Chapter IV: Zoophytes (2)

The fossil Foraminifera are chiefly distinguished from recent and existing species by the size of the former. While the living forms range from one-fourth to the one-hundredth part of an inch, the tertiary strata abound in examples of _Nummulites_ varying from the eighth of an inch to the size of half-a-crown. The engraving is a drawing from Nature, by MM. d'Archaic and Haime, of a piece of nummulitic rock, of Nousse, in the Landes, in which a great variety of sizes and forms are exhibited.

The Nummulina belong to the third family, or Helicostega, in which the outer convolutions completely embrace the earlier-formed ones. Hence it is only by making microscopic sections, or thin slices, that their structure can be fully seen. When such a section is carried horizontally through the centre of the shell, the segments present a spiral arrangement, which, like the convolutions, are remarkable for their small size, and consequent great number.

With respect to the distribution of the Foraminifera according to geological periods, we may briefly state that they have been found in every formation from the Silurian to the Tertiary. The species, at first very simple in their forms, begin to appear in increasing numbers in the carboniferous formations. They become more numerous, and, at the same time, more complex in their forms, in the Cretaceous period; they are still more diversified, and appear to have multiplied much more rapidly in the Tertiary period, where they attain the maximum of their numerical development. In the celebrated quarries of St. Peter, at Maestrecht, the _Siderolites calcitrapoides_ of Lamarck are found in the upper chalk (Fig. 23). In the calcareous formation of Chaussy, in the Seine and Oise district, and other parts of the Paris basin, the _Fabularia discolithes_ (Fig. 24) of Defrance is found. Finally, the _Dactylopora cylindracea_ of Lamarck (Fig. 26) is found in the eocene formation of Valmondois and in the chalk of Grignon. At first, this little creature was thought to be a polype; but d'Orbigny, in his "Prodrome de Paleontologie," has placed it among the Foraminifera, thinking that it appeared to occupy a place between the two classes.

The existing Foraminifera are by no means equally distributed in every ocean. Some genera belong to warm countries, others to temperate and cold climates. They are much more numerous, however, and much more varied in their forms, in warm than in cold climates, and, we may add, larger also, for Sir E. Belcher brought a recent species from Borneo which measured two inches in diameter.

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Before passing on to the study of the Infusoria, a few words may be offered on the _Noctiluca_, a genus of animals usually referred to the class ACALEPHÆ. One species only of this genus has been described, which occurs occasionally on the English coast in prodigious numbers. It is a small creature, scarcely the hundredth part of an inch in diameter, according to Mr. Huxley (Fig. 27, _Noctiluca miliaris_). It was discovered by M. Surriray, in 1810, who describes it as a spherical gelatinous mass, scarcely bigger than a pin's head, with a long filiform tentacular appendage, a mouth, an oesophagus, one or many stomachs, and branching ovaries--thus exhibiting a certain complexity of organization. De Blainville took the same view, and placed it among the _Diphydæ_. Van Beneden and Doyère, on the other hand, deny its relation to the _Acalephæ_, conceiving its organization to be much more simple: they place it with the _Rhizopoda_. Quatrefages adopts the same view, denying the existence of a true mouth or intestinal canal: he considers the so-called stomachs as simple "vacuales," similar to those observed in the Rhizopoda and Infusoria. Mr. Huxley, describing it in the "Journal of Microscopical Science" (vol. iii.), says it has nearly the form of a peach, a filiform tentacle, equal in length to the diameter of the body, occupying the place where the stalk of the peach might be, which depends from it, and exhibits slow wavy motions when the creature is in full activity. "I have even seen a _noctiluca_," he adds, "appear to push against obstacles with this tentacle."

"The body," he continues, "is composed of a structureless and somewhat dense external membrane, which is continued on to the tentacle. Beneath this is a layer of granules, or rather of gelatinous membrane, through whose substance minute granules are scattered without any very definite arrangement; from hence arises a network of very delicate fibrils, whose meshes are not more than one three-hundredth part of an inch in diameter, which gradually pass internally--the reticulation becoming more and more open--into coarser fibres, taking a convergent direction towards the stomach and nucleus. All these fibres and fibrils are covered with minute granules, which are usually larger towards the centre."

