Read Ebook: The Cambridge natural history Vol. 01 (of 10) by Hartog Marcus Hickson Sydney J Sydney John MacBride E W Ernest William Sollas Igerna Br Nhilda Johnson Harmer S F Sidney Frederic Editor Shipley A E Arthur Everett Sir Editor

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Catabolic processes may involve the mere breaking of complex substances into simpler ones, or their combination with oxygen; in either case waste products are formed, which may either be of service to the organism as "secretions" , or of no further use . When nitrogenous substances break down in this way they give rise to "excretions," containing urea, urates, and allied substances; other products of catabolism are carbon dioxide, water, and mineral salts, such as sulphates, phosphates, carbonates, oxalates, etc., which if not insoluble must needs be removed promptly from the organism, many of them being injurious or even poisonous. The energy liberated by the protoplasm being derived through the breakdown of another part of the same or of the food-materials or stored reserves, must give rise to waste products. The exchange of oxygen from without for carbonic acid formed within is termed "respiration," and is distinguished from the mere removal of all other waste products called "excretion." In the fresh-water Amoeba both these processes can be studied.

That the due interchanges may take place between the cell and the surrounding medium, it is obvious that certain limits to the ratio between bulk and surface must exist, which are disturbed by growth, and which we shall study hereafter .

The Protista that live in water undergo a death by "diffluence" or "granular disintegration" on being wounded, crushed, or sometimes after an excessive electric stimulation, or contact with alkalies or with acids too weak to coagulate them. In this process the protoplasm breaks up from the surface inwards into a mass of granules, the majority of which themselves finally dissolve. If the injury be a local rupture of the external pellicle or cuticle, a vacuole forms at the point, grows and distends the overlying cytoplasm, which finally ruptures: the walls of the vacuole disintegrate; and this goes on as above described. Ciliate Infusoria are especially liable to this disintegration process, often termed "diffluence," which, repeatedly described by early observers, has recently been studied in detail by Verworn. Here we have death by "solution," while in the "fixing" of protoplasm for microscopic processes we strive to ensure death by "desolution," so as to retain as much of the late living matter as possible. It would seem not improbable that the unusual contact with water determines the formation of a zymase that acts on the living substance itself.

We have suggested that one function of the contractile vacuole, in naked fresh-water Protists, is to afford a regular means of discharge of the water constantly taken up by the crystalloids in the protoplasm, and so to check the tendency to form irregular disruptive vacuoles and death by diffluence. This is supported by the fact that in the holophytic fresh-water Protista, as well as the Algae and Fungi, a contractile vacuole is present in the young naked stage , but disappears as soon as an elastic cell-wall is formed to counterbalance by its tension the internal osmotic pressure.

DIGESTION is always essentially a catabolic process, both as regards the substance digested and the formation of the digesting substance by the protoplasm. The digesting substance is termed a "zymase" or "chemical ferment," and is conjectured to be produced by the partial breakdown of the protoplasm. In presence of suitable zymases, many substances are resolved into two or more new substances, often taking up the elements of water at the same time, and are said to be "dissociated" or "hydrolysed" as the case may be. Thus proteid substances are converted into the very soluble substances, "proteoses" and "peptones," often with the concurrent or ultimate formation of such relatively simple bodies as leucin, tyrosin, and other amines, etc. Starch and glycogen are converted into dextrins and sugars; fats are converted into fatty acids and glycerin. It is these products of digestion, and not the actual food-materials , that are really taken up by the protoplasm, whether for assimilation, for accumulation, or for the direct liberation of energy for the vital processes of the organism.

The two modes of Anabolism--true "assimilation" in the strictest sense and "accumulation"--may sometimes go on concurrently, a certain proportion of the food material going to the protoplasm, and the rest, after allowing for waste, being converted into reserves.

MOVEMENTS all demand catabolic changes, and we now proceed to consider these in more detail.

