1911 Encyclopædia Britannica/Protozoa

PROTOZOA (Gr. πρῶτος, first, and ζῷον, living thing), the name given by modern zoologists to the animalcules, for the most part microscopic, which were termed by the older naturalists Infusoria, from the manner in which they appear in infusions containing decaying animal and vegetable matter. The name Infusoria is now, however, restricted to one of the four classes which comprise the Protozoa proper. The name Protozoa was coined as far back as 1820 as an equivalent for the German word Urthiere, meaning animals of primitive or archaic nature, the forms of animal life which may be supposed to have been the first that appeared upon our globe. The great naturalist C. T. von Siebold was, however, the first to give a scientific definition to the group. Von Siebold pointed out that in the Protozoa the individual was always a single vital unit or cell, in contrast with the higher division of the animal kingdom, the Metazoa, in which the body is generally, though not universally, regarded as composed of many such units. To put the matter briefly and somewhat technically: the Protozoa are unicellular animals, the Metazoa multicellular animals; in the Protozoa the cell is complete in itself, both morphologically and physiologically, and is capable of maintaining a separate and independent existence in suitable surroundings, like any other organism; in the Metazoa the cells are differentiated for the performance of distinct functions and combined together to form the various tissues of which the body is built up, and the individual cells of the Metazoan body are not capable of maintaining a separate existence apart from their fellows. This is the sense in which the term Protozoa is used by zoologists, whereby certain forms of animal life, which were formerly ranked as Protozoa, such as sponges and rotifers, are now definitely excluded from the group and classed as Metazoa.

The animal kingdom may be divided, therefore, into two sub-kingdoms, the Protozoa and the Metazoa, the first-named characterized by their essentially unicellular nature. This is a criterion by which it is easy to define the Protozoa from a purely zoological standpoint, but which becomes less satisfactory when we take into consideration the whole range of microscopic unicellular organisms. Besides the true Protozoa, which, ex hypothesi, are organisms of animal nature, there are many other organisms of equally simple organization, including the Bacteria and the unicellular plants. The Bacteria stand sharply apart from the other forms of life, not only, in many cases, by their divergent methods of metabolism, but by morphological characteristics, such as the definite body-form limited by a distinct envelope, the absence of organs for locomotion other than the peculiar flagella, and, above all, by the lack of any differentiation of the body-protoplasm into nucleus and cytoplasm, as in all true cells of either animal or vegetable nature. On the other hand, to separate by hard-and-fast definitions the unicellular plants from the unicellular animals is not only difficult but practically impossible. The essential difference between plant and animal is a physiological one, a difference in the method of nutrition. A typical green plant is able to live independently of other organisms and to build up its substance from simple gases in the air and inorganic salts in the soil or water, provided that certain conditions of light and moisture be present in its environment; this is the so-called holophytic method of nutrition. A typical animal, on the other hand, while practically independent of sunlight, is not able to exist apart from other living organisms, since it is not able to build up its substance from simple chemical constituents like a plant, but must be supplied with ready-made proteids in its food, for which it requires other organisms, either plants or animals; this is the so-called holozoic method of nutrition. Intermediate between these two habits of life is the so-called saprophytic habit, exemplified by the fungi amongst plants; in this method of nutrition the organism cannot build up its substance entirely from inorganic substances, but absorbs the organic substances present in solutions containing organic salts or decaying animal or vegetable matter.

If we regard the organisms termed collectively Protozoa from the point of view of their methods of nutrition (considering for the present only free-living, non-parasitic forms), we find in one class, the Flagellata, examples of the three methods mentioned above, the holozoic, holophytic and saprophytic habit of life, not only in species closely allied to each other, but even combined in one and the same species at different periods of its life or in different surroundings. An individual of a given species may contain chlorophyll, with which it decomposes carbonic acid gas in the sunlight, like a plant, while possessing a definite mouth-aperture, by means of which it can ingest solid food, like an animal. Such instances show clearly that in the simplest forms of life the difference between plant and animal is but a difference of habit and of mode of nutrition, to which the organism is not at first irrevocably committed, and which are not at first accompanied by distinctive morphological characteristics. Only when the organism becomes specialized for one or the other mode of life exclusively does it acquire such definite morphological characters that the difference between plant and animal can be used for the purpose of a natural classification, as in the higher forms of life. In the lowest forms it is not possible to base natural subdivisions on their vegetable or animal nature. For this reason it has been proposed by E. Haeckel to unite all the primitive forms of life in which the body is morphologically equivalent to a single cell into one group, the Protista, irrespective of their animal or vegetable nature. In this method of dealing with the problem the Protista are regarded as a distinct kingdom (Reich), more or less intermediate between, but distinct from, the animal and vegetable kingdoms, and representing the ancestral stock from which both animals and plants have sprung. Many authorities have followed Haeckel’s lead in the matter, and the science of Protistology or Protistenkunde has already a special journal devoted to the publication of researches upon it. But though it may be more scientific, from a theoretical point of view, to group all these primitive organisms together in the way suggested by Haeckel, in practice it is inconvenient, on account of the vast number of forms of life to be comprised as Protista, their diversity in habit of life and organization, and, above all, the difference in the technical methods required for their study, which becomes too complicated for a single worker. Hence Protistology becomes split up in practice by its own mass into three sciences: the Bacteria are the objects of the science of bacteriology; botanists deal with the unicellular plants; and the zoologists with those Protista which are more distinctly animal in their characters.

Hence the Protozoa are to be regarded as a convenient rather than a natural group, and may be characterized generally as follows: Organisms in which the individual is a single cell, that is to say, consists of a single undivided mass of protoplasm which is capable of independent existence in a suitable environment; if many such individuals be combined together to form a colony, as frequently occurs, there is no differentiation of the individuals except for reproductive purposes, and never for tissue-formation as in the Metazoa. The body always contains chromatin or nuclear substance, which may be disposed in various ways, but usually forms one or more concentrated masses termed nuclei, which can be distinguished sharply from the general body-protoplasm or cytoplasm. The protoplasmic body may be naked at the surface, or may be limited and enclosed by a distinct envelope or cell-membrane, which is not usually of the nature of cellulose, except in holophytic forms. Organs serving for locomotion and for the capture and assimilation of solid food are usually present, but may be wanting altogether when the mode of nutrition is other than holozoic; chlorophyll, on the other hand, is only found as a constituent of the body-substance in the holophytic Flagellata.[1] To these characters it may be added that reproduction is effected by some form of fission, or division of the body into smaller portions, and that in the vast majority of Protozoa, if not in all, a process of conjugation or syngamy occurs at some period in the life-cycle, the essential feature of the process being fusion of nuclear matter from distinct individuals. The foregoing definition does not distinguish the Protozoa sharply from the primitive forms of plant-life, with which, as stated above, they are connected by many transitions; but the differentiation of the body-substance into nucleus and cytoplasm separates them at once from the Bacteria, in which the chromatin is distributed evenly through the body-protoplasm.

Protozoa and Disease.—The study of the Protozoa has acquired great practical importance from the fact that many of them live as parasites of other animals, and as such may be the cause of dangerous diseases and epidemics in the higher forms of animal life and in man (see Parasitic Diseases). Examples of parasitic forms are to be found in all the four classes into which, as will be stated below, the Protozoa are divided, and one class, the Sporozoa, is composed entirely of endoparasitic forms. Hence Protozoology, as it is termed, is rapidly assuming an importance in medical and veterinary science almost equal to that of bacteriology, although the recognition of Protozoa as agents in the production of disease is hardly older than a decade. The most striking instances of Protozoa well established as pathogenic agents are the malarial parasites, the species of Piroplasma causing haemoglobinuria of cattle and other animals, the trypanosomes causing tsetse-fly disease, surra, sleeping sickness, and other maladies, the species of Leishmania causing kala azar and oriental sore, and the Amoeba responsible for the so-called amoebic dysentery. Other diseases referred, but as yet doubtfully, to the agency of Protozoa are syphilis, smallpox, hydrophobia, yellow fever, and even cancer.

