# 1911 Encyclopædia Britannica/Parasitism

See also Parasitism on Wikipedia; and our 1911 Encyclopædia Britannica disclaimer.
​PARASITISM, in biology, the condition of an organism which obtains its nourishment wholly or partially from the body of another living organism, and which usually brings about extensive modifications in both guest and host, a phenomenon widespread amongst animals and plants. The term has been appropriated by biologists as a metaphor from the Greek (see Parasite). The lives of organisms are so closely intermeshed that if dependence on other organisms for food be the criterion of parasitism it is doubtful if any escape the taint. Green plants, it is true, build up their food from the inorganic elements of the air and the 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 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 plain, therefore, that if parasitism is to be employed as a scientific term it must connote something more than mere dependence on another living organism for nutrition. The necessary additional conceptions are two: the bodies of ​ host and parasite must be in temporary or permanent 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 definition of parasitism that will 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 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 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 chlorophyll, which had always been associated only with vegetable organisms, was detected by Max Schultze in 1851 in the animals Hydra and Vortex, and later on by Ray Lankester in Spongilla and by P. Geddes in some Turbellarian worms. On the theory that the chlorophyll occurs in independent vegetable cells embedded in the animal tissues, such cases form other instances of symbiosis, for the oxygen liberated by the green cells enables their animal hosts to live in fouler water, whilst the hosts provide shelter and possibly nitrogenous food to their guests.
The term commensalism was introduced in 1876 by P. J. Van Beneden to cover a large number of cases in which “animals have established themselves on each other, and live together on a good understanding and without injury.” The most familiar instance is that of fishes of the genus Fierasfer which live in the digestive tube of sea-cucumbers (Holuthuria; see Echinoderma). A variety of commensalism was termed mutualism by Van Beneden and applied to cases where there appeared to be an exchange of benefits. A well-known instance of mutualism is the relation between sea-anemones and hermit crabs. The hermit crab occupies the discarded shell of a mollusc, and anemones such as Sagartia or Adamsia are attached to the outside of the shell. The bright colours of the anemone advertise its distasteful capacity for stinging, and secure 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 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 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 flea, the bed-bug and the leech, which usually abandon their hosts when they have obtained their object. Many ectoparasites, however, pass their whole lives attached to their hosts; lice, for instance, lay their eggs on the hairs or feathers or in rugosities of the skin of birds and mammals; the development of the egg, the larval stages and the adult life are all parasitic. Permanent or stationary parasites are in the most cases endoparasitic, inhabiting the internal organs; bacteria, gregarines, nematodes and tapeworms are familiar instances. But here also there are no sharp lines of demarcation. Leuckart divided endoparasites according to the nature and duration of their strictly parasitic life: (1) Some have free-living and 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 part of their host, to another host, or to a free life before becoming mature; (3) others again are parasitic in every stage of their lives, remaining in the same host, and being without a migratory stage.
Origin of Parasitism.—Now that the theory of spontaneous generation has been disproved, the problem of parasitism is no more than detection of the various causes which may have led organisms to change their environment. Every kind of parasite has relations more or less closely akin which have not acquired the parasitic 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 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 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 supply, the 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 recent occurrence; E. Metchnikoff, indeed, has suggested that amongst parasites we are to look for the latest products of evolution. In any case it is impossible to suppose that parasites form a natural 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 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.
List of Parasites A.—Animals.
Vertebrata.—These are rarely parasitic, and cases are unknown amongst mammals, birds, reptiles and amphibia. Amongst fish and cyclostomes, Myxine burrows into codfish, Remora attaches itself to the external surface of sharks; Rhodens amarus, the bitterling, a small, carp-like fresh-water fish, injects its eggs into the mantle-cavity of pond-mussels, where the fry develop, whilst the mollusc reciprocates by throwing off its embryos on the parent fish; Stegophilus insidiosus, a small colourless fish from Brazil and the Argentine, lives parasitically in the gill-cavity of large cat-fishes and sucks the blood in the gills of a large Silurid; Vandellia cirrhosa, the candiru of Brazil, a minute fish 60 mm. in length, enters and ascends the urethra of people bathing, being attracted by the urine; it cannot be withdrawn, owing to the erectile spines on its gill-covers. The natives in some parts of the Amazon protect themselves whilst in the water by wearing a sheath of minutely perforated coco-nut shell.
