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as the vehicle of heredity and to transmit the characters of of the protoplasm, and only temporarily. Hence many authori. parent to offspring. In the present state of our knowledge, ties have regarded the minute granules-the “ microsomes" therefore, the peculiar chromatin-granules must be regarded as of Hanstein-as themselves the ultimate living units of protoan integral part, perhaps even the inost essentially and primarily plasm, in opposition to those who would regard them merely important portion, of the living substance. At the same time as “metaplastic ” substances, i.e. as thc heterogeneous by. it must be borne in mind that the term “chromatin " does not products of metabolism and vital activity. The granular theory, denote a definite chemical substance, to be recognized universally as this conception of the living substance is called, has received by hard and fast chemical tests. The chromatin of dillerent its extreme elaboration at the hands of Altmann, whose stand

point may be taken as typical of this class of theories. After stains or other reactions; and if it be true that it is the chromatin demonstrating the universal occurrence of granules in protoplasm, which determines the nature and activities of the cell, it follows Altmann has compared each individual granule to a free-living that no two cells which differ from one another in any way can bacterium, and thus regards a cell as a colony of minute organisms, have their chromatin exactly similar. The conception of chrom- namely the granules or bioblasts, as he has termed them, living atin is one based upon its relations to the vital activities and life embedded in a common matrix, like a zoogloea colony of bacteria. cycle, as a whole, of the organism or cell, and not upon any of this theory it may be remarked, firstly, that it brings us dennable material, that is chemical and physical, properties. I no nearer to an explanation of vital phenomena than do the

The importance of protoplasm, as the physical and material plasomes of Wiesner; secondly, that to consider bacteria as basis of life, has caused it to be the subject in recent years of equivalent, not to cells, but to cell granules, is to assume for much minute and laborious research. It seems obvious that this class of organisms a position with regard to the cell theory matter so peculiarly endowed must possess a complexity of which is, to say the least, doubtful; and, thirdly, that the structure and organization far exceeding that which at first observations of the vast majority of competent microscopists sight meets the eye. Some biologists have attacked the furnish abundant support for the statement that granules of problem of the ultimate constitution of protoplasm from a protoplasm do not lie free in a structureless matrix, but are purely theoretical standpoint, and have framed hypotheses embedded in the substance of a minute and delicate framework of an ultramicroscopic constitution sufficient, in their opinion, or mor phoplasm, which in its turn is bathed by a watery fluid to explain, or at least to throw light upon, the vital activities or enchylema permeating the whole substance. The upholders of the living substance. Others, proceeding by more empirical of the granular theory deny the existence of the framework, methods, have attempted to lay bare the structure of protoplasm or explain it as due to an arrangement of the granules, or as by means of the refinements of modern microscopical technique, an optical effect produced by the matrix between the granules or to solve the question of its constitution by means of chemical

means of chemical | Amongst those, on the other hand, who assert the existence of and physiological investigation. Hence a convenient distinction, a framework distinct from granules and enchylema, the utmost not always easy, however, to maintain in practice, is drawn diversity of opinion prevails with regard to the true structural between speculative and empirical theories of protoplasm. relations of these three parts and the rôle played by each in the

I. Speculative thicorics have come with the greatest frequency exercise of vital functions. Some have regarded the framework from those who have attempted to find a material explanation as made up of a tangle of separate fibrillae (hilar Thcory)-a view for the phenomena of heredity (9.0.). As instances may be more especially connected with the name of Flemming--but most mentioned more particularly the “ gemmules" of Darwin, the are agreed that it represents the appearance of a reliculum or “pangenes" of de Vries, the “plastidules" of Haeckel, and the network with excessively fine meshes, usually from 1 to iu in " biophores” of Weismann. These theories have been ably diameter. The reticulum carries the granules at its nodal brought together and discussed by Delage, who has included points, and is bathed everywhere by the enchylema. Even with them all under the term “micromerism,” since they agree in so much in common, however, opinions are still greatly ai the assumption that the living substance contains, or consists variance. In the first place, the majority of observers interpret of, a vast number of excessively minute particles-i.e. aggregates the reticulum as the expression of an actual spongy framework, or combinations of molecules, which give to the protoplasm its a network of minute hbrillae ramilying in specific properties and tendencies (“ idioplasm" of Nägeli). | however, Heitzmann, following the speculations of Brücke, In other cases the assumption of invisible protoplasmic units considered the framework itself to be actively contractile and has been inspired by a desire either to explain the general vital the seat of all protoplasmic movement, an opposite point of view and assimilative powers of protoplasm, as, for example, the is represented by the writings of Leydig, Schäfer and others, “ micellae" of Nägeli and the “ plasomes" of Wiesner, or to who regard the reticulum merely as a kind of supporting frameelucidate the mechanism of some one function, such as the work or spongio piasm, in which is lodged the enchylema or “inotagmas" of Engelmann, assumed to be the agents of hyaloplasm,

