LESSONS IN ELEMENTARY BIOLOGY LESSONS IN ELEMENTARY BIOLOGY BY T. JEFFERY PARKER, B.Sc., F.R.S. I'KOFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, NEW ZEALAND WITH EIGHTY-NINE ILLUSTRATIONS o\ I -li. ii CD p- i CD m CD Hontion MACMILLAN AND CO. AND NEW YORK he Right of Translation and Reproduction is Resetted RICHARD CLAY AND SONS, LIMITED, LONDON' AND BUNGAY. I 6 SI '1 To THOMAS H. HUXLEY D.C.L., LL.D., F.X.S. DEAR PROFESSOR HUXLEY, To you I owe my scientific training, my choice of a profession, and every advance I have made in my career. It is therefore only fitting that J should ask you to accept, as a slight token of affection and gratitude, the dedication of this little book, of which I can ho7iestly say that, while its faults are my own, any good features it may possess are mainly due to my having had the advantage of being your pupil. Believe me to be, Yours very faithfully, T. f. PARKER. OTAGO UNIVERSITY MUSEUM, August 22nd, 1890. PREFACE J:\ his preface to the new edition of the well-known Practical Biology, Professor Huxley gives his reasons for beginning the study of organized nature with the higher forms of animal life, to the abandonment of his earlier method of working from the simpler to the more complex organisms. He says in effect that experience has taught him the unwisdom of taking the beginner at once into the new and strange region of microscopic life, and the advantage of making him com- mence his studies with a subject of which he is bound to know something — the elementary anatomy and physiology of a vertebrate animal. Most teachers will probably agree with the general truth of this opinion. The first few weeks of the beginner in natural science are so fully occupied in mastering an unfamiliar and difficult terminology and in acquiring the art of using his eyes and fingers, that he is simply incapable for a time of grasping any of the principles of the science ; and, this being the case, the more completely his new work can via PREFACE be connected with any knowledge of the subject, however vague, he may already possess, the better for his progress. On the other hand, the advantage to logical treatment of proceeding from the simple to the complex — of working upwards from protists to the higher plants and animals — is so immense that it is not to be abandoned without very good and sufficient reasons. In my own experience I have found that the difficulty may be largely met by a compromise, namely, by beginning the work of the class by a comparative study of one of the higher plants (flowering plant or fern) and of one of the higher animals (rabbit, frog, or crayfish). If there were no limitations as to time, and if it were possible to avoid alto- gether the valley of the shadow of the coming examination, this preliminary work might be extended with advantage, and made to include a fairly complete although elementary study of animal physiology, with a minimum of anatomical detail, and a somewhat extensive study of flowering plants with special reference to their physiology and to their relations to the rest of nature. In any case by the time this introductory work is over, the student of average intelligence has overcome pre- liminary difficulties, and is ready to profit by the second and more systematic part of the course in which organisms are studied in the order of increasing complexity. It is such a course of general elementary biology which 1 have attempted to give in the following Lessons, my aim having been to provide a book which may supply in the PREFACE ix study the place occupied in the laboratory by " Huxley and Martin," by giving the connected narrative which would be .out of place in a practical handbook. I also venture to hope that the work may be of some use to students who have studied zoology and botany as separate subjects, as well as to that large class of workers whose services to English science often receive but scant recognition — I mean amateur microscopists. As to the general treatment of the subject I have been guided by three principles. Firstly, that the main object of teaching biology as part of a liberal education is to familiarize the student not so much with the facts as with the ideas of science. Secondly, that such ideas are best understood, at least by beginners, when studied in connection with concrete types of animals and plants. And, thirdly, that the types chosen should illustrate without unnecessary complication the particular grade of organization they are intended to typify, and that exceptional cases are out of place in an elementary course. The types have therefore been selected with a view of illustrating all the more important modifications of structure and the chief physiological processes in plants and animals ; and, by the occasional introduction of special lessons on such subjects as biogenesis, evolution, £c., the entire work is so arranged as to give a fairly-connected account of the general principles of biology. It is in obedience to the last of the principles just enunciated that I have described so many of the Protozoa, omitted all but a brief reference to x PREFACE the development of Hydra and to the sexual process in Penicillium, and described Nitella instead of Chara, and Polygordius instead of the earthworm. The last-named substitution is of course only made possible by the book being intended for the study and not for the laboratory, but I feel convinced that the student who masters the structure of Polygordius, even from figures and descriptions alone, will be in a far better position to profit by a practical study of one of the higher worms. Lessons XXVI I. and XXX. are mere summaries, and can only be read profitably by those who have studied the organisms described, or allied forms, in some detail. Such abstracts were however necessary to the plan of the book, in order to show how all the higher animals and plants may be described, so to speak, in terms of Polygordius and of the fern. For many years I have been convinced of the urgent need for a modification of nomenclature in biology, and have now attempted to carry out a consistent scheme, as will be seen by referring to the definitions in the glossary. Many of Mr. Harvey Gibson's suggestions are adopted and three new words are introduced — phyllula, gamobium, and agamo- bium. I expect and perhaps deserve to be criticised, or, what is worse, let alone, for the somewhat extreme step of using the word ovary in its zoological sense throughout the vegetable kingdom ; and for describing as the venter of the pistil the so-called ovary of Angiosperms. I would only beg my critics before finally pronouncing judgment to try and look at the book, from the point of view of the begin- PREFACE xi ner, as a graduated course of instruction, and to consider the effect upon the entire scheme of using a term of funda- mental importance in two utterly different senses. A large proportion of the figures are copied either from original sources or from my own drawings — the latter when no authority is mentioned. The majority, even of those which have previously appeared in text-books, have been specially engraved for the work, the draughtsman being my brother, Mr. M. P. Parker. In order to facilitate reference the illustrations referring to each subject have, as far as possible, been grouped together, so that the actual is considerably larger than the nominal number of figures. Full descriptions are given instead of mere lists of reference- letters : these will, I hope, be found useful as abstracts of the subjects illustrated. I have to thank my friends, Mr. A. Dillon Bell and Pro- fessor J. H. Scott, M.D., for constant and valuable help in criticising the manuscript. To Dr. Paul Meyer, of the Zoological Station, Naples, I am indebted for specimens of Polygordius ; and to Professor Sale, of this University, Professor Haswell, of Sydney, Professor Thomas, of Auck- land, and Professors Howes and D. H. Scott, of South Kensington, for important information and criticism on special points. My brother, Professor W. Newton Parker, has kindly promised to undertake a final revision for the press. DUNEDIN, N.Z., August, 1890. TABLE OF CONTENTS PAGE PREFACE vii LIST OF LLUSTKATIONS xix LESSON I. AMCEBA I LESSON II. H/EMATOCOCCUS 23 LESSON III. HETEROMITA- 36 LESSON IV. EUGLENA 44 LESSON V. PROTOMYXA 49 THE MYCETOZOA . 52 xiv TABLE OF CONTENTS LESSON VI. PAGE A COMPARISON OF THE FOREGOING ORGANISMS WITH CERTAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS AND PLANTS ........... ...... 56 ANIMAL AND PLANT CELLS ........ 56 MINUTE STRUCTURE AND DIVISION OF CELLS AND NUCLEI . . ........... 62 OVA OF ANIMALS AND PLANTS ....... 67 LESSON VII. SACCHAROMYCES ........... ..... 70 LESSON VIII. BACTERIA ................. gj LESSON IX. BIOGENESIS AND ABIOGENESIS ......... 93 HOMOGENESIS AND HETEROGENESIS ...... ICO LESSON X. PARAMCECIUM ............... STYLONYCHIA ................. OXYTRICHA LESSON XI. OPALINA LESSON XII. VORTICELLA 124 ZOOTHAMNIUM TABLE OF CONTENTS xv LESSON XIII. PAG F. SPECIES AND THEIR ORIGIN : THE PRINCIPLES OF CLASSIFICA- TION .................. 135 LESSON XIV. THE FORAMIN'IFERA ............... 146 THE RADIOLARIA ........... ..... THE DIATOMACE/E ............... 153 LESSON XV. MUCOR ....... » ........... 156 LESSON XVI. VAUCHERIA . . . ............... 167 CAULERPA .......... ........ 173 LESSON XVII. THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS . . 174 LESSON XVIII. PENICILLIUM . . ............... AGARICUS . , , ............... LESSON XIX. SPIROGYRA . . .............. • 192 LESSON XX MONOSTROMA ...... ..... • • • J99 ULVA. ... ..... ..... ... 201 LAMINARIA, &C ....... ......... 2OI xvi TABLE OF CONTENTS LESSON XXI. PAGE NITELLA 204 LESSON XXTT. HYDRA 219 LESSON XXITT. HYDROID POLYPES 234 BOUGAINVILLEA, &C 234 DIPHYES 249 PORPITA . . 250 LESSON XXIV. SPERMATOGENESIS AND OOGENESIS . . . 252 THE MATURATION AND IMPREGNATION OF THE OVUM . . . 255 THE CONNECTION BETWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS 26l LESSON XXV. POLYGORDIUS 267 LESSON XXVI. POLYGORDIUS (continued) 289 LESSON XXVII. THE GENERAL CHARACTERS OF THE HIGHER ANIMALS . . . 303 THE STARFISH 305 THE CRAYFISH . ^Io \J THE MUSSEI .... 316 THE DOGFISH TABLE OF CONTENTS xvii LESSON XXVIII. PAGE MOSSKS 328 LESSON XXIX. FERNS 34° LESSON XXX. THE GENERAL CHARACTERS OF THE HIGHER PLANTS . . . 359 EQUISETUM 362 SALVINIA • 364 SELAGINELLA 3^7 GYMNOSPERMS 369 ANGIOSPERMS ... 374 SYNOPSIS 383 INDEX AND GLOSSARY 395 b LIST OF ILLUSTRATIONS FIG- PAGE 1. Amoeba, various species 2 2. Protamceba pri mitiva g 3. Hitmatococcus pluvialis and H. lacustris 24 4. Heteromita rostrata 38 5. Euglena viridis ... 45 6. Protomyxa aurantiaca 50 7. Badhamia and Chondrioderma 53 8. Typical animal and plant cells 57 9. Typical animal cell 62 10. Stages in the binary fission of an animal cell ... . . 64 1 1 . Stages in the binary fission of a plant cell 66 12. Ova of Car marina and Gymnadenia . . ... 68 13. Saecharomyces cerevisia 71 14. Bacterium termo 82 15. Bacterium termo, showing flagella ... .... 83 16. Micrococcus 85 17. Bacillus subtilis 86 1 8. Vibrio serpens, Spirillum tenue, and S. volittans .... 87 19. Bacillus anthrads 89 20. Beaker with culture-tubes 98 21. Paramcecium aurelia 106 xx LIST OF ILLUSTRATIONS FIG. PAGE 22. Paramcecium aurelia, conjugation 112 23. Stylonychia mytilus • Ir5 24. Oxytricha flava • IJ7 25. Opalina rananim . 120 26. Vorticella ... 125 27. Zoothamnium arbuscula . . 132 28. Zoothamnium, various species 136 29. Diagram illustrating the Origin of the Species of Zootham- nium by Creation 14° 30. Diagram illustrating the Origin of the Species of Zootham- nium by Evolution *42 31. Rotalia 147 32. Diagrams of Foraminifera ... 148 33. Alveolina quoii .... 149 34. Lithocircus annularis ... .150 35. Actinomma asteracanthion 151 36. Diagrams of a Diatom and shells of Navicula and Aulaco discus 154 37. Alucor mucedo and M. stolonifer 15° 38. Moist chamber 161 39. Vaucheria 1 68 40. C aider pa scalpelliformis 172 41. Penicillium glaucum 184 42. Agaricus campestris 19° 43. Spirogyra ....... .... 193 44. Monostroma bullosum and J\L laceration 200 45. Laminaria claustoni and Lessoniafiitcescens 202 46. Nitclla, general structure 205 47. Nitclla, terminal bud 210 48. Nitella, spermary 213 49. Nitella, ovary 215 50. Chara, pro-embryo 217 51. Hydra viridis and H. fitsca, external form 220 52. Hydra, minute structure 224 53. Hydra viridis, ovum 232 LIST OF ILLUSTRATIONS xxi FIG. PAGE 54. Bougainvillea ramosa 235 55. Diagrams illustrating derivation of Medtisa from Hydranth . 238 56. Eucopella campanularia, muscle-fibres and nerve-cells . . . 242 57. Laomedeaf.exuosa^\\^.Eudendriumrainosum, development. 246 58. Diphyes campanulata 248 59. For pita pacifica and P. mediterranea 250 60. Spermatogenesis in the Rat . 253 6 1. Ovum of Toxopneustes lividus 255 62. Maturation and impregnation of the animal ovum .... 256 63. The gastrula ... 261 64. JHagosphczra platiula . . 262 65. Volvox globator 263 66. Diagram illustrating the hypothetical origin of the gastrula from a colony of unicellular zooids 264 67. Diagram illustrating the hypothetical origin of the gastrula from a solitary multinucleate form 265 68. Polygordius neapolitanus, external form . 268 69. Polygordius neapolitanus, anatomy .... .... 270 70. Polygordius neapolitamis, nephridium 281 71. Polygordius, diagram illustrating the relations of the nervous system 283 72. Polygordius neapolitanus ', reproductive organs 290 73. Polygordius neapolitanus, larva in the trochosphere stage . . 292 74. Diagram illustrating the origin of the trochosphere from the gastrula 294 75. Polygordius neapolitanus, advanced trochosphere .... 296 76. Polygordiiis neapolitanus, larva in a stage intermediate be- tween the trochosphere and the adult .... . 299 77. Starfish, diagrammatic sections .... . . 3°6 78. Crayfish, diagrammatic sections . • 312 79. Mussel, diagrammatic sections . • • 3X7 80. Dogfish, diagrammatic sections 322 81. Mosses, various genera, anatomy and histology 329 82. Funaria, reproduction and development . . . 334 83. Pteris and Aspidium, anatomy and histology . • 342 xxii LIST OF ILLUSTRATIONS PAGE 84. Ferns, various genera, reproduction and development . . .352 85. Equisetum^ reproduction and development ...... 363 86. Salvinia, reproduction and development ....... 365 87. Selaginella, reproduction and development . . . . . . 368 88. GymnospermS) reproduction and development ..... 370 89. Angiosperms, reproduction and development ...... 375 LESSONS IN ELEMENTARY BIOLOGY LESSONS IN ELEMENTARY BIOLOGY LESSON I AMCEBA IT is hardly possible to make a better beginning of the systematic study of Biology than by a detailed examination of a microscopic animalcule often found adhering to weeds and other submerged objects in stagnant water, and known to naturalists as Amoeba. Amoebae are mostly invisible to the naked eye, rarely exceeding one-fourth of a millimetre (y^ inch) in dia- meter, so that it is necessary to examine them entirely by the aid of the microscope. They can be seen and re- cognized under the low power of an ordinary student's microscope which magnifies from twenty-five to fifty dia- meters ; but for accurate examination it is necessary to employ a far higher power, one in fact which magnifies about 300 diameters. Seen under this power an Amoeba appears like a little B AMCEBA A FlG. I. — A. Amoeba quartet, a living specimen, showing granular endosarc surrounded by clear ectosarc, and several pseudopods (psd), GENERAL CHARACTERS 3 some formed of ectosarc only, others containing a core of endosarc. The larger bodies in the endosarc are mostly food-particles (X 300). l B. The same species, killed and stained with carmine to show the numerous nuclei (mt) (X 300). C. Amceba proteus, a living specimen, showing large irregular pseudopods, nucleus (mt), contractile vacuole (c.vac), and two food vacuoles (f.vac), each containing a small infusor (see Lesson X.) which has been ingested as food. The letter a to the right of the figure in- dicates the place where two pseudopods have united to inclose the food vacuole. The contractile vacuole in this figure is supposed to be seen through a layer of granular protoplasm, whereas in the succeeding figures (D, E, and G) it is seen in optical section, and therefore appears clear. D. An encysted Amceba, showing cell-wall or cyst (cy\ nucleus (mi), clear contractile vacuole (c.vac), and three diatoms (see Lesson XIV. ) ingested as food. E. Amoeba proteus i a living specimen, showing several large pseudo- pods (psd), single nucleus (nu), and contractile vacuole (c.vac), and numerous food particles imbedded in the granular endosarc ( X 330). F. Nucleus of the same after staining, showing a ground substance or achromatin, containing deeply-stained granules of chromatin, and surrounded by a distinct membrane (X 1010). G. Amceba vemtcosa, living specimen, showing wrinkled surface, nucleus (mi), large contractile vacuole (c. vac) and several ingested organisms (X 330). H. Nucleus of the same, stained, showing the chromatin aggregated in the centre to form a nucleolus (X 1010). I. Amceba proteus, in the act of multiplying by binary fission (X5oo). (A, B, E, F, G, and H after Gruber ; c and I after Leidy ; D after Howes.) shapeless blob of jelly, nearly or quite colourless. The central part of it (Fig. i, A, c, and E) is granular and semi- transparent — something like ground glass — while surround- ing this inner mass is a border of perfectly transparent and colourless substance. So clear, indeed, is this outer layer that it is easily overlooked by the beginner, who is apt to take the granular internal substance for the whole Amoeba. If in any way the creature can be made to turn over, or if a number of specimens are examined in various positions, 1 A number preceded by the sign of multiplication indicates the number of diameters to which the object is magnified. B 2 4 AMCEBA these two constituents will always be found to have the same relations, whence we conclude that an Amoeba con- sists of a granular substance the endosarc, completely surrounded by a clear transparent layer or ectosarc. One very noticeable thing about Amoeba is that it is never quite the same shape for long together. Often the changes of form are so slow as to be almost imperceptible, like the movements of the hour-hand of a watch, but by examining it at successive intervals the alteration becomes perfectly obvious, and at the end of half an hour it will probably have altered so much as to be hardly like the same thing. In an active specimen the way in which the changes of form are brought about is easily seen. At a particular point the ectosarc is pushed out in the form of a small pimple-like elevation (Fig. i. A, left side) : this increases in size, still consisting of ectosarc only, until at last granules from the endosarc stream into it, and the projection or pseudopod (A, c, E, psd) comes to have the same structure as the rest of the Amoeba. It must not be forgotten that as the animal does not alter perceptibly in volume during the process, every pseudopod thus protruded from one part of the body necessitates the withdrawal of an equal volume from some other part. This peculiar mode of movement may be illustrated by taking an irregular lump of clay or putty and squeezing it between the ringers. As it is compressed in one direction it will elongate in another, and the squeezing process may be regulated so as to cause the protrusion of comparatively narrow portions from the solid lump, when the resemblance to the movements described in the preceding paragraph will be fairly close. Only it must be borne in mind that in Amoeba there is no external compression, the " squeezing " being done by the animalcule itself. •>•> 15 COMPOSITION OF PROTOPLASM 5 The occurrence of these movements is alone sufficient to show that Amoeba is an organism or living thing, and no mere mass of dead matter. The jelly-like substance of which Amoeba is composed is called protoplasm. It is shown by chemical analysis 1 to consist mainly of certain substances known as proteids, bodies of extreme complexity in chemical constitution, the most familiar example of which is white of egg or albumen. They are compounds of carbon, hydrogen, oxygen, nitrogen, and sulphur, the five elements being combined in the following proportions : — Carbon . . from 51*5 to 54-5 per cent. Hydrogen . „ 6-9 „ 7-3 „ Oxygen 20-9 „ 23-5 „ Nitrogen . 15-2 „ 17-0 „ Sulphur . ,, 0*3 ,, 2*0 ,, ,, Besides proteids, protoplasm contains small proportions of mineral matters, especially phosphates and sulphates of potassium, calcium, and magnesium. It also contains a considerable quantity of water which being as essential a constituent of it as the proteids and the mineral salts, is called water of organization. Protoplasm is dissolved by prolonged treatment with weak acids or alkalies. Strong alcohol coagulates it, />., causes it to shrink by withdrawal of water and become comparatively hard and opaque. Coagulation is also produced by raising the temperature to about 40° C. ; the reader will remember how the familiar proteid white of egg is coagulated and rendered hard and opaque by heat. 1 Accurate analyses of the protoplasm of Amoeba have not been made, but the various micro-chemical tests which can be applied to it leave no doubt that it agrees in all essential respects with the protoplasm of other organisms, the composition of which is known (see p. 7). 6 AMCEBA There is another important property of proteids which is tested by the instrument called a dialyser. This consists essentially of a shallow vessel, the bottom of which is made of bladder or vegetable parchment, or some other organic (animal or vegetable) membrane. If a solution of sugar or of salt is placed in a dialyser and the instrument floated in a larger vessel of distilled water, it will be found after a time that some of the sugar or salt has passed from the dialyser into the outer vessel through the membrane. On the other hand, if a solution of white of egg is placed in the dialyser no such transference to the outer vessel will take place. The dialyser thus allows us to divide substances into two classes : crystalloids — so called because most of them like salt and sugar are capable of existing in the form of crystals — which, in the state of solution, will diffuse through an organic membrane ; and colloids or glue-like substances which will not diffuse. Protoplasm, like the proteids of which it is largely composed, is a colloid, that is, is non- diffusible. Another character of proteids is their i?istability. A lump of salt or of sugar, a piece of wood or of chalk, may be preserved unaltered for any length of time, but a proteid if left to itself very soon begins to decompose ; it acquires an offensive odour, and breaks up into simpler and simpler compounds, the most important of which are water (H2O), carbon dioxide or carbonic acid (CO2), ammonia (NH3), and sulphuretted hydrogen (H2S) 1. In this character of instability or readiness to decompose protoplasm notoriously agrees with its constituent proteids ; any dead organism will, 1 For a more detailed account of the phenomena of putrefaction see Lesson VIII., in which it will be seen that the above statement as to the instability of (dead) proteids requires qualification ; as a matter of fact they only decompose in the presence of living Bacteria. CHARACTERS OF THE NUCLEUS 7 unless special means are taken to preserve it, undergo more or less speedy decomposition. Many of these properties of protoplasm can hardly be verified in the case of Amoeba, owing to its minute size and the difficulty of isolating it from other organisms (water- weeds, &c.) with which it is always associated ; but there are some tests which can be readily applied to it while under observation beneath the microscope. One of the most striking of these micro-chemical tests depends upon the avidity with which protoplasm takes up certain colouring matters. If a drop of a neutral or slightly alkaline solution of carmine or logwood, or of some aniline dye, or a weak solution of iodine, is added to the wTater con- taining Amoeba, the animalcule is killed, and at the same time becomes more or less deeply stained. The theory is that protoplasm has a slightly acid reaction, and thus pro- duces precipitation of the colouring matter from the neutral or alkaline solution. The staining is, however, not uniform. The endosarc, owing to the granules it contains, appears darker than the ectosarc, and there is usually to be seen, in the endosarc, a rounded spot more brightly stained than the rest. This structure, which can sometimes be seen in the living Amoeba (Fig. i, c, E, and G, mi\ while frequently its presence is only revealed by staining (comp. A and B), is called the nucleus. But when viewed under a sufficiently high power, the nucleus itself is seen to be unequally stained. It has lately been shown, in many Amoebae, to be a globular body, en- closed in a very delicate membrane, and made up of two constituents, one of which is deeply stained by colouring matters, and is hence called chromatin, while the other, or achromatin, takes a lighter tint (Fig. i, F). The rela- tive arrangement of chromatin and achromatin varies in 8 AMCEBA different Amoebae : sometimes there are granules of chroma- tin in an achromatic ground substance (F) : sometimes the chromatin is collected towards the surface or periphery of the nucleus : sometimes, again, it becomes aggregated in the centre (G, H). In the latter case the nucleus is seen to have a deeply-stained central portion, which is then distinguished as the nucleolus. When it is said that Amoebae sometimes have one kind of nucleus and sometimes another, it must not be inferred that the same animalcule varies in this respect. What is meant is that there are found in stagnant water many kinds or species of Amoeba which are distinguished from one another, amongst other things, by the character of their nuclei, just as the various species of Felis — the cat, lion, tiger, lynx, &c. — are distinguished from one another, amongst other things, by the colour and markings of their fur. According to the method of binomial nomenclature intro- duced into biology by Linnaeus, the same generic name is applied to all such closely allied species, while each is specially distinguished by a second or specific name of its own. Thus under the genus Amoeba are included Amoeba proteus (Fig. i, c, E, and F) with long lobed pseudopods and a nucleus, containing evenly-disposed granules of chromatin ; A. quarta (A and B), with short pseudopods and numerous nuclei ; A. verrucosa (G and H) with crumpled or folded surface, no well-marked pseudopods, and a nucleus with a central aggregation of chromatin, or nucleolus ; and many others. Besides the nucleus, there is another structure frequently visible in the living Amoeba. This is a clear, rounded space in the ectosarc (c, E, and G, c. vac], which periodically dis- appears with a sudden contraction and then slowly re-appears, its movements reminding one of the beating of a minute PROTAMCEBA colourless heart. It is called the contractile vacuole, and consists of a cavity in the ectosarc containing a watery fluid. Occasionally Amoebae — or more strictly Amoeba-like organisms — are met with which have neither nucleus * nor contractile vacuole, and are therefore placed in the separate genus Protamosba (Fig. 2). They may have been looked upon as the simplest of living things. The preceding paragraph may be summed up by saying that Amoeba is a mass of protoplasm produced into tempo- rary processes or pseudopods, divisible into ectosarc and FIG. 2 — Protamosba primitiva ; A, B, the same specimen drawn at short intervals of time, showing changes of form. C — E. Three stages in the process of binary fission. (After Haeckel.) endosarc, and containing a nucleus and a contractile vacuole : that the nucleus consists of two substances, chromatin and achromatin, enclosed in a distinct membrane : and that the contractile vacuole is a mere cavity in the protoplasm con- taining fluid. All these facts come under the head of Morphology, the division of biology which treats of form and structure : we must now study the Physiology of our animalcule — that is, consider the actions or functions it is capable of performing. 1 Judging from the analogy of the Infusoria it seems very probable that such apparently non-nucleate forms as Protamoeba contain chromatin diffused in the form of minute granules throughout their substance (see end of Lesson X., p. 118), or that they are forms which have lost their nuclei. io AMCEBA First of all, as we have already seen, it moves, the move- ment consisting in the slow protrusion and withdrawal of pseudopods. This may be expressed generally by saying that Amoeba is contractile, or that it exhibits contracti- lity. But here it must be borne in mind that contraction does not mean the same thing in biology as in physics. When it is said that a red-hot bar of iron contracts on cooling, what is meant is that there is an actual reduction in volume, the bar becoming smaller in all dimensions. But when it is said that an Amoeba contracts, what is meant is that it diminishes in one dimension while increasing in another, no perceptible alteration in volume taking place : each time a pseudopod is protruded an equivalent volume of protoplasm is withdrawn from some other part of the body. We may say then that contractility is a function of the protoplasm of Amoeba — that is, that it is one of the actions which the protoplasm is capable of performing. A contraction may arise in one of two ways. In most cases the movements of an Amoeba take place without any obvious external cause ; they are what would be called in the higher animals voluntary movements — movements dictated by the will and not necessarily in response to any external stimulus. Such movements are called automatic. On the other hand, movements may be induced in Amoeba by external stimuli, by a sudden shock, or by coming into contact with an object suitable for food : such movements are the result of irritability of the protoplasm, which is thus both automatic and irritable — that is, its contractility may be set in action either by internal or by external stimuli. Under certain circumstances an Amoeba temporarily loses its power of movement, draws in its pseudopods, and MODE OF FEEDING u becomes a globular mass around which is formed a thick, shell-like coat, called the cyst or cell-wall (Fig. i, D, cy). The composition of this is not known ; it is certainly not protoplasmic, and very probably consists of some nitrogenous substance allied in composition to horn and to the chitin which forms the external shell of Crustacea, insects, &c. After remaining in this encysted condition for a time, the Amoeba escapes by the rupture of its cell-wall, and resumes its active life. Very often an Amoeba in the course of its wanderings comes in contact with a still smaller organism, such as a diatom (see Lesson XIV., Fig. 36) or a small infusor (see Lessons X. — XII.). When this happens the Amoeba may be seen to send out pseudopods which gradually creep round the prey, and finally unite on the far side of it, as in Fig. i, c, a. The diatom or other organism becomes in this way completely enclosed in a cavity or food-vacuole (f. vac), which also contains a small quantity of water neces- sarily included with the prey. The latter is taken in by the Amoeba as food : so that another function performed by the animalcule is the reception of food, the first step in the process of nutrition. It is to be noted that the reception of food takes place in a particular way, viz. by ingestion— i.e. it is enclosed raw and entire in the living protoplasm. It has been noticed that Amoeba usually ingests at its hinder end — that is, the end directed backwards in progression. Having thus ingested its prey, the Amoeba continues its course, when, if carefully watched, the swallowed organism will be found to undergo certain changes. Its protoplasm is slowly dissolved ; if it contains chlorophyll — the green colouring matter of plants— this is gradually turned to brown ; and finally nothing is left but the case or cell-wall in which many minute organisms, such as diatoms, are enclosed. 12 AMCEBA Finally, the Amoeba as it creeps slowly on leaves this empty cell-wall behind, and thus gets rid of what it has no further use for. It is thus able to ingest living organisms as food ; to dissolve or digest their protoplasm ; and to egest or get rid of any insoluble materials they may contain. Note that all this is done without either ingestion aperture (mouth), digestion cavity (stomach), or egestion aperture (anus) ; the food is simply taken in by the flowing round it of pseudopods, digested as it lies enclosed in the protoplasm, and got rid of by the Amoeba flowing away from it. It has just been said that the protoplasm of the prey is dissolved or digested : we must now consider more particu- larly what this means. The stomachs of the higher animals — ourselves, for instance — produce in their interior a fluid called gastric juice. When this fluid is brought into contact with albumen or any other proteid a remarkable change takes place. The proteid is dissolved and at the same time rendered diffusible, so as to be capable, like a solution of salt or sugar, of passing through an organic membrane (see p. 6). The diffusible proteids thus formed by the action of gastric juice upon ordinary proteids are called peptones : the transformation is effected through the agency of a constituent of the gastric juice called pepsin. There can be little doubt that the protoplasm ot Amoeba is able to convert that of its prey into a soluble and diffusible form, possibly by the agency of some substance analogous to pepsin, and that the dissolved matters diffuse through the body of the Amoeba until the latter is, as it were, soaked through and through with them. Under these circumstances the Amoeba may be compared to a sponge which is allowed to absorb water, the sponge itself representing the living protoplasm, the water the solution of proteids which per- GROWTH 13 meates it. It has been proved by experiment that proteids are the only class of food which Amoeba can make use of : it is unable to digest either starch or fat — two very important constituents of the food of the higher animals. Mineral matters must, however, be taken with the food in the form of a weak watery solution, since the water in which the animalcule lives is never absolutely pure. The Amoeba being thus permeated, as it were, with a nutrient solution, a very important process takes place. The elements of the solution, hitherto arranged in the form of peptones, mineral salts, and water, become re-arranged in such a way as to form new particles of living protoplasm, which are deposited among the pre-existing particles. In a word, the food is assimilated or converted into the actual living substance of the Amoeba. One effect of this formation of new protoplasm is obvious : if nothing happens to counteract it, the Amoeba must grow, the increase in size being brought about in much the same way as that of a heap of stones would be by continually thrusting new pebbles into the interior of the heap. This mode of growth — by the interposition of new particles among old ones — is called growth by intussusception, and is very characteristic of the growth of protoplasm. It is neces- sary to distinguish it, because there is another mode of growth which is characteristic of minerals and occurs also in some organized structures. A crystal of alum, for instance, suspended in a strong solution of the same substance grows, but the increase is due to the deposition of successive layers on to the surface of the original crystal, in much the same way as a candle might be made to grow by repeatedly dipping it into melted grease. This can be proved by colouring the crystal with logwood or some other dye before suspending it, when a gradually-increasing colour- i4 AMOEBA less layer will be deposited round the coloured crystal : if growth took place by intussusception we should have a gradual weakening of the tint as the crystal increased in size. This mode of growth — by the deposition of successive layers -is called growth by accretion. It is probable that the cyst of Amoeba referred to above (p. n) grows by accretion. Judging from the analogy of other organisms it would seem that, after rounding itself off, the surface of the sphere of protoplasm undergoes a chemical change resulting in the formation of a thin super- ficial layer of non-protoplasmic substance. The process is repeated, new layers being continually deposited within the old ones until the cell-wall attains its full thickness. The cyst is therefore a substance separated or secreted from the protoplasm ; it is the first instance we have met with of a product of secretion. From the fact that Amoeba rarely attains a greater dia- meter than \ mm., it follows that something must happen to counteract the constant tendency to grow, which is one of the results of assimilation. We all know what happens in our own case : if we take a certain amount of exercise — walk ten miles or lift a series of heavy weights — we undergo a loss of substance manifested by a diminution in weight and by the sensation of hunger. Our bodies have done a certain amount of work, and have undergone a proportional amount of waste, just as a fire every time it blazes up consumes a certain weight of coal. Precisely the same thing happens on a small scale with Amoeba. Every time it thrusts out or withdraws a pseudo- pod, every time it contracts its vacuole it does a certain amount of work — moves a definite weight of protoplasm through a given space. And every movement, however slight, is accompanied by a proportional waste of substance, POTENTIAL OF KINETIC ENERGY 15 a certain fraction of the protoplasm becoming oxidized, or in other words undergoing a process of low temperature combustion. When we say that any combustible body is burnt what we usually mean is that it has combined with oxygen, forming certain products of combustion due to the chemical union of the oxygen with the substance burnt. For instance, when carbon is burnt the product of combustion is carbon dioxide or carbonic acid (C + O2 = CO2) : when hydrogen is burnt water (H9 + O = H2O). The products of the slow com- bustion which our own bodies are constantly undergoing are these same two bodies ; carbon dioxide given off mainly in the air breathed out ; and water given off mainly in the form of perspiration and urine ; together with two com- pounds containing nitrogen, urea (CH4N2O) and uric acid (C5H4N4O3), both occurring mainly in the urine. In some animals urea and uric acid are replaced by other com- pounds such as guanin (C5H5N5O), but it may be taken as proved that in all living things the products of combustion are carbon dioxide, water, and some nitrogenous substance of simpler constitution than proteids, and allied to the three just mentioned. With this breaking down of proteids the vital activities of all organisms are invariably connected. Just as useful mechanical work may be done by the fall of a weight from a given height to the level of the ground, so the work done by the organism is a result of its complex proteids falling, so to speak, to the level of simpler substances. In both instances potential energy or energy of position is converted into kinetic or actual energy. In the particular case under consideration we have to rely upon analogy and not upon direct experiment. We may, however, be quite sure that the products of combustion 16 AMCEBA or waste matters of Amoeba include carbon dioxide, water and some comparatively simple (as compared with proteids) compound of nitrogen. These waste matters or excretory products are given off partly from the general surface of the body, but partly, it would seem, through the agency of the contractile vacuole. It appears that the water taken in with the food, together in all probability with some of that formed by oxidation of the protoplasm, makes its way to the vacuole, and is ex- pelled by its contraction. We have here another function performed by Amoeba, that of excretion, or the getting rid of waste matters. In this connection the reader must be warned against a possible misunderstanding arising from the fact that the word excretion is often used in two senses. We often hear, for instance, of solid and liquid "excreta." In Amoeba the solid excreta, or more correctly faces, consist of such things as the indigestible cell-walls, starch-grains, &c., of the organisms upon which it feeds ; but the rejection of these is no more a process of excretion than the spitting out of a cherry-stone, since they are simply parts of the food which have never been assimilated — never formed part and parcel of the organism. True excreta, on the other hand, are invariably products of the waste or combustion of protoplasm. The statement just made that the protoplasm of Amoeba constantly undergoes oxidation presupposes a constant sup- ply of oxygen. The water in which the animalcule lives invariably contains that gas in solution : on the other hand, as we have seen, the protoplasm is continually forming carbon dioxide. Now when two gases are separated from one another by a porous partition, an interchange takes place between them, each diffusing into the space occupied by the EVOLUTION OF HEAT 17 other. The same process of gaseous diffusion is continually going on between the carbon dioxide in the interior of Amoeba and the oxygen in the surrounding water, the proto- plasm acting as the porous partition. In this way the carbon dioxide is got rid of, and at the same time a supply of oxygen is obtained for further combustion. The taking in of oxygen might be looked upon as a kind of feeding process, the food being gaseous instead of solid or liquid, just as we might speak of "feeding" a fire both with coals and with air. Moreover, as we have seen, the giving out of carbon dioxide is a process of excretion. It is, however, usual and convenient to speak of this process of exchange of gases as respiration or breathing, which is therefore another function performed by the protoplasm of Amceba. The oxidation of protoplasm in the body of an organism, like the combustion of wood or coal in a fire, is accompanied by an evolution of heat. That this occurs in Amoeba can- not be doubted although it has never been proved. The heat thus generated is, however, constantly being lost to the surrounding water, so that the temperature of Amoeba, if we could but measure it, would probably be found, like that of a frog or a fish, to be very little if at all above that of the medium in which it lives. We thus see that a very elaborate series of chemical pro- cesses is constantly going on in the interior of Amoeba. These processes are divisible into two sets : those which begin with the digestion of food and end with the manufac- ture of living protoplasm, and those which have to do with the destruction of protoplasm and end with excretion. The whole series of processes are spoken of collectively as metabolism. We have, first of all, digested food diffused through the protoplasm and finally converted into fresh c i8 AMCEBA living protoplasm : these are processes of constructive meta- bolism or anabolism. Next we have the protoplasm gradually breaking down and undergoing conversion into excretory products : this is the process of destructive metabolism or katabolism. There can be little doubt that both are pro- cesses of extreme complexity : it seems probable that after the food is once dissolved there ensues the successive formation of numerous bodies of gradually increasing complexity (anabolic mesostates or anastates\ culminating in protoplasm ; and that the protoplasm, when once formed, is decomposed into a series of substances of gradually diminishing complexity (katabolic mesostates or katastates\ the end of the series being formed by the comparatively simple products of excretion. The granules in the endosarc are probably to be looked upon as various mesostates im- bedded in the protoplasm proper. Living protoplasm is thus the most unstable of substances ; it is never precisely the same thing for two consecutive seconds: it "decomposes but to recompose," and recom- poses but to decompose : its existence, like that of a water- fall or a fountain, depends upon the constant flow of matter into it and away from it. It follows from what has been said that if the income of an Amoeba, i.e., the total weight of substances taken in (food plus oxygen plus water) is greater than its expenditure or the total weight of substances given out (faeces plus excreta proper plus carbon dioxide) the animacule will grow : if less it will dwindle away : if the two are equal it will remain of the same weight or in a state of physiological equilibrium. We see then that the fundamental condition of existence of the individual Amoeba is that it should be able to form new protoplasm out of the food supplied to it. But some- REPRODUCTION 19 thing more than this is necessary. Amoebae are subject to all sorts of casualties ; they may be eaten by other organ- isms or the pool in which they live may be dried up ; in one way or another they are constantly coming to an end. From which it follows that if the race of Amoebae is to be pre- served there must be some provision by which the indi- viduals composing it are enabled to produce new individuals. In other words Amoeba must, in addition to its other functions, perform that of reproduction. An Amoeba reproduces itself in a very simple way. The nucleus first divides into two : then the whole organism elongates, the two nuclei at the same time travelling away from one another : next a furrow appears across the middle of the drawn-out body between the nuclei (Fig. i, I ; fig. 2, C, D) : the furrow deepens until finally the animalcule sepa- rates into two separate Amoebae (Fig. 2, E) which hence- forward lead an independent existence. This, the simplest method of reproduction known, is called simple fission. Notice how strikingly different it is from the mode of multiplication with which we are familiar in the higher animals. A fowl, for instance, multi- plies by laying eggs at certain intervals, in each of which, under favourable circumstances and after a definite lapse of time, a chick is developed : moreover, the parent bird, after continuing to produce eggs for a longer or shorter time, dies. An Amoeba, on the other hand, simply divides into two Amoebae, each exactly like itself, and in doing so ceases to exist as a distinct individual. Instead of the successive production of offspring from an ultimately dying parent, we have the simultaneous production of offspring by the divi- sion of the parent, which does not die, but becomes simply merged in its progeny. There can be no better instance of the fact that reproduction is discontinuous growth. c 2 20 AMCEBA From this it seems that an Amoeba, unless suffering a violent death, is practically immortal, since it divides into two completely organized individuals, each of which begins life with half of the entire body of its parent, there being therefore nothing left of the latter to die. It would appear, however, judging from the analogy of the Infusoria (see Lesson X.) that such organisms as Amoeba cannot go on' multiplying indefinitely by simple fission, and that occasion- ally two individuals come into contact and undergo complete fission. A conjugation of this kind has been observed in Amoeba, but has been more thoroughly studied in other forms (see Lessons III. and X.). Whether it is a necessary condition of continued existence in our animalcule or not, it appears certain that " death has no place as a natural recurrent phenomenon ': in that organism. If an Amoeba does happen to be killed and to escape being eaten it will undergo gradual decomposition, becoming converted into various simple substances of which carbon dioxide, water, and ammonia are the chief. (See p. 29.) In conclusion, a few facts may be mentioned as to the conditions of life of Amoeba — the circumstances under which it will live or die, flourish or do otherwise. In the first place it will only live within certain limits ot temperature. In moderately warm weather the temperature to which it is exposed may be taken as about 15° C. If gradually warmed beyond this point the movements at first show an increased activity, then become more and more sluggish, and at about 30°-— 35° C. cease altogether, re- commencing, however, when the temperature is lowered. If the heating is continued up to about 40° C. the animalcule is killed by the coagulation of its protoplasm (see p. 5) : it is then said to suffer heat-rigor or death-stiffening pro- CONDITIONS OF LIFE 21 duced by heat. Similarly when cooled below the ordinary temperature the movements become slower and slower, and at the freezing point (o° C.) cease entirely. But freezing, unlike over-heating does not kill the protoplasm, but only renders it temporarily inert ; on thawing the movements re- commence. We may, therefore, distinguish an optimum temperature at which the vital actions are carried on with the greatest activity ; maximum and minimum tempera- tures above and below which respectively they cease ; and an ultra-maximum temperature at which death ensues. There is no definite ultra-minimum temperature known in the case of Amoeba. The quantity of water present in the protoplasm — as water of organization (see p. 5) — is another matter of importance. The water in which Amoeba lives, although fresh, always contains a certain percentage of salts in solution, and the protoplasm is affected by any alteration in the density of the surrounding medium : for instance, by replacing it by dis- tilled water and so reducing the density, or by adding salt and so increasing it. The addition of common salt (sodium chloride) to the amount of 2 per cent, causes Amoeba to withdraw its pseudopods and undergo a certain amount of shrinkage : it is then said to pass into a con- dition of dry-rigor. Under these circumstances it may be restored to its normal condition by adding a sufficient proportion of water to bring back the fluid to its original density. In this connection it is interesting to notice that the dele- terious effects of an excess of salt are only produced when the salt is added suddenly. By the very gradual addition of sodium chloride Amoebae have been brought to live in a 4 per cent, solution, i.e., one twice as strong as would, if added suddenly, produce dry-rigor. 22 AMCEBA From what has been said above on the subject of respira- tion (p. 17) it follows that free oxygen is necessary for the existence of Amoeba. Light, on the other hand, appears to be unnecessary, amoeboid movements having been shown to go on actively in darkness. LESSON II H^EMATOCOCCUS THE rain-water which collects in puddles, open gutters, &c., is frequently found to have a green colour. This colour is due to the presence of various organisms — plants or animals — one of the commonest of which is called Hcema- tococcus (or sometimes Protococcus or Sphcerella) pluvialis. Like Amoeba, Hsematococcus is so small as to require a high power for its examination. Magnified three or four hundred diameters it has the appearance (Fig. 3, A) of an ovoidal body, somewhat pointed at one end, and of a bright green colour, more or less necked with equally bright red. Like Amoeba, also, it is in constant movement, but the character of the movement is very different in the two cases. An active Haematococcus is seen to swim about the field of the microscope in all directions and with considerable appa- rent rapidity. We say apparent rapidity because the rate of progression is magnified to the same extent as the organ- ism itself, and what appears a racing speed under the micro- scope is actually a very slow crawl when divided by 300. It has been found that such organisms as Haematococcus travel at the rate of one foot in from a quarter of an hour to an hour : or, to express the fact in another and fairer way, H^MATOCOCCUS that they travel a distance equal to 2^ times their own diameter in one second. In swimming the pointed end is always directed forwards and the forward movement is ac- i 200 771-772 FIG. 3. — A. Hcematococcus pluvialis, motile phase. Living speci- men, showing protoplasm with chromatophores (chr) and pyrenoids (//r), cell-wall (c.w) connected to cell-body by protoplasmic filaments, and flagellay?. The scale to the left applies to Figs. A — D. B. Resting stage of the same, showing nucleus (mi) with nucleolus (mi'}, and thick cell-wall (c.w) in contact with protoplasm. c. The same, showing division of the cell-body in the resting stage into four daughter-cells. D. The same, showing the development of flagella and detached cell- wall by the daughter-cells before their liberation from the inclosing mother-cell-wall. E. Hcematococciis lacustris, showing nucleus (mi), single large pyrenoid (//>'), and contractile vacuole (c.vac. ). F. Diagram illustrating the movement of a flagellum : ab, its base ; c, c', c", different positions assumed by its apex. (E, after Biitschli. ) FLAGELLA 25 companied by a rotation of the organism upon its longer axis. Careful watching shows that the outline of a swimming Haematococcus does not change, so that there is evidently no protrusion of the pseudopods, and at first the cause of the movement appears rather mysterious. Sooner or later, however, the little creature is sure to come to rest, and there can then be seen projecting from the pointed end two exces- sively delicate colourless threads (Fig. 3, A, fl\ each about half as long again as the animalcule itself : these are called flagella or sometimes cilia.