The _Hydra_, with which we started, had always the power of producing eggs; each animal could do so, besides producing buds. But in our Colonial Coralline this is not necessarily so. Some individuals lose the power of producing eggs. Others can do nothing else, and become greatly altered in structure, often losing the power of developing tentacles, and exhibiting other changes. So much are they altered sometimes that they seem to be mere buds, not separate animals at all. In other cases a still more surprising thing happens. The bud that is destined to produce eggs falls off, and becomes quite independent of the colony; more than this, it becomes quite different in appearance from the members of the colony: and instead of being a Hydra-like animal it becomes a jelly-fish. But the eggs of this jelly-fish do not produce jelly-fishes: they produce a more or less Hydra-like animal which gives rise by budding to a fresh colony. This is what is known to Zoologists as "alternation of generations." Now comes a puzzling question--Which part of this family group shall we select and call it an "animal"? Is each Hydroid of the colony an animal, and the jelly-fish another animal? Zoologists say "No": from the development of one egg, to the production of another, is the cycle that constitutes an individual animal. So we have the puzzling result in nomenclature, that an "individual" consists of a very large colony of creatures in one place, together with a perfect shoal of creatures quite unlike it, floating miles away from it on the ocean. What name must we give to the units, so curiously connected with one another? Zoologists call them "Zooids" (animal-like parts) or "persons." This is the story of the jelly-fish as originally told. But there are innumerable variations upon it. There are kinds of jelly-fish that produce jelly-fish and have no Hydroid stage at all. Sometimes the "persons" of the colony present many varieties, each taking up some different task for the community. Some may be "nutritive persons," _i.e._ commonplace Zooids that have mouths and eat food; some "protective persons," reduced to mere folds or sheathing processes to guard the others; some are "stinging persons" armed with enormous quantities of thread cells. Then the whole colony may be like the jelly-fish, a floating affair, and not fixed at all. [Illustration: FIG. 9.--An example of the Hydrozoa. A, branch of a Coralline, _Sertularia Ellisii_, magnified. B, the same, more highly magnified.]