Thus far, these aggregates of the second order exhibit but indefinite individualities. The integration is physical; but not physiological. Though, in the _Polycytharia_, there is a shape that has some symmetry; and though, in the _Foraminifera_, the formation of successive chambers proceeds in such methodic ways as to produce quite-regular and tolerably-specific shells; yet no more in these than in the Sponges or the compound _Vorticellæ_, do we find such co-ordination as gives the whole a life predominating over the lives of its parts. We have not yet reached an aggregate of the second order, so individuated as to be capable of serving as a unit in still higher combinations. But in the class _Cœlenterata_, this advance is displayed. The common _Hydra_, habitually taken as the type of the lowest division of this class, has specialized parts performing mutually-subservient functions, and thus exhibiting a total life distinct from the lives of the units. Fig. 146 represents one of these creatures in its contracted state and in its expanded state; while Fig. 147 is a diagram showing the wall of this creature’s sac-like body as seen in section under the microscope: _a_ and _b_ being the outer and inner cellular layers; while between them is the “mesoglœa” or “structureless lamella,” the supporting or skeletal layer. But this lowly-organized tissue of the Hydra, illustrates a phase of integration in which the lives of the minor aggregates are only partially-subordinated to the life of the major aggregate formed by them. For a _Hydra’s_ substance is separable into _Amœba_-like portions, capable of moving about independently. If we bear in mind how analogous are the extreme extensibility and contractility of a _Hydra’s_ body and tentacles, to the properties displayed by the sarcode among Rhizopods; we may infer that probably the movements and other actions of a _Hydra_, are due to the half-independent co-operation of the _Amœba_-like individuals composing it.