CHAPTER II

PROTOZOA TO WORMS: CELLS, TISSUES, AND ORGANS


The first and lowest form in our ancestral series is the amoeba, a
little fresh-water animal from 1/500 to 1/1000 of an inch in
diameter. Under the microscope it looks like a little drop of
mucilage. This semifluid, mucilaginous substance is the Protoplasm.
Its outer portion is clear and transparent, its inner more granular.
In the inner portion is a little spheroidal body, the nucleus. This
is certainly of great importance in the life of the animal; but just
what it does, or what is its relation to the surrounding protoplasm
we do not yet know. There is also a little cavity around which the
protoplasm has drawn back, and on which it will soon close in again,
so that it pulsates like a heart. It is continually taking in water
from the body, or the outside, and driving it out again, and thus
aids in respiration and excretion. The animal has no organs in the
proper sense of the word, and yet it has the rudiments of all the
functions which we possess.

A little projection of the outer, clearer layer of protoplasm, a
pseudopodium, appears; into this the whole animal may flow and thus
advance a step, or the projection may be withdrawn. And this power
of change of form is a lower grade of the contractility of our
muscular cells. Prick it with a needle and it contracts. It
recognizes its food even at a microscopic distance; it appears
therefore to feel and perceive. Perhaps we might say that it has a
mind and will of its own. It is safer to say that it is irritable,