The Cambridge natural history, Vol. 01 (of 10)

Chapter XXI: Echinodermata (continued): Development and Phylogeny 601 (1)

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INDEX 625

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

_The names of extinct groups are printed in italics._

PROTOZOA (pp. 1, 48).

SPOROZOA (p. 94)
+-- TELOSPORIDIA (p. 97)
| +-- Gregarinidaceae (pp. 97, 98)
| | +-- Schizogregarinidae (p. 97).
| | +-- Acephalinidae (p. 97).
| | +-- Dicystidae (p. 97).
| +-- Coccidiaceae (pp. 97, 99)
| +-- Coccidiidae (pp. 97, 99).
| +-- Haemosporidae (pp. 97, 102).
| +-- Acystosporidae (pp. 97, 102).
+-- NEOSPORIDIA (p. 97)
+-- Myxosporidiaceae (pp. 98, 106).
+-- Actinomyxidiaceae (p. 98).
+-- Sarcosporidiaceae (pp. 98, 108).

FLAGELLATA (p. 109) -----+
+-- Pantostomata (p. 109).
+-- Protomastigaceae (p. 110)
| +-- Distomatidae (p. 110).
| +-- Oikomonadidae (p. 111).
| +-- Bicoecidae (p. 111).
| +-- Craspedomonadidae (pp. 111, 121).
| +-- Phalansteridae (p. 111).
| +-- Monadidae (p. 111).
| +-- Bodonidae (p. 111).
| +-- Amphimonadidae (p. 111).
| +-- Trimastigidae (p. 111).
| +-- Polymastigidae (p. 111).
| +-- Trichonymphidae (pp. 111, 123).
| +-- Opalinidae (pp. 111, 123).
+-- Chrysomonadaceae (pp. 110, 125)
| +-- Coccolithophoridae (p. 114).
+-- Cryptomonadaceae (p. 110).
+-- Volvocaceae (pp. 110, 111)
| +-- Chlamydomonadidae (pp. 111, 125).
| +-- Volvocidae (pp. 111, 126).
+-- Chloromonadaceae (p. 110).
+-- Euglenaceae (pp. 110, 124).
+-- Silicoflagellata (pp. 110, 114).
+-- Cystoflagellata (pp. 110, 132).
+-- Dinoflagellata (pp. 110, 130).

INFUSORIA (p. 136)
+-- CILIATA (p. 137)
| +-- Gymnostomaceae (pp. 137, 152).
| +-- Aspirotrichaceae (pp. 137, 153).
| +-- Heterotrichaceae (pp. 137, 153).
| +-- Oligotrichaceae (pp. 137, 155).
| +-- Hypotrichaceae (pp. 137, 138).
| +-- Peritrichaceae (pp. 138, 155).
+-- SUCTORIA = TENTACULIFERA (p. 158).

PORIFERA (p. 163)

Class. Sub-Class. Order. Family. Sub-Family.
MEGAMASTICTORA (pp. 183, 184)
+-- CALCAREA (p. 184)
+-- HOMOCOELA (p. 185)
| +-- Leucosoleniidae (p. 185).
| +-- Clathrinidae (p. 185).
+-- HETEROCOELA (p. 187)
+-- Sycettidae (p. 187).
+-- Grantiidae (p. 192).
+-- Heteropidae (p. 192).
+-- Amphoriscidae (p. 192).
+-- Pharetronidae (p. 192)
| +-- Dialytinae (p. 192).
| +-- Lithoninae (p. 193).
+-- Astroscleridae (p. 194).

MICROMASTICTORA (pp. 183, 195)
+-- MYXOSPONGIAE (p. 196).
+-- HEXACTINELLIDA (p. 197)
| +-- AMPHIDISCOPHORA (p. 203).
| +-- HEXASTEROPHORA (p. 203).
| +---------------------- _Receptaculitidae_ (p. 207).
+-- _OCTACTINELLIDA_ (p. 208).
+-- _HETERACTINELLIDA_ (p. 208).
+-- DEMOSPONGIAE (p. 209)
+-- TETRACTINELLIDA (pp. 211, 212)
| +-- Choristida (p. 212).
| +-- Lithistida (pp. 212, 215).
+-- MONAXONIDA (pp. 211, 216)
| +-- Halichondrina (p. 217).
| +-- Spintharophora (p. 217).
+-- CERATOSA (pp. 211, 220)
+-- Dictyoceratina (p. 220)
| +-- Spongidae (p. 220).
| +-- Spongelidae (p. 220).
+-- Dendroceratina (pp. 220, 221).

COELENTERATA (p. 243)

SCYPHOZOA = SCYPHOMEDUSAE (pp. 249, 310)
+-- CUBOMEDUSAE (p. 318)
| +-- Charybdeidae (p. 318).
| +-- Chirodropidae (p. 319).
| +-- Tripedaliidae (p. 319).
+-- STAUROMEDUSAE (p. 320)
| +-- Lucernariidae (p. 320).
| +-- Depastridae (p. 321).
| +-- Stenoscyphidae (p. 321).
+-- CORONATA (p. 321)
| +-- Periphyllidae (p. 322).
| +-- Ephyropsidae (p. 322).
| +-- Atollidae (p. 322).
+-- DISCOPHORA (p. 323)
+-- Semaeostomata (p. 323)
| +-- Pelagiidae (p. 323).
| +-- Cyanaeidae (p. 324).
| +-- Ulmaridae (p. 324).
+-- Rhizostomata (p. 324)
+-- Cassiopeidae (p. 324) --+ = Arcadomyaria
| --+ (p. 324).
+-- Cepheidae (p. 324) --+ = Radiomyaria
| --+ (p. 324).
+-- Rhizostomatidae (p. 325)--+
+-- Lychnorhizidae (p. 325) | = Cyclomyaria
+-- Leptobrachiidae (p. 325) | (p. 325).
+-- Catostylidae (p. 325) --+

CTENOPHORA (p. 412).

NUDA (p. 423) Beroidae (p. 423).

ECHINODERMATA (p. 425).

PELMATOZOA (pp. 430, 579)
+-- CRINOIDEA (p. 580)
| +---------------------- Hyocrinidae (p. 590).
| +---------------------- Rhizocrinidae (p. 590).
| +---------------------- Pentacrinidae (p. 591).
| +---------------------- Holopodidae (p. 592).
| +---------------------- Comatulidae (p. 594).
| +-- _INADUNATA_ (p. 595).
| +-- ARTICULATA (p. 595).
| +-- _CAMERATA_ (p. 595).
+-- _THECOIDEA_ = _EDRIOASTEROIDEA_ (pp. 580, 596).
+-- _CARPOIDEA_ (pp. 580, 596).
+-- _CYSTOIDEA_ (pp. 580, 597).
+-- _BLASTOIDEA_ (pp. 580, 599).

PROTOZOA

MARCUS HARTOG, M.A., TRINITY COLLEGE (D.SC. LOND.)

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

{3}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.

{4}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]

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).

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.

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]

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]

{12}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.

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.

{20}(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.

{21}(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 CO_{2} 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.

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The Cambridge natural history, Vol. 01 (of 10)Chapter XXI: Echinodermata (continued): Development and Phylogeny 601 (1)

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