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Phylum V: Platyhelminthes (Flat worms)

(Greek platys meaning flat + helmins meaning worm)
This phylum constitutes the flatworms. Some 12,700 species are known.

General Characteristics
  1. Bilaterally symmetrical, body flattened dorsoventrally.
  2. Triploblastic animals; acoelomate.
  3. Animals are free living (mostly marine, some fresh water or terrestrial) or parasitic.
  4. Suckers and hooks are present in the parasitic forms.
  5. In free living forms the ectoderm is covered by cilia; in parasitic forms the ectoderm is covered with cuticle.
  6. Branched or unbranched alimentary canal with only one opening (mouth). In certain parasitic forms like tapeworm there is no alimentary canal.
  7. No skeleton.
Planaria, Fasciola (liverfluke), Schistosoma (blood fluke), Taenia (tapeworm).

Examples of Flatworms: A. Planaria; B. Fasciola (liverfluke); C. Taenia (tapeworm); D. Schistosoma (blood fluke)

Parasitic Adaptations in Flatworms

The parasitic flatworms were known to man from very ancient times. Out of the three classes of phylum, two (Trematoda and Cestoda) consist of flatworms which are exclusively parasitic in habit. The third class Turbellaria comprises mostly free-living flatworms. The common liver fluke (Fasciola hepatica) which causes liver rot in sheep, belongs to class Trematoda. The tapeworm (Taenia solium) occurring in the human intestine belongs to Cestoda. Parasitic flatworms are very well adapted to their mode of life. The parasitic adaptations of flatworms involve both modifications in structure (morphological adaptations) and function (physiological adaptations).

Morphological Adaptations

  1. Exoskeleton: Parasitic flatworms have developed a fairly thick protective covering or cuticle which is often raised into minute spines or scales. The cuticle provides greater protection to the parasites from the action of body fluids or digestive enzymes of the host.

    Organs which help the parasite to attach to the host tissues, are well developed and these take the form of suckers, hooks, etc.
  2. Absence of Locomotory Organs: The organs of movement such cilia or flagella are absent in parasitic flatworms although their free living larval forms may posses them.
  3. Simplification or Total Absence of Digestive Tract: It is another adaptive feature exhibited by parasitic flatworms. Since they depend on the semi digested or digested food of the host many of the parasites do not need a digestive system. In certain flatworms there is a pharynx which suck in liquid food and pumps it into a much branched intestine.
  4. Less Developed Nervous and Sensory Systems: As they live in a totally protected environment inside the host most of the parasite flatworms have poorly developed nervous system and sense organs. However, nerves going to the organs like suckers are well developed.
  5. Excessive Development of the Reproductive Systems: It is one of the features of parasitic flatworms and is best developed in tapeworms in which each of the segments (proglottis) of the parasite has a complete set of reproductive organs. The parasites produce enormous number of offspring which is necessary for the perpetuation of the species. The occurrence of very complex life histories is another remarkable feature of parasitic flatworms.

Physiological Adaptations

  1. The osmotic pressure of the body fluids of the parasites become the same as that of the host.
  2. The parasites derive energy from the process of anaerobic respiration. This is because the parasites environment is always deficient of oxygen.
  3. The parasites are able to secrete antienzymes which neutralise the digestive juices of the host.

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