. Explain parasitism in detail

Parasitic mode of life ensure free lodging and meals.

It has evolved in so many taxonomic groups from plants to higher vertebrates.

They have evolved to be host - specific (they can parasitises only a single species of host) in such a way that both host and parasite tend to co-evolve.

Host evolves mechanisms to counteract and neutralise them.

The life cycle of parasites is complex.

Involves one or two intermediate host or vectors to facilitate parasitism in its primary host.

The human liver fluke (a trematode parasite) depends on two intermediate hosts(snail and fish) to completes its life cycle.

The malarial parasite needs vector (mosquito) to spread to other hosts.

Majority of parasites harm the host; reduce the survival, growth and reproduction of host and reduce its population.

Ectoparasites : Parasites feeding externally on the host. e.g., lice on man, ticks on dogs, copepods on marine fish, cuscuta on hedge plants in due course of time lose their chlorophyll.

Endoparasites : Parasites living inside the host body at different sites (liver, kidney, lungs, $RBC$). They are more complex with their extreme specialisation.

Brood parasitism : Parasitic bird lays egg in the nest of its host to incubate in due course of evolution its egg resemble the host egg. Cuckoo bird(koel) lays egg in crows's nest.