It is only possible here to discuss briefly in a general way the relations of these parasites to their hosts. When two organisms stand habitually in the relation of host and parasite, an equilibrium tends to become established gradually between them, so that a condition is brought about in which, after many generations, the host becomes “tolerant” of the parasite, and the parasite is not lethal to the host, though perhaps capable of setting up considerable disturbance in its vital functions. Many animals are found to contain almost constantly certain internal parasites without being, apparently, in the least affected by them; and it should be borne in mind that in most cases it is not to the interest of the parasite to destroy the host or to overtax its resources. But when the parasite is transferred naturally or artificially to a species or race of host which does not ordinarily harbour it, and which therefore has not acquired powers of resisting its attacks, the parasites may be most deadly in their effects. Thus the white traveller in the tropics is exposed to far greater dangers from the indigenous disease-producing organisms than are the natives of those climes.

In some cases two organisms have become mutually adapted to each other as host and parasite to such an extent that the parasite is not capable of flourishing in any other host. An instance of this is Trypanosoma lewisi of the rat, which cannot live in any other species of animal but a rat, and which is not as a rule lethal to a rat, at least not to one otherwise healthy. Contrasting in an instructive manner with this species is Trypanosoma brucii, which occurs as a natural parasite of buffaloes and other big game in Africa, and is, apparently, harmless to them, but which is capable of being transferred to other animals by inoculation. The transference may take place naturally, by the bite of a tsetse-fly, or may be effected artificially; in either case T. brucii is extremely lethal to certain animals, such as imported cattle, horses and dogs, or to rats and guinea-pigs. Other animals, however, may be quite “repellent”[2] to this parasite, that is to say, if it be inoculated into their blood it dies out without producing ill effects, just as T. lewisi does when injected into an animal other than a rat. Thus it is seen that T. brucii, when introduced into the blood of an animal which is specifically or racially distinct from its natural hosts in the region where it is indigenous, is either unable to maintain itself in its new host, or flourishes in it to such an extent as to be the cause of its death.

We may assume, therefore, at least as a working hypothesis, that a lethal parasite is one that is new to its host, and that a harmless parasite is one long established. Since all parasites must have been new to their proper hosts at some period, recent or remote, in the history of the species, it would follow that the first commencement of parasitism would be in almost all cases a life and death struggle, as it were, between the two organisms concerned, and it is quite conceivable that the host might succumb in the struggle and so be exterminated. Ray Lankester has suggested that the extinction of many species of animals in the past may have been due, in some cases, to their having been attacked by a species of parasite to which they did not succeed in becoming adapted, and by which they became, in consequence, exterminated entirely.

Organization of the Protozoa.—The body-form may be constant or inconstant in the Protozoa, according as the body-substance is or is not limited at the surface by a firm envelope or cuticle. When the surface of the protoplasm is naked, as in the common amoeba and allied organisms, the movements of the animal bring about continual changes of form. The protoplasm flows out at any point into processes termed pseudopodia, which are being continually retracted and formed anew. Such movements are known as amoeboid, and may be seen in the cells of Metazoa as well as in Protozoa. The pseudopodia serve both for locomotion and for the capture of food. If equally developed on all sides of the body, the animal as a whole remains stationary, but if formed more on one side than the other, the mass of the body shifts its position in that direction, but the movement of translation is generally slow. If the animal remains perfectly quiescent and inactive, the laws of surface-tension acting upon the semi-fluid protoplasmic body cause it to assume a simple spherical form, which is also the type of body-form generally characteristic of Protozoa of floating habit (Radiolaria, Heliozoa, &c.).

In the majority of Protozoa, however, the protoplasm is limited at the surface by a firm membrane or cuticle, and in consequence the body has a definite form, which varies greatly in different species, according to the habit of life. As a general rule those forms that are fixed and sedentary in habit tend towards a radially symmetrical structure; those that are free-swimming approach to an ovoid form, with the longest axis of the body placed in the direction of movement; and those that creep upon a firm substratum have the lower side of the body flattened, so that dorsal and ventral surfaces can be distinguished; it is very rare, however, to find a bilaterally symmetrical type of body-structure amongst these organisms. In some cases the cuticle may be too thin to check completely the changes of form due to the movements of the underlying protoplasm; instances of this are seen amongst the so-called “metabolic” Flagellata, in which the body exhibits continually changes of form, termed by Lankester “euglenoid” movements, due to the activity of the superficial contractile layer of the body manifesting itself in ring-like contractions passing down the body in a manner similar to the peristaltic movements of the intestine.

The body-substance of the Protozoa is protoplasm, or, as it was originally termed by Dujardin, sarcode, which is finely alveolar in structure, the diameter of the alveoli varying generally between ½ and 1 μ. At the surface of the body the alveoli may take on a definite honeycomb-like arrangement, forming a special “alveolar layer” which in optical section appears radially striated. Besides the minute protoplasmic alveoli, the protoplasm often shows a coarse vacuolation throughout the whole or a part of its substance, giving the body a frothy structure. When such vacuoles are present they must be carefully distinguished from the contractile vacuoles and food-vacuoles described below; from the former they differ by their non-contractile nature, and from the latter by not containing food-substances.

In many Protozoa and especially in those forms in which there is no cuticle, the body may be supported by a skeleton. The material of the skeleton differs greatly in different cases, and may be wholly of an organic nature, or may be impregnated with, or almost entirely composed of, inorganic mineral salts, in which case the skeletal substance is usually either silica or carbonate of lime. From the morphological point of view the skeletons of Protozoa may be divided into two principal classes, according as they are formed internal to, or external to, the body in each case. Instances of internal skeletons are best seen in the spherical floating forms comprised in the orders Radiolaria and Heliozoa; such skeletons usually take the form of spicules, radiating from the centre to the circumference, and often further strengthened by the formation of tangential bars, producing by their union a lattice-work, which in species of relatively large size may be formed periodically at the surface as the animal grows so that the entire skeleton takes the form of concentric hollow spheres held together by radiating beams. The architectural types of these skeletons show, however, an almost infinite diversity, and cannot be summarized briefly. External skeletons have usually the form of a shell or house, into which the body can be retracted for protection, and from which the protoplasm can issue forth during the animal's phases of activity. Shells of this kind, which must be carefully distinguished from cuticles or other membranes that invest the body closely, are well seen in the order Foraminifera; in the simplest cases they are monaxon in architecture, that is to say, with one principal axis round which the shell is radially symmetrical, and at one pole is a large aperture through which the protoplasm can creep out. In addition to the principal aperture, the shell may or may not be pierced all over by numerous fine pores, through which also the protoplasm can pass out. For further details concerning these shells and their very numerous varieties of structure the reader is referred to the article Foraminifera.

The protoplasmic body of the Protozoa is frequently differentiated into two zones or regions: a more external, termed the ectoplasm or ectosarc, and a more internal, termed the endoplasm or endosarc. The ectosarc is distinguished by being more clear and hyaline in appearance, and more tough and viscid in consistence; the endoplasm, on the other hand, is more granular and opaque, and of a more fluid nature. The ectoplasm is the protective layer of the body, and is also the portion most concerned in movement, in excretion, and perhaps also in sensation and in functions similar to those performed by the nervous systems of higher animals. The endoplasm, on the other hand, is the chief seat of digestive and reproductive functions.

As the protective layer of the body, the ectoplasm forms the envelopes or membranes which invest the surface of the body, and which are differentiations of the outermost layer of the ectoplasm. Thus in most Flagellata the ectoplasm is represented only by the more or less firm outer covering or periplast. Even when such envelopes are absent, however, the ectoplasm can still be seen to exert a protective function; as, for instance, in those Myxosporidia which are parasitic in the gall-bladders or urinary bladders of their hosts, and which can resist the action of the juices in which they live so long as the ectoplasm is intact, but succumb to the action of the medium if the ectoplasm be injured. In many Infusoria the ectoplasm contains special organs of offence termed trichocysts, each a minute ovoid body from which, on stimulation, a thread is shot out, in a manner similar to the nematocysts of Coelenterata. Similar organs are seen also in the spores of Myxosporidia, as the so-called polar capsules; but in this case the organs are not specially ectoplasmic, and appear to serve for adhesion and attachment, rather than for offence.

