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