=Diatoms= are unicellular plants, isolated or aggregated together, that have the power of constructing flint coverings, often of great complexity and charming pattern. The tracings on many of these flint coverings are so constant and small, that they are frequently employed for the purpose of testing the power of modern compound microscopes. In various parts of the world vast deposits of Diatoms have been discovered. The most remarkable of these for extent, as well as for the beauty and number of the species contained in it, is that on which the city of Richmond, in Virginia, is built, which is over thirty feet deep and extends for many miles. [Illustration: FIG. 6.—Noctiluca miliaris. A, dorsal view; B, side view; n, nucleus; f, flagellum; a, entrance to atrium; b, atrium; o, œsophagus; r, superficial ridge.] =Noctiluca miliaris= (Fig. 6) is a very large unicellular, flagellate animal. It is spheroidal in form, and has an average diameter of not quite one-half a millimeter. It is just large enough to be observed by the unaided eye when the water in which the animal may be swimming is contained in a glass jar held up to the light. It has a tail-like appendage (flagellum) by which the animal moves about. Along one side of the cell is a meridional groove resembling that of a peach, and leading into a deep depression of the surface termed the atrium (Fig. 6, B, b). It is from the shallow commencement of this depression that the flagellum (Fig. 6, f) originates. At the base of the flagellum the depression sinks down to the mouth (o). A slightly elevated ridge (r) extends along the opposite meridian and commences with a bifurcation at that end of the atrium farthest from the flagellum. The mouth opens into a short œsophagus, which leads down directly to the central protoplasmic mass. The central protoplasmic mass sends off branching prolongations of its substance in all directions, the ramifications of which freely inosculate. The farther these ramifications extend out to the periphery, the thinner they become, until finally a protoplasmic network of extreme tenuity is formed immediately under the enveloping membrane of the cell. In addition to these ramifying prolongations, the central protoplasmic mass sends off a thin, broad, irregular extension to the superficial ridge and coalesces with it. Near the central protoplasmic mass is seen the nucleus (n). The flagellum is a flattened, whip-like filament, having a striated appearance, and gradually tapers from the base to its extremity. It slowly bends over five or six times a minute to the mouth, and then, more slowly still, bends away again. It is through the movements of the flagellum that particles of food are driven into the mouth and down the œsophagus into the central protoplasmic mass. In this mass and its extensions the food is digested and assimilated.