This crowd of beings united in a common life began as one such little Hydra-like animal as I have described above,--floating free at first, then becoming attached, and growing into a populous stock by putting out buds at different heights along the length of the stem. The formation of such a bud is very simple, produced by the folding outwardly of the double wall of the body, appearing first as a slight projection of the stem sideways, which elongates gradually, putting out tentacles as it grows longer, while at the upper end an aperture is formed to make the mouth. This is one of the lower group of Radiates, known as Hydroids, and long believed to be Polyps, from their mode of living in communities and reproducing their kind by budding, after the fashion of Corals. But if such a little tuft of Hydroids has been gathered in spring, a close observer may have an opportunity of watching the growth of another kind of individual from it, which would seem to show its alliance with the Acalephs rather than the Polyps. At any time late in February or early in March, bulb-like projections, more globular than the somewhat elongated buds of the true Hydroid heads, may be seen growing either among the tentacles of one of these little animals, or just below the head where it merges in the stem,[3] Very delicate and transparent in substance, it is hardly perceptible at first; and the gradual formation of its internal structure is the less easily discerned, because a horny sheath, forming the outer covering of the Hydroid stock, extends to inclose and shield the new-comer, whom we shall see to be so different from the animal that gives it birth that one would suppose the Hydroid parent must be as much surprised at the sight of its offspring as the Hen that has accidentally hatched a Duck's egg. At the right moment this film is torn open by the convulsive contractions of the animal, which, thus freed from its envelope, begins at once to expand. By this time this little bud has assumed the form of a Medusoid or Jelly-Fish disk, with its four tubes radiating from the central cavity. The proboscis, so characteristic of all Jelly-Fishes, hangs from the central opening; and the tentacles, coiled within the internal cavity up to this time, now make their appearance, and we have a complete little Medusa growing upon the Hydroid head. Gradually the point by which it is attached to the parent-stock narrows and becomes more and more contracted, till the animal drops off and swims away, a free Jelly-Fish. [Illustration: Little Jelly-Fish, commonly called Sarsia, the free Medusa, of Coryne mirabilis.]