Mr. Huxley is inclined to think, from all he has observed, that the animal has a definite alimentary cavity, and that this cavity has an excretory aperture distinct from the mouth.

Surriray discovered the _noctiluca_ while investigating the cause of phosphorescence of sea water at Havre, where it was abundant in the basins; sometimes in such abundance as to form a crust on the surface of the water of considerable thickness. "This singular little creature," says M. Frédol, "offers here and there in its interior certain granules, probably germs, and also luminous points, which appear and disappear with great rapidity--the least agitation determining their lustre." The _noctiluca_ are so abundant in the Mediterranean and in some parts of the channel, that in a cubic foot of sea water, which has been rendered phosphorescent by their presence, it is calculated that there exist about twenty-five thousand.

INFUSORIA.

With the Infusoria we return to the domain of the infinitely little. Of this very interesting group a large proportion are marine, and numerous varieties of them are found in British seas. In their minuteness and variety they almost baffle the attempts of naturalists to classify them.

The waters, both fresh and salt, are inhabited by legions of active, ever-moving beings, of dimensions so small as to be inappreciable to the naked eye; these minute creatures are disseminated by millions and thousands of millions in the great deep, and all knowledge of them would have escaped us, as they escaped the knowledge of the ancients, but for the discovery of the microscope, the sixth sense of man, as it has been happily expressed by the historian and poet Michelet. Another writer of equally poetical mind, M. Frédol, tells us that "the infusorial animalcules are so small that a drop of water may contain them in many millions. They exist in all waters, the fresh as well as the salt, the cold as well as the hot. The great rivers are continually discharging them in vast quantities into the sea."

The Ganges transports them in the course of one year in masses equal to six or eight times the size of the great pyramid of Egypt. Among these animalcules, according to Ehrenberg, we may reckon seventy-one different species.

The water collected in vases between the Philippine and the Marianne Isles at the depth of twenty-two thousand feet (making some allowance for erroneous soundings), have yielded a hundred and sixteen species. Near the Poles, where beings of higher organization could not exist, the Infusoria are still met with in myriads; those which were observed in the Antarctic Seas, during the voyages of Captain Sir James Ross, offer a richness of organization, often accompanied by elegance of form, quite unknown in more northern regions. In the residuum of the blocks of ice floating about in latitude seventy-eight degrees ten minutes, nearly fifty different species were found. Many of them had ovaries, according to Ehrenberg, still green, which proved that they had struggled successfully with the rigours of the climate in searching for food.

At a depth in the sea which exceeds the height of the loftiest mountain, Humboldt asserts that each bed of water is animated by an innumerable phalanx of inhabitants imperceptible to the human eye. These microscopic creatures are, in short, the smallest and the most numerous creations in Nature. They constitute with human beings one of the wheels of that very complicated machine, the globe. They are in the rank and at the station willed for them, as determined in the great First Thought. Suppress these microscopic beings, and the world would be incomplete. It was said, and wisely said, long, long ago, "there is nothing so small to the view but that it may become great by reflection."

The Infusoria, in short, abound everywhere. We find their remains on the loftiest mountain ridges, and in the profoundest depths of the sea. They increase and multiply alike under the Equator, and towards the polar regions. The seas, rivers, ponds--the flower vase which rests upon the casement--even our tissues, and the fluids of our bodies--all contain infusorial animalcules. Whole beds of strata, often many feet thick, and covering a surface of considerable extent, are almost exclusively formed of their accumulated débris. It is to the Infusoria that the mud of the Nile and other fluviatile and lacustrine deposits owe their prodigious fertility. To them also is due the red or green layer of colouring matter found in ponds and tanks at certain seasons. When exposed to great solar heat, in order to extract the salt, as it is in the vast artificial basins hollowed out for the purpose in the salt marshes near the sea-shore in the south of France, the salt water, when it reaches a certain degree of concentration, acquires a fine rose colour, which is due to the presence of innumerable masses of small Infusoria having a reddish shell. Finally, let us add that the solid débris of certain fossil Infusoria, of surprising minuteness, have formed the stone so much used by workers in metal, which is known as _tripoli_.

The study of these creatures is intensely interesting to the naturalist, the philosopher, the physician, and the general reader. They have had a great part assigned to them in Nature, as is evident in the formation of certain beds of rock of immense extent, in which the geologist traces their action.

