GIFT OF 
 MICHAEL REESE 
 
 BIOLOGY 
 UBffARY 
 
 G 
 
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, DUNEDIN, NEW ZEALAND 
 
 
 WITH EIGHTY-EIGHT ILLUSTRATIONS 
 
 Eonton 
 MACMILLAN AND CO. 
 
 AND NEW YORK 
 1893 
 
 The Right of Translation and Reproduction is Reserved 
 
BiOLOGY 
 
 LIBRARY 
 
 G 
 
 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 
 
vi 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 vii 
 
 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 
 
viii 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. 
 
 D UNEDIN , N.2., 
 
 8 9 0. 
 
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^MATOCOCCUS 
 
 HETEROMITA 
 
 EUGLENA 
 
 PROTOMYXA . . 
 THE MYCETOZOA 
 
 LESSON I. 
 
 LESSON II. 
 
 LESSON III. 
 
 LESSON IV. 
 
 LESSON V. 
 
 PAGE 
 V 
 
 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. 
 
 PARAMCECIUM j IO6 
 
 STYLONYCHIA /. Il6 
 
 OXYTRICHA 120 
 
 LESSON XI. 
 
 OPALINA 121 
 
 LESSON XII. 
 
 VORTICELLA 126 
 
 ZOOTHAMNIUM 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 igi 
 
 LESSON XIX. 
 
 SPIROGYRA 194 
 
 LESSON XX. 
 
 MONOSTROMA 2OI 
 
 ULVA 203 
 
 LAMINARIA, &C 2O3 
 
xvi TABLE OF CONTENTS 
 
 LESSON XXI. 
 
 I'AGE 
 
 NITELLA . 206 
 
 LESSON XXII. 
 
 HYDRA ; 221 
 
 LESSON XXIII. 
 
 HYDROID POLYPES 237 
 
 BOUGAINVILLEA, &C 237 
 
 DIPHYES 250 
 
 PORPITA 253 
 
 ^LESSON XXIV. 
 
 SPERMATOGENESIS AND OOGENESIS 255 
 
 THE MATURATION AND IMPREGNATION OF THE OVUM .... 259 
 THE CONNECTION BETWEEN UNICELLULAR AND DIPLOBLASTIC 
 
 ANIMALS 264 
 
 LESSON XXV. 
 
 POLYGORDIUS 271 
 
 LESSON XXVI. 
 POLYGORDIUS (continued} 293 
 
 LESSON XXVII. 
 
 THE GENERAL CHARACTERS OF THE HIGHER ANIMALS .... 307 
 
 THE STARFISH 309 
 
 THE CRAYFISH 314 
 
 THE FRESH-WATER MUSSEL 32O 
 
 THE DOGFISH 324 
 
TABLE OF CONTENTS xvii 
 LESSON XXVIII. 
 
 PACK 
 
 MOSSES 332 
 
 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. Amoeba, various species 2 
 
 2. Protamceba primitiva . , 9 
 
 3. Hfzmatococcits pluvialis and H. lacnstris 24 
 
 4. Heteromita rostrata 38 
 
 5. Euglena viridis 45 
 
 6. Protomyxa atirantiaca 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 
 
 1 2. Saccharomyces cerevisice 72 
 
 13. Bacterium termo . 83 
 
 14. Bacteririm termo, showing flagella 84 
 
 15. Micrococcus 86 
 
 16. Bacillus subtilis 87 
 
 1 7. Vibrio serpens, Spirillum tenue, and S. volutans 88 
 
 1 8. Bacillus anthracii, 90 
 
 19. Beaker with culture- tubes 100 
 
xx LIST OF ILLUSTRATIONS 
 
 FIG. PAGE 
 
 20. Faramoecium aurelia . 108 
 
 21. Paramcecium aurelia, conjugation 115 
 
 22. Stylonychia mytiltis 117 
 
 23. Oxytricha fava 120 
 
 24. Opalina ranarum 122 
 
 25. Vorticella 127 
 
 26. Zoothamnium arbuscida 134 
 
 27. Zoothamnium, various species 138 
 
 28. Diagram illustrating the Origin of the Species of Zootham- 
 
 nium by Creation 142 
 
 29. Diagram illustrating the Origin of the Species of Zootham- 
 
 nium by Evolution 144 
 
 30. Rotalia 149 
 
 31. Diagrams of Foraminifera 150 
 
 32. Alveolina quoii , I5 1 
 
 33. Lithocircus annularis 152 
 
 34. Actinomma asteracanthion 153 
 
 35. Diagrams of a Diatom and shells of Navicula and Attlaco- 
 
 di setts 156 
 
 36. Mucor mucedo and M. stolonifer 159 
 
 37. Moist Chamber 163 
 
 38. Vaucheria 170 
 
 39. Caulerpa scalpelliformis 174 
 
 40. Penicillinm glaucum 186 
 
 41. Agarictts campestris 192 
 
 42. Spirogyra . 195 
 
 43. Monostroma Imllosum and M. laceratum 202 
 
 44. Laminaria claustoni and Lessonia frtsccscens . . . ... 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. Bougainvillca ramosa -. 238 
 
 55. Diagrams illustrating derivation of Medusa from Hydrautli . 242 
 
 56. Eucopella campanularia, muscle fibres and nei've-cells. . . . 245 
 57- Laomedta flexuosa and Eudendrimn ramosum, development . 249 
 
 58. DipJiycs campanulata 252 
 
 59- Porpita pacifica and P. mediterranea 253 
 
 60. Spermatogenesis in the Mole-Cricket 256 
 
 61. Ovum of Toxopneustes lividus 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 neapolitanus, external form 272 
 
 68. Polygordius neapolitanus, anatomy 274 
 
 69. Polygordius neapolitanus, nephridium 285 
 
 70. Polygordius, diagram illustrating the relations of the nervous- 
 
 system 287 
 
 71. Polygordius neapolitanus, 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 neapolitamis, advanced trochosphere 300 
 
 75. Polygordius neapolitanus, 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 3 21 
 
 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. Equisetum, reproduction and development 367 
 
 85. Salvinia, reproduction and development . 369 
 
 86. Selaginella, reproduction and development 372 
 
 87. Gymnosperms, reproduction and development 374 
 
 88. Angiosperms, reproduction and development 379 
 
LESSONS 
 
 IX 
 
 ELEMENTARY BIOLOGY 
 
 ; UNIT? 
 
