Chapter XXI: Echinodermata (continued): Development and Phylogeny 601 (2)

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]

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.)]

{26}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.

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).

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.

In many cases of brood-formation the greater part of the food-supply of
the brood-mother-cell has been stored as reserve-products, which
accumulate in quantity in the cell; this is notably seen in the ovum or
egg of the Higher Animals. How great such an accumulation may be is
indeed well seen in the enormous yolk of a bird's egg, gorged as it were
to repletion. When a cell has entered on such course of "miserly"
conduct, it may lose all power of drawing on its own supplies, and
finally that of accumulating more, and passes into the state of "rest."
To resume activity there is needed either a change in the internal
conditions—demanding the lapse of time—or in the external conditions, or
in both.[46] We may call this resumption "germination."

Very often in the study of a large and complex organism we are able to
find processes in action on a large scale which, depending as they must
do on the protoplasmic activities of its individual cells, reveal the
nature of similar processes in simple unicellular beings: such a clue to
the utilisation of reserves by a cell which has gorged itself with them
so as to pass into a state of rest is to be found in that common
multicellular organism, the Potato. This stores up reserves in its
underground stems (tubers); if we plant these immediately on the
completion of their growth, they will not start at once, even under what
would outwardly seem to be most appropriate conditions. A certain lapse
of time is an essential factor for sprouting. It would appear that in the
Potato the starch can only be digested by a definite ferment, which does
not exist when it is dug, but which is only formed very slowly, and not
at all until a certain time has supervened; and that sprouting can only
{33}take place when soluble material has been provided in this way for
the growth of the young shoots. We have also reason to believe that these
ferments are only formed by the degradation of the protoplasm itself. Now
obviously this degradation must be very slow in a resting organism; and
any external stimulus that will tend to protoplasmic activity will
thereby tend to form at the same time the digestive ferments and dissolve
the stored supplies, to render them available for the life-growth and
reproduction of the being. We now see why inactive "miserly" cells so
often pass into a resting state before dividing, and why they go on
dividing again and again when once they re-enter upon an active life, the
living protoplasm growing at the expense of the reserves.[47] Resting
cells of this type occur of course only at relatively rare intervals in
the animal-feeding Protozoa, that have to take into their substance the
food they require for their growth and life-work, and cannot therefore
store up much reserves. For they are constantly producing in the narrow
compass of their body those very ferments that would dissolve the
reserves when formed. Internal parasites and "saprophytes," that is,
beings which live on dead and decayed organic matter, on the other hand,
live surrounded by a supply of dissolved food; and rarely do we find
larger cells, richer in reserves, than in the parasitic Sporozoa, which
owe their name to the importance of brood-formation in their
life-history. In Radiolaria (p. 75 f.) a central capsule separates off an
inner layer of protoplasm; the outer layer is the one in which digestion
is performed, while the inner layer stores up reserves; and here
brood-formation appears to be the rule. But the largest cells of all are
the eggs of the Metazoa, which in reality lead a parasitic life, being
nurtured by the animal as a whole, and contributing nothing to the
welfare of it as an individual. Their activity is reduced to a minimum,
and the consequent need for a high ratio of surface to volume is also
reduced, which accounts for their inordinate size. But directly the
reserve materials are rendered available by the formation of a digestive
ferment, then protoplasmic growth takes place, and the need for an
extended surface is felt; cell-division follows cell-division with
scarcely an interval in the process of segmentation.

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.

We know that in _many_ cases, and have a right to infer that in _all_,
chemiotaxy plays an important part in attracting the pairing-cells to one
another. In Mammals and Sauropsida there seems also to be a rheotactic
action of the cilia lining the oviducts, which work downwards, and so
induce the sperms to swim upwards to meet the ovum, a condition of things
that was most puzzling until the nature of rheotaxy was understood. A
remarkable fact is that equal gametes rarely appear to be attracted by
members of the same brood, though they are attracted by those of any
other brood of the same species.[50] It may well be that each brood has
its own characteristic secretion, or "smell," as it were, slightly
different from that of other broods, just as every dog has his, so easily
recognisable by other dogs; and that the cells only react to different
"smells" to their own. Such a secretion from the surface of the female
cell would lessen its surface tension, and thereby render easier the
penetration of the sperm into its substance.

As a rule, one at least of the pair-cells is fresh from division, and it
would thus appear that the union of the nuclei is facilitated when one at
least of them is a "young" one. Of the final mechanism of the union of
the nuclei, we know nothing: they may unite in any of the earlier phases
of mitosis, or even in the "resting state." A fibrillation of the
cytoplasm during the process, radiating around a centrosome or two
centrosomes indicates a strained condition.[51]

{35}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.).

{42}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]

{45}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]

A. Pseudopodia the principal means of locomotion and feeding; {50}
flagella absent or transitory I. SARCODINA

(1) Plastogamy only leading to an increase in size, never to the
formation of "fructifications."

(_a_) Pseudopodia never freely coalescing into a network nor
fine to the base RHIZOPODA.

(*) Ectoplasm clear, free from granules; pseudopodia, usually
blunt RHIZOPODA LOBOSA

(**) Ectoplasm finely granular; pseudopodia slender,
branching, but not forming a network, passing into the
body by basal dilatation RHIZOPODA FILOSA

(_b_) Pseudopodia branching freely and coalescing to form
networks; ectoplasm granular; test usually calcareous or
sandy FORAMINIFERA

(_c_) Pseudopodia fine to the very base; radiating, rarely
coalescing.

(i.) Pseudopodia with a central filament HELIOZOA

(ii.) Pseudopodia without a central filament.

(*) Body divided into a central and a peripheral part by a
"central capsule" RADIOLARIA

(**) Body without a central capsule PROTEOMYXA

(2) Cells aggregating or fusing into plasmodia before forming a
complex "fructification" MYCETOZOA

B. Cells usually moving by "euglenoid" wriggling or by excretion of a
trail of viscid matter; reproduction by alternating modes of
brood-formation, rarely by Spencerian fission II. SPOROZOA

C. Flagella (rarely numerous) the chief or only means of motion and
feeding III. FLAGELLATA

D. Cilia the chief organs of motion, in the young state at least; nuclei
of two kinds IV. INFUSORIA

{51}CHAPTER III

PROTOZOA (_CONTINUED_): SARCODINA

I. SARCODINA.

_Protozoa performing most of their life-processes by pseudopodia; nucleus
frequently giving off fragments (chromidia) which may play a part in
nuclear reconstitution on division; sometimes with brood-cells, which may
be at first flagellate; but never reproducing in the flagellate
state.[64]_

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]

I. Ectoplasm distinct, clear; pseudopodia blunt or tapering, but not
branching at the apex

LOBOSA

_Amoeba_, Auctt.; _Pelomyxa_, Greeff; _Trichosphaerium_, A. Schneid.;
_Dinamoeba_, Leidy; _Amphizonella_, Greeff; _Centropyxis_, Stein;
_Arcella_, {52}Ehr.; _Difflugia_, Leclercq; _Lecqueureusia_,
Schlumberger; _Hyalosphenia_, Stein; _Quadrula_, F. E. Sch.;
_Heleopera_, Leidy; _Podostoma_, Cl. and L.; _Arcuothrix_, Hallez.

II. Ectoplasm undifferentiated, containing moving granules; pseudopodia
branching freely towards the tips

FILOSA

_Euglypha_, Duj.; _Paulinella_, Lauterb.; _Cyphoderia_, Schlumb.;
_Campascus_, Leidy; _Chlamydophrys_, Cienk.; _Gromia_, Duj. =
_Hyalopus_, M. Sch.