hyalo plasm, considered to be itself the primary motile and living contractility. In gencral, it may be said of all these speculations substance. Butschli, on the other hand, has pointed out the either that they can only be extended to all vital phenomena grave difficulties that attend the interpretation of the reticulum by the help of so many subordinate hypotheses and assumptions as a fibrillar framework, in view of the distinctly fluid consistence that they become unworkable and unintelligible, or that they of, at any rate, most samples of protoplasm. For if the subonly carry the difficulties a step further back, and really explain stance of the framework be assumed to be of a firm, solid nature, nothing. Thus it is postulated for Wiesner's hypothetical then the protoplasm as a whole could not behave as a fluid, any plasomes that they possess the power of assimilation, growth and more than could a sponge soaked in water. On the other hand, reproduction by division; in other words, that they are endowed the hypothesis of a fluid 6brillar framework leads to a physical with just those properties which constitute the unexplained impossibility, since one liquid cannot be permanently suspended mystery of living matter.

in another in the form of a network. Bütschli therefore interprets 2. Empirical theories of protoplasm differ according as their the universally present reticulum as a meshwork of minute authors seck to find one universal type of structure or constitu lamellae, forming a honeycombed or alucolar structure, similar tion common to all conditions or disserentiations of the living to the arrangement of fluid lamellae in a fine foam or lather, substance, or, on the contrary, are of opinion that it may vary in which the interstices are filled, not with air fundamentally in different places or at different times. From fuid; in other words, the structure of protoplasm is that of an these two points of view protoplasm may be regarded either exceedingly fine emulsion of two liquids not miscible with one as monomorphic or polymorphic (Fischer). The microscopical | another. investigation of protoplasm reveals at the first glance a viscid, It may be claimed for the alveolar theory of Bütschli that it throws slimy or mucilaginous substance, in which is embedded an

light upon many known facts relating to protoplasm. It interprets

the reticuluin as the optical section of a minute loam-like structure, immense number of granules, for the most part very tiny. Very I and permits the formation of protoplasmic striations and of apparent rarely are these granules absent, and then only from a portion i fibrillae as the result of linear or radiating dispositions of the alveolar a criterion by which it is easy to denne the Protozoa from a purely zoochlorellae or zooxanthellae, in their body-protoplasm; for

to there the ultimueset with actions of

framework; it reconciles with the laws of physics the combination The question may be raised how far it is probable that there is of a framework with a fluid or semi-fluid aggregate condition, while one universal living substance which could conceivably be isolated variations in the fluidity of the framework are compatible with a or prepared in a pure state, and which would then exhibit the stiffening of the protoplasm almost to the pitch of rigidity, as seen, phenomena characteristic of vital activity. It is sufficiently obvious, for example, in nervous tissue; and, finally, it explains many charac in the first place, that protoplasm, as we know it, exhibits infinite teristic structural peculiarities of protoplasm, such as the superficial diversity of character, and that no two samples of protoplasm are layer of radiately arranged alveoli, the spherical form of vacuoles, absolutely similar in all respects. Chemical differences must be the continuous wall or pellicle which limits both the vacuoles and the assumed to exist not only between the vital fabrics of allied species protoplasm as a whole, and many other points not intelligible on of organisms, but even between those of individuals of the same the theory of a sponge-like structure. Bütschli has succeeded, species. Kassowitz regards this variability as compatible with the moreover, in producing artificial foams of minute structure, which assumption of a gigantic protoplasmic molecule in which endless not only mimic the appearance of protoplasın, but can be made to variations arise by changes in the combinations of a vast number exhibit streaming and amoeboid movements very similar to those of of ators and atom complexes. It is difficult to conceive, however; simple protoplasmic organisms. Incidentally these experiments of any single substance, however complex in its chemical constitu. have shown that many of the apparent granulations and " micro tion, which could perform all the functions of life. To postulate a somes " are an optical effect produced by the nodes of the minute universal living substance is to proceed along a path which leads framework. In his most recent works Bütschli has extended his inevitably to the assumption of biophores, plastidules or other theory of alveolar structure to many other substances, and has tried similar units, since the ultimate living particles must then be to prove that it is a universal characteristic of colloid bodies, a imagined as endowed at the outset with many, is not all, of the view strongly combated, however, by Fischer. While it cannot fundamental properties and characteristic actions of living bodies. be claimed that Bütschli's thcory furnishes in any way a complete Such a conception has as its logical result a vitalistic standpoint, cxplanation of life, leaving untouched, as it does, the fundamental which may or may not embody the correct mental attitude with question of assimilation and metabolism, he at least draws attention regard to the study of life, but which at any rate tends to check any to a very important class of facts, which, if demonstrated to be of further advance towards an explanation or analysis of elementary universal occurrence, must be reckoned with in future treatment vital phenomena. We may rather, with Kölliker, Verworn and of the protoplasm question, and would form an indispensable others, ascribe the activities of protoplasm to the mutual interaction preliminary to all speculations upon the mechanism of the living of many substances, no single one of which can be considered as substance.

