This distinction, you see, is one of structure, as must be all the subdivisions that follow, if they are to be natural; and it is the clearest possible illustration of what we mean in zoölogy when we speak of "lower" and "higher" rank, for it is evident that it is a step upward, an advance from utter simplicity to greater and greater complexity, to proceed from a single-celled, all but helpless animalcule to one composed of many cells, with so vast a division of labor and extensive power of action as belong to such a combination of forces. I do not propose to describe the Protozoa, because both of lack of space and lack of popular interest; anyone may learn about them in any good zoölogical textbook. But I do want to mention one very important point, on account of its bearing on the history of the higher animals. The protozoans reproduce their kind by simply splitting into two individuals, and these again split into another two, and so on; the process is called "fission." There comes a time, however, when the ability to do this ceases, and the protozoans of this strain will die out unless one or more of them meets with the same kind of animalcule, and the two "conjugate," or merge into one another, thus renewing their power to go on dividing. Turning now to the Metazoa, or animals in general, we may say that they are flexible and usually motile beings, needing a supply of solid food which they convert by digestion into a fluid form, and then diffuse through their tissues. This accounts for the fact that all animals consist essentially of a tube, which in the simpler forms is very apparent. This typical tube consists of at least two layers--an outer, protective, and sensitive coat (ectoderm), and an inner, digestive one (endoderm). This two-layered condition is the limit for a few fresh-water and a vast number of marine animals therefore called "coelenterata," of which the jellyfish and corals are examples. The two coats are separated, and at the same time connected, by a greater or less amount of a jellylike filling called the "mesenchyme." Into this intermediate mesenchyme both ectoderm and endoderm bud off cells which have certain functions--that is, they circulate the digested food, perform the creeping movements when such occur, expel the waste of the body, and most important of all, provide the germ cells by which the race is perpetuated. Now in animals superior to the jellyfishes and the flatworms, the mesenchyme is replaced by a definite hollow tissue that produces a more efficient system of muscular, excretory, and reproductive organs. This hollow tissue is the "coelom," and in the most advanced animals, such as the chordates, "the coelom and its products are of the greatest importance, for they give rise to the vertebræ and the muscles, and in so doing mold the shape of the fish, amphibian, reptile, bird, and mammal."