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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.