This type, however,
which is known as the Flagellate, may be derived from the next,
which we will take as the primitive and fundamental animal type.
It is best seen in the common and familiar Amoeba, a minute sac
of liquid or viscid plasm, often not more than a hundredth of an
inch in diameter. As its "skin" is merely a finer kind of the
viscous plasm, not an impenetrable membrane, it takes in food at
any part of its surface, makes little "stomachs," or temporary
cavities, round the food at any part of its interior, ejects the
useless matter at any point, and thrusts out any part of its body
as temporary "arms" or "feet."
Now it is plain that in an age of increasing microbic cannibalism
the toughening of the skin would be one of the first advantages
to secure survival, and this is, in point of fact, almost the
second leading principle in early development. Naturally, as the
skin becomes firmer, the animal can no longer, like the Amoeba,
take food at, or make limbs of, any part of it. There must be
permanent pores in the membrane to receive food or let out rays
of the living substance to act as oars or arms. Thus we get an
immense variety amongst these Protozoa, as the one-celled animals
are called.
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