Class IV.—Infusoria.
The Infusoria, which form the second group of the Protozoa, are microscopic animals of a higher grade than any of the preceding creatures, although they go through their whole lives as isolated single cells of innumerable forms. They invariably appear in stagnant pools and infusions of animal and vegetable matter when in a state of rapid decomposition. Every drop of the green matter that mantles the surface of pools in summer teems with the most minute and varied forms of animal life. The species called Monas corpusculus by the distinguished Professor Ehrenberg, has been estimated to be 1⁄2000 part of a line in diameter. ‘Of such infusoria a single drop of water may contain 500,000,000 of individuals, a number equalling that of the whole human species now existing upon the face of the earth. But the varieties in size of these animalcules invisible to the naked eye are not less than that which prevails in almost any other natural class of animals. From the Monad to the Loxades or Amphileptus, which are the fourth and sixth part of a line in diameter, the difference in size is greater than between a mouse and an elephant; within such narrow bounds might our ideas of the range in animal life be limited if the sphere of our observation was not augmented by artificial aid.’[[18]]
This singular race of beings has given rise to the erroneous hypothesis of equivocal or spontaneous generation, that is to say, the production of living animalcules by a chemical or even fortuitous combination of the elements of inert matter. That question has been decided by direct experiment, for Professor Schultz kept boiled infusions of animal and vegetable matter for weeks in air which had passed through a red-hot tube, and no animalcules were formed, but they appeared in a few hours when the same infusions were freely exposed to the atmosphere, which shows clearly that the germs of the lowest grade of animal life float in the air, waiting as spores do, till they find a nidus fit for their development.
M. Pasteur, Director of the Normal School in Paris, in a series of lectures published in the ‘Comptes Rendus,’ has not only proved that the atmosphere abounds in the spores of cryptogamic fungi and moulds, but with infusoria of the form of globular monads, the Bacteria, and vibrios, which are like little rods round at their extremities and extremely active. The Bacteria mona and especially the Bacteria terma, are exceedingly numerous. These minute beings are the principal agents in the decomposition of organic matter. They are more numerous in dry than in wet weather, in towns than in the country, on plains than on mountains.
In a memoir read at the Academy of Sciences, Paris, Mr. J. Samuelson mentions that he had received rags from Alexandria, Japan, Melbourne, Tunis, Trieste and Peru. He sifted dust from the rags from each of these localities respectively through fine muslin into vases of distilled water. Life was most abundant in the vases containing dust from Egypt, Japan, Melbourne, and Trieste. The development of the different forms was very rapid, and consisted of protophytes, Rhizopods and true Infusoriæ. In most of the vases monads and vibrios appeared first, and from these Mr. Samuelson traced a change first into one then into another species of infusoria. In the dust from Japan he followed the development of a monad into what appeared to be a minute Paramœcium, then into Lexodes cucullus, and finally into Colpoda cucullus. From these and other experiments it is proved that many infusoria now classed as distinct types are really one and the same animal in different states of development. That appears to be the case also with the Amœbæ. In the dust from Egypt Mr. Samuelson found a new Amœba whose motions were very rapid; as to shape and mode of motion he compared it to soap bubbles blown with a pipe. He traced the gradual changes of the globular form of this Amœba until its pseudopodia were in full action, its increase by conjugation, and other circumstances of its life. In the same dust and in that only, the development of the Protococcus viridis was seen, and that in such abundance that at last the water was tinged green by that plant. In the dust from Egypt a vibrio was changed into a vermiform segmented infusoria of an entirely new type. Its length varied from the 1⁄150 to 1⁄100 of an inch, each ring was ciliated, and the whole series of cilia extending along the body acted in concert; a circlet of them surrounded the anterior segment; a canal seemed to extend throughout the body. It was propagated by bisection; the two parts remained attached to one another; an independent ciliary motion was observed in each which did not interfere with the motion of the whole. It was supposed to be a larval form or series of forms. Mr. Samuelson’s observations show, that the atmosphere in all the great divisions of the globe is charged with representatives of the three kingdoms of nature, animal, vegetable, and mineral: that the animal germs not only include the obscure types of monads, vibrios, and Bacteria, but also the Glaucoma, Cyclides, Vorticella, and other superior Infusoriæ, and occasionally though very rarely germs of the Nematode worms.