The connexion of the ectoplasm with movement is seen in the simplest forms, such as Amoeba, by the fact that all pseudopodia arise from it in the first instance. In forms with a definite cuticle, on the other hand, the ectoplasm usually contains contractile fibres or myonemes, forming, as it were, the muscular system of the organism. The dependence of the motility of the animal upon the development of the ectoplasm is well seen in Gregarines, in which other organs of locomotion are absent; in forms endowed with active powers of locomotion a distinct ectoplasmic layer is present below the cuticle; in those Gregarines incapable of active movement, on the other hand, the ectoplasm is absent or scarcely recognizable.

From the ectoplasm arise the special organs of locomotion, which, when present, take the form of pseudopodia, flagella or cilia. Pseudopodia, as already explained, are temporary protoplasmic organs which can be extruded or retracted at any point; they fall naturally into two principal types, between which, however, transitions are to be found: first, slender, filamentous or filose pseudopodia, composed of ectoplasm alone, which may remain separate from one another, or may anastomose to form networks, and are then termed reticulose; secondly, thick, blunt, so-called lobose pseudopodia, which are composed of ectoplasm with a core of endoplasm, and never form networks. In forms showing active locomotor powers the pseudopodia are usually more lobose in type; filose pseudopodia, on the other hand, are more adapted for the function of capturing food.

Flagella are long, slender, vibratile filaments, generally few in number when present, and usually placed at the pole of the body which is anterior in progression. Each flagellum performs peculiar lashing movements which cause the body, if free, to be dragged along after the flagellum in jerks or leaps; if, however, the body be fixed, the action of the flagellum or flagella causes a current towards it, by which means the animal obtains its food-supply. A flagellum which is anterior in movement has been distinguished by Lankester by the convenient term tractellum; sometimes, however, the flagellum is posterior in movement and acts as a propeller, like the tail of a fish; for this type Lankester has proposed the term pulsellum. The flagellum appears to arise in all cases from a distinct basal granule, and in some cases, as in the genus Trypanosoma, there is a portion of the nuclear apparatus set apart as a distinct kinetic nucleus, with the function, apparently, of governing the activities of the flagellum.

Cilia are minute, hair-like extensions of the ectoplasm, which pierce the cuticle and form typically a furry covering to the body. Though perhaps primitively derived from flagella, cilia, in their usual form, are distinguished from flagella by being of smaller size, by being present, as a rule, in much greater numbers, and above all by the character of their movements. In the place of the complicated lashing movements of the flagella, each cilium performs a simple stroke in one direction, becoming first bowed on one side, by an act of contraction, and then straightened out again when relaxed. The movements of the cilia are co-ordinated and they act in concert, though not absolutely in unison, each one contracting just before or after its neighbour, so that waves of movement pass over a ciliated surface in a given direction, similar to what may be seen in a cornfield when the wind is blowing over it. Primitively coating the whole surface of the body evenly, the cilia may become modified and specialized in various ways, which cannot be described in detail here (see Infusoria).

Besides the organs of locomotion already mentioned, there may be present so-called undulating membranes, in the form of thin sheets of ectoplasm which are capable of performing sinuous, undulating movements by their inherent contractility. In some cases distinct contractile threads or myonemes have been described in these membranes. Undulating membranes appear to be formed either by the fusion together of a row of cilia, side by side, or by the attachment of a flagellum to the body by means of an ectoplasmic web, in which case the flagellum forms the free edge of the membrane, as in the genus Trypanosoma.

Returning to the ectoplasm, the excretory function exerted by this layer is seen by the formation in it of the peculiar contractile vacuoles found in most free-living Protozoa. A contractile vacuole is a spherical drop of watery fluid which makes its appearance periodically at some particular spot near the surface of the animal’s body, or, if more than one such vacuole is present, at several definite and constant places. Each vacuole grows to a certain size, and when it has reached the limit of its growth it discharges its contents to the exterior by a sudden and rapid contraction. There is, apparently, in most if not in all cases, a definite pore through which the contractile vacuole empties itself to the exterior. On account of the relatively large size which the contractile vacuole attains it bulges inwards beyond the limits of the ectoplasm and comes to lie chiefly in the endoplasm, to which it is sometimes, but erroneously, ascribed. In the most highly differentiated Protozoa, for instance, the Ciliata, the ectoplasm contains an apparatus of excretory channels, situated in its deeper layers, and forming as it were a drainage-system, from which the contractile vacuoles are fed. The fluid discharged by the contractile vacuoles appears to be chiefly water which has been absorbed at the surface of the protoplasmic body, and which has filtered through the protoplasm, taking up the soluble waste nitrogenous products of the metabolism and the gaseous products of respiration; hence the contractile vacuoles may be compared in a general way to the urinary and respiratory organs of the Metazoa.

One of the first consequences of the parasitic habit of life is the disappearance of the contractile vacuoles, which are hardly ever found in truly parasitic Protozoa, that is to say, in forms which live in the interior of other animals and nourish themselves at their expense. They are also very frequently absent in marine forms.

Mechanisms of a nervous nature are very seldom found in Protozoa, but in some Ciliata special tactile bristles are found, and it is possible that flagella, and perhaps even pseudopodia, may be sometimes tactile rather than locomotor in function. Pigment-spots, apparently sensitive to light, may also occur in some Flagellata.

The endoplasm, as already stated, is the chief seat of nutritive and reproductive processes. In many Flagellata the ectoplasm is represented only by the thin envelope or periplast, so that the whole body is practically endoplasm. When the two layers are well differentiated the endoplasm is more fluid and coarsely granular, and contains various organs, chief amongst them in importance being the nucleus, which must be considered specially and may be put aside for the present.

In considering the functions of ingestion and assimilation of food a distinction must be drawn between those Protozoa which absorb solid food-particles, that is to say, which are holozoic in habit, and those which, being holophytic, saprophytic or parasitic in habit, absorb their nourishment in a state of solution. Only in holozoic forms is a special apparatus found for ingestion or digestion of food; in all other forms nutriment is absorbed by osmosis through the body-wall, presumably at any point of the surface. In holozoic forms we must distinguish further those in which the protoplasm is naked at the surface from those in which the body is clothed by a firm cuticle or cell-membrane. In naked forms food-particles are taken in at any point of the body-surface, either by means of the pseudopodia, or by the action of flagella causing them to impinge upon the surface of the body. In either case the food is absorbed by the protoplasm simply flowing round it and engulfing it, and the food passes into the interior of the body in a tiny droplet of water forming what is termed a food-vacuole. Into the food-vacuole the surrounding protoplasm secretes digestive enzymes, so that each such vacuole represents a minute digestive cavity, in which the food is slowly digested, rendered soluble, and absorbed by the surrounding protoplasm. The insoluble residue of the food is finally rejected by expelling the food-vacuole and its contents from the surface of the body at any convenient point.

The simple process of food-absorption described above for the more primitive naked forms is necessarily modified in detail, though not in principle, in corticate Protozoa, that is to say, in forms provided with a cuticle. In the first place, it becomes necessary to have a special aperture for the ingestion of food, a cell-mouth or cytostome. Primitively the cytostome is a simple pore or interruption of the cuticle, but in forms more highly evolved the aperture is prolonged inwards in the form of a tube lined by ectosarc and cuticle, forming a gullet or oesophagus which ends in the endoplasm. Food-particles are forced by the action of cilia or flagella down the oesophagus and collect at the bottom of it in a droplet of water which, after reaching a certain size, passes into the endoplasm as a food-vacuole in which the food is digested. For rejection of the insoluble residue of the food-vacuoles, a special pore or cell-anus (cytopyge) may be present. In the Ciliata there is often a distinct anal tube visible at all times, but as a rule the anus is only visible at the moment that faecal matter is being ejected from it, though fine sections show that the pore is a constant one. In the higher Flagellata, on the other hand, the oesophageal ingrowth forms commonly a sort of cloacal cavity, into which the contractile vacuole or vacuoles discharge themselves, and into which also the food-vacuoles evacuate their residues.

Besides the food-vacuoles already described, and the nuclear apparatus presently to be dealt with, the endoplasm may contain various metaplastic products, that is to say, bodies to be regarded as stages in the upward or downward metabolism of the protoplasmic substance. Such substances may take the form of coarse granules of various kinds, crystals, vacuoles or droplets of fatty or oily nature, pigment-grains, and other bodies. In the holophytic Flagellata the endoplasm contains also various organs proper to the vegetable cell, such as chlorophyll-bodies (chromatophores), pyrenoids, grains of a starchy nature (paramylum), and so forth, which need not be described here in detail.

