host and parasite must be in temporary or permanent physical
contact other than the mere preying of the latter on the former;
and the presence of the parasite must not be beneficial, and is
usually detrimental to the host.
It is obvious that within the limits of the strictest definition of parasitism that will cover the facts many degrees occur. The terms symbiosis and commensalism have been applied to conditions really outside the definition of parasitism, but closely related and usually described in the same connexion. Both terms cover the physical consorting of organisms in such a fashion that mutual service is rendered.
The name symbiosis was invented by the botanist A. de Bary in 1879, and is applied to such an extraordinary community as the thallus of a lichen, which is composed of a fungus and an alga so intimately associated, physically and physiologically, that it was not until 1868 that the dual nature of the whole was discovered. The presence of chlorophyll, which had always been associated only with vegetable organisms, was detected by Max Schultze in 1851 in the animals Hydra and Vortex, and later on by Ray Lankester in Spongilla and by P. Geddes in some Turbellarian worms. On the theory that the chlorophyll occurs in independent vegetable cells embedded in the animal tissues, such cases form other instances of symbiosis, for the oxygen liberated by the green cells enables their animal hosts to live in fouler water, whilst the hosts provide shelter and possibly nitrogenous food to their guests.
The term commensalism was introduced in 1876 by P. J. Van Beneden to cover a large number of cases in which “animals have established themselves on each other, and live together on a good understanding and without injury.” The most familiar instance is that of fishes of the genus Fierasfer which live in the digestive tube of sea-cucumbers (Holuthuria; see Echinoderma). A variety of commensalism was termed mutualism by Van Beneden and applied to cases where there appeared to be an exchange of benefits. A well-known instance of mutualism is the relation between sea-anemones and hermit crabs. The hermit crab occupies the discarded shell of a mollusc, and anemones such as Sagartia or Adamsia are attached to the outside of the shell. The bright colours of the anemone advertise its distasteful capacity for stinging, and secure protection for the crab, whilst the anemone gains by vicarious locomotion and possibly has the benefit of floating fragments from the food of the crab.
It is plain that such terms as symbiosis, commensalism and mutualism cannot be sharply marked off from each other or from true parasitism, and must be taken as descriptive terms rather than as definite categories into which each particular association between organisms can be fitted.
R. Leuckart has made the most useful attempt to classify true parasites. Occasional, or temporary, parasites are to be distinguished from permanent, or stationary, parasites. The former seek their host chiefly to obtain food or shelter and are comparatively little modified by their habits when compared with their nearest unparasitic relatives. They may infest either animals or plants, and as they attack only the superficial surfaces of their hosts, or cavities easy of access from the exterior, they correspond closely with another useful term introduced by Leuckart. They are Epizoa or Ectoparasites, as distinguished from Entozoa or Endoparasites. They include such organisms as plant-lice, and caterpillars which feed on the green parts of plants, and animals such as the flea, the bed-bug and the leech, which usually abandon their hosts when they have obtained their object. Many ectoparasites, however, pass their whole lives attached to their hosts; lice, for instance, lay their eggs on the hairs or feathers or in rugosities of the skin of birds and mammals; the development of the egg, the larval stages and the adult life are all parasitic. Permanent or stationary parasites are in the most cases endoparasitic, inhabiting the internal organs; bacteria, gregarines, nematodes and tapeworms are familiar instances. But here also there are no sharp lines of demarcation. Leuckart divided endoparasites according to the nature and duration of their strictly parasitic life: (1) Some have free-living and self-supporting embryos that do not become sexually mature until they have reached their host; (2) others have embryos which are parasitic but migratory, moving either to another part of their host, to another host, or to a free life before becoming mature; (3) others again are parasitic in every stage of their lives, remaining in the same host, and being without a migratory stage.
Origin of Parasitism.—Now that the theory of spontaneous generation has been disproved, the problem of parasitism is no more than detection of the various causes which may have led organisms to change their environment. Every kind of parasite has relations more or less closely akin which have not acquired the parasitic habit, and every gradation exists between temporary and permanent parasites, between creatures that have been only slightly modified and those that have been profoundly modified in relation to this habit. There are many opportunities for an animal or plant in its adult or embryonic stage to be swallowed accidentally by an animal, or to gain entrance to the tissues of a plant, whilst in the case of ectoparasites there is no fundamental difference between an organism selecting a dead or a living environment for food or shelter. If the living environment in the latter case prove to have special advantages, or if the interior of the body first reached accidentally in the former case prove not too different from the normal environment and provide a better shelter, a more convenient temperature, or an easier food supply, the accident may pass into a habit. From the extent to which parasitism exists amongst animals and plants it is clear that it must have arisen independently in an enormous number of cases, and it may be supposed that there must be many cases in which it has been of recent occurrence; E. Metchnikoff, indeed, has suggested that amongst parasites we are to look for the latest products of evolution. In any case it is impossible to suppose that parasites form a natural group; no doubt in many cases the whole of a group, as for instance the group of tapeworms, is parasitic, but indications point clearly to the tapeworms having had free-living ancestors. Parasitism is in short a physiological habit, which theoretically may be assumed by any organism, and which actually has been assumed by members of nearly every living group.
List of Parasites
A.—Animals.
Vertebrata.—These are rarely parasitic, and cases are unknown amongst mammals, birds, reptiles and amphibia. Amongst fish and cyclostomes, Myxine burrows into codfish, Remora attaches itself to the external surface of sharks; Rhodens amarus, the bitterling, a small, carp-like fresh-water fish, injects its eggs into the mantle-cavity of pond-mussels, where the fry develop, whilst the mollusc reciprocates by throwing off its embryos on the parent fish; Stegophilus insidiosus, a small colourless fish from Brazil and the Argentine, lives parasitically in the gill-cavity of large cat-fishes and sucks the blood in the gills of a large Silurid; Vandellia cirrhosa, the candiru of Brazil, a minute fish 60 mm. in length, enters and ascends the urethra of people bathing, being attracted by the urine; it cannot be withdrawn, owing to the erectile spines on its gill-covers. The natives in some parts of the Amazon protect themselves whilst in the water by wearing a sheath of minutely perforated coco-nut shell.
Mollusca.—Few if any are true parasites. The Gasteropods, Eulimae, Styliferae and Entoconchae lodge in Echinoderms, the latter at least being truly parasitic.
Protochorda and Hemichorda.—Most of these are sessile and may lodge on other animals, but are not parasitic.
Arachnida.—Mites and Ticks are Arachnids, the vast majority of which are parasitic, and species of which infest almost every vertebrate group, but there are some free-living forms. Pycnogonids are parasitic in their youthful stages on Hydroids, whilst the Pentastomids have been so much modified by parasitism that they were long regarded as worms; they may occur in most vertebrates.
Crustacea.—These contain an immense number of forms in all stages of parasitism. Some Copepods are amongst the most degenerate parasites known, the so-called fish-lice being for the most part Copepods with piercing mouth-organs, elaborate clinging apparatus, and degenerate organs of locomotion. In Lernea, the female, after becoming attached to its host, undergoes a retrogressive metamorphosis, losing almost completely the segmentation of the body and discarding the appendages and sense-organs, whilst the male, although not so degenerate in structure, is dwarfed in size and itself becomes a parasite of the female. The Cirripeds
