THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge; Superintendent of the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge; University Lecturer on the Morphology of Invertebrates

VOLUME I

PROTOZOA

PORIFERA (SPONGES)

COELENTERATA & CTENOPHORA

ECHINODERMATA

London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1906

All rights reserved

CONTENTS

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

The names of extinct groups are printed in italics.

PROTOZOA

BY

MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)

Professor of Natural History in the Queen's College, Cork.

CHAPTER I

PROTOZOA—INTRODUCTION—FUNCTIONS OF PROTOPLASM—CELL-DIVISION—ANIMALS AND PLANTS

The Free Amoeboid Cell.—If we examine under the microscope a fragment of one of the higher animals or plants, we find in it a very complex structure. A careful study shows that it always consists of certain minute elements of fundamentally the same nature, which are combined or fused into "tissues." In plants, where these units of structure were first studied, and where they are easier to recognise, each tiny unit is usually enclosed in an envelope or wall of woody or papery material, so that the whole plant is honeycombed. Each separate cavity was at first called a "cell"; and this term was then applied to the bounding wall, and finally to the unit of living matter within, the envelope receiving the name of "cell-wall." In this modern sense the "cell" consists of a viscid substance, called first in animals "sarcode" by Dujardin (1835), and later in plants "protoplasm"^[1] by Von Mohl (1846). On the recognition of its common nature in both kingdoms, largely due to Max Schultze, the latter term prevailed; and it has passed from the vocabulary of biology into the domain of everyday life. We shall now examine the structure and behaviour of protoplasm and of the cell as an introduction to the detailed study of the Protozoa, or better still Protista,^[2] the lowest types of living beings, and of Animals at large.

It is not in detached fragments of the tissues of the higher animals that we can best carry on this study: for here the cells are in singularly close connexion with their neighbours during life; the proper appointed work of each is intimately related to that of the others; and this co-operation has so trained and specially modified each cell that the artificial severance and isolation is detrimental to its well-being, if not necessarily fatal to its very life. Again, in plants the presence of a cell-wall interferes in many ways with the free behaviour of the cell. But in the blood and lymph of higher animals there float isolated cells, the white corpuscles or "leucocytes" of human histology, which, despite their minuteness (1⁄3000 in. in diameter), are in many respects suitable objects. Further, in our waters, fresh or salt, we may find similar free-living individual cells, in many respects resembling the leucocytes, but even better suited for our study. For, in the first place, we can far more readily reproduce under the microscope the normal conditions of their life; and, moreover, these free organisms are often many times larger than the leucocyte. Such free organisms are individual Protozoa, and are called by the general term "Amoebae." A large Amoeba may measure in its most contracted state 1⁄100 in. or 250 µ in diameter,^[3] and some closely allied species (Pelomyxa, see p. 52) even twelve times this amount. If we place an Amoeba or a leucocyte under the microscope (Fig. 1), we shall find that its form, at first spherical, soon begins to alter. To confine our attention to the external changes, we note that the outline, from circular, soon becomes "island-shaped" by the outgrowth of a promontory here, the indenting of a bay there. The promontory may enlarge into a peninsula, and thus grow until it becomes a new mainland, while the old mainland dwindles into a mere promontory, and is finally lost. In this way a crawling motion is effected.^[4] The promontories are called "pseudopodia" (= {5}"false-feet"), and the general character of such motion is called "amoeboid."^[5]

Fig. 1.—Amoeba, showing clear ectoplasm, granular endoplasm, dark nucleus, and lighter contractile vacuole. The changes of form, a-f, are of the A. limax type; g, h, of the A. proteus type. (From Verworn.)

The living substance, protoplasm,^[6] has been termed a "jelly," a word, however, that is quite inapplicable to it in its living state. It is viscid, almost semi-fluid, and may well be compared to very soft dough which has already begun to rise. It resembles it in often having a number of spaces, small or large, filled with liquid (not gas). These are termed "vacuoles" or "alveoles," according to their greater or their lesser dimensions. In some cases a vacuole is traversed by strands of plasmic substance, just as we may find such strands stretching across the larger spaces of a very light loaf; but of course in the living cell these are constantly undergoing changes. If we "fix" a cell (i.e. kill it by {6}sudden heat or certain chemical coagulants),^[7] and examine it under the microscope, the intermediate substance between the vacuoles that we have already seen in life is again found either to be finely honeycombed or else resolved into a network like that of a sponge. The former structure is called a "foam" or "alveolar" structure, the latter a "reticulate" structure. The alveoles are about 1 µ in diameter, and spheroidal or polygonal by mutual contact, elongated, however, radially to any free surface, whether it be that of the cell itself or that of a larger alveole or vacuole. The inner layer of protoplasm ("endoplasm," "endosarc") contains also granules of various nature, reserve matters of various kinds, oil-globules, and particles of mineral matter^[8] which are waste products, and are called "excretory." In fixed specimens these granules are seen to occupy the nodes of the network or of the alveoli, that is, the points where two or three boundaries meet.^[9] The outermost layer ("ectoplasm" or "ectosarc") appears in the live Amoeba structureless and hyaline, even under conditions the most favourable for observation. The refractive index of protoplasm, when living, is always well under 1.4, that of the fixed and dehydrated substance is slightly over 1.6.

Again, within the outer protoplasm is found a body of slightly higher refractivity and of definite outline, termed the "nucleus" (Figs. 1, 2). This has a definite "wall" of plasmic nature, and a substance so closely resembling the outer protoplasm in character, that we call it the "nucleoplasm" (also "linin"), distinguishing the outer plasm as "cytoplasm"; the term "protoplasm" including both. Within the nucleoplasm are granules of a substance that stains well with the commoner dyes, especially the "basic" ones, and which has hence been called "chromatin." The linin is {7}usually arranged in a distinct network, confluent into a "parietal layer" within the nuclear wall; the meshes traversing a cavity full of liquid, the nuclear sap, and containing in their course the granules; while in the cavity are usually found one or two droplets of a denser substance termed "nucleoles." These differ slightly in composition from the chromatin granules^[10] (see p. 24 f.).

The movements of the leucocyte or Amoeba are usually most active at a temperature of about 40° C. or 100° F., the "optimum." They cease when the temperature falls to a point, the "minimum," varying with the organism, but never below freezing-point; they recommence when the temperature rises again to the same point at which they stopped. If now the temperature be raised to a certain amount above 40° they stop, but may recommence if the temperature has not exceeded a certain point, the "maximum" (45° C. is a common maximum). If it has been raised to a still higher point they will not recommence under any circumstances whatever.

Again, a slight electric shock will determine the retraction of all processes, and a period of rest in a spherical condition. A milder shock will only arrest the movements. But a stronger shock may arrest them permanently. We may often note a relation of the movements towards a surface, tending to keep the Amoeba in contact with it, whether it be the surface of a solid or that of an air-bubble in the liquid (see also p. 20).

Fig. 2.—Ovum of a Sea-Urchin, showing the radially striated cell-membrane, the cytoplasm containing yolk-granules, the large nucleus (germinal vesicle), with its network of linin containing chromatin granules, and a large nucleole (germinal spot). (From Balfour's Embryology, after Hertwig.)

If a gentle current be set up in the water, we find that the movements of the Amoeba are so co-ordinated that it moves upstream; this must of course be of advantage in nature, as keeping the being in its place, against the streams set up by larger creatures, etc. (see also p. 21).

If substances soluble in water be introduced the Amoeba will, {8}as a rule, move away from the region of greater concentration for some substances, but towards it (provided it be not excessive) for others. (See also pp. 22, 23.) We find, indeed, that there is for substances of the latter category a minimum of concentration, below which no effect is seen, and a maximum beyond which further concentration repels. The easiest way to make such observations is to take up a little strong solution in a capillary tube sealed at the far end, and to introduce its open end into the water, and let the solution diffuse out, so that this end may be regarded as surrounded by zones of continuously decreasing strength. In the process of inflammation (of a Higher Animal) it has been found that the white corpuscles are so attracted by the source of irritation that they creep out of the capillaries, and crowd towards it.

We cannot imagine a piece of dough exhibiting any of these reactions, or the like of them; it can only move passively under the action of some one or other of the recognised physical forces, and that only in direct quantitative relation to the work that such forces can effect; in other words, the dough can have work done on it, but it cannot do work. The Amoeba or leucocyte on the contrary does work. It moves under the various circumstances by the transformation of some of its internal energy from the "potential" into the "kinetic" state, the condition corresponding with this being essentially a liberation of heat or work, either by the breaking down of its internal substances, or by the combination of some of them with oxygen.^[11] Such of these changes as involve the excretion of carbonic acid are termed "respiratory."

This liberation of energy is the "response" to an action of itself inadequate to produce it; and has been compared not inaptly to the discharge of a cannon, where foot-tons of energy are liberated in consequence of the pull of a few inch-grains on the trigger, or to an indefinitely small push which makes electric contact: the energy set free is that which was stored up in the charge. This capacity for liberating energy stored up within, in response to a relatively small impulse from without, is termed "irritability"; the external impulse is termed the "stimulus." The responsive act has been termed "contractility," because it so often means an obvious contraction, but is better termed {9}"motility "; and irritability evinced by motility is characteristic of all living beings save when in the temporary condition of "rest."

Again, in the case of the cannon, the gunner after its discharge has to replenish it for future action with a fresh cartridge; the Amoeba or leucocyte can replenish itself—it "feeds." When it comes in contact with a fragment of suitable material, it enwraps it by its pseudopodia (Fig. 3), and its edges coalesce where they touch on the far side as completely as we can join up the edges of dough round the apple in a dumpling. It dissolves all that can be dissolved—i.e. it "digests" it, and then absorbs the dissolved material into its substance, both to replace what it has lost by its previous activity and to supply fuel for future liberation of energy; this process is termed "nutrition," and is another characteristic of living beings.

Fig. 3.—Amoeba devouring a plant cell; four successive stages of ingestion. (From Verworn.)

Again, as a second result of the nutrition, part of the food taken in goes to effect an increase of the living protoplasm, and that of every part, not merely of the surface—it is "assimilated"; while the rest of the food is transformed into reserves, or consumed and directly applied to the liberation of energy. The increase in bulk due to nutrition is thus twofold: part is the increase of the protoplasm itself—"assimilative growth," part is the storage of reserves—"accumulative growth": these reserves being available in turn by digestion, whether for future true growth or for consumption to liberate energy for the work of the cell.

We can conceive that our cannon might have an automatic feed for the supply of fresh cartridges after each shot; but not that it could make provision for an increase of its own bulk, so as to gain in calibre and strength, nor even for the restoration {10}of its inner surface constantly worn away by the erosion of its discharges. Growth—and that growth "interstitial," operating at every point of the protoplasm, not merely at its surface—is a character of all living beings at some stage, though they may ultimately lose the capacity to grow. Nothing at all comparable to interstitial growth has been recognised in not-living matter.^[12]

Fig. 4.—Amoeba polypodia in successive stages of equal fission; nucleus dark, contractile vacuole clear. (From Verworn, after F. E. Schulze.)

Again, when an Amoeba has grown to a certain size, its nucleus divides into two nuclei, and its cytoplasmic body, as we may term it, elongates, narrows in the middle so as to assume the shape of a dumb-bell or finger-biscuit, and the two halves, crawling in opposite directions, separate by the giving way of the connecting waist, forming two new Amoebas, each with its nucleus (Fig. 4). This is a process of "reproduction"; the special case is one of "equal fission" or "binary division." The original cell is termed the "mother," with respect to the two new ones, and these are of course with respect to it the "daughters," and {11}"sisters" to one another. We must bear in mind that in this self-sacrificing maternity the mother is resolved into her children, and her very existence is lost in their production. The above phenomena, IRRITABILITY, MOTILITY, DIGESTION, NUTRITION, GROWTH, REPRODUCTION, are all characteristic of living beings at some stage or other, though one or more may often be temporarily or permanently absent; they are therefore called "vital processes."

If, on the other hand, we violently compress the cell, if we pass a very strong electric shock through it, or a strong continuous current, or expose it to a temperature much above 45° C., or to the action of certain chemical substances, such as strong acids or alkalies, or alcohol or corrosive sublimate, we find that all these vital processes are arrested once and for all; henceforward the cell is on a par with any not-living substance. Such a change is called "DEATH," and the "capacity for death" is one of the most marked characters of living beings. This change is associated with changes in the mechanical and optical properties of the protoplasm, which loses its viscidity and becomes opaque, having undergone a process of de-solution; for the water it contained is now held only mechanically in the interstices of a network, or in cavities of a honeycomb (as we have noted above, p. 5), while the solid forming the residuum has a refractive index of a little over 1.6. Therefore, it only regains its full transparency when the water is replaced by a liquid of high refractive index, such as an essential oil or phenol. A similar change may be effected by pouring white of egg into boiling water or absolute alcohol, and is attended with the same optical results. The study of the behaviour of coagulable colloids has been recently studied by Fischer and by Hardy, and has been of the utmost service in our interpretation of the microscopical appearances shown in biological specimens under the microscope.^[13]

The death of the living being finds a certain analogy in the breaking up or the wearing out of a piece of machinery; but in no piece of machinery do we find the varied irritabilities, all conducive to the well-being of the organism (under ordinary conditions), or the so-called "automatic processes"^[14] that enable the living being to go through its characteristic functions, to grow, and as we shall see, even to turn conditions unfavourable for active life and growth to the ultimate weal of the species (see p. 32). At the same time, we fully recognise that for supplies of matter and energy the organism, like the machine, depends absolutely on sources from without. The debtor and creditor sheet, in respect of matter and energy, can be proved to balance between the outside world and Higher Organisms with the utmost accuracy that our instruments can attain; and we infer that this holds for the Lower Organisms also. Many of the changes within the organism can be expressed in terms of chemistry and physics; but it is far more impossible to state them all in such terms than it would be to describe a polyphase electrical installation in terms of dynamics and hydraulics. And so far at least we are justified in speaking of "vital forces."

The living substance of protoplasm contains a large quantity of water, at least two-thirds its mass, as we have seen, in a state of physical or loose chemical combination with solids: these on death yield proteids and nucleo-proteids.^[15] The living protoplasm {13}has an alkaline reaction, while the liquid in the larger vacuoles, at least, is acid, especially in Plant-cells.^[16]

Metabolism.—The chemical processes that go on in the organism are termed metabolic changes, and were roughly divided by Gaskell into (1) "anabolic," in which more complex and less stable substances are built up from less complex and more stable ones with the absorption of energy; and (2) "catabolic" changes in which the reverse takes place. Anabolic processes, in all but the cells containing plastids or chromatophores (see p. 36) under the influence of light, necessarily imply the furnishing of energy by concurrent catabolic changes in the food or reserves, or in the protoplasm itself.

Again, we have divided anabolic processes into "accumulative," where the substances formed are merely reserves for the future use of the cell, and "assimilative," where the substances go to the building of the protoplasm itself, whether for the purpose of growth or for that of repair.

Catabolic processes may involve (1) the mere breaking of complex substances into simpler ones, or (2) their combination with oxygen; in either case waste products are formed, which may either be of service to the organism as "secretions" (like the bile in Higher Animals), or of no further use (like the urine). When nitrogenous substances break down in this way they give rise to "excretions," containing urea, urates, and allied substances; other products of catabolism are carbon dioxide, water, and mineral salts, such as sulphates, phosphates, carbonates, oxalates, etc., which if not insoluble must needs be removed promptly from the organism, many of them being injurious or even poisonous. The energy liberated by the protoplasm being derived through the breakdown of another part of the same or of the {14}food-materials or stored reserves, must give rise to waste products. The exchange of oxygen from without for carbonic acid formed within is termed "respiration," and is distinguished from the mere removal of all other waste products called "excretion." In the fresh-water Amoeba both these processes can be studied.

Respiration,^[17] or the interchange of gases, must, of course, take place all over the general surface, but in addition it is combined in most fresh-water Protista with excretion in an organ termed the "contractile" or "pulsatile vacuole" (Figs. 1, 4, etc.). This particular vacuole is exceptional in its size and its constancy of position. At intervals, more or less regular, it is seen to contract, and to expel its contents through a pore; at each contraction it completely disappears, and reforms slowly, sometimes directly, sometimes by the appearance of a variable number of small "formative" vacuoles that run together, or as in Ciliata, by the discharge into it of so-called "feeding canals." As this vacuole is filled by the water that diffuses through the substance, and when distended may reach one-third the diameter of the being, in the interval between two contractions an amount of water must have soaked in equal to one-twenty-seventh the bulk of the animal, to be excreted with whatever substances it has taken up in solution, including, not only carbon dioxide, but also, it has been shown, nitrogenised waste matters allied to uric acid.^[18]

That the due interchanges may take place between the cell and the surrounding medium, it is obvious that certain limits to the ratio between bulk and surface must exist, which are disturbed by growth, and which we shall study hereafter (p. 23 f.).

The Protista that live in water undergo a death by "diffluence" or "granular disintegration" on being wounded, crushed, or sometimes after an excessive electric stimulation, or contact with alkalies or with acids too weak to coagulate them. In this process the protoplasm breaks up from the surface inwards into a mass of granules, the majority of which themselves finally dissolve. If the injury be a local rupture of the external pellicle or {15}cuticle, a vacuole forms at the point, grows and distends the overlying cytoplasm, which finally ruptures: the walls of the vacuole disintegrate; and this goes on as above described. Ciliate Infusoria are especially liable to this disintegration process, often termed "diffluence," which, repeatedly described by early observers, has recently been studied in detail by Verworn. Here we have death by "solution," while in the "fixing" of protoplasm for microscopic processes we strive to ensure death by "desolution," so as to retain as much of the late living matter as possible. It would seem not improbable that the unusual contact with water determines the formation of a zymase that acts on the living substance itself.

We have suggested^[19] that one function of the contractile vacuole, in naked fresh-water Protists, is to afford a regular means of discharge of the water constantly taken up by the crystalloids in the protoplasm, and so to check the tendency to form irregular disruptive vacuoles and death by diffluence. This is supported by the fact that in the holophytic fresh-water Protista, as well as the Algae and Fungi, a contractile vacuole is present in the young naked stage (zoospore), but disappears as soon as an elastic cell-wall is formed to counterbalance by its tension the internal osmotic pressure.

Digestion is always essentially a catabolic process, both as regards the substance digested and the formation of the digesting substance by the protoplasm. The digesting substance is termed a "zymase" or "chemical ferment," and is conjectured to be produced by the partial breakdown of the protoplasm. In presence of suitable zymases, many substances are resolved into two or more new substances, often taking up the elements of water at the same time, and are said to be "dissociated" or "hydrolysed" as the case may be. Thus proteid substances are converted into the very soluble substances, "proteoses" and "peptones," often with the concurrent or ultimate formation of such relatively simple bodies as leucin, tyrosin, and other amines, etc. Starch and glycogen are converted into dextrins and sugars; fats are converted into fatty acids and glycerin. It is these products of digestion, and not the actual food-materials (save certain very simple sugars), that are really taken up by the protoplasm, {16}whether for assimilation, for accumulation, or for the direct liberation of energy for the vital processes of the organism.

Not only food from without, but also reserves formed and stored by the protoplasm itself, must be digested by some zymase before they can be utilised by the cell. In all cases of the utilisation of reserve matter that have been investigated, it has been found that a zymase is formed by the cell itself (or sometimes, in complex organisms, by its neighbours); for, after killing the cell in which the process is going on by mechanical means or by alcohol, the process of digestion can be carried on in the laboratory.^[20] The chief digestion of all the animal-feeding Protista is of the same type as in our own stomachs, known as "peptic" digestion: this involves the concurrent presence of an acid, and Le Dantec and Miss Greenwood have found the contents of food-vacuoles, in which digestion is going on, to contain acid liquid. The ferment-pepsin itself has been extracted by Krukenberg from the Myxomycete, "Flowers of tan" (Fuligo varians, p. 92), and by Professor Augustus Dixon and the author from the gigantic multinucleate Amoeba, Pelomyxa palustris (p. 52).^[21] The details of the prehension of food will be treated of under the several groups.

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

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

The movements of an Amoeboid^[22] cell are of two kinds: "expansion," leading to the formation and enlargement of {17}outgrowths, and "contraction," leading to their diminution and disappearance within the general surface.^[23] Expansion is probably due to the lessening of the surface-tension at the point of outgrowth, contraction to the increase of surface-tension. Verworn regards these as due respectively to the combination of the oxygen in the medium with the protoplasm in diminishing surface-tension, and the effect of combination with substances from within, especially from the nucleus in increasing it. Besides these external movements, there are internal movements revealed by the contained granules, which stream freely in the more fluid interior. Those Protista that, while exhibiting amoeboid movements, have no clear external layer, such as the Radiolaria, Foraminifera, Heliozoa, etc., present this streaming even at the surface, the granules travelling up and down the pseudopodia at a rate much greater than the movements of these organs themselves. In this case the protoplasm is wetted by the medium, which it is not where there is a clear outer layer: for that behaves like a greasy film.

Motile organs.—Protoplasm often exhibits movements much more highly specialised than the simple expansion or retraction of processes, or the general change of form seen in Amoeba. If we imagine the activities of a cell concentrated on particular parts, we may well suppose that they would be at once more precise and more energetic than we see them in Amoeba or the leucocyte. In some free-swimming cells, such as the individual cells known as "Flagellata," the reproductive cells of the lower Plants, or the male cells ("spermatozoa") of Plants as high as Ferns, and even of the Highest Animals, there is an extension of the cell into one or more elongated lash-like processes, termed "flagella," which, by beating the water in a reciprocating or a spiral rhythm, cause the cell to travel through it; or, if the cell be attached, they produce currents in the water that bring food particles to the surface of the cell for ingestion. Such flagella may, indeed, be seen in some cases to be modified pseudopodia. In other cases part, or the whole, of the surface of the cell may be covered with regularly arranged short filaments of similar activity (termed "cilia," from their resemblance to a diminutive eyelash), which, however, instead of whirling round, bend sharply {18}down to the surface and slowly recover; the movement affects the cilia successively in a definite direction in waves, and produces, like that of flagella, either locomotion of the cell or currents in the medium. We can best realise their action by recalling the waves of bending and recovery of the cornstalks in a wind-swept field; if now the haulms of the corn executed these movements of themselves, they would determine in the air above a breeze-like motion in the direction of the waves (Fig. 5).^[24] Such cilia are not infrequent on those cells of even the Highest Animals that, like a mosaic, cover free surfaces ("epithelium cells"). In ourselves such cells line, for instance, the windpipe. One group of the Protozoa, the "Ciliata," are, as their name implies, ciliated cells pure and simple.

Fig. 5.—Motion of a row of cilia, in profile. (From Verworn.)

The motions of cilia and of flagella are probably also due to changes of surface tension—alternately on one side and the other in the cilium, but passing round in circular succession in the flagellum,^[25] giving rise to a conical rotation like that of a weighted string that is whirled round the head. This motion is, however, strongest at the thicker basal part, which assumes a spiral form like a corkscrew of few turns, while the thin lash at the tip may seem even to be quietly extended like the point of the corkscrew. If the tip of the flagellum adhere, as it sometimes does, to any object, the motions induce a jerking motion, which in this case is reciprocating, not rotatory. When the organism is free, the flagellum is usually in advance, and the cell follows, rotating at the same time round its longitudinal axis; such an anterior flagellum, called a "tractellum," is the common form in Protista that possess a single one (Figs. 29, 7, 8; 30, C). In the spermatozoa of Higher Animals (and some Sporozoa) the flagellum is posterior, and is called a "pulsellum."

The cilium or flagellum may often be traced a certain distance into the substance of the cytoplasm to end in a dot of denser, {19}readily-staining plasm, which corresponds to a "centrosome" or centre of plasmic forces (see below, pp. 115, 121, 141); it has been termed a "blepharoplast."^[26]

Again, the cytoplasm may have differentiated in it definite streaks of specially contractile character; such streaks within its substance are called "myonemes"; they are, in fact, muscular fibrils. A "muscle-cell," in the Higher Animals, is one whose protoplasm is almost entirely so modified, with the exception of a small portion of granular cytoplasm investing the nucleus, and having mainly a nutritive function.

Definite muscular fibrils in action shorten, and at the same time become thicker. It seems probable that they contain elongated vacuoles, and that the contents of these vary, so that when they have an increased osmotic equivalent, the vacuoles absorb water, enlarge, and tend to become more spherical, i.e. shorter and thicker, and so the fibril shortens as a whole. The relaxation would be due to the diffusion outwards of the solution of the osmotically active substances which induced expansion.^[27]

The Motile Reactions of the Protozoa^[28] require study from another point of view: they are either (1) "spontaneous" or "arbitrary," as we may say, or (2) responsive to some stimulus. The latter kind we will take first, as they are characteristic of all free cells. The stimuli that induce movements of a responsive character are as follows:—(i.) MECHANICAL: such as agitation and contact; (ii.) force of GRAVITY, or CENTRIFUGAL FORCE; (iii.) CURRENTS in the water; (iv.) RADIANT ENERGY (LIGHT); (v.) changes in the TEMPERATURE of the medium; (vi.) ELECTRIC CURRENTS through the medium; (vii.) the presence of CHEMICAL SUBSTANCES in the medium.

These, or some of them, may induce one of three different results, or a combination thereof: (1) a single movement or an arrest of motion; (2) the assumption of a definite position; (3) movement of a definite character or direction.

(i.) Mechanical stimuli.—Any sudden touch with another body tends to arrest all motion; and if the shock be protracted or severe, the retraction of the pseudopodia follows. It is to this reaction that we must ascribe the retracted condition of the pseudopodia of most Rhizopods when first placed on the slide and covered for microscopic examination. Free-swimming Protista may, after hitting any body, either remain in contact with it, or else, after a pause, reverse their movement, turn over and swim directly away. This combination of movements is characteristic as a reaction of what we may term "repellent" stimuli in general.^[29] Another mechanical reaction is that to continuous contact with a solid; and the surface film of water, either at the free surface or round an air-bubble, may play the part of a solid in exciting it; we term it "thigmotaxy" or "stereotaxy." When positive it determines a movement on to the surface, or a gliding movement along it, or merely the arrest of motion and prolongation of contact; when negative, a contact is followed by the retreat of the being. Thus Paramecium (Fig. 55, p. 151) and many other Ciliates are led to aggregate about solid particles or masses of organic débris in the water, which indeed serve to supply their food. On contact, the cell ceases to move its cilia except those of the oral groove; as these lash backwards, they hold the front end in close contact with the solid, at the same time provoking a backward stream down the groove, which may bring in minute particles from the mass.

(ii.) Most living beings are able to maintain their level in water by floating or crawling against Gravity, and they react in virtue of the same power against centrifugal force. This mode of irritability is termed (negative) "geotaxy" or "barotaxy." We can estimate the power of resisting such force by means of a whirling machine, since when the acceleration is greater than the resistance stimulated thereby in the beings, they are passively sent to the sides of the vessel. The Flagellates, Euglena and Chlamydomonas, begin to migrate towards the centre when exposed to a centrifugal force about equal to ½ G (G = 32.2 feet or 982 cm. per second); they remain at the centre until the centrifugal force is increased to 8 G; above that they yield to the force, and are driven passively to the sides. The reaction ceases or is reversed at high temperatures.

(iii.) Rheotaxy.—This is the tendency to move against the stream in flowing water. It is shown by most Protists, and can be conveniently studied in the large amoeboid plasmodia of the Myxomycetes, which crawl against the stream along wet strips of filter paper, down which water is caused to flow. Most animals, even of the highest groups, tend to react in the same way; the energetic swimming of Fishes up-stream being in marked contrast with their sluggishness the other way; and every student of pond-life knows how small Crustacea and Rotifers, no less than Ciliates, swim away from the inrush of liquid into the dipping-tube, and so evade capture. (See Vol. II. p. 216.)

(iv.) The movements of many Protozoa are affected greatly by Light. These movements have been distinguished into "photopathic," i.e. to or from the position of greatest luminosity; and "phototactic," along the direct path of the rays.^[30] Those Protozoa that contain a portion of their cytoplasm, known as a "plastid" or "chromatophore" (see pp. 36, 39), coloured by a green or yellow pigment are usually "phototactic." They mostly have at the anterior end a red pigment spot, which serves as an organ of sight, and is known as an "eye-spot." In diffused light of low intensity they do not exhibit this reaction, but in bright sunlight they rise to the surface and form there a green or yellow scum.

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

Here, as in the case of other stimuli,^[31] the absolute intensity of the light is of importance; for as it increases from a low degree, different organisms in turn cease to be stimulated, and {22}then are repelled instead of being attracted. The most active part of the spectrum in determining reactions of movement are the violet and blue rays of wave-length between 40 µ/10 and 49 µ/10, while the warmer and less refractive half of the spectrum is inert save in so far as it determines changes in the temperature of the medium.

(v.) The movements of many Protozoa are rendered sluggish by cold, and active by a rise of Temperature up to what we may term the "optimum"; the species becomes sluggish again as the temperature continues to rise to a certain point when the movements are arrested, and the being is said to be in a state of "heat-rigor." Most Protozoa, again, tend to move in an unequally heated medium to the position nearest to their respective optimum temperature. This is called "thermotaxy." The temperature to which Amoeba is thermotactic is recorded as 35° C. (95° F.); that of Paramecium is 28° C. (82° F.).

(vi.) Most active Protozoa tend to take up a definite position in respect to a current of Electricity passing through the medium, and in the majority of cases, including most Ciliates, Amoeba, and Trachelomonas, they orient their long diameters in the direction of the lines of force and swim along these to assemble behind the cathode. The phenomenon is called "galvanotaxy," and this particular form is "negative." Opalina (Fig. 41, p. 123), however, and most Flagellates are "positively galvanotactic," and move towards the anode. H. H. Dale^[32] has shown that the phenomenon may be possibly in reality a case of chemiotaxy, for the direction of motion varies with the nature and concentration of the medium. It would thus be a reaction to the "ion" liberated in contact with the one or other extremity of the being. Induction shocks, as we have seen, if slight, arrest the movements of Protozoa, or if a little stronger determine movements of contraction; if of sufficient intensity they kill them. No observation seems to have been made on the behaviour of Protista in an electric field. A magnetic field of the highest intensity appears to be indifferent to all Protista.

(vii.) We have already referred to the effect of dissolved Chemical Substances present in the water. If the substance is in itself not harmful, and the effect varies with the concentration, we term the reaction one of "tonotaxy," which combines {23}with that of "chemiotaxy" for substances that in weak solution are attractive or repellent to the being. Paramecium, which feeds on bacteria, organisms of putrefaction, is positively chemiotactic to solutions of carbon dioxide, and as it gives this off in its own respiration, it is attracted to its fellows. The special case of reaction to gases in solution is termed "aerotaxy," or "pneumotaxy," according as the gas is oxygen or carbon dioxide. We find that in this respect there are degrees, so that a mixed culture of Flagellates in an organic infusion sorts itself out, under the cover of a microscopic preparation, into zones of distinct species, at different distances from the freely aerated edge, according to the demands of each species for oxygen and CO₂ respectively.

Finally, we must note that the apparently "spontaneous movements" of Protists can hardly be explained as other than due either to external stimuli, such as we have just studied, or to internal stimuli, the outcome of internal changes, such as fatigue, hunger, and the like. Of the latter kind are the movements that result in REPRODUCTION.

Reproduction.—We have noted above that the growth of an organism which retains its shape alters the ratio of the surface area to the whole volume, so necessary for the changes involved in life. For the volume of an organism varies as the cube of any given diameter, whereas the surface varies with the square only. Without going into the arithmetical details, we may say that the ratio of surface to volume is lessened to roughly four-fifths of the original ratio when the cell doubles its bulk. As Herbert Spencer and others have pointed out, this must reduce the activities of the cell, and the due ratio is restored by the division of the cell into two.^[33] This accounts for what we must look on as the most primitive mode of reproduction, as it is the simplest, and which we term "fission" at Spencer's "limit of {24}growth." Other modes of reproduction will be studied later (p. 30), after a more detailed inquiry into the structure of the nucleus and of its behaviour in cell-division. All cell-division is accompanied by increased waste, and is consequently catabolic in character, though the anabolic growth of living protoplasm, at the expense of the internal reserves, may be concurrent therewith.

Cell-Division

In ordinary cases of fission of an isolated cell the cell elongates, and as it does so, like other viscid bodies, contracts in the middle, which becomes drawn out into a thread, and finally gives way. In some cases (e.g. that of the Amoeba, Fig. 4) the nucleus previously undergoes a similar division by simple constriction, which is called direct or "amitotic" division. But usually the division of the nucleus prior to cell-division is a more complex process, and involves the co-operation of the cytoplasm; and we must now study in detail the nucleus and its structure in "rest" and in fission.^[34]

We have noted above (p. 6, Fig. 2) the structure of the so-called "resting nucleus,"^[35] when the cell is discharging the ordinary functions of its own life, with its wall, network of linin, chromatin-granules, and nucleole or nucleoles. The chromatin-granules are most abundant at two periods in the life of the cell, (1) when it is young and fresh from division, and (2) at the term of its life, when it is itself preparing for division. In the interim they are fewer, smaller, and stain less intensely. In many Protista the whole or greater part of the chromatin is densely aggregated into a central "nuclein-mass" or karyosome {25}suspended in the linin network (long regarded as a mere nucleole). Such a nucleus is often termed a "vesicular nucleus".^[36]

Fig. 6.—Changes in nucleus and cell in indirect (mitotic) nuclear division. A, resting nucleus with two centrioles^[37] in single centrosphere (c); B, centrosphere divided, spindle and two asters (a) forming; C, centrospheres separated, nuclear wall disappearing; D, resolution of nucleus into chromosomes; E, mature plasmic spindle, with longitudinal fission of chromosomes; F, chromosomes forming equatorial plate (ep) of spindle. (From Wilson.)

When cell-division is about to take place the linin, or at least the greater part of it, assumes the character of a number of distinct threads, and the whole of the chromatin granules are distributed at even distances along these (Fig. 6, A, B, C), so as to appear like so many strings of beads. Each such thread is called a "chromosome." Then each bead divides longitudinally into two. The thread flattens into a ribbon, edged by the two lines of chromatin beads. Finally, the ribbon splits longitudinally into two single threads of beads (Fig. 6, E). During these changes the nucleole or nucleoles diminish, or even disappear, as if they had contributed their matter to the growth of the chromatin proper. In Higher Animals and Plants the nuclear wall next disappears, and certain structures become obvious, especially in the cytoplasm of Metazoa. Two minute spheres of plasm (themselves often showing a concentric structure), the "centrosomes,"^[38] which hitherto lay close together at the side of the nuclear wall, now separate; but they remain connected by a spindle of clear plasmic threads (Fig. 6, B-E) which, as the centres diverge, comes to lie across the spot the nucleus occupied, and now the chromosomes lie about the equator of this spindle (Fig. 6, F). Moreover, the surrounding cytoplasm shows a radiating structure, diverging from the centrosome, so that spindle and external radiations together make up a "strain-figure," like that of the "lines of force" in relation to the poles of a magnet. Such we can demonstrate in a plane by spreading or shaking iron filings on a piece of paper above the poles of a magnet, or in space by suspending finely divided iron in a thick liquid, such as mucilage or glycerin, and bringing the vessel with the mixture into a strong magnetic field;^[39] the latter mode has the advantage {27}of enabling us to watch the changes in the distribution of the lines under changing conditions or continued strain.

Fig. 7.—Completion of mitotic cell-division. G, splitting of equatorial plate (ep); H, recession of daughter chromosomes; I, J, reconstitution of these into new nuclei, fission of the centrioles and of the cytoplasm. if, Central fibres of spindle; n, remains of old nucleole. (From Wilson.)

The chromosomes are now completely split, each into its two daughter-segments, which glide apart (Fig. 7, G, ep), and pass each to its own pole of the spindle, stopping just short of the centrosome (I). Thus, on the inner side of either centrosome is found an aggregation of daughter-segments, each of which is sister to one at the opposite pole, while the number at either pole is identical with that of the segments into which the old nucleus had resolved itself at the outset. The daughter-segments shorten and thicken greatly as they diverge to the poles, and on their arrival crowd close together.

A distinct wall now forms around the aggregated {28}daughter-chromosomes (J), so as to combine them into a nucleus for the daughter-cell. The reorganisation of the young nucleus certainly varies in different cases, and has been ill-studied, probably because of the rapidity of the changes that take place. The cytoplasm now divides, either tapering into a "waist" which finally ruptures, or constricting by the deepening of a narrow annular groove so as to complete the formation and isolation of the daughter-cells.

We might well compare the cell-division to the halving of a pumpkin or melon, of which the flesh as a whole is simply divided into two by a transverse cut, while the seeds and the cords that suspend them are each singly split to be divided evenly between the two halves of the fruit; the flesh would represent the cytoplasm, the cords the linin threads of the nucleus, and the seeds the chromatin granules. In this way the halving of the nucleus is much more complete and intimate than that of the cytoplasm; and this is the reason why many biologists have been led to regard the nuclear segments, and especially their chromatic granules, as the seat of the hereditary properties of the cell, properties which have to be equally transmitted on its fission to each daughter-cell.^[40] But we must remember that the linin is also in great part used up in the formation of these segments, like the cords of our supposed melon; and it is open to us to regard the halving in this intimate way of the "linin" as the essence of the process, and that of the chromatin as accessory, or even as only part of the necessary machinery of the process. The halving or direct splitting lengthwise of a viscid thread is a most difficult problem from a physical point of view; and it may well be that the chromatin granules have at least for a part of their function the facilitation of this process. If such be the case, we can easily understand the increase in number, and size and staining power of these granules as cell-division approaches, and their atrophy or partial disappearance during their long intervening periods of active cell life. Hence we hesitate to accept the views so commonly maintained that the chromatin represents a {29}"germ-plasm" or "idioplasm" of relatively great persistence, which gives the cell its own racial qualities.^[41]

The process we have just examined is called "mitosis," "karyomitosis," or "karyokinesis"; and the nucleus is said to undergo "indirect" division, as compared to "direct" division by mere constriction. In an intermediate mode, common to many Protista, the nuclear wall persists throughout the whole process, though a spindle is constituted within, and chromosomes are formed and split: the division of the nucleus takes place, however, by simple constriction, as seen in the Filose Rhizopod Euglypha (Fig. 8).