The movements of an Amoeboid cell are of two kinds: "expansion," leading to the formation and enlargement of outgrowths, and "contraction," leading to their diminution and disappearance within the general surface. Expansion is probably due to the lessening of the surface-tension at the point of outgrowth, contraction to the increase of surface-tension. Verworn regards these as due respectively to the combination of the oxygen in the medium with the protoplasm in diminishing surface-tension, and the effect of combination with substances from within, especially from the nucleus in increasing it. Besides these external movements, there are internal movements revealed by the contained granules, which stream freely in the more fluid interior. Those Protista that, while exhibiting amoeboid movements, have no clear external layer, such as the Radiolaria, Foraminifera, Heliozoa, etc., present this streaming even at the surface, the granules travelling up and down the pseudopodia at a rate much greater than the movements of these organs themselves. In this case the protoplasm is wetted by the medium, which it is not where there is a clear outer layer: for that behaves like a greasy film.

MOTILE ORGANS.--Protoplasm often exhibits movements much more highly specialised than the simple expansion or retraction of processes, or the general change of form seen in Amoeba. If we imagine the activities of a cell concentrated on particular parts, we may well suppose that they would be at once more precise and more energetic than we see them in Amoeba or the leucocyte. In some free-swimming cells, such as the individual cells known as "Flagellata," the reproductive cells of the lower Plants, or the male cells of Plants as high as Ferns, and even of the Highest Animals, there is an extension of the cell into one or more elongated lash-like processes, termed "flagella," which, by beating the water in a reciprocating or a spiral rhythm, cause the cell to travel through it; or, if the cell be attached, they produce currents in the water that bring food particles to the surface of the cell for ingestion. Such flagella may, indeed, be seen in some cases to be modified pseudopodia. In other cases part, or the whole, of the surface of the cell may be covered with regularly arranged short filaments of similar activity , which, however, instead of whirling round, bend sharply down to the surface and slowly recover; the movement affects the cilia successively in a definite direction in waves, and produces, like that of flagella, either locomotion of the cell or currents in the medium. We can best realise their action by recalling the waves of bending and recovery of the cornstalks in a wind-swept field; if now the haulms of the corn executed these movements of themselves, they would determine in the air above a breeze-like motion in the direction of the waves . Such cilia are not infrequent on those cells of even the Highest Animals that, like a mosaic, cover free surfaces . In ourselves such cells line, for instance, the windpipe. One group of the Protozoa, the "Ciliata," are, as their name implies, ciliated cells pure and simple.

The motions of cilia and of flagella are probably also due to changes of surface tension--alternately on one side and the other in the cilium, but passing round in circular succession in the flagellum, giving rise to a conical rotation like that of a weighted string that is whirled round the head. This motion is, however, strongest at the thicker basal part, which assumes a spiral form like a corkscrew of few turns, while the thin lash at the tip may seem even to be quietly extended like the point of the corkscrew. If the tip of the flagellum adhere, as it sometimes does, to any object, the motions induce a jerking motion, which in this case is reciprocating, not rotatory. When the organism is free, the flagellum is usually in advance, and the cell follows, rotating at the same time round its longitudinal axis; such an anterior flagellum, called a "tractellum," is the common form in Protista that possess a single one . In the spermatozoa of Higher Animals the flagellum is posterior, and is called a "pulsellum."

The cilium or flagellum may often be traced a certain distance into the substance of the cytoplasm to end in a dot of denser, readily-staining plasm, which corresponds to a "centrosome" or centre of plasmic forces ; it has been termed a "blepharoplast."

The MOTILE REACTIONS of the Protozoa require study from another point of view: they are either "spontaneous" or "arbitrary," as we may say, or responsive to some stimulus. The latter kind we will take first, as they are characteristic of all free cells. The stimuli that induce movements of a responsive character are as follows:-- MECHANICAL: such as agitation and contact; force of GRAVITY, or CENTRIFUGAL FORCE; CURRENTS in the water; RADIANT ENERGY ; changes in the TEMPERATURE of the medium; ELECTRIC CURRENTS through the medium; the presence of CHEMICAL SUBSTANCES in the medium.

These, or some of them, may induce one of three different results, or a combination thereof: a single movement or an arrest of motion; the assumption of a definite position; movement of a definite character or direction.

RHEOTAXY.--This is the tendency to move against the stream in flowing water. It is shown by most Protists, and can be conveniently studied in the large amoeboid plasmodia of the Myxomycetes, which crawl against the stream along wet strips of filter paper, down which water is caused to flow. Most animals, even of the highest groups, tend to react in the same way; the energetic swimming of Fishes up-stream being in marked contrast with their sluggishness the other way; and every student of pond-life knows how small Crustacea and Rotifers, no less than Ciliates, swim away from the inrush of liquid into the dipping-tube, and so evade capture.

