It is an interesting fact, that while the direction in which the cilia propel fluids and particles is generally toward the interior of the organism, it is sometimes _reversed_, and, instead of beating the particles inward, the cilia energetically beat them back if they attempt to enter. Fatal results would ensue if this were not so. Our air-passages would no longer protect the lungs from particles of sand, coal-dust, and filings flying about the atmosphere; on the contrary, the lashing hairs which cover the surface of these passages would catch up every particle and drive it onward into the lungs. Fortunately for us, the direction of the cilia is reversed, and they act as vigilant janitors, driving back all vagrant particles with a stern “No admittance, _even_ on business!” In vain does the whirlwind dash a column of dust in our faces--in vain does the air, darkened with coal-dust, impetuously rush up the nostrils; the air is allowed to pass on, but the dust is inexorably driven back. The swimming apparatus of the Opalina has led us far away from the little animal who has been feeding while we have been lecturing. At the mention of feeding you naturally look for the food that is eaten, the mouth and stomach that eat. But I hinted just now that this ethereal creature dispenses with a stomach, as too gross for its nature, and, of course, by a similar refinement, dispenses with a mouth. Indeed, it has no organs whatever except the cilia just spoken of. And this leads us to consider what biologists mean by an _organ_: it is a particular portion of the body set apart for the performance of some particular function. The whole process of development is this setting apart for special purposes. The starting-point of Life is a single cell--that is to say, a microscopic sac, filled with liquid and granules, and having within it a nucleus, or smaller sac. Paley has somewhere remarked that in the early stages there is no difference discernible between a frog and a philosopher. It is very true--truer than he conceived. In the earliest stage of all, both the Batrachian and the Philosopher are nothing but single cells, although the one cell will develop into an Aristotle or a Newton, and the other will get no higher than the cold, damp, croaking animal which boys will pelt, anatomists dissect, and Frenchmen eat. From the starting-point of a single cell this is the course taken: the cell divides itself into two, the two become four, the four eight, and so on, till a mass of cells is formed not unlike the shape of a mulberry. This mulberry-mass then becomes a sac, with double envelopes or walls; the inner wall, turned toward the yelk, or food, becomes the _assimilating_ surface for the whole; the outer wall, turned toward the surrounding medium, becomes the surface which is to bring frog and philosopher into contact and relation with the external world--the Non-Ego, as the philosopher in after life will call it.