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.)
PARASNATH, a hill and place of Jain pilgrimage in British India, in Hazaribagh district, Bengal; 4480 ft. above the sea; 18 m. from Giridih station on the East Indian railway. It derives its name from the last of the twenty-four Jain saints, who is believed to have here attained nirvana or beatific annihilation. It is crowded with temples, some of recent date; and the scruples of the Jains have prevented it from being utilized as a sanatorium, for which purpose it is otherwise well adapted
