JmFM I - /? -Crt-^U oj LT) a r- a -q a a a m a a *jr LESSONS IN ELEMENTARY BIOLOGY / LESSONS IN ELEMENTARY BIOLOGY BY T. JEFFERY PARKER, D.Sc, F.R.S. PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO. DUXEDIN. NEW ZEALAND WITH EIGHTY-EIGHT ILLUSTRATIONS Hem ton -___ M ACM ILL AN AND CO. AND NEW YORK 1893 The Rig/it of Translation and Reproduction is Reserved RICHARD CLAY AND SONS, LIMITED. LONDON AND BUNGAY. First Edition, 1891. Second Edition Revised, 1893. PREFACE TO THE FIRST EDITION IN 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 un- familiar 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 v i 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 I have attempted to give in the following Lessons, my aim having been to provide a book which may supply in the PREFACE vn 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 via PREFACE the development of Hydra and to the so-called sexual pro- cess 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 XXVII. 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 simplification 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 ix 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 Professer 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., 1890. PREFACE TO THE SECOND EDITION IN addition to a thorough revision, Lessons VI. and XXIV. have been largely re-written. Figs. 9, 10, 52, 60, 64, and 66 are new, and Figs. 9, 10, n, 64, 66, and 67 of the first edition have been withdrawn. I have received valuable help from Professors W. N. Parker and G. B. Howes, Miss M. Greenwood, and Mr. J. E. S. Moore. Much of the proof-correcting has, as before, fallen upon my brother. March 1893. TABLE OF CONTENTS PREFACE TO THE FIRST EDITION PREFACE TO THE SECOND EDITION LIST OF ILLUSTRATIONS AMCEBA . H^EMATOCOCCUS HETEROMITA EUGLENA PROTOMYXA THE MYCETOZOA LESSON I. LESSON II. LESSON III. LESSON IV. LESSON V. 31198 PAGE v xi xix 44 49 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 68 LESSON VII. SACCHAROMYCES 71 LESSON VIII. BACTERIA 82 LESSON IX. BIOGENESIS AND ABIOGENESIS 95 HOMOGENESIS AND HETEROGENESIS IO2 LESSON X. PAKAMCECIUM IO6 STYLONYCHIA Il6 oXYTRICHA I2O LESSON XL OPALINA 121 LESSON XII. VORTICELLA 126 /'M.I IIAMNIUM ... .... 135 TABLE OF CONTENTS xv LESSON XIII. PAGE SPECIES AND THEIR ORIGIN : THE PRINCIPLES OF CLASSIFICA- TION ...................... 137 LESSON XIV. THE FORAMINIFERA .................. 148 THE RADIOLARIA ........ .... 152 THE DIATOMACE^E ................. 155 LESSON XV. MUCOR ....... 158 LESSON XVI. VAUCHERIA ......... ..... 169 CAULERPA .......... .... 175 LESSON XVII. THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS . 176 LESSON XVIII. PENICILLIUM . ......... .... 184 AGARICUS ...................... LESSON XIX. SPIROGYRA ........ .......... 194 LESSON XX. MONOSTROMA ... ......... 2OI ULVA ................. ..... 2O3 LAMINARIA, &C. .............. 203 PAGE 206 xvi TABLE OF CONTENTS LESSON XXI. NITELLA ................... LESSON XXII. HYDRA ......... 221 LESSON XXIII. HYDROID POLYPES ........ 2 37 BOUGAINVILLEA, &C ....... ... 237 DIPHYES ........ 250 PORPITA . . LESSON XXIV. SPERMATOGENESIS AND OOGENESIS .... 255 THE MATURATION AND IMPREGNATION OF THE OVUM . . 259 THE CONNECTION BETWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS .................. 2 64 LESSON XXV. POLYGORDIUS ........... 271 LESSON XXVI. POLYGORDIUS (continued} ................ 293 LESSON XXVII. I HE GENERAL CHARACTERS OF THE HIGHER ANIMALS . . . 307 THE STARFISH ................... 39 THE CRAYFISH . . 3 X 4 THE FRESH-WATER MUSSEL ............. 32O I UK DOGFISH . ...... 324 TABLE OF CONTENTS xvii LESSON XXVIII. PACK MOSSES 33 2 LESSON XXIX. FERNS 344 LESSON XXX. THE GENERAL CHARACTERS OF THE HIGHER PLANTS 363 EQUISETUM 366 SALVINIA 368 SELAGINELLA 371 GYMNOSPERMS 373 ANGIOSPERMS 378 SYNOPSIS ... 385 INDEX AND GLOSSARY . 395 LIST OF ILLUSTRATIONS FIG. PAGE 1. Amceba, various species 2 2. Prot amoeba primitiva 9 3. Hcematococcus pluvialis and //. lacustris 24 4. Heteromita rostrata 38 5. Euglena viridis 45 6. Protomyxa aurantiaca 5 7. Badhamia and Chondrioderma 53 8. Typical animal and vegetable cells 57 9. Animal and plant cells, detailed structure 62 10. Stages in the binary fission of a cell 64 11. Ova of Carmarina and Gymnadenia 69 12. Saccharomyces cerevisice 7 2 13. Bacteritim termo 83 14. Bacterium termo, showing flagella 84 15. Micrococcus 86 16. Bacillus subtilis 87 1 7. Vibrio serpens, Spirillum tcnue, and S. vohitans 88 1 8. Bacillus anthracis 90 19. Beaker with culture-tubes 100 xx LIST OF ILLUSTRATIONS FIG. PAGE 20. Paramcecium attrelia . . 108 21. Paramcecium aurelia, conjugation 115 22. Stylonychia my till is IJ 7 23. Oxytricha flava .... I2 24. Opalina raiiantni I22 25. Vorticella . . 127 26. Zoothamnium arbusatla 134 27. Zoothamnium, various species 138 28. Diagram illustrating the Origin of the Species of Zootham- nium by Creation H 2 29. Diagram illustrating the Origin of the Species of Zootham- nhim by Evolution J 44 30. Rotalia 149 31. Diagrams of Foraminifera 15 32. Alveolina quoii J5 1 33. Lithocircus anmdaris I5 2 34. Actinomma asteracanthion 153 35. Diagrams of a Diatom and shells of Navicula and Aulaco- discus 156 36. Mucor mucedo and M. stolonifer 159 37. Moist Chamber 163 38. Vaucheria 170 39. Caulerpa scalpelliformis 174 40. Penicillinm gla/ucum 186 41. Agaricus campestris 192 42. Spirogyra . 195 43. Monostroma Imllosum and M. laceratum 202 44. Laminaria claustoni and Lessonia fuscescens 204 45. Nitella, general structure 207 46. Nitella, terminal bud 212 47. Nitella, spermary 215 48. Nitella, ovary 217 49. Chara, pro-embryo 219 50. Hydra viridis and H. fusca, external form 222 51. Hydra, minute structure 226 LIST OF ILLUSTRATIONS xxi FIG. PAGE 52. Hydra, nematocyst and nerve-cell 228 53. Hydra viridis, ovum 235 54- BougainviUea ramosa 238 55. Diagrams illustrating derivation of Medusa from Hydranth . 242 56. EncopcUa campanularia, muscle fibres and nerve-cells. . . . 245 57. Laomedta flexuosa and Endendrinm ramosum, development . 249 58. Diphyes campanulata 252 59. Porpita pacifica and P. mediterranea 253 60. Spermatogenesis in the Mole-Cricket 256 61. Ovum of Toxopneustes lividns 259 62. Maturation and impregnation of the animal ovum 260 63. The gastrula 265 64. Pandorina morum 266 65. Volvox globator 268 66. Volvox globator 269 67. Polygordius neapolifanus, external form 272 68. Polygordius ncapolitanus, anatomy 274 69. Polygordius neapoliianus, nephridium 285 70. Polygordius, diagram illustrating the relations of the nervous- system 287 71. Polygordius neapolitamts, reproductive organs ... . . 294 72. Polygordius neapolitanus, larva in the trochosphere stage . . 296 73- Diagram illustrating the origin of the trochosphere from the gastrula 298 74. Polygordius neapolitanus, advanced trochosphere 300 75. Polygordius neapolitamts, larva in a stage intermediate be- tween the trochosphere and the adult 303 76. Starfish, diagrammatic sections 310 77. Crayfish, diagrammatic sections 316 78. Mussel, diagrammatic sections . 321 79. Dogfish, diagrammatic sections 326 80. Mosses, various genera, anatomy and histology 333 81. Funaria, reproduction and development 338 82. Pteris and Aspidium, anatomy and histology 346 83. Ferns, various genera, reproduction and development . . . 356 xxii LIST OF ILLUSTRATIONS FIG. PAGE 84. Eqidsetum, reproduction and development 367 85. Salvinia, reproduction and development 369 86. Selaginella, reproduction and development 372 87. Gymnosperms, reproduction and development 374 88. Angiosperms, repi - oduction and development 379 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 Amceba. Amoebae are mostly invisible to the naked eye, rarely exceeding one-fourth of a millimetre ( T ^-g- 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 AMCEBA LESS. A )- ^ii-yS^&^&vl^^v.' >, . .'.: ifisft^^d**?.- 1 %&3!iM ';& i:*&/tf :;-,. E JIiii8lilbr' ia l{s&.ilmm&j / - v x ^ O-'v ^>- .-- -^ > -- ?r " * " - 6 >? -' FlG I. A. Amccba qttarta, a living specimen, showing granular endosarc surrounded by clear ectosarc, and several pseudopods (psd), i GENERAL CHARACTERS 3 some formed of ectosarc only, others containing a core ot endosarc. The larger bodies in the endosarc are mostly food-particles ( x 300). x B. The same species, killed and stained with carmine to show the numerous nuclei (mi) ( x 300). c. Amceba proteus, a living specimen, showing large irregular pseudopods, nucleus (mi), 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 Amoeba, 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 ) a living specimen, showing several large pseudo- pods (psd). single nucleus (mi), and contractile vacuole (c. vac), and numerous food-particles embedded 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 verrucosa, 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. Amah a protens, in the act of multiplying by binary fission ( x 500). (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 Amceba. If in any way the creature can be made to turn over, or if a number of specimens are examined in various positions, these two constituents will always be found to have the 1 A number preceded by the sign of multiplication indicates the number of diameters to which the object is magnified. B 2 4 AMCEBA LESS. same relations, whence we conclude that an Amoeba con- sists of a granular substance the endosan; completely surrounded by a clear transparent layer or ectosarc. One very noticeable thing about Amoeba is that it is never of 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 Amceba. It must not be forgotten that the animal does not alter perceptibly in volume during the process, every pseudopod thus protruded from one part of the body necessitating 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 fingers. 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 Amceba there is no external compression, the " squeezing " being done by the animalcule itself. i 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, i.e., 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). 55 55 55 5? 55 55 55 55 6 AMCEBA LESS. 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 instability. 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 (H O), carbon dioxide or carbonic acid (CO,,), ammonia (NH :5 ), and sulphuretted hydrogen (H.,S) 1 . In this character of instability or readiness to decompose protoplasm notoriously agrees with its constituent proteids ; any dead organism will, 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 l;u:t they only decompose in the presence of living Bacteria. T 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 water 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, nu\ while frequently its presence is re- vealed only 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, the nuclear matrix or achromatin, takes a lighter tint (Fig. I, F). The relative arrangement of chrornatin and matrix varies 8 AMCEBA LESS. in different Amoebae : sometimes there are granules of chromatin in an achromatic ground substance (F) ; some- times 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 mideolus. 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 Amceba 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 o o MORPHOLOGY AND PHYSIOLOGY 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 J nor contractile vacuole, and are therefore placed in the separate genus Protamceba (Fig. 2). They may be looked upon as the simplest of living things. The preceding paragraphs 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 Protamceba 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 chroma- tin diffused in the form of minute granules throughout their substance (see end of Lesson X., p. nS), or that they are forms which have lost their nuclei. io AMOEBA LESS. 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 contractility. 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 or other 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 I MODE OF FEEDING n 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. 35) 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 (/. 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 seen 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 LESS. 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 ingestive aperture (mouth), digestive cavity (stomach), or egestive 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 of 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- r 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 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- 14 AMCEBA . LESS. 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 fron 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 lire 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, i POTENTIAL AND 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 + O 2 = COo) : when hydrogen is burnt, water (H + O == H 2 O). 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 (CH 4 N 2 O) and uric acid C 5 H 4 N 4 O 3 ), both occurring mainly in the urine. In some animals urea and uric acid are replaced by other com- pounds such as guanin (C 5 H 5 N 5 O), but it may be taken as proved that in all living things the product 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 activity 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 LESS. 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 decomposition 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 i METABOLISM T y 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 Amoeba. 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 LESS. living protoplasm : these are processes of constructive meta- bolism or andbolism. 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 imbedded 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 phis oxygen phis water) is greater than its expenditure or the total weight of substances given out (faeces plus excreta proper plus carbon dioxide) the animalcule 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- i 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 preserved there must be some provision by which the individuals composing it are enabled to produce new in- dividuals. 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 or binary 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 LESS. 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 Amceba cannot go on multiplying indefinitely by simple fission, and that occasion- ally two individuals come into contact and undergo complete fusion. 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. 90.) In conclusion, a few facts may be mentioned as to the conditions of life of Amceba the circumstances under which it will live or die, flourish or otherwise. In the first place, it will live only within certain limits of 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- i CONDITIONS OF LIFE 21 duced by heat. Similarly when it is 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 pro- toplasm, but only renders it temporarily inert ; on thawing, the movements recommence. We may therefore distin- guish an optimum temperature at which the vital actions are carried on with the greatest activity ; maximum and minimum temperatures above and below which respect- ively 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 certaiji 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 produced only 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, /.) connected to cell-body by protoplasmic filaments, and flagella fl. The scale to the left applies to Figs. A- D. B. Resting stage of the same, showing nucleus (mi) with nucleolus ('), and thick cell-wall (c.iv) 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. Hamatococcus lacustris, showing nucleus (), single large pyrenoid (f>yr), and contractile vacuole (c.vac), F. Diagram illustrating the movement of a flagellum : ab, its base 1 ; c, c', c", different positions assumed by its apex. (E, after Biitschli. ) ii FLAGELLA 25 the forward movement is accompanied by a rotation of the organism upon its longer axis. Careful watching shows that the outline of a swimming Hsematococcus does not change, so that there is evidently no protrusion of 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.' 1 In a Hsematococcus 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 Haematococcus is not amczboid, i.e., produced by the pro- trusion and withdrawal 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 H^ematococcus, 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 cilitim 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 each cell bearing only a limited number one or two, or occasionally as many as four. 26 H/EMATOCOCCUS LESS. with the rlagella goes 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 or 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 hcEmato chrome. 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 (n, nit.) 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 II CELL-WALL 27 organism. These are small ovoidal structures (A, 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 characteristic test of the well-known substance starch, as can be seen by letting a few drops of a weak solution of 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 C 6 H 10 5" In Hsematococcus pluvialis there is no contractile vacuole, but in another species, H. laciistris, 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 focusing is seen to be really an extremely thin globular shell (A, c.w.) composed of 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 Amceba, as stated in the preceding lesson (p. n) is very probably nitrogenous : 28 H^MATOCOCCUS LESS. that of Hgematococcus, on the other hand, is formed of a carbohydrate called cellulose, allied in composition to starch, sugar, and gum, and having the formula C 6 H 10 O 5 . 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, Haematococcus has no pseudopods, and therefore cannot take in solid food after the manner of Amosba : 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. Never- theless it must take in food in some way or other, or the de- composition of its protoplasm would soon bring it to an end. IT DECOMPOSITION OF CARBON DIOXIDE 29 Hasmatococcus lives in rain-water. This is never pure water, but always contains certain mineral salts in solution, especially nitrates, ammonia salts, and often sodium chloride or common table salt. 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 Hsematococcus is exposed to sunlight, minute bubbles are found to appear in it, and these bubbles, if collected 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, />., some compound of carbon, hydrogen, and oxygen, with a comparatively small number of atoms to the molecule. The next step seems to be that the carbohydrate thus formed unites with the ammonia salts or the nitrates absorbed from the surrounding water, the result being the formation of some comparatively simple nitrogenous compound, prob- 30 H^MATOCOCCUS LESS. ably belonging to the class of amides, one of the best known of which asparagin has the formula C 4 H 8 N 2 O 3 . Then further combinations take place, substances of greater and greater complexity are produced, sulphur from the ab- sorbed sulphates enters into combination, and proteids are formed. From these, finally, fresh living protoplasm arises. From the foregoing account, which only aims at giving the very briefest outline of a subject as yet imperfectly un- derstood, it will be seen that, as in Amoeba, the final result of the nutritive process is the manufacture of protoplasm, and that this result is attained by the formation of various substances of increasing complexity or anastates (see p. 18). But it must be noted that the steps in this process of con- structive metabolism are widely different in the two cases. In Amoeba we start with living protoplasm that of the prey which is killed and broken up into diffusible proteids, these being afterwards re-combined to form new molecules of the living protoplasm of Amoeba. So that the food of Amoeba is, to begin with, as complex as itself, and is first broken down by digestion into simpler compounds, these being afterwards re-combined into more complex ones. In Haematococcus, on the other hand, we start with extremely simple compounds, such as carbon dioxide, water, nitrates, sulphates, &c. Nothing which can be properly called diges- tion, i.e., a breaking up and dissolving of the food, takes place, but its various constituents are combined into sub- stances of gradually increasing complexity, protoplasm, as before, being the final result. To express the matter in another way : Amoeba can only make protoplasm out of proteids already formed by some other organism : Haematococcus can form it out of simple liquid and gaseous inorganic materials. Speaking generally, it may be said that these two methods IT DESTRUCTIVE METABOLISM 31 of nutrition are respectively characteristic of the two great groups of living things. Animals require solid food con- taining ready-made proteids, and cannot build up their pro- toplasm out of simpler compounds. Green plants, i.e., all the ordinary trees, shrubs, weeds, &c., take only liquid and gaseous food, and built up their protoplasm 'out of carbon dioxide, water, and mineral salts. The first of these methods of nutrition is conveniently distinguished as holozoic, or wholly-animal, the second as holophytic, or wholly-vegetal. It is important to note that only those plants or parts of plants in which chlorophyll is present are capable of holo- phytic nutrition. Whatever may be the precise way in which the process is effected, it is certain that the decomposition of carbon dioxide which characterizes this form of nutrition is a function of chlorophyll, or to speak more accurately, of chromatophores, since there is reason for thinking that it is the protoplasm of these and not the actual green pigment which is the active agent in the process. Moreover, it must not be forgotten that the decomposition of carbon dioxide is carried on only during daylight, so that organisms in which holophytic nutrition obtains are depend- ent upon the sun for their very existence. While Amoeba derives its energy from the breaking down of the proteids in its food (see p. 12), the food of Hsematococcus is too simple to serve as a source of energy, and it is only by the help of sunlight that the work of constructive metabolism can be carried on. This may be expressed by saying that Hsematococcus, in common with other organisms, contain- ing chlorophyll, is supplied with kinetic energy (in the form of light or radiant energy) directly by the sun. As in Amoeba, destructive metabolism is constantly going on side by side with constructive. The protoplasm becomes oxidized, water, carbon dioxide, and nitrogenous waste 32 ELEMATOCOCCUS LESS. matters being formed and finally got rid of. Obviously, then, absorption of oxygen must take place, or in other words, respiration must be one of the functions of the pro- toplasm of Hsematococcus as of that of Amoeba. In many green, i.e., chlorophyll-containing, plants, this has been proved to be the case ; respiration, i.e., the taking in of oxygen and giving out of carbon dioxide, is constantly going on, but during daylight is obscured by the converse process the taking in of carbon dioxide for nutritive purposes and the giving out of the oxygen liberated by its decomposition. In darkness, when this latter process is in abeyance, the occurrence of respiration is more readily ascertained. Owing to the constant decomposition, during sunlight, of carbon dioxide, a larger volume of oxygen than of carbon dioxide is evolved ; and if an analysis were made of all the ingesta of the organism (carbon dioxide plus mineral salts plus respiratory oxygen) they would be found to con- tain less oxygen than the egesta (oxygen from decomposition of carbon dioxide plus water, excreted carbon dioxide and nitrogenous waste) ; so that the nutritive process in Hsema- tococcus is, as a whole, a process of deoxidation. In Amoeba, on the other hand, the ingesta (food plus respi- ratory oxygen) contain more oxygen than the egesta (faeces plus carbon dioxide, water, and nitrogenous excreta), the nutritive process being therefore on the whole one of oxidation. This difference is, speaking broadly, character- istic of plants and animals generally ; animals, as a rule, take in more free oxygen than they give out, while green plants always give out more than they take in. But destructive metabolism is manifested not only in the formation of waste products, but in that of substances simpler than protoplasm which remain an integral part of the organism, viz., cellulose and starch. The cell-wall is ir CILIARY MOVEMENT 33 probably formed by the conversion of a thin superficial layer of protoplasm into cellulose, the cyst attaining its final thickness by frequent repetition of the process (see p. 14). The starch of the pyrenoids is apparently formed by a similar process of decomposition or destructive metabolism of pro- toplasm, growth taking place in both instances by accretion and not by intussusception. We see then that destructive metabolism may result in the formation of (a) waste products and (b) plastic products, the former being got rid of as of no further use, while the latter remain an integral part of the organism. Let us now turn once more to the movements of Hsemato- coccus, and consider in some detail the manner of their performance. Each flagellum (Fig. 3, A,y?) is a thread of protoplasm of uniform diameter except at its distal or free end where it tapers to a point. The lashing movements are brought about by the flagellum bending successively in different directions ; for instance, if in Fig. 3 F, abc represents it in the position of rest, abc' will show the form assumed when it is deflected to the left, and abc " when the bending is towards the right. In the position abc the two sides ab, ac are obviously equal to one another, but in the flexed positions it is equally obvious that the concave sides ac', be" are shorter than the convex sides be' , ac" ; in other words, as the flagellum bends to the left side ac becomes shortened, as it bends to the right the side be. This may be otherwise expressed by saying that, in bend- ing to the left the side ac contracts (see p. 10), in bending to the right the side be, or that the movement is performed by the alternate contraction of opposite sides of the flagellum. D 34 H^MATOCOCCUS LESS. Thus the ciliary movement of Haematococcus, like the amoeboid movement of Amoeba, is a phenomenon of con- tractility. Imagine an Amoeba to draw in all its pseudo- pods but two, and to protrude these two until they became mere threads ; imagine further these threads to contract regularly and rapidly instead of irregularly and slowly ; the result would be the substitution of pseudopods by flagella, i.e., of temporary slow-moving processes of protoplasm by permanent rapidly-moving ones. To put the matter in another way : in Amoeba the function of contractility is performed by the whole organism ; in Haematococcus it is discharged by a small part only, viz., the