CDS (I) 2021 General Knowledge · Question 111 of 120

Correct — D, Planaria. Planarians are free-living flatworms — they occur in fresh water, in the sea and in damp soil, moving over surfaces on their own and feeding on small prey and dead matter rather than living inside another animal. They belong to the phylum Platyhelminthes, class Turbellaria, and they are best known for regeneration: cut a planarian and each piece can rebuild the whole animal, from fragments estimated at as little as one part in a few hundred of the original. That is why they turn up in the reproduction chapter as the standard example of regeneration. Every other name in this question is a parasite that cannot complete its life without a host.

• (a)Liver fluke — A parasitic flatworm of the same phylum as Planaria, living in the bile ducts of sheep, cattle and sometimes humans, with a freshwater snail as its intermediate host.
• (b)Wuchereria — Wuchereria bancrofti is a parasitic roundworm that blocks the lymphatic vessels and causes filariasis, the disease whose severe swelling is called elephantiasis. It is spread by Culex mosquitoes.
• (c)Plasmodium — A parasitic protozoan that spends part of its life in human red blood cells and part in the female Anopheles mosquito. It cannot live independently in either.

An animal is free-living when it feeds and reproduces on its own account, and parasitic when it lives on or inside another organism at that host's expense. The distinction cuts across the classification: the flatworm phylum contains both the free-living planarians and the parasitic flukes and tapeworms, so the group name does not settle the way of life. Parasites usually show reduced sense organs and locomotion, simplified guts or none at all, and hooks or suckers for holding on.

Three of the four names are so closely tied to a disease that a candidate who has met them at all has met them as parasites — liver rot, filariasis and malaria. Planaria is the only one that appears in the syllabus for something it does rather than something it causes, which is a useful signal in itself. The instructive part of the answer is that Planaria and the liver fluke sit in the same phylum, so the item is really testing whether the free-living and parasitic ways of life have been separated from the taxonomy. Planaria also carries a second exam-worthy fact, its regeneration, which is often asked as a mode of reproduction alongside budding in Hydra and fragmentation in Spirogyra.

• Planarians are free-living flatworms found in marine, freshwater and terrestrial habitats.
• They belong to the phylum Platyhelminthes, class Turbellaria — the same phylum as the parasitic flukes and tapeworms.
• A very small fragment of a planarian can regenerate into a complete animal, which is why it is the textbook example of regeneration.
• Liver fluke and tapeworm are parasitic flatworms; Wuchereria is a parasitic roundworm causing filariasis.
• Plasmodium is a parasitic protozoan needing both a human host and a female Anopheles mosquito.

• Assuming every flatworm is a parasite, when the phylum contains free-living planarians as well.
• Confusing Wuchereria, a roundworm, with the flatworm parasites in the same list.
• Reading 'free living' as 'lives in fresh water'; a parasite can live in fresh water too, inside its host.

As an odd-one-out on the parasitic way of life, or as a regeneration item where Planaria is the answer instead.

• practice — not a real PYQ

  Regeneration, in which a full organism grows back from a small piece of the body, is best illustrated by

  • (a)Planaria
  • (b)Ascaris
  • (c)Plasmodium
  • (d)Wuchereria

  Answer(a) Planaria — the free-living flatworm that can rebuild an entire animal from a fragment, and the standard textbook example of regeneration.

• practice — not a real PYQ

  Elephantiasis, marked by gross swelling of the limbs, is caused by

  • (a)Trypanosoma
  • (b)Wuchereria bancrofti
  • (c)Entamoeba histolytica
  • (d)Fasciola hepatica

  Answer(b) Wuchereria bancrofti — a parasitic roundworm that blocks the lymphatic vessels, spread by Culex mosquitoes.