Distinguished from Shelter-associations.—While the contrast between a parasitic relation and an epizoic or epiphytic one is in most cases clear, there may be some difficulty with what are called "shelter-associations." Thus the little pea-crab Pinnotheres pisum is often found off English coasts sheltering in the mantle cavity of the Norway cockle; and other members of the pinno therid family occur in other bivalve molluscs, as well as in worm tubes and corals. A small bivalve is commonly embedded in the cellulose tunic of ascidians. The slender fish called Fierasfer insinuates itself tail-foremost into the end of the food-canal of sea-cucumbers (e.g., Stichopus), and of some large bivalves and starfishes. When the holothurian is placed in water with insufficient aeration, Fierasfer comes out and rises to the surface, taking gulps of air. A small fish, Amphiprion, with resplendent colours, lives inside a large sea-anemone, hiding itself deeply when disturbed. It does not seem to do either good or harm to the sea-anemone, but it is said to die when dissociated from its "host." Similarly some insects find shelter in plants on which they do not feed. A spider may frequent a particular flower, another makes its web in a pitcher-plant. Many ants live in hollow stems and thorns, but feed elsewhere ; they get nothing but shelter from the plant and confer no benefit. But in other cases the shelter ing ants feed on secretions exuded by glands of the plant, and in return they form a "bodyguard" which is regarded by some, on perhaps insufficient evidence, as useful in warding off the attacks of leaf-cutting ants and other unwelcome visitors. Some plant mites live in little shelters on the plants they frequent. The moist spaces between the leaves of the epiphytic bromeliads of tropical forests afford shelter to an astonishing number of more or less epiphytic insects, and the inter-relations are sometimes so subtle that firm classification becomes, as might be expected, almost im possible. It is an unscientific pedantry that seeks to put every grade of inter-relation into a separate pigeon-hole. There are also puzzling epiphytic shelter-associations between plant and plant, one of the best known being the occurrence of the alga, Nostoc, in certain parts of the water-fern, Azolla, and in the liverworts Anthoceros and Blasia.
Distinguished from Commensalism and Symbiosis.—From com mensalism, if defined as a mutually beneficial external partner ship between two organisms of different kinds, ectoparasitism is distinguished by being one-sided. From symbiosis, if defined as a mutually beneficial internal partnership between two organisms of different kinds, endoparasitism is distinguished by being one sided. It is always more or less prejudicial to the host. As the definitions here adopted are historically justified and are very convenient, it seems undesirable to use "symbiosis" loosely for the intimate living together of two kinds of organisms, and then to subdivide it into parasitism and commensalism, as has been sometimes proposed. On this usage a mutually beneficial nutri tive partnership, e.g., between clover and its tubercle-forming bacteria, is called "reciprocal parasitism," and "commensalism" is used to include "those cases of symbiosis in which two or more organisms live together with possible benefit to some or all of the symbionts, but with injury to none." The usage adopted in this article is to distinguish endoparasitism from symbiosis and ectoparasitism from commensalism, and to recognize other link ages, such as the epiphytic and epizoic habits.
But emphasis must also be laid on the fact that parasitism is a nutritive relation, and there may be utility in following the botanists referred to in distinguishing among plants (a) the inde pendent autophytes, which obtain all their food from inorganic sources, and (b) the dependent heterophytes, "whose existence depends upon antecedent or coexistent organic forms, because they derive at least a part of their food from organic sources."
These heterophytes are then divided into saprophytes, which obtain food from dead organic matter, and parasites which obtain food or food materials from living organisms. On this classifica tion of nutritive habits, a special corner would need to be found for carnivorous plants which obtain their food partly from in organic sources and partly from the animals they capture. Among the bacteria and other plants that live in the alimentary tract of animals, it is sometimes difficult to draw a clear line between those that live on non-living material and those that attack living tissue. But all biologists are agreed that a parasite is in a rela tion of more or less nutritive dependence on its host.
Similarly for endoparasites, there is great diversity. Many infusorians and nematodes live in the large intestine on putrefy ing undigested food-material ; they can hardly be called symbionts except in special cases, but they do little harm. Tapeworms have their heads attached to the wall of the intestine, but this has no nutritive significance, for it is the whole long surface of the body that absorbs the digested food of the host. Here is a passive mode of life, almost quite away from the struggle for existence, and here also is unmistakable degeneration. Another grade is illus trated by many parasites which depend not on the food of their host, but on the living tissues, and here again a distinction may be drawn between those, like growing bladder worms and Sacculina, that absorb lymph or other fluids from the surrounding tissue, and those, like larval ichneumon flies, that directly devour the living tissues of their hosts. The formidable hookworm sucks blood from the intestinal wall, the liver-fluke feeds on the blood of the sheep's liver, but the malaria organisms and many others live in the blood-stream itself, destroying the red blood-corpuscles. Not a few cases besides rhizocephalid crustaceans are known where the parasite castrates its host. The concept of parasitism would be clearer if there were excluded from it all cases where the infesting organism lives an energetic predatory life.