flagella, the rest of the protoplasm being incapable of movement. We have therefore in Haematococcus a dif- ferentiation of structure accompanied by a differentiation of function or division of physiological labour. The expression "division of physiological labour'' was invented by the great French physiologist, Henri Milne- Edwards, to express the fact that a sort of rough correspond- ence exists between lowly and highly organized animals and plants on the one hand, and lowly and highly organized human societies on the other. In primitive communities there is little or no division of labour : every man is his own butcher, baker, soldier, doctor, &c., there is no distinc- tion between " classes " and " masses," and each individual is to a great extent independent of all the rest. Whereas in complex civilized communities society is differentiated into politicians, soldiers, professional men, mechanics, labourers, and so on, each class being to a great extent dependent on every other. This comparison of an advanced society with a high organism is at least as old as ^Esop, who gives expression to it in the well-known fable of " the Belly and Members,' ii DIMORPHISM 35 We see the very first step towards a division of labour in the minute organism now under consideration. If we could cut off a pseudopod of Amoeba the creature would be little or none the worse, since every part would be capable of sending off similar processes, and so movement would be in no way hindered. But if we could amputate the flagella of Haematococcus its movements would be absolutely stopped. Haematococcus multiplies only in the resting condition (p. 28, and Fig. 3, B) ; as in Amoeba its protoplasm undergoes simple or binary fission, but with the peculiarity that the process is immediately repeated, so that four daughter-cells are produced within the single mother-cell-wall (Fig. 3 c). By the rupture of the latter the daughter-cells are set free as the ordinary motile form ; sometimes they acquire their flagella and detached cell-wall before making their escape (D). Under certain circumstances the resting form divides into eight instead of four daughter-cells, and these when liberated are found to be smaller than the ordinary motile form, and to have no cell-wall. Haematococcus is therefore dimorphic, i.e., occurs, in the motile condition, under two distinct forms : the larger or ordinary form with detached cell-wall is called a megazooid, the smaller form without a cell-wall a microzooid. D 2 LESSON III HETEROMITA WHEN animal or vegetable matter is placed in water and allowed to stand at the ordinary temperature, the well-known process called decomposition sooner or later sets in, the water becoming turbid and acquiring a bad smell. A drop of it examined under the microscope is then found to teem with minute organisms. To one of these, called "the Springing Monad," or in the language of zoology, Hetero- mita rostrata, we must now direct our attention : it is found in infusion of cod's head which has been allowed to stand for two or three months. Heteromita (Fig. 4, A) is considerably smaller than either Amoeba or Hsematococcus, being only T J T mm. (3^0 o inch) in average length. It has a certain resemblance in general form to Haematococcus, being somewhat ovoidal and pointed at one end. Like Haematococcus also it has two flagella, but only one of these (fl. i) proceeds from its beak-like anterior end and is directed forwards as the creature swims : the other (ft. 2) springs a short distance from the beak, and in the ordinary swimming position is trailed after the organism as in A 2 and F 4 . Thus in Heteromita, besides an anterior and a posterior end, we may distinguish a ventral LESS. TTI NUTRITION 37 surface which is directed downwards in the ordinary position, and bears the second or trailing flagellum, and an opposite or dorsal surface directed upwards. Often instead of swimming freely in the fluid a Hetero- mita is found anchored as it were to a bit of the decompos- ing substance by its ventral flagellum as in A 1 . Under these circumstances it is in constant movement, springing backwards and forwards by alternately coiling and uncoiling the attached ventral flagellum. The general character of the movement will be readily understood from the figure, in which A 1 shows the monad with coiled flagellum, A 2 after it has sprung forward to the full extent of the flagellum, It is from this curious habit that the name " springing monad " is derived. Towards the posterior end of the body is a nucleus (), and at the anterior end a contractile vacuole (c. vac}. There is no trace of an investing membrane or cell-wall, and the protoplasm is colourless. Also, as is invariably the case with organisms devoid of chlorophyll, there is no starch. In considering the nutrition of Heteromita it is necessary, first of all, to take into consideration the precise nature of its surroundings. It lives, as already stated, in decomposing infusions of animal matter. Such infusions contain proteids in solution, in part split up by the process of decomposition into simpler compounds some of which are diffusible ; this process is due, as we shall see hereafter (Lesson VIII.), to the action of the minute organisms known as Bacteria, which are always present in vast numbers in putrescent substances. As Heteromita contains no chlorophyll its nutrition is obviously not holophytic. Observation seems to show pretty conclusively that it is not holozoic ; apart from the nu c.vac FIG. 4. Heteromita rostrata. A 1 , Ilie living organism, showing nucleus (), contractile vacuole LESS. TIT NUTRITION 39 (c. vac], anterior ilagellum (fl. i), and coiled ventral flagellum (fl. 2) by which the organism is anchored ; A- shows the position at the forward limit of the spring, the ventral flagellum being fully extended. it 1 B 3 , three stages in the longitudinal fission of the anchored form. c l c 3 . Three stages in the transverse fission of the same : fl. I 1 , rudiment of newly formed anterior flagellum. D 1 D 3 , three stages in the fission of the free-swimming form : fl. 2 1 , rudiment of the newly-formed ventral flagella. E 1 , free-swimming and anchored forms about to conjugate : E 2 , com- mencement of conjugation : E 3 , E 4 , two stages in the development of the zygote : E 5 , the fully formed zygote : E e , dehiscence of the zygote and emission of spores. F 1 F 4 , four stages in the development of the spores. (After Dallinger. ) fact that it possesses neither mouth nor pseudopods, examples have been kept under observation for hours together by trained microscopists, and have never been observed to ingest the bacteria or other particles, dead or alive, contained in the fluid. There remains only one way in which nutrition can take place, namely, by absorption of the proteids and other nutrient substances in the solution, i.e., by these substances diffusing into the water of organization of the monad. Whether the proteids are rendered diffusible by the process of decomposition alone, i.e., by the action of bacteria (see p. 91), or whether a kind of surface digestion takes place, the protoplasm of Heteromita con- verting the proteids in immediate contact with it into pep- tones or allied compounds, is not certain. Thus Heteromita feeds neither by taking solid pro- teinaceous food into its interior (holozoic nutrition) nor by decomposing carbon dioxide and combining the carbon with water and mineral salts (holophytic nutrition), but by absorb- ing decomposing proteids and other nutrient substances in the liquid form ; this is the saprophytic mode of nutrition. It will be seen that the main difference between saprophytic and holozoic nutrition is that in the former digestion, i.e., the process of rendering food-stuffs soluble and diffusible, 40 HETEROMITA LESS. takes place outside the body so that constructive meta- bolism can begin at once. It is worthy of notice that while the process of feeding is strictly intermittent in Amoeba, which only takes in food at intervals, and largely intermittent in Haematococcus, in which the decomposition of carbon dioxide takes place only during daylight, in Heteromita it is continuous, the organism living in a solution of putrefying proteids" which it is constantly absorbing. It may be said to live immersed in an immense cauldron of broth which it is for ever imbibing, not by its mouth, for it has none, but by the whole surface of its body. Respiration and excretion probably take place in the same manner as in Amoeba. It has been shown that the optimum temperature for saprophytic monads is about 18 C., the ultra-maximum or thermal death-point about 60 C. But it is an interesting fact that by very slowly increasing the temperature, Dr. Dallinger was able in the course of several months to accustom some of these forms not Heteromita itself but closely allied genera to live at a temperature exceeding 68 C. The ordinary method of reproduction is by simple fission, the process affecting not only the body but the flagella as well. In Fig 4, B 1 , the commencement of fission is shown ; the anterior flagellum has undergone complete longitudinal division, while the split has only extended about a third of the length of the body and ventral flagellum. In B- the process has gone further, and in ir the products of division are on the point of separating. Mure frequently, however, fission instead of being longitudinal, i.e., in tin- direction of the long axis of the monad, is transverse, i.e., at right angles to the long axis. This process is shown in C 1 c 3 , and is een to differ from that described in the preceding paragraph in the cir- in CONJUGATION 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 C 1 ,^. 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 (D 1 ), becoming almost amoeboid inform and finally (D 2 ) globular. Division then takes place : the flagella of the parent become each the anterior flagellum of one of the daughter-cells (compare D 1 , D' 2 , and D 3 ), while their ventral flagella are formed by the splitting of a little outgrowth of the dividing body (D 2 ,^. 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 (E 1 ). 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 (E 2 ) 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 (E S ), 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 (E 4 ) : then it comes to rest and loses its flagella, appearing as a clear, homogeneous, three-cornered sac with slightly convex sides (E 5 ). This body, formed by the conjugation of the two monads, is called a zygote, the t\vo conjugating individuals being distinguished as gametes. 42 IIETEROMITA LESS. The zygote remains quiescent for some time, and then, after undergoing wave-like movements of its surface, bursts at its three angles (E G ), 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 (r 1 ) are found to grow into clear visibility and to take on a distinctly oval shape (p 2 ). Still increasing in size they develop a ventral flagellum (r 3 ) which is at first quite quiescent : finally, the pointed end sends out a process which becomes an anterior flagellum (r 4 ). 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 might perhaps be allowable to consider the active, free- swimming 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.). TIT 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 special, 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 Amceba and Hcematococcus. 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 frequently due to the presence of vast numbers of an organism known as Euglena viridis. Although microscopic, Euglena is considerably larger than either Haematococcus or Heteromita, its length varying from ^ 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 (_/?) by the action of which the organism swims with great rapidity, the flagellum being, as in Hsematococcus, 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 first at the anterior end, then in the middle, then at the LESS. IV 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 genus that the name euglenoid is applied to them. A. / rvii- FIG. 5. Englena 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 (/m), reservoir of the con- tractile vacuole (c.vac}, with adjacent pigment spot, and gullet with a single fiagellum springing from it. F, enlarged view of the anterior end of E, showing pigment-spot (fig) and reservoir (c. vac}, mouth (m), 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 (p). (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 LESS. 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 chromatophpre 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, /), a carbohydrate of the same composition as starch (C 6 H 10 O-,), 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 (a>. s) 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 internal 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 powdered carmine in the water, w r hen the iv MOUTH AND GULLET 47 coloured {(articles 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 do so only 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 haematochrome (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 O o 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 4^ EUGLENA LESS, iv 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. A process of multiple fission (p. 42) has also been de- scribed, 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 Spirula, and was at once noticeable from the bright orange colour which suggested its specific name. Appar- ently no one has since been fortunate enough to find it. 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 holozoic, 1 See p. 9, note. E A p.td **" /~y < X ''*-/~x' - v - FIG. 6. Protomyxa aurantiaca. A, the living organism (plasmodium), showing fine branched pseudo- pods (psd) and several ingested organisms, li, the same, encysted : cy the cell-wall, c, the protoplasm of the encysted form breaking up into spores. D, dehiscence of the cyst and emergence of E, flagellulre which afterwards become converted into F, amcebulse. G, amcebulse uniting to form a plasmodium. (After Haeckel.) IKSS. v LIFE-HISTORY 5r 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 shell 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 flagellulcE ; 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 anmbula. The process just described may be taken as a practical E 2 52 PROTOMYXA AND THE MVCETOZOA LESS. 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 amcebula, the flagellum of the former becoming one of the pseudopods of the latter. The amoebulae 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 amcebula but by the conjugation or fusion of a variable number of amoebulae. A body formed in this way by the fusion of amcebulae is called a plasmodiuiu, 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 consists of sheets or networks of proto- plasm 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 FIG. 7. A, part of the plasmodium of Badhamia(^ 3^); /' 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, amoebulae formed by metamorphosis of flagellulce. F, two amoebulas 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.) a 54 PROTOMYXA AND THE MYCETOZOA LESS. other substance : and in doing 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. They can also digest starch which lias 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 l 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 proto- plasm 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 flagellulas lose their cilia and pass into the condition of amcebulas (E), which finally fuse to form the plasmodium with which we started (F H). In the young plasmodia (n 1 ) the nuclei of the constituent amoebulse 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 amoebulce to form the plasmodium of Mycetozoa the cell-bodies (protoplasm) alone coalesce, not the nuclei. There is a suggestive analogy between this process of plasmodium-formation and that of conjugation as seen in Heteromita. Two Heteromitse fuse and form a zygote the The process of formation of the cyst or sporangium is a compli- cated one, and will not be described here. See De Bary, Mycetozoa, and Bacteria (Oxford, 1887). v PL ASMODIUM FORMATION AND CONJUGATION 55 protoplasm of which divides into spores. In Protomyxa and the Mycetozoa not two but several amoebulce 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 the product of their union subsequently, in all probability, breaks up to form the nuclei of the spores. In the Myce- tozoa neither fusion nor apparent disappearance of the nuclei of the amoebulee 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 on careful watching the resemblance becomes closer still, for the corpuscle is seen to put out and withdraw pseudopods (A 1 A 4 ) and 50 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 (A 5 , A, 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 (B 1 ), a reptile, or a bird contains quite similar leucocytes, but in addition there are found in the blood of these red-blooded animal bodies called red corpuscles. They are flat oval discs of protoplasm (B 5 , B) 1) FIG. 8. Typical Animal and Vegetable Cells. A 1 A 4 , living leucocyte (blood corpuscle) of a crayfish showing amoeboid movements : A 5 , A 6 , the same, killed and stained, showing the nucleus (nu}. B 1 , leucocyte of the frog, nu the nucleus ; B 2 , two leucocytes beginning to conjugate : B 3 , the same after conjugation, a binucleate plasmodium being formed : B 4 , a leucocyte undergoing binary fission : B 5 , surface view and B 6 , edge view of a red corpuscle of the same, ;///, the nucleus. C 1 , C-, leucocytes of the newt ; in c 1 particles of vermilion, repre- sented by black dots, have been ingested. C 3 , surface view and C 4 , edge view of a red corpuscle of man. D 1 , columnar epithelial cells from intestine of frog : D-, a similar 58 EPITHELIAL CELLS LESS. cell showing striated distal border from which in D 3 pseudopods are protruded. E 1 , ciliated epithelial cell from mouth of frog : E 2 , E 3 , similar cells from windpipe of dog. F 1 , parenchyma cell from root of lily, showing nucleus (mi), vacuoles (vac), and cell-wall : F 2 , a similar cell from leaf of bean, showing nucleus, vacuoles, cell-wall and chromatophores (chr). (B, D 1 , and E 1 , after Howes : C, E 2 , and E 3 , after Klein and Noble Smith : D 2 , D 3 , after Wiedersheim : F 1 , after Sachs : F 2 , after Behrens.) coloured by a pigment called hcemogloMn, and provided each with a large nucleus (mi) 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 (c 3 ,c 4 ) have the form of biconcave discs, and are devoid of nuclei. In many animals the leucocytes have been observed to ingest solid particles (c 1 ), to multiply by simple fission (B 4 ) and to coalesce with one another forming plasmodia (B 2 ) (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,"/.*., 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 (D 1 , D 2 ) consists of a rod-like mass of protoplasm, contain- ing a large nucleus, and is therefore comparable to an vi 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 (o 3 ), 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 (E 1 E 3 ) 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 for the fact 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 of oysters, mussels, &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 (r 1 ) 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 (p 2 , ckr). 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 Hsematococcus or Euglena, the main differences being that in the plant cell the form is polyhedral owing to the pressure of neighbouring cells and that the chromato- phores 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 vi MINUTE STRUCTURE OF CELLS 61 leaves 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 multicellular or are to be considered as cell-aggregates, while in the case of such beings as Amoeba, Haematococ- 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, B 1 c 1 ,) the ciliated (E), or the encysted (F) condition, and that a plasmodial phase (B 2 ) is sometimes produced by the union of two or more amceboid 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 continual division of their constituent cells. The process of division in animal and vegetable cells is frequently accompanied by certain very characteristic and complicated changes in the nucleus to which we must now 62 MINUTE STRUCTURE OF CELLS LESS. 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. c.m nu- tri C.b ^ >w m ^mi N*^ ** :/ c w chr nu.m FIG. 9. A, Cell from the genital ridge of a young salamander, showing cell-membrane (c. m), protoplasm or cell-body (c. b) with directive sphere (s) and central particle (c), and nucleus with membrane (mi. m) and irregular network of chromatin (chr). B. Cell from the immature stamen of a lily, showing cell-wall (c. ?c), protoplasm with two directive spheres (j), and nucleus as in A. Both figures very highly magnified. (A, from a drawing by Mr. J. E. S. Moore : B, after Guignard.) There seems to be a good deal of variation in the precise structure of various animal and plant cells, but the more recent researches show that in the cell-body or protoplasm (Fig. 9, c. b) two constituents may be distinguished, a clear semi-fluid substance, traversed by a delicate sponge-work. Now under the microscope the whole cell is not seen at once but only an optical section of it, that is all the parts which are in focus at one time : by altering the focus we view the object at successive depths, each view being practically a slice parallel to the lenses of the instrument. This being the case, protoplasm presents the microscopic appearance of a clear or slightly granular vi MINUTE STRUCTURE OF NUCLEI 63 matrix traversed by a delicate network. In the epithe- lial cells of animals the protoplasm is bounded exter- nally by a cell-membrane (Fig. 9, A, c. m] of extreme tenuity, in plants by a cell-wall (B, c. w) of cellulose : in amoeboid cells the ectosarc or transparent non-granular portion of the cell consists of clear protoplasm only, the granular endosarc alone possessing the sponge work. In the majority of full-grown plant cells (Fig. 8, F) and in some animal cells the protoplasm is more or less exten- sively vacuolated, but in the young growing parts as well as in the ordinary cells of animals the foregoing description holds good. It is quite possible that the reticular character of the cell may be merely the optical expression of an extensive but minute vacuolation, or may be due to the presence of innumerable minute granules developed in the protoplasm as products of metabolism. The nucleus is usually spherical in form : it is enclosed in a delicate nuclear membrane (n.m) and contains, as in Amoeba (p. 7) two constituents, the nuclear matrix and the chromatin which exhibit far more striking differences than the two constituents of the cell-body. The nuclear matrix is a homogeneous semi-fluid substance which forms the ground-work of the nucleus : it resembles the clear cell- protoplasm in its general characters, amongst other things in being unaffected by dyes. The chromatin (chr} takes the form of a network or sponge-work of very variable form, and is distinguished from all other constituents of the cell by its strong affinity for aniline and other dyes. Frequently one or more minute globular structures, the nudeoli (B, nu'\ occur in the nucleus either connected with the network or lying freely in its meshes : they also have a strong affinity for dyes although they often differ considerably from the chromatin in their micro-chemical reactions. H FlG. 10. Diagrams illustrating the process of indirect cell division or karyokinesis. A, The resting cell : the nucleus shows a nuclear membrane (ttu.m), chromatin (c/ir] arranged in loops united into a network (the latter shown on the right side only), and two nucleoli (mi 1 ) : near the nucleus is a directive sphere (s), containing a centrosome (c) and surrounded by radiating protoplasmic filaments. B, The chromatin has resolved itself into distinct loops or chromo- somes (chr) which have divided longitudinally : the nuclear membrane has begun to disappear : there are two directive spheres and between them is seen the commencement of the nuclear spindle (sp). C, The nuclear membrane has disappeared : the chromosomes are vi CELL DIVISION 65 arranged irregularly : the spindle has increased in size and is situated definitely within the nuclear area. D, The chromosomes are arranged round the equator of the fully formed nuclear spindle. E, The daughter-loops of the chromosomes are passing in opposite directions towards the poles of the spindle, each having a spindle-fibre attached to it. F, Later stage of the same process. G, The chromosomes are now arranged in two distinct groups one at each pole of the spindle. H, The daughter-cells are partly separated by constriction and the chromosomes of each group are uniting to form the network of the daughter-nucleus. I, Shows the division of a plant cell by the formation of a cell-plate (c. pi} : the daughter nuclei are fully formed. (Altered from Flamming, Rabl, &C. ) In the body of some cells and possibly of all there is found a globular body, surrounded by a radiating arrange- ment of the protoplasm and called the directive sphere (s) : it lies close to the nucleus, and contains a minute granule known as the central particle or centrosome (c]. In many plant cells two directive spheres have been found in each cell (B, s). The precise changes which take place during the fission of a 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 (Fig. 10). First of all, the directive sphere divides (B, s) and the products of its division gradually separate from one another (f), ultimately passing to opposite poles of the nucleus (D). At the same time the network of chromatin divides into a number of separate filaments called chromosomes (B, chr\ the number of which appears to be constant in any given species of animal or plant, although it may vary in different species from two to twenty-four. Soon after this the nuclear membrane and the free nucleoli disappear (B, c) and the F 66 MINUTE STRUCTURE OF CELLS LESS. nucleus is seen to contain a spindle-shaped body (sp] formed of excessively delicate fibres which converge at each pole to the corresponding directive sphere. The precise origin of this nuclear spindle is uncertain : it may arise either from the nuclear matrix or, more probably, from the protoplasm of the cell : it is not affected by colouring matters. At the same time each chromosome splits, sometimes transversely, but usually along its whole length so as to form two parallel rods or loops in close contact with one another (B) : in this way the number of chromosomes is doubled, each one being now represented by a pair. The divided chromosomes now pass to the equator of the spindle (D) and assume the form either of V- shaped loops, or of short rods, which arrange themselves in a radiating manner so as to present a star-like figure when the cell is viewed in the direction of the long axis of the spindle. Everything is now ready for division to which all the fore- going processes are preparatory. The two chromosomes of each pair now gradually pass to opposite poles of the spindle (E, F), two distinct groups being thus produced (G) and each chromosome of each group being the twin of one in the other group. Probably the fibres of the spindle are the active agents in this process, the chromosomes being dragged in opposite directions by their contraction. After reaching the poles of the spindle the chromosomes of each group unite with one another to form a network (H) around which a nuclear membrane finally makes its appear- ance (i). In this way two nuclei are produced within a single cell, the chromosomes of the daughter-nuclei, as well as their attendant directive spheres, being formed by the binary fission of those of the mother-nucleus. vi CELL-DIVISION 67 But pan 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 (H) 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 as a row of granules formed from the equatorial part of the nuclear spindle (i) : the granules extend until they form a complete equatorial plate dividing the cell-body into two halves : fission then takes place by the cell-plate split- ting into two along a plane parallel with its flat surfaces. 