Our earliest knowledge of the Infusoria is traceable to the seventeenth century; to the celebrated naturalist, Leuwenhoek, we are indebted for their discovery. On the 24th of April, 1676, this observer saw for the first time some infusorial animalcules. Fifty years later, Baker and Trembley studied them anew. In 1752, Hill essayed the first attempt at their classification. In 1764, Wiesberg gave them the name of Infusoria, because he found them in such great abundance in animal and vegetable _infusions_. Müller published a special book upon them.

From that time the Infusoria have been considered as forming a special group among the radiate animals; afterwards, in the pages of Baer and of De Blainville, we see in these creatures, so imperfect in appearance, only the indeterminate prototype of other classes. But ideas changed altogether respecting them when microscopes of great power, and armed with achromatic lens, were employed in their study. Thanks to the labours of Ehrenberg and Dujardin, we have arrived at a better comprehension of the organization of these infinitely small beings. Naturalists have established, with more exactness, the limits of the zoological group to which they belong.

Some stagnant waters are so filled with Infusoria that it is only necessary to dip at random into the liquid medium to procure them in abundance. In other waters they form a bed, occupying the whole basin. In general, it is necessary to search for them where the water is calm, and occupied by vegetation of some kind, such as the _confervæ_, or _lemna_, &c., in the marshes, and _ceramium_ if in the sea. Certain Infusoria live not only in water, but also in places habitually moist, as among tufts of mosses; in beds of _oscillaria_, on moist soil, or on air-damp walls. Others live as parasites on the exterior or in the interior of animals, such as _hydra_, _lombrics_, and _naïads_. Quantities of them are found in the liquid excrements and other products of certain organisms, and they have been noted even in women's milk.

But, as their name indicates, it is in aqueous infusions, vegetable or animal, that these animalcules abound. Armed with a microscope, the reader may, with very little trouble, afford himself the pleasure of studying these animals. It is only necessary to place some organic débris--the white of an egg, or some grass, for example--in a vase with a large mouth, filled with water, and expose it to the light and air. Certain reagents, as phosphate of soda, the phosphates, nitrates, or oxalates of ammonia, or carbonate of soda added to these infusions, will singularly favour the development of Infusoria.

There are also some accidental infusions which seem to furnish these microscopic beings in great abundance. Water which stagnates in garden soil or in vegetable mould, in the watering-cart or in flower vases, is filled with myriads of these beings.

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So much for the medium in which they live, move, and have their being. Let us pass on to their organization. We have already dwelt on their extreme minuteness; their mean size is a fifth of a line or the sixtieth part of an inch; the largest species scarcely reveal themselves to the naked eye. They are generally colourless; some of them are, nevertheless, green, blue, red, brown, and even blackish. Seen on the object-glass of the microscope, they appear to be gelatinous, transparent, and naked, or invested with an envelope more or less resistant, which we shall designate after Dujardin by the term _Sarcoda_, a substance which is homogeneous, diaphanous, elastic, contractile, and, above all, destitute of every kind of organization. They are usually ovoid or globular. Those most frequently met with, and which attract the most attention from observers, are furnished with _vibratile cilia_, which cover the whole body, acting as paddles. These organs are evidently intended to propel the animal from one place to another. At other times they appear to be employed in conveying food to the mouth, if we may use the expression. Some Infusoria are without these cilia, having only one or many very slender filaments, the undulating movement of which suffices to determine their progression through the liquid which surrounds them.

Authors who have written on the Infusoria have sometimes, like Leuwenhoek, Ehrenberg, and Pouchet, attributed to them a very complex structure. Others, like Müller, Cuvier, and Lamarck, have considered them to be gifted with an organization extremely simple. We shall probably find that the truth lies between these two extremes.

In the superior Infusoria, besides the granules, the interior globules, vesicles full of liquid, vibratile cils, and a tegumentary system, more or less complex, we find the substance which is called _Sarcoda_.

The digestive apparatus of the Infusoria has been the subject of numerous observations, and has been provocative of very animated discussions. In the inferior order of the class, which comprehends the very smallest animalcules, it has not been found possible to observe the organization of the digestive apparatus in a satisfactory manner. Some writers think they have no mouth, what has been taken for that organ being only hollow dimples on the surface of the body; others recognize the existence of a buccal orifice, sometimes furnished with a solid armature. As to the arrangements of the interior cavities in which digestion takes place, we know nothing certain.

