The Vorticella nebulifera and some others of the Infusoria are remarkable for the diversity of their reproductive powers; for, besides division and gemmation, they are reproduced by a kind of alternate generation, accompanied by singular metamorphoses. The Vorticella, one of the most beautiful animals of its class, lives in pools of fresh water: groups of them are found on almost every mass of duckweed like little blue bells upon slender stalks, creating active currents in the water by the vibrations of long and powerful cilia with which the margin of the bell is fringed. The lip or edge of the bell is bent outwards into a permanent rim, and a deep groove cleaves the rim on one side, in which a wide cavity forming the mouth is placed. The mouth, the short throat or gullet, and the whole bell, are bristled with vibratile cilia.
Fig. 107. Vorticellæ.
The Vorticellæ feed on vegetable organisms, chiefly diatoms, and are exceedingly voracious. The cilia round the rim of the bell entangle the food, draw it into the mouth, and those in the gullet force particle after particle mixed with water into vacuoles which they make in the interior of the soft sarcode which fills the bell, and there the particles undergo rotation till digested and absorbed, and, if refuse remain, it is ejected through a softer part in the outer layer of the bell.
Fig. 108. Acineta.
The stem that fixes the animal to a solid object is a tubular continuation of its outer membrane, containing a highly contractile filament; and, as the creature is extremely sensitive to external impressions, it folds up the ciliated rim of its bell, and its stalk shrinks down in a spiral on the slightest alarm, but the bell opens and the stalk stretches out again as soon as the alarm is over. When a Vorticella is reproduced by division, the bell separates longitudinally into two parts; one is often smaller than the other, and separates from its parent, swims about till it gets a stem, and fixes itself to an object. When the two parts are of equal size, the division extends to a greater or less distance down the stalk, and as each of these become perfect bells, and do not fall off but subdivide in the same manner, it follows that, by successive divisions, a whole group of these beautiful animals may spring from the same stem, as in [fig. 107].
The Vorticella has a most wonderful mode of reproduction common to a few other Infusoria. A gelatinous substance is secreted by the bell, which hardens and envelopes it in a cyst; the encysted bell then separates from its stalk, and is transformed into an infusorial animal called an Acineta ([fig. 108]), closely resembling the Actinophrys sol with radiating filaments which it continually stretches out and draws in. A motile ciliated embryo, or Vorticella bud, is then formed within the Acineta, which, after a time, comes out at a slit in its side, swims about, gets a stem, fixes to some object, and is developed into a Vorticella. The slit closes again, and the Acineta keeps moving its filaments as usual, and another motile embryo is formed within it, which is emitted by a slit in the same manner, and is also developed into a Vorticella. As these young Vorticellæ, or bell animals, may undergo the same transformations, there may be an indefinite alternation of the two forms. The Vorticella-bud, when it issues from the slit in the Acineta, has an oval form, with a circlet of long cilia at its narrow end, a mouth at the more obtuse, a nucleus, and contractile vesicles, and, after swimming about till it finds a suitable place, it becomes fixed by one end of its oval body, a style or stem is formed, which rises rapidly, and the adult shape is developed. The Acinetæ are said to live upon Infusoria: they apply the dilated apex of their rays as sucking discs to the animal, and suck its contents till it dies. The Tricoda linceus undergoes metamorphoses analogous to those of the Vorticella, but more numerous and complicated.[[21]]
Most of the Vorticellæ, and probably the majority of Infusoria, remain unchanged for a time within their cysts, being then in a state analogous to the hybernal sleep of some of the reptiles. The cyst shelters them from cold and draught, and, when heat and moisture are restored, they resume their active vitality. The motions of the Infusoria are probably automatic, and in some instances consensual; they have neither true eyespecks, though their whole body seems to be conscious of light and darkness; nor have they ears; and, with the exception of touch, which the Vorticellæ have in a marvellous degree, it may be doubted whether the Infusoria have any organs of sense whatever, though they avoid obstacles and never jostle one another. The vibrations of their cilia are involuntary as in plants, an instance of the many analogies which perpetually occur between the lowest tribes of the two great kingdoms of nature. In both there are examples of propagation by bisection, conjugation, budding, and the alternation of generation, which occurs more frequently among Protozoa than among any other class of animals. There is a perfect resemblance between Zoospores and Protozoa; they both cease to move, the Zoospore when it secretes its cellulose coat and becomes a winter or resting spore, the Protozoon previous to encysting, a process presumed to be universal among that class of animals, before subdivision or reproduction begins. It is the dried cysts or germs of the Infusoria that float in the atmosphere as winter spores do, and it is believed that, like the fungi, the same germs may develope themselves into several different forms according to the nature of the liquid into which they may chance to be deposited; consequently, it is not necessary that the variety of germs should be very great, although the Infusoria themselves are of numerous forms.[[22]]
The Infusoria, the smallest of beings, apparently so insignificant, and for the most part invisible to the unaided eye, have high functions assigned to them in the economy of nature. They ‘are useful for devouring and assimilating the particles of decaying animal and vegetable matter from their incredible numbers, universal distribution, and insatiable voracity—they are the invisible scavengers for the salubrity of the atmosphere. They perform a still more important office in preventing the gradual diminution of the present amount of organic matter upon the earth. For, when this matter is dissolved or suspended in water in that state of comminution and decay, which immediately precedes its final decomposition into the elementary gases, and its consequent return from the organic to the inorganic world, these wakeful members of Nature’s invisible police are everywhere ready to arrest the fugitive organic particles, and turn them back into an ascending stream of animal life. Having converted the dead and decomposing matter into their own living tissues, they themselves become the food of larger Infusoria, as the Rotifera and numerous other small animals, which, in their turn, are devoured by larger animals as fishes, and thus a pabulum fit for the nourishment of the highest organized beings is brought back by a short route from the extremity of the realms of organized matter.’[[23]]