SYMBIOSIS, a biological term intro duced by De Bary to denote certain kinds of physiological partnership between or ganisms of different kinds. Consortism is synonymous. As there are many kinds of organic association, it is convenient to restrict the term symbiosis to such intimate and complementary partner ships as exist between algoid and fungoid elements in lichens, or between unicellu lar alga and radiolarians. In organic nature there is no isolation; no organism lives or dies to itself; there are countless vital associations, some very indirect and external—e. g., the mutual dependence of some flowers and insects—others very direct and intimate, as in the symbiosis of alga and radiolarians. It often hap pens that two organisms live together without there being any apparent vital bonds between them; thus diatoms may be "epiphytic" on alga; alga, lichens, mosses, ferns, orchids, etc., are often epiphytic on trees; many alga are "epi zoic" on animals—c. g., those which live among the hairs of sloths; and one ani mal may be epizoic on another, as sponges often are on zoophytes. Again, there may be external partnerships, such as those between pilot fish and shark, or between beef-eater birds and wild cattle. These suggest cases of mutualism or commensalism, such as the partnership between certain hermit erabs and sea an emones. Probably the constant occur rence of colonies of the alga anabana in the leaves of the aquatic plant azolla is a similar partnership. Alike in symbi osis and in commensalism the partner ships are advantageous to both of the associated organisms, and are therefore to be distinguished from parasitism, in which the benefit is all on one side. It is useful to distinguish these different grades of association, but it cannot be pretended that the distinctions are rigid.
Apart from lichens, the partnership of unicellular alga with raidiolarians is the best-known case of symbiosis. The partner alga—known for a long time as "yellow cells"—used to be variously inter preted as reproductive cells, secretory cells, reserve stores, parasites, and so on; but the researches of Geddes, Brandt, and others demonstrate their algoid and truly symbiotic nature. They have a cellulose wall (except in the Acanthomet ridx among radiolarians), a nucleus, two pigments, of which one is at least closely analogous to the ordinary chlorophyll of plants; they are able to live and multiply after removal from their host or after its death; in sunlight they evolve oxygen and form starch; they multiply as do free unicellular alga. From his experi
ments Geddes inferred that the starch formed by the alga may be absorbed by the radiolarians; that when they die the alga are digested by their partners; that during life they absorb carbonic acid and nitrogenous waste from the radiolarians and in turn liberate oxygen which may accelerate the vital functions of their bearers. It seems that the partnership is distinctly advantageous, for the alga flourish and multiply, and those radiolar ians which are without alga are few and much less common than the vast ma jority which exhibit symbiosis. Brandt's results are for the most part in agree ment with those of Geddes, though diver gent on some points of details. The alga may belong to a distinct genus (Zooxan thella of Brandt, Philozoon of Geddes), or may be simply the swarmspores of various olive-green seaweeds.
Similar symbiotic alga occur in some Foraminifers, in several Ccelenterates, especially otherwise colorless sea ane mones, and, according to Geddes, in some species of the Turbelletrian Con voluta. Brandt maintains that in the fresh-water sponge and in the fresh-wa ter hydra there are symbiotic alga of the genus Zoochlorellct, but Ray Lankes ter has shown to the satisfaction of most naturalists that the pigmented bodies in those animals are no more symbiotic al ga than are green corpuscles in the leaf of a buttercup. Many marine sponges are infested by various kinds of alga, but we do not know that they exhibit any real symbiosis.
In regard to some green protozoa there is much dispute whether the green color is due to chlorophyll bodies or to sym biotic alga. Some forms—e. g., Stentor polymorphus, Coleps viridis, Ophrydiurn viride, and Vorticella chlorostigma—also occur in a colorless state. Geza Entz re gards the bodies as alga, Miss Salitt as chlorophyll corpuscles. Famintzin finds Zoochlorellx in species of Paramecium, Stentor, and Stylonichia, which he re gards as symbiotic forms of the Protoc occus-like Chlorella which he and Beyer inck have discovered living freely. Thus it appears that, while many cases of sym biosis are indubitable, there are other cases in regard to which judgment must be for a time suspended.