Fig. 8.—Fission with modified karyokinesis in the Filose Rhizopod Euglypha. A, outgrowth of half of the cytoplasm, passage of siliceous plates for young shell outwards; B, completion of shell of second cell, formation of intra-nuclear spindle; C, D, further stages. (From Wilson, after Schewiakoff.)

In many Sarcodina and some Sporozoa the nucleus gives off small fragments into the cytoplasm, or is resolved into them; {30}they have been termed "chromidia" by E. Hertwig. New nuclei may be formed by their growth and coalescence, the original nucleus sometimes disappearing more or less completely.

In certain cases the division of the nucleus is not followed by that of the cytoplasm, so that a plurinucleate mass of protoplasm results: this is called an "apocyte"; and we find transitional forms between this and the uninucleate or true cell. Thus in one species of Amoeba (A. binucleata) there are always two nuclei, which divide simultaneously to provide for the outfit of the daughter-cells on fission. Again, we find in some cases that similar multinucleate masses may be formed by the union of two or more cells by their cytoplasm only: such a union is termed "permanent plastogamy," and the plurinucleate mass a "plasmodium."^[42] Here again we find intermediate forms between plasmodium and apocyte, for the nuclei of the former may divide and so increase in number, without division of the still growing mass. Both kinds of plurinucleate organisms are termed "coenocytes" without reference to their mode of origin.

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

Multiple fission or brood-formation.—We may now turn to a new rhythm, in strong contrast to the former: a cell after having attained a size, often notably greater than its parents, divides: without any interval for growth, the daughter-cells again divide, and this may be repeated as many as ten times, or even more, so as to give rise to a number of small cells—4, 8, 16—1024,^[43] etc., respectively. Such an assemblage of small cells so formed is called a brood, and well deserves this name, for they never separate until the whole series of divisions is completed. By this process the number of individuals is rapidly {31}increased, hence it has received the name of "sporulation." The term spores is especially applied to the reproductive bodies of Cryptogams, such as Mosses, Fungi, etc.: the resulting cells are called "spores," "zoospores" if active ("amoebulae" if provided with pseudopodia, "flagellulae" if flagellate), "aplanospores," if motionless. We prefer to call them by the general term "brood-cells," the original cell the "brood-mother-cell," and the process, "multiple fission" or "brood-formation." As noted, the brood-mother-cell usually attains an exceptionally large size, and it in most cases passes into a state of rest before entering on division: thus brood-formation is frequently the ultimate term of a long series of Spencerian divisions. Two contrasting periods of brood-formation may occur in the life cycle of some beings, notably the Sporozoa.^[44]

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

Some such colonial Protista must have been the starting-points for the Higher Animals and Plants; probably apocytial Protista were the starting-points of the Fungi. In the Higher Animals and Plants, the spermatozoa and the oospheres (the male and female pairing-cells) are alike the offspring of brood-formation: and the coupled-cell (fertilised egg) starts its new life by segmentation, which is a brood-formation in which the cells do not separate, but remain in colonial union, to differentiate in due course into the tissue-cells of the organism.

Retarded brood-formation.—The nuclear divisions may alternate with cell-divisions, as above stated, or the former may be {32}completed before the cytoplasm divides; thus the brood-mother-cell becomes temporarily an apocyte,^[45] which is then resolved simultaneously into the 1-nucleate brood-cells.

A temporary apocytial condition is often passed through in the formation of the brood of cells by repeated divisions without any interval for enlargement; for the nuclear divisions may go on more rapidly than those of the cytoplasm, or be completed before any cell-division takes place (Figs. 31, 34, 35, pp. 95, 101, 104), the nuclear process being "accelerated" or the cytoplastic being "retarded," whichever we prefer to say and to hold. Thus as many as thirty-two nuclei may have been formed by repeated binary subdivisions before any division of the cytoplasm takes place to resolve the apocyte into true 1-nucleate cells.

Syngamy.^[48]—The essence of typical syngamy is, that two cells ("pairing-cells," "gametes") of the same species approach one another, and fuse, cytoplasm with cytoplasm, and nucleus with nucleus, to form a new cell ("coupled-cell," "zygote "). This process is called also "conjugation" or "cytogamy." In the simplest cases the two cells are equal and attract one another equally ("isogamy"), and have frequently the character of zoospores.

In an intermediate type, the one cell is larger and more sluggish (female), "megagamete," "oogamete," "oosphere," "egg"; the other smaller, more active (male), "microgamete," "spermogamete," "spermatozoon," "sperm"; and in the most specialised {34}cases which prevail among the Higher Animals and Plants, the larger cell is motionless, and the smaller is active, ciliate, flagellate, or amoeboid: the coupled-cell or zygote is here termed the "oosperm."^[49] It encysts immediately in most Protista except Infusoria, Acystosporidae, Haemosporidae, and Trypanosomatidae.

As the size of the two gametes is so disproportionate in most cases that the oosphere may be millions of times bigger than the sperm, and the latter at its entrance into the oosphere entirely escape unaided vision, the term "egg" is applied in lax speech, both (1) to the female cell, and (2) to the oosperm, the latter being distinguished as the "fertilised egg," a survival from the time when the union of two cells, as the essence of the process, was not understood.

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

1. A nucleated portion may regenerate completely, if of sufficient size. Consequently, multinucleate forms, such as Actinosphaerium (Heliozoa, Fig. 19, p. 72), may be cut into a number of fragments, and regenerate completely. In Ciliata, such as Stentor (Fig. 59, p. 156), each fragment must possess a portion of the meganucleus, and at least one micronucleus (p. 145), and, moreover, must possess a certain minimum size. A Radiolarian "central-capsule" (p. 75) with its endoplasm and nucleus may regenerate its ectoplasm, but the isolated ectoplasm being non-nucleate is doomed. A "central capsule" of one species introduced into the ectoplasm of another, closely allied, did well. All non-nucleate pieces may exhibit characteristic movements, but appear unable to digest; and they survive only a short time.^[52]

"Animals" and "Plants"

Hitherto we have discussed the cell as if it were everywhere an organism that takes in food into its substance, the food being invariably "organic" material, formed by or for other cells; such nutrition is termed "holozoic." There are, however, limits to the possibilities in this direction, as there are to the fabled capacities of the Scillonians of gaining their precarious livelihood by taking in one another's washing. For part of the food material taken in in this way is applied to the supply of the energies of the cell, and is consequently split up or oxidised into simpler, more stable bodies, no longer fitted for food; and of the matter remaining to be utilised for building up the organism, a certain proportion is always wasted in by-products. Clearly, then, the supply of food under such conditions is continually lessening in the universe, and we have to seek for a manufactory of food-material from inorganic materials: this is to be found in those cells that are known as "vegetal," in the widest sense of {36}the word. In this, sense, vegetal nutrition is the utilisation of nitrogenous substances that are more simple than proteids or peptones, together with suitable organic carbon compounds, etc., to build up proteids and protoplasm. The simplest of organisms with a vegetal nutrition are the Schizomycetes, often spoken of loosely as "bacteria" or "microbes," in which the differentiation of cytoplasm and nucleus is not clearly recognisable. Some of these can build up their proteids from the free uncombined nitrogen of the atmosphere, carbon dioxide, and inorganic salts, such as sulphates and phosphates. But the majority of vegetal feeders require the nitrogen to be combined at least in the form of a nitrate or an ammonium salt—nay, for growth in the dark, they require the carbon also to be present in some organic combination, such as a tartrate, a carbohydrate, etc. Acetates and oxalates, "aromatic" compounds^[53] and nitriles are rarely capable of being utilised, and indeed are often prejudicial to life. In many vegetal feeders certain portions of the protoplasm are specialised, and have the power of forming a green, yellow, or brown pigment; these are called "plastids" or "chromatophores." They multiply by constriction within the cell, displaying thereby a certain independent individuality. These plastids have in presence of light the extraordinary power of deoxidising carbon dioxide and water to form carbohydrates (or fats, etc.) and free oxygen; and from these carbohydrates or fats, together with ammonium salts or nitrates, etc., the vegetal protoplasm at large can build up all necessary food matter. So that in presence of light of the right quality^[54] and adequate intensity, such coloured vegetal beings have the capacity for building up their bodies and reserves from purely inorganic materials. Coloured vegetal nutrition, then, is a process involving the absorption of energy; the source from which this is derived in the bacteria being very obscure at present. Nutrition by means of coloured plastids is {37}distinguished as "holophytic," and that from lower substances, which, however, contain organically combined carbon, as "saprophytic," for such are formed by the death and decomposition of living beings. The third mode of nutrition (found in some bacteria) from wholly inorganic substances, including free nitrogen, has received no technical name. All three modes are included in the term "autotrophic" (self-nourishing).

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

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

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

The cells of a plant colony are usually connected by very fine threads of protoplasm, passing through minute pores where the new party-wall is left incomplete after cell-division.^[55] In a few plants, such as most Fungi, the cell-partitions are {38}in abeyance for the most part, and there is formed an apocyte with a continuous investment, sometimes, however, chambered at intervals by partitions between multinucleate units of protoplasm. We started with a purely physiological consideration, and we have now arrived at a morphological distinction, very valid among higher organisms.

Higher Plants consist of cells for the most part each isolated in its own cell-cavity, save for the few slender threads of communication.

Higher Animals consist of cells that are rarely isolated in this way, but are mostly in mutual contact over the greater part of their surface.

Again, Plants take in either food or else the material for food in solution through their surface, and only by diffusion through the cell-wall. Insectivorous Plants that have the power of capturing and digesting insects have no real internal cavity. Animal-feeding Protista take in their food into the interior of their protoplasm and digest it therein, and the Metazoa have an internal cavity or stomach for the same purpose. Here again there are exceptions in the case of certain internal parasites, such as the Tapeworms and Acanthocephala (Vol. II. pp. 74, 174), which have no stomachs, living as they do in the dissolved food-supplies of their hosts, but still possessing the general tissues and organs of Metazoa.

Corresponding with the absence of mouth, and the absorption instead of the prehension of food, we find that the movements of plant-beings are limited. In the higher Plants, and many lower ones, the colonial organism is firmly fixed or attached, and the movements of its parts are confined to flexions. These are produced by inequalities of growth; or by inequalities of temporary distension of cell-masses, due to the absorption of liquid into their vacuoles, while relaxation is effected by the cytoplasm and cell-wall becoming pervious to the liquid. We find no case of a differentiation of the cytoplasm within the cell into definite muscular fibrils. In the lower Plants single naked motile cells disseminate the species; and the pairing-cells, or at least the males, have the same motile character. In higher Cryptogams, Cycads, and Ginkgo (the Maiden-hair Tree), the sperms alone are free-swimming; and as we pass to Flowering Plants, the migratory character of the male cells is restricted to the smallest limits. {39}though never wholly absent. Intracellular movements of the protoplasm are, however, found in all Plants.

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

We may then define Plants as beings which have the power of manufacturing true food-stuffs from lower chemical substances than proteids, often with the absorption of energy. They have the power of surrounding themselves with a cell-wall, usually of cellulose, and of growing and dividing freely in this state, in which animal-like changes of form and locomotion are impossible; their colonies are almost always fixed or floating; free locomotion is only possible in the case of their naked reproductive cells, and is transitory even in these. The movements of motile parts of complex plant-organisms are due to the changes in the osmotic powers of cells as a whole, and not to the contraction of differentiated fibrils in the cytoplasm of individual cells. Plants that can form carbohydrates with liberation of free oxygen have always definite plastids coloured with a lipochrome^[56] pigment, or else (in the Phycochromaceae) the whole plasma is so coloured. Solid food is never taken into the free plant-cell nor into an internal cavity in complex Plants. If, as in insectivorous Plants, it is digested and absorbed, it is always in contact with the morphological external surface. In the complex Plants apocytes and syncytes are rare—the cells being each invested with its own wall, and, at most, only communicating by minute threads with its neighbours. No trace of a central nervous system with differentiated connexions can be made out.

Animals all require proteid food; their cyst-walls are never formed of cellulose; their cells usually divide in the naked condition only, or if encysted, no secondary party-walls are formed between the daughter-cells to unite them into a vegetative colony. Their colonies are usually locomotive, or, if fixed, their parts largely retain their powers of relative motion, and are often provided on their free surfaces with cilia or flagella; and many cells have differentiated in their cytoplasm contractile muscular fibrils. Their food (except in a few parasitic groups) is always taken {40}into a distinct digestive cavity. A complex nervous system, of many special cells, with branched prolongations interlacing or anastomosing, and uniting superficial sense-organs with internal centres, is universally developed in Metazoa. All Metazoa fulfil the above conditions.

But when we turn to the Protozoa we find that many of the characters evade us. There are some Dinoflagellates (see p. 130) which have coloured plastids, but which differ in no other respect (even specific) from others that lack them: the former may have mouths which are functionless, the latter have functional mouths. Some colourless Flagellates are saprophytic and absorb nutritive liquids, such as decomposing infusions of organic matter, possibly free from all proteid constituents; while others, scarcely different, take in food after the fashion of Amoeba. Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. Indeed, a strong natural line of demarcation lies between Metazoa and Protista. Of the Protozoa, certain groups, like the Foraminifera and Radiolaria and the Ciliate and Suctorial Infusoria are distinctly animal in their chemical activities or metabolism, their mode of nutrition, and their locomotive powers. When we turn to the Proteomyxa, Mycetozoa, and the Flagellates we find that the distinction between these and the lower Fungi is by no means easy, the former passing, indeed, into true Fungi by the Chytridieae, which it is impossible to separate sharply from those Flagellates and Proteomyxa which Cienkowsky and Zopf have so accurately studied under the name of "Monadineae." Again, many of the coloured Flagellates can only (if at all) be distinguished from Plants by the relatively greater prominence and duration of the mobile state, though classifiers are generally agreed in allotting to Plants those coloured Flagellates which in the resting state assume the form of multicellular or apocytial filaments or plates.

On these grounds we should agree with Haeckel in distinguishing the living world into the Metazoa, or Higher Animals, which {41}are sharply marked off; the Metaphyta, or Higher Plants, which it is not so easy to characterise, but which unite at least two or more vegetal characters; and the Protista, or organisms, whose differentiation is limited to that within the cell (or apocyte), and does not involve the cells as units of tissues. These Protista, again, it is impossible to separate into animal and vegetal so sharply as to treat adequately of either group without including some of the other: thus it is that every text-book on Zoology, like the present work, treats of certain Protophyta. The most unmistakably animal group of the Protista, the Ciliata, is, as we have seen, by the complex differentiation of its protoplasm, widely removed from the Metazoa with their relatively simple cells but differentiated cell-groups and tissues. The line of probable origin of the Metazoa is to be sought, for Sponges at least, among the Choanoflagellates (pp. 121 f. 181 f.).

CHAPTER II

PROTOZOA (CONTINUED): SPONTANEOUS GENERATION—CHARACTERS OF PROTOZOA—CLASSIFICATION

The Question of Spontaneous Generation

From the first discovery of the Protozoa, their life-history has been the subject of the highest interest: yet it is only within our own times that we can say that the questions of their origin and development have been thoroughly worked out. If animal or vegetable matter of any kind be macerated in water, filtered, or even distilled, various forms of Protista make their appearance; and frequently, as putrefaction advances, form after form makes its appearance, becomes abundant, and then disappears to be replaced by other species. The questions suggested by such phenomena are these: (1) Do the Protista arise spontaneously, that is, by the direct organisation into living beings of the chemical substances present, as a crystal is organised from a solution: (2) Are the forms of the Protista constant from one generation to another, as are ordinary birds, beasts, and fishes?

The question of the "spontaneous generation" of the Protista was readily answered in the affirmative by men who believed that Lice bred directly from the filth of human skins and clothes;^[57] and that Blow-flies, to say nothing of Honey-bees, arose in rotten flesh: but the bold aphorism of Harvey "omne vivum ex ovo" at once gained the ear of the best-inspired men of science, and set them to work in search of the "eggs" that gave rise to the organisms of putrefaction. Redi (ob. 1699) showed that Blow-flies never arise save when other Blow-flies gain access to meat and deposit their very visible eggs thereon. Leeuwenhoek, his {43}contemporary, in the latter half of the seventeenth century adduced strong reasons for ascribing the origin of the organisms of putrefaction to invisible air-borne eggs. L. Joblot and H. Baker in the succeeding half-century investigated the matter, and showed that putrefaction was no necessary antecedent of the appearance of these beings: that, as well as being air-borne, the germs might sometimes have adhered to the materials used for making the infusion; and that no organisms were found if the infusions were boiled long enough, and corked when still boiling. These views were strenuously opposed by Needham in England, by Wrisberg in Germany, and by Buffon, the great French naturalist and philosopher, whose genius, unballasted by an adequate knowledge of facts, often played him sad tricks. Spallanzani made a detailed study of what we should now term the "bionomical" or "oecological" conditions of Protistic life and reproduction in a manner worthy of modern scientific research, and not attained by some who took the opposite side within living recollection. He showed that infusions kept sufficiently long at the boiling-point in hermetically sealed vessels developed no Protistic life. As he had shown that active Protists are killed at much lower temperatures, he inferred that the germs must have much higher powers of resistance; and, by modifying the details of his experiments, he was able to meet various objections of Needham.

Despite this good work, the advocates of spontaneous generation were never really silenced; and the widespread belief in the inconstancy of species in Protista added a certain amount of credibility to their cause. In 1845 Pineau put forward these views most strongly; and from 1858 to 1864 they were supported by the elder Pouchet. Louis Pasteur, who began life as a chemist, was led from a study of alcoholic fermentation to that of the organisms of fermentation and of putrefaction and disease. He showed that in infusions boiled sufficiently long and sealed while boiling, or kept at the boiling-point in a sealed vessel, no life manifested itself: objections raised on the score of the lack of access of fresh air were met by the arrangement, so commonly used in "pure cultures" at the present day, of a flask with a tube attached plugged with a little cotton-wool, or even merely bent repeatedly into a zigzag. The former attachment filtered off all germs or floating solid particles from the air, the latter brought about the settling of such particles in the elbows {44}or on the sides of the tube: in neither case did living organisms appear, even after the lapse of months. Other observers succeeded in showing that the forms and characters of species were as constant as in Higher Animals and Plants, allowing, of course, for such regular metamorphoses as occur in Insects, or alternations of generations paralleled in Tapeworms and Polypes. The regular sequences of such alternations and metamorphoses constitute, indeed, a strong support of the "germ-theory"—the view that all Protista arise from pre-existing germs. It is to the Rev. W. H. Dallinger and the late Dr. Charles Drysdale that we owe the first complete records of such complex life-histories in the Protozoa as are presented by the minute Flagellates which infest putrefying liquids (see below, p. 116 f.). The still lower Schizomycetes, the "microbes" of common speech, have also been proved by the labours of Ferdinand Cohn, von Koch, and their numerous disciples, to have the same specific constancy in consecutive generations, as we now know, thanks to the methods first devised by De Bary for the study of Fungi, and improved and elaborated by von Koch and his school of bacteriologists.

And so to-day the principle "omne vivum ex vivo" is universally accepted by men of science. Of the ultimate origin of organic life from inorganic life we have not the faintest inkling. If it took place in the remote past, it has not been accomplished to the knowledge of man in the history of scientific experience, and does not seem likely to be fulfilled in the immediate or even in the proximate future.^[58]

PROTOZOA

Organisms of various metabolism, formed of a single cell or apocyte, or of a colony of scarcely differentiated cells, whose organs are formed by differentiations of the protoplasm, and its secretions and accretions; not composed of differentiated multicellular tissues or organs.^[59]

This definition, as we have seen, excludes Metazoa (including Mesozoa, Vol. II. p. 92) sharply from Protozoa, but leaves an uncertain boundary on the botanical side; and, as systematists share with nations the desire to extend their sphere of influence, we shall here follow the lead of other zoologists and include many beings that every botanist would claim for his own realm. Our present knowledge of the Protozoa has indeed been largely extended by botanists,^[60] while the study of protoplasmic physiology has only passed from their fostering care into the domain of General Biology within the last decade. The study of the Protozoa is little more than two centuries old, dating from the school of microscopists of whom the Dutchman Leeuwenhoek is the chief representative: and we English may feel a just pride in the fact that his most important publications are to be found in the early records of our own Royal Society.

Baker, in the eighteenth century, and the younger Wallich, Carter, Dallinger and Drysdale, Archer, Saville Kent, Lankester, and Huxley, in the last half-century, are our most illustrious names. In France, Joblot, almost as an amateur, like our own Baker, flourished in the early part of the eighteenth century. Dujardin in the middle of the same century by his study of protoplasm, or sarcode as he termed it, did a great work in laying the foundations of our present ideas, while Balbiani, Georges Pouchet, Fabre-Domergue, Maupas, Léger, and Labbé in France, have worthily continued and extended the Gallic traditions of exact observation and careful deduction. Otto Friedrich Müller, the Dane, in the eighteenth century, was a pioneer in the exact study and description of a large number of forms of these, as of other microscopic forms of life. The Swiss collaborators, Claparède and Lachmann, in the middle of the nineteenth century, added many facts and many descriptions; and illustrated them by most valuable figures of the highest merit from every point of view. Germany, with her large population of students and her numerous universities, has given many names to our list; among these, Ehrenberg and von Stein have added {46}the largest number of species to the roll. Ehrenberg about 1840 described, indeed, an enormous number of forms with much care, and in detail far too elaborate for the powers of the microscope of that date: so that he was led into errors, many and grave, which he never admitted down to the close of a long and honoured life. Max Schultze did much good work on the Protozoa, as well as on the tissues of the Metazoa, and largely advanced our conceptions of protoplasm. His work was continued in Germany by Ernst Haeckel, who systematised our knowledge of the Radiolaria, Greeff, Richard Hertwig, Fritz Schaudinn, and especially Bütschli, who contributed to Bronn's Thier-Reich a monograph of monumental conception and scope, and of admirable execution, on which we have freely drawn. Cienkowsky, a Russian, and James-Clark and Leidy, both Americans, have made contributions of high quality.

Lankester's article in the Encyclopædia Britannica was of epoch-making quality in its philosophical breadth of thought.

Delage and Hérouard have given a full account of the Protozoa in their Traité de Zoologie Concrète, vol. i. (1896); and A. Lang's monograph in his Vergleichende Anatomie, 2nd ed. (1901), contains an admirable analysis of their general structure, habits, and life-cycles, together with full descriptions of well-selected types. Calkins has monographed "The Protozoa" in the Columbia University Biological series (1901). These works of Bütschli, Delage, Lang, and Calkins contain full bibliographies. Doflein has published a most valuable systematic review of the Protozoa parasitic on animals under the title Die Protozoen als Parasiten und Krankheitserreger (1901); and Schaudinn's Archiv für Protistenkunde, commenced only four years ago, already forms an indispensable collection of facts and views.

The protoplasm of the Protozoa (see p. 5 f.) varies greatly in consistency and in differentiation. Its outer layer may be granular and scarcely altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria, Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the Acrasieae; it is continuous with a firm but delicate superficial pellicle of membranous character in most Flagellates and Infusoria; and this pellicle may again be consolidated and locally thickened in some members of both groups so as to form a coat of mail, even with definite spines and hardened polygonal plates (Coleps, Fig. 54, {47}p. 150). Again, it may form transitory or more or less permanent pseudopodia,^[61] (1) blunt or tapering and distinct, with a hyaline outer layer, or (2) granular and pointed, radiating and more or less permanent, or (3) branching and fusing where they meet into networks or perforated membranes. Cilia or flagella are motile thread-like processes of active protoplasm which perforate the pellicle; they may be combined into flattened platelets or firm rods, or transformed into coarse bristles or fine motionless sense-hairs. The skeletons of the Protozoa, foreign to the cytoplasm, will be treated of under the several groups.

Most of the fresh-water and brackish forms (and some marine ones) possess one or more contractile vacuoles, often in relation to a more or less complex system of spaces or canals in Flagellates and Ciliates.

The Geographical Distribution of Protozoa is remarkable for the wide, nay cosmopolitan, distribution of the terrestrial and fresh-water forms;^[62] they owe this to their power of forming cysts, within which they resist drought, and can be disseminated as "dust." Very few of them can multiply at a temperature approaching freezing-point; the Dinoflagellates can, however, and therefore present Alpine and Arctic forms. The majority breed most freely at summer temperatures; and, occurring in small pools, can thus achieve their full development in such as are heated by the sun during the long Arctic day as readily as in the Tropics. In infusions of decaying matter, the first to appear are those that feed on bacteria, the essential organisms of putrefaction. These, again, are quickly followed and preyed upon by carnivorous species, which prefer liquids less highly charged with organic matters, and do not appear until the liquid, hitherto cloudy, has begun to clear. Thus we have rather to do with "habitat" than with "geographical {48}distribution," just as with the fresh-water Turbellaria and the Rotifers (vol. ii. pp. 4 f., 226 f.). We can distinguish in fresh-water, as in marine Protista, "littoral" species living near the bank, among the weeds; "plankton," floating at or near the surface; "zonal" species dwelling at various depths; and "bottom-dwellers," mostly "limicolous" (or "sapropelic," as Lauterborn terms them), and to be found among the bottom ooze. Many species are "epiphytic" or "epizoic," dwelling on plants or animals, and sometimes choice enough in their preference of a single genus or species as host. Others again are "moss-dwellers," living among the root-hairs of mosses and the like, or even "terrestrial" and inhabiting damp earth. The Sporozoa are internal parasites of animals, and so are many Flagellates, while many Proteomyxa are parasitic in plant-cells. The Foraminifera (with the exception of most Allogromidiaceae) are marine, and so are the Radiolaria; while most Heliozoa inhabit fresh water. Concerning the distribution in time we shall speak under the last two groups, the only ones whose skeletons have left fossil remains.

Classification.—The classification of the Protozoa is no easy task. We omit here, for obvious reasons, the unmistakable Plant Protists that have a holophytic or saprophytic nutrition; that are, with the exception of a short period of locomotion in the young reproductive cells, permanently surrounded with a wall of cellulose or fungus-cellulose, and that multiply and grow freely in this encysted state: to these consequently we relegate the Chytridieae, which so closely allied to the Proteomyxa and the Phycomycetous Fungi; and the Confervaceae, which in the resting state form tubular or flattened aggregates and are allied to the green Flagellates; besides the Schizophyta. At the opposite pole stand the Infusoria in the strict sense, with the most highly differentiated organisation found in our group, culminating in the possession of a nuclear apparatus with nuclei of two kinds, and exhibiting a peculiar form of conjugation, in which the nuclear apparatus is reorganised. The Sporozoa are clearly marked off as parasites in living animals, which mostly begin life as sickle-shaped cells and have always at least two alternating modes of brood-formation, the first giving rise to aplanospores, wherein is formed the second brood of sickle-shaped, wriggling zoospores. The remainder, comprising the Sarcodina, or Rhizopoda in the old wide sense (including all {49}that move by pseudopodia during the great part of their active life), and the Flagellata in the widest sense, are not easy to split up; for many of the former have flagellate reproductive cells, and many of the latter can emit pseudopodia with or without the simultaneous retraction of their flagella. The Radiolaria are well defined by the presence in the body plasm of a central capsule marking it off into a central and a peripheral portion, the former containing the nucleus, the latter emitting the pseudopodia. Again, on the other hand, we find that we can separate as Flagellata in the strict sense the not very natural assemblage of those Protista that have flagella as their principle organs of movement or nutrition during the greater part of their active life. The remaining groups (which with the Radiolaria compose the Sarcodina of Bütschli), are the most difficult to treat. The Rhizopoda, as we shall limit them, are naked or possess a simple shell, never of calcium carbonate, have pseudopodia that never radiate abundantly nor branch freely, nor coalesce to form plasmatic networks, nor possess an axial rod of firmer substance. The Foraminifera have a shell, usually of calcium carbonate, their pseudopodia are freely reticulated, at least towards the base; and (with the exception of a few simple forms) all are marine. The Mycetozoa are clearly united by their tendency to aggregate more or less completely into complex resting-groups (fructifications), and by reproducing by unicellular zoospores, flagellate or amoeboid, which are not known to pair. The Heliozoa resemble the Radiolaria in their fine radiating pseudopodia, but have an axial filament always present in each, and lack the central capsule; and are, for the most part, fresh-water forms. Finally, the Proteomyxa forms a sort of lumber-room for beings which are intermediate between the Heliozoa, Rhizopoda, and Flagellata, usually passing through an amoeboid stage, and for the most part reproducing by brood-formation. Zoospores that possess flagella are certainly known to occur in some forms of Foraminifera, Rhizopoda, Heliozoa, and Radiolaria, though not by any means in all of each group.^[63]

CHAPTER III

PROTOZOA (CONTINUED): SARCODINA

I. Sarcodina.

1. Rhizopoda

Sarcodina of simple form, whose pseudopodia never coalesce into networks (1),^[65] nor contain an axial filament (2), which commonly multiply by binary fission (3), though a brood-formation may occur; which may temporarily aggregate, or undergo temporary or permanent plastogamic union, but never to form large plasmodia or complex fructifications as a prelude to spore-formation (4); test when present gelatinous, chitinous, sandy, or siliceous, simple and 1-chambered (5).

Classification.^[66]

We have defined this group mainly by negative characters, as such are the only means for their differentiation from the remaining Sarcodina; and indeed from Flagellata, since in this group zoospores are sometimes formed which possess flagella. Moreover, indeed, in a few of this group (Podostoma, Arcuothrix), as in some Heliozoa, the flagellum or flagella may persist or be reproduced side by side with the pseudopodia. The subdivision of the Rhizopoda is again a matter of great difficulty, the characters presented being so mixed up that it is hard to choose: however, the character of the outer layer of the cytoplasm is perhaps the most obvious to select. In Lobosa there is a clear layer of ectosarc, which appears to be of a greasy nature at its surface film, so that it is not wetted. In the Filosa, as in most other Sarcodina, this film is absent, and the ectoplasm is not marked off from the endoplasm, and may have a granular surface. Corresponding to this, the pseudopodia of the Lobosa are usually blunt, never branching and fraying out, as it were, at the tip, as in the Filosa; nay, in the normal movements of Amoeba limax (Fig. 1, p. 5) the front of the cell forms one gigantic pseudopodium, which constantly glides forward. Apart from this distinction the two groups are parallel in almost every respect.

There may be a single contractile vacuole, or a plurality; or none, especially in marine and endoparasitic species. The nucleus may remain single or multiply without inducing fission, thus leading to apocytial forms. It often gives off "chromidial" fragments, which may play an important part in reproduction.^[67] In Amoeba binucleata there are constantly two nuclei, both of which divide as an antecedent to fission, each giving a separate nucleus to either daughter-cell. Pelomyxa palustris, the giant of the group, attaining a diameter of 1‴ (2 mm.), has very blunt pseudopodia, an enormous number of nuclei, and no contractile vacuole, though {53}it is a fresh-water dweller, living in the bottom ooze of ponds, etc., richly charged with organic débris. It is remarkable also for containing symbiotic bacteria, and brilliant vesicles with a distinct membranous wall, containing a solution of glycogen.^[68] Few, if any, of the Filosa are recorded as plurinuclear.

The simplest Lobosa have no investment, nor indeed any distinction of front or back. In some forms of Amoeba, however, the hinder part is more adhesive, and may assume the form of a sucker-like disc, or be drawn into a tuft of short filaments or villi, to which particles adhere. Other species of Lobosa and all Filosa have a "test," or "theca," i.e. an investment distinct from the outermost layer of the cell-body. The simplest cases are those of Amphizonella, Dinamoeba, and Trichosphaerium, where this is gelatinous, and in the two former allows the passage of food particles through it into the body by mere sinking in, like the protoplasm itself, closing again without a trace of perforation over the rupture. In Trichosphaerium (Fig. 9) the test is perforated by numerous pores of constant position for the passage of the pseudopodia, closing when these are retracted; and in the "A" form of the species (see below) it is studded with radial spicules of magnesium carbonate. Elsewhere the test is more consistent and possesses at least one aperture for the emission of pseudopodia and the reception of food; to avoid confusion we call this opening not the mouth but the "pylome": some Filosa have two symmetrically placed pylomes. When the test is a mere pellicle, it may be recognised by the limitation of the pseudopodia to the one pylomic area. But the shell is often hard. In Arcella (Fig. 10, C), a form common among Bog-mosses and Confervas, it is chitinous and shagreened, circular, with a shelf running in like that of a diving-bell around the pylome: there are two or more contractile vacuoles, and at least two nuclei. Like some other genera, it has the power of secreting carbonic acid gas in the form of minute bubbles in its cytoplasm, so as to enable it to float up to the surface of the water. The chitinous test shows minute hexagonal sculpturing, the expression of vertical partitions reaching from the inner to the outer layer.

Fig. 9.—Trichosphaerium sieboldii. 1, Adult of "A" form; 2, its multiplication by fission and gemmation; 3, resolution into 1-nucleate amoeboid zoospores; 4, development (from zoospores of "A") into "B" form (5); 6, its multiplication by fission and gemmation; 7, its resolution after nuclear bipartition into minute 2-flagellate zoospores or (exogametes); 8, liberation of gametes; 9, 10, more highly magnified pairing of gametes of different origin; 11, 12, zygote developing into "A" form. (After Schaudinn.)

Several genera have tests of siliceous or chitinous plates, formed in the cytoplasm in the neighbourhood of the nucleus, and connected by chitinous cement. Among these Quadrula (Fig. 10, A) is Lobose, with square plates, Euglypha (Fig. 8, p. 29), and Paulinella^[69] are Filose, with hexagonal plates. In the latter they are in five longitudinal rows, with a pentagonal oral plate, perforated by the oval pylome. In other genera again, such as Cyphoderia (Filosa), the plates are merely {55}chitinous. Again, the shell may be encrusted with sand-grains derived directly from without, or from ingested particles, as shown in Centropyxis, Difflugia (Fig. 10, D), Heleopera, and Campascus when supplied with powdered glass instead of sand. The cement in Difflugia is a sort of organic mortar, infiltrated with ferric oxide (more probably ferric hydrate). In Lecqueureusia spiralis (formerly united with Difflugia) the test is formed of minute sausage-shaped granules, in which may be identified the partly dissolved valves of Diatoms taken as food; it is spirally twisted at the apex, as if it had enlarged after its first formation, a very rare occurrence in this group. The most frequent mode of fission in the testaceous Rhizopods (Figs. 8, 10) is what Schaudinn aptly terms "bud-fission," where half the protoplasm protrudes and accumulates at the mouth of the shell, and remains till a test has formed for it, while the other half retains the test of the original animal. The materials for the shell, whether sand-granules or plates, pass from the depths of the original shell outwards into the naked cell, and through its cytoplasm to the surface, where they become connected by cementing matter into a continuous test. The nucleus now divides into two, one of which passes into the external animal; after this the two daughter-cells separate, the one with the old shell, the other, larger, with the new one.

Fig. 10.—Test-bearing Rhizopods. A, Quadrula symmetrica: B, Hyalosphenia lata; C, Arcella vulgaris; D, Difflugia pyriformis. (From Lang's Comparative Anatomy.)

If two individuals of the shelled species undergo bud-fission in close proximity, the offspring may partially coalesce, so that a monstrous shell is produced having two pylomes.

Reproduction by fission has been clearly made out in most members of the group; some of the multinucleate species often abstrict a portion, sometimes at several points simultaneously, so that fission here passes into budding^[70] (Fig. 9, 2, 6).

Brood-division, either by resolution in the multinucleate species, or preceded by multiple nuclear division in the habitually 1-nucleate, though presumably a necessary incident in the life-history of every species, has only been seen, or at least thoroughly worked out, in a few cases, where it is usually preceded by encystment, and mostly by the extrusion into the cyst of any undigested matter.^[71]

In Trichosphaerium (Fig. 9) the cycle described by Schaudinn is very complex, and may be divided into two phases, which we may term the A and the B subcycles. The members of the A cycle are distinguished by the gelatinous investment being armed with radial spicules, which are absent from the B form. The close of the A cycle is marked by the large multinucleate body resolving itself into amoeboid zoospores (3), which escape from the gelatinous test, and develop into the large multinucleate adults of the B form. These, like the A form, may reproduce by fission or budding. At the term of growth, however, they retract their pseudopodia, expel the excreta, and multiply their nuclei by mitosis (7). Then the body is resolved into minute 2-flagellate microzoospores (8), which are exogamous gametes, i.e. they will only pair with similar zoospores from another cyst. The zygote (9-11) resulting from this conjugation is a minute amoeboid; its nucleus divides repeatedly, a gelatinous test is formed within which the spicules appear, and so the A form is reconstituted. In many of the test-bearing forms, whether Lobose or Filose, plastogamic unions occur, and the two nuclei may remain distinct, leading to plurinucleate monsters in their offspring by fission, or they may fuse and form a giant nucleus, a process which has here no relation to normal syngamy, as it is not associated with any marked change in the alternation of feeding and fission, etc. In Trichosphaerium also plastogamic unions between small individuals have for their only result the increase of size, enabling the produce to deal with {57}larger prey. Temporary encystment in a "hypnocyst" is not infrequent in both naked and shelled species, and enables them to tide over drought and other unfavourable conditions.

Schaudinn has discovered and worked out true syngamic processes, some bisexual, some exogamous, in several other Rhizopods. In Chlamydophrys stercorea the pairing-cells are equal, and are formed by the aggregation of the chromidia into minute nuclei around which the greater part of the cytoplasm aggregates, while the old nucleus (with a little cytoplasm) is lost. These brood-cells are 2-flagellate pairing-cells, which are exogamous: the zygote is a brown cyst; if this be swallowed by a mammal, the original Chlamydophrys appears in its faeces.^[72]

Centropyxis aculeata, a species very common in mud or moss, allied to Difflugia, also forms a brood by aggregation around nuclei derived from chromidia. The brood-cells are amoeboid, and secrete hemispherical shells like those of Arcella; some first divide into four smaller ones, before secreting the shell. Pairing takes place between the large and the small forms; and the zygote encysts. Weeks or months afterwards the cyst opens and its contents creep out as a minute Centropyxis. Finally, Amoeba coli produces its zygote in a way recalling that of Actinosphaerium (pp. 73-75, Fig. 21): the cell encysts; its nucleus divides, and each daughter divides again into two, which fuse reciprocally. Thus the cyst contains two zygote nuclei. After a time each of these divides twice, so that the mature cyst contains eight nuclei. Probably when swallowed by another animal they liberate a brood of eight young amoebae. Thus in different members of this group we have exogamy, both equal and bisexual, and endogamy.