^ In a Haematococcus which has come to rest these can often be seen gently waving from side to side : when this slow movement is exchanged for a rapid one the whole organism is propelled through the water, the flagella acting like a pair of extremely fine and flexible fins or paddles. Thus the movement of Hsematococcus is not amoeboid, i.e., produced by the pro- trusion and withdrawl of pseudopods, but is ciliary, i.e., due to the rapid vibration of cilia or flagella. The flagella are still more clearly seen by adding a drop of iodine solution to the water : this immediately kills and stains the organism, and the flagella are seen to take on a distinct yellow tint. By this and other tests it is shown that Haematococcus, like Amoeba, consists of protoplasm, and that the flagella are simply filamentous processes of the protoplasm. It was mentioned above that in swimming the pointed end 1 The word cilium is sometimes used as a general term to include any delicate vibratile process of protoplasm : often, however, it is used in a restricted sense for a rhythmically vibrating thread, of which each cell bears a considerable number (see Fig. 8, E, and Fig. 21) ; a flagel- lum is a cilium having a whip-lash-like movement, and of which each cell bears only a limited number — one or two, or occasionally as many as four. 26 H^MATOCOCCUS with the flagella went first ; this may therefore be distin- guished as the anterior extremity, the opposite or blunt end being posterior. So that, as compared with Amoeba, Haematococcus exhibits a differentiation of structure : an anterior and a posterior end can be distinguished, and a part of the protoplasm is differentiated set apart as flagella. The green colour of the body is due to the presence of a special pigment called chlorophyll, the substance to which the colour of leaves is due. That this is something quite distinct from the protoplasm may be seen by treatment with alcohol, which simply kills and coagulates the protoplasm, but completely dissolves out the chlorophyll producing a clear green solution. The solution, although green by trans- mitted light, is red under a strong reflected light, and is hence fluorescent : when examined through the spectro- scope it has the effect of absorbing the whole of the blue and violet end of the spectrum as well as a part of the red. The red colour which occurs in so many individuals, some- times almost replacing the green, is due to a colouring- matter closely allied in its properties to chlorophyll and called h&matochrome. At first sight the chlorophyll appears to be evenly distri- buted over the whole body, but accurate examination under a high power shows it to be lodged in a variable number of irregular structures called chromatophores (Fig. 3, A, chr.\ which together form a layer immediately beneath the surface. Each chromatophore consists of a protoplasmic substance impregnated with chlorophyll. After solution of the chlorophyll with alcohol a nucleus (B, nu.) can be made out ; like the nucleus of Amoeba it is stained by iodine, magenta, &c. Other bodies which might easily be mistaken for nuclei are also visible in the living CELL-WALL 27 organism. These are small ovoidal structures (A, pyr.\ with clearly defined outlines occurring in varying numbers in the chromatophores. When treated with iodine they assume a deep, apparently black, but really dark blue, colour. The assumption of a blue colour with iodine is the charac- teristic test of the well-known substance starch, as can be seen by letting a few drops of a weak solution of tincture or iodine fall upon some ordinary washing starch. The bodies in question have been found to consist of a proteid substance covered with a layer of starch, and are called pyrenoids. Starch itself is a definite chemical compound belonging to the group of carbo-hydrates, i.e., bodies containing the elements carbon, hydrogen, and oxygen : its formula is CG Hio °5- In Hcematococcus pluvialis there is no contractile vacuole, but in another species, H. lacustris, this structure is pre- sent as a minute space near the anterior or pointed end (Fig. 3, E, c. vac.}. There is still another characteristic structure to which no reference has yet been made. This appears at the first view something like a delicate haze around the green body, but by careful focussing is seen to be really an extremely thin globular shell (A, c.w.) composed ot some colourless trans- parent material and separated by a space containing water from the body, to which it is connected by very delicate radiating strands of protoplasm. It is perforated by two extremely minute apertures for the passage of the flagella. Obviously we may consider this shell as a cyst or cell- wall differing from that of an encysted Amoeba (Fig. i, D) in not being in close contact with the protoplasm. A more important difference, however, lies in its chemical composition. The cyst or cell-wall of Amoeba, as stated in the preceding lesson (p. n) is very probably nitrogenous : 28 H^MATOCOCCUS that of Haematococcus, on the other hand, is formed of a carbohydrate called cellulose, allied in composition to starch, sugar, and gum, and having the formula C6H10O5. Many vegetable substances, such as cotton, consist of cellulose, and wood is a modification of the same com- pound. Cellulose is stained yellow by iodine, but iodine and sulphuric acid together turn it blue, and a similar colour is produced by a solution of iodine and potassium iodide in zinc chloride known as Schulze's solution. These tests are quite easily applied to Haematococcus : the proto- plasm stains a deep yellowish-brown, around which is seen a sort of blue cloud due to the stained and partly-dissolved cell-wall. It has been stated that in stagnant water in which it has been cultivated for a length of time Haematococcus some- times assumes an amoeboid form. In any case, after leading an active existence for a longer or shorter time it comes to rest, loses its flagella, and throws around itself a thick cell- wall of cellulose (Fig. 3, B), thus becoming encysted. So that, as in Amoeba, there is an alternation of an active or motile with a stationary or resting condition. In the matter of nutrition the differences between Haema- tococcus and Amoeba are very marked and indeed funda- mental. As we have seen, Hsematococcus has no pseudopods, and therefore cannot take in solid food after the manner of Amoeba : moreover, even in its active condition it is usually surrounded by an imperforate cell-wall, which of course quite precludes the possibility of ingestion. As a matter of observation, also, however long it is watched it is never seen to feed in the ordinary sense of the word. Nevertheless, it must take in food in some way or other, or the oxidation of its protoplasm would soon bring it to an end. DECOMPOSITION OF CARBON DIOXIDE 29 Haematococcus lives in rain-water. This is never pure water, but always contains certain mineral salts, especially nitrates, ammonia salts, and often sodium chloride or common table salt in solution. These salts, being crystalloids, can and do diffuse into the water of organization of the ani- malcule, so that we may consider its protoplasm to be con- stantly permeated by a very weak saline solution, the most important elements contained in which are oxygen, hydro- gen, nitrogen, potassium, sodium, calcium, sulphur, and phosphorus. If water containing a large quantity of Haematococcus is exposed to sunlight, minute bubbles are found to appear in it, and these bubbles, if connected and properly tested, are found to consist largely of oxygen. Accurate chemical analysis has shown that this oxygen is produced by the de- composition of the carbon dioxide contained in solution in rain-water, and indeed in all water exposed to the air, the gas, which is always present in small quantities in the atmosphere, being very soluble in water. As the carbon dioxide is decomposed in this way, its oxygen being given off, it is evident that its carbon must be retained. As a matter of fact, it is retained by the organism but not in the form of carbon : in all probability a double decomposition takes place between the carbon dioxide ab- sorbed and the water of organization, the result being the liberation of oxygen in the form of gas and the simultaneous production of some extremely simple form of carbohydrate, /.ed in the preceding paragraph in the cir- MULTIPLICATION BY FISSION 41 cumstance that the anterior flagellum of the parent form is unaffected, and becomes without alteration the anterior flagellum of one of the daughter- forms — that to the right in the figures. The anterior flagellum of the other product of division— that to the left — is a new structure formed as an outgrowth from the body : its commencement is shown in c1,^?. i'. These two modes of fission — longitudinal and transverse — both occur in the anchored form of Heteromita, i.e., in individuals attached by the ventral flagellum. The free-swimming form presents a third variety of the process. It comes to rest, loses its regular outline (D1) becoming almost amceboicl in form and finally (D2) globular. Division then takes place : the flagella of the parent become each the anterior flagellum of one of the daughter cells (compare D1, D2, and D3), while their ventral flagella are formed by the splitting of a little outgrowth of the dividing body (D2, Jl. 2'). As in Amoeba fission is invariably preceded by division of the nucleus. But in Heteromita fission is not the only mode of repro- duction. Under certain circumstances a free-swimming form approaches an anchored form, and applies itself to it in such a way that the posterior ends of the two are in contact (E1). The two individuals then fuse with one another as completely as two drops of gum on a plate unite when brought into contact. Fusion of the nuclei also takes place, and there is formed an irregular body (E2) with a single nucleus and with two flagella at each end. This swims about freely, and as it does so the last trace of distinction between the two monads of which it is formed is lost, and a triangular form is assumed (ES), the two pairs of cilia being situated at two of the angles. Still later the protoplasm of this triangular body loses all trace of nucleus, granules, &c., and becomes perfectly clear (E4) : then it comes to rest and loses its flagella, appearing as a clear, homogeneous, three-cornered sac with slightly convex sides (E5). This body, formed by the conjugation of the two monads, is called a zygote, the two conjugating individuals being distinguished as g 42 HETEROMITA The zygote remains quiescent for some time, and then after undergoing wave-like movements of its surface, bursts at its three angles (EG), its contents escaping in the form of granules called spores, so minute as to be barely visible even under the highest powers of the best modern microscopes. They are formed by the protoplasm of the zygote dividing into an immense number of separate masses, a process known as multiple fission. Carefully watched, these almost ultra-microscopic particles (p1) are found to grow into clear visibility and to take on a distinctly oval shape (p2). Still increasing in size they develop a ventral flagellum (r3) which is at first quite quiescent : finally, the pointed end sends out a process which becomes an anterior flagellum (p4). The spore has now become a Heteromita resembling the parent form in all but size. It will be seen that this remarkable mode of multiplication by conjugation differs from multiplication by fission in the fact that it requires the co-operation of two individuals which undergo complete fusion. As we shall see more plainly later on (Lessons XV. and XVI.) conjugation is the simplest case of sexual reproduction , differing from the sexual repro- duction of the higher organisms in that the two conjugating bodies or gametes are each an entire individual, and in the further circumstance that the gametes resemble one another in form and size, so that there is no distinction of sex,1 but each takes an equal and similar share in the production of the zygote. Binary fission, on the other hand, is an example of asexual reproduction. It iiii^ht perhaps be allowable to consider the active, free- \\iinuiing monad which seeks and attaches itself to the anchored form as a male, and the passive anchored form as a female gamete (see Lesson XII). LIFE HISTORY 43 Notice also another important fact. The spores when first emitted from the ruptured zygote are mere granules of protoplasm, approaching as nearly as anything in nature to the mathematical definition of a point, " without parts and without magnitude." And during its growth a spore increases not only in size but also in complexity, in other words undergoes a progressive differentiation or development. This is an instance of the principle known as Von Baer's law, according to which " development is a progress from the simple to the complex, from the general to the particular, from the homogeneous to the heterogeneous." In Heteromita, then, we have our first instance of development, since in simple fission there is no development, each product of division being from the first similar to the parent in all but size. Lastly, Heteromita is the first instance we have had of an organism with a definite life-history. It multiplies asexually by simple fission producing free-swimming and anchored forms : these conjugate in pairs forming a zygote, in which, by multiple fission, numerous spores are formed : the spores develop into the adult form, asexual multiplica- tion begins once more, and so the cycle of existence is completed. It must be borne in mind that further researches may reveal the occurrence of a true sexual process in Amoeba and Haematococcus. LESSON IV EUGLENA THE] rain-water collected in puddles by the road-side, on roofs, &c., is often found to have a bright green colour : this is sometimes due to the presence of delicate water weeds visible to the naked eye (Lesson XVI.), but frequently the water when held up to the light in a glass vessel appears uniformly green, no suspended matter being visible to the unaided sight. Under these circumstances the green colour is usually due to the presence of vast numbers of an organism known as Englena viridis. Although microscopic, Euglena is considerably larger than either Hsematococcus or Heteromita, its length varying from -o\- mm. to \ mm. The body is spindle-shaped, wide in the middle and narrow at both ends (Fig. 5, A — E) : one extremity is blunter than the other, and from it proceeds a single long flagellum (fl} by the action of which the organism swims with great rapidity, the flagellum being as in Haematococcus, directed forwards. Besides its rapid swimming movements Euglena frequently performs slow movements of contraction and expansion, something like those of a short worm, the body becoming broadened out In si at l he anterior end, then in the middle, then at the GENERAL CHARACTERS 45 posterior end, twisting to the right and left, and so on (Fig. 5, A — D). These movements are so characteristic of the germs that the name euglenoid is applied to them. B c.vac cvuc FIG. 5. — Euglena viridis. A — D, four views of the living organism, showing the changes of form produced by the characteristic euglenoid movements. E, enlarged view, showing the nucleus (mi), reservoir of the con- tractile vacuole (c. vac), with adjacent pigment spot, and gullet with a single flagellum springing from it. F, enlarged view of the anterior end of E, showing pigment-spot (pg) and reservoir (c. vac], mouth (ni), and gullet (a>s), and origin of flagellum (/). G, resting form after binary fission, showing cyst or cell-wall (cy), and the nuclei (nu) and reservoirs (c. vac] of the daughter-cells. H, active form showing contractile vacuole (c. vac], reservoir ( r], and paramylum-bodies (/>). (A — G, after Saville Kent : H, from Biitschli after Klebs. ) The body consists of protoplasm covered with a very delicate skin or cuticle which is often finely striated, and is to be looked upon as a superficial hardening of the protoplasm. The green colour is due to the presence of 46 EUGLENA chlorophyll which tinges all the central part of the body, the two ends being colourless. It is difficult to make out whether the chlorophyll is lodged in one chromatophore or in several. In Haematococcus we saw that chlorophyll was asso- ciated with starch (p. 27). In Euglena there are, near the middle of the body, a number of grains of paramylum (H, p) a carbohydrate of the same composition as starch (CfiH10O5), but differing from it in remaining uncoloured by iodine. Water containing Euglena gives off bubbles of oxygen in sunlight : as in Hsematococcus the carbon dioxide in solution in the water is decomposed in the presence of chlorophyll, its oxygen evolved, and its carbon combined with the elements of water and used in nutrition. For a long time Euglena was thought to be nourished entirely in this way, but there is a good deal of reason for thinking that this is not the case. When the anterior end of a Euglena is very highly magnified it is found to have the form shown in Fig. 5, F. It is produced into a blunt snout-like extremity at the base of which is a conical depression (ees) leading into the soft internal protoplasm : — just the sort of depression one could make in a clay model of Euglena by thrusting one's finger or the end of a pencil into the clay. From the bottom of this tube the flagellum arises, and by its continual movement gives rise to a sort of whirlpool in the neighbourhood. By the current thus produced minute, solid food-particles are swept down the tube and forced into the soft interna protoplasm, where they doubtless become digested in the same way as the substances ingested by an Amoeba. That solid particles are so ingested by Euglena has been proved by diffusing finely produced carmine in the water, when the MOUTH AND GULLET 47 coloured particles were seen to be swallowed in the way described. The depression in question is therefore a gullet, and its external aperture or margin (;;/) is a mouth. Euglena, like Amoeba, takes in solid food, but instead of ingesting it at almost any part of the body, it can only do so at one particular point where there is a special ingestive aperture or mouth. This is clearly a case of specialization or differentiation of structure : in virtue of the possession of a mouth and gullet Euglena is more highly organized than Amoeba. It thus appears that in Euglena nutrition is both holozoic and holophytic : very probably it is mainly holophytic during daylight and holozoic in darkness. Near the centre of the body or somewhat towards the posterior end is a nucleus (E, nu) with a well-marked nucleolus, and at the anterior end is a clear space (c. vac) looking very like a contractile vacuole. It has been shown, however, that this space is in reality a non-contractile cavity or reservoir (H, r) into which the true contractile vacuole (c.vac) opens, and which itself discharges into the gullet. In close relation with the reservoir is found a little bright red speck (pg) called the pigment spot or stigma. It con- sists of hgematochrome (see p. 26) and is curiously like an eye in appearance, so much so that it is sometimes known as the eye-spot. There seems, however, to be no reason for assigning a visual function to it : indeed it has been shown that the greatest sensitiveness to light is manifested by the colourless anterior end of the body. As in Haematococcus a resting condition alternates with the motile phase : the organism loses its flagellum and 48 EUGLENA surrounds itself with a cyst of cellulose (Fig. 5, G, cy) from which, after a period of rest, it emerges to resume active life. Reproduction takes place by simple fission of the resting form, the plane of division being always longitudinal (G). Sometimes each product of division or daughter-cell divides again : finally, the two or four, or sometimes even eight daughter-cells emerge from the cyst as active Euglenae, Recently a process of multiple fission (p. 42) has been described, numerous minute active spores being produced which gradually assume the ordinary form and size. LESSON V PROTOMYXA AND THE MYCETOZOA WHEN Professor Haeckel was investigating the zoology of the Canary Islands more than twenty years ago he discovered a very remarkable organism which he named Protomyxa aurantiaca. It was found in sea-water attached to a shell called Spirilla, and was at once noticeable from the bright orange colour which suggested its specific name. In its fully developed stage Protomyxa is the largest of all the organisms we have yet studied, being fully imm. (^- inch) in diameter, and therefore visible to the naked eye as a small orange speck. In general appearance (Fig. 6, A) it is not unlike an immense Amoeba, the chief difference lying in the fact that the pseudopods (psd) instead of being short, blunt processes, few in number (comp. Fig. i, p. 2) are very numerous, slender, branching threads which often unite with one another so as to form networks. No nucleus was ob- served x and no contractile vacuole, but it is quite possible that a renewed examination might prove the presence of one or both of these structures. The figure (A) is enough to show that nutrition is holozoir 1 See p, 9, note, £ psd FIG. 6. — Protomyxa aurantiaca. A, the living organism (plasmodium), showing fine branched pseudo- pods (pscf) and several ingested organisms. ]?, the same, encysted : cy the cell-wall. c, the protoplasm of the encysted form breaking up into spores, i), dehiscence of the cyst and emergence of K3 flagellulec which afterwards become converted into F, amcubulce. G, amcebulcC uniting to form a plasmodium. (After liacckel.) LIFE-HISTORY 51 the specimen has ingested several minute organisms and is in the act of capturing another. But the main interest of Protomyxa lies in its very curious and complicated life-history. After crawling over the Spirula shells for a longer or shorter time it draws in its pseudopods, comes to rest, and surrounds itself with a cyst (B, cy). The composition of the cyst is not known, but it is apparently not cellulose, since it is not coloured by iodine and sulphuric acid (p. 28). Next, the encysted protoplasm undergoes multiple fission, dividing into a number of spores (c) : soon the cyst bursts and its contents emerge (D) as bodies which differ utterly in appearance from the amoeboid form from which we started. Each spore has in fact become a little ovoid body of an orange colour, provided with a single flagellum (E, fl] by the lashing of which it swims through the water after the manner of a monad. It is convenient to have a name by which to distinguish these flagellate bodies, just as we have special names for the young of the higher animals such as tadpoles or kittens. From the fact of their distinguishing character being the possession of a flagellum they are called flagellultE ; the same name will be applied to the flagellate young of various other organisms which we shall study hereafter. After swimming about actively for a time each flagellula settles down on some convenient substratum and undergoes a remarkable change : its movements become sluggish, its outline irregular, and its flagellum short and thick, until it finally takes on the form of a little Amoeba (F). For this stage also a name is required : it is not an Amoeba but an amoeboid phase in the life-history of a totally different organism : it is called an amotbida. The process just described may be taken as a practical 52 PROTOMYXA AND THE MYCETOZOA proof of the statement made in a previous Lesson (p. 34) that a flagellum is nothing more than a delicate and rela- tively permanent pseudopod. In Protomyxa we have a flagellula directly converted into an amoebula, the flagellum of the former becoming one of the pseudopods of the latter. The amoebulse thus formed may simply increase in size and send out numerous delicate pseudopods, thus becoming converted into the ordinary Protomyxa-form. Frequently, however, they attain this form by a very curious process : they come together in twos and threes until they are in actual contact with one another, when they undergo complete and permanent fusion (G). In this case the Protomyxa-form is produced not by the development of a single amoebula but by the conjugation or fusion of a variable number of amoebulae. A body formed in this way by the fusion of amoebulae is called a plasmodium^ so that in the life-history of Protomyxa we can distinguish an encysted, a ciliated or flagellate, an amoeboid, and a plasmodial phase. The nature of a plasmodium will be made clearer by a short consideration of the strange group of organisms known as Mycetozoa or sometimes " slime- fungi." They occur as gelatinous masses on the bark of trees, on the surface of tan-pits, and sometimes in water. It must be remembered that Mycetozoa is the name not of a genus but of -a class in which are included several genera, such as Badhamia, Chondrioderma, &c., (see Fig. 7) : a general account of the class is all that is necessary for our present purpose. The Mycetozoa consist of sheets or networks of protoplasm which may be as much as 30 cm. (ift.) in diameter, and throughout the substance of which are found numerous nuclei. In this condition they creep about over bark or some THE PLASMODIUM OF BADHAMIA 53 H FIG. 7. — A, part of the plasmodium of Badhamia (X 3i) ; £, short pseudopod enclosing a bit of mushroom stem. B, spore of Chondrioderma. c, the same, undergoing dehiscence. D, flagellulse liberated from spores of the same. E, amoebula formed by metamorphosis of flagellula. F, two amoebulae about to fuse : F', the same after complete union. G, G', two stages in the formation of a three-celled plasmodium. H, a small plasmodium. (A, after Lister : B— H, from Sachs after Cienkowski. ) 54 PROTOMYXA AND THE MYCETOZOA other substance : and as they do so ingest solid food (Fig. 7, A). It has been proved that they digest protoplasm : and in one genus pepsin — the constituent of our own gastric juice by which the digestion of proteids is effected (see p. 12) -has been found to exist. They can also digest starch which has been swollen by a moderate heat—as in our own bread and rice-puddings — but are unable to make use of raw starch. After living in this free condition, like a gigantic terrestrial Amoeba, for a longer or shorter time, either a part or the whole of the protoplasm becomes encysted 1 and breaks up into spores. These (B) consist of a globular mass of proto- plasm covered with a wall of cellulose : the cysts are also formed of cellulose. By the rupture of the cell-wall of the spore (c) the protoplasm is liberated as a flagellula (D) provided with a nucleus and a contractile vacuole, and frequently exhibiting amoeboid as well as ciliary movements. After a time the flagellulse lose their cilia and pass into the condition of amcebulae (E), which finally fuse to form the plasmodium with which we started (F — H). In the young plasmodia (c1) the nuclei of the constituent amcebulje are clearly visible, and from them the nuclei of the fully developed plasmodia are probably derived. It would seem, therefore, that in the fusion of amcebulae to form the plasmodium of Mycetozoa the cell-bodies (protoplasm) only coalesce, not the nuclei. There is a suggestive analogy between this process of plasmodium-formation and that of conjugation as seen in Heteromita. Two Heteromitoe fuse and form a zygote the 1 The process of formation of the cyst or sporangium is a compli- cated one, and will not be described here. See De Bary, Fungi, Mycetozoa, and Bacteria (Oxford, 1887). PLASMODIUM FORMATION AND CONJUGATION 55 protoplasm of which divides into spores. In Protomyxa and the Mycetozoa not two but several Amcebulae unite to form a plasmodium which after a time becomes encysted and breaks up into spores. So that we might look upon the conjugation of Heteromita as an extremely simple plasmo- dial phase in its life-history, or upon the formation of a plasmodium by Protomyxa and the Mycetozoa as a process of multiple conjugation. There is, however, an important difference between the two cases by reason of which the analogy is far from complete. In Heteromita the nuclei of the two gametes are no longer visible (p. 41) : they coalesce during conjugation, and subsequently, in all probability, break up to form the nuclei of the spores. In the Mycetozoa neither fusion nor apparent disappearance of the nuclei of the amoebulae has been observed. LESSON VI A COMPARISON OF THE FOREGOING ORGANISMS WITH CER- TAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS AND PLANTS WHEN a drop of the blood of a crayfish, lobster or crab is examined under a high power, it is found to consist of a nearly colourless fluid, the plasma, in which float a number of minute solid bodies, the blood-corpuscles or leucocytes. Each of these (Fig. 8, A) is a colourless mass of proto- plasm, reminding one at once of an Amoeba, and if it is watched carefully the resemblance becomes closer still, for the corpuscle is seen to put out and withdraw pseudopods (A1 — A4) and so gradually to alter its form completely. Moreover the addition of iodine, logwood, or any other suitable colouring matter reveals the presence of a large nucleus (A5, A6, mi) \ so that, save for the absence of a con- tractile vacuole in the leucocyte, the description of Amoeba in Lesson I. would apply almost equally well to it The blood of a fish, a frog (B1), a reptile, or a bird contains quite similar leucocytes, but in addition there are found in the blood of these red-blooded animals bodies called red corpuscles. They are flat oval discs of protoplasm (B5, B6) D FIG. 8. — Typical Animal and Vegetable Cells. A1 — A4, living leucocyte (blood corpuscle) of a crayfish showing amoeboid movements : A5, A6, the same, killed and stained, showing the nucleus (mi). B1, leucocyte of the frog, nu the nucleus : B2, two leucocytes beginning to conjugate : B3, the same after conjugation, a binucleate plasmodium being formed : B4, a leucocyte undergoing binary fission : B5, surface view and B6 edge view of a red corpuscle of the same, nu, the nucleus. c1, c2, leucocytes of the newt ; in c1 particles of vermilion, repre- sented by black dots, have been ingested. C3, surface view and c4 edge view of a red corpuscle of man. D1, column or epithelium from intestine of frog : D2, a similar cell 58 EPITHELIAL CELLS showing striated distal border from which in r>3 pseudopods are protruded. E1, ciliated epithelial cell from mouth of frog : E'2, E3, similar cells from windpipe of dog. F1, parenchyma cell from root of lily, showing nucleus («#)» vacuoles (vac), and cell-wall : F", a similar cell from leaf of bean, showing nucleus, vacuoles, cell-wall and chromatophores (chr}. (B, D1 and E1, after Howes : c, E2, and E3, after Klein and Noble Smith : D2, D3, after Wiedersheim : F1, after Sachs : F'2, after Behrens. ) coloured by a pigment called h&moglobin, and provided each with a large nucleus (nu) which, when the corpuscle is seen from the edge produces a bulging of its central part. These bodies may be compared to Amoebae which have drawn in their pseudopods, assumed a flattened form, and become coloured with haemoglobin. In the blood of mammals, such as the rabbit, dog, or man, similar leucocytes occur, but their red blood corpuscles (c3, c4) have the form of biconcave discs and are devoid of nuclei. In many animals the leucocytes have been observed to ingest solid particles (c1), to multiply by simple fission (B4) and to coalesce with one another forming plasmodia (B2) (P- 52). The stomach and intestines of animals are lined with a sort of soft slimy skin called mucous membrane. If a bit of the surface of this membrane — in a frog or rabbit for instance — is snipped off and "teased out," i.e., torn apart with needles, it is found when examined under a high power to be made up of an immense number of microscopic bodies called epithelial cells, which in the living animal, lie close to one another in the inner layer of mucous mem- brane in something the same way as the blocks of a wood pavement lie on the surface of a road. An epithelial cell (D1, D2) consists of a rod-like mass of protoplasm, contain- ing a large nucleus, and is therefore comparable to an PARENCHYMA CELLS 59 elongated Amoeba without pseudopods. In some animals the resemblance is still closer : the epithelial cells have been observed to throw out pseudopods from their free surfaces (DS), that is, from the only part where any such movement is possible, since they are elsewhere in close contact with their fellow cells. The mouth of the frog and the trachea or windpipe of air- breathing vertebrates such as reptiles, birds, and mammals, are also lined with mucous membrane, but the epithelial cells which constitute its inner layer differ in one important respect from those of the stomach and intestine. If ex- amined quite fresh each is found to bear on its free surface, i.e., the surface which bounds the cavity of the mouth or windpipe, a number of delicate protoplasmic threads or cilia (E1 — E3) which are in constant vibratory movement. In the process of teasing out the mucous membrane some of the cells are pretty sure to become detached, and are then seen to swim about in the containing fluid by the action of their cilia. These ciliated epithelial cells remind one strongly of Heteromita : except that they bear numerous cilia in constant rhythmical movement instead of two only — in this case distinguished as flagella — presenting an irregular lashing movement. Similar ciliated epithelial cells are found on the gills ot oyster, mussel, &c., and in many other situations. The stem or root of an ordinary herbaceous plant, such as a geranium or sweet-pea, is found when cut across to consist of a central mass of pith, around which is a circle of woody substance, and around this again a soft greenish material called the cortex. A thin section shows the latter to be made up of innumerable polyhedral bodies called 60 PARENCHYMA CELLS parenchyma cells, fitting closely to one another like the bricks in a wall. A parenchyma cell examined in detail (r1) is seen to consist of protoplasm hollowed out internally into one or more cavities or vacuoles (vac) containing a clear fluid. These vacuoles differ from those of Amoeba, Heteromita, or Euglena in being non-contractile ; they are in fact mere cavities in the protoplasm containing a watery fluid : the layer of protoplasm immediately surrounding them is denser than the rest. Sometimes there is only one such space occupying the whole interior of the cell, sometimes, as in the example figured, there are several, separated from one another by delicate bands or sheets of protoplasm. The cell contains a large nucleus (nu) and is completely enclosed in a moderately thick cell-wall composed of cellulose. The above description applies to the cells composing the deeper layers of the cortex, i.e., those nearest the woody layer : in the more superficial cells, as well as in the internal cells of a leaf, there is something else to notice. Imbedded in the protoplasm, just within the cell-wall, are a number of minute ovoid bodies of a bright green colour (r2, chr). These are chromatophores or chlorophyll corpuscles ; they consist of protoplasm coloured with chlorophyll which can be proved experimentally to have the same properties as the chlorophyll of Haematococcus and Euglena. Such a green parenchyma cell is clearly comparable with an encysted Haematococcus or Euglena, the main difference being that in the plant cell the form is polyhedral owing to the pressure of neighbouring cells, and the chromatophores are relatively small and numerous. Similarly a colourless parenchyma cell resembles an encysted Amoeba. The pith, the epidermis or thin skin which forms the outer surface of herbaceous plants, the greater part of the leaves, MINUTE STRUCTURE OF CELLS 61 and other portions of the plant may be shown to consist of an aggregation of cells agreeing in essential respects with the above description. We come therefore to a very remarkable result. The higher animals and plants are built up — in part at least — of elements which resemble in their essential features the minute and lowly organisms studied in previous lessons. Those elements are called by the general name of cells : hence the higher organisms, whether plants or animals, are multicelhdar or are to be considered as cell-aggregates, while in the case of such beings > as Amoeba, Hsematococ- cus, Heteromita, or Euglena, the entire organism is a single cell, or is unicellular. Note further that the cells of the higher animals and plants, like entire unicellular organisms, may occur in either the amoeboid (Fig. 8, A, B1, c1,) the ciliated (E), or the encysted (F) condition, and that a plasmodial phase (e2) is sometimes produced by the union of two or more amoeboid cells. One of the most characteristic features in the unicellullar organisms described in the preceding lessons is the con- stancy of the occurrence of binary fission as a mode of multiplication. The analogy between these organisms and the cells of the higher animals and plants becomes still closer when we find that in the latter also simple fission is the normal mode of multiplication, the increase in size of growing parts being brought about by the continued division of their constituent cells. The process of division in animal and vegetable cells is frequently accompanied by certain very characteristic and 62 MINUTE STRUCTURE OF CELLS complicated changes in the nucleus to which we must now direct our attention. First of all, however, it will be neces- sary to describe the exact microscopic structure of cells and their nuclei as far as it is known at present. There seems to be a good deal of variation in the precise structure of various animal cells, but the more recent re- searches show that the cell-protoplasm (Fig. 9, cell-plsm) nucl. mem b. nucl.plsm chrom. FIG. 9. — A typical animal cell, showing cell-membrane (cell-memb.), cell-protoplasm (cell-plsni), nuclear membrane (micl-meinb.}, nuclear protoplasm or achromatin (uncl-phm], and coil of chromatin (chrom}. (After Carnoy. ) consists of a finely granular substance traversed by an extremely delicate network, the constituent threads of which are of almost inconceivable fineness, and bounded externally by a membrane (cell-memb.) of excessive tenuity. As the granules of the protoplasm are to be looked upon as pro- ducts of metabolism (anastates and katastates, p. 18) it is NUCLEAR DIVISION 63 clear that the precise appearances are sure to vary with the state of nutrition of the cell. The nucleus contains the same elements as in Amoeba (see p. 7). It is bounded externally by a delicate mem- brane (iiud. memb.} within wrhich is a granular substance traversed by a fine network, the nuclear protoplasm or achromatin. The chromatin or deeply-staining element presents various appearances in different cells : sometimes it takes the form of a network, sometimes of isolated granules or nucleoli : but in some instances, at any rate, it consists of a long tangled thread (ckrom.} which is said by some observers to be in reality a tube filled with the deeply stain- ing substance to which the name chromatin is properly applied. It should be noticed that a coil of this kind some- what loosely woven might easily be mistaken for a network, and that if it were alternately constricted and dilated instead of being regularly cylindrical it would present the appear- ance of isolated granules or nucleoli. The cells in the young growing parts of many plants have much the same structure as this (Fig. n, A) except that the delicate cell-membrane is replaced by a true cell-wall of cellulose. In the older portions of the plant the protoplasm is usually vacuolated (Fig. 8, F). The precise changes which take place during the fission of an animal cell are, like the structure of the cell itself, subject to considerable variation. We will consider what may probably be taken as a typical case. First of all, when the cell is about to divide, the achro- matic fibres of the nuclear protoplasm become arranged in the form of a spindle (Fig. 10, A). At the same time the chromatin filament unwinds itself, as it were, forming a loose coil (A) : it then becomes broken up (B), and forms a series 64 CHANGE IN CELL-PROTOPLASM of longitudinal bands of chromatin arranged along the meridians of the nuclear spindle (c). While this is going on the network of the cell-protoplasm undergoes a change, some of its fibres becoming arranged in the form of two radiating bundles of filaments one at each pole of the spindle (B, c). And at about this stage (c) the FIG. 10. — Stages in the binary fission ot an animal cell. A — c, formation of the nuclear spindle and breaking up of the chro- matin coil (black). D, E, aggregation of the chromatin at the equator of the spindle. F, fission of the chromatin-masses. G, accumulation of the chromatin at the poles of the spindle and ormation of the cell-plate. H, reconstruction of the daughter-nuclei. (After Carnoy.) nuclear membrane disappears so that the cell-protoplasm mingles with the nuclear protoplasm or achromatin. Next the longitudinal bands of chromatin gradually con- centrate at the equator of the spindle, where they form a ring of somewhat elongated masses (D). Each of these undergoes a splitting, and may thus become ring-like (E) ; CELL DIVISION 65 it then divides into two (F), the separate segments there- upon travelling to opposite poles of the spindle where they unite (G). The spindle now elongates, carrying the two masses of chromatin further away from each other (H). Around each of them a membrane is formed (lower half of H) enclosing a portion of protoplasm, which thus becomes the achromatin or nuclear protoplasm of one of the two daughter-nuclei into which the original or mother-nucleus has now com- pletely divided. We thus get two completely formed nuclei in a single cell. But pari passu with this process of nuclear division, fission of the cell-body is also going on. This may take place by a simple process of constriction — in much the same way as a lump of clay or dough would divide if a loop of string were tied round its middle and then tightened — or by the formation of what is known as a cell-plate. This arises in a row of granules formed from the equatorial part of the nuclear spindle (G) : the granules extend until they form a complete equatorial plate dividing the cell-body into two halves (H) : fission then takes place by the cell-plate splitting into two along a plane parallel with its flat surfaces.1 In some cases the dividing nucleus instead of going through the complicated processes just described divides by simple constriction. We have therefore to distinguish between direct and indirect nuclear division. To the latter very elaborate method the name karyokinesis is often applied. In plant cells many similar changes are gone through during the division of the nucleus. A nuclear spindle is 1 It must not be forgotten that the cells which are necessarily repre- sented in such diagrams as Figs. 8 — II as planes are really solid bodies, and that consequently the cell-plate represented in the figures as a line is actually a plane at right angles to the plane of the paper. F 66 STAGES IN BINARY FISSION formed (Fig. n, B) : the chromatin, at first arranged in a coil (A), breaks up, and its segments become arranged along the equator of the spindle (B), divide transversely (c) and travel to the poles (D E), where they form the chromatin-coils of the daughter-nuclei. At the same time the fibres of the spindle give rise, across the equator of the cell, to a cell- FIG. II. — Stages in the binary fission of a plant cell. A, cell with resting nucleus. B — D, formation of nuclear spindle and division of chromatin. E, reconstruction of daughter nuclei and formation of cell-plate. (After Strasburger). plate (E) along which division takes place. But in the plant- cell the cell-plate gives rise to a partition wall of cellulose which divides the two daughter-cells from one another.1 1 The nucleoli of the plant-cell appear to be independent of the chromatin of its nuclear thread, from which they differ markedly in structure. They disappear during karyokinesis, and it seems probable that they are active agents in the formation of the cell-wall. COMPLEXITY OF CELL STRUCTURE 67 In this connection the reader will not fail to note the extreme complexity of structure revealed in cells and their nuclei by the highest powers of the microscope. When the constituent cells of the higher animals and plants were discovered, during the early years of the present century, by Schleiden and Schwann, they were looked upon as the ultima Thule of microscopic analysis. Now the demonstration of the cells themselves is an easy matter, the problem is to make out their ultimate constitution. What would be the result if we could get microscopes as superior to those of to-day as those of to-day are to the primitive instruments of eighty or ninety years ago, it is impossible even to conjecture. But of one thing we may feel confident — of the enormous strides which our knowledge of the constitution of living things is destined to make during the next half century. The striking general resemblance between the cells of the higher animals and plants and entire unicellular organisms has been commented on as a very remarkable fact : there is another equally significant circumstance to which we must advert. All the higher animals begin life as an egg, which is either passed out of the body of the parent, as such, as in most fishes, frogs, birds, &c., or undergoes the first stages of its development within the body of the parent, as in sharks, some reptiles, and nearly all mammals. The structure of the egg is, in essential respects, the same in all animals from the highest to the lowest. In a jelly-fish for instance, it consists (Fig. 12, A) of a globular mass of protoplasm (gd\ in which are deposited granules of a pro- teinaceous substance known as yolk-spherules. Within the protoplasm is a large clear nucleus, (g.v.\ the chromatin of which is aggregated into a central mass or nucleolus (g. m). F 2 68 STRUCTURE OF THE EGG An investing membrane may or may not be present. In other words the egg is a cell : it is convenient, for reasons which will appear immediately, to speak of it as the ovum or egg-cell. The young or immature ova of all animals present this structure, but in many cases certain modifications are under- gone before the egg is mature, i.e., capable of development into a new individual. For instance, the protoplasm may throw out pseudopods, the egg becoming amoeboid (see Fig. 53) ; or the surface of the protoplasm may secrete a thick cell-wall (see Fig. 61). The most extraordinary modification FIG. 12. — A, ovum of an animal (Carmarina hastata, one of the jelly fishes), showing protoplasm (gd), nucleus (gv], and nucleolus (gm). B, ovum of a plant (Gymnadenia conopsea, one of the orchids), showing protoplasm (plsni), nucleus (mi), and nucleolus (mi1). (A, from Balfour after Haeckel : B, after Marshall Ward. ) takes place in some vertebrata such as birds. In a hen's egg, for instance, the yolk-spherules increase immensely, swelling out the microscopic ovum until it becomes what we know as the "yolk' of the egg: around this layers of albumen or "white' are deposited, and finally the shell membrane and the shell. Hence we have to distinguish carefully in eggs of this character between the entire " egg " in the ordinary acceptation of the term, and the ovum or egg-cell. But complexities of this sort do not alter the fundamental fact that all the higher animals begin life as a single cell, or in other words that multicellular animals, however large and THE PLANT OVUM 69 complex they may be in their adult condition, originate as unicellular bodies of microscopic size. The same is the case with all the higher plants. The pistil or seed-vessel of an ordinary flower contains one or more little ovoidal bodies, the so-called " ovules " (more accurately megasparangia (see Lesson XXX., and Fig. 89), which, when the flower withers, develop into the seeds. A section of an ovule shows it to contain a large cavity, the embryo-sac or megaspore (see Fig. 89, F), at one end of which is a microscopic cell (Fig. 12, B), consisting as usual of protoplasm (plsni), nucleus («?