as the vehicle of heredity and to transmit the characters of parent to offspring. In the present state of our knowledge, therefore, the peculiar chromatin-granules must be regarded as an integral part, perhaps even the most essentially and primarily important portion, of the living substance. At the same time it must be borne in mind that the term "chromatin " does not denote a definite chemical substance, to be recognized universally by hard and fast chemical tests. The chromatin of different organisms or cells may behave quite differently in relation to stains or other reactions; and if it be true that it is the chromatin which determines the nature and activities of the cell, it follows that no two cells which differ from one another in any way can have their chromatin exactly similar. The conception of chromatin is one based upon its relations to the vital activities and life cycle, as a whole, of the organism or cell, and not upon any definable material, that is chemical and physical, properties.

The importance of protoplasm, as the physical and material basis of life, has caused it to be the subject in recent years of much minute and laborious research. It seems obvious, that matter so peculiarly endowed must possess a complexity of structure and organization far exceeding that which at first sight meets the eye. Some biologists have attacked the problem of the ultimate constitution of protoplasm from a purely theoretical standpoint, and have framed hypotheses of an ultramicroscopic constitution sufficient, in their opinion, to explain, or at least to throw light upon, the vital activities of the living substance. Others, proceeding by more empirical methods, have attempted to lay bare the structure of protoplasm by means of the refinements of modern microscopical technique, or to solve the question of its constitution by means of chemical and physiological investigation. Hence a convenient distinction, not always easy, however, to maintain in practice, is drawn between speculative and empirical theories of protoplasm.

1. Speculative theories have come with the greatest frequency from those who have attempted to find a material explanation for the phenomena of heredity (q.v.). As instances may be mentioned more particularly the " gemmules" of Darwin, the pangenes" of de Vries, the "plastidules" of Haeckel, and the biophores" of Weismann. These theories have been ably brought together and discussed by Delage, who has included them all under the term "micromerism," since they agree in the assumption that the living substance contains, or consists of, a vast number of excessively minute particles-i.e. aggregates or combinations of molecules, which give to the protoplasm its specific properties and tendencies (" idioplasm' " of Nägeli). In other cases the assumption of invisible protoplasmic units has been inspired by a desire either to explain the general vital and assimilative powers of protoplasm, as, for example, the "micellae" of Nägeli and the " plasomes" of Wiesner, or to elucidate the mechanism of some one function, such as the "inotagmas" of Engelmann, assumed to be the agents of contractility. In general, it may be said of all these speculations either that they can only be extended to all vital phenomena by the help of so many subordinate hypotheses and assumptions that they become unworkable and unintelligible, or that they only carry the difficulties a step further back, and really explain nothing. Thus it is postulated for Wiesner's hypothetical plasomes that they possess the power of assimilation, growth and reproduction by division; in other words, that they are endowed with just those properties which constitute the unexplained mystery of living matter.

2. Empirical theories of protoplasm differ according as their authors seek to find one universal type of structure or constitution common to all conditions or differentiations of the living substance, or, on the contrary, are of opinion that it may vary fundamentally in different places or at different times. From these two points of view protoplasm may be regarded either as monomorphic or polymorphic (Fischer). The microscopical investigation of protoplasm reveals at the first glance a viscid, slimy or mucilaginous substance, in which is embedded an immense number of granules, for the most part very tiny. Very rarely are these granules absent, and then only from a portion

