Owing to parasitic mode of life, Liver fluke has undergone morphological, anatomical, physiological adaptations etc. to suit its existence as an endoparasite in the bile duct of sheep.
These are the following:
Type # 1. Morphological Adaptations:
i. The body is flat and leaf like so that it can reside within the bile duct of sheep.
ii. Oral sucker, acetabulum, hooks and rostellum of adult worm serve as organs of attachment in the host’s body.
iii. Cilia or any kind of locomotory appendages are absent in adult but the larva (Miracidium) possesses cilia as it requires to move about in search of its host (snail). Cercaria has a locomotory tail.
Type # 2. Anatomical Adaptations:
i. On the body wall presence of thick cuticle acts as enzyme-resistant, so that the parasite is not digested by the digestive juices of the host.
ii. The digestive system is simple as the flukes feed on digested food. The intestine is highly branched so that the digested food can be distributed to all parts of the body. In liver fluke anus is absent as there is no undigested food for egestion.
iii. Adult liver fluke lacks circulatory system so that the parasite receives digested food by the process of diffusion.
iv. It respires anaerobically because free oxygen is not available.
v. In Fasciola excretory system is well-developed, the longitudinal excretory canals can receive excretory products from different parts of the body by means of flame cells.
vi. Nervous system is poorly developed. There are no special sense organs as they are not needed.
vii. Well-developed reproductive system is the characteristic feature for the parasitic mode of life. They are hermaphrodites; i.e. both male and female reproductive organs are present in the same individual.
Type # 3. Physiological Adaptations:
i. The osmotic pressure of the body fluid of the host and the medium in which it resides is the same which prevents unnecessary loss of water.
ii. The cuticle is non-living and contains anti-enzymes on its surface which ensures protection against host’s enzymes.
iii. Liver fluke is hyperosmotic to the bile of the host.
iv. The digestive glands in Liver fluke are absent as they are not necessary because the food is in a state of absorption.
Type # 4. Adaptations in Life Cycle:
Parasites are always in danger of extinction unless they reach new host and the life cycle is completed.
i. Each liver fluke produces large number of eggs. This is necessary to offset several hazards resulting in great mortality.
ii. Fertilized eggs are protected by resistant egg-shells which provide further safety from unfavourable environmental conditions.
iii. Presence of a secondary host ensures the continuity as well as dispersal of the species from one place to another place.
iv. Sometimes hermaphroditism ensures self-fertilization which is necessary for survival of the species.
v. As Cercaria is to move, it is provided with tail. With the help of this tail it forces its way out of the body of the intermediate host.
vi. In the life cycle of Fasciola, various types of larvae are formed. Miracidium can move about by cilia and can enter the secondary host.
The other forms of larvae like Sporocyst and Redia which can exhibit the phenomenon of Paedogenesis i.e., they reproduce the next larval forms from germ cells within their body. They also show some sort of adaptations to overcome the unfavourable situations. For example when Cercaria is formed it gets itself encysted for protection. As a result the Cercariae change into Metacercariae which become attached to the aquatic plants so that they can be swallowed by sheep.