1 In plants the cell-plate gives rise to a partition wall of cellulose which divides the two daughter-cells from one another. 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 this connection the reader will not fail to note tin- 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 Tfmle 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 1 It must not be forgotten that the cells which are necessarily repre- sented in such diagrams as Fig. 10 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 2 68 COMPLEXITY OF CELL STRUCTURE LESS. 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 livinsr *J o 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 now 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. n, 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 (.?'.), the chromatin of which is aggregated into a central mass or nucleolus (g.w.}. 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 ot 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 (sec- vi STRUCTURE OF THE EGG 69 Fig. 53) ; or the surface of the protoplasm may secrete a thick cell-wall (see Fig. 61). The most extraordinary modification 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 FIG. ll. A, ovum of an animal {Car marina hasfata, one of the jelly fishes), showing protoplasm (gd], nucleus (gv), andnucleolus (;v//). B, ovum of a plant (Gymnadcnia conopsca, one of the orchids), showing protoplasm (flsin}, nucleus (////), and nucleolus (////'). (A, from LJalfour after Haeckel : P., after Marshall Ward.) fact that all the higher animals begin life as a single cell, or in other words that multicellular animals, however large and 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 megasporangia (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 70 THE PLANT OVUM LESS, vi embryo-sac or megaspore (see Fig. 89, D), at one end of which is a microscopic cell (ov, and Fig. 1 2 B), consisting as usual of protoplasm (plsm), nucleus (nu\ and nucleolus (nu). 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 cerevisicz. Saccharomyces consists of a globular or ellipsoidal mass of protoplasm (Fig. 12), about -^ 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 SACCHAROMYCES LESS. 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 11 11 11 lOO'OO vii EXPERIMENTS IN NUTRITION 77 The composition of this fluid is not a matter ot guess- work, but is 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 can 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 libera- 1 It is a matter of indifference whether cane-sugar or grape-sugar is used. 78 SACCHAROMYCES LESS. 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 (SO.,). 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 solti- o 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. vii EXPERIMENTS IN NUTRITION 79 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 \vell-beingofthe 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 O 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. 75) 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. 8o SACCHAROMYCES LESS. 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 /.*., 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 34C : at low temperatures it is com- paratively slow, and at 38C. 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 vii FERMENTS Si the Mycetozoa (p. 52), and probably it or some similar pep- fotiizing or proteolytic ferment effects this change in all organisms which have the power of digesting proteids. Another instance is furnished by diastase, which effects the conversion of starch into grape sugar : it is present in ger- minating barley (see p. 73), 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. 52), are able to 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 LESS, vin BACTERIUM TERMO 83 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 fluid is free from organisms, and indeed, if properly filtered, A ' , / * t FIG. 13. Bacterium termo. A, motile stage : B, resting stage or zooglaea. (From Klein.) 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 or an ordinary student's microscope Bacterium termo has the appearance show r n in Fig. 13, A : it is like a minute finger-biscuit, i.e. has the form G 2 84 BACTERIA LESS. of a rod constricted in the middle. But it is only by using the very highest powers of the microscope that its precise form and structure can be satisfactorily made out. It is then seen (Fig. 14) to consist of a little double spindle, showing neither nucleus, vacuole, nor other internal structure. It stains very deeply with aniline dyes, and from this and other circumstances there is reason for thinking that the whole cell consists of chromatin covered with a membrane of extreme tenuity formed of cellulose. It may therefore be considered as a cell consisting of cell-wall and nucleus only, the cell-body being absent. 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 FIG. 14. Bacterium tcrmo ( x 4000), showing the terminal fiagella. (After Dallinger.) easier to deal with whole numbers than with fractions, its size is best expressed by taking as a standard the one- 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 /JL, i.e. about the -$ mm. or the T O-TOTT 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 lias been estimated by Dallinger to be about -| p or ^o-jVuo 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. vin BACILLUS 85 14 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. ; this multiplied by 4,000 gives 72,000 cm. == 7 20 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. 14 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 a proteid material (Fig. 1.3, B). After continuing in the active con- dition 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. 15) is a minute form, the cells of which are about 2^ (~^ 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 zoogtea condition (Fig. 15, 4). Bacillus is commonly found in putrescent infusions in which the process of decay has gone on for some days : as 8.j BACTERIA LESS. its numbers increase those of Bacterium terrno diminish, until Bacillus becomes the dominant form. Its cells (Fig. 1 6) are rod-shaped and about 6/x, (y-1-^ mm.) in length in the commonest species. Both motionless and active forms are found, the latter having a flagellum at each end. The zoogloea condition ; s 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. 1 7, A) are wavy instead of straight. They are actively motile and when highly magnified are found to be provided with a 2 * *'. ' * * \ *&:. ..... : :: ...& " /. ..--. ""I: ' ..... ' "" FIG. 15. Micrococcits. I, single and double (dumb-bell shaped) forms : 2 and 3, chain-forms : 4, a zooglaea. flagellum at each end. Yibriones vary from 8/x to 25^ in length. Spirillum is at once distinguished by its spiral form, the cells resembling minute corkscrews (Fig. 17, E &: c) and being provided with a flagellum at each end (c). The smaller species, such as S. tenue (B) are from 2 to 5 /A in length, but the larger forms, such as S. volutans (c) attain a length of from 25 to 30^. 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 VIII BINARY FISSION long axis, like Hsematococcus (p. 25), and this double move- ment produces the appearance of undulation. Most Bacteria are colourless, but three species (Bacterium riride, I), chlorimun, and Bacillus vir ens} 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 FIG. 16.- Bacillus subtilis, showing various stages between single orms and long filaments (Lcptothrix). on slices of boiled potato, hard-boiled egg, &c., forming brilliantly coloured patches ; and the yellow colour often 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 si mole 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 88 BACTERIA loosely attached, forming chains. These are very common in some species of micrococcus (see Fig. 15). 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 I I I-'IG. 17. A, Vibrio. B, Spirillum temte. C, St>irilinm vonitans. 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 process is repeated again and again, and the daughter-cells vin NATURE OF GENERIC FORMS 89- remaining in contact form a long wavy or twisted filament called Leptothrix (Fig. 16) the separate elements of which are usually only visible after staining. Bacillus also multiplies by a peculiar process ot 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. 18) : 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 t j 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 ordinary cells, which become thick walled (arthrospores). The spores differ from the 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. The conditions of life of Bacteria are very various. Some live in water, such as that of stagnant ponds, and of these 1 See especially De Bary, Fungi, Myceiozoa, and Bacteria (Oxford, 1887). and Klein, Micro-organisms and Disease (London. 1886). -90 BACTERIA three species, as already stated (p. 85), 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. But this mode of nutrition is rare among the Bacteria : nearly all of those to which reference has been made are FIG. 18. Spore-formation in Bacillus. (From Klein.) 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 forms such as Bacterium termo, and species of Bacillus, Vibrio, &rc., will, however, flourish in Pasteur's solution, in which they obtain their nitrogen in the form of ammonium vin BACTERIA AS FERMENTS <>r tartrate instead of decomposing proteid. It lias also been shown that some Bacteria can go further and make use of nitrates as a source of nitrogen, and of a carbonate or even of carbon dioxide as a source of carbon : in other words, they are able to live upon purely inorganic matter in spite of the fact that they contain no chlorophyll. Some species may even multiply to a considerable extent in distilled water. But pari 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. C 6 H 12 6 = 2(C S H 6 8 ), Sugar. Lactic Acid. The transformation is brought about by the agency of 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 C 2 H 6 O + Oo = H 2 O + C,H 4 Oo, Alcohol. Oxygen. Water. Acetic Acid. The ferment in this instance is Bacterium aceti, often called Mycoderma 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 or 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 92 BACTERIA LESS. (NH 3 ), sulphuretted hydrogen (H.,8), and ammonium sulphide ( (NH 4 ).,S), the evolution of which produces the characteristic odour of putrefaction. The final stage in putrefaction is the formation of nitrates and nitrites. The process is a double one, both stages being due to special forms of Bacteria. In the first place, by the agency of the nitrous ferment^ ammonia is converted into nitrous acid- NH 3 + 30 = H 2 O + HNO 2 Ammonia. Oxygen. Water. Nitrous Acid. The nitric ferment then comes into action, converting the nitrous into nitric acid- NHOo + O = HNO S Nitrous Acid. Oxygen. Nitric Acid. This process is one of vast importance, since by its agency the soil is constantly receiving fresh supplies of nitric acid which is one of the most important substances used as food by plants. 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, typhoid fever, and cholera, 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 ]). 78), are quite independent of free oxygen and must there- fore be able to take the oxygen, without which their metabolic vin CONDITIONS OF LIFE 93 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 flourish only 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 modem 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- arly it is a matter of common observation that a moderately igh 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 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 one as high as 130 C. On the other hand, putrefactive Bacteria retain their power of development after being exposed to a temperature of -mC., 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 aces of the dried bodies of animals in such countries as O Egypt and Peru depends at least as much upon the moisture- less air as upon the antiseptics used in embalming. 94 BACTERIA LESS, vni 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 ABIOGENESIS I HOMOGENESIS AND HETERO- GENESIS THE study of the foregoing living things and especially ot 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, 96 BIOGENESIS AND HOMOGENESIS LESS. 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-w^orms 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, commenting 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. ix PROBLEM LIMITED TO MICROSCOPIC FORMS 97 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 was opened with the in- vention of the microscope. In 1683, Anthony van Leeuwen- hoek 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 later, 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 repeating the process, the vessel will soon con- H 98 BIOGENESIS AND HOMOGENESIS LESS. tain millions 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 Avithin 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 sup- porting 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, ix EXPERIMENTS ON BIOGENESIS 99 after these two conditions have been rigorously complied with, living organisms appear in the fluid, such organisms must have originated abiogenetically. 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, i.e., 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 19). 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 2 IOO BIOGENESIS AND HOMOGENESIS LESS. 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. 19. 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 IX EXPERIMENTS ON BIOGENESIS 101 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- ~38C., 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 thereupon 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 102 BIOGENESIS AND HOMOGENESIS LESS. 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 Heter agenesis, according to which the offspring of a given animal or plant may be something utterly different from itself, a plant giving rise to an animal or vice versa, 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 ix HETEROGENESIS 103 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. 45) to Amoeba and Infusoria (see p. 107), Euglense 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- cellas 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. 20, p. 108). 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 104 BIOGENESIS AND HOMOGENESIS LESS. 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 : Euglenae always giving rise to Euglenae 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. 53), 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 Euglenae or Infusoria from Bacteria? To this it is sufficient to answer that the evolution of one form from another takes place by a series ix HETEROGENESIS 105 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 /.., leading to either a less or a more highly organized form and in the course of a single generation. LESSON X PARAMCECIUM, 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. 15, p. 86) 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 or 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, LESS, x GENERAL CHARACTERS 107 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," Param&cium 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 i \ mm. (200 26o/x.) in length : in fact it is just visible to the naked eye as a minute whitish speck. Its form (Fig. 20 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.vac T>uc. r/7? c.rac. FIG. 20. Paramcecium aurelia. A, the living animal from the ventral aspect, showing the covering of cilia, the buccali groove (to the right) ending posteriorly in the mouth LESS, x MOVEMENTS 109 (/nth} and gullet (gtil} ; several food vacuoles (/. vac}, and the two contractile vacuoles (c. vac}. B, the same in optical section, showing cuticle (en}, cortex (cort}, and medulla (wed] ; buccal groove (buc. gr}, mouth, and gullet (gul) ; numerous food vacuoles (/ vac} circulating in the direction indicated by the arrows, and containing particles of indigo, which are finally ejected at an anal spot ; meganucleus (nu), micronucleus (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 micronucleus (pa. mi} has already divided, the nucleus (mi) 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 grove is called the buccal groove (Fig. 20, 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. 20 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 perma- nence of contour is due to the presence of a tolerably firm though delicate cuticle (cu} which invests the whole surface. i io PAKAMCECIUM, STYLONYCHIA, OXYTRICHA LESS, The protoplasm thus enclosed by the cuticle is distinctly divisible into two portions an external somewhat dense layer, the cortical layer or cortex (cort\ 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 (/. vac, see below, p. 112,) 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, ]). 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 myophan layer. The mouth (mt/i) leads into a short funnel-like tube, the gullet (gut], 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 Hsematococcus, Euglena, &c., which exhibit more or less intermittent lashing move- x CONTRACTILE VACUOLES in 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 nuclear matrix 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. nu) which is also deeply stained by magenta or carmine. This is the micronucleus : it is to be considered as a second, smaller nucleus, the larger body being distinguished as the megamideus. 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 Paramcecium 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 then seen that during the diastole, or 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 in A & B) but rapidly appearing once more. There seems to be no doubt that the U2 PARAMCECIUM, STYLONYCHIA, OXYTRICHA LESS. water taken in with the food is collected into these canals, emptied into the vacuole, and finally discharged into the surrounding medium. The process of feeding can be very conveniently studied in Paramoecium 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. 20, 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 x TRICHOCYSTS 113 ejected.- The spot in question is therefore to be looked 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 Paramcecium 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 (C 6 H 10 O 5 ) but soluble and diffusible. Oils or fats seem to be partly converted into fatty acids and glycerine. The nutrition of Paramcecium 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, trcfi) closely arranged in a single layer and perpen- dicular to the surface. These are called trichocysts. When a Paramcecium 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 or- dinary circumstances in the trichocysts, probably coiled up ; and by the contraction of the cortex consequent upon any i ii4 PARAMCECIUM, STYLONYCHIA, OXYTRICHA LESS.X sudden irritation they are projected in the way indicated. In Fig. 20 B, a few trichocysts (trcK) are shown in the ex- ploded condition, i.e. with the threads protruded. Most likely these bodies are weapons of offence like the very similar structures (nematocysts) found in polypes (see Lesson XXII. Fig 51). Paramcecium multiplies by simple fission, the division of the body being always preceded by the elongation and subsequent division of the mega- and micronucleus (Fig. 20, D). Division of the meganucleus is direct, that of the micronucleus indirect, i.e. takes place by karyokinesis. 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. 21, A) and the mega- nucleus (mg. nu) of each gradually breaks up into minute fragments (D G) which are either absorbed into the proto- plasm or ejected. At the same time the micronucleus (mi. nu] divides, by karyokinesis, and the process is repeated, the result being that each gamete contains four micro- nuclei (B). Two of these become absorbed and disappear, (c mi. mi ', mi. nu") of the remaining two one is now distin- guished as the active pronucleus, the other as the stationary pronucleus. Next, the active pronucleus of each gamete passes into the body of the other (c) and fuses with its stationary pronucleus (D): in this way each gamete con- tains a single nuclear body, the conjugation-nucleus (E), formed by the union of two similar pronuclei one of which is derived from another individual. It is this fusion of two nuclear bodies, one from each of the con- jugating cells, which is the essential part of the whole Mg.nn roi-nu FlG. 21. Stages in the Conjugation of Paranuecium. A, Commencement of conjugation : the meganuclei (mg. mi) of the two gametes are almost unaltered : the micronuclei (mi. mi) are in an early stage of karyokinesis. B, The micronuclei have divided twice, each gamete now containing four. C, Two of the micronuclei (mi. mi', mi. w")of each gamete are degenerating : of the remaining two one the active pronucleus is passing into the other gamete. D, The active pronucleus of each gamete has passed into the other gamete and is conjugating with its stationary pronucleus. The mega- nucleus (mg. mi] has begun to break up. E, Each gamete contains a single conjugation-nucleus formed by the union of its own stationary pronucleus with the active pronucleus of the other gamete. On the right side the conjugation-nucleus is beginning to divide. F, Conjugation is over and only one of the separated gametes is shown. It contains the fragments of the meganucleus (dotted) and four nuclear bodies (mi. mi) produced by the division and re-division of the con- jugation-nuc eus. G, Two of the products of division of the conjugation-nucleus (Mg. mi) are enlarging to form mega-nuclei, the other two (Mi.nu} are taking on the characters of micronuclei. (After Hortwig. ) I 2 ii6 PARAMOECIUM, STYLONYCHIA, OXYTRICHA LESS. process. Soon after this the gametes separate from one another and begin once more to lead an independent existence ; the conjugation nucleus of each undergoing a twice repeated process of division, the infusor thus acquiring four small nuclei (F). Two of these enlarge and take on the character of meganuclei (G, Mg. nu\ the other two remaining unaltered and having the character of micronuclei (Mi. nu\ Thus shortly after the completion of conjugation each individual contains two mega- and two micronuclei all derived from the conjugation-nucleus. Ordinary transverse fission now takes place, as described in the preceding paragraph, each of the two daughter cells having one mega- and one micronucleus, and thus the normal form of the species is re-acquired. It will be noticed that, in the present instance, conjuga- tion is not a process of multiplication : it has been ascertained that during the time two infusors are conju- gating each might have produced several thousand offspring by continuing to undergo fission at the usual rate. The importance of the process lies in the exchange of nuclear material between the two conjugating individuals : without such exchange these organisms have been shown to undergo a gradual process of senile decay characterized by diminution in size and degeneration in structure. Another ciliated infusor common in stagnant water and organic infusions is Stylonychia mytilus, an animalcule vary- ing from -j^inm. to mm. Like Paramcecium it is often to be seen swimming rapidly in the fluid, but unlike that genus 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 X CILIA OF STYLONYCHIA 117 cylindrical, it is flattened on one the ventral side, and is thus irregularly plano-convex in transverse section (Fig. 22, c). It resembles Paramoecium in general structure (compare in.c i llllC.t FIG. 22. A, Stylonychia my til us, ventral aspect, showing the buccal groove (buc. gr.} and mouth (mt/i), two nuclei (nit, nu), contractile vacuole (c.vac), and cilia differentiated into hook-like (//. ci], bristle- like (b. ci), plate-like (p. ci), and fan-like (m. ci) organs. B, one of the plate-like cilia of the same (p. ci in A), showing its frayed extremity. C, transverse section of Gastrostyla, a form allied to Stylonychia, showing buccal groove (btic. gr.}, small dorsal cilia (d. ci}, hook-like cilium (/i. ci), and the various cilia of the buccal groove, including an expanded fan-like organ (/;/. ci). A and B after Claparede and Lach- mann : c after Sterki. Fig. 22, A, with Fig. 20, A) ; but owing to the absence of trichocysts the distinction between cortex and medulla is less obvious : moreover, it has two nuclei (;///, mi) and only one contractile vacuole (c. vac}. iiS PARAMCECIUM, STYLONYCHIA, OXYTRICHA LESS. But it is in the character of its cilia that Stylonychia is most markedly distinguished from Paramcecium : 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. a.) set in longi- tudinal grooves and exhibiting little movement. It seems probable that these are to be looked upon as vestigial or rudimentary cilia, /.., 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 dimorphic forms, including Z. arbuscula, alternans, and xin CREATION AND EVOLUTION 141 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 a 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 he 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 by a natural process of descent from a single indi- 1 4 2 SPECIES AND THEIR ORIGIN LESS. vidual, or from a pair of individuals, in each case precisely resembling, in all essential respects, their existing descend- ants, which came into existence 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. 