The digestive apparatus is better understood in the superior Infusoria, called _ciliate_, namely, those provided with _vibratile cils_. These cils seem to determine the currents of the liquid, leading the nutritive corpuscles suspended in the water towards the entrance of the digestive apparatus. They form, in some sort, the prehensile organs which seize the aliment. The cils are, at the same time, the organs intended to facilitate respiration; in short, these little whips playing upon the water unceasingly round the Infusoria, is just the action required for the absorption of the oxygen contained in the water. These cils, then, serve at once for the propulsion of the animal, for its nutrition, and for its respiration, presenting a remarkable example of cumulative functions in physiology.

The corpuscles of nutritive substances directed towards the buccal orifice by the vibratile cils soon disappear in the interior of the animal. Availing himself of this fact and the transparency of the animal, Herr Gleichen, a German physiologist of the last century, conceived the happy idea of colouring the water which contained these animalcules with a finely-powdered carmine; he traced the colouring matter in the bodies of some of them. But it was reserved for Ehrenberg to avail himself of the same artifice in order to study the internal structure and mode of absorbing nutritive matter in these minute creatures. This physiologist fed many groups of Infusoria, some of them with water coloured with carmine, others with indigo and other colouring matters. He saw, besides, some coloured globules, nearly uniform in size, in different individuals of the same species. From this he arrived at the conclusion that the colouring matter was deposited in many of the surrounding dimples. Ehrenberg thought that each of these dimples was a stomach, and that the introduction of the food into the interior of these reservoirs, as well as the evacuations, were produced by means of an intestine around which these stomachs are arranged. In some cases he even thought he could distinguish the outlines of this intestinal canal, and its connection with numbers of ampula or bladders. Generalizing the conclusions drawn from his observations, in short, we find that his class, Infusoria, embraced two very different forms of animal life, which he divided into _Infusoria_, _Polygastrica_, and _Rotifera_, the latter division including those known as Wheel animalcules; the _Polygastrica_ being so called from his idea that the typical forms possessed a number of stomachs. In some, Ehrenberg counted four stomachs, an organization which brings these microscopic beings into a strange kind of comparison with the ox and the goat. In others he counted two stomachs.

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 oesophagus, 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."

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.

The prodigious number to which the calculation would reach, if we were to add the other modes of propagation, viz., by germs and by budding, we dare not mention: it would only be necessary to place a single germ in a favourable condition for its development, in order to produce myriads of these microscopic animalcules in a very few days.

We have seen three modes of reproduction in the Infusoria; it is possible that a fourth mode exists, to which its partisans give the name of _spontaneous generation_. According to their views, an infusoria can be produced without egg-germ or pre-existent parent. It would be sufficient to expose organic matter, animal or vegetable, to the action of the air and water at a suitable temperature, in order to see this matter organize itself, and form itself into living infusorial animals.

Such is the general enumeration of the question of spontaneous or _heterogeneous_ generation, on which so much has been written in the last ten years. The great expounders of the doctrine have been the two French naturalists, MM. Pouchet and Joly. Their views have, however, made little progress; they have, on the contrary, met with vigorous opposition from the generality of French naturalists, and from most of the members of the Académie des Sciences of Paris, who have raised their voices against a doctrine which is contrary to the ordinary course of nature. In short, the direct observations made upon the theory of "primitive generation" are as yet wanting in necessary exactness; those observers who profess to have witnessed the sudden origin of the minutest of the infusoria from elementary substances have in all probability overlooked the organic structure of these elementary bodies. The wonderful changes of form undergone by many infusoria have their limits, and the laws governing them have still to be defined. With the poet we may say:

"Grammatici certant et adhuc sub judice lis est."

Many of the Infusoria are subject to metamorphoses, and it has already been ascertained that certain species which have been considered as distinct are only transition forms of the same species depending on age.

We know that it is common for insects to enclose themselves in protecting envelopes, and to remain for whole months shut up in this their retreat, to all appearance dead. Similar facts have been observed in the Infusoria. We have even seen some of these beings surrounding strange bodies as if in a mass of jelly, forming a sort of living envelope around them.

