Other observers were not slow in raising objections to these views. Dujardin, especially, was much opposed to the batch of stomachs attributed to these creatures by the German physiologist. He attempted to establish the fact that the coloured globules which appeared in the bodies of the Infusoria, while subjected to a regimen of carmine and indigo, are not confined by a membrane; that is to say, they are not contained in intestinal sacs. According to Milne Edwards, "they are a species of basins, constituted," he says, "by the alimentary matter with which each is gorged, united into a rounded pasty mass, where it could no longer be dispersed, but would continue to advance, still preserving its form. We have, in short, seen these spherules changing their places, and passing one another in their progress from the mouth to the intestinal canal. That they could not do this is evident, if many stomachs were attached to the intestinal canal!"
This opinion, due to the patient and precise studies of Dujardin, has been adopted by most naturalists of eminence. Besides, this learned microscopist does not admit that there was in the sarcodic mass of Infusoria any pre-existent cavity destined to receive the food. In a word, he does not recognise any stomach whatever. This view of the extreme simplicity of structure in the Infusoria has, however, met with much opposition. To accord them neither four nor two stomachs, it is not necessary to deprive them of the organ altogether. Meyen represents them as having one great hollow stomach occupied by a pulpy matter, into which the alimentary masses are successively absorbed. "All recent observations," says Milne Edwards, "tend to establish the fact that the digestive apparatus of the ciliate Infusoria consists of—first, a mouth; second, of a pharyngeal canal, in which the food often assumes the form of a bolus; third, of one great stomach with distinct walls, and more or less distant from the common tegumentary membrane; fourth, of an excretory orifice."
This mouth presents sensible differences both as to its position and conformation, often occupying the bottom of a hollow, the edges of which are furnished with well-developed cilia, the action of which attracts the aliment; in short, the mouth is a sort of decoy at the bottom of a simple pit, being at once contractile and prehensile, the interior part being sometimes capable, according to Milne Edwards, of being turned inside out in the form of a trumpet, while in a great many species it is provided with a peculiar armature, consisting of a band of rigid bristles disposed in the form of a bow-net, and susceptible of dilatation and contraction, according to the wants of the animal. The œsophagus, which is connected by means of the canal with the mouth, has generally an oblique direction backwards, often terminating in a great undivided stomach.
The reproduction of the Infusoria exhibits some very surprising phenomena, while it offers another proof of the wonderful means Nature employs for perpetuating the races of animals. They can be reproduced by three different processes: 1. By gemmation, or budding, somewhat after the manner of plants. 2. By sexual reproduction; for in these little creatures it has recently been discovered that sexual differences exist. 3. By the spontaneous division of the animal into two individuals—a process known to zoologists as fissiparism or fission.
Among these three processes, that which appears best understood is the last. The singular phenomenon of spontaneous division may be witnessed by any one having patience to examine the creature long enough, isolated from its innumerable companions, under the microscope. The oblong body of the animal will soon be observed to contract at the middle, the compression becoming more and more marked. The lower segment soon begins to show a few vibratile cils, thus indicating the place which will soon be a new mouth; the organ soon becomes more and more distinct, and now the Infusoria literally cuts itself into two parts. We see, at first, the fragment of glutinous substance fluttering on the edge of the plate; the two halves then separate from each other very quickly, each moiety having finally a perfect resemblance to the primitive animal. This process is represented in Fig. 28, a and b being the adult, c the same in course of separation, d after its completion. Assuredly this is one of the most remarkable phenomena which the study of living beings can present. "By this mode of propagation," says Dujardin, "an infusoria is the half of the one which preceded it, the fourth of the parent of that, the eighth of its grand-parent, and so on, if we can apply the terms father or mother to animals which must see in its two halves the grandfather himself by a new division again living in his four parts. We might imagine such an infusoria to be an aliquot part of one like it, which had lived years, and even ages before, and which by continued subdivision into pairs might continue to live for ever by its successive development."
Fig. 28. Propagation of an Infusoria by spontaneous division.
This mode of generation, however, enables us to comprehend the miraculous fecundity of these beings. The process defies calculation, if we wished to be precise. We may, however, arrive at a proximate estimate of the number which may be derived from a single individual by this process of fission. It has been found that at the end of a month two Stylonichiæ had a progeny of more than one million and forty-eight thousand individuals, and that in a lapse of forty-two days a single Paramecium had produced more than one million three hundred and sixty-four thousand forms like itself.
Life is spread over Nature in such abundance that the smallest infusoria has its parasite a little smaller; these in their turn serving as "a dwelling and pasture ground," to use Humboldt's words, for still smaller animalcules, as represented in Fig. 29—a being parasites in various stages; b, the larger animalcule on which they have established themselves.