Most of the colourless Protista are negatively phototactic or photopathic; but those which are parasitic on the coloured ones are positively phototactic, like their hosts.

Here, as in the case of other stimuli, the absolute intensity of the light is of importance; for as it increases from a low degree, different organisms in turn cease to be stimulated, and then are repelled instead of being attracted. The most active part of the spectrum in determining reactions of movement are the violet and blue rays of wave-length between 40 u/10 and 49 u/10, while the warmer and less refractive half of the spectrum is inert save in so far as it determines changes in the temperature of the medium.

CELL-DIVISION

In ordinary cases of fission of an isolated cell the cell elongates, and as it does so, like other viscid bodies, contracts in the middle, which becomes drawn out into a thread, and finally gives way. In some cases the nucleus previously undergoes a similar division by simple constriction, which is called direct or "amitotic" division. But usually the division of the nucleus prior to cell-division is a more complex process, and involves the co-operation of the cytoplasm; and we must now study in detail the nucleus and its structure in "rest" and in fission.

When cell-division is about to take place the linin, or at least the greater part of it, assumes the character of a number of distinct threads, and the whole of the chromatin granules are distributed at even distances along these , so as to appear like so many strings of beads. Each such thread is called a "chromosome." Then each bead divides longitudinally into two. The thread flattens into a ribbon, edged by the two lines of chromatin beads. Finally, the ribbon splits longitudinally into two single threads of beads . During these changes the nucleole or nucleoles diminish, or even disappear, as if they had contributed their matter to the growth of the chromatin proper. In Higher Animals and Plants the nuclear wall next disappears, and certain structures become obvious, especially in the cytoplasm of Metazoa. Two minute spheres of plasm , the "centrosomes," which hitherto lay close together at the side of the nuclear wall, now separate; but they remain connected by a spindle of clear plasmic threads which, as the centres diverge, comes to lie across the spot the nucleus occupied, and now the chromosomes lie about the equator of this spindle . Moreover, the surrounding cytoplasm shows a radiating structure, diverging from the centrosome, so that spindle and external radiations together make up a "strain-figure," like that of the "lines of force" in relation to the poles of a magnet. Such we can demonstrate in a plane by spreading or shaking iron filings on a piece of paper above the poles of a magnet, or in space by suspending finely divided iron in a thick liquid, such as mucilage or glycerin, and bringing the vessel with the mixture into a strong magnetic field; the latter mode has the advantage of enabling us to watch the changes in the distribution of the lines under changing conditions or continued strain.

The chromosomes are now completely split, each into its two daughter-segments, which glide apart , and pass each to its own pole of the spindle, stopping just short of the centrosome . Thus, on the inner side of either centrosome is found an aggregation of daughter-segments, each of which is sister to one at the opposite pole, while the number at either pole is identical with that of the segments into which the old nucleus had resolved itself at the outset. The daughter-segments shorten and thicken greatly as they diverge to the poles, and on their arrival crowd close together.

A distinct wall now forms around the aggregated daughter-chromosomes , so as to combine them into a nucleus for the daughter-cell. The reorganisation of the young nucleus certainly varies in different cases, and has been ill-studied, probably because of the rapidity of the changes that take place. The cytoplasm now divides, either tapering into a "waist" which finally ruptures, or constricting by the deepening of a narrow annular groove so as to complete the formation and isolation of the daughter-cells.

