Parasitism

PARASITISM Animal Parasitism.

THE problems suggested by the occurrence of parasites not only in the intestines or the kidneys but even in flesh and blood, in eye or brain, have occupied alike physician and naturalist from the earliest times. From ancient Egyptian and Jewish sanitary and religious codes we may perhaps infer considerable knowledge of the distribution and danger of parasites,—unclean animals like the pig, rabbit, and dog being peculiarly infested with them. The schoolmen, too, perplexed themselves with quaint hypotheses as to the time and place and mode of the introduction of the parasites of man, while the long per sistence of mediaeval myths is evidenced by the “Furia infernalis” of the Systema Naturx. The spontaneous generation of parasites seems never to have been doubted until the commencement of the 18th century, when Redi proved the origin of maggots from eggs of the blow-fly, and Swammerdam announced the similar origin of lice and other insect parasites. Both naturalists, however, opposed the extension of their results to the Entozoa, but the discovery of microscopic animalcules, and the reflexion that these must readily be introduced into the body, induced Boerhaave to suggest the origin of parasites from free-living worms and infusorians. The sexuality and characteristics of a few Entozoa gradually became better known, while Linnaeus, though little dreaming of their complex form-history, expelled the spontaneous generation theory by the in-so-far fortunate mistake of identifying the free Bothriocephalus of the stickleback as the young stage of B. latus of man, and certain free Planarians and Nematoids as the young of liver flukes and thread worms. His school vastly increased the hitherto scanty catalogue of known forms, while their exacter knowledge rendered his hypothesis improbable. The origin of Entozoa from eggs which leave the body of their host, enter new hosts in food or drink, and when developing in other organs than the alimentary are carried thither by the circulation, was clearly put forward by Pallas, who also revived the early view of inheritance, which had been propounded before by the contemporaries of Leeuwenhoek (then, however, to avoid the apparently insoluble difficulty of tracing the origin of the parasite from its innumerable yet apparently wasted ova). With the enormous labours of Rudolphi and Bremser hel minthology rose to the rank of an important special study, yet the degeneration of the Linnaean school had nowhere fuller course: observation of faunistic and systematic detail excluded all physiological or morphological research, and the knotty problem of origin was simply cut by a return to the hypothesis of spontaneous generation. This view seemed supported by the absence of reproductive organs in cystic parasites, and reigned almost undisputed until the accumulation of a new chain of evidence. Of this the main links were the discovery of the ciliated larva of a Trematode {Monostomunι) by Mehlis in 1831, of the Redia or C er caria stages of the same genus, and of the six-hooked embryo of Txnia by Siebold in 1835, and the renewed study of Bothriocephalus latus by Eschricht, who maintained that the encysted forms were persistently larval, and that the life history of the Entozoa should be viewed as broadly parallel to that of parasitic insects. Yet in spite of all this, and of the corroborative researches of Valentin, many helminthologists remained obstinate, until these incredible life-histories had been confirmed and treated as so many other cases of the “Alternation of Generations” in the epoch-making work of Steenstrup (1842). Dujardin next observed the wanderings of Mermis, and Siebold those of Gordius·, the latter, however, advanced the doctrine that cysts were not larval stages, but mere pathological modifications of those worms which had chanced to “wander” into situations unfitted for their normal life. Meanwhile were commencing the important labours of Van Beneden, who traced the actual development of the cystic parasites of the bony fishes into the tape-worms of the rays and dogfishes which had devoured them, so proving that the transmission of the parasites depended upon the mode of feeding. These results were soon confirmed by Küchenmeister, who not only transmuted cyst into tapeworm by transmission in food, but redeveloped the cystic form by feeding with eggs from the adult tape-worm, thus (1852-53) commencing the modern era of experimental helminthology. Häubner and Leuckart eagerly followed for the same group; Filippi, Valette, Pagenstecher, and Cobbold .made similar investigations on Trematodes; while Leuckart transferred Pentastomum from rabbit to dog, and traced the formidable Trichina from pig to man. From this time (1860) the advances of our knowledge have been no longer in principle, though numerous and important, but in detail. To Küchenmeister, Cobbold, Davaine, and others, but more especially to Leuckart, we owe valuable general works; to the last the present article is especially indebted.

Any discussion of parasitism with its difficulties and wide theoretic bearings should naturally be preceded by an account of the known facts. This would involve the preparation of two systematic lists,—the first enumerating the parasitic members of each animal group, while the second, from the point of view not of parasites but of hosts, would indicate the forms which are infested, stating by what parasites. Of these lists the following scanty outlines must suffice.

A. List of Parasites. Protozoa (see Protozoa). —Amoeboid organisms are occasionally detected in dysentery and kindred diseases; the best known of these is Amoeba coli. Parasitic Infusoria occur much more frequently: thus in the paunch of sheep and oxen six species (0p½- ryoscolex, Entodinium, lsotrichd) are constant; similarly in the rectum of the frog are invariably present 0palina, Nyctotherus, and Balantidium ; while B. coli, first described from man, inhabits the pig. Trichodina infests Planarians. Flagellate parasites are more numerous: Cercomonas intestinalis is frequently observed in choleraic affections; Trichomonas intestinalis and vaginalis are also described in diseases. In perhaps all invertebrates and cold-blooded vertebrates ciliate and flagellate parasites seem to occur. Acinetae are sometimes parasitic on other Infusoria.

By far the most important group, however, are the exclusively parasitic Gregarinicla. These are very widely distributed among the tissues of invertebrates, especially worms and insects, and their normal life-history is readily observed in the species infesting the tissues of the common earthworm. Their spores or pseudo- navicellae are apparently closely related to the psorosperms frequently detected in both vertebrate and invertebrate tissues, and even in the liver and hair of the human subject. Dicyemida.— This group contains only one entirely parasitic genus, various species of which live in the renal organs of Cephalopoda. The adult consists essentially of a simple sac of finely ciliated ectodermal cells enclosing a single elongated endodermal cell, which discharges nutritive and reproductive functions. Some have attempted to demonstrate a mesoderm. The embryos are of two kinds, nematogenic or vermiform, and rhombogenic or infusiform, differing in origin, structure, and life-history, but of still uncertain relations and import. The infusiform embryo which becomes free is of complicated structure, and probably completes its development in some new host. Some have connected the Dicyemida with such higher forms as the Rotifera or Trematodes, and have regarded the simplicity of the adult as the result of that degeneration which is suggested both by development and habit. Haeckel, while acknowledging degeneration, regards Dicyema as a survivor of the originally simple Gastraeada from which the Metazoa have sprung. Orthonectida. — This group consists of a number of minute parasites, such as Rhopalura, infesting some Nemertines, Turbellaιians, and Ophiuroids. Although moving in linear direction, as their name implies, they exhibit radiate structure. The ciliated and segmented ectoderm encloses an inner endoderm layer and a central cavity which usually contains embryos. They exhibit a well-marked sexual dimorphism, the males being smaller and with fewer segments. Their position is as problematic as that of the Dicyemida ; they may be regarded as degraded forms allied to the Turbcllaria, Trematoda, or Rotatoria, or as survivors of the Gastrseada. Coelenterata.— In this group (see Corals, Hydrozoa), while the fixed forms are frequently indebted for support to other organisms or to each other, and although such associations occasionally seem tolerably constant, true parasitism is remarkably rare. Young Narcomedusae (Cunina) are parasitic within the mouth of Carmarina, and the hydroid Lafoea parasitica grows like ivy on Aglaophenia.

Similar remarks apply to the Mollusca, where, with one or two exceptions {e.g., Entoconcha mirabilis discovered by Johannes Miiller in Synapta, and another Philippine species described by Semper) parasitism is unknown.

Echinodermata.— There are no parasitic Echinoderms.

Vermes.— To this sub-kingdom belong the majority of parasites, but the greater groups are treated in separate articles. See Nematoidea, and for Cestoids and Trematodes see Tapeworm. Acanthocephala.— This group, usually regarded as degenerate from Nematoidea (q.υ.), is represented by various species of Echinorhynchus. These parasites possess a muscular elongated body with a retractile proboscis armed with hooks, which serves to fix the animal to its host. Sense organs, mouth, alimentary canal, and anus are wanting; but the muscles, nerves, and generative organs are well developed. There is a complicated subcutaneous canal system; the sexes are distinct, and the reproduction is viviparous. The embryo, well provided with ensheathing membranes and with hooks, is expelled with the excreta of its vertebrate host and swallowed by some Arthropod, such as Asellus or Gammarus. There a remarkable metamorphosis takes place: the adult is formed within the body of the larva, the skin being the only part of the larva which passes over to the adult. The young Echinorhynchus finally passes with its invertebrate host into the alimentary canal of some vertebrate, e.g., fish or bird or even pig, and there attains sexual maturity.

Rotatoria.— Such forms as Albertia, found externally on certain worms {Nais, &c.), and Balatro inside the same, are distinctly parasitic, and are not improbably differentiations of the same form.

Among the Nemerteans various parasites occur, such as Ponto-l·della, Branchellion, Piscicola, found especially on fish. The Chaetopoda are never parasitic, and but rarely commensal. The Myzostomata are probably, however, degenerate Chaetopods, represented by the genus Myzostoma living ectoparasitically on Crinoids.

Crustacea.— This group includes an immense number of forms in varying degree parasitic. The Copepoda include all grades from free-living forms to such degenerated parasites as Achtheres, Lernaea, Chondracanthus, and Argulus. Many Entomostraca are parasitic, and among the Isopoda we find such forms as Boρyrus and Cryptoniscus. Among the Cirripedia again are various grades of parasitism from some of the Lepadidae to the ne plus ultra of degeneration —the Rhizocephala. Insecta. — Insects furnish a large proportion of ectoparasites, but comparatively few endoparasites, for very obvious reasons. The Strepsiptera, parasitic on bees, the ichneumon-flies, Platygaster, and allied Hymenopterous forms, the Pediculinae {Hemiptera) and the Mallophaga are the more important parasites. Many of the other groups also include parasitic members. See Insects. Arachnida. — The majority of Acarina (see Mite) are parasitic, and there are many other Arachnida of similar habit. To the Arachnida the Pycnogonida and the Pentastomida are often referred. The former are parasitic in their youth at least on Hydroids. Pentastomum exhibits considerable divergence from the Arachnoid type, and has a life-history closely parallel to that of the Cestoids. The adult form is found in the frontal sinus of the dog or wolf; the embryos pass through the nose to the exterior, and if eaten by a hare or rabbit lose their investment, penetrate to the liver, encyst, and pass through a complicated series of changes, finally attaining maturity and sexuality when the flesh of the rodent is eaten by the original host.