living in itself, but only in so far as it forms an indispensable consti

tuent of a living body. From this point of view life is to be regarded, In opposition to the above-mentioned monomorphic theories not as the property of a single definite substance, but as the exof protoplasm, all of which agree in assuming the existence of pression of the ever-changing relations existing between the many some fundamental type of structure in all living substance,

substances which make up the complex and variable congeries known

to us as protoplasm. attempts have been made at various times to show that the

AUTHORITIES.--For exhaustive historical summaries of the protostructural appearances seen in protoplasm are in reality artificial | plasm question, with full biblio products, due to precipitation or coagulation caused by reagents be referred to the following works, especially the first five: Bütschli, used in the study or preparation of living objects. These views

Investigalions on Microscopic Fooms and Protoplasm (London, 1894):

Untersuchungen über Strukturen (Leipzig, 1898); " Meine Ansicht have been developed by Fischer, who by experimenting upon über die Struktur des Protoplasmas und einige ihrer Kritiker," various proteids with histological fixatives, has shown that it Arch. . Entwickelungsmechanik d. Org. (1901): xi. 499-584, pl. xx.; is possible to produce in them a granular, reticular or alveolar | Dclage, La Structure du protoplasme et les théories Sur 1 héridice structure, according to treatment, and, further, that granules

(Paris, 1895): Wilson, The Cell (2nd ed., London, 1900): Fischer,

Fixirung. Färbung, und Bau des Protoplasmas (Leipzig, 1899); 50 produced may be differentially stained according to their size

Kassowitz, Allgemeine Biologie (Vienna, 1899); G. Mann, Protoplasm, and absorptive powers. Fischer therefore suggests that many | ils Definition, Chemistry and Structure (Oxford, 1906), p. 59. structural appearances seen in protoplasm may be purely

(E. A. M.) artificial, but docs not extend this view to all such structures, PROTOZOA (Gr. T AWTOS, first, and swov, living thing), the which would indeed be impossible, in view of the frequency with name given by modern zoologists to the animalcules, for the which reticular or alveolar structures have been observed during most part microscopic, which were termed by the older naturallife. He suggests, however, that such structures may be lists Infusoria, from the manner in which they appear in infusions temporary results of vital precipitation of proteids within the containing decaying animal and vegetable matter. The name organism, and that protoplasm may have at different times a Insusoria is now, however, restricted to one of the four classes granular reticular or alveolar structure, or may be homogeneous. which comprise the Protozoa proper. The name Protozoa was Fischer's conception of living protoplasm is therefore that of coined as far back as 1820 as an equivalent for the German word a polymorphic substance, and a similar view is held at the Urthiere, meaning animals of primitive or archaic nature, the present time by Flemming, Wilson and others. Strassburger | forms of animal life which may be supposed to have been the also regards protoplasm as composed of two portions: a motile first that appeared upon our globe. The great naturalisi C. T. kinoplasm which is fibrillar, and a nutritive trophoplasm which von Siebold was, however, the first to give a scientific definition is alveolar, in structure.

