The last of the diatoms which we shall be able to mention in this work is that represented on Fig. 31. The members of this genus have the name of Navícula, on account of their boat-like shape and their habit of gliding through the water in a canoe-like fashion. There are many species of this genus, all of which are notable for the graceful and varied courses formed by their outlines, and the extreme delicacy of their markings. In many species the markings are so extremely minute that they can only be made out with the highest powers of the microscope and the most careful illumination, so that they serve as test objects whereby the performance of a microscope can be judged by a practical man. * * * * * The large spherical figure in the centre of Plate IV. represents an example of a family belonging to the confervoid algæ, and known by the name of Volvox globator. There seems to be but one species known. This singular plant has been greatly bandied about between the vegetable and animal kingdoms, but seems now to be satisfactorily settled among the vegetables. In the summer it may be found in pools of water, sufficiently large to be visible to the naked eye, like a little green speck proceeding slowly through the water. When a moderate power is used, it appears as shown in the figure, and always contains within its body a number of smaller individuals, which after a while burst through the envelope of the parent and start upon an independent existence. On a closer examination, a further generation may be discovered even within the bodies of the children. The whole surface is profusely covered with little green bodies, each being furnished with a pair of movable cilia, by means of which the whole organism is moved through the water. These bodies are analogous to the zoospores already mentioned, and are connected with each other by a network of filaments. Reproduction also takes place by conjugation as in other algæ. A more magnified representation of one of the green bodies is shown immediately above the larger figure. The volvox is apt to die soon when confined in a bottle. Fig. 25 is the common yeast-plant, consisting simply of a chain of cells, which increase by budding, and only form spores when they have exhausted the nutriment in the fluid in which they live. Fig. 26 is a curious object, whose scientific name is Sárcina ventrículi. It is found in the human stomach. Similar forms are often to be found in the air; for instance, a piece of cocoa-nut will exhibit this, and many other kinds of Bacteria and moulds, after a few days’ exposure to the air, preferably in a dark cupboard.