Vertebrata. — The Vertebrata are rarely parasitic. The best case of incipient parasitism is that of Myxinc, which burrows into the codfish. With this may be compared the well-known Remora, which attaches itself externally to sharks, &c. Commensalism is, however, more common, many small Teleosteans living with Medusae, sea anemones, and such like. Fierasfer finds a lodgment inside the respiratory tree of Holothurians; and Semper describes a Philippine species which actually devours the viscera of its Holothurian host.

B. Distribution of Parasites and List of Hosts. Protozoa are of course rarely infested; Coelenterata also rarely; species of Distomum have been taken on Physophora, Velella, Pelagia, Beroe, and Cestum ; a scolex and a nematoid have been described from Ctenophores, while various Arthropods occur ectoparasitically. Echinoderms are also very free from parasites; on Echinus, however, despite its pedicellariae, occur occasionally the semi-planariform Trematode Syndesmis, and the molluscs Stylifer, Anaplodium, and Eulima (the latter occurs also on starfishes). The Comatulae of all seas bear Myzostoma. Holothurians from the Pacific occasionally contain crustaceans, such as the crab Pinnotheres, and several Copepods. Their respiratory tree lodges Fierasfer, while Synapta contains the mollusc Entoconcha mirabilis. Mollusca are more largely infested. Pinnotheres and other Crustacea frequently inhabit the mantle cavity of marine Lamellibranchs, as the Arachnid Atax does the fresh-water mussel. The Lamellibranchs also have their peculiar Trematodes like Aspido∙ gaster and Bucephalus, besides Cercariae, from which probably few Gasteropods, whether marine, fresh water, or terrestrial, are ever free. The Cephalopods not only contain certain Dicyemae in their renal organs, but through their piscivorous habits acquire Tetra-rhynchus and Ascarids. Among the Chaetopods not only are Protozoan parasites frequent, but parasitic worms are occasionally described.

Crustaceans frequently contain Gregarines; and a few Cestoids, Trematodes, and Nematoids (and Branchiobdella y ) have been described, as well as the cystic Echinorhynchus, from Gammarus pulex. More formidable, however, are the Copepods, like the familiar Nicothoe of the lobster's gills; and, worst of all, the Rhizocephala, like Peltogaster and Sacculina of the hermit and shore crab respectively.

Centipedes often contain Nematoids, and spiders Mermis and Gordius. Insects are preyed upon by ichneumons, are largely plagued by ticks externally, and internally by Gregarines and worms, most frequently Gordius and Mermis ; but also by larval Hymenoptera of many families, by certain Diptera, and by the Strepsiptera. See Insects.

The Tunicata harbour many crustaceans, &c., chiefly in the test.

It is among vertebrates, however, that parasitism is most frequent and most fatal. Fishes swarm externally with Trematodes, leeches, and parasitic crustaceans, internally with cysts and intestinal worms all too numerous for enumeration. Nothing gives a more vivid idea of the extent to which parasitism has reached than an examination of a ray, or even better, the common sunfish (f)rthagoriscus'). Amphibians are inhabited by many parasites,—the common frog having almost constantly Ascaris nigrovenosa in its lungs, and infusorial parasites in its rectum, and may also yield Distomum, Echinorhynchus, &c., twenty species in all. Lizards harbour tapeworms, Nematoids, including species of Trichina, more rarely Trematodes. Ophidians have all kinds of parasitic worms, Chelonians chiefly Nematoids and Trematodes. The parasites of birds are of extraordinary number and variety; preying, fishing, and omnivorous birds serve, of course, very constantly as intermediate hosts; but graminivorous birds are hardly more exempt. The number of parasites is often so vast as to occasion the most serious disease; thus the “gapes” of poultry is due to the choking of the bronchial passages by multitudes of Nematoids (Sclerostoma syngamus), and the grouse disease to a similar cause (Strongylus pergracilis).

Yet a great number of parasites may be borne without apparent injury: thus the post-mortem examination of a single stork has yielded twenty four Filaria and sixteen Strongylus from the lungs and air passages, one hundred Spiroptera from the coats of the stomach, more than a hundred of various species of Distomum, and many hundreds oiHolostomum from the gullet and intestine. Ticks and insect parasites are also common; of these the most remarkable are the feather-eating Mallophaga. The majority of the Mammalia have as internal parasites many different species of worms either in adult or cystic form, which are fully described in veterinary works. The special parasites of man are estimated by Cobbold at as many as 121 species (13 Trematodes, 16 Cestodes, 21 Nematoids, 10 Leeches, 17 Arachnids, 44 Insects); many of these, especially among insects, have occurred only very rarely, and should not be reckoned, e.g., Musca vomitoria and Blaps mortisaga, while a considerable number of the truly parasitic forms have only been once or twice described,—the above estimate thus becoming reduced well-nigh to half. Taxonomy.— Far then from there being, as was formerly thought, one great group of Entozoa by itself, we have seen that most invertebrate groups have their parasitic members and exhibit transitions or grades connecting these with free-living forms. The systematic position of many parasitic species is, however, not yet clear, many have been named by accident or according to habitat, and great concentration seems necessary. It is, for example, extremely probable that a careful systematic study of genera like Gordius, Distomum, and Tetrarhynckus, of which innumerable species have been described from as many different hosts, would result in proving the identity of many forms described as distinct, and that experiment would show that many of the forms still apparently specifically distinct are really only individuals of the same species more or less modified by the host upon whom the lotterv of nature has chanced to quarter them.

With the increasing completeness of our knowledge of parasitic forms the transitions from free to parasitic species are becoming more prominent, and the relationships of the parasitic to the non-parasitic groups more definite. Among the Nematoidea, for example, as Leuckart indicates, we are able to construct a series, starting from free-living forms, and through such cases as Leptodera (a Rhabditis like form, sometimes free, sometimes parasitic), thence to parasitic Nematodes hardly to be distinguished from their free-living relations, but passing gradually through 0xyuris, Trichocephalus, Spiroptera, <fcc., to such highly parasitic forms as Trichina, where all relation to the outer world is lost. The Acardkocephala Leuckart has taught us to regard as Nematodes highly modified by parasitism, and he points out how Gordius, with its atrophied alimentary canal, terminal position of female reproductive organs, and other persistent and embryonic characters in which it differs from the typical Nematoid, really leads up to Echinorhynchus. As Echinorhynchus is related to the Nematodes, so are the Cestoids to the Trematodes. The close alliance suggested by numerous points of anatomical correspondence, and by the close parallelism in life-history, is corroborated by such intermediate forms as Caryophyllaeus and Amphilina, from which we pass with ever-increasing parasitic adaptation through the Ligulidx to Bothriocephalus and Taenia. Leuckart further points out how closely the Trematodes are united by intermediate forms to the Planarians. The affinities of Myzostoma and Pentastomum are not yet precisely determined, though the former is most plausibly regarded as a degenerate Chaetopod and the latter as similarly degenerated from some low Arachnid or at least Arthropod type. In the Copepoda, Cirripedia, and other crustaceans all degrees in intimacy of association may be observed, making the relations of the parasitic to the free forms sufficiently obvious. Everywhere, in short, we find a morphological and physiological gradation from free to parasitic forms.

Nature and Degree of Parasitism—Commensalism.— From the foregoing necessarily much abbreviated lists we observe not only the enormously wide prevalence of parasitism—the number of parasitic individuals, if not indeed that of species, probably exceeding that of non-parasitic forms—but its very considerable variety in degree and detail. The majority indeed derive their main support from their host, but of these some are free, wandering about from animal to animal, some are attached permanently to the exterior of their victim, while others again are concealed within its body. In some cases the parasitism is only temporary, with others it is a life-long habit. The majority are free in their youth, while some pass their early life as parasites, becoming free in their mature state, and others again spend their whole life on their host.

In some cases there is the very slightest association; every student of marine forms is familiar with the complex incrustations and intergrowths of sessile forms, and has seen how almost any surface or cranny may afford a lodgment. Parasitism for support is not infrequent; it may be temporary or permanent; in the former case it is useful in diffusion,—the glochidium-larva of the freshwater mussel, for example, being transported on the fins of fishes. From cases like those of many Cirripedes, which occur indifferently on rocks or on animals, we pass readily to permanent associations like that of Loxosoma on the posterior end of Phascolosoma. Vague and loose associations, if useful to one or both participants, may become perpetuated by natural selection. Thus sea anemones may settle on any surface,—occasionally therefore on shells inhabited by hermit crabs; hence have arisen permanent associations. Of these there are many familiar instances, such as the hermit crabs bearing Sagartia parasitica (fig. 1), or having their shell-mouth enveloped by Adamsia. One of the quaintest instances is a lately described species of crab which wields an anemone firmly grasped in either claw. In such cases the association is obviously useful: the crab is protected from the octopus and other enemies by the nematocysts of its comrade, which also aid in holding the prey, while the Actinia too gains its share of the food, and vicariously acquires means of locomotion. To such cases where two animals are associated together for mutual support and advantage the term “Commensalism” is applied. In the struggle for existence increased complexity of needs, and difficulty in satisfying them, evokes in the individual organism a certain specialization of function and consequent differentiation of structure. Similar causes result not so much in the differentiation of each individual of a species as in the specialization of certain individuals for certain specific functions, resulting again in that specialization of structure which is called polymorphism. Thus in a Hydractinia or Siphonophore colony many different individuals of the same species have been specialized in each to perform a certain function. The same purpose is served by those associations, not of individuals of the same species, but of two individuals of different species, united as we have just seen for mutual advantage, and each working out some definite part of the common life-problem. Just as polymorphism in the same species is physiologically equivalent to differentiation in the individual organism, so is commensalism between different species the physiological equivalent of polymorphism in a single species.

But cases of co-operation on equal terms are rare; size constitutes the most frequent disparity, and the smaller tends to become first wholly dependent upon the other for support, then for concealment, and finally perhaps for sustenance. The reverse may occasionally occur, the weaker being utilized for the purposes of the stronger; thus a species of Dromia adapts a colony of sponge or ascidian as a removable upper garment for concealment. Parasitism within the same Species.— In some cases even within the morphological unity of the species a physiological relation is established analogous to commensalism if not to parasitism. Thus in Boneilia the diminutive and degenerate male lives in the uterus of the female, in Trichosomum crassicauda of the rat three or four male are found within the spermatheca of the female, while in Bilharzia the incipient reciprocal of these cases is found, the male being host. Many of the most remarkable cases are also afforded by the Cirripedia, in which a female may bear males in various states of dependence and degeneration. In viviparous animals a certain absorption by the young from the tissues of the parent can hardly avoid taking place; this is therefore so far an analogy to endoparasitism. This advantage is clearly retained and developed if absorption take place by an organ specialized for the purpose. Thus in the well-known shark Mustel·us laevis the young are attached to the oviduct by a placenta developed on the yolk sac; and the like arrangement, though morphologically different, is physiologically the same among the Mammalia. Hyper parasitism.— Not only are very few animals altogether free from parasites, but even parasites themselves find their nemesis in being themselves infested by lesser parasites, though not “ad infinitum.” Thus Leuckart mentions that water-lice and thread-worms are found on parasitic crustaceans, and the endoparasitic larvae of some Hymenoptera are themselves preyed upon by other larvae (Pteromalinaf). Nematodes are found in Nicothoe, and associated with Sacculina are frequently found two other crustacean parasites, one of which, after destroying the greater part of its host, continues to subsist upon the nourishment afforded by its root-like processes which survive the operation.