to the group. Von Siebold pointed out that in the Protozoa The chemical investigation of protoplasm labours at the outset the individual was always a single vital unit or cell, in contrast under the disadvantage that it cannot deal with the living with the higher division of the animal kingdom, the Metazoa, substance as a whole, since no analysis can be performed upon in which the body is generally, though not universally, regarded it without destroying the life. Protoplasm consists, to the as composed of many such units. To put the matter briefly extent of about 60% of its total mass, of a mixture of various and somewhat technically: the Protozoa are unicellular animals, nucleo-proteids--that is to say, of those substances which, in the Metazoa multicellular animals; in the Protozoa the cell is molecular structure and chemical composition, are the most complete in itsell, both morphologically and physiologically, complex bodies known. In association with them are always and is capable of maintaining a separate and independent exist. found varying amounts of fats, carbohydrates, and other bodies, ence in suitable surroundings, like any other organism; in the and such compounds are always present in the living substance | Metazoa the cells are differentiated for the performance of disto a greater or less degree as products of both upward and down-tinct functions and combined together to form the various tissues ward metabolism. Protoplasm also contains a large but variable of which the body is built up, and the individual cells of the percentage of water, the amount of which present in any given | Metazoan body are not capable of maintaining a separate case affects largely its fluid or viscid aggregate condition. existence apart from their fellows. This is the sense in which Especial interest attaches to the remarkable class of bodies known the term Protozoa is used by zoologists, whereby certain forms as ferments or enzymes, which when prepared and isolated from of animal life, which were formerly ranked as Protozoa, such the living body are capable of effecting in other substances as sponges and rotifers, are now definitely excluded from the chemical changes of a kind regarded as specifically vital. It is group and classed as Metazoa, from their study, and from that of the complex proteids found The animal kingdom may be divided, therefore, into two in the living body, that the greatest advances towards an sub-kingdoms, the Protozoa and the Metazoa, the first named explanation of the properties of living matter may be expected characterized by their essentially unicellular nature. This is at the present time.

zoological standpoint, but which becomes less satisfactory when / vast number of forms of life to be comprised as Protista, their we take into consideration the whole range of microscopic diversity in habit of life and organization, and, above all, the unicellular organisms. Besides the true Protozoa, which, ex ditlerence in the technical methods required for their study, hypothesi, are organisms of animal nature, there are many other which becomes too complicated for a single worker. Hence organisms of equally simple organization, including the Protistology becomes split up in practice by its own mass into Bacteria and the unicellular plants. The Bacteria stand sharply three sciences: the Bacteria are the objects of the science of apart from the other forms of life, not only, in many cases, by bacteriology; botanists deal with the unicellular plants; and their divergent methods of metabolism, but by morphological the zoologists with those Protista which are more distinctly characteristics, such as the definite body-form limited by a animal in their characters. distinct envelope, the absence of organs for locomotion other Hence the Protozoa are to be regarded as a convenient rather than the peculiar flagella, and, above all, by the lack of any than a natural group, and may be characterized generally as differentiation of the body-protoplasm into nucleus and cyto-follows: Organisms in which the individual is a single cell, that plasm, as in all true cells of either animal or vegetable nature. is to say, consists of a single undivided mass of protoplasm which On the other hand, to separate by hard-and-fast definitions the is capable of independent existence in a suitable environment; unicellular plants from the unicellular animals is not only difficult if many such individuals be combined together to form a colony, but practically impossible. The essential difference between as frequently occurs, there is no differentiation of the individuals plant and animal is a physiological one, a difference in the except for reproductive purposes, and never for tissue-formation method of nutrition. A typical green plant is able to live as in the Metazoa. The body always contains chromatin or independently of other organisms and to build up its substance nuclear substance, which may be disposed in various ways, but from simple gases in the air and inorganic salts in the soil or usually forms one or more concentrated masses termed nuclei, water, provided that certain conditions of light and moisture which can be distinguished sharply from the general bodybe present in its environment; this is the so-called holophytic protoplasm or cytoplasm. The protoplasmic body may be method of nutrition. A typical animal, on the other hand, while naked at the surface, or may be limited and enclosed by a distinct practically independent of sunlight, is not able to exist apart envelope or cell-membrane, which is not usually of the nature from other living organisms, since it is not able to build up its of cellulose, except in holophytic forms. Organs serving for substance from simple chemical constituents like a plant, but locomotion and for the capture and assimilation of solid food must be supplied with ready-made proteids in its food, for which are usually present, but may be wanting altogether when the it requires other organisms, either plants or animals; this mode of nutrition is other than holozoic; chlorophyll, on the is the so-called holozoic method of nutrition. Intermediate other hand, is only found as a constituent of the body-substance between these two habits of life is the so-called saprophytic in the holophytic Flagellata. To these characters it may be habit, exemplified by the fungi amongst plants; in this method | added that reproduction is effected by some form of fission, oi of nutrition the organism cannot build up its substance entirely division of the body into smaller portions, and that in the vast from inorganic substances, but absorbs the organic substances majority of Protozoa, if not in all, a process of conjugation or present in solutions containing organic salts or decaying animal syngamy occurs at some period in the life-cycle, the essential or vegetable matter.