Most of the Rhizopoda live among filamentous Algae in pools, ponds, and in shallow seas, etc.; some are "sapropelic" or mud-dwellers (many species of Amoeba, Pelomyxa, Difflugia, etc.), others frequent the roots of mosses. Amoeba coli is often found as a harmless denizen of the large intestine of man. Amoeba histolytica, lately distinguished therefrom by Schaudinn, is the cause of tropical dysentery. It multiplies enormously in the gut, and is found extending into the tissues, and making its way into the abscesses that so frequently supervene in the liver and other organs. Chlamydophrys stercorea is found in the {58}faeces of several mammals. The best monograph of this group is that of Penard.^[73]

2. Foraminifera^[74]

Sarcodina with no central capsule or distinction of ectosarc; the pseudopodia fine, branching freely, and fusing where they meet to form protoplasmic networks, or the outermost in the pelagic forms radiating, but without a central or axial filament: sometimes dimorphic, reproducing by fission and by rhizopod or flagellate germs in the few cases thoroughly investigated: all marine (with the exception of some of the Allogromidiaceae), and usually provided with a test of carbonate of lime ("vitreous" calcite, or "porcellanous" aragonite?), or of cemented particles of sand ("arenaceous"); test-wall continuous, or with the walls perforated by minute pores or interstices for the protrusion of pseudopodia.

The classification of Carpenter (into Vitreous or Perforate, Porcellanous or Imperforate, and Arenaceous), according to the structure of the shell, had proved too artificial to be used by Brady in the great Monograph of the Foraminifera collected by the "Challenger" Expedition,^[75] and has been modified by him and others since then. We reproduce Lister's account of Brady's classification.^[76] We must, however, warn the tyro that its characterisations are not definitions (a feature of all other recent systems), for rigid definitions are impossible: here as in the case, for instance, of many Natural Orders of Plants, transitional forms making the establishment of absolute boundaries out of the question. In the following classification we do not think it, therefore, necessary to complete the characterisations by noting the extremes of variation within the orders:—

The Allogromidiaceae are a well-marked and distinct order, on the whole resembling the Rhizopoda Filosa, and are often found with them in fresh water, while all other Foraminifera are marine. The type genus, Allogromia (Fig. 14, 1), has an oval chitinous shell. Microgromia socialis (Fig. 11) is often found in aggregates, the pseudopodia of neighbours fusing where they meet into a {60}common network. This is due to the fact that one of the two daughter-cells at each fission, that does not retain the parent shell, remains in connexion with its sister that does: sometimes, however, it retracts its pseudopodia, except two which become flagella, wherewith it can swim off. The test of Pamphagus is a mere pellicle. In Lieberkühnia (Fig. 12) it is hardly that; though the body does not give off the fine pseudopodia directly, but emits a thick process or "stylopodium"^[78] comparable to the protoplasm protruded through the pylome of its better protected allies; and from this, which often stretches back parallel to the elongated body, the reticulum of pseudopodia is emitted. Diaphorodon has a shell recalling that of Difflugia (Fig. 10, D, p. 55), formed of sandy fragments, but with interstices between them through which as well as through the two pylomes the pseudopodia pass. In all of these the shell is formed as in the Rhizopods once for all, and does not grow afterwards; and the fresh-water forms, which are the majority, have one or more contractile vacuoles; in Allogromia they are very numerous, scattered on the expanded protoplasmic network.

Fig. 11.—Microgromia socialis. A, entire colony; B, single zooid; C, zooid which has undergone binary fission, with one of the daughter-cells creeping out of the shell; D, flagellula. c.vac, Contractile vacuole; nu, nucleus; sh, shell. (From Parker and Haswell, after Hertwig and Lesser.)

Fig. 12.—Lieberkühnia, a fresh-water Rhizopod, from the egg-shaped shell of which branched pseudopodial filaments protrude. (From Verworn.)

The remaining marine families may all be treated of generally, before noting their special characters. Their marine habitat is variable, but in most cases restricted. A few extend up the brackish water of estuaries: a large number are found between tide-marks, or on the so-called littoral shelf extending to deep water; they are for the most part adherent to seaweeds, or lie among sand or on the mud. Other forms, again, are pelagic, such as Globigerina (Figs. 13, 6, 16, 17) and its allies, and float as part of the plankton, having the surface of their shells extended by delicate spines, their pseudopodia long and radiating, and the outer part of their cytoplasm richly vacuolated ("alveolate"), and probably containing a liquid lighter than sea water, as in the Radiolaria. Even these, after their death and the decay of the protoplasm, must sink to the bottom (losing the fine spines by solution as they fall); and they accumulate there, to form a light oozy mud, the "Globigerina-ooze" of geographers, at depths where the carbonic acid under pressure is not adequate to dissolve the more solid calcareous matter. Grey Chalk is such an ooze, consolidated by {62}the lapse of time and the pressure of superincumbent layers. Some Foraminifera live on the sea bottom even at the greatest depths, and of course their shell is not composed of calcareous matter. Foraminifera may be obtained for examination by carefully washing sand or mud, collected on the beach at different levels between tide-marks, or from dredgings, or by carefully searching the surface of seaweeds, or by washing their roots, or, again, by the surface or deep-sea tow-net. The sand used to weight sponges for sale is the ready source of a large number of forms, and may be obtained for the asking from the sponge-dealers to whom it is a useless waste product. If this sand is dried in an oven, and then poured into water, the empty shells, filled with air, will float to the surface, and may be sorted by fine silk or wire gauze.

From the resemblance of the shells of many of them to the Nautilus they were at first described as minute Cephalopods, or Cuttlefish, by d'Orbigny,^[79] and their true nature was only elucidated in the last century by the labours of Williamson, Carpenter, Dujardin, and Max Schultze. At first they possess only one nucleus, but in the adult stage may become plurinucleate without dividing, and this is especially the case in the "microsphaeric" states exhibited by many of those with a complex shell; the nucleus is apt to give off fragments (chromidia) which lie scattered in the cytoplasm. At first, too, in all cases, the shell has but a single chamber, a state that persists through life in some. When the number of chambers increases, their number has no relation to that of the nuclei, which remains much smaller till brood-formation sets in.

The shell-substance, if calcareous, has one of the two types, porcellanous or vitreous, that we have already mentioned, but Polytrema, a form of very irregular shape, though freely perforated, is of a lovely pink colour. In the calcareous shells sandy particles may be intercalated, forming a transition to the Arenacea. In these the cement has an organic base associated with calcareous or ferruginous matter; in some, however, the cement is a phosphate of iron. The porcellanous shells are often deep brown by transmitted light.

Fig. 13.—Shells of Foraminifera. In 3, 4, and 5, a shows the surface view, and b a section; 8a is a diagram of a coiled cell without supplemental skeleton; 8b of a similar form with supplemental skeleton (s.sk); and 10 of a form with overlapping whorls; in 11a half the shell is shown in horizontal section; b is a vertical section; a, aperture of the shell; 1-15, successive chambers, 1 being always the oldest or initial chamber. (From Parker and Haswell, after other authors.)

Despite the apparent uniformity of the protoplasmic body in this group, the shell is infinitely varied in form. As Carpenter writes, in reference to the Arenacea, "There is nothing more wonderful in nature than the building up of these elaborate and symmetrical structures by mere jelly-specks, presenting no traces {64}whatever of that definite organisation which we are accustomed to regard as necessary to the manifestations of conscious life.... The tests (shells) they construct when highly magnified bear comparison with the most skilful masonry of man. From the same sandy bottom one species picks up the coarsest quartz grains, unites them together with a ferruginous cement, and thus constructs a flask-shaped test, having a short neck and a single large orifice; another picks up the finer grains and puts them together with the same cement into perfectly spherical tests of the most extraordinary finish, perforated with numerous small pores disposed at pretty regular intervals. Another species selects the minutest sand grains and the terminal portions of sponge-spicules, and works them up together—apparently with no cement at all, but by the mere laying of the spicules—into perfect white spheres like homoeopathic globules, each showing a single-fissured orifice. And another, which makes a straight, many-chambered test, the conical mouth of each chamber projecting into the cavity of the next, while forming the walls of its chambers of ordinary sand grains rather loosely held together, shapes the conical mouths of the chambers by firmly cementing together the quartz grains which border it." The structure of the shell is indeed variable. The pylome may be single or represented by a row of holes (Peneroplis, Orbitolites), or, again, there may be several pylomes (Calcituba); and, again, there are in addition numerous scattered pores for the protrusion of pseudopodia elsewhere than from the stylopodium, in the whole of the "Vitrea" and in many "Arenacea"; and, as we shall see, this may exercise a marked influence on the structure of the shell.

In some cases the shell is simple, and in Cornuspira and Spirillina increases so as to have the form of a flat coiled tube. In Calcituba the shell branches irregularly in a dichotomous way, and the older parts break away as the seaweed on which they grow is eaten away, and fall to the bottom, while the younger branches go on growing and branching. The fallen pieces, if they light on living weed, attach themselves thereto and repeat the original growth; if not, the protoplasm crawls out and finds a fresh weed and forms a new tube. In the "Polythalamia" new chambers are formed by the excess of the protoplasm emerging and surrounding itself with a shell, organically united with the existing chamber or chambers, and in a space-relation which follows definite laws characteristic of the species or of its stage of growth, so as to give rise to circular, spiral, or irregular complexes (see Fig. 13).

Fig. 14.—Various forms of Foraminifera. In 4, Miliola, a, shows the living animal; b, the same killed and stained; a, aperture of shell; f, food particles; nu, nucleus; sh, shell. (From Parker and Haswell, after other authors.)

In most {66}cases the part of the previously existing chamber next the pylome serves as the hinder part of the new chamber, and the old pylome becomes the pore of communication. But in some of the "Perforata" each new chamber forms a complete wall of its own ("proper wall," Fig. 13, 8b), and the space between the two adjacent walls is filled with an intermediate layer traversed by canals communicating with the cavities of the chambers ("intermediate skeleton"), while an external layer of the same character may form a continuous covering. The shell of the Perforata may be adorned with pittings or fine spines, which serve to increase the surface of support in such floating forms as Globigerina, Hastigerina, and the like (Fig. 17). In the "Imperforata" the outer layer is often ornamented with regular patterns of pits, prominences, etc., which are probably formed by a thin reflected external layer of protoplasm. In some of the "Arenacea" a "labyrinthine" complex of laminae is formed.

A very remarkable point which has led to great confusion in the study of the Foraminifera, is the fact that the shell on which we base our characters of classification, may vary very much, even within the same individual. Thus in the genus Orbitolites the first few chambers of the shell have the character of a Milioline, in Orbiculina of a Peneroplis. The arrangements of the Milioline shell, known as Triloculine, Quinqueloculine, and Biloculine respectively, may succeed one another in the same shell (Figs. 14 4, 15). A shell may begin as a spiral and end by a straight continuation: again, the spherical Orbulina (Fig. 16 1) is formed as an investment to a shell indistinguishable from Globigerina, which is ultimately absorbed. In some cases, as Rhumbler has pointed out, the more recent and higher development shows itself in the first formed chambers, while the later, younger chambers remain at a lowlier stage, as in the case of the spiral passing into a straight succession; but the other cases we have cited show that this is not always the case. In Lagena (Fig. 13 2) the pylome is produced into a short tube, which may protrude from the shell or be turned into it, so that for the latter form the genus Entosolenia was founded. Shells identical in minute sculpture are, however, found with either form of neck, and, moreover, the polythalamial shells (Nodosaria, Fig. 13 3), formed of a nearly straight succession of Lagena-like chambers, may have these chambers with their {67}communications on either type. Rhumbler goes so far as to suggest that all so-called Lagena shells are either the first formed chamber of a Nodosaria which has not yet become polythalamian by the formation of younger ones, or are produced by the separation of an adult Nodosaria into separate chambers.

Fig. 15.—A, Megalospheric; B, microspheric shell of Biloculina. c, The initial chamber. The microspheric form begins on the Quinqueloculina type. (From Calkins' Protozoa.)

Many of the chambered species show a remarkable dimorphism, first noted by Schlumberger, and finally elucidated by J. J. Lister and Schaudinn. It reveals itself in the size of the initial chamber; accordingly, the two forms may be distinguished as "microspheric" and "megalospheric" respectively (Fig. 15), the latter being much the commoner. The microspheric form has always a plurality of nuclei, the megalospheric a single one, except at the approach of reproduction. Chromidial masses are, however, present in both forms. The life-history has been fully worked out in Polystomella by Schaudinn, and in great part in Polystomella, Orbitolites, etc., by Lister; and the same scheme appears to be general in the class, at least where the dimorphism noted occurs. The microspheric form gives birth only to the megalospheric, but the latter may reproduce megalospheric broods, or give rise to swarmers, which by their (exogamous) {68}conjugation produce the microspheric young. The microspheric forms early become multinucleate, and have also numerous chromidia detached from the nuclei, which they ultimately replace. These collect in the outer part of the shell and aggregate into new nuclei, around which the cytoplasm concentrates, to separate into as many amoeboid young "pseudopodiospores" as there are nuclei. These escape from the shell or are liberated by its disintegration, and invest themselves with a shell to form the initial large central chamber or megalosphere.

Fig. 16.—1, Orbulina universa. Highly magnified. 2, Globigerina bulloides. Highly magnified. (From Wyville Thomson, after d'Orbigny.)

In the ordinary life of the megalospheric form the greater part of the chromatic matter is aggregated into a nucleus, some still remaining diffused. At the end of growth the nucleus itself disintegrates, and the chromidia concentrate into a number of small vesicular nuclei, each of which appropriates to itself a small surrounding zone of thick plasm and then divides by mitosis twice; and the 4-nucleate cells so formed are resolved into as many 1-nucleate, 2-flagellate swarmers, which conjugate {69}only exogamously.^[80] The fusion of their nuclei takes place after some delay: ultimately the zygote nucleus divides into two, a shell is formed, and we have the microsphere, which is thus pluri-nucleate ab initio. As we have seen, the nuclei of the microsphere are ultimately replaced by chromidia, and the whole plasmic body divides into pseudopodiospores, which grow into the megalospheric form.

Fig. 17.—Shell of Globigerina bulloides, from tow-net, showing investment of spines. (From Wyville Thomson.)

In the Perforate genera, Patellina and Discorbina, plastogamy precedes brood formation, the cytoplasms of the 2-5 pairing individuals contracting a close union; and then the nuclei proceed to break up without fusion, while the cytoplasm aggregates around the young nuclei to form amoebulae, which acquire a shell and separate. In both cases it is the forms with a single nucleus, corresponding to megalospheric forms that so pair, and the brood-formation is, mutatis mutandis, the same as in these forms. Similar individuals may reproduce in the same way, in both genera, without this plastogamic pairing, which is therefore, though probably advantageous, not essential. If pseudopodiospores form their shells while near one another, they may coalesce to form monsters, as often happens in Orbitolites.^[81]

The direct economic uses of the Foraminifera are perhaps greater than those of any other group of Protozoa. The Chalk is {70}composed largely of Textularia and allied forms, mixed with the skeletons of Coccolithophoridae (pp. 113-114), known as Coccoliths, etc. The Calcaire Grossier of Paris, used as a building stone, is mainly composed of the shells of Miliolines of Eocene age; the Nummulites of the same age of the Mediterranean basin are the chief constituent of the stone of which the Pyramids of Egypt are built. Our own Oolitic limestones are composed of concretions around a central nucleus, which is often found to be a minute Foraminiferous shell.

The palaeontology of the individual genera is treated of in Chapman's and Lister's recent works. They range from the Lower Cambrian characterised by perforated hyaline genera, such as Lagena, to the present day. Gigantic arenaceous forms, such as Loftusia, are among the Tertiary representatives; but the limestones formed principally of their shells commence at the Carboniferous. The so-called Greensands contain greenish granules of "glauconite," containing a ferrous silicate, deposited as a cast in the chambers of Foraminifera, and often left exposed by the solution of the calcareous shell itself. Such granules occur in deep-sea deposits of the present day.^[82]

3. Heliozoa

Sarcodina with radiate non-anastomosing pseudopodia of granular protoplasm, each with a stiff axial rod passing into the body plasma; no central capsule, nor clear ectoplasm; skeleton when present siliceous; nucleus single or multiple; contractile vacuole (or vacuoles) in fresh-water species, superficial and prominent at the surface in diastole; reproduction by fission or budding in the active condition, or by brood-formation in a cyst, giving rise to resting spores; conjugation isogamous in the only two species fully studied; habitat floating or among weeds, mostly fresh water.

This class were at first regarded and described as fresh-water Radiolaria, but the differences were too great to escape the greatest living specialist in this latter group, Ernst Haeckel, who in 1866 created the Heliozoa for their reception. We owe our knowledge of it mainly to the labours of Cienkowsky, the late William Archer, F. E. Schulze, R. Hertwig, Lesser, and latterly to Schaudinn, who has monographed it for the "Tierreich" (1896); and Penard has published a more recent account.

Fig. 18.—Actinophrys sol. a, Axial filament of pseudopod; c.v, contractile vacuole; n, nucleus. (From Lang's Comparative Anatomy, after Grenacher.)

Actinophrys sol Ehrb. (Fig. 18) is a good and common type. It owes its name to its resemblance to a conventional drawing of the sun, with a spherical body and numerous close-set diverging rays. The cytoplasm shows a more coarsely vacuolated outer layer, sometimes called the ectosarc, and a denser internal layer the endosarc. In the centre of the figure is the large nucleus, to which the continuations of the rays may be seen to converge; the pseudopodia contain each a stiffish axial filament,^[83] which is covered by the fine granular plasm, showing currents of the granules. The axial filament disappears when the pseudopodia are retracted or bent, and is regenerated afterwards. This bending occurs when a living prey touches and adheres to a ray, all its neighbours bending in like the tentacles of a Sundew. The prey is carried down to the surface of the ectoplasm, and {72}sinks into it with a little water, to form a nutritive vacuole. Fission is the commonest mode of reproduction, and temporary plastogamic unions are not uncommon. Arising from these true conjugations occur, two and two, as described by Schaudinn. A gelatinous cyst wall forms about the two which are scarcely more than in contact with their rays withdrawn. Then in each the nucleus divides into two, one of which passes to the surface, and is lost (as a "polar body"), while the other approaches the corresponding nucleus of the mate, and unites with it, while at the same time the cytoplasms fuse. Within the gelatinous cyst the zygote so formed divides to produce two sister resting spores, from each of which, after a few days, a young Actinophrys escapes, as may take place indeed after encystment of an ordinary form without conjugation.

Fig. 19.—Actinosphaerium eichornii. A, entire animal with two contractile vacuoles (c.vac); B, a portion much magnified, showing alveolate cytoplasm, pseudopodia with axial rods, non-nucleate cortex (cort), multiple nuclei (nu) of endoplasm (med), and food-vacuole (chr). (From Parker and Haswell.)

The axial rods of the pseudopodia may pass either to the circumference of the nucleus or to a central granule, corresponding, it would appear, to a centrosome or blepharoplast; or again, {73}in the plurinucleate marine genus Camptonema, each rod abuts on a separate cap on the outer side of each nucleus. The nucleus is single in all but the genera Actinosphaerium, Myxastrum, Camptonema, and Gymnosphaera. The movements of this group are very slow, and are not well understood. A slow rolling over on the points of the rays has been noted, and in Camptonema they move very decidedly to effect locomotion, the whole body also moving Amoeba-fashion; but of the distinct movements of the species when floating no explanation can be given. The richly vacuolate ectoplasm undoubtedly helps to sustain the cell, and the extended rays must subserve the same purpose by so widely extending the surface. Dimorpha (Fig. 37, 5, p. 112) has the power of swimming by protruding a pair of long flagella from the neighbourhood of the eccentric nucleus; and Myriophrys has an investment of long flagelliform cilia. Actinomonas has a stalk and a single flagellum in addition to the pseudopodia; these genera form a transition to the Flagellata.

Several species habitually contain green bodies, which multiply by bipartition, and are probably Zoochlorellae, Chlamydomonadidae of the same nature as we shall find in certain Ciliata (pp. 154, 158) in fresh-water Sponges (see p. 175), in Hydra viridis (p. 256), and the marine Turbellarian Convoluta (Vol. II. p. 43).

Reproduction by fission is not rare, and in some cases (Acanthocystis) the cell becomes multinuclear, and buds off 1-nucleate cells. In such cases the buds at first lack a centrosome, and a new one is formed first in the nucleus, and passes out into the cytoplasm. These buds become 2-flagellate before settling down. In Clathrulina the formation of 2-flagellate zoospores has long been known (Fig. 20, 3). In Actinosphaerium (Figs. 19, 21), a large species, differing from Actinophrys only in the presence of numerous nuclei in its endoplasm, a peculiar process, which we have characterised as endogamy, results in the formation of resting spores. The animal retracts its rays and encysts; and the number of nuclei is much reduced by their mutual fusion, or by the solution of many of them, or by a combination of the two processes. The body then breaks up into cells with a single nucleus, and each of these surrounds itself with a wall to form a cyst of the second order.

Fig. 20.—Various forms of Heliozoa. In 3, a is the entire animal and b the flagellula; c.vac, contractile vacuole; g, gelatinous investment; nu, nucleus; psd, pseudopodia; sk, siliceous skeleton; sp, spicules. (From Parker and Haswell, after other authors.)

Each of these divides, and the two sister cells then conjugate after the same fashion as in Actinophrys, but the nuclear divisions to form the coupling nucleus are two in number, i.e. the nucleus divides into two, one of which goes to the surface as the first polar body, and the sister of this again divides to form a second polar body (which also passes to {75}the surface) and a pairing nucleus.^[84] The two cells then fuse completely, and surround themselves with a second gelatinous cyst wall, separated from the outer one by a layer of siliceous spicules. The nucleus appears to divide at least twice before the young creep out, to divide immediately into as many Actinophrys-like cells as there were nuclei; then each of these multiplies its nuclei, to become apocytial like the adult form.

Fig. 21.—Diagram illustrating the conjugation of Actinosphaerium. 1, Original cell; 2, nucleus divides to form two, N₂N₂; 3, each nucleus again divides to form two, N₃ and n₃, the latter passing out with a little cytoplasm as an abortive cell; 4, repetition of the same process as in 3; 5, the two nuclei N₄ have fused in syngamy to form the zygote nucleus N_z.

Schaudinn admits 24 genera (and 7 doubtful) and 41 species (and 18 doubtful). None are known fossil. Their geographical distribution is cosmopolitan, as is the case with most of the minute fresh-water Protista; 8 genera are exclusively marine, and Orbulinella has only been found in a salt-pond; Actinophrys sol is both fresh-water and marine, and Actinolophus has 1 species fresh-water, the other marine. One of the 14 species of Acanthocystis is marine; the remaining genera and species are all inhabitants of fresh water.^[85]

4. Radiolaria

Sarcodina with the protoplasm divided by a perforated chitinous central capsule into a central mass surrounding the nucleus, and an outer layer; the pseudopodia radiate, never anastomosing enough to form a marked network; skeleton either siliceous, of spicules, or perforated; or of definitely arranged spicules of proteid matter (acanthin), sometimes also coalescing into a latticed shell; reproduction by fission and by zoospores formed in the central capsule. Habitat marine, suspended at the surface (plankton), at varying depths (zonarial), or near the bottom (abyssal).

Fig. 22.—Collozoum inerme. A, B, C, three forms of colony; D, small colony with central capsules (c.caps), containing nuclei, and alveoli (vac) in ectoplasm; E, isospores, with crystals (c); F, anisospores; nu, nucleus. (From Parker and Haswell.)

The following is Haeckel's classification of the Radiolaria:—

Fig. 23.—Actinomma asteracanthion. A, the shell with portions of the two outer spheres broken away; B, section showing the relations of the skeleton to the animal, cent.caps, Central capsule; ex.caps.pr, extra-capsular protoplasm: nu, nucleus; sk.1, outer, sk.2, middle, sk.3, inner sphere of skeleton. (From Parker and Haswell, after Haeckel and Hertwig.)

A. Spumellaria.

Fig. 24.—Xiphacantha (Acantharia). From the surface. The skeleton only, × 100, (From Wyville Thomson.)

B. Acantharia.

C. Nassellaria.

D. Phaeodaria.

We exclude Haeckel's Dictyochida, with a skeleton recalling that of the Stephoidea, but of the impure hollow substance of the Phaeodaria (p. 84). They rank now as Silicoflagellates (p. 114).

The Radiolarian is distinguished from all other Protozoa by the chitinous central capsule, so that its cytoplasm is separated into an outer layer, the extracapsular protoplasm (ectoplasm), and a central mass, the intracapsular, containing the nucleus.^[88]

The extracapsular layer forms in its substance a gelatinous mass, of variable reaction, through which the plasma itself ramifies as a network of threads ("sarcodictyum"), uniting at the surface to constitute the foundation for the pseudopodia. This gelatinous matter constitutes the "calymma." It is largely vacuolated, the vacuoles ("alveoli"), of exceptional size, lying in the nodes of the plasmic network, and containing a liquid probably of lower specific gravity than seawater; and they are especially abundant towards the surface, where they touch and become polygonal. On mechanical irritation they disappear, to be formed anew after an interval, a fact that may explain the sinking from the surface in disturbed water. This layer may contain minute pigment granules, but the droplets of oil and of albuminous matter frequent in the central layer are rare here. {80}The "yellow cells" of a symbiotic Flagellate or Alga, Zooxanthella, are embedded in the jelly of all except Phaeodaria, and the whole ectosarc has the average consistency of a firm jelly.

The pseudopodia are long and radiating, with a granular external layer, whose streaming movements are continuous with those of the inner network. In the Acantharia they contain a firm axial filament, like that of the Heliozoa, which is traceable to the central capsule; and occasionally a bundle of pseudopodia may coalesce to form a stout process like a flagellum ("sarcoflagellum"). Here, too, each spine, at its exit from the jelly, is surrounded by a little cone of contractile filaments, the myophrisks, whose action seems to be to pull up the jelly and increase the volume of the spherical body so as to diminish its density.

Fig. 25.—Skeletons of Radiolaria. A, Dorataspis; B, Theoconus. (After Haeckel.)

The intracapsular protoplasm is free from Zooxanthella except in the Acantharia. It is less abundantly vacuolated, and is finely granular. In the Porulosa it shows a radial arrangement, with pyramidal stretches of hyaline plasma separated by intervals rich in granules. Besides the alveoli with watery contents, others are present with albuminoid matter in solution. Oil-drops, often brilliantly coloured, occur either in the plasma or floating in either kind of vacuole; and they are often luminous at night. Added to these, the intracapsular plasm contains pigment-granules, most frequently red or orange, {81}passing into yellow or brown, though violet, blue, and green also occur. The "phaeodium,"^[89] however, that gives its name to the Phaeodaria, is an aggregate of dark grey, green, or brown granules which are probably formed in the endoplasm, but accumulate in the extracapsular plasm of the oral side of the central capsule. Inorganic concretions and crystals are also found in the contents of the central capsule, as well as aggregates of unknown composition, resembling starch-grains in structure.

In the Monopylaea, or Nassellaria (Figs. 25, B, 26, A), the endoplasm is differentiated above the perforated area of the central capsule into a cone of radiating filaments termed the "porocone," which may be channels for the communication between the exoplasm and the endoplasm, or perhaps serve, as Haeckel suggests, to raise, by their contraction, the perforated area: he compares them to the myophane striae of Infusoria. In the Phaeodaria (Fig. 26, B), a radiating laminated cone is seen in the outermost layer of the endoplasm above the principal opening ("astropyle"), and a fibrillar one around the two accessory ones ("parapyles"); and in some cases, continuous with these, the whole outer layer of the endoplasm shows a meridional striation.

The nucleus is contained in the endoplasm, and is always at first single, though it may divide again and again. The nuclear wall is a firm membrane, sometimes finely porous. If there are concentric shells it at first occupies the innermost, which it may actually come to enclose, protruding lobes which grow through the several perforations of the lattice-work, finally coalescing outside completely, so as to show no signs of the joins. In the Nassellaria a similar process usually results in the formation of a lobed nucleus, contained in an equally lobed central capsule. The chromatin of the nucleus may be concentrated into a central mass, or distributed into several "nucleoli," or it may assume the form of a twisted, gut-like filament, or, again, the nuclear plasm may be reticulated, with the chromatin deposited at the nodes of the network.

Fig. 26.—A, Lithocercus annularis, with sagittal ring (from Parker and Haswell). B, Aulactinium actinastrum. C, calymma; cent.caps., km, central capsule; Ext.caps.pr., Extracapsular, and Int.caps.pr., intracapsular protoplasm; n, nu, nucleus; op, operculum; ph, phaeodium; psd, pseudopodium; Skel., skeleton; z, Zooxanthella. (From Lang's Comparative Anatomy, after Haeckel.)

The skeleton of this group varies, as shown in our conspectus, in the several divisions.^[90] The Acantharia (Figs. 24, 25, A) have a skeleton of radiating spines meeting in the centre of figure of the endoplasm, and forcing the nucleus to one side. The spines are typically 20 in number, and emerge from the surface of the regular spherical forms (from which the others may be readily derived) radially, in five sets of four in the regions corresponding to the equator and the tropics and polar circles of our world. {83}The four rays of adjacent circles alternate, so that the "polar" and "equatorial" rays are on one set of meridians 90° apart, and the "tropical" spines are on the intermediate meridians, as shown in the figures. By tangential branching, and the meeting or coalescence of the branches, reticulate (Figs. 23, 24, 25) and latticed shells are formed in some families, with circles of openings or pylomes round the bases of the spines. In the Sphaerocapsidae the spines are absent, but their original sites are inferred from the 20 circles of pylomes.

In the Spumellaria the simplest form of the (siliceous) skeleton is that of detached spicules, simple or complex, or passing into a latticed shell, often with one or more larger openings (pylomes). Radiating spines often traverse the whole of the cavity, becoming continuous with its latticed wall, and bind firmly the successive zones when present (Fig. 23).

Calcaromma calcarea was described by Wyville Thomson as having a shell of apposed calcareous discs, and Myxobrachia, by Haeckel, as having collections of the calcareous Coccoliths and Coccospheres. In both cases we have to do with a Radiolarian not possessing a skeleton, but retaining the undigested shells of its food, in the former case (Actissa) in a continuous layer, in the latter (Thalassicolla) in accumulations that, by their weight, droop and pull out the lower hemisphere into distinct arms.

The (siliceous) skeleton of the Nassellaria is absent only in the Nassoidea, and is never represented by distinct spicules. Its simplest form is a "tripod" with the legs downward, and the central capsule resting on its apex. The addition of a fourth limb converts the tripod into a "calthrop," the central capsule in this case resting between the upturned leg and two of the lower three regarded as the "anterolateral"; the odd lower leg, like the upturned one, being "posterior." Again, the skeleton may present a "sagittal ring," often branched and spiny (Fig. 26, A), or combined with the tripod or calthrop, or complicated by the addition of one or more horizontal rings. Another type is presented by the "latticed chamber" surrounding the central capsule, with a wide mouth ("pylome") below. This is termed the "cephalis"; it may be combined in various ways with the sagittal ring and the tripod or calthrop; and, again, it may be prolonged by the addition of one, two, or three chambers below, {84}the last one opening by a pylome (Fig. 25, B). These are termed "thorax," "abdomen," and "post-abdomen" respectively.

In the Phaeodaria the skeleton may be absent, spicular (of loose or connected spicules) or latticed, continuous or bivalve. It is composed of silica combined with organic matter, so that it chars when heated, is more readily dissolved, and is not preserved in fossilisation. The spicules or lattice-work are hollow, often with a central filament running in the centre of the gelatinous contents. The latticed structure of the shell of the Challengeridae (Fig. 28) is so fine as to recall that of the Diatomaceae. In the Phaeoconchida the shell is in two halves, parted along the "frontal" plane of the three apertures of the capsule.

Fig. 27.—Scheme of various possible skeletal forms deposited in the meshes of an alveolar system, most of which are realised in the Radiolaria. (From Verworn, after Dreyer.)

The central capsule (rarely inconspicuous and difficult, if not impossible to demonstrate) is of a substance which resembles chitin, though its chemical reactions have not been fully studied hitherto, and indeed vary from species to species. It is composed of a single layer, except in Phaeodaria, where it is double. The operculum in this group, i.e. the area around the aperture, is composed of an outer layer, which is radially thickened, and a thin inner layer; the former is produced into the projecting tube ("proboscis").

Reproduction in the Radiolaria may be simple fission due to the binary fission of the nucleus, the capsule, and the ectoplasm in succession. If this last feature is omitted we have a colonial organism, composed of the common ectoplasm containing numerous central capsules; and the genera in which this occurs, all belonging to the Peripylaea, were formerly separated (as Polycyttaria) from {85}the remaining Radiolaria (Monocyttaria). They may either lack a skeleton (Collozoidae, Fig. 22), or have a skeleton of detached spicules (Sphaerozoidae), or possess latticed shells (Collosphaeridae) one for each capsule, and would seem therefore to belong, as only differentiated by their colonial habit, to the several groups having these respective characters. Fission has been well studied in Aulacantha (a Phaeodarian) by Borgert.^[91] He finds that in this case the skeleton is divided between the daughter-cells, and the missing part is regenerated. In cases where this is impossible one of the daughter-cells retains the old skeleton, and the other escapes as a bud to form a new skeleton.

Fig. 28.—Shells of Challengeridae: A, Tuscarora; B, Pharyngella; C, Haeckeliana. (From Wyville Thomson.)

Two modes of reproduction by flagellate zoospores have been described (Fig. 22). In the one mode all the zoospores are alike—isospores—and frequently contain a crystal of proteid nature as well as oil-globules. In the Polycyttaria alone has the second mode of spore-formation been seen, and that in the same species in which the formation of isospores occurs. Here "anisospores" are formed, namely, large "mega-," and small "micro-zoospores." They probably conjugate as male and female respectively; but neither has the process been observed, nor has any product of such conjugation (zygote) been recognised. In every case the formation of the zoospores only involves the {86}endoplasm: the nucleus first undergoes brood division, and the plasma within the capsule becomes concentrated about its offspring, and segregates into the spores; the extracapsular plasm disintegrates.^[92]

The Yellow Cells (Zooxanthella), so frequently found in the Radiolaria were long thought to be constituents of their body. Cienkowsky found that when the host died from being kept in unchanged water, the yellow cells survived and multiplied freely, often escaping from the gelatinised cell-wall as biflagellate zoospores. The cell-wall is of cellulose. The cell contains two chloroplastids, or plates coloured with the vegetal pigment "diatomin." Besides ordinary transverse fission in the ordinary encysted state in the ectoplasm of the host, when free they may pass into what is known as a "Palmella-state," the cell-walls gelatinising; in this condition they multiply freely, and constitute a jelly in which the individual cells are seen as rounded bodies. They contain starch in two forms—large hollow granules, not doubly refractive, and small solid granules which polarise light. We may regard them as Chrysomonadaceae (p. 113). Similar organisms occur in many Anthozoa (see pp. 261, 339, 373 f., 396). Diatomaceae (yellow Algae with silicified cell-walls) sometimes live in the jelly of certain Collosphaera. Both these forms live in the state known as "symbiosis" with their host; i.e. they are in mutually helpful association, the Radiolarian absorbing salts from the water for the nutrition of both, and the Alga or Flagellate taking up the CO₂ due to the respiration of the host, and building up organic material, the surplus of which is doubtless utilised, at least in part, for the nutrition of the host. A similar union between a Fungus and a coloured vegetal ("holophytic") organism is known as a Lichen.

The Suctorian Infusorian Amoebophrya is parasitic in the ectoplasm of certain Acantharia, and in the peculiar genus Sticholonche which appears to be intermediate between this group and Heliozoa.

The Silicoflagellate family Dictyochidae are found temporarily {87}embedded in the ectoplasm of some of the Phaeocystina, and have a skeleton of similar nature. Their true nature was shown by Borgert.

The Amphipod crustacean Hyperia^[93] may enter the jelly of the colonial forms, and feed there at will on the host.^[94]

Haeckel, in his Monograph of the Radiolaria of the Challenger enumerated 739 genera, comprising 4318 species; and Dreyer has added 6 new genera, comprising 39 species, besides 7 belonging to known genera. Possibly, as we shall see, many of the species may be mere states of growth, for it is impossible to study the life-histories of this group; on the other hand, it is pretty certain that new forms are likely to be discovered and described. The Radiolaria are found living at all depths in the sea, by the superficial or deep tow-net; and some appear to live near the bottom, where the durable forms of the whole range also settle and accumulate. They thus form what is known as Radiolarian ooze, which is distinguished from other shallower deposits chiefly through the disappearance by solution of all calcareous skeletons, as they slowly fell through the waters whereon they originally floated at the same time with the siliceous remains of the Radiolaria. The greatest wealth of forms is found in tropical seas, though in some places in cold regions large numbers of individuals of a limited range of species have been found.

Radiolaria of the groups with a pure siliceous skeleton can alone be fossilised, even the impure siliceous skeleton of the Phaeodaria readily dissolving in the depths at which they live: they have been generally described by Ehrenberg's name Polycystineae. Tripolis (Kieselguhr) of Tertiary ages have been found in many parts of the globe, consisting largely or mainly of Radiolaria, and representing a Radiolarian ooze. That of the Miocene of Barbados contains at least 400 species; that of Gruppe at least 130. In Secondary and Palaeozoic rocks such oozes pass into Radiolarian quartzites (some as recent as the Jurassic). They occur also in fossilised excrement (coprolites), and in flint or chert concretions, as far down as the lowest fossiliferous rocks, {88}the Cambrian. The older forms are simple Sphaerellaria and Nassellaria. From a synopsis of the history of the order in Haeckel's Monograph (pp. clxxxvi.-clxxxviii.) we learn that while a large number of skeletal forms had been described by Ehrenberg, Huxley in 1851 published the first account of the living animal. Since then our knowledge has been extended by the labours of Haeckel, Cienkowsky, R. Hertwig, Karl Brandt, and A. Borgert.