Classification. — Some classification of these various parasitic forms is necessary. Van Beneden introduced the useful term commensals or messmates, under which he includes (1) oikosites or fixed and (2) coinosites or free partners. These he distinguishes not only from parasites but from mutualists where two species are associated, but neither share a common food nor does one prey on the other. Parasites he divides according to the duration of their state of attachment to a host, distinguishing (1) those which are free all their life (leeches, bugs, fleas, &c. ); (2) those free as adults but parasitic when young (ichneumon, Mermis, &c. ); (3) those free only in youth, and attaining their adult form either directly in the first host entered, or only after a migration from one host to another (most parasitic worms); (4) those which pass all phases of their life on or in their host, c.g., Strepsiptera, Tristomum, &c. In this classification there is no attempt to define the degree of dependence or the closeness of the association, except in the general distinction between parasites and commensals; the group of mutualists is entirely superfluous and confused, no clear definition being given, and in the examples of the various groups the limits of his own definitions are not adhered to.

Leuckart distinguishes parasites as ecto-and endo-parasitic, and divides the former into temporary and permanent. Endoparasites he divides according to the nature and duration of their strictly parasitic life. (1) Some have free-living and self-supporting embryos which become sexually mature either in their freedom or only after assuming the parasitic habit. (2) Others have embryos which, without having a strictly free life, yet pass through a period of active or passive wandering, living for a while in an intermediate host. They may either (a) escape to pass their adult life in freedom (Archigetes and Aspidogaster), or (ft) they may become sexual, or (c) they may bore their way to another part of the body (Trichina), or (d) most frequently they pass to their final host either directly when their intermediate host is devoured as food, or indirectly seeking for themselves another intermediate host, or producing asexual forms which do so (Trematodes and Cestoids). (3) Others again have no free-living or even migratory embryonic stage, but pass through their complete life-cycle in one host (Trichocephalus, Oxyuris, &c. ). This somewhat detailed classification has at least the advantage of clearness, and of showing to some extent the various degrees of parasitism.

Rossmann has proposed a more physiological classification dealing with the organization and habit of the parasite. This he has applied to the Crustacea :—I. Diosmotici, or vegetative without independent digestive organs, e.g., Rhizocephala; II. Digestorii, with independent digestive system, and including (1) Sedentarii, Copepod(i ateletmeta, Bopyridae, Euboniscidae, Cryptoniscidae ; (2) Vagantes, Copepoda holotmeta, Branchiura, Cymothoidae. The great variety of details, however, makes it almost impossible to establish any logically accurate division. Any strict classification of such a variety of organisms having only in common the physiological correspondence of their mode of life is almost impossible, and the most that can be done is to point out the existence of series of adaptations varying with the intimacy and constancy of the association and the degree of dependence.

Origin of Parasitism and Transmission of Parasites.— With the dismissal of the theory of generatio aequivoca, the question of the origin of parasites is limited to the discussion of the causes which might induce such a change of habit and environment. There are obviously many opportunities for one animal either in adult or larval state being swallowed by another in food or drink, in which case, if the environment were not too utterly different from that previously enjoyed, parasitism might arise in a purely unconscious way. It is again easily conceivable that animals which have sought a host for temporary protection from climate or enemies, or for safety and seclusion in the bearing and breeding of the young, might, finding the environment congenial and a supply of food at hand, remain there during a large portion of their life. It is worth noticing, as corroboratory of the idea that the host was in many cases resorted to primarily as a sort of maternity asylum, that we find many parasitic females with free males, e.g., Nicothoe. Given an animal with a carnivorous habit, it is intelligible enough that during a period of scarcity of food or of extreme pressure from enemies, various methods of solving the problem of life would be attempted, the successful results of which in a few cases persist especially in ectoparasitism, not the least obvious mode of retaliation on stronger foes. The degree of the parasitism is, as we have seen, not of primary moment, and its intimacy may be increased. There are naturally some physiological limits of respiration, &c., determining the possibilities of parasitism—air-breathing insects are found on land animals or at most on some amphibian forms, water-breathing Arthropods on water-breathers, waterbreathing worms only in the interior of land animals; but even these limits may be overstepped by adaptation when, for example, the respiration becomes cutaneous in Pentastomum, Sarcoptes, &c.

The various modes of transmission of parasites, though of great practical importance, do not call for much discussion here. They may be summarized as follows after Leuckart :—(1) the majority of parasites reach their hosts through the medium of food or drink; (2) eggs are in some cases transferred from one animal to another by actual bodily contact, e.g., the eggs of Pentastomum by the licking of dogs; (3) sometimes the eggs are deposited in or on the host by the mother, for example, by insect parasites, such as Ichneumons, Oestridae, &c.; (4) in some rare cases parasites are transmitted by self-infection,—for example, young Trichinae, born free in the alimentary canal of their host, bore their way thence directly into the muscles, there to grow into the well-known encapsuled worms. Eggs or proglottides of tape-worm may, on gaining the exterior, be transmitted inadvertently to the mouth, and so recommence their life-cycle within the same host.

The mode of diffusion of the ova of parasites presents many analogies to that of seeds in the vegetable kingdom: thus wind and water are alike utilized, passing animals may serve as unconscious bearers, and the like. Though well protected by a usually thickened egg-shell and an often remarkable degree of vitality, so as to resist prolonged drought, burial, and other vicissitudes, the parasite has an exceedingly small chance of success in finding a host; to preserve the species from extinction an enormous number of eggs must be produced, far exceeding that of free-living organisms. Thus Leuckart points out that as a tapeworm has an average lifetime of two years, and produces in that time about 1500 proglottides, each containing say 57,000 ova, and since the species is not increasing in numbers, an ovum has thus only one chance in 85,000,000 of reaching maturity. The difficulties are of course increasingly greater as the life-history becomes more complicated, demanding an increasing number of hosts. Given a sufficient number of eggs, however, no difficulty is insuperable, and few parasitic forms accordingly seem in any risk of disappearance, except, it is to be hoped, in the case of civilized man and the domestic animals, where the large consumption of cooked food, aided by conscious hygienic precautions and medical aid, tends to exclude or remove them.

Effects of Parasitic Life on Parasites.— So far from treating the phenomena of parasitic life as highly aberrant, and the peculiarities of parasitic form as differentiations sui generis, it becomes evident that we have to do with only one of the many cases in which the influence of environment on organism is clearly marked. The aetiology of parasitism is only a fraction of a vaster general question; and we shall never fully understand the adaptation of the parasite to its host until the relation of environment to organism has been far more profoundly analysed and completely experimented on—inquiries which have only recently begun to be seriously set on foot. The most cursory consideration of the action of environment shows how profoundly it determines form; of this ho better examples can be found than those furnished by the habit of plants. It is easy to see how submerged leaves must become dissected, or desert plants tend to become succulent; how evergreens are only possible in certain conditions of climate, or thorns are only useful where herbivorous mammals abound. In the same way we can broadly see that the conditions of life profoundly influence animal form. Before considering how the abnormal parasitic environment affects the parasite, we should know how the normal environment affects the non-parasite, and how the two cases differ. The environment thus needs analysis into its factors, the organism similarly into its constituent systems of organs; and the influence of any factor of the environment upon each system and organ demands determination, species by species, before safe and exhaustive generalizations can be obtained. Pending these inquiries, which are destined to take so large a place in the biology of the future, and within the present narrow limits, only the merest outline can be attempted.

Morphological science has but slowly and with difficulty disentangled itself from the primitive classifications of plants and animals by habit and resemblances of external form; the physiologist, however, needs to reassert the claims of these and develop them in detail; as for the child so for him whales are in a sense fishes, and bats birds,—just as the swimming organs of the former, like those of the penguin or cuttlefish, are all fins, or the flying organs of the latter and those of insects are wings alike. Such considerations show too the first importance of the mechanical conditions, primarily those of locomotion or rest, and whether in water, or land, or in air, since these determine, not only external form, but muscular and skeletal disposition and structure. These determined, conditions of heat and light play an obvious part; copious supplies of heat energy to the organism have a distinct result in stimulating plant growth, and accelerating that of animals; light too, a primal necessity for green plants, has also the most marked effect on animals, which develop tracts of absorbent pigments in its presence, these becoming locally evolved for perception into eyes; while in relation to sound-vibrations and impressions of contact other sense organs develop. Quantity of food has its influence mainly on size, but nature of food and mode of feeding demand many appropriate specializations of details of form. Expressing the same adaptations from the other point of view, that of the organism, we see how not only the general form but the integument with its colour and texture, and also the respiratory and alimentary organs, are necessarily fitted to avail themselves of the given conditions; how the circulatory and how the reproductive systems must comply; how the sensory organs must take note more and more of the changes in the environment; and how the whole series of complex adaptations demands a similarly complex internal mechanism for their co-ordination through the nervous system.

From the slightest analysis then of the relation of organism to environment, the theory of evolution might almost have been predicted, since, if the details of environment and organism be indeed obviously and precisely adapted one to another, change in the former must either be followed by the extinction of the latter or its modification in the requisite details. To explain the modus operandi of change in the organism, we have mainly to bear in mind Dohrn’s admirably expressed “principle of functional change,”—the simple conception that any living tissue, however specialized, still retains traces of all the functions of living protoplasm, and that any one of these traces may be indefinitely increased by favourable conditions, and the specialized function similarly reduced to a trace. Along with this, or rather as a corollary of it, comes the conception of economy of unused structure; our notions of specialization become henceforth associated with a corresponding possibility of simplification, and our idea of progress must be for ever accompanied by the corresponding possibility of degeneration.

