates in a stable climax community, which is usually a forest.
Ecological succession is defined as an orderly process of changes in the community structure and function with time mediated through modifications in the physical environment and ultimately culminating in a stabilized ecosystem known as climax. The whole sequence of communities which are transitory are known as Seral stages or seres whereas the community establishing first of all in the area is called a pioneer community. Ecological successions starting on different types of areas or substrata are named differently as follows:
(i) Hydrarch or Hydrosere: Starting in watery area like pond, swamp, bog (ii) Mesarch: starting in an area of adequate moisture.
(iii) Xerarch or Xerosere: Starting in a dry area with little moisture. They can be of the following types:
Lithosere : starting on a bare rock
Psammosere : starting on sand
Halosere : starting on saline soil
Process of Succession
The process of succession takes place in a systematic order of sequential steps as follows:
(i) Nudation: It is the development of a bare area without any life form. The bare area may be caused due to landslides, volcanic eruption etc. (topographic factor), or due to drought, glaciers, frost etc. (Climatic factor), or due to overgrazing, disease outbreak, agricultural/ industrial activities (biotic factors).
(ii) Invasion: It is the successful establishment of one or more species on a bare area through dispersal or migration, followed by ecesis or establishment. Dispersal of the seeds, spores etc. is brought about by wind, water, insects or birds. Then the seeds germinate and grow on the land. As growth and reproduction start, these pioneer species increase in number and form groups or aggregations.
(iii) Competition and coaction: As the number of individuals grows there is competition, both inter-specific (between different species) and intra-specific (within the same species), for space, water and nutrition. They influence each other in a number of ways, known as coaction.
(iv) Reaction: The living organisms grow, use water and nutrients from the substratum, and in turn, they have a strong influence on the environment which is modified to a large extent and this is known as reaction. The modifications are very often such that they become unsuitable for the existing species and favour some new species, which replace them. Thus, reaction leads to several seral communities.
(v) Stabilization: The succession ultimately culminates in a more or less stable community called climax which is in equilibrium with the environment. The climax community is characterized by maximum biomass and symbiotic (mutually beneficial) linkages between organisms and are maintained quite efficiently per unit of available energy. Let us consider very briefly two types of succession. A. Hydrosere (Hydrarch): This type of succession starts in a water body like pond. A number of intermediate stages come and ultimately it culminates in a climax community which is a forest. The pioneer community consists of phytoplanktons, which are free floating algae, diatoms etc. Gradually these are replaced by rootedsubmerged plants followed by rooted-floating plants. Growth of these plants keep on adding organic matter to the substratum by death and decay and thus a layer of soil builds up and shallowing of water takes
place. Then Reed swamp (marshy) stage follows in which the plants are partly in water and partly on land. This is followed by a sedgemeadow stage of grasses then by a woodland consisting of shrubs and trees and finally by a forest acting as climax. (Fig. 3.15)
B. Xerosere (Xerarch): This type of succession originates on a bare rock, which lacks water and organic matter. Interestingly, here also the climax community is a forest, although the intermediate stages are very different.
The pioneer community here consists of crustose and foliose lichens. These lichens produce some weak acids and help in disintegrating the rock, a process known as weathering. Their growth helps in building up gradually some organic matter, humus and soil.
Then comes the community of mosses, followed by herbs, shrubs and finally the forest trees. Throughout this gradual process there is a slow build up of organic matter and water in the substratum.
Thus, succession tends to move towards mesic conditions (moderate condition), irrespective of the fact, whether it started from a dry (Xeric) condition or a moist (hydric) condition and it culminates in a stable climax community, which is usually a forest.
Saturday, January 2, 2016
FASCIOLA NOTES
External morphology and life history of Fasciola hepatica
PHYLUM-PLATYHELMINTHES
CLASS-TREMATODA
ORDER-DIGENEA
1. Habit and Habitat of Fasciola Hepatica:
Fasciola hepatica (L., fasciola = small bandage; Gr., hepar = liver), the sheep liver fluke, lives as an endoparasite in the bile passages of sheep.
Fasciola hepatica, in addition to sheep, also infects other vertebrates like goat, deer, horse, dog, ass, ox and occasionally man. Fasciola hepatica is worldwide in distribution, particularly sheep and cattle raising areas are the primary zones where human beings are also infected.
Its other Indian species, F. gigantica (= indica) is found in the bile passages of buffaloes, cow, goats and pigs.
2. Shape, Size and Colour:
F. hepatica has a thin, dorsoventrally flattened, leaf-shaped, elongated and oval body. It measures about 25 to 30 mm in length and 4 to 12 mm in breadth.
