They are very small and circular, and the interior is clothed with cells very similar to the cilia cells in higher animal life. These cells are arranged around the ciliated chambers in the form of a collar, and from each cell flagella protrude, which are in continual motion.

It will have struck you that the new organism enters upon its career with only two flagella, and the normal organism is possessed of four. But in a few minutes, three or four at most, the full complement were always there. How they were acquired it was the work of months to discover, but at last the mystery was solved.

But they all agree in certain characters; all take their food and oxygen and carry on excretory processes by osmosis, i.e., through the body-wall; all are capable of some kind of locomotion, some have one or more flagella, others move by a pseudopod movement. Some are capable of moving from cell to cell in the body as do the white blood-corpuscles.

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.

Rapid growth set in, and at the end of another hour, how has entirely baffled us, they acquired flagella and swam freely; in thirty-five minutes more they possessed a nucleus and rapidly developed, until at the end of nine hours after emission a sporule was followed to the parent condition and left in the act of fission.

Meanwhile, the diffluence causes a spreading and flattening of the sarcode and swimming gives place to creeping, while the flagella violently lash. In this condition two forms meet by apparent accident, the protrusions touch, and instant fusion supervenes.

These flagella keep up a lashing to and fro in the liquid, and the lashing serves to propel the bacteria through the liquid. It is hardly possible to say much about the structure of the bacteria beyond the description of their external forms. With all the variations in detail mentioned, they are extraordinarily simple, and about all that can be seen is their external shape.

In all its structure it tells us of something earlier and far simpler, but what this earlier ancestor was we do not know. Rather more highly organized relatives of the amoeba, the flagellata, have produced a membrane, and swim by means of vibratile, whiplash-like flagella.

This condition tensified, the amoeboid action quickened as here depicted, the agility of motion ceased, the nucleus body became strongly developed, and the whole sarcode was in a state of vivid and glittering action. If now it be sharply and specially looked for, it will be seen that the root of the flagella splits, dividing henceforth into two separate pairs.

There still remains the difficulty of explaining why the male gametocytes after reduction develop into similar sperms, with their heads and long flagella, although half of them possess one X chromosome each and the other half none.