Attention has already been drawn to the difference between those lowly organisms, each of which is composed of a single cell--the protozoa, as they are termed--and those higher organisms, called metazoa, in which there are many cells with varied functions. Confining our attention at first to the former group of unicellular animals, we find considerable diversities of form and habit, from the relatively large, sluggish, parasitic _Gregarina_, to the active slipper-animalcule, or _Param[oe]cium_, or the beautiful, stalked bell-animalcule, or _Vorticella_; and from the small, slow-moving am[oe]ba to the minute, intensely active monad. In many cases reproduction is by simple fission, as in the am[oe]ba, where the nucleus first undergoes division; and then the whole organism splits into two parts, each with its own nucleus. In other cases, also numerous, the organism passes into a quiescent state, and becomes surrounded with a more or less toughened cyst. The nucleus then disappears, and the contents of the cyst break up into a number of small bodies or spores. Eventually the cyst bursts, and the spores swarm forth. In the case of some active protozoa the minute creatures that swarm forth are more or less like the parent; but in the more sluggish kinds the minute forms are more active than the parent. Thus in the case of the gregarina, the minute spore-products are like small am[oe]bæ; while in other instances the embryos, if so we may call them, have a whip-like cilium like the monads. Very frequently, however, there is, in the protozoa, a further process, which would seem to be intimately associated with fission or the formation of spores, as the case may be. This is known as conjugation. Among monads, for example, two individuals may meet together, conjugate, and completely fuse the one into the other. A triangular cyst results. After a while, the cyst bursts, and an apparently homogeneous fluid escapes. The highest powers of the microscope fail to disclose in it any germ of life; and there, at first sight, would seem to be an end of the matter. But wait and watch; and there will appear in the field of the microscope, suddenly and as if by magic, countless minute points, which prolonged watching shows to be growing. And when they have further grown, each distinct point is seen to be a monad.