PARASITISM, a one-sided nutritive relation between two organisms of different kinds, a relation which is more or less in jurious, yet not usually fatal, to the host, a relation, moreover, that relieves the parasite from most of the activity or struggle which is usually associated with procuring food, and thus tends to favour or induce some degree of simplification or degeneracy. The parasitism may be (a) between one animal and another, e.g., between tapeworm and dog; or (b) an animal as host and a plant as parasite, e.g., the salmon infested with a Saprolegnia fungus; or (c) between one plant and another, e.g., dodder on clover; or (d) between a plant as host and an animal as parasite, e.g., the ears of wheat infested by a minute threadworm, Tylen chus tritici, which causes "ear-cockles"; or (e) in rare cases, e.g., Bonellia, the male is a parasite of the female.
The parasite may be externally associated with the host, like mange-mites on dogs, but this ectoparasitism has also its grades, varying with the extent to which the host is punctured or penetrated by the parasite. Thus the rhynchocephalid crustacean Sacculina pro trudes visibly on the ventral surface of the parasitized crab, but its root-like absorp tive processes penetrate through and through its host, and the bean-like adult stage is actually a burrowing endoparasite that has come to bulge out like a hernia.
The larval stage of Sacculina is a free swimming nauplius. It might seem an easy matter to distinguish between outside and inside, but when the ectoparasite is seden tarily fixed to the skin and absorbs food by an intruded portion of its body, it becomes difficult to determine, as in some of the sedentary plant-mites, how far in it must go before becoming an endoparasite. It is no easy matter to decide whether small ani mals, such as mites, that wander about on the surface of an animal's body are to be regarded as parasites. Some are only scav engers, others draw blood, others deposit eggs in their host. On the whole the dis tinction between ectoparasites and endo parasites is convenient.
Distinguished from Epiphytic and Epizoic Relations.—Parasit ism must be distinguished from epiphytic or epizoic relations. An epiphytic plant grows on another plant without deriving any nourishment from it, as in the case of orchids perched on trees. Similarly microscopic green Algae live on the surface of the coarse hairs of the Brazilian tree-sloths, and many a seashore crab carries a garden of seaweeds on its back. But if the crab such as Hyas araneus, has itself implanted these Algae, and if there is evidence that the crustacean is usefully masked, while the plant is benefited by being carried about, then the relation passes into a mutually beneficial external partnership.
An epizoic animal may live attached to another animal with out deriving any nourishment from it, as a bunch of barnacles to the flattened tail of a sea-snake, or as a tunicate, a false oyster (Anomia), a serpulid worm, a polyzoon colony, and a sponge may all be found together on the shell of a whelk. But if the sponge (e.g., Suberites) should mask a hermitcrab ensconced in an empty periwinkle shell, and should be benefited by its asso ciation with the vigorously active animal, then the epizoic rela tion becomes a commensalism. Various marine animals, such as hydroids and even sea-anemones, live attached to large laminarian seaweeds, but without any nutritive relation.