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Wednesday, 13 April 2011

Parasitic adaptation in platyhelminthes

                  Parasitic adaptation in platyhelminthes

      Fitness of an organism to its environment
      It is the characteristic which results in suitable & convenient morphological & functional correlation between an organism & its environment

                                          Parasitic adaptation

      Platyhelminthes have undergone profound adaptation to suit their parasitic modes of life
      These adaptations- parasitic adaptations
      Are of morphological & physiological nature

1.                              Morphological adaptations

       Body covering
      Organs of adhesion
      Organs of locomotion
      Organs of nutrition (Trophic organs)
      Neurosensory system
       Reproductive system

                                          Body covering

Thick tegument frequently provided with scales affords suitable protection to the parasite
This thick protoplasmic layer is continually renewed by mesenchymal cells forming it

                                    B. Organs of adhesion

      For a firm grip on/in the host’s body, some special organs of adhesion are needed
      Flatworms are variously armed with suckers, hooks & spines
      Suckers may be with/without hooks/spines
                                       Organs of locomotion

      Locomotion is actually an effort of procuring food
      But parasites habitually inhabit such places in host’s body, where sufficient food is available without effort
      Thus, organs of locomotion such as cilia of turbellarians- absent in parasitic forms
      Locomotory organs present in free living larvae of parasitic forms
      Miracidium possess cilia & cercaria bears a tail for locomotion
                     D. Organs of nutrition (Trophic organs)

      Food of parasite comprises readily available & digested/ semi digested food of the host
      Elaborate organs of nutrition not needed
      Trematodes have an incomplete gut & in most cases a suctorial pharynx for sucking food
      An eversible pharynx is present in free living turbellarians
      In cestodes, parasite freely bathes in digested food of host which is absorbed directly
      Thus, total absence of alimentation in tapeworms

                                     E. Neurosensory system

      Need for quick & efficient “response to stimuli” is associated with free active life & not with a quiet parasitic life in a safe environment
      In parasites therefore, there is preferred reduction of nervous system & a total absence of sense organs
      But the free living miracidium possesses eye spots

                                    F. Reproductive system

      Best developed system in helminth parasites, designed & preferred to meet the need for tremendous egg production
      Parasitic flatworms with a few exceptions like Schistosoma, are monoecious (hermaphrodite)
      Hermaphroditism is of distinct advantage to the parasite because:
1. It ensures copulation even when a few individuals are present
2. After copulation both individuals lay eggs, doubling the rate of production
3. In absence of companion parasite can reproduce offspring

      In cestodes reproductive system is much more elaborate & each mature proglottid possesses 1 or2 complete sets of male & female genitalia
      In gravid proglottid all other organs of the system degenerate to make room for the uterus which becomes highly enlarged & branched to accommodate large number of eggs

2.                                   Physiological adaptations

 a. Protective mechanism                                      b. Anaerobic respiration                                                                                                                                                                           c. Osmoregulation                                                 d. High fertility

a.                                    Protective mechanism

      Inside the alimentary canal the parasites have to protect  themselves from the action of digestive juices of host
      Tapeworms accomplish this:
1. By stimulating walls of gut to secrete mucus, which then forms a protective clothing around parasite
2. By secreting antienzymes to neutralize the digestive enzymes of host
3. By probably continually renewing their protective body covering i.e., tegument

b.                                Anaerobic respiration

      Environment in gut & bile ducts is devoid of free oxygen
      Flatworms inhabiting these places, therefore, respire anaerobically by breaking down glycogen
c.                                   Osmoregulation

      Osmotic pressure of endoparasite’s body fluids, especially in case of trematodes is almost the same as that of host
      This renders osmoregulation unnecessary
      But in intestinal tapeworms, osmotic pressure is little higher
      This permits ready absorption of host’s digested food by tapeworms
d.                                        High fertility

      Eggs produced by a parasitic flatworm face a very uncertain future while passing through the complex life cycle, these potential offsprings face several hazards as a result of which a very small percentage of total eggs produced reaches adulthood
      This threat to the very existence of species is suitably met by parasite which in its life time may produce eggs in millions
      Reproductive organs of flatworms are accordingly developed



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