We might well compare the cell-division to the halving of a pumpkin or melon, of which the flesh as a whole is simply divided into two by a transverse cut, while the seeds and the cords that suspend them are each singly split to be divided evenly between the two halves of the fruit; the flesh would represent the cytoplasm, the cords the linin threads of the nucleus, and the seeds the chromatin granules. In this way the halving of the nucleus is much more complete and intimate than that of the cytoplasm; and this is the reason why many biologists have been led to regard the nuclear segments, and especially their chromatic granules, as the seat of the hereditary properties of the cell, properties which have to be equally transmitted on its fission to each daughter-cell. But we must remember that the linin is also in great part used up in the formation of these segments, like the cords of our supposed melon; and it is open to us to regard the halving in this intimate way of the "linin" as the essence of the process, and that of the chromatin as accessory, or even as only part of the necessary machinery of the process. The halving or direct splitting lengthwise of a viscid thread is a most difficult problem from a physical point of view; and it may well be that the chromatin granules have at least for a part of their function the facilitation of this process. If such be the case, we can easily understand the increase in number, and size and staining power of these granules as cell-division approaches, and their atrophy or partial disappearance during their long intervening periods of active cell life. Hence we hesitate to accept the views so commonly maintained that the chromatin represents a "germ-plasm" or "idioplasm" of relatively great persistence, which gives the cell its own racial qualities.

In many Sarcodina and some Sporozoa the nucleus gives off small fragments into the cytoplasm, or is resolved into them; they have been termed "chromidia" by E. Hertwig. New nuclei may be formed by their growth and coalescence, the original nucleus sometimes disappearing more or less completely.

The rhythm of cell-life that we have just studied is called the "Spencerian" rhythm. Each cell in turn grows from half the bulk of its parent at the time it was formed to the full size of that parent, when it divides in its own turn. Rest is rare, and assumed only when the cell is exposed to such unfavourable external conditions as starvation, drought, etc.; it has no necessary relation to fission.

COLONIAL UNION.--In certain cases, the brood-cells instead of separating remain together to form a "colony"; and this may enlarge itself again by binary division of its individual cells at their limit of growth. Here, certain or all of the cells may undergo brood-formation: such cells are often termed "reproductive cells" in contrast with the "colonial cells."

A temporary apocytial condition is often passed through in the formation of the brood of cells by repeated divisions without any interval for enlargement; for the nuclear divisions may go on more rapidly than those of the cytoplasm, or be completed before any cell-division takes place , the nuclear process being "accelerated" or the cytoplastic being "retarded," whichever we prefer to say and to hold. Thus as many as thirty-two nuclei may have been formed by repeated binary subdivisions before any division of the cytoplasm takes place to resolve the apocyte into true 1-nucleate cells.

In many cases of brood-formation the greater part of the food-supply of the brood-mother-cell has been stored as reserve-products, which accumulate in quantity in the cell; this is notably seen in the ovum or egg of the Higher Animals. How great such an accumulation may be is indeed well seen in the enormous yolk of a bird's egg, gorged as it were to repletion. When a cell has entered on such course of "miserly" conduct, it may lose all power of drawing on its own supplies, and finally that of accumulating more, and passes into the state of "rest." To resume activity there is needed either a change in the internal conditions--demanding the lapse of time--or in the external conditions, or in both. We may call this resumption "germination."

Very often in the study of a large and complex organism we are able to find processes in action on a large scale which, depending as they must do on the protoplasmic activities of its individual cells, reveal the nature of similar processes in simple unicellular beings: such a clue to the utilisation of reserves by a cell which has gorged itself with them so as to pass into a state of rest is to be found in that common multicellular organism, the Potato. This stores up reserves in its underground stems ; if we plant these immediately on the completion of their growth, they will not start at once, even under what would outwardly seem to be most appropriate conditions. A certain lapse of time is an essential factor for sprouting. It would appear that in the Potato the starch can only be digested by a definite ferment, which does not exist when it is dug, but which is only formed very slowly, and not at all until a certain time has supervened; and that sprouting can only take place when soluble material has been provided in this way for the growth of the young shoots. We have also reason to believe that these ferments are only formed by the degradation of the protoplasm itself. Now obviously this degradation must be very slow in a resting organism; and any external stimulus that will tend to protoplasmic activity will thereby tend to form at the same time the digestive ferments and dissolve the stored supplies, to render them available for the life-growth and reproduction of the being. We now see why inactive "miserly" cells so often pass into a resting state before dividing, and why they go on dividing again and again when once they re-enter upon an active life, the living protoplasm growing at the expense of the reserves. Resting cells of this type occur of course only at relatively rare intervals in the animal-feeding Protozoa, that have to take into their substance the food they require for their growth and life-work, and cannot therefore store up much reserves. For they are constantly producing in the narrow compass of their body those very ferments that would dissolve the reserves when formed. Internal parasites and "saprophytes," that is, beings which live on dead and decayed organic matter, on the other hand, live surrounded by a supply of dissolved food; and rarely do we find larger cells, richer in reserves, than in the parasitic Sporozoa, which owe their name to the importance of brood-formation in their life-history. In Radiolaria a central capsule separates off an inner layer of protoplasm; the outer layer is the one in which digestion is performed, while the inner layer stores up reserves; and here brood-formation appears to be the rule. But the largest cells of all are the eggs of the Metazoa, which in reality lead a parasitic life, being nurtured by the animal as a whole, and contributing nothing to the welfare of it as an individual. Their activity is reduced to a minimum, and the consequent need for a high ratio of surface to volume is also reduced, which accounts for their inordinate size. But directly the reserve materials are rendered available by the formation of a digestive ferment, then protoplasmic growth takes place, and the need for an extended surface is felt; cell-division follows cell-division with scarcely an interval in the process of segmentation.