 \ c ^ 
 
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 Anuzba. 
 
 Amoebae are mostly invisible to the naked eye, rarely 
 exceeding one-fourth of a millimetre (^-^ inch) in dia- 
 meter, so that it is necessary to examine them entirely by 
 the aid of the microscope. They can be seen and re- 
 cognized under the low power of an ordinary student's 
 microscope which magnifies from twenty-five to fifty dia- 
 meters ; but for accurate examination it is necessary to 
 employ a far higher power, one in fact which magnifies 
 about 300 diameters. 
 
 Seen under this power, an Amoeba appears like a little 
 
 B 
 
AMCEBA 
 
 LESS. 
 
 FIG i. A. Amasba quarta, a living specimen, showing granular 
 endosarc surrounded by clear ectosarc, and several pseudopods (fist*), 
 
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). 1 
 
 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 (nu), 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. Amceba proleus, 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 constitutents 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. 
 
 13 2 
 
4 AMCEBA LESS. 
 
 same relations, whence we conclude that an Amoeba con- 
 sists of a granular substance the endosarc, completely 
 surrounded by a clear transparent layer or ectosarc. 
 
 One very noticeable thing about Amoeba is that it is never 
 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 Amoeba. 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 
 Amoeba 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_ o .which Amoeba is composed 
 is called protoplasm. It is shown by chemical analysis x 
 to consist mainly of certain substances known %s> protcids, 
 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 alcofiol coagulates it, f.e.j causes it 
 to shrink by withdrawal of water and become comparatively 
 hard and opaque. Coagulation is also produced by raising 
 the temperature to about 40 C. ; the reader will remember 
 how the familiar proteid white of egg is coagulated and 
 rendered hard and opaque by heat. 
 
 1 Accurate analyses of the protoplasm of Amoeba have not been 
 made, but the various micro-chemical tests which can be applied to it 
 leave no doubt that it agrees in all essential respects with the protoplasm 
 of other organisms, the composition of which is known (see p. 7). 
 
6 AMCEBA 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^jat, 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 2 O), 
 carbon dioxide or carbonic acid (CO 2 ), ammonia (NH 3 ), 
 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, 
 
 1 For a more detailed account of the phenomena of putrefaction see 
 Lesson VIII., in which it will be seen that the above statement as to 
 the instability of (dead) proteids requires qualification ; as a matter of 
 fact they onty 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 chromatin 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 nucleates. 
 
 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 
 
i MORPHOLOGY AND PHYSIOLOGY 9 
 
 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 
 
 ' A B C D ^HP^F 
 
 FrHv.^2 Protam&ba primitive*, ; 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. 118), 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 11 
 
 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 (f. 
 vac), which also contains a small quantity of water neces- 
 sarily included with the prey. The latter is taken in by the 
 Amoeba as food : so that another function performed by the 
 animalcule is the reception of food, the first step in the 
 process of nutrition. It is to be noted that the reception 
 of food takes place in a particular way, viz. by ingestion 
 i.e. it is enclosed raw and entire in the living protoplasm. It 
 has been noticed that Amoeba usually ingests at its hinder 
 end that is, the end directed backwards in progression. 
 
 Having thus ingested its prey, the Amoeba continues its 
 course, when, if carefully watched, the swallowed organism 
 will be 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 
 mice. 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- 
 
i 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 ne.w 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 fire every time it blazes up 
 consumes a certain weight of coal. 
 
 Precisely the same thing happens on a small scale with 
 Amoeba. Every time it thrusts out or withdraws a pseudo- 
 pod, every time it contracts its vacuole, it does a certain 
 amount of work moves a definite weight of protoplasm 
 through a given space. And every movement, however 
 slight, is accompanied by a proportional waste of substance, 
 
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 = CO 2 ) : when hydrogen is burnt, 
 water (H 2 + 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 Jby 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 17 
 
 other. The same process of gaseous diffusion is continually 
 going on between the carbon dioxide in the interior of 
 Amoeba and the oxygen in the surrounding water, the proto- 
 plasm acting as the porous partition. In this way the carbon 
 dioxide is got rid of, and at the same time a supply of 
 oxygen is obtained for further combustion. 
 
 The taking in of oxygen might be looked upon as a kind 
 of feeding process, the food being gaseous instead of solid 
 or liquid, just as we might speak of "feeding" a fire both 
 with coals and with air. Moreover, as we have seen, the 
 giving out of carbon dioxide is a process of excretion. It 
 is, however, usual and convenient to speak of this process 
 of exchange of gases as respiration or breathing, which 
 'is therefore another function performed by the protoplasm of 
 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 
 
1 8 AMOEBA LESS. 
 
 living protoplasm : these are processes of constructive meta- 
 bolism or anabolism. Next we have the protoplasm gradually 
 breaking down and undergoing conversion into excretory 
 products : this is the process of destructive metabolism or 
 katabolism. There can be little doubt that both are pro- 
 cesses of extreme complexity : it seems probable that 
 after the food is once dissolved there ensues the successive 
 formation of numerous bodies of gradually increasing 
 complexity (anabolic mesostates or anastates), culminating 
 in protoplasm ; and that the protoplasm, when once formed, 
 is decomposed into a series of substances of gradually 
 diminishing complexity (katabolic mesostates or katastates], 
 the end of the series being formed by the comparatively 
 simple products of excretion. The granules in the endosarc 
 are probably to be looked upon as various mesostates 
 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, *.*., the total weight of substances taken in (food 
 plus oxygen plus water) is greater than its expenditure or 
 the total weight of substances given out (feces 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 
 otjier 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 Amoeba 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 Amoeba 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 certain 
 amount of shrinkage : it is then said to pass into a con- 
 dition of dry-rigor. Under these circumstances it may 
 be restored to its normal condition by adding a sufficient 
 proportion of water to bring back the fluid to its original 
 density. 
 
 In this connection it is interesting to notice that the dele- 
 terious effects of an excess of salt are 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, i.e., one twice as strong as would, if added 
 suddenly, produce dry-rigor. 
 
22 AMCEBA 
 
 From what has been said above on the subject of respira- 
 tion (p. 17) it follows that free oxygen is necessary for the 
 existenoe of Amoeba. Light, on the other hand, appears to 
 be unnecessary, amoeboid movements having been shown" to 
 go on actively in darkness. 
 