28) ; each of the species being derived from a single individual which came into Existing Individuals A Z.arbuscula Z.alternans Z.dichotomum Z. simplex Z.affine Z.nutant Ancestral Individuals FlG. 28. Diagram illustrating the origin of the species of Zoothamnium by creation. existence, independently of the 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 species than between any two independently manufactured xin EVOLUTION 143 chairs or tables. The words affinity, relationship, (Sec., as applied to different species are, on the theory of Creation 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 w^e 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 gener- ation 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 faciltated by framing a working hypothesis. Suppose that at some distant period of the world's history 144 SPECIES AND THEIR ORIGIN LESS. there existed a Vorticella-like organism which we will call A (Fig. 29), having the general characters of a single stalked zooid of Zoothamnium (compare Fig. 26, F 2 ), and suppose that, of the numerous descendants of this form, represented by the lines diverging from A, there were some in which both the zooids formed by the longitudinal division of the body remained attached to the stalk instead of one of them swimming off as in Vorticella. The result it matters Brancliiiig dichotonwus Branching Brandling umbellate nwnopodial dichotomurn Z. arbiescitla DIMORPHIC HOMOMORPHIC FlG. 29. Diagram illustrating the origin of the species of Zoothamnium by evolution. not for our present purpose how it may have been caused would be a simple colonial organism consisting of two zooids attached to the end of a single undivided stalk. Let us call this form B. Next let us imagine that in some of the descendants of B, represented as before by the diverging lines, the plane of division was continued downwards so as to include the distal end of the stalk : this would result in the production xiii DIVERGENCE OF CHARACTER 145 of a form (c) consisting of two zooids borne on a forked stem and resembling Z. nutans. If in some of the descend- ants of c this process were repeated, each of the two zooids again dividing into two fixed individuals and the division as before affecting the stem, we should get a species (D) con- sisting of four zooids on a dichotomous stem, like Z. affine. Let the same process continue from generation to genera- tion, the colony becoming more and more complex ; we should finally arrive at a species E, consisting of numerous zooids on a complicated dichotomously branching stem, and therefore resembling Z. dichotomum. Let us further suppose that, in some of the descendants of our hypothetical form B, repeated binary fission took place without affecting the stem : the result would be a new form F, consisting of numerous zooids springing in a cluster from the end of the undivided stem, after the manner of Z. simplex. From this a more complicated umbellate form (G), like Z. arbuscula, may be supposed to have originated, and again starting from B with a different mode of branch- ing a monopodial form (H) might have arisen. Finally, let it be assumed that while some of the descend- ants of the forms c, D, and F became modified into more and more complex species, others survived to the present time with comparatively little change, forming the existing species nutans, affine, and simplex : and that, in the similarly surviving representatives of E, G, and H, a differentiation of the individual zooids took place resulting in the evolution of the dimorphic species dichotomum, arbuscula, and alternans. It will be seen that, on this hypothesis, the relative like- ness and unlikeness of the species of Zoothamnium are explained as the result of their descent with greater or less modification or divergence of character from the ancestral form A. And that we get an arrangement or classification L 146 SPECIES AND THEIR ORIGIN LESS. in the form of a genealogical tree, which on the hypothesis is a strictly natural one, since it shows accurately the relationship of the various species to one another and to the parent stock. So that, on the theory of evolution, a natural classification of any given group of allied organisms is simply a genealogical tree, or as it is usually called, a phytogeny. It must not be forgotten that the forms A, B, c, D, E, F, G, and H are purely hypothetical : their existence has been assumed in order to illustrate the doctrine of descent by a concrete example. The only way in which we could be perfectly sure of an absolutely natural classification of the species of Zoothamnium would be by obtaining specimens as far back as the distant period when the genus first came into existence ; and this is out of the question, since minute soft-bodied organisms like these have no chance of being preserved in the fossil state. It will be seen that the theory of evolution has the advantage over that of creation of offering a reasonable explanation of certain facts. First of all the varying degrees of likeness and unlikeness of the species are explained by their having branched off from one another at various periods : for instance, the greater similarity of structure between Z. affiine and Z. dichotomum than between either of them and any other species is due to these two species having a common ancestor in D, whereas to connect either of them, say with Z. arbuscula, we have to go back to B. Then again the fact that all the species, however complex in their fully developed state, begin life as a simple zooid which by repeated branching gradually attains the adult complexity, is a result of the repetition by each organism, in the course of its single life, of the series of changes passed through by its ancestors in the course of ages. In other words ontogeny, xin HEREDITY AND VARIABILITY 147 or the evolution of the individual, is, in its main features, a recapitulation of phytogeny or the evolution of the race. One other matter must be referred to in concluding the present lesson. It is obvious that the evolution of one species from another presupposes the occurrence of varia- tions in the ancestral form. As a matter of fact such individual variation is of universal occurrence : it is a matter of common observation that no two leaves, shells, or human beings are precisely alike, and in our type genus Zootham- nium the number of zooids, their precise arrangement, the details of branching, &c., are all variables. This may be expressed by saying that heredity, according to which the offspring tends to resemble the parent in essentials, is modified by variability, according to which the offspring tends to differ from the parent in details. If from any cause an individual variation is perpetuated there is produced what is known as a variety of the species, and, according to the theory of the origin of species by evolution, such a variety may in course of time become a new species. Thus a variety is an incipient species, and a species is a (relatively) permanent variety. It does not come within the scope of the present work to discuss either the causes of variability or those which deter- mine the elevation of a variety to the rank of a species : both questions are far too complex to be adequately treated except at considerable length, and anything of the nature of a brief abstract could only be misleading. As a preliminary to the study of Darwin's Origin of Species, the student is recommended to read Romanes's Evidences of Organic Evolution, in which the doctrine of Descent is expounded as briefly as is consistent with clearness and accuracy. L 2 LESSON XIV FORAMINIFERA, RADIOLARIA, AND DIATOMS IN the four previous lessons we have learnt how a uni- cellular organism may attain very considerable complexity by a process of differentiation of its protoplasm. In the present lesson we shall consider briefly certain forms of life in which, while the protoplasm of the unicellular body un- dergoes comparatively little differentiation, an extraordinary variety and complexity of form is produced by the develop- ment of a skeleton, either in the shape of a hardened cell- wall or by the formation of hard parts within the protoplasm itself. The name Foraminifera is given to an extensive group of organisms which are very common in the sea, some living near the surface, others at various depths. They vary in size from a sand-grain to a shilling. They consist of variously- shaped masses of protoplasm, containing nuclei, and pro- duced into numerous pseudopods which are extremely long and delicate, and frequently unite with one another to form networks, as at x in Fig. 30. The cell-body of these organisms is therefore very simple, and may be compared to that of a multinucleate Amoeba with fine radiating pseudopods. LESS. XIV THE SHELL 149 But what gives the Foraminifera their special character is the fact that around the protoplasm is developed a cell-wall, sometimes membranous, but usually impregnated with cal- cium carbonate, and so forming a shell. In some cases, as in the genus Rotalia (Fig. 30), this is perforated by nume- rous small holes, through which the pseudopods are pro- truded, in others it has only one large aperture (Fig. 31), FIG. 30. A living Foraminifer (Rotalia), showing the tine radiating pseudopods passing through apertures in the chambered shell : at x several of them have united. (From Gegenbaur. ) through which the protoplasm protrudes, sending off its pseudopods and sometimes flowing over and covering the outer surface of the shell. Thus while in some cases the shell has just the relations of a cell-wall with one or more holes in it, in others it becomes an internal structure, being covered externally as well as filled internally by protoplasm. The mode of growth of Foraminifera is largely determined by the hard and non-distensible character of the cell-wall, 150 FORAMINIFERA, RADIOLARIA, DIATOMS LESS. which when once formed is incapable of being enlarged. In the young condition they consist of a simple mass of proto- plasm covered by a more or less globular shell, having at least one aperture. But in most cases as the cell-body grows, it protrudes through the aperture of the shell as a mass of protoplasm at first naked, but soon becoming covered by the secretion around it of a second compartment or chamber of the shell. The latter now consists of two FIG. 31. A, diagram of a Foraminifer in which new chambers are added in a straight line : the smallest first-formed chamber is below, the newest and largest is above and communicates with the exterior. B, diagram of a Foraminifer in which the chambers are added in a flat spiral : the oldest and smallest chamber is in the centre, the newest and largest as before communicates with the exterior. (From Carpenter.) chambers communicating with one another by a small aperture, and one of them the last formed communi- cating with the exterior. This process may go on almost indefinitely, the successive chambers always remaining in communication by small apertures through which continuity of the protoplasm is maintained, while the last formed chamber has a terminal aperture placing its protoplasm in free communication with the outer world. XIV COMPLEXITY OF SHELL The new chambers may be added in a straight line (Fig. 31, A) or in a gentle curve, or in a flat spiral (Fig. 31, B), or like the segments of a Nautilus shell, or more or less irregularly. In this way shells of great variety and beauty FlG. 32. Section of one of the more complicated Foraminifera (Aveolina), showing the numerous chambers containing protoplasm (dotted), separated by partitions of the shell (white). x 60. (From Gegenbaur after Carpenter. ) of form are produced, often resembling the shells of Mol- lusca, and sometimes attaining a marvellous degree of com- plexity (Fig. 32). The student should make a point of examining mounted slides of some of the principal genera and of consulting the plates in Carpenter's Introduction to the Study of Foraminifera (Ray Society, 1862), or in Brady's Report on the foraminifera of the " Challenger' 1 '' Expedition, in order to get some notion of the great amount of dif- ferentiation attained by the shells of these extremely simple organisms. 152 FORAMINIFERA, RADIOLARIA, DIATOMS LESS. The Radiolaria form another group of marine animal- cules, the numerous genera of which are, like the Foram- inifera, amongst the most beautiful of microscopic objects. They also (Fig. 33) consist of a mass of protoplasm giving off numerous delicate pseudopods (psd) which usually have a radial direction and sometimes unite to form networks. In the centre of the protoplasmic cell-body one or more nuclei (nu) of unusual size and complex structure are found. Stel. Int. caps, pr cent caps FIG. 33. Lithocircus annularis, one of the Radiolaria, showing central capsule (cent, caps.}, intra- and extra capsular protoplasm (int. caps, pi'., ext.caps.pr.}, nucleus (nu), pseudopods (psd}, silicious skeleton, (skel}, and symbiotic cells of Zooxanthella (2). (After Biitschli.) In the interior of the protoplasm, surrounding the nucleus, is a sort of shell, called the central capsule (cent. caps.\ formed of a membranous material, and perforated by pores which place the inclosed or intra-capsular protoplasm (/;//. caps, pr.] in communication with the surrounding or extra- capsular protoplasm (ext. caps. pr.). But besides this simple membranous shell there is often developed, mainly in the extra-capsular protoplasm, a skeleton (skel) formed in the majority of cases of pure silica, and often of surpassing xiv COMPLEXITY OF SHELL 153 beauty and complexity. One very exquisite form is shown in Fig. 34 : it consists of three perforated concentric spheres connected by radiating spicules : the material of which it is composed resembles the clearest glass. The student should examine mounted slides of the silicious shells of these organisms sold under the name of Poly- cystinea and should consult the plates of Haeckel's Die FIG. 34. Skeleton of a Radiolarian (Actinomma), consisting of three concentric perforated spheres the two outer partly broken away to show the inner connected by radiating spicules. (From Gegenbaur after Haeckel. ) Radiolarien : he cannot fail to be struck with the complexity and variety attained by the skeletons of organisms which are themselves little more complex than Amoebae. Before leaving the Radiolaria, we must touch upon a matter of considerable interest connected with the physio- 154 FORAMINIFERA, RADIOLARIA, DIATOMS LESS. logy of the group. Imbedded usually in the extra-capsular protoplasm are found certain little rounded bodies of a yellow colour, often known as " yellow cells " (Fig. 33, 2). Each consists of protoplasm surrounded by a cell-wall of cellulose, and coloured by chlorophyll, with which is asso- ciated a yellow pigment of similar character called diatomin, For a long time these bodies were a complete puzzle to biologists, but it has now been conclusively proved that they are independent organisms resembling the resting condition of Haematococcus, and called Zooxanthella nutricola. Thus an ordinary Radiolarian, such as Lithocircus (Fig. 33), consists of two quite distinct things, the Lithocircus in the strict sense of the word plus large numbers of Zooxan- thellae associated with it. The two organisms multiply quite independently of one another : indeed Zooxanthella has been observed to multiply by fission after the death of the associated Radiolarian. This living together of two organisms is known as Sym- biosis. It differs essentially from parasitism (see p. 121), in which one organism preys upon another, the host deriving no benefit but only harm from the presence of the parasite. In symbiosis, on the contrary, the two organisms are in a condition of mutually beneficial partnership. The carbon dioxide and nitrogenous waste given off by the Radiolarian serve as a constant food-supply to the Zooxanthella : at the same time the latter by decomposing the carbon dioxide provides the Radiolarian with a constant supply of oxygen, and at the same time with two important food-stuffs starch andproteids, which, after solution, diffuse from the protoplasm of the Zooxanthella into that of the Radiolarian. The Radiolarian may therefore be said to keep the Zooxanthellae constantly manured, while the Zooxanthellae in return supply the Radiolarian with abundance] of oxygen and oi ready- xiv STRUCTURE OF CELL-WALL 155 digested food. It is as if a Hsematococcus ingested by an Amoeba retained its vitality instead of being digested : it would under these circumstances make use of the carbon dioxide and nitrogenous waste formed as products of kata- bolism by the Amoeba, at the same time giving off oxygen and forming starch and proteids. The oxygen evolved would give an additional supply of this necessary gas to the Amceba, and the starch after conversion into sugar and the proteids after being rendered diffusible would in part diffuse through the cell-wall of the Haematococcus into the surrounding protoplasm of the Amceba, to which they would be a valuable food. Thus, as it has been said, the relation between a Radio- larian and its associated yellow-cells are precisely those which obtain between the animal and vegetable kingdoms generally. The DiatomacecR^ or Diatoms, as they are often called for the sake of brevity, are a group of minute organisms, in- cluded under a very large number of genera and species, and so common that there is hardly a pond or stream in which they do not occur in millions. Diatoms vary almost indefinitely in form : they may be rod- shaped, triangular, circular, and so on. Their essential structure is, however, very uniform : the cell-body contains a nucleus (Fig. 35, A, mi] and vacuoles (vac\ as well as two large chromatophores (chr) of a brown or yellow colour ; these are found to contain chlorophyll, the characteristic green tint of which is veiled, as in Zooxanthella, by diatomin. The cell is motile, executing curious, slow, jerky or gliding movements, the cause of which is still obscure. The most interesting feature in the organization of diatoms is however the structure of the cell-wall : it consists of two 156 FORAMINIFERA, RADIOLARIA, DIATOMS LESS. parts or valves (B, c, c. w, c. w'), each provided with a rim or girdle, and so disposed that in the entire cell the girdle of one valve (c. w) fits over that of the other (c. w'} like the -mi u_U c.u*> r.w FIG. 35. A, semi-diagrammatic view of a diatom from its flat face, showing cell-wall (c. iv] and protoplasm with nucleus (mi], two vacuoles (vac), and two chromatophores (chr\ B, diagram of the shell of a diatom from the side, i.e., turned on its long axis at right angles to A, showing the two valves (c. "w, c. iv f ) with their overlapping girdles. C, the same in transverse section. D, surface view of the silicious shell of Navicula truncata. ]:, surface view of the silicious shell of \tilacodiscus sollittianns. (n, after Donkin ; !:, after Norman.) lid of a pill-box. The cell-wall is impregnated with silica, so that diatoms can be boiled in strong acid or exposed to the heat of a flame without losing their form : the protoplasm xiv MARKINGS OF DIATOMS 157 is of course destroyed, but the flinty cell-wall remains uninjured. Moreover, the cell-walls of diatoms are remarkable for the beauty and complexity of their markings, which are in some cases so delicate that even now microscopists are not agreed as to the precise interpretation of the appearances shown by the highest powers of the microscope. Two species are shown in Fig. 35, D and E, but, in order to form some con- ception of the extraordinary variety in form and ornamenta- tion, specimens of the mounted cell-walls should be ex- amined and the plates of some illustrated work consulted. See especially Schmidt's Atlas fur Diatomaceenkundb and the earlier volumes of the Quarterly Journal of Micro- scopical Science. We see then that while Diatoms are in their essential structure as simple as Haematococcus, they have the power of extracting silica from the surrounding water, and of forming from it structures which rival in beauty of form and intricacy of pattern the best work of the metal-worker or the ivory-carver. LESSON XV MUCOR THE five preceding lessons have shown us how complex a cell may become either by internal differentiation of its protoplasm, or by differentiation of its cell-wall. In this and the following lesson we shall see how a considerable degree of specialization may be attained by the elongation of cells into filaments. Mucor is the scientific name of the common white or grey mould which every one is familiar with in the form of a cottony deposit on damp organic substances, such as leather, bread, jam, &c. For examination it is readily obtained by placing a piece of damp bread or some fresh horse-dung under an inverted tumbler or bell-jar so as to prevent evapo- ration and consequent drying. In the course of two or three days a number of delicate white filaments will be seen shooting out in all directions from the bread or manure ; these are filaments of Mucor. The species which grows on bread is called Mucor stolonifer, that on horse-dung, M. mucedo. The general structure and mode of growth of the mould can be readily made out with the naked eye. It first appears, as already stated, in the form of very fine white threads projecting from the surface of them ouldy substance ; and these free filaments (Fig. 36, A, a. hy] can be easily FIG. 36. Mucor. A, portion of mycelium of M. mucedo (my} with two aerial hy price (a. hy}, each ending in a sporangium (spg). B, small portion of an aerial hypha, highly magnified, showing pro- toplasm (^/jw)and cell-wall (c iv}. The scale above applies to this figure only. C 1 , immature sporangium, showing septum (sep} and undivided pro- toplasm : C-, mature sporangium in which the protoplasm has divided into spores ; the septum (sep) has become very convex distally, forming the columella. D 1 , mature sporangium in the act of dehiscence, showing the spores (sp} surrounded by mucilage (g) ; D 2 , 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. The scale above c 2 applies to c 1 c 2 , D 1 , and E. 160 MUCOR LESS. F, spores. G 1 , G a , G 3 , three stages in the germination of the spores. H, a group of germinating spores forming a small mycelium. i 1 , i 6 , five stages in conjugation, showing two gametes (gam) uniting to form the zygote (zyg). K 1 , K 2 , development of ferment cells from submerged hyphu\ (A, c' 2 D, E, F, G, and K, after Howes ; I, after De Bary. ) ascertained to be connected with others (my) which form a network ramifying through the substance of the bread or horse-dung. This network is called a mycelium ; the threads of which it is composed are mycelial hyphce ; and the fila- ments which grow out into the air and give the characteristic fluffy appearance to the growth are aerial hyphtz. The aerial hyphae are somewhat thicker than those which form the mycelium, and are at first of even diameter through- out : they continue to grow until they attain a length, in M. mucedo, of 6-8 cm. (two or three inches). As they grow their ends are seen to become dilated, so that each is termi- nated by a minute knob (A, spg) : this increases in size and darkens in tint until it finally becomes dead black. In its earlier stages the knobs may be touched gently without injury, but when they have attained their full size the slightest touch causes them to burst and apparently to dis- appear their actual fate being quite invisible to the naked eye. As we shall see, the black knobs contain spores, and are therefore called sporangia or spore-cases. Examined under the microscope, a hypha is found to . be a delicate more or less branched tube, with a clear trans- parent wall (B, c. w) and slightly granular contents (flsm) : its free end tapers slightly (H), and the wall is somewhat thinner at the extremity than elsewhere. If a single hypha could be obtained whole and unbroken, its opposite end would be found to have much the same structure, and each of its branches would also be seen to end in the same way. xv ASEXUAL REPRODUCTION 161 So that the mould consists of an interlacement of branched cylindrical filaments, each consisting of a granular substance completely covered by a kind of thin skin of some clear transparent material. 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 vacnoles 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 of the mould only 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. 36, c 1 ) : 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. Let us consider precisely what this process implies. Before it takes place the protoplasm is continuous throughout the M 1 62 MUCOR whole organism, which is therefore comparable to the un- divided plant-cell shown in Fig. 9, B. 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. 10, i), 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, bicelhdar : if, as is ordi- narily the case, it bears numerous aerial hyphas, 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 hyphK : 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 (c 2 ) : at the same time the protoplasm of the sporangium under- goes multiple fission, becoming divided into numerous ovoid masses each of which surrounds itself with a cellulose coat and becomes a spore (D T , r/ 2 , 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, and give it the appearance of being closely covered with short cilia (o 2 ). XV GERMINATION OF SPORES 16- 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 consisting of protoplasm containing a nucleus and sur- FIG. 37. 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 under side of which is a hanging drop of nutrient fluid (r). The upper figure shows the apparatus in perspective, the lower in vertical section. (From Klein. ) rounded 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. 37, 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 of Mucor is touched with the point of a needle, which is M 2 1 64 MUCOR LKSS. 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 (c 1 , Fig. 36), 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 (c 2 ). These filaments continue to grow, giving off as they do so side branches (o :! ) which interlace with similar threads from adjacent spores (H). The filaments are obviously hyphit, and the interlacement is a mycelium. Thus the statement made in a previous paragraph (p. 161), 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 hyphre constituting the Mucor-] >lant 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 hyphce 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 the centrifugal mode of growth the mycelium is always xv CONJUGATION 165 thicker at the centre than towards the circumference, since it is the older or more central portions of the hyphse 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 hyphse send out short branches (Fig. 36, i 1 ), which come into contact with one another by their somewhat swollen free ends (i 2 ). In each a septum appears so as to shut off a separate terminal cell (i 3 , gam} from the rest of the hypha. The opposed w r alls of the two cells then become absorbed (i 4 ) and their contents mingle, forming a single mass of protoplasm (i 5 , syg), 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 Heteromi.ta 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. 