For a long time now we have comprised under the general name of amoebae a number of microscopic unicellular organisms, which are very widely distributed, especially in fresh-water, but also in the ocean; in fact, they have lately been discovered in damp soil. There are also parasitic amoebae which live inside other animals. When we place one of these amoebae in a drop of water under the microscope and examine it with a high power, it generally appears as a roundish particle of a very irregular and varying shape (Figures 1.16 and 1.17). In its soft, slimy, semi-fluid substance, which consists of protoplasm, we see only the solid globular particle it contains, the nucleus. This unicellular body moves about continually, creeping in every direction on the glass on which we are examining it. The movement is effected by the shapeless body thrusting out finger-like processes at various parts of its surface; and these are slowly but continually changing, and drawing the rest of the body after them. After a time, perhaps, the action changes. The amoeba suddenly stands still, withdraws its projections, and assumes a globular shape. In a little while, however, the round body begins to expand again, thrusts out arms in another direction, and moves on once more. These changeable processes are called "false feet," or pseudopodia, because they act physiologically as feet, yet are not special organs in the anatomic sense. They disappear as quickly as they come, and are nothing more than temporary projections of the semi-fluid and structureless body. (FIGURE 1.17. Division of a unicellular amoeba (Amoeba polypodia) in six stages. (From F.E. Schultze.) the dark spot is the nucleus, the lighter spot a contractile vacuole in the protoplasm. The latter reforms in one of the daughter-cells.) FIGURE 1.18. Ovum of a sponge (Olynthus). The ovum creeps about in a body of the sponge by thrusting out ever-changing processes. It is indistinguishable from the common amoeba.)