Q. 2. Describe parasitic adaptations in Platyhelminthes.
What do you understand by parasitism and parasitic adaptations? Discuss parasitic adaptations in Platyhelminthes.
Parasitism
Parasitism is an association between two organisms when one (the parasite) lives on another (host) receiving nourishment and shelter without any compensation for the host. The parasite lives at the cost of the host, causing it harm and sometimes proves so harmful that it ultimately results in its death.
Parasitic Adaptations
The parasitic adaptations in the organisation and life-history of a parasite can be defined as the profound changes and modifications occurring in pursuit of successful parasitism so that the parasite is fully adapted inside the body of the host.
Parasitic Adaptations in Platyhelminthes
The parasitic trematodes and cestodes have undergone profound morphological and physiological adaptations to suit their parasitic mode of life:
Morphological Adaptations
1. Body shape: The shape of the body is flattened, leaf-like or ribbon-like to fit in the spaces of their host.
2. Body covering: Body is covered with a many-layered thick cuticle which protects the parasite from the digestive juices of the host.
3. Organs of adhesion: To have a firm grip on or in the host’s body, special organs of adhesion in the form of suckers, hooks or spines are present.
4. Organs of locomotion: Since sufficient food is present in the host’s body, the organs of locomotion such as cilia, are absent.
5. Organs of nutrition: Food of the parasite comprises the readily available digested or semi-digested food of the host. Therefore, there is a reduction in trophic organs. In trematodes, there is an incomplete gut and in most of them, a suctorial pharynx is present. In cestodes, the mouth and alimentary canal are lacking because digested food is readily available in the gut of the host.
6. Neuro-sensory system: There is complete absence of sense organs and nervous system is greatly reduced.
7. Reproductive system: It is highly developed and the production of eggs is prolific. The parasitic flatworms are hermaphrodite. In cestodes the reproductive organs are much more elaborate and are repeated in each proglottid. Each mature proglottid possesses one (Taenia solium) or two sets (Diphylidium) of male and female reproductive organs. In each gravid proglottid uterus is greatly enlarged and branched to accomodate a large number of eggs.
8. Eggs are shelled and open.
Physiological Adaptations
9. Protective mechanism: In the alimentary canal of the host the parasite protects itself from the action of digestive juices of the host by (a) stimulating the walls of the gut to secrete mucus which forms a protective covering around the body of the parasite, (b) continually renewing its body covering, and (c) secreting anti-enzymes to neutralize the effect of digestive fluids or enzymes of the host.
10. Anaerobic respiration: The alimentary canal and bile ducts are devoid of free oxygen. Therefore, the parasites found in these places respire anaerobically by breaking down the glycogen.
11. Osmoregulation: The osmotic pressure of the body fluids in case of endo-parsites is almost the same as that of the host. To have ready absorption of host’s digested food, the intestinal tapeworms have a little higher osmotic pressure of their body fluid.
12. High fertility: To cover the risk of finding individuals of opposite sex in the same host, the, parasite and moreover, each segment is hermaphrodite and as a rule self-fertilization takes place. The over-production of eggs is very necessary to maximise the chances of reaching the proper host and ensure the perpetuation of host.
Several multiplicative phases in the life-cycle of some flatworms further ensure the perepetuation of race.
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