“All nature widens upward. Evermore
The simpler essence lower lies:
More complex is more perfect—owning more
Discourse, more widely wise.”
You are now familiarized with the words “differentiation” and “development,” so often met with in modern writers; and have gained a distinct idea of what an “organ” is; so that on hearing of an animal without organs, you will at once conclude that in such an animal there has been no setting apart of any portion of the body for special purposes, but that all parts serve all purposes indiscriminately. Here is our Opalina, for example, without mouth, or stomach, or any other organ. It is an assimilating surface in every part; in every part a breathing, sensitive surface. Living on liquid food, it does not need a mouth to seize, or a stomach to digest, such food. The liquid, or gas, passes through the Opalina’s delicate skin, by a process which is called endosmosis; it there serves as food; and the refuse passes out again by a similar process, called exosmosis. This is the way in which many animals and all plants are nourished. The cell at the end of a rootlet, which the plant sends burrowing through the earth, has no mouth to seize, no open pores to admit the liquid that it needs; nevertheless the liquid passes into the cell, through its delicate cell-wall, and passes from this cell to other cells, upwards from the rootlet to the bud. It is in this way, also, that the Opalina feeds: it is all-mouth, no-mouth; all-stomach, no-stomach. Every part of its body performs the functions which in more complex animals are performed by organs specially set apart. It feeds without mouth, breathes without lungs, and moves without muscles.
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.
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!
That phrase, generation within generation, was not a careless phrase; it is exact. Take the tiny insect (Aphis) which, with its companions, crowds your rose-tree; open it, in a solution of sugar-water, under your microscope, and you will find in it a young insect nearly formed; open that young insect with care, and you will find in it, also, another young one, less advanced in its development, but perfectly recognizable to the experienced eye; and beside this embryo you will find many eggs, which would in time become insects!