5. Proteomyxa

Sarcodina without a clear ectoplasm, whose active forms are amoeboid or flagellate, or pass from the latter form to the former; multiplying chiefly, if not exclusively, by brood-formation in a cyst. No complete cell-pairing (syngamy) known, though the cytoplasms may unite into plasmodia; pseudopodia of the amoeboid forms usually radiate or filose, but without axial filaments. Saprophytic or parasitic in living animals or plants.

This group is a sort of lumber-room for forms which it is hard to place under Rhizopoda or Flagellata, and which produce simple cysts for reproduction, not fructifications like the Mycetozoa. The cyst may be formed for protection under drought ("hypnocyst"), or as a preliminary to spore-formation ("sporocyst"). The latter may have a simple wall (simple sporocyst), or else two or three formed in succession ("resting cyst"), so as to enable it to resist prolonged desiccation, etc.: both differing from the hypnocyst in that their contents undergo brood formation. On encystment any indigestible food materials are extruded into the cyst, and in the "resting cysts," which are usually of at least two layers, this faecal mass lies in the space between them. The brood-cells escape, either as flagellate-cells, resembling the simpler Protomastigina, called "flagellulae," and which often become amoeboid (Fig. 29); or already furnished with pseudopodia, and called "amoebulae," though they usually recall Actinophrys rather than Amoeba. In Vampyrella and some others the amoebulae fuse, and so attain a greater size, which is most probably advantageous for feeding purposes. But usually it is as a uninucleate cell that the being encysts. They may feed either by ingestion by the pseudopodia, by the whole surface contained in a living host-cell, or by passing a pseudopodium into a host-cell (Fig. 29 5). They may be divided as follows:—

Fig. 29.—Pseudospora lindstedtii. 1, 2, Flagellate zoospores; 3, young amoebula, with two contractile vacuoles, one being reconstituted by three minute formative vacuoles; 4, 5, an amoebula migrating to a fungus hypha through the wall of which it has sent a long pseudopodium; 6, amoebula full-grown; 7, 8, mature cells rounded off, protruding a flagellum, before encysting; 9, young sporocyst; 10, the nucleus has divided into a brood of eight; 11-14, stages of formation of zoospores. cv, Contractile vacuole; e, mass of faecal granules; fl, flagellum; n, nucleus, × about 750⁄1.

Plasmodiophora infests the roots of Crucifers, causing the disease known as "Hanburies," or "fingers and toes," in turnips, etc. Serumsporidium dwells in the body cavity of small Crustacea. Many of this group were described by Cienkowsky under the name of "Monadineae" (in Arch. Mikr. Anat. i. 1865, p. 203). Zopf has added more than anyone else since then to our knowledge. He monographed them under Cienkowsky's name, as a subordinate group of the Myxomycetes, "Pilzthiere oder Schleimpilze," in Schenk's Handb. d. Bot. vol. iii. pt. ii. (1887). To Lankester (Encycl. Brit., reprint 1891) we owe the name here adopted. Zopf has successfully pursued their study in recent {90}papers in his Beitr. Nied. Org. The Chytridieae, usually ascribed to Fungi, are so closely allied to this group that Zopf proposes to include at least the Synchytrieae herein.

This group is very closely allied to Sporozoa; for the absence of cytogamy, and of sickle-germs,^[96] and of the complex spores and cysts of the Neosporidia, are the only absolute distinctions.

6. Mycetozoa (Myxomycetes, Myxogastres)

Sarcodina moving and feeding by pseudopodia, with no skeleton, aggregating more or less completely into complex "fructifications" before forming 1-nucleate resting spores; these may in the first instance liberate flagellate zoospores, which afterwards become amoeboid, or may be amoeboid from the first; zoospores capable of forming hypnocysts from which the contents escape in the original form.

I. The Acrasieae are a small group of saprophytes, often in the most literal sense, though in some cases it has been proved that the actual food is the bacteria of putrefaction. In them, since no cell-division takes place in the fructification, it is certain that the multiplication of the species must be due to the fissions of the amoeboid zoospores, which often have the habit of Amoeba limax (Fig. 1, p. 5).

II. Filoplasmodieae.—Chlamydomyxa^[97] is a not uncommon inhabitant of the cells of bog-mosses and bog-pools, and its nutrition may be holophytic, as it contains chromoplasts; but it {91}can also feed amoeba-fashion. Labyrinthula is marine, and in its fructification each of the component cells forms four spores. Leydenia has been found in the fluid of ascitic dropsy, associated with malignant tumour.

III. Myxomycetes.—The fructification in this group is not formed by the mere aggregation of the zoospores, but these fuse by their cytoplasm to form a multinucleate body, the "plasmodium," which, after moving and growing (with nuclear division) for some time like a great multinucleate Reticularian, passes into rest, and develops a fructification by the formation of a complex outer wall; within this the contents, after multiplication of the nuclei, resolve themselves into uninucleate spores, each with its own cyst-wall. The fructifications of this group are often conspicuous, and resemble those of the Gasteromycetous fungi (e.g., the Puffballs), whence they were at first called Myxogastres. De Bary first discovered their true nature in 1859, and ever since they have been claimed by botanist and zoologist alike.

The spore on germination liberates its contents as a minute flagellate, with a single anterior lash and a contractile vacuole (Fig. 30, C). It soon loses the lash, becomes amoeboid, and feeds on bacteria, etc. (Fig. 30, D, E). In this state it can pass into hypnocysts, from which, as from the spores, it emerges as a flagellula. After a time the amoeboids, which may multiply by fission, fuse on meeting, so as to form the plasmodium (Fig. 30, F). This contains numerous nuclei, which multiply as it grows, and numerous contractile vacuoles. When it attains full size it becomes negatively hydrotactic, crawls to a dry place, and resolves itself into the fructification. The external wall, and sometimes a basal support to the fruit, are differentiated from the outer layer of protoplasm; while the nuclei within, after undergoing a final bipartition, concentrate each around an independent portion of plasma, which again is surrounded as a spore by a cyst-wall. Often the maturing plasmodium within the wall of the fruit is traversed by a network of anastomosing tubes filled with liquid, the walls of which become differentiated into membrane like the fruit-wall, and are continuous therewith. As the fruit ripens the liquid dries, and the tubes now form a network of hollow threads, the "capillitium," often with external spiral ridges (Fig. 30, A, B). These are very hygroscopic, and by their expansion and contraction {92}determine the rupture of the fruit-wall and the scattering of the spores.

Fig. 30.—Didymium difforme. A, two sporangia (spg 1 and 2) on a fragment of leaf (l); B, section of sporangium, with ruptured outer layer (a), and threads of capillitium (cp); C, a flagellula with contractile vacuole (c.vac) and nucleus (nu); D, the same after loss of flagellum; b, an ingested bacillus; E, an amoebula; F, conjugation of amoebulae to form a small plasmodium; G, a larger plasmodium accompanied by numerous amoebulae; sp, ingested spores. (After Lister.)

Again, in some cases the plasmodia themselves aggregate in the same way as the amoeboids do in the Acrasieae, and combine to form a compound fruit termed an "aethalium,"^[98] with the regions of the separate plasmodia more or less clearly marked off. The species formerly termed Aethalium septicum is now known as Fuligo varians. It is a large and conspicuous species, common on tan, and is a pest in the tanpits. Its aethalia may reach a {93}diameter of a foot and more, and a thickness of two inches. Chondrioderma diffusum, often utilised as a convenient "laboratory type," is common on the decaying haulms of beans in the late autumn. The interest of this group is entirely biological, save for the "flowers of tan."^[99]

CHAPTER IV

PROTOZOA (CONTINUED): SPOROZOA^[100]

II. Sporozoa.

This group, of which seven years ago no single species was known in its complete cycle, has recently become the subject of concentrated and successful study, owing to the fact that it has been recognised to contain the organisms which induce such scourges to animals as malarial fevers, and various destructive murrains. Our earliest accurate, if partial knowledge, was due to von Siebold, Kölliker, and van Beneden. Thirty years ago Ray Lankester in England commenced the study of species that dwell in the blood, destined to be of such moment for the well-being of man and the animals in his service; and since then our knowledge has increased by the labours of Manson, Ross and Minchin at home, Laveran, Blanchard, Thélohan, Léger, Cuénot, Mesnil, Aimé Schneider in France, Grassi in Italy, Schaudinn, Siedlecki, L. and R. Pfeiffer, Doflein in Central Europe, and many others.

Fig. 31.—Lankesteria ascidiae, showing life-cycle. a, b, c, Sporozoites in digestive epithelium cells of host; d, e, growth stages; f, free gregarine; g, association; h, encystment; i, j, brood-divisions in associated mates; k, pairing-cells; l, syngamy; m, zygote; n, o, p, nuclear divisions in spores; q, cyst with adult spores, each containing 8 sickle-germs. (After Luhe, modified from Siedlecki.)

As a type we will take a simple form of the highest group, the Gregarinidaceae, Monocystis, which inhabits the seminal vesicles of the earthworm. In its youngest state, the "sporozoite," it is a naked, sickle-shaped cell, which probably makes its way from the gut into one of the large radial cells of the seminal funnel, where it attains its full size, and then passes out into the vesicles or reservoirs of the semen, to lie among the sperm morulae and young spermatozoa. The whole interior is formed of the opaque endosarc, which contains a large central nucleus, and is full of refractive granules of paramylum or paraglycogen,^[101] a carbohydrate allied to glycogen or animal starch, so common in the liver and {96}muscles of Metazoa; besides these it contains proteid granules which stain with carmine, and oil-drops. The ectosarc is formed of three layers: (1) the outer layer or "cuticle"^[102] is, in many cases if not here, ribbed, with minute pores in the furrows, and is always porous enough to allow the diffusion of dissolved nutriment; (2) a clear plasmatic layer, the "sarcocyte"; (3) the "myocyte," formed of "myonemes," muscular fibrils disposed in a network with transverse meshes, which effect the wriggling movements of the cell. The endosarc contains the granules and the large central nucleus. The adult becomes free in the seminal vesicles; here two approximate, and surround themselves with a common cyst: a process which has received the name of "association" (Fig. 31, g-i). Within this, however, the protoplasms remain absolutely distinct. The nucleus undergoes peculiar changes by which its volume is considerably reduced. When this process of "nuclear reduction" is completed, each of the mates undergoes brood-divisions (j), so as to give rise to a large number of rounded naked 1-nucleate cells—the true pairing-cells. These unite two and two, and so form the 1-nucleate spores (k-m), which become oat-shaped, form a dense cyst-wall, and have been termed "pseudonavicellae" from their likeness to the Diatomaceous genus Navicella. Some of the cytoplasm of the original cells remains over unused, as "epiplasm," and ultimately degenerates, as do a certain number of the brood-cells which presumably have failed to pair. It is believed that the brood-cells from the same parent will not unite together. The contents of each spore have again undergone brood-division to form eight sickle-shaped zoospores, or "sporozoites" (n-q), and thus the developmental cycle is completed. Probably the spores, swallowed by birds, pass out in their excrement, and when eaten by an earthworm open in its gut; the freed sickle-germs can now migrate through the tissues to the seminal funnels, in the cells of which they grow, ultimately becoming free in the seminal vesicles.^[103]

We may now pass to the classification of the group.

Fig. 32.—Gregarina blattarum Sieb. A, two cephalonts, embedded by their epimerite (ep), in cells of the gut-epithelium; deu, deutomerite; nu, nucleus; pr, protomerite; B¹, B², two free specimens of an allied genus; the epimerite is falling off in B², which is on its way to become a sporont; C, cyst (cy) of A, with sporoducts (spd) discharging the spores (sp), surrounded by an external gelatinous investment (g). (From Parker and Haswell.)

Monocystis offers us the simplest type of Gregarinidaceae. In most Gregarines (Figs. 31, 32) the sporozoite enters the epithelium-cell of the gut of an Arthropod, Worm or Mollusc, and as it enlarges protrudes the greater part of its bulk into the lumen, and may become free therein, or pass into the coelom. The attached part is often enlarged into a sort of grapple armed with spines, the "epimerite"; this contains only sarcocyte, the other layers being absent. The freely projecting body is usually divided by an ingrowth of the myocyte into a front segment ("protomerite"), and a rear one ("deutomerite"), with the nucleus usually in the latter. In this state the cell is termed a "cephalont." Conjugation is frequent, but apparently is not always connected with {99}syngamy or spore-formation; sometimes from two to five may be aggregated into a chain or "syzygy." The number of cases in which a syngamic process between two cells has been observed is constantly being increased. In Stylorhynchus (Fig. 33) the conjugation at first resembles that of Monocystis, but the actual pairing-cells are bisexually differentiated into sperms in the one parent, and oospheres in the other; it is remarkable that here the pear-shaped sperms are apparently larger than the oospheres. In Pterocephalus the chief difference is that the sperms are minute.^[106] In all cases of spore-formation the epimerite is lost and the septum disappears; in this state the cell is termed a sporont. Sometimes the epiplasm of the sporont forms tubes ("sporoducts"), which project through the cyst-wall and give exit to the spores, as in Gregarina (Fig. 32, C), a parasite in the beetle Blaps.

Gregarines infest most groups of Invertebrates except Sponges and perhaps Coelenterates, the only exception cited being that of Epizoanthus glacialis, a Zoantharian (p. 406). They appear to be relatively harmless and are not known to induce epidemics.

The Coccidiaceae never attain so high a degree of cellular differentiation as the Gregarines, which may be due to their habitat; for in the growing state they are intracellular parasites. Their life-history shows a double cycle, which has been most thoroughly worked out in Coccidiidae by Schaudinn and Siedlecki in parasites of our common Centipedes. We take that of Coccidium schubergi (in Lithobius forficatus^[107]), beginning with the sporozoite, which is liberated from the spores taken in with the food, in the gut of the Centipede. This active sickle-shaped cell (Fig. 34, l) enters an epithelial cell of the mid-gut, and grows therein till it attains its full size (a), when it is termed a "schizont"; for it segments (Gk. σχίζω, "I split") superficially into a large number of sickle-shaped zoospores, the "merozoites" (c), resembling the sporozoites. The segmentation is superficial, so that there may remain a large mass of residual epiplasm. The merozoites are set free by the destruction of the epithelium-cell in which they were formed, and which becomes disorganised, like the residual epiplasm. Each merozoite may repeat the {100}behaviour of the sporozoite, so that the disease spreads freely, and becomes acute after several reinfections. After a time the adult parasites, instead of becoming schizonts and simply forming merozoites by division, differentiate into cells that undergo a binary sexual differentiation. Some cells, the "oocytes" (d, e), escape into the gut, and the nucleus undergoes changes by which some of its substance (or an abortive daughter-nucleus) is expelled to the exterior (f), such a cell is now an "oogamete" or oosphere. Others, again, are spermatogones (h): each when full grown on escaping into the gut commences a division (i, j), like that of the schizonts. The products of this division or segment-cells are the flagellate sperms (s): they are more numerous and more minute than the merozoites produced by the schizonts, and are attracted to the oosphere by chemiotaxy (p. 23), and one enters it and fuses with it (g). The oosperm, zygote or fertilised egg, thus formed invests itself with a dense cyst-wall, as a "oospore" (k), its contents form one or more (2, 4, 8, etc.) spores; and each spore forms again one, two, or four sickle-shaped zoospores ("sporozoites"), destined to be liberated for a fresh cycle of parasitic life when the spores are swallowed by another host.

Fig. 33.—Bisexual pairing of Stylorhynchus. a, Spermatozoon; b-e, fusion of cytoplasm of spermatozoon and oosphere; f, g, fusion of nuclei; h-j, development of wall to zygote; k, l, formation of four sporoblasts; l, side view of spore; m, mature sporozoites in spore. (After Léger.)

Fig. 34.—Life-history of Coccidium schubergi. a, Penetration of epithelium-cell of host by sporozoite; b-d, stages of multiple cell-formation in naked state (schizogony); e, f, formation of oogamete; g, conjugation; h-j, formation of sperms (s); k, development of zygote (fertilised ovum) to form four spores; l, formation of two zoospores (or sickle germs) in each spore. (From Calkins's Protozoa, after Schaudinn.)

In some cases the oogametes are at first oblong, like ordinary merozoites, and round off in the gut. The microgametocyte, or spermatogone, has the same character, but is smaller; it applies itself like a cap to one pole of the oogamete, which has rounded off; it then divides into four sperms, whose cytoplasm is not sharply separated; one of these then separates from the common mass, enters the oogamete, and so conjugation is effected, with an oosperm as its result. This latter mode of conjugation is that of Adelea ovata and Coccidium lacazei: the former is probably the more primitive and the commoner. The sperms {102}of Coccidiidae, when free, usually possess two long flagella, either both anterior, or a very long one in front and a short one behind, both turned backwards.

The genus Coccidium affects many animals, and one species in particular, C. cuniculi Rivolta, attacks the liver of young rabbits,^[108] giving rise to the disease "coccidiosis." Coccidium may also produce a sort of dysentery in cattle on the Alpine pastures of Switzerland; and cases of human coccidiosis are by no means unknown. Coccidium-like bodies have been demonstrated in the human disease, "molluscum contagiosum," and the "oriental sore" of Asia; similar bodies have also been recorded in smallpox and vaccinia, malignant tumours and even syphilis, but their nature is not certainly known; some of these are now referred to Flagellata (see p. 121).

Closely allied to the Coccidiidae are the Haemosporidae, dwellers in the blood of various cold-blooded Vertebrates,^[109] and entering the corpuscles as sporozoites or merozoites to attain the full size, when they divide by schizogony; they are freed like those of the next family by the breaking up of the corpuscle. The merozoites were described by Gaule (1879) as "vermicles" ("Würmchen"), and regarded by him as peculiar segregation-products of the blood; though Lankester had described the same species in the Frog's blood as early as 1871, with a full recognition of its true character. His name, Drepanidium, has had to give way, having been appropriated to another animal, and has been aptly replaced by that of Lankesterella. The sexual process of Karyolysus has been found to take place in a Tick, that of Haemogregarina in a Leech, thus presenting a close analogy to the next group, which only differs in its less definite form in the active state, and in the lack of a cell-wall during brood-formation.

Laveran was the first to describe a member of the {103}Acystosporidae, in 1880, as an organism always to be found in the blood of patients suffering from malarial fever; this received the rather inappropriate name of Plasmodium, which, by a pedantic adherence to the laws of priority, has been used by systematists as a generic name. Golgi demonstrated the coincidence of the stages of the intermittent fever with those of the life-cycle of the parasite in the patient, the maturation of the schizont and liberation of the sporozoites coinciding with the fits of fever. Manson, who had already shown that the Nematodes of the blood that give rise to Filarial haematuria (see Vol. II. p. 149) have an alternating life in the gnats or mosquitos of the common genus Culex,^[110] in 1896 suggested to Ronald Ross that the same might apply to this parasite, and thus inspired a most successful work. The hypothesis had old prejudices in its favour, for in many parts there was a current belief that sleeping under mosquito-netting at least helped other precautions against malaria. Ross found early in his investigations that Culex was a good host for the allied genus Haemoproteus or Proteosoma, parasitic in birds, but could neither inoculate man with fever nor be inoculated from man. He found, however, that the malaria germs from man underwent further changes in the stomach of a "dappled-wing mosquito," that is, as we have since learned, a member of the genus Anopheles. Thenceforward the study advanced rapidly, and a number of inquirers, including Grassi, Koch, MacCallum (who discovered the true method of sexual union in Halteridium^[111]), and Ross himself, completed his discovery by supplying a complete picture of the life-cycles of the malaria-germs. Unfortunately, there has been a most unhappy rivalry as to the priority of the share in each fragment of the discovery, whose history is summarised by Nuttall, we believe, with perfect fairness.^[112]

The merozoite is always amoeboid, and in this state enters the blood corpuscle; herein it attains its full size, as a schizont, becoming filled with granules of "melanin" or black pigment, probably a decomposition product of the red colouring matter (haemoglobin).

Fig. 35.—Life-history of Malarial Parasites. A-G, Amoebula of quartan parasite to sporulation; H, its gametocyte; I-M, amoebula of tertian parasite to sporulation; N, its gametocyte; O, T, "crescents" or gametocytes of Laverania; P-S, sperm-formation; U-W, maturation of oosphere; X, fertilisation; Y, zygote. a, Zygote enlarging in gut of Mosquito; b-e, passing into the coelom; f, the contents segmented into naked spores; g, the spores forming sickle-germs or sporozoites; h, sporozoites passing into the salivary glands. (From Calkins's Protozoa, after Ross and Fielding Ould.)

The nucleus of the schizont now divides repeatedly, and then the schizont segments into a flat brood of germs (merozoites), relatively few in the parasite of quartan fever (Haemamoeba malariae, Fig. 35, E-G), many in that of tertian (H. vivax, Fig. 35, M). These brood-cells escape and behave for the most part as before. But after the disease has persisted for some time we find that in the genus Haemamoeba, {105}which induces the common malarial fevers of temperate regions, certain of the full-grown germs, instead of behaving as schizonts, pass, as it were, to rest as round cells; while in the allied genus Laverania, (Haemomenas, Ross) these resting-cells are crescentic, with blunt horns, and are usually termed half-moons (Fig. 35, O, T), characteristic of the bilious or pernicious remittent fevers of the tropics and of the warmer temperate regions in summer. These round or crescent-shaped cells are the gametocytes, which only develop further in the drawn blood, whether under the microscope, protected against evaporation, or in the stomach of the Anopheles: the crescents become round, and then they, like the already round ones of Haemamoeba, differentiate in exactly the same way as the corresponding cells of Coccidium schubergi. The female cell only exhibits certain changes in its nucleus to convert it into an oosphere: the male emits a small number of sperms, long flagellum-like bodies, each with a nucleus; and these, by their wriggling, detach themselves from the central core, no longer nucleated. The male gametogonium with its protruded sperms was termed the "Polymitus form," and was by some regarded as a degeneration-form, until MacCallum discovered that a "flagellum" regularly undergoes sexual fusion with an oosphere in Halteridium, as has since been found in the other genera. The oosperm (Y) so formed is at first motile ("ookinete"), as it is in Haemosporidae, and passes into the epithelium of the stomach of the gnat and then through the wall, acquiring a cyst-wall and finally projecting into the coelom (a-e). Here it segments into a number of spheres ("zygotomeres" of Ross) corresponding to the Coccidian spores, but which never acquire a proper wall (f). These by segmentation produce at their surface an immense quantity of elongated sporozoites (the "zygotoblasts" or "blasts" of Ross, Fig. 35, g), these are ultimately freed by the disappearance of the cyst-wall of the oosperm, pass through the coelom into the salivary gland (h), and are discharged with its secretion into the wound that the gnat inflicts in biting. In the blood the blasts follow the ordinary development of merozoites in the blood corpuscle, and the patient shows the corresponding signs of fever. This has been completely proved by rearing the insect from the egg, feeding it on the blood of a patient in whose blood there were ascertained to be the germs of a definite species of {106}Haemamoeba, sending it to England, where it was made to bite Dr. Manson's son, who had never had fever and whose blood on repeated examination had proved free from any germs. In the usual time he had a well-defined attack of the fever corresponding to that germ, and his blood on examination revealed the Haemamoeba of the proper type. A few doses of quinine relieved him of the consequences of his mild martyrdom to science. Experiments of similar character but of less rigorous nature had been previously made in Italy with analogous results. Again, it has been shown that by mere precautions against the bites of Anopheles, and these only, all residents who adopted them during the malarious season in the most unhealthy districts of Italy escaped fever during a whole season; while those who did not adopt the precautions were badly attacked.^[113]

Anopheles flourishes in shallow puddles, or small vessels such as tins, etc., the pools left by dried-up brooks and torrents, as well as larger masses of stagnant water, canals, and slow-flowing streams. Sticklebacks and minnows feed freely on the larvae and keep down the numbers of the species; where the fish are not found, the larvae may be destroyed by pouring paraffin oil on the surface of the water and by drainage. A combination of protective measures in Freetown (Sierra Leone) and other ports on the west coast of Africa, Ismailia, and elsewhere, has met with remarkable success during the short time for which it has been tried; and it seems not improbable, that as the relatively benign intermittent fevers have within the last century been banished from our own fen and marsh districts, so the Guinea coast may within the next decade lose its sad title of "The White Man's Grave."

So closely allied to this group in form, habit, and life-cycle are some species of the Flagellate genus Trypanosoma, that in their less active states they have been unhesitatingly placed here (see p. 119). Schaudinn has seen Trypanosomic characters in the "blasts" of this group, which apparently is the most primitive of the Sporozoa and a direct offshoot of the Flagellates.

The Myxosporidiaceae (Fig. 36) are parasitic in various {107}cold-blooded animals. They are at least binucleate in the youngest free state, and become large and multinucleate apocytes, which may bud off outgrowths as well as reproduce by spores. The spores of the apocyte are not produced by simultaneous breaking up, but by successive differentiation. A single nucleus aggregates around itself a limited portion of the cytoplasm, and this again forms a membrane, becoming an archespore or a "pansporoblast," destined to produce two spores; within this, nuclear division takes place so as to form about eight nuclei, two of which are extruded as abortive, and of the other six, three are used up in the formation of each of the two spores. Of these three nuclei in each spore, two form nematocysts, like those of a Coelenterate (p. 246 f.), at the expense of the surrounding plasm; while the third nucleus divides to form the two final nuclei of the reproductive body. The whole aggregate of the reproductive body and the two nematocysts is enveloped in a bivalve shell. In what we may call germination, the nematocysts eject a thread that serves for attachment, the valves of the shell open, and the binucleate mass crawls out and grows afresh. Nosema bombycis Nägeli (the spore of which has a single nematocyst) is the organism of the "Pébrine" of the silkworm, which was estimated to have caused a total loss in France of some £40,000,000 before Pasteur investigated the malady and prescribed the effectual cure, or rather precaution against its spread. This consisted in crushing each mother in water after it had laid its eggs and seeking for pébrine germs. If the mother proved to be infected, her eggs were destroyed, as the eggs she had laid were certain to be also tainted. Balbiani completed the study of the organism from a morphological standpoint. Some Myxosporidiaceae produce destructive epidemics in fish.

Fig. 36.—A, Myxidium lieberkühnii, amoeboid phase; B, Myxobolus mülleri, spore with discharged nematocysts (ntc); C, spores (psorosperms) of a Myxosporidian. ntc, nematocysts. (From Parker and Haswell.)

The Dolichosporidia or Sarcosporidiaceae are, in the adult state, elongated sacs, often found in the substance of the voluntary muscles, and known as "Rainey's" or "Miescher's Tubes"; they are at first uninucleate, then multinucleate, and then break up successively into uninucleate cells, the spores, in each of which, by division, are formed the sickle-shaped zoospores.^[114]

CHAPTER V

PROTOZOA (CONTINUED): FLAGELLATA

III. Flagellata.

The Flagellates thus defined correspond to Bütschli's group of the Mastigophora. The lowest and simplest forms, often loosely called "Monads," are only distinguishable from Sarcodina (especially Proteomyxa) and Sporozoa by the above characters: their artificial nature is obvious when we remember that many of the Sarcodina have a flagellate stage, and that the sperms of bisexual Sporozoa are flagellate (as are indeed those of all Metazoa except Nematodes and most Crustacea). Even as thus limited the group is of enormous extent, and passes into the Chytridieae and Phycomycetes Zoosporeae on the one hand, and by its holophytic colonial members into the Algae, on the other.^[115]

Classification.

The Protomastigaceae and Volvocaceae are so extensive as to require further subdivision.

Protomastigaceae

Volvocaceae

Fig. 37.—Various forms of Flagellata. 2, 6-8, 10, 13, 14, Protomastigaceae; 11, 12, Chrysomonadaceae; 9, Cryptomonadaceae; 1, 3, Euglenaceae; 4, Pantostomata: note branched stalk in 13; branched tubular theca in 14; distinct thecae in 11; stalk and theca in 10. In 2, flagellate (a) and amoeboid (b) phases are shown; in 5, flagellate (a) and Heliozoan (b) phases^[116]; in 8 are shown two stages in the ingestion of a food particle (f); chr, plastoids; c.vac, contractile vacuole; f, food particle; g, gullet; l, theca; nu, nucleus; p, protoplasm; per, peristome; v.i, vacuole of ingestion. (From Parker and Haswell, mostly from Bütschli's Protozoa.)

The modes of nutrition are threefold: the simplest forms live in liquids containing decaying organic matter which they absorb through their surface ("saprophytic"): others take in food either Amoeba fashion, or into a vacuole formed for the purpose, or into a definite mouth ("holozoic"): others again have coloured plastids, green or brown or yellow ("holophytic"), having the plant's faculty of manufacturing their own food-supply. But we meet with species that show chromatophores at one time and lack them at another; or, again, the same individual (Euglena) may pass from holozoic life to saprophytic (Paramoeba, some Dinoflagellates) as conditions alter.

Many secrete a stalk at the hinder end: by "continuous" formation of this, without rupture at fission, a branching colony is formed (Polyoeca). This stalk may have a varying consistency. In Anthophysa (Fig. 37, 13) it appears to be due to the welding of excrementitious particles voided at the hinder end of the body with a gelatinous excretion; but the division of the stalk is here occasional or intermittent, so that the cells are found in tufts at the apex of the branches. A corresponding secretion, gelatinous or chitinous, around the body of the cell forms a cup or "theca," within which the cell lies quite free or sticking to it by its surface, or attached to it by a rigid or contractile thread. The theca, again, may assume the form of a mere gelatinous mass in which the cell-bodies may be completely plunged, so that only the flagella protrude, as in Volvocidae, Proterospongia (Fig. 75, p. 182), and Rhipidodendron (Fig. 37, 14). Often this jelly assumes the form of a fan (Phalansterium), the branching tubes of which it is composed lying for some way alongside, and ultimately diverging. In Hydrurus, the branching jelly assumes the form of a branching Confervoid.^[117]

The cell-body may be bounded by an ill-defined plasmatic layer in Chrysomonadaceae and some Protomastigaceae,^[118] or it may form a plasmatic membrane or "pellicle," sometimes very firm and tough, or striated as in Euglenaceae, or it may have a separate "cuticle" (in the holophytic species formed of cellulose), or even a bivalve or multivalve shell of distinct plates, hinged or overlapping (Cryptoglena, Phacotus, Dinoflagellates). The wall of the {114}Coccolithophoridae, a family of Chrysomonadaceae, is strengthened by embedded calcareous spicules ("coccoliths," "cyatholiths," "rhabdoliths"), which in the most complex forms (cyatholiths) are like a shirt-stud, traversed by a tube passing through the stem and opening at both ends. These organisms^[119] constitute a large proportion of the plankton; the spicules isolated, or in their original state of aggregation ("coccospheres," "rhabdospheres"), enter largely into the composition of deep-sea calcareous oozes. They occur fossil from Cambrian times (Potsdam sandstone of Michigan and Canada), and are in some strata extremely abundant, 800,000 occurring to the mm. cube in an Eocene marl.

The Silicoflagellates have siliceous skeletons resembling that of many Radiolaria, to which they were referred until the living organism was described (see pp. 79, 86 f.).

The flagellum has been shown by Fischer to have one of two forms: either it is whip-like, the stick, alone visible in the fresh specimen, being seen when stained to be continued into a long lash, hitherto invisible; or the whole length is fringed with fine ciliiform lateral outgrowths. If single it is almost always protruded as a tugging organ ("tractellum");^[120] the chief exceptions are the Craspedomonads, where it is posterior and acts as a scull ("pulsellum"), and some Dinoflagellates, where it is reversible in action or posterior. In addition to the anterior flagellum there may be one or more posterior ones, which trail behind as sense organs, or may anchor the cell by their tips. Dallingeria has two of these, and Bodo saltans a single anterior anchoring lash, by which they spring up and down against the organic débris among which they live, and disintegrate it. The numerous similar long flagella of the Trichonymphidae afford a transition in the genus Pyrsonympha to the short abundant cilia of Opalina, usually referred to the Ciliate Infusoria.

An undulating membrane occurs, sometimes passing into the flagellum in certain genera, all parasitic, such as Trypanosoma (incl. Herpetomonas), Trichomonas, Hexamitus, and Dinenympha.

In some cases the flagellum (or flagella) is inserted into a definite pit, which in allied forms is the mouth-opening. The contractile vacuole is present in the fresh-water forms, but not in all the marine ones, nor in the endoparasites. It may be single or surrounded by a ring of minute "formative" vacuoles or discharge into a permanently visible "reservoir." This again may discharge directly to the surface or through the pit or canal in which the flagellum takes origin (Euglena).

The "chromatophore" may be a single or double plate, or multiple.^[121] In the peculiar form Paramoeba the chromatophore may degenerate and be reproduced anew. It often encloses rounded or polygonal granules of uncoloured plasma, very refractive, known as "pyrenoids." These, like the chromatophores, multiply by direct fission. The "reserves" may be (1) fat-globules; (2) granules of a possibly proteid substance termed "leucosin"; (3) a carbohydrate termed "paramylum," differing slightly from starch (see p. 95); (4) true starch, which is usually deposited in minute granules to form an investment for the pyrenoid when such is present.

A strongly staining granule is usually present in the plasma near the base of the flagellum. This we may term a "blepharoplast" or a "centrosome" in the wider sense.

Fission is usually longitudinal in the active state; a few exceptions are recorded. Encystment is not uncommon; and in the coloured forms the cyst-wall is of cellulose. Division in the cyst is usually multiple;^[122] in the coloured forms, however, vegetative growth often alternates with division, giving rise to plant-like bodies. Polytoma and other Chlamydomonadidae multiply by "brood-formation" in the active state; the blepharoplast, as Dangeard suggests, persisting to continue the motion of the flagella of the parent, while the rest of the plasm divides to form the brood. Conjugation has been observed in many species. In some species of Chlamydomonas it takes place after one or both of the two {116}cells have come to rest, but in most cases it occurs between active cells. We find every transition between equal unions and differentiated sexual unions, as we shall see in discussing the Volvocaceae.^[123] The "coupled-cell" differs in behaviour in the different groups, but almost always goes to rest and encysts at once, whatever it may do afterwards.

The life-history of many Flagellates has been successfully studied by various observers, and has shed a flood of light on many of the processes of living beings that were hitherto obscure. The first studies were carried through by the patient labours of Drysdale and Dallinger. A delicate mechanical stage enabled the observer to keep in the field of view a single Flagellate, and, when it divided into two, to follow up one of the products. A binocular eye-piece saved much fatigue, and enabled the observers to exchange places without losing sight of the special Flagellate under observation; for the one who came to relieve would put one eye to the instrument and recognise the individual Flagellate under view as he passed his hand round to the mechanism of the stage before the first watcher finally relinquished his place at the end of the spell of work. Spoon-feeding by Mrs. Dallinger enabled such shifts to be prolonged, the longest being one of nine hours by Dr. Dallinger.

Fig. 38.—Bodo saltans. A, the positions assumed in the springing movements of the anchored form; B, longitudinal fission of anchored forms; C, transverse fission of the same; D, fission of free-swimming form; E¹-E⁴, conjugation of free-swimming with anchored form; E⁵, zygote; E⁶, emission of spores from zygote; F, growth of spores: c.vac, contractile vacuole; fl.1, anterior; fl.2, ventral flagellum; nu, nucleus. (From Parker's Biology, after Dallinger.)

The life-cycles varied considerably in length. It was in every case found that after a series of fissions the species ultimately underwent conjugation (more or less unequal or bisexual in character);^[124] the zygote encysted; and within the cyst the protoplasmic body underwent brood-formation, the outcome of which was a mass of {118}spores discharged by the rupture of the cyst (Fig. 38). These spores grow from a size too minute for resolution by our microscopes into the ordinary flagellate form. They withstand the effects of drying, if this be effected immediately on their escape from the ruptured cyst; so that it is probable that each spore has itself a delicate cyst-wall and an aplanospore, from which a single zoospore escapes. The complex cycle, of course, comprises the whole course from spore-formation to spore-formation. Such complete and regular "life-histories," each characteristic of the species, were the final argument against those who held to the belief that spontaneous generation of living beings took place in infusions of decomposing organic matter.

Previous to the work of these observers it had been almost universally believed that the temperature of boiling water was adequate to kill all living germs, and that any life that appeared in a closed vessel after boiling must be due to spontaneous change in its contents. But they now showed that, while none of the species studied resisted exposure in the active condition to a temperature of 138°-140° F., the spores only succumbed, in liquid, to temperatures that might even reach 268° F., or when dry, even 300° F. or more. Such facts explain the constant occurrence of one or more such minute species in liquids putrefying under ordinary conditions, the spores doubtless being present in the dust of the air. Very often several species may co-exist in one infusion; but they separate themselves into different zones, according to their respective need for air, when a drop of the liquid is placed on the slide and covered for examination. Dallinger^[125] has made a series of experiments on the resistance of these organisms in their successive cycles to a gradual rise of temperature. Starting with a liquid containing three distinct species, which grew and multiplied normally at 60° F., he placed it under conditions in which he could slowly raise the temperature. While all the original inmates would have perished at 142° F., he succeeded in finally producing races that throve at 158° F., a scalding heat, when an accident put an end to that series of experiments. In no instance was the temperature raised so much as to kill off the beings, so that the increased tolerance of their descendants was due not, as might have been anticipated, to selection of those that best resisted, but to the inheritance of {119}an increased toleration and resistance from one generation or cycle to another.