/), and nucleolus (;///). This is the ovum or egg-cell of the plant : from it the new plant, which springs from the germinating seed, arises. Thus the higher plants, like the higher animals, are, in their earliest stage of existence, microscopic and unicellular. LESSON VII SACCHAROMYCES EVERY one is familiar with the appearance of the ordinary brewer's yeast — the light-brown, muddy, frothing substance which is formed on the surface of the fermenting vats in breweries — and is used in the manufacture of bread to make the dough "rise." Examined under the microscope yeast is seen to consist of a fluid in which are suspended immense numbers of minute particles, the presence of which produces the mud- diness of the yeast. Each of these bodies is a unicellular organism, the yeast-plant, or in botanical language Sac- charomyces cerevisice. Saccharomyces consists of a globular or ellipsoidal mass of protoplasm (Fig. 13), about TJ^ mm. in diameter, and surrounded with a delicate cell- wall of cellulose (c, c.w.). In the protoplasm are one or more non-contractile vacuoles (vac) — mere spaces filled with fluid and varying according to the state of nutrition of the cell. Granules also occur in the protoplasm which are products of metabolism, some of them being of a proteid material, others fat globules. Under ordinary circumstances no nucleus is to be seen ; but recently, by the employment of a special mode of GENERAL CHARACTERS 71 staining, a small rounded nucleus has been shown to exist near the centre of the cell. The cell-wall is so thin that it is difficult to be sure of its presence unless very high powers are employed. It can however be easily demonstrated by staining yeast with vu C.U' X. x • E E FIG. 13. — Saccharomyces cerevisite. A, a group of cells under a moderately high power. The scale to the left applies to this figure only. B, several cells more highly magnified, showing various stages of budding, vac, the vacuole. C, a single cell with two buds (bd, bd') still more highly mag- nified : c.w, cell-wall : vac, vacuole. D, cells, crushed by pressure : c.iv, the ruptured cell- walls : flsm, the squeezed out protoplasm. E, E', starved cells, showing large vacuoles and fat globules (/). F, F', formation of spores by fission of the protoplasm of a starved cell : in F the spores are still enclosed in the mother-cell-wall, in F1 they are free. magenta, and then applying pressure to the cover-glass so as to crush the cell. Under this treatment the cell-walls are burst and appear as crumpled sacs, split in various ways and unstained by the magenta (D, c. «/), while the squeezed-out protoplasm is seen in the form of irregular masses (pis///) stained pink by the dye. 72 SACCHAROMYCES The mode of multiplication of Saccharomyces is readily made out in actively fermenting yeast, and is seen to differ from anything we have met with hitherto. A small pimple- like elevation (c, bd) appears on the surface of a cell and gradually increases in size : examined under a high power this bud is found to consist of an offshoot of the protoplasm of the parent cell covered with a very thin layer of cellulose : it is formed by the protoplasm growing out into an offshoot -like a small pseudopod — which pushes the cell-wall before it. The bud increases in size (bd'} until it forms a little globular body touching the parent cell at one pole : then a process of fission takes place along the plane of junction, the protoplasm of the bud or daughter-cell becoming sepa- rated from that of the mother-cell and a cellulose partition being secreted between the two. Finally the bud becomes completely detached as a separate yeast-cell. It frequently happens that a Saccharomyces buds in several places and each of its daughter-cells buds again, before detachment of the buds takes place. In this way chains or groups of cells are produced (B), such cell- colonies consisting of two or more generations of cells, the central one standing in relation of parent, grandparent, or great-grandparent to the others. It must be observed that this process ot budding or gemmation is after all only a modification of simple fission. In the latter the two daughter-cells are of equal size and both smaller than the parent cell, while in gemma- tion one — the mother-cell — is much larger than the other daughter-cell or bud — and is of the same size as, indeed is practically identical with, the original dividing-cell. Hence in budding, the parent form does not, as in simple fission, lose its individuality, becoming wholly merged in its twin offspring, but merely undergoes separation of a small portion MULTIPLE FISSION 73 of its substance in the form of a bud, which by assimilation of nutriment gradually grows to the size of its parent, the latter thus retaining its individuality and continuing to produce fresh buds as long as it lives. Multiplication by budding only goes on while the Sac- charomyces is well supplied with food : if the supply of nutriment fails, a different mode of reproduction obtains. Yeast can be effectually starved by spreading out a thin layer of it on a slab of plaster-of-Paris kept moist under a bell-jar : under these circumstances the yeast is of course supplied with nothing but water. In a few days the yeast-cells thus circumstanced are found to have altered in appearance : larger vacuoles appear in them (Fig. 13, E,E') and numerous fat-globules (/) are formed. The protoplasm has been undergoing destructive meta- bolism, and, there being nothing to supply new material, has diminished in quality, and at the same time been partly converted into fat. Both in plants and animals it is found that fatty degeneration, or the conversion of protoplasm into fat by destructive metabolism, is a constant phenomenon of starvation. After a time the protoplasm collects towards the centre of the cell and divides simultaneously into four masses arranged like a pyramid of four billiard balls, three at the base and one above (F). Each of these surrounds itself with a thick cellulose coat and becomes a spore, the four spores being sooner or later liberated by the rupture of the mother-cell wall (F'). The spores being protected by their thick cell-walls are able to withstand starvation and drought for a long time ; when placed under favourable circumstances they develop into the ordinary form of Saccharomyces. So that repro- 74 SACCHAROMYCES duction by multiple fission appears to be, in the yeast-plant, a last effort of the organism to withstand extinction. The physiology of nutrition 01 Saccharomyces has been studied with great care by several men of science and notably by Pasteur, and is in consequence better known than that of any other low organism. For this reason it will be advisable to consider it somewhat in detail. The first process in the manufacture of beer is the pre- paration of a solution of malt called " sweet-wort." Malt is barley which has been allowed to germinate or sprout, i.e., the young plant is allowed to grow to a certain extent from the seed. During germination the starch which forms so large a portion of the grain of barley is partly converted into sugar : barley also contains soluble proteids and mineral salts, so that when malt is infused in hot water the sweet- wort formed may be looked upon as a solution of sugar, proteid, and salts. Into this wort a quantity of yeast is placed. Very soon the liquid begins to froth, the quantity of yeast increasing enormously : this means of course that the yeast-cells are budding actively, as can be readily made out by microscopic examination. If while this frothing is going on a lighted candle is lowered into the vat the flame will be immediately extinguished : if an animal were placed in the same position it would be suffocated. Chemical analysis shows that the extinction of the candle's flame or of the animal's life is caused by a rapid evolution of carbon dioxide from the fermenting wort, the frothing being due to the escape of the gas from the liquid. After a time the evolution of gas ceases, and the liquid is then found to be no longer sweet but to have acquired what we know as an alcoholic or spirituous flavour. Analysis ALCOHOLIC FERMENTATION 75 shows that the sugar has nearly or quite disappeared, while a new substance, alcohol, has made its appearance. The sweet-wort has, in fact, been converted into beer. Expressed in the form of a chemical equation what has happened is this : — C6H1206 = 2(C2H00) + 2(C02) Grape sugar. Alcohol. Carbon dioxide. One molecule of sugar has, by the action of yeast, been split up into two molecules of alcohol which remain in the fluid, and two of carbon dioxide which are given off as gas. This is the process known as alcoholic fermentation. It has been shown by accurate analysis that only about 95 per cent, of the sugar is thus converted into alcohol and carbon dioxide : 4 per cent, is decomposed, with the for- mation of glycerine, succinic acid, and carbon dioxide, and i per cent, is used as nutriment by the yeast cells. For the accurate study of fermentation the sweet-wort of the brewer is unsuitable, being a fluid of complex and un- certain composition, and the nature of the process, as well as the part played in it by Saccharomyces, becomes much clearer if we substitute the artificial wort invented by M. Pasteur, and called after him Pasteur's solution. It is made of the following ingredients :- Water, H2O 8376 per cent. Cane sugar, C12H2.2On . 15*00 Ammonium tartrate, (NH4)2C4H4O0 . 1*00 Potassium phosphate, K3PO4 . . . 0-20 Calcium phosphate, Ca3 (PO4)2 . . 0-02 Magnesium sulphate, MgSO4 . . . 0-02 11 11 11 11 11 11 11 11 11 11 lOO'OO 76 SACCHAROMYCES The composition of this fluid is not a matter of guess- work, but the result of careful experiments, and is deter- mined by the following considerations. It is obvious that if we are to study alcoholic fermentation sugar must be present,1 since the essence of the process is the formation of alcohol from sugar. Then nitrogen in some form as well as carbon, oxygen, and hydrogen must be present, since these four elements enter into the composition of protoplasm, and all but the first-named (nitrogen) into that of cellulose, and they are thus required in order that the yeast should live and multiply. The form in which nitrogen could best be assimi- lated was found out by experiment. We saw that in the manufacture of beer the yeast cells obtain their nitrogen largely in the form of soluble proteids : green plants obtain theirs largely in the simple form of nitrates. It was found that while proteids are, so to say, an unnecessarily complex food for Saccharomyces, nitrates are not complex enough, and an ammonia compound is necessary, ammonium tartrate being the most suitable. Thus while Saccharomyces can build up the molecule of protoplasm from less complex food- stuffs than are required by Amoeba, it cannot make use of such comparatively simple compounds as suffice for Haema- tococcus : moreover it appears to be indifferent whether its nitrogen is supplied to it in the form of ammonium tartrate or in the higher form of proteids. Then as to the remaining ingredients of the fluid- potassium and calcium phosphate and magnesium sulphate. If a quantity of yeast is burnt, precisely the same thing happens as when one of the higher animals or plants is subjected to the same process. It first chars by the deposi- 1 It is ;i matter of indifference whether cane-sugar ur grape-sugar is used. EXPERIMENTS IN NUTRITION 77 tion of carbon, then as the heat is continued the carbon is completely consumed, going off by combination with the oxygen of the air in the form of carbon dioxide ; at the same time the nitrogen is given off mostly as nitrogen gas, the hydrogen by union with atmospheric oxygen as water- vapour, and the sulphur as sulphurous acid or sulphur dioxide (SO2). Finally, nothing is left but a small quantity of white ash which is found by analysis to contain phos- phoric acid, potash, lime, and magnesia ; i.e., precisely the ingredients of the three mineral constituents of Pasteur's solu- tion with the exception of sulphur, which, as already stated, is given off during the process of burning as sulphur dioxide. Thus the principle of construction of an artificial nutrient solution such as Pasteur's is that it should contain all the elements existing in the organism it is designed to support ; or in other words, the substances by the combination of which the waste of the organism due to destructive meta- bolism may be made good. That Pasteur's solution exactly fulfils these requirements may be proved by omitting one or other of the constituents from it, and finding out how the omission affects the well- being of Saccharomyces. If the sugar is left out the yeast-cells grow and multiply, but with great slowness. This shows that sugar is not necessary to the life of the organism, but only to that active condition which accompanies fermentation. A glance at the composition of Pasteur's solution will show that all the necessary elements are supplied without sugar. Omission of ammonium tartrate is fatal : without it the cells neither grow nor multiply. This, of course, is just what one would expect since, apart from ammonium tartrate, the fluid contains no nitrogen without which the molecules of protoplasm cannot be built up. 78 SACCHAROMYCES It is somewhat curious to find that potassium and calcium phosphates are equally necessary ; although occurring in such minute quantities they are absolutely essential to the well-being of the yeast-cells, and without them the organism, although supplied with abundance of sugar and ammonium tartrate, will not live. This may be taken as proving that phosphorus, calcium, and magnesium form an integral part of the protoplasm of Saccharomyces, although existing in almost infinitesimal proportions. Lastly, magnesium sulphate must not be omitted if the organism is to flourish : unlike the other two mineral constituents it is not absolutely essential to life, but without it the vital processes are sluggish. Thus by growing yeast in a fluid of known composition it can be ascertained exactly what elements and combina- tions of elements are necessary to life, what advantageous though not absolutely essential, and what unnecessary. The precise effect of the growth and multiplication of yeast upon a saccharine fluid, or in other words the nature of alcoholic fermentation, can be readily ascertained by a simple experiment with Pasteur's solution. A quantity of the solution with a little yeast is placed in a flask the neck of which is fitted with a bent tube leading into a vessel of lime-water or solution of calcium oxide. When the usual disengagement of carbon dioxide (see p. 74) takes place the gas passes through the tube into the lime-water and causes an immediate precipitation of calcium carbonate as a white powder which effervesces with acids. This proves the gas evolved during fermentation to be carbon dioxide since no other converts lime into carbonate. When fermentation is complete the presence of alcohol may be proved by distil- lation : a colourless, mobile, pungent, and inflammable liquid being obtained. CONDITIONS OF ALCOHOLIC FERMENTATION 79 By experimenting with several flasks of this kind it can be proved that fermentation goes on as well in darkness as in light, and that it is quite independent of free oxygen. Indeed the process does not go on if free oxygen — i.e., oxygen in the form of dissolved gas — is present in the fluid ; from which it would seem that Saccharomyces must be able to obtain the oxygen, which like all other organisms it requires for its metabolic processes, from the food supplied to it. The process of fermentation goes on most actively between 28° and 34°C : at low temperature it is com- paratively slow, and at 38°C. multiplication ceases. If a small portion of yeast is boiled so as to kill the cells, and then added to a flask of Pasteur's solution, no fermentation takes place, from which it is proved that the de- composition of sugar is effected by the living yeast-cells only. There seems to be no doubt that the property of exciting alcoholic fermentation is a function of the living protoplasm of Saccharomyces. The yeast-plant is therefore known as an organized ferment : when growing in a saccharine solu- tion it not only performs the ordinary metabolic processes necessary for its own existence, but induces decomposition of the sugar present, this decomposition being unaccom- panied by any corresponding change in the yeast-plant itself. It is necessary to mention in this connection that there is an important group of not-living bodies which produce striking chemical changes in various substances with- out themselves undergoing any change : these are distin- guished as unorganized ferments. A well-known example is pepsin, which is found in the gastric juice of the higher animals, and has the function of converting proteids into peptones (see p. 12) : its presence has been proved in 8o SACCHAROMYCES the Mycetozoa (p. 54), and probably it or some similar pep- tonizing or proteolytic ferment effects this change in all organisms which have the power of digesting proteids. Another instance is diastase, which effects the conversion of starch into grape sugar : it is present in germinating barley (see p. 74), and an infinitesimal quantity of it can convert immense quantities of starch. The ptyalin of our own saliva has a like action, and probably some similar diastatic or amylolytic ferment is present in the Mycetozoa which, as we saw (p. 54), can digest cooked starch. LESSON VIII BACTERIA IT is a matter of common observation that if certain moist organic substances, such as meat, soup, milk, &c., are allowed to stand at a moderate temperature for a few days — more or fewer according as the weather is hot or cold — they " go bad " or putrefy; i.e. they acquire an offensive smell, a taste which few are willing to ascertain by direct experiment, and often a greatly altered appearance. One of the most convenient substances for studying the phenomena of putrefaction is an infusion of hay, made by pouring hot water on a handful of hay and straining the resultant brown fluid through blotting paper. Pasteur's solution may also be used, or mutton-broth well boiled and filtered, or indeed almost any vegetable or animal infusion. If some such fluid is placed in a glass vessel covered with a sheet of glass or paper to prevent the access of dust, the naked-eye appearances of putrefaction will be found to manifest themselves with great regularity. The fluid, at first quite clear and limpid, becomes gradually dull and turbid. The opacity increases and a scum forms on the surface : at the same time the odour of putrefaction arises, and, G 82 BACTERIA especially in the case of animal infusions, quickly becomes very strong and disagreeable. The scum after attaining a perceptible thickness breaks up and falls to the bottom, and after this the fluid slowly clears again, becoming once more quite transparent and losing its bad smell. If exposed to the light patches of green appear in it sooner or later, due to the presence of microscopic organisms containing chlorophyll. The fluid has acquired, in fact, the characteristics of an ordinary stagnant pond, and is quite incapable of further putrefaction. The whole series of changes may occupy many months. Microscopic examination shows that the freshly-prepared I f t FIG. 14.— Bacterium termo. A, motile stage : B, vesting stage or zooglsea. (From Klein.) fluid is free from organisms, and indeed, if properly filtered, from particles of any sort. But the case is very different when a drop of infusion in which turbidity has set in is placed under a high power. The fluid is then seen to be crowded with incalculable millions of minute specks, only just visible under a power of 300 or 400 diameters, and all in active movement. These specks are Bacteria, or as they are sometimes called, microbes or micro-organisms ; they belong to the particular genus and species called Bacterium termo. Seen under the high power of an ordinary student's microscope Bacterium termo has the appearance shown in BACTERIUM TERMO 83 Fig. 14, A : it is like a minute finger-biscuit, i.e. has the form of a rod constricted in the middle. It is only by using the very highest powers of the microscope that its form and structure can be satisfactorily made out. It is then seen (Fig. 15) to consist of a little double spindle, showing neither nucleus, vacuole, nor other internal structure. It is com- posed of a particular variety of protoplasm, and is sur- rounded by a membrane of extreme tenuity formed of cellulose. At each end is attached a flagellum about as long as the cell itself. Bacterium termo is much smaller than any organism we have yet considered, so small in fact that, as it is always easier to deal with whole numbers than with fractions, its size is best expressed by taking as a standard the one- FIG. 15. — Bacterium termo (X 4000), showing the terminal flagella. (After Dallinger.) thousandth of a millimetre, called a micromillimetre and expressed by the symbol //,. The entire length of the organism under consideration is from i'5 to 2 /z, i.e. about the y^Q- mm. or the jo-Jo-Q- inch. In other words, its entire length is not more than one-fourth the diameter of a yeast- cell or of a human blood-corpuscle. The diameter of the flagellum has been estimated by Dallinger to be about | //. or 2 orVoir mcn> a smallness of which it is as difficult to form any clear conception as of the distances of the fixed stars. Some slight notion of these almost infinitely small dimen- sions may, however, be. obtained in the following way. Fig. 15 shows a Bacterium termo magnified 4000 diameters, the scale above the figure representing -^ mm. magnified to the same amount. The height of this book is a little over 18 cm.; G 2 84 BACTERI this multiplied by 4,000 gives 72,000 cm. = 720 metres = 2362 feet. We therefore get the proportion — as 2362 feet, or nearly six times the height of St. Paul's, is to the height of the present volume, so the length of Fig. 15 is to that of Bacterium termo. It was mentioned above that at a certain stage of putre- faction a scum forms on the surface of the fluid. This film consists of innumerable motionless Bacteria imbedded in a transparent gelatinous substance formed of mycoprotein (Fig. 14, B). After continuing in the active condition for a time the Bacteria rise to the surface, lose their flagella, and throw out this gelatinous substance in which they lie imbedded. The bacterial jelly thus formed is called a zooglcea. Thus in Bacterium termo, as in so many of the organisms we have studied, there is an alternation of an active with a resting condition. During the earlier stages of putrefaction Bacterium termo is usually the only organism found in the fluid, but later on other microbes make their appearance. Of these the com- monest are distinguished by the generic names Micrococcus, Bacillus, Vibrio, and Spirillum. Micrococcus (Fig. 16) is a minute form, the cells of which are about 2/x (5-5-^ mm.) in diameter. It differs from Bacterium in being globular instead of spindle-shaped and in having no motile phase. Like Bacterium it assumes the zooglaea condition (Fig. 16, 4). Bacillus is commonly found in putrescent infusions in which the process of decay has gone on for some days : as its numbers increase those of Bacterium termo diminish, until Bacillus becomes the dominant form. Its cells (Fig. 17) are rod-shaped and about 6/x (Tiy- mm.) in length in the BACILLUS 85 commonest species. Both motionless and active forms are found, the latter having a flagellum at each end. The zooglaea condition is often assumed, and the rods are fre- quently found united end to end so as to form filaments. Vibrio resembles Bacillus, but the rod-like cells (Fig. 18, A) are wavy instead of straight. They are actively motile and when highly magnified are found to be provided with a flagellum at each end. Vibriones vary from 8/x to 2^ in length. Spirillum is at once distinguished by its spiral form, the V * •. • \ ::•&:. 4 \ : :?#•& • • •••••»••••» % • •.?; ••'••••• V / ••••.•;:•• ::: ...... ••• : Pft •** O •«•* FIG. 1 6. — Micrococcus. I, single and double (dumb-bell shaped) forms : 2 and 3, chain-forms : 4, a zooglsea. cells resembling minute corkscrews (Fig. 18, B & c) and being provided with a flagellum at each end (c). The smaller species, such as S. tenue (B) are from 2 to 5/x in length, but the larger forms, such as S. volutans (c) attain a length of from 25 to 3o/x. In swimming Spirillum appears on a superficial examination to undulate like a worm or a serpent, but this is an optical illusion : the spiral is really a permanent one, but during progression it rotates upon its long axis, like Hsematococcus (p. 25) and this double move- ment produces the appearance of undulation. 86 BACTERIA Most Bacteria are colourless, but three species (Bacterium viride, B. chlornium, and Bacillus virens] contain chlorophyll, and several others form pigments of varying tints and often of great intensity. For instance, there are red, yellow, brown, blue, and violet species of Micrococcus which grow on slices of boiled potato, hard-boiled egg, &c., forming brilliantly coloured patches ; and the yellow colour often FIG. 17. — Bacillus subtilis, showing various stages between single forms and long filaments (Leptothrix). assumed by milk after it has been allowed to stand for a considerable time is due to the presence of Bacterium xanthinum. All Bacteria multiply by simple transverse fission, the process taking place sometimes during the motile, sometimes during the resting condition. Frequently the daughter cells do not separate completely from one another but remain I'.INARY FISSION 87 loosely attached, forming chains. These are very common in some species of micrococcus (see Fig. 1 6). Bacillus when undergoing fission behaves something like Heteromita : the mother-cell divides transversely across the middle, and the two halves gradually wriggle away from one another, but remain connected for a time by a very fine thread -MG. 1 8. — A, Vibrio. B, Spirillum temic. c, Spirillum volutaus. (From Klein.) of protoplasm which extends between their adjacent ends. This is drawn out by the gradual separation of the two cells until it attains twice the length of a flagellum when it snaps in the middle, thus providing each daughter-cell with a new flagellum. Bacillus may, however, divide while in the resting condition and, under certain circumstances, the 88 BACTERIA process is repeated again and again, and the daughter-cells remaining in contact form a long wavy or twisted filament called Leptothrix (Fig. 17) the separate elements of which are usually only visible after staining. Bacillus also multiplies by a peculiar process of spore- formation which may take place either in the ordinary resting form or in a leptothrix filament. A bright dot appears at one place in the protoplasm (Fig. 1 9) : this increases in size, the greater part of the protoplasm being used up in its formation, and finally takes on the form of a clear oval spore which remains for some time enclosed in the cell-wall of the Bacillus, by the rupture of which it is finally liberated. Spores of this kind are termed endospores, In other Bacteria spores are formed directly from the vegetative cells, which become thick walled (arthrospores). The spores differ from ordinary Bacilli in being unstained by aniline dyes. After a period of rest the spores, under favourable cir- cumstances, germinate by growing out at one end so as to become rod-like, and thus finally assuming the form of ordinary Bacilli. There are other genera often included among Bacteria for the description of which the student is referred to the more special treatises.1 One remark must, however, be made in concluding the present brief account of the morphology of the group. There is a great deal of evidence to show that what have been spoken of as genera (Bacterium, Bacillus, Spirillum, &c.) may merge into one another and are therefore to be looked upon as phases in the life-history of various microbes rather than as true and distinct genera. But this is a point which cannot at present be considered as settled. 1 See especially De Bary, Fungi, Mycetozoa, and Bacteria (Oxford, 1887), and Klein, Micro-organisms and Disease (London, 1886). NATURE OF GENERIC FORMS 89 The conditions of life of Bacteria are very various. Some live in water such as that of stagnant ponds, and of these three species as already stated (p. 86), contain chlorophyll. The nutrition of such forms must obviously be holophytic, and in the case of Bacterium chlorinum the giving off of oxygen in sunlight has actually been proved. FIG. 19. — Spore-formation in Bacillus. (From Klein.) But this mode of nutrition is rare among the Bacteria : nearly all of those to which reference has been made are saprophytes, that is, live upon decomposing animal and vegetable matters. They are, in fact, nourished in precisely the same way as Heteromita (see p. 37). Many of these 90 BACTERIA forms such as Bacterium termo, and species of Bacillus, Vibrio, &c., will, however, flourish in Pasteur's solution, in which they obtain their nitrogen in the form of ammonium tartrate instead of decomposing proteid. It has also been shown that some Bacteria can go further and make use of nitrates as a source of nitrogen. But part passu with their ordinary nutritive processes, many Bacteria exert an action on the fluids on which they live comparable to that exerted on a saccharine solution by the yeast-plant. Such microbes are, in fact, organized ferments. Every one is familiar with the turning sour of milk. This change is due to the conversion of the milk-sugar into lactic acid. CGH120(! = 2(C3H60)3. Sugar. Lactic Acid. The transformation is brought about by the agency 01 Bacterium lactis, a microbe closely resembling B. termo. Beer and wine are two other fluids which frequently turn sour, there being in this case a conversion of alcohol into acetic acid, represented by the equation — C2H60 + 02 = H20 + C2H402, Alcohol. Oxygen. Water. Acetic Acid. The ferment in this instance is Bacterium aceti, often called My coder ma aceti, or the " vinegar plant." It will be noticed that in this case oxygen enters into the reaction : it is a case of fermentation by oxidation. Putrefaction itself is another instance of fermentation induced by a microbe. Bacterium termo — the putrefactive ferment — causes the decomposition of proteids into simpler compounds, amongst which are such gases as ammonia (NH.,), sulphuretted hydrogen (H2S), and ammonium BACTERIA AS FERMENTS 91 sulphide ( (NH4)2S), the evolution of which produces the characteristic odour of putrefaction. Besides holophytes and saprophytes there are included among Bacteria many parasites, that is, species which feed not on decomposing but on living organisms. Many of the most deadly infectious diseases, such as tuberculosis, diph- theria, and typhoid fever, are due to the presence in the tissues or fluids of the body of particular species of microbes, which feed upon the parts affected and give rise to the morbid symptoms characteristic of the disease. Some Bacteria, like the majority of the organisms pre- viously studied, require free oxygen for their existence, but others, like Saccharomyces during active fermentation (see p. 79), are quite independent of free oxygen and must there- fore be able to take the oxygen without which their metabolic processes could not go on, from some of the compounds contained in the fluid in which they live. Bacteria are for this reason divided into aerobic species which require free oxygen, and anaerobic species which do not. As to temperature, common observation tells us that Bacteria only flourish within certain limits. We know for instance that organic substances can be preserved from putrefaction by being kept either at the freezing-point, or at or near the boiling-point. One important branch of modern industry, the trade in frozen meat, depends upon the fact that the putrefactive Bacteria, like other organisms, are rendered inactive by freezing, and every housekeeper knows how easily putrefaction can be staved off by roasting or boiling. Simi- larly it is a matter of common observation that a moderately high temperature is advantageous to these organisms, the heat of summer or of the tropics being notoriously favourable to putrefaction. In the case of Bacterium termo, it has been 92 BACTERIA found that the optimum temperature is from 30° to 35° C., but that the microbe will flourish between 5° and 40° C. Although fully-formed Bacteria, like other organisms, are usually killed by exposure to heat several degrees below boiling-point, yet the spores of some species will withstand, at any rate for a limited time, a much higher temperature — • even as high as i3o°C. On the other hand, putrefactive Bacteria retain their power of development after being exposed to a temperature of — m°C., although during the time of exposure all vital activity is of course suspended. Bacteria also resemble other organisms in being unable to carry on active life without a due supply of water : no perfectly dry substance ever putrefies. The preservation for ages of the dried bodies of animals in such countries as Egypt and Peru depends at least as much upon the moisture- less air as upon the antiseptics used in embalming. For the most part Bacteria are unaffected by light, since they grow equally well in darkness and in ordinary daylight. Many of them, however, will not bear prolonged exposure to direct sunlight, and it has been found possible to arrest the putrefaction of an organic infusion by insolation, or exposure to the direct action of the sun's rays. It has also been proved that it is the light-rays and not the heat-rays which are thus prejudicial to the life of micro-organisms. LESSON IX BIOGENESIS AND HOMOGENESIS : HOMOGENESIS AND HETERO- GENESIS THE study of the foregoing living things and especially of Bacteria, the smallest and probably the simplest of all known organisms, naturally leads us to the consideration of one of the most important problems of biology — the problem of the origin of life. In all the higher organisms we know that each individual arises in some way or other from a pre-existing individual : no one doubts that every bird now living arose by a process of development from an egg formed in the body of a parent bird, and that every tree now growing took its origin either from a seed or from a bud produced by a parent plant. But there have always — until quite recently, at any rate- been upholders of the view that the lower forms of life, bacteria, monads, and the like, may under certain circum- stances originate independently of pre-existing organisms : that, for instance, in a flask of hay-infusion or mutton-broth, boiled so as to kill any living things present in it, fresh forms of life may arise de novo, may in fact be created then and there. We have therefore two theories of the lower organisms 94 BIOGENESIS AND HOMOGENESIS the theory of Biogenesis, according to which each living thing, however simple, arises by a natural process of bud- ding, fission, spore-formation, or what not, from a parent organism : and the theory of Abiogenesis, or as it is some- times called Spontaneous or Equivocal Generation, accord- ing to which fully formed living organisms sometimes arise from not-living matter. In former times the occurrence of abiogenesis was uni- versally believed in. The expression that a piece of meat has "bred maggots" ; the opinion that parasites such as the gall-insects of plants or the tape-worms in the intestines of animals originate where they are found ; the belief still held in some rural districts in the occurrence of showers of frogs, or in the transformation of horse -hairs kept in water into .eels ; all indicate a survival of this belief. Aristotle, one of the greatest men of science of antiquity, explicitly teaches abiogenesis. He states that some animals " spring from putrid matter," that certain insects " spring from the dew which falls upon plants," that thread-worms " originate in the mud of wells and running waters," that fleas "originate in very small portions of corrupted matter," and that " bugs proceed from the moisture which collects on the bodies of animals, lice from the flesh of other creatures." Little more than 200 years ago one Alexander Ross, com- menting on Sir Thomas Browne's doubt as to " whether mice may be bred by putrefaction," says, " so may he doubt whether in cheese and timber worms are generated ; or 'if beetles and wasps in cow's dung ; or if butterflies, locusts, grasshoppers, shell- fish, snails, eels, and such like, be pro- created of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense, and experience. THE PROBLEM LIMITED TO MICROSCOPIC FORMS 95 If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of Nylus, to the great calamity of the inhabitants." As accurate inquiries into these matters were made, the number of cases in which equivocal generation was sup- posed to occur was rapidly diminished. It was a simple matter — when once thought of — to prove, as Redi did in 1638, that no maggots were ever "bred " in meat on which flies were prevented by wire screens from laying their eggs. Far more difficult was the task, also begun in the seventeenth century, of proving that parasites, such as tape-worms, arise from eggs taken in with the food ; but gradually this pro- position was firmly established, so that no one of any scientific culture continued to believe in the abiogenetic origin of the more highly organized animals any more than in showers of frogs, or in the origin of geese from barnacles. But a new phase of the question opened with the inven- tion of the microscope. In 1683, Anthony van Leuwenhoek discovered Bacteria, and it was soon found that however carefully meat might be protected by screens or infusions, by being placed in well-corked or stoppered bottles, putrefaction always set in sooner or latter, and was invariably accom- panied by the development of myriads of bacteria, monads, and other low organisms. It was not surprising, considering the rapidity with which these were found to make their appearance, that many men of science imagined them to be produced abiogenetically. Let us consider exactly what this implies. Suppose we have a vessel of hay-infusion, and in it a single Bacterium. The microbe will absorb the nutrient fluid and convert it into fresh protoplasm : it will divide repeatedly, and its progeny doing the same the vessel will soon contain millions 96 BIOGENESIS AND HOMOGENESIS of Bacteria instead of one. This means, of course, that a certain amount of fresh living protoplasm has been formed out of the constituents of the hay-infusion, through the agency in the first instance of a single living Bacterium. The question naturally arises — Why may not the formation of protoplasm take place independently of this insignificant speck of living matter ? It must not be thought that this question is in any way a vain or absurd one. That living protoplasm has at some period of the world's history originated from not-living matter seems a necessary corollary of the doctrine of evolution, and is obviously the very essence of the doctrine of special creation, and there is no a priori reason why it should be impossible to imitate the unknown conditions under which this took place. At present, however, we have absolutely no data towards the solution of this fundamental problem. But however insoluble may be the question as to how life first dawned upon our planet, the origin of living things at the present day is capable of investigation in the ordinary way of observation and experiment. The problem may be stated as follows : — any putrescible infusion, — i.e. any fluid capable of putrefaction — will be found after a longer or shorter exposure to swarm with bacteria and monads : do these organisms or the spores from which they first arise reach the infusion from without, or are they generated within it ? And the general lines upon which an investigation into the problem must be conducted are simple : given a vessel of any putrescible infusion ; let this be subjected to some process which, without rendering it incapable of supporting life, shall kill any living things contained in it ; let it then be placed under such circumstances that no living particles, however small, can reach it from without. If, after these two condi- METHOD OF FILTERING AIR 97 tions have been rigorously complied with, living organisms appear in the fluid, such organisms have originated abio- genetically. To kill any microbes contained in the fluid it is usually quite sufficient to boil it thoroughly. As we have seen, protoplasm enters into heat-rigor at a temperature consider- ably below the boiling-point of water, so that, with an exception which will be referred to presently, a few minutes' boiling suffices to sterilize all ordinary infusions, z>., to kill any organisms they may contain. Then as to preventing the entrance of organisms or their spores from without. This may be done in various ways. One way is to take a flask with the neck drawn out into a very slender tube, to boil the fluid in it for a sufficient time, and then, while ebullition is going on, to close the end of the tube by melting the glass in the flame of a Bunsen-burner or spirit-lamp, thus hermetically sealing the flask. By this method not only organisms and their spores are excluded from the flask but also air. But this is obviously unnecessary : it is evident that air may be admitted to the fluid with perfect impunity if only it can be filtered, that is, passed through some substance which shall retain all solid particles however small, and therefore of course, bacteria, monads, and their spores. A perfectly efficient filter for this purpose is furnished by cotton-wool. A flask or test-tube is partly filled with the infusion : the latter is boiled, and during ebullition cotton- wool is pushed into the mouth of the vessel until a long and firm plug is formed (Fig. 20). When the source of heat is removed, and, by the cooling of the fluid, the steam which filled the upper part of the tube condenses, air passes in to supply its place, but as it does so it is filtered of even the H 98 BIOGENESIS AND HOMOGENESIS smallest solid particles by having to pass through the close meshes of the cotton-wool. Experiments of this sort conducted with proper care have been known for many years to give negative results in the great majority of cases : the fluids remain perfectly sterile for any length of time. But in certain instances, in spite of the most careful precautions, bacteria were found to appear FIG. 20. — A Beaker with a number of test-tubes containing putres- cible infusions and plugged with cotton-wool. (From Klein.) in such fluids, and for years a fierce controversy raged between the biogenists and the abiogenists, the latter in- sisting that the experiments in question proved the occurrence of spontaneous generation, while the biogenists considered that all such cases were due to defective methods — either to imperfect sterilization of the fluid or to imperfect exclusion of germ-containing atmospheric dust. The matter was finally set at rest, and the biogenists SOME SPORES NOT KILLED BY BOILING 99 proved to be in the right, by the important discovery that the spores of bacteria and monads are not killed by a tem- perature many degrees higher than is sufficient to destroy the adult forms : that in fact while the fully developed organisms are killed by a few minutes' exposure to a temperature of 70° C. the spores are frequently able to survive several hours' boiling, and must be heated to 130° — 150° C. in order that their destruction may be assured. It was also shown that the more thoroughly the spores are dried the more difficult they are to kill, just as well dried peas are hardly affected by an amount of boiling sufficient to reduce fresh ones to a pulp. This discovery of the high thermal death-point or ultra- maximum temperature of the spores of these organisms has necessitated certain additional precautions in experiments with putrescible infusions. In the first place the flask and the cotton-wool should both be heated in an oven to a temperature of 150° C., and thus effectually sterilized. The flask being filled and plugged with cotton-wool is well boiled and then kept for some hours at a temperature of 32°- -38° C., the optimum temperature for bacteria. The object of this is to allow any spores which have not been killed by boiling to germinate, in other words to pass into the adult con- dition in which the temperature of boiling water is fatal. The infusion is then boiled again, so as to destroy any such freshly germinated forms it may contain. The same process is repeated once or twice, the final result being that the very driest and most indurated spores are induced to ger- minate, and are therefore slain. It must not be forgotten that repeated boiling does not render the fluid incapable of supporting life, as may be seen by removing the cotton-wool plug, when it will in a short time swarm with microbes. Experiments conducted with these precautions all tell the H 2 ioo HOMOGENESIS AND HETEROGENESIS same tale : they prove conclusively that in properly sterilized putrescible infusions, adequately protected from the entrance of atmospheric germs, no micro-organisms ever make their appearance. So that the last argument for abiogenesis has been proved to be fallacious, and the doctrine of biogenesis shown, as conclusively as observation and experiment can show it, to be of universal application as far as existing conditions known to us are concerned. It is also necessary to add that the presence of microbes in considerable quantities in our atmosphere has been proved experimentally. By drawing air through tubes lined with a solid nutrient material Prof. Percy Frankland showed that the air of South Kensington contained about thirty-five micro-organisms in every ten litres, and by ex- posing circular discs coated with the same substance he was further able to prove that in the same locality 279 micro- organisms fall upon one square foot of surface in one minute. There is another question intimately connected with that of Biogenesis, although strictly speaking quite independent of it. It is a matter of common observation that, in both animals and plants, like produces like ; that a cutting from a willow will never give rise to an oak, nor a snake emerge from a hen's egg. In other words, ordinary observation teaches the general truth of the doctrine of Homogenesis. But there has always been a residuum of belief in the opposite doctrine of Heterogenesis^ according to which the offspring of a germ, animal, or plant may be something utterly different from itself, a plant giving rise to an animal or vice versd, a lowly to a highly organized plant or animal and so on. Perhaps the most extreme case in which hetero- genesis was once seriously believed to occur is that of SUPPOSED CASES OF HETEROGENESIS 101 the " barnacle-geese." Buds of a particular tree growing near the sea were said to produce barnacles, and these falling into the water to develop into geese. This sounds absurd enough, but within the last twenty years two or three men of science have described, as the result of repeated observations, the occurrence of quite similar cases among microscopic organisms. For instance, the blood-corpuscles of the silkworm have been said to give rise to fungi, the protoplasm of the green weed Nitella (see Fig. 46) to Amoebae and Infusoria (see p. 105), Euglenas to thread- worms, and so on. It is proverbially difficult to prove a negative, and it might not be easy to demonstrate, what all competent naturalists must be firmly convinced of, that every one of these sup- posed cases of heterogenesis is founded either upon errors of observation or upon faulty inductions from correct observations. Let us take a particular case by way of example. Many years ago Dr. Dallinger observed among a number of Vorti- cellse or bell-animalcules (Fig. 26) one which appeared to have become encysted upon its stalk. After watching it for some time, there was seen to emerge from the cyst a free- swimming ciliated Infusor called Amphileptus, not unlike a long-necked Paramcecium (Fig. 21, p. 106). Many ob- servers would have put this down as a clear case of hetero- genesis : Dallinger simply recorded the observation and waited. Two years later the occurrence was explained : he found the same two species in a pond, and watched an Amphileptus seize and devour a Vorticella, and, after finish- ing its meal, become encysted upon the stalk of its victim. It is obvious that the only way in which a case of hetero- genesis could be proved would be by actually watching the transformation, and this no heterogenist has ever done ; at 102 HOMOGENESIS AND HETEROGENESIS the most, certain supposed intermediate stages between the extreme forms have been observed — say, between a Euglena and a thread-worm — and the rest of the process inferred. On the other hand, innumerable observations have been made on these and other organisms, the result being that each species investigated has been found to go through a definite series of changes in the course of its development, the ultimate result being invariably an organism resembling in all essential respects that which formed the starting-point of the observations : Euglense always giving rise to Euglense and nothing else, Bacteria to Bacteria and nothing else, and so on. There are many cases which imperfect knowledge might class under heterogenesis, such as the origin of frogs from tadpoles or of jelly-fishes from polypes (Lesson XXIII. Fig. 54), but in these and many other cases the apparently anomalous transformations have been found to be part of the normal and invariable cycle of changes undergone by the organism in the course of its development ; the frog always gives rise ultimately to a frog, the jelly-fish to a jelly- fish. If a frog at one time produced a tadpole, at another a trout, at another a worm : if jelly-fishes gave rise sometimes to polypes, sometimes to infusoria, sometimes to cuttle- fishes, and all without any regular sequence — that would be heterogenesis. It is perhaps hardly necessary to caution the reader against the error that there is any connection between the theory of heterogenesis and that of organic evolution. It might be said — if, as naturalists tell us — dogs are descended from wolves and jackals and birds from reptiles, why should not, for instance, thread-worms spring from Euglenas or Infusoria from Bacteria ? To this it is sufficient to answer that the evolution of one form from another takes place by a series EVOLUTION AND HETEROGENESIS 103 of slow, orderly progressive changes going on through a long series of generations (see Lesson XIII.) ; whereas heterogenesis presupposes the casual occurrence of sudden transformations in any direction — i.e., leading to either a less or a more highly organized form — and in the course of a single generation. LESSON X PARAMOECIUM, STYLONYCHIA, AND OXYTRICHA IT will have been noticed with regard to the simple uni- cellular organisms hitherto considered that all are not equally simple : that Protamceba (Fig. 2, p. 9) and Micrococcus (Fig. 1 6, p. 85), may be considered as the lowest of all, and that the others are raised above these forms in the scale of being in virtue of the possession of nucleus or contractile vacuole, or of flagella, or even, as in the case of Euglena (Fig. 5, p. 45), of a mouth and gullet. Thus we may speak of any of the organisms already studied as relatively " high " or " low " with regard to the rest : the lowest or least differentiated forms being those which approach most nearly to the simplest conception of a living thing — a mere lump of protoplasm : the highest or most differentiated those in which the greatest complication of structure has been attained. It must be remembered, too, that this increase in structural complexity is always accompanied by some degree of division of physiological labour, or, in other words, that morphological and physio- logical differentiation go hand in hand. We have now to consider certain organisms in which this differentiation has gone much further ; which have, in fact, OCCURRENCE OF INFUSORIA 105 acquired many of the characteristics of the higher animals and plants while remaining unicellular. The study of several of these more or less highly differentiated though unicellular forms will occupy the next seven Lessons. It was mentioned above that, in the earlier stages of the putrefaction of an organic infusion, bacteria only were found, and that later, monads made their appearance. Still later organisms much larger than monads are seen, generally of an ovoidal form, moving about very quickly, and seen by the use of a high power to be covered with innumerable fine cilia. These are called dilate Infusoria, in contradistinction to monads, which are often known as flagellate Infusoria : many kinds are common in putrefying infusions, some occur in the intestines of the higher animals, while others are among the commonest inhabitants of both fresh and salt water. Five genera of these infusoria will form the subjects of this and the four following Lessons. A very common ciliate infusor is the beautiful " slipper- animalcule," Paramoedum aurelia, which from its compara- tively large size and from the ease with which all essential points of its organization can be made out is a very con- venient and interesting object of study. Compared with the majority of the organisms which have come under our notice it may fairly be considered as gigantic being no less than \ — J mm. (200 — 260 //,) in length : in fact it is just visible to the naked eye as a minute whitish speck. Its form (Fig. 21 A) can be fairly well imitated by making out of clay or stiff dough an elongated cylinder rounded at one end and bluntly pointed at the other ; then giving the broader end a slight twist ; and finally making on the side B A C.TttC , (/?: C.TCIC. FIG. 21. — Paramcecium aiirelia. A, the living animal from the ventral aspect, showing the covering of cilia, the buccal groove (to the right) ending posteriorly in the mouth CORTEX AND MEDULLA 107 (mlh] and gullet (gul) ; several food vacuoles (/. vac], and the two contractile vacuoles (c. vac). B, the same in optical section, showing cuticle (cti), cortex (cort), and medulla (med) ; buccal groove (buc. gr), mouth, and gullet (gul) ; numerous food vacuoles (f. vac) circulating in the direction indicated by the arrows, and containing particles of indigo, which are finally ejected at an anal spot ; nucleus (nu) and paranucleus (pa. mi), and trichocysts, some of which (trch) are shown with their threads ejected. The scale to the right of this figure applies to A and B. C, a specimen killed with osmic acid, showing the ejection of tricho- cyst-threads, which project considerably beyond the cilia. D, diagram of binary fission : the paranucleus (fa. mi) has already divided, the nucleus (nu) is in the act of dividing. (D after Lankester.) rendered somewhat concave by the twist a wide shallow groove beginning at the broad end and gradually narrowing to about the middle of the body, where it ends in a tolerably deep depression. The groove is called the buccal groove (Fig. 21, A & B, buc. gr.