Mollusca.—Few if any are true parasites. The Gasteropods, Eulimae, Styliferae and Entoconchae lodge in Echinoderms, the latter at least being truly parasitic.
Protochorda and Hemichorda.—Most of these are sessile and may lodge on other animals, but are not parasitic.
Arachnida.—Mites and Ticks are Arachnids, the vast majority of which are parasitic, and species of which infest almost every vertebrate group, but there are some free-living forms. Pycnogonids are parasitic in their youthful stages on Hydroids, whilst the Pentastomids have been so much modified by parasitism that they were long regarded as worms; they may occur in most vertebrates.
Crustacea.—These contain an immense number of forms in all stages of parasitism. Some Copepods are amongst the most degenerate parasites known, the so-called fish-lice being for the most part Copepods with piercing mouth-organs, elaborate clinging apparatus, and degenerate organs of locomotion. In Lernea, the female, after becoming attached to its host, undergoes a retrogressive metamorphosis, losing almost completely the segmentation of the body and discarding the appendages and sense-organs, whilst the male, although not so degenerate in structure, is dwarfed in size and itself becomes a parasite of the female. The Cirripeds ​ are all sessile in the adult condition. The Lepadidae are the least modified and are rarely parasitic; the Balanidae are more modified and frequently become embedded in the skin of whales. The Abdominalia live as parasites buried in the shells of other Cirripeds and of molluscs. The Apoda live as parasites in the mantle of other Cirripeds, whilst the Rhizocephala live chiefly on the abdomen of Decapod Crustacea, sending burrowing root-like nutritive processes into their tissues.
Insecta.—A very large number of insects are temporary or permanent parasites of animals or plants, the adult stages being chiefly ectoparasitic, the larval stages endoparasitic. The Hemimeridae, allies of the earwigs, are ectoparasites on rats. The Mallophaga or bird-lice are degenerate wingless insects spending their whole lives as ectoparasites on birds and mammals. The larvae of Hemerobiinae are parasitic on Aphides. The saw-flies are parasitic on plants. There are over 200,000 species known of the Hymenoptera parasitica or Terebrantia. The adults deposit their eggs in the eggs, caterpillars or adults of other insects, particularly Lepidoptera. The clothes-moth, for instance, is known to be subject to the attack of over sixty species of Hymenoptera. To such an extent has parasitism been developed in this group, that the parasites themselves are attacked by other parasites, giving rise to the phenomena known as hyperparasitism. The gall-flies (see Galls) are included amongst the Terebrantia, but in their case the early stages are passed in vegetable galls more frequently than in the bodies of other insects. The ruby-flies (Hymenoptera Tubulifera), in the larval condition are parasitic on the larvae of wasps and bees. The Denudatae are bees that in the larval stage are parasitic on other bees, the larvae of the parasites being deposited in the food-cells prepared for their own larvae by other bees. Many of the fossorial Hymenoptera form no special nests for their young, but take advantage of the abodes and food-stores prepared by other insects. The very large number of Hymenopterous insects that collect living larvae to be shut up as provender for their developing young are in a sense parasitic. The complex relations of ants with other insects must be referred to in this connexion. The nests of many species are inhabited by foreign insects of various orders, such insects being termed myrmecophilous or ants’-nest insects. The relations between the ants and their guests are very complex, and the guests migrate with their hosts. Aphidae, Coccidae and other bugs that secrete sugary matter are cherished and tended by ants; so also the caterpillars of some Lycaenid butterflies are kept as a kind of domesticated animal for some useful purpose. There are also many Orthoptera, Hemiptera, and other insects, as well as some acarids and wood-lice found only in ants’-nests as cherished or tolerated guests. The relations between ants and plants is also interesting; the ants live as parasites on the plants or trees, but in return protect them from more harmful intruders. Such phenomena are on the border-line between symbiosis and true parasitism. Although most beetles live on decaying animal or vegetable matter, a large number are parasitic in the adult or larval condition on animals or plants. The curious beetle known as Platypsyllus castoris is known only as an ectoparasite of the beaver, whilst the Leptinidae are parasites of several species of mammals. The minute beetles of the families Mordellidae and Rhipiphoridae are endoparasites of wasps and cockroaches, whilst the larvae of many of the Cantharidae are parasites of locusts. The