(1) Complicated life history:
The life history of liver fluke is much complicated due to the occurrence of several larval forms in a series. It is digenetic, i.e. requiring two alternating hosts for its completion. This helps in dispersal of the worm as well as reduces the period of exposure to many hazards of the environment. It should be looked upon as an adaptation to be parasitic mode of life and enabling the fluke to find new hosts in spite of many mishaps.
Vast reproduction: Reproductive organs in fluke are extremely well developed and the number of eggs and larvae produced is very great. The liver of an infected sheep may contain about 200 sexually mature flukes, and each fluke may produce about 2, 00,000 eggs. It means there may be produced 700 million eggs within a single parasitized host. A single egg may lead to the formation of about 100 cercaria larvae, so that the total number of them formed is simply countless. The great number of eggs and larvae formed by a single fluke is necessary because many eggs do not reach water and most larvae do not encounter the proper host for further development. The vast reproduction ensures the infection of new hosts and the continuance of the race of the parasite.
Considerable diversity and controversy exist in the explanation of reproduction of a digenetic trematode. The origin and development of the germinal cells in successive larval stages present the main controversy. Various views and phenomena have been put forward to explain the life cycle in Digenea.
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Metagenesis:
Steenstsrup (1842) first propounded the hypothesis of alternation of sexual and asexual stages. He regarded cereariae as pupae and rediae as ‘nurses’ developing from ova. The hypothesis of metagenesis, in the light of life cycle of Fasciola hepatica, is based mainly upon superficial homologies between adult and larva stages.
Heterogamy:
Arobben (1882) formulatged the opinion that germinal cells or germ balls in the sporocyst and redia are ova which develop pathenogenetically under the influence of the hypothesis three terms were introduced (1) Parthenitac for sporocysts and rediac, (2) Adolescariae for cercariae and metacercaria and (3) Maritae for adult. According to this theory, algternation takes place between sexual (adult) and a series of parthenogenetic (larval) generations. This phenomenon, where larval stages reproduce asexually by parthenogenesis is known as heterogamy. This view is discarded in favour of the theory of polyembryony.
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Polyembryony:
It is now believed that the germ cells present inside the larval forms are not ova. They are said to be diploid cells obtained by mitotic division from the propagatory cells of the zygote. Thus, the germ cells are the parts of the zygote. It means, all the larval stages are derived from a single zygote. This is known as polye mbryony.
Further, the various larval forms do not arise simultaneously bust at regular intervals from the zygote. This phenomenon has been termed as delayed polyembryony. Only miracidium and cercaria, which are free motile stages, are the actual larvae in the strict sense of the term. The sporocyst, redia and metacercaria are not true larval stages, but rather and developmental or embryonic stages.
Polyembryony is a special kind of reproduction involving a continuous line of germinal cells from the miracidium, through all the larval generation, to the gonads of the adult. This is known germinal lineage hypothesis. This view is widely accepted.
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Extended metamorphosis: Since in some forms life cycle involves more than one intermediate hosts for the development of larval stages, it is supposed to be extended metamorphosis.