feature of the process being fusion of nuclear matter from distinct If we regard the organisms termed collectively Protozoa from individuals. The foregoing definition does not distinguish the the point of view of their methods of nutrition (considering for Protozoa sharply from the primitive forms of plant-lise, with

ent only free. living, non-parasitic forms), we find in which, as stated above, they are connected by many transitions: one class, the Flagellata, examples of the three methods men- but the differentiation of the body-substance into nucleus tioned above, the holozoic, holophytic and saprophytic habit and cytoplasm separates them at once from the Bacteria, in of life, not only in species closely allied to each other, but even which the chromatin is distributed evenly through the bodycombined in one and the same species at different periods of its protoplasm. life or in different surroundings. An individual of a given Protozoa and Disease. The study of the Protozoa has acspecies may contain chlorophyll, with which it decomposes quired great practical importance from the fact that many of carbonic acid gas in the sunlight, like a plant, while possessing them live as parasites of other animals, and as such may be the a definite mouth-aperture, by means of which it can ingest solid cause of dangerous diseases and epidemics in the higher forms of food, like an animal. Such instances show clearly that in the animal life and in man (see Parasitic DISEASES). Examples simplest forms of life the difference between plant and animal of parasitic forms are to be found in all the four classes into is but a difference of habit and of mode of nutrition, to which which, as will be stated below, the Protozoa are divided, and one the organism is not at first irrevocably committed, and which class, the Sporozoa, is composed entirely of endoparasitic forms. are not at first accompanied by distinctive morphological | Hence Protozoology, as it is termed, is rapidly assuming an characteristics. Only when the organism becomes specialized importance in medical and veterinary science almost equal to for one or the other mode of life exclusively does it acquire such that of bacteriology, although the recognition of Protozoa as definite morphological characters that the difference between agents in the production of disease is hardly older than a decade. plant and animal can be used for the purpose of a natural | The most striking instances of Protozoa well established as classification, as in the higher forms of life. In the lowest forms pathogenic agents are the malarial parasites, the species of it is not possible to base natural subdivisions on their vegetable Piroplasma causing haemoglobinuria of cattle and other animals, or animal nature. For this reason it has been proposed by the trypanosomes causing tsetse-fly disease, surra, sleeping E. Haeckel to unite all the primitive forms of life in which the sickness, and other maladies, the species of Leishmania causing body is morphologically equivalent to a single cell into one kala azar and oriental sore, and the Amoeba responsible for group, the Protista, irrespective of their animal or vegetable the so-called amoebic dysentery. Other discases referred, but nature. In this method of dealing with the problem the Protista as yet doubtfully, to the agency of Protozoa are syphilis, smallare regarded as a distinct kingdom (Reich), more or less inter pox, hydrophobia, yellow fever, and even cancer. mediate between, but distinct from, the animal and vegetable It is only possible here to discuss brictly in a general way the kingdoms, and representing the ancestral stock from which relations of these parasites to their hosts. When lwo organisms both animals and plants have sprung. Many authorities have stand habitually in the relation of host and parasite, an equifollowed Haeckel's lead in the matter, and the science of Pro- | librium tends to become established gradually between them, so tistology or Protistenkunde has already a special journal devoted to the publication of researches upon it. But though it may Many Protozoa contain symbiotic green organisms, so-called be more scientific, from a theoretical point of view, to group

instance, Radiolaria, and Ciliata such as Paramecium bursaria, &c. all these primitive organisms together in the way suggested | This condition must be carefully distinguished from chlorophyll by Haeckel, in practice it is inconvenient, on account of the l occurring as a cell-constituent.