The nucleus in Protozoa is usually a compact, fairly conspicuous structure, composed of chromatin combined in various ways with an achromatic substance or substances. Sometimes the chromatin is distributed in smaller masses through the nucleus, producing a granular type of nucleus; more often the chromatin is more or less concentrated in a central mass forming a so-called karyosome, consisting of an achromatic plastinoid substance impregnated with chromatin. If the karyosome is large and there is very little chromatin between it and the nuclear membrane, the nucleus is of the type termed vesicular. A nuclear membrane is not, however, always present, and true nucleoli, of the type found in the nuclei of metazoan cells, are not found in Protozoa.

A given individual may have more than one nucleus, and the number present may amount to many thousands, as in the plasmodia of Mycetozoa. In such cases the nuclei may be all of one kind, that is to say, not markedly different in size, structure or function, so far as can be seen; or there may be a pronounced morphological differentiation of the nuclei correlated with a difference of function. Thus in the class Infusoria two nuclei are found in each individual; a macronucleus which is somatic in function, that is to say, which regulates the metabolism and vital processes of the body generally, and the micronucleus, which is generative in function, that is to say, which remains in reserve during the ordinary, “vegetative” life of the organism and becomes active during the act of syngamy, after which the effete macronucleus is absorbed or cast out and a new somatic nucleus is formed from portions of the micronuclei which have undergone fusion in the sexual act. Thus the micronucleus of the Infusoria can be compared in a general way with the germ-plasm of the Metazoa, like which it remains inactive until the sexual union. On the other hand, in some Flagellata a differentiation of the nucleus of quite a different type is seen, a smaller, kinetic nucleus being separated off from the larger, trophic or principal nucleus. The kinetic nucleus has the function, apparently, of controlling the locomotor apparatus, so that the specialization of these two nuclei is of a kind quite different from that seen in the Infusoria.

Besides the nuclear substance which is concentrated to form the principal nucleus or nuclei, there may be present also extranuclear granules of chromatin, so-called chromidia, scattered throughout the whole or some part of the protoplasmic body. Chromidia may be normally present in addition to the principal nucleus, or may be formed from the principal nucleus during certain phases of the life-cycle. In some cases the entire nucleus may become resolved temporarily into chromidia, from which a new nucleus may be formed again later by condensation and concentration of the scattered granules. When the chromidia are numerous and closely packed they may form a so-called chromidial network (Chromidial-Netz). Recent observations on the reproduction of some Sarcodina have shown that the chromidia may possess great importance in the life-cycle as representing generative chromatin which, like the micronucleus of the Infusoria mentioned above, remains in reserve until, by the process of syngamy, the nuclear apparatus is renewed; while the principal nuclei represent, like the macronuclei, somatic or vegetative chromatin which becomes effete and is cast off or absorbed when syngamy takes place. These questions will be discussed further below.

It was formerly supposed that the lowest Protozoa were entirely without a nucleus, and on this supposition E. Haeckel attempted to establish a class named by him Monera, defined as Protozoa consisting of protoplasm alone, in which a nucleus was not differentiated. To this class were referred various organisms whose alleged archaic nature was expressed by such names as Protogenes primordialis, organisms which, like so many other of the primitive forms of animal life described by Haeckel, have been seen by that naturalist alone up to the present. In all Protozoa that have been examined by modern methods a nucleus in some form has been demonstrated to exist, and it must be supposed, until proof to the contrary be forthcoming, that in the case of the so-called Monera either the nucleus was overlooked owing to defective technique, or it had been temporarily resolved into chromidia.

The nuclear apparatus may be supplemented by other bodies of which the nature is not always clear. Such is the so-called “Nebenkern” of Paramoeba eilhardi, apparently of the nature of a centrosome. Sometimes the karyosome acts like a centrosome during the division of the nucleus, and sometimes true centrosomes are present. Flagella also commonly arise from basal granules of a centrosomic nature, blepharoplasts in the correct sense of the term[3]; these blepharoplasts are always in connexion with the nucleus, or with the kinetic nucleus if there is one distinct from the trophic nucleus, as in the genus Trypanosoma and allied forms.

Reproduction of the Protozoa.—The mode of reproduction in these organisms is the same as that of the cell generally, and takes always the form of fission of some kind; that is to say, of division of the body into smaller portions, each of which represents a young individual. The division of the body is preceded by that of the nucleus, if single, or of each nucleus in the cases where there are two different nuclei; if, however, more than one nucleus of the same kind be present, the nuclei may be simply shared amongst the daughter-individuals, this mode of division being known as plasmotomy. Other organs of the body may either, like the nucleus, undergo fission, or may be formed afresh in the daughter-individuals.

The division of the nucleus in Protozoa may take place by the direct method or by means of mitosis. Direct division, without mitosis, is of very common occurrence; the division may be simple or multiple, that is to say, into only two parts, or into a number of fragments formed simultaneously. An extreme case of multiple fission is seen in the formation of the microgametes of Coccidium schubergi, where the nucleus breaks up into a great number of chromidia, which become concentrated in patches to form the several daughter-nuclei. In some cases, on the other hand, multiple daughter-nuclei are formed by rapidly repeated simple division of the parent nucleus. The mode of division may be different in different nuclei of the same individual; thus in the Infusoria the macronucleus divides by direct division, the micronucleus by mitosis.

The mitosis of the Protozoa is far from being of the uniform stereotyped pattern seen in the Metazoa, but, as might have been expected, often shows a much simpler and more primitive condition. Centrosomes are often absent, and their place may be taken, as stated above, by other bodies. The nuclear membrane may be retained throughout the mitosis. Definite chromosomes can, as a rule, be made out, but the chromosomes are often very numerous and minute, without definite form, and divide irregularly. Much remains to be done in studying the mitosis of the Protozoa, but it is probable that wider knowledge will show many conditions intermediate between direct division and perfect mitosis.

The simplest method of fission in Protozoa is that termed binary, where the body divides into two halves, which may be equal and similar, so that the result is two sister-individuals impossible to distinguish as parent and offspring. In many cases of binary fission, however, the resulting daughter-individuals may be markedly unequal in size, so that one may be distinguished as the parent, the other as the offspring. If the daughter-individual be relatively very small, and formed in a more or less imperfect condition at first, the process is termed gemmation or budding. The buds formed in this way may be either external, formed on the surface of the body, or internal, that is, formed in special internal cavities, from which the offspring are later set free, as in many Acinetaria. Gemmation may be correlated with multiple nuclear fission in such a way that buds are formed over the whole body surface of the organism, which thereby undergoes a process of simultaneous multiple fission into numerous daughter-individuals. Rapid multiple fission of this kind is termed sporulation, and is a form of reproduction which is of common occurrence, especially in parasitic forms. Usually the central portion of the parent body remains over as a residual body (Restkörper) , but sometimes the parent organism is entirely resolved into the daughter-individuals, which are termed spores in a general way, but can be given special names in special cases (see Gregarines, Coccidia, &c.).

Life-cycles of the Protozoa.—It is probable that in all Protozoa, as in the Metazoa, the life-history takes its course in a series of recurrent cycles of greater or less extent, a fixed point, as it were, in the cycle being marked by the act of syngamy, or conjugation, which represents, apparently, a process for recuperation of the waning vital powers of the organism. It is true that in many types of Protozoa syngamy is not known as yet to occur, but in all species which have been thoroughly investigated syngamy in some form has been observed, and there is nothing to lead to the belief that the sexual process is not of universal occurrence in the Protozoa.

The life-cycle of a given species may be very simple or it may be extremely complex, the organism occurring under many different forms at different phases or periods of its development. The polymorphism of the Protozoa is best considered under three categories, according to the three main causes to which it is due, namely, first, polymorphism due to adaptation to different conditions of existence; secondly, polymorphism due to differences of size and structure during growth; thirdly, polymorphism due to the differentiation of individuals in connexion with the process of syngamy or sexual conjugation.