SYNGAMY.--The essence of typical syngamy is, that two cells of the same species approach one another, and fuse, cytoplasm with cytoplasm, and nucleus with nucleus, to form a new cell . This process is called also "conjugation" or "cytogamy." In the simplest cases the two cells are equal and attract one another equally , and have frequently the character of zoospores.

In an intermediate type, the one cell is larger and more sluggish , "megagamete," "oogamete," "oosphere," "egg"; the other smaller, more active , "microgamete," "spermogamete," "spermatozoon," "sperm"; and in the most specialised cases which prevail among the Higher Animals and Plants, the larger cell is motionless, and the smaller is active, ciliate, flagellate, or amoeboid: the coupled-cell or zygote is here termed the "oosperm." It encysts immediately in most Protista except Infusoria, Acystosporidae, Haemosporidae, and Trypanosomatidae.

As a rule, one at least of the pair-cells is fresh from division, and it would thus appear that the union of the nuclei is facilitated when one at least of them is a "young" one. Of the final mechanism of the union of the nuclei, we know nothing: they may unite in any of the earlier phases of mitosis, or even in the "resting state." A fibrillation of the cytoplasm during the process, radiating around a centrosome or two centrosomes indicates a strained condition.

REGENERATION.--Finally, experiments have been made by several observers as to the effects of removing parts of Protozoa, to see how far regeneration can take place. The chief results are as follows:--

"ANIMALS" AND "PLANTS"

Hitherto we have discussed the cell as if it were everywhere an organism that takes in food into its substance, the food being invariably "organic" material, formed by or for other cells; such nutrition is termed "holozoic." There are, however, limits to the possibilities in this direction, as there are to the fabled capacities of the Scillonians of gaining their precarious livelihood by taking in one another's washing. For part of the food material taken in in this way is applied to the supply of the energies of the cell, and is consequently split up or oxidised into simpler, more stable bodies, no longer fitted for food; and of the matter remaining to be utilised for building up the organism, a certain proportion is always wasted in by-products. Clearly, then, the supply of food under such conditions is continually lessening in the universe, and we have to seek for a manufactory of food-material from inorganic materials: this is to be found in those cells that are known as "vegetal," in the widest sense of the word. In this, sense, vegetal nutrition is the utilisation of nitrogenous substances that are more simple than proteids or peptones, together with suitable organic carbon compounds, etc., to build up proteids and protoplasm. The simplest of organisms with a vegetal nutrition are the Schizomycetes, often spoken of loosely as "bacteria" or "microbes," in which the differentiation of cytoplasm and nucleus is not clearly recognisable. Some of these can build up their proteids from the free uncombined nitrogen of the atmosphere, carbon dioxide, and inorganic salts, such as sulphates and phosphates. But the majority of vegetal feeders require the nitrogen to be combined at least in the form of a nitrate or an ammonium salt--nay, for growth in the dark, they require the carbon also to be present in some organic combination, such as a tartrate, a carbohydrate, etc. Acetates and oxalates, "aromatic" compounds and nitriles are rarely capable of being utilised, and indeed are often prejudicial to life. In many vegetal feeders certain portions of the protoplasm are specialised, and have the power of forming a green, yellow, or brown pigment; these are called "plastids" or "chromatophores." They multiply by constriction within the cell, displaying thereby a certain independent individuality. These plastids have in presence of light the extraordinary power of deoxidising carbon dioxide and water to form carbohydrates and free oxygen; and from these carbohydrates or fats, together with ammonium salts or nitrates, etc., the vegetal protoplasm at large can build up all necessary food matter. So that in presence of light of the right quality and adequate intensity, such coloured vegetal beings have the capacity for building up their bodies and reserves from purely inorganic materials. Coloured vegetal nutrition, then, is a process involving the absorption of energy; the source from which this is derived in the bacteria being very obscure at present. Nutrition by means of coloured plastids is distinguished as "holophytic," and that from lower substances, which, however, contain organically combined carbon, as "saprophytic," for such are formed by the death and decomposition of living beings. The third mode of nutrition from wholly inorganic substances, including free nitrogen, has received no technical name. All three modes are included in the term "autotrophic" .