that a condition is brought about in which, after many general form, which is also the type of body-form generally characteristic tions, the host becomes “tolerant " of the parasite, and the of Protozoa of floating habit (Radiolaria, Heliozoa, &c.). parasite is not lethal to the host, though perhaps capable of In the majority of Protozoa, however, the protoplasm is setting up considerable disturbance in its vital functions. Many limited at the surface by a firm membrane or cuticle, and in animals are found to contain almost constantly certain internal consequence the body has a definite form, which varice greatly parasites without being, apparently, in the least affected by in different species, according to the habit of life. Ris a general them; and it should be borne in mind that in most cases it is rule those forms that are fixed and sedentary w habit tend not to the interest of the parasite to destroy the host or to over- towards a radially symmetrical structure; those that are freetax its resources. But when the parasite is transferred naturally swimming approach to an ovoid form, with the longes or artificially to a species or race of host which does not ordinarily the body placed in the direction of movement; and those that harbour it, and which therefore has not acquired powers of creep upon a firm substratum have the lower side of the body resisting its attacks, the parasites may be most deadly in their nattened, so that dorsal and ventral surfaces can be distinguished; effects. Thus the white traveller in the tropics is exposed to it is very rare, however, to find a bilaterally symmetrical type far greater dangers from the indigenous disease-producing of body-structure amongst these organisms. In some cases organisms than are the natives of those climes.

the cuticle may be too thin to check completely the changes In some cases two organisms have become mutually adapted of form due to the movements of the underlying protoplasm; to each other as host and parasite to such an extent that instances of this are seen amongst the so-called “metabolic " parasite is not capable of flourishing in any other host. An Flagellata, in which the body exhibits continually changes of instance of this is Trypanosoma lewisi of the rat, which cannot form, termed by Lankester " euglenoid ” movements, due to hve in any other species of animal but a rat, and which is not the activity of the superficial contractile layer of the body as a rule lethal to a rat, at least not to one otherwise healthy. manifesting itself in ring-like contractions passing down the Contrasting in an instructive manner with this species is Trypano- body in a manner similar to the peristaltic movements of the soma brucii, which occurs as a natural parasite of buffaloes and intestine. other big game in Africa, and is, apparently, harmless to them, The body-substance of the Protozoa is protoplasm, or, as it

ch is capable of being transferred to other animals by was originally termed by Dujardin, sarcode, which is fine inoculation. The transference may take place naturally, by alveolar in structure, the diameter of the alveoli varying the bite of a tsetse-fly, or may be effected artificially; in either generally between } and id. At the surface of the body casc T. brucii is extremely lethal to certain animals, such as the alveoli may take on a definite honeycomb-like arrangeimported cattle, horses and dogs, or to rats and guinea-pigs. ment, forming a special “alveolar layer" which in optical Other animals, however, may be quite “repellent "! to this section appears radially striated. Besides the minute protoparasite, that is to say, if it be inoculated into their blood it dies plasmic alveoli, the protoplasm often shows a coarse vacuolaout without producing ill effects, just as T. lewisi does when tion throughout the whole or a part of its substance, giving injected into an animal other than a rat. Thus it is seen that the body a frothy structure. When such vacuoles are present T. brucii, when introduced into the blood of an animal which they must be carefully distinguished from the contractile vacuoles is specifically or racially distinct from its natural hosts in the and food-vacuoles described below; from the former they differ region where it is indigenous, is either unable to maintain itself by their non-contractile nature, and from the latter by not in its new host, or flourishes in it to such an extent as to be the containing food-substances. cause of its death.

In many Protozoa and especially in those forms in which there We may assume, therefore, at least as a working hypothesis, is no cuticle, the body may be supported by a skeleton. The that a lethal parasite is one that is new to its host, and that a material of the skeleton differs greatly in different cases, and harmless parasite is one long established. Since all parasites may be wholly of an organic nature, or may be impregnated with, must have been new to their proper hosts at some period, recent or almost entirely composed of, inorganic mineral salts, in or remote, in the history of the species, it would follow that the which case the skeletal substance is usually either silica or first commencement of parasitism would be in almost all cases carbonate of lime. From the morphological point of view a life and death struggle, as it were, between the two organisms the skeletons of Protozoa may be divided into two principal concerned, and it is quite conceivable that the host might classes, according as they are formed internal to, or external succumb in the struggle and so be exterminated. Ray Lankester to the body in each case. Instances of internal skeletons has suggested that the extinction of many species of animals are best scen in the spherical floating forms comprised in in the past may have been due, in some cases, to their having the orders Radiolaria and Heliozoa; such skeletons usually been attacked by a species of parasite to which they did not take the form of spicules, radiating from the centre to the succeed in becoming adapted, and by which they became, in circumference, and often further strengthened by the forconsequence, exterminated entirely.