As we noted above (p. 40), the study of the Flagellates has been largely in the hands of botanists. After the work of Bütschli in Bronn's Thier-Reich, Klebs^[126] took up their study; and the principal monographs during the last decade have appeared in Engler and Prantl's Pflanzenfamilien, where Senn^[127] treats the Flagellates generally, Wille^[128] the Volvocaceae, and Schütt the "Peridiniales" or Dinoflagellata;^[129] while only the Cystoflagellata, with but two genera, have been left to the undisputed sway of the zoologists.^[130]

Among this group the majority are saprophytes, found in water containing putrefying matter or bacteria. The forms so carefully studied by Dallinger and Drysdale belong to the genera Bodo, Cercomonas, Tetramitus, Monas, and Dallingeria. Many others are parasites in the blood or internal cavities of higher animals, some apparently harmless, such as Trichomonas vaginalis, parasitic in man, others of singular malignity. Costia necatrix, infesting the epithelial scales of fresh-water fish, often devastates hatcheries. The genus Trypanosoma, Gruby, contributes a number of parasites, giving rise to deadly disease in man and beast.^[131] T. lewisii is common in Rodents, but is relatively harmless. T. evansii is the cause of the Surra disease of Ruminants in India, and is apparently communicated by the bites of "large brown flies" (almost certainly Breeze Flies or Tabanidae, Vol. VI. p. 481). T. brucei, transferred to cattle by the Tsetse Fly, Glossina morsitans (see Vol. VI. Fig. 244, p. 513) in Equatorial Africa, is the cause of the deadly Nagana disease, which renders whole tracts of country impassable to ox or horse. Other Trypanosomic diseases of animals are, in Algeria and the Punjab, "dourine," infecting horses and dogs; in South America, Mal de Caderas (falling-sickness), an epidemic paralysis of cattle. During the printing of this book, much additional knowledge has been gained on this genus and the diseases it engenders. The Trypanosomic {120}fever recently recognised on the West Coast has been found to be the early stage of the sleeping-sickness, that well-known and most deadly epidemic of Tropical Africa. Through the researches of Castellani, Nabarro, and especially Colonel and Mrs. Bruce, we know now that the parasite T. gambiense is transferred by an intermediate host, a kind of Tsetse Fly (Glossina palpalis). Schaudinn's full study of a parasite of the blood corpuscles of the Owl has shown that while in its intracorpuscular state it resembles closely the malarial parasites in behaviour, and in its schizogenic multiplication, so that it was considered an Acystosporidian, under the name of Halteridium, it is really a Trypanosoma;^[132] for the accomplishment of successful sexual reproduction it requires transference to the gut of a gnat (Culex). The germs may infect the ovary, and give the offspring of the insect the innate power of infecting Owls. Thus a new light is shed on the origin of the Coccidiaceae, whose "blasts" in the insect host resemble Trypanosoma in their morphology.

Fig. 39.—Morphology of Trypanosoma. a-f, Stages in development of Trypanosoma noctuae from the active zygote ("ookinete"); b, first division of nucleus into larger (trophic) and smaller (kineto-) nucleus; c, d, division of smaller nucleus and its transformations to form "blepharoplast" and myonemes; f, adult Trypanosoma; g, h, i, Treponema zeemannii of Owl; g, Trypanosome form; h, Spirochaeta form; i, rosette aggregate. (After Schaudinn.)

The human Tick fever of the Western United States and the epizootic Texas fever are known to be due to blood parasites of the genus Piroplasma (Babesia), of which the free state is that of a Trypanosome. It appears certain that Texas fever, though due to Tick bites, is not transferred directly from one beast to another by the same Tick; but the offspring of a female Tick that has sucked an infected ox contains Trypanosome germs, and will by their bites infect other animals. {121}It would seem probable that the virulence of the Persian Tick (Argas persica) is due to similar causes. The Indian maladies known as "Kala Azar" and "Oriental Sore" are characterised by blood parasites, at first called after their discoverer the "Leishman bodies," which have proved to be the effects of a Piroplasma.

Trypanosoma is distinguished by the expansion of its flagellum into an undulating membrane, that runs down the edge of the body, and may project behind as a second lash. In this membrane run eight fine muscular filaments, or myonemes, four on either surface, within the undulating membrane; at their lower end they are all connected with a rounded body, the "blepharoplast," which is here in its origin, as well as in its behaviour in reproductive processes, a true modified nucleus, comparable in some respects, as was first noted by Plimmer and Rose Bradford,^[133] with the micronucleus of the Infusoria. Part of the segmentation spindle persists in the form of a filament uniting the blepharoplast with the large true functional nucleus (Fig. 39, a-f).

The blood of patients suffering from relapsing fever contains a fine wriggling parasite, which was described as a Schizomycete, allied to the bacteria, and hitherto termed Spirochaeta obermeieri. Schaudinn has shown that this and other similar blood parasites are closely allied to Trypanosoma; and since the original genus was founded on organisms of putrefaction which are undoubtedly Schizomycetes, Vuillemin has suggested the name Treponema. T. pallidum is found in syphilitic patients, and appears to be responsible for their illness.^[134]

The Craspedomonadidae (often called Choanoflagellates, Fig. 40) are a group whose true nature was elucidated some forty years ago by the American zoologist, H. James-Clark. They are attached either to a substratum, by a stalk produced by the base of the cell, or to other members of the same colony; they are distinguished by the protrusion of the cytoplasm around the base of the single flagellum into a pellucid funnel,^[135] in which the plasma is in constant motion, though the funnel retains its shape and size, except when, as sometimes happens, it is retracted.

Fig. 40.—Various forms of Craspedomonadidae. 2, a, Adult cell; 2, b, longitudinal fission; 2, c, the production of flagellulae by brood-formation; c, collar; c.vac, contractile vacuole; fl, flagellum; l, theca; nu, nucleus; s, stalk. (After Saville Kent.)

The agitation of the flagellum determines a stream of water upwards along the outer walls of the funnel; and the food-particles brought along adhere to the outside of the funnel, and are carried by its streaming movement to the basal constriction, where they are swallowed by the plasma, which appears to form a swallowing vacuole at that point. Longitudinal fission is the ordinary mode of reproduction, extending up through the funnel. If the two so formed continue to produce a stalk, the result is the formation of a tree-like stem, whose twigs bear at the ends the funnelled cells, or "collar-cells" as they are usually called. In Salpingoeca, as in so many other Flagellates, each cell forms a cup or theca, often of most graceful vase-like outline, the rim being elegantly turned back. Proterospongia (Fig. 75, p. 182) secretes a gelatinous investment for the colony, which is attached to solid bodies. In this species, according to Saville Kent, the central members of the colony retract their collar, lose their flagellum, become amoeboid, and finally undergo brood-formation to produce minute zoospores. This is the form which by its differentiation recalls the Sponges, and has been regarded as a {123}transition towards them; for the flagellate, nutritive cells of the Sponges are provided with a collar, which exists in no other group of Metazoa (see pp. 171, 181, and Fig. 70, p. 176). The most recent monographer of the family is Raoul Francé, but James-Clark and Saville Kent did the pioneering work.

Fig. 41.—Opalina ranarum. A, living specimen; B, stained specimen showing nuclei; C, stages in nuclear division; D-F, stages in fission; G, final product of fission; H, encysted form; I, young form liberated from cyst; K, the same after multiplication of the nucleus has begun. nu, Nucleus. (From Parker's Biology, after Saville Kent and Zeller.)

Of the life-history of the Trichonymphidae,^[136] all of which are parasitic in the alimentary canal of Insects, especially Termites or White Ants (Vol. V. p. 356), nothing is known. Some of them have a complete investment of motile flagella, like enormously long cilia, which in Dinenympha appear to coalesce into four longitudinal undulating membranes. Lophomonas inhabits the gut of the Cockroach and Mole-cricket. The Opalinidae have also a complete investment of cilia, which are short, and give the aspect of a Ciliate to the animal, which is common in the rectum of Amphibia, and dies when transferred to water. But despite the outward resemblance, the nuclei, of which there may be as many as 200, are all similar, and consequently this group cannot be placed among the Infusoria at all. Opalina has no mouth nor contractile vacuole. It multiplies by dividing {124}irregularly and at intervals, resolving finally into 1-nucleate fragments, which encyst and pass into the water. When swallowed the cyst dissolves, its contents enlarge, and ultimately assume the adult form.^[137]

Maupasia has a partial investment of cilia, a single long flagellum and mouth, a contractile vesicle, and a single simple nucleus. It seems to find an appropriate place near the two above groups, though it is free, and possesses a mouth.

Fig. 42.—Longitudinal Fission of Eutreptia viridis (Euglenaceae), showing chloroplasts, nucleus, and flagella arising from pharynx-tube. (After Steuer.)

Among the Euglenaceae, Euglena viridis is a very common form, giving the green colour to stagnant or slow-flowing ditches and puddles in light places, especially when contaminated by a fair amount of dung, as by the overflow of a pig-sty, in company with a few hardy Rotifers, such as Hydatina senta (Vol. II. Fig. 106, p. 199) and Brachionus. Euglena is about 0.1 mm. in length when fully extended, oval, pointed behind, obliquely truncate in front, with a flagellum arising from the pharyngeal pit. It shows a peculiar wriggling motion, waves of transverse constriction passing along the body from end to end, as well as flexures in different meridians. Such motions are termed "euglenoid." The front part is colourless, but under a low {125}power the rest of the cell is green, owing to the numerous chlorophyll bodies or chloroplasts. The outermost layer of the cytoplasm shows a somewhat spiral longitudinal striation, possibly due to muscular fibrils. The interior contains many laminated plates of paramylum, and a large single nucleus. At the front of the body at the base of the flagellum is a red "eye-spot" on the dorsal side of the pharynx-tube or pit, from which the flagellum protrudes. Wager has shown that this tube receives, also on its dorsal side, the opening of a large vacuole, sometimes called the reservoir, for into it discharges the contractile vacuole (or vacuoles). The eye-spot is composed of numerous granules, containing the vegetal colouring matter "haematochrome." It embraces the lower or posterior side of the communication between the tube and the reservoir. The flagellum has been traced by Wager through the tube into the reservoir, branching into two roots where it enters the aperture of communication, and these are inserted on the wall of the reservoir at the side opposite the eye-spot. But on one of the roots near the bifurcation is a dilatation which lies close against the eye-spot, so that it can receive the light reaction. Euglena is an extremely phototactic organism. It shows various wrigglings along the longitudinal axis, and transverse waves of contraction and expansion may pass from pole to pole.^[138]

Among the Chrysomonadaceae the genus Zooxanthella, Brandt, has already been described under the Radiolaria (p. 86), in the jelly of which it is symbiotic. It also occurs in similar union in the marine Ciliates, Vorticella sertulariae and Scyphidia scorpaenae, and in Millepora (p. 261) and many Anthozoa (pp. 373 f., 396).

Of the Chlamydomonadidae, Sphaerella (Haematococcus, Ag.) pluvialis (Fig. 43), and S. nivalis, in which the green is masked by red pigment, give rise to the phenomena of "red snow" and "bloody rain." The type genus, Chlamydomonas, is remarkable for the variations from species to species in the character and behaviour of the gametes. Sometimes they are equal, at other times of two sizes. In some species they fuse immediately on approximation, in the naked active state; in others, they encyst on approaching, and unite by the emission of a fertilising tube, {126}as in the Algal Conjugatae. Zoochlorella is symbiotic in green Ciliata (pp. 153 f., 158), Sponges (p. 175), Hydra (p. 256), and Turbellaria (Vol. II. p. 43).

Fig. 43.—Sphaerella pluvialis. A, motile stage; B, resting stage; C, D, two modes of fission; E, Sphaerella lacustris, motile stage. chr, Chromatophores; c.vac, contractile vacuole; c.w, cell-wall; fl, flagella; nu, nucleus; nu', nucleolus; pyr, pyrenoids. (From Parker's Biology.)

Of the Volvocidae, Volvox (Fig. 44) is the largest and most conspicuous genus. Its colony forms a globe the size of a pin's head, floating on the surface of ponds, drains, or even puddles or water-barrels freely open to the light. It has what may be called a skeleton of gelatinous matter,^[139] condensed towards the surface into a denser layer in which the minute cells are scattered. These have each an eye-spot, a contractile vacuole, and two flagella, by the combined action of which the colony is propelled. Delicate boundary lines in the colonial wall mark out the proper investment of each cell. The cells give off delicate plasmic threads which meet those of their neighbours, and form a bond between them. In that half of the hemisphere which is posterior in swimming, a few (five to eight) larger cells ("macrogonidia" of older writers) are evenly distributed, protruding as they increase in size into the central jelly. These as they grow segment to form a new colony.

Fig. 44.—Volvox globator. A, entire colony, enclosing several daughter-colonies; B, the same during sexual maturity; C, four zooids in optical section; D¹-D⁵, development of parthenogonidium; E, ripe spermogonium; F, sperm; G, ovum; H, oosperm. a, Parthenogonidia; fl, flagellum; ov, ovum; ovy, ovaries; pg, pigment spot; sp, sperms; Spy, spermogonia dividing to form sperms. (From Parker's Biology, after Cohn and Kirchner.)

The divisions are only in two planes at right angles, so that the young colony is at first a plate, but as the cells multiply the plate bends up (as in the gastrulation of the double cellular plate of the Nematode Cucullanus, Vol. II. p. 136), and finally forms a hollow sphere bounded by a single layer of cells: the site of the original orifice may be traced even in the adult as a blank space larger than exists elsewhere. Among the cells of the young colony some cease to divide, but continue to grow at an early period, and these are destined to become in turn the mothers ("parthenogonidia") of a new colony; they begin segmenting before the colony of which they are cells is freed. The young colonies are ultimately liberated by the rupture of the sphere as small-sized spheres, which henceforth only grow by enlargement of the sphere as a whole, and the wider separation of the vegetative cells. Thus the vegetative cells soon cease to grow; all the supply of food material due to their living activities goes to the nourishment of the parthenogonidia, or the young colonies, as {128}the case may be. These vegetative cells have therefore surrendered the power of fission elsewhere inherent in the Protist cell. Moreover, when the sphere ruptures for the liberation of the young colonies, it sinks and is doomed to death, whether because its light-loving cells are submerged in the ooze of the bottom, or because they have no further capacity for life. When conjugation is about to take place, it is the cells that otherwise would be parthenogonidia that either act as oospheres or divide as "spermogonia" to form a flat brood of minute yellow male cells ("sperms"). These resemble vegetative cells, in the possession of an eye-spot and two contractile vacuoles, but differ in the enormously enlarged nucleus which determines a beaked process in front. After one of these has fused with the female cell ("oosphere") the product ("oosperm") encysts, passes into a stage of profound rest, and finally gives rise to a new colony. The oospheres and sperm-broods may arise in the same colony or in distinct ones, according to the species.

Before we consider the bearings of the syngamic processes of Volvox, we will study those presented by its nearer allies, which have the same habitat, but are much more minute. Three of these are well known, Stephanosphaera, Pandorina, and Eudorina, all of which have spherical colonies of from eight to thirty-two cells embedded at the surface of a sphere, and no differentiation into vegetative cells and parthenogonidia (or reproductive cells).

Stephanosphaera has its eight cells spindle shaped, and lying along equidistant meridians of its sphere; in vegetative reproduction each of these breaks up in its place to form a young colony, and the eight daughter-colonies are then freed. In conjugation, each cell of the colony breaks up into broods of 4, 8, 16, or 32 small gametes, which swim about within the general envelope, and pair and fuse two and two: this is "isogamous," "endogamous" conjugation. In Pandorina (Fig. 45) the cells are rounded, and are from 16 to 32 in each colony. The vegetative reproduction in this, as in Eudorina, is essentially the same as in Stephanosphaera. In conjugation the cells are set free, and are of three sizes in different colonies, small (S), medium (M), and large (L). The following fusions may occur: S × S, S × M, S × L, M × M, M × L. Thus the large are always female, as it were, the medium may play the part of male to the large, female to the small; the small are males to the medium and to the large. The medium {129}and small are capable, each with its like, of equal, undifferentiated conjugation; so that we have a differentiation of sex far other than that of ordinary, binary sex. Eudorina, however, has attained to "binary sex," for the female cells are the ordinary vegetative cells, at most a little enlarged, and the male cells are formed by ordinary cells producing a large flat colony of sixty-four minute males or sperms. In some cases four cells at the apex of a colony are spermogonia, producing each a brood of sperms, while the rest are the oospheres. The transition to Volvox must have arisen through the sterilisation of the majority of cells of a colony for the better nutrition of the few that are destined alone for reproduction.

Fig. 45.—Pandorina morum. A, entire colony; B, asexual reproduction, each zooid dividing into a daughter-colony; C, liberation of gametes; D-F, three stages in conjugation of gametes; G, zygote; H-K, development of zygote into a new colony. (From Parker's Biology, after Goebel.)

Volvox, as we have seen, has attained a specialisation entirely comparable to that of a Metazoon, where the segmentation of the fertilised ovum results in two classes of cells: those destined {130}to form tissues, and condemned to ultimate death with the body as a whole, and those that ultimately give rise to the reproductive cells, ova, and sperms. But this is a mere parallelism, not indicating any sort of relationship: the oospores of the Volvocaceae show that tendency to an encysted state, in which fission takes place, that is so characteristic of Algae, and these again show the way to Cryptogams of a higher status. Thus, Volvox, despite the fact that in its free life and cellular differentiation it is the most animal of all known Flagellates, is yet, with the rest of the Volvocaceae, inseparable from the Vegetable Kingdom, and is placed here only because of the impossibility of cleaving the Flagellates into two.

The Dinoflagellata (Figs. 46, 47) are often of exceptionally large dimensions in this class, attaining a maximum diameter of 150 µ (1⁄160") and even 375 µ (1⁄67") in Pyrocystis noctiluca. The special character of the group is the presence of two flagella; the one, filiform, arises in a longitudinal groove, and extending its whole length projects behind the animal, and is the conspicuous organ of motion: the other, band-like, arises also in the longitudinal groove, but extends along a somewhat spiral transverse groove,^[140] and never protrudes from it in life, executing undulating movements that simulate those of a girdle of cilia, or a continuous undulating membrane (Fig. 46). This appearance led to the old name "Cilioflagellata," which had of course to be abandoned when Klebs discovered the true structure.^[141] There is a distinct cellulose membrane, sometimes silicified, to the ectoplasm, only interrupted by a bare space in the longitudinal groove, whence the flagella take origin. This cuticle is usually hard, sculptured, and divided into plates of definite form, bevelled and overlapping at their junction; occasionally the cell has been seen to moult them.

A large vacuolar space, traversed by plasmic strings, separates the peripheral cytoplasm from the central, within which is the large nucleus. There are in most species one or more chromatophores, coloured by a yellowish or brownish pigment, which is a mixture of lipochromes, distinct from diatomin. In a few species the presence of these is not constant, and these species {131}show variability as to their nutrition, which is sometimes holozoic. Under these conditions the cell can take in food-particles as bulky as the eggs of Rotifers and Copepods, by the protrusion of a pseudopod at the junction of the two grooves. As in most coloured forms an eye-spot is often present, a cup-shaped aggregation of pigment, with a lenticular refractive body in its hollow. A contractile vacuole, here termed a "pusule," occurs in many species, communicating with the longitudinal groove by a canal. Nematocysts (see p. 246 f.) are present in Polykrikos, trichocysts (see p. 142) in several genera.

Fig. 46.—Peridinium divergens. a, Flagellum of longitudinal groove; b, flagellum of transverse groove; cr.v, contractile vacuole surrounded by formative vacuoles; n, nucleus. (After Schütt.)

Division is usually oblique, dividing the body into two dissimilar halves, each of which has to undergo a peculiar growth to reconstitute the missing portion, and complete the shell. The incomplete separation of the young cells leads to the formation of chains, notably in Ceratium and Polykrikos, the latter dividing transversely and occurring in chains of as many as eight. The process of division may take place when the cell is active, or in a cyst, as in Pyrocystis (Fig. 47). Again, encystment may precede multiple fission, resulting in the formation of a brood of minute swarmers. It has been suggested that these are capable of playing the part of gametes, and conjugating in pairs.^[142]

The Dinoflagellates are for the most part pelagic in habit, floating at the surface, and when abundant tinge the water of fresh-water lakes or even ponds red or brown. Peridinium (Fig. 46) and Ceratium (the latter remarkable for the horn-like backward prolongations of the lower end) are common genera both in the sea {132}and fresh-waters. Gymnodinium pulvisculus is sometimes parasitic in Appendicularia (Vol. VII. p. 68). Polykrikos^[143] has four transverse grooves, each with its flagellum, besides the terminal one. Many of the marine species are phosphorescent, and play a large part in the luminosity of the sea, and some give it a red colour.

Several fossil forms have been described. Peridinium is certainly found fossil in the firestone of Delitzet, belonging to the Cretaceous. A full monograph of the group under the name "Peridiniales" was published by Schütt.^[144]

Fig. 47.—Pyrocystis fusiformis, Murray. × 100. From the surface in the Guinea Current. (From Wyville Thomson.)

The Cystoflagellates contain only two genera,^[145] Noctiluca, common at the surface of tranquil seas, to which, as its name implies, it gives phosphorescence, and Leptodiscus, found by R. Hertwig in the Mediterranean. Noctiluca is enormous for a Flagellate, for with the form of a miniature melon it measures about 1 mm. (1⁄25") or more in diameter. In the depression is the "oral cleft," from one end of which rises, by a broad base, a large coarse flagellum, as long as the body or longer and transversely striated. In front of the base of the flagellum are two lip-like {133}prominences, of which one, a little firmer than the other, and transversely ridged, is called the tooth; at the junction of the two is a second, minute, flagellum, usually called the cilium. Behind these the oral groove has an oval space, the proper mouth; behind this, again, the oral groove is continued for some way, with a distinct rod-like ridge in its furrow. The whole body, including the big flagellum, is coated by a strong cuticular pellicle, except at the oblong mouth, and the lips and rod are mere thickenings of this. The cytoplasm has a reticulate arrangement: the mouth opens into a central aggregate, from which strands diverge branching as they recede to the periphery, where they pass into a continuous lining for the cuticular wall, liquid filling the interspaces. The whole arrangement is not unlike that found in many plant-cells, but the only other Protists in which it occurs are the Ciliata Trachelius (Fig. 56, p. 153) and Loxodes. The central mass contains the large nucleus. Noctiluca is an animal feeder, and expels its excreta through the mouth. The large flagellum is remarkable for the transverse striation of its plasma, especially on the ventral side. The cuticle may be moulted as in the Dinoflagellates. As a prelude to fission the external differentiations disappear, the nucleus divides in the plane of the oral groove, and a meridional constriction parts the two halves, the new external organs being regenerated. Conjugation occurs also, the two organisms fusing by their oral region; the locomotive organs and pharynx disappear; the conjoined cytoplasms unite to form a sphere, and the nuclei fuse to form a zygote or fertilisation nucleus. This conjugation is followed by sporulation or brood-formation.^[146]

Fig. 48.—Noctiluca miliaris, a marine Cystoflagellate. (From Verworn.)

The nucleus passes towards the surface, undergoes successive fissions, and as division goes on the numerous daughter-nuclei occupy little prominences formed by the upgrowth of the cytoplasm of the upper pole. The rest of the cytoplasm atrophies, and the hillocks formed by the plasmic outgrowths around the final daughter-nuclei become separate as so many zoospores (usually 256 or 512); each of these is oblong with a dorsal cap-like swelling, from the edge of which arises a flagellum pointing backwards; parallel to this the cap is prolonged on one side into a style also extending beyond the opposite pole of the animal.^[147] In this state the zoospore is, to all outward view, a naked Dinoflagellate, whence it seems that the Cystoflagellates are to be regarded as closely allied to that group. Leptodiscus is concavo-convex, circular, with the mouth central on the convex face, 1-flagellate, and attains the enormous size of 1.5 mm. (1⁄16") in diameter.

The remarkable phosphorescence of Noctiluca is not constant. It glows with a bluish or greenish light on any agitation, but rarely when undisturbed. A persistent stimulus causes a continuous, but weak, light. This light is so weak that several teaspoonsful of the organism, collected on a filter and spread out, barely enable one to read the figures on a watch a foot away. As in other marine phosphorescence, no rise of temperature can be detected. The luminosity resides in minute points, mostly crowded in the central mass, but scattered all through the cytoplasm. A slight irritation only produces luminosity at the point touched, a strong one causes the whole to flash. Any form of irritation, whether of heat, touch, or agitation, electricity or magnetism, is stated to induce the glow. By day, it is said, Noctiluca, when present in abundance, may give the sea the appearance of tomato soup.

The earliest account of Noctiluca will be read with interest. Henry Baker writes in Employment for the Microscope:^[148]—"A curious Enquirer into Nature, dwelling at Wells upon the Coast of Norfolk, affirms from his own Observations that the Sparkling of Sea Water is occasioned by Insects. His Answer to a Letter wrote to him on that Subject runs thus, 'In the Glass of Sea Water I send with this are some of the Animalcules which cause the Sparkling Light in Sea Water; they may be seen by holding {135}the Phial up against the Light, resembling very small Bladders or Air Bubbles, and are in all Places of it from Top to Bottom, but mostly towards the Top, where they assemble when the Water has stood still some Time, unless they have been killed by keeping them too long in the Phial. Placing one of these Animalcules before a good Microscope, an exceeding minute Worm may be discovered, hanging with its Tail fixed to an opake Spot in a Kind of Bladder, which it has certainly a Power of contracting or distending, and thereby of being suspended at the Surface, or at any Depth it pleases in the including Water.'"

"The above-mentioned Phial of Sea Water came safe, and some of the Animalcules were discovered in it, but they did not emit any Light, as my Friend says they do, upon the least Motion of the Phial when the Water is newly taken up. He likewise adds, that at certain Times, if a Stone be thrown into the Sea, near the Shore, the Water will become luminous as far as the Motion reacheth: this chiefly happens when the Sea hath been greatly agitated, or after a Storm." Obviously what Mr. Sparshall, Baker's correspondent, took for a worm was the large flagellum.

The chief investigators of this group have been Huxley, Cienkowski, Allman, Bütschli, and G. Pouchet, while Ischikawa and Doflein have elucidated the conjugation.

CHAPTER VI

PROTOZOA (CONTINUED): INFUSORIA (CILIATA AND SUCTORIA)

IV. Infusoria.

The name Infusoria was formerly applied to the majority of the Protozoa, and included even the Rotifers. For the word signifies organisms found in "infusions" of organic materials, including macerations. Such were made with the most varied ingredients, pepper and hay being perhaps the favourites. They were left for varying periods exposed to the air, to allow the organisms to develop therein, and were then examined under the microscope.^[150] With the progress of our knowledge, group after group was split off from the old assemblage until only the ciliate or flagellate forms were left. The recognition of the claims of the Flagellates to independent treatment left the group more natural;^[151] while it was enlarged by the admission of the Acinetans (Suctoria), which had for some time been regarded as a division of the Rhizopoda.

I. Ciliata

Infusoria, with a mouth, and cilia by which they move and feed; usually with undulating membranes, membranellae, cirrhi, or some of these. Genera about 144: 27 exclusively marine, 50 common to both sea and fresh water, 27 parasitic on or in Metazoa, the rest fresh water. Species about 500.

We divide the Ciliata thus:^[152]—

The Ciliata have so complex an organisation that, as with the Metazoa, it is well to begin with the description of a definite type. For this purpose we select Stylonychia mytilus, Ehrb. (Fig. 49), a species common in water rich in organic matter, and relatively large (1⁄75" = ⅓ mm.). It is broadly oval in outline, with the wide end anterior, truncate, and sloping to the left side behind; the back is convex, thinning greatly in front; the belly flat. It moves through the water either by continuous swimming or by jerks, and can either crawl steadily over the surface of a solid or an air surface such as an air bubble, or advance by springs, which recall those of a hunting spider. The boundary is everywhere a thin plasmic pellicle, very tender, and readily undergoing diffluence like the rest of the cell. From the pellicle pass the cilia, which are organically connected with it, though they may be traced a little deeper; they are arranged in slanting longitudinal rows, and are much and variously modified, according to their place and function. On the edge of the dorsal surface they are fine and motionless, probably only sensory (s.h.); except three, which protrude well over the hinder end (c.p.), stout, pointed, and frayed out at the ends, and possibly serving as oars or rudders for the darting movements. These are distinguished from simple cilia as "cirrhi."

Fig. 49.—Ventral view of Stylonychia mytilus. a.c, Abdominal cirrhi; an, anus discharging the shell of a Diatom; c.c, caudal cirrhi; c.p, dorsal cirrhi; cv, contractile vacuole; e, part of its replenishing canal; f.c, frontal cirrhi; f.v, food vacuoles; g, internal undulating membrane; l, lip; m, mouth or pharynx; mc, marginal cirrhi; N, N, lobes of meganucleus; n, n, micronuclei; o, anterior end; per, adoral membranellae; poc, preoral cilia; p.om, preoral undulating membrane; s.h, sense hairs. (Modified from Lang.)

At the right hand of the frontal area there begins, just within the dorsal edge, a row of strong cilium-like organs (Fig. 49, per); these, on careful examination, prove to be transverse triangular plates, which after death may fray into cilia.^[154] They are the "adoral membranellae." This row passes to the left blunt angle, and there crosses over the edge of the body to the ventral aspect, and then curves inwards towards the median line, which it reaches about half-way back, where it passes into the pharynx (m). It forms the front and left-hand boundary of a wedge-shaped depression, the "peristomial area," the right-hand boundary being the "preoral ridge" or lip (l), which runs nearly on the median line, projecting downward and over the depression. This ridge bears on its inner and upper side a row of fine "preoral cilia" (poc) and a wide "preoral undulating membrane" (p.om), which extends horizontally across, below the peristomial area. The roof of this area bears along its right-hand edge an "internal undulating membrane" (g), and then, as we pass across to the left, first an "endoral membrane" and then an "endoral" row of cilia. In some allied genera (not in Stylonychia), at the base and on the inner side of each adoral membranella, is a "paroral" cilium. {140}All these motile organs, with the exception of the preoral cilia, pass into the pharynx; but the adoral membranellae soon stop short for want of room. There are some seventy membranellae in the adoral wreath.

The rest of the ventral surface is marked by longitudinal lines, along which the remaining appendages are disposed. On either side is a row of "marginal cirrhi" (mc.), which, like the membranellae, may fray out into cilia, but are habitually stiff spine-like, and straight in these rows; these are the chief swimming organs. Other cirrhi, also arranged along longitudinal rows, with so many blank spaces that the arrangement has to be carefully looked for, occur in groups along the ventral surface. On the right of the peristome are a group which are all curved—the "frontal cirrhi" (f.c.). Behind the mouth is a second group—the "abdominal cirrhi" (a.c.), also curved hooks; and behind these again the straight spine-like "caudal" or "anal" cirrhi (c.c), which point backwards. These three sets of ventral cirrhi are the organs by which the animal executes its crawling and darting movements. Besides the mouth there are two other openings, both indistinguishable save at the very moment of discharge; the anus (an) which is dorsal, and the pore of the contractile vacuole, which is ventral.

The protoplasm of the body is sharply marked off into a soft, semi-fluid "endoplasm" or "endosarc," and a firmer "ectoplasm" or "ectosarc." The former is rich in granules of various kinds, and in food-vacuoles wherein the food is digested. The mode of ingestion, etc., is described below (p. 145). The ectoplasm is honeycombed with alveoli of definite arrangement, the majority being radial to the surface or elongated channels running lengthwise; inside each of these lies a contractile plasmic streak or myoneme. The contractile vacuole (cv) lies in this layer, a little behind the mouth, and is in connexion with two canals, an anterior (e) and a posterior, from which it is replenished.

The nuclear apparatus lies on the inner boundary of the ectoplasm; it consists of (1) a large "meganucleus" formed of two ovoid lobes (N, N), united by a slender thread; and (2) two minute "micronuclei" (n, n), one against either lobe of the meganucleus.

Stylonychia multiplies by transverse fission, the details of which are considered on pp. 144, 147.

The protoplasm of Ciliata is the most differentiated that we {141}find in the Protista, and we can speak without exaggeration of the "organs" formed thereby.

The form of the body, determined by the firm pellicle or plasmic membrane, is fairly constant for each species, though it may be subject to temporary flexures and contractions. The pellicle varies in rigidity; where the cilia are abundant it is proportionately delicate, and scarcely differs from the ectoplasm proper, save for not being alveolate. In the Peritrichaceae it is especially resistant and proof against decay. In Coleps (Gymnostomaceae) it is hardened and sculptured into the semblance of plate-armour, and the prominent points of the plates around the mouth serve as teeth to lacerate other active Protista, its prey; but, like the rest of the protoplasm, this disappears by decay soon after the death of the Coleps. Where, as in certain Oligotrichaceae, cilia are absent over part of the body, the pellicle is hardened; and on the dorsal face and sides of Dysteria it even assumes the character of a bivalve shell, and forms a tooth-like armature about the mouth.

From the pellicle protrude the cilia, each of which is continued inwards by a slender basal filament to end in a "basal granule" or "blepharoplast." The body-cilia are fine, and often reversible in action, which is exceptional in the organic world. They may be modified or combined in various ways. We have seen that in Stylonychia some are motionless sensory hairs. The cirrhi and setae sometimes fray out during life, and often after death, into a brush at the tip, and have a number of blepharoplasts at their base. The same holds good for the membranellae and undulating membranes. They are thus comparable to the "vibratile styles" of Rotifers (Vol. II. p. 202) and the "combs" or "Ctenophoral plates" of the Ctenophora (p. 412 f.).^[155]

Fig. 50.—Ectosarc of Ciliata. a-f, from Stentor coeruleus; g, Holophrya discolor. a, Transverse section, showing cilia, pellicle, canals, and myonemes; b, surface view below pellicle, showing myonemes alternating with blue granular streaks; c, more superficial view, showing rows of cilia adjacent to myonemes; d, myoneme, highly magnified, showing longitudinal and transverse striation; e, two rows of cilia; f, g, optical sections of ectosarc, showing pellicle, alveolar layer (a), myonemes (m), and canals in ectosarc. (From Calkins, after Metschnikoff, Bütschli, and Johnson.)

The ectosarc has a very complex structure. Like other protoplasm it has a honeycombed or alveolate structure, but in this case the alveoli are permanent in their arrangement and position. Rows of these alveoli run under the surface; and the cilia are given off from their nodal points where the vertical walls of several unite, and wherein the basal granule or blepharoplast is contained. Longitudinal threads running along the inner walls of the alveoli of the superficial layer are differentiated into muscular fibrils or "myonemes," to which structures so many owe their marked longitudinal striation on the one hand, and their power of sudden contraction on the other. The appearance of transverse striation may be either due to transverse myonemes, or produced by the folds into which the contraction of longitudinal fibrils habitually wrinkles the pellicles, when it is fairly dense (Peritrichaceae); circular muscular fibrils, however, undoubtedly exist in the peristomial collar of this group. Embedded in the ectosarc are often found trichocysts,^[156] analogous {143}to the nematocysts of the Coelenterata (p. 247), and doubtless fulfilling a similar purpose, offensive and defensive. A trichocyst is an oblong sac (4 µ long in Paramecium) at right angles to the surface, which on irritation, chemical (by tannin, acids, etc.) or mechanical, emits or is converted into a thread several times the length of the cilia (33 µ), often barbed at the tip. In the predaceous Gymnostomaceae, such as Didinium, the trichocysts around (or even within) the mouth are of exceptional size, and are ejected to paralyse, and ultimately to kill, the active Infusoria on which they feed. In most of the Peritrichaceae they are, when present, limited to the rim around the peristome, while in the majority of species of Ciliata they have not been described. Fibrils, possibly nervous,^[157] have been described in the deepest layer of the ectosarc in Heterotrichaceae.

The innermost layer of the ectosarc is often channelled by a system of canals,^[158] usually inconspicuous, as they discharge continuously into the contractile vacuole; but by inducing partial asphyxia (e.g. by not renewing the limited supply of air dissolved in the drop of water on the slide under the cover-glass), the action of the vacuole is slackened, and these canals may be more readily demonstrated. The vacuole, after disappearance, forms anew either by the coalescence of minute formative vacuoles, or by the enlargement of the severed end of the canal or canals. The pore of discharge to the surface is visible in several species, even in the intervals of contraction.^[159] The pore is sometimes near that of the anus, but is only associated with it in Peritrichaceae, where it opens beside it into the vestibule or first part of the long pharynx, often through a rounded reservoir (Fig. 60, r) or elongated canal.

The endosarc, in most Ciliates well differentiated from the ectosarc, is very soft; though it is not in constant rotation like that of a Rhizopod, it is the seat of circulatory movements alternating with long periods of rest. Thus it is that the food-vacuoles, after describing a more or less erratic course, come to discharge their undigested products at the one point, the anus. {144}In a few genera (Didinium, for instance) the course from mouth to anus is a direct straight line, and one may almost speak of a digestive tract. In Loxodes and Trachelius (Fig. 56) the endosarc, as in the Flagellate Noctiluca (Fig. 48, p. 133), has a central mass into which the food is taken, and which sends out lobes, which branch as they approach and join the ectoplasm. The endosarc contains excretory granules, probably calcium phosphate, droplets of oil or dissolved glycogen, proteid spherules, paraglycogen grains, etc.

The nuclear apparatus lies at the inner boundary of the ectoplasm. The "meganucleus" may be ovoid, elongated, or composed of two or more rounded lobes connected by slender bridges (Stentor, Stylonychia). The "micronucleus" may be single; but even when the meganucleus is not lobed it may be accompanied by more than one micronucleus, and when it is lobed there is at least one micronucleus to each of its lobes.^[160] The meganucleus often presents distinct granules of more deeply staining material, varying with the state of nutrition; these are especially visible in the band-like meganuclei of the Peritrichaceae (Figs. 51, 60). At the approach of fission it is in many cases distinctly fibrillated.^[161] But all other internal differentiation, as well as any constriction, then disappears; and the ovoid or rounded figure becomes elongated and hour-glass shaped, and finally constricts into two ovoid daughter-meganuclei, which, during and after the fission of the cell, gradually assume the form characteristic of the species. The micronuclei (each and all when they are multiple) divide by modification of karyokinesis (or "mitosis") as a prelude to fission: in this process the chromatin is resolved into threads which divide longitudinally, but the nuclear wall {145}remains intact. If an Infusorian be divided into small parts, only such as possess a micronucleus and a fragment of the meganucleus are capable of survival. We shall see how important a part the micronuclei play in conjugation, a process in which the old meganuclei are completely disorganised and broken up and their débris expelled or digested.

The mouth of the Gymnostomaceae is habitually closed, opening only for the ingestion of the living Protista that form their prey. It usually opens into a funnel-shaped pharynx, strengthened with a circle of firm longitudinal bars, recalling the mouth of an eel-trap or lobster-pot ("Reusenapparat" of the Germans); and this is sometimes protrusible. In Dysteria the rods are replaced by a complicated arrangement of jaw- or tooth-like thickenings, which are not yet adequately described. We have above noted the strong adoral trichocysts in this group.

