l-box. As the plant grows, the
cell enlarges and forms two more valves, one _c_ fitting into the
cover _a_, so as to make a complete box _ac_, and a second, _d_, back
to back with _c_, fitting into the valve _b_, and making another
complete _bd_. This goes on very rapidly, and in this plant each new
cell separates as it is formed, and the free diatoms move about quite
actively in the water.

If you consider for a moment, you will see that, as the new valves
always fit into the old ones, each must be smaller than the last, and
so there comes a time when the valves have become too small to go on
increasing. Then the plant must begin afresh. So the two halves of the
last cell open, and throwing out their flinty skeletons, cover
themselves with a thin jelly layer, and form a new cell which grows
larger than any of the old ones. These, which are spore-cells, then
form flinty valves inside, and the whole thing begins again.

Now, though the plants themselves die, or become the food of minute
animals, the flinty skeletons are not destroyed, but go on
accumulating in the waters of the ponds, lakes, rivers, and seas, all
over the world. Untold millions have no doubt crumbled to dust and
gone back into the waters, but untold millions also have survived. The
towns of Berlin in Europe and of Richmond in the United States are
actually built upon ground called "infusorial earth," composed almost
entirely of valves of these minute diatoms which have accumulated to a
thickness of more than eighty feet! Those under Berlin are fresh-water
forms, and must have lived in a lake, while those of Richmond belong
to salt-water forms. Every inch of the ground under those cities
represents thousands and thousands of living plants which flourished
in ages long gone by, and were no larger than those you will see
presently under the microscope.

These are a very few of the microscopic inhabitants of my pond, but,
as you will confuse them if I show you too many, we will conclude with
two rather larger specimens, and examine them carefully. The first,
called the Cydippe, is a lovely, transparent living ball, which I want
to explain to you because it is so wondrously beautiful. The second,
the Sea-mat or Flustra, looks like a crumpled drab-colored seaweed,
but is really composed of many thousands of grottos, the homes of tiny
sea-animals.

[Illustration: FIG. 9. _Cydippe Pileus_.

1, Animal with tentacles _t_, bearing small tendrils _t'_. 2, Body