Vegetal feeders have a great tendency to accumulate reserves in insoluble forms, such as starch, paramylum, and oil-globules on the one hand, and pyrenoids, proteid crystals, aleurone granules on the other.

When an animal-feeding cell encysts or surrounds itself with a continuous membrane, this is always of nitrogenous composition, usually containing the glucosamide "chitin." The vegetal cell-wall, on the contrary, usually consists, at least primarily, of the carbohydrate "cellulose"--the vegetal cell being richly supplied with carbohydrate reserves, and drawing on them to supply the material for its garment. This substance is what we are all familiar with in cotton or tissue-paper.

Again, not only is the vegetal cell very ready to surround itself with a cell-wall, but its food-material, or rather, speaking accurately, the inorganic materials from which that food is to be manufactured, may diffuse through this wall with scarcely any difficulty. Such a cell can and does grow when encysted: it grows even more readily in this state, since none of its energies are absorbed by the necessities of locomotion, etc. Growth leads, of course, to division: there is often an economy of wall-material by the formation of a mere party-wall dividing the cavity of the old cell-wall at its limit of growth into two new cavities of equal size. Thus the division tends to form a colonial aggregate, which continues to grow in a motionless, and, so far, a "resting" state. We may call this "vegetative rest," to distinguish it from "absolute rest," when all other life-processes are reduced to a minimum or absolutely suspended.

Again, Plants take in either food or else the material for food in solution through their surface, and only by diffusion through the cell-wall. Insectivorous Plants that have the power of capturing and digesting insects have no real internal cavity. Animal-feeding Protista take in their food into the interior of their protoplasm and digest it therein, and the Metazoa have an internal cavity or stomach for the same purpose. Here again there are exceptions in the case of certain internal parasites, such as the Tapeworms and Acanthocephala , which have no stomachs, living as they do in the dissolved food-supplies of their hosts, but still possessing the general tissues and organs of Metazoa.

In Plants we find no distinct nervous system formed of cells and differentiated from other tissues with centres and branches and sense-organs. These are more or less obvious in all Metazoa, traces being even found in the Sponges.

We may then define Plants as beings which have the power of manufacturing true food-stuffs from lower chemical substances than proteids, often with the absorption of energy. They have the power of surrounding themselves with a cell-wall, usually of cellulose, and of growing and dividing freely in this state, in which animal-like changes of form and locomotion are impossible; their colonies are almost always fixed or floating; free locomotion is only possible in the case of their naked reproductive cells, and is transitory even in these. The movements of motile parts of complex plant-organisms are due to the changes in the osmotic powers of cells as a whole, and not to the contraction of differentiated fibrils in the cytoplasm of individual cells. Plants that can form carbohydrates with liberation of free oxygen have always definite plastids coloured with a lipochrome pigment, or else the whole plasma is so coloured. Solid food is never taken into the free plant-cell nor into an internal cavity in complex Plants. If, as in insectivorous Plants, it is digested and absorbed, it is always in contact with the morphological external surface. In the complex Plants apocytes and syncytes are rare--the cells being each invested with its own wall, and, at most, only communicating by minute threads with its neighbours. No trace of a central nervous system with differentiated connexions can be made out.

