COLONIAL, or COMPOUND, ANI MALS, organisms which cannot be fairly re garded as unities, 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 infre quently 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 repro ducing discontinuously, retains its daughter cells in union with itself or with one another, much like the segments of the egg-cell of a higher animal. By sacrifice of individuality at the time of reproduction, a higher unity is formed. In the same way a simple cup-shaped sponge, by continuous budding, fortns a colony of similar forms, which may possess more or less distinct individuality. The common fresh water Hydra, to mount a ste,p higher, buds off daughter Hydra., which remain for a while con nected with the parent organism and make it temporarily colonial.
Many marine hydroids retain their polyps permanently as constituents of such a complex organism, and, as a differentiation of these into nutritive and reproductive, and often into still other kinds, takes place, a higher order of more complex individuality arises; the polyp life is in part subordinated to the colony life. Among
the most perfectly org-anized of such colonies are the floating Stphonophora (such as the Por tuguese man-of-war), in which five or six per fectly differentiated and specialized kinds of zooids act together in mutual interdependence to support the whole. In most cases such zooids are incapable of performing any but the one function, or set of functions, to which they are adapted, and consequently cannot sustain inde pendent life. This is one way of approaching the questions of individuality and of organiza tion of the higher Metazoa. Among the higher animals that occur in colonial form are Poly zoa, Ascidia and Chetopoda. See COMPARATIVE