In all other Ciliates^[162] the "mouth" is a permanent depression lined by a prolongation of the pellicle, and containing cilia and one or more undulating membranes, and when adoral membranellae are present, a continuation of these. In some species, such as Pleuronema (Fig. 57), one or two large membranes border the mouth right and left. In Peritrichaceae the first part of the pharynx is distinguished as the "vestibule," since it receives the openings of the contractile vacuole or its reservoir and the anus. The pharynx at its lower end (after a course exceptionally long and devious in the Peritrichaceae; Figs. 51, 60) ends against the soft endosarc, where the food-particles accumulate into a rounded pellet; this grows by accretion of fresh material until it passes into the endosarc, which closes up behind it with a sort of lurch. Around the pellet liquid is secreted to form the food-vacuole. If the material supplied be coloured and insoluble, like indigo or carmine, the vacuoles may be traced in a sort of irregular, discontinuous circulation through the endosarc until their remains are finally discharged as faeces through the anus. No prettier sight can be watched under the microscope than that of a colony of the social Bell-animalcule (Carchesium) in coloured water—all producing food-currents brilliantly shown up by the wild eddies of the pigment granules, and the vivid blue or crimson colour of {146}the food-vacuoles, the whole combining to present a most attractive picture. Ehrenberg fancied that a continuous tube joined up the vacuoles, and interpreted them as so many stomachs threaded, as it were, along a slender gut; whence he named the group "Polygastrica."

Fig. 51.—Carchesium polypinum. Scheme of the path taken by the ingested food in digestion and expulsion of the excreta. The food enters through the pharynx and is transported downward (small circles), where it is stored in the concavity of the sausage-shaped meganucleus (the latter is recognised by its containing darker bodies). It remains here for some time at rest (small crosses). Then it passes upward upon the other side (dots) and returns to the middle of the cell, where it undergoes solution. The excreta are removed to the outside, through the vestibule and cell mouth. The black line with arrows indicates the direction of the path. (From Verworn, after Greenwood.)

We owe to Miss Greenwood^[163] a full account of the formation and changes of the food-vacuoles in Carchesium polypinum. The vacuole passes steadily along the endosarc for a certain time after its sudden admission into it, and then enters on a phase of quiescence. A little later the contents of the vacuole aggregate together in the centre of the vacuole, where they are surrounded by a zone of clear liquid; this takes place in the hollow of the meganucleus, in this species horseshoe-shaped. The vacuole then slowly passes on towards the peristome, lying deep in the endosarc, and the fluid peripheral zone is absorbed. {147}For some time no change is shown in the food-material itself: this is the stage of "storage." Eventually a fresh zone of liquid, the true digestive vacuole, forms again round the food-pellet, and this contains a peptic juice, of acid reaction. The contents, so far as they are capable of being digested, liquefy and disappear. Ultimately the solid particles in their vacuole reach the anal area of the vestibule, and pass into it, to be swept away by the overflow of the food-current. The anus is seated on a transverse ridge about a third down the tube, the remaining two-thirds being the true pharynx.

Fission is usually transverse; but is oblique in the conical Heterotrichaceae, and longitudinal in the Peritrichaceae. It involves the peristome, of which one of the two sisters receives the greater, the other the lesser part; each regenerates what is missing. When there are two contractile vacuoles, as in Paramecium, either sister receives one, and has to form another; where there is a canal or reservoir divided at fission, an extension of this serves to give rise to a new vacuole in that sister which does not retain the old one. In some cases the fission is so unequal as to have the character of budding (Spirochona). We have described above (p. 144) the relations of the nuclear apparatus in fission.

Several of the Ciliata divide only when encysted, and then the divisions are in close succession, forming a brood of four, rarely more. This is well seen in the common Colpoda cucullus. In the majority, however, encystment is resorted to only as a means of protection against drought, etc., or for quiet rest after a full meal (Lacrymaria).

Maupas^[164] has made a very full study of the life-cycles of the Ciliata. He cultivated them under the usual conditions for microscopic study, i.e. on a slide under a thin glass cover supported by bristles to avoid pressure, preserved in a special moist chamber; and examined them at regular intervals.

Fig. 52.—Paramecium caudatum, stages in conjugation. gul, Gullet; mg.nu, meganucleus; Mg.nu, reconstructed meganucleus; mi.nu, micronucleus; Mi.nu, reconstructed micronucleus; o, mouth. (From Parker and Haswell, after Hertwig.)

The animals collect at that zone where the conditions of aeration are most suitable, usually just within the edge of the cover, and when well supplied with food are rather sluggish, not swimming far, so that they are easily studied and counted. When well supplied with appropriate food they undergo binary fission at frequent intervals, dividing as often as five times in the twenty-four hours at a temperature of 65-69° F. (Glaucoma scintillans), so that in this period a single individual has resolved itself into a posterity of 32; but such a rapid increase is exceptional. At a minimum and a maximum temperature multiplication is arrested, the optimum lying midway. If the food-supply is cut off, encystment occurs in those species capable of the process; but when there is a mixture of members of different broods of the same species, subject to the limitations that we shall learn, conjugation ensues. Under the conditions of Maupas' investigations he found a limit to the possibilities of continuous fissions, even when interrupted by occasional encystment. The individuals of a series ultimately dwindle in size, their ciliary apparatus is reduced, and their nuclear apparatus degenerates. Thus the ultimate members of a fission-cycle show a progressive decay, notably in the nuclear apparatus, which Maupas has aptly compared to "senility" or "old age" in the Metazoan. If by the timely mixture of broods conjugation be induced, these senile degenerations do not occur.^[165] In Stylonychia {149}mytilus the produce of a being after conjugation died of senility after 336 fissions; in Leucophrys after 660.

Save in the Peritrichaceae (p. 151) conjugation takes place between similar mates, either of the general character and size of the species, or reduced by fissions, in rapid succession, induced by the same conditions as those of mating. The two mates approach, lying parallel and with their oral faces or their sides (Stentor) together, and partially fuse thereby; though no passage of cytoplasm is seen it is probable that there is some interchange or mixture.^[166]

Fig. 53.—Diagram of conjugation in Colpidium colpoda. Horizontal line means degeneration; parallel vertical lines, separation of gametes; broken lines (above), boundary between pairing animals; (below), first fission; single vertical line, continuity or enlargement. M, Meganucleus; µ, micronucleus; Z, zygote-nucleus.

Fig. 54.—Four individuals of Coleps hirtus (Gymnostomaceae) swarming about and ingesting a Vorticella (?) (From Verworn.)

The meganucleus lengthens, becomes irregularly constricted, and breaks up into fragments, which are ultimately extruded or partially digested. The micronucleus enlarges (Fig. 52, A) and undergoes three successive divisions, or, strictly speaking, two fissions producing four nuclei, of which one only undergoes the third. The other three nuclei of the second fission degenerate like the meganucleus.^[167] Of the two micronuclei of this last division one remains where it is as a "stationary" pairing nucleus, while its sister passes as a "migratory" pairing-nucleus into the other mate, and fuses with its stationary pairing-nucleus. Thus in either mate is formed a "zygote-nucleus," or "fusion-nucleus." All these processes are simultaneous in the two mates; and the migratory nuclei cross one another on the bridge of junction of the two mates (Fig. 52, C). Each mate now has its original cytoplasm (subject to the qualification above), {151}but its old nuclear apparatus is replaced by the fusion-nucleus. This new nucleus undergoes repeated fissions; its offspring enlarge unequally, the larger being differentiated as mega-, the smaller as micro-nuclei. The mates now separate (Fig. 52, F, G), and by the first (or subsequent) fission of each, the new mega- and micro-nuclei are distributed to the offspring. Colpidium colpoda offers the simplest case, on which we have founded our diagram showing the nuclear relations. During conjugation the oral apparatus often atrophies, and is regenerated; and in some cases the pellicle and ciliary apparatus are also "made over."

Fig. 55.—Paramecium caudatum (Aspirotrichaceae). A, The living animal from the ventral aspect; B, the same in optical section, the arrows show the course taken by food-particles. buc.gr, Buccal groove; cort, cortex; cu, cuticle; c.vac, contractile vacuole; f.vac, food vacuole; gul, gullet; med, medulla; mth, mouth; nu, meganucleus; pa.nu, micronucleus; trch, trichocysts discharged. (From Parker's Biology.)

In the Peritrichaceae the mates are unequal; the larger is the normal cell, and is fixed; the smaller, mobile, is derived from an ordinary individual by brood-divisions, which only occur under the conditions that induce conjugation (Fig. 60). Here, though the two pairs of nuclei are formed, it is only the migratory {152}nuclei that unite, the stationary ones aborting in both mates. During the final processes of conjugation the smaller mate is absorbed into the body of the larger, and so plays the part of male there. But this process, though one of true binary sex, is clearly derived from the peculiar type of equal reciprocal conjugation of the other Infusoria.

The Ciliata are almost all free-swimming animals with the exception of most of the Peritrichaceae, and of the genera we now cite. Folliculina forms a sessile tube open at either end; and Schizotricha socialis inhabits the open mouths of a branching gelatinous tubular stem, obviously secreted by the hinder end of the animal, and forking at each fission to receive the produce. A similar habit to the latter characterises Maryna socialis; all three species are marine, and were described by Gruber.^[168] Stentor habitually attaches itself by processes recalling pseudopodia, and often forms a gelatinous sheath.

The majority of the Oligotrichaceous Tintinnidae inhabit free chitinous tests often beautifully fenestrated, as in Dictyocystis.

Many genera are parasitic in the alimentary canal of various Metazoa, but none appear to be seriously harmful except Ichthyophtheirius, which causes an epidemic in fresh-water fish. Quite a peculiar fauna inhabit the paunch of Ruminants. Nyctotherus and Balantidium are occasionally found in the alimentary canal of Man.^[169]

The Gymnostomaceae are predaceous, feeding for the most part on smaller Ciliates. We have described the peculiar character of the mouth and pharynx in this group, and the mail-like pellicle of Coleps (Fig. 54). Loxophyllum is remarkable for the absence of cilia from one of the sides of its flattened body, and the tufts of trichocysts studding its dorsal edge at regular intervals. Actinobolus has numerous tentacles, exsertile and retractile, each bearing a terminal tuft of trichocysts, which serve to paralyse such active prey as Halteria. Ileonema has one tentacle overhanging the mouth; and Mesodinium has four short sucker-like projections around it.^[170] It has only two girdles {153}of cilia, which are stout and resemble fine-pointed cirrhi. In Dysteria the cilia are exclusively ventral, and the naked dorsal surface has its pellicle condensed into a bivalve shell; a posterior motile process ("foot") and a complex pharyngeal armature add to the exceptional characters of the genus.

The Aspirotrichaceae are well known to every student of "Elementary Biology" by the "type" Paramecium (Fig. 55), so common in infusions, especially when containing a little animal matter. P. bursaria often contains in its endosarc the green symbiotic Flagellate Zoochlorella. Colpoda cucullus, very frequent in vegetable infusions, usually only divides during encystment, and forms a brood of four. Pleuronema chrysalis (Fig. 57) is remarkable for its habit of lying for long periods on its side and for its immense undulating membrane, forming a lip on the left of its mouth; Glaucoma has two, right and left.

Fig. 56.—Trachelius ovum. A, general view; B, section through sucker; C, section through contractile vacuole and its pore of discharge. al, Alveolar layer of ectoplasm; cil, cilia; c.v, contractile vacuole; m, mouth; N, meganucleus; s, sucker, from which pass inwards retractile myonemes. (After Clara Hamburger.)

Fig. 57.—Pleuronema chrysalis (Aspirotrichaceae). A, Unstimulated, lying quiet; B, stimulated, in the act of springing by the stroke of its cilia. (From Verworn.)

The Heterotrichaceae present very remarkable forms. Spirostomum is nearly cylindrical, and, a very giant, may attain a length of 4 mm. (1⁄6"). Stentor can attach itself by its hinder end, which is then finely tapered and prolonged into a few pseudopodia; its body is trumpet-shaped, with a spiral peristome forming a coil round its wide end, and leading on the left side into the mouth. Many species when attached secrete a gelatinous sheath or tube. S. polymorphus is often coloured green by Zoochlorella (p. 125); S. coeruleus^[171] and S. igneus owe their names to the brilliant pigment, blue or scarlet, deposited in granules in lines between the conspicuous longitudinal myonemes. From their large size and elongated meganucleus accompanied by numerous micronuclei, these two genera have frequently been utilised for experiments on regeneration. In Metopus sigmoides the peristomial area forms a dome above its wreath of membranellae; and in M. pyriformis this is so great as to form the larger part of the cell, which is top-shaped, tapering behind to a point. Caenomorpha (Fig. 58) has the same general form, with a peg-like tail, and possesses a girdle of cirrhi.^[172] The converse occurs in {155}Bursaria; the cell is a half ellipse, something like a common twin tobacco-pouch when closed: a deep depression thus occupies the whole ventral surface, and opens by a wide slit extending along the anterior end. The peristomial area occupies the dorsal side of the pocket so formed, and the mouth is in the hinder left-hand corner. Blepharisma sp. is parasitic in the Heliozoon Raphidiophrys viridis (Fig. 20, 1, p. 74).

Fig. 58.—Caenomorpha uniserialis. crh, Zone of cirrhi; c.t, cilia of tail; c.v, contractile vacuole; c.w, ciliary wreath; g, granular aggregate; m, zone of membranellae; N, meganucleus; n, micronucleus; oe, pharynx; t, tail-spine; t¹, accessory spine; u.m, undulating membrane; v, vacuole; z, precaudal process. (After Levander.)

Among Oligotrichaceae, Halteria, common among the débris at the bottom of pools in woods containing dead leaves, is remarkable for an equatorial girdle of very long fine setae, and for its rapid erratic darting movements, alternating with a graceful bird-like hover. The Tintinnidae are mostly marine, pelagic, with the general look of a stalkless Vorticella; some have a latticed chitinous shell.^[173]

Fig. 59.—Stentor polymorphus. I, Young individual attached, extended; II, adult in fission, contracted; cv in I, afferent canal of contractile vacuole; in II, contractile vacuole; N, moniliform meganucleus (micronuclei omitted); o, mouth; the fine lines are the myoneme fibrils. (From Verworn.)

Among Peritrichaceae, Vorticella (Fig. 60) and its allies have long been known as Bell-animalcules to every student of pond-life. The body has indeed the form of an inverted bell, closed at its mouth by the "peristome," or oral disc; this is a short, inverted truncate cone set obliquely so that its wide base hardly projects at one side, but is tilted high on the other; the edge of the bell is turned out into a rim or "collar," separated from the disc by a deep gutter. The collar, habitually everted, or even turned down, contracts over the retracted disc when the animal is retracted (E²), which is brought about by any sort of shock, or when it swims freely backwards. For the latter purpose a posterior ring of cilia (or rather membranellae) is developed round the hinder end of the bell (A, cr, E³). The cilia of the adoral wreath are very strong, united at the base into a continuous membrane, and indeed themselves partake of the composite nature of membranellae. The wreath forms more than one turn of a right-handed spiral, the innermost turn ending abruptly on the disc, the outer leading down into the mouth at the point where the disc is most tilted and the groove deepest.^[174] The pharynx (p) is long, and contains an undulating membrane (u.m) on its inner side projecting out through the mouth, and numerous cilia; it leads deep into the body (p). The first part is distinguished as the "vestibule" (v), as into it opens the anus, and the contractile vacuole (c.v.), the latter sometimes opening by a reservoir (r). The body contains in the ectoplasm {157}myonema-fibrils which, by their contraction, withdraw the disc, and at the same time circular fibrils close the peristome over it. In the type-genus the pellicle is continued into a long, slender elastic stalk (s), of which the longitudinal myoneme fibrils of the ectoplasm converge to the stalk, and are prolonged into it as a spirally winding fibre, sometimes transversely striated.^[175] The effect of the contraction of this is to pull the stalk into a helicoid spiral (like a coil-spring), with the line of insertion of the muscle along the inner side of the coils, which is, of course, the shortest path from one end to the other (Fig. 60, B).

Fig. 60.—Vorticella. A, expanded; B, stalk in contraction; c, eversible collar below peristome; cr, line of posterior ciliary ring; c.v, contractile vacuole; m, muscle of stalk; N, meganucleus; n, micronucleus; p, pharynx; r, reservoir of contractile vacuole; s, tubular stalk; u.m, undulating membrane in vestibule; v, hinder end of vestibule. E¹, E², two stages in binary fission; E³, free zooid, with posterior wreath; F¹, F² division into mega- and micro-zooids (m); G¹, G², conjugation; m, microzooid. (Modified from Bütschli, from Parker and Haswell.)

The members of the Vorticellidae are very commonly attached to weeds or to various aquatic Metazoa, each species being more or less restricted in its haunts. Vorticella, the type, is singly {158}attached to a contractile stalk; fission takes place in the vertical plane, and one of the two so formed retains the original stalk, while the other swims off (Fig. 60, E¹-E³), often to settle close by, so that the individuals are found in large social aggregates, side by side, fringing water-weeds with a halo visible to the naked eye, which disappears on agitation by the sudden contraction of all the stalks. Carchesium and Zoothamnium differ from Vorticella in the fact that the one daughter-cell remains attached by a stalk coming off a little below the body of the other, so as to give rise to large branching colonies.

In Carchesium (Fig. 51) the muscular threads of each cell are separate, while in Zoothamnium they are continuous throughout the colony. Epistylis has a solid, rigid stalk, and may give rise to branching colonies, which often infest the body of the Water-Fleas (Copepoda) of the genus Cyclops. Opercularia is characterised by the depth of the gutter, the height of the collar, and the tapering downward of the elongated disc. Vaginicola, Pyxicola, Cothurnia, Scyphidia, all inhabit tubes, some of extreme elegance. Ophrydium is a colonial form, found in ponds and ditches, resembling Opercularia, but inhabiting tubes of jelly^[176] that coalesce by their outer walls into a large floating sphere; it usually contains the green symbiotic Flagellate Zoochlorella. Trichodina is free, short, and cylindrical, with both wreaths permanently exposed, and is provided with a circlet of hooks within the aboral wreath. It is often parasitic, or perhaps rather epizoic, on the surface of Hydra (see p. 254), gliding over its body^[177] with a graceful waltzing movement; it occurs also in the bladder and genito-urinary passages of Newts, and even in their body-cavity and kidneys.

II. Suctoria = Tentaculifera

Infusoria with cilia only in the young state,^[178] without mouth or anus, but absorbing food (usually living Ciliates) by one or more tentacles, perforated at the apex; mostly attached, frequently epizoic, rarely parasitic in the interior of other Protozoa.

This group, despite a superficial resemblance to the Heliozoa, show a close affinity to the Ciliata; the nuclear apparatus is usually double though a micronucleus is not always seen; the young are always ciliated, and the mode of conjugation is identical in all cases hitherto studied. Most of the genera are attached by a chitinous stalk (Fig. 61), continued in Acineta into a cup or "theca" surrounding the cell. The pellicle is firm, often minutely shagreened or "milled" in optical section by fine radial processes, whether superficial rods or the expression of the meeting edges of radial alveoli is as yet uncertain. The pellicle closely invests the ectosarc, is continued down into a tubular sheath, from the base of which the tentacle rises, and upwards to invest the tentacle, and is even prolonged into its cavity in Choanophrya, the only genus where the tentacles are large enough for satisfactory demonstration. These organs may be one or more, and vary greatly in character. They may be (1) pointed for prehension, puncture, and suction (Ephelota, Fig. 61, 5); (2) nearly cylindrical, with a slightly "flared" truncate apex (Podophrya, Fig. 61, 1a); (3) filiform with a terminal knob; (4) "capitate" (Acineta, Fig. 61, 2); (5) bluntly truncate and capable of opening into a wide funnel for the suction of food^[179] (Choanophrya, Fig. 62; Rhyncheta, Fig. 61, 3). Their movements, too, are varied, including retraction and protrusion, and a degree of flexion which reaches a maximum in Rhyncheta (Fig. 61, 3), whose tentacle is as freely motile as an elephant's trunk might be supposed to be were it as slender in proportion to its length. They are continued into the body, and in Choanophrya may extend right across it. In Podophrya trold the pellicle rises into a conical tube about the base of the tentacle, which is retracted through it completely with the prey in deglutition. In Dendrocometes, Dendrosoma, and Ophryodendron (Fig. 61, 4, 9, 7), the tentacles arise from outgrowths of the cell-body.

Fig. 61.—Various forms of Suctoria. 1, a and b, two species of Podophrya; c, a tentacle much enlarged; 2, a, Acineta jolyi; 2, b, A. tuberosa, with four ciliated buds; in 6 the animal has captured several small Ciliata; 8, a, a specimen multiplying by budding; 8, b, a free ciliated bud; 9, a, the entire colony; 9, b, a portion of the stem; 9, c, a liberated bud. a, Organism captured as food; b.c, brood-cavity; bd, bud; c.vac, contractile vacuole; l, test; mg.nu, meganucleus; mi.nu, micronucleus; nu, nucleus; t, tentacle. (From Parker and Haswell, after Bütschli and Saville Kent.)

The mechanism of suction is doubtful; but from the way particles from a little distance flow into the open funnels of Choanophrya, it may be the result of an increase of osmotic pressure. The external pellicle of the tentacles is marked by a spiral constriction,^[180] which may be prolonged over the part included in the sheath. The endosarc is rich in oil-drops, often coloured, and in proteid granules which sometimes absorb stains so readily as to have been named "tinctin bodies." It usually contains at least one contractile vacuole.

In Dendrocometes (and perhaps others) the whole cell may become ciliated, detach itself and swim off; this it does when its host (Gammarus) moults its cuticle.

In fission or budding we have to distinguish many modes. (1) In the simplest, after the nuclear apparatus has divided, the cell divides transversely; the distal half acquires cilia and swims off to attach itself elsewhere, while the proximal remains attached. The tentacles have previously disappeared and have to be formed afresh in both. (2) More commonly fission passes into budding on the distal face; a sort of groove deepens around a central prominence which becomes the ciliated larva (Fig. 62, em); the tentacles of the "parent" are retained. This process passes into (3) "internal budding," where a minute pit leads into a bottle-shaped cavity.^[181] (4) Again, the budding may be multiple, the meganucleus protruding a branch for each bud, while the micronucleus, by successive divisions, affords the supply requisite. Sphaerophrya (Fig. 61, 6) and Endosphaera multiply freely by fission within their Ciliate hosts, and were indeed described by Stein as stages in their life-cycle. Conjugation of the same type as in most Ciliates has been fully worked out in Dendrocometes alone, by Hickson,^[182] who has found the meganuclei (though destined to disorganisation) conjugate for a short time by the bridge of communication before the reciprocal conjugation of the micronuclei.

We have referred to the endoparasitism of two genera. Amoebophrya lives in several Acanthometrids, and in the aberrant Radiolarian Sticholonche (see p. 86). The attached species are {162}some of them indifferent to their base; others are only found on Algae, or again only epizoic on special Metazoan hosts, or even on special parts of these. Thus Rhyncheta is only found on the couplers of the thoracic limbs of Cyclops, and Choanophrya on the ventral surface of its head and the adjoining appendages.

Fig. 62.—Choanophrya infundibulifera. A, adult; B-D, tentacles in action in various stages; E, tentacle at rest; F, young, just settled down, a, a, a, Tentacles in various stages of activity; c, central cavity; c.v, contractile vacuole; em, ciliated embryo showing contractile vacuole and nucleus; f, spiral ridge; m, muscular wall of funnel; n, nucleus; tr, opening of funnel. (A-D, F, modified after Zenker; E, original.)

We owe our knowledge of this group to the classical works of Ehrenberg, Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed much light on Dendrocometes, and Hickson has studied its conjugation. Ischikawa^[183] has utilised modern histological methods for the cytological study of Ephelota bütschliana. René Sand has written a useful, but unequal, and not always trustworthy monograph of the Order,^[184] containing an elaborate bibliography.

PORIFERA (SPONGES)

BY

IGERNA B. J. SOLLAS, B.Sc. (Lond.)

Lecturer on Zoology at Newnham College, Cambridge.

CHAPTER VII

PORIFERA (SPONGES)^[185]

INTRODUCTION—HISTORY—DESCRIPTION OF HALICHONDRIA PANICEA AS AN EXAMPLE OF BRITISH MARINE SPONGES AND OF EPHYDATIA FLUVIATILIS FROM FRESH WATER—DEFINITION—POSITION IN THE ANIMAL KINGDOM.

Sponges occupy, perhaps, a more isolated position than any other animal phylum. They are not only the lowest group of multicellular animals, but they are destitute of multicellular relatives. They are all aquatic and—with the exclusion of a few genera found in fresh water—marine, inhabiting all depths from between tide marks to the great abysses of the ocean. They depend for their existence on a current of water which is caused to circulate through their bodies by the activity of certain flagellated cells. This current contains their food, it is their means of respiration, and it carries away effete matters. Consequently sponges cannot endure deprivation of oxygenated water except for short periods, and only the hardiest inhabit regions where the supply is intermittent, as between tide marks. This also renders useless attempts to keep specimens in tanks, unless the water is frequently renewed.

The outward appearance of sponges has an exceptionally wide range, so that it is difficult to give a novice any very definite picture of what he is to expect when searching for these animals. This diversity is in part due to the absence of organs of sufficient size to determine the shape of the whole or limit its variation, {166}that is to say, the separate organs are of an order of size inferior to that of the entire body. The animals are fixed or lie loose on the sea bottom; there are in no case organs of locomotion, and again no sense-organs, no segregated organs of sex, and as a rule no distinction into axis and lateral members. It is by these negative characters that the collector may easily recognise a sponge.

History.—Sponges are, then, in many of their characters unique; and they present a variety of problems for solution, both of special and general interest, they are widely distributed in time and space, and they include a host of forms. It therefore causes no little surprise to learn that they have suffered from a long neglect, even their animal nature having been but recently established. Though known to naturalists from the time of Aristotle, sponges have been left for modern workers as a heritage of virgin soil: it has yielded to them a rich harvest, and is as yet far from exhausted.

The familiar bath sponge was naturally the earliest known member of the phylum. It is dignified by mention in the Iliad and in the Odyssey, and Homer, in his choice of the adjective "full of holes," πολύτρητος, shows at least as much observation as many a naturalist of the sixteenth and seventeenth centuries. Aristotle based his ideas of sponges entirely upon the characters of the bath sponge and its near allies, for these were the only kinds he knew. With his usual perspicuity he reached the conclusion that sponges are animals, though showing points of likeness to plants.

The accounts of sponges after Aristotle present little of scientific interest until the last century. Doubtless this is in part due to the absence of organs which would admit of dissection, and the consequent necessity of finer methods of study. Like other attached forms, sponges were plant or animal as it pleased the imagination of the writer, and sometimes they were "plant animals" or Zoophyta: those who thought them animal were frequently divided among themselves as to whether they were "polypous" or "apolypous." An opinion which it is somewhat difficult to classify was that of Dr. Nehemiah Grew,^[186] who says: "No Sponge hath any Lignous Fibers, but is wholly composed of those which make the Pith and all the pithy parts {167}of a Plant, ... So that a Sponge, instead of being a Zoophyton, is but the one-half of a Plant."

Sponges figure in herbals beside seaweeds and mushrooms, and Gerarde says:^[187] "There is found growing upon rockes near unto the sea a certaine matter wrought together of the foame or froth of the sea which we call Spunges ... whereof to speak at any length would little benefit the reader ... seeing the use thereof is so well known." About the middle of the eighteenth century, authors, especially Peyssonnel, suggested that sponges were but the houses of worms, which built them much as a bee or wasp builds nests and cells. This was confuted by Ellis in 1765,^[188] when he pointed out that the sponge could not be a dead structure, as it gave proof of life by "sucking and throwing out water." To Ellis, then, is due the credit of first describing, though imperfectly, a current set up by sponges. He mentions that Count Marsigli^[189] had already made somewhat similar observations.

It was not till 1825 that attention was again turned to the current, when Robert Grant approached the group in a truly scientific manner, and was ably supported by Lieberkühn. It would be impossible to do justice to Grant in the brief summary to which we must limit ourselves. The most important of his contributions was the discovery that water enters the sponge by small apertures scattered over the surface, and leaves it at certain larger holes, always pursuing a fixed course. He made a few rough experiments to estimate the approximate strength of the current, and, though he failed to detect its cause, he supposed that it was probably due to ciliary action. Grant's suggestion was afterwards substantiated by Dujardin (1838), Carter (1847), Dobie (1852), and Lieberkühn (1857). These five succeeded in establishing the claims of sponges to a place in the animal kingdom, claims which were still further confirmed when James-Clark^[190] detected the presence of the protoplasmic collar of the flagellated cells (see pp. 171, 176). Data were now wanted on which to base an opinion as to the position of sponges within the animal kingdom. In 1878 Schulze^[191] furnished valuable embryological facts, in a description agreeing with an earlier one of Metschnikoff's, of the amphiblastula larva (p. 226) and its metamorphosis. {168}Then Bütschli^[192] (1884) and Sollas^[193] on combined morphological and embryological evidence (1884) concluded that sponges were remote from all the Metazoa, showing bonds only with Choanoflagellate Protozoa (p. 121). This the exact embryological work of Maas, Minchin, and Delage has done much to prove, but it has to be admitted that unanimity on the exact position of the phylum has not yet been attained, some authorities, such as Haeckel, Schulze, and Maas still wishing to include sponges in the Metazoa.

In this short history we have been obliged to refer only to work helping directly to solve the problem of the nature of a sponge, hence many names are absent which we should have wished to mention.

Halichondria panicea.

One of the commonest of British sponges, which may be picked up on almost any of our beaches, and which has also a cosmopolitan distribution, is known by the clumsy popular name of the "crumb of bread sponge," alluding to its consistency; or by the above technical name, with which even more serious fault may be found.^[194]

In its outward form H. panicea affords an excellent case of a peculiarity common among sponges. Its appearance varies according to the position in which it has lived. In fact, Bowerbank remarks that it has no specific form. It may grow in sheets of varying thickness closely attached to a rock, when it is "encrusting," or it is frequently massive and lying free on the sea bottom; again, it may be fistular, consisting of a single long tube, or it may be ridge-like, apparently in this case consisting of a row of long tubes fused laterally. In this last form it used to be called the "cockscomb sponge," having been taken for a distinct species.

Bidder has proposed to call the different forms of the same species "metamps" of the species. Figures of the metamps of H. panicea will be found in Bowerbank's useful Monograph.^[195]

The colour of the species is as inconstant as its form, ranging from green to light brown and orange. MacMunn concludes from spectroscopic work that H. panicea, contains at least three pigments, a chlorophyll, a lipochrome, and a histohaematin.^[196] Lipochromes vary from red to yellow, chlorophyll is always associated with one or more of them. Histohaematin is a respiratory pigment. Proof has not yet been adduced that the chlorophyll is proper to the sponge and is not contained in symbiotic algae.

In spite of all this inconstancy H. panicea, is one of the most easily determined species. It is only necessary to dry a small fragment, including the upper surface; a beautiful honeycomb-like structure is then visible on this surface, and among British sponges this is a property peculiar to the species (Bowerbank). Whatever the form of the sponge, one or more large rounded apertures are always present on the exterior; these are the "oscula." In the encrusting metamp the oscula are flush with the general surface, while in the other cases they are raised on conical projections; fistular specimens carry the osculum at the distal end, and the cockscomb has a row of them along its upper edge. Much more numerous than the oscula are smaller apertures scattered over the general surface of the sponge, and known as "ostia."

Fig. 63.—Portion of the surface of H. panicea, from dried specimen. A, natural size; B, magnified. The large shaded patches are ostia.

If the sponge be placed in a shallow glass dish of sea water the function of the orifices can be made out with the naked eye, especially if a little powdered chalk or carmine be added to the water. If the specimen has been gathered after the retreating tide has left it exposed for some time, this addition is unnecessary, for as soon as it is plunged into water its current bursts vigorously forth, and is rendered visible by the particles of detritus that have accumulated in the interior during the period of {170}exposure and consequent suspended activity. The oscula then serve for the exit of currents of water carrying particles of solid matter, while the entrance of water is effected through the ostia.

Sections show that the ostia lead into spaces below the thin superficial layer or "dermal membrane"; these are continued down into the deeper parts of the sponge as the "incurrent canals," irregular winding passages of lumen continually diminishing as they descend. They all sooner or later open by numerous small pores—"prosopyles"—into certain subspherical sacs termed flagellated chambers. Each chamber discharges by one wide aperture—"apopyle"—into an "excurrent canal." This latter is only distinguishable from an incurrent canal by the difference in its mode of communication with the chambers.

Fig. 64.—H. panicea: the arrows indicate the direction of the current, which is made visible by coloured particles. (After Grant.)

The excurrent canals convey to the osculum the water which has passed through the ostia and chambers. All the peripheral parts of the sponge from which chambers are absent are termed the "ectosome," while the chamber-bearing regions are the "choanosome."

The peculiar crumb-of-bread consistency is due to the nature of the skeleton, which is formed of irregular bundles and strands of minute needles or spicules composed of silica hydrate, a substance familiar to us in another form as opal: they are clear and transparent like glass. They are scattered through the tissues in great abundance.

The classes of cellular elements in the sponge are as follows: Flattened cells termed "pinacocytes" cover all the free surfaces, that is to say, the external surface and the walls of the {171}excurrent and incurrent canals. The flagellated chambers are lined by "choanocytes" (cf. Fig. 70, p. 176); these are cells provided at their inner end with a flagellum and a collar surrounding it. They resemble individuals of the Protozoan sub-class Choanoflagellata, and the likeness is the more remarkable because no other organisms are known to possess such cells. Taken together the choanocytes constitute the "gastral layer," and they are the active elements in producing the current. The tissue surrounding the chambers thus lying between the excurrent and incurrent canals consists of a gelatinous matrix colonised by cells drawn from two distinct sources. In the first place, it contains cells which have a common origin with the pinacocytes, and which together with them make up the "dermal layer"; these are the "collencytes" and "scleroblasts"; secondly, it contains "archaeocytes," cells of independent origin.

Collencytes are cells with clear protoplasm and thread-like pseudopodial processes; they are distinguished as stellate or bipolar, according as these processes are many or only two. Scleroblasts or spicule cells are at first rounded, but become elongated with the growth of the spicule they secrete, and when fully grown are consequently fusiform.

Fig. 65.—Diagrammatic section of a siliceous Sponge. a.p, Apopyle; d.o, dermal ostia; ex.c, excurrent, or exhalant canal; in.c, incurrent canal; o, osculum. (Modified from Wilson.)

Each spicule consists of an organic filamentar axis or axial fibre around which sheaths of silica hydrate are deposited successively by the scleroblast. Over the greater length of the spicule the sheaths are cylindrical, but at each end they taper to a point. The axial canal in which the axial fibre lies is open at both ends, and the fibre is continuous at these two points with an organic sheath, which invests the entire spicule. From this structure we may conclude that the spicule grows at both ends—i.e. it grows in two opposite directions along one line—it has two rays lying in one axis, and is classed among uniaxial diactinal spicules. Being {172}pointed at both ends it receives the special name oxea. The lamination of the spicule is rendered much more distinct by heating or treatment with caustic potash.^[197]

Fig. 66.—Cut end of a length of a siliceous spicule from Hyalonema sieboldii, with the lamellar structure revealed by solution. × 104. (After Sollas.)

The archaeocytes are rounded amoeboid cells early set apart in the larva; they are practically undifferentiated blastomeres. Some of them become reproductive elements, and thus afford a good instance of "continuity of germ plasm," others probably perform excretory functions.^[198]

Fig. 67.—Free-swimming larva of Gellius varius, in optical section. a, Outer epithelium; pi, pigment; x, hinder pole. (After Maas.)

The reproductive elements are ova and spermatozoa, and are to be found in all stages in the dermal jelly. Dendy states that the eggs are fertilised in the inhalant canals, to which position they migrate by amoeboid movements, and there become suspended by a peduncle.

The larva has unfortunately not been described, but as the course of development among the near relatives of H. panicea is known to be fairly constant, it will be convenient to give a description of a "Halichondrine type" of larva based on Maas' account of the development of Gellius varius.^[199] The free-swimming larvae escape by the osculum; they are minute oval bodies moving rapidly by means of a covering of cilia. The greater part of the body is a dazzling white, while the hinder pole is of a brown violet colour. This coloured patch is non-ciliate, the general covering of cilia ending at its edge in a ring of cilia twice the length of the others. Forward {173}movement takes place in a screw line; when this ceases the larva rests on its hinder pole, and the cilia cause it to turn round on its axis.

Sections show that the larva is built up of two layers:—

1. "The inner mass," consisting of various kinds of cells in a gelatinous matrix.

2. A high flagellated epithelium, which entirely covers the larva with the exception of the hinder pole.

Fig. 68.—Longitudinal section through the hinder pole of the larva of G. varius. a, Flagellated cells; ma¹, undifferentiated cell; ma², differentiated cell; pi, pigment; x, surface of hinder pole. (After Maas.)

The cells in the inner mass are classified into (1) undifferentiated cells, recognised by their nucleus, which possesses a nucleolus; these are the archaeocytes; (2) differentiated cells, of which the nucleus contains a chromatin net; these give rise to pinacocytes, collencytes, and scleroblasts. Some of them form a flat epithelium, which covers the hinder pole. Some of the scleroblasts already contain spicules. Fixation occurs very early. The front pole is used for attachment, the pigmented pole becoming the distal end (Fig. 69). The larva flattens out, the margin of the attached end is produced into radiating pseudopodial processes. The flagellated cells retreat to the interior, leaving the inner mass exposed, and some of its cells thereupon form a flat outer epithelium. This is the most important process of the metamorphosis; it is followed by a pause in the outward changes, coinciding in time with rearrangements of the internal cells to give rise to the canal system; that is to say, lacunae arise in the inner mass, pinacocytes pass to the surface of the lacunae, and form their lining; the flagellated cells, which have lain in confusion, become grouped in small clusters. These become flagellated chambers, communications are established between the various portions of the canal system, and its external apertures arise. There is at first only one osculum. The larvae may be obtained by keeping the parent sponge in a dish of sea water, shielded from too bright a light, and surrounded by a second dish of water to keep the temperature constant. They will undergo {174}metamorphosis in sea water which is constantly changed, and will live for some days.