The average duration of life with them is only a few hours; but certain species present, in relation to the duration of life, phenomena which are only imperfectly known, but which never fail to excite the surprise and admiration of the naturalist. By drying certain infusoria with care, it is possible to suspend and indefinitely prolong its life. Thus dried, and covered with a powder, which shelters it from every breath of wind, it may be carried to any given distance, through any indefinite period of time--abandoned on some ledge of rock, on a housetop, in the cleft of a wall, or under the capital of a column; but let a drop of water approach it, and the dormant being awakes immediately--the microscopic Lazarus springs again into existence: feeds and multiplies as before: and its life, suspended possibly for years, resumes its interrupted course!

Into what a world of reflection does not a revelation of this mysterious property of a living creature plunge us!

The physiologist Müller has noted another peculiarity in infusorial life. These animalcules can lose a part of their bodies without being destroyed; the dead part disappears, and the individual, diminished by one-half, or reduced to a fourth of its former size, continues to live as if nothing had happened. Müller has observed a kalpode (_Kolpoda meleagris_) thus melt before his eyes until scarcely a sixteenth part of its body remained. After its loss, this sixteenth part of an animal continued to swim about without troubling itself as to its diminished proportions. "The infusoria," says Frédol, in "La Monde de la Mer," "present yet another kind of decomposition. If we approach the drop of water in which it swims with the barb of a feather dipped in ammonia, the animalcule is arrested in its movement, but its cils continue to move rapidly. All at once, upon some point of its circumference, a notch is formed, which increases bit by bit until the whole animal is dissolved. If a drop of pure water is added, the decomposition is suddenly stopped, and what remains of the animalcule recommences its swimming movements." (Dujardin.)

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.

The same, magnified 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 foetid 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.

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.

The _Monads_ are other infusorial animalcules which make an early appearance in vegetable infusions. They constitute a family that are destitute of any covering. The substance of their bodies can swallow itself, or draw itself out more or less; many of the whip-like filaments serve as organs of locomotion. They are sometimes provided with lateral appendages disposed as a kind of tail. Their organization is extremely simple; their whip-like filaments are so fine as to be scarcely perceptible, their length being sometimes double and even quadruple the length of the animal itself.

The _Lentille Monad_ (Fig. 33) is a species which is frequently met with in vegetable and animal infusions. The older microscopists had it indicated under the form of a globule, moving in a slow and vacillating manner. The globule is formed of a homogeneous transparent substance, swollen into tubercles on its surface, and throws out obliquely a whip-like filament, three, four, or even five times the length of the body of the Monad.

The _Cercomonad_ of Davaine was discovered by this gentleman in the still warm ejections of cholera patients. Its body is pyriform, having, in front, a vibratile filament, very long, very flexible, and easily agitated. Behind the body there is a thicker straight filament attaching itself sometimes to neighbouring corpuscles, round which, in this case, the _Cercomonad_ oscillates like the ball of a pendulum round its stem.

The _Volvocineæ_ are inhabitants of fresh limpid water full of confervæ and other aquatic plants. The _Volvocineæ_ are, according to Dujardin, animalcules of a green or yellowish brown colour, regularly disseminated in the thickness and near the surface of a gelatinous and transparent globe, which would become hollow and be filled with water in its perfect state. In this state, from five to eight smaller globules, with the same organization, appear destined to undergo the same changes when they are released by the rupture of the globule. These animalcules are each furnished with one or two flagelliform filaments, which, by their agitation, determine the movement by rotation of the mass.

A very remarkable phenomenon is recorded in the Transactions of the Microscopic Society, namely, the conversion of the contents of an ordinary vegetable cell into a free moving mass of Protoplasm, bearing a strong resemblance to the animal _Amoebæ_ (Fig. 20). This, it is affirmed by Dr. Hicks, takes place in Volvox, under circumstances which suggest a vegetable transformation. But Dr. Carpenter does not consider that this involves any real confusion in the boundaries of Animal and Vegetable Life.