Strepsiptera are endoparasites of Hymenoptera and Hemiptera. The habits of the Diptera easily pass over into parasitism, and a very large number are temporary or permanent parasites in the adult or larval stages. Most of the larvae of the Cecidomyiidae live in plants and form galls or other deformities. The bloodsucking habits of mosquitoes and gnats and sand-flies have not led to any special development in the direction of parasitism. The larvae of Bombyliidae are endoparasites of the larvae of mason-bees, and some of the Cyrtidae similarly infest spiders, whilst the Tachinidae deposit their larvae in other living insects, caterpillars being especially selected. The larvae of some of the Sarcophagidae may be deposited in the nostrils of man and other animals, where they may cause death, whilst those of the South American genus Lucilia infest the nasal fossae and frontal sinuses of man, producing great suffering, and the larvae of the numerous kinds of bot-fly attack man and many animals. The very large group of Pupipara live by sucking the blood of mammals and birds, and many of them are reduced to wingless permanent parasites. The single member of the family Braulidae is a parasite of the bee. All the known fleas (Aphaniptera or Siphonaptera) are ectoparasites in the adult condition; the larval stages are usually to be found in organic refuse. The larvae of most Lepidoptera are temporary ectoparasites of plants, but a few attack other insects, such as coccids and aphids. All the Hemiptera (bugs) have sucking-mouth organs and the majority of them are temporary parasites of plants or other animals. Some, such as the bed-bug, have been so modified by parasitism as to be found only in human dwellings, others, such as the aphids or plant-lice, are permanent parasites of plants, many of them producing galls. The coccids. or scale insects, have been still further modified as plant ectoparasites. The Pediculidae, or lice, are the most completely parasitic of insects, and are degraded wingless insects found on almost any kind of bird or mammal, but in most cases so highly modified as to be capable of existence only on the particular species with which they are associated.
Lower Invertebrates.—No true Chaetopods are parasitic, but a few are commensal. The leeches are probably Chaetopods modified by parasitism; and Myzostomes are still more highly modified relatives of the group, very degenerate and parasitic on Crinoids. A few rotifers are ecto- and endo-parasites. No Brachiopods, Polyzoa or Echinoderms are true parasites. The flat-worms and round-worms contain the most characteristic endoparasites, and parasitism is so characteristic a feature of most of the groups that it is discussed in the separate articles dealing with the various natural assemblages of such worms. All the Cestodes (see Tape-worm), most of the Trematodes (q.v.), and a few of the Planarians (q.v.) are parasites of animals. Most Nemertines are free-living, but Cephalothrix galatheae is endoparasitic in the ovaries of the Crustacean Galathea strigosa, whilst Eunemertes and Tetrastemma occur on Ascidians, and Malacobdella in lamellibranch Molluscs. The degraded Mesozoa (q.v.) are endoparasites of Planarians, Nemertines and Ophiurids. The Nematoda (q.v.) or typical round-worms, exhibit every degree from absolute free-life to absolute parasitism in animals and plants. The Echiuroidea (q.v.) are mostly free-living, but the male of Bonellia lives as a very degenerate parasite in the uterus and pharynx of the female. Although Coelentera and Porifera are usually sessile, very few are true parasites; young stages of the Narcomedusae are parasitic in the mouth of adults of different species, whilst Mnestra parasites is a degenerate medusa living on the pelagic mollusc Phyllirhoe. The Protozoa, from their minute size and capacity to live in fluids, naturally include an enormous number of parasitic forms, the importance of which in producing disease in their hosts is so great that a very large special literature on parasitic protozoology is being formed (see Pathology). Of the Sarcodina (q.v.) many forms of Amoeba such as Amoeba coli are associated with dysentery and kindred diseases. A very large number of the Mastigophora (q.v.), including such forms as the trypanosome of sleeping-sickness, are parasitic; in fact, observation by adequate means of the juices of almost any animal reveals the occasional presence of some kind of mobile protozoon, provided with a whip-like process. The enormous group of Sporozoa (q.v.) are entirely parasitic, and have been found in every group of animals except the Protozoa and Coelentera. Infusoria (q.v.) contain a considerable number of parasitic forms, some endoparasitic; others like the Suctoria, ectoparasitic.
B.—Plants.
Bacteria.—Every degree of adaptation to parasitism occurs amongst bacteria, a majority of which pass at least some stage of their lives in a parasitic condition.
