Perhaps the lowest form of life is the _Amœba_, shown in Plate IX. Fig. 1, a mere lump of jelly, which flows along, and when it comes into contact with any likely subject for digestion flows round it, encloses it, absorbs what it can from it, and leaves it behind. A near relative of the Amœba is the _Arcella_ (Fig. 2), which is simply an Amœba with a shell. Being unable to swim, these organisms are naturally to be most often found at the bottom of the collecting bottle, and it is always advisable to take up a portion of the débris with a dipping tube, which is then held upright on a slide with the finger upon it until the dirt settles on to the slide, when it is removed, a cover-glass put upon the dirt, and a quarter-inch power used for examination. Many forms will be discovered in this way which would otherwise escape observation. Another curious organism, of great size (comparatively) and extreme beauty, is the sun animalcule (_Actínophrys_), which has a round body and long tentacles (Fig. 3), to which free-swimming organisms adhere, and by the combined action of the neighbouring ones are drawn to the body and received into it; one cannot say swallowed. Fig. 6, Plate IX., shows the curious arithmetical process whereby the Infusoria multiply by division, a groove appearing at one point, rapidly deepening, and finally separating the animal completely into two. The species is the _Chílodon_, a flattened creature, ciliated all over, having a set of teeth arranged in the form of a tube, and at its fore-part a kind of membranous lip. A similar phenomenon, in an earlier stage, is shown in Fig. 26, Plate XIII., the organism in this case being _Euplótes_. It has been said that sponges are colonies of extremely minute organisms, each furnished with a membranous collar or funnel, the whole looking like an exquisite wine-glass without a foot. These organisms are not always grouped in colonies, however. Many are free-growing, and may be found attached to the stems of water-plants, but they are extremely minute, and will hardly be noticed until the microscopist has acquired considerable experience, nor even then--with such an instrument as we have postulated--will he see more than a tiny pear, with a straight line, the margin of the cup, on each side of its summit. The flagellum will be quite invisible.