Individuum

INDIVIDUUM (from Latin individuus — indivisible), or organism, a term used to denote an indivisible vital unit, independent in both morphological and physiological respects. The word "indivisible" designates the necessity of the integrity of the individuum for its normal existence. Each part of an individuum is merely one of the organs of an integral organism and is incapable of independent existence. Such a concept of the individuum is fully applicable mainly to the higher representatives of the animal kingdom. Several categories of individualities of different orders can be outlined. The simplest situation is found in Protozoa, where the animal is unicellular and the concept of the individuum coincides with the concept of the cell. However, quite a few protozoa during asexual reproduction do not bring the latter to completion, as a result of which the daughter individuals resulting from division retain both anatomical and physiological connection with each other. In this way, colonies of unicellular organisms arise, composed of several equivalent individua. The individuality of the units making up the colony is subordinated here to the individuality of the entire colony, just as a known system of organs is subordinated to the whole organism. Consequently, in relation to unicellular individuals, such a colony can be regarded as an individuum of a higher order. In all other animals, except Protozoa, the individuum as a rule is composed of a multitude of cells specialized in various directions. It is highly probable that such multicellular individua originated from protozoan colonies through a closer union of the individuals that composed them. In this case, the individuum of multicellular organisms morphologically corresponds not to an individual, but to a colony of unicellular organisms. However, in a physiological respect, the individuum is equivalent in both groups of animals; only in one of them are the organs of the individual multicellular complexes, while in Protozoa they are separate regions of the single cell forming the individual, or organelles. — In turn, the individua of many multicellular animals, through incomplete asexual reproduction, can give rise to colonies, i.e., individuals of the next higher order. Each individual in such a colony once again acts as it were as an organ in relation to the colony taken as a whole. In most cases, the individuality of the individuals making up the colony is more or less well preserved and clearly discernible. Thus, in hydroid polyps, usually each individual of the colony, or hydranth, independently captures prey, digests food, and is more or less sharply demarcated from other hydranths. In other cases, for example, in Bryozoa, some individuals of the colony lose most of their organs and turn into simple pincers (avicularia) serving as defensive organs for the entire colony. The individuality of such individuals is strongly suppressed, and they turn in the full sense of the word into organs of the colony (siphonophores). A similar change is experienced in some hydroid polyps by medusae formed on their colonies through budding. In some cases, medusae, from isolated and even capable of separating from the colony and leading an independent life, are reduced to the degree of sacks with germ cells sitting on the colony. Finally, in a number of lower invertebrates, especially in many sponges, individual members of the colony merge with each other so closely that the boundaries between them become completely indistinguishable and the individuality of the colony's individuals is destroyed. It is even more difficult to apply the concept of the individuum in relation to higher multicellular plants, which is why the term organism is little used in botany.