LESSON II 
 
 H.EMATOCOCCUS 
 
 THE rain-water which collects in puddles, open gutters, 
 &c., is frequently found to have a green colour. This colour 
 is due to the presence of various organisms plants or 
 animals one of the commonest of which is called H<zma~ 
 tococcus (or as it is sometimes called Protococcus or Sphcerella) 
 pluvialis. 
 
 Like Amoeba, Haematococcus is so small as to require a 
 high power for its examination. Magnified three or four 
 hundred diameters it has the appearance (Fig. 3, A) of an 
 ovoidal body, somewhat pointed at one end, and of a bright 
 green colour, more or less flecked with equally bright red. 
 
 Like Amoeba, moreover, it is in constant movement, but 
 4he character of the movement is very different in the two 
 cases. An active Haematococcus is seen to swim about 
 the field of the microscope in all directions and with 
 considerable apparent rapidity. We say apparent rapidity 
 because the rate of progression is magnified to the same 
 extent as the organism itself, and what appears a racing 
 speed under the microscope is actually a very slow crawl 
 when divided by 300. It has been found that such 
 organisms as HDematococcus travel at the rate of one foot 
 in from a quarter of an hour to an hour : or, to express 
 
2 4 
 
 H^MATOCOCCUS 
 
 LESS. 
 
 the fact in another and fairer way, that they travel a distance 
 equal to 2\ times their own diameter in one second. In 
 swimming the pointed end is always directed forwards and 
 
 FIG. 3. A. Hamatococcus pluvialis, motile phase. Living speci- 
 men, showing protoplasm with chromatophores (chr} and pyrenoids 
 (pyr), cell-wall (c.w] 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 (nu) with nucleolus 
 (nu'} ) and thick cell-wall (c.vv) 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. Hannalococcus laciistris, showing nucleus (>*&} single large 
 pyrenoid (pyr}> and contractile vacuole (c.vac), 
 
 F. Diagram illustrating the movement of a flagellum : ab, its base ; 
 c, c', c", different positions assumed by its apex. (E, after Biitschli. ) 
 
IT 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 
 Haematococcus 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,y7), each about 
 half as long again as the animalcule itself : these are called 
 flagella or sometimes cilia}- In a Haematococcus which 
 has come to rest these can often be seen gently waving 
 from side to side : when this slow movement is exchanged 
 for a rapid one the whole organism is propelled through 
 the water, the flagella acting like a pair of extremely fine 
 and flexible fins or paddles. Thus the movement of 
 Haematococcus is not amceboid, 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 
 Haematococcus, like Amoeba, consists of protoplasm, and 
 that the flagella are simply filamentous processes of the 
 protoplasm. 
 
 It was mentioned above that in swimming the pointed end 
 
 1 The word cilium is sometimes used as a general term to include 
 any delicate vibratile process of protoplasm : often, however, it is used 
 in a restricted sense for a rhythmically vibrating thread, of which each 
 cell bears a considerable number (see Fig. 8, E, and Fig. 21) ; a flagel- 
 lum is a cilium having a whip-lash-like movement, and each cell 
 bearing only a limited number one or two, or occasionally as many 
 as four. 
 
 ^ 
 
26 . HdEMATOCOCCUS LESS. 
 
 with the flagella goes first ; this may therefore be distin- 
 guished as the antja<jf3j^tremity, the opposite or blunt 
 end being posterior. So that, as compared with Amoeba, 
 Hsematococcus 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 hcematochrome. 
 
 At first sight the chlorophyll appears to be evenly distri- 
 buted over the whole body, but accurate examination under 
 a high power shows it to be lodged in a variable number 
 of irregular structures called chromatophores (Fig. 3, A, 
 chr.\ which together form a layer immediately beneath the 
 surface. Each chromatophore consists of a protoplasmic 
 substance impregnated with chlorophyll. 
 
 After solution of the chlorophyll with alcohol a nucleus 
 (B, nu.) can be made out ; like the nucleus of Amoeba it is 
 stained by iodine, magenta, &c. Other bodies which might 
 easily be mistaken for nuclei are also visible in the living 
 
ii CELL-WALL 27 
 
 organism. These are small ovoidal structures (A, pyr.}, 
 with clearly defined outlines occurring in varying numbers 
 in the chromatophores. When treated with iodine they 
 assume a deep, apparently black but really dark blue, 
 colour. The assumption of a blue colour with iodine is the 
 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 Haematococcus pluvialis there is no c^ntra^ti^vaciiokv^ 
 but in another species, H. lacustris, this structure is pre- 
 sent as a minute space near the anterior or pointed end 
 (Fig. 3, E, c. vac.). 
 
 There is still another characteristic structure to which no 
 reference has yet been made. This appears at the first view 
 something like a delicate haze around the green body, but 
 by careful 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 Amoeba, as stated in 
 the preceding lesson (p. n) is very probably nitrogenous : 
 
28 H^EMATOCOCCUS LESS. 
 
 that of Haematococcus, on the other hand, is formed of a 
 carbohydrate called cellulose, allied in composition to 
 starch, sugar, and gum, and having the formula 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 Hsematococcus : 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 Amceba 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 Amoeba : moreover, even in its active condition it is 
 usually surrounded by an imperforate cell-wall, which of 
 course quite precludes the possibility of ingestion. As a 
 matter of observation, also, however long it is watched it is 
 never seen \.Q 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. 
 
TT DECOMPOSITION OF CARBON DIOXIDE 29 
 
 Hsematococcus 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, 
 i.e., 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^EMATOCOCCUS 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 
 Hsematococcus, 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 
 
ii 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, /.<?., 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 Haematococcus 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 
 Haematococcus, 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 H^EMATOCOCCUS 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 Haematococcus 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 Haema- 
 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 
 
ii 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 (fr) 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, \,fl) 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. r ln 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 ^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^EMATOCOCCUS 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 Amceba 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 JEsop, 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 
 Hsematococcus 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 daufhter-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. Hsematococcus 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 ^ mm. (^oV o mcn ) 
 in average length. It has a certain resemblance in general 
 form to Hsematococcus, 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 (fl. 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, in 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 (nu\ 
 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. 
 
 T 
 
 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 , the living organism, showing nucleus (nu), contractile vacuole 
 
LESS, in NUTRITION 39 
 
 (c. vac), anterior flagellum (ft. i), and coiled ventral flagellum (ft. 2) 
 by which the organism is anchored ; A? shows the position at the 
 forward limit of the spring, the ventral flagellum being fully extended. 
 
 B 1 B 3 , three stages in the longitudinal fission of the anchored form. 
 
 c 1 c 3 . Three stages in the transverse fission of the same : ft. I 1 , 
 rudiment of newly formed anterior flagellum. 
 
 D 1 D 3 , three stages in the fission of the free-swimming form : ft. 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 K , 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 2 the process has gone further, and in B S the products of 
 division are on the point of separating. 
 
 More frequently, however, fission instead of being longitudinal, i.e., 
 in the 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 
 seen to differ from that described in the preceding paragraph in the cir- 
 
TII 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 in form 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 , fl. 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 
 two conjugating individuals being distinguished as gametes. 
 
42 HETEROMITA 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 
 (p 1 ) are found to grow into clear visibility and to take on a 
 distinctly oval shape (r 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. 
 
 1 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.). 
 
m 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 Amoeba 
 and Haematococcus. 
 
LESSON IV 
 
 EUGLENA 
 
 THE rain-water collected in puddles by the road-side, on 
 roofs, &c., is often found to have a bright green colour : 
 this is sometimes due to the presence of delicate water 
 weeds visible to the naked eye (Lesson XVI.), but frequently 
 the water when held up to the light in a glass vessel appears 
 uniformly green, no suspended matter being visible to the 
 unaided sight. Under these circumstances the green colour 
 is 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 
 ff mm. to J mm. The body is spindle-shaped, wide in the 
 middle and narrow at both ends (Fig. 5, A E) : one 
 extremity is blunter than the other, and from it proceeds 
 a single long flagellum (fl) by the action of which the 
 organism swims with great rapidity, the flagellum being, 
 as in Haematococcus, directed forwards. Besides its rapid 
 swimming movements Euglena frequently performs slow 
 movements of contraction and expansion, something like 
 those of a short worm, the body becoming broadened out 
 first at the anterior end, then in the middle, then at the 
 
GENERAL CHARACTERS 
 
 45 
 
 posterior end, twisting to the right and left, and so on (Fig. 
 5, A D). These movements are so characteristic of the 
 genus that the name euglenoid is applied to them. 
 
 FIG. 5. Euglena viridis. 
 
 A u, four views of the living organism, showing the changes of form 
 produced by the characteristic euglenoid movements. 
 
 E, enlarged view, showing the nucleus (nu}, reservoir of the con- 
 tractile vacuole (c.vac}, with adjacent pigment spot, and gullet with a 
 single flagellum springing from it. 
 
 F, enlarged view of the anterior end of E, showing pigment-spot 
 (t>g) and reservoir (c. vac\ mouth (m), gullet (ce. s), and origin of 
 flagellum (/). 
 
 G, resting form after binary fission, showing cyst or cell- wall (n/), 
 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 chromatophore or 
 in several. 
 
 In Haematococcus we saw that chlorophyll was asso- 
 ciated with starch (p. 27). In Euglena there are, near the 
 middle of the body, a number of grains of paramylum 
 (H, p\ a carbohydrate of the same composition as starch 
 (C 6 H 10 O 5 ), 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 (<x. 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, when the 
 
iv MOUTH AND GULLET 47 
 
 coloured particles were seen to be swallowed in the way 
 described. 
 
 The depression in question is therefore a gullet, and its 
 external aperture or margin (m) 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 
 that the greatest sensitiveness to light is manifested by the 
 colourless anterior end of the body. 
 
 As in Haematococcus a resting condition alternates with 
 the motile phase: the organism loses its flagellum and 
 
48 EUGLENA 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 1 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. 
 
psd 
 
 FIG 6 Frotomyxa aurantiaca. 
 
 A, the living organism (plasmodium), showing fine branched pseudo- 
 pods (psd] and several ingested organisms. 
 B 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, flagellulse which afterwards become converted into 
 
 F, amcebulae. ___ , , 
 
 G, amcebulse uniting to form a plasmodium. (After Haeckel. ) 
 
LESS, v LIFE-HISTORY 51 
 
 the specimen has ingested several minute organisms and is 
 in the act of capturing another. 
 
 But the main interest of Protomyxa lies in its very curious 
 and complicated life-history. After crawling over the Spirula 
 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, fi] 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 flagellula ; 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 amcebula. 
 
 The process just described may be taken as a practical 
 
 K 2 
 
52 PROTOMYXA AND THE MYCETOZOA 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 amoebula, the flagellum 
 of the former becoming one of the pseudopods of the 
 latter. 
 
 The amcebulse 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 
 amcebulae. A body formed in this way by the fusion of 
 amoebulae is called a plasmodium, so that in the life-history 
 of Protomyxa we can distinguish an encysted, a ciliated or 
 flagellate, an amoeboid, and a plasmodial phase. 
 
 The nature of a plasmodium will be made clearer by a 
 short consideration of -the strange group of organisms known 
 as Mycetozoa or sometimes " slime-fungi." They occur 
 as gelatinous masses on the bark of trees, on the surface of 
 tan-pits, and sometimes in water. It must be remembered 
 that Mycetozoa is the name not of a genus but of a class 
 in which are included several genera, such as Badhamia, 
 Chondrioderma, &c. (see Fig. 7) : a general account of the 
 class is all that is necessary for our present purpose. 
 
 The Mycetozoa 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 
 
 H 
 
 FIG. 7. A, part of the plasmodium of Badhamia (X 3^) ', l> 
 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. 
 
 K, amcebulse formed by metamorphosis of flagellulse. 
 
 F, two amcebulse about to fuse : F', the same after complete union. 
 
 G, G', two stages in the formation of a three-celled plasmodium. 
 H, a small plasmodium. 
 
 (A, after Lister : B H, from Sachs after Cienkowski.) 
 
54 PROTOMYXA AND THE MYCETOZOA 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 
 has been swollen by a moderate heat as in our own bread 
 and rice-puddings but are unable to make use of raw 
 starch. 
 
 After living in this free condition, like a gigantic terrestrial 
 Amoeba, for a longer or shorter time, either a part or the 
 whole of the protoplasm becomes encysted 1 and breaks up 
 into spores. These (B) consist of a globular mass of proto- 
 plasm covered with a wall of cellulose : the cysts are also 
 formed of cellulose. 
 
 By the rupture of the cell-wall of the spore (c) the 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 flagellulse 
 lose their cilia and pass into the condition of amoebulse (E), 
 which finally fuse to form the plasmodium with which 
 we started (F H). In the young plasmodia (c 1 ) the 
 nuclei of the constituent amcebulse are clearly visible, and 
 from them the nuclei of the fully developed plasmodia are 
 probably derived. It would seem, therefore, that in the 
 fusion of amcebulae to form the plasmodium of Mycetozoa the 
 cell-bodies (protoplasm) 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 Heteromitae fuse and form a zygote the 
 
 1 The process of formation of the cyst or sporangium is a compli- 
 cated one, and will not be described here. See De Bary, Fun%i, 
 Mycetozoa, and Bacteria (Oxford, 1887). 
 
v PLASMODIUM FORMATION AND CONJUGATION 55 
 
 protoplasm of which divides into spores. In Protomyxa and 
 the Mycetozoa not two but several amoebulae 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 amcebulae 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 so gradually to alter its form completely. 
 Moreover the addition of iodine, logwood, or any other 
 suitable colouring matter reveals the presence of a large 
 nucleus (A 5 , A 6 , nu) : 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 , u) 
 
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 e , edge view of a red corpuscle of the same, 
 nu, 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 2 , 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 8 , D 3 , after Wiedersheim : F 1 , after Sachs : F 2 , after Behrens. ) 
 
 coloured by a pigment called haemoglobin, and provided 
 each with a large nucleus (nu) which, when the corpuscle is 
 seen from the edge, produces a bulging of its central part. 
 These bodies may be compared to Amoebae which have 
 drawn in their pseudopods, assumed a flattened form, and 
 become coloured with haemoglobin. 
 
 In the blood of mammals, such as the rabbit, dog, or man, 
 similar leucocytes occur, but their red blood corpuscles (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. 53). 
 
 The stomach and intestines of animals are lined with a 
 sort of soft slimy skin called mucous membrane. If a 
 bit of the surface of this membrane in a frog or rabbit for 
 instance is snipped off and "teased out," i.e., torn apart 
 with needles, it is found when examined under a high power 
 to be made up of an immense number of microscopic 
 bodies called epithelial cells, which in the living animal, lie 
 close to one another in the inner layer of mucous mem- 
 brane in something the same way as the blocks of a wood 
 pavement lie on the surface of a road. An epithelial cell 
 (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 T 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^^^i^^le . polyhedral bodies called 
 
 ' UNIVERSITY 
 
 V ^, 
 
60 PARENCHYMA CELLS LESS. 
 
 parenchyma cells, fitting closely to one another like the 
 bricks in a wall. 
 
 A parenchyma cell examined in detail (F 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 (F 2 , chr). 
 These are chromatophores or chlorophyll corpuscles ; they 
 consist of protoplasm coloured with chlorophyll which can 
 be proved experimentally to have the same properties as 
 the chlorophyll of Hsematococcus 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. 
 
 
 chr 
 
 nu.m 
 
 -c.b 
 
 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 (<:), and nucleus with membrane 
 (nu. m) and irregular network of chromatin (chr). 
 
 B. Cell from the immature stamen of a lily, showing cell-wall (c. 7t), 
 protoplasm with two directive spheres (s), 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 01 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 protoplasnJjjJj^ the 
 granular endosarc alone possessing the sponge ^Vork. 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 
 chroinatin 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. 
 
f A 
 
 D 
 
 B 
 
 x E 
 
 H 
 
 FIG. 10. Diagrams illustrating the process of indirect cell division 
 or karyokinesis. 
 
 A, The resting cell : the nucleus shows a nuclear membrane (#./#), 
 chromatin (chr] arranged in loops united into a network (the latter 
 shown on the right side only), and two nucleoli (nu 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, fc^ 
 
 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. pl] : the daughter nuclei are fully formed! 
 
 (Altered from Flemming, 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 
 (c), 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 chroinoAOines (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 
 
 T? T? C \ T Y 
 
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 the 
 extreme complexity of structure revealed in cells and their 
 nuclei by the highest powers of the microscope. When the 
 constituent cells of the higher animals and plants were 
 discovered, during the early years of the present century, by 
 Schleiden a.nd Schwann, they were looked upon as the ultima 
 Thule of microscopic analysis. Now the demonstration of 
 the cells themselves is an easy matter, the problem is to 
 make out their ultimate constitution. What would be the 
 
 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. 
 
 !' 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 living 
 things is destined to make during the next half century. 
 
 The striking general resemblance between the cells of the 
 higher animals and plants and entire unicellular organisms 
 has been commented on as a very remarkable fact : there is 
 another equally significant circumstance to which we must 
 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 (g.v.\ the chromatin of 
 which is aggregated into a central mass or nucleolus (g.vi.}- 
 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 (see 
 
YI STRUCTURE OF THE EGG 69 
 
 Fig. 53) ; or the surface of the protoplasm may secrete a thick 
 cell-wall (see Fig. 6 1). 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. ii. A, ovum of an animal {Car marina hastata, one of the 
 jelly fishes), showing protoplasm (gd), nucleus (gv), andnucleolus (,^-w). 
 
 B, ovum of a plant (Gymnadcnia conofsca, one of the orchids), 
 showing protoplasm (pis in), nucleus (nit), and nucleolus (mi). 
 
 (A, from Balfour after Ilaeckel : B, 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 rdl 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 (0v 9 and Fig. 1 2 B), consisting as 
 usual of protoplasm (plsm], nucleus (), 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- 
 char omyces cerevisice. 
 
 Saccharomyces consists of a globular or ellipsoidal mass 
 of protoplasm (Fig. 12), about T Jy- 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 rilled 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 
 
72 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 
 
 ~bcl' 
 
 FIG. 12. Sacckaromyces cerevisia. 
 
 A, a group of cells under a moderately high power. The scale to 
 the left applies to this figure only. 
 
 B, several cells more highly magnified, showing various stages ot 
 budding, vac, the vacuole. 
 
 C, a single cell with two buds (bd, bd'} still more highly mag- 
 nified : c.w, cell -wall : vac, vacuole. 
 
 D, cells, crushed by pressure : c.iv, the ruptured cell-walls : plsm, 
 the squeezed out protoplasm. 
 
 E, E', starved cells, showing large vacuoles and fat globules (/). 
 
 F, F', formation of spores by fission of the protoplasm of a starved 
 cell : in F the spores are still enclosed in the mother-cell-wall, in F' 
 they are free. 
 
 magenta, and then applying pressure to the cover-glass so as 
 to crush the cells. Under this treatment the cell-walls are 
 burst and appear as crumpled sacs, split in various ways and 
 unstained by the magenta (D, c.w), while the squeezed-out 
 protoplasm is seen in the form of irregular masses (plsm) 
 stained pink by the dye. 
 
vii GEMMATION 73 
 
 The mode of multiplication of Saccharomyces is readily 
 made out in actively fermenting yeast, and is seen to differ 
 from anything we have met with hitherto. A small pimple- 
 like elevation (c, bd) appears on the surface of a cell and 
 gradually increases in size : examined under a high power 
 this bud is found to consist of an offshoot of the protoplasm 
 of the parent cell covered with a very thin layer of cellulose : 
 it is formed by the protoplasm growing out into an offshoot 
 like a small pseudopod which pushes the cell-wall before 
 it. The bud increases in size (bdf ) until it forms a little 
 globular body touching the parent cell at one pole : then a 
 process of fission takes place along the plane of junction, 
 the protoplasm of the bud or daughter-cell becoming sepa- 
 rated from that of the mother-cell and a cellulose partition 
 being secreted between the two. Finally the bud becomes 
 completely detached as a separate yeast-cell. 
 
 It frequently happens that a Saccharomyces buds in 
 several places and each of its daughter-cells buds again, 
 before detachment of the buds takes place. In this way 
 chains or groups of cells are produced (B), such cell- 
 colonies consisting of two or more generations of cells, the 
 central one standing in relation of parent, grandparent, or 
 great-grandparent to the others. 
 
 It must be observed that this process of budding or 
 gemmation is after all only a modification of simple 
 fission. In the latter the two daughter-cells are of equal size 
 and are both smaller than the parent-cell, while in gemma- 
 tion one the mother-cell is much larger than the other 
 the daughter-cell or bud and is of the same size as, indeed is 
 practically identical with, the original dividing-cell. Hence 
 in budding, the parent form does not, as in simple fission, 
 lose its individuality, becoming wholly merged in its twin 
 offspring, but merely undergoes separation of a small portion 
 
74 SACCHAROMVCES LESS. 
 
 of its substance in the form of a bud, which by assimilation 
 of nutriment gradually grows to the size of its parent, 
 the latter thus retaining its individuality and continuing to 
 produce fresh buds as long as it lives. 
 
 Multiplication by budding goes on only while the Sac- 
 charomyces is well supplied with food : if the supply of 
 nutriment fails, a different mode of reproduction obtains. 
 Yeast can be effectually starved by spreading out a thin 
 layer of it on a slab of plaster-of-Paris kept moist under 
 a bell-jar : under these circumstances the yeast is of course 
 supplied with nothing but water. 
 
 In a few days the yeast-cells thus circumstanced are found 
 to have altered in appearance : large vacuoles appear in 
 them (Fig. 1 2. E ; 'E') and numerous fat-globules (/) are formed. 
 The protoplasm has been undergoing destructive meta- 
 bolism, and, there being nothing to supply new material, has 
 diminished in quantity, and at the same time been partly 
 converted into fat- Both in plants and in animals it is found 
 that fatty degeneration, or the conversion of protoplasm 
 into fat by destructive metabolism, is a constant phenomenon 
 of starvation. 
 
 After a time the protoplasm collects towards the centre 01 
 the cell and divides simultaneously into four masses arranged 
 like a pyramid of four billiard balls, three at the base and 
 one above (F). Each of these surrounds itself with a thick 
 cellulose coat and becomes a spore, the four spores being 
 sooner or later liberated by the rupture of the mother-cell 
 wall (F'). 
 
 The spores being protected by their thick cell-walls are 
 able to withstand starvation and drought for a long time ; 
 when placed under favourable circumstances they develop 
 into the ordinary form of Saccharomyces. So that repro- 
 
vii ALCOHOLIC FERMENTATION 75 
 
 duction by multiple fission appears to be, in the yeast-plant, 
 a last effort of the organism to withstand extinction. 
 
 The physiology of nutrition of Saccharomyces has been 
 studied with great care by several men of science and 
 notably by Pasteur, and is in consequence better knownthan 
 that of any other low organism. For this reason it will be 
 advisable to consider it somewhat in detail. 
 
 The first process in the manufacture of beer is the pre- 
 paration of a solution of malt called " sweet-wort." Malt 
 is barley which has been allowed to germinate or sprout, i.e., 
 the young plant is allowed to grow to a certain extent from 
 the seed. During germination the starch which forms so 
 large a portion of the grain of barley is partly converted into 
 sugar : barley also contains soluble proteids and mineral 
 salts, so that when malt is infused in hot water the sweet- 
 wort formed may be looked upon as a solution of sugar, 
 proteid, and salts. 
 
 Into this wort a quantity of yeast is placed. Very soon 
 the liquid begins to froth, the quantity of yeast increasing 
 enormously : this means of course that the yeast-cells are 
 budding actively, as can be readily made out by microscopic 
 examination. If while the frothing is going on a lighted 
 candle is lowered into the vat the flame will be immediately 
 extinguished : if an animal were placed in the same position 
 it would be suffocated. 
 
 Chemical examination shows that the extinction of the 
 candle's flame or of the animal's life is caused by a rapid 
 evolution of carbon dioxide from the fermenting wort, the 
 frothing being due to the escape of the gas from the liquid. 
 
 After a time the evolution of gas ceases, and the liquid 
 is then found to be no longer sweet but to have acquired 
 what we know as an alcoholic or spirituous flavour. Analysis 
 
76 SACCHAROMYCES 
 
 shows that the sugar has nearly or quite disappeared, while 
 a new substance, alcohol, has made its appearance. The 
 sweet-wort has, in fact, been converted into beer. 
 
 Expressed in the form of a chemical equation what has 
 happened is this : 
 
 C,.H 12 O (; 2(C 2 H 6 O) + 2(CO 2 ) 
 Grape sugar. Alcohol. Carbon dioxide. 
 
 One molecule of sugar has, by the action of yeast, been 
 split up into two molecules of alcohol which remain in the 
 fluid, and two of carbon dioxide which are given off as gas. 
 This is the process known as alcoholic fermentation. 
 
 It has been shown by accurate analysis that only about 95 
 per cent, of the sugar is thus converted into alcohol and 
 carbon dioxide : 4 per cent, is decomposed, with the for- 
 mation of glycerine, succinic acid, and carbon dioxide, and 
 i per cent, is used as nutriment by the yeast cells. 
 
 For the accurate study of fermentation the sweet-wort of 
 the brewer is unsuitable, being a fluid of complex and un- 
 certain composition, and the nature of the process, as well 
 as the part played in it by Saccharomyces, becomes much 
 clearer if we substitute the artificial wort invented by 
 M. Pasteur, and called after him Pasteur's solution. It 
 is made of the following ingredients : 
 
 Water, H 2 O 8376 per cent. 
 
 Cane sugar, C 12 R 22 O U iS' 
 
 Ammonium tartrate (NH 4 ) 2 C 4 H 4 O . i - oo ,, ,, 
 Potassium phosphate, KoPO 4 . . . . 0-20 ,, ,, 
 Calcium phosphate, Ca 3 (PO 4 ) 2 . . . 0-02 ,, 
 Magnesium sulphate, MgSO 4 . . . o'02 ,, ,, 
 
 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 Hsema- 
 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-su^ar 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 solu- 
 tion with the exception of sulphur, which, as already stated, 
 is given off during the process of burning as sulphur dioxide: 
 
 Thus the principle of construction of an artificial nutrient 
 solution such as Pasteur's is that it should contain all the 
 elements existing in the organism it is designed to support ; 
 or in other words, the substances by the combination of 
 which the waste of the organism due to destructive meta- 
 bolism may be made good. 
 
 That Pasteur's solution exactly fulfils these requirements 
 may be proved by omitting one or other of the constituents 
 from it, and finding out how the omission affects the well- 
 being of Saccharomyces. 
 
 If the sugar is left out the yeast-cells grow and multiply, 
 but with great slowness. This shows that sugar is not 
 necessary to the life of the organism, but only to that active 
 condition which accompanies fermentation. A glance at 
 the composition of Pasteur's solution will show that all the 
 necessary elements are supplied without sugar. 
 
 Omission of ammonium tartrate is fatal : without it the 
 cells neither grow nor multiply. This, of course, is just 
 what one would expect since, apart from ammonium tartrate, 
 the fluid contains no nitrogen without which the molecules of 
 protoplasm cannot be built up. 
 
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 
 well-being of the yeast-cells, and without them the organism, 
 although supplied with abundance of sugar and ammonium 
 tartrate, will not live. This may be taken as proving that 
 phosphorus, calcium, and magnesium form an integral part 
 of the protoplasm of Saccharomyces, although existing in 
 almost infinitesimal proportions. 
 
 Lastly, magnesium sulphate must not be omitted if the 
 organism is to flourish : unlike the other two mineral 
 constituents it is not absolutely essential to life, but without 
 it the vital processes are sluggish. 
 
 Thus by growing yeast in a fluid of known composition 
 it can be ascertained exactly what elements and combina- 
 tions of elements are necessary to life, what advantageous 
 though not absolutely essential, and what unnecessary. 
 
 The precise effect of the growth and multiplication of 
 yeast upon a saccharine fluid, or in other words the nature 
 of alcoholic fermentation, can be readily ascertained by a 
 simple experiment with Pasteur's solution. A quantity of 
 the solution with a little yeast is placed in a flask the neck 
 of which is fitted with a bent tube leading into a vessel of 
 lime-water or solution of calcium oxide. When the usual 
 disengagement of carbon dioxide (see p. 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 SACCHAROMYCE8 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 i.e., 
 oxygen in the form of dissolved gas is present in the fluid ; 
 from which it would seem that Saccharomyces must be able 
 to obtain the oxygen, which like all other organisms it 
 requires for its metabolic processes, from the food supplied 
 to it. 
 
 The process of fermentation goes on most actively, 
 between 28 and 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 har been proved in 
 
vii FERMENTS 81 
 
 the Mycetozoa (p. 52), and probably it or some similar pep- 
 tonizing or proteolytic ferment effects this change in all 
 organisms which have the power of digesting proteids. 
 Another instance is 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. 
 
 *%: 
 
 [v- r 
 
 UNIVERSITY 
 
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. VIII 
 
 BACTERIUM TERMO 
 
 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 01 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, 
 
 1 
 
 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 ter7no. 
 
 Seen under the high power 01 an ordinary student's 
 microscope Bacterium termo has the appearance shown 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- termo ( x 4000), showing the terminal flagella. 
 (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 /,t. The entire length of the 
 organism under consideration is from i '5 to 2 /x, i.e. about 
 the T Jo mm. or the T^TTTTT mc ^- In otner words, its entire 
 length is not more than one-fourth the diameter of a yeast- 
 cell or of a human blood-corpuscle. The diameter of the 
 flagellum has been estimated by Dallinger to be about \ //. 
 or -^-L-^jj inch, 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. 
 
vni BACILLUS 85 
 
 14 shows a Bacterium termo magnified 4000 diameters, the 
 scale above the figure representing T ^- mm. magnified to the 
 same amount. The height of this book is a little over 18 cm. ; 
 this multiplied by 4,000 gives 7 2,000 cm. = 720 metres = 2362 
 feet. We therefore get the proportion as 2362 feet, or 
 nearly six times the height of St. Paul's, is to the height of 
 the present volume, so the length of Fig. 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. 13, 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 zooglaa. 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/x (j-J-^- 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 
 zooglsea condition (Fig. 15, 4). 
 
 Bacillus is commonly found in putrescent infusions in 
 which the process of decay has gone on for some days : as 
 
86 BACTERIA LESS. 
 
 its numbers increase those of Bacterium termo diminish, 
 until Bacillus becomes the dominant form. Its cells (Fig. 
 1 6) are rod-shaped and about 6/x ( T y<) mm.) in length in the 
 commonest species. Both motionless and active forms are 
 found, the latter having a flagellum at each end. The 
 zooglaea 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 
 
 "**: 
 
 FIG. 15. Micrococcus. I, single and double (dumb-bell shaped) 
 forms : 2 and 3, chain-forms : 4, a zooglaea. 
 
 flagellum at each end. Vibriones vary from 8/x to 25^, in 
 length. 
 