} : at the narrow end is a small aperture the mouth (mth\ which, like the mouth of Euglena (Fig. 5) leads into the soft internal protoplasm of the body. The surface of the creature on which the groove is placed is distinguished as the, ventral surface, the opposite surface being upper or dorsal : the broad end is anterior, the narrow end posterior, the former being directed forwards as the animalcule swims. These descriptive terms being decided upon, it will be seen from Fig. 21 A, that the buccal groove begins on the left side of the body, and gradually curves over to the middle of the ventral surface. As the animal swims its form is seen to be permanent, exhibiting no contractions of either an amoeboid or a euglenoid nature. It is however distinctly flexible, often being bent in one or other direction when passing between obstacles such as entangled masses of weed. This permanence of contour is due to the presence of a tolerably firm though delicate cuticle (cii) which invests the whole surface. io8 PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA The protoplasm thus enclosed by the cuticle is distinctly divisible into two portions — an external somewhat dense layer, the cortical layer or cortex (corf], and an internal more fluid material, the medullary substance or medulla (med}. It will be remembered that a somewhat similar distinction of the protoplasm into two layers is exhibited by Amoeba (p. 3), the ectosarc being distinguished from the endosarc simply by the absence of granules. In Paramcecium the distinction is a far more fundamental one : the cortex is radially striated and is comparatively firm and dense, while the medulla is granular and semi-fluid, as may be seen from the fact that food particles (f. vac, see below, p. no,) move freely in it, whereas they never pass into the cortex. It has recently been found that the medulla has a reticular structure similar to that of the protoplasm of the ordinary animal cell (Fig. 9, p. 62), consisting of a delicate granular network the meshes of which are filled with a transparent material. In the cortex the meshes of the network are closer, and so form a comparatively dense substance. The cortex also exhibits a superficial oblique striation, forming what is called the my op h an layer. The mouth (mth) leads into a short funnel-like tube, the gullet (#«/), which is lined by cuticle and passes through the cortex to end in the soft medulla, thus making a free com- munication between the latter and the external water. The cilia with which the body is covered are of approxi- mately equal size, quite short in relation to the entire animal, and arranged in longitudinal rows over the whole outer surface. They consist of prolongations of the cortex, and each passes through a minute perforation in the cuticle. They are in constant rhythmical movement, and are thereby distinguished from the flagella of Haematococcus, Euglena, &c., which exhibit more or less intermittent lashing move- CONTRACTILE VACUOLE 109 ments (see p. 25, note, and p. 59). Their rapid motion and minute size make them somewhat difficult to see while the Paramoecium is alive and active, but after death they are very obvious, and look quite like a thick covering of fine silky hairs. Near the middle of the body, in the cortex, is a large oval nucleus (B, nu\ which is peculiar in taking on a uniform tint when stained, showing none of the distinction into chroma- tin and achromatin which is so marked a feature in many of the nuclei we have studied (see especially Fig. i, p. 2, and Fig. 9, p. 62). It has also a further peculiarity : against one side of it is a small oval structure (pa, mi) which is also deeply stained by magenta or carmine. This is the paramicleus ; it is to be considered as a kind of second, smaller nucleus. There are two contractile vacuoles (c. vac\ one situated at about a third of the entire length from the anterior end of the body, the other at about the same distance from the posterior end : they occur in the cortex. The action of the contractile vacuoles is very beautifully seen in a Paramoecium at rest : it is particularly striking in a specimen subjected to slight pressure under a cover glass, but is perfectly visible in one which has merely temporarily suspended its active swimming movements. It is thus seen that during the diastole, a phase of expansion of each vacuole, a number — about six to ten — of delicate radiating, spindle- shaped spaces filled with fluid appear round it, like the rays of a star (upper vacuole in A & B) : the vacuole itself contracts or performs its systole, completely disappearing from view, and immediately afterwards the radiating canals flow together and re-fill it, becoming themselves emptied and therefore invisible for an instant (lower vacuole) but rapidly appearing once more. There seems to be no doubt that the water taken in with the food is collected into these canals, emptied i io PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA into the vacuole, and finally discharged into the surrounding medium. The process of feeding can be very conveniently studied in Paramcecium by placing in the water some finely-divided carmine or indigo. When the creature comes into the neighbourhood of the coloured particles, the latter are swept about in various directions by the action of the cilia : some of these are however certain to be swept into the neighbour- hood of the buccal groove and gullet, the cilia of which all work downwards, i.e. towards the inner end of the gullet. The grains of carmine are thus carried into the gullet, where for an instant they lie surrounded by the water of which it is full : then, instantaneously, probably by the contraction of the tube itself, the animalcule performs a sort of gulp, and the grains with an enveloping globule of water or food-vacuole are forced into the medullary protoplasm. This process is repeated again and again, so that in any well-nourished Paramoecium there are to be seen numerous globular spaces filled with water and containing particles of food — or in the present instance of carmine or indigo. At every gulp the newly formed food-vacuole pushes, as it were, its predecessor before it : contraction of the medullary protoplasm also takes place in a definite direction, and thus a circulation of food- vacuoles is produced, as indicated in Fig. 21, B, by arrows. After circulating in this way for some time, the water of the food-vacuoles is gradually absorbed, being ultimately excreted by the contractile vacuoles, so that the contained particles come to lie in the medulla itself (refer to figure). The circu- lation still continues, until finally the particles are brought to a spot situated about half-way between the mouth and the posterior end of the body : here if carefully watched they are seen to approach the surface and then to be suddenly ejected. The spot in question is therefore to be looked TRICHOCYSTS in upon as a potential anus, or aperture, for the egestion of faeces or undigested food-matters. It is a potential and not an actual anus, because it is not a true aperture, but only a soft place in the cortex through which by the contractions of the medulla solid particles are easily forced. Of course when Paramoecium ingests, as it usually does, not carmine but minute living organisms, the latter are digested as they circulate through the medullary protoplasm, and only the non-nutritious parts cast out at the anal spot. It has been found by experiment that this infusor can digest not only proteids but also starch and perhaps fats. The starch is probably converted into dextrin, a carbo-hydrate having the same formula (C6H10O5) but soluble and diffusible. Oils or fats seem to be partly converted into fatty acids and glycerine. The nutrition of Paramoecium is therefore characteristically holozoic. It was mentioned above (p. 108) that the cortex is ra- dially striated in optical section. Careful examination with a very high power shows that this appearance is due to the presence in the cortex of minute spindle-shaped bodies (A and B, trcJi) closely arranged in a single layer and perpen- dicular to the surface. These are called trichocysts. When a Paramoecium is killed, either by the addition of osmic acid or some other poisonous reagent or by simple pressure of the cover glass, it frequently assumes a remark- able appearance. Long delicate threads suddenly appear, projecting from its surface in all directions (c) and looking very much as if the cilia had suddenly protruded to many times their original length. But these filaments have really nothing 'to do with the cilia; they are contained under ordinary circumstances in the trichocysts, probably coiled up ; and by the contraction of the cortex consequent upon any sudden irritation they are projected in the way indicated. ii2 PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA In Fig. 21, B, a few trichocysts (trcti) are shown in the ex- ploded condition, i.e. with the threads protruded. Most likely these bodies are weapons of offence like the very FIG. 22. — Stages in the conjugation of Paramcecium aurelia. gul, gullet : n, nuclei of gametes ; p, paranuclei of gametes ; pl, /2, products of division of paranucleus ; p. n, those products of division of the paranucleus which afterwards unite to form the reconstituted nucleus ; /. /, those forming the reconstituted paranucleus. For full explanation see text below, p. 113. (After Gruber). similar structures (nematocysts) found in polypes (see Lesson XXII. Fig. 52). Paramoecium multiplies by simple fission, the division of the body being always preceded by the elongation and subsequent division of the nucleus and paranucleus (Fig. 21, D). As shown in the figure nuclear division is direct, there being no formation of karyokinetic figures. CONJUGATION 113 Conjugation also occurs, usually after multiplication by fission has gone on for some time, but the details and the results of the process are very different from what are found to obtain in Heteromita (p. 62). Two Paramcecia come into contact by their ventral faces (Fig. 22, A) the nuclei and paranuclei break up (B — N) and are afterwards reconstituted (o, P) : at a certain stage in the process the two conjugating individuals or gametes separate from one another (H, i), the union being thus a temporary one, and not followed by the production of a zygote or by spore-formation. It is certain, however, that conjugation has a beneficial effect, which in all probability is due to an exchange of nuclear material. The process is exactly comparable to that which occurs in the sexual reproduction of multicellular animals (Lesson XXIV.). The conjugation of the ciliate Infusoria presents so many peculiarities and is so difficult of observation, that very various accounts of it have been given by skilled observers. The following abstract of Gruber's elaborate researches on the process as it occurs in Paramoecium aurelia may not be out of place, since the student may at any time meet with specimens in conjugation exhibiting one or other of the complicated phases shown in Fig. 22. Two individuals become applied by their ventral faces (Fig. 22, A) the paranucleus of each (/) separates from the nucleus (n}, and, after forming the spindle characteristic of a dividing nucleus (B, /), divides into two (c, /\ p1}. One of the products of division (pl) in each gamete approaches the ventral face and becomes flattened out as it were against the cuticle (D, p1} : in this way two paranuclei, one from each gamete, are brought into intimate relations with one another, and in all probability an exchange of nuclear material takes place between them, although this has not been actually proved. Next, these two paranuclei take on the form of rounded homogeneous bodies and retreat towards the interior of the Paramoecium (E). The same process is there gone through with the second pair of paranuclei (F, p-), so that foui homo- geneous bodies are produced, two in each gamete (G). Each of these takes on the spindle form (H), and divides, and at this phase the two I ii4 PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA gametes separate from one another (i), and are found to differ from normal Paramcecium in having each four clear paranuclei. Division of the latter again takes place (K), eight paranuclei being formed (L), and at the same time the nucleus becomes band-like (K, n), and finally breaks up into a number of separate masses (L, M). Next, four of the eight paranuclei unite with one another, forming a single, rounded, granular body (N, p. p), which becomes the permanent paranncleus of the cell: at the same time the remaining four (/. n) increase in size part passu with the gradual disappearance of the nucleus, so that at last a Paramoecium is produced (N, o) having a single normal para- nucleus (p, p) and four large nuclear bodies (/. «), ivhich finally unite to form the permanent nucleus (P). Another ciliated infusor common in stagnant water and organic infusions is Stylonychia mytilus, an animalcule vary- ing from yT mm. to \ mm. Like Paramcecium it is often to be seen swimming rapidly in the fluid, but unlike it frequently creeps about, almost like a wood-louse or a caterpillar, on the surface of the plants or other solid objects among which it lives. In correspondence with this, instead of being nearly cylindrical, it is flattened on one — the ventral — side, and is thus irregularly plano-convex in transverse section (Fig. 23, c). It resembles Paramoecium in general structure (compare Fig. 23, A, with Fig. 21, A) ; but owing to the absence of trichocysts the distinction between cortex and medulla is less obvious : moreover, it has two nuclei («», ««) and only one contractile vacuole (c. vac}. But it is in the character of its cilia that Stylonychia is most markedly distinguished from Paramoecium : these structures, instead of being all alike both in form and size, are modified in a very extraordinary way. On the dorsal surface the cilia are represented only by very minute processes of the cortex (c, d. ci.) set in longi- tudinal grooves and exhibiting little movement. It seems GENERAL CHARACTERS 115 probable that these are to be looked upon as vestigial or rudimentary cilia, i.e., as the representatives of cilia which were of the ordinary character in the ancestors of Stylo- u Ji.ci - FIG. 23. — A, Stylonychia mytilus, ventral aspect, showing the buccal groove (buc. gr.) and mouth (mth)t two nuclei (nu, mt), contractile vacuole (c. vac), and cilia differentiated into hook-like (h. d), bristle- like (b. ci), plate-like (/. ci), and fan-like (m. d) organs. B, one of the plate-like cilia of the same (/. d in A), showing its frayed extremity. c, transverse section of Gastrostyla, an allied form to Stylonychia, showing buccal groove (buc. gr.), small dorsal cilia (d. d), hook-like cilium (h. d), and the various cilia of the buccal groove, including an expanded fan-like organ (m. d). A and B after Claparede and Lach- mann ; C after Sterki. nychia, but which have undergone partial atrophy, or diminution beyond the limits of usefulness, in correspond- ence with the needs of an animalcule which has taken to I 2 ii6 PARAMOECIUM, STYLONYCHIA, AND OXYTRICHA creeping on its ventral surface, instead of swimming freely and so using all its cilia equally. On the other hand, the cilia on the ventral surface have undergone a corresponding enlargement or hypertrophy. Near the anterior and posterior ends and about the middle are three groups of cilia of comparatively immense size, shaped either like hooks (h. «'.), or like flattened rods frayed at their ends ( p. ci. and B). All these structures neither vibrate rhythmically like ordinary cilia nor perform lashing move- ments like flagella, but move at the base only like one- jointed legs. The movement is under the animal's control, so that it is able to creep about by the aid of these hooks and plates in much the same way as a caterpillar uses its legs. Notice that we have here a third form of contractility : in amoeboid movement there is an irregular flowing of the pro- toplasm (pp. 4 and 10) ; in ciliary movement a flexion of a protoplasmic filament from side to side (p. 33) ; while in the present case we have sudden contractions taking place at irregular intervals. The movements of these locomotor hooks and plates is therefore very similar to the muscular contraction to which the movements of the higher animals are due : it cannot be said that definite muscles are present in Stylonychia, but the protoplasm in certain regions of the unicellular body is so modified as to be able to perform a sudden contraction in a definite direction. The nature of muscular contraction will be further discussed in the next Lesson (see p. 128). The remainder of the ventral surface, with the exception of the buccal groove is bare, but along each side of the margin is a row of large vibratile cilia, of which three at the posterior end are modified into long, stiff, bristle-like processes (A, b. d ). DIFFERENTIATION OF CILIA 117 There is also a special differentiation of the cilia of tj^e buccal groove (buc.gr!). On its left side is a single row of very large and powerful cilia (A and c, m a.) which are the chief organs for causing the food-current as well as the main swimming-organs : each has the form of a triangular fan-like plate (c, m. a.}. On the right side of the buccal groove is a row of smaller but still large cilia of the ordinary form, and in the interior of the gullet a row of extremely delicate cilia which aid in forcing particles of food down the gullet into the medulla. In Stylonychia and allied genera intermediate forms are found between these peculiar hooks, plates, bristles, and FIG. 24. — Oxytncha flava, killed and stained, showing the frag- mentation of the nuclei. (After Gruber.) fans and ordinary cilia ; from which we may conclude that these diverse appendages are to be looked upon as highly modified or differentiated cilia. Probably they have been evolved in the course of time from ordinary cilia, and on the principle that the more complicated or specialized organisms are descended from simpler or more generalized forms (see Lesson XIII.), we may consider Stylonychia as the highly-specialized descendant of some uniformly-ciliated progenitor. A third series of ciliated Infusoria must be just referred to in concluding the present Lesson. We have seen how n8 PARAMCECIUM, STYLONYCHIA, AND OXYTRTCHA the nucleus of a Paramoecium which has just conjugated breaks up and apparently disappears (Fig. 22, K — o). In Oxytricha, a genus closely resembling Stylonychia, the two nuclei have been found to break up into a large number of minute granules (Fig. 24), which can be seen only after careful staining and by the use of high magnifying powers. This process is called fragmentation of the nucleus : in other cases it goes even further, and the nucleus is reduced to an almost infinite number of chromatic granules only just visible under the highest powers. From this it seems very probable that organisms which, like Protamceba (p. 9) and Protomyxa (p. 49), appear non-nucleate, are actually pro- vided with a nucleus in this pulverized condition, and that a nucleus in some form or other is an essential constituent of the cell. LESSON XI OPALINA THE large intestine of the common frog often contains numbers of ciliate Infusoria belonging to two or three genera. One of these parasitic animalcules, called Opalina ranarum, will now be described. It is easily obtained by killing a frog, , opening the body, making an incision in the rectum, and spreading out a little of its blackish contents in a drop of water on a slide. Opalina has a flattened body with an oval outline (Fig. 25, A, B), and full-sized specimens may be as much as one millimetre in length. The protoplasm is divided into cortex and medulla, and is covered with a cuticle, and the cilia are equal-sized and uniformly arranged in longitudinal rows -over the whole surface (A). On a first examination no nucleus is apparent, but after staining a large number of nuclei can be seen (B. nu\ each being a globular body (c, i), consisting of achromatin sur- rounded by a membrane and containing a coil or network of chromatin. These nuclei multiply within the body of the infusor, and in so doing pass through the various changes characteristic of karyokinesis or indirect nuclear division (compare Fig. 10, p. 64, with Fig. 25, c) : the I2O OPALINA chromatin breaks up (c, 2), a spindle is formed with the chromatin across its equator (3), the chromatin passes to the FIG. 25. — Opalina ranarum. A, living specimen, surface view, showing longitudinal rows of cilia. B, the same, stained, showing numerous nuclei (nu) in various stages of division. C, I — 6, stages in nuclear division. D, longitudinal fission. E, transverse fission. F, the same in a specimen reduced in size by repeated division. G, final product of successive divisions. H, encysted form. I, uninucleate form produced from cyst. K, the same after multiplication of the nucleus has begun. (A— c, after Pfitzner ; D— K, from Saville Kent after Zeller.) poles of the spindle (4, 5), and the nucleus becomes con- stricted (5), and finally divides into two (6). PARASITIC NUTRITION 121 The presence of numerous nuclei in Opalina is a fact worthy of special notice. The majority of the organisms we have studied are uninucleate as well as unicellular : the higher animals and plants we found (Lesson VI.) to consist of numerous cells each with a nucleus, so that they are multicellular and multinucleate : Opalina, on the other hand, is multinucleate but unicellular. An approach to this condition of things is furnished by Stylonychia, which is unicellular and binucleate (Fig. 23, A), but the only organisms we have yet studied in which numerous nuclei of the ordi- nary character occur in an undivided mass of protoplasm are the Mycetozoa (p. 52), and in them the multinucleate con- dition of the plasmodium is largely due to its being formed by the fusion of separate cells, while in Opalina it is due, as we shall see, to the repeated binary fission of an originally single nucleus. There is no contractile vacuole, and no trace of either mouth or gullet, so that the ingestion of solid food is impos- sible. The creature lives, as already stated, in the intestine of the frog : it is therefore an internal parasite, or endo- parasite, having the frog as its host. The intestine contains the partially digested food of the frog, and it is by the ab- sorption of this that the Opalina is nourished. Having no mouth, it feeds solely by imbibition : whether it performs any kind of digestive process itself is not certainly known, but the analogy of other mouthless parasites leads us to expect that it simply absorbs food ready digested by its host, upon which it is dependent for a constant supply of soluble and diffusible nutriment. Thus Opalina, in virtue of its parasitic mode of life, is saved the performance of certain work — the work of diges- tion, that work being done for it by its host. This is the essence of internal parasitism : an organism exchanges a free 122 OPALINA life, burdened with the necessity of finding food for itself, for existence in the interior of another organism, on which in one way or another it levies blackmail. Note the close analogy between the nutrition of an internal parasite like Opalina and the saprophytic nutrition of a monad (p. 39). In both the organism absorbs proteids rendered soluble and diffusible, in the one case by the digestive juices of the host, in the other by the action of putrefactive bacteria. The reproduction of Opalina presents certain points of interest, largely connected with its peculiar mode of life. It is obvious that if the Opalinse simply went on multiplying, by fission or otherwise, in the frog's intestine, the population would soon outgrow the means of subsistence : moreover, when the frog died there would be an end of them. What is wanted in this as in other internal parasites is some mode of multiplication which shall serve as a means of dispersal, or in other words, enable the progeny of the parasite to find their way into the bodies of other hosts, and so start new colonies instead of remaining to impoverish the mother country. Opalina multiplies by a somewhat peculiar process of binary fission : an animalcule divides in an oblique direction (Fig. 25, D), and then each half, instead of growing to the size of the parent cell, divides again transversely (E). The process is repeated again and again (F), the plane of division being alternately oblique and transverse, until finally small bodies are produced (G), about -^-"FO mm. in length, and containing two to four nuclei. If the parent cell had divided simultaneously into a num- ber of these little bodies the process would have been one of multiple fission : as it is it forms an interesting link between simple and multiple fission. MEANS OF DISPERSAL 123 Opalina ranarum multiplies in this way in the spring — i.e. during the frog's breeding season. Each of the small pro- ducts of division (G) becomes encysted (H), and in this passive condition is passed out with the frog's excrement, probably falling on to a water weed or other aquatic object. Nothing further takes place unless the cyst is swallowed by a tadpole, as must frequently happen when these creatures, produced in immense numbers from the frogs' eggs, browse upon the water-weeds which form their chief food. Taken into the tadpole's intestine, the cyst is burst or dissolved, and its contents emerge as a lanceolate mass of protoplasm (i), containing a single nucleus and covered with cilia. This, as it absorbs the digested food in the intestine of its host, grows, and at the same time its nucleus divides repeatedly (K) in the way already described, until by the time the animalcule has attained the maximum size it has also acquired the large number of nuclei characteristic of the genus. Here, then, we have another interesting case of develop- ment (see p. 43) : the organism begins life in a very small uninucleate mass of protoplasm, and as it increases in size increases also in complexity by the repeated binary fission of its nucleus. LESSON XII VORTICELLA AND ZOOTHAMNIUM THE next organism we have to consider is a ciliated infusor even commoner than those described in the two previous lessons. It is hardly possible to examine the water of a pond with any care without finding in it, sometimes attached to weeds, sometimes to the legs of water-fleas, sometimes to the sticks and stones of the bottom, numbers of exquisitely beautiful little creatures, each like an inverted bell with a very long handle, or a wine-glass with a very long stem. These are the well-known " bell-animalcules ; ' the com- monest among them belong to various species of the genus Vorticella. The first thing that strikes one about Vorticella (Fig 26, A) is the fact that it is permanently fixed, like a plant, the proximal or near end of the stalk being always firmly fixed to some aquatic object, while to the distal or far end the body proper of the animalcule is attached. But in spite of its peculiar form it presents certain very obvious points of resemblance to Paramcecium, Stylonychia, and Opalina. The protoplasm is divided into cortex (Fig. 26, c, corf} and medulla (ined\ and is invested with a pe H' FIG. 26. — Vorticella. A, living specimen fully expanded, showing stalk (st) with axial fibre (ax.f), peristome (per), disc (d), mouth (mth), gullet (gull), and contractile vacuole. B, the same, bent on its stalk and with the disc turned away from the observer. c, optical section of the same, showing cuticle (CM), cortex (corf), medulla (med), nucleus (mi), gullet (gull), several food-vacuoles, and anus (an), as well as the stractures shown in A. D1, a half-retracted and D2 a fully-retracted specimen, showing the coiling of the stalk and overlapping of the disc by the peristome. E1, commencement of binary fission ; E2, completion of the process ; 126 VORTICELLA AND ZOOTHAMNIUM E3, the barrel-shaped product ol division swimming freely in the direction indicated by the arrow. F1, a specimen dividing into a megazooid and several microzooids (m) ; F2, division into one mega- and one microzooid. G1, Gl, two stages in conjugation showing the gradual absorption of the microgamete (m) into the megagamete. H1, multiple fission of encysted form, the nucleus dividing into nume- rous masses : H2, spore formed by multiple fission ; H3 — H7, development of the spore ; H4 is undergoing binary fission. (E — H after Saville Kent. ) delicate cuticle (cu). There is a single contractile vacuole (c. vac) the movements of which are very readily made out owing to the ease with which the attached organism is kept under observation. There is a nucleus (nu) remarkable for its elongated band-like form, and having in its neighbour- hood a small rounded paranucleus. Cilia are also present, but the way in which they are disposed is very peculiar and characteristic. To understand it we must study the form of the body a little more closely. The conical body is attached by its apex or proximal end to the stalk : its base or distal end is expanded so as to form a thickened rim, the peristome (per), within which is a plate- like body elevated on one side, called the disc (S) in all but size, and like it become detached and swim freely by means of a basal circlet of cilia. After swimming about for a time, one of these microzooids comes in contact with an ordinary form or megazooid, when it attaches itself to it near the proximal end (c1), and under- goes gradual absorption (c2), the mega- and microzooids becoming completely and permanently fused. As in Para- moecium, conjugation is followed by increased activity in feeding and dividing (p. 113). Notice that in this case the conjugating bodies or gametes are not of equal size and similar characters, but one, which is conveniently distinguished as the microgamete ( = micro- zooid) is relatively small and active, while the other or megagamete ( = megazooid, or ordinary individual) is rela- tively large and passive. As we shall see in a later lesson, this differentiation of the gametes is precisely what we get in almost all organisms with two sexes : the microgamete being the male, the megagamete the female conjugating body (see Lesson XVI.). The result of conjugation is strikingly different in the three cases already studied : in Heteromita (p. 41) the two gametes METAMORPHOSIS 131 unite to form a zygote, a motionless body provided with a cell-wall, the protoplasm of which divides into spores : in Paramcecium (p. 113) no zygote is formed, conjugation being a mere temporary union : in Vorticella the zygote is an actively moving and feeding body, indistinguishable from an ordinary individual of the species. Vorticella sometimes encysts itself (Fig. 26, H1), and the nucleus of the encysted cell has been observed to break up into a number of separate masses, each doubtless surrounded by a layer of protoplasm. After a time the cyst bursts, and a number of small bodies or spores (n2) emerge from it, each containing one of the products of division of the nucleus. These acquire a circlet of cilia (H3), by means of which they swim freely, sometimes multiplying by simple fission (H4). Finally, they settle down (H5) by the end at which the cilia are situated, the attached end begins to elongate into a stalk (H°), this increases in length, the basal circlet of cilia is lost, and a ciliated peristome and disc are formed at the free end (HT), and in this way the ordinary form is assumed by a process of development recalling that we found to occur in Heteromita (p. 42), but with an important difference : the free-swimming young of Vorticella (HS), to which the spores formed by division of the encysted protoplasm give rise, differ strikingly in form and habits from the adult. This is expressed by saying that development is in this case accom- panied by a metamorphosis, this word literally meaning simply a change, being always used in biology to express a striking and fundamental difference in form and habit between the young and the adult ; as, for instance, between the tadpole and the frog, or between the caterpillar and the butterfly. It is obvious that in the present instance metamorphosis is another means of ensuring dispersal. In Vorticella, as we have seen, fission results not in the K 2 132 VORTICELLA AND ZOOTHAMNIUM production of equal and similar daughter-cells, but of one stalked and one free-swimming form. It is however quite possible to conceive of a Vorticella-like organism in which the parent cell divides into two equal and similar products, each retaining its connection with the stalk. If this process //.z. FIG. 27. — Zoothamnium arlmscula. A, entire colony, magnified, showing nutritive (n. z) and reproductive (r. z) zooids ; (ax.f) axial fibre of the stem. B, the same, natural size. c, the same, magnified, in the condition of retraction. D, nutritive zooid, showing nucleus (mi), contractile vacuole (c. vac], gullet, and axial fibre (ax.f}. E, reproductive zooid, showing nucleus (nit) and contractile vacuole (c. vac), and absence of mouth and gullet. F1, F'2, two stages in the development of the reproductive zooid. (After Saville Kent.) were repeated again and again, and if further the plane of fission were extended downwards so as to include the dista- end of the stalk, the result would be a branched, tree-like stem with a Vorticella-like body at the end of every branch. As a matter of fact, this process takes place not in Vorti- DIMORPHISM 133 cella itself, but in a nearly allied infusor, the beautiful Zoothamnium, a common genus found mostly in sea-water attached to weeds and other objects. Zoothamnium arbuscula (Fig. 27, A) consists of a main stem attached by its proximal end and giving off at its distal end several branches, on each of which numerous shortly- stalked bell-animalcules are borne, like foxgloves or Canter- bury-bells on their stem. The entire tree is about i cm. high, and so can be easily seen by the naked eye : it is shown of the natural size in Fig. 27, B. We see, then, that Zoothamnium differs from all our previous types in being a compound organism. The entire " tree " is called a colony or stock, and each separate bell animalcule borne thereon is an individual or zooid, morphologically equivalent to a single Vorticella or Paramcecium. As in Vorticella the stem consists of a cuticular sheath with an axial muscle-fibre (ax. /), which, at the distal end of the main stem, branches like the stem itself, a prolonga- tion of it being traceable to each zooid (D). So that the muscular system is common to the whole colony, and any shock causes a general contraction, the tree-like structure assuming an almost globular form (c). It will be noticed from the figure that all the zooids of the colony are not alike : the majority are bell-shaped and resemble Vorticellae (A, n. z, and D), but here and there are found larger bodies (A, r. z, and E) of a globular form, with- out mouth, peristome, or disc, and with a basal circlet of cilia. The characteristic band-like nucleus (nu) and the contractile vacuole (c. vac) are found in both the bell-shaped and the globular zooids. It is to these globular, mouthless zooids that the functions of reproducing the whole colony and of ensuring dispersal 134 VORTICELLA AND ZOOTHAMNIUM are assigned. They become detached, swim about freely for a time, then settle down, develop a stalk and mouth (r1, F2), and finally, by repeated fission, give rise to the adult, tree-like colony. The Zoothamnium colony is thus dimorphic, bearing indi- viduals of two kinds : nutritive zooids, which feed and add to the colony by fission, but are unable to give rise to a new colony, and reproductive zooids, which do not feed while attached, but are capable, after a period of free existence, of developing a mouth and stalk, and finally producing a new colony. Dimorphism is a differentiation of the individuals of a colony, just as the formation of axial fibre, gullet, con- tractile vacuole, and cilia are cases of differentiation of the protoplasm of a single cell. LESSON XIII SPECIES AND THEIR ORIGIN — THE PRINCIPLES OF CLASSIFICATION MORE than once in the course of the foregoing lessons we have had occasion to use the word species — for instance, in Lesson I. (p. 8) it was stated that there were different kinds or species of Amoebae, distinguished by the characters of their pseudopods, the structure of their nuclei, &c. We must now consider a little more in detail what we mean by a species, and, as in all matters of this sort, the study of concrete examples is the best aid to the formation of clear conceptions, we will take, by way of illustration, some of the various species of Zoothamnium. The kind described in the previous lesson is called Zoothamnium arbuscula. As Fig. 27, A, shows, it consists of a tolerably stout main stem, from the distal end of which spring a number of slender branches diverging in a brush- like manner, and bearing on short secondary branchlets the separate individuals of the colony : these are of two kinds, bell-shaped nutritive zooids, and globular reproductive zooids, so that the colony is dimorphic. Zoothamnium (or, for the sake of brevity, Z.) alternans, 136 SPECIES AND THEIR ORIGIN (Fig. 28, A) is found also in sea-water, and differs markedly from Z. arbuscula in the general form of the colony. The main stem is continued to the extreme distal end of the colony and terminates in a zooid ; from it branches are given off right and left, and on these the remaining zooids are borne. To use Mr. Saville Kent's comparison, Z. arbus- FIG. 28. — Species of Zoothamnium. A, Z, alternans. B, Z. dichotomum. c, Z. simplex. D, Z. ajfine. E, Z. nutans. (After Saville Kent.) cula may be compared to a standard fruit tree, Z. alternans to an espalier. In this species also the colony is dimorphic. Z. dichotomum (Fig. 28, B) is also dimorphic and presents a third mode of branching. The main stem divides into two, and each of the secondary branches does the same, so that a repeatedly forking stem is produced. The branching of this species is said to be dichotomous^ while that of Z. alter- nans is monopodial) and that of Z. arbuscula umbellate, Another mode of aggregation of the zooids is found in Z. GENERIC AND SPECIFIC CHARACTERS 137 simplex (Fig. 28, c) in which the stem is unbranched and bears at its distal end about six zooids in a cluster. The zooids are more elongated than in any of the preceding species, and there are no special reproductive individuals, so that the colony is homomorphic. In Z. affine (Fig. 28, D) the stalk is dichotomous but is proportionally thicker than in the preceding species, and bears about four zooids all alike. It is found in fresh water attached to insects and other aquatic animals. The last species we shall consider is Z. nutans (Fig. 28, E), which is the simplest known, never bearing more than two zooids, and sometimes only one. A glance at Figs. 27 and 28 will show that these six species agree with one another in the general form of the zooids, in the characters of the nucleus, contractile vacuole, £c., in the arrangement of the cilia, and in the fact that they are all compound organisms, consisting of two or more zooids attached to a common stem, having an axial fibre which branches with the stem, i.e., is continuous throughout the colony. On account of their possessing these important characters in common, the species described are placed in the single genus Zoothamnium, and the characters summarized in the preceding paragraph are called generic characters. On the other hand the points of difference between the various species, such as the forking of the stem in Z. dichotomum, the presence of only two zooids in Z. nutans, and so on, are called specific characters. Similarly the name Zoothamnium^ which is common to all the species, is the generic name, while those which are applied only to a particular species, such as arbuscula, simplex, &c., are the specific names. As was mentioned in the first lesson (p. 8), this is the Linnean system of binomial nomenclature. I38 SPECIES AND THEIR ORIGIN It will be seen from the foregoing account that by a species we understand an assemblage of individual or- ganisms, whether simple or compound, which agree with one another in all but unessential points, such as the precise number of zooids in Zoothamnium, which may vary con- siderably in the same species, and come, therefore, within the limits of individual variation. Similarly, what we mean by a genus is a group of species agreeing with one another in the broad features of their organization, but differing in detail, the differences being constant. A comparison of the six species described brings out several interesting relations between them. For instance, it is clear that Z. arbuscula and Z. alternans are far more complex, i.e., exhibit greater differentiation of the entire colony, than Z. simplex or Z. nutans ; so that, within the limits of the one genus, we have comparatively low or generalized, and comparatively high or specialized species. Nevertheless, a little consideration will show that we cannot arrange the species in a single series, beginning with the lowest and ending with the highest, for, although we should have no hesitation in placing Z. nutans at the bottom of such a list, it would be impossible to say whether Z. affine was higher or lower than Z. simplex, or Z. arbuscula than Z. alternans. It is, however, easy to arrange the species into groups according to some definite system. For instance, if we take the mode of branching as a criterion, Z. nutans, affine, and dichotomum will all be placed together as being dichoto- mous, and Z. simplex and arbuscula as being umbellate - the zooids of the one and the branches of the other all springing together from the top of the main stem : on this system Z. alternans will stand alone on account of its mono- podial branching. Or, we may make two groups, one of CREATION 139 dimorphic forms, including Z. arbuscula, alternans, and dichotomum, and another of homomorphic species, including Z. affine, simplex, and nutans. We have thus two very obvious ways of arranging or classifying the species of Zoothamnium, and the question arises — which of these, if either, is the right one ? Is there any standard by which we can judge of the accuracy of a given classification of these or any other organisms, or does the whole thing depend upon the fancy of the classifier, like the arrangement of books in a library ? In other words, are all possible classi- fications of living things more or less artificial, or is there such a thing as natural classification ? Suppose we were to try and classify all the members of a given family — parents and grandparents, uncles and aunts, cousins, second cousins, and so on. Obviously there are a hundred ways in which it would be possible to arrange them — into dark and fair, tall and short, curly-haired and straight-haired, and so on. But it is equally obvious that all these methods would be purely artificial, and that the only natural way, i.e. the only way to show the real connection of the various members of the family with one another would be to classify them according to blood-relationship, in other words, to let our classification take the form of a genea- logical tree. It may be said — what has this to do with the point under discussion, the classification of the species of Zoothamnium ? There are two theories which attempt to account for the existence of the innumerable species of living things which inhabit our earth : the theory of creation and the theory of evolution. According to the theory of creation, all the individuals of every species existing at the present day- -the tens of thousands of dogs, oak trees, amoebae, and what not — are derived a natural process of descent from a single indi- 140 SPECIES AND THEIR ORIGIN vidual, or from a pair of individuals, in each case precisely resembling the existing descendants, which came into exist- ance by a process outside the ordinary course of nature and known as Creation. On this hypothesis the history of the genus Zoothamnium would be represented by the diagram (Fig. 29) ; each of the species being derived from a single individual which came into existence independently of the Existing Individuals Z.arbuscula Z.alternans Z.dichotomum Z. simplex Z.affine Z.nutans Ancestral Individuals FIG. 29. — Diagram illustrating the origin of the species of Zoothamnium by creation.1 progenitors of all the other species at some distant period of the earth's history. Notice that on this theory the various species are no more actually related to one another than is either of them to Vorticella or for the matter of that to Homo. The in- dividuals of any one species are truly related since they all share a common descent, but there is no more relationship between the individuals of any two independently created 1 As the original drawing of the above figure could not be found just as the book was ready for publication, the above was inserted to avoid delay, for which the author is not responsible. — W. N. P., April 4, 1891. ORGANIC EVOLUTION 141 species than between any two independently manufactured chairs or tables. The words affinity, relationship, £c., as applied to different species are, on the Creation Theory purely metaphorical, and mean nothing more than that a certain likeness or community of structure exists ; just as we might say that an easy chair was more nearly related to a kitchen chair than either of them to a three-legged stool. We see therefore that on the hypothesis of creation the varying degrees of likeness and unlikeness between the species receive no explanation, and that we get no absolute criterion of classification : we may arrange our organisms, as nearly as our knowledge allows, according to their resem- blances and differences, but the relative importance of the characters relied on becomes a purely subjective matter. According to the rival theory — that of Descent or Organic Evolution — every species existing at the present day is derived by a natural process of descent from some other species which lived at a former period of the world's history. If we could trace back from generation to gene- ration the individuals of any existing species we should, on this hypothesis, find their characters gradually change, until finally a period was reached at which the differences were so considerable as to necessitate the placing of the ancestral forms in a different species from their descendants at the present day. And in the same way if we could trace back the species of any one genus, we should find them gradually approach one another in structure until they finally con- verged in a single species, differing from those now existing but standing to all in a true parental relation. Let us illustrate this by reference to Zoothamnium. As a matter of fact we know nothing of the history of the genus, but the comprehension of what is meant by the evolution of species will be greatly facilitated by framing a working hypothesis. 142 SPECIES AND THEIR ORIGIN Suppose that at some distant period of the world's history there existed a Vorticella-like organism which we will call A (Fig. 30), having the general characters of a single stalked zooid of Zoothamnium (compare Fig. 27, F2), and suppose that, of the numerous descendants of this form, represented by the lines diverging from A, there were some Branching dichotomous Branching umbellate L^ "*-i 5? ?•*£%) «t ) has become very convex, distally forming the columella. D\ mature sporangium in the act of dehiscence, showing the spores (sp) surrounded by mucilage (g] ; D'!, small portion of the same, more highly magnified, showing spicules of calcium oxalate attached to wall. E, a columella, left by complete dehiscence of a sporangium, showing the attachment of the latter as a black band. FORMATION OF SPORANGIA 159 The scale above c- applies to c1, c2, D1, and E. F, spores. G1, G2, G3, three stages in the germination of the spores. H, a group of germinating spores forming a small mycelium. I1 — 16; five stages in conjugation, showing two gametes (gam) uniting to form the zygote (zyg). K1, K'2, development of ferment cells from submerged hyphse. (A, C'J, D, E, F, G, and K, after Howes ; I, after De Bary. ) By the employment of the usual reagents, it can be ascer- tained that the granular substance is protoplasm, and the surrounding membrane cellulose. The protoplasm moreover contains vacuoles at irregular intervals, and numerous small nuclei. Thus a hypha of Mucor consists of precisely the same constituents as a yeast cell — protoplasm, containing nuclei and vacuoles, surrounded by cellulose. Imagine a yeast cell to be pulled out — as one might pull out a sphere of clay or putty — until it assumed the form of a long, narrow cylin- der, and suppose it also to be pulled out laterally at intervals so as to form branches : there would be produced by such a process a very good imitation of a hypha of Mucor. We may therefore look upon a hypha as an elongated and branched cell, so that Mucor is, like Opalina, a multinucleate but unicellular organism. We shall see directly however that this is strictly true only of the mould in its young state. As stated above, the aerial hyphse are at first of even calibre, but gradually swell at their ends, forming sporangia. Under the microscope the distal end of an aerial hypha is found to dilate (Fig. 37, c1) : immediately below the dilata- tion the protoplasm divides at right angles to the long axis of the hypha, the protoplasm in the dilated portion thus becoming separated from the rest. Between the two a cellulose partition or septum (sep) is formed, as in the ordi- nary division of a plant-cell (Fig. n, p. 66). The portion thus separated is the rudiment of a sporangium. 