Animals all require proteid food; their cyst-walls are never formed of cellulose; their cells usually divide in the naked condition only, or if encysted, no secondary party-walls are formed between the daughter-cells to unite them into a vegetative colony. Their colonies are usually locomotive, or, if fixed, their parts largely retain their powers of relative motion, and are often provided on their free surfaces with cilia or flagella; and many cells have differentiated in their cytoplasm contractile muscular fibrils. Their food is always taken into a distinct digestive cavity. A complex nervous system, of many special cells, with branched prolongations interlacing or anastomosing, and uniting superficial sense-organs with internal centres, is universally developed in Metazoa. All Metazoa fulfil the above conditions.

But when we turn to the Protozoa we find that many of the characters evade us. There are some Dinoflagellates which have coloured plastids, but which differ in no other respect from others that lack them: the former may have mouths which are functionless, the latter have functional mouths. Some colourless Flagellates are saprophytic and absorb nutritive liquids, such as decomposing infusions of organic matter, possibly free from all proteid constituents; while others, scarcely different, take in food after the fashion of Amoeba. Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. Indeed, a strong natural line of demarcation lies between Metazoa and Protista. Of the Protozoa, certain groups, like the Foraminifera and Radiolaria and the Ciliate and Suctorial Infusoria are distinctly animal in their chemical activities or metabolism, their mode of nutrition, and their locomotive powers. When we turn to the Proteomyxa, Mycetozoa, and the Flagellates we find that the distinction between these and the lower Fungi is by no means easy, the former passing, indeed, into true Fungi by the Chytridieae, which it is impossible to separate sharply from those Flagellates and Proteomyxa which Cienkowsky and Zopf have so accurately studied under the name of "Monadineae." Again, many of the coloured Flagellates can only be distinguished from Plants by the relatively greater prominence and duration of the mobile state, though classifiers are generally agreed in allotting to Plants those coloured Flagellates which in the resting state assume the form of multicellular or apocytial filaments or plates.

On these grounds we should agree with Haeckel in distinguishing the living world into the Metazoa, or Higher Animals, which are sharply marked off; the Metaphyta, or Higher Plants, which it is not so easy to characterise, but which unite at least two or more vegetal characters; and the Protista, or organisms, whose differentiation is limited to that within the cell , and does not involve the cells as units of tissues. These Protista, again, it is impossible to separate into animal and vegetal so sharply as to treat adequately of either group without including some of the other: thus it is that every text-book on Zoology, like the present work, treats of certain Protophyta. The most unmistakably animal group of the Protista, the Ciliata, is, as we have seen, by the complex differentiation of its protoplasm, widely removed from the Metazoa with their relatively simple cells but differentiated cell-groups and tissues. The line of probable origin of the Metazoa is to be sought, for Sponges at least, among the Choanoflagellates .

PROTOZOA : SPONTANEOUS GENERATION--CHARACTERS OF PROTOZOA--CLASSIFICATION

THE QUESTION OF SPONTANEOUS GENERATION

From the first discovery of the Protozoa, their life-history has been the subject of the highest interest: yet it is only within our own times that we can say that the questions of their origin and development have been thoroughly worked out. If animal or vegetable matter of any kind be macerated in water, filtered, or even distilled, various forms of Protista make their appearance; and frequently, as putrefaction advances, form after form makes its appearance, becomes abundant, and then disappears to be replaced by other species. The questions suggested by such phenomena are these: Do the Protista arise spontaneously, that is, by the direct organisation into living beings of the chemical substances present, as a crystal is organised from a solution: Are the forms of the Protista constant from one generation to another, as are ordinary birds, beasts, and fishes?

The question of the "spontaneous generation" of the Protista was readily answered in the affirmative by men who believed that Lice bred directly from the filth of human skins and clothes; and that Blow-flies, to say nothing of Honey-bees, arose in rotten flesh: but the bold aphorism of Harvey "omne vivum ex ovo" at once gained the ear of the best-inspired men of science, and set them to work in search of the "eggs" that gave rise to the organisms of putrefaction. Redi showed that Blow-flies never arise save when other Blow-flies gain access to meat and deposit their very visible eggs thereon. Leeuwenhoek, his contemporary, in the latter half of the seventeenth century adduced strong reasons for ascribing the origin of the organisms of putrefaction to invisible air-borne eggs. L. Joblot and H. Baker in the succeeding half-century investigated the matter, and showed that putrefaction was no necessary antecedent of the appearance of these beings: that, as well as being air-borne, the germs might sometimes have adhered to the materials used for making the infusion; and that no organisms were found if the infusions were boiled long enough, and corked when still boiling. These views were strenuously opposed by Needham in England, by Wrisberg in Germany, and by Buffon, the great French naturalist and philosopher, whose genius, unballasted by an adequate knowledge of facts, often played him sad tricks. Spallanzani made a detailed study of what we should now term the "bionomical" or "oecological" conditions of Protistic life and reproduction in a manner worthy of modern scientific research, and not attained by some who took the opposite side within living recollection. He showed that infusions kept sufficiently long at the boiling-point in hermetically sealed vessels developed no Protistic life. As he had shown that active Protists are killed at much lower temperatures, he inferred that the germs must have much higher powers of resistance; and, by modifying the details of his experiments, he was able to meet various objections of Needham.

