The Protozoa are then essentially unicellular animals. The individual or person in this grade of the animal kingdom is a single cell; and, although we find Protozoa which consist of aggregates of such cells, and are entitled to be called “multicellular,” yet an examination of the details of structure of these cell-aggregates and of their lifehistory establishes the fact that the cohesion of the cells in these instances is not an essential feature of the life of such multicellular Protozoa but a secondary and non-essential arrangement. Like the budded “persons” forming, when coherent to one another, undifferentiated “colonies” among the Polyps and Corals, the coherent cells of a compound Protozoon can be separated from one another and live independently; their cohesion has no economic significance. Each cell is precisely the counterpart of its neighbour; there is no common life, no distribution of function among special groups of the associated cells, and no cor- Γ3sponding differentiation of structure. As a contrast to this we find even in the simplest Enterozoa that the cells are functionally and structurally distinguishable into two groups—those which line the enteron or digestive cavity and those which form the outer body wall. The cells of these two layers are not interchangeable; they are fundamentally different in properties and structure from one another. The individual Enterozoon is not a single cell; it is an aggregate of a higher order consisting essentially of a digestive cavity around which two layers of cells are disposed. The individual Protozoon is a single cell; a number of these individuals may, as the result of the process of fission (cell-division), remain in contact with one another, but the compound individual which they thus originate has not a strong character. The constituent cells are still the more important individualities; they never become differentiated and grouped in distinct layers differing from one another in properties and structure; they never become subordinated to the individuality of the aggregate produced by their cohesion; hence we are justified in calling even these exceptional aggregated Protozoa unicellular.

By far the larger number of Protozoa are absolutely single isolated cells, which, whenever they duplicate themselves by that process of division common to these units of structure (whether existing as isolated organisms or as constituents of the tissues of plants or of animals), separate at once into two distinct individuals which move away from one another and are thenceforward strangers.

Whilst it is easy to draw the line between the Protozoa and the Enterozoa or Metazoa which lie above them, on account of the perfectly definite differentiation of the cells of the latter into two primary tissues, it is more difficult to separate the Protozoa from the parallel group of unicellular plants.

Theoretically there is no difficulty about this distinction. There is no doubt that organisms present themselves to us in two great series starting in both cases from simple unicellular forms. The one series, the plants, can take up the carbon, hydrogen, oxygen, and nitrogen necessary to build up their growing protoplasm from mineral compounds soluble in water, compounds which constitute the resting stage of those elements in the present physical conditions of our planet. Plants can take their nitrogen in the form of ammonia or in the form of nitrates and their carbon in the form of carbonic acid. Accordingly they require no mouths, no digestive apparatus; their food being soluble in water and diffusible, they absorb at all or many points of their surface. The spreading diffuse form of plants is definitely related to this fact. On the other hand the series of organisms which we distinguish as animals cannot take the nitrogen, necessary to build up their protoplasm, in a lower state of combination than it presents in the class of compounds known as albumens; nor can they take carbon in a lower state of combination than it presents when united with hvdroeen or with

hydrogen and oxygen to form fat, sugar, and starch. Albumens and fats are not soluble in water and diffusible; they have to be seized by the animal in the condition of more or less solid particles, and by chemical processes superinduced in the living protoplasm of the animal by the contact of these particles they are acted upon, chemically modified, and rendered diffusible. Hence the animal is provided with a mouth and a digestive cavity, and with organs of locomotion and prehension by which it may search out and appropriate its scattered nutriment. Further the albumens, fats, sugars, and starch which are the necessary food of an animal are not found in nature excepting as the products of the life of plants or of animals; accordingly all animals are in a certain sense parasitic upon either plants or other animals. It would therefore seem to be easy to draw the line between even the most minute unicellular plants and the similarly minute unicellular animals—assigning those which feed on the albumens, &c., of other organisms by means of a mouth and digestive apparatus to the animal series, and those which can appropriate the elements of ammonia, nitrates, and carbonates to the plants.

Such absolute distinctions lending themselves to sharp definitions have, however, no place in the organic world; and this is found to be equally true whether we attempt to categorically define smaller groups in the classification of plants and animals or to indicate the boundaries of the great primary division which those familiar names imply. Closely allied to plants which are highly and specially developed as plants, and feed exclusively upon ammonia, nitrates, and carbonates, we find exceptionally modified kinds which are known as “insectivorous plants” and are provided with digestive cavities (the pitchers of pitcherplants, &c.), and actually feed by acting chemically upon the albumens of insects which they catch in these digestive receptacles. No one would entertain for a moment the notion that these insectivorous plants should be considered as animals. The physiological definition separating plant from animal breaks down in their case; but the consideration of the probable history of their evolution as indicated by their various details of structure suffices at once to convince the most sceptical observer that they actually belong to the vegetable line of descent or family tree, though they have lost the leading physiological characteristic which has dominated the structure of other plants. In this extreme case it is made very obvious that in grouping organisms as plants or as animals we are not called upon to apply a definition but to consider the multifarious evidences of historical evolution. And we find in the case of the Protozoa and the Protophyta that the same principle holds good, although, when dealing with extremely simple forms, it becomes much more difficult to judge of the genetic relationship of an organism in proportion as the number of detailed points of possible agreement with and divergence from other forms to which it may be supposed to be related are few.

