Describe the structure and life history of Fasciola heptica (Liver
fluke).
Ans. Habit and Habitat : It lives as an endophosite in the bile duct of
sheep and is scientifically known as Fasciola hepatica. The adult flukes are
typical parasites of vertebrate animals but are stage of their life history is
invariably spent in an invertebrate host-a Molluse. This alternation of hosts
suggests that they have digenetic life histories. The disease caused by the
parasites is known as liver rot.
Structure :
Liver fuke is a soft bodied, flattened leaf-like animal and
exhibits bilateral summetry. The size varies from about half to one inch in
length but the width does not generally exceed half an inch. The anterior end
has a conical projection. The head lobe and at its apex is situated the oralor
anterior sucker perforated by the mouth. On the ventral surface, a little behind
the head lobe, the ventral sucker is situated. Between the two suckers and
close to the posterior sucker there is the genital opening through which the
penis sometimes protrudes. A single excretory aperture lies at the extreme
posterior tip of the body. The body, surface is marked by the presence of a
number of conical projections, the spicules or panillae.
Digestive system :
The mouth is situated in the middle of the anterior
sucker and opens into pharynx which is muscular and suctorial. Next to the
pharynx is the oesophagus which is very short and followed by the intestine.
The intestine and its branches are very prominent as they remain filled with
biliary matter, desquamated epithelium and blood on which the animal feeds.
Excretory system :
The excretory system consists of a median longitudinal
excretory canal or protonephredial tubule which opens to the body. Each canal
gives repeated branches. These branches finallally end in five micro scopic
vessels which enter the large flame cell.
Nervous system :
The nervous system consists of a pair of prominent
nerve ganglia situated are on either side of the oesophagies. The ganglia are
joined together by a nerve ring around the oesophagus. From the nerve ganglia
nerves are given off to the head lobe and to the posterior part of the body.
Reproductive system :
Fasciola reproduces sexually and some individual
bears both the sexes. Male reproductive system consists of a pair of testes
from each testis is given off a vast deferens which runs towards the anterior
region.
The two deferentia unite and form a median coiled and dilated vascular
seminales. From it a narrow tube the ejaculatory duct leads to the mole aperture
situated at the tip of penis.
Female reproductive organ is a single overy. The tubules of the very open
into a single median oviduct. The uterus is a wide convoluted tube formed by
the oviduct and the median vitelline duct and it opens in the genital atrium near
the base of the cirrus. The common genital apertures is situated on the mid
ventral line between the suckers and close to the ventral suckers.
Life history:
Fasciola is hermaphroditebut crossfertilization is the rule
The completion of life-cycle depends upon the transference of the parasite
from one host to the other through an intermediate host. The cgg can survive i
only it fall on damp soil.
Miracidium :
Development within the eggs ccases with the emergcnce of
miracidium larva through the operculum ofthe egg. The miracidium is camicat
in appearance and bcars cilia round the body. A distinct head lobe is situated at
the broad end of the miracidium behind which there is a pair of eyes. The
miracidium swims freely in water or crawls over damp surface for syometime
and dies in case it does not find it.
Sccond host : The fresh water shell.
Sporocyst:
On meeting the nail hte miracidium bores into it by the
operation of the penetration glands and reaches in the internal organs. specially
the pulmonary sac. It metamorphoses into sporocyst. The sporocyst is clongatcd
in appearance and its internal cavity is lined with cells. The germ cells and the
lining cells are budded off which undergo segmentation and produce redia.
Redia:
A redia is provided with mouth, pharynx and a simple intestine. A
redia is clanged in appearance and bears a pair of short muscular projections
at the posterior ridi many germ cells are found in the body cavity. The germ
cells within the body of of redia undergo segmentation and give rise to cerearia.
Cerearia :
The cercaria is provided a long tail and oral and ventral suckers
Alimentary canal consists of mouth pharynx and a billed intestine. The cercaria
comes out of the body of the snail,. It swims for sometime and then takes refuse
on green leaves, Ultimately the tail is shed and it becomes eneysted which is
called necto cercaria.
Transform of host :
When the sheep feeds on the infested green leave,
metacercaria eats the gut. The young fluke cmerges from the emigrates the bile
duct where it crawls rapidly.
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