Fungi.—As in the case of Bacteria, the absence of chlorophyll from the tissues of fungi makes it necessary that they should take up carbon compounds already assimilated by other organisms, and accordingly they are either saprophytes or parasites. The mycelium is, so to say, the parasitic organ of the fungus, ramifying in the tissues of the host. The plant may obtain access to its host by means of spores which enter usually by wounds in the case of animal and plant hosts, but occasionally by natural apertures such as the stomata of plants. The fungi that develop in the organs of warm-blooded animals reach the blood-stream through wounds, and thence spread to the tissues where germination takes place. Many fungi, especially those that are epiphytic, reach the tissues of their host by germ-tubes which emerge from the spore and penetrate either by a natural or artificial aperture, whilst in other cases the germ-tubes or hyphae actually penetrate uninjured tissues or membranes.
The fungi parasitic on animals are in most cases little known, and additions to the list, of which the pathological rather than the botanical features have been worked out, are constantly being made. A number of species of Eurotium and Aspergillus, usually saprophytic, may migrate to the bodies of animals, spreading in the tissues and exciting a disease known as mycosis or aspergillosis. They were first discussed in the disease of the human ear known as otomycosis, but they occur also in lungs and air-passages of mammals and birds. Recent pathological investigations conducted at the Prosectorium of the Zoological Society of London, show that mycosis is extremely frequent and fatal in birds and reptiles, and rather less frequent in mammals. Almost any organ of the body is liable to attack. The Laboulbenieae are probably Ascomycetes restricted to parasitism on insects, chiefly beetles and flies, sometimes forming a thick fur on the bodies and spreading by spores. The Entomophthoreae, possibly Mucorini, are also restricted to insects, the fungus that kills the common house-fly being the most familiar example. Cordyceps militaris and Botrytis bassii are familiar examples of Ascomycete fungi that attack the caterpillars of insects, the latter producing the fatal disease “muscardine” of the silkworm. The group of Saprolegnieae usually vegetate as saprophytes but readily settle on aquatic animals such as goldfish, salmon, salamanders and frogs, with fatal results. It is not yet entirely certain if diseases of this kind, of which the salmon disease is the most notorious, are produced on healthy animals, by the ​ attacks of the fungi, or if some antecedent predisposing condition be necessary. There are a number of well-known fungi that produce diseases of the skin in man and other vertebrates. Achorion Schoenleinii produces favus in man, rabbits, cats, fowls and other birds and mammals. Trichophyton tonsurans (Malmsten), is the fungus of tinea or ringworm in man, oxen, horses, dogs and rabbits. Saccharomyces albicans (Reess), produces thrush of the mouth in young herbivora and birds. Actinomyces bovis (Harz) is associated with swellings on the jaw-bone of cattle and kangaroos, but has been found in pigs and human beings.
The fungi parasitic on plants are much better known and are responsible for a large number of diseases. They display every gradation from occasional to complete parasitism. Amongst the Pyrenomycetes, the group Erysipheae contain a large number of common parasites; the main body of the fungus is usually epiphytic as in various mildews (q.v.). Ergot (q.v.) is the most familiar example of the group. The Discomycetes are chiefly saprophytic, being common on dead fruits, roots and so forth, but many of them kill living plants: Exoascus on plums, peaches and cherries. Sclerotinia is most common on dead juicy fruits, but will destroy turnips in store, and has been known to attack living Phaseolus and Petunia. The Hymenocytes are naturally saprophytes, but when they gain access through wounds are the most destructive parasites of living timber. The Ustilagineae are endoparasites in Phanerogams, and are especially notorious for their attacks on grain-crops and grasses. The species of Ustilego set up hypertrophy in the tissues of their hosts, and the enlarged spaces thus formed become filled with the spores of the parasite. The Uredineae are also endoparasites of the higher plants and produce the diseases known as rusts which specially affect cultivated plants. The Peronosporeae are all parasites of plants and are the most destructive enemies of agriculture and horticulture. Phytophthora infestans (de Bary), the potato-disease fungus, is a typical example.
Algae.—The chlorophyll-containing green and yellow cells found in Hydroids and Planarians referred to in connexion with symbiosis and the small green algae that infest the hairs of sloths are on the border-line of parasitism. A species of Nostoc occurs in the intercellular spaces of other plants; Chlorochytrium is found in the tissues of Lemna, and Phyllosiphon arisari (Kühn) infests the parenchyma of Arum arisarum.