1. Polymorphism in Relation to Life-conditions.—As a protection against unfavourable conditions, or for other reasons, most Protozoa have the power of passing into a resting condition, during which the vital functions may be wholly or in part suspended. In the resting phase the animal usually becomes enveloped in a resistant membrane or cyst secreted by it, and is then said to be encysted. The formation of a cyst may be a response to conditions of various kinds. Very commonly it is formed to protect the organism against a change of medium, as in the case of freshwater forms liable to desiccation, or of parasites about to pass out of the bodies of their hosts. In other cases the organism passes into the resting state in order to absorb ingested nutriment or in order to enter upon reproductive phases.

As a preparation for encystment, organs of locomotion, if present, are retracted or cast off; contractile vacuoles cease to be formed; and the food-vacuoles disappear, usually by digestion of their contents and rejection of the waste residue. The body becomes rounded off and more or less spherical in form, and the protoplasm becomes denser, that is, less fluid and more opaque, but at the same time of diminished specific gravity, by loss of water. The cyst is then secreted at the surface as a layer of varying thickness and toughness. In the encysted condition many Protozoa are capable of being transported by the wind, a fact which explains their appearance in infusions and liquids exposed to the air. In favourable conditions the cysts germinate, that is to say, the envelope is dissolved and the contained organism or organisms are set free to enter upon the strenuous life once more.

In the Mycetozoa, organisms adapted to a semi-terrestrial life in moist surroundings, the protoplasm is capable, when desiccated, of passing into a tough condition resembling sealing-wax, which, when moistened, assumes again its normal appearance and active condition.

Resting phases, analogous to encystment, are seen in the spores of various forms, especially those of parasitic habit, which are commonly enclosed in tough, resistant envelopes or sporocysts, and enveloped as a protection against change of medium or of host. Within the sporocyst multiplication of the sporoplasm may take place to form more or fewer sporozoites. The sporocysts usually show definite symmetry and structure, infinitely variable in different species. In a suitable medium the spores germinate by rupture of the sporocysts and escape of the contents.

2. Polymorphism in Relation to Growth and Development.—In many species of Protozoa there is hardly any difference to be observed between different individuals during their active phases except in size. Those individuals about to multiply by fission are slightly above the normal in dimensions: on the other hand, those resulting from recent fission will be smaller than the average; and such differences are, it need hardly be said, more pronounced when the fission is of the unequal binary type, or in cases of gemmation or multiple fission. In cases also where a given strain of a species is becoming senile, it is sometimes observed that the individuals are markedly undersized on the average.

On the other hand, it is often the case that the young individuals resulting from a recent act of multiplication may differ from adult individuals of the species, not merely in size, but in structural characters, to such an extent that their relationship to the adult forms could not be determined by simple inspection without other evidence. This is especially true of those species in which multiplication by sporulation occurs, giving rise to numerous small spores which may at first be in a resting condition, enveloped in protective sporocysts, but which sooner or later become free, motile individuals known technically as swarm-spores. Thus in many Sarcodina the adult is a large amoeboid organism which produces by sporulation a great number of relatively minute swarm-spores. These may be either, as in the common Amoeba proteus, amoeboid organisms, so-called amoebulae or pseudopodiospores, or, as in the Foraminifera and Radiolaria, flagellated organisms, so-called flagellulae or flagellispores. Sometimes, as in many Mycetozoa, amoeboid and flagellated phases may succeed each ether rapidly in the development of the swarm-spores. The familiar Noctiluca miliaris is another instance of a species which produces by sporulation numerous tiny swarm-spores quite different from the parent form in their characters. Such instances could be multiplied indefinitely amongst the Protozoa.

When the young individuals differ greatly from the adults in structure and appearance they may be regarded as larval forms, and it is interesting to note that such forms appear to be just as much recapitulative, in the phylogenetic sense, as are the larvae of many Metazoa. A striking instance is that of the Acinetaria, in which the swarm-spores produced by gemmation are ciliated, and thus betray affinities with the Ciliata which could hardly be suspected from a study of the adult forms alone. Similarly, in the genus Trypanosoma, the young forms often show a Herpetomonas-like structure which is probably of phyletic significance. The swarm-spores of Sarcodina and of Noctiluca mentioned above can, perhaps, be regarded in the same light. On the other hand, many larval forms cannot be considered as exhibiting recapitulative characters, but merely as adaptations to environment or other special life-conditions. This is especially true, as in Metazoa, of parasitic forms, subject as they are to great vicissitudes, to cope with which the most finely adjusted adaptations are necessary on the part of the organism.

3. Polymorphism in Relation to Sex.—In all Protozoa of which the life-cycle has been made known in its entire course, a process of syngamy or sexual union has been found to occur. There are still many forms in which syngamy remains to be discovered: this is true even of some groups of considerable extent. It is quite possible, therefore, that Protozoa exist in which syngamy does not occur. In view, however, of the widespread occurrence of sexual processes amongst unicellular organisms, both of animal and vegetable nature, and the fact that extended observation continually brings to light new instances of this kind, it is safer, in cases amongst the Protozoa in which syngamy is not known to occur, to explain its apparent absence by the imperfections of the present state of our knowledge, than to suppose that in such forms sexual phenomena are entirely lacking in the life-cycle.[4]

The process of syngamy, though greatly diversified in different forms, consists essentially of one and the same process in all cases; namely, the fusion of nuclear matter from two distinct individuals. Plus ça change, plus c’est la même chose! Hence true syngamy may be distinguished as karyogamy from the process of plastogamy, or fusion of the protoplasmic bodies, of frequent occurrence in many forms of Protozoa. The individuals whose nuclei undergo fusion are termed gametes. They may be in no way different from each other or from ordinary individuals of the species, or, on the other hand, they may be highly differentiated in size, form and structure. The two gametes may undergo complete fusion into one body, thus giving rise to an individual termed generally a zygote or copula, but which may bear special names in special cases (e.g. vermicule or oökinete of the malarial parasites, &c.); such a process is termed sometimes copulation. On the other hand, the bodies of the two gametes may remain distinct, and portions of the nucleus of each be exchanged between them; to this condition the term conjugation is sometimes specially applied. The act of syngamy may be performed in the free condition, or in the resting state, within a cyst.

The significance of syngamy has been much discussed, and it is very difficult to make positive statements upon this point. By comparing the life-cycles of different forms it is found that syngamy sometimes precedes, sometimes follows, a period of great reproductive activity on the part of the organism. Thus in such a form as Noctiluca, syngamy between two full-grown individuals is followed by rapid sporulation and the production of a swarm of young individuals; on the other hand, in Foraminifera and Radiolaria, rapid sporulation of adult individuals produces a numerous progeny of young forms which may go through the process of syngamy and produce zygotes that simply grow into the adult form. Comparing these two types of development, instances of which might be greatly multiplied, it is seen that in one case syngamy follows a period of growth and precedes a period of proliferation in the life-cycle, and that in the other case exactly the reverse is true. Hence it follows that syngamy must not be regarded as in any way specially connected with reproduction, but must be considered in its relation to the life-cycle as a whole, and in those instances in which syngamy is followed by increased reproductive activity the explanation must be sought in the general physiological effects of the sexual process upon the vital powers of the organism.

In the Metazoa the sexual process is always related to the production of a new individual, that is to say, of a multicellular organism for which there is no analogy amongst the Protozoa, although an approach to the Metazoan condition is seen in colony-forming Flagellata, such as Volvox and its allies. The reproduction of Protozoa is analogous to the ordinary process of cell-division and multiplication which is going on at all times in the bodies of the Metazoa, and which can be observed in the production of the gametes; that is to say, in the period of the life-cycle immediately preceding the sexual process in the Metazoa, just as much as in the developmental phases which follow syngamy and result in the building up of a new Metazoan individual. Hence, so far as the Protozoa are concerned, the phrase “sexual reproduction” is an incongruous combination of words; reproduction and sex are two distinct things, not necessarily related or in any direct causal connexion; and in order to arrive at any theory of sex it is necessary first of all to clear away all misconceptions or preconceived notions arising from analogies with the multicellular Metazoan individual.