The feeding of plants upon carbonic acid is invariably accompanied by the presence of a peculiar green-colouring matter—chlorophyll. In virtue of some direct or indirect action of this chlorophyll the protoplasm of the plant is enabled to seize the carbon of the mineral world—the carbon which has sunk to the lowest resting stage of combination—and to raise it into combination with hydrogen and oxygen and ultimately with nitrogen. There are plants which have no chlorophyll and are thus unable to feed upon carbonic acid. They are none the less plants since they agree closely with particular chlorophyll-bearing plants in details of form and structure, mode of growth and reproduction. A large series of these are termed Fungi. Though unable to feed on carbonic acid, they do not feed as do animals. They can take their carbon from acetates and tartrates, which animals cannot do, and their nitrogen from ammonia. Even when it is admitted that some of these colourless plants, such as the Bacteria (Schizomycetes), can act upon albumens so as to digest them and thus nourish themselves, it is not reasonable to place the Bacteria among animals, any more than it would be reasonable so to place Nepenthes, Sarracenia, and Drosera (insectivorous Phanerogams). For the structure and mode of growth of the Bacteria is like that of well-known chlorophylligerous minute Algae from which they undoubtedly differ only in having secondarily acquired this peculiar mode of nutrition, distinct from that which has dominated and determined the typical structure of plants.

So we find in a less striking series of instances amongst animals that here and there the nutritional arrangements which we have no hesitation in affirming to be the leading characteristic of animals, and to have directly and perhaps solely determined the great structural features of the animal line of descent, are largely modified or even altogether revolutionized. The green Hydra, the freshwater Sponge, and some Planarian worms produce chlorophyll corpuscles in the protoplasm of their tissues just as green plants do, and are able in consequence to do what animals usually cannot do—namely, feed upon carbonic acid. The possibilities of the protoplasm of the plant and of the animal are, we are thus reminded, the same. The fact that characteristically and typically plant protoplasm exhibits one mode of activity and animal protoplasm another does not prevent the protoplasm of even a highly developed plant from asserting itself in the animal direction, or of a thoroughly characterized animal, such as the green Hydra, from putting forth its chlorophylligenous powers as though it belonged to a plant.

Hence it is not surprising that we find among the Protozoa, notwithstanding that they are characterized by the animal method of nutrition and their forms determined by the exigencies of that method, occasional instances of partial vegetable nutrition such as is implied by the development of chlorophyll in the protoplasm of a few members of the group. It would not be inconsistent with what is observed in other groups should we find that there are some unicellular organisms which must, on account of their structural resemblances to other organisms, be considered as Protozoa and yet have absolutely given up altogether the animal mode of nutrition (by the ingestion of solid albumens) and have acquired the vegetable mode of absorbing ammonia, nitrates, and carbonic acid. Experiment in this matter is extremely difficult, but such “vegetable” or “holophytic nutrition” appears to obtain in the case of many of the green Flagellata, of the Dinoflagellata, and possibly of other Protozoa.

On the other hand there is no doubt that we may fall into an error in including in the animal line of descent all unicellular organisms which nourish themselves by the inception of solid nutriment. It is conceivable that some of these are exceptional creophagous Protophytes parallel at a lower level of structure to the insectivorous Phanerogams. In all cases we have to balance the whole of the evidence and to consider probabilities as indicated by a widely-reaching consideration of numerous facts.

The mere automatic motility of unicellular organisms was at one time considered sufficient indication that such organisms were animals rather than plants. We now know that not only are the male reproductive cells of ferns and similar plants propelled by vibratile protoplasm, but such locomotive particles are recognized as common products (“swarm-spores” and “zoospores ”) of the lowest plants.

The danger of dogmatizing erroneously in distinguishing Protozoa from Protophyta, and the insuperable difficulty in really accomplishing the feat satisfactorily, has led at various times to the suggestion that the effort should be abandoned and a group constituted confessedly containing both unicellular plants and unicellular animals and those organisms which may be one or the other. Haeckel has proposed to call this group the Protista (I). On the whole, it is more satisfactory to make the attempt to discriminate those unicellular forms which belong to the animal line of descent from those belonging to the vegetable line. It is, after all, not a matter of much consequence if the botanist should mistakenly claim a few Protozoa as plants and the zoologist a few Protophyta as animals. The evil which we have to avoid is that some small group of unattractive character should be rejected both by botanist and zoologist and thus our knowledge of it should unduly lag. Bearing this in mind the zoologist should accord recognition as Protozoa to as wide a range of unicellular organisms as he can without doing violence to his conceptions of probability.

 very interesting and very difficult subject of speculation forces itself on our attention when we attempt to draw the line between the lowest plants and the lowest animals, and even comes again before us when we pass in review the different forms of Protozoa.

That subject is the nature of the first protoplasm which was evolved from not-living matter on the earth’s surface. Was that first protoplasm more like animal or more like vegetable protoplasm as we know it to-day? By what steps was it brought into existence?

