Download PARASITISM Definition. Parasitism is an association of two

PARASITISM
Definition.
Parasitism is an association of two organisms of different sizes and species in which the smaller one
is benefited and the larger one is harmed. In a parasitic association, that organism, which is
benefited, is called parasite; and the organism, that suffers, is term the host.
According to the nature of the host the parasites are of 4 kinds:
(A) Phytophagous parasites
The parasites occur on or in the plants. They chiefly come from the groups Protozoa, nematoda and
insecta. The common examples are Tylenchus tritici, a round bowl of wheat, and aphis brassicae, an
aphid of mustard plants, lac insect, red cotton bug, stem borer, etc.
(B) Zoophagous parasites
This parasites live on or in the animals. They are very numerous. Almost every animal shelters
parasites.
(C) Social parasites
The animals that parasite as a social organisation rather than an individual are called social parasites.
The cow bird of the new world and the Cuckoo of the old world do not building nests of their own.
They lay their eggs in the nest of the Other species of birds and leave. The unwilling Foster parents
incubate the eggs and care for the young ones as their own. The young of parasitic birds are stronger
than the young of host birds, and either take the Lion's share of the food or push the real owners out
of the nest.
(D) Hyperparasitism
The existence of a parasite on another parasite is called hyperparasitism. Even the parasites of
parasites may have parasites. A virus maybe a parasite in a Protozoan that itself is a parasite in some
metazoan.
Modifications for parasitic life:
Simplification or loss of locomotory, digestive and sensory organs
The parasites have enough food at hand and no enemies nearby. Therefore they do not need well
developed locomotory organs, which have consequently become reduced.
The gut parasites lie in the digested food that awaits absorption and assimilation. That have,
therefore, lost the digestive glands.
The parasites, particularly the internal ones live in the uniform environment with practically no
changes to be perceived. Their sense organs are therefore simple or lacking. Liverfluke, for instance
loses eyespots of the larval stage on becoming a parasite.
Development of adhesive organs
The parasites have developed adhesive organs to avoid their dislodging from the host. Such organs
include adhesive disc of Giardia, suckers of tapeworms, grasping claws of lice, etc.
Excessive multiplication
The parasites, especially in the internal ones, face a great problem of migration from host to host.
Many are likely to Perish in this effort. Numerous young ones are, therefore, produced so that
enough survive to continue the race.
Capacity of anaerobic respiration
Many endoparasites have a Remarkable ability to live in media that contain little or no oxygen,
respiring anaerobically.
Resistant Cysts and Eggs
Many internal parasites from highly resistant cysts and eggs for the safe transfer of their progeny to
fresh hosts. The cysts and eggs successfully survive the adverse conditions, such as desiccation,
temperature extremes, chemicals of the outside world. Eggs of Ascaris remain viable in the soil for
several years.
Piercing and Sucking Apparatus
Certain ectoparasites have developed special piercing-sucking apparatus to pierce the skin and suck
animal blood or plants sap. Some of them, such as, leeches and mosquitoes, have anticoagulants in
their salivary secretion to check clotting of blood.
Resistant cuticle
The gut parasites have thick cuticle to resist the action of digestive juices. Others have body wall
resistant to the host’s antibodies.
Suitable Body Form
Certain parasites have a form that suits parasitic life. Spindle-like sporozoites of Plasmodium can
easily penetrate into host cells. Literally compressed bodies of fleas enable them to move through
narrow spaces among the hair.
Origin of parasitism
Different parasites evidently arose separately in various phyla from free living ancestors and have
become variously specialised or degenerate for the parasitic mode of life. Parasitism may arise by
way of predation, commensalism, saprozoic nutrition, bacteriophagy and coprophagy.