Colonial Animals, organisms which cannot be fairly regarded as units, but consist of numerous more or less similar individuals united in a common life. Among the usually single-celled simplest animals or Protozoa, loose colonies not unfrequently occur, and are of not a little importance as suggestions of the bridge between the single-celled and many-celled animals. Such colonies arise when the original cell, instead of reproducing discontinuously, retains its daughter-cells in union with itself or with one another, just like the egg-cell of a higher animal. By sacrifice of individuality at the epoch of reproduction, a higher unity is formed. In the same way a simple cup-shaped sponge, by continuous budding, forms a colony of similar forms, which may possess more or less distinct individuality. The common fresh-water Hydra, to mount a step higher, buds off daughter Hydræ, which remain for a while connected with the parent organism, and make it temporarily colonial. This becomes constant in the myriad colonies of hydra-like forms which are known as Zoophytes, or Hydroids (q.v.). The same formation of colonies is well illustrated in the higher polyps of the sea-anemone type, where in dead-men's fingers and in most corals, compound or colonial organisms are beautifully illustrated. Some jellyfish-like or medusoid types also become compound, and lead on to the order Siphonophora, where, as in the Portuguese man-of-war (Physalia) or in Velella, the occurrence of compound forms is a constant character. But in this last case a further step has been made, for the individuals not only share a common nutritive life, but are bound together in a more intimate way into a true unity of a higher order. Among simple worm-like forms, chains of individuals are occasionally formed, as in Catenula, and these point to the probable origin of the higher or segmented worms which consist of a series of similar 'joints.' Even among the higher worms, temporary compound forms occasionally occur, as in the fresh-water Nais or the marine Syllis. The Polyzoa afford good illustration of colonial life as an almost constant character of a class. Finally, passing beyond the limit of invertebrate animals, we find the last examples of true colonial organisms in the Ascidians (q.v.) or Tunicata. The occasional development of double or multiple forms from a single ovum is among higher animals the only suggestion of compound organisms.
From the above illustrations a few general conclusions may be drawn. The process of asexual budding, which leads to the formation of compound organisms, is commonest in relatively passive vegetative animals, like sponges, hydroids, corals, polyzoa, and ascidians, and is an expression of their general physiological constitution. When the colonial organism is free-living, as in Siphonophora and some Tunicata, the individual members are more closely knit together, and the colony is more perfectly integrated. As above suggested, some colonial forms are of importance as illustrating in loose union an order of individuality, which in higher types becomes more firmly unified. We first find loose 'aggregates;' at a higher level these become 'integrated.' The loose colonies of some medusoid types are thus succeeded by more perfectly integrated forms in the Siphonophora. Finally, colonial organisms are of the highest physiological interest in illustrating division of labour. In a compound organism the internal and external conditions of life are not uniform for all the members; certain 'persons,' as the individuals are technically called, exhibit the predominance of one function, and others of another; the differences in function thus started bring about more or less marked difference of structure as its consequence; and thus division of labour and 'polymorphism' or difference in form are established. See Perrier, Les Colonies Animales (1882).