132) ; here both conjugating bodies exhibit only the slow r 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 conjugation is merely increased activity in feeding and fissive 166 MUCOR LESS. 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 hyphas 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 accordance 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 Amceba 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. 1 In Mucor no distinction can be drawn betueen the conjugating body (gamete) and the organ which produces it (gonad). See the de- scription of the sexual process in Yauchcria (Lesson XVI.) and in Spirogyra (Lesson XIX.). An exception is formed by colonial forms such as Zoothamnium, in \\hich life is cairied on from generation to generation by the reproduc- tive /ooids only. In all probability the colony itself, like an annual plant, dies down after a longer or shorter time. .Moreover the ciliate infuviria are found, as already stated (p. 116), to sink into deevepitude after multiplying by fission for a long series of generations. xv NUTRITION 167 lively small part of the organism is set apart for repro- 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 in 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 cndo-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- 1 68 MUCOR LESS, xv ditions, Mucor is capable of exciting alcoholic fermentation in a saccharine solution. When the hyphse are submerged in such a fluid they have been found to break up, forming rounded cells (Fig. 36, K 1 , K 2 ), 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 Algtz, a group of plants which includes most of the smaller fresh-water weeds, and the vast majority of sea-weeds. One of these filamentous Algce, 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. 38, 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 . ;-,..*:< -' "ss FlG. 38. }'anclicria. A, tangled filaments of the living plant, shm\ ing mode of branching. K, extremity of a filament, showing cell -wall (c. TC-) and protoplasm with chromatophores (c/ir), and oil-drops (o). The scale above applies t< i ihis ligurc only. <', immature sjiorangium (>/.;') separated from the filament by a sep- tum -, C 2 , mature sporangium with the spore (sfi) in the act of escaping ; lree->\\i ling --pore, ^h(i\\ing cilia, colourless ectoplasm containing LESS, xvi ASEXUAL REPRODUCTION 171 nuclei, and endoplasm containing the green chromatophores ; c 4 , the same at the commencement of germination. D 1 , early, and D-, later stages in the development of the gonads, the spermary to the left, the ovary to the right ; D 3 , the fully-formed spermary (spy] and ovary (ovy), each separated by a septum (sep) from the filament. D 4 , the ovary after dehiscence, showing the ovum (oz>), with small detached portion of protoplasm ; D 5 , sperms ; D 6 , distal end of ripe ovary, showing sperms (sp) passing through the aperture towards the ovum (ov). D 7 , the gonads after fertilization, showing the oosperm (osp] still inclosed in the ovary and the dehisced spermary. E 1 , oosperm nbout to germinate : E 2 , further stage in germination. (C 1 and C ;J , after Strasburger ; c' 2 and C 4 , after Sachs ; D and E, after rringsheim.) character of a membrane lining the cellulose coat. Numerous small nuclei occur in the protoplasm, as well as oil-globules ( 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 appearances 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 LF.SS. xvin MYCELIUM 185 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 white 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. 40, A (iny\ from the upper surface of which delicate threads, the aerial hyphtz (a. //y.), grow vertically upwards into the air, while from its lower surface similar but shorter threads, the sub- merged hyphcc (s. //r.), hang vertically downwards into the fluid. B FIG. 40. Penicillium ^ A, Diagrammatic vertical section of a young growth ( x 5), showing mycelium (my], submerged hyphrc (s. hy], and aerial hypha? (a. hy}. n, group of spores : I, before commencement of germination ; 2, after imbibition of fluid : the remaining three have begun to germinate. C, very young mycelium formed by a srnall group of germinating spores. LESS. XVIIT MULTICELLULAR IIYPIL'E 187 n, more advanced mycelium : the hypha; have increased in length and begun to branch, and septa (scp) have appeared. K, germinating spore (s_/>) 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 hyphre contain vacuoles (va<:) in their protoplasm. F J -F 4 , 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. F 5 , a fully-developed brush with a row of spores developed from each sterigma (stg). F 6 , a single sterigma (stg) with its spores (sp\ F 7 , an over-ripe brush in which the structure is obscured by spores which have dropped from the sterigmata. B-D, F^F-', and F 7 x 150 : F 6 *: : 200 : E : 500. As long as the growths are white or blue in colour no powder can be detached by touching the aerial hyphse, 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 hyphen, : they are regularly cylindrical, about yjpjj mm. in diameter, frequently branched, and differ in an important particular from the somewhat similar hyphce of Mucor (p. 161). 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. 162), imtlticelhdar, 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 88 PENICILLIUM AND AGARICUS LESS. cillium 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. 120), or a filament of Mucor or Vaucheria, is multinucleate. The submerged hyphse 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 (r 1 F 4 ). Each sends off from its distal or upper end a larger or smaller number of branches which remain short and grow parallel to one another : the primary branches (F 1 , F 2 ) form secondary ones (F 3 ), and the secondary tertiary (p 4 ), 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 s/er/'s- *> mata (F S , stg). Next, the ends of the sterigmata become constricted, exactly as if a thread were tied round them and gradually tightened (r 1 , F <; ), 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). xviu GERMINATION OF SPORES 189 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 (r 5 , F), of which the proximal or lower one is the youngest, the distal or upper one the oldest. 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 (F G ). 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 (F'). It is at the period of complete formation of the spores that the growth turns green. The colour is not due to the pres- ence of chlorophyll. Under a high power the spores appeal- quite colourless, whereas a cell of the same size coloured 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. 37, p. 163). The spores, several of which usually adhere together, are at first clear and bright (B 1 ) : soon they swell considerably, and the protoplasm becomes granular and vacuolated (B-) : 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 190 PENICILLIUM AND AGARICUS LESS. formed ( D, E. sep) dividing the young hypha into two cells (compare Fig. 36, H, p. 159). The distal cell then elongates and divides again, and in this way the hyphae 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 furthest 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 hyphae, and the subsequent development of spores in the resulting branched zygote. But as the details of the process 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 Eurotiiiin 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. 167). 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 xvin FILEUS AND LAMELLJ-: 191 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 (Agaricus) consists of a stout vertical stalk (Fig. 41, A, .>-/), on the upper or distal end of which is borne an umbrella-like disc or pileus (/). The lower or proximal end of the stalk is in connection with an underground mycelium (tny\ from which it springs. On the underside of the pileus are numerous radiating vertical plates or lamella (/) extending a part or the whole of the distance from the circumference of the pileus to the stalk. In the common edible mushroom (Agaricus cam- 192 PENICILLIUM AND AGARICUS LESS. 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. 41. Agaricus campcslns. A, Diagrammatic vertical section, showing the stalk (st) springing from a mycelium (;//?'), and expanding into the pileus (/), on the under side of which are the radiating lamelke (/). B, transverse vertical section of a lamella, showing the hyphaj (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 t\\<> sterigmata (iV^), each bearing a spore (.t/h (p. and c after Sachs. the mycelium of Penicillium, and its structure is bust in- vestigated by cutting thin sections of various parts and examining them under a high power. XVITI HISTOLOGY OF MUSHROOM 193 Such sections show the whole mushroom to be composed of immense numbers of closely interwoven, branched hyph?e (ii) divided by numerous septa into cells. In the stalk the hyphce 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 hyphse 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, a), 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. 175) 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. o LESSON XIX SPIROGYRA AMONGST the numerous weeds which form a green scum in stagnant ponds and slowly-flowing streams, one, called Spirogyra, is perhaps the commonest. It is recognised 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. 42, A) are cylindrical, covered with a cellulose cell-wall (c. w], 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 ph-m tram- FIG. 42. Spirogyra. A, small portion of a living filament, showing a single cell, with cell- wall (c. w], septa (sep) separating it from adjacent cells, peripheral layer of protoplasm (plsm) connected by threads with a central mass contain- O 2 196 SPIROGYRA ing the nucleus (nu), two spiral chvomatophores (chr\ and pyrenoids (pyr). i; 1 , B 2 , middle portion of a cell, showing two stages in binary fission. c, four stages in dioecious conjugation : in c 1 the gonads (go>^, gori*) are connected by short processes of their adjacent sides : in c 2 the active or male gamete (gam 1 ) has separated from the wall of the gonad (gon 1 ) preparatory to passing across the connecting bridge to the stationary or female gamate (gam"*) which has not yet separated from its containing gonad (gon 2 ) : in c 3 the female gamete (gar/P) has undergone separa- tion, and the male gamete (gam 1 ) is in the act of conjugating with it : in C 4 the two have united to form a zygote (zyg) lying in the female gonad. D, two stages in monoecious conjugation : in D 1 the adjacent cells (gonads) have sent out conjugating processes (a) : in D 2 conjugation is complete, the male gamete having passed through the aperture between the conjugating processes and united with the female gamete to form the zygote (zyg). E, parthenogenetic formation of zygotes. F, fully developed zygote (zygospore). G, early stage in the germination of the zygote. (B after Sachs : c after Strasburger : F and G from Sachs after Pringsheim.) larger cells of plants the development of vacuoles is so ex- tensive that the protoplasm is reduced to a thin layer in contact with the cell-wall (see pp. 169 and 188). This state of things is carried to excess in Spirogyra : the central vacuole is so large that the protoplasm (A, plsm) has the character of a mere delicate colourless membrane within the cell-wall : to make it out clearly the specimen should be treated with a fluid of greater density than water, such as a 10 per cent, solution of sodium chloride, which by absorbing the water in the vacuole causes the protoplasm to shrink away from the cell-wall and so brings it clearly into view. It is to this layer of protoplasm that the name primordial utricle is applied by botanists, but the student should remember that a primordial utricle is not a special constituent of those cells in which it occurs, but is merely the protoplasm of a vegetable cell in which the vacuole is inordinately large. The protoplasm of the cell of Spirogyra is not, however, xix INTERSTITIAL GROWTH 197 confined to the primordial utricle ; towards the centre of the vacuole is a small irregular mass of protoplasm connected to the peripheral layer by extremely delicate protoplasmic strands. Imbedded in this central mass is the nucleus (), which has the form of a biconvex lens and contains a distinct nucleolus. The chromatophores differ from anything we have yet considered, having the form of green spiral bands (chr\ of which each cell may contain one (o 1 ) or two coiled in oppo- site directions (A). Imbedded in the chromatophores are numerous pyrenoids (pyr, see p. 27), to which the strands of protoplasm proceeding from the central nucleus-containing mass can be traced. The process of growth in Spirogyra is brought about by the binary fission of its constituent cells. It takes place under ordinary circumstances during the night (i i--i2 P.M.), but by keeping the plant cold all night maybe delayed until morning. The nucleus divides by the complicated process (karyo- kinesis) already described in general terms (p. 67), so that two nuclei are found at equal distances from the centre of the cell. The cell-body with its chromatophores then begins to divide across the centre (B 1 ), the process commencing near the cell-wall and gradually proceeding inwards : as it goes on cellulose is secreted between the halves of the dividing protoplasm so that a ring of cellulose is formed lying transversely across the middle of the cell, and in con- tinuity externally with the wall (B 2 ). The ring is at first very narrow, but as the annular furrow across the dividing cell- body deepens, so the ring increases in width, until by the time the protoplasm has divided it has become a complete partition separating the newly-formed daughter-cells from one another. 198 vSPIROGYRA LESS. Any of the cells of a Spirogyra-filament may divide in this way, so that the filament grows by the intercalation of new cells between the old ones. This is an example of interstitial growth. Note its difference from the apical growth which was found to take place in Penicillium (p. 190), a difference which explains the fact mentioned above (p. 194) that there is no distinction between the two ends of a filament of Spirogyra, while in Penicillium the proximal and distal ends can always be distinguished in a complete hypha. The sexual reproduction of Spirogyra is interesting, as being intermediate between the very different processes which were found to obtain in Mucor (p. 165) and in Vaucheria (p. 172). In summer or autumn adjoining filaments become arranged parallel to one another and the opposite cells of each send out short rounded processes which meet (Fig. 43, c 1 ), and finally become united by the absorption of the adjacent walls, thus forming a free communication between the two connected cells or gonads (gon 1 , gvri 2 ). As several pairs of cells on the same two filaments unite simultaneously a ladder-like ap- pearance is produced. The protoplasmic cell-bodies (c 2 , gam 1 , gam 2 ) of the two gonads become rounded off and form gametes or conjugating bodies (see p. 166, note 1 ) : it is observable that this process of separation from the wall of the gonad always takes place earlier in one gamete (c 2 , gam 1 ) than in the other (c 2 , c 3 , gam 2 ). Then the gamete which is ready first (gam 1 ) passes through the connecting canal (c 3 ) and conjugates with the other (gam 2 ), forming a zygote (c 4 , zyg) which soon surrounds itself with a thick cell-wall. It has been ascertained that the nuclei of the gametes unite to form the single nucleus of the zygote. xix CONJUGATION 199 Thus, as in Mucor, the gametes are similar and equal- sized, and the result of the process is a resting zygote or zygospore. But while in Mucor each gamete meets the other half way, so that there is absolutely no sexual differentiation, in Spirogyra, as in Vaucheria, one gamete remains passive, and conjugation is effected by the activity of the other. So that we have here the very simplest case of sexual differen- tiation : the gametes, although of equal size and similar ap- pearance, are divisible into an active or male cell, correspond- ing with the sperm of Vaucheria, and a passive or female cell corresponding with the ovum. It will be seen that in Spirogyra the whole of the protoplasm of each gonad is used up in the formation of a single gamete, whereas in Vaucheria, while this is the case with the ovary, numerous gametes (sperms) are formed from the protoplasm of the spermary. In some forms of Spirogyra conjugation takes place not between opposite cells of distinct filaments, but between adjacent cells of the same filament. Each of the gonads sends out a short process (D 1 , a) which abuts against a corresponding process from the adjoining cell : the two processes are placed in communication with one another by a small aperture (o 2 ) through which the male gamete makes its way in order to conjugate with the female gamete and form a zygote (zyg). In the ordinary ladder-like method of conjugation the conjugating filaments appear to be of opposite sexes, one producing only male, the other only female gametes : the plant in this case is said to bediozdous, i.e., has the sexes lodged in distinct individuals, and conjugation is a process of cross- fertilization. But in the method described in the preceding paragraph the individual filaments are monoecious, i.e., produce both male and female cells, and conjugation is a process of self-fertilization. 200 SPIROGYRA LESS, xix Sometimes filaments are found in which the protoplasm of certain cells separates from the wall, and surrounds itself with a thick coat of cellulose forming a body which is quite indistinguishable from a zygote (E). There seems to be some doubt as to whether such cells ever germinate, but they have all the appearance of female cells which for some reason have developed into zygote-like bodies without fertil- ization. Such development from an unfertilized female gamete, although it has not been proved in Spirogyra is known to occur in many cases, and is distinguished as parthenogenesis. When the zygote is fully developed (F) its cell wall is divided into three layers, the middle one undergoing a peculiar change which renders it waterproof : at the same time the starch in its protoplasm is replaced by oil. In this condition it undergoes a long period of rest, its structure enabling it to offer great resistance to drought, frost, &c. Finally it germinates : the two outer coats are ruptured, and the protoplasm covered by the inner coat protrudes as a club-shaped process (G) which gradually takes on the form of an ordinary Spirogyra filament, dividing as it does so into numerous cells. Thus in the present case, as in Penicillium and the mushroom, the multicellular adult organism is originally unicellular. The nutrition of Spirogyra is purely holophytic : like Ha^matococcus and Vaucheria it lives upon the carbon dioxide and mineral salts dissolved in the surrounding water. Like these organisms also it decomposes carbon dioxide and forms starch only under the influence of sunlight. LESSON XX MONOSTROMA, ULVA, LAMINARIA, &C. IT was pointed out in a previous lesson (p. 193) that the highest and most complicated fungi, such as the mushrooms, are found on analysis to be built up of linear aggregates of cells to consist of hyphae so interwoven as to form struc- tures often of considerable size and of definite and regular form. This is not the case with the Algae or lower green plants the group to which Vaucheria, Caulerpa, Spirogyra, the diatoms, and in the view of some authors Haematococcus and Euglena, belong. These agree with fungi in the fact that the lowest among them (e.g. Zooxanthella) are unicellu- lar, and others (e.g. Spirogyra) simple linear aggregates, but the higher forms, such as the majority of sea-weeds, have as it were gone beyond the fungi in point of structure and attained a distinctly higher stage of morphological differen- tiation. This will be made clear by a study of three typical genera. Amongst the immense variety of seaweeds found in rock- pools between high and low water-marks are several kinds having the form of flat irregular expansions, of a bright green 202 MONOSTROMA, ULVA, LAMINARIA, &c. LESS. colour and very transparent. One of these is the genus Monostroma, of which M. bullosum is a fresh-water species. Examined microscopically the plant (Fig. 43) is found to consist of a single layer of close-set, green cells, the cell-walls of which are in close approximation, so that the cell-bodies appear as if embedded in a continuous layer of transparent cellulose. Thus Monostroma, like Spirogyra, is only one cell thick (B), but unlike that genus it is not one but many vlOJ B FIG. 43. Monostroma. A, surface view of M. bullosum, showing the cells embedded in a common layer of cellulose : many of them are in various stages of division. B, vertical section of M. laceratum, showing the arrangement of the cells in a single layer. (A after Reinke : B after Cooke. ) cells broad. In other words, instead of being a linear it is a superficial aggregate. To use a geometrical analogy : a unicellular organism like Hsematococcus may be compared to a point ; a linear aggregate like Penicillium or Spirogyra to a line ; a superficial aggregate like Monostroma to a plane. Growth takes place by the binary fission of the cells (A), but here again there is a marked and important difference from Spirogyra. In the latter the plane of division is always at right angles to the long axis of the filament, so that growth xx SOLID AGGREGATES 203 takes place in one dimension of space only, namely in length. In Monostroma the plane of division may be inclined in any direction provided it is perpendicular to the surface of the plant, so that growth goes on in two dimensions of space, namely in length and breadth. Another of the flat, leaf-like, green sea-weeds is the very common genus Ulva, sometimes called "sea-lettuce." It consists of irregular, more or less lobed expansions with crinkled edges, and under the microscope closely resembles Monostroma, with one important difference : it is formed not of one but of two layers of cells, and is therefore not a superficial but a solid aggregate. To return to the geometrical analogy used above it is to be compared not to a plane but to a solid body. As in Monostroma growth takes place by the binary fission of the cells. But these divide not only along variously inclined planes at right angles to the surface of the plant but also along a plane parallel to the surface, so that growth takes place in all three dimensions of space in length, breadth, and thickness. Ulva may be looked upon as the simplest example of a solid aggregate : the largest and most complicated sea-weeds are the great olive-brown forms known as "tangles" or "kelp," so common at low water-mark. They belong to various genera, of which the commonest British form is Laminaria. Laminaria (Fig. 44, A) consists of a cylindrical stem, which may be as much as two metres (6 ft.) in length and 5 or 6 cm. in diameter : its proximal end is fastened to the rocks by a branched, root-like structure, while distally it expands into a great, flat, irregularly-cleft, leaf-like body, 204 MONOSTROMA, ULVA, LAMINARIA, &c. LESS. which may be as much as 2-3 metres long and 70-80 cm. wide. Other genera of tangles attain even greater dimensions. A common New Zealand genus, Lessonia (Fig. 44, B) is a gigantic tree-like weed, the trunk of which is sometimes more than three metres (9-10 ft.) long, and as thick as a P^IG. 44. A, Laininaria claustoni, a young plant, showing stem with branched root-like organ of attachment, and deeply-cleft leaves (about ^-th natural size). B, Lessonia fuscescens, showing tree-like form (about ^Vth natural size). (A after Sachs : .B after Le Maout and Decaisne. ) man's thigh, while the graceful Macrocystis, another southern genus, is believed to attain a length of over 200 metres (700 ft), and is known to grow as much as 5^ metres (over 1 8 ft.) in six months. But in spite of their immense size these olive sea-weeds are comparatively simple solid aggregates of cells. Ex- amined with the naked eye the difference between them xx HISTOLOGY OF TANGLES 205 and a tree or shrub is quite obvious : when cut across they are seen to consist of a nearly homogeneous substance of the consistency of soft gristle, neither bark, wood, nor pith being distinguishable. Under the microscope, however, the cells of which they are composed are seen to vary considerably in form and size, some of them even assuming the characters of what we shall learn in our studies of the higher plants (Lesson XXIX) to distinguish as sieve-tubes. LESSON XXI NITELLA IN the linear, superficial, and solid aggregates discussed in the three previous lessons, the organism was seen to be composed of cells which in most cases differed but little from one another, all complications of structure being due to a continued repetition of the process of cell-multiplica- tion accompanied, except in Laminaria and its allies, by little or no cell-differentiation. In the present lesson we shall make a detailed study of a solid aggregate in which the constituent cells differ very considerably from one another in form and size. Nitella (Fig. 45, A) is a not uncommon fresh-water weed, found in ponds and water-races, and distinguished at once from such low Algae as Vaucheria and Spirogyra by its ex- ternal resemblance to one of the higher plants, since it presents structures which may be distinguished as stem, branches, leaves, &c. A Nitella plant consists of a slender cylindrical stem, some 15-20 cm. and upwards in length, but not more than about \ mm - m diameter. The proximal end is loosely rooted to the mud at the bottom of the stream or pond by delicate root- filaments or rhizoids (A, /-//) : the distal end is FlG. 45. Nitella.^ A, the entire plant (nat. size), showing the segmented stem, each seg- 1 This and the following figures are taken from a New Zealand species closely allied to, if not identical with, the British N. flexilis. 208 NITELLA. LESS. ment (seg) consisting of a proximal internode (int. nd) and distal node (nd) : the leaves (/) arranged in whorls and ending in leaflets (/') : the rhizoids (r/i) : and two branches (br), each springing from the axil of a leaf and ending, like the main stem, in a terminal bud (term. bud}. B, distal end of a shoot with gonads attached to the leaves : ovy, the ovaries ; spy, the spermaries. C, distal end of a rhizoid. D, distal end of a leaf (/) with two leaflets (/), showing the chroma- tophores and the white line. The arrows indicate the direction of rota- tion of the protoplasm. E, distal end of a leaflet, showing the general structure of a typical cell of Nitella in optical section : c. w, the cell-wall ; plsm 1 , the quies- cent outer layer of protoplasm containing chromatophores (ckr) ; plsni 1 , the inner layer, rotating in the direction indicated by the arrows, and containing nuclei (mi) ; vac, the large vacuole. F, terminal bud, partly dissected, showing the nodes (nd], internodes (int. nd), and leaf-whorls (/), numbered from I to 4, starting from the proximal end ; gr. pt, growing point. G, distal end of a leaf (/) with two leaflets (/' ), at the base of which are attached a spermary (spy) and two ovaries (ovy}. free. Springing from it at intervals are circlets or whorls of delicate, pointed leaves (/). Owing to the regular arrangement of the leaves the stem is divisible into successive sections or segments (seg), each consisting of a very short distal division or node (nd} from which the leaves spring,, and of an elongated proximal division or internode (int. nd\ which bears no leaves. Throughout the greater part of the stem the whorls ot leaves are disposed at approximately equal distances from one another, so that the internodes are of equal length, but towards the distal end the internodes become rapidly shorter and the whorls consequently closer together, until, at the actual distal end, a whorl is found the leaves of which, in- stead of spreading outwards like the rest, are curled upwards so that their points are in contact. In this way is formed the terminal bud (term. bud\ by which the uninjured stem is always terminated distally. The angle between the stem and a leaf, above (distad of) the attachment of the latter, is called the axil of the leaf. xxi HISTOLOGY 209 There is frequently found springing from the axil of one of the leaves in a whorl a branch or shoot (br) which repeats the structure of the main stem, i.e. consists of an axis from which spring whorls of leaves, the whole ending in a ter- minal bud. The axis or stem of a shoot is called a second- ary axis, the main stem of the plant being the primary axis. It is important to notice that both primary and secondary axes always end in terminal buds, and thus differ from the leaves which have pointed extremities. The rhizoids or root-filaments (rJi) arise, like the leaves and branches, exclusively from nodes. In the autumn the more distal leaves present a peculiar appearance, owing to the development on them of the gonads or sexual reproductive organs (Fig. 45, B and G) : of these the spennaries (antheridia) look very like minute oranges, being globular structures (spy) of a bright orange colour : the ovaries (oogonia) are flask-shaped bodies (ovy) of a yellowish brown colour when immature, but turning black after the fertilization of the ova. Examined under the microscope each internode is found to consist of a single gigantic cell (F, int. nd' 2 ) often as much as 3 or 4 cm. long in the older parts of the plant. A node on the other hand is composed of a transverse plate of small cells (nd 1 ) separating the two adjacent internodes from one another. The leaves consist each of an elongated proximal cell like an internode (D, /; F, 7 1 ), then of a few small cells having the character of a node, and finally of two or three leaflets (D, G, /'), each consisting usually of three cells, the distal one of which is small and pointed. Thus the Nitella plant is a solid aggregate in which the cells have a very definite and characteristic arrangement. The details of structure of a single cell are readily made 210 NITELLA LESS. out by examining a leaflet under a high power. The cell is surrounded by a wall of cellulose (E, c,w] of considerable thickness. Within this is a layer of protoplasm (primordial utricle, p. 196), enclosing a large central vacuole (vac), and clearly divisible into two layers, an outer (plsm 1 } in im- mediate contact with the cell-wall, and an inner (plsm* 2 -) bounding the vacuole.- In the outer layer of protoplasm are the chromatophores or chlorophyll-corpuscles (chr) to which the green colour of the plant is due. They are ovoidal bodies, about y^- mm. long, and arranged in obliquely longitudinal rows (D). On opposite sides of the cylindrical cell are two narrow oblique bands devoid of chromatophores and consequently colourless (D). The chromatophores contain minute starch grains. The inner layer of protoplasm contains no chlorophyll corpuscles, but only irregular, colourless granules, many of which are nuclei (E, nu : see below, p. 213). If the tem- perature is not too low this layer is seen to be in active rotating movement, streaming up one side of the cell and down the other (E), the boundary between the upward and downward currents being marked by the colourless bands just mentioned, along which no movement takes place (D). This rotation of protoplasm is a form of contractility very common in vegetable cells in which, owing to the confining cell-wall, no freer movement is possible. The numerous nuclei (E, mi] are rod-like and often curved : they can be seen to advantage only after staining (Fig. 46). Lying as they do in the inner layer of protoplasm, they are carried round in the rotating stream. In the general description of the plant it was mentioned that the stem ended distally in a terminal bud (Fig. 45, A, term, bud} formed of a whorl of leaves with their apices curved towards one another. If these leaves (F, / T ) are dis- xxi APICAL GROWTH 211 sected away, the node from which they spring (nd l ) is found to give rise distally to a very short internode (inf. nd z }, above which is a node (nd 2 } giving rise to a whorl of very small leaves (/ 2 ), also curved inwards so as to form a bud. Within these is found another segment consisting of a still smaller internode (int. nd^} and node, bearing a whorl of extremely small leaves (/ 3 ), and within these again a segment so small that its parts (int. nd*, / 4 ) are only visible under the microscope. The minute blunt projections (/ 4 ), which are the leaves of this whorl, surround a blunt, hemispherical projection (gr. pf), the actual distal extremity of the plant the growing point or punctum vegetationis. The structure of the growing point and the mode of growth of the whole plant is readily made out by examining vertical sections of the terminal bud in numerous specimens (Fig. 46). The growing point is formed of a single cell, the apical cell (A, ap. c\ approximately hemispherical in form and about -5^5- mm. in diameter. Its cell-wall is thick, and its cell-body formed of dense granular protoplasm containing a large rounded nucleus (;/?/) but no vacuole. In the living plant the apical cell is continually undergoing binary fission. It divides along a horizontal plane, i.e., a plane parallel to its base, into two cells, the upper (distal) of which is the new apical cell (B, ap. \\ ^ ^^ & '- - -'"; ;.r^ w ; ?.'*,{*' , ; . '. '." ' ' ,-V \ i; ; '" I' FIG. 50. A, Two living specimens of H. viridis attached to a bit of weed. The larger specimen is fully expanded, and shows the elongated body ending distally in the hypostome (hyp], surrounded by tentacles (/), and three buds (?>ifl, bd' 2 , bd' A } in different stages of development : a small water-flea (a) has been captured by one tentacle. The smaller specimen (to the right and above) is in a state of complete retraction, the tentacles (0 appearing like papillae. B, H. fiesca, showing the mouth (mth] at the end of the hypostome (hyp], the circlet of tentacles (/), two spermaries (spy), and an ovary (ovy}. C, a Hydra creeping on a flat surface by looping movements, D, a specimen crawling on its tentacles, (C and D after W. Marshall.) LESS, xxn MOVEMENTS 223 ation with a pocket lens shows that from the free extremity a number of very delicate filaments, barely visible to the naked eye, are given off. Under the low power of a compound microscope, a Hydra (Fig. 50, B) is seen to have a cylindrical body attached by a flattened base to a weed or other aquatic object, and bearing at its opposite or distal end a conical structure, the hypostome (]iyp\ at the apex of which is a circular aperture, the mouth (mth.). At the junction of the hypostome with the body proper are given off from six to eight long delicate ten- tacles (/) arranged in a circlet or whorl. A longitudinal section shows that the body is hollow, containing a spacious cavity, the enteron (Fig. 51, A, ent. cav), which communicates with the surrounding water by the mouth. The tentacles are also hollow, their cavities communicating with the enteron. There are three kinds of Hydra commonly found : one, H. vulgaris, is colourless or nearly so ; another, H. fusca, is of a pinkish-yellow or brown colour ; the third, H. viridis, is bright green. In the two latter it is quite evident, even under a low power, that the colour is in the inner parts of the body-wall, the outside of which is formed by a transparent colourless layer (Fig. 50, A, B). It is quite easy to keep a Hydra under observation on the stage of the microscope for a considerable time by placing it in a watch-glass or shallow " cell " with weeds, &c., and in this way its habits can be very profitably studied. It will be noticed, in the first place, that its form is continually changing. At one time (Fig. 50, A, left-hand figure) it extends itself until its length is fully fifteen times its diameter and the tentacles appear like long delicate filaments : at another time (right-hand figure) it contracts itself into an almost globular mass, the tentacles then appearing like little blunt knobs. 224 HYDRA LESS. Besides these movements of contraction and expansion, Hydra is able to move slowly from place to place. This it usually does after the manner of a looping caterpillar (Fig. 50, c) : the body is bent round until the distal end touches the surface ; then the base is detached and moved nearer the distal end, which is again moved forward, and so on. It has also been observed to crawl like a cuttle fish (D) by means of its tentacles, the body being kept nearly vertical. It is also possible to watch a Hydra feed. It is a very voracious creature, and to see it catch and devour its prey is a curious and interesting sight. In the water in which it lives are always to be found numbers of " water-fleas," minute animals from about a millimetre downwards in length, belonging to the class Crustacea, a group which includes lobsters, crabs, shrimps, &c. Water-fleas swim very rapidly, and occasionally one may be seen to come in contact with a Hydra's tentacle. Instantly its hitherto active movements stop dead, and it remains adhering in an apparently mysterious manner to the tentacle. If the Hydra is not hungry it usually liberates its prey after a time, and the water-flea may then be seen to drop through the water like a stone for a short distance, but finally to expand its limbs and swim off. If however the Hydra has not eaten recently it gradually contracts the tentacle until the prey is brought near the mouth, the other tentacles being also used to aid in the process. The water-flea is thus forced against the apex of the hypostome, the mouth expands widely and seizes it, and it is finally passed down into the digestive cavity. Hydrse can often be seen with their bodies bulged out in one or more places by recently swallowed water-fleas. The precise structure of Hydra is best made out by cutting xxii MINUTE STRUCTURE 225 it into a series of extremely thin sections and examining them under a high power. The appearance presented by a vertical section through the long axis of the body is shown in Fig. 51. The whole animal is seen to be built up of cells, each consisting of protoplasm with a large nucleus (p, nu\ and with or without vacuoles. As in the case of most animal cells, there is no cell-wall. Hydra is therefore a solid aggre- gate ; but the way in which its constituent cells are arranged is highly characteristic and distinguishes it at once from a plant. The essential feature in the arrangement of the cells is that they are disposed in two layers round the central digestive cavity or enteron (A, ent. aw) and the cavities of tentacles (ent. cav). So that the wall of the body is formed throughout of an outer layer of cells, the tctoderm (ect\ and of an inner layer, the endoderm (end\ which bounds the enteric cavity. Between the two layers is a delicate trans- parent membrane, the mesoglcsa, or supporting lamella (msgl). A transverse section shows that the cells in both layers are arranged radially (B). Thus Hydra is a two-layered or diploblastic animal, and may be compared to a chimney built of two layers of radially arranged bricks with a space between the layers filled with mortar or concrete. Accurate examination of thin sections, and of specimens teased out or torn into minute fragments with needles, shows that the structure is really much more complicated than the foregoing brief description would indicate. The ectoderm cells are of two kinds. The first and most obvious (B, ect and c), are large cells of a conical form, the bases of the cones being external, their apices internal. Spaces Q nfc ~cnc I TMS_ 755- m m SCALE F03 A FIG. 51. Hydra. A, Vertical section of the entire animal, showing the body-wall corny posed of ectoderm (ect) and endodenn (end), enclosing an enteric cavit- LESS, xxn ECTODERM 227 (cut. cav), which, as well as the two layers, is continued (ent. cav') into the tentacles, and opens externally by the mouth (mth) at the apex of the hypostome (hyp). Between the ectoderm and endoderm is the mesogloea (msgl], represented by a black line. In the ectoderm are seen large (ntc} and small (ntc') nematocysts : some of the endoderm cells are putting out pseudopods (psd), others flagella (/?). Two buds (bd l , bcF) in different stages of development are shown on the left side, and on the right a spermary (spy] and an ovary (ovy) containing a single ovum (ov). B, portion of a transverse section more highly magnified, showing the large ectoderm cells (cct] and intei - stitial cells (inf. c} : two cnidoblasts (cnbl) enclosing nematocysts (ntc), and one of them produced into a cnidocil (cnc) : the layer of muscle- processes (;;/. pr} cut across just external to the mesogloea (msgl) : endoderm cells (end) with large vacuoles and nuclei (mi}, pseudopods (psd), and flagella (ft). The endoderm cell to the right has ingested a diatom (a), and all enclose minute black granules. c, two of the large ectoderm cells, showing nucleus (nu) and muscle- process (m. pr}. D, an endoderm cell of H. viridis, showing nucleus (mi}, numerous chromatophores (chr], and an ingested nematocyst (ntc}. E, one of the larger nematocysts with extruded thread barbed at the base. F, one of the smaller nematocysts. G, a single sperm. (D after Lankester : F and G after Howes.) are necessarily left between their inner or narrow ends, and these are filled up with the second kind of cells (int. c), small rounded bodies which lie closely packed between their larger companions and are distinguished as interstitial cells. The inner ends of the large ectoderm cells are continued into narrow, pointed prolongations (c, m. pr\ placed at right angles to the cells themselves and parallel to the long axis of the body. There is thus a layer of these longitudinally- arranged muscle-processes lying immediately external to the mesogloea (B, ;;/. pr). They appear to possess, like the axial fibre of Vorticella (p. 129), a high degree of contractility, the almost instantaneous shortening of the body being due, in great measure at least, to their rapid and simultaneous contraction. It is probably correct to say that, while the ectoderm cells are both contractile and irritable, a special Q 2 A FIG. 52. Hydra. A, A nematocyst contained in its cnidoblast (cnl>), showing the coiled filament and the cnidocil (cnc). B, The same after extrusion of the thread, showing the larger and smaller barbs at the base of the thread. nu, the nucleus of the cnidoblast. c, A cnidoblast, with its contained nematocyst, connected with one of the processes of a nerve-cell (nv. c}. (After Schneider.) LESS, xxn NEMATOCYSTS 229 degree of contractility is assigned to the muscle-processes while the cells themselves are eminently irritable, the slightest stimulus applied to them being followed by an immediate contraction of the whole body. Imbedded in some of the large ectoderm cells are found clear, oval sacs (A and B, ntc\ with very well defined walls, and called nematocysts. Both in the living specimen and in sections they ordinarily present the appearance shown in Fig. 51, B, ntc, and Fig. 52 A, but are frequently met with in the condition shown in Fig. 51 E, and Fig. 52 B, that is, with a short conical tube protruding from the mouth of the sac, armed near its distal end with three recurved barbs, besides several similar processes of smaller size, and giving rise distally to a long, delicate, flexible fila- ment. Accurate examination of the nematocysts shows that the structure of these curious bodies is as follows : each con- sists of a tough sac (Fig. 52, A), one end of which is turned in as a hollow pouch : the free end of the latter is continued into a hollow coiled filament, and from its inner surface project the barbs. The whole space between the wall of the sac and the contained pouch and thread is tensely filled with fluid. When pressure is brought to bear on the outside of the sac the whole apparatus goes off like a harpoon-gun (B), the compression of the fluid forcing out first the barbed pouch and then the filament, until finally both are turned inside out. It is by means of the nematocysts the resemblance of which to the trichocysts of Paramoecium (p. 113) should be noted that the Hydra is enabled to paralyze its prey. Prob- ably some specific poison is formed and ejected into the wound with the thread : in the larger members of the group to which Hydra belongs, such as jelly-fishes, the nematocysts 230 HYDRA LESS. produce an effect on the human skin quite like the sting of a nettle. The nematocysts are formed in special interstitial cells called cnidoblasts (Fig. 51, B, ), radial (rad. c) and circular (dr. c) canals, and eye-spots (oc). (After Allman.) LESS, xxiii STRUCTURE OF A HYDRANT!! 239 shows a colony of the natural size, n a part of it magnified : it consists of a much-branched stem of a yellowish colour attached by root-like fibres to the support. The branches terminate in little Hydra-like bodies called hydranths (B, hyd\ each with a hypostome (hyp) and circlet of tentacles (/). Lateral branchlets bear bell-shaped structures or medusae (med] : these will be considered presently. Sections show that the hydranths have just the structure of a Hydra, consisting of a double layer of cells ectoderm and endoderm separated by a supporting lamella or mesogloea and enclosing a digestive cavity (ent. cav.) which opens externally by a mouth placed at the summit of the hypostome. The stem is formed of the same layers and contains a cavity (ent. cav'} continuous with those of the hydranths, and thus the structure of a hydroid polype is, so far, simply that of a Hydra in which the process of budding has gone on to an indefinite extent and without separation of the buds. There is however an additional layer added in the stem for protective and strengthening purposes. It is evident that a colony of the size shown in Fig. 54, A would, if formed only of soft ectodermal and endodermal cells, be so weak as to be hardly able to bear its own weight even in water. To remedy this a layer of transparent, yellowish substance of horny consistency, called the cuticle, is developed outside the ectoderm of the stem, extending on to the branches and only stopping at the bases of the hydranths and medusae. It is this layer which, when the organism dies and decays, is left as a semi-transparent branched structure resembling the living colony in all but the absence of hydranths and medusae. The cuticle is therefore a supporting organ or skeleton, not like our own bones formed in the interior of 240 HYDROID POLYPES LESS. the body (endoskehton\ but like the shell of a crab or lobster lying altogether outside the soft parts (exoskeleton). As to the mode of formation of the cuticle : we saw that many organisms, such as Amoeba and Haematococcus, form, on entering into the resting condition, a cyst or cell-wall, by secreting or separating from the surface of their protoplasm a succession of layers either of cellulose or of a transparent horn-like substance. But Amoeba and Haematococcus are unicellular, and are therefore free to form this protective layer at all parts of their surface. The ectoderm cells of Bougainvillea on the other hand are in close contact with their neighbours on all sides and with the mesogloea at their inner ends, so that it is not surprising to find the secretion of skeletal substance taking place only at their outer ends. As the process takes place simultaneously in adjacent cells, the result is a continuous layer common to the whole ectoderm instead of a capsule to each individual cell. It is to an exoskeletal structure formed in this way, i.e. by the secretion of successive layers from the free faces of adjacent cells, that the name cuticle is in strictness applied in multi- cellular organisms. The medusae (B, med. and c), mentioned above as occur- ring on lateral branches of the colony, are found in various stages of development, the younger ones having a nearly globular shape, while when fully formed each resembles a bell attached by its handle to one of the branches of the colony and having a clapper in its interior. When quite mature the medusae become detached and swim off as little jelly-fishes (c). The structure of medusa must now be described in some detail. The bell (c) is formed of a gelatinous sub- stance (Fig. 55, D, msgl) covered on both its inner and xxiir STRUCTURE OF A MEDUSA 241 outer surfaces by a thin layer of delicate cells (ect}. The clapper-like organ or manubrium (Fig. 54, c and Fig. 550 and D', mnb) is formed of two layers of cells, precisely resembling the ectoderm and endoderm of Hydra, and separated by a thin mesoglcea ; it is hollow, its cavity (Fig. 55, D, ent. cav] opening below, i.e. at its distal or free end, by a rounded aperture, the mouth (mth\ used by the medusa for the ingestion of food. At its upper (attached or proxi- mal) end the cavity of the manubrium is continued into four narrow, radial canals (Fig. 54, c, rad. c, and Fig. 55, D and D' rad} which extend through the gelatinous substance of the bell at equal distances from one another, like four meridians, and finally open into a circular canal (dr. c} which runs round the edge of the bell. The whole system of canals is lined by a layer of cells (Fig. 55, D and D', end] continuous with the inner layer or endoderm of the manubrium ; and extending from one canal to another in the gelatinous sub- stance of the bell, is a delicate sheet of cells, the endoderui- lamella (D', end. la]. From the edge of the bell four pairs of tentacles (Fig. 54, c and Fig. 55, D, /) are given off, one pair corresponding to each radial canal, and close to the base of each tentacle is a little speck of pigment (Fig. 54, oc], the ocellus or eye-spot. Lastly, the margin of the bell is continued inwards into a narrow circular shelf, the velum (v}. At first sight there appears to be very little resemblance between a medusa and a hydranth, but it is really quite easy to derive the one form from the other. Suppose a short hydranth or Hydra-like body with four tentacles (Fig. 55, A, A') to have the region from which the tentacles spring pulled out so as to form a hollow, trans- versely extended disc (B). Next, suppose this disc to become bent into the form of a cup with its concavity towards the R eel A D eel FIG. 55. Diagrams illustrating the derivation of the medusa from the hydranth. In the whole series of figures the ectoderm (ect) is dotted, the endoderm (end) striated, and the mesoglcea (msgl) black. A, longitudinal section of a Hydra-like body, showing the tubular body with enteric cavity (cut. cav), hypostome (hyp} , mouth (//////), and tentacles (/). LESS, xxin MEDUSA DERIVED FROM HYDRANTH 243 A', transverse section of the same through the plane a b. B, the tentacular region is extended into a hollow disc. C, the tentacular region has been further extended and bent into a bell-like form, the enteric cavity being continued into the bell (ent. cav'} : the hypostome now forms a manubrium (mnb}. c', transverse section of the same through the plane a b, showing the continuous cavity (ent. cav'} in the bell. D, fully formed medusa : the cavity in the bell is reduced to the radiating (rad) and circular (dr. c} canals, the velum (v) is formed, and a double nerve-ring (nv, nv'} is produced from the ectoderm. D', transverse section of the same through the plane a b, showing the four radiating canals (rad} united by the endoderm-lamella (end. /a), produced by partial obliteration of the continuous cavity ent. cav' in C' hypostome, and to undergo a great thickening of its meso- glcea. A form would be produced like c, i.e. a medusa-like body with bell and manubrium, but with a continuous cavity (c', ent. cav'} in the thickness of the bell instead of four radial canals. Finally, suppose the inner and outer walls of this cavity to grow towards one another and meet, thus obliterating the cavity, except along four narrow radial areas (D, rad} and a circular area near the edge of the bell (D, dr. c]. This would result in the substitution for the continuous cavity of four radial canals opening on the one hand into a circular canal and on the other into the cavity of the manubrium (ent. cav), and connected with one another by a membrane the endoderm-lamella (end. la) indi- cating the former extension of the cavity. It follows from this that the inner and outer layers of the manubrium are respectively endoderm and ectoderm : that the gelatinous tissue of the bell is an immensely thickened mesoglcea : that the layer of cells covering both inner and outer surfaces of the bell is ectodermal : and that the layer of cells lining the system of canals, together with the endoderm-lamella, is endodermal. Thus the medusa and the hydranth are similarly con- structed or homologous structures, and the hydroid colony, R 2 244 HYDROID POLYPES LESS. like Zoothamnium (p. 136), is dimorphic, bearing zooids of two kinds. The ectoderm cells of the hydranth bear muscle-processes like those of Hydra (p. 227, Fig. 51, c) : in the medusae similar processes are found on the inner concave side of the bell and in the velum. Sometimes, however, the place of these processes is taken by a layer of spindle-shaped fibres (Fig. 56, A), many times longer than broad, and provided each with a nucleus. Such muscle-fibres are obviously cells greatly extended in length, so that the ectoderm cell of Hydra with its continuous muscle-process is here represented by an ectoderm cell with an adjacent muscle-^//. We thus get a partial intermediate layer of cells between the ectoderm and endoderm, in addition to the gelatinous mesoglcea, and so, while a hydroid polyp is, like Hydra, diploblastic (p. 225), it shows a tendency towards the as- sumption of a three-layered or triploblastic condition. Both the muscle-processes and muscle-cells of the medusae differ from those of the hydranths in exhibiting a delicate transverse striation (Fig. 56). Sooner or later the medusae separate from the hydroid colony and begin a free existence. Under these circum- stances the rhythmical contraction i.e. contraction taking place at regular intervals of the muscles of the bell causes an alternate contraction and expansion of the whole organ, so that water is alternately pumped out of and drawn into it. The obvious result of this is that the medusa is propelled through the water by a series of jerks. There is still another important matter in the structure of the medusa which has not been referred to. At the junction of the velum with the edge of the bell there lies, imme- diately beneath the ectoderm, a layer of peculiar branched XXIII NERVOUS SYSTEM 245 cells (Fig. 56, B, ;/. c], containing large nuclei and produced into long fibre-like processes. These nerve-cells (see p. 230) are so disposed as to form a double ring round the margin of the bell, one ring (Fig. 55, D, nv) being immediately above, the other (nv'} immediately below the insertion of the velum. An irregular network of similar cells and fibres B m.c n.c m.c FIG. 56. A, Muscle fibres from the inner face of the bell of the medusa of a hydroid polype (Eucopella campamdaria}, showing nucleus and transverse striation. B, portion of the nerve-ring of the same, showing two large nerve- cells (;/. c] and muscle-fibres (in. c] on either side. (After von Len- denfeld. ) occurs on the inner or concave face of the bell, between the ectoderm and the layer of muscle-fibres. The whole consti- tutes the nervous system of the medusa ; the double nerve-ring is the central, the network the peripheral nervous system. Some of the processes of the nerve-cells are connected with ordinary ectoderm-cells, which thus as it were connect the nervous system with the external world : others, in some instances at least, are probably directly connected with muscle-fibres. 246 HYDROID POLYPES LESS. We thus see that while the manubrium of a medusa has the same simple structure as a hydranth, or what comes to the same thing, as a Hydra, the bell has undergone a very remarkable differentiation of its tissues. Its ordinary ecto- derm cells instead of being large and eminently contractile form little more than a thin cellular skin or epithelium over the gelatinous mesogloea : they have largely given up the function of contractility to the muscle processes or fibres, and have taken on the functions of a protective and sensitive layer. Similarly the function of automatism, possessed by the whole body of Hydra, is made over to the group of specially modified ectodermal cells which constitute the central nervous system. If a Hydra is cut into any number of pieces, each of them is able to perform the ordinary move- ments of expansion and contraction, but if the nerve-ring of a medusa is removed by cutting away the edge of the bell, the rhythmical swimming movements stop dead : the bell is in fact permanently paralysed. It is not, however, rendered incapable of movement, for a sharp pinch, i.e. an external stimulus, causes a single con- traction, showing that the muscles still retain their irritability. But no movement takes place without such external stimulus, each stimulus giving rise infallibly to one single contraction : the power possessed by the entire animal of independently originating movement, i.e. of supplying its own stimuli, is lost with the central nervous system. Another instance of morphological and physiological differentiation is furnished by the pigment spots or ocelli (Fig. 54, c, oc) situated at the bases of the tentacles. They consist of groups of ectoderm cells in which are deposited granules of deep red pigment. Their function is proved by the following experiment. If a number of medusae are placed in a glass vessel of xxin GONADS 247 water in a dark room, and a beam of light from a lantern is allowed to pass through the water, the animals are all found to crowd into the beam, thus being obviously sensitive to and attracted by light. If however the ocelli are removed this is no longer the case : the medusae do not make for the beam of light, and are incapable of distinguishing light from darkness. The ocelli are therefore organs of sight. In Zoothamnium we saw that the two forms of zooid were respectively nutritive and reproductive in function, the re- productive zooids becoming detached and swimming off to found a new colony elsewhere (p. 135). This is also the case with Bougainvillea : the hydranths are purely nutritive zooids, the medusae, although capable of feeding, are specially distinguished as reproductive zooids. The gonads are found in the walls of the manubrium, between the ectoderm and endoderm, some medusae reproducing ovaries, others spermaries only. Thus while Hydra is monoecious, both male and female gonads occurring in the same individual, Bougainvillea is dioecious, certain individuals producing only male, others only female products. In some Hydroids it has been found that the sexual cells from which the ova and sperms are developed do not originate in the manubrium of a medusa, but apparently arise in the endoderm of the stem of the hydroid colony, afterwards migrating, while still small and immature, to their permanent situation where they undergo their final development. 1 In Bougainvillea, however, the reproductive products are said to originate in the manubrium. 1 This migration of the sexual cells renders the question of their origin in many cases a very difficult one. In some Hydroids, at any rate, they arise in the ectoderm, but migrate into the endoderm at a very early stage. 248 HYDROID POLYPES LESS. The medusae, when mature, become detached and swim away from the hydroid colony. The sperms of the males are shed into the water and carried to the ovaries of the females, where they fertilize the ova, converting them, as usual, into oosperrns. The changes by which the oosperm or unicellular embryo of a hydroid polype is converted into the adult are very remarkable. The process is begun by the oosperm, still enclosed within the body of the parent (Fig. 57, A), undergoing binary fission, so that a two-celled embryo is formed (B). Each of the two cells again divides (c), and the process is repeated, the embryo consisting successively of 2, 4, 8, 16, 32, &c., cells, until a solid globular mass of small cells is produced (D, E) by the repeated division of the one large cell which forms the starting-point of the series. The embryo in this stage has been compared to a mulberry, and is called the morula or polyplast. So far all the cells of the polyplast are alike globular nucleated masses of protoplasm squeezed into a polyhedral form by mutual pressure. But before long the cells lying next the surface alter their form, becoming cylindrical, with their long axes disposed radially (F) . In this way a superficial layer of cells, or ectoderm, is differentiated from an internal mass, or endoderm. The embryo now assumes an elongated form (G) and begins to exhibit slow, worm-like movements, finally escaping from the parent and beginning a free existence (H). The ectoderm cells are now found to be ciliated, and before long a cavity appears in the previously solid mass of endoderm cells : this is the first appearance of the enteron or digestive cavity. In this stage the embryo is called a plamila : it xxin DEVELOPMENT 249 swims slowly through the water by means of its cilia, the A B FIG. 57. vStages in the development of two hydroid polypes, Lao- uiedeaflexuosa (A-H) and Eitdendrium ramosnm (I-M). A, oosperm. B, two-celled, and c, four- celled stage. D, E, polyplast. F, G, formation of planula by differentiation of ectoderm and endoderm. In A-G the embryo is embedded in the maternal tissues. H, free swimming planula, showing ciliated ectoderm, and endoderm enclosing a narrow enteric cavity. I, planula, after loss of its cilia, about to affix itself. K, the same after fixation. I,, Hydra-like stage, still enclosed in cuticle. M, the same after rupture of the cuticle and liberation of the tentacles. (After Allman.) broader end being directed forwards in progression. It then loses its cilia and settles down on a rock, shell, sea-weed, or 250 HYDROID POLYPES LESS. other submarine object, assuming a vertical position with its broader end fixed to the support (i). The attached or proximal end widens into a disc of attach- ment, a dilatation is formed a short distance from the free or distal end, and a thin cuticle is secreted from the whole surface of the ectoderm (K). From the dilated portion short buds arise in a circle : these are the rudiments of the tentacles : the narrow portion beyond their origin becomes the hypostome (L). Soon the cuticle covering the distal end is ruptured so as to set free the growing tentacles (M) : an aperture, the mouth, is formed at the end of the hypostome, and the young hydroid has very much the appearance of a Hydra with a broad disc of attachment, and with a cuticle covering the greater part of the body. Extensive budding next takes place, the result being the formation of the ordinary hydroid colony. Thus from the oosperm or impregnated egg-cell of the medusa the hydroid colony arises, while the medusa is produced by budding from the hydroid colony. The analogy w r ith Nitella (p. 219) will be at once obvious : in each case there is an alternation of generations, the asexual genera- tions or agamobium (hydroid colony, pro-embryo of Nitella) giving rise by budding to the sexual generation or gamobium (medusa, Nitella-plant), which in its turn produces the agamobium by a sexual process, i.e. by the conjugation of ovum and sperm. Two other Hydroids must be briefly referred to in con- cluding the present lesson. Floating on the surface of the ocean in many parts of the world is found a beautiful transparent organism called Diphyes. It consists of a long, slender stem (Fig. 58, A, FlG. 58. DipJiyes campamilata. A, the entire colony, natural size, showing stem (a) bearing groups 01 zooicls (e) and two swimming bells (;//, /;/), the apertures of which are marked o. B, one of the groups of zooids marked c in A, showing common stem (a), hydranth (;/), medusoid (), bract (/), and branched tentacle or grappling line (?'). (From Gegenbaur. ) XXI II tNDIVIDUATION 253 logical and physiological differentiation are thus carried much further than in such a form as Bougainvillea. FIG. 59. A. Porpita pacific a (nat. size), from beneath, showing disc- like stem surrounded by tentacles (/), a single functional hydranth (hy], and numerous mouthless hydranths (hy'). B, vertical section of P. mcditen-anca, showing the relative positions of the functional (hy} and mouthless (Iiy'} hydranths, the tentacles, and the chambered shell (s/i). (A after Duperrey ; B from Huxley after Kolliker.) Porpita is another free-swimming Hydroid, presenting at 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, 254 HYDROID POLYPES LESS, xxm 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, s/i). Accurate examination shows that the manubrium-like body (hy) on the under surface is a hydranth, that the short, hollow, tentacle-like bodies (hy') surrounding it are mouthless hydranths, and that the disc represents the common stem oi 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 char- acter of a single physiological individual. It was pointed out in the previous lesson (p. 233) that Hydra, while morphologically 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 (spermato gene sis] and of ova (pogenesis], 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 : recent researches show that essentially similar processes take place 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 (Fig. 60, A) undergo repeated fission, forming what are known as the sperm-mother-cells (B). These have been found in several instances to be 256 SPERMATOGENESIS AND OOGENESIS LESS. distinguished by a peculiar condition of the nucleus. We saw (p. 65) that the number of chromosomes is constant in A c C *** FIG. 60. Spermatogenesis in the Mole-Cricket (Gryllotaipa}. A. Primitive sex-cell, just preparatory to division, showing twelve chromosomes (c/ir] ; c, the centrosome. B. Sperm-mother-cell, formed by the division of A, and containing twenty-four chromosomes. The centrosome has divided into two. C. The sperm-mother-cell has divided into two by a reducing division, each daughter cell containing twelve chromosomes. D. Each daughter cell has divided again in the same manner, a group of four sperm-cells being produced, each with six chromosomes. E. A single sperm-cell about to elongate to form a sperm. F. Immature sperm ; the six chromosomes are still visible in the head. G. Fully formed sperm. (After vom Rath.) xxiv REDUCING DIVISION 257 any given animal, though varying greatly in different species. In the formation of the sperm-mother-cells from the primitive sex-cells the number becomes doubled : in the case of the mole-cricket, for instance, shown in Fig. 60, while the ordinary cells of the body, including the primitive sex- cells, contain twelve chromosomes, the sperm-mother-cells contain twenty-four. The sperm-mother-cell now divides (c), but instead of its chromosomes splitting in the ordinary way (p. 66, Fig. 10) half of their total number in the present instance twelve- passes into each daughter cell : in this way two cells are produced having the normal number of chromosomes. The process of division is immediately repeated in the same peculiar way (D), the result being that each sperm-mother- cell gives rise to a group of four cells having half the normal number of chromosomes in the present instance six. The four cells thus produced are the immature sperms (E) : in the majority of cases the protoplasm of each undergoes a great elongation, being converted into a long vibratile thread, the tail of the sperm (F, G), while the nucleus becomes its more or less spindle-shaped head. Thus the sperm or male gamete is a true cell, specially modified in most cases for active movement : its head, representing the nucleus, is directed forwards in progres- sion, its long tail, formed from the protoplasm, backwards. The direction of movement is thus the precise opposite of that of a monad (p. 36) to which a sperm presents a certain resemblance. This actively motile tailed form is, however, by no means essential : in many animals the sperms are non-motile and in some they resemble ordinary cells. The peculiar variety of karyokinesis described above, by which the number of chromosomes in the sperm-mother-cells is reduced by one-half, is known as a reducing division. b 258 SPERMATOGENESIS AND OOGENESIS LESS As already stated, the ova arise from primitive sex-cells, precisely resembing those which give rise to sperms. These divide and give rise to the egg-mother-cells in which, as in the sperm-mother-cells, the number of chromosomes is doubled. The egg-mother-cells do not immediately undergo division but remain passive and increase, often enormously, in size, by the absorption of nutriment from surrounding parts : in this way each egg-mother-cell becomes an ovum. Sometimes this nutriment is simply taken in by osmosis, in other cases the growing ovum actually ingests neigh- bouring 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. 216), 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-formed 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. 68), the "yolk " is simply an immense egg-cell. When fully formed, the typical animal ovum (Fig. 61) consists of a more or less globular mass of protoplasm, generally exhibiting a reticular structure and enclosing a xxiv STRUCTURE OF THE OVUM 259 larger or smaller quantity of yolk-granules. Surrounding the cell-body is usually a cell-wall or cuticle, often of con- siderable thickness and known as the vitelline membrane. The nucleus is large and has the usual constituents (p. 63) nuclear membrane, nuclear matrix, and chromatin. As a rule there is a very definite 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 FIG. 61. Ovum of a Sea-urchin (Toxopnenstes lividus], showing the radially-striated cell-wall (vitelline membrane), the protoplasm contain- ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its network of chromatin, and a large nucleolus (germinal spot). (From Balfour after Hertwig. ) 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. Maturation consists essentially in a twice-repeated process of cell-division. The nucleus (Fig. 62, A, nu) loses its mem- brane, travels to the surface of the egg, and takes on the S 2 f* o ^E/* HW? -*\ *y ~ x SU^ ^fr t / ., ^^ f _v \ / // b-t-J >' " FIG. 62. The Maturation and Impregnation of the Animal Ovum. A, portion of the ovum of a Round worm (Ascaris megaioccpJiala), showing the sperm (sp) in the act of conjugation, and the unaltered LESS, xxiv POLAR CELLS 261 nucleus (mi] of the egg, Ascaris being an animal in which the conjuga- tion of ovum and sperm takes place before the maturation of the former. In the nucleus, the nuclear membrane and matrix, and a band-like mass of chromatin are visible. The sperm of Ascaris is of peculiar form, and is non-motile. B, the same at the commencement of maturation : the nucleus (mi] has travelled to the periphery of the egg and taken on the spindle form. In this and the two next figures the vitelline membrane is shown. c, formation of the first polar cell (p. c. i). D, the entire egg after the completion of maturation, showing the two polar cells, the first (p. c. i) adhering to the vitelline membrane, the second, (p. c. 2) to the surface of the protoplasm : the female pronucleus (pr. mi. ? ) : and the sperm (st>), which has penetrated into the cell- protoplasm, but has not yet become converted into the male pro- nucleus. E 1 , E 2 , two stages in the conjugation of the pronuclei in Molluscs (E 1 , Pterotrachea, E 2 , Phyllirhoc}. In E 1 the male {pr. nu. <5 ) and female (pr. nil. ? ) pronuclei are separated : in E 2 they are applied by their flattened adjacent faces : in connection with each the cell -protoplasm has a radiating arrangement around one of the directive spheres ; the polar cells (p.c.i, p.c. 2) are shown. F 1 -? 3 , three stages in the development of the nucleus of the oosperm in a Sea-urchin (Echinus microtuberculatns] : in F 1 the nucleus contains nine chromatin -fibres (chrom. ? ) derived from the female pronucleus, and a globular mass of the same (chrom, 3 ) derived from the male pro- nucleus : the two directive spheres are now situated one at each end of the nucleus. In F' 2 the male chromatin (chrom. $ ) has begun to unwind itself: in F there is no longer any distinction between male and female elements, the nucleus containing eighteen similar chromatin-threads. G, central portion of the egg of a Hermit-Crab (Eiipagums prideauxit), showing the conjugation of the pronuclei. The male and female chro- matin-networks appear to be fused along the plane of union. The pro- nuclei are surrounded by finely-granular protoplasm devoid of yolk- spheres. (A-F after Boveri ; G after Weismann and Ischikawa. ) form of an ordinary nuclear spindle (B, nu, see p. 65). Next, the protoplasm grows out into a small projection or bud, into which one end of the spindle projects (c). The usual pro- cess of nuclear division then takes place (Fig. 10, p. 64), one of the daughter nuclei remaining in the bud, the other in the ovum itself. Nuclear division is followed as usual by division of the protoplasm, and the bud becomes separated 262 SPERMATOGENESIS AND OOGENESIS LESS. as a small cell distinguished as the first polar cell (c E p.c. i). It was mentioned in a previous lesson (p. 200) that in some cases development from an unfertilized female gamete took place, the process which is not uncommon among insects and crustaceans being distinguished as partheno- genesis. It has been proved in many instances and may be generally true that in such cases the egg begins to develop after the formation of the first polar cell. Thus in partheno- genetic ova it appears that maturation is completed by the separation of a single polar cell. In the majority of animals, however, development takes place only after fertilization, and in such cases maturation is not complete until a second polar cell (D and E, p.c. 2) has been formed in the same manner as the first. The ovum has now lost a portion of its protoplasm together with three- fourths of its chromatin, half having passed into the first polar cell and half of what remained into the second : the remaining one-fourth of the chromatin takes on a rounded form and is distinguished as the female pronucleus (D, pr. nu. 9 ). The formation of both polar cells takes place by a reducing division, so that, while the immature ovum con- tains double the number of chromosomes found in the ordinary cells of the species, the mature ovum, like the sperm, contains only one-half the normal number. In some animals the first polar body has been found to divide after separating from the egg. In such cases the egg- mother-cell or immature ovum gives rise to a group of four cells the mature ovum and three polar-cells ; just as the sperm-mother-cell gives rise to a group of four cells, all of which, however, become sperms. Shortly after, or in some cases before maturation the xxiv FUSION OF PRONUCLEI 263 ovum is fertilized by the conjugation with it of a single sperm. As we have found repeatedly, sperms are produced in vastly greater numbers than ova, and it often happens that a single egg is seen quite surrounded with sperms, all apparently about to conjugate with it. It has however been found to be a general rule that only one of these actually conjugates : the others, like the drones in a hive, perish without fulfilling the one function they are fitted to perform. The successful sperm (A, sp) takes up a position at right angles to the surface of the egg and gradually works its way through the vitelline membrane until its head lies within the egg protoplasm (D, sp). The tail is then cast off, and the head, penetrating deeper into the protoplasm, takes on the form of a rounded nucleus-like body, the male pronucleus (E 1 , pr. nit. $ ). The two pronuclei, each accompanied by its directive sphere and centrosome, approach one another (E 1 , E 2 ) and finally unite to form the single nucleus (r 1 F R ) of what is now not the ovum but the oosperm the impregnated egg or unicellular embryo. The fertilizing process is thus seen to consist of the union of two nuclear bodies, one contributed by the male gamete or sperm, the other by the female gamete or ovum. It follows from this that the essential nuclear matter or chromatin of the oosperm is derived in equal proportions from each of the two parents. Moreover, as both male and female pronuclei contain only half the number of chromosomes found in the ordinary cells of the species, the union of the pronuclei results in the restoration of the normal number to the oosperm. There is reason for thinking that the directive spheres of the sperm and ovum as well as their nuclei unite with one another : in this way the directive sphere of the oosperm 264 SPERMATOGENESIS AND OOGENESIS LESS. is derived, like its nucleus, in equal proportions from the two parents. Fertilization being thus effected, the process of segmenta- tion or division of the oosperm takes place as described in the preceding lesson (p. 248). In concluding the present lesson, we shall consider briefly a point which has probably already struck the reader. Among the plant-forms which have come under our notice there has been a very complete series of gradations from the simple cell, through the branched cell, linear aggregate, and superficial aggregate, to the solid aggregate, whilst among the animals already discussed there has so far been no attempt to fill up the very considerable gap between the unicellular Infusoria and Hydra, which is not only a solid aggregate, but has its cells arranged in two definite layers enclosing a digestive cavity. When we say that no attempt has been made to fill up this gap, we mean as far as adult forms are concerned. If the reader will turn to the account, in the previous lesson, of the development of hydroid polypes (p. 248), he will see that the facts there described do as a matter of fact help us to see a possible connection between unicellular animals and multicellular two-layered forms with mouth and digestive cavity. The oosperm of the hydroid (Fig. 57, A) has the essential character of an Amoeba, the polyplast (E) is practically a colony of Amoebae, and the planula (H) a similar colony in which the zooids (cells) are dimorphic, being arranged in two layers with a central digestive cavity which finally communicates with the exterior by a mouth. In hydroids the mouth is not formed until after the appearance of the tentacles, but in a large propor- tion of the higher animals the polyplast stage is succeeded XXIV THE GASTRULA 265 not by a mouthless planula but by a two-layered embryo with a mouth at one end, called & gastrula (Fig. 63). This is a very important stage, since it exhibits in the simplest possible way the essential characteristic of a diploblastic animal a two-layered sac with mouth (J3lp) and stomach (7), the outer layer of cells (Ekt) being protective and sensory, the inner (Enf) having a digestive function. The Wct- FIG. 63. A typical animal gastrula in vertical section, showing ectoderm (Ekt), endoclerm (Ent), enteron or digestive cavity (/), and mouth (Blp). (From Wiedersheim. ) planula of a hydroid may be looked upon as a gastrula in which the mouth has not yet appeared. Another very important difference is the fact that in uni- cellular organisms reproduction is effected either asexually by the fission of the entire individual, or, in the case of sexual reproduction, by two entire individuals undergoing conjugation. In multicellular forms, on the other hand, single cells are set apart for sexual reproduction. B FIG. 64. Panaorina morum. A. The entire colony, consisting of sixteen flagellate zooids, enclosed in a gelatinous envelope. B. Asexual reproduction ; each zooid has divided into sixteen, forming as many daughter families, still enclosed within the original gelatinous envelope. c. Sexual reproduction ; zooids are being set free from the colony, forming gametes. D. Conjugation of two gametes. E. The same after complete fusion. F. The immature zygote. G. The fully-formed zygote. H. Protoplasm of zygote escaping from cell-wall. I. The same after acquisition of flagella. K. The same undergoing division and forming a young colony. (From Goebel.) LESS, xxvi PANDORINA 267 Thjre are several interesting organisms which help to bridge this gulf. Two of the more accessible and well- known forms will now be described. Pandorina (Fig. 64, A) is a colony consisting of sixteen zooids closely packed in a gelatinous case of a globular form. Each zooid resembles in general characters a mo- tile Hasmatococcus or Euglena, having an ovoid cell-body coloured green by chlorophyll, a red pigment spot, and two flagella, which protrude through the gelatinous wall of the colony, and by their action impart to it a rotatory movement. In asexual reproduction each of