hollow cylinder closed at one end, by which it attaches itself, while at
the upper end, surrounded by a group of tentacles, is the mouth which
leads to the central cavity. The wall of this simple body is composed of
two layers of cells, between which there is a gelatinous layer rarely
invaded by cells. The inner layer lines the central space into which food
organisms are thrust by the tentacles, and it is concerned primarily with
digestion. The outer layer comprises cells for protection and sensation
primarily. Cells of both layers have muscular prolongations which by their
operation enable the whole animal to change its form and to move from one
place to another.

It may seem that such an animal is totally unlike any of the higher and
more complex types. In certain respects, however, it is identical with the
other forms inasmuch as it performs all of the eight biological tasks
demanded by nature. It is also similar in so far as its inner layer, like
the innermost sheet of cells in higher forms, is concerned with problems
of taking and preparing food, while the protective outer layer resembles
in function the outermost covering of all animals higher in the scale.
Beyond these a still more fundamental agreement is found in its cellular
composition.

At the lower end of the animal scale are organisms which consist of one
cell and nothing more. _Amoeba_, to which we must refer again and again,
is an example of this group which possesses an overwhelming importance to
the comparative student because the origins of all the characteristics of
animals higher in the scale are to be found within it. _Amoeba_ itself
is a naked mass of protoplasm, about 1/100 of an inch in diameter,
enclosing a nucleus. Its form is not constant during activity, for
fingerlike processes called pseudopodia are pushed out tentatively in many
directions to be followed as circumstances direct by the materials of the
whole cell body. Other protozoa differ in possessing constant forms, or in
having constant vibratile processes, or shells of some kind, while in
still other cases like individuals combine to make colonies which are more
or less definite and permanent. Here at the very foot of the organic scale
are found animals which seem to be entirely different from those above.
Upon examination they, like _Hydra_, prove to be the same as regards the
number and kind of functions they perform, but in structural regards their
evolutionary relation to all hig