260 PARASITISM [ANIMAL having almost constantly Ascaris ?iigrovenosa in its lungs, and infusorial parasites in its rectum, and may also yield Distomum, Echinorhynchus, &c., twenty species in all. Lizards harbour tape worms, Nematoids, including species of Trichina-, more rarely Trematodes. Ophidians have all kinds of parasitic worms, Chelo- niaus 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 (Sderostoma syngamus], and the grouse disease to a similar cause (Stronyylus pcryracilis). 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 of Holostomum 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 laps mortisaga, while a con siderable number of the truly parasitic forms have only been once or twice described, the above estimate thus becoming reduced well-nigh to half. 1 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 con centration seems necessary. It is, for example, extremely probable that a careful systematic study of genera like G ordius, Distomum, and Tetrarhynchm, of which innumer able 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 lottery 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 Nematoidfd, for example, as Leuckart indicates, we are able to construct a series, starting from free-living forms, and through such cases as Leptodera (a fthabditis-like form, .sometimes free, sometimes parasitic), thence to parasitic Xematodes hardly to be distinguished from their free-living relations, but passing gradually through Oryuris, Tricho- cephalus, Spiroptera, &c., to such highly parasitic forms as Trichina, where all relation to the outer world is lost. The Acanthocephila Leuckart has taught us to regard as Nematodes highly modified by parasitism, and he points out how G ordius, with its atrophied alimentary canal, terminal position of female reproductive organs, and other persistent and embryonic characters in which it differs from the typical Xematoid, really leads up to Echinorhynchus. As Echino rhynchus 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 inter mediate forms as Caryophyllaus and Amphilina, from which we pass with ever-increasing parasitic adaptation i Kor lists see, in addition to general authorities. Linstow Compendium <l. He.lmiiitholorjic. Hanover. 1878; V. Beneden, Animal Parasites and Messmates; Cobbold, Human / ara. itts (1832), anil Parasites of Domestic Animalt (1S74); Zlegler s Patholoyij, English ed., London, 188:;. through the Ligulidx to Bothriocephalw and Txnia. 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 Chajtopod 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 para sitism 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 per manently 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 FIG. 1. Colony of sea -anemones (Sagartia paraiftica) on s cl! < f hermit crab. 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 fresh water 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 associa tions, 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