of the protoplasm, and only temporarily. Hence many authori ties have regarded the minute granules-the "microsomes of Hanstein-as themselves the ultimate living units of protoplasm, in opposition to those who would regard them merely as "metaplastic" substances, i.e. as the heterogeneous byproducts of metabolism and vital activity. The granular theory, as this conception of the living substance is called, has received its extreme elaboration at the hands of Altmann, whose standpoint may be taken as typical of this class of theories. After demonstrating the universal occurrence of granules in protoplasm, Altmann has compared each individual granule to a free-living bacterium, and thus regards a cell as a colony of minute organisms, namely the granules or bioblasts, as he has termed them, living embedded in a common matrix, like a zoogloea colony of bacteria. Of this theory it may be remarked, firstly, that it brings us no nearer to an explanation of vital phenomena than do the plasomes of Wiesner; secondly, that to consider bacteria as equivalent, not to cells, but to cell granules, is to assume for this class of organisms a position with regard to the cell theory which is, to say the least, doubtful; and, thirdly, that the observations of the vast majority of competent microscopists furnish abundant support for the statement that granules of protoplasm do not lie free in a structureless matrix, but are embedded in the substance of a minute and delicate framework or morphoplasm, which in its turn is bathed by a watery fluid or enchylema permeating the whole substance. The upholders of the granular theory deny the existence of the framework, or explain it as due to an arrangement of the granules, or as an optical effect produced by the matrix between the granules. Amongst those, on the other hand, who assert the existence of a framework distinct from granules and enchylema, the utmost diversity of opinion prevails with regard to the true structural relations of these three parts and the rôle played by each in the exercise of vital functions. Some have regarded the framework as made up of a tangle of separate fibrillae (filar theory)—a view more especially connected with the name of Flemming-but most are agreed that it represents the appearance of a reticulum or network with excessively fine meshes, usually from to 1 μ in diameter. The reticulum carries the granules at its nodal points, and is bathed everywhere by the enchylema. Even with so much in common, however, opinions are still greatly at variance. In the first place, the majority of observers interpret the reticulum as the expression of an actual spongy framework, a network of minute fibrillae ramifying in all planes. While, however, Heitzmann, following the speculations of Brücke, considered the framework itself to be actively contractile and the seat of all protoplasmic movement, an opposite point of view is represented by the writings of Leydig, Schäfer and others, who regard the reticulum merely as a kind of supporting framework or spongioplasm, in which is lodged the enchylema or hyaloplasm, considered to be itself the primary motile and living substance. Bütschli, on the other hand, has pointed out the grave difficulties that attend the interpretation of the reticulum as a fibrillar framework, in view of the distinctly fluid consistence of, at any rate, most samples of protoplasm. For if the substance of the framework be assumed to be of a firm, solid nature, then the protoplasm as a whole could not behave as a fluid, any more than could a sponge soaked in water. On the other hand, the hypothesis of a fluid fibrillar framework leads to a physical impossibility, since one liquid cannot be permanently suspended in another in the form of a network. Bütschli therefore interprets the universally present reticulum as a meshwork of minute lamellae, forming a honeycombed or alveolar structure, similar to the arrangement of fluid lamellae in a fine foam or lather, in which the interstices are filled, not with air but with another fluid; in other words, the structure of protoplasm is that of an exceedingly fine emulsion of two liquids not miscible with one another.

It may be claimed for the alveolar theory of Bütschli that it throws light upon many known facts relating to protoplasm. It interprets the reticuluin as the optical section of a minute foam-like structure, and permits the formation of protoplasmic striations and of apparent fibrillae as the result of linear or radiating dispositions of the alveolar

framework; it reconciles with the laws of physics the combination of a framework with a fluid or semi-fluid aggregate condition, while variations in the fluidity of the framework are compatible with a stiffening of the protoplasm almost to the pitch of rigidity, as seen, for example, in nervous tissue; and, finally, it explains many characteristic structural peculiarities of protoplasm, such as the superficial layer of radiately arranged alveoli, the spherical form of vacuoles, the continuous wall or pellicle which limits both the vacuoles and the protoplasm as a whole, and many other points not intelligible on the theory of a sponge-like structure. Bütschli has succeeded, moreover, in producing artificial foams of minute structure, which not only mimic the appearance of protoplasm, but can be made to exhibit streaming and amoeboid movements very similar to those of simple protoplasmic organisms. Incidentally these experiments have shown that many of the apparent granulations and "microsomes" are an optical effect produced by the nodes of the minute framework. In his most recent works Bütschli has extended his theory of alveolar structure to many other substances, and has tried to prove that it is a universal characteristic of colloid bodies, a view strongly combated, however, by Fischer. While it cannot be claimed that Bütschli's theory furnishes in any way a complete explanation of life, leaving untouched, as it does, the fundamental question of assimilation and metabolism, he at least draws attention to a very important class of facts, which, if demonstrated to be of universal occurrence, must be reckoned with in future treatment of the protoplasm question, and would form an indispensable preliminary to all speculations upon the mechanism of the living substance.

In opposition to the above-mentioned monomorphic theories of protoplasm, all of which agree in assuming the existence of some fundamental type of structure in all living substance, attempts have been made at various times to show that the structural appearances seen in protoplasm are in reality artificial products, due to precipitation or coagulation caused by reagents used in the study or preparation of living objects. These views have been developed by Fischer, who by experimenting upon various proteids with histological fixatives, has shown that it is possible to produce in them a granular, reticular or alveolar structure, according to treatment, and, further, that granules so produced may be differentially stained according to their size and absorptive powers. Fischer therefore suggests that many structural appearances seen in protoplasm may be purely artificial, but does not extend this view to all such structures, which would indeed be impossible, in view of the frequency with which reticular or alveolar structures have been observed during life. He suggests, however, that such structures may be temporary results of vital precipitation of proteids within the organism, and that protoplasm may have at different times a granular reticular or alveolar structure, or may be homogeneous. Fischer's conception of living protoplasm is therefore that of a polymorphic substance, and a similar view is held at the present time by Flemming, Wilson and others. Strassburger also regards protoplasm as composed of two portions: a motile kinoplasm which is fibrillar, and a nutritive trophoplasm which is alveolar, in structure.