mation of tangential bars, producing by their union a Organization of thc Protozoa.--The body-form may be constant | lattice-work, which in species of relatively large size may be or inconstant in the Protozoa, according as the body-substance formed periodically at the surface as the animal grows so that is or is not limited at the surface by a firm envelope or cuticle. the entire skeleton takes the form of concentric hollow When the surface of the protoplasm is naked, as in the common spheres held together by radiating beams. The architec. amoeba and allied organisms, the movements of the animal bring tural types of these skeletons show, however, an almost about continual changes of form. The protoplasm flows out infinite diversity, and cannot be summarized briefly. External at any point into processes termed pseudo podia, which are being skeletons have usually the form of a shell or house, into which continually retracted and formed anew. Such movements are the body can be retracted for protection, and from which the known as amoeboid, and may be seen in the cells of Metazoa protoplasm can issue forth during the animal's phases of activity. as well as in Protozoa. The pseudopodia serve both for locomo- Shells of this kind, which must be carefully distinguished from tion and for the capture of food. If equally developed on all cuticles or other membranes that invest the body closely, are sides of the body, the animal as a whole remains stationary, but well seen in the order Foraminifera; in the simplest cases they if formed more on one side than the other, the mass of the body are monaxon in architecture, that is to say, with one principal shifts its position in that direction, but the movement of transla- axis round which the shell is radially symmetrical, and at one tion is generally slow. If the animal remains perfectly quiescent pole is a large aperture through which the protoplasm can creep and inactive, the laws of surface-tension acting upon the semi-out. In addition to the principal aperture, the shell may or fluid protoplasmic body cause it to assume a simple spherical may not be pierced all over by numerous fine pores, through

which also the protoplasm can pass out. For further details The use of the terms " tolerant " and "repellent" is taken from the excellent article on " Sleeping Sickness," by E. Ray Lankester, in concerning these shells and their very numerous varieties of the Quarterly Review (July 1904), No. 399, pp. 113-138.

structure the reader is referred to the article FORAMINIFERA.

The protoplasmic body of the Protozoa is frequently differ- of the nuclear apparatus set apart as a distinct kinctic nucleus, entiated into two zones or regions: a more external, termed the with the function, apparently, of governing the activities of ectoplasm or ectosarc, and a more internal, termed the endo- the flagellum. plasm or endosarc. The ectosarc is distinguished by being Cilia are minute, hair-like extensions of the ectoplasm, which more clear and hyaline in appearance, and more tough and viscid pierce the cuticle and form typically a furry covering to the body, in consistence; the endoplasm, on the other hand, is more Though perhaps primitively derived from Nagella, cilia, in their granular and opaque, and of a more fluid nature. The ecto usual form, are distinguished from Nagella by being of smaller plasm is the protective layer of the body, and is also the portion size, by being present, as a rule, in much greater numbers, and most concerned in movement, in excretion, and perhaps also above all by the character of their movements. In the place in sensation and in functions similar to those persormed by the of the complicated lashing movements of the Nagella, each cilium nervous systems of higher animals. The endoplasm, on the performs a simple stroke in one direction, becoming first bowed other hand, is the chief seat of digestive and reproductive on one side, by an act of contraction, and then straightened functions.

out again when relaxed. The movements of the cilia are coAs the protective layer of the body, the ectoplasm forms ordinated and they act in concert, though not absolutely in the envelopes or membranes which invest the surface of the body, unison, each one contracting just before or after its neighbour, and which are differentiations of the outermost layer of the so that waves of movement pass over a ciliated surface in a ectoplasm. Thus in most Flagellata the ectoplasm is represented given direction, similar to what may be seen in a cornfield when only by the more or less firm outer covering or periplast. Even the wind is blowing over it. Primitively coating the whole when such envelopes are absent, however, the ectoplasm can surface of the body evenly, the cilia may become modified and still be seen to exert a protective function; as, for instance, in specialized in various ways, which cannot be described in detail those Myxosporidia which are parasitic in the gall-bladders or here (sce INFUSORIA). urinary bladders of their hosts, and which can resist the action Besides the organs of locomotion already mentioned, there o long as the ectoplasm is intact, I may be present so-called undulating membranes, in une 10

led undulating membranes, in the form but succumb to the action of the medium is the ectoplasm bc | of thin sheets of ectoplasm which are capable of performing injured. In many Infusoria the ectoplasm contains special sinuous, undulating movements by their inherent contractility. organs of offence termed trichocysts, each a minute ovoid body In some cases distinct contractile threads or myonemcs have from which, on stimulation, a thread is shot, out, in a manner been described in these membranes. Undulating membranes similar to the nematocysts of Coelenterata. Similar organs appear to be formed cither by the fusion together of a row of are seen also in the spores of Myxosporidia, as the so-called polar cilia, side by side, or by the attachment of a flagellum to the

psules: but in this case the organs are not specially ectoplasmic, body by means of an ectoplasmic web, in which c and appear to serve for adhesion and attachment, rather than lum, forms the free edge of the membrane, as in the genus for offence.

