Such an organism as a _Hydra_ consists of an aggregate of cells which are not all of the same kind. The outer layer, or ectoderm, is sensory and protective, and contains organs of aggression; while the inner layer consists of cells which subserve the functions of digestion and assimilation. All these parts are, in general, necessary for the life of the _Hydra_. They can be mutilated; the animal can be cut into two parts, and each of these parts may regenerate, by growth, the part that was removed. Yet the existence of ectoderm and endoderm, in a certain minimum of mass, is necessary for this regeneration. The higher animal, or Metazoon, is therefore an aggregate of cells, each of which is equivalent to the individual Protozoon; but these cells are not all alike--that is, there is differentiation of tissues in the multicellular organism. Again, the Cœlenterates provide examples of animals which are aggregates of parts, each of which is the morphological equivalent of a single _Hydra_. Such an animal as a Siphonophore, for instance, consists of zooids, and each of these units has the essential structure of a _Hydra_. But the zooids are not all alike: some of them subserve the function of locomotion, others of aggression, others of digestion and assimilation, and so on. Here, again, the whole organism may be mutilated; parts may be removed and regeneration may occur; but, as a Siphonophore, all of the different zooids must be present if the characteristic functioning of the animal is to continue.