160 MUCOR Let us consider precisely what this process implies. Before it takes place the protoplasm is continuous throughout the whole organism, which is therefore comparable to the un- divided plant cell shown in Fig. n, A. As in that case, the protoplasm divides into two and a new layer of cellulose is formed between the daughter-cells. Only whereas in the ordinary vegetable cell the products of division are of equal size (Fig. n, E), in Mucor they are very unequal, one being the comparatively small sporangium, the other the rest of the hypha. Thus a Mucor-plant with a single aerial hypha becomes, by the formation of a sporangium, bicellular : if, as is ordi- narily the case, it bears numerous aerial hyphae, each with its sporangium, it is multicellular. Under unfavourable conditions of nutrition, septa fre- quently appear at more or less irregular intervals in the mycelial hyphse : the organism is then very obviously multi- cellular, being formed of numerous cylindrical cells arranged end to end. The sporangium continues to grow, and as it does so the septum becomes more and more convex upwards, finally taking the form of a short, club-shaped projection, the colu- mella, extending into the interior of the sporangium (c2). At the same time the protoplasm of the sporangium undergoes multiple fission, becoming divided into numerous ovoid masses, each of which surrounds itself with a cellulose coat and becomes a spore (p1, D2, sp). A certain amount of the protoplasm remains unused in the formation of spores, and is converted into a gelatinous material (g), which swells up in water. The original cell-wall of the sporangium is left as an exceedingly delicate, brittle shell around the spores : minute needle-like crystals of calcium oxalate are deposited in it. STRUCTURE OF SPORES 161 and give it the appearance of being closely covered with short cilia (D2). In the ripe sporangium the slightest touch suffices to rupture the brittle wall and liberate the spores, which are dispersed by the swelling of the transparent intermediate substance. The aerial hypha is then left terminated by the columella (E), around the base of which is seen a narrow black ring indicating the place of attachment of the sporangium. The spores (F) are clear, bright-looking, ovoidal bodies FIG. 38. — Moist chamber formed by cementing a ring of glass or metal (c) on an ordinary glass slide (A), and placing over it a cover-slip (B) on the underside of which is a hanging drop of nutrient fluid (p). The upper figure shows the apparatus in perspective, the lower in vertical section. (From Klein.) consisting of protoplasm containing a nucleus and surround- ed by a thick cell-wall. A spore is therefore an ordinary encysted cell, quite comparable to a yeast-cell. The development of the spores is a very instructive process, and can be easily studied in the following way : A glass or metal ring (Fig. 38, c) is cemented to an ordinary microscopic slide (A) so as to form a shallow cylindrical chamber. The top of the ring is oiled, and on it is placed a cover glass (B), with a drop of Pasteur's solution on its under surface. Before placing the cover-glass in position a ripe sporangium M 162 MUCOR of Mucor is touched with the point of a needle, which is then stirred round in the drop of Pasteur's solution, so as to sow it with spores. By this method the drop of nutrient fluid is prevented from evaporating, and the changes under- gone by the spores can be watched by examination from time to time under a high power. The first thing that happens to a spore under these con- ditions is that it increases in size by imbibition of fluid, and instead of appearing bright and clear becomes granular and develops one or more vacuoles. Its resemblance to a yeast-cell is now more striking than ever. Next the spore becomes bulged out in one or more places (o1, Fig. 37) looking not unlike a budding Saccharomyces. The buds, however, instead of becoming detached increase in length until they become filaments of a diameter slightly less than that of the spore and somewhat bluntly pointed at the end (c2). These filaments continue to grow, giving off as they do so side branches (c3) which interlace with similar threads from adjacent spores (H). The filaments are obviously hyphae, and the interlacement is a mycelium. Thus the statement made in a previous paragraph (p. 159), that Mucor was comparable to a yeast-cell pulled out into a filament, is seen to be fully justified by the facts of develop- ment, which show that the branched hyph?e constituting the Mucor-plant are formed by the growth of spores each strictly comparable to a single Saccharomyces. It will be noticed that the growth of the mycelium is cen- trifugal : each spore or group of spores serves as a centre from which hyphae radiate in all directions (H), continuing to grow in a radial direction until, in place of one or more spores quite invisible to the naked eye, we have a white patch more or less circular in outline, and having the spores from which the growth proceeded in its centre. Owing to CONJUGATION 163 the centrifugal mode of growth the mycelium is always thicker at the centre than towards the circumference, since it is the older or more central portions of the hyphae which have had most time to branch and become interlaced with one another. Under certain circumstances a peculiar process of con- jugation occurs in Mucor. Two adjacent hyphae send out short branches (Fig. 37, i1) which come into contact with one another by their free somewhat swollen ends (i2). In each a septum appears so as to shut off a separate terminal cell (i3, gam) from the rest of the hypha. The opposed walls of the two cells then become absorbed (i4) and their contents mingle, forming a single mass of protoplasm (i5, zyg), the cell-wall of which becomes greatly thickened and divided into two layers, an inner delicate and trans- parent, and an outer dark in colour, of considerable thick- ness, and frequently ornamented with spines. Obviously the swollen terminal cells (gam) of the short lateral hyphae are gametes or conjugating bodies, and the large spore-like structure (zyg) resulting from their union is a zygote. The striking feature of the process is that the gametes are non-motile, save in so far as their growth towards one another is a mode of motion. In Heteromita both gametes are active and free-swimming (p. 41) : in Vorticella one is free-swimming, the other fixed but still capable of active movement (p. 130) ; here both conjugating bodies only exhibit the slow movement in one direction due to growth. There are equally important differences in the result of the process in the three cases. In Heteromita the proto- plasm of the zygote breaks up almost immediately into spores ; in Vorticella the zygote is active and the result of M 2 1 64 MUCOR conjugation is merely increased activity in feeding and fissive multiplication ; in Mucor the zygote remains inactive for a longer or shorter time, and then under favourable conditions germinates in much the same way as an ordinary spore? forming a mycelium from which sporangium-bearing aerial hyphse arise. A resting zygote of this kind, formed by the conjugation of equal-sized gametes, is often distinguished as •» a zygospore. Notice that differentiation of a very important kind is exhibited by Mucor. In relation with its comparatively large size the function of reproduction is not performed by the whole organism, as in all previously studied types, but a certain portion of the protoplasm becomes shut off from the rest, and to it — as spore or gamete — the office of reproduc- ing the entire organism is assigned. So that we have for the first time true reproductive organs, which may be of two kinds, asexual — the sporangia, and sexual — the gametes.1 In describing the reproduction of Amoeba it was pointed out (p. 20) that as the entire organism divided into two daughter-cells, each of which began an independent life, an Amoeba could not be said ever to die a natural death. The same thing is true of the other unicellular forms we have considered in the majority of which the entire organism produces by simple fission two new individuals.2 But in Mucor the state of things is entirely altered. A compara- tively small part of the organism is set apart for repro- 1 In Mucor no distinction can be drawn between the conjugating body (gamete) and the organ which produces it (gonad). See the de- scription of the sexual process in Vaucheria (Lesson XVI.) and in Spirogyra (Lesson XIX.). 2 An exception is formed by colonial orms such as Zoothamnium, in which life is carried on from generation to generation by the reproduc- tive zooids only. In all probability the colony itself, like an annual plant, dies down after a longer or shorter time. FERMENT CELLS 165 duction, and it is only the reproductive cells thus formed- spores or zygote — which carry on the life of the species : the remainder of the organism, having exhausted the available food supply and produced the largest possible number of reproductive products, dies. That is, all vital manifestations, such as nutrition, cease and decomposition sets in, the protoplasm becoming converted into pro- gressively simpler compounds, the final stages being chiefly carbon dioxide, water, and ammonia. Mucor is able to grow either in Pasteur's or some similar nutrient solution, or on various organic matters such as bread, jam, manure, &c. In the latter cases it appears to perform some fermentative action, since food which has become " mouldy " is found to have experienced a definite change in appearance and flavour without actual putre- faction. When growing on decomposing organic matter, as it often does, the nutrition of Mucor is saprophytic, but in some instances, as when it grows on bread, it seems to approach very closely to the holozoic method. M. stolo- nifer is also known to send its hyphae into the interior of ripe fruits, causing them to rot, and thus acting as a para- site. The parasitism in this case is, however, obviously not quite the same thing as that of Opalina (p. 121) : the Mucor feeds not upon the ready digested food of its host but upon its actual living substance which it digests by the action of its own ferments. Thus a parasitic fungus such as Mucor, unlike an endo-parasitic animal such as Opalina or a tape- worm, is no more exempted from the work of digestion than a dog or a sheep : the organism upon which it lives is to be looked upon rather as its prey than as its host. It is a remarkable circumstance that, under certain con- ditions, Mucor is capable of exciting alcoholic fermentation 1 66 MUCOR in a saccharine solution. When the hyphse are submerged in such a fluid they have been found to break up, forming rounded cells (Fig. 37, K1, K2), which not only resemble yeast-cells in appearance but are able like them to set up alcoholic fermentation. The aerial hyphse of Mucor exhibit in an interesting way what is known as heliotropism, i.e., a tendency to turn to- wards the light. This is very marked if a growth of the fungus is placed in a room lighted from one side : the long aerial hyphae all bend towards the window. This is due to the fact that growth is more rapid on the side of each hypha turned away from the light than on the more strongly illuminated aspect. LESSON XVI VAUCHERIA AND CAULERPA STAGNANT ponds, puddles, and other pieces of still, fresh water usually contain a quantity of green scum which in the undisturbed condition shows no distinction of parts to the naked eye, but appears like a homogeneous slime full of bubbles if exposed to sunlight. If a little of the scum is spread out in a saucer of water, it is seen to be com- posed of great numbers of loosely interwoven green filaments. There are many organisms which have this general naked- eye character, all of them belonging to the Algce, a group of plants which includes most of the smaller fresh-water weeds, and the vast majority of sea-weeds. One of these filamentous Alg^e, occurring in the form of dark-green thickly-matted threads is called Vaucheria. Besides occur- ring in water it is often found on the surface of moist soil, e.g., on the pots in conservatories. Examined microscopically the organism is found to consist of cylindrical filaments with rounded ends and occasionally branched (Fig. 39, A). Each filament has an outer cover- ing of cellulose (B, c.w} within which is protoplasm con- taining a vacuole so large that the protoplasm has the ths mm FIG. 39. — Vauckeria. A, tangled filaments of the living plant, showing mode of branching. B, extremity of a filament, showing cell-wall (c. w] and protoplasm with chromatophores (chr), and oil-drops (o). The scale above applies to this figure only. c1, immature sporangium (.<•/><;•) separated from the filament by a sep- tum ; C-, mature sporangium with the spore (.«/>) in the act of escaping ; c:;, free-swimming spore, showing cilia, colourless ectoplasm containing ASEXUAL REPRODUCTION 169 nuclei, and endoplasm containing the green chromatophores ; c4, the same at the commencement of germination. D1, early, and D2, later stages in the development of the gonads, the spermary to the left, the ovary to the right ; Da, the fully-formed spermary (spy) and ovary (ovy], each separated by a septum (sep) from the filament. D4, the ovary after dehiscence, showing the ovum (ov), with small detached portion of protoplasm ; D5, sperms ; D6, distal end of ripe ovary, showing sperms (sp] passing through the aperture towards the ovum (ov). D7, the gonads after fertilization, showing the oosperm (osp) still inclosed in the ovary and the dehisced spermary. E1, oosperm about to germinate : E", further stage in germination. (c1 and C3, after Strasburger ; c2 and C4, after Sachs ; D and E, after Pringsheim.) character of a membrane lining the cellulose coat. Numerous small nuclei occur in the protoplasm, as well as oil-globules (o), and small, close-set, ovoid chromatophores (chr) coloured with chlorophyll and containing starch. Thus a Vaucheria-plant, like a Mucor-plant, is comparable to a single multinucleate cell, extended in one dimension of space so as to take on the form of a filament. Various modes of asexual reproduction occur in different species of Vaucheria : of these we need only consider that which obtains in V. sessilis. In this species the end of a branch swells up (c1) and becomes divided off by a septum (sep), forming a sporangium (spg) in principle like that of Mucor, but differing in shape. The protoplasm of the sporangium does not divide, but separates itself from -the wall, and takes on the form of a single naked ovoidal spore (c3), formed of a colourless cortical layer containing nume- rous nuclei and giving off cilia arranged in pairs, and of an inner or medullary substance containing numerous chroma- tophores. The wall of the sporangium splits at its distal end (c2), and the contained spore (sp) escapes and swims freely in the water for some time by the vibration of its cilia (c3). After 170 VAUCHERIA AND CAULERPA a short active life it comes to rest, develops a cell-wall, and germinates (c4), i.e., gives out one or more processes which extend and take on the form of ordinary Vaucheria-filaments. so that in the present case, as in Mucor (p. 162), the de- velopment of the plant shows it to be a single immensely elongated multinucleate cell. In its mode of sexual reproduction Vaucheria differs strikingly not only from Mucor, but from all the organisms we have hitherto studied. The filaments are often found to bear small lateral pro- cesses arranged in pairs (D1), and each consisting of a little bud growing from the filament and quite continuous with it. These are the rudiments of the sexual reproductive organs or gonads. The shorter of the two becomes swollen and rounded (D2), and afterwards bluntly pointed (DS, ovy) : its protoplasm becomes divided from that of the filament, and a septum (o3, sep'} is formed between the two : the new cell thus constituted is the ovary.1 The longer of the two buds undergoes further elongation and becomes bent upon itself (o2) its distal portion is then divided off by a septum (DS, sep) forming a separate cell (spy), the spermary? Further changes take place which are quite different in the two organs. At the bluntly-pointed distal end of the ovary the cell-wall becomes gelatinized and the protoplasm protrudes through it as a small prominence which divides off and is lost (D4). The remainder of the protoplasm then separates from the wall of the ovary and becomes a naked cell, the ovum '•'' or egg-cell (D4, ov), which, by the gelatiniza- tion and subsequent disappearance of a portion of the 1 Usually called the oogoninm. Usually called the anthcriiliiun. I'Yequciitly called oosfhcrc. OOSPERM 171 wall of the ovary, is in free contact with the surrounding water. At the same time the protoplasm of the spermary under- goes multiple fission, becoming converted into numerous minute green bodies (D5), each with two flagella, called sperms}- These are liberated by the rupture of the spermary (DT) at its distal end, and swim freely in the water. Some of the sperms make their way to an ovary, and, as it has been expressed, seem to grope about for the aperture, which they finally pass through (DG), and are then seen moving actively in the space between the aperture and the colourless distal end of the ovum. One of them, and pro- bably only one, then attaches itself to the ovum and be- comes completely united with it, forming the oosperm^ a body which we must carefully distinguish from the ovum, since, while agreeing with the latter in form, and size, it differs in having incorporated with it the substance of a sperm. Almost immediately the oosperm (D", osp) surrounds itself with a cellulose wall, and numerous oil-globules are formed in its interior. It becomes detached from the ovary, and, after a period of rest, germinates (E1, E2) and forms a new Vaucheria plant. It is obvious that the fusion of the sperm with the ovum is a process of conjugation in which the conjugating bodies differ strikingly in form and size, one — the megagamete or ovum — being large, stationary, and more or less amoeboid ; the other — the microgamete or sperm — small, active, and flagellate. In other words, we have a more obvious case of sexual differentiation than was found to occur in Vorticella (p. 130) : the large inactive egg-cell which furnishes by far 1 Often called spermatozooids or antherozooids. • Often called oospore. 172 VAUCHERIA AND CAULERPA the greater portion of the material of the oosperm is the female gamete ; the small active sperm-cell, the function of which is probably (see Lesson XXIV.) to furnish additional nuclear material, is the male gamete. Similarly the oosperm is evidently a zygote, but a zygote formed by the union of the highly differentiated gametes, FlG. 40. — Caulerpa scalpelliformis (§ nat. size), showing the stem- like, root-like, and leaf-like portions of the unicellular plant. (After Hervey.) ovum and sperm, just as a zygospore (p. 164) is one formed by the union of equal-sized gametes. As we shall see, this form of conjugation — often distin- guished as fertilization — occurs in a large proportion of flowerless plants, such as mosses and ferns (Lessons XXVIII. and XXIX.), as well as in all animals but the very lowest. From lowly water-weeds up to ferns and club-mosses, and from sponges and polypes up to man, the process of sexual reproduction is essentially the same, consisting in the conju- gation of a microgamete or sperm with a megagamete or ovum, a zygote, the oosperm or unicellular embryo, being MAXIMUM DIFFERENTIATION 173 produced, which afterwards develops into an independent plant or animal of the new generation. It is a truly remark- able circumstance that what we may consider as the highest form of the sexual process should make its appearance so low down in the scale of life. The nutrition of Vaucheria is purely holophytic : its food consists of a watery solution of mineral salts and of carbon dioxide, the latter being split up, by the action of the chro- matophores, into carbon and oxygen. Mucor and Vaucheria are examples of unicellular plants which attain some complexity by elongation and branching. The maximum differentiation attainable in this way by a unicellular plant may be illustrated by a brief description of a sea-weed belonging to the genus Caulerpa. Caulerpa (Fig. 40) is commonly found in rock-pools between tide-marks, and has the form of a creeping stem from which root-like fibres are given off downwards and branched leaf-like organs upwards. These " leaves " may attain a length of 30 cm. (i ft.) or more. So that, on a superficial examination, Caulerpa appears to be as complex an organism as a moss (compare Fig. 40 with Fig. 81, A). But microscopical examination shows that the plant consists of a single continuous mass of vacuolated protoplasm, containing numerous nuclei and green chromatophores and covered by a continuous cell-wall. Large and complicated in form as it is, the whole plant is therefore nothing more than a single branched cell, or, as it may be expressed, a continuous mass of protoplasm in which no cellular structure has appeared. LESSON XVII THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS HITHERTO the words "animal' and "plant" have been either avoided altogether or used incidentally without any attempt at definition. We are now however in a position to consider in some detail the precise meaning of the two words, since in the last half-dozen lessons we have been dealing with several organisms which can be assigned without hesi- tation to one or other of the two great groups of living things. No one would dream of calling Paramcecium and Stylonychia plants, or Mucor and Vaucheria animals, and we may there- fore use these forms as a starting-point in an attempt to form a clear conception of what the words plant and animal really signify, and how far it is possible to place the lowly organisms described in the earlier lessons in either the vegetable or the animal kingdom. Let us consider, first of all, the chief points of resemblance and of difference between the indubitable animal Paramcecium on the one hand, and the two indubitable plants Mucor and Vaucheria on the other. In the first place, the essential constituents of all three organisms is protoplasm, in which is contained one or more nuclei. But in Paramcecium the protoplasm is invested COMPARISON OF TYPICAL FORMS 175 only by a delicate cuticle interrupted at the mouth and anus, while in Mucor and Vaucheria the outer layer is formed by a firm, continuous covering of cellulose. We thus have as the first morphological difference between our selected animal and vegetable organisms the absence of a cellulose cell-wall in the former and its presence in the latter. This is a fundamental distinction, and applies equally well to the higher forms. The constituent cells of plants are in nearly all cases covered with a cellulose coat (p. 60), while there is no case among the higher animals of cells being so invested. Next, let us take a physiological character. In all three organisms there is constant waste of substance which has to be made good by the conversion of food material into proto- plasm : in other words, constructive and destructive meta- bolism are continually being carried on. But when we come to the nature of the food and the mode of its reception, we meet at once with a very fundamental difference. In Para- mcecium the food consists of living organisms taken whole into the interior of the body, and the digestion of this solid proteinaceous food is the necessary prelude to constructive metabolism. In Vaucheria the food consists of a watery solution of carbon dioxide and mineral salts — i.e. it is liquid and inorganic, its nitrogen being in the form of nitrates or of simple ammonia compounds. Mucor, like Paramcecium, contains no chlorophyll, and is therefore unable to use carbon dioxide as a food : like Vaucheria, it is prevented by its continuous cellulose investment from ingesting solid food, and is dependent upon an aqueous solution. It takes its carbon in the form of sugar or some such compound, while it can make use of nitrogen either in the simple form of a nitrate or an ammonia salt, or in the complex form of proteids or peptones. 1 76 CHARACTERS OF ANIMALS AND PLANTS In this case also our selected organisms agree with animals and plants generally. Animals, with the exception of some internal parasites, ingest solid food, and they must all have their nitrogen supplied in the form of proteids, being unable to build up their protoplasm from simpler compounds. Plants take their food in the form of a watery solution : those which possess chlorophyll take their carbon in the form of carbon dioxide and their nitrogen in that of a nitrate or ammonia salt : those devoid of chlorophyll cannot make use of carbon dioxide as a food, and are able to obtain nitrogen either from simple salts or from proteids. Chloro- phyll-less plants are therefore nourished partly like green plants, partly like animals. This difference in the character of the food is connected with a morphological difference. Animals have, as a rule, an ingestive aperture or mouth, and some kind of digestive cavity, either permanent (stomach) or temporary (food- vacuole). In plants neither of these structures exists. Another difference which was referred to at length in an early lesson (p. 32), is not strictly one between plants and animals, but between organisms with and organisms without chlorophyll. It is that in green plants the nutritive processes result in deoxidation, more oxygen being given out than is taken in : while in animals and not-green plants the precise contrary is the case. There is also a difference in the method of excretion. In Paramcecium there is a special structure, the contractile vacuole, which collects the superfluous water taken in with the food and expels it, doubtless along with nitrogenous and other waste matters. In Vaucheria and Mucor there is no contractile vacuole, and excretion is simply performed by diffusion from the general surface of the organism into the surrounding medium. DEFINITION 177 This character also is of some general importance. The large majority of animals possess a special organ of excre- tion, plants have nothing of the kind. Another difference has to do with the general form of the organism. Paramoecium has a certain definite and constant shape, and when once formed produces no new parts. Vaucheria and Mucor are constantly forming new branches, so that their shape is always changing and their growth can never be said to be complete. Finally, we have what is perhaps the most obvious and striking distinction of all. Paramoecium possesses in a con- spicuous degree the power of automatic movement ; in both Mucor and Vaucheria the organism, as a whole, exhibits no automatism but only the slow movements of growth. The spores and sperms of Vaucheria are, however, actively motile. % Thus, taking Paramcecium as a type of animals, and Mucor and Vaucheria as types of plants, we may frame the following definitions : — Animals are organisms of fixed and definite form, in which the cell-body is not covered with a cellulose wall. They ingest solid proteinaceous food, their nutritive pro- cesses result in oxidation, they have a definite organ of excretion, and are capable of automatic movement. Plants are organisms of constantly varying form in which the cell-body is surrounded by a cellulose wall ; they cannot ingest solid food, but are nourished by a watery solution of nutrient materials. If chlorophyll is present the carbon dioxide of the air serves as a source of carbon, nitrogen is obtained from simple salts, and the nutritive processes result in deoxidation ; if chlorophyll is absent carbon is obtained from sugar or some similar compound, nitrogen, N 178 CHARACTERS OF ANIMALS AND PLANTS either from simple salts or from proteids, and the process of nutrition is one of oxidation. There is no special excretory organ, and, except in the case of certain reproductive bodies there is no locomotion as a rule. Let us now apply these definitions to the simple forms described in the first eight lessons, and see how far they will help us in placing those organisms in one or other of the two " kingdoms " into which living things are divided. Amoeba has a cell-wall, probably nitrogenous, in the resting condition : it ingests solid proteids, its nutrition being therefore holozoic : it has a contractile vacuole : and it performs amoeboid movements. It may therefore be safely considered as an animal. Haematococcus has a cellulose wall : it contains chloro- phyll and its nutrition is purely holophytic : a contractile vacuole is present in H. lacustris but absent in H. pluvialis : and its movements are ciliary. Euglena has a cellulose wall in the encysted state : in virtue of its chlorophyll it is nourished by the absorption of carbon dioxide and mineral salts, but it can also ingest solid food through a special mouth and gullet : it has a contractile vacuole, and performs both euglenoid and ciliary move- ments. In both these organisms we evidently have conflicting characters : the cellulose wall and holophytic nutrition would place them both among plants, while from the con- tractile vacuole and active movements of both genera and from the holozoic nutrition of Euglena we should group them with animals. That the difficulty is by no means easily overcome may be seen from the fact that both genera are claimed at the present day both by zoologists and by DISCUSSION OF DOUBTFUL FORMS 179 botanists. For instance, Prof. Huxley considers Hasma- tococcus as a plant, and expresses doubts about Euglena : Mr. Saville Kent ranks Haematococcus as a plant and Euglena as an animal ; Prof. Sachs and Mr. Thiselton Dyer place both genera in the vegetable kingdom : while Profs. Ray Lankester and Biitschli group them both among animals. In Heteromita the only cell-wall is the delicate cuticle wrhich in the zygote is firm enough to hold the spores up to the moment of their escape : food is taken exclusively by absorption and nutrition is wholly saprophytic : there is a contractile vacuole, and the movements are ciliary. Here again the characters are conflicting : the probable absence of cellulose, the contractile vacuole and the cilia all have an "animal" look, but the mode of nutrition is that of a fungus. In Protomyxa there is a decided preponderance of animal characteristics — ingestion 01 living prey, and both amoeboid and ciliary movements. There is no chlorophyll, and the composition of the cell-wall is not known. In the Mycetozoa, the life history of which so closely resembles that of Protomyxa, the cyst in the resting stage consists of cellulose, and so does the cell-wall of the spore : nutrition is holozoic, a contractile vacuole is present in the flagellulae, and both amoeboid and ciliary movements are performed. Here again we have a puzzling combination of animal and vegetable characters, and as a consequence we find these organisms included among plants — under the name of Myxomycetes or " slime-fungi ' -by Sachs and Goebel, while De Bary, Biitschli, and Ray Lankester place them in the animal kingdom. In Saccharomyces there is a clear preponderance of vegetable characters. The cell-wall consists of cellulose, N 2 i8o CHARACTERS OF ANIMALS AND PLANTS nutrition takes place by absorption and proteids are not essen- tial, there is no contractile vacuole, and no motile phase. Lastly, in the Bacteria the cell-wall is composed of cellu- lose, nutrition is saprophytic, there is no contractile vacuole, and the movements are ciliary. So that in all the characters named, save in the presence of cellulose and the absence of a contractile vacuole, the Bacteria agree with Heteromita, yet they are universally — except by Prof. Glaus — placed among plants, while Heteromita is as constantly included among animals. We see then that while it is quite easy to divide the higher organisms into the two distinct groups of plants and animals, any such separation is by no means easy in the case of the lowest forms of life. It was in recognition of this fact that Haeckel proposed many years ago to institute a third "kingdom," called Protista, to include all unicellular organ- isms. Although open to many objections in practice, there is a great deal to be said for the proposal. From the strictly scientific point of view it is quite as justifiable to make three subdivisions of living things as two : the line between animals and plants is quite as arbitrary as that between protists and plants or between protists and animals, and no more so : the chief objection to the change is that it doubles the difficulties by making two artificial boundaries instead of one. The important point for the student to recognize is that these boundaries are artificial, and that there are no scientific frontiers in Nature. As in the liquefaction of gases there is a " critical point " at which the substance under experiment is neither gaseous nor liquid : as in a mountainous country it is impossible to say where mountain ends and valley begins : as in the development of an animal it is futile to argue about the exact period when, for instance, the egg EVOLUTION OF THE TWO KINGDOMS 181 becomes a tadpole or the tadpole a frog : so in the case under discussion. The distinction between the higher plants and animals is perfectly sharp and obvious, but when the two groups are traced downwards they are found gradually to merge, as it were, into an assemblage of organ- isms which partake of the characters of both kingdoms, and cannot without a certain violence be either included in or excluded from either. Where any given " protist " has to be classified the case must be decided on its individual merits : the organism must be compared in detail with all those which resemble it closely in structure, physiology, and life-history : and then a balance must be struck and the doubtful form placed in the kingdom with which it has, on the whole, most points in common. It will no doubt occur to the reader that, on the theory of evolution, we may account for the fact of the animal and vegetable kingdoms being related to one another like two trees united at the root, by the hypothesis that the earliest organisms were protists, and that from them animals and plants were evolved along divergent lines of descent. LESSON XVIII PENICILLIUM AND AGARICUS ONE of the commonest and most familiar of the lower organisms is the " green mould ': which so quickly covers with a thick sage-green growth any organic substances ex- posed to damp, such as paste, jam, cheese, leather, &c. This mould is a plant belonging, like Mucor, to the group of Fungi, and is called Penicillium glaucum. Examined with the naked eye a growth of Penicillium is seen to have a powdery appearance, and if the finger is passed over it a quantity of extremely fine dust of a sage- green colour comes away. This dust consists, as we shall see, of the spores of Penicillium. The best way to study the plant is to sow some of the spores in a saucer of Pasteur's solution by drawing a needle or brush over a growth of the mould and stirring it round in the fluid. It is as well to study the naked-eye appearance first. If the quantity of spores taken is not too large and they are sufficiently well diffused through the fluid, little or no trace of them will be apparent to the naked eye. After a few days, however, extremely small white dots appear on the surface of the fluid : these increase in size and are seen, especially by the aid of a hand-magnifier, to consist of little NAKED-EYE CHARACTERS 183 • discs, circular or nearly so in outline, and distinctly thicker in the centre than towards the edge : they float on the fluid so that their upper surfaces are dry. Each of these patches is a young Penicillium-growth, formed, as will be seen hereafter, by the germination of a group of spores. As the growths are examined day by day they are found to increase steadily in size, and as they do so to become thicker and thicker in the middle : their growth is evidently centrifugal. The thicker central portion acquires a fluffy appearance, and, by the time the growth has attained a diameter of about 4 or 5 mm., a further conspicuous change takes place : the centre of the patch acquires a pale blue tint, the circumference still remaining pure white. When the diameter has increased to about 6-10 mm. the colour of the centre gradually changes to dull sage-green : around this is a ring of light blue, and finally an outer circle of white. In all probability some of the growths, several of which will most likely occur in the saucer, will by this time be found to have come together by their edges : they then become completely interwoven, their original boundaries remaining evident for some time by their wrhite tint. Sooner or later, however, the white is replaced by blue and the blue by sage- green, until the whole surface of the fluid is covered by a single growth of a uniform green colour. Even when they are not more than 2 — 3 mm. in diameter the growths are strong enough to be lifted up from the fluid, and are easily seen under a low power to be formed of a tough, felt-like substance, the mycelium, Fig. 41 A, (my), from the upper surface of which delicate threads, the aerial hyphcz (a. hy.) grow vertically upwards into the air, while from its lower surface similar but shorter threads, the sub- merged hyphen (s. /iy.) hang vertically downwards into the fluid. B FIG. 41. — Penicillium glaucitm. A Diagrammatic vertical section of a young growth (X 5)> showing mycelium (my], submerged hyphae (s. hy], and aerial hyphse (a. hy). B, group of spores : I, before commencement of germination ; 2, after inhibition of fluid : the remaining three have begun to germinate. C, very young mycelium formed by a small group of germinating spores. A LINEAR AGGREGATE 185 D, more advanced mycelium : the hypha; have increased in length and begun to branch, and septa (sep) have appeared. E, germinating spore (sp) very highly magnified, sending out one short and one long hypha, the latter with a short lateral branch and several septa (sep). Both spore and hyphae contain vacuoles (vac] in their protoplasm. F^F4, development of the spore-bearing brushes by repeated branch- ing of an aerial hypha : the short terminal branches or sterigmata are already being constricted to form spores. F5, a fully-developed brush with a row of spores developed from each sterigma (stg). F6, a single sterigma (stg) with its spores (sp). F7, an over-ripe brush in which the structure is obscured by spores which have dropped from the sterigmata. B-D, F^F5, and F7 X 150 : F6 X 200 : E X 500. As long as the growths are white or blue in colour no powder can be detached by touching the aerial hyphae, showing that the spores are not yet fully formed, but as soon as the permanent green hue is attained the slightest touch is sufficient to detach large quantities of spores. A bit of the felt-like mycelium is easily teased out or torn asunder with two needles, and is then found, like actual felt, to be formed of a close interlacement of delicate threads (D). These are the mycelial hyp/ice : they are regularly cylindrical, about TjQ- mm. in diameter, frequently branched, and differ in an important particular from the somewhat similar hyphae of Mucor (p. 159). The protoplasm is not continuous, but is interrupted at regular intervals by transverse partitions or septa (D, E, sep). In other words, a hypha of Penicillium is normally, what a hypha of Mucor becomes under un- favourable conditions (p. 160), multicellular^ the septa dividing it into separate portions, each of which is morphologically comparable to a single yeast-cell. Penicillium shows therefore a very important advance in structure over the organisms hitherto considered. While in these latter the entire organism is a single cell ; in Peni- 1 86 PENICILLIUM AND AGARTCUS cellium it is a cell-aggregate — an accumulation of numerous cells all in organic connection with one another. As the cells are arranged in a single longitudinal series, Penicillium is an example of a linear aggregate. Each cell is surrounded, as already described, by a wall of cellulose : its protoplasm is more or less vacuolated (E, vac), sometimes so much so as to form a mere thin layer within the cell-wall, the whole interior of the cell being occupied by one large vacuole. Recently, by staining with logwood, numerous nuclei have been found, so that the Penicillium cell, like an Oxytricha (p. 118), or a filament of Mucor or Vaucheria, is multinucleate. The submerged hyphas have the same structure, but it is easier to find their actual ends than those of the mycelial hyphae. The free extremity tapers to a blunt point where the cellulose wall is thinner than elsewhere (see E). « The aerial hyphae from the youngest (white) part of a growth consist of unbranched filaments, but taken from a part which is just beginning to turn blue they are found to have a very characteristic appearance (r1 — F4). Each sends off from a distal or upper end a larger or smaller number of branches which remain short and grow parallel to one another : the primary branches (p1, F2) form secondary ones (FS), and the secondary tertiary (F4), so that the hypha finally assumes the appearance of a little brush or pencil, or more accurately, of a minute cactus, with thick-set forking branches. The ultimate or distal branches are short cells called sterig- mata (r5, stg). Next, the ends of the sterigmata become constricted, exactly as if a thread were tied round them and gradually tightened (p1 F6), the result being to separate the distal end of the sterigma as a globular daughter-cell, in very much the same way as a bud is separated in Saccharomyces (p. 72). GERMINATION OF SPORES 187 In this way a spore is produced. The process is repeated, the end of the sterigma is constricted again and a new spore formed, the old one being pushed further onwards. By a continual repetition of the same process a longitudinal row of spores is formed (r5, F°), of which the proximal or lower one is the youngest, the distal or upper one the eldest. The spores grow for some time after their formation, and are therefore found to become larger and larger in passing from the proximal to the distal end of the chain (FG). Sooner or later they lose their connection with each other, become detached, and fall, covering the whole growth with a fine dust which readily adheres to all parts owing to the some- what sticky character of the spores. In this stage it is by no means easy to make out the structure of the brushes, since they are quite obscured by the number of spores adhering to them (FT). It is at the period of complete formation of the spores that the growth turns green. The colour is not due to the presence of chlorophyll. Under a high power the spores appear quite colourless, whereas a cell of the same size covered with chlorophyll would appear bright green. The germination of the spores can be readily studied by sowing them in a drop of Pasteur's solution in a moist chamber (Fig. 38, p. 161). The spores, several of which usually adhere together, are at first clear and bright (B1) : soon they swell considerably, and the protoplasm becomes granular and vacuolated (B2) : in this stage they are hardly distinguishable from yeast-cells (compare Fig. 13, p. 71). Then one or more buds spring from each and elongate into hyphse (B, c), just as in Mucor. But the difference between the two moulds is soon apparent : by the time a hypha has grown to a length equal to about six or eight times its own diameter, the pro- toplasm in it divides transversely and a cellulose septum is i88 PENICILLIUM AND AGARICUS formed (D, E, sef) dividing the young hypha into two cells (compare Fig. 37, H, p. 158). The distal cell then elongates and divides again, and in this way the hyphse are, almost from the first, divided into cells of approximately equal length. The mode of growth of the distal or apical cell of a hypha is probably as follows : — the free end tapers slightly (E) and the cellulose wall thins out as it approaches the apex. The protoplasm performing constructive more rapidly than de- structive metabolism increases in volume, and its tendency is to grow in all directions : as, however, the cellulose mem- brane surrounding it is thinner at the apex than elsewhere, it naturally, on the principle of least resistance, extends in that direction, thus increasing the length of the cell without adding to its thickness. Thus the growth of a hypha of Penicillium is apical, i.e. takes place only at the distal end, the cells once formed ceasing to grow. Thus also the oldest cells are those nearest the original spore from which the hypha sprang, the youngest those further removed from it. A process which has been described as sexual, sometimes, but appa- rently very rarely, occurs in Penicillium, and is said to consist essentially in the conjugation of two gametes having the form of twisted hyphse, and the subsequent development of spores in the resulting branched zygote. But as the details of the prdcess are complicated and its sexual character is doubtful, it is considered best to do no more than call attention to it. The student is referred to Brefeld's original account of the process in the Quarterly Journal of Microscopical Science, vol. xv., p. 342. The so-called sexual reproduction of the closely-allied Eurotium is described in Huxley and Martin's Elementary Biology (new edition), p. 419, and figured in Howes's Atlas of Elementary Biology ; pi. xix., figs, xxvi and xxvii. The nutrition of Penicillium is essentially like that of Mucor (p. 165). But, as it has been remarked, "it is often content with the poorest food which would be too bad for higher fungi. It lives in the human ear : it does not shun cast-off GENERAL FEATURES 189 clothes, damp boots, or dried-up ink. Sometimes it contents itself with a solution of sugar with a very little [nitrogenous] organic matter, at other times it appears as if it preferred the purest solution of a salt with only a trace of organic matter. It will even tolerate the hurtful influence of poisonous solutions of copper and arsenious acid." It flourishes best in a solution of peptones and sugar. This eclecticism in matters of diet is one obvious ex- planation of the universal occurrence of Penicillium ; another is the extraordinary vitality of the spores. They will ger- minate at any temperature between 1-5° and 43° C., the optimum being about 22° C. They are not killed by a dry heat of 1 08° C., and some will even survive a temperature of 120°. And lastly, they will germinate after being kept for two years. We have seen that the form of a Penicillium growth is ir- regular, and is determined by the surface on which it grows. There are, however, certain fungi which are quite constant and determinate both in form and size, and are yet found on analysis to be formed exclusively of interlaced hyphae, that is, to belong to the type of linear aggregates. Among the most striking of these are the mushrooms and toad- stools. A mushroom (Agancus) consists of a stout vertical stalk (Fig. 42, A, st\ on the upper or distal end of which is borne an umbrella-like disc or p ileus (p). The lower or proximal end of the stalk is in connection with an underground mycelium (my), from which it springs. On the underside of the pileus are numerous radiating vertical plates or lamellae (I) extending a part or the whole of the distance from the circumference of the pileus to the stalk. In the common edible mushroom (Agariats cam- PENICILLIUM AND AGARICUS pestris) these lamellae are pink in young specimens, and afterwards become dark brown. The mushroom is too tough to be readily teased out like FIG. 42. — Agarictts cainpestris. A, Diagrammatic vertical section, showing the stalk (sf) springing from a mycelium (my), and expanding into the pileus (/), on the under side of which are the radiating lamella. B, transverse vertical section of a lamella, showing the hyphce (hy) turning outwards to form the layer of club-shaped cells (a) from which the sterigmata spring. c, one of the club-shaped cells (a), highly magnified, showing its two sterigmata (sty), each bearing a spore (sp). (B and c after Sachs.) the mycelium of Penicillium, and its structure is best in- vestigated by cutting thin sections of various parts and examining them under a high power. FORMATION OF SPORES 191 Such sections show the whole mushroom to be composed of immense numbers of closely interwoven branched hyphae (B), divided by numerous septa into cells. In the stalk the hyphae take a longitudinal direction ; in the pileus they turn outwards, passing from the centre to the circumference, and finally send branches downwards to form the lamellae. Fre- quently the hyphae are so closely packed as to be hardly distinguishable one from another. At the surfaces of the lamellae the hyphae turn outwards, so that their ends are perpendicular to the free surfaces of those plates. Their terminal cells become dilated or club- shaped (B, c, #), and give off two small branches or sterig- mata (c, stg\ the ends of which swell up and become constricted off as spores (sp). These fall on the ground and germinate, forming a mycelium from which more or fewer mushrooms are in due course produced. Thus in point of structure a mushroom bears much the same relation to Penicillium as Caulerpa (p. 173) bears to Vaucheria. Caulerpa shows the extreme development of which a single branched cell is capable, the mushroom how complicated in structure and definite in form a simple linear aggregate may become. LESSON XIX SPIROGYRA AMONGST the numerous weeds which form a green scum in stagnant ponds and slowly-flowing streams, one, called Spiro- gyra, is perhaps the commonest. It is recognized at once under a low power by the long delicate green filaments of which it is composed being marked with a regular green spiral band. Examined under the microscope the filaments are seen to be, like the hyphae of Penicillium, linear aggregates, that is, to be composed of a single row of cells arranged end to end. But in Penicillium the hyphae are frequently branched, and it is always possible in an entire hypha to distinguish the slightly tapering distal end from the proximal end, which springs either from another hypha or from a spore. In Spirogyra the filaments do not branch, and there is no distinction between their opposite ends. The cells of which the filaments are composed (Fig. 43, A) are cylindrical, covered with a cellulose cell-wall (c. iv), and separated from adjacent cells by septa (sep} of the same substance. The protoplasmic cell-body presents certain characteristic peculiarities. It has been noticed in more than one instance that in the tram;*- c3 FIG. 43. — Spirogyra. A, small portion of a living filament, showing a single cell, with cell- wall (c. TV), septa (sep) separating it from adjacent cells, peripheral layer of protoplasm (plsm) connected by threads with a central mass contain- O 194 SPIROGYRA ing the nucleus (««), two spiral chromatophores (chr), and pyrenoids (pyr). B1, B2. middle portion of a cell, showing two stages in binary fission. c, four stages in dioecious conjugation : in c1 the gonads (gon1, gon^} are connected by short processes of their adjacent sides : in c- the active or male gamete (gam1} has separated from the wall of the gonad (gon1) preparatory to passing across the connecting bridge to the stationary or female gamete (gam1), which has not yet separated from its containing gonad (gon-) : in c3 the female gamete (gV *?*..•' ^'-V^^ . .- - ^$'%$'h$$l'{i $ '> i'.'f ^ '• ;,i) Hi &ife\!^.' : ' • •:•";,'' ^llil; &yW --,;/., • ; FIG. 59. — A, Porpita pacified (nat. size), from beneath, showing disc- like stem surrounded by tentacles (/), a single functional hydranth (/$j), and numerous mouthless hydranths (/y1). B, vertical section of P. mediterranea, showing the relative positions of the functional (hy) and mouthless (Ay1) hydranths, the tentacles (/), and the chambered shell. (A after Duperrey ; B from Huxley after Kolliker.) logical and physiological differentiation are thus carried much further than in such a form as Bougainvillea. Porpita is another free-swimming Hydroid, presenting at INDIVIDUATION 251 first sight no resemblance whatever to Diphyes. It has much the appearance of a flattened medusa (Fig. 59), consisting of a circular disc, slightly convex above and concave below, bearing round its edge a number of close-set tentacles, and on its under side a central tubular organ (hy) with a ter- minal mouth, like the manubrium of a medusa, surrounded by a great number of structures like hollow tentacles (hy). The discoid body is supported by a sort of shell having the consistency of cartilage and divided into chambers which contain air (B). Accurate examination shows that the manubrium-like body (hy) on the under surface is a hydranth, that the short, hollow, tentacle-like bodies (hy1) surrounding it are mouthless hydranths, and that the disc represents the common stem of Diphyes or Bougainvillea. So that Porpita is not what it appears at first sight, a single individual, like a Medusa or a Hydra, but a colony in which the constituent zooids have become so modified in accordance with an extreme division of physiological labour, that the entire colony has the character of a single physiological individual. It was pointed out in the previous lesson (p. 229) that Hydra, while metaphorically the equivalent of an indefinite number of unicellular organisms, was yet physiologically a single individual, its constituent cells being so differentiated and combined as to form one whole. A further stage in this same process of individuation is seen in Porpita, in which not cells but zooids, each the morphological equivalent of an entire Hydra, are combined and differentiated so as to form a colony which, from the physiological point of view, has the characters of a single individual. LESSON XXIV SPERMATOGENESIS AND OOGENESIS. THE MATURATION AND IMPREGNATION OF THE OVUM. THE CONNECTION BE- TWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS IN the preceding lessons it has more than once been stated that sperms arise from ordinary undifferentiated cells in the spermary, and that ova are produced by the enlargement of similar cells in the ovary. Fertilization has also been de- scribed as the conjugation or fusion of ovum and sperm. We have now to consider in greater detail what is known as to the precise mode of development of sperms (spermatogenesis) and of ova (oogenesis), as well as the exact steps of the pro- cess by which an oosperm or unicellular embryo is formed by the union of the two sexual elements. The following description applies to animals in which all these processes have been worked out with greater detail than in plants. Both ovary and spermary are at first composed of cells of the ordinary kind, the primitive sex-cells, and it is only by the further development of these that the sex of the gonad is determined. In the spermary the sex-cells undergo repeated fission, forming a group of small cells, each of which (Fig. 60, A) ORIGIN OF HEAD AND TAIL OF SPERM 253 becomes differentiated into a sperm in the following way. A delicate filament of protoplasm (B, /) grows out on one side, and at the same time the nucleus (nit) retreats to the opposite extremity (c — F). Next the protoplasm draws itself away, as it were, from the nucleus and forms a kind of swel- ling (G and H, x) round the base of the filament, which in- creases considerably in length. The filament is now recog- FIG. 60. — Spermatogenesis in the Rat. A, Sperm-cell : nu, the nucleus. B, first indication of the tail (/) as a protoplasmic filament. C-F, further growth of the tail, and retreat of the nucleus (mt) to the opposite end of the cell to form the head. G, H, separation of protoplasmic globule (it), afterwards cast off. I, fully formed sperm. (After H. H. Brown.) nizable as the flagellum or tail of the sperm, the nucleus as its head. The lump of protoplasm (x} at the junction of head and tail assumes the form of a globule attached only by one side (H) : this gradually separates itself from the now fully-developed sperm, and is finally completely detached (i). Thus a sperm is a true cell, the nucleus being represented by the head, and the cell-body by the tail. But it is to be 254 SPERMATOGENESIS AND OOGENESIS noted that the whole cell is not used up in the formation of the sperm, a part of it being cast off in the form of a protoplasmic globule as the mature form is assumed. As already stated, the ova arise from primitive sex-cells, precisely resembling those which give rise to sperms. Very soon however the behaviour of the sex-cells in the ovary distinguishes them from the similar cells of the spermary. Instead of actively dividing they remain passive and increase, often enormously, in size, by the absorption of nutriment from surrounding parts. Sometimes this nutriment is simply taken in by osmosis, in other cases the growing ovum actually ingests neighbouring cells after the manner of an Amoeba. Thus in the developing egg the processes of constructive are vastly in excess of those of destructive metabolism. We saw in the second lesson (p. 33) that the products of destructive metabolism might take the form either of waste products which are got rid of, or of plastic products which are stored up as an integral part of the organism. In the developing egg, in addition to increase in the bulk of the protoplasm itself, a formation of plastic products usually goes on to an immense extent. In plants the stored-up materials may take the form of starch, as in Nitella (p. 214), of oil, or of proteid substance : in animals it consists of rounded or angular grains of proteid material, known as yolk-granules. These being deposited, like plums in a pudding, in the protoplasm, have the effect of rendering the fully-framed egg opaque, so that its structure can often be made out only in sections. When the quantity of yolk is very great the ovum may attain a comparatively enormous size, as for instance in birds, in which as already mentioned (p. 69) the " yolk " is simply an immense egg-cell. When fully formed the typical animal ovum (Fig. 61) STRUCTURE OF THE OVUM 255 consists of a more or less globular mass of protoplasm, generally exhibiting a reticular structure and enclosing a larger or smaller quantity of yolk-granules. Surrounding the cell-body is usually a cell-wall or cuticle, often of considerable thickness and known as the vitelline membrane. The nucleus is large and has the usual constituents (p. 62), nuclear mem- brane, achromatin, and chromatin. As a rule the chromatin or a portion of it is aggregated in the form of a very definite FIG. 6 1. — Ovum of a Sea-urchin (Toxopneustes lividtts], showing the radially-striated cell-wall (vitelline membrane), the protoplasm contain- ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its protoplasmic network, and a large nucleolus (germinal spot). (From Balfour after Hertwig.) nucleolus, which is often known as the germinal spot, the entire nucleus being called the germinal vesicle. Such a fully-formed ovum is, however, incapable of being fertilized or of developing into an embryo : before it is ripe for conjugation with a sperm or able to undergo the first stages of yolk division it has to go through a process known as the maturation of the egg. B fiAro7/i-. Ccelomic epithelium n "S. \^-/ V^*^ A V_/ J. J. i J. ^^ V_. \J J. L JL 1 V^ -1 J. V-l ( (parietal layer). Splanchnic ( Ccelomic epithelium Mesoderm . . (rudimentary) layer ( (visceral layer). Endoderm Enteric epithelium. 