The Opalina, as I said, is a parasite. It may be found in various animals, and almost always in the frog. You will, perhaps, ask why it should be considered a parasite; why may it not have been swallowed by the frog in a gulp of water? Certainly, nothing would have been easier. But to remove your doubts, I open the skull of this frog, and carefully remove a drop of the liquid found inside, which, on being brought under the microscope, we shall most probably find containing some animalcules, especially those named _Monads_. These were not swallowed. They live in the cerebro-spinal fluid, as the Opalina lives in the digestive tube. Nay, if we extend our researches, we shall find that various organs have their various parasites. Here, for instance, is a parasitic worm from the frog’s bladder. Place it under the microscope, with a high power, and behold! It is called _Polystomum_--many-mouthed, or, more properly, many-suckered. You are looking at the under side, and will observe six large suckers with their starlike clasps (_e_), and the horny instrument (_f_), with which the animal bores its way. At _a_ there is another sucker, which serves also as a mouth; at _b_ you perceive the rudiment of a gullet, and at _d_ the reproductive organs. But pay attention to the pretty branchings of the digestive tube (_c_) which ramifies through the body like a blood-vessel. [Illustration: Fig. 3. POLYSTOMUM INTEGERRIMUM, Magnified.] This arrangement of the digestive tube is found in many animals, and is often mistaken for a system of blood-vessels. In one sense this is correct; for these branching tubes are carriers of nutriment, and the only circulating vessels such animals possess; but the nutriment is _chyme_, not blood: these simple animals have not arrived at the dignity of blood, which is a higher elaboration of the food, fitted for higher organisms. Thus may our frog, besides its own marvels, afford us many “authentic tidings of invisible things,” and is itself a little colony of Life. Nature is economic as well as prodigal of space. She fills the illimitable heavens with planetary and starry grandeurs, and the tiny atoms moving over the crust of earth she makes the homes of the infinitely little. Far as the mightiest telescope can reach, it detects worlds in clusters, like pebbles on the shore of Infinitude; deep as the microscope can penetrate, it detects Life within Life, generation within generation; as if the very Universe itself were not vast enough for the energies of Life!