The Revolving Volvox, _V. globator_ (Figs. 34 and 35), is found in great abundance, during summer, in tanks and ponds of stagnant water. It consists of green or brownish-yellow globules about the eighth part of an inch, formed of animalcules scattered round a gelatinous and diaphanous spherical membrane, each furnished with a flagelliform filament and with a reddish interior point, which Ehrenberg took for an eye. Leuwenhoek first observed this Volvox in marshy waters. This eminent naturalist has left a very interesting account of his observations on these microscopic inhabitants of the waters, displaying an amount of patience and address which cannot be too much admired; his observations were made with a simple lens, which he constructed himself. In one hand he held his instrument, which was very coarse if we compare it to the more perfect and infinitely more powerful instruments now in use; whilst, in the other hand, he carried to his eye the glass tube full of water which contained the object under observation. "The microscopes of Leuwenhoek," says Dujardin, "were the very smallest bi-convex lenses, mounted in a silver frame. He made a collection of twenty-six, which he bequeathed to the Royal Society of London. These instruments, subject to all the inconveniences of a maximum of spherical aberration and a total want of stability, were only fit for use in the hands of Leuwenhoek himself, who had acquired, in his labour of twenty years, habits of observation which compensated, in great part, for the want of perfection in his instruments."

The _Eugleniæ_ are infusoria usually coloured green or red. Their form is very variable. They are oblong or fusiform in shape, swelling at the middle during action, and contracted or bowl-shaped in repose, or after death. They are furnished with the usual whip-shaped filament, which issues from an opening in front, and from one or many reddish points irregularly placed anteriorly.

_Euglenia viridis_ (Fig. 36) is the most common species, and, perhaps, the most widely diffused of all the Infusoria. It is this animalcule which habitually covers stagnant pools with its floating surface of green, and which forms, on the surface of marshy waters, the shining pellicle so strongly coloured, which, collected upon paper, so long preserves its brilliant tint.

The _Euglenia sanguinea_, at first green, becomes subsequently of a blood colour. It has often been met with by microscopists. Ehrenberg, who first described it, attributes to its great abundance the red colour of some stagnant waters. Its presence explains the pretended miracle of water changing into blood, which was frequently invoked by the Egyptian priests.

CILIATE INFUSORIA.

Let us now take a glance at some of the more remarkable species of Ciliate Infusoria. The bodies of these creatures are all more or less translucent. They have not substance enough, in fact, to reach a state of opacity. Their bodies are more or less globular or ovoid, sometimes fashioned like a shuttle, or curved while growing, sometimes swollen in the middle like an ampulla, or bell-shaped, and flattened into a discoid shape; some slightly resemble a tadpole, a thimble, a shoe, a rose-bud, a flower, even a seed.

The Paramecians have a soft flexible body, usually of oblong form, and more or less depressed. They are provided with a loose reticulated covering, through which issue numerous vibratile cilia, arranged in a regular series. They were known to the older naturalists; and it is in this group that organization is carried to the highest perfection it attains among the Infusoria. The Paramecium possess, besides their reticulated and contractile tegument, cilia disposed in such a manner as to serve at once for locomotion, for prehension, that is, for seizing its food, and as a means of respiration. They are furnished with a mouth, at the bottom of which the whorl excited by the cilia determines, according to Dujardin, the hollowing out of a cavity, formed after the manner of a cul-de-sac, and also the formation of _vacuoles_ with permanent partitions, in which are enclosed the substances which the animalcules have swallowed along with the water.

The Paramecium are propagated by spontaneous division, as already described. They abound, as we have said, in stagnant water, or in pure water which is occupied by aquatic plants, sometimes in such prodigious quantities that they become troublesome. They occur also in flower vases where the water is not frequently renewed.

The species of this genus have an oblong compressed body, with an oblique longitudinal fold, directed towards the mouth, which is lateral. They are sufficiently large to be observed by the common lens, or eye-glass. _Paramecium aurelia_ appears chiefly in vegetable infusions. It is common in ditches and moats with aquatic plants.

Humboldt's assertion is fully verified in the case of the Infusoria under consideration, which is often found with its parasites. These are small creatures, cylindrical in form, and provided with suckers. Swimming vigorously in the water, they devote themselves to chasing the Paramecium. When they have overtaken the fugitive, they throw themselves upon it, and establish themselves there. They soon multiply in the interior of its body, and their starving progeny suck and devour the unfortunate animalcule, which serves them at once for dwelling-house and larder.

Another of the parasites which prey upon the Paramecium, in place of pursuing it, remains perfectly quiet until one of these approach, when it throws itself upon its victim, and is carried along with it. It buries itself in the body of the Paramecium, and, in a short time, multiplies to such a degree, that sometimes fifty of them are found on a single individual. Poor victim!