Trypanosoma. The connexion of the ectoplasm with movement is seen in the Returning to the ectoplasm, the excretory function exerted simplest forms, such as Amoeba, by the fact that all pseudopodia by this layer is seen by the formation in it of the peculiar conarise from it in the first instance. In forms with a definite tractile vacuoles found in most frec-living Protozoa. A concuticle, on the other hand, the ectoplasm usually contains tractile vacuole is a spherical drop of watery fluid which makes contractile fibres or myonemes, forming, as it were, the muscular its appearance periodically at some particular spot near the system of the organism. The dependence of the motility of surface of the animal's body, or, if more than one such vacuole the animal upon the development of the ectoplasm is well scen is present, at several definite and constant places. Each in Gregarines, in which other organs of locomotion are absent; | vacuole grows to a certain size, and when it has reached the in forms endowed with active powers of locomotion a distinct limit of its growth it discharges its contents to the exterior by ectoplasmic layer is present below the cuticle; in those Gregarines a sudden and rapid contraction. There is, apparently, in most incapable of active movement, on the other hand, the ectoplasm is not in all cases, a definite pore through which the contractile is absent or scarcely recognizable.

vacuole empties itself to the exterior. On account of the From the ectoplasm arise the special organs of locomotion, relatively large size which the contractile vacuole attains it which, when present, take the form of pseudopodia, flagella or bulges inwards beyond the limits of the ectoplasm and comes cilia. Pseudopodia, as already explained, are temporary proto to lie chiefly in the endoplasm, to which it is sometimes, but plasmic organs which can be extruded or retracted at any point; crroneously, ascribed. In the most highly differentiated Protozoa, they fall naturally into two principal types, between which, for instance, the Ciliata, the ectoplasm contains an apparatus however, transitions are to be found: first, slender, filamentous of excretory channels, situated in its deeper layers, and forming or filose pseudopodia, composed of ectoplasm alone, which may as it were a drainage-system, from which the contractile vacuoles remain separate from one another, or may anastomose to form are fed. The fluid discharged by the contractile vacuoles appears networks, and are then termed reticulose; secondly, thick, blunt, to be chiefly water which has been absorbed at the surface of so-called lobose pseudopodia, which are composed of ectoplasm the protoplasmic body, and which has filtered through the with a core of endoplasm, and never form networks. In forms protoplasm, taking up the soluble waste nitrogenous products showing active locomotor powers the pseudopodia are usually of the metabolism and the gaseous products of respiration; hence more lobose in type; filose pseudopodia, on the other hand, are the contractile vacuoles may be compared in a general way to more adapted for the function of capturing food.

the urinary and respiratory organs of the Metazoa. Flagella are long, slender, vibratile filaments, generally few One of the first consequences of the parasitic habit of life is in number when present, and usually placed at the pole of the the disappearance of the contractile vacuoles, which are hardly body which is anterior in progression. Each flagellum performs ever found in truly parasitic Protozoa, that is to say, in forms peculiar lashing movements which cause the body, is free, to which live in the interior of other animals and nourish thembe dragged along after the flagellum in jerks or leaps; is, however, selves at their expense. They are also very frequently absent the body be fixed, the action of the flagellum or flagella causes in marine forms. a current towards it, by which means the animal obtains its Mechanisms of a nervous nature are ve food-supply. A flagellum which is anterior in movement has Protozoa, but in some Ciliata special tactile bristles are found, been distinguished by Lankester by the convenient term and it is possible that flagella, and perhaps even pseudopodia, tracicllum; sometimes, however, the flagellum is posterior in may be sometimes tactile rather than locomotor in function. movement and acts as a propeller, like the tail of a fish; for this Pigment-spots, apparently sensitive to light, may also occur type Lankester has proposed the term pulsellum. The flagellum in some Flagellata. appears to arise in all cases from a distinct basal granule, and The endoplasm, as already stated, is the chief seat of nutritive in some cases, as in the genus Trypanosoma, there is a portion I and reproductive processes. In many Flagellata the ectoplasm

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