The conditions of parasitic life are readily seen to differ primarily from those of independent organisms in negative characters, i.e., in the simplification of the factors of the environment; let us therefore .briefly consider the results of such progressive simplification upon organisms in general. Let the mechanical conditions be simplified by the cessation of active movement; the specialized body-form necessary for locomotion then becomes unnecessary; locomotor muscles and their skeletal attachments are simplified or disappear; organs of sense are far less needed; and nervous adaptations and structures become correspondingly reduced. In all these respects then sessile parasites simply agree with other sessile animals. Again, let us simplify the environment by the deprivation of light; eyes and pigment are useless, and our organism, whether cave-dwelling insect or crustacean or internal parasite, becomes blanched and blind; and similarly with other senses. Or let us subtract as far as possible the element of danger from other animals by special protection or concealment in one of the “nooks of life”; here again for shelled mollusc, sand buried Amphioxus, or hidden parasite the diminished need of nervous adaptations is a similar degenerative factor. Let food become abundant, the same nervous economy follows; let it be highly nutritive, and digestive structures and functions may be simplified; thus the examples of progressive degeneration of the alimentary system up to its complete replacement by superficial absorption, afforded by various parasitic series, are natural enough. The soft integument unprotected and blanched, the reduced muscular activities, the simple or absent alimentary tube, the reduced circulatory and respiratory organs consequent upon diminished waste and softened integument, are all intelligible enough, as also is the increase in reproductive activity demanded by increased risk of failure to find the appropriate conditions. The few adaptive conditions are readily understood: given the continuous application of a flat muscular surface to resist detachment from the host, and atmospheric pressure helps the development of the sucker; given either a clutching limb or a portion of the body-wall thrust for support into the host, and the mechanical conditions aid the differentiation of a hook; here, if anywhere, function in fact may be said to make the organ, and such curious resemblances of superficial form as those between say a gregarine, a tapeworm, and an Eeh,inorhynchus are not hard to explain.

Further details of the process of retrograde metamorphosis and of the enormously important phenomena of degeneration cannot here be attempted; it must suffice if the general dependence of such changes upon simplification of environment—freedom from danger, abundant alimentation and complete repose, &c. (in short, the conditions commonly considered those of complete material wellbeing)—has been rendered clearer, and if the phenomena of parasitism, however apparently aberrant, become intelligible as new evidences of the unity of organic nature. Effects of Parasite on Host.— As the result of the association of two organisms with more or less constancy, various mutual modifications of form and function must obviously occur. The more important effects of parasite on host may be briefly outlined. Semper cites numerous cases where the commensal or parasite has a mechanically trans forming effect on the host. Thus a horny coral with which an annelid is constantly associated has become permanently modified to form an encasing tube. Worms inside corals have enlarged the base of the cavity by stimulating growth, and may also produce permanent pores. Pycnogonids on Campanularia produce galls, which acquire specific characters, and various species of crab parasitic on corals form galls, two of which coalescing, form a sort of “cave dwelling” with two fissures which are kept open by the respiratory currents of the crab, which thus both stimulates and checks the growth of the polyps. In higher animals, and with more intimate parasitism, the mechanical influences of the parasite on the host are more serious and more markedly pathological. Thus parasitic worms, by their size and number, frequently close up passages such as arteries, windpipe, àc., causing often fatal results. But many parasites are also actively destructive to certain tissues of their host—thus, as Semper points out, Peltogaster destroys the female reproductive organsof Pagurus, a Trematode those of Limnaea stagnalis, the larva of a fly (Cuterebra emasculator) the testes of various species of American squirrel. In none of these cases, however, is the general vitality of the host affected. The results of active motion within the host are productive of still more serious mischief; thus the internal migration and burrowing of such parasites as Trichina and Bilharzia is well known to produce violent inflammation. The perforation of vessels, the consequent extravasation of blood, and the destruction of tissue often end fatally for the host. Leuckart distinguishes pathological effects as due either to growth and increase of parasites, or to their wanderings within the host, or thirdly to the very considerable loss of nourishment which a number of parasites of appreciable size necessarily entails. Some blood-sucking parasites are specially dangerous, and many less ferocious forms doubtless poison their host to some extent by their waste products. Roux also notes how parasites—an Echinococcus, for example—by inducing a flow of nutritive material, may develop a net work of capillaries and produce other histological changes

It is probable that many of the most remarkable integumentary specializations of the animal kingdom are defences against parasites (somewhat as the stings or thorns which protect foliage, or the hairs which keep ants from flowers); thus the nematocysts of coelenterates, the molluscan shell or the crustacean mail, the vigilant pedicellariae of the echinoderm, or the scales of the fish are alike largely specialized as defences against the never-ceasing attacks of swarms of larval parasites, eagerly struggling to gain entrance or footing anywhere

The history of the medical aspects of parasitism can only be very briefly alluded to. From the time of the ancient Arabian physicians some diseases, such as itch, have been referred to parasites. With the increasing knowledge as to the prevalence and importance of parasitism there arose a distinct parasitic theory of disease, and in the 17th and 18th centuries such questions were discussed as “an mors naturalis sit substantia verminosa.” In spite of the gradual unravelling of the mysteries of origin and lifehistory, physicians long clung conservatively to the old hypothesis of spontaneous generation, even Bremser regarding the pathological states of the host not as caused by the parasites, but as causing and in fact creating them. It was not till within the last thirty years that, with the rise of experimental helminthology, medical science shook itself free from superstition and ignorance, and devoted close attention to aetiology and treatment, culminating in that systematic warfare against all forms of parasitism which now occupies so important a place in medicine and the veterinary art (see p. 269 infra and Veterinary Science). (p. ge.) Vegetable Parasitism.

The name of parasites has been given to those plants which are nourished wholly or partially at the expense of other living organisms. The degree and nature of the benefit thus obtained varies greatly with different plants, and the effect produced upon the host ranges from an almost imperceptible one to complete destruction. At one extreme are certain forms which, while drawing the nourishment necessary for life from their hosts, yet do so in such fashion that both organisms continue to live in intimate association, and, it may be, rendering mutual help. From these by a series of gradations we come to parasites of such destructive influence as to cause widespread death to certain animal and vegetable forms of life. This physiological group is closely related to another, the saprophytes, which obtain their nourishment from the dead remains of organisms. True parasites belong exclusively to the dicotyledonous flowering plants and the fungi. A few algae are partial parasites.

The remarkable appearance presented by most parasitic flowering plants undoubtedly attracted notice in remote times. They are frequently mentioned by early writers, but there is no evidence sufficient to enable us to determine whether they were regarded as independent plants or merely as pathological excrescences—unless in the one case of the mistletoe, which was recognized as the former by Pliny, who gives an account of its reproduction by seed. The effects of the attack of parasitic fungi were also observed in very early times, as there is abundant evidence to show, but the plants themselves which caused the damage were necessarily not detected as such from their minute size and obscure nature. We must come to the middle of the 18th century for the first attempt to establish a botanical group of flowering parasites. Pfeiffer, in his treatise on the Fungus melitensis (Cynomorium coccineum), divides all flowering parasites into three groups, according as they infested the whole plant or attacked but one place or were confined to the root; but he includes many epiphytes, such as ivy, lichens, &c. After this remarkable classification a knowledge of native and exotic forms grew up, and nothing noteworthy occurred in the history of the subject until the end of last century and beginning of the present one, when there was a relapse to the old theory that parasites were no more than degenerate outgrowths from their hosts. For example, Meyen attempted to account on anatomical grounds for the existence of Lathraea squamaria on its host, and more absurdly still, Trattinick, in a letter to Schlechtendal, gave a short list of plants to which parasites bear a very superficial resemblance, and gravely affirmed his belief that the latter are but specific degenerations of these. Thus he contended that Balanophora is but an Arum, C'y tinus a Cotyledon, Raffl,esia, a cabbage, &c. De Candolle made the first genuine attempt in 1832 to establish a classification of parasites on morphological and physiological grounds; Unger followed in 1840 with a purely morphological arrangement, and, though he advanced matters considerably, his treatise contains much speculation not borne out by facts. Martins’s classification of about the same time is on much the same lines as De Candolle’s. The knowledge of parasitic fungi has advanced gradually with the improvement of the microscope, and the accumulation of the life-histories of forms has grown up under the hands of numerous observers, among the earliest of whom Knight performed admirable service. With increasing knowledge of native and exotic forms, and the advance made in the fields of vegetable anatomy and physiology, the whole group of vegetable parasites has become more strictly defined,—the last noteworthy service being the establishment by De Bary {Morph, u. Physiol, der Pilze, Flechten u. Myxomyceten) of the physiological group of “saprophytes” to receive those plants which differ from the parasites in obtaining their nourishment from the dead bodies of organisms and from soil rich in humus. Phanerogamia. —The parasitic flowering plants are exclusively dicotyledons confined to natural orders falling under the two divisions of Gamopetalae and Monochlamydeae. Among the Gamopétales there are the (Monolropese↑) Lennoaceae, Cuscuteae (Convolvulacées), certain genera of Scrophulariaceae (such as Rhinanthus, Melampyrum, Euphrasia, and Pedicularis), and the Orobancheae. Among the Monochlamydeae there are the Cytinaceae, Cassytha (Laurineae), Loranthaceae, Santalaceae, and Balanophoraceae. The vegetative bodies of these exhibit various degrees of degradation, and this process may go so far that, excepting the parts concerned in reproduction, not only the external appearance but the whole structure of the tissues characteristic of a vascular plant may be lost to the parasite. The roots in particular undergo considerable change of form and structure in adaptation to their peculiar function, and the typical root of a vascular plant may lose all its characteristics, retaining only its physiological properties. A degraded root or part of a root so adapted is termed a haustorium, and the mistletoe, dodder, Thesium, Balano- ])hora, and Rafflesia exhibit such in various degrees of removal from the true type.

The arrangement of the orders as follows is that adopted in systematic botany. Their physiological relations will be afterwards indicated.

The Monotropeae, which are allied to the heaths, possess no chlorophyll and only small scale-like leaves. Monotropa, which may be taken as a type of the group, undoubtedly subsists as a saprophyte on organic matter derived from the soil. There has been some controversy as to the parasitism of these plants. Perhaps the strongest evidence in its favour was offered by Drude, who stated that he found a parasitic connexion between Monotropa and the roots of Abies excelsa. Monotropa was then generally regarded as both parasite and saprophyte. More recently, however, Kamienski has denied the accuracy of Drude’s interpretation of the case, and, affirming that Monotropa possesses no haustoria, upholds the view that it is no true parasite. Upon the evidence it may be taken that no case has yet been satisfactorily made out for the parasitism of this group. The suborder consists of ten or twelve species included in nine genera occurring in north temperate regions. Monotropa Hypopitys, L., is distributed through Europe; var. glabra, Roth, mostly among deciduous trees; and var. hirsuta, Roth, commonly among conifers.