1 In the majority of the higher animals there is a layer of muscle between the enteric and coelomic epithelia : in such cases the body-wall and enteric canal consist of the same layers but in reverse order, the coloemic epithelium being internal in the one, external in the other. GENERAL STRUCTURE 275 Strictly speaking, this comparison does not hold good of the anterior and posterior ends of the worm : at both mouth and anus the deric passes insensibly into the enteric epithe- lium, and the study of development shows (p. 294) that the cells lining both the anterior and posterior ends of the canal are, as indicated in the diagram (Fig. 69, B), ectodermal. For this reason the terms deric and enteric epithelium are not mere synomyms of ectoderm and endoderm respectively. It is important that the student should, before reading further, understand clearly the general composition of a triploblastic animal as typified by Polygordius, which may be summarized as follows : It consists of two tubes formed of epithelial cells, one within and parallel to the other, the two being continuous at either end of the body where the inner tube (enteric epithelium) is in free communication with the exterior ; the outer tube (deric epithelium) is lined by a layer of muscle plates within which is a thin layer of ccelornic epithelium, the three together forming the body- wall ; the inner tube (enteric epithelium) is covered ex- ternally by a layer of ccelomic epithelium which forms with it the enteric canal ; lastly, the body-wall and enteric canal are separated by a considerable space, the ccelome. The enteric canal is not, as might be supposed from the foregoing description, connected with the body-wall only at the mouth and anus, but is supported in a peculiar and somewhat complicated way. In the first place there are thin vertical plates, the dorsal and ventral mesenteries (Fig. 69, A and c, D. Mes, V. Mes\ which extend longitudinally from the dorsal and ventral surfaces of the canal to the body wall, dividing the coelome into right and left halves. The structure of the mesenteries is seen in a transverse section (Fig. 69, c, and Fig. 72, A) which shows that at the middle T 2 276 POLYGORDIUS dorsal line the parietal layer of coelomic epithelium becomes deflected downwards forming a two-layered membrane, the dorsal mesentery : the two layers of this on reaching the enteric canal diverge and pass one on each side of it, form- ing the visceral layer of coelomic epithelium : uniting again below the canal they are continued downwards as the ventral mesentery, and on reaching the body-wall diverge once more to join the parietal layer. Thus the mesenteries are simply formed of a double layer of ccelomic epithelium, continuous on the one hand with the parietal and on the other with the visceral layer of that membrane. Besides the mesenteries the canal is supported by trans- verse vertical partitions or septa (Fig. 69, A and B, sef) which extend right across the body-cavity each being perforated by the canal. The septa are regularly arranged and correspond with the external grooves by which the body is divided into metameres. Thus the transverse or metameric segmen- tation affects the coelome as well as the body-wall. Each septum is composed of a sheet of muscle covered on both sides with coelomic epithelium (B sep). Where the septa come in contact with the enteric canal the latter is more or less definitely constricted so as to pre- sent a beaded appearance (A and B) : thus we have segmen- tation of the canal as well as of the body-wall and ccelome. The digestive canal, moreover, is not a simple tube of even calibre throughout, but is divisible into four portions. The first or pharynx (PJi) is very short, and can be pro- truded during feeding : the second, called the gullet or oesophagus (ats\ is confined to the peristomium and is distin- guished by its thick walls and comparatively great diameter : the third or intestine (Int) extends from the first metamere to the last — i.e., from the segment immediately following the peristomium to that immediately preceding the anal DIGESTION 277 segment ; it is laterally compressed so as to have an elongated form in cross section (c, and Fig. 72, A) : the fourth portion or rectum (Ret) is confined to the anal seg- ment ; it is somewhat dilated and is not laterally compressed. The epithelium of the intestine is as indicated in the diagram (B) endodermal ; that of the remaining division of the canal is ectodermal. The large majority of the cells in all parts of the canal are ciliated. The cells of the enteric canal and especially those of the gullet are very granular, and like the endoderm cells of the hypstome of Hydra (p. 228) are to be considered as gland cells. They doubtless secrete a digestive juice which, mixing with the various substances taken in by the mouth, dissolves the proteids and other digestible parts, so as to allow of their absorption. There is no evidence of intra- cellular digestion such as occurs in Hydra (p. 229), and it is very probable that the process is purely extra-cellular or enteric, the food being dissolved and rendered diffusible entirely in the cavity of the canal. By the movements of the canal — caused partly by the general movements of the body and partly by the contraction of the muscles of the septa — aided by the action of the cilia, the contents are gradually forced backwards and the sand and other indi- gestible matters are expelled at the anus. The coelome is filled with a colourless, transparent ccslomic fluid in which are suspended minute, irregular, colourless bodies, as well as oval bodies containing yellow granules. From the analogy of the higher animals one would expect these to be leucocytes (p. 56), but their cellular nature has not been proved. The function of the ccelomic fluid is probably to distribute the digested food in the enteric canal to all parts of the 278 POLYGORDIUS body. In Hydra, where the lining wall of the digestive cavity is in direct contact with the simple wall of the body, the products of digestion can pass at once by diffusion from endoderm to ectoderm, but in the present case a means of communication is wanted between the enteric epithelium and the comparatively complex and distant body-wall. The peptones and other products of digestion diffuse through the enteric epithelium into the ccelomic fluid and by the con- tinual movement of the latter — due to the contractions of the body-wall — are distributed to all parts. Thus the external epithelium and the muscles, as well as the nervous system and reproductive organs not yet described, are wholly dependent upon the enteric epithelium for their supply of nutriment. We have now to deal with structures which we find for the first time in Polygordius, namely blood-vessels. Lying in the thickness of the dorsal mesentery is a delicate tube (Fig. 69, A and c, D. V] passing along almost the whole length of the body : this is the dorsal vessel. A similar ventral vessel (V.V) is contained in the ventral mesentery1 and the two are placed in communication with one another in every segment by a pair of commissural vessels (A, com.v.} which spring right and left from the dorsal trunk, pass downwards in or close behind the corresponding septum, following the contour of body-wall, and finally open into the ventral vessel. Each commissural vessel, at about the middle of its length, gives off a recurrent vessel (R.V.) which passes backwards and 1 The statement that the dorsal and ventral vessels lie in the thickness of the mesenteries requires qualification. As a matter of fact, these vessels are simply spaces formed by the divergence of the two layers of epithelium composing the mesentery (Fig. 69, C, and Fig. 72, A) : only their anterior ends have proper walls. HEMOGLOBIN 279 ends blindly. The anterior pair of commissural vessels lie in the peristomium and have an oblique direction, one on each side of the gullet. The whole of these vessels form a single closed vascular system, there being no communication between them and any of the remaining cavities of the body. The vascular system contains a fluid, the blood, which varies in colour in the different species of Polygordius, being either colourless, red, green, or yellow. In one species cor- puscles (? leucocytes) have been found in it. The function of the blood has not been actually proved in Polygordius, but is well known in other worms. In the common earthworm, for instance, the blood is red, the colour being due to the same pigment, hcemoglobin, which occurs in our own blood and in that of other vertebrate animals. Haemoglobin is a nitrogenous compound containing in addition to carbon, hydrogen, nitrogen, oxygen, and sulphur, a minute quantity of iron. It can be obtained pure in the form of crystals which are soluble in water. Its most striking and physiologically its most important property is its power of entering into a loose chemical combination with oxygen. If a solution of haemoglobin is brought in contact with oxygen it acquires a bright scarlet colour : the solution is then found to have a characteristic spectrum distinguished by two absorption-bands, one in the yellow, another in the green. Loss of oxygen changes the colour from scarlet to purple, and the spectrum then presents a single broad absorption-band intermediate in position between the two of the oxygenated solution. This property is of use in the following way. All parts of the organism are constantly undergoing destructive meta- bolism and giving off carbon dioxide : this gas is absorbed by the blood, and at the same time the haemoglobin gives up 28o POLYGORDIUS its oxygen to the tissues. On the other hand whenever the blood is brought sufficiently near the external air — or water in the case of an aquatic animal — the opposite process takes place, oxygen being absorbed and carbon dioxide given off. Haemoglobin is therefore to be looked upon as a respiratory or oxygen-carrying pigment : its function is to provide the various parts of the body with a constant supply of oxygen, while the carbon dioxide formed by their oxidation is given up to the blood. The particular part of the body in which the carbon dioxide accumulated in the blood is exchanged for the oxygen is called a respiratory organ : in Polygordius, as in the earthworm and many others of the lower animals there is no specialized respiratory organ — lung or gill — but the necessary exchange of gases is performed by the entire surface of the body, In discussing in a previous lesson the differences between plants and animals we found (p. 176) that, in the unicellular organisms previously studied, the presence of an excretory organ in the form of a contractile vacuole was a characteristic feature of such undoubted animals as the ciliate infusoria, but was absent in such undoubted plants as Vaucheria and Mucor. But the reader will have noticed that Hydra and its allies have no specialized excretory organ, waste products being apparently discharged from any part of the surface. In Polygordius we meet once more with an animal in which excretory organs are present, although, in correspondence with the complexity of the animal itself, they are very different from the simple contractile vacuoles of Paramce- cium or Vorticella. The excretory organs of Polygordius consist of little tubes called mphridia, of which each metamere possesses a pair, one on each side (Fig. 69, A, B, and c, Nphui). Each NEPHRIDIA 281 nephridium (Fig. 70) is an extremely delicate tube consisting of two divisions bent at right angles. The outer division is placed vertically, lies in the thickness of the body-wall, and opens externally by a minute aperture, the nephridiopore (Figs. 69 and 70, Nph. f). The inner division is horizontal and lies in the ccelomic epithelium : passing forward it pierces the septum which bounds the segment in front (Fig. 69, A and B), and then dilates into a funnel-shaped extremity or nephrostome (Nph. si) which places its cavity in free com- munication with the ccelome. The whole interior of the Np~h.sb FIG. 70. — A nephridium of Polygordius, showing the cilia lining the tube, the ciliated funnel or nephrostome (Nph. st), and the external aperture or nephridospore (Nph. p). (After Fraipont.) tube as well as the inner face of the nephrostome is lined with cilia wrhich work outwards. A nephridium may therefore be defined as a ciliated tube, lying in the thickness of the body-wall and opening at one end into the ccelome and at the other on the exterior of the body. In the higher worms, such as the earthworm, the nephridia are lined in part by gland-cells, and are abundantly supplied with blood-vessels. Water and nitrogenous waste from all parts of the body pass by diffusion into the blood and are conveyed to the nephridia, the gland-cells of which withdraw the waste products and pass them into the cavities of the tubes, whence they are finally discharged into the surround- 282 POLYGORDIUS ing medium. In all probability some such process as this takes place in Polygordius. In discussing the hydroid polypes we found that one of the most important points of difference between the loco- motive medusa and the fixed hydranth was the presence in the former of a well-developed nervous system (p. 242) con- sisting of an arrangement of peculiarly modified cells, to which the function of automatism was assigned. It is natural to expect in such an active and otherwise highly organized animal as Polygordius a nervous system of a considerably higher degree of complexity than that of a medusa. The central nervous system consists of two parts, the brain and the ventral nerve-cord. The brain (Fig. 69, A and B, Br^] is a rounded mass occupying the whole interior of the prostomium and divided by a transverse groove into two lobes, the anterior of which is again marked by a longitu- dinal groove. The ventral nerve cord (V. Nv. Cd.) is a longitudinal band extending along the whole middle ventral line of the body from the peristomium to the anal segment. The posterior lobe of the brain is connected with the anterior end of the ventral nerve-cord by a pair of nervous bands, the cesophageal connectives (CES. con.} which pass respectively right and left of the gullet. It is to be noted that one division of the central nervous system — the brain — lies altogether above and in front of the enteric canal, the other division — the ventral nerve-cord- altogether beneath it, and that, in virtue of the union of the two divisions by the cesophageal connectives, the enteric canal perforates the nervous system. It is also important to notice that the nervous system is throughout in direct contact with the epidermis or ectoderm, PERIPHERAL NERVOUS SYSTEM 283 the ventral cord appearing in sections (Fig. 69, c, and Fig. 72, A) as a mere thickening of the latter. Both brain and cord are composed of delicate nerve-fibres Drr aer. Ep th m- FIG. 71. — Diagram illustrating the relations of the nervous system of Polygordius. The deric epithelium (Der. Epthm} is either in direct contact with the central nervous system (lower part of figure), or is connected by afferent nerves (af. nv.} with the inter-muscular plexus (int. muse, pi ex.} : the latter is connected to the muscle-plates (M. PI] by efferent nerves (Ef. nv). The central nervous system consists of nerve-fibres (Nv. F) and nerve-cells (Nv. C) : other nerve-cells (Nv. C') occur at intervals in the inter-muscular plexus. The muscle-plates (M. PI], one of which is entire, while only the middle part of the other is shown, are invested by a delicate protoplasmic network, containing nuclei (««), to which the efferent nerves can be traced. (The details copied from Fraipont.) (Fig. 71, Nv. F.} interspersed with nerve-cells (Nv. C.}. In the cord the fibres are arranged longitudinally, and the nerve-cells are ventral in position forming a layer in imme- 284 POLYGORDIUS diate contact with the deric epithelium. In the posterior lobe of the brain the nerve-cells are superficial and the central part of the organ is formed of a finely punctate substance in which neither cells nor fibres can be made out. Ramifying through the entire muscular layer of the body- wall is a network of delicate nerve-fibres (Int. Muse. Fix.} with nerve-cells (Nv. C') at intervals, the inter-muscular plexus. Some of the branches of this plexus are traceable to nerve-cells in the central nervous system, others (of. Nv.) to epidermic cells, others (Ef. Nv.) to the delicate proto- plasmic layer covering the muscle-plates. The superficial cells of both brain and cord are also, as has been said, in direct connection with the overlying epidermis, and from the anterior end of the brain a bundle of nerve-fibres (Fig. 69, B, t. nv.) is given off on each side to the corresponding tentacle, constituting the nerve of that organ, to the epidermic cells of which its fibres are distributed. We see then that, apart from the direct connection of nerve-cells with the epidermis, the central nervous system is connected, through the intermediation of nerve fibres (a) with the sensitive cells of the deric epithelium and (b) with the contractile muscle-plates. And we can thus distinguish two sets of nerve-fibres, (a) sensory or afferent (of. Nv.) which connect the central nervous system with the epidermis, and (b) motor or efferent (Ef. Nv.) which connect it with the muscles. Comparing the nervous system of Polygordius with that of medusa (p. 242) there are two chief points to be noticed. Firstly, the concentration of the central nervous system in the higher type, and the special concentration at the anterior end of the body to form a brain. Secondly, the important fact that the intermuscular plexus is not, like the peripheral nervous system of a medusa which it resembles, situated FUNCTIONS OF NERVOUS SYSTEM 285 immediately beneath the epidermis (ectoderm) but lies in the muscular layer, or in other words, has sunk into the mesoderm. It is obvious that direct experiments on the nervous system would be a very difficult matter in so small an animal as Polygordius. But numerous experiments on a large number of other animals, both higher and lower, allow us to infer with considerable confidence the functions of the various parts in this particular case. If a muscle be laid bare or removed from the body in a living animal it may be made to contract by the application of various stimuli, such as a smart tap (mechanical stimulus), a drop of acid or alkali (chemical stimulus), a hot wire (thermal stimulus), or an electric current (electric stimulus). If the motor nerve of the muscle is left intact the application to it of any of these stimuli produces the same effect as its direct application to the muscle, the stimulus being con- ducted along the eminently irritable but non-contractile nerve. Further, if the motor nerve is left in connection with the central nervous system, /.&, with one or more nerve-cells, direct stimulation of these is followed by a contraction, and not only so, but stimulation of a sensory nerve connected with such cells produces a similar result. And finally, stimulation of an ectoderm cell connected, either directly or through the intermediation of a sensory nerve, with the nerve-cells, is also followed by muscular contraction. An action of this kind, in which a stimulus applied to the free sensitive surface of the body is transmitted along a sensory nerve to a nerve-cell or group of such cells and is thus, as it were, reflected along a motor nerve to a muscle, is called a reflex action; the essence of the arrangement is the inter- 286 POLYGORDIUS position of nerve-cells between sensory or afferent nerves connected with sensory cells, and motor or efferent nerves connected with muscles. The diagram (Fig. 71) serves to illustrate this matter. The muscle plate (M. PL) may be made to contract by a stimulus applied (a) to itself directly, (b) to the motor-fibre (Ef. Nv.), (c) to the nerve-cells (Nv. C.) in the central nervous system, or to those (Nv. C) in the inter-muscular plexus, (d) to the sensory fibre (of. Nv.), or (e) to the epidermic cells (Der. JEpthm.). In all probability the whole central nervous system of Polygordius is capable of automatic action. It is a well- known fact that if the body of an earthworm is cut into several pieces each performs independent movements ; in other words, the whole body is not, as in the higher animals, paralysed by removal of the brain. There can, however, be little doubt that complete co-ordination, i.e., the regulation of the various movements to a common end, is lost when the brain is removed. The nervous system is thus an all-important means of communication between the various parts of the organism and between the organism and the external world. The outer or sensory surface is by its means brought into connection with the entire muscular system with such perfection that the slightest touch applied to one end of the body may be followed by the almost instantaneous contrac- tion of muscles at the other. In some species of Polygordius the prostomium bears a pair of eye-specks, but in the majority of species the adult animal is eyeless, and, save for the ciliated pits (Fig. 68, B, c.p) the function of which is not known, the only definite organs of sense are the tentacles which have a tactile PHYSIOLOGICAL DIFFERENTIATION: ORGANS 287 function, their abundant nerve-supply indicating that their delicacy as organs of touch far surpasses that of the general surface of the body. They are beset with short, fine pro- cesses of the cuticle called setce. (Fig. 68 and 69, s) which probably, like the whiskers of a cat, serve as conductors of external stimuli to the sensitive epidermic cells. There are two matters of general importance in connec- tion with the structure of Polygordius to which the student's attention must be drawn in concluding the present lesson. Notice in the first place how in this type, far more than in any of those previously considered, we have certain definite parts of the body set apart as organs for the performance of particular functions. There is a mouth for the reception of food, an enteric canal for its digestion, and an anus for the extrusion of fasces : a coelomic fluid for the transport of the products of digestion to the more distant parts of the body : a system of blood-vessels for the transport of oxygen to and of carbon dioxide from all parts : an epidermis as organ of touch and of respiration : nephridia for getting rid of water and nitrogenous waste : and a definite nervous system for regulating the movements of the various parts and forming a means of communication between the organism and the external world. It is clear that differentiation of structure and division of physiological labour play a far more obvious and important part than in any of the organisms hitherto studied. Notice in the second place the vastly greater complexity of microscopic structure than in any of our former types. The adult organism can no longer be resolved into more or less obvious cells. In the deric, enteric, and coelomic epithelia we meet with nothing new, but the muscle plates are not cells, the nephridia show no cell-structure, neither do 288 POLYGORDIUS the nerve-fibres nor the punctate substance of the brain. The body is thus divisible into tissues or fabrics each clearly distinguishable from the rest. We have epithelial tissue, cuticular tissue, muscular tissue, and nervous tissue : and the blood and ccelomic fluid are to be looked upon as liquid tissues. One result of this is that, to a far greater extent than in the foregoing types, we can study the morphology of Polygordius under two distinct heads : anatomy, dealing with the general structure of the parts, and histology, dealing with their minute or microscopic structure. One point of importance must be specially referred to in connection with certain of the tissues. It has been pointed out (p. 272) that the epidermis has rather the character of a sheet of protoplasm with regularly arranged nuclei than of a layer of cells, and that the muscle-plates are covered with a layer of protoplasm with which the ultimate nerve-fibres are continuous (p. 273). Thus certain of the tissues of Polygordius exhibit continuity of the protoplasm, a fact of considerable interest in connection with the question of the origin of multicellular animals discussed in the previous lesson (p. 266). LESSON XXVI POLYGORDIUS (Continued] ASEXUAL reproduction is unknown in Polygordius, and the organs of sexual reproduction are very simple. The animal is dioecious, gonads of one sex only being found in each individual. In the species which has been most thoroughly investi- gated (P. neapolitanus) the reproductive products are formed in each metamere from the fourth to the last. Crossing these segments obliquely are narrow bands of muscle (Fig. 72, A, O.M) and certain of the cells of coelomic epithelium covering these bands multiply by fission and form little heaps of cells (spy) each of which is to be looked upon as a gonad. There is thus a pair of gonads to each segment with the exception of the prostomium, the peristomium, the first three metameres, and the anal segment, the reproductive organs exhibiting the same simple metameric arrangement as the digestive, excretory, and circulatory organs. It will be noticed that the primitive sex-cells, arising as they do from ccelomic epithelium, are mesodermal structures, not ectodermal as in hydroids (pp. 231 and 244). In the male the primitive sex-cells divide and sub-divide, the ultimate products being converted into sperms (Fig. 72, u D.V DerEpthm B M.Pl V.Nv.Cd Cosl.Epknm. i« FIG. 72- — Polygordius neapolitanus. A, transverse section of a male specimen to show the position of the immature gonads (spy} and the precise form and arrangement of the various layers represented diagrammatically in Fig. 69, C. The body-wall consists of cuticle (Cu), deric epithelium (Der. Epthm), muscle-plates (M. PI), and parietal layer of ccelomic epithelium (C&l. Epthm). The ventral nerve-cord ( V. Nv. Cd) is shown to be continu- ous with the deric epithelium. The enteric canal consists of ciliated enteric epithelium (Ent. Epthm} covered by the visceral layer of ccelomic epithelium (Ccel. Epthm') : connecting it with the body-wall are the dorsal and ventral mesenteries formed of a double layer of ccelomic epithelium, and containing respec- tively the dorsal (D. V) and ventral (V. V] blood-vessels. Passing obliquely across the ccelome are the oblique muscles (0. M) DEVELOPMENT 291 covered with coelomic epithelium : by differentiation of groups of cells of the latter the spermaries (spy] are formed. B, a single sperm, showing expanded head and delicate tail. C, horizontal section of a sexually mature female. The body- wall (Cu, Der. Eptkm, M. PI} has undergone partial histological degeneration, and is ruptured in two places to allow of the escape of the ova (ov) which still fill the coelomic spaces enclosed between the body-wall, the enteric canal (Ent. Epthni), and the septa (sep). (After Fraipont.) B : see p. 252) : in the female they enlarge immensely, and take on the character of ova (c, ov). Multiplication of the sexual products takes place to such an extent that the whole ccelome becomes crammed full of either sperms or ova (c). In the female the growth of the eggs takes place at the expense of all other parts of the body, which undergo more or less complete atrophy : the epidermis, for instance, be- comes liquefied and the muscles lose their contractility. Finally rupture of the body-wall takes place in each segment (c), and through the slits thus formed the eggs escape. So that Polygordius, like an annual plant, produces only a single brood : death is the inevitable result of sexual maturity. Whether or not the same dehiscence of the body-wall takes place in the male is not certain : it has been stated that the sperms make their escape through the nephridia. Thus while there are no specialized gonaducts, or tubes for carrying off the sexual products, it is possible that the neph- ridia may, in addition to their ordinary function, serve the purpose of male gonaducts or spermiducts. Female gona- ducts or oviducts are however entirely absent. The ova and sperms being shed into the surrounding water impregnation takes place, and the resulting oosperm under- goes segmentation or division (see p. 245), a polyplast being formed. By the arrangement of its cells into two layers and U 2 POLYGORDIUS the formation of an enteron or digestive cavity the polyplast becomes a gastrula (see p. 260) which by further develop- ment is converted into a curious, free-swimming creature shown in Fig. 73, A, and called a trochosphere. oo A •and Pt-.or.CL dm FIG. 73. — A, larva of Polygordius neapolitanus in the trochosphere stage : from a living specimen. B, diagrammatic vertical section of the same : the ectoderm is dotted, the endoderm radially striated, the mesoderm evenly shaded, and the nervous system finely dotted. C, transverse section through the plane ab in B. The body-wall consists of a single layer of ectoderm cells, which, at the apex of the prostomium (upper hemisphere) are modified to form the brain (Br) and a pair of ocelli (oc). The enteric canal consists of three parts : the stomodaeum (st. dm], opening externally by the mouth (Mth), and lined by ectoderm ; the enteron (Ent) lined by endoderm ; and the proctodseum (Prc. dm], opening by the anus (An) and lined by ectoderm. Between the body-wall and the enteric canal is the larval body-cavity or blastoccele (BL cccl). The mesoderm is confined to two narrow bands of cells (B and C, Msd) in the blastoccele, one on either side of the proctodaeum ; slender mesodermal bands (Msd' ) are also seen in the prostomium in A. The cilia consists of a prse-oral circlet (Pr. or. ci) above the mouth, a post-oral circlet (PL or. ci) below the mouth, and an anal circlet (An. ci} around the anus. (A after Fraipont.) The trochosphere, or newly-hatched larva of Polygordius (Fig. 73, A) is about \ mm. in diameter, and has something the form of a top, consisting of a dome-like upper portion, the prostomium, produced into a projecting horizontal rim ; THE TROCHOSPHERE 293 of an intermediate portion or peristomium^ having the form of an inverted hemisphere ; and of a lower somewhat conical anal region. Around the projecting rim is a double circlet of large cilia (Pr. or. d.) by means of which the larva is propelled through the water. Beneath the edge of the ciliated rim is a rounded aperture, the mouth (Mt/i.) ; this leads by a short, nearly straight gullet (St. dm.) into a spacious stomach (Ent.\ from the lower side of which proceeds a short slightly curved intestine (Prc. dm.), opening at the extremity of the conical inferior region by an anus (An.). Between the body-wall and the enteric canal is a space filled with fluid (Bl. cceL), but, as we shall see, this does not correspond with the body-cavity of the adult. The body-wall and the enteric canal consist each of a single layer of epithelial cells, all the tissues included in the adult under the head of mesoderm (p. 274) being absent or so poorly developed that they may be neglected for the present. Leaving aside all details it will be seen that the trocho- sphere of Polygordius is comparable in the general features of its organization to a medusa (compare Fig. 55, p. 238), consisting as it does of an outer layer of cells forming the external covering of the body and of an inner layer lining the digestive cavity. There are, however, two important differences : the space between the two layers is occupied by the mesoglcea in the medusa, while in the worm it is a cavity filled with fluid ; and the digestive cavity of the trochosphere has two openings instead of one. But still, in order to compare the medusa with the trocho- sphere, it is necessary to fill up, by the help of other types, an important gap in our knowledge of the development of Polygordius — the passage from the gastrula to the trocho- sphere. From what we know of the development of other 294 POLYGORDIUS worms, the process, in its general features, is probably as follows : — .The ectoderm and endoderm of the gastrula (Fig. 74, A) are not in close contact with one another as in Fig. 63 (p. 261), but are separated by a space filled with fluid — the blastoccele or larval body-cavity. The mouth of the gastrula closes (B) the enteron (Ent\ being thus converted into a shut sac. At about the same time the ectoderm is tucked GaslMlh. FIG. 74. — Diagram illustrating the origin of the trochosphere from the gastrula. The ectoderm is dotted, the endoderm striated. A, gastrula, with enteron (Ent) and gastrula-mouth (G. Mth), and with the ectoderm and endoderm separated by the larval body-cavity or blastoccele (Bl. ccel). B, the gastrula-mouth has closed, the enteron (Ent) becoming a shut sac. c, two ectodermal pouches, the stomodseum (St. dm) and proctodseum (Prc. dm) have appeared. D, the stomodseum (St. dm) and proctodaeum (Prc. dm) have opened into the enteron (Ent), forming a complete enteric canal with mouth (Mth) and anus (An). in or invaginated at two places (C), and the two little pouches (St. dm, Prc. dm) thus formed grow inwards until they meet with the closed enteron and finally open into it (Z>), so that a complete enteric canal is formed — formed, we must not fail to notice, of three distinct parts : (i) an anterior ectodermal pouch, opening externally by the mouth, and distinguished as the stomodaum ; (2) the enteron, lined with endoderm; and (3) a posterior ectodermal pouch, opening externally by the anus, and called the proctodaum. METAMORPHOSIS 295 In the trochosphere (Fig. 73) the gullet is derived from the stomodaeum, the stomach from the enteron, and the intestine from the proctodaeum ; so that only the stomach of the worm-larva corresponds with the digestive cavity of a medusa : the gullet and intestine are structures not repre- sented in the latter form. Two or three other points in the anatomy of the trocho- sphere must now be referred to. At the apex of the dome-shaped prostomium the dcto- derm is greatly thickened forming a rounded patch of cells (Figs. 73 and 75, Br\ the rudiment of the brain. On the surface of the same region and in close relation with the brain is a pair of small patches of black pigment, the eye-spots or ocelli (Oc). On each side of the intestine between its epithelium and the external ectoderm is a row of cells forming a band which partly blocks up the blastoccele (B and c, Msd). These two bands are the rudiments of the whole of the meso- dermal tissues of the adult — muscle, coelomic epithelium, &c. — and hence called mesodermal bands. Finally on each side of the lower or posterior end of the stomach is a delicate tube (Fig. 75, A, Np]i] opening by a small aperture on to the exterior, and by a wide funnel- shaped extremity into the blastoccele : it has all the relations of a nephridium, and is distinguished as the head-kidney. As the larva of Polygordius is so strikingly different from the adult, it is obvious that development must, in this as in several cases which have come under our notice, be accom- panied by a metamorphosis. The first obvious change is the elongation of the conical anal region of the trochosphere into a tail-like portion which 296 POLYGORDIUS may be called the trunk (Fig. 75, A). The stomach (enteron), which was formerly confined to the pro- and peri- stomium, has now grown for a considerable distance into the trunk (B, ent) so that the proctodseum (Prc. dm} occupies only the portion in proximity to the anus. Br B An ci FIG. 75. — A, living specimen of an advanced trochosphere-larva of Polygordius neapolitanus, showing the elongation of the anal region to form the trunk. B, diagrammatic vertical section of the same : the ectoderm is coarsely, the nervous system finely dotted, the endoderm radially striated, and the mesoderm evenly shaded. C, transverse section through the plane ab in B. The pre-oral (Pr. or. «'), post-oral (Pt. or. ci], and anal (An. ci) cilia, brain (£r), ocelli (oc), blastoccele (Bl. ccel). mouth (Mth}> stomo- dseum (St. dm), proctodaeum (Prc. dm], and anus (An) as in Fig. 73, A : the enteron (Ent} has extended some distance into the trunk. In A, slender mesodermal bands (Msd. bd) in the prostomium and the branched head-kidney (Nph) are shown. In B and C the mesoderm (Msd} is seen to have obliterated the blasto- ccele in the trunk-region : the ectoderm has undergone a thickening, forming the ventral nerve-cord ( V. Nv. Cd}. (A after Fraipont.) Important internal changes have also taken place. The deric epithelium or external ectoderm is for the most part composed, as in the preceding stage, of a single layer of DEVELOPMENT OF METAMERES 297 cells, but on that aspect of the trunk which lies on the same side as the mouth — i.e., to the left in Fig. 75, A and B — this layer has undergone a notable thickening, being now com- posed of several layers of cells. This ectodermal thickening is the rudiment of the ventral nerve-cord ( V. Nv. Cd\ and the side of the trunk on which it appears is now definitely marked out as the ventral aspect of the future worm, the opposite aspect — that to the right in the figures — being dorsal. At a later stage two ectodermal cords, the cesopha- geal connectives, are formed connecting the anterior end of the ventral nerve-cord with the brain. Note that the two divisions of the central nervous system are originally quite distinct. The mesodermal bands which were small and quite separate in the preceding stage (Fig. 73, B and c, Msd) have now increased to such an extent as to surround com- pletely the enteron and obliterate the blastoccele (Fig. 75, B and B, Msd). At this stage therefore there is no body- cavity in the trunk, but the space between the deric and enteric epithelia is occupied by a solid mass of mesoderm. In a word, the larva is at present, as far as the trunk is con- cerned, triploblastic but acotlomate. Development continues, and the larva assumes the form shown in Fig. 76, A. The trunk has undergone a great increase in length and at the same time has become divided, by a series of annular grooves, into segments or metameres like those of the adult worm but more distinct (compare Fig. 68, D, p. 268). By following the growth of the larva from the preceding to the present stage it is seen that these segments are formed from before backwards, i.e., the seg- ment next the peristomium is the oldest, and new ones are continually being added between the last formed and the 298 POLYGORDIUS extremity of the trunk or what may now be called the anal segment. By this process the larva'has assumed the appear- ance of a worm with an immense head and a very slender trunk. The original larval stomach (enteron) has extended with the formation of the metameres so as to form the greater portion of the intestine : the proctodseum (Prc. dni) is confined to the anal segment. Two other obvious changes are the appearance of a pair of small slender processes (A, /) — the rudiments of the tentacles — on the apex of the prostomium, and of a circlet of cilia (Pr. An. ci) round the posterior end of the trunk. The internal changes undergone during the assumption of the present form are very striking. In every fully formed metamere the mesoderm — solid, it will be remembered, in the previous stage — has become divided into two layers, a somatic layer (B and c, Msd (soni) ) in contact with the ectoderm and a splanchnic layer (Msd (spl) ) in contact with the endoderm. The space between the two layers (Ccel) is the permanent body-cavity or ccelome, which is thus quite a different thing from the larval body-cavity or blastoccele, being formed, not as a space between ectoderm and endoderm, but by the splitting of an originally solid mesoderm. The division of the mesoderm does not however extend quite to the middle dorsal and middle ventral lines : in both these situations a layer of undivided mesoderm is left (c) and in this way the dorsal and ventral mesenteries are formed. Spaces in these, apparently the remains of the blastoccele, form the dorsal and ventral blood-vessels. More- over the splitting process takes place independently in each segment and a transverse vertical layer of undivided mesoderm (B, Sep) is left separating each segment from the Msd (fern) v.m fr.an.ci- An.ci FIG. 76. — A, larva of Polygordius neapolitanus in a condition inter- mediate between the trochosphere and the adult worm, the trunk-region having elongated and divided into metameres. B, diagrammatic vertical section of the same : the ectoderm is coarsely, the nervous system finely dotted, the endoderm radially striated, and the mesoderm evenly shaded. c, transverse section along the plane ab in B. The pre-oral (Pr. or. ci), post-oral (PL or. cz), and anal (An. ci) cilia, the blastoccele (Bl. cosl}, stomodaeum (St. dm], and proctodaeum (Prc. dm] are as in Fig. 73, A and B : the enteron now extends through- out the segmented region of the trunk. A pair of tentacles (t] has appeared on the prostomium near the ocelli (oc), and a pre-anal circlet of cilia (Pr. an. ci} is developed. The mesoderm has divided into somatic (J\hd (som) ) and splanchnic (Msd (spl) ) layers with the ccelome (Ccet) between : the septa (sep) are formed by undivided plates of mesoderm separating the segments of the ccelome from one another. D1-D3, three stages in the development of the somatic mesoderm. In D1 it (Msd (som} } consists of a single layer of cells in contact with the deric epithelium (Der. Epthm} : in D2 the cells have begun to split up in a radial direction : in D3 each has divided into a number of radially arranged sections of muscle-plates (M. PI} and a single cell of ccelome epithelium (Ccel. Epthm}. (A after Fraipont.) 300 POLYGORDIUS adjacent ones before and behind : in this way the septa arise. The nephridia appear to have a double origin, the super- ficial portion of each being formed from ectoderm, the deep portion including the nephrostome from the somatic layer of mesoderm. In the ventral nerve cord the cells lying nearest the outer surface have enlarged and formed nerve-cells, while those on the dorsal aspect of the cord have elongated longitudinally and become converted into nerve fibres. This process has already begun in the preceding stage. But the most striking histological changes are those which gradually take place in the somatic layer of mesoderm. At first this layer consists of ordinary nucleated cells (o1, Msd (soni) ), but before long each cell splits up in a radial direction (D2) from without inwards — i.e.t from the ectoderm (Der. Epthm} towards the coelome — finally taking on the form of a book with four or more slightly separated leaves directed outwards or towards the surface of the body, and with its back — the undivided portion of the cell — bounding the coelome. The cells being arranged in longitudinal series we have a number of such books placed end to end in a row with the corresponding leaves in contact — page one of the first book being followed by page one of the second, third, fourth, £c., page two by page two, and so on through one or more segments of the trunk. Next what we have compared with the leaves of the books — the divided portions of the cells — become separated from the backs- the undivided portions (DS) — and each leaf (M. PI) fuses with the corresponding leaves of a certain number of books in the same longitudinal series. The final result is that the undivided portions of the cells (backs of the books, CceL Epthni) become the parietal layer of coelomic epithelium, the SIGNIFICANCE OF DEVELOPMENTAL STAGES 301 longitudinal bands formed by the union of the leaves (M. PI] becoming the muscle-plates which are thus, cell- fusions, each being formed by the union of portions of a series of longitudinally arranged cells. At the same time the cells of the splanchnic layer of mesoderm thin out and become the visceral layer of ccelomic epithelium. We see then that by the time the larva has reached the stage shown in Fig. 76, it is no longer a mere aggregate of simple cells arranged in certain layers. The cells them- selves have undergone differentiation, some becoming modi- fied into nerve-fibres, others by division and subsequent fusion with their neighbours forming muscle-plates, while others, such as the epithelial cells, remain almost unaltered. Thus, in the course of the development of Polygordius, cell-multiplication and cell-differentiation go hand in hand, the result being the formation of those complex tissues the presence of which forms so striking a difference between the worm and the simpler types previously studied. It is important to notice that this comparatively complex animal is in one stage of its existence — the oosperm — as simple as an Amoeba ; in another — the polyplast — it is com- parable to a Magosphsera or a Volvox ; in a third — the gastrula — it corresponds in general features with a Hydra ; while in a fourth — the trochosphere — it resembles in many respects a Medusa. As in other cases we have met with, the comparatively highly organized form passes through stages in the course of its individual development similar in general characters to those which, on the theory of evolution, its ancestors may be considered to have passed through in their gradual ascent from a lower to a higher stage of organization. 302 POLYGORDIUS The rest of the development of Polygordius may be summarized very briefly. The trunk grows so much faster than the head (pro- plus peri-stomium) that the latter under- goes a relative diminution in size, finally becoming of equal diameter with the trunk as in the adult. The ciliated rings are lost, the tentacles grow to their full size, the eye-spots atrophy, and thus the adult form is assumed. LESSON XXVII1 THE GENERAL CHARACTERS OF THE HIGHER ANIMALS THE student who has once thoroughly grasped the facts of structure of such typical unicellular animals as Amoeba and the Infusoria, of such typical diploblastic animals as Hydra and Bougainvillea, and of such a typical triploblastic animal as Polygordius, ought to have no difficulty in understanding the general features of the organization of any other members of the animal kingdom. When once the notions of a cell, a cell-layer, a tissue, body-wall, enteron, stomodaeum, procto- daeum, ccelome, somatic and splanchnic mesoderm are fairly understood, all other points of structure become hardly more than matters of detail. If we turn to any text-book of Zoology we shall find that the animal kingdom is divisible into seven primary sub- divisions, called sub-kingdoms, types, or phyla. These are as follows :— Protozoa. Coslenterata. Verities. Echinodermata. A rth ropoda. Mollusca. Vertebrata. 1 Readers who have not studied zoology, or at least examined a series of selected animal types, should omit this lesson and go on to the next. 304 GENERAL CHARACTERS OF THE HIGHER ANIMALS With a few exceptions, the discussion of which would be out of place here, the vast number of animals known to us can be arranged in one or other of these groups. The Protozoa are the unicellular animals : they have been represented in previous lessons by Amoeba and Protamceba, Hsematococcus, Heteromita, Euglena, the Mycetozoa, Para- moecium, Stylonychia, Oxytricha, Opalina, Vorticella, Zooth- amnium, the Foraminifera, the Radiolaria, Magosphaera, and Volvox. The reader will therefore have no difficulty in grasping the general features of this phylum. The Coeltnterata are the diploblastic animals, and have also been well represented in the foregoing pages, namely, by Hydra, Bougainvillea, Diphyes, and Porpita. The sea- anemones, corals, and sponges also belong to this phylum. The Vermes, or Worms, are a very heterogeneous assem- blage. They are all triploblastic, but while some are ccelomate, others have no body-cavity ; some, again, are segmented, others not. Still, if the structure of Polygordius is thoroughly understood, there will be little difficulty in understanding that of a fluke, a tape-worm, a round-worm, an earthworm, or one of the ordinary marine worms. Of the remaining four sub-kingdoms we have, so far, studied no example, but a brief description of a single typical form of each will show how they all conform to the general plan of organization of Polygordius, being all triplo- blastic and ccelomate. Under the Echinodermata are included the various kinds of starfishes — sand-stars, brittle-stars, and feather-stars, as well as sea-urchins, sea-cucumbers, &c. A starfish will serve as an example of the group. The phylum Arthropoda includes crayfishes, lobsters, crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions, spiders, and mites ; centipedes and millipedes ; and all GENERAL STRUCTURE 305 kinds of insects, such as cockroaches, beetles, flies, ants, bees, butterflies, and moths. A crayfish forms a very fair type of the group. In the phylum Mollusca are included the ordinary bi- valves, such as mussels and oysters ; snails, slugs, and other univalves or one-shelled forms ; sea-butterflies ; and cuttlefish, squids, and Octopi. An account of a fresh-water mussel will serve to give a general notion of the character of this group. Finally, under the head of Vertebrata are included all the backboned animals : the lampreys and hags ; true fishes, such as the shark, skate, sturgeon, cod, perch, trout, &c. ; amphibians, such as frogs, toads, newts, and salamanders ; true reptiles, such as lizards, crocodiles, snakes, and tor- toises ; birds ; and mammals, or creatures with a hairy skin which suckle their young, such as the ordinary hairy quadrupeds, whales and porpoises, apes, and man. The essential structure of a vertebrate animal will be understood from a brief description of a dog-fish. THE STARFISH.1 A common starfish consists of a central disc-like portion, from which radiate five arms or rays. It crawls over the rocks with its ventral surface downwards, its dorsal surface upwards. It can move in any direction, so that, in the ordinary sense of the words, anterior and posterior extremi- ties cannot be distinguished. Radial symmetry such as this, /. l < ^ •- o _ fi K^ *& **§ .