Despite this good work, the advocates of spontaneous generation were never really silenced; and the widespread belief in the inconstancy of species in Protista added a certain amount of credibility to their cause. In 1845 Pineau put forward these views most strongly; and from 1858 to 1864 they were supported by the elder Pouchet. Louis Pasteur, who began life as a chemist, was led from a study of alcoholic fermentation to that of the organisms of fermentation and of putrefaction and disease. He showed that in infusions boiled sufficiently long and sealed while boiling, or kept at the boiling-point in a sealed vessel, no life manifested itself: objections raised on the score of the lack of access of fresh air were met by the arrangement, so commonly used in "pure cultures" at the present day, of a flask with a tube attached plugged with a little cotton-wool, or even merely bent repeatedly into a zigzag. The former attachment filtered off all germs or floating solid particles from the air, the latter brought about the settling of such particles in the elbows or on the sides of the tube: in neither case did living organisms appear, even after the lapse of months. Other observers succeeded in showing that the forms and characters of species were as constant as in Higher Animals and Plants, allowing, of course, for such regular metamorphoses as occur in Insects, or alternations of generations paralleled in Tapeworms and Polypes. The regular sequences of such alternations and metamorphoses constitute, indeed, a strong support of the "germ-theory"--the view that all Protista arise from pre-existing germs. It is to the Rev. W. H. Dallinger and the late Dr. Charles Drysdale that we owe the first complete records of such complex life-histories in the Protozoa as are presented by the minute Flagellates which infest putrefying liquids . The still lower Schizomycetes, the "microbes" of common speech, have also been proved by the labours of Ferdinand Cohn, von Koch, and their numerous disciples, to have the same specific constancy in consecutive generations, as we now know, thanks to the methods first devised by De Bary for the study of Fungi, and improved and elaborated by von Koch and his school of bacteriologists.

PROTOZOA

This definition, as we have seen, excludes Metazoa sharply from Protozoa, but leaves an uncertain boundary on the botanical side; and, as systematists share with nations the desire to extend their sphere of influence, we shall here follow the lead of other zoologists and include many beings that every botanist would claim for his own realm. Our present knowledge of the Protozoa has indeed been largely extended by botanists, while the study of protoplasmic physiology has only passed from their fostering care into the domain of General Biology within the last decade. The study of the Protozoa is little more than two centuries old, dating from the school of microscopists of whom the Dutchman Leeuwenhoek is the chief representative: and we English may feel a just pride in the fact that his most important publications are to be found in the early records of our own Royal Society.

The PROTOPLASM of the Protozoa varies greatly in consistency and in differentiation. Its outer layer may be granular and scarcely altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria, Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the Acrasieae; it is continuous with a firm but delicate superficial pellicle of membranous character in most Flagellates and Infusoria; and this pellicle may again be consolidated and locally thickened in some members of both groups so as to form a coat of mail, even with definite spines and hardened polygonal plates . Again, it may form transitory or more or less permanent pseudopodia, blunt or tapering and distinct, with a hyaline outer layer, or granular and pointed, radiating and more or less permanent, or branching and fusing where they meet into networks or perforated membranes. Cilia or flagella are motile thread-like processes of active protoplasm which perforate the pellicle; they may be combined into flattened platelets or firm rods, or transformed into coarse bristles or fine motionless sense-hairs. The skeletons of the Protozoa, foreign to the cytoplasm, will be treated of under the several groups.

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