Briefly stated the present writer’s view is that the earliest protoplasm did not possess chlorophyll and therefore did not possess the power of feeding on carbonic acid. A conceivable state of things is that a vast amount of albuminoids and other such compounds had been brought into existence by those processes which culminated in the development of the first protoplasm, and it seems therefore likely enough that the first protoplasm fed upon these antecedent steps in its own evolution just as animals feed on organic compounds at the present day, more especially as the large creeping plasmodia of some Mycetozoa feed, on vegetable refuse. It indeed seems not at all improbable that, apart from their elaborate fructification, the Mycetozoa represent more closely than any other living forms the original ancestors of the whole organic world. At subsequent stages in the history of this archaic living matter chlorophyll was evolved and the power of taking carbon from carbonic acid. The “green” plants were rendered possible by the evolution of chlorophyll, but through what ancestral forms they took origin or whether more than once, i.e., by more than one branch, it is difficult even to guess. The green Flagellate Protozoa (Volvocineae) certainly furnish a connecting point by which it is possible to link on the pedigree of green plants to the primitive protoplasm; it is noteworthy that they cannot be considered as very primitive and are indeed highly specialized forms as compared with the naked protoplasm of the Mycetozoon’s plasmodium.

Thus then we are led to entertain the paradox that though the animal is dependent on the plant for its food yet the animal preceded the plant in evolution, and we look among the lower Protozoa and not among the lower Protophyta for the nearest representatives of that first protoplasm which was the result of a long and gradual evolution of chemical structure and the starting point of the development of organic form.

The Protozoon Cell-Individual compared with the Typical Cell of Animal and Vegetable Tissues. Morphology.

The Protozoon individual is a single corpuscle of protoplasm, varying in size when adult from less than the ττ> 1 σt7^ °f an i nc h in diameter (some Sporozoa and Flagellata) up to a diameter of an inch (Nummulites), and even much larger size in the plasmodia of Mycetozoa. The substance of the Protozoa exhibits the same general properties —irritability, movement, assimilation, growth, and division —and the same irremediable chemical alteration as the result of exposure to a moderate heat, which are observed in the protoplasm constituting the corpuscles known as cells which build up the tissues of the larger animals and plants. There is therefore no longer any occasion to make use of the word “sarcode” which before this identity was established was very usefully applied by Dujardin (2) to the substance which mainly forms the bodies of the Protozoa. Like the protoplasm which constitutes the “cells” of the Enterozoa and of the higher plants, that of the Protozoon body is capable of producing, by chemical processes which take place in its substance (over and above those related merely to its nutrition), a variety of distinct chemical compounds, which may form a deposit in or beyond the superficial protoplasm of the corpuscle or may accumulate centrally. These products are therefore either ectoplastic or entoplastic. The chemical capacities of protoplasm thus exhibited are very diverse, ranging from the production of a denser variety of protoplasm, probably as the result of dehydration, such as we see in the nucleus and in the cortical substance of many cells, to the chemical separation and deposition of membranes of pure chitin or of cellulose or of shells of pure calcium carbonate or quasicrystalline needles of silica.

Nucleus. —The nucleus is probably universally present in the Protozoon cell, although it may have a very simple structure and be of very small size in some cases. The presence of a nucleus has recently been demonstrated by means of appropriate staining reagents in some Protozoa (shellbearing Reticularia or Foraminifera and many Mycetozoa) where it had been supposed to be wanting, but we are not yet justified in concluding absolutely that there are not some few Protozoa in which this central differentiation of the protoplasm does not exist; it is also a fact that in the young forms of some Protozoa which result from the breaking up of the body of the parent into many small “spores” there is often no nucleus present.

In contrast to this it is the fact that the cells which build up the tissues of the Enterozoa are all derived from the division of a nucleated egg-cell and the repeated division of its nucleated products, and are invariably nucleated. The same is true of tissue-forming plants,— though there are a few of the lowest plants, such as the Bacteria, the protoplasm of which presents no nucleus. In spite of recent statements (3) it cannot be asserted that the cells or protoplasmic corpuscles of the yeast-plant (Saccharomyces) and of the hyphae of many simple moulds contain a true nucleus. We are here brought to the question “What is a true nucleus ?” The nucleus which is handed on from the egg-cell of higher plants and Enterozoa to the cells derived from it by fission has lately been shown to possess in a wide variety of instances such very striking characteristics that we may well question whether every more or less distinctly outlined mass or spherule of protoplasm which can be brought into view by colouring or other reagents, within the protoplasmic body of a Protozoon or a Protophyte, is necessarily to be considered as quite the same thing as the nucleus of tissueforming egg-cell-derived cells.

Researches, chiefly due to Flemming (4), have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of “achromatin” not deeply stained by reagents, ramifying in which is a reticulum of “chromatin” consisting of fibres which readily take a deep stain (Fig. I., A). Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in an, arrangement of its fibres first in the form of a wreath (Fig. I., D) and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed

as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of loops of chromatin is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin substance precedes the separation of the loops (G). The striated achromatin is then termed a “nucleusspindle,” and the group of chromatin loops (Fig. I., G, α) is known as “the equatorial plate.” At...