§§ +-» -*—> 2 t/3 i^ :ie ^ rC C3 O '<3 .S -— ^ M3 .Jx C/3 c ^^» ^ *-R rt O\ t/3 g ^ ^"C" CU -~ rt •a « \ If, O rt O jU GENERAL CHARACTERS 313 The body is divided into a head (Hd] and thorax ( Th), together constituting the cephalo-thorax (C. Th}, and seven free abdominal segments (Abd. seg. I, Abd. seg. 7) : the head is produced in front into a rostrum. The body- wall consists of cuticle (Cu), partly calcified to form the exoskeleton, deric epithelium (Der. Epthni), dermis (Derm.}, and a very thick layer of muscle (M) which in the abdomen is distinctly segmented. The mouth (Mth} leads by a short gullet (Gul} into a large stomach (6Y), from which a short small intestine (S. Ini} leads into a large in- testine (L. Itit}, ending in the anus (An}. Opening into the small intestine are the digestive glands (D. Gl}. The epithelium of the small intestine and digestive glands is endodermal, that of the rest of the canal is ectodermal and secretes a cuticle : the outer layer throughout is meso- dermal (connective tissue and muscle). The cavity (B. S) between the enteric canal and the body-muscles is a blood- sinus. The heart (Hi} is enclosed in the pericardial sinus (Per. S) : the chief ventral blood-vessel or sternal artery (St. A} is shown in B. The gills (B, Gill} are enclosed in a cavity formed by a fold of the thoracic body-wall called the branchiostigite (Brstg] : they are formed of the same layers as the body-wall, of which they are offshoots. The kidneys (A, K] are situated in the head. The brain (Br) lies in the prostomium : the ventral nerve-cord ( V. Nv. Cd} consists of a chain of ganglia (Gn) united by connectives. The ovary (ovy] is a hollow organ opening by an oviduct (B, ovd} on the base of one of the legs (Leg). stomach (67), and a straight intestine divisible into a short anterior division or small intestine (S. Int.] and a long posterior division or larger intestine (L. Int.] : the latter opens by an anus (An) on the ventral surface of the last segment. The study of development shows that the only part of the canal derived from the enteron of the embryo is the small intestine : the gullet and stomach arise from the stomodseum, the large intestine from the proctodaeum. Thus the only portion of the enteric epithelium which is endodermal is that of the small intestine : the epithelium of gullet, stomach, and large intestine is ectodermal, and, like the deric epithelium secretes a cuticle. The outer layer of the whole enteric canal consists of connective tissue and muscle : there is no coelomic epithelium. 3H THE CRAYFISH On each side of the small intestine is a large organ, the digestive gland (D. Gl) : it consists of numberless glove- finger-like processes or cozca which open by a short tube or duct into the small intestine (B, D. Gl}. Both coeca and duct are lined with epithelium derived from the endo- derm, and the whole digestive gland is to be looked upon as a branched lateral outgrowth of the enteron. The secretion of digestive juice is performed exclusively by the epithelium of the digestive glands. Between the enteric canal and the body-wall are a series of spaces (jB.S) containing blood and having the general relations of a coelome, but very probably only representing a number of enlarged blood-spaces or sinuses. Respiration is performed by special organs, the gills (B, Gill, see p. 313) developed in the thoracic region as out- growths of the body-wall, and containing the same layers (cuticle, epithelium, and connective tissue) as the latter. They have a brush-like form and are protected by a fold of the body-wall (Brstg). The blood-system is constructed on the same general lines as that of Polygordius but is greatly modified. A portion of the dorsal vessel is enlarged to form a muscular dilatation, the heart (Ht\ and the rest of the vessels, now called arteries (B, St. A), instead of forming by themselves a closed system, ramify extensively over the body, their ulti- mate branches opening into larger cavities or sinuses between the muscles. One of these cavities — the pericardial sinus (Pcd. S) — surrounds the heart. The heart, arteries, and sinuses together form a closed system through which the blood is propelled in a definite direction by the contractions of the heart. Renal excretion is performed by a pair of glandular bodies, the kidneys (A, K}, situated in the front part of the ABSENCE OF CILIA 315 head and opening by ducts on the bases of antennae. They consist of convoluted tubes lined by epithelium, and are probably to be looked upon as greatly modified nephridia. The Crayfish is dioecious. The ovaries (Ovy) are a pair of hollow organs, united in the middle line in some genera, situated in the thorax, and opening by oviducts (B, Ovd] on the bases of the third pair of legs. The spermaries (testes) are also frequently united in the middle line and open by spermiducts (vasa deferentia) on the bases of the fifth pair of legs. There is some reason for thinking that the gonaducts represent modified nephridia, and the cavities of the hollow gonads a greatly reduced coslome from the epithelium of which the sex-cells are produced. The nervous system is formed on quite the same plan as that of Polygordius, consisting of a dorsal brain (Br) united by oesophageal connectives to a ventral nerve-cord (V. Nv. Cd). In the cord, however, the nerve-cells, instead of being evenly distributed, are aggregated into little enlarge- ments or ganglia (Gn), of which there is primitively a pair to each metamere, the number being reduced in the adult by concrescence. The portions of the ventral nerve-cord between the ganglia consist of nerve-fibres only, and are called connectives. In the embryo the nervous system is, as in Polygordius, in direct connection with the epidermis, but in the adult it has sunk inwards so as to be entirely surrounded by mesoderm. A striking feature in the histology of the Crayfish, and one in which it agrees with the vast majority of Arthropoda, is the entire absence of cilia. Another peculiarity — also shared by the greater part of the phylum — is that the sperms are non-motile. The laid eggs become attached to the swimmerets of the mother, and in this situation undergo their development. In 316 THE FRESH-WATER MUSSEL the fresh-water crayfish the young is hatched in a condition closely resembling the adult, but in the lobster and the sea- crayfish there is a metamorphosis. THE FRESH-WATER MussEL.1 * The body is bilaterally symmetrical, and is greatly com- pressed from side to side. Its dorsal margin is produced into paired flaps, the mantle-lobes (Fig. 79, A and B, Mant\ which pass downwards one on each side of the body. Closely applied to the outer surface of the mantle-lobes, and formed as a cuticular secretion of their deric epithelium, are the two valves of the bivalved strongly calcified shell (J3., Sh). The ventral region of the body is produced into a laterally compressed muscular structure, the foot (A and B, foot), by the contraction of which the animal can move slowly through the sand or mud in which it lives partly buried. The possession of a mantle formed as a prolongation of the dorsal region, of a calcareous shell secreted by the mantle, and of a muscular foot formed as an impaired prolongation of the ventral region, are the most characteristic features of the Mollusca generally. Posteriorly the edges of the mantle-lobes are greatly thickened and are united to one another in such a way as to form two apertures, a large ventral inhalent (Ink. Ap\ and a small dorsal exhalent aperture (Exh. Ap). By means of the cilia of the gills (see below) a current of water is pro- duced which enters at the inhalent aperture carrying 1 For detailed descriptions of the fresh-water Mussel see Rolleston and Jackson, Forms of Animal Life, pp. 124 and 285 : Huxley and Martin, Elementary Biology, p. 305 : and Marshall and Hurst, Practical Zoology, p. 76. l'a.t.£]>llnn CaLEnthm* -K,., L 2 ^ CP.Un FIG. 79. — Diagrammatic sections of the Fresh-water Mussel. A, longitudinal section : the right mantle-lobe (Mant) and gills (/. G. O. G) are shown in perspective. B, transverse section. The cuticular shell (S/i), shown only in B, is black, the ectoderm dotted, the nervous system finely dotted, the endoderm radially striated, the mesoderm evenly shaded, and the ccelornic epithelium represented by a beaded line. 'The dorsal region is produced into the right and left mantle-lobes (Mant), attached to which are the valves of the shell (Sh) joined dorsally by an elastic ligament (lig). The mantle-lobes are partly united so as to form the inhalent (Ink. Ap) and exhalent (Exk. Ap) apertures at the posterior end. The body is produced ventrally into the foot (Foot), on each side of which are the gills, an inner (/. G) and an outer (0. G), each formed of an inner and an outer lamella. The body is covered externally by deric epithelium (Der. Epthin], within which is mesoderm (Msd) largely differentiated into muscles, of which the anterior (A. Ad) and posterior (P. Ad) adductors are indi- cated in A. The mouth (Mth) leads by the short gullet (Gtil) into the stomach (St), from which proceeds the coiled intestine (Int), ending in the anus 318 THE FRESH-WATER MUSSEL (An): the enteric epithelium is mostly endodermal. The digestive gland (D. Gl) surrounds the stomach. The ccelome (C&l) is reduced to a small dorsal chamber enclosing part of the intestine and the heart : the parietal (Ccel. Epthni) and visceral (Ccel. Epthn^} layers of ccelomic epithelium are shown. The heart consists of a median ventricle ( Vent), enclosing part of the intestine, and of paired auricles (Aur). The paired nephridia (Nphni) open by apertures into the ccelome (Nph. si] and on the exterior (Nph. p). The gonads (Gon) are imbedded in the solid mesoderm, and open on the exterior by gonaducts (Gnd). The nervous system consists of a pair of cerebro-pleural ganglia (C. P. Gn) above the gullet, a pair of pedal ganglia (Pd. Gn) in the foot, and a pair of visceral ganglia ( V. Gn} below the posterior adductor muscle. abundant oxygen and the minute organisms used as food, and makes its escape at the exhalent aperture, taking with it the various products of excretion and faecal matter. The mouth (MtJi) is anterior and ventral, lying just in front of the foot : it is bounded on each side by a pair of triangular bodies, the labial palpi, and leads by a short gullet (Gul) into a stomach (St) from which proceeds a long coiled intestine (Int) : this makes several turns in the ventral region of the trunk, then passes to the dorsal region, and finally backwards in the median plane to open by an anus (An) at the posterior end of the body just within the exhalent aperture. The enteric canal is formed almost exclusively from the enteron, the stomodaeum and procto- daeum being both insignificant, hence the enteric epithelium is almost wholly endodermal. There is a large digestive gland (D. Gl) surrounding the stomach and opening into it by several ducts. The coelome (Ccel) is a small cavity in the dorsal region containing a portion of the intestine : the rest of the enteric canal is embedded in solid mesoderm. The mesoderm, as usual, is largely differentiated into muscle, There are numerous muscles connected with the REPRODUCTION AND DEVELOPMENT 319 foot, and two very large ones (A. Ad, P. Ad) pass trans- versely from valve to valve of the shell, one immediately above the gullet, the other immediately below the anal end of the intestine ; these latter are called adductors, and serve to close the shell. On either side of the body, between the trunk and the mantle, are two gills (/. G, O. G\ each having the form of a double plate (B) nearly as long as the body. They serve, in conjunction with the mantle, as respiratory organs, but their main function is to produce the current of water re- ferred to above by means of the cilia with which they are covered. There is an extensive system of blood-vessels. The heart lies in the ccelome, and consists of three chambers, a median ventricle (Vent), which surrounds the intestine, and paired auricles (Aur). Excretion is performed by a single pair of nephridia (Nphm) which open at one end (Nph. st] into the ccelome and at the other (Nph. p) on to the exterior. The nervous system consists of three pairs of ganglia, the two ganglia of each pair being united by transverse com- missures. The cerebro-pleural ganglia (C. P. Gn) lie above the gullet, and represent, in a general way, the brain of Polygordius and the crayfish ; they are united by longitu- dinal connectives with the pedal ganglia (P. Gn} which lie in the foot, and may be taken as representing the ventral nerve-cord of worms and arthropods, and with the visceral ganglia ( V. Gn) which are placed beneath the posterior adductor muscle. The gonads (Gon) are large irregular organs, very similar in appearance in the two sexes, situated among the coils of the intestine and opening by a duct (Gnd) on each side of the trunk, close to the nephridiopore. The impregnated 320 THE DOG-FISH eggs are passed into the cavity of the outer gill of the female, where they undergo the early stages of their develop- ment. The larva of the fresh-water mussel is a peculiar bivalved form, very unlike the adult, and called a glochidium^ but in the more typical molluscs the embryo leaves the egg as a trochosphere, closely resembling that of Polygordius. THE Doc-FisH.1 A dog-fish is bilaterally symmetrical, the nearly cylin- drical body (Fig. 80, A) terminating in front in a blunt snout and behind passing insensibly into an upturned tail. Externally there is no appearance of segmentation. The mouth (MtK) is on the ventral surface of the head or anterior region of the body ; it is transversely elongated, and is supported by jaws which are respectively anterior (upper) and posterior (lower). They thus differ funda- mentally from the jaws of arthropods, which are modified appendages and are therefore disposed right and left. A short distance behind the mouth are five vertical slits (B, Ext. br. ap) arranged in a longitudinal series, the external branchial apertures or gill-clefts. The vent, or cloacal aperture (An) is situated on the ventral surface a considerable distance from the end of the tail. That part of the body lying in front of the last gill-cleft is counted as the head, all behind the vent as the tail, the intermediate portion as the trunk. 1 For a detailed description of a dog-fish see Marshall and Hurst, Practical Zoology (London, 1888), p. 196. For descriptions of other fishes, equally suitable in some respects as types of Vertebrata, see Rolleston and Jackson, Forms of Animal Life (Oxford, 1888), pp. 83 and 273 : and Parker, Zootomy (London, 1884), pp. I, 27, 86. APPENDAGES 321 Appendages are present, but in a very different form from those .of the crayfish. They consist of flat processes of the body- wall called fins. Two of them (D.F1, D.F") are situated in the middle line of the back (dorsal fins] : one ( V.F} in the middle ventral line behind the anus (ventral fin\ and one (C.J?) is attached to the up-turned end of the tail (caudal fiii) : all these being unpaired structures or median fins. Then there is a pair of pectoral fins situated one on each side just behind the last gill-cleft, and a pair of pelvic fins placed one on each side of the vent : these are the lateral or paired fins. It is characteristic of Vertebrata that the number of lateral appendages never exceeds two pairs. The skin or external layer of the body-wall consists of an outer epidermis (Der. Rpthm) composed of several layers of cells, and of an inner connective tissue layer or dermis (Demi}. In the latter are found innumerable bony scales (Derm. Sp) constituting a dermal exoskeleton. The muscular layer of the body-wall (M) is of great thickness, especially in the dorsal region, and is distinctly segmented, indicating that the body of the dog-fish, like that of Polygordius and the crayfish, is divisible into metameres, although there is no indication of these externally. The large coelome (cceV) is confined to the trunk : it is characteristic of vertebrates that both head and tail are accelomate in the adult. The coelomic epithelium (Ccel. Epthm, Ccvl. Epthm1) is underlaid by a distinct layer of connective tissue, the two together forming the peritoneum. Another important vertebrate character is that the dorsal region of the body-wall contains a median longitudinal canal (C. Sp. cav.) extending from shortly behind the snout to near the end of the tail. This is the cerebro-spinal cavity and contains the central nervous system. Y APPENDAGES 323 A, longitudinal vertical section. B, horizontal section through the pharynx and gills. C, transverse section through the trunk. The ectoderm is dotted, the nervous system finely dotted, the endo- derm radially striated, the mesoderm evenly shaded, the coelomic epithelium represented by a beaded line, and all skeletal structures black. The body gives origin to the dorsal (D. F1, D. F-), ventral ( V. F), and caudal (C. F) fins : the paired fins are not shown. The body-wall consists of deric epithelium (Der. Epthm}, dermis (Derm], and muscle (M) : the latter is metamerically segmented and is very thick, especially dorsally, where it forms half the total vertical height (C). The exoskeleton consists of calcified dermal spines (Derm, sp} in the dermis, and of dermal fin-rays (Derm. F. R) in the fins. The endoskeleton consists of a row of vertebral centra ( V. Cent} below the spinal cord (sp. cd), giving rise to neural arches (n. a), which enclose the cord, and in the caudal regions to haemal arches (h. a) : a cranium (Cr) enclosing the brain (Br) : upper and lower jaws : branchial arches (Br. A) and rays (Br. R, Br. Rl), shown only in B, supporting the gills : shoulder (Sh. G) and pelvic (Pelv. G] girdles : and pterygiosphores (Ptgph] supporting the fins. The mouth (Mtfi) leads into the oral cavity (Or. cav], from which the pharynx (P/i) and gullet (Gttl} lead to the stomach (St) : this is con- nected with a short intestine (Int) opening into a cloaca (C!) which communicates with the extei'ior by the vent (An). The oral cavity and cloaca are the only parts of the canal lined by ectoderm. Connected with the enteric canal are the liver (Lr) with the gall- bladder (G. Bl) and bile-duct (B. D}, the pancreas (/"«), and the spleen (Spl). The mouth is bounded above and below by teeth ( T). The respiratory organs consist of pouches (shown in B) communicating with the pharynx by internal (Int. br. ap} and with the exterior by external (Ext. br. ap} branchial apertures, and lined by mucous mem- brane raised into branchial filaments (br. fil). 1'he heart (Ht) is ventral and anterior, and is situated in a special compartment of the ccelome (Ped). Six of the most important blood- vessels, the dorsal vessel (dorsal aorta, D. Ao}, the cardinal veins (Card. V), the lateral vessels (lateral veins, Lat. F), and the ventral vessel (intra-intestinal vein, /. int. V] are shown in c. The whole ccelome is lined by epithelium, showing parietal (Cat. Epthni) and visceral (Cal. Epthm1} layers. The ovaries (Ovy) are connected with the dorsal body- wall : the oviducts (ovd) open anteriorly into the ccelome (ovd'} and posteriorly into the cloaca. The kidneys (K] are made up of nephridia (NpJi] and open by ureters ( Ur) into the cloaca. The nervous system is lodged in the cerebro-spinal cavity ( C. Sp. cav} hollowed out in the dorsal body-wall : it consists of brain (Br} and spinal cord (Sp. cd}, and contains a continuous cavity, the neuroccele (n. cce). Y 2 324 THE DOG-FISH Still another characteristic feature is the presence, in addition to the dermal exoskeleton, of an endoskeleton^ or system of internal supporting structures. Between the cerebro-spinal cavity above and the coelome below is a longitudinal series of biconcave discs or vertebral centra ( V. Cent) : they are formed of a peculiar tissue called cartilage or gristle, and are strongly impregnated with lime- salts : in the young condition their place is occupied by a gelatinous rod, the notochord. The centra, which alternate with the muscle-segments, are connected with a series of cartilaginous arches (n.a) which extend over the cerebro- ' spinal cavity and with the centra constitute the vertebral column. In the tail there is also a ventral series of arches (h.a.) enclosing a space (H.C] which indicates a backward extension of the ccelome in the embryo. Anteriorly the vertebral column is continued into a cartilaginous box, the cranium (Cr) which encloses the brain and the organs of smell and hearing. The jaws, referred to above, are cartilaginous rods which bound the mouth above and below. The gills are supported by a complicated system of cartilages (Br. A, Br. R, Br. JR.') and both median and paired fins by parallel rods of the same material (Ptgph). All these cartilages are strengthened by a more or less extensive superficial deposit of bony matter. The mouth (MtJi) leads into a large oral cavity (Or. cav) which passes insensibly into a wide throat or pharynx (P/i) : from this a short gullet (Gut) leads into a large U-shaped stomach (St\ whence is continued a short wide intestine (Int) opening on to the exterior through the intermediation of a small chamber, the cloaca (6V). From the gullet backwards the enteric canal is contained in the coelome. The greater part of the enteric epithelium is endodermal : GILLS AND HEART 325 only the oral cavity arises from the stomodaeum and the cloaca from the proctodaeum. In the skin covering the jaws dermal ossicles of unusual size are developed and constitute the teeth (T). The chief digestive glands are two in number, an immense liver (Lr) occupying the whole anterior and ventral region of the ccelome, and a small pancreas (Pn), attached to the anterior end of the intestine. The ducts of both glands open into the intestine, and their secreting cells are, as in former cases, endodermal. Gland-cells are also found in the walls of the stomach and intestine. The respiratory organs or gills (B) consist of five pairs of pouches opening on the one hand into the pharynx (P/i) and on the other to the exterior by the branchial clefts already noticed : they have their walls raised into ridges, the branchial filaments (Br. fil) which are covered with epithelium and are abundantly supplied with blood-vessels. The gills are developed as offshoots of the pharynx, and the respiratory epithelium is therefore endodermal, not ecto- dermal as in the crayfish and mussel. The heart (fit) lies below the pharynx in a separate anterior compartment of the ccelome, the pericardial cavity. It is composed of four chambers arranged in a single longi- tudinal series (sinus venosus, auricle, ventricle, and conus arteriosus), and is to be looked upon as a muscular dilatation of a ventral blood-vessel. The blood is propelled by the heart from the conus arteriosus into a paired series of hoop-like vessels (aortic arches) resembling the transverse commissures of Polygordius (Fig. 69, A, p. 270), which take it through the gills and pour it, in a purified condition, into the dorsal vessel (dorsal aorta, D. Ao) whence it is taken to all parts of the body to be finally returned by thin-walled vessels, called veins, to the sinus venosus. The ventral 326 THE DOG-FISH position of the heart and the fact that the blood is sent directly from the heart to the respiratory organs are characteristic vertebrate features : so also is the circumstance that the blood from the stomach, intestine, &c., is taken by a specially modified portion of the ventral vessel (portal vein) through the liver on its way to the heart. The blood is red, containing, in addition to leucocytes, oval corpuscles coloured by haemoglobin (see p. 56). The excretory organs are a pair of kidneys (K) situated at the posterior end of the dorsal region of the co3lome, and opening by ducts, the ureters (Ur), into the cloaca. De- velopment shows that they consist of an aggregation of nephridia (Nph\ the nephrostomes of which open in the young and sometimes throughout life, into the ccelome, while the nephridiopores discharge not directly on the exterior, but into a common tube. The gonads (ovaries, Ovy, or spermaries) are situated in the anterior part of the ccelome attached by peritoneum to its dorsal wall. The sex-cells are differentiated from ccelomic epithelium. The gonaducts of both sexes (Ovd) are de- veloped from the nephridial system of the embryo. As already stated the central nervous system is contained in a cavity ( C. Sp. cav) of the dorsal body-wall, and is therefore far removed from the ectoderm from which it originates. It consists of a long cylindrical rod, the spinal cord (Sp. cd) which is continued in front into a complicated brain (Br). It has the further peculiarity of being hollow, a more or less cylindrical cavity, the neurocosle (n. cce) ex- tending through its whole length. The possession of a hollow nervous system lying altogether dorsal to the enteric canal and ccelome, of either a noto- chord or a chain of vertebral centra below the nervous system, and of pharyngeal pouches communicating with the DEVELOPMENT 327 exterior, are the three most characteristic features of the vertebrate phylum. The organs of sense are highly developed, and consist of paired olfactory sacs, eyes, and auditory sacs situated in the head, together with an extensive system of integumentary organs. Their sensory cells are in every case ectodermal. The eggs are very large, and are impregnated within the body of the female. In the common Dog-fish (Scyllium) they are laid shortly after impregnation, each enclosed in a horny egg-shell : in the Piked Dog-fish (Acanthias] and the Smooth Hound (Mustelus) they are retained in the oviduct until the adult form is assumed. LESSON XXVIII MOSSES IN the four previous lessons we have traced the advance in organization of animals from the simple diploblastic Hydra to the complicated triploblastic forms which con- stitute the five higher phyla of the animal kingdom. We have now to follow in the same way the advance in structure of plants. The last member of the vegetable kingdom with which we were concerned was Nitella (p. 204), a solid aggregate, exhibiting a certain differentiation of form and structure, but yet composed of what were clearly recognizable as cells, there being, as in Hydra, none of that formation of well-marked tissues which is so noticeable a feature in Polygordius as in other animals above the Ccelenterata. Taking Nitella as a starting point, we shall see that among plants as among animals there is an increasing differentiation in structure and in function as we ascend the series. The first steps in the process are well illustrated by a considera- tion of that very abundant and beautiful group of plants, the Mosses. In spite of the variations in detail met with in different genera of the group the essential features of their organization are so constant that the following description will be found to apply to any of the common forms. B te°.w>? ^Q^L.ma.r FIG. 81. — The Anatomy and Histology of Mosses. A, Entire plant of Funaria hygrometrica, showing stem (st), leaves (/), and rhizoids (r/i). (X 6.) B, leaf of the same, showing midrib (md. r) and lateral portions. (X 25.) C, semi-diagrammatic vertical section of a moss, showing the arrange- 330 MOSSES ment of the tissues. The stem is formed externally of sclerenchyma (scl), and contains an axial bundle (ax. b) : in some of the leaves (/) the section passes through the midrib, in others (71) through the lateral portion : the stem ends distally in an apical cell (ap. c), from which segmental cells (seg. c) are separated. D, transverse section of the stem of Bryum rosettm, showing scleren- chyma (scl), axial bundle (ax. b). and rhizoids (r/i). (X 60.) E, transverse section of a leaf of Funaria, showing the midrib (md. r) formed of several layers of cells, and the lateral portions one cell thick. (X 150.) F, small portion of the lateral region of the same, showing the form of the cells and the chromatophores (chr). (X 150.) G, distal end of the stem of Fontinalis antipyretica in vertical section, showing the apical cell (ap. c) giving rise to segmental cells (seg. c), which by subsequent division form the segments of the stem with the leaves : the thick lines show the boundaries of the segments. H, diagram of the apical cell of a moss in the form of a tetrahedron with rounded base abc and three flat sides abd, bed, acd. (D after Sachs ; G after Leitgeb. ) The plant consists of a short slender stem (Fig. 81, A, st\ from which are given off structures of two kinds, rhizoids or root-hairs (r/i), which pass downwards into the soil, and leaves (/), which are closely set on the stem and its branches. As in Nitella (p. 206) the portion of the stem from which a leaf arises is called a node and the part intervening between any two nodes an internode, while the name segment is applied to a node with the internode next below it. At the upper or distal end of the stem the leaves are crowded, forming a terminal bud. Owing to the opacity of the stem its structure can only be made out by the examination of thin sections (c and D). It is a solid aggregate of close-set cells which are not all alike but exhibit a certain amount of differentiation. In the outer two or three rows the cells (sd} are elongated in the direction of length of the stem, so as to have a spindle-shape, and their walls are greatly thickened and of a reddish colour. They thus form a protective and supporting tissue to which the name sclerenchyma is applied. Running longitudinally STRUCTURE OF LEAF 331 through the centre of the stem is a mass of tissue (ax. b) distinguished by its small, thin-walled cells, and constituting the axial bundle. The leaves (B) are shaped like a spear-head, pointed distally, and attached proximally by a broad base to the stem. The axial portion (B and E, md. r., c. /) consists of several layers of somewhat elongated cells and is called the midrib : the lateral portions (E and F : c, I1} are formed of a single layer of short cells. Thus the leaf has, for the most part, the character of a superficial aggregate. The cells contain oval chromatophores (F, chr). The rhizoids (c and D, rh) are linear aggregates, being formed of elongated cells, devoid of chlorophyll, arranged, end to end. In the terminal bud the leaves, as in Nitella (pp. 206 and 208), arch over the growing point of the stem, which in this case also is formed of a single apical cell (c and G, ap. c). But in correspondence with the increased complexity of the plant the apical cell is not a hemisphere from which new segments are cut off parallel to its flat base, but has the form (H) of an inverted, three-sided pyramid or tetrahedron, the rounded base of which (a be} forms the apex of the stem while segments (seg. c} are cut off from each of its three triangular sides in succession. The best way to understand the apical growth of a moss is to cut a tetrahedron with rounded base out of a carrot or turnip : this represents the apical cell (H) : then cut off a slice parallel to the side abd^ a second parallel to bed, and a third parallel to acd : these represent three successively formed segments. Now imagine that after every division the tetrahedron grows to its original size, and a very fair notion will be obtained of the way in which the successive segments of the moss-stem are formed by the fission in three 332 MOSSES planes of the apical cell. Each segment (c and G, seg. c] immediately after its separation divides and subdivides, pro- ducing a mass of cells from which a projection grows out forming a leaf, and in this way the stem increases in length and the leaves in number. Asexual reproduction takes place in various ways : all of them are, however, varieties of budding, and the buds always arise in the form of a linear aggregate of cells called a protonema : from this the moss-plant develops in the same way as from the protonema arising from a spore (p. 335). The gonads are developed at the extremity of the main stem or of one of its branches, and are enclosed in a tuft of leaves often of a reddish colour — the terminal bud of the fertile shoot or so-called " flower " of the moss. The spermary (Fig. 82, A1, A2) is an elongated club-shaped body consisting of a solid mass of cells, the outermost of which form the wall of the organ, while the inner (AS) become converted into sperms. The latter (A4) are spirally coiled and provided with two cilia : they are liberated by the rupture of the wall of the spermary at its distal end (A2). The ovaries1 (see Preface, p. x, and p. 377) (B1, B2) may or may not occur on the same plant as the spermaries, some mosses being monoecious, others dioecious. Like the sperm- aries they consist at first of a solid mass of cells which assumes the form of a flask, having a rounded basal portion or venter (v) and a long neck (n). The outer layer of cells in the neck and the two outer layers in the venter form the wall of the ovary, the internal cells are arranged in a single axial row at first similar to those of the wall. As the ovary develops the proximal or lowermost cell of the axial row 1 The ovary of mosses, ferns, &c., is usually called an archegonium : the spermary, as in the lower plants, an antheridium. DEVELOPMENT OF SPOROGONIUM 333 takes on the character of an ovum (B2, ov\ the others, called canal-cells (en. c) are converted into mucilage, which by its expansion forces open the mouth of the flask and thus makes a clear passage from the exterior to the ovum (BS). Through the passage thus formed a sperm makes it way and conjugates with the ovum, producing as usual an oosperm or unicellular embryo. The development of the embryo is at first remarkably like what we have found to take place in Hydroids (p. 245). The oosperm divides into two cells by a wall at right angles to the long axis of the ovary : each of these cells divides again repeatedly, and there is produced a solid multicellular embryo or polyplast (c1 spgnm). Very early, however, the moss-polyplast exhibits a striking difference from the animal polyplast or morula : one of its cells— that nearest the neck of the ovary — takes on the character of an apical cell, and begins to form fresh seg- ments like the apical cell of the stem. Thus the plant embryo differs almost from the first from the animal embryo. In the animal there is no apical cell : all the cells of the polyplast divide and take their share in the formation of the permanent tissues. In the plant one cell is at a very early period differentiated into an apical cell, and from it all cells thereafter produced are, directly or indirectly, derived. The embryo continues to grow, forming a long rod-like body (c2, spgnm) the base of which becomes sunk in the tissue of the moss-stem, while its distal end projects vertically upwards, covered by the distended venter (v) of the ovary. Gradually it elongates more and more and its distal end dilates : the embryo has now become a sporogonium^ con- sisting of a slender stalk (c4, st) bearing a vase-like capsule or urn (?/) at its distal end. In the meantime the elonga- tion of the stalk has caused the rupture of the enveloping A* FIG. 82. — Reproduction and Development of Mosses. A1, A spermary of Funaria in optical section, showing the wall en- closing a central mass of sperm-cells : A2, the same from the surface discharging its sperms. (X 300.) PROTONEMA 335 A3, a sperm-cell with enclosed sperm : A4, a free-swimming sperm. (X 800.) B1, an ovary of Funaria, surface view, showing venter (v) and neck (n) : B2, the same in optical section, showing ovum (ov) and canal cells (en. c) : B3, the same after disappearance of the canal cells ; the neck is freely open, and the ovum (ov} exposed. (X 200.) C1, ovary with withered neck containing an embryo (spgnm) in the polyplast stage ( X 200) : in c2 the ovary, consisting of swollen venter (v) and shrivelled neck (n), encloses a young sporogonium (spgnm) ; the distal end of the stem is shown with bases of leaves (/) ; in c3 the venter has ruptured, forming a proximal portion or sheath and a distal portion or calyptra which is carried up by the growth of the sporogonium. (X 10.) C4, a small plant of Funaria with ripe sporogonium consisting of seta (si], with urn (u) and lid (/) covered by the calyptra (c}. C5, diagrammatic vertical section of urn (it,}, showing lid (/), airspaces (a), and spores (sp}. D1, a germinating spore of Funaria, showing ruptured outer coat (sp) and young protonema (pr) with rhizoid (;-//). (X 550.) D2, portion of protonema of the same, showing lateral bud (bd), from which the leafy plant arises. (X 90.) (A and D after Sachs ; B, c1, and C5 altered from Sachs.) venter of the ovary (c3 ) : its proximal part remains as a sort of sheath round the base of the stalk, while its distal portion, with the shrivelled remains of the neck (n) is carried up by the elongation of the sporogonium and forms an extinguisher- like cap or calyptra (c4, c) over the urn. As development goes on the distal end of the urn be- comes separated in the form of a lid (c4, c5, /) and certain of the cells in its interior, called spore-mother-cells, divide each into four daughter cells, which acquire a double cell- wall and constitute the spores (c5, sp) of the moss. When the spores are ripe the calyptra falls off or is blown away by the wind, the lid separates from the urn, and the spores are scattered. In germination the protoplasm of the spore covered by the inner layer of the cell-wall protrudes through a split in the outer layer (D1, sp.) and grows into a long filament, the protonema (pr.), divided by oblique septa into a row of cells. The protonema — which it will be observed is a simple linear 336 MOSSES aggregate — branches, and may form a closely-matted mass of filaments. Sooner or later small lateral buds (D2, bd) appear at various places on the protonema : each of these takes on the form of a three-sided pyramidal apical cell, which then proceeds to divide in the characteristic way (p. 331) forming three rows of segments from which leaves spring. In this way each lateral bud of the protonema gives rise to a moss-plant. Obviously we have here a somewhat complicated case of alternation of generations (see p. 218). The gamobium or sexual generation is represented by the moss-plant, which originates by budding and produces the sexual organs, while the agamobium consists of the sporogonium, developed from the oosperm and reproducing by means of spores. The protonema arising from a spore and producing the leafy plant by budding, is merely a stage of the gamobiurn. The nutrition of mosses is holophytic ; but there is a striking differentiation of function correlated with terrestrial habits. In Nitella the entire organism is submerged in water and all the cells contain chlorophyll, so that decom- position of carbon dioxide and absorption of an aqueous solution of salts is performed by all parts alike, every cell being nourished independently of the rest. In the moss, on the other hand, the rootlets are removed from the influence of light and contain no chlorophyll : hence they cannot decompose carbon dioxide ; but, being sur- rounded by moist soil, are in the most favourable position for absorbing water and mineral salts. The stem, again, is converted into an organ of support : the thickness of its external cells prevents absorption and it contains no chlo- rophyll. Hence the function of decomposing carbon dioxide is confined to the leaves. DISTRIBUTION OF FOOD MATERIALS 337 We have thus as an important fact in the nutrition of an ordinary terrestrial plant that its carbon is taken in at one place, its water, nitrogen, sulphur, potassium, &c., at another. But as all parts of the plant require all these substances it is evident that there must be some means by which the root can obtain a supply of carbon, and the leaves a supply of elements other than carbon. Tn other words, we find for the first time in the ascending series of plants, just as we did in ascending from the simple Hydra to the complex Polygordius (p. 277) the need for some contrivance for the distribution of food-materials. The way in which this distributing process is performed has been studied chiefly in the higher plants, but its essential features are probably the same for mosses. Water is continually evaporating from the surface of the leaves, its place being as constantly supplied by water — with salts in solution — taken in by the rhizoids. This trans- piration, or the giving off of water from the leaves, is one important factor in the process under consideration, since it ensures a constant upward current of water, or, more ac- curately, of an aqueous solution of mineral salts. The withering of a plucked moss-plant is of course due to the fact that where the roots are not imbedded in moist soil or in water, transpiration is no longer balanced by absorption.1 In the higher plants it has been found that the root-hairs have an absorbent action independent of transpiration, so that water may be absorbed in the absence of leaves. By the transpiration current, then, the leaves are kept constantly supplied with a solution of mineral salts derived from the soil, and are thus nourished like any of the aquatic green plants considered in previous lessons : by the double 1 Mosses, however unlike most higher plants, can absorb water by their leaves. Z 338 MOSSES decomposition of water and carbon dioxide a carbo-hydrate is formed : this, by further combination with the nitrogen of the absorbed ammonium salts or nitrates, forms simple nitrogenous compounds, and from these, probably through a long series of mesostates or intermediate products, proto- plasm is finally manufactured. In this way the food supply of the green cells of the leaves is accounted for, but we have still to consider that of the colourless cells of the stem and rhizoids, which, as we have seen, are supplied by the transpiration current with everything they require except carbon, and this, owing to their possessing no chlorophyll, they are unable to take in the form of carbon dioxide. As a matter of fact the chlorophyll-containing cells of the leaves have to provide not only their own food, but also that of their not-green fellows. In addition to making good the waste of their own protoplasm they produce large quantities of plastic products (see p. 337) such as grape sugar, and simple nitrogenous compounds like asparagin, and these pass by diffusion from cell to cell until they reach the uttermost parts of the plant, such as the centre of the stem and the extremities of the rhizoids. The colourless cells are in this way provided not only with the salts contained in the ascending transpiration current, but with carbo-hydrates and nitrogenous compounds. From these they derive their nutriment, living therefore like yeast-cells in Pasteur's solution, or like Bacteria in an organic infusion. We see then that the colourless cells of the stem and rhizoids are dependent upon the green cells of the leaves for their supplies. Like other cells devoid of chlorophyll they are unable to make use of carbon dioxide as a source of carbon but require ready-made carbo-hydrates, the DISTRIBUTION OF FOOD MATERIALS 339 manufacture of which is continually going on, during daylight, in the chlorophyll-containing cells of the leaves. This striking division of labour is the most important physiological difference between mosses and the more lowly organized green plants described in previous lessons. Z 2 LESSON XXIX FERNS WE saw in the previous lesson that in mosses there is a certain though small amount of histological differentation, some cells being modified to form sclerenchyma, others to form axial bundles. We have now to consider a group of plants which may be considered to be, in this respect, on much the same morphological level as Polygordius, the adult organism being composed not of a mere aggregate of simple cells, but of various well-marked tissues. A fern-plant has a strong stem which in some forms, such as the common Bracken (Ptei'is aquilina) is a horizontal underground structure, and is hence often incorrectly con- sidered as a root : in others it creeps over the trunks of trees or over rocks : in others again, such as the tree-ferns, it is vertical, and may attain a height of three or four metres. From the stem are given off structures of two kinds, the leaves, which present an almost infinite variety of form in the various species, and the numerous slender roots. In some cases, such as the tree-ferns and the common Male Shield-fern (Aspidium filix-mas\ the plant ends distally in a terminal bud, consisting, as in Nitella and mosses, of the growing end of the stem over-arched by leaves : in others TISSUES OF THE STEM 341 such as Pteris, the stem ends in a blunt, knob-like extremity quite uncovered by leaves. On the proximal portion of the stem are usually found the withered remains of the leaves of previous seasons, or the scars left by their fall. The roots are given off from the whole surface of the stem, often covering it with a closely-matted mass of dark brown fibres. When the stem is cut across transversely (Fig. 83, A) it is seen, even with the naked eye, to consist of three well- marked tissues. The main mass of it is formed of a wrhitish substance, soft and rather sticky to the touch, and called ground-parenchyma (par) : this is covered by an external layer of very hard tissue, dark brown or black in colour, the hypodermis (hyp) : bands of a similar hard brown substance are variously distributed through the parenchyma, and con- stitute the sdereHchyma (set] : and interspersed with these are rounded or oval patches of a yellowish colour (V.B) harder than the parenchyma, but not so hard as the sclerenchyma, and called vascular bundles, The general distribution of these tissues can be made out O by making longitudinal sections of the stem in various planes or by cutting awray the hypodermis, and then scraping the parenchyma from the vascular bundles and bands of sclerenchyma. The hypodermis is found to form a more or less complete hard sheath or shell to the stem, while the internal sclerenchyma and vascular bundles form longi- tudinal bands and rods imbedded in the parenchyma, and serve as a sort of supporting framework or skeleton. The minute structure of the stem can be made out by the examination either of very thin longitudinal and trans- verse sections, or of a bit of stem which has been reduced to a pulp by boiling in nitric acid with the addition of a few crystals of potassium chlorate : by this process the various 1/.B par st FIG. 83.— Anatomy and Histology of Ferns. GENERAL CHARACTERS 343 A, Transverse section of the stem of Pteris aquilina, showing hypo- dermis (hyp], ground parenchyma (par), sclerenchyma (scl), and vascular bundles (V. B). (X 2.) B, transverse section of a vascular bundle, showing bundle-sheath (b. s/i), sieve-tubes (sv. t). scalariform vessels (sc. v), and spiral vessels (sp.v). (X 6.) C, semi-diagrammatic vertical section of the growing point of the stem, showing apical cell (ap. c), segmental cells (seg. c), and apical meristem (ap. mer) passing into permanent tissue consisting of epidermis (ep), hypodermis (hyp), ground parenchyma (par), sclerenchyma (scl), and vascular bundles in which the sheath (b. sh), sieve-tubes (sv. t), scalariform vessels (sc. v), and spiral vessels (sp. v) are indicated. D, a single parenchyma cell, showing nucleus (mi), and vacuole (vac). E, cell of hypodermis. F, portion of a sieve-tube, showing sieve-plates (sv. pi). G, portion of a spiral vessel with the spiral fibre partly unrolled at the lower end. H, fibre-like cell of sclerenchyma. I, portion of a scalariform vessel, part of the wall being supposed to be removed. K, vertical section of a leaf of Pteris, showing upper and lower epi- dermis (ep), mesophyll cells (msph), with intercellular spaces (i. c. sp), a stoma (st) in the lower epidermis, and hairs (h). L, surface view of epidermis of leaf of Aspidium, showing two stomata (st) with their guard-cells (gd. c). M, vertical section of the end of a root, showing apical cell (ap. c), segmental cells (seg. c), and root-cap (r. cp) with its youngest cap-cells marked cp. c. (A, B, and D-K after Howes ; M from Sachs, slightly altered.) tissue elements are separated from one another, and can be readily examined under a high power. By combining these two methods of sectioning and dissociation the parenchyma is found to consist of an aggregate of polyhedral cells (D) considerably longer than broad, their long axes being parallel with that of the stem itself. The cells are to be considered as right cylinders which have been converted into polyhedra by mutual pres- sure. They have the usual structure, and their protoplasm is frequently loaded with large starch-grains. They do not fit quite closely together, but spaces are left between them, especially at the angles, called inter-cellular spaces. 344 FERNS The cells of the hypodermis (E) are proportionally longer than those of the parenchyma, and are pointed at each end : they contain no starch. Their walls are greatly thickened, and are composed not of cellulose but of lignin, a carbo- hydrate allied in composition to cellulose, but containing a larger proportion of carbon. Schulze's solution, which, as we have seen, stains cellulose blue, imparts a yellow colour to lignin. Outside the hypodermis is a single layer of cells (c, ep) not distinguishable by the naked eye and forming the actual external layer of the stem : the cells have slightly thickened, yellowish-brown walls, and constitute the epidermis. From many of them are given off delicate filamentous processes con- sisting each of a single row of cells : these are called hairs. In the sclerenchyma the cells (H) are greatly elongated and pointed at both ends, so as to have the character rather of fibres than of cells. Their walls are immensely thickened and lignified, and present at intervals oblique markings due to narrow but deep clefts : these are produced by the deposition of lignin from the surface of the protoplasm (see p. 32) being interrupted here and there instead of going on continually as is the case of a cell-wall of uniform thickness. The vascular bundles have in transverse section (B) the appearance of a very complicated network with meshes of varying diameter. In longitudinal sections (c) and in dis- sociated specimens they are found to be partly composed of cells, but to contain besides structures which cannot be called cells at all. In the centre of the bundle are a few narrow cylindrical tubes (B and c, sp. v.) characterized at once by a spiral marking, and hence called spiral vessels. Accurate exam- ination shows that their walls (G) are for the most part thin, but are thickened by a spiral fibre, just as a paper tube SIEVE-TUBES 345 might be strengthened by gumming a spiral strip of paste- board to its inner surface. These vessels are of considerable length, and are open at both ends : moreover they contain no protoplasm, but are filled with either air or water : they have therefore none of the characteristics of cells. They are showrn, by treatment with Schulze's solution, to be com- posed of lignin. Surrounding the group of spiral vessels, and forming the large polygonal meshes so obvious in a transverse section, are wide tubes (B and c, sc. v) pointed at both ends and fitting against one another in longitudinal series by their oblique extremities. They have transverse markings like the rungs of a ladder, and are hence called scalariform vessels. The markings (i) are due to wide transverse pits in the otherwise thick lignified walls : in the oblique ends by which the vessels fit against one another, the pits are frequently replaced by actual slits, so that a longitudinal series of such vessels forms a continuous tube containing, like the spiral vessels, air or water, but no protoplasm. In most ferns the terminal walls are not thus perforated, and the elements are then called tracheides. The presence of these vessels — spiral and scalariform- is the most important histological character separating ferns and mosses. The latter group and all plants below them are composed exclusively of cells : ferns and all plants above them contain vessels in addition, and are hence called vas- cular plants. The vessels, together with small parenchyma-cells inter- spersed among them, make up the central portion of the vascular bundle, called the wood or xylem. The peripheral portion is formed of several layers of cells composing the bast or phloem, and surrounding the whole is a single layer of small cells, the bundle-sheath (p. s/i). 