The maximum width is at about anterior third of the body from where the body tapers anteriorly as well as posteriorly, however, the anterior end is somewhat rounded, while it is bluntly pointed posteriorly.
3. External Morphology:
Shape and size: body of F. hepatica is soft, oval in outline, dorso-ventrally flattened and leaf like. It is about 1.8 to 3cm, which is a little infront of the middle region of body. From this region body tapers away anteriorly as well as posteriorly. Anterior end is somewhat broad and rounded, while posterior end is bluntly pointed.
Colouration: Colour is usually pinkish, but the transparency of body wall enables the observer to see the blackish or brownish vitelline glands along the lateral margins, and the alimentary canal, which appears brownish due to ingested bile of the host.
Oral cone: anterior end of body is drawn out into a prominent conical projection, termed the oral cone or head lobe, bearing at its tip a somewhat triangular aperture, the mouth.
Suckers: there are two small suckers, anterior and ventral. Both are devoid of hooks and spines.
Anterior sucker: Mouth is situated at the centre and bottom of a cup-shaped muscular organ, known as the anterior or oral sucker. It has a diameter of about 1 mm. Muscles of oral sucker radiate from margin of mouth to the periphery of sucker. Oral sucker acts as an ideal suctorial organ serving for adhesion as well as ingestion.
Ventral sucker: About 3 to 4 mm. behind the oral sucker, situated mid-ventrally, is another bowl-like adhesive sucker, the ventral or posterior sucker, also known as acetabulum. It is without an aperture and has a diameter of about 1.6mm.
4. Life cycle and development:
Digenetic life cycle. Life cycle of F. hepatica is complex and completed in two hosts. Primary host, in which the adult fluke lives, is sheep. While the intermediate host, in which numerous larval stages are passed, is a snail (Lymnaea, Planorbis, etc.). This type of life cycle, involving two different kinds of hosts, is termed digenetic.
Copulation. Self-fertilization is of rare occurrence in liver flukes though they are hermaphrodite. In F. hepatica, copulation takes place in bile ducts of the host.
Two flukes in copulation bring their genital pores in opposition. Cirrus of one fluke, everted through its gonopore, penetrates the Laurer’s canal of the other through the latter’s temporary opening, and injects spermatozoa. Secretion of prostate glands, and perhaps also of the Mehlis’s glands, keep the sperm active for fertilization.
Fertilization. Fertilization is internal. In cross-fertilization, sperms received in Laurer’s canal during copulation, enter the distal end of oviduct where fertilization occurs. During self-fertilization, sperms enter the uterus of same fluke through female genital aperture and pass down to fertilize the egg.
Capsule formation. Each fertilized egg or zygote is surrounded by yolk cells, which provide yolk and shell material. Shell- globules of yolk cells contain proteins and a phenol.
According to Stephenson (1947), phenol is oxidized to a quinine in the proximal part of uterus. Quinine then tans the protein, producing a hard, resistant and lethary sclerotin like that of insects. This sclerotin forms the shell around fertilized eggs.
Capsules. Shelled eggs are termed capsules or simply eggs. A shell or capsule is yellow or brown, in colour and oval in shape. It is about 130 to 150µ long and 60 to 90µ wide.
It is operculate, i.e., provided with a lid or operculum. Situated immediately beneath the operculum, at the terminal end of egg is a viscous and granular cushion. About 3000or more such capsules may occur at a time in the uterus of a single fluke.
There may be as many as 200 flukes in the liver of one sheep. If each fluke produces 500,000 eggs, a single infected sheep may disperse 100 million fertile eggs. This vast capacity for egg production is necessary in view of the complicated life cycle and slim chances of survival.
Cleavage and embryonic development. Cleavage starts while eggs are still in uterus. Cleavage is holoblastic and unequal. First division of zygote results in two unequal cells, a larger somatic cell and a smaller propagatory cell. Subsequent divisions of somatic cell form larval ectoderm and tissues. Propagatory cell divides further into two daughter cells.
One daughter cell by its divisions finally produces the larval body. Other daughter cell divides several times to form a mass of smaller germ cells which cluster in the posterior part of larval body.
Encapsulated embryos or capsules or simply eggs do not develop further in fluke’s uterus.
A very large number of capsules leave fluke’s body through its gonopore into host’s intestine, and finally ejected out with its faeces.
Further development takes place when capsules come in contact with water (or Damp areas with at least 60% moisture content) which is slightly acidic (pH 6.5). Optimum temperature for development ranges from 220C to 250C.
