ogical history of the globe. These are the protozoa. They
include, first of all, the foraminifera, which usually have shells
composed of carbonate of lime. These shells, settling to the bottom
of the ocean, have accumulated in vast beds, and when compacted and
raised above the surface, form chalk, limestone, or marble,
according to the degree and mode of their hardening.

The protozoa include also the flagellata, a great, very poorly
defined mass of forms occupying the boundary between the plant and
animal kingdoms. They are usually unicellular, and their protoplasm
is surrounded by a thin, structureless membrane. This prevents their
putting out pseudopodia as organs of motion. Instead of these they
have at one end of the ovoid or pear-shaped body a long,
whiplash-like process or thread, a flagellum, and by swinging this
they propel themselves through the water. These flagellata seem to
have a rather marked tendency to form colonies. The first individual
gives rise to others by division. But the division is not complete;
the new individuals remain connected by the undivided rear end of
the body. And such a colony may come to contain a large number of
individuals.

[Illustration: 2. MAGOSPHAERA PLANULA. LANG, FROM HAECKEL.]

Such a colony is represented by magosphaera. This is a microscopic
globular form, discovered by Professor Haeckel on the coast of
Norway. It consists of a large number of conical or pear-shaped
individual cells, whose apices are turned toward the centre of the
sphere. The cells are cemented together by a mucilaginous substance.
Around their exposed larger ends, which form the surface of the
sphere, are rows of flagella, by whose united action the colony
rolls through the water. After a time each individual absorbs its
flagella, the colony is broken up, the different individuals settle
to the bottom, and each gives rise by division to a new colony. This
group of cells may be considered as a colony or as an individual.
Each term is defensible.

Volvox is also a spheroidal organism, composed often of a very large
number of flagellated cells. But it differs from magosphaera in
certain important respects. In the first place its cells have
chlorophyl, the green coloring matter of plants. It lives therefore
on unorganized fluid nourishment, carbon dioxide, nitrates, etc. It
is a plant. But certain characteristics render it probable that it
once lived on solid food and was therefore an animal. For whe