The flowering plants have a considerable number of representatives which have become epiphytes and which exhibit various degrees of parasitic degeneration. The Monotropeae allied to the heaths, are degenerate, with no chlorophyll and with scale-like leaves but the evidence as to their parasitism is more than doubtful; they are possibly only saprophytic. The allied Lennoaceae, a small group also devoid of chlorophyll and with scale-leaves, are true root-parasites. The genus Cuscuta of the Convolvulaceae consists of the true parasites known as dodders. They are destitute of chlorophyll and attach themselves to other plants by twining stems on which occur haustoria that penetrate the tissues of the host and absorb nutritive material. Cuscuta europaea, the great dodder, is a parasite of nettles and hops; Cuscuta epilinum is the flax dodder; Cuscuta epithymum attacks a number of low-growing plants; and Cuscutum trifolii is very destructive to clover. Several genera of Scrophulariaceae are partially parasitic; they contain chlorophyll but have degenerate roots with haustoria. Euphrasia, the eyebright, attacks the roots of grasses; Pedicularis, the lousewort, Rhinanthus, the rattle, Melampyrum, the cow-wheat and Bartsia are all partly parasitic on the roots of other plants. The Orobanchaceae or broomworts, are all destitute of chlorophyll and have scale-leaves; they are parasitic on the roots of other plants, species attacking various Leguminosae, ivy, hemp and hazel. The Cytinaceae are true parasites devoid of chlorophyll and leaves, with deformed bodies and conspicuous flowers or inflorescences. Most of them are tropical, and the group is widely scattered throughout the world. The Santalales are all parasitic; some members like Thesium linophyllum (the bastard toad-flax), a root parasite, and Viscum album (the mistletoe), parasitic on branches, have chlorophyll, but rather degenerate leaves; others like the tropical Balanophoraceae are devoid of chlorophyll and foliage leaves and have deformed bodies. Of the Lauraceae, a few genera such as Cassytha (the tropical “dodder-laurels,”) are true parasites, without chlorophyll and with twining stems.
Effect of Parasitism on Parasites.—The phenomena of parasitism occur so generally in the animal and vegetable kingdoms and are repeated in degrees so varying that no categorical statements can be laid down as to the effects produced on the organisms concerned. All living creatures have a certain degree of correspondence with the conditions of their environment, and parasitism is only a special case of such adaptation. The widest generalization that can be made regarding it is that parasitism tends towards a rigid adaptation to a relatively limited and stable environment, whilst free life tends towards a looser correspondence with a more varying environment. The summum bonum of a parasite is to reach and maintain existence in the limited conditions afforded by its host; the goal of the free-living organism is a varying or experimental fitness for varying surrounding conditions. And, if the metaphor be continued, the danger of parasitism for the parasite, is that if it become too nicely adjusted to the special conditions of its host, and fail to attain these, it will inevitably perish. The degeneration of parasites is merely a more precise adaptation; in the favourable environment the degenerate, or specialized parasite is best equipped for successful existence, but the smallest change of environment is fatal. Such a generalization as has been formulated covers nearly all the peculiarities of parasitism. Organs of prehension are notably developed; parasitic plants have twining stems, boring roots and special clinging organs; parasitic animals display hooks, suckers and boring apparatus. The normal organs of locomotion tend to disappear, whether these be wings or walking legs. Organs of sense, the chief purpose of which is to make animals react quickly to changes in the environment, become degenerate in proportion as the changes which the parasite may have to encounter are diminished. The changes correlated with nutrition equally conform with the generalization. The chlorophyll of the plant becomes unnecessary and tends to disappear; the stem has no longer to thrust a spreading crown of leaves into the tenuous air or groping rootlets into the soil, but absorbs already prepared nourishment from the tissues of its host through compact conduits. And so the parasitic higher plant tends to lose its division into stem and leaves and roots, and to acquire a compact and amorphous body. The animal has no longer to seek its food, and the lithe segmentation of a body adapted for locomotion becomes replaced by a squat or insinuating form. Jaws give place to sucking and piercing tubes, the alimentary canal becomes simplified, or may disappear altogether, the parasite living in the juices of its host, and absorbing them through the skin. So, also, parasites obtaining protection from the tissues of their host lose their intrinsic protective mechanisms.