Larval stages of Fasciola hepatica
Miracidium Larva. It is the first larval stage involved in life cycle. When suitable conditions become available, the encapsulated embryo, in 4-15 days, differentiates into a miracidium larva. It hatches out and swims in water. Hatching is initiated by a proteolytic hatching enzyme. It dissolves the cementing material by which operculum is attached, thus releasing the operculum.
Characters:
1) It is 150 microns in length. It is small. It is conical in shape.
2) It is covered by ciliated epidermal cells. ,
3) The body is covered by 21 ciliated cells which are arranged in five rows.
a) First row - six cells
b) Second row - six cells
c) Third row - three cells
d) Fourth row - four cells
e) Fifth row - two cells
4) With the help of cilia it swims in the water
5) At the apex of the larva an apical papilla or boring papilla is present.
6) An apical gland is present in the miracidium larva which opens into the apical papilla. On either side of it two penetration glands are present.
7) A brain is present. Above the brain two eye spots are present.
8) A pair of flame cells are present which open out laterally towards the posterior end. The larva shows a number of germ cells.
9) The miracidium larva lives only for 8 hours. During this time it will swim in search of the secondary host.
Transmission to secondary host:
The secondary host of liver fluke is Limnea truncatula or Planorbis (fresh water snailj.When the miracidium larva comes in contact with the snail it pierce into the soft body of snail. Apical papilla and secretions of penetration gland will help the larva to bore into the body of snail. In the body of snail miracidium develops into sporocyst stage.
Sporocyst:
In the body of snail miracidium enters into pulmonary sac. There miracidium will loose its ciliated epidermis. It becomes a bag like structure. It looses all the structures except flame cells and germ cells. The germ cells will undergo parthenogenesis and give rise to the next larvae called Redia. The sporocyst absorbs nourishment from the host tissues and often causes destruction to the host.
Redia:
In the sporocyst five to eight redia larvae are produced. They come out of the sporocyst by rupturing the wall of the sporocyst.
This larva is elongated in structure. It is covered by thin cuticle. It shows a collar which is muscular. It helps in locomotion.
Near the collar a birth pore is present. The next larval stage will go out through the birth pore. The larva shows a gut which opens out through mouth. Mouth opens into pharynx which leads into intestine.
Many flame cells are present. The flame cells of one side will open into a common excretory duct which opens out through a single nephridiopore. The mesenchyme of the larva shows germ cells.
The germ cells will undergo parthenogenesis and give rise to the next larval stage called cercaria in the winter season. These cercaria larvae will come out of the redia through birth pore.
Cercaria:
The redia larva will give 15 to 20 cercaria larvae. They are liberated from the redia larva through birth pore.
1) It is oval in shape with tail.
2) It is 0.25mm to 0.35mm in length.
3) The cuticle covering will show backwardly directed spines.
4) Two suckers are present, a) Oral sucker around mouth, b) ventral sucker.
5) The digestive system starts with mouth, opens into pharynx, oesophagus and intestine. Intestine divided into two branches.
6) More flame cells are present. All of them open into excretory tubules. The two excretory tubules will unite at the posterior end and become excretory bladder. It gives an excretory tube. It divides into two, which opens out through nephridiopore.
7) Germ cells are present.
The completely developed cercaria will enter into water from the body of snail. They swim for 2 or 3days in the water and settle on a water plant.
Metacercaria:
As many as a thousand metacercariae may be found attached to a single grass blade. They have a rounded form with a diameter of about 0.2mm.
Meta cercaria is in fact the juvenile fluke, also called marita. It differs from cercaria in that it has a rounded form, a thick hard cyst and large number of flame cells. It lacks a tail and cysogenous gland cells and its excretory bladder opens out directly through a single pore.
Germ cells or the genital rudiments are present as such. Cyst provides protection against short periods of desiccation.
Infection of primary host.
When the sheep eats the plants with metacercaria stages they enter into its digestive system. The cyst wall is digested in intestine, finally dissolves in proximal part of intestine and liberate the larva. It penetrates through the intestine wall and gets onto coelomic cavity.
Now it infect the liver, feeds on its tissue, and grows in size in five to six weeks. It then takes up its position in the bile duct, where it finally attains sexual maturity. In 11 to 13 weeks. After entering the body of host, it starts laying eggs.
PHYLUM-PLATYHELMINTHES
CLASS-TREMATODA
ORDER-DIGENEA
1. Habit and Habitat of Fasciola Hepatica:
Fasciola hepatica (L., fasciola = small bandage; Gr., hepar = liver), the sheep liver fluke, lives as an endoparasite in the bile passages of sheep.
