Class 12 Biology Organisms and Populations Parasitism


Parasitism is the association between two organisms in which one organism is benefitted and another organism is harmed.

Some of the parasitic adaptations are-

  • Loss of unnecessary sense organs
  • Presence of adhesive organs or suckers to cling on to the host.
  • Loss of digestive system.
  • High reproductive capacity.
  • The life cycles of parasites are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host.
  • The human liver fluke depends on two intermediate hosts, a snail and a fish to complete its life cycle.

Effects of parasites on the host-

  • Majority of the parasites harm the host.
  • They may reduce the survival, growth and reproduction of the host and reduce its population density.
  • They might render the host more vulnerable to predation by making it physically weak.

Parasites that feed on the external surface of the host organism are called ectoparasites. Examples-

  • The lice on humans and ticks on dogs
  • Many marine fish are infested with ectoparasiticcopepods.
  • Cuscuta, a parasitic plant that is commonly found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution and derives its nutrition from the host plant which it parasitizes.

Endoparasites are those that live inside the host body at different sites. Example- tape worm, liver fluke.

Brood parasitism is the process in which the parasitic bird lays its eggs in the nest of its host and lets the host incubate them.

The eggs of the host is similar with the egg of the host. For example- cuckoo lays eggs in the nest of the crow.

Fig. lice on human

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