>REPRODUCTIVE SYSTEM
Fasciolahepatica is hermaphrodite bearing both male and female gonads.
Male Reproductive System consists of testes, vasa deferentia, seminal vesicle, ejaculatory duct, cirrus or penis, prostate glands and genital atrium. Testes are two in number, one anterior and another posterior. They are connected to seminal vesicle via two vasa deferentia which join to open into seminal vesicle. The cells lining the walls of testes give rise to spermatozoa. The seminal vesicle continues anteriorly into a very narrow and coiled duct called ejaculatory duct. The cirrus opens by male genital aperture in a common genital atrium. The ejaculatory duct is surrounded by numerous unicellular prostate glands. These glands open into the ejaculatory duct and their secretion helps in free movement of sperms during copulation. The genital atrium is a common chamber for male and female genital apertures. It opens externally by a gonopore.
Female Reproductive System consists of ovary, oviduct, uterus, vitelline glands, Mehlis’s glands and Laurer’s canal. Ovary is single, highly branched, situated anterior to testes on the right side in anterior one-third of the body. Ovaries open into a short and narrow tube called oviduct. Uterus is connected to the yolk reservoir and opens by female genital aperture into the genital atrium. Vitelline glands are present on lateral sides and produce albumin, yolk and shell material for the eggs. The vitelline glands open by means of minute ducts into a lateral longitudinal vitelline duct on each side, which is connected together by a transverse vitelline duct that forms the yolk reservoir. Numerous unicellular Mehlis’s glands surround the junction of median vitelline duct, oviduct and uterus. The secretion of Mehlis’s glands lubricates the eggs in the uterus.
A Laurer’s canal develops during breeding season that opens on the dorsal side and connects with the oviduct and serves as copulation canal.
LIFE CYCLE
Life cycle of F. hepatica is completed in two hosts. Primary host, in which the adult fluke lives, is sheep, while the intermediate host, in which larval stages occur, is a snail (Lymnaea, Planorbis, etc).
Copulation takes place by inserting the cirrus into Laurer’s canal of other Fasciola and sperms are deposited into the oviduct where fertilization takes place. The eggs, fertilized in the lower part of oviduct then pass on to the uterus, where yolk is deposited around them. Then eggs are enclosed in a proteinaceous capsule secreted by shell glands. The shell becomes hard in the uterus by the action of quinone. One pole of the egg shell bears an operculum for the exit of larva. The eggs are excreted from the bile duct to the intestine and finally out of the body along with the faeces of host. One fluke may lay about 500,000 egg capsules. In two weeks a small ciliated miracidium larva comes out of the shell by forcing the operculum open.
Miracidium is the first larval stage and is conical in appearance, bearing cilia around the body. A distinct apical papilla is situated on the anterior end of miracidium, behind which there is a pair of eyes and a pair of protonephridia with flame cells that open on the lateral sides by two excretory pores. The germ cells in blocks are present on the posterior part of the body. The apical papilla bears penetration glands that help in penetrating into the secondary host. The larva swims freely in water in search of snails and dies in case it does not find one.
Sporocyst: Miracidium bores into the pulmonary sac and metamorphoses into sporocyst by casting off the ciliated skin. The sporocyst is elongated bag-like in appearance and has a pair of protonephridia each with two pair of flame cells. It also contains germ cells which produce 5-8 redia larvae.
Redia is elongated with two ventral lappets that help in movement through the snail tissues. There is a birth pore near the anterior end through which cercaria larvae emerge. Redia has mouth, pharynx and a simple intestine. The germ cells of redia may give rise to second generation daughter rediae or may produce cercaria larvae which escape from the birth pore of the redia into the snail tissues.
Cercaria has a long tail and oral and ventral suckers. Alimentary canal is well-developed and consists of mouth, pharynx and a bifurcated intestine. Paired excretory tubules with flame cells, germ cells and peripheral cyst-forming cells are also present. Cercaria comes out of the body of snail, swims for some time and then climbs on grass leaves, where it becomes encysted and is called metacercaria.
Metacercaria: The encysted cercaria is called a metacercaria which is about 0.2 mm in diameter and can survive for a few weeks on the blades of grasses. It waits to be eaten along with grasses by the primary host, sheep.
When a sheep feeds on infested green leaves, metacercaria enters the gut. The young fluke emerges from the metacercaria in the intestine, penetrates its lining and migrates to the bile duct where it grows rapidly and becomes sexually matures adult.
PARASITIC ADAPTATIONS
Liver fluke has the following adaptations to suit its existence as an endoparasite in the bile ducts of sheep and in the secondary host that is snail: