ANIMALCULE.

Animalcule, a diminutive term (from the word animaD, applied by naturalists to those minute beings which become apparent in various fluids when subjected to the microscope. They were named infusory animals (Infusoria) by Müller, one of the most celebrated observers in this department of zoology; and the appellation, however inapplicable, now occurs in the majority of scientific publications. Of course it applies with propriety only to such species as arc developed through the medium of infused substances. Now we know, that of 400 species of Infusoria (commonly so called) described by Müller himself, not a sixth part were observed in any kind of infusions; whilst the remainder inhabited the most translucent waters, and speedily died when placed in impure or corrupted liquids. Even the word animalcule (or little animaD does not convey a positive or sufficiently restricted idea in relation to this particular class; because mites and certain polypi are extremely minute in their dimensions, and equally require the aid of microscopical investigation; and thus the term microscopies (microscopiques), recently proposed by Μ. Bory de St Vincent, is not less faulty than its predecessors. The size of an animal, in fact, bears no essential relation to the other conditions of its organization; and therefore we cannot infer its nature with any certainty from a knowledge of its dimensions. At the same time it must be admitted, that the most simply organized, both of plants and animals, are also the most minute; and thus the Infusoria may be regarded as possessed of certain characters in common. We here adopt the word Animalcule, chiefly because it is the most familiar to the English reader.

The subjects of our present observations may be thus defined :— Animals invisible to the naked eye; The Volvox globator, and a few others, which are just discernible without the aid of a microscope, form exceptions to the above character. more or less translucent ; unprovided with members (the caudal, and other appendices, with which certain species are furnished, being scarcely regardable as such); no perceptible eyes; contractile in whole or in part ; endowed with the sense of touch ; deriving nourishment by absorption ; generation (when not apparently spontaneous, and consequently incomprehensible) effected by division, or by the emission of gemmules or oviform bodies ; inhabitants of liquids. They are the smallest and most simple of living creatures, but not less perfect than the other tribes; for though they possess the fewest faculties, their means are in every way adequate to their wants, and their vital energies proportioned to their sphere of enjoyment.

Among microscopical animals we find many species which, in their aspect and structure, present no analogy [7:3:181]to other forms of animal life: they are merely moving molecules of the simplest organization, the exact nature of which it is sometimes difficult to determine, and which involve in deeper obscurity the mysterious line of demarcation by which we so often seek in vain to separate the animal from the vegetable kingdom. If, however, the true distinction between plants and animals consists chiefly in the irritability and power of contraction possessed by the latter, then the Infusoria, which are strongly endowed with these attributes, are indeed so far removed from the vegetable kingdom, that the name of Zoophytes, or animal plants, is inapplicable to the class to which they belong. In the extreme simplicity of their structure, they no doubt present some analogy to the least complicated tribes of plants, such as the algae and others; but it is a mere analogy, and not a connection of affinity,—for no alliance beween these kingdoms has ever been demonstrated, although certain obscure phenomena may have presented difficulties in the way of our investigations. “We need not be surprised,” Mr Macleay observes in his Horoe Entomologicae, “that several of the Linnaean algae should be still hovering in a state of uncertainty between the two kingdoms; but, on the contrary, be prepared to expect additional proofs of the analogy which the two great divisions of organized matter bear to each other. The Agastria, or Agastraires of De Blainville, are indeed animals, though they have neither distinct organs of sense, alimentary canal, nor even mouth,—though they have, in short, so far as our present knowledge of them would lead us to believe, no internal digestion whatever to execute, but trust for nourishment, like plants, to the absorption of their external pores. They must be esteemed animals on account of their peculiar irritability; but are vegetables in almost every other respect.”

Our knowledge of the history of animalcules resulted from the improvement of the microscope by Hartzocker and Lecuwenhoeck. The ancients were consequently unacquainted with the mysteries of this “invisible world ;” and we are thus saved the tedium of a lengthened bibliographical investigation. Notwithstanding the observations of Hill, Baker, Ledermuller, Goeze, Gleichen, Roësel, Pallas, Needham, Spallanzani, and several other minute and laborious inquirers, it may be said that this branch of zoology only assumed a truly scientific form in consequence of the labours of a distinguished Danish naturalist, Otho Frederic Müller. His earlier works, such as the Vermium Terrestrium, et Fluviatilium, Historia, and the Zoologiae Danicae Prodromus, presented very decided improvements in the knowledge and classification of animalcules. These emendations were transferred by Gmelin to the 13th edition of the Systema Natures of Linnaeus, in which animalcules form the fifth order of the class of Vermes. But the work which, had the rest been wanting, would alone have immortalized the name of Müller, appeared (posthumously) in 1786, under the title of Animalcula Infusoria, Fluviatilia et Marina. It is illustrated by 50 plates, containing figures of 360 species variously represented. A later and very useful compendium of knowledge regarding microscopical animals forms a portion of the Encyclopédie Méthodique (46th livraison), in which Bruguière has reproduced the plates of Müller, with the addition of several others of equal accuracy from the third volume of Roësel’s lnsecten-belustigungen. The reader will there find descriptions of 28 plates, containing nearly 1100 figures representing 385 animalcular species.

Before proceeding further, we shall present a brief view of the system of Müller, as of high importance in itself, and the fondamental basis of the more recent and improved arrangements. It includes, however, several genera which are not now classed among the animalcular tribes. He divides the Class of Animalcula Infusoria as follows:
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In the year 1815 Lamarck published the first part of his Animaux sans Vertèbres, a work which forms an epoch in the history of the inferior tribes. In this signal publication the author adopts a different course from that usually followed by systematic writers; and pursuing an ascending rather than a descending scale, he commences with the lowest and least complex species, viz. the Infusoria. From this class, however, he rejects all those species in which any kind of complication of organs is apparent. All the genera so distinguished are referred by him to the first order of the second class of the animal kingdom, called Polypi, under the title of Polypi ciliati; and the true and restricted Infusoria are thus defined: Microscopical animals, gelatinous, transparent, polymorphous, contractile ; having no distinct mouth, nor constant, determinable, internal organs ; generation fissiparous or sub-gemmiparous. The genus Kerona, it may be further remarked, is in this system united to the Himantopus of Müller, while the genus Ccrearia of that author is divided into two. Thus the class Infusoria of Lamarck, composed of two great sections, the naked and the appendiculated, may be said to correspond to the first 14 genera of the Danish naturalists.

Cuvier, in the Pègne Animal (1817), places the Infusoria as a part of his fourth great division, the zoophytical or radiated animals. The term radiated was originally bestowed on a numerous tribe of animals, such as the Asterias and others, on account of their arms or tentacula being extended in a radiated or star-like form; but it certainly applies unfitly to the true Infusoria of Lamarck, which possess nothing resembling a radiated structure. It cannot, however, be always expected that a general term of wide import should apply with etymological accuracy to every part of the extensive series which it is intended to embrace. Cuvier then divides his Infusoria into two orders, Les Rotifères, and Les Infusoires homogènes, the former of which correspond to the ciliated polypi of La-march, [7:3:182]the latter to the Infusoria properly so called of that author.

' According to Μ. de Blainville, the class Infusoria can scarcely be regarded as established upon a natural foundation. The organization of its component tribes is so various , as to lead to the belief that a more precise knowledge would show that several of those tribes belong to different types of the animal kingdom. Some, as the genus Brachionus, are symmetrically formed both as regards their bodies and appendages, and are protected by a homy or crustaceous covering. Others, as Vibrio, Paramaecium, &c. have the body elongated, depressed, vermiform, and without appendages. A third division exhibit a radiated structure, as for example the Vorticellae, which, however, we have already stated, are now seldom classed among the Infusoria. Many genera, such as Proteus, Volvox, Monas, are amorphous, or without determinate form, and cannot be referred to any other known type of the animal kingdom. They are regarded by many as the elementary molecules of all animal life, and in their structure no other than the cellular tissue is observable. They may be said to be dependent on external circumstances, instead of being able, like other animals, to modify or control them; and their usually spherical form is the necessary result of an equal pressure of water .on all sides of a frail and yielding texture. Μ. de Blainville considers the genera Brachionus, Urcelaria, Cercaria, Fur-cularia, Kerona, Trichocerca, and Himantopus, as belonging to the type of Enlomozoaires or articulated animals, and especially to the class Heteropoda, order Entomostraca. Many species of Vibrio he regards as Apodes, as well as Paramoecium and Kolpoda. Other species of the genera Vibrio and Cyclidium ought rather to be ranged with the Planaria; and in the genus Leucophra there is even a species which Μ. de Blainville is inclined to look upon as an Ascidia! Finally, the genera Gonium, Proteus, Volvox, and Monas, if they are really animals, appear to form a distinct type, which may be called Amorphes or Agastraires; so named from the circumstance of their having neither determinate form nor reduplication of the external envelope for the formation of a stomach, as in all other true animals.

Such is a brief exposition of the views of one of the most distinguished physiological inquirers of the present day. It may serve, if for nothing more, at least to show the unsettled state of opinion concerning these extraordinary creatures. In regard to this, however, we may rest assured that, in the future progress of science, the class Infusoria, as at present constituted, will suffer an entire dismemberment, and its component parts will be referred to various groups of the animal kingdom, some of them widely distant from each other.

In the year 1826 a full and most elaborate classification of microscopical animals was given to the world by Μ. Bory de St Vincent. As it is the singular mode of existence of animalcular beings, their general economy in the field of nature, the actual conditions of their organization, and the state of their limited faculties so far as these can be ascertained, with which we are chiefly interested—so, in our systematic view of this extraordinary class, we shall merely present to our readers the characters of the principal genera, and of a few of the most remarkable species which they contain. But, as some may be desirous to possess at least a sketch of the full extent and condition of this intricate subject, we have constructed the accompanying tabular scheme of the orders, families, and genera of microscopical animals, according to the views of Μ. Bory de St Vincent, the latest and most assiduous writer on this department with whose labours we are acquainted. We have thought it advisable to retain the terms of the original language,'lest, by inadvertence or misconception on the part of the translator, any additional obscurity should rise around a subject already sufficiently encumbered. (See Tabular View on the opposite page.) J

The order Gymnodes of Bory de St Vincent nearly cor. responds to the entire class Infusoria of Lamarck; and although the observations by which he illustrates his arrangement partake of the accustomed defects of the French philosophy, the facts which he details, if not the theore∙ tical views which he inculcates, are worthy of an atte∏. tive consideration. These mysterious creatures are observed to swim with astonishing rapidity; and although their bodies are usually diaphanous, it has hitherto proved impossible, even by the aid of the most powerful glasses, to ascertain by what natural mechanism these movements are effected. They direct their courses by a discretionary power, in one direction rather than another, avoiding and turning round opposing obstacles, according to the necessities of the case—discerning, as the process of evaporation proceeds, the points in which they may prolong their existence, and flocking in crowds to those places where they are best screened from the overpowering bril· hancy of the reflecting mirror. They thus appear to possess volition, which we are accustomed to regard as a result dependent on the faculties of perception and comparison.

The principal obstacle to our understanding the essential nature of animalcules results from their want of a nervous system, which, in ourselves, and in all the intermediate classes of the nature of whose’ consciousness we have even a vague idea, we regard as the sine qua non of sensation and intelligence. Voluntary motion without muscular action is also a circumstance which we cannot very clearly comprehend. But as there may be “more things in heaven and earth than are dream’t of in our philosophy,” we must not reject facts, that is to say appearances which present themselves under the same determinate and uniform aspect to various unprejudiced observers, merely because they do not coincide, or may possibly controvert or interfere, with a previous hypothesis. On the other hand, the extreme softness of texture, and excessive minuteness, of most of the animalcular species, render anatomical investigation almost impossible; and naturalists may have erred in suppo⅞ing the absence of what they are merely unable to perceive and demonstrate.

It is in truth impossible to discover any traces of the nervous system, even among several tribes of animals in other respects much more highly organized than the subjects of our present inquiry. Trembley’s examination of the Polypus threw no positive light upon the matter; nor did Gade’s dissections of the larger Medusae enable him to discover either muscular or nervous fibres. According to Μ. Bory de St Vincent, the nervous system is one of the last to be developed. To the perfect simplicity of the Monads, the first perceptible addition is that of a central cavity, or rudimentary intestinal sac, which we find to occur even before the existence of a mouth. Next appears an opening to this canal, which serves both for the reception of nourishment, and the rejection of excrementitious parts where such exist. The hairs andcirrated appendages which ensue in still more complicated species have been regarded as the early rudiments of the respiratory system; and even a heart, or central organ of a circulating fluid, is partly developed before the appearance of any nervous chords. The earliest, most general, and perhaps the only indispensable function of animal life, is that of nutrition· But the materials of nutrition are so different, and their modes of reception so various, that the exercise of this
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function by no means necessitates the existence of a mouth, a stomach, or an alimentary' canal; for an increase of parts may be effected even through the medium of imponderable or elastic fluids, and by imperceptible and superficial pores.

The exterior of an infusory animal may be compared to the interior of one of the higher classes, in which nutrition is carried on by the reception of the chyle by the absorbent pores. These pores are external among the Infusoria, and the process of absorption is with them analogous to that of plants, in which there is a direct reception and appropriation of fluids from the earth and air, without any previous preparation in a central cavity or stomach. Zoophytes in general have indeed been called the cryptogamia of the animal kingdom. According to Carus, the Infusoria ought to be regarded merely as little cells, partially filled with lymph, and possessed of the powers of nutrition and locomotion; and thus the infinite changes and variations perceptible in their forms may be supposed to be produced by the various degrees in which this fluid is collected at one or other of the points of their bodies. In the opinion of that anatomist, a more complete development of the organs of motion, and indeed of the whole organization, is inseparably united with the appearance of a distinct nervous system. This may be true as a general rule, but not as a universal principle; for the Medusa has more apparent voluntary motion than the Asterias, though the former is destitute of those nerves which in the latter make their first appearance in the shape of a pale thread-like ring surrounding the oesophagus. It is this ring around the upper extremity of the alimentary canal which, in the molluscous and articulated classes, we shall afterwards find to constitute the most uniform and most essential portion of the nervous system. The Medusae, just referred to, being almost of the same specific gravity with water, are easily carried by currents, and moved about from place to place by the action of the waves, or even (as in the case of Holothuria physalis) by the winds; but Carus and other writers have assuredly erred in doubting that they execute a voluntary locomotion; for that they do so in a very decided and graceful manner must be obvious to all who have attended to these animals in their native haunts along the shores, or among the land-locked waters of the beautiful firths of Scotland.

The mysteries revealed by the glasses of Lecuwenhoeck were at first regarded as beyond belief. The uncertainty of microscopical investigations, in consequence of which so much was supposed to depend on the imagination of the beholder, was alleged against them; and even at an after-period, when men of sober judgment and the most industrious application had confirmed the experience of the indefatigable Dutchman, the wit of Voltaire did not disdain to throw its cutting sarcasm over the disciples of the “anguilles de la pâte et du vinaigre.” We hope it is now admitted, that however frequently those who endeavour to expound the mysteries of nature may fail in their attempts at elucidation, yet that there is nothing in the manifold works of Omnipotent Wisdom which, if duly studied and rightly understood, would not conduce to our wellbeing and happiness; and that a single square inch of water, with its many millions of animalcular atoms, is in truth as wonderful a work of divine intelligence, and as interesting a field for human investigation, as the starry galaxy of heaven.

In his tam parvis, atque tam nullis, quae ratio! Quanta vis! quam inextricabilis perfectio!

And if a heathen philosopher (Pliny) has so expressed his almost reverential admiration, is it pot to be deplored that those whose labours might be carried on under the ⅛fl u . ence of a purer light, seem as often degraded as exalted'' by the contemplation of their Creator’s works; and, referring all to the powers of nature, or some other indefinite abstraction, refuse to recognise, amid so many wonders, the “Good Supreme” from whom these and other mightier wonders have proceeded? It is in the study of the subject with which we are now engaged, and the analogous pursuits of physiology, where the completion of the most perfect design and the happiest results of superhuman forethought are so constantly manifested, that w e frequently meet, where we should least expect it, with the sneer of the sceptic, or the impious ridicule of the unbeliever. ' How different are the sentiments ∙ of one who combines the piety of the Christian with the genius of the poet and philosopher. “But about the time of its invention (the invention of the telescope), another instrument was formed, which laid open a scene no less wonderful, and rewarded the inquisitive spirit of man with a discovery which serves to neutralize the whole of this argument. This was the microscope. The one led me to see a system in every star; the other leads me to see a world in every atom. The one taught me that this mighty globe, with the whole burden of its people and of its countries, is but a grain of sand on the high field of immensity; the other teaches me that every grain of sand may harbour within it the tribes and the families of a busy population. The one told me of the insignificance of the world I tread upon; the other redeems it from all its insignificance; for it tells me, that in the leaves of every forest, and in the flowers of every garden, and in the waters of every rivulet, there are worlds teeming with life, and numberless as are the glories of the firmament. The one has suggested to me, that beyond and above all that is visible to man, there may lie fields of creation which sweep immeasurably along, and carry the impress of the Almighty’s hand to the remotest scenes of the universe; the other suggests to me, that within and beneath all that minuteness which the aided eye of man has been able to explore, there may be a region of invisibles; and that, could we draw aside the mysterious curtain which shrouds it from our senses, we might then see a theatre of as many wonders as astronomy has unfolded, a universe within the compass of a point so small as to elude all the powers of the microscope, but where the wonder-working God finds room for the exercise of all his attributes, where he can raise another mechanism of worlds, and fill and animate them all with the evidence of his glory.”^[2. Chalmers’s Astronomical Discourses, p. 112. ]

Although we cannot hope to derive the same amusement or advantage from the study of each of the animalcular species considered separately, as we do from the consideration of the history of many of the higher animals, yet, in a philosophical point of view, a knowledge of the general attributes of the class presents several highly important objects; and their obscure origin, their singular organization, and more singular mode of existence, cannot fail to excite our unfeigned wonder and admiration. They can scarcely be described otherwise than by a negation of all those characters which constitute the life, power, and activity of other beings; they have no head, no eyes, no muscles, no blood-vessels, no nerves, no determinate organs for respiration, generation, or digestion—and yet they are endowed with life.

The animal nature of the Infusoria has indeed been denied by many; but such is the regular gradation from the most simply organized of the monadal forms to the much [7:3:185]more complicated structure of the Polypi, which present, under a remarkable aspect, such unequivocal characters of animality, that it is impossible to draw the line of demarcation; and if we admit the life of the one we can scarcely doubt that of the other. Yet many of the Infusoria appear to present the very lowest conceivable point to which animal life can be reduced.

The structure of an animal, the individual existence of which is preserved by the absorption of a circumambient fluid, and the continuance of whose species is effected by the division or separation of a part of its own body, might, a priori, be supposed to be of the most simple kind. “We may rest assured,” observes Lamarck, “that whenever an organic function is itself unnecessary, the special organ by which it is usually performed will not be found to exist.” It is indeed by considering the nature of the Infusoria that we are enabled to form a proper idea of the simplest condition of animal life; and the invention of the telescope was not of higher importance to the astronomer, than that of the microscope to the physiological naturalist. There are few subjects of reflection more interesting than the uses which philosophers of an enlightened age have deduced and matured from the scanty knowledge of a barbarous people. Glass, a material known it an early period to the Asiatic nations, and once estimated at its weight in gold, has become in the hands of Europeans of more value than the finest gold. Whoever polished the first lens may be said to have laid the foundation of an instrument destined to discover thousands of celestial worlds above and around us, and an unseen world of wonders beneath our feet. “Indeed,” says Cuvier, “it could not be expected that those Phoenician sailors vho saw the sand of the shores of Boetica transformed by fire into a transparent glass, should have at once foreseen that this new substance would prolong the pleasures of sight to the old; that it would one day assist the astronomer in penetrating the depths of the heavens, and in numbering the stars of the milky way; that it would lay open to the naturalist a miniature world as populous, as rich in wonders, as that which alone seemed to have been granted to his senses and his contemplation; in fine, that the most simple and direct use of it would enable the inhabitants of the coast of the Baltic Sea to build palaces more magnificent than those of Tyre and Memphis, and to cultivate, almost under the frost of the polar circle, the most delicious fruits of the torrid zone.”^[3. Réflexions sur la Marche actuelle des Sciences, &c. read to the Institute of France in April 1816. ]

The faculties of the most simple infusory animals, it has been observed, are reduced to such as are common to all living beings, and to that irritability which results from their animal nature; and their bodies are destitute of special organs, precisely because their extremely limited faculties neither require nor admit of such organs being exercised. The chief interest to be derived from the study of this class of beings results, according to Lamarck, from the view with which such study presents us of the ultimate point to which the organization of an animal is capable of being reduced; and, among all the the wonders of the creation, he regards as the most surprising the existence of animal life in such inconceivably frail and simple bodies as the least complicated of the animalcular species. It is not, however, to be said that “nature was incapable of forming special organs from the naterials of these frail gelatinous bodies,” but rather that he all-wise Author and Ruler of Nature has seen fit to form them in what to us may appear a destitute and incomplete condition, merely because their structure does not fulfil those other conditions which, erroneously, we have sought to establish as the indispensable bases of animal life. They truly show how confined a knowledge our limited faculties enable us to gain of His unlimited power; for they not only present no analogy to other more accustomed forms of life, but almost seem to exist in direct opposition to those laws in accordance with which we “live, move, and have our being.”

Infusory animals, commonly so called, are found in the fresh and saline waters of all countries. They occur both naturally, if we may use the term in a contradistinctive sense, and as the apparent result of infused animal and vegetable substances. According to Lecuwenhoeck, the milt of a eod-fish contains more animalcules than there are individuals of the human race existing on the face of the earth; and he calculated that 10,000 might be held within the bulk of a grain of sand. The size of many bears the same relation to that of a mite as the dimensions of a bee do to those of an elephant; and the most powerful microscopes frequently discover nothing more than merely perceptible points in motion. Flour and water made to the consistence of book-binders’ paste, exposed in an open vessel, and frequently stirred to prevent the surface from growing hard, will in a few days be found to contain millions of animalcules. The thin pellicle which grows on the surface of an infusion of common black pepper also produces an innumerable congregation of minute beings. Of these and others the real origin is still exceedingly obseure; and both Müller and Spallanzani maintained the improbable opinion that they fell from the air. Their subsequent increase or multiplication is obviously effected in different and very singular ways. Such as are spherical are usually propagated by minute portions, which, though they burst from the anterior of the animalcule itself, cannot be called eggs; and such as are of a depressed or flattened form continue their race by cuttings or divisions of their own bodies. We first observe a line or groove, longitudinal or transverse, according to the species; and ere long a notch or incision is perceptible at one or other or both of the ends of that apparent line. This notch increases across or longitudinally, till at last a portion is separated or cut off, or the original animal is divided into two, and each assumes the form and nature of their mutual predecessor. These new objects retain for some time their natural shape, and then in their turn give rise to one or more individuals by a similar separation of parts. Lamarek seems to be of opinion (Philosophie Zoologique, tome ii. p. 120 and 150) that this multiplication by division, and that by the emission of gemmules or oviform portions, are modifications of one and the same natural process ;—that substantially each is the result of an extension and separation of parts, which take place when the parent individual has reached the final term of its increase. It is in faet the same excess of nourishment’ and growth of particular parts that, even in the higher classes, give rise to the germ of separate life, physically considered, although in regard to these the additional process of fecundation is required. It is the new light which may be gathered from the observation of the minutest of the animalcular tribes that renders their study both interesting and important to the physiologist; and it is the belief of some, that a persevering study of the history of microscopical animals will one day enable us to withdraw the mysterious veil which still conceals from our view the most important secrets of nature.

The systematic arrangement of animalcules which we propose to follow in this place is nearly that of Lamarck, [7:3:186]which is itself founded on the systems of. Müller and Bruguičre. The French naturalist includes in his system only such species of the same class described by Müller as are destitute of a mouth.

Order I.— Naked Infusoria. Body extremely simple, apparently homogeneous, unprovided with organs or external appendages.

The naked Infusoria are the smallest and simplest of those animals which are cognizable by the senses of man. When we expose water to air and light, especially if it contains an infusion of animal or vegetable remains, we speedily perceive, by the assistance of the microscope, a variety of animalcules. These are divided into two sections.

Section I.— Body Thick. Of this section the bodies present such a perceptible degree of thickness as removes them from the simply membranous state. Genus Monas. —Body extremely small, of the simplest construction, transparent, punctiform.

The Monads are the smallest and least organized of living creatures. We have indeed scarcely any proof of their animal life, except that they are moving corpuscles, allied to the genus Volvox, the animality of which is undoubted. They have neither mouth nor alimentary canal, nor any apparent organs. They live by absorption, and arc found in tranquil, but rarely in limpid waters. They are produced in infusions of animal and vegetable substances.

Sp. 1. Monas termo. — An extremely minute gelatinous point, scarcely perceptible even with the aid of a powerful lens, and frequently disappearing under a strong light in consequence of its perfect transparency. This species is common in ditch-water, and in numerous infusions. See Plate XLVI. fig. 1. These minute creatures being the earliest discernible evidence of animal life, and the last result to which the higher and more perfect forms can be reduced by infusion, have been called the alpha and omega of all organized existence. Their history has given rise to many curious views, and more absurd speculations.

Sp. 2. Monas atomus. — White, with a variable darkcoloured spot, which appears to change its position. This species was found in sea-water which had been kept an entire winter, but was not very fetid. See Plate XLVI. fig. 2 and 3.

Sp. 3. Monas punctum. — Nearly black, of a sub-cylindrical form. Found in the infusion of the pulp of a pear.

Sp. 4. Monas lens. — Hyaline, of an ovoid form. Found in all kinds of waters. Multiplies by spontaneous incision.

Bp. 5. Monas pulvisculus. — Hyaline, with a greenish margin. Found in the waters of marshes. This species has been lately regarded as an enchelis. Indeed, so great is the difficulty of microscopical investigation, and such indefatigable patience is required in order to see things as, they really are, that the species and genera of this class of beings are frequently transposed and altered in their relative position and arrangement, in consequence of the very dissimilar views which different observers have taken of the same object.

As it would be inconsistent with our present limits to describe more than a few species of each genus, we shall content ourselves with the preceding Monads. “How many kinds,” observes Mr Adams, “there may be of these invisibles, is yet unknown, as they are discerned of all sizes, from those which are barely invisible to the naked eye, to such as resist the force of the microscope, as the fixed stars do that of the telescope, and, with the greatest powers hitherto invented, appear only as so many moviι√ points.”

Genus Volvox. —Body very simple, transparent, spherical or ovoid, revolving on itself as on an axis.

With the exception of one species (V. globator) the vol-voces are invisible to the naked eye. Under the micro· scope they assume the aspect of small gelatinous masses, which sometimes present an oval form. In some the body seems composed of numerous smaller globules united in one common mass. There is reason to suppose that these interior bodies are afterwards propelled outwardly, and become separate individuals. The species occur both in fresh and salt waters, and in vegetable infusions. They derive their generic name from the manner in which they turn or revolve upon their axis. Lecuwenhoeck describes an animalcule “a thousand times smaller than a louse’s eye, which exceeded all the rest in briskness,” and turned itself round as it were upon a point, with the celerity of a spinning-top. The genus is divisible into two sections.

* Interior of the body apparently simple and homogeneous. Sp. 1. Volvox punctum. — Spherical, blackish, with a central lucid point. Of this species many are sometimes seen together in their passage through the water. They occasionally move as if subjected to the influence of a whirlpool, and then separate. Numerous in fetid seawater.

Sp. 2. Volvox granulum. — Spherical, green, periphery hyaline. Dwells in the water of marshes.

Sp. 3. Volvox globulus. — Globular, sub-obscure behind. This species is ten times larger than the Monas lens. It occurs in most vegetable infusions, and moves with a slow fluttering motion. Plate XLVI. fig. 4.

* * Interior of the body exhibiting smaller corpuscles. Sp. 4. Volvox pilula. — Spherical, with greenish internal globules. In those pure waters which nourish the Lenina minor. Plate XLVI. fig. 5.

>Sp. 5. Volvox lunula. — Roundish and transparent, and composed of an innumerable assemblage of homogeneous crescent-shaped molecules, without any common margin. Its movements are of two kinds, that of the molecules among themselves, and that of the whole revolving mass. It occurs in marshy places in the early spring. Plate XLVI. fig. 6.

Sp. 6. Volvox globator. — Commonly called the globeanimal. Spherical, membranous, the internal globules distant or scattered. Abundant in the infusions of hemp and tremella, and in stagnant pools during spring and summer. The following is an account of it by Mr Baker. “There is no appearance of either head, tail, or fins, and yet it moves in every direction, backwards, forwards, up or down, rolling over and over like a bowl, spinning horizontally like a top, or gliding along smoothly without turning itself at all; sometimes its motions are very slow, sometimes very swift; and, when it pleases, it can turn round as upon an axis very nimbly, without moving out of its place. The body is transparent, except where the circular spots are placed, which are probably its young.” Another authority states that this species is at first very small, but increases to such a size that it may be discerned by the naked eye, and that its interior is filled with small globules, which are smaller animalcules, each of which contains within itself a still smaller generation, all perceptible by means of powerful glasses. The lesser globules may be seen escaping from the parent, and increasing ® size.
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Genus Proteus.— Body very small, simple, transparent, of varying form, changing itself instantaneously into ». different lobated shapes.

This genus is more obviously contractile than the preceding. It is seldom seen above a minute under the same form, but is continually passing from a simple oval or oblong to an irregular or sinuated shape, and vice versa. A species described by Roësel is so remarkable for this faculty, that it has been compared to a drop of water thrown upon oil. Hence also the generic name.

Sp. 1. Proteus diffluens. — Body diverging into branches. Occurs in the water of marshes. Plate XLVI. fig. 7, 8, ad 9.

Sp. 2. Proteus tenax. — Body prolonged to a fine point. Occurs in rivers and in sea-water. There are only two species described as belonging to this genus.

Genus Enciielis. —Body very small, simple, oblong, cylindrical, slightly variable.

There is a marked analogy between this genus and the following. The Enchelides are, however, short and thick compared with the Vibriones, which are slender and lengthened. To the genus now under consideration belong those animals which, if the recorded observations on the subject have been accurately made and faithfully reported, more than any other confound our preconceived ideas regarding the distinction between animal and vegetable life. The species alluded to are named Zoocarpes by M. Boryde St Vincent, or animated seeds, which appear reciprocally to give rise to and proceed from certain aquatic plants of the confervae kind. They are formed in a bulbous-shaped part or swelling of the plant, are ejected when ripe, swim about for some time with a voluntary motion, throw out a root and a branch, become genuine vegetables, poduce living seeds, and give birth to animals which, after a similar change of form, speedily return again to the vegetable state. These facts are vouched for by Μ. Bory de St Vincent, andare credited and confirmed by M. Dutrochet and several other continental inquirers, some of whom declare that they kept so watchful an eye upon the same individual as never to lose sight of it for a moment till they had witnessed the singular transformation above mentioned. We recommend it to our readers’ consideration.

Sp. 1. Enchelis viridis. — Sub cylindrical, obliquely truncated anteriorly. This species has an obtuse tail or terminal part. It continually varies its motion, turning from right to left. Occurs in long-kept water.

Sp. 2. Enehelis pimctifera. — Subcylindrical, green, obtuse anteriorly, pointed posteriorly. This species is opaque, with a small pellucid spot in the fore part, in which two black points are seen, and a kind of double band crosses the middle of the body. It occurs in marshes. Plate XLVI.fig. 10, 11.

Sp. 3. Enchelis pulvisculus. — This species bears a great resemblance to the Monas pulvisculus of Müller, which is the E. monadina of Bory de St Vincent. It is, however, double the size, deeper tinted, and more ovoid. It is found in the waters of marshes, and accumulates around the sides of jars or vases in which confervae have been kept. It forms on the surface of water a slight pellicle of a delicate green colour, which is supposed to have been erroneously regarded by many botanists as a egetable production, and described under the name of /Byssus flos aquae. On dying it becomes more lengthened nd pellucid, or at least retains only a slight central spot of green. Plate XL VI. fig. 12.

Sp. 4. Enchelis amcena. — This is a new species, of a lively green colour, discovered by Bory de St Vincent. In swimming it appears to elongate itself, and advances with the more slender end foremost. Two individuals are sometimes observed to unite and form one animal, of a perfectly spherical form, and similar in aspect to a Volvox.

Sp. 5. Enchelis tìrcsias. — This species was also discovered by the above-named writer, and led to his peculiar views regarding those apparently animated seeds which he has named Zoocarpes. He asserts that he has seen this animalcule formed in the articulations of a true conferva; that it burst from its vegetable envelope with a gyration or circular movement; that it soon produced a translucent prolongation of its body, which may be called anterior, as it then swam in the direction of that new organ, which, with the body itself, became visibly longer, till the creature finally acquired the exact form of the Encnelis deses of Müller. The chief difference seemed to be that it always moved with the slender end foremost, whereas the species just mentioned swims with its blunt end in advance. It is described by recent French writers as an “animal extraordinaire qui n’est certainement que la grain vivante d’un végétal.” (See Diet. Class. (IHist. Nat. tome vi. p. 156.)

Sp. 6. Enchelis deses. — This species is of an obseure green, much elongated, and moves with the thick end anteriorly. “Celui-ci (the obtuse portion) parait comme tronqué dans certains aspects; et en examinant attentivement cette sorte de troncature, on la reconnait formée par un cercle en forme de disque moins foncé que le reste de l’animal. La pointe postérieure est parfaitement hyaline. Dans la pensée où nous sommes que les Enchelides vertes ne sont que des Zoocarpes, ou propagules animés de quelques genres d’Arthrodiées, nous croyons que le disque obscurément transparent de la partie antérieure n’est que la marque du point sur lequel doit se développer l’article par lequel doit s’allonger en filament confervoide le Zoocarpe, lorsque, arrivé au terme de sa carrière animale, il doit se fixer et prendre racine par le point hyalin de la partie postérieure.” (Loc. cit. p. 157.) We present the above passage to our readers without note or comment, as we do not ourselves understand the zoocarpal nature of an Enchelis.

Genus Vibrio. —Body very small, simple, cylindrical, elongated.

Animalcules have been described as constituents of this genus, which probably do not at all belong to it, being too complicated in their structure. If the V. aceti, for example, commonly called the vinegar eel, is furnished with a mouth, lips, and alimentary canal, it does not even pertain to the class Infusoria, however small its dimensions. But many of the species are undoubtedly of the simplest construction; and although they may present some appearance of an internal cavity or sac, they yet exhibit neither mouth nor other external orifice of any kind.

Sp. 1. Vibrio lineola. — Body linear, extremely minute. Occurs in many vegetable infusions in such numbers as apparently to occupy their entire space. It is so small, that with the best magnifiers little more can be discerned than an obscure tremulous motion. It is supposed to exceed even the Monas termo in tenuity. Plate XLVI. fig. 13.

Sp. 2. Vibrio spirillum. — Filiform, and twisted spirally, which seems to be its natural shape, as it is never observed to unbend, but moves forwards with a vibratory motion at both ends. Found in an infusion of Sonchus arveĭisis. Plate XLVI. fig. 14.

Sp. 3. Vibrio vermiculus. — Presents a milky aspect, with a blunt apex, and moves with a languid vermicular motion. It has been found in marshy water in November, but is seldom seen. It agrees with the animal mentioned by Lecuwenhoeck as occurring in the dung of frogs.

Sp. 4. Vibrio paxillifer. — “Animal culum,” says Mül- [7:3:188]1er, a vel congeries animalculorum mirabilis. Pluries in guttulis aquae marinae vidi corpuscula linearia flavescentia (solitaria paleas, in quadrangula disposita scobes referebant), granulaque seminalia qualiscunque vegetabilis diu credidi; demum nocte inter 6 et 7 Octobrem 1781 as-spectu fili flavescentis, sese in longum producentis et m breve contrahentis, ac ex his paxillis compositi, obstupefactus, novoque phaenomeno gavisus, ejusdem variis evolutionibus incubui.” A salt-water species, abundant in ulva latissima. Observed during the months of September and October. Plate XLVI. fig. 15, 16, 17, 18, 19.

Sp. 5. Vibrio serpens.— Slender and gelatinous, with obtuse windings or flexures, resembling a serpentine line. It is rare, and occurs in river-water.

Section II.— Body Membranaceous. Of scarcely perceptible thickness, whether flat or concave.

The organization of the animalcules of this section is scarcely less simple than that of the preceding; but their form, being in a small degree resistant, is less subject to variation from the pressure or other action of the surrounding fluids, which has been regarded as the proof of a certain progress or advance in the scale of structure. Genus Gonium. —Body very small and simple, flattened, short, angular. Some species of this genus appear to be composed of several corpuscles united together under a common membrane. This appearance probably results from their cellular tissue, or from certain lines which are the rudiments of those spontaneous divisions formerly mentioned, by which their propagation is affected. Their movements are oscillatory.

Sp. 1. Gonium pectorale. — Quadrangular and pellucid, with sixteen globules of a greenish colour set in a quadrangular membrane, “like jewels in the breast-plate of the high-priest, reflecting light on both sides.” Occurs in pure waters. Plate XLVI. fig. 20.

Sp. 2. Gonium pulvinatum. — Quadrangular and opaque. Found in dunghills.

Sp. 3. G onium corrugatum. — Sub-quadrangular, whitish, marked by a longitudinal line. This species is found in various infusions, particularly that of the pear.

Sp. 4 ι . G onium truncatum. — Internal molecules dark green. Anterior extremity forming a straight line, with which the sides produce an obtuse angle, terminated posteriorly by a curved line. This species exhibits a languid motion. It is much larger than the preceding, and occurs, though rarely, in pure water.

Genus Cyclidium. —Body very small and simple, transparent, flattened, orbicular or oval.

The motions of this genus are oscillatory, circulatory, or demi-circulatory, more or less interrupted, and languid or lively, according to the species.

⅛. 1. Cyclidium bulla.— Orbicular and hyaline. General appearance pale and pellucid, with the edges somewhat darker than the rest. It moves slowly in a semicircular direction, and occurs in the infusion of hay.

Sp. 2. Cyclidium hyalinum. — Oval, depressed, perfectly transparent, terminated by a tail-like elongation. This species is very common, and is produced in many infusions, particularly in those of the cerealia. It swims ™ a filiating manner, and as if continually trembling. Plate XLVI. fig. 21. ; fe Sp. 3. Cyclidium Nucleus. — Of a brownish tinge, deeper behind, and shaped exactly like an apple pippinĩ

Genus Paramaecium.— Body very small, simple, transparent, membranous, oblong.

The species of this genus, according to Lamarck, scarcely differ from those of the preceding, except in their more lengthened forms and a slight increase of animal development. They appear to. vary instantaneously according to their position in relation to the eye of the observer; but their real form is tolerably determinate. The mode of increasing the species by transverse and longitudinal divisions, or natural cuttings, is very obvious in this genus. They are nearly related to the following, but are less sinuous and irregular. Their movements are for the most part slow and indefinite. They swim horizontally on one of their flattened surfaces, after the manner of flounders.

Sp. 1. Paramaecium aurelia. — Body compressed, with a kind of plait or fold towards the apex, acute behind. Very common in water where confervae grow. Plate XLV1. fig. 22, 23, 24.

Sp. 2. Paramaeevum chrysalis. — Plicated anteriorly, obtuse behind. Occurs during the autumn in sea-water.

Sp. 3. Paramaecium versutum. — Cylindrical, thickened posteriorly, obtuse at both ends. Found in ditches.

Genus Kolpoda. —Body very small, simple, flattened, oblong, sinuous, irregular, transparent.

This genus is nearly allied to the preceding, and differs from it chiefly in its more varied forms. It is also less subject to the influence of pressure by the medium in which it lives. An Italian naturalist of the name ofLosanahas lately published a monograph on Kolpoda; but his figures are somewhat exaggerated, and not very naturally expressed.

Sp. 1. Kolpoda lamella. — Elongated, membranaceous, curved anteriorly. This species is seldom met with. It has a singular vacillatory mode of movement, and advances on its sharp edge, instead of its flattened side, the more usual position.

Sp. 2. Kolpoda gallinula. — Oblong, the anterior por. tion of the back membranaceous and hyaline. In cor. rupted sea-water.

Sp. 3. Kolpoda crassa. —Yellow, thickish, somewhat opaque, curved a little in the centre, kidney-shaped. This species has a quick vacillatory motion, and becomes apparent in the infusion of hay generally in about 13 hours. When the water is nearly evaporated it assumes an oval form, becomes compressed, and bursts.

Sp. 4. Kolpoda rostrum. —Oblong, hooked anteriorly. The movements of this species are slow and horizontal. It is found, but not frequently, in water where the lemna grows.

Sp. 5. Kolpoda cucullus. — Ovate, ventricoe, the top bent into a kind of beak, and an oblique incision beneath the apex. This species is found in vegetable infusions, and in fetid hay, and usually moves with great velocity. It is pellucid, and appears as if filled with little bright vesicles, which differ in size. Some have supposed them to be lesser animalcules which the Kolpoda has swallowed; but as it has no mouth wherewith to swallow, Müller is more probably right in regarding them in the light of germs. When about to die in consequence of evaporation, it protrudes its contents, whether food or offspring, with great violence. Plate XLVII. fig. 25, 26, 27, 28.

Genus Bursa ria. —Body simple, membranaceous, concave.

This genus occurs in fresh, saline, and stagnant waters-It appears to vary its form beneath the eye of the observer, and, from a rounded flattened shape, assumes a concave or somewhat purse-shaped aspect.

Sp. 1. Bursaria truncatella. — Follicular, with a tru∏∙ cated apex. This species is visible to the naked eye-It moves from left to right, and from right to left; ascends to the surface in a straight line, and sometimes rolls about as it descends.
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Sp. 2. Bursaria butina.— Boat-shaped, labiated anteriorly. This species is pellucid and crystalline, with brilliant globules within it. It is concave on the upper side, and convex below.

Sp. 3. Bursaria hirundella. —With two small wing-like projections, which give it somewhat of the appearance of a bird. It is invisible to the naked eye, but appears under the microscope like a pellucid hollow membrane. Recent micrographical observers doubt if this species really pertains to the present genus. Plate XLVII. fig. 29.

Order II.— Appendiculated Infusoria. Furnished exteriorly with projecting parts.

The animalcules of this second principal order or divsion, though still infinitely small, gelatinous, and transparent, are so far less simply organized than their predecessors, inasmuch as they are furnished with salient parts, such as hairs, horns, or tail-like appendages, or at least with such projecting organs as, for want of more appropriate terms, we are obliged to designate by those names. They multiply both by division and by the bursting forth of internal germs. Though their essential fluids, and the living tissue which contains them, are probably of a more compound nature than those of the naked Infusoria, they have not yet reached that point of organization in which special organs are developed for the performance of particular functions; and it is not till we reach the higher class of Polypi that these are distinctly perceptible.

“Il parait,” observes Lamarck, “par les nombreuses espèces déjà connues et publiées, que les Infusoires de cet ordre sont bien plus nombreux dans la nature que les Infusoires nus. Cela doit être ainsi d’après les principes que je me suis cru fondé à établir.

“En effet, dans les Infusoires nus, l’origine encore trop récente des races qui proviennent de celles, en petit nombre, qui furent générées spontanément, n’a permis à la durée de la vie et aux circonstances qui ont influé sur ces races, qu’une diversité peu considérable. Mais à mesure que la durée de la vie, que sa transmission dans les individus qui se sont succédés en se multipliant, et que les circonstances ont eu plμs de temps pour exercer leurs influences, les races se sont diversifiées de plus en plus, et sont devenues plus nombreuses.

“Cet ordre des choses, qu’il est facile de reconnoitre pour celui même de la nature, nous fait sentir pourquoi les Infusoires sont bien moins diversifiés et moins nombreux que les Polypes. Effectivement, quoique nous ne connaissions pas probablement tous les Infusoires, et que nous connaissions bien moins encore tous les Polypes, ce qui est déjà connu de part et d’autre indique que la diversité des Polypes est considérablement plus grande que des Infusoires. Aussi les Polypes sont plus éloignés de leur origine que les Infusoires.” (Animaux sans Vertèbres, tome i. p. 433.)

The first genus of this order (Trichoda), as constituted by Müller, contains several species which manifest the rudiments of a mouth and the commencement of an alimentary canal: these, according to the negative characters of the class, do not belong to the Infusoria.

Genus Trichoda. —Body very small, transparent, diversiform, without caudal appendage, but garnished with soft hairs either on the whole or on part of its surface.

According to the views of Lamarck, this genus contains not only a great proportion of the genus Tricloda of Müller, but also the whole of the genus Leucophra of the Danish author. It is distinguished from Kerona by the want of the long, stiff, distant, corniform hairs which characterize the latter.

A. Body ciliated over its entire surface. (Leucophra of Müller.)

Sp. 1. Trichoda viridiscens. — Greenish, cylindrical, opaque, thicker posteriorly. Found in sea-water.

Sp. 2. Trichoda dilata.— Body flattened, variable, with sinuated margins. Inhabits sea-water, and swims like a Planaria. It scarcely differs from the genus Kolpoda, except in being ciliated.

Sp. 3. Trichoda scintillans. —Of a green colour, oval, slender, and opaque. Occurs in stagnant water. A doubtful species, closely allied to Volvox. Sp. 4. Trichoda acuta. — Ovate, with a sharpened point. Colour yellowish. Of this species the form is very variable. It occurs in sea-water among ulvae. Plate XLVII. fig. 30, 31, 32.

Sp. 5. Trichoda signata. — Oblong, sub-depressed, with a blackish margin. Common in sea-water. This species is distinguished by a curved line in its centre, shaped like the letter S, one end of which sometimes assumes a spiral form.

Sp. 6. Trichoda mammilla. — Spherical, opaque, with an exsertile papilla. Occurs in the waters of marshes. It is of a dark colour, its short hairs are curved inwards, and it occasionally appears to project and draw in a little white protuberance.

B. Body ciliated, or covered with short hair only over a part of its surface.

(The greater proportion of the genus Trichoda of Müller.)

Sp. 7. Trichoda grandinella. — Spherical, pellucid, haired above. The minute ciliary appendages of this species are not easily discovered, as it seems to possess the power of withdrawing them at pleasure and instantaneously. It occurs both in pure water and that of infusions.

Sp. 8. Trichoda cometa. — Spherical, ciliated anteriorly, with one or more globular appendages behind. Found in pure water in the autumnal season. Plate XLVII. fig. 33, 34.

Sp. 9. Trichoda solaris. — Spherical and crystalline, its edges beset with diverging rays, which exceed in length the diameter of the body. This animalcule contracts and dilates, but is stationary in the same spot. In marine infusions. Plate XLVII. fig. 35.

Sp. 10. Trichoda pubes. — Oval oblong, gibbous, depressed anteriorly. The apex of this species is furnished with hairs, which are seldom visible till it is about to expire, when it protrudes and extends them vehemently, as if in a vain attempt to secure and detain a remaining particle of water.

Sp. 11. Trichodaproteus. — Oval, obtuse behind, with an elongated retractile neck. Apex haired. This species, according to Müller and Lamarck, is found in river water. It appears, however, to agree in general character and appearance with the Proteus described by Mr Baker, which usually occurs in the slimy matter adhering to the sides of vessels in which either animal or vegetable substances have been some time kept. That of which an account is given by Mr Adams was found in the slime produced by water containing small fishes, snails, <⅛c. The body was something similar to that of a snail, but pointed at one end, while from the other proceeded a long, slender, “and finely proportioned neck, of a size suitable to the rest of the animal.” If we credit Mr Baker, this animalcule, though its eyes are not discernible, plainly demonstrates by its actions that it can see; for though multitudes swim about in the same water, and its own progressive motion is [7:3:190]very swift, it never strikes against its neighbours, but directs its course with a dexterity “wholly unaccountable should we suppose it destitute of sight. Its entire shape bears a resemblance to that of a swan. See Plate XLVII. fig. 36 and 37. When alarmed, it draws in its supposed neck, becomes more opaque, and moves about slowly with the large end foremost. See fig. 38. After continuing for some time under this form, it will put forth a kind of wheel machinery, the motions of which are alleged to draw a current of water towards it from a considerable distance. After frequently pushing out and pulling in this shorter head, sometimes with and sometimes without the wheel-work, it will remain motionless, as if wearied or worn out; and then its long head and neck or apex will be again slowly protruded, after which it generally resumes its accustomed agility.

Genus Kerona. —Body very small, diversiform, without tail-like prolongation, and furnished with scattered, stiff, corniform hairs on some parts of its body.

To this genus Lamarck has united the Himantopus of Müller. The species are rare. They seldom occur in infusions, and are most frequent in the purer kinds of fresh and salt waters.

Sp. I. Kerona rostellum. — Orbicular and membranaceous; one side angulated, the other furnished with a series of triple horns. Inhabits sea and river water. This species is alleged by Bory de St Vincent to be entirely destitute of hairs and cirri, and he therefore proposes to remove it to the order Gymnodes, which corresponds with our first order, the naked Infusoria. It is sometimes difficult to identify species in this department, or to ascertain, in a doubtful or contested case, that the same animalcule has been the subject of observation by two or more disputants. Plate XLVII. fig. 39.

Sp. 2. Kerona cypris. — Somewhat pear-shaped, compressed, the front furnished with hairs or vibrating points, inserted beneath the edge, shorter behind, and partly extended straight forward, partly bent downwards. Motion retrograde. Inhabits fresh water.

Sp. 3. Kerona ludia. — Smooth, pellucid, full of small points, the fore part clubbed and a little bent, the hinder part narrow; the base obliquely truncated, and terminating in a tail stretched out transversely. The top of that part which may be called the head, and the centre of the back, are furnished with long hairs. When this animalcule is at rest, its tail is curled; when in motion it is drawn tight and extended upwards. The movements of this species are lively and diverting.

Genus Cercaria. —Body very small, transparent, diversiform, furnished with a distinct but very simple

This genus, as constituted by Müller, contains many species which bear no natural relation to each other; but his characters are precise and definite, and strictly applicable to those species which now form the genus as limited by modern observers. They occur more rarely among annual and vegetable infusions than in running streams and the waters of marshes. Their movements are for the most part circular and very rapid. With the exception of a well-marked tail, their organization is in every other respect extremely simple. If a mouth and the rudiment of a stomach or alimentary canal exist in any of these animals, such characters would remove them not only from the genus Cercaria, but from the class Infusoria, as defined at the commencement of this treatise. A fortiori, the existence of eyes (a fact assumed by some inaccurate observers) in any of the animalcular species, would entirely alter their position and arrangement in the animal kingdom. We cannot do better than report the observations of the venerable Lamarck (himself unfortunately now deprived of sight) on this obscure subject "Ici, comme dans le genre suivant, l’on est expose, d’après la petitesse extreme des individus, à rapporter à la classe des Infusoires des animaux qui, par leur organisation, appar tiennent à d’autres points de l’échelle animale.”