It has been already mentioned that many of the microscopic fungi are ferments, aiding greatly in the decomposition of organic matter. They however are by no means the only agents in decomposition. The moment life is extinct in an animal or vegetable, Infusoria of the lowest grade seize upon the inanimate substance, speedily release its atoms from their organic bond, and restore them to the inorganic world whence they came. The ferment which transforms lactic acid into butyric acid is a species of vibrio which abounds in the liquid, isolated or united in chains; they glide, pirouette, undulate, and float in all directions, and multiply by spontaneous division. Vibrios possess the unprecedented property of living and propagating without an atom of free oxygen; they not only live without air, but air kills them. This singular property forms an essential difference between the Vibrios and the Mycoderms: the former cannot live in oxygen; the latter cannot live without it, and as soon as it is exhausted within the infusion, they go to the surface to borrow it from the atmosphere.
There are also two groups of Infusoria which possess these opposite characters, one being unable to live in oxygen, while the other cannot live without it; sometimes they even inhabit the same liquid. When the tartrate of lime is put into water along with some ammoniacal and alkaline phosphates, a Monad, the Bacteria terma, and other Infusoria appear after a time. These little animals bud rapidly in an infusion of animal matter, then a slight motion is produced by the appearance of the Monas corpusculum and the Bacterium terma, which glide in wavy lines in all directions in quest of the oxygen dissolved in the liquid, and as soon as it is exhausted they go to the surface in such numbers as to form a pellicle, where by aid of the oxygen they form the simple binary compounds water, ammonia, and carbonic acid. In the meantime the Vibrios, which are without oxygen, are developed below, and keep up the fermentation, and between the two, the work of decomposition is completed.
It is not the worm that destroys our dead bodies; it is the Infusoria, the least of living beings. The intestinal canal of the higher animals, and of man, is always filled during life not only with the germs of vibrios, but with adult and well-grown vibrios themselves. M. Leewenhoeck had already discovered them in man, a fact which has since been confirmed. They are inoffensive as long as life is an obstacle to their development, but after death their activity soon begins. Deprived of air and bathed in nourishing liquid, they decompose and destroy all the surrounding substances as they advance towards the surface. During this time, the little Infusoria, whose germs from the air had been lodged in the wrinkles and pores of the skin, are developed, and work their way from without inwards, till they meet the vibrios, and after having devoured them, they perish, or are eaten by maggots.
Of all the Infusoria and ferments the Vibrios are the most tenacious of life; their germs resist the destructive effect of a temperature of 100° Cent. The spores of the Mucedines are still more vivacious; they grow after being exposed to a heat of 120° Cent., and are only killed by a temperature of 130° Cent. As neither spores of the fungi nor the germs of the Infusoria are ever exposed to so high a temperature while in the atmosphere, they are ready to germinate as soon as they meet with a substance that suits them.
M. Ehrenberg has estimated that the Monas corpusculum is not more than the 1⁄24,000th part of an inch in diameter; whence Dr. M. C. White, assuming that the ova of the Infusoria and the spores of minute fungi are only the 1⁄10th part in linear dimensions of their parent organisms, concludes that there must be an incalculable amount of germs no larger than the 1⁄240,000th or 1⁄100,000th part of an inch in diameter; and since according to MM. Sullivant and Wormley, vision with the most powerful microscope is limited to objects of about the 1⁄80,000th part of an inch in diameter, we need not be surprised if Infusoria and other organisms appear in putrescible liquids in far greater numbers than the germs in atmospheric dust visible by the aid of microscopes would lead us to expect.