·add. ·noun The living together in more or less imitative association or even close union of two dissimilar organisms. In a broad sense the term includes parasitism, or antagonistic, / antipathetic, symbiosis, in which the association is disadvantageous or destructive to one of the organisms, but ordinarily it is used of cases where the association is advantageous, or often necessary, to one or both, and not harmful to either. When there is bodily union (in extreme cases so close that the two form practically a single body, as in the union of algae and fungi to form lichens, and in the inclusion of algae in radiolarians) it is called conjunctive symbiosis; if there is no actual union of the organisms (as in the association of ants with myrmecophytes), disjunctive symbiosis.