“Une bouche, quoique d’abord inapercue, et conséquemment l’ébauchc d’un sac alimentaire, peuvent exister dans certains de ces animaux, et dès-lors ils appartiennent au premier ordre des Polypes; mais des yeux, comme on en a supposé dans certains Cercaires, cela est impossible.

“Avant de dire que le fait lui-même vaut mieux que la raisonnement, il faut, Imo, constater que les points que l’on a pris pour des yeux, en sont réellement, et qu’ils ont chacun un nerf optique qui se rend à one masse médullaire, centre de rapports pour des sensations; 2do, il faut ensuite établir positivement que des animalcules réellement pourvus d’yeux, sont néanmoins, par leur organisation, de la même classe que les Infusoires.” (Animaux sans Vertèbres, tome i. p. 444.)

This genus forms the nucleus of the new family of Infusoria proposed by Μ. Bory de St Vincent under the name of Cercariées, and which contains in all seven genera, the names and nature of which will be seen by referring to the tabular view. Müller, who was not practically acquainted with the spermatic animalcules, was attracted by the resemblance which some of the Cercariae bore to the figures of those organic molecules in the works of his predecessors. He did not, however, assert their identity; and probably perceived that, although in their general aspect and mode of movement they resembled each other, their peculiar and very different localities, and even the details of their structure, rendered it advisable that they should be assigned to separate genera.^[4. The following are the characters of the genus Zoosperma, as recently established: “Corps non contractile, ovoide, très-conn- primé, avec une queue ssetiforme, aussi longue ou beaucoup plus longue, implantée a la partie postérieure, qui est peu ou point amincie. Ce genre, dont nous possédons un trés-grand nombre d’especes, se compose d’animaux spermatiques." (Dict. Class d’Hist. Nat. tome iii. p. 356) The production and existence of these animals, their nature and uses, are still among the many inscrutabler mysteries of nature. ] Sp. 1. Cercaria inquieta. — Changeable, convex, with a slender tail. This species occurs in salt water, and is remarkable for assuming a variety of different forms. It is sometimes oval, sometimes cylindrical, sometimes shaped like a sphere. Plate XLVII. fig. 40.

Sp. 2. Cercaria gyrinus. —Body of a rounded form, with an acuminated tail. In swimming, this animalcule moves its tail like a tadpole.

Sp. 3. Cercaria lemna. — Changeable, sub-depressed, with an annulated tail. The C. lemna varies the form of its body in a manner almost as singular as that exhibited by the Proteus, already described. The body is triangular, or oblong, or kidney-shaped. Its tail is at times thick, short, annulated; at others it is long, cylindrical, and without rings; and when stretched out it sometimes vibrates with such velocity as to appear double. A small pellucid globule, which Müller regarded as the mouth, is perceptible near the apex; and there are also two excessively minute black points, which, whatever they may really be, are by some called eyes. It advances slowly by a few steps or movements at a time, and frequently shakes and bends its tail, in which position it bears a great resemblance to a [7:3:191]lemna leaf in, miniature. This animalcule exhibits an advanced organization. Plate XLVII. fig. 41, 42, 43.

Genus Fuhcocerca.- Body very small, transparent, rarely ciliated, furnished with a bipartite tail.

This terminal genus, according to the views of Lamarck, conducts us to the limits of the infusorial class, and we become thus more liable to deception in regard to the non-existence of a mouth than in the preceding genera. It is a dismemberment of the Cercaria of Müller, and probably contains many species which will be placed else-where when future and more continuous observation shall have thrown additional light upon their nature and atributes.

Sp. 1. Furcocerca podura.— Cylindrical, acuminated posteriorly. This species is pellucid, and seems to consist of a head, trunk, and tail, the first of which, in the view of some observers, “resembles that of a herring.” It turns round as if upon an axis when it moves, and is usually found in the months of November and December, in places where the lemna abounds. The tail frequently, but not always, appears to be divided into two. One of Müller’s figures of this species is probably erroneous. He represents it as covered with short hairs; whereas, to more recent observers, it appears perfectly smooth. Plate XLVII. fig.44, 45.

Sp. 2. Furcocerca viridis. — Cylindrical, variable, divided and acuminated behind. Occurs in spring in ditches and standing pools. It frequently contracts its anterior and posterior portions, so as to assume a spherical form. It is difficult to determine the genus to which this species belongs. Lamarck is supposed to have erred in placing it where it now stands. In truth, the genus appears to have been rather established provisionally, than upon an assured and natural foundation. The varying forms of the species which it contains render it extremely difficult either to discern or describe them with precision. Plate XLVII. fig, 46, 47, 48.

We have now endeavoured to present a general view, and a systematic exposition, of the principal features of the animalcular world; and if our statements have been less explicit, and our arrangement less complete and methodical, than accords with the reader’s expectation, these defects must in part be attributed to the uncertainty which still prevails regarding a subject of which many of the essential characters scarcely lie within the limits of human intelligence. The observations and experiments of the English microscopical observers of last century, thouugh they might amuse the general student, are too vague and fanciful to be now regarded as parts of the assured history of animalcules. The ultra-analogical reasoning on subjects of natural science with which we have been lately favoured by such men as Oken and Geoffroy St Hilaire, are tame in comparison with the inferences deduced by some of our older observers, who describe with minuteness the head, eyes, mouth, jaws, throat, stomach, intestines, and other parts of animalcules, which the improved glasses of modern times do not reveal to the vision of not less patient inquirers. The recent observations on the motions of the pollen of plants, which have puzzled the modern philosophers, would have opposed but feeble barriers in the way of our predecessors. “To discover,” says Buffon, “whether all the parts of animals, and all the seeds of plants, contained moving organic particles, I made infusions of the flesh of different animals, and of the seeds of more than twenty different species of vegetables; and after remaining some days in close glasses, I had the pleasure of seeing organic moving particles in all of them. In some they appeared sooner, in others later; some preserved their motions for months, and others soon lost it. Some at first produced large moving globules resembling animals, which changed their figure, split, and became gradually smaller; others produced only small globules, whose motions were extremely rapid; and others produced filaments, which grew longer, seemed to vegetate, and then swelled and poured forth torrents of moving globules.” It was from these and similar observations that the theory arose proposed by Baron Munchausen (an ominous name !). The Baron perceiving that these moving globules, after taking a little exercise, began again to vegetate, drew the conclusion that they were first animals and then plants; thus anticipating by more than half a century the supposed discoveries of some modern physiologists. Which of them was first in error it is perhaps of little consequence to inquire; and we allude to the subject here rather in connection with some singular observations by Mr Ellis, recorded in the 59th volume of the Philosophical Transactions, than from its own intrinsic importance. His object was to overturn Munchausen’s hypothesis, by showing that the supposed Zoo-carpes were nothing more than “the seeds of that genus of fungi called mucor or mouldiness, ” and that their motions were caused by the attacks of myriads of animalcules! “Having at the request of Dr Linnaeus made several experiments on the infusion of mushrooms in water, in order to prove the theory of Baron Munchausen, that these seeds are first animals and then plants, it appeared evidently that the seeds were put in motion by very minute animalcules, which proceeded from the putrefaction of the mushroom: for by pecking at these seeds, which are reddish, light, round bodies, they moved them about with great agility in a variety of directions; while the little .animals themselves were scarcely visible till the food they had eaten had discovered them. The satisfaction I received from clearing up this point led me into many other curious and interesting experiments. The ingenious Mr Needham supposes these little transparent ramified filaments, and jointed or coralloid bodies, which the microscope discovers to us on the surface of most animal and vegetable infusions when they become putrid, to be zoophytes, or branched animals; but to me they appear, after a careful scrutiny with the best glasses, to be of that genus of fungi called mucor or mouldiness, many of which Michellius has figured, and Linnaeus has accurately described. Their vegetation is so amazingly quick, that they may be perceived in the microscope even to grow and feed under the eye of the observer. Mr Needham has pointed out to us a species that is very remarkable for its parts of fructification (See Phil. Trans. vol. xlv. tab. 5, fig. 3, a, A). This, he says, proceeds from an infusion of bruised wheat. I have seen the same species proceed from the body of a dead fly, which was become putrid by lying floating for some time in a glass of water where some flowers had been, in the month of August 1768. This species of mucor sends forth a mass of transparent filamentous roots; from whence arise hollow stems, that support little oblong oval seed-vessels, with a hole on the top of each. From these I could plainly see minute globular seeds issue forth in great abundance, with an elastic force, and turn about in the water as if they were animated. Continuing to view them with some attention, I could just discover that the putrid water which surrounded them was full of the minutest animalcula; and that these little creatures began to attack the seeds of the mucor for food, as I have observed before in the experiment on the seeds of the larger kind of fungi or mushrooms. This new motion continued the appearance of their being alive for some time longer; but soon after many of them arose to the surface of the water, remaining there without [7:3:192]motion; and a succession of them afterwards coming up, they united together in little thin masses, and floated to 'the edge of the water, remaining there quite inactive during the time of observation.” In like manner, the movements of the jointed coralloid bodies which Mr Needham has named chaplets and pearl necklaces are attributed to the attacks of their animalcular enemies. “When a small portion of these branches and seeds are put into a drop of the same putrid water upon which the scum floats, many of these millions of little animalcula with which it abounds immediately seize them as food, and turn them about with a variety of motions, as in the experiments on the seeds of the common mushrooms, either singly, or two or three seeds connected together; answering exactly to Mr Needham’s description, but evidently without any motion of their own, and consequently not animated !”