Many observations indicate that the vital powers of the Protozoa become gradually weakened, and the individual tends to become senile and effete, unless the process of syngamy intervenes. The immediate result of the sexual union is a renewal of the vitality, a rejuvenescence, which manifests itself in enhanced powers of metabolism, growth and reproduction. These facts have been most studied in the Ciliata. It is observed that if these organisms be prevented from conjugating with others of their kind they become senile and finally die off. It has been found by G. N. Calkins, however, that if the senile individuals be given a change of medium and nourishment, their vigour may be renewed and their life prolonged for a time, though not indefinitely; there comes a period when artificial methods fail and only the natural process of syngamy can enable them to prolong their existence. The results obtained by Calkins are of great interest, as indicating that under special conditions of the environment the necessity for the sexual process may be diminished and the event may be deferred for a long time, if not indefinitely. Hence it is quite possible that in many Protozoa the process of syngamy may be in abeyance, just as there are plants which can be propagated indefinitely by suckers or cuttings without ever setting seed; and it is possible that the inoculative or artificial transmission of parasitic Protozoa from one host to another, as in the case of pathogenic trypanosomes, without any apparent diminution in their vital powers, is an instance of this kind.

As a general rule, in order that syngamy may be attended by beneficial results to the organism, it is necessary that the two conjugating individuals should be from different strains, that is to say, they should not be nearly related by descent and parentage. Thus F. Schaudinn found that in order to observe the sexual union of the gametes of Foraminifera it was necessary to bring together gametes of distinct parentage. On the other hand it has been observed that in many Protozoa, especially in parasitic forms, syngamy takes place between individuals of common parentage. Thus in Amoeba coli, according to F. Schaudinn, a single individual becomes encysted and its nucleus divides into two; after each nucleus has undergone certain maturative changes they give rise to pronuclei which conjugate and initiate a new developmental cycle. Syngamy between sister individuals, or autogamy, as it has been termed, is not, however, confined to parasitic Protozoa; it has been observed in Actinosphaerium by R. Hertwig. The benefit to the organism, if any, arising from autogamy can only be supposed to result from the rearrangement and reconstitution of the nuclear apparatus. The frequent occurrence of autogamy suggests that in many Protozoa the nature of the environment diminishes the importance of the sexual process, at least so far as the mixture of nuclear material from distinct sources is concerned; and, since autogamy is most common in parasitic forms, this result may, in the light of G. N. Calkins's experiments, be ascribed in great part to the frequent changes of environment and nutrition to which parasitic forms, above all, are subject.

True syngamy consists, as has been said, of nuclear fusion or karyogamy. It rarely, if ever, happens, however, that such fusion takes place without the conjugating nuclei having undergone some process of reduction by elimination of a portion of the nuclear substance, in a manner analogous to the maturation of the germ-cells in the Metazoa. The chromatin thus eliminated may be cast out from the body of the organism as one or more so-called polar bodies; or may be absorbed in the cytoplasm; or may remain in the cytoplasm and be left over in the residual protoplasm in cases where syngamy is followed by a process of rapid multiplication by sporulation; but in all cases the chromatin removed from the nucleus is rejected in some way or other and plays no part in the subsequent development of the organism. The nuclei of the gametes which have completed this process of épuration nucléaire are then ripe for syngamic fusion and are termed pronuclei; the union of two pronuclei produces a single nucleus termed a synkaryon.

It is certain that in many, if not in all, cases the nuclear substance that is rejected as a preliminary to syngamy consists of somatic or vegetative chromatin; that is to say, of chromatin that has been functional in regulating the ordinary vital functions, metabolism, growth, reproduction, &c., during previous generations, and has become effete; while on the other hand the chromatin that persists to form the pronuclei is generative chromatin which has remained in reserve for the sexual act and has retained its peculiar powers and properties unimpaired. The truth of this explanation is extremely obvious in such forms as the Infusoria, where somatic and generative chromatin are concentrated into two distinct and entirely separate nuclei. In some Rhizopoda also the body contains one or more principal nuclei and a mass of chromidia, and it has been observed that as a preparation for syngamy the principal nuclei are eliminated and the pronuclei are formed from the chromidia; in such cases, therefore, it is reasonable to regard the principal nuclei as representing somatic chromatin, the chromidia as generative chromatin. In other cases, however, for example Actinosphaerium, the chromidia must be interpreted, from their behaviour, as somatic chromatin, and the principal nuclei as generative chromatin; hence R. Goldschmidt has proposed the special term sporetia for those chromidia which represent reserve generative chromatin. In the majority of Protozoa, however, the nuclear substance is not differentiated in such a way that it can be distinguished by any visible peculiarities into somatic and generative chromatin.

The process of reduction is not limited, apparently, to the elimination of somatic chromatin, but a portion of the generative chromatin is also cast off. Thus in the Infusoria not only the somatic macronucleus, but also a considerable portion of the generative micronucleus, is absorbed at each act of conjugation. The elimination of generative chromatin is perhaps of importance as a factor in heredity and the production of variations, or possibly for sex determination, as will be discussed below; it is difficult to suggest any other explanations for it, unless it be supposed that during the exercise of ordinary vital functions a portion of the generative chromatin be rendered effete as well as the somatic chromatin.

From the considerations set forth in the foregoing paragraphs it must be supposed that the synkaryon, the fusion-product of the two pronuclei in syngamy, consists at first purely of generative chromatin, which must speedily become differentiated into the regulative somatic chromatin of the ensuing generations and the generative chromatin held in reserve for the next act of syngamy. Such a differentiation can be actually observed in the Infusoria, where immediately after conjugation the synkaryon divides into one or more pairs of nuclei, each pair becoming the two unequally sized nuclei of an ordinary individual, sometimes with, even at this stage, an apparently wanton elimination of nuclear substance. Thus the somatic and generative chromatin of the Protozoa offer a certain analogy with the soma and germ-plasm of Metazoa; but in making such comparisons the distinction between a physiological analogy and a morphological homology should be borne clearly in mind.

It has been stated above that the two gametes of a given species of Protozoa may be perfectly similar and indistinguishable, or may be very different one from the other. The condition with similar gametes is termed isogamy, that with differentiated gametes anisogamy. Every transition can be found from complete isogamy and pronounced anisogamy in the Protozoa; in tracing, however, the evolution of specialized gametes it must be remembered that we are dealing only with visible morphological differences mainly of an adaptive nature, without prejudice to the question of the possible existence of a fundamental sexual antithesis in all gametes, present even when not perceptible. The sex philosopher O. Weininger has urged that sex is a fundamental attribute of living things, and that the living substance, protoplasm, consists of arrhenoplasm and thelyplasm united in varying proportions. Certain observations of F. Schaudinn tend to support this view; in Trypanosoma noctuae, for example, Schaudinn found that the process of reduction in one gamete took an opposite course to that which it took in the other gamete. In one gamete certain portions of the nucleus were retained and certain other portions rejected; in the maturation of the other gamete the portions rejected and the portions retained were the reverse. Hence Schaudinn was led to regard the indifferent individuals as essentially hermaphrodite in nature, and therefore capable of giving rise to gametes of either order by elimination of one or the other set of sexual elements; a theory which throws further light on the elimination of generative chromatin mentioned above. It is possible, therefore, that the gametes of Protozoa may possess sexual characters intrinsically different even when perfectly similar so far as can be perceived. It is very probable, for instance, that the isogamy in Gregarines is a state of things derived secondarily from a primitive condition of anisogamy (see Gregarines).

The simplest possible condition of the gametes is seen in the free-swimming Ciliata, forms which in other respects are the most highly organized of Protozoa; here the individuals which conjugate are only distinguished from ordinary individuals of the species by the fact that their nuclei have undergone very complicated processes of reduction and nuclear elimination. In these forms there is also no difference between young and adult individuals, beyond scarcely perceptible differences of size between individuals about to divide and those that are the products of recent division, so that these species are practically monomorphic in the active condition. In forms, however, which, like Vorticelia, are of sessile habit, small free-swimming individuals are liberated which seek out and conjugate with the ordinary sessile individuals. Here we have an instance of a morphological differentiation of the gametes which is clearly adaptive to the life-conditions of the species. In other Protozoa there may be, as already stated, differences, more or less pronounced, between young and adult individuals, and syngamy may take place either between young individuals (microgamy) or between adults (macrogamy); the gametes may be in either case ordinary individuals of the species, not specially differentiated in any way, or on the other hand they may be differentiated from ordinary individuals, while still similar and isogamic amongst themselves; or, finally, they may be anisogamic; that is to say, differentiated into two distinct types. Thus in the Radiolaria, for example, an adult individual breaks up by a process of sporulation into numerous minute flagellated swarm-spores; these may be all of one kind, termed isospores, which develop directly without undergoing syngamy; or they may be of two kinds, termed anisospores, both different in their character from the isospores, and incapable of development without syngamy.

When the gametes are differentiated the divergence between them almost always follows parallel paths. One gamete is distinguished by its smaller size, its greater activity, and its comparative poverty in granules of reserve food-material; hence it is termed the microgamete. The other gamete is distinguished by its greater bulk, its pronounced sluggishness and inertness, and its tendency to form and store up in the cytoplasm reserve nutriment of one kind or another; hence it is termed the macrogamete, or, as some prefer to write it, the megagamete (better megadogamete). When these differences are very pronounced, as, for instance, in the Coccidia and other Sporozoa, a condition is reached which is practically indistinguishable from that seen in the sperm and ova of the Metazoa. Hence the microgamete is generally regarded as male, the macrogamete as female; and these terms may be conveniently used, although they do not in themselves imply more than would the words positive and negative, or any other pair of terms expressive of a fundamental contrast. The microgamete may become reduced to a mere thread of chromatin, which may possess one or two flagella for purposes of locomotion, as in Coccidia, &c., or may move by serpentine movements of the whole body, which resembles in its entirety a flagellum, and is often wrongly so termed. In contrast with the microgamete, its correlative, the macrogamete, tends to become a bulky, inert body, often with great resemblance to an ovum, its cytoplasm dense and granular, packed with reserve food-materials as an egg contains yolk, and without organs of locomotion or capacity for movement of any kind. Hence the macrogamete is the passive element in syngamy, which requires to be sought out and “fertilized” by the active microgamete, a division of labour perfectly analogous to that seen in the male and female gametes of Metazoa. In those cases where syngamy takes place by interchange of nuclear substance between two gametes which remain separate from one another, as in the Infusoria, each gamete forms two pronuclei, which are distinguished by their behaviour as the active and passive pronuclei respectively. The active pronucleus of each gamete passes over into the body of the other and fuses with its passive pronucleus to form a synkaryon. A similar method of procedure occurs also in Amoeba coli, according to F. Schaudinn.

When gametes are not very highly specialized they may still retain the power of multiplication by division possessed by ordinary individuals, so long as they have not undergone the process of nuclear reduction preliminary to syngamy. If, however, the gametes are highly specialized they may forfeit the power of multiplication. In this respect the microgametes are worse off than the other sex; on account of the great reduction of the body-protoplasm, and the entire absence of any reserve materials, they must either fulfil their destiny as gametes or die off. The macrogametes, on the other hand, with their great reserves of cytoplasm and nutriment, are more hardy than any other forms of the species, and are able to maintain their existence in periods of famine and starvation when all other forms are killed off. Moreover they may regain the power of multiplication by a process of parthenogenesis, a term originally applied in the Metazoa to cases where a germ-cell of definitely female character, that is to say an ovum, acquires the power of reproduction without fertilization by syngamy. A macrogamete multiplying by parthenogenesis first goes through certain nuclear changes whereby it is set back, as it were, from the female to the indifferent condition, and it is then able to multiply by fission like any ordinary, non-sexual individual of the species. Parthenogenesis has been described by F. Schaudinn in the malarial parasites and in Trypanosoma noctuae. In both cases the female forms are able to persist under adverse conditions after all other forms have perished, and then by parthenogenesis they may multiply when conditions are more favourable, overrun the host again, and cause a relapse of the disease of which they are the cause. S. v. Prowazek has described in Herpetomonas muscae-domesticae an analogous process of multiplication on the part of male individuals, and has coined the term etheogenesis for this process, but the statement needs confirmation, and as a general rule the microgamete is quite incapable of independent reproduction under any circumstances.

It is often found that not only are the gametes differentiated, but that their immediate progenitors may also exhibit characters which mark them off from the ordinary or indifferent individuals of the species. In such cases the parent-forms of the gametes are termed gametocytes, and they may differ amongst themselves in characters which render it possible to distinguish those destined to produce microgametes from those which will produce the other sex. The parent-individuals of the microgametes, or microgametocytes, are distinguished as a general rule by clearer protoplasm, free from coarse granulations, and a larger nucleus, more rich in chromatin. The macrogametocytes, on the other hand, usually have coarsely granular cytoplasm, rich in reserve food-stuffs, and a relatively small nucleus. The gametocytes produce the gametes by methods that vary according to the degree of specialization of the gametes. In isogamous forms, of which good examples are furnished by many Gregarines (q.v.), the gametes are produced by a process of sporulation on the part of the gametocytes, a certain amount of residual protoplasm being left over. In forms with pronounced anisogamy, for instance, Coccidia or Haemosporidia, the microgametes are produced by sporulation in which almost the whole mass of the body of the gametocyte may be left over as residual protoplasm, together with some portion of the nucleus; in the other sex, however, the process of sporulation may be altogether in abeyance, and the macrogametocyte becomes simply converted into the macrogamete after going through a process of nuclear reduction.

The gametocytes may, however, possess the power of multiplication without change of character for many generations; or, to put the matter in other words, the sexual differentiation may be apparent not merely in the generation immediately preceding the gametes, but in many generations prior to this. Thus a given species may consist of three different types of adult individuals, male, female and indifferent, each multiplying in its own line. Complicated alternations of generations are the result, and if at the same time there is a well-marked difference between young and adult forms of the species the height of polymorphism is reached. Very commonly a double series of generations occurs, the non-sexual or indifferent forms multiplying apart from the sexually differentiated individuals and the generations immediately descended from them; in such cases the series of non-sexual generations is termed schizogony, the series of sexual generations gametogony or sporogony. Schizogony and sporogony usually occur as adaptations to, or at least in relation with, distinct conditions of life. Thus in parasitic forms, as well illustrated by the Coccidia, the organisms multiply by schizogony when overrunning the host, that is to say, when nutriment is abundant; sporogony begins as a preparation for passing into the outer world, in order to infect new hosts. In the Haemosporidia, in which transmission from one vertebrate host to another is effected by means of blood-sucking ectoparasites (Diptera, ticks, leeches, &c.), the schizogony goes on in the vertebrate host, the sporogony in the invertebrate host. In free-living, non-parasitic forms, schizogony may go on under ordinary conditions, while sporogony supervenes as a preparation for a marked change in the life-conditions; for instance, a change of medium, or at the approach of winter. It is interesting to note that, as a general rule, the differentiation of sexual forms seems to be a preliminary to the production of more resistnt forms capable of braving adverse conditions or violent changes in the conditions of life; a phenomenon which is in support of the hypothesis that syngamy has a strengthening effect on the vitality of the species.

Classification of the Protozoa.

Various attempts have been made to separate the Protozoa into two primary subdivisions. E. Ray Lankester divided them into two main groups, the Gymnomyxa, with naked protoplasm and indefinite form, and the Corticata, with the protoplasm limited by a firm membrane, and consequently with a definite body-form. In many of the corticate groups, however, there must be placed amoeboid, non-corticate forms, such as Mastigamoeba amongst the Flagellata, or the malarial parasites amongst the Sporozoa. Hence if Lankester’s classification be used, it must be without a hard and fast verbal definition. F. Doflein, on the other hand, has divided the Protozoa into Plasmodroma, with organs of locomotion derived from protoplasmic processes, i.e. pseudopodia or flagella, and Ciliophora, with locomotion by cilia. It may be doubted, however, if the distinction between flagella and cilia is so fundamental and sharply defined as this mode of classification would imply. W. H. Jackson has proposed to unite the forms bearing flagella and cilia into one section, Plegepoda, and distinguishes two other sections, Rhizopoda ( = Sarcodina) and Endoparasita ( = Sporozoa).

Four main groups of Protozoa, of the rank of classes, are universally recognized, however they may be combined into larger categories; these are the Sarcodina, Mastigophora, Sporozoa and Infusoria.

