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PARASITISM
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PARASITISM , in See also:biology, the See also:condition of an organism which obtains its nourishment wholly or partially from the See also:body of another living organism, and which usually brings about extensive modifications in both See also:guest and See also:host, a phenomenon widespread amongst animals and See also:plants. The See also:term has been appropriated by biologists as a See also:metaphor from the See also:Greek (see See also:PARASITE). The lives of organisms are so closely intermeshed that if dependence on other organisms for See also:food be the criterion of parasitism it is doubtful if any See also:escape the taint. See also:Green plants, it is true, build up their food from the inorganic elements of the See also:air and the See also:soil, and are farthest removed from the suspicion of dependence; but most, if not all, thrive only by the aid of living microbes either actually attached to their roots or swarming in the nutrient soil. Saprophytes, organisms that live on organic See also:matter, are merely parasites of the dead, whilst all animals derive their nourishment from the bodies of plants, either directly or indirectly through one or more sets of other animals. It is See also:plain, therefore, that if parasitism is to be employed as a scientific term it must connote something more. than See also:mere dependence on another living organism for See also:nutrition. The necessary additional conceptions are two: the bodies of 794 host and parasite must be in temporary or permanent See also:physical contact other than the mere preying of the latter on the former; and the presence of the parasite must not be beneficial, and is usually detrimental to the host. It is obvious that within the limits of the strictest See also:definition of parasitism that will See also:cover the facts many degrees occur. The terms symbiosis and commensalism have been applied to conditions really outside the definition of parasitism, but closely related and usually described in the same connexion. Both terms cover the physical consorting of organisms in such a See also:fashion that mutual service is rendered. The name symbiosis was invented by the botanist A. de Bary in 1879, and is applied to such an extraordinary community as the thallus of a See also:lichen, which is composed of a fungus and an alga so intimately associated, physically and physiologically, that it was not until 1868 that the dual nature of the whole was discovered. The presence of See also:chlorophyll, which had always been associated only with See also:vegetable organisms, was detected by Max See also:Schultze in 1851 in the animals See also:Hydra and Vortex, and later on by See also:Ray Lankester in Spongilla and by P. See also:Geddes in some Turbellarian See also:worms. On the theory that the chlorophyll occurs in See also:independent vegetable cells embedded in the See also:animal tissues, such cases See also:form other instances of symbiosis, for the See also:oxygen liberated by the green cells enables their animal hosts to live in fouler See also:water, whilst the hosts provide shelter and possibly nitrogenous food to their guests. The term commensalism was introduced in 1876 by P. J. See also:Van Beneden to cover a large number of cases in which " animals have established themselves on each other, and live together on a See also:good understanding and without injury." The most See also:familiar instance is that of fishes of the genus Fierasfer which live in the See also:digestive See also:tube of See also:sea-cucumbers (Holuthuria; see Eon-taro-See also:DERMA). A variety of commensalism was termed mutualism by Van Beneden and applied to cases where there appeared to be an See also:exchange of benefits. A well-known instance of mutualism is the relation between sea-anemones and See also:hermit crabs. The hermit crab occupies the discarded See also:shell of a mollusc, and anemones such as Sagartia or Adamsia are attached to the out-See also:side of the shell. The See also:bright See also:colours of the See also:anemone advertise its distasteful capacity for stinging, and secure See also:protection for the crab, whilst the anemone gains by vicarious locomotion and possibly has the benefit of floating fragments from the food of the crab. It is plain that such terms as symbiosis, commensalism and mutualism cannot be sharply marked off from each other or from true parasitism, and must be taken as descriptive terms rather than as definite categories into which each particular association between organisms can be fitted. R. Leuckart has made the most useful See also:attempt to classify true parasites. Occasional, or temporary, parasites are to be distinguished from permanent, or stationary, parasites. The former seek their host chiefly to obtain food or shelter and are comparatively little modified by their habits when compared with their nearest unparasitic relatives. They may infest either animals or plants, and as they attack only the superficial surfaces of their hosts, or cavities easy of See also:access from the exterior, they correspond closely with another useful term introduced by Leuckart. They are Epizoa or Ectoparasites, as distinguished from Entozoa or Endoparasites. They include such organisms as plant-lice, and caterpillars which feed on the green parts of plants, and animals such as the See also:flea, the See also:bed-See also:bug and the See also:leech, which usually abandon their hosts when they have obtained their See also:object. Many ectoparasites, however, pass their whole lives attached to their hosts; lice, for instance, See also:lay their eggs on the hairs or feathers or in rugosities of the skin of birds and mammals; the development of the See also:egg, the larval stages and the adult See also:life are all parasitic. Permanent or stationary parasites are in the most cases endoparasitic, inhabiting the See also:internal See also:organs; bacteria, See also:gregarines, nematodes and See also:tapeworms are familiar instances. But here also there are no See also:sharp lines of demarcation. Leuckart divided endoparasites according to the nature and duration of their strictly parasitic life: (1) Some have See also:free-livingand self-supporting embryos that do not become sexually mature until they have reached their host; (2) others have embryos which are parasitic but migratory, moving either to another See also:part of their host, to another host, or to a free life before becoming mature; (3) others again are parasitic in every See also:stage of their lives, remaining in the same host, and being without a migratory stage. Origin of Parasitism.—Now that the theory of spontaneous See also:generation has been disproved, the problem of parasitism is no more than detection of the various causes which may have led 'organisms to See also:change their environment. Every See also:kind of parasite has relations more or less closely akin which have not acquired the parasitic See also:habit, and every gradation exists between temporary and permanent parasites, between creatures that have been only slightly modified and those that have been profoundly modified in relation to this habit. There are many opportunities for an animal or plant in its adult or embryonic stage to be swallowed accidentally by an animal, or to gain entrance to the tissues of a plant, whilst in the See also:case of ectoparasites there is no fundamental difference between an organism selecting a dead or a living environment for food or shelter. If the living environment in the latter case prove to have See also:special advantages, or if the interior of the body first reached accidentally in the former case prove not too different from the normal environment and provide a better shelter, a more convenient temperature, or an easier food See also:supply, the See also:accident may pass into a habit. From the extent to which parasitism exists amongst animals and plants it is clear that it must have arisen independently in an enormous number of cases, and it may be supposed that there must be many cases in which it has been of See also:recent occurrence; E. Metchnikoff, indeed, has suggested that amongst parasites we are to look for the latest products of See also:evolution. In any case it is impossible to suppose that parasites form a natural See also:group; no doubt in many cases the whole of a group, as for instance the group of tapeworms, is parasitic, but indications point clearly to the tapeworms having had free-living ancestors. Parasitism is in See also:short a physiological habit, which theoretically may be assumed by any organism, and which actually has been assumed by members of nearly every living group. Additional information and CommentsThere are no comments yet for this article.
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