We have said that the young sponge has only one osculum. This is the only organ which is present in unit number, and it is natural to ask whether perhaps the osculum may not be taken as a mark of the individual; whether the fistular specimens, for example, of H. panicea may not be solitary individuals, and the cockscomb and other forms colonies in which the individuals are merged to different degrees. Into the metaphysics of such a view we cannot enter here. We must be content to refer to the views of Huxley and of Spencer on Individuality.

But it is advisable to avoid speaking of a multi-osculate sponge as a colony of many individuals, even in the sense in which it is usual to speak of a colony of polyps as formed of individuals. The repetition of oscula is probably to be regarded as an example of the phenomenon of repetition of parts, the almost universal occurrence of which has been emphasised by Bateson.^[200] Delage^[201] has shown that when two sponge larvae fixed side by side fuse together, the resulting product has but one osculum. This, though seeming to bear out our point of view, loses weight in this connexion, when it is recalled that two Echinoderm larvae fused together give rise in a later stage to but one individual.

Fig. 69.—Larva of Gellius varius shortly after fixation. The pigmented pole, originally posterior, is turned towards the reader. R, Marginal membrane with pseudopodia; x, hinder pole. (After Maas.)

Ephydatia fluviatilis.

In the fresh water of our rivers, ponds, and lakes, sponges are represented very commonly by Ephydatia (Spongilla) fluviatilis, a cosmopolitan species. The search for specimens is most likely {175}to be successful if perpendicular timbers such as lock-gates are examined, or the underside of floating logs or barges, or overhanging branches of trees which dip beneath the surface of the water.

The sponge is sessile and massive, seldom forming branches, and is often to be found in great luxuriance of growth, masses of many pounds weight having been taken off barges in the Thames. The colour ranges from flesh-tint to green, according to the exposure to light. This fact is dealt with in a most interesting paper by Professor Lankester,^[202] who has shown not only that the green colour is due to the presence of chlorophyll, but that the colouring matter is contained in corpuscles similar to the chlorophyll corpuscles of green plants, and, further, that the flesh-coloured specimens contain colourless corpuscles, which, though differing in shape from those which contain the green pigment, are in all probability converted into these latter under the influence of sufficient light. The corpuscles, both green and colourless, are contained in amoeboid cells of the dermal layer;^[203] and in the same cells but not in the corpuscles are to be found amyloid substances.

The anatomy of Ephydatia fluviatilis is very similar to that of Halichondria panicea, differing only in one or two points of importance. The ectosome is an aspiculous membrane of dermal tissue covering the whole exterior of the sponge and forming the roof of a continuous subdermal space. This dermal membrane is perforated by innumerable ostia, and is supported above the subdermal cavity by means of skeletal strands, which traverse the subdermal cavity and raise the dermal membrane into tent-like elevations, termed conuli. The inhalant canals which arise from the floor of the subdermal cavity are as irregular as in H. panicea, and interdigitate with equally irregular exhalant canals; these latter communicate with the oscular tubes. Between the two sets of canals are the thin folds of the choanosome with its small subspherical chambers provided with widely open apopyles (Fig. 70). The soft parts are supported on a siliceous skeleton of oxeas, which may have a quite smooth surface or may {176}be covered in various degrees with minute conical spines (Fig. 72, a, b). These spicules are connected by means of a substance termed spongin deposited around their overlapping ends, so as to form an irregular network of strands, of which some may be distinguished as main strands or fibres, others as connecting fibres. In the main fibres several spicules lie side by side, while in the connecting fibres fewer or frequently single spicules form the thickness of the fibre. The fibres are continuous at the base with a plate or skin of spongin, which is secreted over the lower surface of the sponge and intervenes between it and the substratum. Of the chemical composition of spongin we shall speak later (see p. 237). It is a substance which reaches a great importance in some of the higher sponges, and forms the entire skeleton of certain kinds of bath sponge. Lying loose in the soft parts and hence termed flesh spicules, or microscleres, are minute spicules of peculiar form. These are the amphidiscs, consisting of a shaft with a many-rayed disc at each end (Fig. 72).

Fig. 70.—Ephydatia fluviatilis. Section of flagellated chamber, showing the choanocytes passing through the apopyle. (After Vosmaer and Pekelharing.)

In addition to its habitat the fresh-water sponge is worthy of attention on account of its methods of reproduction, which have arisen in adaptation to the habitat. A similar adaptation is widespread among fresh-water members of most aquatic invertebrates.^[204]

Ephydatia fluviatilis normally produces not only free-swimming larvae of sexual origin, but also internal gemmules arising asexually. These bodies appear in autumn, distributed throughout the sponge, often more densely in the deeper layers, and they come into activity only after the death of the parent, an event which happens in this climate at the approach of winter.

Fig. 71.—Portion of the skeletal framework of E. fluviatilis. a, Main fibres; b, connecting fibres. (After Weltner.)

Fig. 72.—Spicules of E. fluviatilis. a. b. c. Oxeas, spined and smooth; d. e, amphidiscs, side and end views. (After Potts.)

Weltner^[205] has shown that on the death and disintegration of the mother sponge some of the gemmules remain attached to the old skeleton, some sink and some float. Those which remain attached are well known to reclothe the dead fibres with living tissue. They inherit, as it were, the advantages of position, which contributed to the survival of the parent, as one of the selected fittest. The gemmules which sink are doubtless rolled short distances along the bottom, while those which float have the opportunity of widely distributing the species with the risk of being washed out to sea. But even these floating gemmules are exposed to far less dangers than the delicate free-swimming larvae, for their soft parts are protected from shocks by a thick coat armed with amphidiscs.

The gemmules are likewise remarkable for their powers of {178}resistance to climatic conditions, powers which must contribute in no small way to the survival of a species exposed to the variable temperatures of fresh water. Thus, if the floating gemmules or the parent skeleton with its attached and dormant offspring should chance to be included in the surface layer of ice during the winter, so far from suffering any evil consequences they appear to benefit by these conditions. Both Potts and Weltner have confirmed the truth of this statement by experiments. Weltner succeeded in rearing young from gemmules which had suffered a total exposure of 17 days to a temperature "under 0° C."

Of important bearing on the question of the utility of the gemmules are certain instances in which E. fluviatilis has been recorded as existing in a perennial condition.^[206] The perennial individuals may or may not bear gemmules, which makes it evident that, with the acquisition of the power to survive the winter cold, the prime necessity of forming these bodies vanishes.

The perennial specimens are described as exhibiting a diminished vegetative activity in winter, the flagellated chambers may be absent (Lieberkühn), or present in unusually small numbers (Weltner), the entire canal system may be absent (Metschnikoff), or, on the other hand, it may be complete except for the osculum.

Fig. 73.—Gemmule of E. fluviatilis. b, Amphidisc. (After Potts.)

In tropical countries gemmulation occurs as a defence against the ravages caused by the dry season when the waters recede down their banks, exposing all or most of their sponge inhabitants to the direct rays of the sun. The sponges are at once killed, but the contained gemmules being thoroughly dried, become efficient distributing agents of the species; they are light enough to be carried on the wind. It is probable that those individual sponges which escape desiccation survive the dry season without forming gemmules.

It has been shown experimentally that gemmules are not injured by drying—Zykoff found that gemmules kept dry for a period of two years had not lost the power of germination.

The mature gemmules consist of a more or less spherical mass of cells, which we shall refer to as yolk cells, and of a complex coat. The latter is provided with a pore or pore tube (Fig. 74) which is closed in winter by an organic membrane.

There are three layers in the coat: an inner chitinous layer surrounded by an air-chamber layer, which is finely vesicular, showing a structure recalling plant tissue, and containing amphidiscs arranged along radii passing through the centre of the gemmule. One of the discs of each amphidisc lies in the inner chitinous coat, while the other lies in a similar membrane which envelopes the air-chamber layer and is termed the outer chitinous coat.

Marshall has suggested that one function of the amphidiscs is to weight the gemmules and thus protect them against the force of the river current; and no doubt the sinking or floating of individual gemmules depends on the relative degree of development of the air-chambers and of the amphidiscs.

A study of the development of Ephydatia gemmules vividly illustrates various characters of the inner processes of sponges. Specially noteworthy are the migrations of cells and the slight extent to which division of labour is carried: one and the same cell will be found to perform various functions.

Fig. 74.—Part of a longitudinal section of a gemmule of Ephydatia sp. passing through the pore (a). (After Potts.)

The beginning of a gemmule is first recognisable^[207] as a small cluster of amoeboid archaeocytes in the dermal membrane. These move into the deeper parts of the sponge to form larger groups. They are the essential part of the gemmule, the yolk cells, which, when germination takes place, give rise to a new sponge. They are followed by two distinct troops of actively moving cells. Those forming the first troop arrange themselves round the yolk cells and ultimately assume a columnar form so that they make an epithelioid layer. They then secrete the inner chitinous coat. The cells of the second troop are entrusted with the nutrition of the gemmule. Consequently they pass in among the yolk cells, distribute their food supplies, and make their escape {180}by returning into the tissues of the mother sponge, before the columnar cells have completed the chitinous coat. Yet another migration now occurs, the cells—"scleroblasts"—which have been occupied in secreting amphidiscs at various stations in the sponge, carry the fully formed spicules to the gemmules and place them radially round the yolk cells between the radially lying cells of the columnar layer. The scleroblasts themselves remain with the amphidiscs, and becoming modified, contribute to the formation of the air-chamber layer. The columnar cells now creep out between the amphidiscs till their inner ends rest on the outer ends of these spicules. They then secrete the outer chitinous coat and return to the mother sponge.

Carter gives directions^[208] for obtaining young sponges from the gemmules. The latter should be removed from the parent, cleaned by rolling in a handkerchief, and then placed in water in a watch-glass, protected with a glass cover and exposed to sunlight. In a few days the contents of the gemmule issue from the foramen and can be seen as a white speck. A few hours later the young sponge is already active and may be watched producing aqueous currents. At this age the sponge is an excellent object for studying in the living condition: being both small and transparent it affords us an opportunity of watching the movements of particles of carmine as they are carried by the current through the chambers.

Potts^[209] describes how he has followed the transportal of spicules by dermal cells, the end of each spicule multiplying the motion, swaying like an oscillating rod.

In E. fluviatilis reproduction also occurs during the warmer months in this climate by means of sexual larvae. These are interesting for certain aberrant features in their metamorphosis.^[210] While some of the flagellated chambers are formed in the normal way from the flagellated cells of the larva, others arise each by division of a single archaeocyte. This, it is suggested, is correlated with the acquisition of the method of reproduction by gemmules, the peculiarities (i.e. development of organs from archaeocytes) of which are appearing in the larvae.

Definition.—We may now define sponges as multicellular, {181}two-layered animals; with pores perforating the body-walls and admitting a current of water, which is set up by the collared cells of the "gastral" layer.

Position in the Animal Kingdom.—Sponges are the only multicellular animals which possess choanocytes, and their mode of feeding is unique. Since they are two-layered it has been sought to associate them with the Metazoan phylum Coelenterata, but they are destitute of nematocysts or any other form of stinging cell, and their generative cells arise from a class of embryonic cells set apart from the first, while the generative cells of Coelenterata are derived from the ectoderm, or in other cases from the endoderm. These weighty differences between sponges and that group of Metazoa to which they would, if of Metazoan nature at all, be most likely to show resemblance, suggest that we should seek a separate origin for sponges and Metazoa. We naturally turn to the Choanoflagellate Infusorian stock (see p. 121) as the source of Porifera, leaving the Ciliate stock as the progenitors of Metazoa.

That both Porifera and Metazoa are reproduced by ova and spermatozoa is no objection to this view, seeing that the occurrence of similar reproductive cells has been demonstrated in certain Protozoa (see pp. 100, 128).

Let us now see which view is borne out by facts of embryology. Suppose, for the moment, we regard sponges as Metazoa, then if the sponge larva be compared with the Metazoan larva we must assign the large granular cells to the endoderm; the flagellated cells to the ectoderm; and we are led to the anomalous statement that the digestive cells in the adult are ectodermal, the covering, outer cells endodermal; or conversely, if we start our comparisons with the adults, then it follows that the larval ectoderm has the characters of an endoderm, and the larval endoderm those of an ectoderm.

Thus both embryology and morphology lead us to the same point, they both show that in the absence of any fundamental agreement between Porifera and Metazoa it is necessary to regard the two stocks as independent from the very first, and hence the name Parazoa (Sollas) has been given to the group which contains the Porifera as its only known phylum.

Interesting in connexion with the phylogeny of Parazoa is the Choanoflagellate genus Proterospongia (Fig. 75), described by {182}Saville Kent, and since rediscovered both in England and abroad.^[211] This is a colony of unicellular individuals embedded in a common jelly. The individuals at the surface are choanoflagellate, while in the interior the cells are rounded or amoeboid, and some of them undergo multiple fission to form reproductive cells. This is just such a creature as we might imagine that ancestral stage to have been of which the free-swimming sponge larva is a reminiscence: for we have seen that the flagellated cells of the larva are potential choanocytes.

Fig. 75.—Proterospongia haeckeli. a, Amoeboid cell; b, a cell dividing; c, cell with small collar; z, jelly. × 800. (After S. Kent.)

CHAPTER VIII

PORIFERA (CONTINUED): FORMS OF SPICULES—CALCAREA—HOMOCOELA—HETEROCOELA—HEXACTINELLIDA—DEMOSPONGIAE—TETRACTINELLIDA—MONAXONIDA—CERATOSA—KEY TO BRITISH GENERA OF SPONGES

Sponges fall naturally into two branches differing in the size of their choanocytes: in the Megamastictora these cells are relatively large, varying from 5µ to 9µ in diameter; in Micromastictora they are about 3µ in diameter.^[212] For further subdivision of the group the spicules are such important weapons in the hands of the systematist that it is convenient to name them according to a common scheme. This has been arrived at by considering first the number of axes along which the main branches of the spicules are distributed, and secondly whether growth has occurred in each of these axes in one or both directions from a point of origin.^[213]

I. Monaxons.—Spicules of rod-like form, in which growth is directed from a single origin in one or both directions along a single axis. The axis of any spicule is not necessarily straight, it may be curved or undulating. The ray or rays are known as actines.

Biradiate monaxon spicules are termed "rhabdi" (Fig. 76, a). A rhabdus pointed at both ends is an "oxea," rounded at both ends a "strongyle," knobbed at both ends a "tylote." By branching a rhabdus may become a "triaene" (Fig. 110, k, l).

Uniradiate monaxon spicules are termed "styli."

II. Tetraxons.—Spicules in which growth proceeds from an {184}origin in one direction only, along four axes arranged as normals to the faces of a regular tetrahedron. Forms produced by growth from an origin in one direction along three axes lying in one plane are classed with tetraxons.

III. Triaxons.—Spicules in which growth is directed from an origin in both directions along three rectangular axes. One or more actines or one or two axes may be suppressed.

IV. Polyaxons.—Spicules in which radiate growth from a centre proceeds in several directions.

V. Spheres.—Spicules in which growth is concentric about the origin.

A distinction more fundamental than that of form is afforded by the chemical composition: all sponges having spicules composed of calcium carbonate belong to a single class, Calcarea, which stands alone in the branch Megamastictora.

Fig. 76.—Types of megascleres. a, Rhabdus (monaxon diactine); b, stylus (monaxon monactine); c, triod (tetraxon triactine); d, calthrop (tetraxon tetractine); e, triaxon hexactine; f, euaster.

BRANCH I. MEGAMASTICTORA

CLASS CALCAREA

Calcarea are marine shallow-water forms attached for the most part directly by the basal part of the body or occasionally by the intervention of a stalk formed of dermal tissue. They are almost all white or pale grey brown in colour. Their spicules are either monaxon or tetraxon or both. The tetraxons are either quadriradiate and then called "calthrops," or triradiate when the fourth actine is absent. The triradiates always lie more or less tangentially in the body-wall; similarly three rays of a calthrop are tangentially placed, the fourth lying across the thickness of the wall. It is convenient to include the triradiate and the three tangentially placed rays of a calthrop under the common {185}term "triradiate system" (Minchin). The three rays of one of these systems may all be equal in length and meet at equal angles: in this case the system is "regular." Or one ray or one angle may differ in size from the other rays or angles respectively, which are equal: in either of these two cases the system is bilaterally symmetrical and is termed "sagittal." A special name "alate" is given to those systems which are sagittal in consequence of the inequality in the angles. Thus all equiangular systems whether sagittal or not are opposed to those which are alate. This is the natural classification.^[214]

Sub-Class I. Homocoela.

The Homocoela or Ascons possess the simplest known type of canal system, and by this they are defined. The body is a sac, branched in the adult, but simple in the young; its continuous cavity is everywhere lined with choanocytes, its wall is traversed by inhalant pores, and its cavity opens to the exterior at the distal end by an osculum. The simple sac-like young is the well-known Olynthus of Haeckel—the starting-point from which all sponges seem to have set out. Two processes are involved in the passage from the young to the adult, namely, multiplication of oscula and branching of the original Olynthus tube or sac. If the formation of a new osculum is accompanied by fission of the sac, and the branching of the latter is slight, there arises an adult formed of a number of erect, well separated main tubes, each with one osculum and lateral branches. Such is the case in the Leucosoleniidae. In the Clathrinidae, on the other hand, branching of the Olynthus is complicated, giving rise to what is termed reticulate body form, that is, a sponge body consisting of a network of tubules with several oscula, but with no external indication of the limits between the portions drained by each osculum. These outward characters form a safe basis for classification, because they are correlated with other fundamental differences in structure and development.^[215]

As in Halichondria, and in fact all sponges, the body-wall is formed of two layers; the gastral layer, as we have said, forming a continuous lining to the Ascon tube and its branches. The {186}dermal layer includes a complete outer covering of pinacocytes, which is reflected over the oscular rim to meet the gastral layer at the distal end of the tube; a deeper gelatinous stratum in which lie scleroblasts and their secreted products—calcareous spicules; and finally porocytes.^[216] These last are cells which traverse the whole thickness of the thin body-wall, and are perforated by a duct or pore. The porocytes are contractile, and so the pores may be opened or closed; they are a type of cell which is known only in Calcarea. It will be noticed that the fusiform or stellate "connective tissue cells" are absent. The layer of pinacocytes as a whole is highly contractile, and is capable of diminishing the size of the sponge to such an extent as quite to obliterate temporarily the gastral cavity.^[217]

The choanocytes show certain constant differences in structure in the families Clathrinidae and Leucosoleniidae respectively. In the former, the nucleus of the choanocyte is basal; in the latter, it is apical, and the flagellum can be traced down to it (Fig. 77).

Fig. 77.—The two types of Asconid collar cells. A, of Clathrina, nucleus basal; B, of Leucosolenia, nucleus not basal, flagellum arising from the nuclear membrane. (A, after Minchin; B, after Bidder.)

The tetraxon spicules have "equiangular" triradiate systems in the Clathrinidae, while in Leucosoleniidae they are "alate." Finally, the larva of Clathrinidae is a "parenchymula" (see p. 226), that of Leucosoleniidae an "amphiblastula."

The fact that it is possible to classify the Calcarea Homocoela largely by means of histological characters is in accordance with the importance of the individual cell as opposed to the cell-layers generally throughout the Porifera, and is interesting in serving to emphasise the low grade of organisation of the Phylum. The organs of sponges are often unicellular (pores), or the products of the activity of a single cell (many skeletal elements); and even in the gastral layer, which approaches nearly to an epithelium, comparable with the epithelia of Metazoa, the component cells {187}still seem to assert their independence, the flagella not lashing in concert,^[218] but each in its own time and direction.

Sub-Class II. Heterocoela.

Fig. 78.—Transverse section of the body-wall of Sycon carteri, showing articulate tubar skeleton, gastric ostia (a.p), tufts of oxeas at the distal ends of the chambers (fl.ch), and pores (p). (After Dendy.)

Fig. 79.—Sycon coronatum. At a a portion of the wall is removed, exposing the paragaster and the gastric ostia of the chambers opening into it.

The Heterocoela present a series of forms of successive grades of complexity, all derivable from the Ascons, from which they differ in having a discontinuous gastral layer. The simplest Heterocoela are included in the family Sycettidae, of which the British representative is Sycon (Fig. 79). In Sycon numerous tubular flagellated chambers are arranged radially round a central cavity, the "paragaster," into which they open (Figs. 78, 79). The chambers, which are here often called radial tubes, are close set, leaving more or less quadrangular tubular spaces, the {188}inhalant canals, between them; and where the walls of adjacent chambers come in contact, fusion may take place. Pores guarded by porocytes put the inhalant canals into communication with the flagellated chambers. The paragaster is lined by pinacocytes; choanocytes are confined to the flagellated chambers.

The skeleton is partly defensive, partly supporting; one set of spicules strengthens the walls of the radial tubes and forms collectively the "tubar skeleton." It is characteristic of Sycettidae that the tubar skeleton is of the type known as "articulate"—i.e. it is formed of a number of successive rings of spicules, instead of consisting of a single ring of large spicules which run the whole length of the tube.

Fig. 80.—Sycon setosum. Young Sponge. × 200. d, Dermal cell; g, gastral cell; o, osculum; p, pore cell; sp₁, monaxon; sp₃, triradiate spicule. (After Maas.)

The walls of the paragaster are known as the "gastral cortex"; they contain quadriradiate spicules, of which the triradiate systems lie tangentially in the gastral cortex, while the apical ray projects into the paragaster, and is no doubt defensive. The distal ends of the chambers bristle with tufts of oxeate spicules, and the separate chambers are distinguishable in surface view. It is interesting to notice that in some species of Sycon, the gaps between the distal ends of the chambers are covered over by a delicate perforated membrane, thus leading on, as we shall see presently, to the next stage of advance.^[219] The larva of Sycon is an amphiblastula (see p. 227). Fig. 80 is a drawing of the young sponge soon after fixation; it would pass equally well for an ideally simple Ascon or, neglecting the arrangement of the spicules, for an isolated radial tube of Sycon. Figs. 81, 82 show the same sponge, somewhat older. From them it is seen that the Sycon type is produced from the young individual, in what {189}may be called its Ascon stage, by a process of outgrowth of tubes from its walls, followed by restriction of choanocytes to the flagellated chambers. Minute observation has shown^[220] that this latter event is brought about by immigration of pinacocytes from the exterior. These cells creep through the jelly of the dermal layer and line the paragaster as fast as its original covering of choanocytes retreats into the newly formed chambers.

Fig. 81.—S. setosum. Young Sponge, with one whorl of radial tubes. o, Osculum; p, pore; sp₁, monaxon; sp₄, quadriradiate spicule. (After Maas.)

With a canal system precisely similar to that of Sycon, Ute (Fig. 83) shows an advance in structure in the thickening of the dermal layers over the distal ends of the chambers. The dermal thickenings above neighbouring chambers extend laterally and {190}meet; and there results a sheet of dermal tissue perforated by dermal ostia, which open into the inhalant canals, and strengthened by stout spicules running longitudinally. This layer is termed a cortex; it covers the whole sponge, compacting the radial tubes so that they form, together with the cortex, a secondary wall to the sponge, which is once more a simple sac, but with a complex wall. The cortex may be enormously developed, so as to form more than half the thickness of the wall (Fig. 84). The chambers taken together are spoken of as the chamber layer.

Fig. 82.—Sycon raphanus. A, Longitudinal section of young decalcified Sponge at a stage somewhat later than that shown in Fig. 81. B, Transverse section of the same through a whorl of tubes. d, Dermal membrane; g, gastral membrane; H, paragaster; sp₄, tetraradiate spicule; T, radial tube. (After Maas.)

Fig. 83.—Transverse section of the body-wall of Ute, passing longitudinally through two chambers. a.p, Apopyle; d.o, dermal ostium; fl.ch, flagellated chamber or radial tube; i.c, inhalant canal; p, prosopyle. (After Dendy.)

We have already alluded to the resemblance between a young Ascon person and a radial tube of Sycon—a comparison which calls to mind the somewhat strange view of certain earlier authors, that the flagellated chambers are really the sponge individuals. If now we suppose each Ascon-like radial tube of Sycon to undergo that same process of growth by which the {191}Sycon itself was derived from the Ascon, we shall then have a sponge with a canal system of the type seen in Leucandra among British forms, but more diagrammatically shown in the foreign genus Leucilla (Fig. 85). The foregoing remarks do not pretend to give an account of the transition from Sycon to Leucilla as it occurred in phylogeny. For some indication of this we must await embryological research.

In Leucandra the fundamental structure is obscured by the irregularity of its canal system. It shows a further and most important difference from Leucilla in the smaller size and rounded form of its chambers. This change of form marks an advance in efficiency; for now the flagella converge to a centre, so that they all act on the same drop of water, while in the tubular chamber their action is more widely distributed and proportionately less intense (see p. 236).

Fig. 84.—Transverse section through the body-wall of Grantiopsis. d.o, Dermal ostium; fl.ch, flagellated chamber; i.c, long incurrent canal traversing the thick cortex to reach the chamber layer; p, apopyle. (After Dendy.)

Fig. 85.—Transverse section through the body-wall of Leucilla. d.o, Dermal ostium; ex.c, exhalant canal; fl.ch, chamber; i.c, inhalant canal. (After Dendy.)

Above are described three main types of canal system—that of Homocoela, of Sycon, and of Leucandra and Leucilla. These are conveniently termed the first, second, and third types respectively, and may be briefly described as related to one another somewhat in the same way as a scape, umbel, and compound umbel among {192}inflorescences. These types formed the basis of Haeckel's famous classification.^[221] It has, however, been concluded^[222] that the skeleton is a safer guide in taxonomy, at any rate for the smaller subdivisions; and in modern classifications genera with canal systems of the third type will be found distributed among various families; while in the Grantiidae, Ute and Leucandra stand side by side. This treatment implies a belief that the third type of canal system has been independently and repeatedly evolved within the Calcarea—an example of a phenomenon, homoplasy, strikingly displayed throughout the group. It is, remarkably enough, the case that all the canal systems found in the remainder of the Porifera are more or less modified forms of one or other of the second two types of canal system above described.

The families Grantiidae, Heteropidae, and Amphoriscidae, all possessing a dermal cortex, are distinguished as follows:—The Grantiidae by the absence of subdermal sagittal triradiate spicules and of conspicuous subgastral quadriradiates; the Heteropidae by the presence of sagittal triradiates; the Amphoriscidae by the presence of conspicuous subgastral quadriradiates.

Two families of Calcarea, possibly allied, remain for special mention—the Pharetronidae, a family rich in genera, and containing almost all the fossil forms of the group, and the Astroscleridae.

The Pharetronidae are with one, or perhaps two exceptions, fossil forms, having in common the arrangement of the spicules of their main skeletal framework in fibres. The family is divided into two sub-families:—

I. Dialytinae.—The spicules are not fused to one another; the exact mode of their union into fibres is unknown, but an organic cement may be present.

Lelapia australis, a recent species, should probably be placed here as the sole living representative. Dendy has shown^[223] that this remarkable species has a skeleton of the same fibrous character as is found in typical Dialytinae, and that the triradiate spicules in the fibres undergo a modification into the "tuning-fork" type (Fig. 86, C), to enable them to be compacted into smooth fibres. {193}"Tuning-forks," though not exclusively confined to Pharetronids, are yet very characteristic of them.

Fig. 86.—Portions of the skeleton of Petrostroma schulzei. A, Framework with ensheathing pellicle; B, quadriradiate spicules with laterally fused rays; C, a "tuning-fork." (After Doederlein.)

II. Lithoninae.—The main skeletal framework is formed of spicules fused together, and is covered by a cortex containing free spicules.

Fig. 87.—A spicule from the skeleton framework of Plectroninia, showing the terminally expanded rays. (After Hinde.)

The sub-family contains only one living genus and a few recently described fossil forms. Petrostroma schulzei^[224] lives in shallow water near Japan; Plectroninia halli^[225] and Bactronella were found in Eocene beds of Victoria; Porosphaera^[226] long known from the Chalk of England and of the Continent, has recently been shown by Hinde^[226] to be nearly allied to Plectroninia; finally, Plectinia^[227] is a genus erected by Počta for a sponge from Cenomanian beds of Bohemia. Doederlein, in 1896, expressed his opinion that fossil representatives of Lithoninae would most surely be discovered. The fused spicules are equiangular quadriradiates; they are united in Petrostroma by lateral fusion of the rays, in Plectroninia (Fig. 87) and Porosphaera by {194}fusion of apposed terminal flat expansions of the rays, and in some, possibly all, genera a continuous deposit of calcium carbonate ensheaths the spicular reticulum. Thus they recall the formation of the skeleton on the one hand of the Lithistida and on the other of the Dictyonine Hexactinellida (see pp. 202, 211). "Tuning-forks" may occur in the dermal membrane.

Fig. 88.—Astrosclera willeyana, Lister. A, the Sponge, × about 3. p, The ostia on its distal surface. B, a portion of the skeleton showing four polyhedra with radiating crystalline fibres. C, an ostium; the surrounding tissue contains young stages of polyhedra. (After Lister.)

The Astroscleridae, as known at present, contain a single genus and species, apparently the most isolated in the phylum. Astrosclera willeyana^[228] was brought back from the Loyalty Islands, and from Funafuti of the Ellice group. Its skeleton is both chemically and structurally aberrant. In other Calcarea the calcium carbonate of the skeleton is present as calcite, in Astrosclera as aragonite, and the elements are solid polyhedra, {195}united by their surfaces to the total exclusion of soft parts (Fig. 88). Each element consists of crystalline fibres radially disposed around a few central granules, and terminating peripherally in contact with the fibres of adjacent elements. Young polyhedra are to be found free in the soft parts at the surface. The chambers are exceptionally minute, especially for a calcareous sponge, comparing with those of other sponges as follows:—

In its outward form Astrosclera resembles certain Pharetronids. The minute dimensions of the ciliated chambers relegate Astrosclera to the Micromastictora, and the fortunate fact that the calcium carbonate of its skeleton possesses the mineral characters not of calcite, but of aragonite, renders it less difficult to conceive that its relations may be rather with the non-calcareous than the calcareous sponges.

BRANCH II. MICROMASTICTORA

All sponges which do not possess calcareous skeletons are characterised by choanocytes, which, when compared with those of Calcarea, are conspicuous for their smaller size. The great majority (Silicispongiae) of the non-calcareous sponges either secrete siliceous skeletons or are connected with siliceous sponges by a nicely graded series of forms. The small remainder are entirely askeletal. All these non-calcareous sponges are included, under the title Micromastictora, in a natural group, opposed to the Megamastictora as of equal value.

The subdivision of the Micromastictora is a matter of some difficulty. The Hexactinellida alone are a well circumscribed group. After their separation there remains, besides the askeletal genera, an assemblage of forms, the Demospongiae, which fall into two main tribes. These betray their relationship by series of intermediate types, but a clue is wanting which shall determine decisively the direction in which the series are to be read. The askeletal genera are the crux of the systematist. It is perhaps safest, while recognising that many of them bear a likeness of {196}one kind or another to various Micromastictora, to retain them together in a temporary class, the Myxospongiae.

CLASS I. MYXOSPONGIAE

The class Myxospongiae is a purely artificial one, containing widely divergent forms, which possess a common negative character, namely, the absence of a skeleton. As a result of this absence they are all encrusting in habit.

One genus, Hexadella, has been regarded by its discoverer Topsent^[229] as an Hexactinellid. The same authority places Oscarella with the Tetractinellida; it is more difficult to suggest the direction in which we are to seek the relations of the remaining type, Halisarca.

Hexadella, from the coast of France, is a remarkable little rose-coloured or bright yellow sponge, with large sac-like flagellated chambers and a very lacunar ectosome.

Oscarella is a brightly coloured sponge, with a characteristic velvety surface; it is a British genus, but by no means confined to our shores. Its canal system has been described by some authors as diplodal, by others as eurypylous. Topsent^[230] has shown, and we can confirm his statement, that though the chambers have usually the narrow afferent and efferent ductules of a diplodal system, yet since each one may communicate with two or three canals, the canal system cannot be described as diplodal. The hypophare attains a great development, and in it the generative products mature. The pinacocytes, like those of Plakinidae, and perhaps of Aplysilla, are flagellated.

Halisarca, also British, is easily distinguished from Oscarella by the presence of a mucus-like secretion which oozes from it, and by the absence of the bright coloration characteristic of Oscarella. It naturally suggests itself that the coloration in the one case and the secretion in the other are protective, and in this respect perform one of the functions of the skeleton of other sponges. The chambers are long, tubular, and branched. There is no hypophare.

CLASS II. HEXACTINELLIDA^[231]

Silicispongiae, defined by their spicules, of which the rays lie along three rectangular axes. The canal system is simple, with thimble-shaped chambers. The body-wall is divided into endosome, ectosome, and choanosome.

Some authors would elevate the Hexactinellida to the position of a third main sub-group of Porifera, thus separating them from other siliceous sponges. In considering this view it is important to realise at the outset that they are deep-water forms. They bear evident traces of the influence of their habitat, and like others of the colonists of the deep sea, are impressed with marked archaic features. Yet they are still bound to other Micromastictora, first by the small size of their choanocytes, and secondly by the presence of siliceous spicules. This second character is really a double link, for it involves not merely the presence of silica in the skeleton, but also the presence in each spicule of a well-marked axial filament. Now this axial filament is a structure which is gaining in importance, for purposes of classification, in proportion as its absence in Calcarea is becoming more probable. The Hexactinellida are the only sponges, other than the bath sponge, which are at all generally known. They have won recognition by their beauty, as the bath sponge by its utility, and, like it, one of their number—the Venus's Flower-Basket—forms an important article of commerce, the chief fishery being in the Philippine Islands. This wonderful beauty belongs to the skeleton, and is greatly concealed when the soft parts are present.

We have said that the Hexactinellids are deep-sea forms; they are either directly fixed to the bottom or more often moored in the ooze by long tufts of rooting spicules. In the "glass-rope sponge," the rooting tuft of long spicules, looking like a bundle of spun glass, is valued by the Japanese, who export it to us. In Monorhaphis the rooting tuft is replaced by a single giant spicule,^[232] three metres in length, and described as "of the thickness of a little finger"! Probably it is as a result of their fixed life in the calm waters of the deep sea^[233] that {198}Hexactinellids contrast with most other sponges by their symmetry. It should not, however, be forgotten that many of the Calcarea which inhabit shallow water exhibit almost as perfect a symmetry.

Fig. 89.—Longitudinal section of a young specimen of Lanuginella pupa O.S., with commencing formation of the oscular area. × 35. d.m, Dermal membrane; g.m, gastral membrane; pg, paragaster; sd.tr, subdermal trabeculae; Sg.tr, subgastral trabeculae. (After F. E. Schulze.)

The structure of the body-wall in Hexactinellida is so constant as to make it possible to give a general description applicable to all members of the group. It is of considerable thickness, but a large part is occupied by empty spaces, for the actual tissue is present in minimum quantity. In the wall the chamber-layer is suspended by trabeculae of soft tissue, between a dermal membrane on the outside and a similar gastral membrane on the inner side (Fig. 89). Thus the water entering the chambers through their numerous pores has first passed through the ostia in the dermal membrane and traversed the subdermal trabecular space; on leaving the chambers it flows through the subgastral trabecular space and the ostia in the gastral membrane, to enter the paragaster and leave the body at the osculum. The trabeculae and the dermal and gastral membranes together constitute the dermal layer. This conclusion is based on comparison with adults of the other groups, for in the absence of embryological knowledge no direct evidence is available. According to {199}the Japanese investigator, Isao Ijima,^[234] the dermal and gastral membranes are but expansions of the trabeculae, and the trabeculae themselves are entirely cellular, containing none of the gelatinous basis met with in the dermal layer of all other sponges. There is no surface layer of pinacocytes, the cells forming the trabeculae being all of one type, namely, irregularly branching cells, connected with one another by their branches to form a syncytium. In the trabeculae are found scleroblasts and archaeocytes.

The chambers have a characteristic shape: they are variously described as "thimble-shaped," "tubular," or "Syconate," and they open by wide mouths into the subgastral trabecular space. Their walls have been named the membrana reticularis from the fact that, when preserved with only ordinary precautions, they are seen as a regular network of protoplasmic strands, with square meshes and nuclei at the nodes. This appearance recently found an explanation when Schulze, for the first time, succeeded in preserving the collared cells of Hexactinellids.^[235] Schulze was then able to show that the choanocytes are not in contact with one another at their bases, where the nuclei are situated, but communicate with one another by stout protoplasmic strands. The form of the choanocyte can be seen in Fig. 91.

Fig. 90.—Portion of the body-wall of Walteria sp., showing the thimble-shaped flagellated chambers, above which is seen the dermal membrane. (After F. E. Schulze.)

To Schulze's description of the chamber, Ijima has added the important contributions that every mesh in the reticulum functions as a chamber pore or prosopyle; and that porocytes, such as are found in Calcarea, are wanting. This structure of the chamber-walls, the absence of gelatinous basis in the dermal layer, and the slight degree of histological differentiation in {200}the same layer, added to the more obvious character of thimble-shaped chambers, are the chief archaic features of Hexactinellid morphology.

Fig. 91.—Portion of a section of the membrana reticularis or chamber-wall of Schaudinnia arctica, × 1500. (After F. E. Schulze.)

The skeleton which supports the soft parts is, like them, simple and constant in its main features. It is secreted by scleroblasts, which lie in the trabeculae, and is made up of only one kind of spicule and its modifications. This is the hexactine, a spicule which possesses six rays disposed along three rectangular axes. Each ray contains an axial thread, which meets its fellow at the centre of the spicule, where they together form the axial cross. Modifications of the hexactine arise either by reduction or branching, by spinulation or expansion of one or more of the rays. The forms of spicule arising by reduction are termed pentactines, tetractines, and so on, according to the number of the remaining rays. Those rays which are suppressed leave the proximal portion of their axial thread as a remnant marking their former position (Fig. 94). Octactine spicules seem to form an exception to the above statements, but Schulze has shown that they too are but modifications of the hexactine arising by (1) branching of the rays of a hexactine, followed by (2) recombination of the secondary rays (Fig. 92).

Fig. 92.—A, discohexaster, in which the four cladi a, a', b, b', c of each ray start directly from a central nodule. B, disco-octaster, resulting from the redistribution of the twenty-four cladi of A into eight groups of three. (After Schulze, from Delage.)

