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...