the sixteen zooids divides and re-divides, forming at last a group of sixteen cells. In this way sixteen daughter colonies are produced within the gelatinous envelope of the original mother colony (B). By the solution of the envelope the daughter colonies are set free, and each begins an independent existence. In sexual reproduction the zooids are set free singly from the colony (c). They swim about actively, approach one another in pairs, and conjugate (D), becoming completely fused together (E) to form a zygote (F). This increases in size and develops a thick cell wall (G). After a period of rest, the protoplasm escapes from the cell ^vall (H), puts out a pair of flagella (i), and swims about. Finally it settles down, divides and re-divides, and so gives rise to a new colony (K). It is obvious that Pandorina resembles the polyplast stage of an embryo : moreover it is produced by the repeated fission of a flagellula, just as the polyplast is formed by the repeated fission of an oosperm. The beautiful Volvox (Figs. 65 and 66), one of the favourite studies of microscopists, is a colony of Haematococcus-like zooids arranged in the form of a hollow sphere containing a H FIG. 65. Volvox globator. A, the entire colony, surface view, showing the biflagellate zooids and several daughter-colonies swimming freely in the interior ; the latter are produced by the repeated fission of non-flagellate reproductive zooids (a\ B, the same during sexual maturity, showing spermaries from the surface (spy\ in profile (spy'} and after complete formation of sperms (spy") : and ovaries from the surface (ovy, ovy", ovy'") and in profile (ovy'). C, four zooids in optical section, showing cell-wall, nucleus, contractile vacuole, with adjacent pigment-spot, and flagella (_/?.) D 1 -D 5 , stages in the formation of a colony by the repeated binary fission of an asexual reproductive zooid. E, a ripe spermary. F, a single sperm, showing pigment-spot (fg) and flagella (_/?). G, an ovary containing a single ovum surrounded by several sperms. H, oosperm enclosed in its spinose cell wall. (A from Geddes and Thomson, after Kirchner ; B-H after Cohn.) LESS. XXIV VOLVOX 269 transparent mucilage. Each cell (c) has a nucleus, a con- tractile vacuole, a large green chromatophore, a small red pigment-spot like that of Euglena (p. 47) and two flagella. The cells are surrounded by thick mucilaginous cell walls which do not give the reaction of cellulose, but are probably formed of an allied carbohydrate. By the combined move- ment of all the flagella a rotating movement is given to the entire colony. Asexual reproduction takes place by certain of the zooids ^ S v-~A FIG. 66. Part of a Volvox-colony showing the structure in greater detail than in Fig. 65 : s, spermaries ; o, ovaries. (After Lang.) which are not ciliated, undergoing a process very like the segmentation of the hydroid egg (p. 248), dividing into 2, 4, 8, 1 6, &c. cells (A, a, and D 1 D 5 ), and so forming a daughter colony which becomes detached and swims freely in the interior of the parent colony (A), by the rupture of which it is finally liberated. In sexual reproduction certain cells enlarge and take on the characters of ovaries (B, ovy, ovy', ovy", ovy'" , and Fig. 66, o) the protoplasm of each forming 270 SPERMATOGENESIS AND . OOGENESIS LESS, xxiv a single ovum : the protoplasm of others divides repeatedly and forms aggregations of sperms (B, spy^ spy' , spy", and Fig. 66, s). By the conjugation of a sperm (F) with an ovum (G) an oosperm (H) is produced, and from this by continued division a new colony arises. Volvox is clearly comparable to a hollow polyplast, and further resembles the higher or multicellular animals in that certain of its cells are differentiated to form true sexual products. LESSON XXV POLYGORDIUS POLYGORDIUS is a minute worm, about 3 or 4 cm. in length, found in the European seas, where it lives in sand at a depth of a few fathoms. It has much the appearance of a tangle of pink thread with one end produced into two delicate processes (Fig. 67, A). These, which are the tentacles, mark the anterior end of the animal the opposite extremity* which in some species also bears a pair of slender processes, is the posterior end. As the creature creeps along, one side is kept constantly upwards and is distinguished as the dorsal aspect ; the lower surface is called -ventral. The anterior end is narrower than the rest of the body, and is marked off behind by a groove (B and c) ; this division is called the prostomium (Pr. st) and bears the tentacles (/) already mentioned in front and above ; and on each side a small oval depression (c. p) lined with cilia. Immediately following the prostomium is a region clearly marked off in front, but ill-defined posteriorly, and known as the peristomium (Per. sf) ; on its ventral surface is a trans- verse triangular aperture the mouth (Mtti). The rest of the body is more or less distinctly marked by annular grooves (D and E, gr] into body-segments or metameres fr.st, An FIG. 67. Polygordius neapolitamts. A, the living animal, dorsal aspect, about five times natural size. B, anterior end of the worm from the right side, more highly magni- fied, showing the prostomium (Pr. st), peristomium (Per. st), tentacles (/), with setoe (s) and ciliated pit (c. p). C, ventral aspect of the same : letters as before except Mth, mouth. D, portion of body showing metameres (Mtmr) separated by grooves (gr). E, posterior extremity from the ventral aspect, showing the last three metameres (Mtmr} separated by distinct grooves (gr), the anal seg- ment (An. scg) bearing the anus (An), and a circlet of papilhe (/>). (After Fraipont.) xxv GENERAL CHARACTERS 273 , the number of which varies considerably. Poly- gordius is thus the first instance we have met with of a trans- versely segmented animal. The last or anal segment (E, An. seg) differs from the others by its swollen form and by bearing a circlet of little prominences or papillae (p) ; it is separated from the preceding segment by a deep groove, and bears at its posterior end a small circular aperture, the anus (An). Polygordius may therefore be described as consisting of a number of more or less distinct segments which follow one another in longitudinal series ; three of these, fasprostomium, which lies altogether in front of the mouth, the peristomium, which contains the mouth, and the anal segment, which contains the anus, are constant ; while between the peri- stomium and the anal segment are intercalated a variable number of metameres which resemble one another in all essential respects. Polygordius feeds in much the same way as an earth- worm : it takes in sand, together with the various nutrient matters contained in it, such as infusoria, diatoms, &c., by the mouth, and after retaining it for a longer or shorter time in the body, expels it by the anus. It is obvious, therefore, that there must be some kind of digestive cavity into which the food passes by the mouth, and from which effete matters are expelled through the anus. Sections (Fig. 68) show that this cavity is not a mere space excavated in the interior of the body, but a definite tube, the enteric canal (A, B), which passes in a straight line from mouth to anus, and is separated in its whole extent from the walls of the body (A, B. W.) by a wide space, the body cavity or ccelome (cat). So that the general structure of Polygordius might be imi- tated by taking a wide tube, stopping the ends of it with corks, boring a hole in each cork, and then inserting through T xxv GENERAL CHARACTERS 275 Between the enteric canal and the body-wall is the ccelome (Ccel}, divided into right and left portions by the dorsal (D. Jfes} and ventral ( V. JA's) mesenteries, and into segmental compartments by the septa (Sept). Lying in the mesenteries are the dorsal (D. V) and ventral ( V. V} blood-vessels, connected by commissural vessels (Com. V) running in the septa ; from the latter go off recurrent vessels (R. V) Nephridia (Nphrn) are shown in the second and third metameres, each consisting of a horizontal portion which perforates a septum and opens in the preceding segment by a nephrostome (Nph. si), and of a vertical portion which perforates the body-wall and opens externally by a nephridiopore (Nph. p}. The brain (JBr) lies in the prostomium and is connected with the ventral nerve-cord ( V. Nv. Cd} by a pair of cesophageal connectives (CEs. Com}. B, diagrammatic longitudinal section showing the cell-layers. The cuticle is represented by a black line, the ectoderm is dotted the endoderm radially striated, the muscle-plates evenly shaded, the ccelomic epithelium represented by a beaded line, and the nervous system finely dotted. The body- wall is composed of cuticle (Cu}, cleric epithelium (Dcr. Epthm}, muscle-plates (J/. PI}, and parietal layer of ccelomic epithe- 1 i u m ( Ccel. Epth in ) . The enteric canal is formed of enteric epithelium (Ent. Epthm] covered by the visceral layer of coelomic epithelium (Ccel. Epthm!} ; in the neighbourhood of the mouth (MtJi) and anus (An} the enteric epithe- lium is ectodermal, elsewhere it is endoclermal ; Ph, pharynx ; Ocs, oesophagus ; /;//, intestine ; Rcl, rectum. The septa (Sept) are formed of muscle covered on both sides by coelomic epithelium. Four nephridia (Nph nt) with nephrostome (Nph. st} and nephridiopore (Nph. p} are shown. The brain (Br} and ventral nerve cord ( V. Nv. Cd) are seen to be in contact with the ectoderm : from the brain a nerve (nv) passes to the tentacle. C, diagrammatic transverse section showing the cell-layers as in B, viz : the cuticle (Cu), cleric epithelium (Der. Epthm}, muscle-plates (M. PL], and parietal layer of coelomic epithelium (Ccel. Epthm}, form- ing the body-wall ; and the enteric epithelium (Ent. Epthm} and visceral layer of coelomic epithelium (Ccel. Epthm'}, forming the enteric canal. The dorsal (D. Mes) and ventral ( V. Mes) mesenteries are seen to be formed of a double layer of ccelomic epithelium, and to enclose respec- tively the dorsal (D. V} and ventral ( V. V} blood-vessels. A nephridium (Nphm} is shown on each side with nephrostome (Nph. st} and nephridiopore Nph. p}. The connection of the ventral nerve-cord with the ectoderm (cleric epithelium) is well shown. Fig. 71, A (p. 294), should be compared with this figure, as it is an accurate representation of the parts here shown diagram - matically. T 2 276 POLYGORDIUS LESS the holes a narrow tube of the same length as the wide one. The outer tube would represent the body-wall, the inner the enteric canal, and the cylindrical space between the two the ccelome. The inner tube would communicate with the ex- terior by each of its ends, representing respectively mouth and anus ; the space between the two tubes, on the other hand, would have no communication with the outside. Polygordius is the first example we have studied of a cwloinate animal : one in which there is a definite body- cavity separating from one another -the body-wall and the enteric canal, and in which therefore a transverse section of the body has the general character of two concentric circles (Fig 68, c). It will be remembered that a transverse section of Hydra has the character of two concentric circles, formed re- spectively of ectoderm and endoderm (Fig. 55, A', p. 242), the two layers being, however, in contact or only separated by the thin mesoglcea. At first sight then, it seems as if we might compare Polygordius to a Hydra in which the ecto- derm and endoderm instead of being in contact were separated by a wide interval ; we should then compare the body-wall of Polygordius with the ectoderm of Hydra and its enteric canal with the endoderm. But this comparison would only express part of the truth. A thin transverse section shows the body-wall of Poly- gordius to consist of four distinct layers. Outside is a thin transparent cuticle (Fig. 68, c, and Fig. 71, A, cu) showing no structure beyond a delicate striation. Next comes a layer of epithelial cells (Der. Epthui], their long axes at right angles to the surface of the body, and the boundaries between them very indistinct, so as to give the whole layer the character of a sheet of protoplasm with regularly dis- posed nuclei : this is the deric epithelium or epidermis. Within it is a rather thick layer of muscle-plates (M. PL], xxv ENTERIC EPITHELIUM 277 having the form of long flat spindles (Fig. 70, p. 287, M. PI.) exhibiting a delicate longitudinal striation and covered on their free services with a fine network of protoplasm con- taining scattered nuclei. Each plate is arranged longitu- dinally, extending through several segments, and with its short axis perpendicular to the surface of the body (Fig. 71, M. PL). It is by the contraction of the muscle-plates that the movements of the body, which resemble those of an earthworm, are produced. Finally, within the muscular layer and lining the ccelome is a very thin layer of cells, the ccelomic epithelium (Ccel. Epthm). A transverse section of the enteric canal shows only two layers. The inner consists of elongated cells (JEnt. Epthm) fringed on their inner or free surfaces with cilia : these con- stitute the enteric epithelium. Outside these is a very thin layer of flattened cells (Coel. Epthm'} bounding the ccelome, and hence called, like the innermost layer of the body-wall, coelomic epithelium. We have, therefore, to distinguish two layers of ccelomic epithelium, an outer or parietal layer (Ccel. Ept/un.) which lines the body-wall, and an inner or vis- ceral layer (C&1. Epthm') which invests the enteric canal. We are now in a better position to compare the transverse section of Hydra and of Polygordius (Fig. 55, A', and Fig. 68, c). The deric epithelium of Polygordius being the outermost cell-layer is to be compared with the ectoderm of Hydra, and its cuticle with the layer of the same name which, though absent in Hydra, is present in the stem of hydroid polypes such as Bougainvillea (p. 239) The enteric epithelium of Polygordius, bounding as it does the digestive cavity, is clearly comparable with the endoderm of Hydra. So that we have the layer of muscle-plates and the two layers of ccelomic epithelium not represented in Hydra, in which their position is occupied merely by the mesoglcea. 278 POLYGORDIUS LESS. But it will be remembered that in Medusae there is some- times found a layer of separate muscle-fibres between the ectoderm and the mesoglcea, and it was pointed out (p. 244) that such fibres represented a rudimentary intermediate cell- layer or mesoderm. We may therefore consider the muscular layer and the ccelomic epithelium of Polygordius as meso- derm, and we may say that in this animal the mesoderm is divisible into an outer or somatic layer, consisting of the muscle-plates and the parietal layer of coelomic epithelium, and an inner or splanchnic layer, consisting of the visceral layer of coelomic epithelium. 1 The somatic layer is in contact with the ectoderm or deric epithelium, and with it forms the body-wall ; the splanchnic layer is in contact with the endoderm or enteric epithelium and with it forms the enteric canal. The ccelome separates the somatic and splanchnic layers from one another, and is lined throughout by coelomic epithelium. The relation between the diploblastic polype and the triploblastic worm may therefore be expressed in a tabular form as follows Hydroid. Cuticle . Ectoderm Mesoderm (rudimentary) Endoderm Somatic layer Splanchnic layer Polygordius. Cuticle. Deric epithelium or epidermis. ,- Muscle-plates. Ccelomic epithelium I (parietal layer). f Ccelomic epithelium ( (visceral layer). . Enteric epithelium. i 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 ccelomic epithelium being internal in the one, external in the other. xxv GENERAL STRUCTURE 279 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. 298) that the cells lining both the anterior and posterior ends of the canal are, as indicated in the diagram (Fig. 68, B), ectodermal. For this reason the terms deric and enteric epithelium are not mere synonyms 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 summarised 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 ccelomic 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. 68, 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 ccelome into right and left halves. The structure of the mesenteries is seen in a transverse section (Fig. 68, c, and Fig. 71, A) which shows that at the middle 28o POLYGORDIUS dorsal line the parietal layer of crelomic 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 either side of it, form- ing the visceral layer of coelomic epithelium ; uniting again below r the canal, they are continued downwards as the ventral mesentery, and on reaching the body-w r all diverge once more to join the parietal layer. Thus the mesenteries are simply formed of a double layer of coelomic 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. 68, A and B, Sept) 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 of metameric segmen- tation affects the ccelome as well as the body-wall. Each septum is composed of a sheet of muscle covered on both sides with coelomic epithelium (B, Sept). 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 (Ph) is very short, and can be pro- truded during feeding ; the second, called the gullet or cesopliagus (Oes\ is confined to the peristomium and is distin- guished by its thick walls and comparatively great diameter ; the third or intestine (hit) extends from the first metamere to the last i.e., from the segment immediately following the peristomium to that immediately preceding the anal xxv DIGESTION 281 segment ; it is laterally compressed so as to have an elongated form in cross section (c, and Fig. 71, 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 hypostome of Hydra (p. 231) 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. 232), 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 cceloniic 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 282 POLYGORDIUS LESS. 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. 68, A and c, D. F} 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 mesentery, 1 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. 68, C, and Fig. 71, A) : only their anterior ends have proper walls. xxv HAEMOGLOBIN 283 ends blindly. The anterior parts of the 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 j o * common earthworm, for instance, the blood is red, the colour being due to the same pigment, haemoglobin, 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 into contact with oxygen it acquires a bright scarlet colour, and the solu- tion is then found to have a characteristic spectrum distin- guished 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 284 POLYGORDIUS LESS. 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 of the surrounding medium is called a respiratory organ ; in Polygordius, as in the earthworm and many others of the lower animals, there is no specialised 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. 178) 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 specialised 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 nepliridia, of which each metamere possesses a pair, one on either side (Fig. 68, A, B, and c, Nphui}. Each xxv NEPHRIDIA 285 nephridium (Fig. 69) 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. 68 and 69, Nph. p). The inner division is horizontal and lies in the ccelomic epithelium ; passing forward it pierces the septum which bounds the segment in front (Fig. 68, A and B), and then dilates into a funnel shaped extremity or nephrostome (Nph. st\ which places its cavity in free com- munication with the ccelome. The whole interior of the tube as well as the inner face of the nephrostome is lined with cilia which work outwards. FIG. 69. A nephridium of Polygordius, showing the cilia lining the tube, the ciliated funnel or nephrostome (Nph. st), and the external aperture or nephridiopore (Nph. p}. (After Fraipont.) 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- ing medium. In all probability some such process as this takes place in Polygordius. 286 POLYGORDIUS LESS. 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. 244) 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. 68, 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 ctsophageal 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 oesophageal 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, the ventral cord appearing in sections (Fig. 68, c, and Fig. 71, A) as a mere thickening of the latter. Both brain and cord are composed of delicate nerve-fibres NERVOUS SYSTEM 287 (Fig. 70, Nv. 1*\) 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- / JJer i ner.Epthm FIG. 70. Diagram illustrating the relations of the nervous system ui Polygordius. The cleric epithelium (Da: Epthni) is either indirect contact with the central nervous system (lower part of figure), or is connected by afferent nerves (af. nv.) with the inter-muscular plexus (////. muse, plex.} : the latter is connected to the muscle-plates (J/. PI) by efferent nerves (Ef. nv). The central nervous system consists of nerve-fibres (Nv. F) and nerve-cells (Av. C) : other nerve-cells (A T v. C') occur at intervals in the inter-muscular plexus. The muscle-plates (Jlf. 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 (mi), to which the efferent nerves can be traced. (The details copied from Fraipont.) 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. 288 POLYGORDIUS LESS. Ramifying through the entire muscular layer of the body- wall is a network of delicate nerve-fibres (int. muse, plx.} with nerve-cells (Nv. C') at intervals, the inter-muscular blexus. Some of the branches of this plexus are traceable to nerve-cells in the central nervous system, others (af. nv.} to epidermic cells, others (Ef. uv.} 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. 68, B, /., 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 (af. nz>.) which connect the central nervous system with the epidermis, and (b) motor or efferent (Ef. m>.} which connect it with the muscles. Comparing the nervous system of Polygordius with that of a medusa (p. 244) 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 inter-muscular plexus is not, like the peripheral nervous system of a medusa which it resembles, situated 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 xxv FUNCTIONS OF NERVOUS SYSTEM 289 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 conducted along the eminently irritable but non-contractile nerve. Further, if the motor nerve is left in connection with the central nervous system, i.e., 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 then, as it were, reflected along a motor nerve to a muscle, is called a reflex action ; the essence of the arrangement is the inter- position of nerve-cells between sensory or afferent nerves connected with sensory cells, and motor or efferent nerves connected with muscles. The diagram (Fig. 70) serves to illustrate this matter. The muscle-plate (M. PL) may be made to contract by a stimulus applied (a) to itself directly, (t>) to the motor fibre u 290 POLYGORDIUS LESS. (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. Epthm.\ 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, /.., to the left in Fig. 74, 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. 72, B and c, Msd\ have now r increased to such an extent as to surround com- pletely the enteron and obliterate the blastoccele (Fig. 74, 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 acmlomate. Development continues, and the larva assumes the form shown in Fig. 75, 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. 67, D, p. 272). 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 302 POLYGORDIUS LESS, xxvi 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 proctodaeum (Prc. d?n] 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. a) 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 (soui) ) in contact with the ectoderm and a splanchnic layer (Msd (spl) ) in contact with the endoderm. The space between the two layers (Ctti) 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 Pr.or.cl Msd (/Sfetnf\ Y.TSf Pr.an.cl An.ci FIG. 75. A, larva of Polygordius neapolitanus in a condition inter- mediate between the trochosphere and the adult worm, the trunk-region being 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 (Pi. or. ci), and anal (An. ci) cilia, the blastocoele (Bl. ccel), stomodaeum (St. dm), and proctodceum (Prc. dm) are as in Fig. 72,. A and B : the enteron now extends through- out the segmented region of the trunk. A pair of tentacles (/) has appeared on the prostomium near the ocelli (o), and a pre-anal circlet of cilia (Pr. an. ci) is developed. The mesoderm has divided into somatic (Msd (som) ) and splanchnic (Msd (spl) ) layers with the ccelome (Ccel) between : the septa (Sep) are formed by undivided plates of mesoderm separating the segments of the ccelome from one another. D^D 3 , three stages in the development of the somatic mesoderm. In D 1 it (Msd (Som) ) consists of a single layer of cells in contact with the deric epithelium (Der. Epthni) : in D 2 the cells have begun to split up in a radial direction : in D 3 each has divided into a number of radially arranged sections of muscle-plates ( M. PI) and a single cell of ccelomic epithelium (Ccel. Epthni), (A after Fraipont. ) 304 POLYGORDIUS LESS. 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 (o 1 , Msd (Soni) ), but before long each cell splits up in a radial direction (o 2 ) from without inwards /.$} containing blood and having the general relations of a ccelome, but very probably only representing a number of enlarged blood-spaces or sinuses. Respiration is performed by special organs, the gills (B, Gift, see p. 317), 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 those of Polygordius, but is greatly modified. A portion of the dorsal vessel is enlarged to form a muscular dilatation, the heart (/fr), 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 head and enclosed in spacious sacs which open by ducts on the bases of the antennae. They consist of convoluted tubes lined by epithelium, and are probably to be looked upon as greatly modified nephridia. xxvn ABSENCE OF CILIA 319 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 deferential 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 ccelome 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 (r) 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 primatively 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 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. 320 THE FRESH-WATER MUSSEL LESS. 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. 78, A and B, Manf), which pass downwards one on either side of the body, Closely applied to the outer surface of the mantle-lobes, and formed as a cuticular secretion of their deric epithelium, ar j the two valves of the bivalved, strongly calcified shell (B., S/i). The ventral region of the body is produced into a laterally compressed muscular structure, the /