The various spicules are named, irrespective of their form, according to their position and corresponding function. The {201}arrangement of the spicules is best realised by means of a diagram (Fig. 93).

Fig. 93.—Scheme to show the arrangement of spicules in the Hexactinellid skeleton. Canalaria, microscleres in the walls of the excurrent canals; Dermalia Autoderm[alia], microscleres in the dermal membrane; D. Hypoderm[alia], more deeply situated dermalia; Dictyonalia, parenchymalia which become fused to form the skeletal framework of Dictyonina; Gastralia Autogastr[alia], microscleres in the gastral membrane; Gastralia Hypogastr[alia], more deeply situated gastralia; Parenchymalia Principalia, main supporting spicules between the chambers; P. Comitalia, slender diactine or triactine spicules accompanying the last; P. Intermedia, microscleres between the P. principalia; Prostalia, projecting spicules; P. basalia, rooting spicules, from the base; P. marginalia, defensive spicules, round the oscular rim; P. pleuralia, defensive spicules, from the sides. (From Delage and Hérouard, after F. E. Schulze.)

The deviations from this ground-plan of Hexactinellid structure are few and simple. They are due to folding of the chamber-layer, or to variations in the shape of the chambers, and to increasing fusion of the spicules to form rigid skeletons. A simple condition of the chamber-layer, like that of the young sponge of Fig. 89, {202}occurs also in some adult Hexactinellids, e.g. in Walteria of the Pacific Ocean (Fig. 90). Thus is represented in this order the second type of canal system described among Calcarea. More frequently, however, instead of forming a smooth sheet, the chamber-layer grows out into a number of tubular diverticula, the cavities of which are excurrent canals; these determine a corresponding number of incurrent canals which lie between them. In this way there arises a canal system resembling the third type of Calcarea. By still further pouching so as to give secondary diverticula, opening into the first, a complicated canal system is formed, as, for example, in Euplectella suberea.

To return to the skeleton, the most complete fusion is attained by the deposit of a continuous sheath of silica round the apposed parallel rays of neighbouring spicules. This may be termed the dictyonine type of union, for it occurs in all those forms originally included under the term Dictyonina, in which the cement is deposited pari passu with the formation of the spicules. In other cases connecting bridges of silica unite the spicules, or there may be a connecting reticulum of siliceous threads, or, again, rays crossing obliquely may be soldered together at the point of contact. These more irregular methods occur in species where the spicules are free at their first formation. Spicules originally free may later be united in a true Dictyonine fashion. The terms Lyssacina and Dictyonina are useful to denote respectively: the former all those Hexactinellida in which the spicules are free at their first formation, and the latter those in which the deposit of the cementing layer goes hand in hand with the formation of the spicules. But the terms do not indicate separateness of origin of the groups denoted by them, for there is evidence that Dictyonine types have been derived repeatedly from Lyssacine types, and that in fact every Dictyonine was once a Lyssacine.

Fig. 94.—Amphidisc, at a are traces of the four missing rays.

The real or natural cleft in the class lies between those genera possessing amphidiscs (Figs. 94, 97) among their microscleres, and all the remainder of the Hexactinellida which bear hexasters (Fig. {203}96). The former set of genera constitute the sub-class Amphidiscophora, the latter the Hexasterophora.

Fig. 95.—Portion of body-wall of Hyalonema, in section, showing the irregular chambers.

Sub-Class 1. Amphidiscophora.—Amphidiscs are present, hexasters absent. A tuft of rooting spicules or basalia is always present. The ciliated chambers deviate more or less from the typical thimble shape, and the membrana reticularis is continuous from chamber to chamber (Figs. 94, 95, 97).

Fig. 96.—Hexasters. A, Graphiohexaster; B, floricome; C, onychaster.

Sub-Class 2. Hexasterophora.—Hexasters are present, amphidiscs absent. The chambers have the typical regular form, and are sharply marked off from one another (Figs. 90, 96).

All the Amphidiscophora have Lyssacine skeletons; in the Hexasterophora both types of skeleton occur. The subdivision of the Hexasterophora is determined by the presence or absence of uncinate spicules. An "uncinatum" is a diactine spicule, pointed at both ends and bearing barbs all directed towards one end. This method of classification gives us a wholly Dictyonine order, Uncinataria, and an order consisting partly of Dictyonine, partly of Lyssacine genera, which may be distinguished as the Anuncinataria. {204}Ova have rarely been found, and sexually produced larvae never; but Ijima has found archaeocyte clusters in abundance, and his evidence is in favour of the view that they give rise asexually to larvae, described by him in this class for the first time (see p. 231).

Both sub-classes are represented in British waters: the Amphidiscophora by Hyalonema thomsoni and Pheronema carpenteri; the Hexasterophora by Euplectella suberea and Asconema setubalense, and of course possibly by others.

Hyalonema thomsoni, one of the glass-rope sponges, was dredged by the Porcupine off the Shetland Islands in water of about 550 fathoms. The spindle-shaped body of the sponge is shown in Fig. 97. Its long rooting tuft is continued right up its axis, to end in a conical projection, which is surrounded by four apertures leading into corresponding compartments of the paragaster.

Fig. 97.—Hyalonema thomsoni. A, Whole specimen with rooting tuft and Epizoanthus crust; B, pinulus, a spicule characteristic of but not peculiar to the Amphidiscophora, occurring in the dermal and gastral membranes; C, amphidisc with axial cross; D, distal end of rooting spicule with grapnel. (After F. E. Schulze.)

The crust of Anthozoa of the genus Epizoanthus (p. 406) on the rooting tuft is a constant feature in this as in other species of Hyalonema. It contributed to make the sponge a puzzle, which long defied interpretation. The earliest diagnosis the genus received was the "Glass Plant." Then the root tuft was thought to be part of the Epizoanthus, which was termed a "most aberrant Alcyonarian with its base inserted in a sponge"; next we hear of the sponge as parasitic {205}on the Sea Anemone. Finally, the root tuft was shown to be proper to the sponge, which was, however, figured upside down, till some Japanese collectors described the natural position, or that in which they were accustomed to find it.

Pheronema carpenteri was found by the Lightning off the north of Scotland in 530 fathoms. The goblet shaped, thick walled body and broad, ill-defined root tuft are shown in Fig. 98, but no figure can do justice to the lustre of its luxuriant prostalia and delicate dermal network with stellate knots at regular intervals. The basalia are two-pronged and anchor-like.

Fig. 98.—Pheronema carpenteri. × ½. (From Wyville Thomson.)

Both the Hexasterophoran genera were dredged off the north of Scotland, and both conform to the Lyssacine type without uncinates. Euplectella suberea is a straight, erect tube, anchored by a tuft {206}of basalia. The upper end of the tube is closed by a sieve plate, the perforations in which are oscula, while the beams contain flagellated chambers, so that the sieve is simply a modified portion of the wall. It is a peculiarity of this as of one or two other allied genera that the lateral walls are perforated by oscula. They are termed parietal gaps, and are regularly arranged along spiral lines encircling the body.

Fig. 99.—Sieve plate of Euplectella imperialis. (After Ijima.)

Ijima, who has dredged Euplectellids from the waters near Tokyo, finds that in young specimens oscula are confined to the sieve plate; parietal gaps are secondary formations. The groundwork of the skeleton is a lattice similar to that shown in Fig. 100. The chamber-layer is much folded. Various foreign species of Euplectella afford interesting examples of association with a Decapod Crustacean, Spongicola venusta, of which a pair lives in the paragaster of each specimen. The Crustacean is light pink, the female distinguished by a green ovary, which can be seen through the transparent tissues. It is not altogether clear what the prisoner gains, nor what fee, if any, the host exacts.

Ijima relates that the skeleton of Euplectella is in great demand in Japan for marriage ceremonies. He also informs us that the Japanese name means "Together unto old age and unto the same grave," while by a slight alteration it becomes "Lobsters in the same cell," and remarks that the Japanese find this an amusing pun.

Fig. 100.—Skeletal lattice of Euplectella imperialis. (After Ijima.)

The same Spongicola lives in pairs in Hyalonema sieboldi. Another case of apparently constant association is that of the Hydroid stocks which inhabit Walteria. F. E. Schulze describes Stephanoscyphus mirabilis (see p. 318) in a specimen of Walteria flemmingi; the presence of the polyp causes the sponge to grow out into little dome-shaped elevations, each of which shelters one polyp; while in W. leuckarti Ijima finds a similar association in every specimen examined.

Fossil Hexactinellida.

This group has the distinction of including among its Lyssacine members the oldest known sponge, Protospongia fenestrata, of Cambrian age (Salter). As preserved it consists of a single layer of quadriradiate, or possibly quinqueradiate spicules, which, arranged as a square meshed lattice, supported the superficial layer of the sponge (Fig. 101). Whether or not the fossil represents the whole of the sponge-skeleton does not appear.^[236]

Fig. 101.—Part of the specimen of Protospongia fenestrata in the Sedgwick Museum, Cambridge. Nat. size. (After Sollas.)

Fig. 102.—A portion of the outer surface of a Receptaculitid, Acanthoconia barrandei, in which the expanded outer rays of the spicules are partially destroyed, revealing the four tangential rays beneath, × 3. (After Hinde.)

The extraordinary Receptaculitidae are probably early Lyssacine forms: they are cup- or saucer-shaped fossils, abundant in Silurian and above all in Devonian strata, and have been "assigned in turn to pine cones, Foraminifera, Sponges, Corals, Cystideans," and Tunicata. Hinde^[237] brings forward important arguments for retaining them among Hexactinellida. The only elements in the skeleton of the simpler genera, e.g. Ischadites, are structures comparable to Hexactinellid spicules. The surface of the fossil presents a series of lozenges forming a regular mosaic. Each lozenge is the expanded end of one of the rays of a spicule; it conceals four rays in one plane, tangential to the wall of the cup-shaped fossil, while the sixth ray projects vertically to the wall into the cavity of the cup. In the genus Receptaculites itself there is an inner layer of plates abutting against the inner {208}ends of the sixth rays, and at present problematic. An axial canal is present in each of the rays—the six canals meeting at the centre of the spicule. Special chinks between the spicules appear to have provided a passage for the water current.

The beautiful Ventriculites, so common in the Chalk and present in the Cambridge Greensand, are historically interesting, for the fact that they are fossil Hexactinellida of which the general and skeletal characters were very minutely described by Toulmin Smith long before recent representatives of the group were known. In common with a number of fossil Dictyonine species they are distinguished by the perforation of the nodes, a character due to the fact that the siliceous investment which unites the spicules together stops short before reaching the centre of each spicule, and bridges across the rays so as to form a skeleton octahedron. This character is rare in recent Hexactinellids, but, as first pointed out by Carter, it is presented by one or two forms, of which Aulocystis grayi Bwk is best known. The majority of the fossil Hexactinellida belong to the Dictyonine section, a fact attributable to the greater coherence of their skeleton. The "Dictyonina" are to be reckoned among the rock-builders of Jurassic and Cretaceous times.

Fig. 103.—A node of the skeleton of Ventriculites from the Cambridge Greensand. (After Sollas.)

The Octactinellida and Heteractinellida are two classes created by Hinde^[238] to contain certain little-known Devonian and Carboniferous sponges, possessing in the one case 8-rayed spicules, of which 6 rays lie in one plane and 2 are perpendicular to this plane; in the other case, spicules with a number of rays varying from 6 to 30. Bearing in mind the manner in which octactine spicules are known to arise in recent Hexactinellida (p. 200), it is clearly possible to derive these 8-rayed spicules from hexactines by some similar method; while the typical {209}spicule of the Heteractinellida is a euaster. Hence we may refer the Octactinellid fossils to the class Hexactinellida, and the Heteractinellid forms either to the Monaxonida or Tetractinellida.

CLASS III. DEMOSPONGIAE

Silicispongiae in which triaxonid spicules are absent.

This class has attained the highest level of organisation known among Porifera; the most efficient current-producing apparatus is met with here, so, too, are protective coverings, stout coherent skeletons, and the highest degree of histological differentiation found in the phylum.

Correspondingly it is the most successful group, the majority of existing sponges coming within its boundaries. A few genera and species are exceedingly specialised, for example, Disyringa dissimilis (p. 215). These, however, contribute only a very small contingent to the Demosponge population, those species which are really prolific and abundant being, as we should expect, the less exaggerated types.

Canal System.—With a few exceptions the representatives of the Demospongiae may be said to have taken up the evolution of the canal system at the stage where it was left in Leucandra aspera—a stage which the ancestral Demosponges must have reached quite independently of the Calcarea. These commoner members are thus already gifted with the advantages pertaining to a spherical form of ciliated chamber, and so, too, is the Rhagon (Fig. 105), an immature stage noteworthy as the simplest form of Demosponge, and thus the starting-point for the higher types of canal system. The exceptions above alluded to are not without interest: they are the Dendroceratina, of doubtful affinities, (p. 220), which possess small tubular Syconate chambers. They may be regarded either as of independent origin from other Demospongiae, thus making the group polyphyletic, or more simply as representing the ancestral condition, and in this case we must look on the possession of spherical chambers by the Rhagon as a secondary feature. Occupying as it does the important position above indicated, the Rhagon merits a brief description. It is a small discoid or hemispherical body attached by a flat base. It contains a central paragaster, with a single osculum at the free end. Into the paragaster open directly a {210}few spherical flagellated chambers, which lie in the lateral walls of the body. The basal wall of the paragaster, the parts of its lateral walls between the openings of neighbouring chambers, and the entire outer surface of the body are covered with pinacocytes. It is convenient to call the basal part of the sponge from which chambers are absent the hypophare, the upper chamber-bearing part the spongophare. In some of the deeper dermal cells spicules may be already present. In the Rhagon, then, the canal system is of the second type, but all the adult Demosponges have advanced to the third type, and the further evolution in this system is in the direction of improving the mode of communication of the chambers with the canal system. The changes involved go hand in hand with increasing bulk of the dermal layer. A glance at the accompanying figures will show at once the connexion between the phenomena. The increase in the dermal layer (1) greatly reduces the extent of the lumen of the excurrent canals; and (2) results in the intervention of a narrow tube or aphodus between the mouth of each chamber and the excurrent canal. The chamber system is then converted from an "eurypylous" to an "aphodal" type. When the incurrent canal also opens into the chamber by way of narrow tubes, one proper to each chamber and termed "prosodus," the canal system is of the "diplodal" type.

Fig. 104.—Diagram of (A) eurypylous and (B) aphodal canal systems. a, Apopyle; a', aphodus; E, excurrent canal; I, incurrent canal; p, prosopyle; p', short prosodus. (After Sollas.)

Cortex.—All the stages in the formation of a cortex are to be seen among the adult members of the group. Certain species (e.g. Plakina monolopha, F.E.S.) are destitute even of an ectosome, {211}others have a simple dermal membrane (Halichondria panicea, Tetilla pedifera) and various others are provided with a cortex, either of simple structure or showing elaboration in one or more particulars. Thus a protective armature of special spicules may be present in the cortex, e.g. in Geodia, or to a less extent in Tethya, or there may be an abundance of contractile elements, and these may be arranged in very definite ways, forming valve-like apparatus that will respond to stimuli.

Everywhere among sponges the goal of the skeleton appears to have been coherence. We have seen how in Calcarea and in Hexactinellida this has been attained by the secretion around the separate elements of a continuous mineral sheath, calcareous in the one case and siliceous in the other. Here we had an excellent instance of the attainment of one end by similar means in two different groups, after their separation from the common stock, and therefore independently. In Demospongiae, on the other hand, the same end—coherence—has been secured by two new methods, each distinct from the former: first the spicules may be united in strands by an organic deposit, spongin; secondly, the spicules may assume irregular shapes and interlock closely with one another, forming dense and stout skeletons. The latter method is that characteristic of the Lithistid Tetractinellida.

Classification.—It is not of great moment which scheme of classification we maintain, seeing that all hitherto proposed are confessedly more or less artificial, and sufficient data for framing a natural one are not yet forthcoming. For convenience, we accept three subdivisions and define them thus:—

But at the same time we admit that some of the Ceratosa are probably descended from some of the families of Monaxonida, so that we should perhaps be justified in separating these families of Monaxonida from the rest, and associating them with the allied families of Ceratosa—a method of classification due to {212}Vosmaer. Again, some Monaxonida approximate to Tetractinellida, and we might, with Vosmaer, unite them under the title Spiculispongiae. This proceeding, though it has the advantage of being at least an attempt to secure a natural classification, involves too much assumption when carried out in detail to be wholly satisfactory.

Sub-Class I. Tetractinellida.^[239]

Tetractinellida appear to flourish best in moderate depths from 50 to 200 fathoms, but they are found to be fairly abundant also in shallower water right up to the coast line, and in deep water up to and beyond the 1000 fathom line. Occasionally they lie free on the bottom, but are far more commonly attached; fixation may be direct or by means of rooting spicules; the occurrence of a stalk is rare. There is great variety in the root tuft, which may be a long loose wisp of grapnel-headed spicules, as in many species of Tetilla, or a massive tangle, as in Cinachyra barbata; in these cases the sponge is merely anchored, so that it rests at the level of the surface of the ooze; in other cases, e.g. Thenea wyvillei, the root tuft consists of a number of pillars of spicules which raise the sponge above the level of the ooze, into which they descend and there become continuous with a large dense and confused mass of spicules. The parachute-like base of Tetilla casula invites comparison with the "Crinorhiza" forms of some Monaxonids (p. 216).

Two Orders are distinguished thus:—

Order I. Choristida.

Plakina monolopha, from the Adriatic and Mediterranean, furnishes a connecting link between the Rhagon stage and other Tetractinellida. The choanosome is simply folded; there is no distinct ectosome; the chambers are eurypylous. The skeleton {213}consists of microcalthrops and their derivatives. The hypophare is well developed. Plakina thus shows a certain amount of resemblance to Oscarella (p. 196), with which it shares the very remarkable possession of flagellated pinacocytes.

One of the species of Tetilla, T. pedifera, continues the series. The folds of its choanosome are more complicated than in P. monolopha, and their outer ends are bridged together by a thin layer of ectosome (cf. species of Sycon among Calcarea); the chambers are still eurypylous.

The skeleton reaches a high level: it includes oxeas and triaenes radiately disposed and microscleres (sigmata) scattered throughout the dermal layer. The British Poecillastra compressa from the north of Scotland and Orkney and Shetland is at about the same stage of development, being without cortex and having eurypylous chambers, but it is not so good an example, as the folds of its choanosome are confused.

Fig. 105.—Diagrammatic vertical sections of A, Rhagon; B, Plakina; C, Tetilla pedifera.

From T. pedifera we pass to the other species of Tetilla and all the higher genera of Choristida; these possess a cortex not of homologous origin in the various cases, but probably to be classified under one of two heads, typified by Stelletta and Craniella respectively (Fig. 106).

Fig. 106.—A, Craniella type; B, Stellettid type. ch, Chone; co, collenchyma; d.o, dermal ostia; fb, fibrous tissue; i.c, intercortical cavity; sd, subdermal cavity; sp, sphincter. (After Sollas.)

In the Stellettids the cortex arises by the centrifugal growth of a dermal membrane such as that of Tetilla pedifera; in Craniella directly from the dermal tissue of the distal ends of the choanosomal folds.

In both cases the end result, after completion of cell differentiation, is a cortex either fibrous throughout or collenchymatous in its outer portion and fibrous in the deeper layers. In the Stellettid type the centrifugal growth of the dermal membrane involves the addition of secondary distal portions to the ends of the inhalant passages. These are the intercortical cavities or canals. Their most specialised form is the "chone." A chone is a passage through the cortex opening to the exterior by one or more ostia, and communicating with the deeper parts of the inhalant system by a single aperture provided with a sphincter (Fig. 106, B).

In the Craniella type the intercortical cavities are parts of the primary inhalant system. They communicate with its deeper parts by sphinctrate apertures. Without any knowledge of the development one would certainly have supposed that the subdermal cavity, pore-sieve and sphinctrate passages of Craniella represented a number of chones, of which the outer portions had become fused (Fig. 106, A).

Fig. 107.—Disyringa dissimilis. Diagrammatic longitudinal section of the Sponge. × ½. a, b, c, Transverse sections at the levels indicated to show subdivision of the lumina of the excurrent and incurrent tubes; e.t, excurrent tube; i.t, incurrent tube; o, osculum. (After Sollas.)

In both Craniella and Stelletta the chamber system is aphodal, and these genera may fairly be taken as representatives of the average level reached by Tetractinellida. The skeleton is of the radiate type: the type which prevails in the Choristida, but which has an erratic distribution, appearing in some genera of {215}each family but not in others. The genus Pachymatisma, of which we have the species P. johnstonia and P. normani in these islands, exemplifies this; it belongs to the highly differentiated family Geodiidae, possesses an elaborate cortex with chones, but its main skeleton is non-radiate.

Disyringa dissimilis is remarkable for the perfection of its symmetry, and for the absence of that multiplication of parts which is so common among sponges. It possesses a single inhalant tube and a single osculum (Fig. 107). Until quite recently it stood alone in the restriction of its inhalant apertures to a single area. Kirkpatrick, however, has now described a sponge—Spongocardium gilchristi^[240]—from Cape Colony, in which the dermal ostia are concentrated in one sieve-like patch at the opposite pole to the single osculum. Disyringa is still without companions in the possession of an inhalant tube. The concentration of ostia into sieve areas occurs again in Cinachyra, each sponge possessing in this case several inhalant areas with or without scattered ostia also.

Order II. Lithistida.

The characteristic spicule of Lithistida—the desma—may be a modified calthrop (tetracrepid desma), or it may be produced by the growth of silica over a uniaxial spicule (rhabdocrepid desma) (Fig. 110, q), or it may be of the polyaxon type. It is probable that the group is polyphyletic,^[241] and that some of its members should remain associated with Tetractinellida, while others should be removed to Monaxonida. Forms with tetracrepid desmas, and those forms with rhabdocrepid desmas which possess triaenes, have Tetractinellid affinities, while forms possessing rhabdocrepid desmas but lacking triaenes, and again those in which the desmas are polyaxon, are probably descendants of Monaxonida.

Owing to the consistency of the skeleton Lithistida are frequently found as fossils. The commonest known example is Siphonia.^[242] As in the case of so many other fossil sponges the skeleton is often replaced by carbonate of lime, a fact which {216}misled some of the earlier investigators but was established by the researches of Sollas and Zittel.

Sub-Class II. Monaxonida.^[243]

The Monaxonida inhabit for the most part shallow water, but they also extend through deep water into the abysses, thirteen species having been dredged from depths of over 2000 fathoms by the "Challenger" Expedition alone. In some cases, e.g. Cladorhiza, Chondrocladia, all the species of a genus may live in deep water, while in others the genus, or in others, again, the species, may have a wide bathymetrical range. Thus Axinella spp. occur in shallow water and in various depths down to 2385 fathoms, Axinella erecta ranges from 90 to 1600 fathoms, Stylocordyla stipitata from 7 to 1600, and so on. The symmetry of the deep-water forms contrasts strikingly with the more irregular shape of their shallow-water allies.^[244] The shallow-water species are almost always directly attached, some few are stalked; those from deep water have either a long stalk or some special device to save them from sinking in the soft ooze or mud. Thus the deep-sea genus Trichostemma has the form of a low inverted cone, round the base of which a long marginal fringe of spicules projects, continuing the direction of the somal spicules, and so forming a supporting rim. The same form has been independently evolved in Halicnemia patera, and an approach to it in Xenospongia patelliformis. A similar and more striking case of homoplasy is afforded by the Crinorhiza form, which has been attained in certain species of the deep-sea genera Chondrocladia, Axoniderma, and Cladorhiza; here the sub-globular body is supported by a vertical axis or root, and by a whorl of stout processes radiating outwards and downwards from it, and formed of spicular bundles together with some soft tissue.

There is recognisable in the order Monaxonida a cleft between one set of genera, typically corticate, and suggesting by their structure a relationship, whether of descent or parentage, with the Tetractinellida, and a second set typically non-corticate: these latter are the Halichondrina, the former are the Spintharophora.

Order I. Halichondrina.

We have already seen typical examples of the Halichondrina in Halichondria panicea and Ephydatia fluviatilis. Within the Halichondrina the development of spongin reaches its maximum among spiculiferous sponges, and accordingly the Ceratosa take their multiple origin here (p. 220). Among Halichondrina spongin co-operates with spicules to form a skeleton in various ways, but always so as to leave some spicules bare or free in the flesh. It may bind the spicules end to end in delicate networks (as in Reniera or Gellius), or into strands, sometimes reaching a considerable thickness (as in Chalina and others). In a few cases there appears to be a kind of division of labour between the spicules and spongin, the latter forming the bulk of the fibre, i.e. fulfilling the functions of support, while the spicules merely beset its surface as defensive organs, rendering the sponge unfit for food. Fibres formed on this pattern are called plumose, and are typical of Axinellidae. The distinctive fibre of the Ectyoninae is as it were a combination of the Axinellid and Chalinine types: a horny fibre both cored with spicules and beset with them. Spicules besetting the surface of a fibre are termed "echinating." Whenever its origin has been investigated, spongin has proved to be the product of secretion of cells; in the great majority of cases it is poured out at the surface of the cell, and Evans showed,^[245] at any rate in one species of Spongilla, that the spongin fibres are continuous with a delicate cuticle at the surface of the sponge. In Reniera spp. occurs a curious case of formation of spongin as an intracellular secretion. A number of spherical cells each secrete within themselves a short length of fibre; they then place themselves in rows, so orientated that their contained rods lie end to end in one line. The rods then fuse and make up continuous threads; the cells diminish in breadth, ultimately leaving the fibre free.^[246]

Order II. Spintharophora.

These corticate forms are further characterised by the arrangement of their megascleres, which is usually, like that of most {218}Tetractinellida, radial, or approximating to radial. The microscleres are, when present, some form of aster. The cortex resembles that of Tetractinellida, and v. Lendenfeld has described chones in Tethya lyncurium.^[247]

The existence of the above points of resemblance between Spintharophora and Tetractinellida suggests a relationship between the two groups as its cause. In judging this possibility the following reflections occur to us. A cortex exists in various independent branches of Tetractinellida. It has in all probability had a different phylogenetic history in each—why not then in these Monaxonida also? Within single genera of Tetractinellida some species are corticate, others not, witness Tetilla. The value of a cortex for purposes of classification may easily be overestimated. If we are to uphold the relationship between these two groups, we must base our argument on the conjunction of similar characters in each.

The genus Proteleia^[248] is interesting for its slender grapnel-like spicules, which project beyond the radially disposed cortical spicules, and simulate true anatriaenes of minute proportions. That they are not anatriaenes is shown by the absence of an axial thread in their cladi. It is not surprising that a form of spicule of such obvious utility as the anatriaene should arise more than once.

Of exceptional interest, on account of their boring habit, are the Clionidae. How the process of boring is effected is not known; the presence of an acid in the tissues was suspected, but has been searched for in vain. The pieces of hard substance removed by the activity of the sponge take their exit through the osculum and have a fixed shape^[249] (Fig. 108).

As borers into oyster shells, Clionidae may be reckoned as pests of practical importance, and in some coasts they even devastate the rocks, penetrating to a depth of some feet, and causing them to crumble away.^[250]

Sponges, however, as agents in altering the face of the earth do not figure as destroyers merely. On the contrary, it has {219}been calculated^[251] that sponge skeletons may give rise with considerable rapidity to beds of flint nodules; in fact, it appears that a period so short as fifty years is sufficient for the formation of a bed of flints out of the skeletons of sponges alone.

Suberites domuncula is well known for its constant symbiosis with the Hermit crab. The young sponge settles on a Whelk or other shell inhabited by a Pagurus, and gradually envelops it, becoming very massive, and completely concealing the shell, without however closing its mouth. The aperture of this always remains open to the exterior, however great the growth of the sponge, a tubular passage being left in front of it, which continues the lumen of the shell and maintains its spiral direction. When the crab has grown too big for the shell, it merely advances a little down this passage. The shell is never absorbed, as was once supposed.^[252] The crab, besides being provided with a continually growing house, and being thus spared the great dangers attending a shift of lodgings, benefits continually by the concealment and protection afforded by the massive sponge; the latter in return is conveyed to new places by the crab.

Fig. 108.—A, calcareous corpuscle detached by Cliona; B, view of the galleries excavated by the Sponge. (After Topsent.)

Ficulina ficus is sometimes, like S. domuncula, found in symbiosis with Pagurus, but the constancy of the association is wanting in this case. The sponge has several metamps, one of which, from its fig-like shape, gives it its name.

Sub-Class III. Ceratosa.

The Ceratosa are an assemblage of ultimate twigs shorn from the branches of the Monaxonid tree. They are therefore related forms, but many of them are more closely connected with their Monaxonid relatives than with their associates in their own sub-class.

The genera Aulena and Phoriospongia, placed by v. Lendenfeld among Ceratosa, by Minchin among Monaxonida, show each in its own way how close is the link between these two sub-classes.

Aulena possesses in its deeper parts a skeleton of areniferous spongin fibres, in fact a typical Ceratose skeleton; but this is continuous with a skeleton in the more superficial parts, which is composed of spongin fibres echinated by spicules proper to the sponge, and precisely comparable to the ectyonine fibres of some Monaxonida.

Phoriospongia, as far as its main skeleton is concerned, is a typical Ceratose sponge, with fibres of the areniferous type, but it possesses sigmata free in the flesh.

The sub-class is confined to shallow water, no horny sponge having been dredged from depths greater than 410 fathoms.^[253] The greatest number occur at depths between 10 and 26 fathoms.

In the majority of the Ceratosa the skeletal fibres are homogeneous, formed of concentric lamellae of spongin, deposited by a sheath of spongoblasts around a filiform axis. In others, however, the axis attains a considerable diameter, so as to form a kind of pith to the fibre, which is then distinguished as heterogeneous. In one or two cases some of the spongoblasts of a heterogeneous fibre are included in the fibre between the spongin lamellae. Ianthella is the best-known example in which this occurs.

Ceratosa are divided into Dictyoceratina and Dendroceratina, distinguished, as their names express, by the nature of the skeleton—net-like, with many anastomoses, in the one; tree-like, without anastomoses between its branches, in the other.

The Dictyoceratina comprise by far the larger number of Ceratosa. They fall into two main families, the Spongidae and Spongelidae, both represented in British waters. The Spongidae {221}are characterised by a granular ground substance and aphodal chamber system; the Spongelidae by a clear ground substance and sac-like eurypylous chambers.

The bath sponge, Euspongia officinalis, belongs to the Spongidae. The finest varieties come from the Adriatic, the coarser ones from the Dalmatian and North African coasts of the Mediterranean, from the Grecian Archipelago, from the West Indies, and from Australian seas. The softer species of the genus Hippospongia also form a source of somewhat inferior bath sponges.

Among Dendroceratina, Darwinella is unique and tempts to speculation, in that it possesses isolated spongin elements, resembling in their forms triaxon spicules.

Key to British Genera of Sponges.

Fig. 109.—Microscleres of Demospongiae. a, b, Sigmaspires viewed in different directions; c, d, bipocilli viewed in different directions; e, toxaspire; f, f', spiraster; g, sanidaster; h, amphiaster; i, sigma; j, diaucistra; k, isochela; l, m, anisochelae viewed in different directions; n, cladotyle; o, toxa; p, forceps; q, oxyaster; r, spheraster; s, oxyaster with 6 actines; t, another with 4 actines; u, another with rays reduced to two (centrotylote microxea); v, tylote microrhabdus; w, trichodragmata; x, oxeate microrhabdus or microxea.

Fig. 110.—Megascleres. a-l and q-s, Modifications of monaxon type. a, Strongyle; b, tylote; c, oxea; d, tylotoxea; e, tylostyle; f, style; g, spined tylostyle; h, sagittal triod (a triaxon form derived from monaxon); j, oxytylote; k, anatriaene; l, protriaene; m, sterraster (polyaxon); n, radial section through the outer part of m, showing two actines soldered together by intervening silica; o, desma of an Anomocladine Lithistid (polyaxon); q, crepidial strongyle, basis of rhabdocrepid Lithistid desma; r, young form of rhabdocrepid desma, showing crepidial strongyle coated with successive layers of silica; s, rhabdocrepid desma.

CHAPTER IX

PORIFERA (CONTINUED): REPRODUCTION, SEXUAL AND ASEXUAL—PHYSIOLOGY—DISTRIBUTION—FLINTS

The reproductive processes of Sponges are of such great importance in leading us to a true conception of the nature of a sponge that we propose to treat them here in a special section. Both sexual and asexual methods are common; the multiplication of oscula we do not regard as an act of reproduction (p. 174).

Fig. 111.—A, amphiblastula larva of Sycon raphanus; B, later stage, showing invagination of the flagellated cells. c.s, Segmentation cavity; ec, ectoderm; en, endoderm. (After F. E. Schulze, from Balfour.)

A cursory glance at a collection of sponge larvae from different groups would suggest the conclusion that they are divisible into two wholly distinct types. One of these is the amphiblastula, and the other the parenchymula. This was the conclusion accepted by zoologists not long ago. We are indebted to Delage, Maas, and Minchin for dispelling it, and showing that {227}these types are but the extreme terms of a continuous series of forms which have all the same essential constitution and undergo the same metamorphosis.

The amphiblastula of Sycon raphanus (Fig. 111) consists of an anterior half, formed of slender flagellated cells, and a posterior half, of which the cells are large, non-flagellate, and rounded. These two kinds of cell are arranged around a small internal cavity which is largely filled up with amoebocytes. The flagellated cells are invaginated into the dome of rounded cells during metamorphosis, in fact, become the choanocytes or gastral cells; the rounded cells, on the other hand, become the dermal cells—an astonishing fact to any one acquainted only with Metazoan larvae.

A typical parenchymula is that of Clathrina blanca (Fig. 112). When hatched it consists of a wall surrounding a large central cavity and built up of flagellated cells interrupted at the hinder pole by two cells (p.g.c)—the mother-cells of archaeocytes. Before the metamorphosis, certain of the flagellated cells leave the wall and sink into the central cavity, and undergoing certain changes establish an inner mass of future dermal cells. By subsequent metamorphosis the remaining flagellated cells become internal, not this time by invagination, but by the included dermal cells breaking through the wall of the larva, and forming themselves into a layer at the outside.

Fig. 112.—Median longitudinal section of parenchymula larva of Clathrina blanca. p.g.c, Posterior granular cells—archaeocyte mother-cells. (After Minchin.)

In the larva of C. blanca, after a period of free-swimming existence, the same three elements are thus recognisable as in that of Sycon at the time of hatching; in the newly hatched larva of C. blanca, however, one set of elements, the dermal cells, are not distinguishable. The difference, then, between the two newly hatched larvae is due to the earlier cell differentiation of the Sycon larva.^[254]

Now consider the larva of Leucosolenia. It is hatched as a {228}completely flagellated larva; its archaeocytes are internal (as in Sycon); future dermal cells, recognisable as such, are absent. They arise, as in C. blanca, by transformation of flagellated cells; but (1) this process is confined to the posterior pole, and (2) the internal cavity is small and filled up with archaeocytes. Consequently the cells which have lost their flagella and become converted into dermal cells cannot sink in as in C. blanca: they accumulate at the hinder pole, and thus arises a larva half flagellated, half not; in fact, an amphiblastula. Or, briefly, in Leucosolenia the larva at hatching is a parenchymula, and when ready to fix is an amphiblastula; and, again, the difference between the newly hatched larva and that of Sycon is due to the earlier occurrence of cell differentiation in the latter. What completer transitional series could be desired?

Turning to the Micromastictora, the developmental history already sketched is fairly typical (p. 172). The differences between Mega- and Micro-mastictoran larvae are referable mainly to the fact that the dermal cells in the latter become at once differentiated among themselves to form the main types of dermal cell of the adult.^[255] The metamorphosis is comparable to that of C. blanca. Among Tetractinellida and Hexactinellida sexually produced larvae have not been certainly identified.

Asexual reproduction takes place according to one of three types, which may be alluded to as (1) "budding," (2) "gemmulation," (3) formation of "asexual larvae."

By budding (Fig. 113) is meant the formation of reproductive bodies, each of which contains differentiated elements of the various classes found in the parent. A simple example of this is described by Miklucho Maclay in Ascons, where the bud is merely the end of one of the Ascon tubes which becomes pinched off and so set free.

In Leucosolenia botryoides^[256] Vasseur describes a similar process; in this, however, a strikingly distinctive feature is present (Fig. 114), namely, the buds have an inverse orientation with respect to that of the parent, so that the budding sponge presents a contrast to a sponge in which multiplication of oscula has occurred. In fact, the free distal end of the bud becomes the base of the young sponge, and the osculum is formed at the opposite extremity, where the bud is constricted from the parent.

Fig. 113.—Lophocalyx philippensis. The specimen bears several buds attached to it by long tufts of spicules. (After F. E. Schulze.)

Fig. 114.—Leucosolenia botryoides. A, a piece of the Sponge laden with buds, a-f; i, the spicules of the buds directed away from their free ends; k, the spicules of the parent directed towards the osculum, j. B, a bud which has been set free and has become fixed by the extremity which was free or distal in A. (After Vasseur.)

Such a reversal of the position of the bud is noteworthy in view of its rarity, and the case is worth reinvestigating, for in other animal groups a bud or a regenerated part retains so constantly the same orientation as the parent that Loeb,^[257] after experimenting on the {230}regeneration of Coelenterata and other forms, concluded that a kind of "polarity" existed in the tissues of certain animals.

In Oscarella lobularis^[258] the buds are transparent floating bladders, derived from little prominences on the surface of the sponge. Scattered in the walls of the bladders are flagellated chambers, which open into the central cavity. The vesicular nature of the buds is doubtless an adaptation, lessening their specific gravity and so enabling them to float to a distance from the parent.

Gemmulation.—Spongilla has already afforded us a typical example of this process. Gemmules very similar to those of Spongilla are known in a few marine sponges, especially in Suberites and in Ficulina. They form a layer attached to the surface of support of the sponge—a layer which may be single or double, or even three or four tiers deep. A micropyle is sometimes present in the spongin coat, sometimes absent; possibly its absence may be correlated with the piling of one layer of gemmules on another, as this, by covering up the micropyle, would of course render it useless. Presumably when a micropyle is present the living contents escape through it and leave the sponge by way of the canal system (Fig. 115).