Thesis Essay

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PARASITISM IN THE ANIMAL KINGDOM A quick survey of the animal kingdom shows that there are parasitic members in practically every major phylum. Among the Protozoa, many species of amoebae, flagellates, ciliates, and all of the Sporozoa are parasitic. It is of interest to note that despite their small sizes, certain protozoa, such as Stentor and Spirostomum can serve as hosts for the flagellate Astasia sp. Small amoebae are known to parasitize opalinid ciliates and Trichodina. The Mesozoa, which are minute, wormlike, acoelomate organisms, include forms that are common parasites in cephalopods and other marine invertebrates. Dicyema, for example, is a common parasite in the nephridia of squids and octopuses. Rhopalura is a less common mesozoan parasite encountered in various tissues and body cavities of marine turbellarians, nemerteans, brittle star, oligochaetes and clams. Although parasitic coelenterates are not common, some are known. One of the most popular subdivisions of Parasitology is helminthology, which deals specifically with the parasitic representatives of the Platyhelminthes, the Nematoda and the Acanthocephala. All Platyhelminthes, except the turbellarians, are parasitic and even a few cases of parasitism have been reported among turbellarians. Numerous species of nematodes are parasitic, many as parasites of animal and others as parasites of plants. The acanthocephala are all parasitic. In addition to these major groups of helminthes, the less commonly encountered Nematomorpha, or horsehair worm, are of interest to helminthologists, because the larvae of most species are parasitic in insects although the adults are free-living in aquatic situations. Some species of the phylum Mollusca, in addition to serving as definitive or as intermediate hosts for protozoan and helminth parasite, may also be ectoparasitic continuously or during some

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