346 FERNS The cells of the phloem are for the most part parenchy- matous, but amongst them are some to which special attention must be drawn. These (B and c, sv. t\ are many times as long as they are broad, and have on their walls irregular patches or sieve-plates (F, sv.pl.) composed of groups of minute holes through which the protoplasm of the cell is continuous with that of an adjacent cell. The transverse or oblique partitions between the cells of a longitudinal series are also perforated, so that a row of such cells forms a sieve- tube in which the protoplasm is continuous from end to end. We have here, therefore, as striking an instance of proto- plasmic continuity as in the deric epithelium and certain other tissues of Polygordius (see p. 272). The distal or growing end of the stem terminates in a blunt apical cone or punctum vegetationis (c), surrounded by the leaves of the terminal bud in the case of vertical stems, or sunk in a depression and protected by close-set hairs in the underground stem of the bracken. A rough longitudinal section shows that, at a short distance from the apical cone, the various tissues of the stem — epidermis, parenchyma, sclerenchyma, and vascular bundles — merge insensibly into a whitish substance, resembling parenchyma to the naked eye, and called apical meristem (ap. mer). Thin sections show that the summit of the apical cone is occupied by a wedge-shaped apical cell (ap. c) which in vertical stems is three-sided like that of mosses (Fig. 81, H, p. 329), while in the horizontal stem of Pteris it is two-sided. As in mosses segmental cells (seg. c) are cut off from the three (or two) sides of the apical cell in succession and by further division form the apical meristem (ap. mer\ which consists of small, close-set cells without intercellular spaces. As the base of the apical cone is reached the meristem is found to APICAL GROWTH 347 pass insensibly into the permanent tissues, the cells near the surface gradually merging into epidermis and hypodermis, those towards the central region into sclerenchyma and the various constituents of the vascular bundles, and those of the intermediate regions into parenchyma. The examination of the growing end of the stem shows us how the process of apical growth is carried on in a compli- cated plant like the fern. The apical cell is continually undergoing fission, forming a succession of segmental cells : these divide and form the apical meristem, which is thus being constantly added to at the growing end by the formation and subsequent fission of new segmental cells : in this way the apex of the stem is continually growing upwards or forwards. But at the same time the meristem cells farthest from the apex begin to differentiate : some elongate but slightly, increasing greatly in size, and become parenchyma cells : others by elongation in the direction of length of the stem and by thickening and lignification of the cell-wall become sclerenchyma cells : others again elongate greatly, become arranged end to end in longitudinal rows, and, by the loss of their protoplasm and of the transverse partitions between the cells of each row, are converted into vessels — spiral or scalariform according to the character of their walls. Thus while the epidermis, parenchyma, and sclerenchyma are formed of cells, the spiral and scalariform vessels are cell- fusions, or more accurately cell-wall-fusions, being formed by the union in a longitudinal series of a greater or less number of cell-walls. It will be remembered that the muscle-plates of Polygordius are proved by the study of development to be cell-fusions (p. 300). We thus see that every cell in the stem of the fern was once a cell in the apical meristem, that every vessel has arisen by the concrescence of a number of such cells, and that the 348 FERNS meristem cells themselves are all derived, by the ordinary process of binary fission, from the apical cell. In this way the concurrent processes of cell-division, cell-differentiation, and cell-fusion result in the production of the various and complex tissues of the fully-formed stem. The leaves vary greatly in form in the numerous genera and species of ferns : they may consist of an unbranched stalk bearing a single expanded green blade : or the stalk may be more or less branched, its ramifications bearing the numerous subdivisions of the blade , ncl\ attached by its proximal end to the sporophyll, and sur- B B 2 372 GYMNOSPERMS rounded by a wall or integument (/) also formed of a small- celled tissue. The integument is in close contact with the nucellus, but is perforated distally by an aperture, the micropyle (mpy\ through which a small area of the nucellus is exposed. Each megasporangium contains only a single megaspore (embryo sac, c and D, mg. sp) in the form of a large ovoidal body embedded in the tissue of the nucellus. It has at first the characters of a single cell, but afterwards, by division of its protoplasm, becomes filled with small cells representing a prothallus (prt/i). As in Vascular Cryptogams, single superficial cells of the prothallus are converted into ovaries which are extremely simple in structure, each consisting of a large ovum (ov\ and of a variable number of neck-cells. The pollen, liberated by the rupture of the microsporangia, is carried to considerable distances by the wind, some of it falling on the female cones of the same or another tree. In this way single microspores (pollen-grains) find their way into the micropyle of a megasporangium (D, mi. sp). This is the process known as pollination, and is the necessary antecedent of fertilization. The microspore now germinates : the outer coat bursts, and the larger of the two cells (B, b) protrudes in the form of a filament resembling a hypha of Mucor, and called a pollen-tube (D, p.f). This forces its way into the tissue of the nucellus, like a root making its way through the soil, and finally reaches the megaspore in the immediate neigh- bourhood of an ovary. A process then grows out from the end of the tube, passes between the neck-cells, and comes in contact with the ovum. In the meantime the nucleus of the large cell (b) of the microspore — that from which the pollen-tube grows — has travelled to the end of the pollen-tube and divided into two FORMATION OF THE SEED 373 Protoplasm collects round each of the daughter nuclei, con- verting them into cells, one of which remains undivided, while the other divides, and its substance, in some way not understood, passes from the pollen-tube into the ovum, where it forms a cell-like body, to which the name of male pronucleus (see p. 259) has been applied. This conjugates with the nucleus of the ovum, or female pronucleus, and thus effects the process of fertilization, or the conversion of the ovum into the oosperm. The mode of formation of cells described in the preceding paragraph should be specially noted. Instead of the ordin- ary process of fission hitherto met with, the products of division of a nucleus become surrounded by protoplasm, cells being produced which lie freely in the interior of the mother-cell. This is called free cell-formation. The development of the oosperm is a very complicated process, and results in the formation not of a single polyplast but of four, each at the end of a long suspensor (D, spsr\ in the form of a linear aggregate of cells, which by its elonga- tion carries the embryo (emb) down into the tissue of the prothallus. As a rule only one of these embryos comes to maturity : it develops a rudimentary stem and root and four or more cotyledons, and so becomes a phyllula. While these processes are going on the female cone increases greatly in size and becomes woody. The mega- sporangia also become much larger, their integuments (E, /), becoming brown and hard, and the megaspore in each enlarges so much as to displace the nucellus : at the same time the cells of the prothallus filling the megaspore develop large quantities of plastic products, such as fat and albumin- ous substances to be used in the nutrition of the embryo : the tissue thus formed is the endosperm (end}. The mega- sporangium is now called a seed (see p. 361). 374 ANGIOSPERMS Under favourable circumstances the seed germinates. By absorption of moisture its contents swell and burst the seed-coat, and the root of the phyllula (r) emerges, followed before long by the stem (st) and cotyledons (. Plants. All exhibit alternation of generations and the series shows the gradual subordination of the gamobium to the agamobium. SYNOPSIS 389 PAGE 39. Equisetum. Sporangia borne on sporophylls arranged in cones : spores homomorphic : prothalli dimorphic (male and female) ... 362 40. Salvinia. Spores dimorphic : microspore produces vestigial male prothallus : megaspore produces greatly reduced female prothallus 364 41. Selaginella. Microspore produces unicellular prothallus and multi- cellular spermary, both endogenously : female prothallus formed in megaspore and is almost endogenous : embryo provided with suspensor . 367 42. Gymnospenns. Cones dimorphic (male and female), with rudimentary perianth : no sperms formed but microspore gives rise to pollen tube, nuclei in \vhich are the active agents in fer- tilization : single megaspore permanently inclosed in each megasporangium : female prothallus purely endo- genous : embryo (phyllula) remains inclosed in mega- sporangium which becomes a seed . 369 43. Angiosperms. Cone modified into flower by differentiation of sporo- phylls and perianth : female sporophyll forms closed cavity in which megasporangia are contained : mega- spore produces a single ovary represented simply by an ovum and two synergidae : formation of prothallus re- tarded until after fertilization . . 374 B.— SUBJECTS OF GENERAL IMPORTANCE DISCUSSED IN SPECIAL LESSONS. I. — CELLS AND NUCLEI a. The higher plants and animals contain cells similar in struc- ture to entire unicellular organism, and like them exist- ing in either the amoeboid, ciliated, encysted, or plas modial condition 56 b. Minute structure of cells : — cell-protoplasm, cell-membrane, nuclear membrane, achromatin, chromatin 6 1 c. Direct and indirect nuclear division .... ... 63 d. The higher plants and animals begin life as a single cell, the ovum 67 390 SYNOPSIS II. — BIOGENESIS. PAGE a. Definition of biogenesis and abiogenesis : brief history of the controversy .... 93 b. Crucial experiment with putrescible infusions : sterilization : germ-filters : occurrence of abiogenesis disproved under known existing conditions ... 96 III. HOMOGENESIS. Definition of homogenesis and heterogenesis : truth of the former firmly established .... 100 IV. — ORIGIN OF SPECIES. a. Meaning of the term Species : the question illustrated by a consideration of certain species of Zoothamnium . . . 135 b. Definition of Creation and Evolution : hypothetical histories of Zoothamnium in accordance with the two theories . 139 c. The principles of Classification : natural and artificial classifications ... . .138 d. The connection between ontogeny and phylogeny . .143 V. — PLANTS AND ANIMALS. a. Attempt to define the words plant and animal, and to place the previously considered types in one or other king- dom ... . . 174 b. Significance of the " third kingdom," Protista 180 VI. — SPERMATOGENESIS AND OOGENESIS. Origin of sperms and ova from primitive sex-cells : differences in structure and development of the sexual elements . 252 VII. — MATURATION AND IMPREGNATION. a. Formation of first and second polar ceils and of female pronucleus ... 257 b. Entrance of sperm and formation of male pronucleus . . 258 c. Conjugation of pronuclei . . 259 SYNOPSIS 391 VIII. — UNICELLULAR AND DIPLOBLASTIC ANIMALS. PAGE a. In plants there is a clear transition from unicellular forms to solid aggregates, but in animals the connection of the gastrula with unicellular forms is uncertain 260 b. Two hypotheses of the origin of the gastrula : (i) from a colony of unicellular zooids ; (2) from a solitary multi- nucleate but unicellular form . 265 C. — Other matters of general importance, such as the composition and properties of protoplasm, cellulose, chlorophyll, starch, &c. : metabolism : holozoic, holophytic, and saprophytic nutrition : intra- and extra-cellular digestion : amoeboid, ciliary, and muscular movements : the elementary physiology of muscle and nerve : parasitism and sym- biosis : asexual and sexual generation : and the elements of embryology — are discussed under the various types and will be most conveniently referred to by consulting the Index. INDEX AND GLOSSARY INDEX AND GLOSSARY AbiOgen'esiS (a, not : |3io?, life : -yeVeo-i?, origin), the origin of organisms from not-living matter : former belief in, 94 Absorption by root-hairs, 337 Accre'tion (ad, to : cresco, to grow), in- crease by addition of successive layers, *4 Acnrom'atin (a, not : xp^>Ma> colour), the constituent of the nucleus which is un- affected or but slightly affected by dyes, 7, 62 Acoelom'ate (a, not : Kot'Aw/xa, a hollow), having no ccelome (9, life), applied to those microbes to which free oxygen is unnecessary, 91 Agamob'ium (a, not : -ya^o?, marriage : /Si'o?, life), the asexual generation in or- ganisms exhibiting alternation of gene- rations (g.i>.) AGAR'ICUS (mushroom) :— Figure, 190 : general characters, 189 : microscopic structure, 189, 190 : spore-formation, 191 Algae (alga, sea-weed), 167 Alternation of Generations, meaning of the phrase explained under Nitella, 218 : Bougainvillea, 247 : Moss, 336 : Fern, 357 : Equisetum, 363, 364 : Salvinia, 366 : Selaginella, 369 : Gymnosperms, 374 : Angiosperms, 380, 381 Ambula'cral (ambulacrujii), a walking place) system, starfish, 309 AMCEB'A (ju.epaj, to bear), a mass of proteid material im- pregnated with chlorophyll or some other colouring matter, 26, 46, 195, 213, 228 Cil'ium (ciliuin, an eye-lash), defined, 25» : comparisons of with pseudopod, 34, 52 : absence of cilia in Arthropoda, 315 Ciliary movement, 25 : a form of con- tractility, 34 Ciliate Infusoria, 105 Classification, natural and artificial, 139 : natural, a genealogical tree, 142 Cnid'Ofolast (wiSy, a nettle : j8A.aoro'?, a bud), the cell in which a nematocyst (g.v.) is developed, 227 Cnid'OCil (wiSr) and ciliiiui), the "trigger- hair " of a cnidoblast, 227 Ccelenterata, the, 304 Ccelome (/coiAu>ju.a, a hollow), the body- cavity : — Polygordius, 269 : Starfish, 307 : Crayfish, 314 : Mussel, 318 : Dogfish, 321 : development of, Polygordius, 298 Ccelom'ate, provided with a coelome, 272 Cffilomic epithelium. See Epithelium. Ccelomic fluid, Polygordius, 277 Colloids (/co'AAa, glue : elSo?, form), pro- perties of, 6 Colony, Colonial organism, meaning of term, 133, 241 : formation of temporary colonies, Hydra, 230 Columel'la (a little column), 160 Com'missure (commissura, a band), 278 Compound organism. See Colony. Concres'cence (cum, together : cresco, to grow), the union of parts during growth Cone, an axis bearing sporophylls : — Equi- setum, 362 : Selaginella, 367 : Gymno- sperms, 369 Conjuga'tion (conjugfitio, a coupling), the union of two cells in sexual reproduc- tion : — Amoeba, 20 : Heteromita, 41, 42 : Paramoscium, 112, 113: Vorticella, 130: Mucor, 163 : Spirogyra, 196 : of ovum and sperm, 259 : monoecious and dioe- cious, 197 : comparison with plasmodium- formation. 54 Connective, ossophageal, 282 Connective tissue, 308 Contractile vac'uole (ziaciiiis, empty) :— Amoeba, 8, 9 : Euglena, 47 : Para- moecium, 109 Contractility (contracts, a drawing to- gether), nature of, 10, 34 : muscular, 128 Contraction, physical and biological, 10 Corolla (corolla, a little wreath), the inner or distal whorl of the perianth in the flower of Angiosperms, 377 Corpuscles. See Blood-corpuscles, and Leucocytes. Cortex, cor'tical layer (cortex, bark), constitution of, 59 : Infusoria, 108 Cotton- wool as a germ-fitter, 97 Cotyle'don (/cOTuATjSioV, a cup or socket), the first leaf or leaves of the phyllula (q.t>.) in vascular plants, 355 Cranium (xpd.vi.ov, the skull), 324 CRAYFISH : — Figure, 312: general charac- ters, 310, 311 : limited number and con- crescence of metameres, 310 : appen- dages, 310: exoskeleton, 311: enteric canal, 311: gills, 314: blood-system, 314 kidney, 314 : nervous system, 315 Creation (creo, to produce), definition of, 1 39 illustrated in connection with species of Zoothamnium (Diagram), 140 Cross-fertilization : applied to the sexual process when the gametes spring from different individuals, 197 Cryst'alloids (Kpuo-raAAo?, crystal : elSos. form), properties of, 6 Cuticle (cuticula, the outer skin), nature of in unicellular animals, 45, 107 : in multicellular animals, 236 Cyst ( KU'CTTIS, a bag), used for cell-wall in many cases, n, 54 D Dallinger, Dr. W. H., observations on an apparent case of heterogenesis, 101 Daughter-cells, cells formed by the fission or gemmation of a mother-cell, 35, 65 Death, phenomena attending, 20, 21, 164, 165 Decomposition, nature of, 6 DermiS (6e'p/xa, skin), the deep or connec- tive tissue layer of the skin, 308 Descent, doctrine of. See Evolution. Development, meaning of the term, 43. For development of the various types see under their names. Dextrin, m Diastase, 80 Diast'Ole (SiourreAAoj, to separate), the phase of dilatation of a heart, contractile vacuole, £c. , 109 DIATOMA'CE-S (5taTe>i/w, to cut across, because of the division of the shell into two valves), 153: Figure^, 154 Diat'omin, the characteristic yellow colour- ing matter of diatoms, 152 DichOt'OmOUS (Sixoro/aeo), to cut in two), applied to branching in which the stem divides into two axes of equal value, 136 Differentiation (diffiro, to carry different ways), explained and illustrated, 34, 117 Diges'tion (digero, to arrange or digest), the process by which food is rendered fit for absorption, 12, 13 : intra- and extra- 398 INDEX AND GLOSSARY cellular, 229 : contrasted with assimila- tion, 229 Digestive gland, 314 Dimorphism, dimorphic (<3is, twice : popart, form), existing under two forms, 35, 134, 241, 364, 366 Dice'ClOUS (<5i's, twice : ot*o?, a dwelling), applied to organisms in which the male and female organs occur in different in- dividuals, 197 DIPH'YES (Si^v-n?, double) : Figure, 248 : occurrence and general characters, 249 : polymorphism, 249 Diploblastic (6177X005, double : £Aao-Tos, a bud), two-layered : applied to animals in which the body consists of ectoderm and endoderm, 241 : derivation of diploblas- tic from unicellular animals (Figures). 264, 265 Disc, Vorticella, 126 Dispersal, means 6f : in internal parasite, 122 : in fixed organisms, 131-134 Distal, the end furthest from the point of attachment or organic base, 124 Distribution of food-materials :— in a complex animal, 277 : in a complex plant, 337 Divergence of character, 143 Division of physiological labour, 34 DOGFISH : — Figure, 322 : general charac- ters, 320 : fins, 321 : exoskeleton, 321 : endoskeleton, 324 : enteric canal, 324 : gills, 325 : blood-system, 325 : kidney, 326 : gonads, 326 : nervous system, 326 Dry-rigor, stiffening of protoplasm due to abstraction of water, 21 E Echinodermata, the, 304 Ect'Oderm (e/cro'?, outside : Septet, skin), the outer cell-layer of diploblastic and triploblastic animals, 223, 227. 274 Ect'OSarC (e/cros, outside : crap£, flesh), the outer layer of protoplasm in the lower unicellular organisms, distinguished by freedom from granules, 4 Egestlon (egero, to expel), the expulsion of waste matters, 12 Egg-cell. See Ovum. Em'bryo (e'/u./Spvoi', an embryo or foetus), the young of an organism before the commencement of free existence. Em'bryo-sac. See Megaspore. Encysta'tion, being enclosed in a cyst (?.,*-) End'oderm (evSov, within : Sep^a, skin), the inner cell-layer of diploblastic and triploblastic animals, 223, 228. 274 End'oderm-lamella, Medusa, 239 EndOg'enOUS (evSov, within : yiyi/o/xai, to come into being), arising from within, e.g. the roots, of vascular plants, 350 End'OSarc (evBov, within: o-ap£, flesh), the inner, granular protoplasm of the lower unicellular organisms, 4 Endoskel'eton(eV5ov, within, and skeleton, from o-Ke'AAto, to dry), the internal skele- ton of animals, 324 End'osperm (evSov, within : 0-irepiJ.a, seed), nutrient tissue formed in the megaspore of Phanerogams, 373, 380 Endospore (iivSov, within : CTTropd, a seed), a spore formed within a vegetative cell, OQ OO Energy, conversion of potential into kinetic, 15 : source of, in chlorophyll- containing organisms, 31 Enteric (eWepoi/, intestine) canal, the entire food-tube from mouth to anus : — Polygordius, 269, 275 ; Starfish, 307 ; Crayfish, 311 ; Mussel, 318 ; Dogfish, 324 Ent'eron or Enteric cavity, the simple digestive chamber of diploblastic ani- mals, 221 Epidermis (eTu, upon : Sep^a, the skin) : in animals synonymous with deric epi- thelium (g.v., under Epithelium) : in vas- cular plants a single external layer of cells, 344, 348, 349 Epithelial cells : columnar, 58 : ciliated, 59 Epithelium (en-t, upon : tfrjA.^ the nipple), a cellular membrane bounding a free surface, 243 : ccelomic, 273, 300 : deric, 272 : enteric, 273, 275 Equatorial plate, 65 EQUISETUM (egnits, a horse: seta, a bristle) : — Figure, 363 : general charac- ters, 362 : cone and sporophylls, 362 : male and female prothalli, 363, 364 : al- ternation of generations, 363, 364 Equiv'ocal generation. See Abiogenesis. EUGLEN'A (euyArjvo?, bright-eyed) :— Figure, 45 : occurrence and general characters, 44 : movements, 44 : struc- ture, 45 : nutrition, 46 : resting stage, 47 : reproduction, 48 : animal or plant ? 178 Euglen'oid movements, 45 Ev'olution (evolvo, to roll out), organic : definition, 139 : illustrated in connection with species of Zoothamnium (Diagram), 142 Excre'tion (cxcerno, to separate), the separation of waste matters derived from the destructive metabolism of the or- ganism, 1 6, 280 Exogenous (ef, out of: -yiy^o^at, to come into being), arising from the exterior, e.g. leaves, 350 Exoskel'eton (e£w, outside, and skeleton, from o-KeAAw, to dry), the external or skin-skeleton : cuticular, 237, 273-275 : cuticular and calcined, 311, 316 : epi- dermal (hair and nails) : dermal, 308, 321 INDEX AND GLOSSARY 399 Eye-spots or Ocelli :— Medusa, 239 Polygordius, 286, 295 Faeces (faex, dregs), solid excrement, consisting of the undigested portions of the food, 1 6 Ferm'ent (fermentum, yeast, from fer- veo, to boil or ferment), a substance which induces fermentation, i.e. a definite chemical change, in certain sub- stances with which it is brought into contact, without itself undergoing change : unorganized and organized ferments, 79 : alcoholic, 75-80 : ace- tous, 90 : diastatic or amylolytic, 80 : lactic, 90 : peptonizing or proteolytic, 80 : putrefactive, 90 : ferment-cells of Mucor, 165 FERNS : — Figures, 342, 352 : general characters, 340, 341 : histology of stem, leaf, and root, 340-350 : nutrition, 350 : spore-formation, 351 : prothallus and gonads, 351-354 : development, 355 : al- ternation of generations, 357 Fertilization (fertilis, bearing fruit) : the process of conjugation of a sperm or sperm-nucleus with an ovum whereby the latter is rendered capable of develop- ment : a special case of conjugation (q.v.), 197 : details of process, 259 : in Vaucheria, 171 : in Gymnosperms, 372, 373 : in Angiosperms, 379 Filtering air, method of, 97 Fins, Dogfish, 321 FiSS'ion (fissio, a cleaving), Simple or binary), the division of a mother-cell into two daughter-cells : in Amoeba, 19 : Heteromita, 40 : animal cells generally, 63-65 : plant-cells generally, 65-67 : Para- moecium, 112 : Vorticella, 129 Fission, multiple, the division of a mother-cell into numerous daughter- cells : — in Heteromita, 42 : Protomyxa, 51 : Saccharomyces, 73 Fission, process intermediate between simple and multiple, Opalina, 122 Flagella. See Cilium. Flag'ellate Infusoria, 105 Flagell'ula (diminutive of flagelluni), the flagellate germ of one of the lower organisms (often called zoospores, 51, 54 Flagell'um (flagellum, a whip) : defined, 25 : transition to pseudopod, 52, 228 Floral receptacle, the abbreviated axis of an angiospermous flower, 376 Flower, a specially modified cone (g.v.), having a shortened axis which bears perianth-leaves as well as sporophylls, 374 : often applied to the cone of Gymno- sperms, 369 Food-current, Mussel, 316, 318 Food-vacuole, a temporary space in the protoplasm of a cell containing water and food-particles, n, no Foot : of Mussel, 316 : of phyllula of fern FORAMINIF ERA (foramen,* hole \fero, to bear), 146 : Figures, 147, 148, 149 Fragmentation of the nucleus, n8 Free cell formation, 373 Fruit of Angiosperms, 380 Function (fttnctio, a performing), mean- ing of the term, 9 Gam'ete (ya/ae'co, to marry), a conjugating cell, whether of indeterminate or deter- minate sex : — Heteromita, 41 : Mucor, 163 : Spirogyra, 196 : Vaucheria, 171 Gamob'ium (ya/aos, marriage : /St'os, life), the sexual generation in organisms ex- hibiting alternation of generations (q.v.) : progressive subordination of, to agamo- bium in vascular plants, 357, 381 Ganglion (yayyAiov, a tumour), a swelling on a nerve-cord in which nerve-cells are accumulated, 315, 319 Gastric juice (yaa-nfip, the stomach), pro- perties of, 12 Gast'mla (diminutive of yacrrxjp, the stomach), the diploblastic stage of the animal embryo in which there is a diges- tive cavity with an external opening : characters and Figure of, 261 : two hypotheses of origin of (Figures), 264, 265 : contrasted with phyllula, 356 Gemmation (gemma, a bud). See Bud- ding. Generation, asexual, See Agamobium. Sexual. See Gamobium. Generations, Alternation of. See Al- ternation of generations. Generalized, meaning of term, 138 Gen'US (genus, a race), generic name, generic characters, 8, 137 Germ-filter, 97 Germinal spot, the nucleolus 01 the ovum, 255 Germination (germinatio, a budding), the sprouting of a spore, zygote, or oosperm to form the adult plant : for germination of the various types see under their names. Gill, an aquatic respiratory organ, 314, 319, 32o> 325 Gland (glans, an acorn), an organ of se- cretion (q.v .) : gland-cells, 227, 229, 281 Glochid'ium, 320 Gon'ad (yo'i/o?, offspring, seed), the essen- tial organ of sexual reproduction, whether of indeterminate or determinate sex, i.e. an organ producing either un- 4OO INDEX AND GLOSSARY differentiated gametes, ova, or sperms : see under the various types, and espe- cially 170, 196, 207, 212 Gon'aduct (gonad, and dftco, to lead), a tube carrying the ova or sperms from the gonad to the exterior, 291, 310, 315, 319, 326 Grappling-lines, Diphyes, 249 Growing point : Nitella, 209 : Moss, 331 : Fern, 346 Growth, 13 Guard-cells of stomates, 349 Gullet, the simple food-tube of Infusoria, 47, 108 : or part of the enteric canal of the higher animals, 276 GYM'NOSPERMS (yufxi/o'?,naked: a-nep^a, seed) : Figure, 370 : general characters, 369 : structure of cones and sporophylls, 369, 371 : reduction of gamobium (pro- thalli and gonads), 372 : pollination and fertilization, 372, 373 : formation of seed and development of leafy plant, 373, 374 Gynoec'ium (yvvr/, a female : uroV, a plant) nutrition, defined, 31 Holozo'ic (oAos, whole : £o>oi/, an animal), nutrition, defined, 31 Homogen'esis (6/x6s, the same : yeVeo-is, origin), meaning of the term, 100 Homol'ogOUS (6/xoAo-yos, agreeing), applied to parts which have had a common origin, 240 Homomorph'ism, homomorph'ic (6/ixos, the same : /u.op<£ij, form), existing under a single form, 137 Host, term applied to the organism upon which a parasite preys, 121 HYDRA (v5pa, a water-serpent) : Figures, 220, 224, 232 : occurrence and general characters, 219 : species, 221 : move- ments, 221, 222 : mode of feeding, 222 : microscopic structure, 223 : digestion, 229 : asexual, artificial, and sexual re- production, 230, 231 : development, 232, Oil Hydr'anth (iiSpa, a water-serpent : avOos, a flower), the nutritive zooid of a hydroid polype, 236, 240 Hydroid (v6pa, a water-serpent : elSos, form) Polypes (TroAuVov?, many-footed), compound organisms, the zooids of which have a general resemblance to Hydra, 234 Hyper'trophy (vTre'p, over: rpo^-tj, nourish- ment), an increase in size beyond the usual limits, 116 Hyph'a (v^atVw, to weave), applied to the separate filaments of a fungus : they may be mycelial (see mycelium), sub- merged, or aerial : Mucor, 157, 166 Penicillium, 183, 186 Hyp'odermiS (UTTO, under : Se'p/ia, skin), Fern, 341, 344 ^ Hyp'ostome (vno, under : 0-To/xa, mouth), 221, 236 I Immortality, virtual, of lower organisms, 21 Income and expenditure of protoplasm, 18 Individual. See Zooid. Individuation, meaning of the term, 230 251 Indus'ium (indusz'um, an under-garment), 35° INDEX AND GLOSSARY 401 Inflores'cence (floresco, to begin to flower), an aggregation of cones or flowers, 369 Infusoria (so called because of their fre- quent occurrence in infusions), 105 Ingesta (ingero, to put into) and Egesta (egero, to expel), balance of, 32 Ingestion (ingero, to put into), the taking in of solid food, n, 58 Insola'tion (insolo, to place in the sun), exposure to direct sunlight, 92 Integ'ument (integiimen.tum, a covering) of megaspore : Gymnosperms, 372 : Angiosperms, 378 Inter-cellular spaces, 343 Inter-muscular plexus (n-Ae'icw, to twine), 284 Internode (inter, between : nodus, a knot), the portion of stem intervening between two nodes, 206 Interstitial (attersttfium, a space be- tween) cells. Hydra, 225 : growth, Spirogyra, 196 Intestine (intcsttnns, internal), part of the enteric canal of the higher animals, 276 IntUS-SUSCep'tion (in/us, into : suscipio, to take up), addition of new matter to the interior, 13 Iodine, test for starch, 27 Irritability (irritabllis, irritable), the property of responding to an external stimulus, 10 Jaws: — Crayfish, 311 : Dogfish, 325 K Karyokinesls (napvov, a kernel _or_ nu- cleus : /ciVrjcri?, a movement), indirect nuclear division, 65 Katab'olism (/cara^o^, a laying down), 18. See Metabolism, destructive. Kat'astateS ((caTa.on-TJi'ai, to sink down), 1 8. See Mesostates, katabolic. Kidney : — Crayfish, 314 : Dogfish, 326 LAMINARTA (lamina, a plate), 201 (Figure), 202 Labial palps, Mussel, 318 Larva, the free-living young of an animal in which development is accompanied by a metamorphosis, 295 Larval stages, significance of, Polygor- dius, 297 Leaf, structure of:— Nitella. 206, 207, 212 : Moss, 331 : Fern, 340, 348 : limited growth of, 207 Leaflet, Nitella, 207 Lept'othrix(A.e7TT09, slender : 0pif, a hair), filamentous condition of Bacillus, 88 : Figure, 86 LESSONIA (after Lesson, the French naturalist), 202 (Figure) Leuc'OCyte (Aev/iyag, large : an-opa, seed : ayyelov, a vessel), the female sporangium in plants with sexually di- morphic sporangia, usually distinguished by its greater size frorn the male or micro-sporangium : — Salvinia, 364 : Sela- ginella, 367 : Gymnosperms, 372 : Angio- sperms, 378 D D 402 INDEX AND GLOSSARY Meg'aspore (/xeyas, large : erTropa, a seed), the female spore in plants with sexually dimorphic spores, always distinguished by its large size from the male or micro- spore : — Salvinia, 364 : Selaginella, 367 : Gymnosperms, 372 : Angiosperms, 378 Megazo'oid (jae'yas, large : £uov, animal : et6os, form), the larger zooid in unicel- lular organisms with dimorphic zooids, 35, 130 Mer'istem (juepurr^a, formed from jaept^io, to divide), indifferent tissue of plants from which permanent tissues are differentiated, 346 Mes'entery (jae'cros, middle : eVrepoi/, in- testine), a membrane connecting the en- teric canal with the body-wall, 275 : de- velopment of, 300 Mes'oderm (/ueVo?, middle : 5e'p/u.a, skin), the middle cell-layer of triploblastic animals, n, 241 : Polygordius, 274 : de- velopment of, 295 : splitting of to form somatic and splanchnic layers, 298 Mesoglce'a (/ueVo?, middle : y\oia, glue), a transparent layer between the ecto- and endo-derm of Ccelenterates : — in Hydra, 223 : in Bougainvillea, 241 Mes'ophyll (/aeVo?, middle : (f>v\\ov, a leaf), the parenchyma of leaves, 348 Mes'OStates (/o.eo-0?, middle : oTTJi'ai, to stand), intermediate products formed during metabolism (g.v.) and divisible into (a) anabolic mesostates or ana- states, products formed during the con- version of food-materials into proto- plasm : and (li) katabolic mesostates or katastates, products formed during the breaking down of protoplasm, 18 Metab'Olism (/aera/SoAry, a change), the entire series of processes connected with the manufacture of protoplasm, and divisible into («) constructive meta- bolism or anabolism, the processes by which the substances taken as food are converted into protoplasm, and (&) de- £ structive metabolism or katabolism, the processes by which the protoplasm i breaks down into simpler products, ex- cretory or plastic, 17 Met'amere OueYa, after : ju.e'pos, a part), a body-segment in a transversely seg- mented animal such as Polygordius, 267, 269 : development of, 297 : limited num- ber and concrescence of in Crayfish, 310 MetamorphOS'iS (/xeTa/xop^wcris), a trans- formation applied to the striking change of form undergone by certain organisms in the course of development after the commencement of free existence : — Vor- ticella, 131 : Polygordius, 302 Mic'robe Campos, small : /St'os, life). See Bacteria. MICROCOC'CUS (jal/cpo?, small : KOKKOS, a berry) (Figure), 85 Microgam'ete (^I/epos, small : ya/xew. to marry), a male gamete (q.v.\ distin- guished by its smaller size from the female or megagamete, 130 Micro-millimetre, the one-thousandth of a millimetre, or i-25,oooth of an inch, 83 Micro-organism. See Bacteria. Micropyle (/xt/cpo?, small : irv\ri, an en- trance), 372 Micro-sporan'gium (jul/cpo?, small : a-iropd. a seed : ayyelov., a vessel), the male sporangium in plants with sexually di- morphic sporangia, usually distinguished by its smaller size from the female or mega-sporangium : — Salvinia, 364 : Se- laginella, 367 : Gymnosperms, 371 : An- giosperms, 377 Mic'rospore (julKpos, small : crnopd, a seed), the male spore in plants with sexually dimorphic spores, always dis- tinguished by its small size from the female or mega-spore : — Salvinia, 364 : Selaginella, 367 : Gymnosperms, 371 '. Angiosperms, 377 Microzo Oid (ju-Z/cpo?, small : giaov, an ani- mal : eiSos, form), the smaller zooid in unicellular organisms with dimorphic zooids, 35, 130 Midrib of leaf, Moss, 331 Minimum temperature for amoeboid move- ments, 21 Mollusca, the, 305 Moncec'iOUS (/j.oVo?, single : oucos, a house), applied to organisms in which the male and female organs occur in the same individual, 197, 231 Monopod'ial Guoi'os, single : TTOV'?, a foot), applied to branching in which the main axis continues to grow in a straight line and sends off secondary axes to the sides, 1^6 MONOSTROMA OAO^OS, single : anything spread out), 200 (Figure) MorphOl'ogy (/xop^, form : Aoyo?, a dis- cussion), the department of biology which treats of form and structure, 9 Mor'ula (diminutive of indnnii, a mul- berry). See Polyplast. MOSSES: — Figures, 329, 334: general characters, 328 : structure of stem, 330 : leaf, 331 : rhizoids, 331 : terminal bud. 331 : reproduction, 332 : development of sporogonium, 333 : of leafy plant, 335, 336 : alternation of generations, 336 : nutrition, 336, 337 Mouth : — Euglena, 47 : Paramoecium, 108 : Hydra, 222 : Medusa, 239 : Polygordius. 267 : backward shifting of in Crayfish, 311 MUCOR (uincor, mould) :— Figure, 158: occurrence and general characters, 156 : INDEX AND GLOSSARY 403 mycelium and aerial hyphae, 157, 159 '. sporangia and spores, 157-159 : transition from uni- to multi-cellular condition, 160 : development of spores, 161 : con- jugation, 163 : death, 165 : nutrition, 165 : parasitism, 165 : ferment-cells, 166 Mucous membrane, 58 Multicellular, formed of many cells, 61, 1 60 Muscle (miescielns,a. little mouse,a muscle), nature of, 128, 129 Muscle-fibre, Bougainvillea, 241 Muscle-plate, Polygordius, development of, 301 Muscle-process. Hydra, 225, 228 Mushroom. See Agaricus. MUSSEL (same root as muscle), Fresh- water : — Figure, 317 : general characters, 316 : mantle, shell, and foot, 316 : food- current, 316, 318 : enteric canal, 318 : gills and blood-system, 319 : nephridia, gonads, and nervous system, 319 Mycelial Iiypha3, the hyphee interwoven to form a mycelium. Mycelium (JUVKT/?, a fungus), a more or less felt-like mass formed of interwoven hyphae : — Mucor, 157 : Penicillium, 185 MYCET'OZOA OXV'/CTJ?, a fungus : &ov, an animal) : — Figure, 53 : occurrence and general characters, 52 : nutrition, 54 : reproduction and life-history, 54, 55 : animals or plants? 179 My'ophan (M^S, mouse, muscle : 4>aii/ci>. to appear), 108 Myxomyce'tes (MV£p6s, a kidney : crroiJ-a, a mouth), the internal or ccelomic aper- ture of a nephridium, 281 Nerve, afferent and efferent, functions of, 284 Nerve-cell, 227, 242 Nervous system, central and peripheral : — Medusa, 242 : Polygordius, 282 : func- tions of, 285 : Starfish, 309 : Crayfish, 315 : Mussel, 319 : Dogfish, 326 Neur'OCCele (vevpov, a nerve : KOI'ATJ, a hollow), the central cavity of the verte- brate nervous system, 326 Ocellus (ocellus, a little eye), structure and functions of, Medusa, 239, 243 CEsoph'agUS (ot6v, an egg : yefe'cris, origin), the development of an ovum from a primitive sex-cell, 252, 254 Oogon ium (u>ov, egg : yovo?, produc- tion), the name usually given to the ovary of many of the lower plants. Oosperm (tioV, egg : o-n-ep/aa, seed), a zygote (q.v.) formed by the ovum and sperm : a unicellular embryo, 171 : origin of nucleus of, 257 Oosphere (uov, an egg : o-<£oupa, a sphere), a name frequently given to the ovum of plants. Oospore (u>6v, an egg : a-rropd, a seed), a 404 INDEX AND GLOSSARY name frequently applied to the oosperm of plants. OPALIN'A (from its opalescent appear- ance) : — Figure, 120: occurrence and general characters, 119-121 : structure and division of nuclei, 119: parasitic nutrition, 121 : reproduction, 122 : means of dispersal, 122 : development, 123 Optimum (pptimus, best) temperature for amoeboid movements, 21 : for sapro- phytic monads, 40 Organ (opyavov, an instrument), a portion of the body set apart for the perform- ance of a particular function, 287 Or'ganism, any living thing, whether animal or plant, 5 Ossicle (diminutive of os, a bone), 308 Ov'ary (dvmii), an egg), the female gonad or ovum-producing organ ; see under the various types and especially Vaucheria, 170 : atrophy of, in Angiosperms, 378. The name is also incorrectly applied to the venter of the pistil of Angiosperms, 377 Ovi'duct (ovuni, an egg : duco, to lead), a tube conveying the ova from the ovary to the exterior, 291 Ov'um (ovum, an egg), the female or megagamete in its highest stage of dif- ferentiation : general structure of, 68, 69 : minute structure and maturation of, 254> 255 : see also under the various types and especially Vaucheria, 170 : formation of, in Angiosperms, 379 Ov'ule (diminutive of ovum\ the name usually applied to the megasporangium of Phanerogams. Oxidation of protoplasm, 15 OXYTRICH A (ofus, sharp : 0pi£, a hair), 1 17 (Figure) Pancreas (Trayxpeas, sweetbread), 325 Param'ylum (Trapa, beside : a^vAoi/, fine meal, starch), 46 PARAM(E'CIUM :— Figures, 106, 112: structure, 104: mode of feeding, no: asexual reproduction, 112: conjugation, 112 Para'nucleus (beside the nucleus), 109 Par'asite, parasitism (Trapao-Zros, one who lives at another's table) : — Opalina, 121 : Bacteria, 91 : Mucor, 165 Paren'chyma (Trape'yxuMa.anything poured in beside, a word originally used to de- scribe the substance of the lungs, liver, and other soft internal organs), applied to the cells of plants the length of which does not greatly exceed their bread tli and which have soft non-liguified walls, 60 : ground-parenchyma, 343. 344 Pari'etal (paries, a wall), applied to the layer of ccelomic epithelium lining the body-wall, 273, 274 Parthenogen'esis (TrapfleVo?, a virgin : yeVecri?, origin), development from an unfertilized ovum or other female gamete, 198 Parthenogenet'ic ova, characteristics of, 258 Pasteur, Louis, researches on yeast, 77- 79 Pasteur's solution, composition of, 77-79 Pedal (pes, the foot) ganglion, Mussel, 3i9 PENICILL'IUM (pcnicillnm, a painter's brush from the form of the fully-deve- loped aerial hypha;) : — Figure, 184: oc- currence and general characters, 182: mode of growth, 183: microscopic structure, 183 : formation and germina- tion of spores, 187, 188 : sexual repro- duction, 188 : nutrition, 188 : vitality of spores, 189 Pepsin (TreTTTw, to digest), the proteolytic or peptonizing ferment of the gastric juice, 12, 79 Peptones, 12 Perianth (n-ept, around : av9os, a flower), the proximal infertile leaves of a flower, 37.7 Perisperm (rrepi, around : o-rrep/^a, seed), nutrient tissue developed in the nucleus of the seed, 376 (description of figure) Peristom'e (irept, around ; oro/ma, the mouth), Vorticella, 126 Peristomlum (jrepi, around : oro/ouoi/, a little mouth), the mouth-bearing segment of worms, 267, 293 Peritone'um (Tre'pn-ovcuoi/), the membrane covering the viscera, 321 Pet'alS (fl-e'TaAof, a leaf), the inner or dis- tal perianth leaves in the flower of Angiosperms, 377 Phar'ynx (apu-y£, the throat) :— Poly- gordius, 276 : Dogfish, 324 Phloem ($A.OA.ov, a tribe) of the animal king- dom, 303 : of the vegetable kingdom, 360 Phyll'ula (diminutive of vXoi^, a race : ytVto-ts, origin), the development of the race, 144 Physiol'Ogy ($uT09, first), composition of, 5 ProtiSt'a (TTPCOTICTTO?, the first of all), the lowest organisms intermediate between the lowest undoubted animals and plants, 1 80 ProtOCOC'CUS (TTpwros, first : KOK/CO?, berry). See Hffimatococcus. PROTOMYXA (TrpdjTos, first :/xi!fa, mucus): Figure, 50 : occurrence and general cha- acters, 49 : life-history, 51 : animal or plant? 179 Protonem'a(7rpu)ro?, first : frj/xa, a thread), Moss, 332, 335 _ Prot'oplasm (Trpwros, first : n\dcriJ.a, any- thing moulded), composition of, 5 : pro- perties of, 5-7 : micro-chemical tests for, 7, 8 : minute structure of, 62, 63 : con- tinuity of in Fern, 346 : in Polygordius, 288_ : intra- and extra-capsular, Radio- laria, 151 Protozoa, the, 304 Prox'imal (proximus, nearest), the end nearest the point of attachment or or- ganic base, e.g. in the stalk of Vorticella, 124 Pseud 'opod ((/>ev6^?, false : 7rou5, foot), described, 4 : comparison of with cilium, 34, 52 : in columnar epithelium, 59 : in endoderm cells of Hydra, 228 PteriS. See Ferns. Punctum vegetationis. See Growing point. Putrefac'tion (putrefacio, to make rotten), nature of, 81 : a process of fermentation, 90 : conditions of temperature, moisture, &c., 91, 92 Putres'cent (putresco, to grow rotten) solution, characters of, 37, 81 Putres'cible infusion, sterilization of, 96- 100 Pyren'oid (nvpriv, the stone of stone-fruit : elfio?, form), a small mass of proteid material invested by starch, 27 Radial symmetry, starfish, 305 RADIOLAR IA (radiits, a spoke or ray):— Figures, 150, 151 : occurrence and general characters, 150 : central capsule, 150 : intra- and extra-capsular protoplasm. 406 INDEX AND GLOSSARY 151 : silicious skeleton, 151 '. symbiotic relations with Zooxanthella, 152 Rect'um (intestinum rectum, the straight gut), the posterior or anal division of the enteric canal, 277 Redi, Francisco (Italian savant), experi- ments on biogenesis, 95 Reflex action, 285 Reproduction, necessity for, 19 Reproductive organ. See Gonad. Reservoir of contractile vacuole, Euglena, 47 Respiration: — Amoeba, 17: Polygordius, 280 Respiratory caeca, Starfish, 308 Rhiz'oid (pufc, root : elfios, form):— Nitella, 204, 212 : Moss, 331 : prothallusof Fern, 35i Root, Fern, 340, 349 Root-cap, 350 Root-hairs, 349, 353 ROSS, Alexander, on abiogenetic origin 01 mice, insects, &c., 94 Rotation of protoplasm, 208 Rudiment, rudimentary (rudimentum, a beginning), the early stage of a part or organ : often used for a structure which has undergone partial atrophy, but in such cases the word vestige (ff-v.) is more suitable. Segment'al cell :— Nitella, 209 : Moss, 331 : Fern, 347 Segmentation, metameric. See Meta- mere. SACCHAROMY CES (a-dKXa.pov, sugar : /XV'KTJS, fungus): — Figure, 71 : occurrence, 70 : structure, 70 : budding, 72, 73 : in- ternal fission, 73 : nutrition, 74 : alco- holic fermentation caused by, 74, 78, 79 : experiments on nutrition of, 77-79 : ani- mal or plant? 179 SALVIN'IA : — Figure, 365 : general cha- racters, 364 : mega- and micro-sporangia and spores, 364 : male and female pro- thalli and gonads, 366 : development and alternation of generations, 366 Saprophyt'iC (craTrpo?, putrid : (^VTOI/, a plant) nutrition, defined, 39 Schulze's solution, test for cellulose, 28 : for lignin, 344, 345 Scleren'chyma (o-/cATjp6?, hard : e-yx^M-a, infusion): — Moss, 330: Fern, 344, 347 Secre'tion (secrctus, separate), nature of, 227 : formation of cell-wall a process of, -, I4 Seed, formation of, 373, 374, 380 : germi- nation of, 374 Seg'ment (scg»icntu)>i, a piece cut off), in plants a node together with the next proximal internode, 206 : in animals the name is variously applied. See Meta- mere, Podomere. SELAGINELL'A (treAa-yew, to shine):— Figure, 368 : general characters, 367: cone, sporangia, and spores, 367 : prothalli and gonads, 367 : development and alterna- tion of generations, 369 Self-fertilization, applied to the sexual process when the gametes spring from the same individual, 197 Sep'alS (separ, separate), the outer or proximal perianth-leaves in the flower of Angiosperms, 377 Sep'tum (septitm, a barrier) : — In plants, 185 : in Polygordius, 276 : development of, 298 Set'a (seta, a bristle), 287 Sex-cells, primitive, 252 : origin of in Hydroids, 244 : in Polygordius, 289 Sexual differentiation, illustrated by Vaucheria, 170: by Spirogyra, 197 Sexual generation. See Gamobium. Sexual reproduction, nature of, 42 Shell, Mussel, 316 Shoot, in plants, an axis of the second or any higher order with its leaves, 207 Sieve-tubes and plates, 346 Sinus (shins, a hollow), a spacious cavity, Skeleton. See Endo- and Exo-skeleton. Slime-fungi. See Mycetozoa. Solid aggregate, 201 Somat'iC (o-w/xa, the body), applied to the layer of mesoderm which is in contact with the ectoderm and with it forms the body-wall, 274 Sor'us (crajpo?, a heap), an aggregation of sporangia, 350, 364 Species (species, a kind), meaning of term illustrated, 8, 135 : definition of, 137 : origin of, 139-142 Specific characters, specific name, 8. 137 Specialized, meaning of, 138 Sperm (a-irepfj-a, seed), the male or micro- gamete in its highest stage of differentia- tion : structure and development of, 252 : see also under the various types, and especially Vaucheria, 170, 171 Spermatozo'id, spermatozo'on (o-Trep/ua. seed : ftoof, animal, from the actively moving sperms of animals having been supposed to be parasites), synonyms of sperm. Spermary (o-Tre'p/xa, seed), the male gonad or sperm-producing organ : see under the various types, and especially Vaucheria, 170 Sperm 'idUCt (a-n-ep^a, seed : duco, to lead), a tube conveying the sperm from the spermary to the exterior, 291 INDEX AND GLOSSARY 407 SpermatOgen'esiS (crTrep/xa, seed : yeVecrts, origin), the development of a sperm from a primitive sex-cell, 252, 253 (Figure). Spinal cord, Dogfish, 326 Spiral vessel. See Vessel. SPIRILL'UM (spira, a coil) (Figure) 86, 87 Spirogyra (spira, a coil : gyms, a revolu- tion) : — Figure, 193 : occurrence and general characters, 192 : microscopic structure, 192 : growth, 195 : conjugation, 196 : development, 198 : nutrition, 198 ' Splanch/niC (crn\6.yxvov> intestine or vis- cus), applied to the layer of mesoderm which is in contact with the endoderm and with it forms the enteric canal, 274 Spontaneous generation. See Abio- genesis. Sporan'gium (v\\ov, leaf), a sporangium-bearing leaf : — Equisetum, 362 : Selaginella, 367 : Gymnosperms, 369, 372 : Angiosperms, 377 Stamen (stamen, a. thread), a male sporo- phyll. 371, 377 _ Starch, composition and properties of, 27 STARFISH :— Figure, 306 : general cha- racters, 305-307 : radial symmetry, 305 : tube-feet and ambulacral system, 307, 309 : exoskeleton, 308 Stem, structure of: — Moss, 330 ; Fern, 341 Sterig'ma (crT^pty^a, a support : — Penicil- lium, 186 ; Agaricus, 190 Sterilization of putrescible infusions, 96- IOO Stigma (o-Ttyjua, a spot), the receptive ex- tremity of the style, 378 Stimulus, various kinds of, 285 Stock. See Colony. Stom'ate (o-ro/xa, mouth), 349 Stomodae'um (o-To/aa, mouth : oSaios, be- longing to a way), an ectodermal pouch which unites with the enteron and forms the anterior end of the enteric canal, its aperture being the permanent mouth, 294 Stone-canal, Starfish, 309 Style (stylus, a column), the distal solid portion of the female sporophyll or of the entire gyncecium in Angiosperms, 378 STYLONYCH'IA (arvAos, a column : ow£ , a claw), Figure, 115: occurrence and general characters, 1 14 *. polymorphism of cilia, 114-116 Sub-apical cell. See Segmental cell. Superficial aggregate, 200 Supporting lamella. See Mesogloca. Suspensor :— Selaginella, 369 : Gymno- sperms, 373 : Angiosperms, 380 Sweet Wort, composition of, 74 Swimming-bell, Diphyes, 249 Symbio'sis (crv/a/Stajo-t?, a living with), an intimate and mutually advantageous association between two organisms, 152 Syner'gidas (crwepyos, a fellow worker), 379 Sys'tole (crvoToAvj, a drawing together, contraction), the phase of contraction of a heart, contractile vacuole, £c., 109 Teeth, Dogfish, 325 Temperature, effects of on protoplasmic movements, 20, 21 Tentacles : — Hydra, 222 : Bougainvillea, 236 : Polygordius, 266 : development of, 298 Terminal bud :— Nitella, 206, 208 : Moss, 331 TestiS (the Latin word), generally used for the spermary in animals. Thermal death-point. See Ultra-maxi- mum temperature. Tissues, differentiation of: — Polygordius, 288 : Fern, 349 Tracheides (rpaxv's, rough : elSos, form). See Vessels of Plants, 345 Transpiration, the giving off of water from the leaves of plants, 337 Trich'ocyst (Qpi£, a hair : KUCTTIS, a bag), in Triploblast'iC (rpiTrAoos, triple : /SAacrros, a bud), three-layered : applied to ani- mals in which the body consists of ecto- derm, mesoderm, and endoderm, 241, 274 Troch'OSphere (rpoxo?, a wheel, in reference to the circlet of cilia : cn/>aipa, a sphere), the free -swimming larva of Polygordius. &c. : — characters of, 292 (Figure) ; origin of from gastrula, 293, 294 ; metamorpho- sis of, 295 Tube-feet, Starfish, 307, 309 U Ultra-maximum temperature, for amoe- boid movements, 21 ; for monads, 40 ; for Bacteria, 92 ULVA (nh'ci, an aquatic plant), 201 Umbell'ate (nmbella, a sun-shade, um- brella), applied to branching in which the primary axis is of limited growth and 408 INDEX AND GLOSSARY sends off a number of secondary axes from its distal end, 136 Unicell'ular, formed of a single cell, 61 ; connection of uni- with multi-cellular organisms, 260-265 Ureter (ovprjnjp, the Greek name), the duct of the kidney, 326 VORTICELLA (diminutive of vortex, an eddy) : — Figure, 125 : occurrence and general characters, 124 : structure, 126 : asexual reproduction, 129 : conjugation, 130: means of dispersal, 130-134: encys- tation, spore-formation, development, and metamorphosis, 131 Vac'uole (vacuus, empty), contractile, n, 109: non-contractile, 70 Variability, 145 Variation, individual, 138, 145 Variety, an incipient species, 145 Vasc'ular (vasculum, a small vessel) bundles, 341, 344 Vascular plants, 361 VAUCHERIA (after J. P. E. Vaucher, a Swiss botanist) : — Figure, 168 : occur- rence and general characters, 167: minute structure, 167 : asexual reproduction, 169: sexual reproduction, 170 : nutrition, 173 Veins of Dogfish, 325 : of leaves, 348 Vel'um (velum, a veil) of medusa, 240 Vent, the aperture of the cloaca, 320 Venter (venter, the belly), of ovary o. Moss, 332, and Fern, 354 : of the female sporophyll or of the entire gynoecium of Angiosperms (so-called ovary), 377 Ventral nerve-cord :— Polygordius, 282 : development of, 297 : Crayfish, 315 Ventricle. See Heart. Venues, the, 304 Ver'tebral (vertebra, a joint) centra and column, Dogfish, 324 Vertebrata, the, 305 Vessels : — of plants, spiral and scalariform, 344, 347 : of animals, see Blood-vessels. Vestige, vestigial (vestigium, a trace), applied to any structure which has become atrophied or undergone reduction beyond the limits of usefulness, 115 Vib'rio (vibro, to vibrate), 85, (Figure) 87 Visc'eral (viscits, an internal organ), ap- plied to the layer of ccelomic epithelium, or of petitoneum, covering the intestine and other internal organs, 273 Visceral ganglion, Mussel, 319 Vitelline (vitellus, yolk) membrane, the cell-membrane of the ovum, 255 VolVQX (volvo, to roll), 263 (Figure) W Waste-products, 33 Water of organization, 5, 29 Whorl of leaves, 206 Wood. See Xylem. Work and Waste, 14 X Xylem (£u/\oi', wood), the inner portion ot vascular bundle, 345 Yeast, 70 Yeast-plant. See Saccharomyces. YellOW-cells of Radiolaria, 152 Yolk-granules or spheres, 68, 232, 254 Zoogl03'a (£of, an animal ; eifios, form), a single individual of a compound organism. 138, 234 Zootham'nium (£u>ov, an animal : Od^vcs. a bush) : — Figures, 132, 136 : occurrence and general characters, 133 : dimorphism of zooids, 133 : means of dispersal, 133 : characters and mutual relations of species. I*"1 *"* T ° A JJ! Ij4 Zooxanthell'a (£u>ov, an animal : £ai/0u?, yellow), 152 Zyg'ospore (£vyoi>, a yoke : cnropd, a seed), applied to a resting zygote formed by the conjugation of similar gametes, 164 Zygote (^vycoTo?, yoked), the products of conjugation of two gametes : — Hetero- mita, 41 : Vorticella, 131 : Mucor, 164 : Vaucheria, 197 : Spirogyra, 198. THE END K1C1IAKD CLAY AND SONS, LIMITED. LONDON AND BUNGAY SSv