The Lennoaceae are a very small order confined to Mexico and California. They are succulent herbs with simple or slightly branched stems bearing small scale-like leaves, and resemble in general habit the Monotropeae, to which they are allied. They possess no chlorophyll, and are probably always parasitic.

The Cuscutaceae (Dodders) are a suborder of Convolvulaceae, and are distinguished by their fibrous, climbing stems bearing very small scale-like leaves. They are entirely without chlorophyll, and are true parasites. The gtoup consists of annual plants reproduced each year from their seed, which commonly ripens about the same time as that of the host plants. The seeds of host and parasite are frequently found mixed, and it consequently happens that they are sown together. When the seed of the dodder germinates it pushes up its stem, which meeting with the stem of the host plant develops a papilla-like body at the point of contact. From the papilla there proceeds the true haustorium, which penetrates the tissues of the host as far as the vascular system, where it expands slightly and terminates in a broad surface. The haustorium is furnished with a central vascular bundle originating in the vascular system of the dodder stem. When this haustorium has been developed the root of the dodder dies off and all connexion with the soil ceases, while the stem above the haustorium continues to wind round its host, producing fresh haustoria at short intervals, and gradually enveloping and destroying the plant. The influences exerted are of two kinds :—(1) a truly parasitic influence, since the dodder, possessing neither connexion with the soil nor chlorophyll, obtains all its nourishment from its host by the action of its haustoria; and (2) a mechanical influence, in depriving its host of air, light, &c., and preventing the development of branches, leaves, &c. (see fig. 2). The commonest species are Cuscuta Epithymum, Murr., distributed throughout Europe, growing on Thymus Serpyllum, Calluna vulgaris, Genista, &c.; var. trifolii on clover, to which crop it is enormously destructive; C. europaea, L., occurring throughout Europe on hops, vines, &c., and C. Epilinum, Weihe, commonly found throughout Europe growing on flax.

There are at least five genera of Scrophulariaceae which are partially parasitic, viz., Ehinanthus, Melampyrum, Pedicularis, Euphrasia, and Striga. They all contain chlorophyll, and possess true roots on which small haustoria are developed. Euphrasia, occurring in both north and south temperate regions, is partially parasitic on roots of grasses. Pedicularis is common in alpine and arctic regions of the northern hemisphere, Melampyrum and Ehinanthus in the north temperate zone, and Striga is a native of Asia, Africa, and Australia. The last possesses perhaps more distinctly parasitic habits than the others—though the cultivation experiments of Decaisne, Cornu, and others tend to show that parasitism is necessary in the cases of Melampyrum, Ehinanthus, and Euphrasia.

The Orobancheae (Broomrapes) possess erect, simple or little-branched stems bearing numerous scale-like leaves, and are variously coloured, but destitute of chlorophyll. They are parasitic on the roots of many different herbs and shrubs by means of their haustoria, which penetrate to the vascular system of the host. They attach themselves thus immediately after germination. There are about one hundred and fifty so-called species of Orobanche, of which the following.are perhaps best known:— Orobanche rubens, Wallr., parasitic on and very destructive to lucerne; 0. minor, Sutt., on red clover; 0. major, L., which attains a height of 2 feet on roots of furze and other leguminous plants; and 0. Eapum, Thuill. Phelipaea ramosa, Mey., attacks particularly hemp and tobacco. Lathraea, which according to Solms Laubach belongs to Scrophulariaceae, is parasitic on the roots of trees such as hazels.

The Cytinaceae are a very remarkable order of truly parasitic plants which are wholly destitute of chlorophyll, and of a very degraded structure. Cytinus possesses a scaly stem bearing sessile flowers, while Eafflesia and Brugmansia consists one may say of a single flower, measuring in the case of Eafflesia as much as 3 feet across. These flowers appear first in the form of knobs emerging from the host plant, and before expanding resemble an unopened cabbage. They remain expanded only for a few days, when putrefaction begins and a smell as of putrescent flesh is emitted, serving thus to attract insects which probably aid in effecting fertilization, since the stamens are in different flowers. There are about twenty-four species in the order, and these are mostly tropical. Cytinus Hypocistis, L., which is parasitic on the roots of Cissus, occurs in southern Europe. Eafflesia and Brugmansia are limited to the Malay Islands, and Sapria has a wider distribution throughout the same region. Eafflesia is parasitic on both roots and stems, the latter generally prostrate. Pilostyles, a native of America and Africa, and Apodanthes, confined to America, are parasitic on branches. Hydnora, found in tropical and south Africa, grows on succulent plants, chiefly Euphorbiaceae ; and closely allied to it is Prosopanche, an American genus.

The genus Cassytha (Laurineae), of which there are about fifteen species occurring in the tropics, but mostly in Australia, strongly resembles Cuscuta. The plants are exceedingly alike in appearance and in parasitic habit, for which reason the name of “dodder laurels” has been given to the Cassythae. They are wholly without chlorophyll, and their thin, twining, cylindrical stems, bearing scaly leaves, envelop their hosts, to which they are attached by means of papilla-like haustoria. The seeds germinate in the soil, and the roots subsequently die off as in Cuscuta.

The Loranthaceae are parasitic on the stems and branches of trees, but, since they bear mostly thick and leathery leaves containing chlorophyll, their parasitism cannot be considered so complete as in those cases where chlorophyll is absent. The order is for the most part a tropical one, but it is represented in Europe by Loranthus europaeus, L., and Viscum album, L., the common mistletoe. Loranthus is a large tropical genus containing upwards of three hundred species. Arceuthobium occurs in southern Europe. The mode of parasitism of Viscum album, L., the mistletoe, may be taken as illustrative of the order. Its seeds adhere to the young shoots of trees by means of the viscid pulp of the fruit (used in the preparation of bird-lime). On germination it shoots out rootlets which traverse the cortex of the host mostly in the direction of the axis, sending down numerous haustoria into the wood, where the cells of the parasite become partly lignified, and thus attain an intimate connexion with the wood-cells of the host. A layer of meristem is formed in the haustorium where it passes through the cambium region of the host stem, thus enabling the parasite to keep pace with the growth in thickness, and gradually to become more deeply fixed. The function of the growing point, which soon passes over into permanent tissue, is thus transferred to this region of the haustorium. Ultimately this layer of meristem is also transformed into permanent tissue, and the activity of the parasite in this direction ceases. The haustoria are commonly situated close together in considerable numbers, and an excessive demand upon the host is thus brought about, causing local death and a hurtful influence throughout the plant, exhibited in its defective development. Where a tree has been attacked by mistletoe a corroded and distorted appearance is presented, owing to the drying up of the tissues and the reparative processes that ensue. "When the mistletoe has thus exhausted one region of supply it frequently sends out adventitious shoots, which, attacking the host in fresh places, give rise to new growths of the parasite. The mistletoe grows on a large number of different trees, such as the apple, lime, elm, maple, willow, thorn, poplar, and even on conifers. Though exceedingly plentiful on the apple, it rarely attacks the pear tree, and the Lombardy poplar seems to be exempt, while other poplars suffer considerably. Very rarely does it attack the oak, and Dr Bull, who made exhaustive inquiry (Journ. Bot., vol. ii.) into the matter, succeeded in discovering only seven authentic cases in England. Loranthus europaeus, L., occurs on the oak in southern Europe.

The Santalaccae are mostly if not all partially parasitic shrubs or herbs—their foliage containing chlorophyll. Santalum (S. album yields sandal wood), distributed throughout the East Indies, Malay Islands, and Australia, and Thesium, a native of Europe, are parasites on the roots of plants, especially monocotyledons. Their haustoria are more or less globular in shape, and emit from the surface in contact with the host a process which penetrates the tissues. Osyris also attacks the roots of trees. Henslowia and Myzodendτon are partially parasitic on the branches of trees. The latter, a native of south temperate climates, attaches itself to its host by means of the feathered processes on its seeds. These retain them in contact with the branches on which they fall until germination (thus performing the same function as the viscid pulp of the mistletoe), when the haustoria penetrate the bark and become, as it were, grafted into the living tissues.

The Balanophoraccae are flowering plants of degraded structure, destitute of chlorophyll, and generally coloured red, yellow, or brown. In appearance they somewhat resemble Cptinaceae, though there is no real affinity in the case. The stems are succulent, somewhat knob-shaped or cylindrical, varying in height from a few inches to a foot, in which latter case they are sometimes branched, and bear imbricated scales in place of leaves. They are true parasites on the roots of woody Dicotyledons, rarely on Monocotyledons. The haustoria vegetate in the tissues, frequently setting up extensive hypertrophy. They occur chiefly in mountainous tropical regions—some in Australia and the Cape. The order ebntains thirty-five species in fourteen genera, of which Balanpphora, Cynomorium, and Langsdorjjia are the best known. Cynomorium coccineum — the Fungus melitensis of old writers—is found in Malta, the Levant, North Africa, and the Canary Islands. ALgae. —Several microscopic algae may very well be partial parasites, though it is'probable that in most cases they are little more than epiphytes in their relation to the plants in which they occur. They all possess chlorophyll and are able to assimilate; but from their situation in the tissues of other plants a degree of parasitism may be inferred. A species of Nostoc. occurs in the intercellular spaces of the roots, leaves, and thalli of other plants; and Chlorwhytriuτn is found in the tissues of Lemna, Cerato- phyllum, and in another alga Schizonema. More distinctly parasitic is the case of Phyllosiphon Arisari, Kühn, which inhabits the parenchymatous tissue of Arum Ari-sarum. i Lichenes. — Mycoidea parasitica, Cunn., was described and figured by Cunningham as a parasitic green alga. It, or a closely allied form, has been recently examined by Ward, who says, “It seems clear that the injury is not due to a direct parasitic action of the thallus; even in the extreme case of Citrus I do not imagine the active development to depend so much on absorption of food from the living leaf as on the sheltered situation enjoyed by the ensconced thallus.” Fungi. —The absence of chlorophyll from all fungi, and the necessity thus thrown upon them of taking up the carbon compounds assimilated by other organisms, determines their mode of life, which is therefore either parasitic or saprophytic. The parasitic organ of the fungal thallus is the mycelium, upon which haustoria are sometimes developed in the form of lateral protuberances of various shapes and sizes. In the same species of parasitic fungus receptacles frequently occur of different kinds, succeeding each other more or less regularly in cycles, and sometimes in their course preying upon hosts of remote affinities among themselves. This course of life is of practical importance when effort is made to limit the ravages of such a parasite (see Mildew, vol. xvi. p. 293). Many indiscriminately attack plants nearly allied to each other; numerous species are peculiar to one host; while others are confined to a single region such as the ovary, the stem, or the leaf of one or more species of the higher plants. The spores, invariably of microscopical -dimensions, represent the infectious agent, as the seeds of flowering parasites commonly do. They are conveyed by the atmosphere, by contact of one plant with another, by insects and other animals, àc., and germinate by the emission of a germ-tube, the production of zoospores sometimes intervening. Access to the host is obtained by the penetration of the epidermal tissue or by way of the open stomata. The main body of the fungus is either endophytic or epiphytic—the spore-producing portion in nearly all cases opening externally. The amount of damage effected by the attack varies from slight local injury to the destruction of the host; in some cases cellcontents only are destroyed, while in others whole tissues perish. The effect produced is often in the direction of abnormal stimulus, and the hypertrophy of whole regions or the production of galls ensues. The parasite commonly prepares the way for the saprophyte, which steps in to break up the dead and decaying remains. In certain rare instances the union of parasitic and saprophytic modes of life in the same species has been observed (see below). The fungi which are concerned in the constitution of lichens maintain with the algal components throughout life relations of consortism which will be dealt with below, under “Symbiosis.”

