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 of animal enlarged. m, Mouth. c, Digestive cavity. s, Sac into which the tentacles are withdrawn. p, Bands with comb-like plates. 3, Portion of a band enlarged to show the moving plates p.]

Let us take the Cydippe first (1, Fig. 9).  I have six here, each in a separate tumbler, and could have brought many more, for when I dipped my net in the pool yesterday such numbers were caught in it that I believe the retreating tide must just have left a shoal behind.  Put a tumbler on the desk in front of you, and if the light falls well upon it you will see a transparent ball about the size of a large pea marked with eight bright bands, which begin at the lower end of the ball and reach nearly to the top, dividing the outside into sections like the ribs of a melon.  The creature is so perfectly transparent that you can count all the eight bands.

At the top of the ball is a slight bulge which is the mouth (m 2, Fig. 9), and from it, inside the ball hangs a long bag or stomach, which opens below into a cavity, from which two canals branch out, one on each side, and these divide again into four canals which go one into each of the tubes running down the bands.  From this cavity the food, which is digested in the stomach, is carried by the canals all over the body.  The smaller tubes which branch out of these canals cannot be seen clearly without a very strong lens, and the only other parts you can discern in this transparent ball are two long sacs on each side of the lower end.  These are the tentacle sacs, in which are coiled up the tentacles, which we shall describe presently.  Lastly you can notice that the bands outside the globe are broader in the middle than at the ends, and are striped across by a number of ridges.