Fasciola hepatica, in addition to sheep, also infects other vertebrates like goat, deer, horse, dog, ass, ox and occasionally man. Fasciola hepatica is worldwide in distribution, particularly sheep and cattle raising areas are the primary zones where human beings are also infected.
Its other Indian species, F. gigantica (= indica) is found in the bile passages of buffaloes, cow, goats and pigs.
2. Shape, Size and Colour:
F. hepatica has a thin, dorsoventrally flattened, leaf-shaped, elongated and oval body. It measures about 25 to 30 mm in length and 4 to 12 mm in breadth.
The maximum width is at about anterior third of the body from where the body tapers anteriorly as well as posteriorly, however, the anterior end is somewhat rounded, while it is bluntly pointed posteriorly.
3. External Morphology:
Shape and size: body of F. hepatica is soft, oval in outline, dorso-ventrally flattened and leaf like. It is about 1.8 to 3cm, which is a little infront of the middle region of body. From this region body tapers away anteriorly as well as posteriorly. Anterior end is somewhat broad and rounded, while posterior end is bluntly pointed.
Colouration: Colour is usually pinkish, but the transparency of body wall enables the observer to see the blackish or brownish vitelline glands along the lateral margins, and the alimentary canal, which appears brownish due to ingested bile of the host.
Oral cone: anterior end of body is drawn out into a prominent conical projection, termed the oral cone or head lobe, bearing at its tip a somewhat triangular aperture, the mouth.
Suckers: there are two small suckers, anterior and ventral. Both are devoid of hooks and spines.
Anterior sucker: Mouth is situated at the centre and bottom of a cup-shaped muscular organ, known as the anterior or oral sucker. It has a diameter of about 1 mm. Muscles of oral sucker radiate from margin of mouth to the periphery of sucker. Oral sucker acts as an ideal suctorial organ serving for adhesion as well as ingestion.
Ventral sucker: About 3 to 4 mm. behind the oral sucker, situated mid-ventrally, is another bowl-like adhesive sucker, the ventral or posterior sucker, also known as acetabulum. It is without an aperture and has a diameter of about 1.6mm.
4. Life cycle and development:
Digenetic life cycle. Life cycle of F. hepatica is complex and completed in two hosts. Primary host, in which the adult fluke lives, is sheep. While the intermediate host, in which numerous larval stages are passed, is a snail (Lymnaea, Planorbis, etc.). This type of life cycle, involving two different kinds of hosts, is termed digenetic.
Copulation. Self-fertilization is of rare occurrence in liver flukes though they are hermaphrodite. In F. hepatica, copulation takes place in bile ducts of the host.
Two flukes in copulation bring their genital pores in opposition. Cirrus of one fluke, everted through its gonopore, penetrates the Laurer’s canal of the other through the latter’s temporary opening, and injects spermatozoa. Secretion of prostate glands, and perhaps also of the Mehlis’s glands, keep the sperm active for fertilization.
Fertilization. Fertilization is internal. In cross-fertilization, sperms received in Laurer’s canal during copulation, enter the distal end of oviduct where fertilization occurs. During self-fertilization, sperms enter the uterus of same fluke through female genital aperture and pass down to fertilize the egg.
Capsule formation. Each fertilized egg or zygote is surrounded by yolk cells, which provide yolk and shell material. Shell- globules of yolk cells contain proteins and a phenol.
According to Stephenson (1947), phenol is oxidized to a quinine in the proximal part of uterus. Quinine then tans the protein, producing a hard, resistant and lethary sclerotin like that of insects. This sclerotin forms the shell around fertilized eggs.
Capsules. Shelled eggs are termed capsules or simply eggs. A shell or capsule is yellow or brown, in colour and oval in shape. It is about 130 to 150µ long and 60 to 90µ wide.
It is operculate, i.e., provided with a lid or operculum. Situated immediately beneath the operculum, at the terminal end of egg is a viscous and granular cushion. About 3000or more such capsules may occur at a time in the uterus of a single fluke.
There may be as many as 200 flukes in the liver of one sheep. If each fluke produces 500,000 eggs, a single infected sheep may disperse 100 million fertile eggs. This vast capacity for egg production is necessary in view of the complicated life cycle and slim chances of survival.
Cleavage and embryonic development. Cleavage starts while eggs are still in uterus. Cleavage is holoblastic and unequal. First division of zygote results in two unequal cells, a larger somatic cell and a smaller propagatory cell. Subsequent divisions of somatic cell form larval ectoderm and tissues. Propagatory cell divides further into two daughter cells.
One daughter cell by its divisions finally produces the larval body. Other daughter cell divides several times to form a mass of smaller germ cells which cluster in the posterior part of larval body.
Encapsulated embryos or capsules or simply eggs do not develop further in fluke’s uterus.