The reproduction of parasites offers many peculiarities, all of which are readily correlated with our generalization. A creature rigidly adapted to a special environment fails if it does not reach that environment, and hence species most successful in reproduction are able to afford the largest number of misses to secure a few hits and so to maintain existence. High reproductive capacity is still more urgent when the parasites tend to bring to an end their own environment by killing their hosts. Reproduction in parasites, so far from being degenerate, displays an exuberance of activity, and an extraordinary efficiency. In parasitic flowering plants the flowers tend to be highly conspicuous, the seeds to be numerous, and specially adapted to ready diffusion. Amongst the fungi, the reproductive processes are most prolific, spores are produced by myriads, and very many special adaptations exist for the protection of the latter during their transference from host to host. It is notorious that the spores of bacteria and the higher fungi resist changes of temperature, desiccation, and the action of physical and chemical agents, to an astonishing extent. Vegetative reproduction is extremely active under favourable conditions, and resting reproductive bodies of varying morphological character are produced in great abundance. Amongst fungi, a phenomenon known as heteroecism is developed as a special adaptation to parasitic conditions, and recalls the similar adaptations in many animal parasites. At one stage of its existence, the fungus is adapted to one host, at another stage to another host. Puccinia graminis, the fungoid rust affecting many grasses, is a typical instance. It inhabits wheat, rye and other grasses, developing a mycelium in the tissues of young plants. During the summer, the mycelium gives rise to large numbers of simple processes which break through the tissues of the host and bud off orange-coloured uredogonidia. These small bodies are scattered by the wind, and reach other plants on which they germinate, enter the new host through the stomata and give rise to new mycelia. Towards autumn, when the tissues of the host are becoming hard and dry, darker-coloured teleutogonidia are produced, and these remain quiescent during the winter. In spring they germinate, produce ​small free-living mycelia on which in due course sporidia are formed. When these, scattered by the wind, fall on the leaf of the barberry-plant, they germinate, and entering the leaf-tissue of the new host by the stomata produce a mycelium bearing reproductive organs so different from those of the phase on the grass-plant, that it was described as a distinct fungus (Aecidium berberidis), before its relation with the rust of grasses was known. The spores of the Aecidium when they reach grasses give rise to the Puccinia stage again.
The reproductive processes of animal parasites are equally exuberant. In the first place, hermaphroditism is very common, and the animals in many cases are capable of self-fertilization. Parthenogenetic reproduction and various forms of vegetative budding are found in all stages of the life-history of animal parasites. The prolificness of many parasites is almost incredible. R. Leuckart pointed out that a human tapeworm has an average life of two years, and produces in that time about 1500 proglottides, each containing between fifty and sixty thousand eggs, so that the single tapeworm has over eighty million chances of successfully reproducing its kind. The devices for nourishing and protecting the eggs and embryos are numerous and elaborate, and many complex cases of larval migration and complicated cases of heteroecism occur. (See Trematode and Tapeworm.)
The physiological adaptations of parasites are notable, especially in cases where the hosts are warm-blooded. The parasites tend to become so specialized as to be peculiar to particular hosts; ectoparasites frequently differ from species to species of host, and the flea of one mammal, for instance, may rapidly die if it be transferred to another although similar host. The larval and adult stages of endoparasites become similarly specialized, and although there are many cases in which the parasites that excite a disease in one kind of animal are able to infect animals of different species, the general tendency is in the direction of absolute limitation of one parasite, and indeed one stage of one parasite to one kind of host. The series of events seems to be a gradual progression from temporary or occasional parasitism to obligatory parasitism and to a further restriction of the obligatory parasite to a particular kind of host.
Effect of Parasitism on Hosts.—The intensity of the effect of parasitism on the hosts of the parasites ranges from the slightest local injury to complete destruction. Most animals and plants harbour a number of parasites, and seem to be unaffected by them. On the other hand, as special knowledge increases, the range of the direct and indirect effect of parasites is seen to be greater. It is probable that in a majority of cases, the tissues of animals and plants resist the entrance of microbes unless there is some abrasion or wound. In the case of plants the actual local damage caused by animal or vegetable ectoparasites may be insignificant, but the wounds afford a ready entrance to the spores or hyphae of destructive endoparasites. So also in the case of animals, it is probable that few microbes can enter the skin or penetrate the walls of the alimentary canal if these be undamaged. But as knowledge advances the indirect effect of parasites is seen to be of more and more importance. Through the wounds caused by biting-insects the microbes of various skin diseases and inflammations may gain entrance subsequently, or the insects may themselves be the carriers of the dangerous endoparasites, as in the cases of mosquitoes and malaria, fleas and plague, tsetse flies and sleeping sickness. Similarly the wounds caused by small intestinal worms may be in themselves trifling, but afford a means of entrance to microbes. It has been shown, for instance, that there is an association between appendicitis and the presence of small nematodes. The latter wound the coecum and allow the microbes that set up the subsequent inflammation to reach their nidus. It has been suggested that the presence of similar wounding parasites precedes tubercular infection of the gut.
