Page:Popular Science Monthly Volume 16.djvu/659

 of the individuals which compose it? We have no need of hypothesis, however, to demonstrate that these colonial organs are the equivalents of true individuals. The buds that give birth to the different sorts of individuals in a colony of Hydractinia all grow alike, and remain



alike for a long time. This is the first presumption in favor of their equivalence. But in the allied type, Podocoryne, we see the humble sac, which represents the sexual individual, replaced by a being more active, more elegant, much more elevated than the Hydra itself, by a transparent medusa, which is detached when it reaches maturity and swims actively in the water, the colony suffering no inconvenience from the change (Fig. 2). These medusæ constitute the most general



form of the sexual individuals in the group of Hydroid Polyps, but they are very polymorphic. Their form is modified from one species