The Sarcodina are characterized by the body being composed of naked protoplasm, not covered by any limiting cuticle, although in many cases a house or shell is secreted into which the protoplasm can be partly or entirely withdrawn. No special organs of locomotion, either flagella or cilia, are ever present in the adult, and locomotion and capture of food are effected in the manner named amoeboid, by more or less temporary extrusions or outflow of the protoplasm which are termed pseudopodia, as in Amoeba.

The Mastigophora are so named because organs of locomotion are always present in the adult in the form of one or more flagella, each flagellum (Gr. μάςτιξ, whip) a delicate, thread-like extension of the protoplasm, endowed with a special contractility which enables it to perform lashing, whip-like movements. The body protoplasm is sometimes naked, in which case it may be amoeboid, but is more usually limited by a cuticle, varying in thickness in different types.

The Sporozoa, with the exception of a few forms of dubious position, are exclusively internal parasites of Metazoa, absorbing their food from the internal juices and secretions of their hosts, and never exhibiting in their trophic phases any organs of locomotion or for the ingestion and digestion of solid food. The body-protoplasm may be naked and amoeboid or limited by a cuticle. The reproduction is specialized in correlation with the parasitic habit, and results typically in the formation of a number of minute germs or spores, by which the infection of fresh hosts is effected. It must not be supposed, however, that spore-formation is confined to this class of Protozoa.

The Infusoria, a name originally of much wider application, is now restricted to denote those Protozoa in which locomotion or capture of food is effected by means of special organs termed cilia, minute hair-like contractile extensions of the protoplasm differing from flagella not only in their usually smaller size and greater number, but also in the mode of contraction and movement. The cilia may be present throughout life or only in an early stage of the individual. The body is always limited by a cuticle and the nucleus seems to be invariably double, being divided into two parts specialized in function and differing in size, termed respectively macronucleus and micronucleus.

Comparing these four subdivisions with one another, it may be said at once that the Sporozoa and Infusoria are highly specialized classes, each well marked off from the other subdivisions. The Sarcodina and Mastigophora, on the other hand, include the most primitive types of Protozoa and are delimited from one another by a somewhat arbitrary character, the presence or absence of a flagellum in the adult. Thus Mastigamoeba is a form which unites the characters of the Sarcodina and Mastigophora, having an amoeboid body which bears a flagellum, and it is classed among the Mastigophora merely because the flagellum is retained throughout life; if the flagellum were absent in the adult condition it would be placed among the Sarcodina, many of which have flagella in their young stages but lack them when adult. Hence Bütschli considered the Rhizomastigina (i.e. Mastigamoeba and its allies) as the most primitive group of Protozoa, representing the common ancestral form of all the classes; and on this view the flagellated young stages of many Sarcodina would represent recapitulative larval stages.

Bütschli’s theory of Protozoan phylogeny implies that a flagellum is an organ of most primitive nature, possessed perhaps by the earliest forms of life; and it must be remembered that flagella are borne by many Bacteria. On the other hand, one would imagine, from general considerations, that living beings possessing a flagellum would have been preceded in evolution by others that did not bear so definite an organ. The flagellum itself is generally regarded as a vibratile process or extension of the protoplasm, comparable in its nature to a slender pseudopodium endowed with peculiar powers of movement. More knowledge with regard to the nature and formation of the flagellum is needed in order to decide this point, and particularly with regard to the question whether the flagella of Bacteria are of the same nature as those of Protozoa.

It has been much debated whether the earliest forms of life were of the nature of plants or animals. Many authors consider the question settled beyond all debate by a process of trenchant deductive reasoning. It is argued that animals require other organisms for their nutriment, and that plants, that is to say green plants, do not; therefore plants must have preceded animals. On the other hand, the morphologist will urge that green plants derive their peculiar powers of metabolism from the possession of very definite cell-organs, namely chromatophores containing chlorophyll; and will argue that living things without such organs must have preceded in evolution those possessing them. The whole dispute is based on the assumption that plant and animal represent the two fundamental modes of metabolism; whereas the study of the Bacteria shows the possibility of many other modes of life. Many Bacteria exhibit processes of metabolism totally different from those generally laid down in textbooks as characteristic of living matter; some are killed by free oxygen; others can absorb free nitrogen, and various other “abnormal” properties are manifested by them. Hence the primitive organisms may have been neither plant nor animal in their nature, but may have possessed, like the Bacteria at present, many different methods of metabolism from which plant and animal are two divergent paths of evolution.

The origin of life is veiled in a mist which biological knowledge in its present state is unable to dispel; and speculations with regard to the nature of the earliest form of life are as yet premature and futile.

The following references are either general treatises on the Protozoa, or memoirs dealing with special points in a general manner.

Bibliography.—O. Bütschli, “Protozoa,” in Bronn’s Klassen und Ordnungen des Thierreichs, Bd. I. (1881-1886); idem., “Investigations on Microscopic Forms and Protoplasm,” translated by E. A. Minchin (London, A. & C. Black, 1894); G. N. Calkins, “Studies on the Life History of the Protozoa, IV.” (and other memoirs cited therein), Journ. Exp. Zool., i. 423-461, 3 plates, 3 text-figs.; idem., “The Protozoa,” Columbia University Biological Series, vi. pp. xvi.+347, 153 text-figs.; Y. Delage and E. Hérouard, Traité de Zoologie Concrète, I. La Cellule et les protozoaires (Paris, 1896); F. Doflein, Die Protozoen als parasiten und Krankheitserreger (Jena, Gustav Fischer, 1901); idem., “Das System der Protozoen,” Arch. f. Protistenkunde, i. 169-192, 3 text-figs.; R. Goldschmidt, “Die Chromidien der Protozoen,” Arch. f. Protistenkunde, (1904), v. 126-144, 1 fig.; R. Goldschmidt and M. Popoff, “Die Karyokinen der Protozoen und der Chromidialapparat der Protozoen—und Metazoenzelle, Arch. f. Protistenkunde (1907), viii., pp. 321-343, 6 text-figs.; M. Hartog, “Protozoa” Cambridge Natural History, vol. i. (London, 1906); R. Hertwig, “Ueber Wesen und Bedeutung der Befruchtung,” SB. Akad. München (1902), xxxii., pp. 57-73; idem., “Die Protozoen und die Zelltheorie,” Arch. f. Protistenkunde, (1902), pp. 1-40; W. H. Jackson, “Protozoa” in Rolleston’s Forms of Animal Life, (rev. ed., pp. 818-922, Oxford, 1888); A. Lang, Lehrbuch der vergleichenden Anatomie der wirbellosen Tiere, Bd. 1. Abt. II. Lief. 2, “Protozoa” (Jena, 1901); E. R. Lankester, article “Protozoa” in Ency. Brit. 9th ed.; idem. (edited by). A Treatise on Zoology by various authors: “Protozoa” in part I., fasc. 2 (1903), and fasc. 1, to be published shortly (London, A. and C. Black); E. A. Minchin, “Protozoa” in Allbutt and Rolleston’s A System of Medicine, rev. ed. vol. ii. pt. 2 (London, 1907); F. Schaudinn, “Neuere Forschungen über die Befruchtung der Protozoen,” Verh. deutsch. zool. Ges. (1905), xv. 16-35, 1 diagram; H. M. Woodcock, “Protozoa” in Zool. Record (1902-1905), vols. xxxix.-xlii.

(E. A. M.)

  1. Many Protozoa contain symbiotic green organisms, so-called zoochlorellae or zooxanthellae, in their body-protoplasm; for instance, Radiolaria, and Ciliata such as Paramecium bursaria, &c. This condition must be carefully distinguished from chlorophyll occurring as a cell-constituent.
  2. The use of the terms “tolerant” and “repellent” is taken from the excellent article on “Sleeping Sickness,” by E. Ray Lankester, in the Quarterly Review (July 1904), No. 399. pp. 113—138.
  3. The kinetic nucleus of Trypanosoma is sometimes, but in the writer’s opinion wrongly, named centrosome or blepharoplast; the bodies to which cytologists give these names are achromatic bodies; the kinetic nucleus is a true chromatic nucleus. The question of the centrosome in Protozoa is discussed by R. Goldschmidt and M. Popoff.
  4. It will be shown below, however, that in some species syngamy may perhaps be secondarily in abeyance.