Z. alternans is a vorticellid which forms a colony having a definite symmetry. All members of the colony are arranged in the same plane, but alternate with one another on the 2 sides of the main axis. The colony originates from a single "ciliospore" which subsequently divides, giving rise to individuals that remain connected by the protoplasmic core of the main stalk and its branches. This fact, together with the arrangement of the individual zooids, permits of the interpretation of the animal as a multicellular organism. The author studied the growth and life-history of the colony from this aspect and found that the "growth-potential" of certain cells varied, the terminal cells (macrozooids) retaining the highest rate and also an apparently unlimited power of multiplication. There was for the colony as a whole a definite growth-rate curve which resembled that of a multicellular organism, and the power of growth of the microzooids was divided among them according to a kind of gradient. The successive divisions by which the colony is formed are often definitely differential, both protoplasm and nucleus being unequally divided, and are described in some detail by the author. In addition to the micro- and macrozooids which make up the major part of the colony certain larger individuals are formed from time to time along the main trunk; these become ciliospores which swim away to produce new colonies. Such individuals go through an endomictic process during their development, and contain certain secretory granules which are not found in any other cells of the colony. It is suggested that these may have something to do with the limitation of growth of the latter, but that whether the growth limiting factor is this or something else it is quite independent of any "factor of senescence.".