We may divide the Infusoria into two orders—the Ciliate Infusoria, namely, those provided with vibratile cilia, and the Flagelliferous Infusoria, those, namely, which have arms or branches. The greater part of Infusoria belong to the first order, which comprehends many families; our space limits us to the mention here of a few typical forms only in each group, selecting those which appear the most interesting, from their size, structure, rarity, or abundance.
Flagelliferous Infusoria.
The family of Vibrionidæ, so named from their darting or quivering motion, includes the eel-like microscopic animalcules which occur in stale paste, vinegar, &c., with some others, which are parasitic on living vegetables, such as Vibrio tritici, which infest the grains of wheat, producing the destructive disease called corn-cockle or purples. They are filiform animals, extremely slender, without appreciable organization, internal stomach, or apparent organs of locomotion. They are the first animalcules which show themselves in any infusion of organic matter. By using microscopes of the highest magnifying power, traces of very thin, short lines can be perceived, either straight or sinuous, the thickest of them not exceeding the thousandth part of the fraction of an inch. They are contractile, and propagated by spontaneous division, or fission. Among them some resemble right lines, more or less distinctly articulated, and endowed with a very slow movement; these are Bacteridæ. Others are flexuous and undulating, and more or less lively; these are true Vibrions. Others have the body fashioned in the form of a corkscrew, turning unceasingly upon themselves with great rapidity; these are the Spirillidæ, having an oblong fusiform or filiform body, which undulates or turns spirally upon itself.
Fig. 30. Bacterium, The same, termo (Müller), magnified magnified 600 times. 1600 times.
The Bacterium termo (Fig. 30) is the smallest of the Infusoria. It is found, at the end of a short time, in all vegetable or animal infusions exposed to the air. It shows itself in infinite numbers, forming swarms of animalcules, which disappear as other species multiply in the liquid, to which animals it serves for nourishment. When the infusion becomes too fœtid for these new species to live in it, in consequence of fermentation or putrefaction, the Bacterium termo reappears. This species was one of the first observed; Leuwenhoek found it in the white matter in the teeth and gums, which is called teeth tartar. It is also found in the fluids of various animals which have been affected by disease.
Fig. 31. Vibrion baguette (Müller), magnified 300 times.
The Wand-like Vibrion (Fig. 31) has the body transparent, filiform, with long articulations, often appearing as if broken at each connection. It moves very slowly in the water. Leuwenhoek observed this second species joined to the first in the teeth tartar, and also in a great number of organic infusions. "There is no microscopic object," says Dujardin, "which excites the admiration of the observer more vividly than the twisting spirillum" (Fig. 32). He is struck with surprise when he first contemplates this little creature, which, under the greatest magnifying power, only presents the appearance of a thin black line, fashioned like a corkscrew, which every instant turns upon itself with marvellous velocity, such as the eye can scarcely follow, or the mind divine the cause which produces this startling phenomenon.