The _Nassula_ have the body entirely covered with cilia; they are ovoid or oblong in form, contractile, the mouth placed laterally and dentate, or surrounded with a band of horny bristles, the band dilating and contracting according to the size of the prey which it would swallow. It either advances to seize the prey, which the movement of vibratile cilia have failed to draw within the vortex of its mouth, or, as in the case of the Paramecium, it is sometimes obliged to seek for its prey. These curious infusoria live in stagnant waters, feeding on the débris of aquatic plants, from which they draw their chief nourishment as well as their colour.

The _Bursarians_ are animals with an oval or oblong contractile body, provided also with vibratile cilia, especially on the surface, having also a large mouth, surrounded with cilia, forming a sort of microscopic moustache, spirally arranged.

Among the species belonging to this group may be noted the _Condylostoma patens_ (Fig. 39), remarkable for its size and voracity. It sometimes attains the twelfth of an inch, and abounds on every shore from the Mediterranean to the Baltic. Another Bursarian, a species of _Plagiostoma_, lives between the intestines and the external muscular bed of the earth-worm, _Lumbricus terrestris_. To the group of _Urceolarians_ belong the _Stentors_, which are in number the most numerous of the Infusoria; they are, for the most part, visible to the naked eye.

The _Stentors_ are inhabitants of fresh, tranquil water, not subject to agitation, and covered with water plants. They are nearly all coloured green, blackish, or blue; their bodies covered with cilia. They are eminently contractile, and very variable in form. They can attach themselves temporarily, by means of the cils at their posterior extremities, when they assume a trumpet-like form, the bell of which is closed by a convex membrane, the edge being furnished with a row of very strong obliquely-placed cilia, ranged in a spiral, meeting at the mouth, which is placed near this edge. When they swim freely, they alternately resemble a club, a spindle, or a sphere. The _Stentor Muelleri_ is seen in ponds in the neighbourhood of Paris and elsewhere; it has been found even in the basins of the Jardins des Plantes (Fig. 40).

The animals which constitute this genus are fixed in the first part of their existence, but free in the second. So long as they are fixed, they resemble, in their expanding state, a bell or funnel, with the edges reversed and ciliate. When they become free, they lose their crown of cilia, take a cylindrical form, more or less ovoid and elongated, and move themselves by means of a new organ. "There is no animal," says Dujardin, "which excites our admiration in a higher degree than the Vorticellate Infusoria, by their crown of cilia, and by the vortex which it produces; by their ever-varying forms; above all, by their _pedicle_, which is susceptible of rapid spiral contraction, by drawing the body backward and again extending it. This _pedicle_ is a flat membranous band, thicker upon one of its edges than the other, and containing on the thicker side a continuous channel, occupied, at least in part, by a fleshy substance, analogous to that of the interior of the body. During contraction, this thick edge is shortened more than the thin side, and hence results the precise form of the spiral of the corkscrew."

We cannot conclude our brief history of these curiously-organized beings without recording the doubt which still exists in the minds of our most eminent naturalists, whether some of those we have named are animal or vegetable in their origin. The _Desmideæ_, long classed among animals, are now generally recognized as plants. The group of _Diatomaceæ_ are still considered doubtful, and the Monads and _Volocina_ are still subjects of discussion, the evidence inclining in favour of those who argue for their vegetable nature. Messrs. Busk, Williamson, and Cohn, have published in the "Microscopical Transactions" minute details of the evolutions of these curiously-organized globules, which seem to prove their vegetable nature. On the other hand, it is difficult to imagine so accurate an observer as Agassiz writing so positively as he does on a doubtful subject. Remarking on a former paper, in which he had shown that the embryo hatched from the egg of a Planaria was a true polygastric animalcule of the genus Paramecium, he adds, that in former writers a link was wanting, viz., tracing the young hatched from the egg of _Distoma_. "This deficiency," he says, "I can now fill. It is another _Infusorium_, a genuine _Opalina_. With such facts before us there is no longer any doubt left respecting the character of all these Polygastria; they are the earliest larvæ condition of worms." Amid these friendly disputes we congratulate ourselves that we have to do with the oceanic creations, both animal and vegetable.