The chemical investigation of protoplasm labours at the outset under the disadvantage that it cannot deal with the living substance as a whole, since no analysis can be performed upon it without destroying the life. Protoplasm consists, to the extent of about 60% of its total mass, of a mixture of various nucleo-proteids-that is to say, of those substances which, in molecular structure and chemical composition, are the most complex bodies known. In association with them are always found varying amounts of fats, carbohydrates, and other bodies, and such compounds are always present in the living substance to a greater or less degree as products of both upward and downward metabolism. Protoplasm also contains a large but variable percentage of water, the amount of which present in any given case affects largely its fluid or viscid aggregate condition. Especial interest attaches to the remarkable class of bodies known as ferments or enzymes, which when prepared and isolated from the living body are capable of effecting in other substances chemical changes of a kind regarded as specifically vital. It is from their study, and from that of the complex proteids found in the living body, that the greatest advances towards an explanation of the properties of living matter may be expected at the present time.

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The question may be raised how far it is probable that there is one universal living substance which could conceivably be isolated or prepared in a pure state, and which would then exhibit the phenomena characteristic of vital activity. It is sufficiently obvious, in the first place, that protoplasm, as we know it, exhibits infinite diversity of character, and that no two samples of protoplasm are absolutely similar in all respects. Chemical differences must be assumed to exist not only between the vital fabrics of allied species of organisms, but even between those of individuals of the same species. Kassowitz regards this variability as compatible with the assumption of a gigantic protoplasmic molecule in which endless variations arise by changes in the combinations of a vast number of atoms and atom complexes. It is difficult to conceive, however, of any single substance, however complex in its chemical constitution, which could perform all the functions of life. To postulate a universal living substance is to proceed along a path which leads inevitably to the assumption of biophores, plastidules or other similar units, since the ultimate living particles must then be imagined as endowed at the outset with many, if not all, of the fundamental properties and characteristic actions of living bodies. Such a conception has as its logical result a vitalistic standpoint, which may or may not embody the correct mental attitude with regard to the study of life, but which at any rate tends to check any further advance towards an explanation or analysis of clementary vital phenomena. We may rather, with Kölliker, Verworn and others, ascribe the activities of protoplasm to the mutual interaction of many substances, no single one of which can be considered as living in itself, but only in so far as it forms an indispensable constituent of a living body. From this point of view life is to be regarded, not as the property of a single definite substance, but as the expression of the ever-changing relations existing between the many substances which make up the complex and variable congeries known to us as protoplasm.

AUTHORITIES.-For exhaustive historical summaries of the protoplasm question, with full bibliographical references, the reader may be referred to the following works, especially the first five; Bütschli, Investigations on Microscopic Foams and Protoplasm (London, 1894) über die Struktur des Protoplasmas und einige ihrer Kritiker,' Untersuchungen über Strukturen (Leipzig, 1898); "Meine Ansicht Arch. f. Entwickelungsmechanik d. Org. (1901); xi. 499-584, pl. xx.; Delage, La Structure du protoplasme et les théories sur l hérédité (Paris, 1895); Wilson, The Cell (2nd ed., London, 1900); Fischer, Fixirung, Farbung, und Bau des Protoplasmas (Leipzig, 1899); Kassowitz, Allgemeine Biologie (Vienna, 1899); G. Mann, Protoplasm, its Definition, Chemistry and Structure (Oxford, 1906), p. 59. (E. A. M.)

PROTOZOA (Gr. pros, first, and for, 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 bodyprotoplasm 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. To these characters it may be added that reproduction is effected by some form of fission, oi 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 bodyprotoplasm.

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

Many Protozoa contain symbiotic green organisms, so-called instance, Radiolaria, and Ciliata such as Paramecium bursaria, &c. zoochlorellae or zooxanthellae, in their body-protoplasm; for This condition must be carefully distinguished from chlorophyll occurring as a cell-constituent.

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 scen 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

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 coordinated 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