A very large number of capsules leave fluke’s body through its gonopore into host’s intestine, and finally ejected out with its faeces.
Further development takes place when capsules come in contact with water (or Damp areas with at least 60% moisture content) which is slightly acidic (pH 6.5). Optimum temperature for development ranges from 220C to 250C.
Larval stages of Fasciola hepatica
Miracidium Larva. It is the first larval stage involved in life cycle. When suitable conditions become available, the encapsulated embryo, in 4-15 days, differentiates into a miracidium larva. It hatches out and swims in water. Hatching is initiated by a proteolytic hatching enzyme. It dissolves the cementing material by which operculum is attached, thus releasing the operculum.
Characters:
1) It is 150 microns in length. It is small. It is conical in shape.
2) It is covered by ciliated epidermal cells. ,
3) The body is covered by 21 ciliated cells which are arranged in five rows.
a) First row - six cells
b) Second row - six cells
c) Third row - three cells
d) Fourth row - four cells
e) Fifth row - two cells
4) With the help of cilia it swims in the water
5) At the apex of the larva an apical papilla or boring papilla is present.
6) An apical gland is present in the miracidium larva which opens into the apical papilla. On either side of it two penetration glands are present.
7) A brain is present. Above the brain two eye spots are present.
8) A pair of flame cells are present which open out laterally towards the posterior end. The larva shows a number of germ cells.
9) The miracidium larva lives only for 8 hours. During this time it will swim in search of the secondary host.
Transmission to secondary host:
The secondary host of liver fluke is Limnea truncatula or Planorbis (fresh water snailj.When the miracidium larva comes in contact with the snail it pierce into the soft body of snail. Apical papilla and secretions of penetration gland will help the larva to bore into the body of snail. In the body of snail miracidium develops into sporocyst stage.
Sporocyst:
In the body of snail miracidium enters into pulmonary sac. There miracidium will loose its ciliated epidermis. It becomes a bag like structure. It looses all the structures except flame cells and germ cells. The germ cells will undergo parthenogenesis and give rise to the next larvae called Redia. The sporocyst absorbs nourishment from the host tissues and often causes destruction to the host.
Redia:
In the sporocyst five to eight redia larvae are produced. They come out of the sporocyst by rupturing the wall of the sporocyst.
This larva is elongated in structure. It is covered by thin cuticle. It shows a collar which is muscular. It helps in locomotion.
Near the collar a birth pore is present. The next larval stage will go out through the birth pore. The larva shows a gut which opens out through mouth. Mouth opens into pharynx which leads into intestine.
Many flame cells are present. The flame cells of one side will open into a common excretory duct which opens out through a single nephridiopore. The mesenchyme of the larva shows germ cells.
The germ cells will undergo parthenogenesis and give rise to the next larval stage called cercaria in the winter season. These cercaria larvae will come out of the redia through birth pore.
Cercaria:
The redia larva will give 15 to 20 cercaria larvae. They are liberated from the redia larva through birth pore.
1) It is oval in shape with tail.
2) It is 0.25mm to 0.35mm in length.
3) The cuticle covering will show backwardly directed spines.
4) Two suckers are present, a) Oral sucker around mouth, b) ventral sucker.
5) The digestive system starts with mouth, opens into pharynx, oesophagus and intestine. Intestine divided into two branches.
6) More flame cells are present. All of them open into excretory tubules. The two excretory tubules will unite at the posterior end and become excretory bladder. It gives an excretory tube. It divides into two, which opens out through nephridiopore.
7) Germ cells are present.
The completely developed cercaria will enter into water from the body of snail. They swim for 2 or 3days in the water and settle on a water plant.
Metacercaria:
As many as a thousand metacercariae may be found attached to a single grass blade. They have a rounded form with a diameter of about 0.2mm.
Meta cercaria is in fact the juvenile fluke, also called marita. It differs from cercaria in that it has a rounded form, a thick hard cyst and large number of flame cells. It lacks a tail and cysogenous gland cells and its excretory bladder opens out directly through a single pore.
Germ cells or the genital rudiments are present as such. Cyst provides protection against short periods of desiccation.
Infection of primary host.
When the sheep eats the plants with metacercaria stages they enter into its digestive system. The cyst wall is digested in intestine, finally dissolves in proximal part of intestine and liberate the larva. It penetrates through the intestine wall and gets onto coelomic cavity.
Now it infect the liver, feeds on its tissue, and grows in size in five to six weeks. It then takes up its position in the bile duct, where it finally attains sexual maturity. In 11 to 13 weeks. After entering the body of host, it starts laying eggs.
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