[Illustration: FIG. 3.--_Amoeba_, a typical unicellular animal: _n_, nucleus; _cv_, contractile vacuole; _ps_, pseudopodia; highly magnified. This represents Grade I. of animal existence.] GRADE I. _The One-Celled Animals._--_Amoeba_, the Mobile animal, is the typical example of these. It consists of a single microscopic cell. In this cell is seen a dark irregular speck, the nucleus, which is an essential character of cells, whether they are independent or form part of the body of a larger animal. There is often visible also a clear rounded space, called the "contractile vacuole," which squeezes out fluid, disappears, and reappears again, serving the purpose of excretion. The cell-substance, called protoplasm, is practically identical in this and in cells of all other kinds. It is jelly-like, and capable of a slow movement, which may be watched under the microscope. It suggests the flowing of treacle or thick gum. The movement may be traced by the change in outline of the cell and by the change in position of any granules that it may have taken in; for particles which touch the creature sink in and are surrounded; thus it obtains its food. These slow flowing movements of the protoplasm result in continual changes of shape; hence the name, Amoeba, the mobile animal. Sometimes the island of protoplasm, as it changes its shape, throws out, as it were, capes and headlands. These projections, which are presently drawn in again, are called pseudopodia or false feet. They are characteristic of the whole group of Amoeba-like animals, which are consequently called Rhizopoda, the root-footed. The production of new individuals is accomplished by the division of the old cell into two. Thus it may be said that there is always a bit of the old cell remaining, though divided into fragments; and for this reason the Amoeba-like Protozoans have been fancifully called "immortal." [Illustration: FIG. 4.--Section, highly magnified, of a two-layered animal, _Hydra_ (Grade II.). _Ec_, outer layer of Ectoderm; _En_, inner layer of Endoderm; _l_, lamella dividing the two, represented by a line; _n_, nuclei of the cells; _v_, thin vacuoles of small interstitial cells; _E_, the Enteron or digestive cavity.]