For the life-histories of the following groups the student is referred to the article Fungus (vol. ix. p. 827), and to the literature therein cited.

Saprolegnieae. — The fungi of this suborder are many of them saprophytes, as their name implies, but some are of distinctly parasitic habits. Certain species of Pythium are parasitic on freshwater algae, on the prothallia of vascular cryptogams, and in the tissues of the higher plants. Several species of Saprolegnia are parasitic on similar hosts, but one in particular, £. fcra·x, Gruith, is well known for the part it plays in the disease of fishes in fresh water—commonly called the salmon disease. That this fungus possesses both parasitic and saprophytic modes of life is established, and the fact is one of remarkable importance, since it stands almost by itself in this respect among the higher fungi.

The Peronosporcae are all parasites on vascular plants of many different orders. The mycelium inhabits the tissues of the host, and, in many of the species, while passing through the intercellular passages, sends globular or irregularly branching filamentous haustoria (see fig. 3) into the adjoining cells. On the other hand the mycelial filaments of certain species, such as Phytophthora infestans, De Bary, the potato disease, possess no true haustoria, but they penetrate the cells, breaking down the cell-walls in their course. In the regions where the oospores of Peronosporcae are formed hypertrophy of the tissues of the host sometimes occurs, and, the normal functions being checked, the parts in question die off. The Peronosporcae are enormously destructive to the higher plants, and may be reckoned among the most dangerous enemies of agriculture and horticulture. Besides the potato disease, Cystopus candidus and Peronospora parasitica, both occurring plentifully on Cruciferae, may be mentioned as typical of the group.

The Chytridieae are a small suborder of parasitic fungi inhabiting rarely the epidermal tissue of higher plants, but commonly attacking fresh-water algae and sometimes Infusoria. Many of these exceedingly simple plants consist merely of a sporangial cell maintained in position and nourishment by a haustorium which penetrates the host cell. The affinities of the group are somewhat uncertain, but probably they are correctly placed among Zygomycetes.

The UrelÀneae are endophytic parasites on vascular plants producing the disease popularly called rust. These fungi occur on very various plants, and in their life-history go through a cycle of generations on at least in many cases two different hosts. Cornmildew is the best known of them, and may be taken as typical of the rest (sec Mildew, vol. xvi. p. 293; and for figures, see Fungus, vol.’ ix.). This suborder, like the Peronosporex, is exceedingly destructive to cultivated and other plants. The Roestelia of the pear tree (which alternates with the Podisoma of junipers) and the Puccinia of Malvaceae may be mentioned as familiar examples of the group. The coffee-leaf disease, Hemileia vastatrix, is considered by Ward to be allied to this group.

The Ustilagineae are all parasites of a very destructive nature on the stems, leaves, ovaries, &c., of the higher plants. The mycelial filaments inhabit the tissues of the host, where hypertrophy is frequently set up, and the enlarged space thus obtained is used by the fungus to contain the masses of spores formed by the breaking up of the hyphae. Their whole life-history is earned out in the same host. Though attacking grain crops particularly, many species infest other plants. Ustilago Carbo, Till., is perhaps the commonest, and is exceedingly destructive to a considerable number of grasses. The Entomophthoreae are a very small group attacking insects.

The mycelium ramifies densely in the body of the insect and breaks out through the skin where spores are produced singly on basidia. Within the body resting spores are formed by means of which the fungus hibernates. Empusa Muscae is very common on the ordinary house fly. ®

The Hymenomycetes is the only suborder of Basidiomycetes certainly known to include parasitic members, and these relatively few in number. Agaricus melleus, Vahl, by means of its subterranean mycelium (Rhizomorpha sιd>terranea of older authors), is exceedingly destructive to the roots of many trees and woody plants. Other Agaricini, such as Nyctalis parasitica, attack members of the same group as themselves, but by far the greater number are saprophytes. Trametes radiciperda, R. Hart., and T. pini, Fr., Polyporus fulvus, Scop., P. vaporarius, Fr., P. mollis, Fr., and P. borealis, Fr., all attack Conifer ae especially, while P. sulphureus, Fr., P. igniarius, Fr., and P. dryadcus, Fr., are parasitic on oaks, poplars, beeches, willows, and other dicotyledonous trees. Thelephora, Stqreum, and Hydnum also include species parasitic on trees.

The Discomycetcs, like the last group, are mostly saprophytes, but a few distinctly parasitic members are to be found in it. Ascomyces, Gymnoascus, and Exoascus (E. Pruni, Fiickel, and E. deformans, Fiickel) are parasitic, the last-named upon plum, peach, and cherry trees. Several species of Peziza, as P. calycina, Schum., on the larch, and a number of those belonging to the section of Pseudo-peziza attack the higher plants. It is highly probable that many Sclerotia, numbers of them parasitic, the positions of which are not definitely known, will be found to belong to such discomycetous forms as Peziza. Peziza scier otioides, Lib., is said to remain living as a saprophyte after the death of its host. Rhytisma is a very common disease of leaves, such as those of Acer, in which it produces large darkly discoloured patches. Pyrcnomycctes.— Of this group the Erysipheae are perhaps the most destructive as parasites. They exhibit in their life-history a cycle of generations each of considerable parasitic acti vity. The main body of the fungus is commonly epiphytic, the mycelium sending down haustoria through the epidermis of the host (see fig. 4). Of the perithecial form of fructification good examples are Sphaerotheca Castagnei, Lev., the hop mildew (see Mildew, vol. xvi. p. 294), Phyllactinia, Uncinula, Calocladia, and Erysiphe (E. graminis, Lev., E. Linkii, Lev., E. Martii, Lev., and E. lamprocarpa, Link. ). The oidium forms are also conspicuous as parasites, a familiar example being found in E. Tuckeri, Berk., the vine-mildew (see Mildew, as above). Claviceps purpurea, Tul., the ergot of grasses (see Ergot, vol. viii. p. 251 ), is the best known and most important of all pyrenomycetous parasites. The group includes a multitude of minor parasites,—some of them, however, doubtfully so—belonging to such genera as Stigmatea, Sphaerella, Fusisporium, Ramularia, Fumago, Polystigma, Pleospora, Nectria, &c. Nectria ditissima, Tul., is reputed to be the cause of canker in certain trees. Cordyceps is well known as a disease of insects. Nature of Vegetable Parasitism. —It has been seen that the dependence of parasites upon their hosts for the means of subsistence varies considerably in degree, but it is equally manifest that underlying this condition of existence there are certain facts which characterize every case. The most important of these is the absence or the inadequate supply of chlorophyll and the consequent loss or deficiency of the power of assimilation. For a comparison of this abnormal condition with the normal state a subject is found ready to hand in the nourishment of one organ by another, as exemplified in the growth of young seedlings, which in the case of seeds containing endosperm (cocoa-nut, date-palm, and many other monocotyledons) absorb by means of a definite organ the nourishment necessary for their development. Young plants nourished from the reserve-materials stored in bulbs and the like, and the young shoots of a tree from winter buds, afford a comparison which is even closer in an anatomical respect, since in this case there is present, as in the intimate association of parasitic haustoria with the host, a continuity of tissues which is not so strongly marked in the union of the absorbing organ of a seedling with the endosperm. Looking at the subject wholly from the point of view of the process of nutrition, there seems to be little essential difference between parasite and saprophyte, since we have not only experimental instances of the nutrition of parasites on artificially prepared solutions, but the natural union of both habits in the same individual (salmon-disease, <fcc.; see also the experiments of Grawitz on the growth of saprophytic fungi in the blood of animals). These are exceptional instances, however, and it is manifest that other properties must be brought into play, since most parasites affect peculiar hosts, and many of them certain regions only of the plant. It is equally true that many saprophytes are able to grow only in peculiar substrata.

That parasitism is often but partial is apparent from such instances as the mistletoe, Rhinanthus, Thesium, <fcc., which probably obtain from their hosts in the main only water and mineral substances in solution, to be prepared for plant food in their green leaves. It is most likely, however, that a small quantity of certain organic compounds is a necessary accompaniment in all such instances. Here again there exist the means for comparison with green saprophytes. The taking up of ash constituents from the soil may occur in such parasites as Orobanche, which possesses rootlets, though undoubtedly the whole of the necessary carbon compounds are obtained from the host.

This mode of life not only acts upon the host, but reacts upon the parasite itself, as is manifested by the aberrant and degraded structure of the parts (directly and indirectly) concerned in nutrition, and even of the reproductive system. This is strongly marked in the case of the embryo. It is apparent that large transpiratory surfaces are unnecessary, and would even be of detriment to a parasite; and with this the formation of wood so intimately connected with the process of transpiration keeps pace in degradation. In the mistletoe, for example, the bulk of wood is in relation to the small transpiratory surface, and in the cases of parasites without chlorophyll it dwindles to insignificance. No other abnormal mode of life so influences the structure of a plant as a parasitic or a saprophytic one, though we see an approach to it in the adaptations existing in insectivorous plants.

The effect upon 'the host ranges from local injury to destruction on the one hand, and, on the other, in the case of stimulus, from the local production of galls to the complete hypertrophy and transformation of at least large regions of a plant. The exciting of definite reparative processes is an indirect effect. It must be noticed also that many parasites, especially fungi, cause in the host enormous destruction of food material far exceeding that necessary to their maintenance. In this way the parasite frequently commits suicide as it were, and the act is in striking contrast to the relations of symbiosis as exemplified in the lichen thallus.

The change of or alternation between two different hosts is adapted to suit the requirements of the parasite. This is notably so in the case of the corn-mildew, which passes an intermediate stage on the barberry until a period when the wheat plant has sufficiently developed to become a suitable host.

