an aggregate of attached individuals that originated as a result of incomplete division or budding. Formation of colonies is characteristic of certain unicellular algae and aquatic invertebrate animals. Colonies are diverse in shape, size, and arrangement of individuals; they may be free-swimming or attached. Among the free-swimming colonies in plants, a spherical shape is most common (for example, Volvox); there are also elliptical and, less frequently, cylindrical, filiform, and branched forms. Among attached colonies in plants there are filiform, saccate, saccate-lamelliform, and dendroid forms. Primitive colonies in plants are characterized by an even distribution of the cells within the body of mucus that unites them. In more highly organized colonies there is some differentiation; some cells move to the periphery of the colony.
In animals a true colony has a common body that does not belong to any one individual (zooid). Sometimes all the zooids in a colony have the same structure (monomorphic colony). More often there is morphological and physiological differentiation (polymorphic colony). Some individuals perform the functions of feeding; others, of defense; and a third group, of reproduction. As a result of this specialization, the zooids depend on one another and cannot exist outside the colony. The colony itself may be considered as an individual of a higher order. The individuality of a colony is determined by its morphology and the distinctive development characteristic of each species of colonial animals. In primitive colonies the individual zooids exchange nutritive matter (bryozoans, hydrozoans, the majority of coral polyps, and colonial ascidians). In highly organized colonies, such as pennatularians, a stimulus is transmitted from one zooid to another. In some, such as the Siphonophora and Pyrosomata, coordinated movements are observed.
These types of colonies must not be confused with the families of social animals (ants, bees, and termites) whose individual members are not attached to one another. Societies of animals with individuals that touch each other but do not have a common body should not be classified as colonies; the individuals of these societies originate from different parents (for example, the pseudocolony of the genus Cephalodiscus of the order Pterobranchia and the pseudocolony of some Mytilus). Sometimes the temporary cooperative settlements of certain birds that appear during the periods of reproduction and feeding of nestlings are called colonies.
D. V. NAUMOV and T. V. SEDOVA