We shall conclude this subject with a short notice of another view of the matter, which has resulted from some recent experiments and observations by our celebrated botanist Robert Brown. While engaged in some inquiries regarding the structure of the pollen of plants, and its mode of action on the pistillum of phaenogamous tribes, that accurate observer had occasion to immerse in water some particles taken from the full-grown anthers (previous to bursting) of Clarkia pulchella. Of these, he perceived by the microscope that many were evidently in motion, and that their motion consisted not only of a change of place in the fluid, but of a change of form in themselves; that is to say, a contraction or curvature about the middle of one side, accompanied by a corresponding enlargement or convexity on the other, frequently occurred. The particles were seen, in a few instances, to turn on their longer axis; and their general motions were of such a nature as to produce the conviction in Mr Brown’s mind that they did not arise either from currents or evaporation of the fluid, but were proper to the particles themselves. Having ascertained that motion existed in the pollen of all the living plants which he examined, he next inquired whether, and for what length of time, this singular property was retained after the death of the plant. Specimens were experimented on, which had been dried and preserved in an herbarium for 100 years, and the moving molecules or small spherical bodies were still perceived in considerable numbers. “The very unexpected fact,” says Mr Brown, “of seeming vitality retained by those minute particles so long after the death of the plant, would not perhaps have materially lessened my confidence in the supposed peculiarity; but I at the same time observed, that on bruising the ovula or seeds of Equisetum, which at first happened accidentally, I so greatly increased the number of moving particles, that the source of the added quantity could not be doubted. I found also, on bruising first the floral •leaves of mosses, and then aU other parts of those plants, that I readily obtained similar particles, not in equal quantity indeed, but equally in motion. My supposed test of the male organ was therefore necessarily abandoned. Reflecting on all the facts with which I had now become acquainted, I was disposed to believe that the minute spherical particles or molecules of apparently uniform size, first seen in the advanced state of the pollen of Onagrariae, and most other phaenogamous plants,—then in the antherae of mosses, and on the surfaces of the bodies regarded as the stamina of Equisetum,—and, lastly, in bruised portions of other parts of the same plants,—were in reality the supposed constituent or elementary molecules of organic bodies, first so considered by Buffon and Needham, then by Wrisberg with greater precision, soon after and still more particularly by Müller, and very recently by Dr Müne Edwards, who has revived the doctrine, and supported it with much interesting detail. I now, therefore, expected to find these molecules in all organic bodies; and accordingly, on examining the var i.∖ ous animal and vegetable tissues, whether living or dead, they were always found to exist; and merely by bruis, ing these substances in water, I never failed to disengage the molecules in sufficient numbers to ascertain their apparent identity in size, form, and motion, with the smaller particles of the grains of the pollen. I e χ. amined also various products of organic bodies, particularly the gum raisins, and substances of vegetable origin, extending my inquiry even to pit-coal; and in all these bodies molecules were found in abundance. I remark here also, partly as a caution to those who may hereafter engage in the same inquiry, that the duct or soot deposited on all bodies in such quantity, especiallyiu London, is entirely composed of these molecules. One of the substances examined was a specimen of fossil wood, found in Wiltshire oolite, in a state to burn with flame; and as I found these molecules abundantly and in motion in this specimen, I supposed that their existence, though in smaller quantity, might be ascertained in mineralized vegetable remains. With this view, a minute portion of silicified wood, which exhibited the structure of coniferas, was bruised, and spherical particles, or molecules in all respects like those so frequently mentioned, were readily obtained from it; in such quantity, however, that the whole substance of the petrifaction seemed to be formed of them. But hence I inferred that these molecules were not limited to organic bodies, nor even to their products. To establish the correctness of the inference, and to ascertain to what extent the molecules existed in mineral bodies, became the next object of inquiry. The first substance examined was a minute fragment of window-glass, from which, when merely bruised on the stage of the microscope, I readily and copiously obtained molecules, agreeing in size, form, and motion, with those which I had already seen. I then proceeded to examine, and with similar results, such minerals as I either had at hand or could readily obtain, including several of the simple earths and metals, with many of their combinations. Rocks of all ages, including those in which organic remains have never been found, yielded the molecules in abundance. Their existence was ascertained in each of the constituent minerals of granite, a fragment of the sphinx being one of the specimens examined. To mention all the mineral substances in which I have found these molecules would be tedious; and I shall confine myself, in this summary, to an enumeration of a few of the most remarkable. These were both of aqueous and igneous origin, as travertine, stalactites, lava, obsidian, pumice, volcanic ashes, andme-teorites from various localities. Of metals I may mention manganese, nickel, plumbago, bismuth, antimony, and arsenic. In a word, in every mineral which I could reduce to powder sufficiently fine to be temporarily suspended in water, I found these molecules more or less copiously; and in some cases, more particularly in siliceous crystals, the whole body submitted to examination appeared to be composed of them.”

There were three points of importance which ΙVIr Brown was anxious to ascertain regarding these molecules, viz. their form, whether they were of uniform size, and their absolute magnitude. He seems, however, not to have been entirely satisfied with his determination on any of these points. As to form, he states the molecules to be spherical. ∙ His manner of estimating the absolute magnitude and uniformity of size of the molecules found in ⅛ e various bodies submitted to examination, γvas l da 9 π ≡ them on a micrometer divided to five thousandths of an inch, the lines of which were very distinct; or, more rarely, [7:3:193]on one divided to ten thousandths, with fainter lines, not readily visible without the application of plumbago, as emplyed by Dr Wollaston, but which in this case was inadmissible. The results can only be regarded as approximate, but Mr Brown is disposed to believe that the molecule is of uniform size, though, as existing in various substances, and examined in circumstances more or less favourable, he regards it necessary to state that its diameter appeared to vary from ⅟ 15000 th to ⅟ 20000 th of an inch.^[5. The following summary from the pen of Mr Brown contains the renewed expression of that gentleman’s opinion, matured by some recent experiments on the subject of active molecules. “That extremely minute particles of solid matter, whether obtained from organic or inorganic substances, when suspended in pure water or in some other aqueous fluids, exhibit motions for which I am unable to account, and which, from their irregularity and seeming independence, resemble in a remarkable degree the less rapid motions of some of the simplest animalcules of infusions. That the smallest moving particles observed, and which I have termed Active Molecules, appear to be spherical, or nearly so, and to be between ⅟ 20000 th and ⅟ 30000 th of an inch in diameter; and that other particles of considerably greater and various size, and either of similar or of very different figure, also present analogous motions in like circumstances. I have formerly stated my belief that these motions of the particles neither arose from currents in the fluid containing them, nor depended on that intestine motion which may be supposed to accompany its evaporation. These causes of motion, however, either singly or combined with others,—as the attractions and repulsions among the particles themselves, their unstable equilibrium in the fluid in which they are suspended, their hygrometrical or capillary action, and in some cases the disengagement of volatile matter, or of minute air-bubbles,—have been considered by several writers as sufficiently accounting for the appearances. Some of the alleged causes here stated, with others which I have considered it unnecessary to mention, are not likely to be overlooked, or to deceive observers of any experience in microscopical researches; and the insufficiency of those enumerated may, I think, be satisfactorily shown by means of a very simple experiment. This experiment consists in reducing the drop of water containing the particles to microscopic minuteness, and prolonging its existence by immersing it in a transparent, fluid of inferior specific gravity, with which it is not miscible, and in which evaporation is extremely slow. If to almond oil, which is a fluid having these properties, a considerably smaller proportion of water, duly impregnated with particles, be added, and the two fluids shaken or triturated together, drops of water of various sizes, from ⅟th to ⅟ 2000 th of an inch in diameter, will be immediately produced. Of these, the most minute necessarily contain but few particles, and some may be occasionally observed with one particle only. In this manner minute drops, which, if exposed to the air, would be dissipated in less than a minute, may be retained for more than an hour. But in all the drops thus formed and protected, the motion of the particles takes place with undiminished activity, while the principal causes assigned for that motion, namely, evaporation and their mutual attraction and repulsion, are either materially reduced or absolutely null.” ] (T.)