Read Ebook: The Cambridge natural history Vol. 01 (of 10) by Hartog Marcus Hickson Sydney J Sydney John MacBride E W Ernest William Sollas Igerna Br Nhilda Johnson Harmer S F Sidney Frederic Editor Shipley A E Arthur Everett Sir Editor

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PAGE

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK ix

PROTOZOA

PROTOZOA--INTRODUCTION--FUNCTIONS OF PROTOPLASM--CELL-DIVISION--ANIMALS AND PLANTS 3

PROTOZOA : SPONTANEOUS GENERATION--CHARACTERS OF PROTOZOA--CLASSIFICATION 42

PROTOZOA : SARCODINA 51

PROTOZOA : SPOROZOA 94

PROTOZOA : FLAGELLATA 109

PROTOZOA : INFUSORIA 136 PORIFERA

PORIFERA : FORMS OF SPICULES--CALCAREA--HOMOCOELA --HETEROCOELA--HEXACTINELLIDA--DEMOSPONGIAE--TETRACTINELLIDA --MONAXONIDA--CERATOSA--KEY TO BRITISH GENERA OF SPONGES 183

PORIFERA : REPRODUCTION, SEXUAL AND ASEXUAL--PHYSIOLOGY --DISTRIBUTION--FLINTS 226

COELENTERATA

COELENTERATA--INTRODUCTION--CLASSIFICATION--HYDROZOA--ELEUTHEROBLASTEA --MILLEPORINA--GYMNOBLASTEA--CALYPTOBLASTEA-- GRAPTOLITOIDEA --STYLASTERINA 245

HYDROZOA : TRACHOMEDUSAE--NARCOMEDUSAE--SIPHONOPHORA 288

ANTHOZOA : ZOANTHARIA 365 CTENOPHORA

CTENOPHORA 412

ECHINODERMATA

ECHINODERMATA--INTRODUCTION--CLASSIFICATION--ANATOMY OF A STARFISH --SYSTEMATIC ACCOUNT OF ASTEROIDEA 427

ECHINODERMATA : DEVELOPMENT AND PHYLOGENY 601

INDEX 625

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

PROTOZOA .

SPOROZOA +-- TELOSPORIDIA | +-- Gregarinidaceae | | +-- Schizogregarinidae . | | +-- Acephalinidae . | | +-- Dicystidae . | +-- Coccidiaceae | +-- Coccidiidae . | +-- Haemosporidae . | +-- Acystosporidae . +-- NEOSPORIDIA +-- Myxosporidiaceae . +-- Actinomyxidiaceae . +-- Sarcosporidiaceae .

PORIFERA

Class. Sub-Class. Order. Family. Sub-Family. MEGAMASTICTORA +-- CALCAREA +-- HOMOCOELA | +-- Leucosoleniidae . | +-- Clathrinidae . +-- HETEROCOELA +-- Sycettidae . +-- Grantiidae . +-- Heteropidae . +-- Amphoriscidae . +-- Pharetronidae | +-- Dialytinae . | +-- Lithoninae . +-- Astroscleridae .

COELENTERATA

CTENOPHORA .

Class. Order. Family. TENTACULATA +-- CYDIPPIDEA | +-- Mertensiidae . | +-- Callianiridae . | +-- Pleurobrachiidae . +-- LOBATA | +-- Lesueuriidae . | +-- Bolinidae . | +-- Deiopeidae . | +-- Eurhamphaeidae . | +-- Eucharidae . | +-- Mnemiidae . | +-- Calymmidae . | +-- Ocyroidae . +-- CESTOIDEA | +-- Cestidae . +-- PLATYCTENEA +-- Ctenoplanidae . +-- Coeloplanidae .

NUDA Beroidae .

ECHINODERMATA .

PROTOZOA

MARCUS HARTOG, M.A., TRINITY COLLEGE

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

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 , and later in plants "protoplasm" by Von Mohl . 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, the lowest types of living beings, and of Animals at large.

It is not in detached fragments of the tissues of the higher animals that we can best carry on this study: for here the cells are in singularly close connexion with their neighbours during life; the proper appointed work of each is intimately related to that of the others; and this co-operation has so trained and specially modified each cell that the artificial severance and isolation is detrimental to its well-being, if not necessarily fatal to its very life. Again, in plants the presence of a cell-wall interferes in many ways with the free behaviour of the cell. But in the blood and lymph of higher animals there float isolated cells, the white corpuscles or "leucocytes" of human histology, which, despite their minuteness , 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 u in diameter, and some closely allied species even twelve times this amount. If we place an Amoeba or a leucocyte under the microscope , 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. The promontories are called "pseudopodia" , and the general character of such motion is called "amoeboid."

The living substance, protoplasm, 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 . 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 , 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 u 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 contains also granules of various nature, reserve matters of various kinds, oil-globules, and particles of mineral matter 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. The outermost layer 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" . 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" , 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 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 .

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" . 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 .

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. .

If substances soluble in water be introduced the Amoeba will, as a rule, move away from the region of greater concentration for some substances, but towards it for others. 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 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.

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 "motility "; and irritability evinced by motility is characteristic of all living beings save when in the temporary condition of "rest."

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 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.

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 . 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 "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."

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. The living protoplasm has an alkaline reaction, while the liquid in the larger vacuoles, at least, is acid, especially in Plant-cells.

METABOLISM.--The chemical processes that go on in the organism are termed metabolic changes, and were roughly divided by Gaskell into "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 "catabolic" changes in which the reverse takes place. Anabolic processes, in all but the cells containing plastids or chromatophores 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 the mere breaking of complex substances into simpler ones, or their combination with oxygen; in either case waste products are formed, which may either be of service to the organism as "secretions" , or of no further use . 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 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.

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