In the sympodial method of budding, on the other hand, the founder-polyp is of limited growth, and forms a bud from its side, which is also of limited growth, and forms a bud in its turn, and so on (figs. 15, 16). Hence, in a colony of calyptoblastic hydroids, the oldest polyp of a system is the lowest; the youngest polyp is the topmost one; and the axis of the system is a false axis composed of portions of each of the consecutive polyps. In this method of budding there are two types. In one, the biserial type (fig. 15), the polyps produce buds right and left alternately, so that the hydranths are arranged in a zigzag fashion, forming a "scorpioid cyme," as in _Obelia_ and _Sertularia_. In the other, the uniserial type (fig. 16), the buds are formed always on the same side, forming a "helicoid cyme," as in _Hydrallmania_, according to H. Driesch, in which, however, the primitively uniserial arrangement becomes masked later by secondary torsions of the hydranths. [Illustration: FIG. 15.--Diagram of sympodial budding, biserial type, shown in five stages (1-5). F, founder-polyp; 1, 2, 3, 4, 5, 6, succession of polyps budded from the founder; a, b, c, second series of polyps budded from the founder; a³, b³, series budded from 3.] [Illustration: FIG. 16.--Diagram of sympodial budding, uniserial type, shown in four stages (1-4). F, founder-polyp; 1, 2, 3, succession of polyps budded from the founder.] In a colony formed by sympodial budding, a polyp always produces first a bud, which contributes to the system to which it belongs, i.e. continues the stem or branch of which its parent forms a part. The polyp may then form a second bud, which becomes the starting point of a new system, the beginning, that is, of a new branch; and even a third bud, starting yet another system, may be produced from the same polyp. Hence the colonies of Calyptoblastea may be complexly branched, and the budding may be biserial throughout, uniserial throughout, or partly one, partly the other. Thus in _Plumularidae_ (figs. 17, 18) there is formed a main stem by biserial budding; each polyp on the main stem forms a second bud, which usually forms a side branch or _pinnule_ by uniserial budding. In this way are formed the familiar feathery colonies of _Plumularia_, in which the pinnules are all in one plane, while in the allied _Antennularia_ the pinnules are arranged in whorls round the main biserial stem. The pinnules never branch again, since in the uniserial mode of budding a polyp never forms a second polyp-bud. On the other hand, a polyp on the main stem may form a second bud which, instead of forming a pinnule by uniserial budding, produces by biserial budding a branch, from which pinnules arise as from the main stem (fig. 18--3, 6). Or a polyp on the main stem, after having budded a second time to form a pinnule, may give