rise to a third bud, which starts a new biserial system, from which uniserial pinnules arise as from the main stem--type of _Aglaophenia_ (fig. 19). The laws of budding in hydroids have been worked out in an interesting manner by H. Driesch [13], to whose memoirs the reader must be referred for further details. [Illustration: FIG. 17.--Diagram of sympodial budding, simple unbranched _Plumularia_-type. F, founder; 1-8, main axis formed by biserial budding from founder; a-e, pinnule formed by uniserial budding from founder; a¹-d¹, branch formed by similar budding from 1; a²-d² from 2, and so forth.] _Individualization of Polyp-Colonies._--As in other cases where animal colonies are formed by organic union of separate individuals, there is ever a tendency for the polyp-colony as a whole to act as a single individual, and for the members to become subordinated to the needs of the colony and to undergo specialization for particular functions, with the result that they simulate organs and their individuality becomes masked to a greater or less degree. Perhaps the earliest of such specializations is connected with the reproductive function. Whereas primitively any polyp in a colony may produce medusa-buds, in many hydroid colonies medusae are budded only by certain polyps termed _blastostyles_ (fig. 10, b). At first not differing in any way from other polyps (fig. 5), the blastostyles gradually lose their nutritive function and the organs connected with it; the mouth and tentacles disappear, and the blastostyle obtains the nutriment necessary for its activity by way of the coenosarc. In the Calyptoblastea, where the polyps are protected by special capsules of the perisarc, the _gonothecae_ enclosing the blastostyles differ from the hydrothecae protecting the hydranths (fig. 54). [Illustration: FIG. 18.--Diagram showing method of branching in the _Plumularia_-type; compare with fig. 17. Polyps 3 and 6, instead of producing uniserial pinnules, have produced biserial branches (3¹, 3², 3³, 3^4; 6¹-6³), which give off uniserial branches in their turn.] In other colonies the two functions of the nutritive polyp, namely, capture and digestion of food, may be shared between different polyps (fig. 10). One class of polyps, the _dactylozoids_ (dz), lose their mouth and stomach, and become elongated and tentacle-like, showing great activity of movement. Another class, the _gastrozoids_ (gz), have the tentacles reduced or absent, but have the mouth and stomach enlarged. The dactylozoids capture food, and pass it on to the gastrozoids, which swallow and digest it.