Most fungi are endophytic, and certain phanerogamic parasites, such as Rafflesia, develop within the cortex of the host, while on the other hand the fungal part of a lichen encloses the algal.

The existence and complete dependence upon its host of a parasite culminating in the production of seed after its kind is one of the most impressive relationships physiology presents. Symbiosis. —This, the consortism of organisms in such fashion that mutual services are rendered sufficient to make the alliance profitable and successful to the whole community of organisms, is a mode of life closely related to parasitism, in which, however, as has been seen, the profit is one-sided and the alliance ends with the exhaustion of the host or the detachment of the parasite. The term was first employed by De Bary (Die Erscheinung der Symbiose, 1879), but the relations expressed by it were first brought into general notice by the epoch-marking discovery of the dual nature of the lichen thallus by Schwendener in 1868, and established after prolonged and searching controversy, more especially by the classical histological researches of Bornet, and the actual artificial lichen synthesis (by sowing fungus on alga) by Stahl. Some theory of reciprocal accommodation was necessary to account for the duration of such relations between a fungal organism and an algal; and, though it is not yet precisely known in what way these relations are maintained, speculation has been active enough. It may safely be inferred that the fungal portion of the thallus is nourished by the exosmose of starch and the like in much the same fashion as the colourless cells of a plant are fed by those bearing chlorophyll; and there can be little doubt that the algal cells benefit in return by the endosmose of the waste products of the fungal protoplasm. In the reproductive process an adaptation exists in certain lichens for the supply of gonidia to the new lichen. Hymenial-gonidia (the offspring of the thallus-gonidia) are present in the apothecia, from which they are cast out along with the spores, and falling with them are subsequently enclosed by the germ tubes (see Fungus, vol. ix. p. 835). It may be noted here that, though the fungal portions of the thallus retain the marks of near relationship to ascomycetous fungi, they are yet considerably modified by this mode of life, and unfitted most probably in nearly every case for the distinctly parasitic or saprophytic life normal to fungi. The algal portion, on the other hand, is capable of independent existence after liberation from the fungal thallus. The complete symbiotic community represents an autonomous whole, living frequently in situations where neither alga nor fungus is known to support existence separately.

The presence of chlorophyll in animals (Hydra and Vortex) was discovered by Max Schultze in 1851, and confirmed more recently (Hydra and Spongilla) by the spectroscopic evidence furnished by Lankester and by Sorby. That a chlorophyll-bearing animal is able to vegetate by means of its own intrinsic chlorophyll was finally established in 1878 by the experiments of Geddes on Convoluta Schultzii, Schm. He found that the analysis of the gas given off by these green animals, under the influence of direct sunlight, “yielded from 45 to 55 per cent, of oxygen.” The discovery of these vegetating animals directed fresh attention to chlorophyll-bearing animals, with much result. The nature and functions of the yellow cells of radiolarians had long been an unsolved enigma. Haeckel had detected in them in 1870 the presence of starch, and regarded them as stores of reserve material. Cienkowski, in the following year, contended for their algal nature without finally deciding the question, and without perceiving the significance such organisms would have in the economy of the radiolarian. Much suggestive observation followed by the Hertwigs, Brandt, Entz, Korotneff, Lankester, Moseley, and others on similar bodies in various organisms; but the subject remained in uncertainty till its reinvestigation by Brandt, and simultaneously and much more conclusively by Geddes, finally supplied the solution of the difficulties. After confirming Haeckel’s discovery of the presence of starch, and the observations of Cienkowski, Brandt, and others on the survival of the yellow cells after the death of the radiolarian, and extending his observation to various other organisms, Geddes demonstrated the truly algal nature of these cells from their cellulose walls, the identity of their yellow colouring-matter with that of diatoms, and the evolution of oxygen (in some instances, such as Anthea Cereas, very copiously) under the influence of sunlight. It was pointed out that the animal protoplasm investing these starch-producing cells (and containing amylolytic ferment) must obtain by osmosis its share of the dissolved starch, and that benefit must accrue to the animal from thedigestion of the dead bodies of the algae. The evolution of oxygen during sunshine into the surrounding animal protoplasm is a respiratory function fittingly compared to that performed by certain stationary deposits of haemoglobin. On the other hand the carbonic acid and nitrogenous waste produced by the animal cell is the nutritive return made to the alga, which in removing them performs an intracel lular renal function. The young gonophores of Velella, after budding off from the parent, start in life with a provision of algae, and in this respect bear interesting resemblance to the function performed by the hymenial-gonidia of lichens described above. The physiological relations are summed up as follows :—“Thus, then, for a vegetable cell no more ideal existence can be imagined than that within the body of an animal cell of sufficient active vitality to manure it with abundance of carbonic anhydride and nitrogenous waste, yet of sufficient transparency to allow the free entrance of the necessary light. And, conversely, for an animal cell there can be no more ideal existence than to contain a sufficient number of vegetable cells, constantly removing its waste products, supplying it with oxygen and starch, and being digestible after death.” The completeness of the case thus established for a symbiotic mode of life marks one of the most interesting and impressive chapters in the history of the biological relations between animals and plants.

A re-discussion of the subject, largely historical and controversial, but with excellent bibliography, has been lately furnished by Brandt, and more recently a further contribution has been made by Oscar Hertwig, who repeats the views of preceding investigators and goes on to speculate as to the nature of symbiosis and its general relations with other modes of life. (g. mu.) Parasitism in Medicine.

Only a limited number of the parasitic diseases of man are included in the present article. Under Tapeworm will be found all that medically relates to that important parasitic group, and under Schizomycetes will be discussed the significance of the parasitic micro-organisms (Bacterium, Bacillus, Spirillum, Vibrio, àc.) in morbid processes, and particularly in the infective diseases. There fall to be considered here (1) the skin-diseases due to filamentous fungi, (2) a peculiar disease called “actinomycosis,” primarily affecting cattle, (3) the itch, and (4) certain diseases caused by various species of nematodes, and one disease caused by a trematode.

1. Skin Diseases due to Parasitic Fungi. Favus (“honeycomb”) is a common disease of the scalp (more rarely of the hairless parts of the skin) in children, primarily of scrofulous or ill-cared-for children, but apt to spread to others, especially in schools. The uncomplicated appearance is that of a number of yellowish circular cup-shaped crusts, grouped in patches like a piece of honeycomb, each about the size of a split pea, with a hair projecting in the centre. This was the first disease in which a fungus was discovered—by Schönlein in 1839; the discovery was published in a brief note of twenty lines in Muller's Archiv for that year (p. 82), the fungus having been subsequently named by Remak Achorion Schönleinii after its discoverer. The achorion consists of slender mycelial threads matted together, bearing oval nucleated gonidia either free or jointed. The spores would appear to enter through the unbroken cutaneous surface, and to germinate mostly in and around the hair-follicle and sometimes in the shaft of the hair. Favus is commonest among the poorer Jews of Russia, Poland, Hungary, Galicia, and the East, and among the same class of Mohammedans in Turkey, Asia Minor, Syria, Persia, Egypt, Algiers, &c. It is not rare in the southern departments of France, in some parts of Italy, and in Scotland.

The treatment of favus is difficult and disappointing. The first requisite is good feeding; meanwhile the crusts are to be removed by poulticing, the hair being cut short. The next thing is to destroy the fungus, to which end a lotion of sulphurous acid (one part to three or four of water) may be applied repeatedly by means of lint, and the scalp kept covered by an oil-silk cap. To prevent the return of the fungus, various agents may be rubbed in, such as codliver oil, oil of cade, or an ointment of iodine or of pitch, the oil-silk cap being worn continuously. It has often been found of advantage to pull out all the broken stumps of hairs with a tweezers (ste Bennett’s Prin. and Pract. of Med., 5th ed., Edin., 1868, ρ. 847).

Ringιcorm, or Tinea Tonsurans, a much more common disease of the scalp (especially within the tropics), consists of bald patches, usually round, and varying in diameter from half an inch up to several inches, the surface showing the broken stumps of hairs and a fine whitish powdering of desquamated epidermic scales. In scrofulous subjects matter is sometimes produced, which forms crusts or glues the hairs together or otherwise obscures the characteristic appearance. The disease is due to a fungus, Trichophyton tonsurans, which exists mostly in the form of innumerable spores (with hardly any mycelium), and is most abundant within the substance of the hairs, especially at their roots. If a piece of the hair near the root be soaked for a time in dilute liquor potassae and pressed flat under a cover-glass, the microscope will show it to be occupied by long rows of minute oval spores, very uniform in size, and each bearing a nucleus. The treatment of ringworm is very much the same as the treatment of favus.

The same fungus sometimes attacks the hairs of the beard, producing a disease called “sycosis.” Sometimes it invades the hairless regions of skin, forming “tinea circinata circular patches of skin disease, if they be sharply defined by a margin of papules or vesicles, may be suspected of depending on the tinea-fungus. Interesting varieties of tinea are found in some of the Pacific and East Indian islands.

A less serious condition of the skin due to a fungus is Pityriasis versicolor, consisting of patches of brownish discolorations of various sizes and shapes, mostly on the front of the body, and often attended with itching, especially after heating exercise. The pigmentation seems to radiate from the orifices of hair-follicles. The epidermis is in a scaly condition over the patch, and among the debris of the epidermic cell there may be seen minute oval spores, which are supposed to belong to a fungus, the Microsporon Furfur. The disease is mostly one of adult age, found all over the world, and not associated in any special way with poor general health. The treatment consists of rubbing in an ointment of sulphuret of potassium, or one of the mercurial ointments, or using sulphur-soap habitually.

The remarkable brown, black, and blue spots of discoloration of the whole body met with endemically in Mexico, Panama, New Granada, and Venezuela, and known under the name of “pinto” or “mal de los pintos,” lιave been claimed by Gastambide {Presse Med. Beige, 1881, Nos. 33-41) as due to the presence of a fungus, whose spores and even mycelial filaments may be detected among the deeper rows of cells of the rete mucosum. The disease, which is somewhat serious from its large superficial area, would appear to be one of the many forms of morbus miseries ; but it is contagious, and is sometimes seen in the well-to-do. In some villages of the western districts of Tabasco (Mexico), it has been estimated that 9 per cent, of the inhabitants suffer from the pinto; M*Clellan says that in 1826 in the city of Mexico he saw a whole regiment of “pintados.”

Before leaving the parasitic fungi of the skin, it should be mentioned that Oidiιιm albicans is apt to plant itself on the mucous membrane of the mouth in young and ailing children, causing whitish patches known as thrush.