The parasites themselves may cause direct mechanical injury, and such injury is greatly aggravated where active reproduction takes place on or in the host, with larval migrations. A tangled mass of Ascarid worms may occlude the gut; masses of eggs, larvae or adults may block bloodvessels or cause pressure on important nerves. The irritation caused by the movements or the secretions of the parasites may set up a reaction in the tissues of the host leading to abnormal growths (e.g. galls and pearls) or hypertrophies. Migrations of the parasites or larvae may cause serious or fatal damage. The abstraction of food-substances from the tissues of the host may be insignificant even if the parasites are numerous, but it is notable that in many cases the effect is not merely that of causing an extra drain on the food-supply of the host which might be met by increased appetite. The action is frequently selective; particular substances, such as glycogen, are absorbed in quantities, or particular organs are specially attacked, with a consequent overthrow of the metabolic balance. Serious anaemia out of all proportion to the mass of parasites present is frequently produced, and the hosts become weak and fail to thrive. A. Giard has worked out the special case which he has designated as “parasitic castration” and shown to be frequent amongst animal hosts. Sometimes by direct attacks on the primary sexual organs, and sometimes by secondary disturbance of metabolism, the presence of the parasites retards or inhibits sexual maturity, with the result that the secondary sexual characters fail to appear. The most usual and serious effect on their hosts of parasites is, however, the result of toxins liberated by them. (See Parasitic Diseases.)
Finally, the attacks of parasites have led to the development by the hosts of a great series of protective mechanisms. Such adaptations range from the presence of thickened cuticles, and hairs or spines, the discharge of waxy, sticky or slimy secretions, to the most elaborate reactions of the tissues of the host to the toxins liberated by the parasites.
History and Literature of Parasitism.—The history and literature of parasitism are inextricably involved with the history and literature of zoology, botany, medicine and pathology. Pliny recognized the mistletoe as a distinct parasitic plant and gave an account of its reproduction by seed. Until the 18th century little more was done. In 1755 Pfeiffer in his treatise on Fungus melitensis (in Linnaeus’s Amoenitat. acad. Dissert. LXV. vol. iv.) made a group of parasitic flowering plants, but included epiphytes like the ivy. In 1832 A. de Candolle (Physiol. végétale, vol. iii.) attempted to divide and classify flowering parasites on morphological and physiological grounds, and since then, the study of parasitism has been a part of all botanical treatises. With regard to Fungi, A. de Bary’s treatise on the Comparative Morphology and Biology of the Fungi, Mycetozoa and Bacteria (Eng. ed., 1887) remains the standard work. There is in addition a large special literature on bacteriology. With regard to animal parasites, the first real steps in knowledge were the refutation of spontaneous generation (see Biogenesis). Linnaeus traced the descent of the liver fluke of sheep from a free-living stage, and although his particular observations were erroneous, they laid the foundation on which later observers worked, and pointed the way towards discovery of larval migrations and heteroecism. O. Fr. Müller in 1773, and L. H. Bojanus in the beginning of the 19th century reached more nearly to a correct interpretation. J. J. Steenstrup in his famous monograph of which an English edition was published by the Ray Society in 1845 (On the Alternation of Generations, or the Propagation and Development of Animals through Alternate Generations) interpreted many scattered observations by a clear and coherent theory. Thereafter there was a steady and consistent progress, and the literature of animal parasites merges in that of general zoology. The two best-known names are those of T. S. Cobbold (Entozoa: an Introduction to the Study of Helminthology, 1869) and R. Leuckart (The Parasites of Man, Eng. trans., 1886), the former describing a very large number of types, and the latter adding enormously to scientific knowledge of the structure and life-history. Of more modern books, G. Fleming’s Eng. ed. of L. G. Neumann’s Parasites and Parasitic Diseases of the Domesticated Animals, and the Eng. ed. of Max Braun’s Animal Parasites of Man (1906), are the most comprehensive.  (P. C. M.)