2. Actinomycosis.

In certain tumour-like formations of cattle, usually growing from the alveoli of the lower molar teeth, and protruding externally near the angle of the jaw, Bollinger in 1877 detected the presence of a number of sulphur-yellow bodies about the size of a hemp seed and of a fatty consistence. These were found to be aggregates of a peculiar radiate fungus {Actinomyces}, which assumed the form of minute rosettes, the mycelial filaments expanding into flask-like swellings at their free or circumferential ends. The yellow seed-like conglomerates lay in spaces of the tumour, and they were also found within cavities on the tongue, fauces, larynx, mucous membrane of the stomach, in lymphatic glands, and (by a later observer) in the lungs. In 1879 Ponfick found the same sulphur-yellow bodies in the body of a man who had died of chronic disease of the chest, and who had a number of sinuses in the skin of the back. Some twenty cases of actinomycosis in man have now been described in Germany; in most of them· there have been centres of chronic inflammation in front of the vertebrae in the cervical, dorsal, or lumbar regions, with numerous sinuses penetrating the muscles and opening on the skin. The yellow conglomerates of Actinomyces are found in or upon the granulations of these sinuses, or in the sero-purulent discharge from them, or in the muscles, or more rarely in centres of granulationlike new growth in some of the viscera. The relation of the fungus to the primary tumour-like new growth of the ox has not yet been made out, and there is hardly any clue to the connexion between the bovine disease and the somewhat modified form of it in man. In some respects there is an analogy between actinomycosis and the fungus-foot of India as described by Vandyke Carter.

3. Scabies.

Of the human diseases due to animal parasites there is only one of any importance affecting the skin, namely, scabies or the itch. The parasite is the Sarcoptes scabiei (see Mite, vol. xvi. p. 529), which burrows under the epidermis at any part of the body, but hardly ever in the face or scalp of adults; it usually begins at the clefts of the fingers, where its presence may be inferred from several scattered pimples, which will probably have been torn at their summits by the scratching of the patient, or have been otherwise converted into vesicles or pustules. The remedy is soap and water, and sulphur ointment.

4. Diseases due to Nematode and Trematode Worms.

The common thread-worm {0xyuris), a small white object about half an inch long, is very frequent in all countries, mostly in children; its habitat is"the lower bowel, but it is often a troublesome irritant outside the bowel as well. The round-worm (Ascaris lumbricoides}, about 6 inches long when full-sized, and not unlike the common earth-worm, is less common in England and other Western countries; but it is enormously common all over the East, and in the tropics generally. Hundreds of them may accumulate in the body, causing an obvious enlargement of the abdomen. The most valuable remedy against them is santonine powder. A third intestinal nematode is the whip-worm (Trichocephalus dispar}, about 2 inches long, having a slender anterior extremity joined on to the body like the thong to the handle of a whip. It is said to be very common in some countries, such as France, but it has no great importance as regards disease.

The nematodes of greatest pathological interest arc Trichina spiralis, causing the serious malady of trichinosis; Anchylostoma duodenale, often associated with the profound anaemiaof men working in mines, making tunnels, and the like; Anguillula stercoratis, associated with a specific kind of diarrhoea in Cochin China; Filaria sanguinis hominis, a blood-worm occurring mostly in China and other parts of the East, and often associated with the disease called lymph-scrotum, and with haemato-chyluria; and Filaria medinensis, the Guinea-worm, very common on the Guinea coast and in many other tropical regions, a long and slender filament like a hair from a horse’s tail, and mostly infesting the skin of the legs.

Trichinosis.— The presence of encysted trichinae in the muscles was discovered in one or more of the London dissecting-rooms in 1828 and following years; but it was not until thirty years later that the clinical characters of the acute disease caused by the invasion of the parasite were discovered. This discovery was made in 1860 by Zenker, on examining the abdominal muscles of a patient who had died at Dresden, with symptoms taken to be those of typhoid fever, the case being afterwards accounted one of trichinosis on the post mortem evidence. Epidemics of this disease occur from time to time, especially in north Germany, from the eating of uncooked swine’s flesh, in which trichinae are not uncommon. The greatest care is now taken in Germany to examine the carcases of swine for trichinae, a piece of the diaphragm of every animal being searched with the microscope by an inspector specially appointed. The symptoms in man are occasioned by the presence of the free parasites in the intestine, by the development of young trichinae from the eggs, and most of all by the migration of the parasites from the intestinal canal to the muscles, where they become quiescent within a calcareous shell. This cycle occupies from four to six λveeks. "When consumed in small quantity, the parasites may give rise to no marked symptoms, and they are sometimes found accidentally in muscular fibre in the bodies of those who had probably experienced no definite symptoms from their invasion. In the more acute and serious cases, sometimes ending fatally, the early symptoms are nausea, failure of appetite, diarrhoea, and fever; later, when the migration to the muscles begins, there is more fever, stiffness, pain, and swelling in the limbs, swelling of the eyelids, continued exhausting diarrhoea, perspirations, and sometimes delirium. During convalescence there is desquamation of the cuticle. If the diagnosis be made early in the case, brisk purgatives, particularly calomel, are the best treatment; if the parasites are already on their way to the muscles, the only thing left to do is to support the patient’s strength.

Anaemia and Cachexia caused by Anchylostoma duodenale. — A disease which caused a great mortality among the negroes in the West Indies towards the end of last century, and of which descriptions were afterwards sent from Brazil and various other tropical and subtropical regions, was identified, chiefly through the labours of Bilharz and Griesinger in Egypt (1854), as being due to the presence in the intestine of nematoid worms from one-third to half an inch long, and variously named Anchylostoma, Sclerostoma, Strongylas, &c. The same disease has subsequently been found in some places among miners, and particularly among the men employed in making the St Gotthard tunnel. Various names have been given to the malady, such as mal d’estomac, mal de coeur, dirt-eating, anaemia intertropicalis, cachexia Africana, and cachexie aqueuse. The symptoms, as first observed among the negroes, were pain in the stomach, capricious appetite, pica (or dirt-eating), obstinate constipation followed by diarrhoea, palpitations, small and unsteady pulse, coldness of the skin, pallor of the skin and mucous membranes, diminution of the secretions, loss of strength, and, in cases running a fatal course, colliquative diarrhoea and dysentery, haemorrhages, and dropsies. The parasites, which cling to the intestinal mucous membrane, draw their nourishment from the blood-vessels of their host, and as they are found in hundreds in the body after death, the disorders of digestion, the increasing anaemia, and the consequent dropsies and other cachectic symptoms are easily explained. It seems probable that the parasite is introduced in its larval stage through the medium of the drinking-water. Male-fern, santonine, or other anthelmintic remedies are prescribed for it; but, inasmuch as it is most apt to lodge in the bodies of the ill-fed and otherwise ailing poor, there is little doubt that the most satisfactory remedy would be to increase the power of resistance by improving the general well-being.

Chyluria and Lymph-Scrotum caused by Filaria sanguinis hominis. — A milky appearance of the urine, due to the presence of a substance like chyle, which forms a clot, had been observed from time to time, especially in tropical and subtropical countries; and it has been proved by the late Dr Wucherer of Bahia, and by Dr Timothy Lewis, that this peculiar condition is uniformly associated with the presence in the blood of minute· eel-like worms, visible only under the microscope, being the embryo forms of a Filaria. The parent worms are very difficult to find, and their characters and habits are imperfectly known; but they are supposed to be about 3 inches long, and to inhabit dilatations of the lymph-carrying vessels. It is not yet clear how the chyle gets into the urine, but it seems probable that the blood in which filariae are present is altered in its constituents, although there is no obvious change in its microscopic characters beyond the presence of the young nematodes. These are also present in the chylous urine. Sometimes the discharge of lymph takes place at one or more points of the surface of the body, and there is in other cases a condition of nae· void elephantiasis of the scrotum, or lymph-scrotum. More or less of blood may occur along with the chylous fluid in the urine. Both the chyluria and the presence of filariae in the blood are curiously intermittent; it may happen that not a single filaria is to be seen during the daytime, while they swarm in the blood at night, and it has been ingeniously shown by Dr S. Mackenzie that they may be made to disappear*if the patient sits up all night, reappearing while he sleeps through the day.

Dr Manson of Amoy has proved that mosquitoes imbibe the embryo filariae from the blood of man; and that many of these reach full development within the mosquito, acquiring their freedom when the latter resorts to water, where it dies after depositing its eggs. Mosquitoes would thus be the intermediate host of the filariae, and their introduction into the human body would be through the medium of water.

Dracontiasis or Guinea-worm.—Filaria medinensis, or Dracunculus, or Guinea-worm, is a very long filarious nematode like a horse hair, whose most frequent habitat is the skin of the legs and feet. It is common on the Guinea coast, and in many other tropical and subtropical regions, and has been familiarly known since ancient times. The condition of dracontiasis due to it is a very common one, and sometimes amounts to an epidemic. The black races are most liable, but Europeans of almost any social rank and of either sex are not altogether exempt. The λvorm lives in water, and, like the Filaria sanguinis hominis, appears to have an intermediate host for its larval stage. It is doubtful whether the worm penetrates the skin of the legs directly; it is not impossible that the intermediate host (a Cyclops) which contains the larvae may be swallowed with the water, and that the larvae of the Dracunculus may be set free in the course of digestion.

Endemic Haematuria and Calculus due to Distoma haematobium. —D. haematobium is a trematode or fluke-worm, which is extensively parasitic in man in northern and southern Africa—in the former along the Nile, and in the latter mostly on a narrow belt of the Natal coast. The parasites live mostly in the blood-vessels of the intestine and of the urinary bladder, whence they reach the mucous membranes; and the most remarkable effects of their parasitism are bleeding from the surface of the bladder and the formation of uratic and phosphatic calculi around the clusters of eggs deposited by the Distoma. The mode of access to their human habitat is still uncertain.

Literature.— The more special memoirs are Γonfick, Die Actinomykose det Menschen, eine neue lnjectionskrankheit (plates, Berlin, 1882); Leuckart, Untersuch. uber Trichina spiralis (plates, Leipsic, 2<1 e<l., 18G6); Virchow, Darstellung der Lehre von den Trichinen (plate, Berlin, 2d ed., 1864); Long. ’ ∙ De l’anémie des mineurs du Gothard, causée par 1 Ankylostome duodénal," in Trans. Jnternat. Med. Congr. l 1881, i. p 437, and papers quoted in Hirsch; T. R. Lewis, On a H<xmatozoon inhabiting Human Blood, its relation to Chyluria, <kc., Calcutta, 1872; Manson, The Filaria Sanguinis Hominis, <fcc. (plates, London, 1884); S. Mackenzie, “Case of filarial haemato-chyluria," in Trans. Path. Soc. Lond., 1882, p. 394; see also Hirsch, Historisch-geographische Pathologie, vol. ii., Stuttgart, 1883 (English translation).