 Spirillum is at once distinguished by its spiral form, the 
 cells resembling minute corkscrews (Fig. 17, B &: c) and 
 being provided with a flagellum at each end (c). The 
 smaller species, such as S. tenue (B) are from 2 to 5 /x in 
 length, but the larger forms, such as S. volutans (c) attain a 
 length of from 25 to 3o/x. In swimming Spirillum appears 
 on a superficial examination to undulate like a worm or a 
 serpent, but this is an optical illusion : the spiral is really a 
 permanent one, but during progression it rotates upon its 
 
VIII 
 
 BINARY FISSION 
 
 long axis, like Haematococcus (p. 25), and this double move- 
 ment produces the appearance of undulation. 
 
 Most Bacteria are colourless, but three species (Bacterium 
 viride, J3. chlorinum, and Bacillus virens] contain chlorophyll, 
 and several others form pigments of varying tints and often 
 of great intensity. For instance, there are red, yellow, 
 brown, blue, and violet species of Micrococcus which grow 
 
 FIG. 16. Bacilhis subtilis, showing various stages between single 
 orms and long filaments (Leptothrix). 
 
 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 simple transverse fission, the 
 process taking place sometimes during the motile, sometimes 
 during the resting condition. Frequently the daughter-cells 
 do not separate completely from one another but remain 
 
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 
 
 FIG. 17. A, Vibrio. 
 (From Klein.) 
 
 B, Spirilhtm tenue. c, S-birihum voiutans. 
 
 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 ne\v 
 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 
 
vni 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 
 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 (a?'throspores). 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 ai'e very various. Some 
 live in water, such as that of stagnant ponds, and of these 
 
 1 See especially De Bary, Fungi, Mycetozoa, and Bacteria (Oxford, 
 1887), and Klein, Micro-organisms and Disease (London, 1 886). 
 
BACTERIA 
 
 LESS. 
 
 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 
 
 FlG. 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, &c., will, however, flourish in Pasteur's solution, in 
 which they obtain their nitrogen in the form of ammonium 
 
vin BACTERIA AS FERMENTS 91 
 
 tartrate instead of decomposing proteid. It has also been 
 shown that some Bacteria can go further and make use of 
 nitrates as a source of nitrogen, 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 8 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 + O 2 = H 2 O + C 2 H 4 O 2 , 
 
 Alcohol. Oxygen. Water. Acetic Acid. 
 
 The ferment in this instance is Bacterium aceti, often 
 called My coder ma aceti, or the " vinegar plant." It will 
 be noticed that fti this case oxygen enters into the reaction : 
 it is a case of fermentation by oxidation. 
 
 Putrefaction itself is another instance 01 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^S), and ammonium 
 sulphide ( (NH 4 ) 2 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 + 3 O = H 2 O + HNO 2 
 Ammonia. Oxygen. Water. Nitrous Acid. 
 
 The nitric ferment then comes into action, converting the 
 nitrous into nitric acid 
 
 NHO 2 + O = HNO.> 
 
 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 
 p. 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 modern 
 industry, the trade in frozen meat, depends upon the fact that 
 the putrefactive Bacteria, like other organisms, are rendered 
 inactive by freezing, and every housekeeper knows how easily 
 putrefaction can be staved off by roasting or boiling. Simi- 
 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 
 ages of the dried bodies of animals in such countries as 
 Egypt and Peru depends at least as much upon the moisture- 
 less air as upon the antiseptics used in embalming. 
 
94 BACTERIA LESS, vin 
 
 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 : 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-worms in the intestines of 
 animals originate where they are found ; the belief still held 
 in some rural districts in the occurrence of showers of frogs, 
 or in the transformation of horse-hairs kept in water into 
 eels ; all indicate a survival of this belief. 
 
 Aristotle, one of the greatest men of science of antiquity, 
 explicitly teaches abiogenesis. He states that some animals 
 "spring from putrid matter," that certain insects "spring 
 from the dew which falls upon plants," that thread-worms 
 "originate in the mud of wells and running waters," that 
 fleas "originate in very small portions of corrupted matter," 
 and that " bugs proceed from the moisture which collect:* 
 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 clung ; 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 within 
 it? And the general lines upon which an investigation 
 into the problem must be conducted are simple : given a 
 vessel of any putrescible infusion ; let this be subjected to 
 some process which, without rendering it incapable of 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 putu 
 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 HeUrogcnesis, 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- 
 cellse or bell-animalcules (Fig. 26) one which appeared to 
 have become encysted upon its stalk. After watching it for 
 some time, there was seen to emerge from the cyst a free- 
 swimming ciliated Infusor called Amphileptus, not unlike a 
 long-necked Paramcecium (Fig. 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 : Euglense always giving rise to Euglense 
 and nothing else, Bacteria to Bacteria and nothing else, and 
 so on. 
 
 There are many cases which imperfect knowledge might 
 class under heterogenesis, such as the origin of frogs from 
 tadpoles or of jelly-fishes from polypes (Lesson XXIII. Fig. 
 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 i.e., leading to either a less 
 or a more highly organized form and in the course of a 
 single generation. 
 
LESSON X 
 
 PARAMGECIUM, 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 ovoidai form, moving about very quickly, and seen by 
 the use of a high power to be covered with innumerable fine 
 cilia. These are called ciliate 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," Paramczcium aurelia, which from its compara- 
 tively large size and from the ease with which all essential 
 points of its organization can be made out is a very con- 
 venient and interesting object of study. 
 
 Compared with the majority of the organisms which have 
 come under our notice it may fairly be considered as gigantic, 
 being no less than \ J mm. (200 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 
 
 FIG. 20. Paramcecium aurelia. 
 
 A, the living animal from the ventral aspect, showing the covering of 
 cilia, the buccal groove (to the right) ending posteriorly in the mouth 
 
LESS, x MOVEMENTS 109 
 
 (mth} and gullet (gut] ; several food vacuoles (/. vac), an'd the two 
 contractile vacuoles (c. vac}. 
 
 B, the same in optical section, showing cuticle (at}, cortex (cort], and 
 medulla (med) ; buccal groove (buc. gr}, mouth, and gullet (gul) ; 
 numerous food vacuoles (f. vac) circulating in the direction indicated 
 by the arrows, and containing particles of indigo, which are finally 
 ejected at an anal spot ; meganucleus (nu), micronucleus (pa. nu), 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. nu} 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 PARAMCECIUM, 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 Paramoecium 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, 
 p. 62), consisting of a delicate granular network the meshes 
 of which are filled with a transparent material. In the 
 cortex the meshes of the network are closer, and so form a 
 comparatively dense substance. The cortex also exhibits 
 a superficial oblique striation, forming what is called the 
 myophan layer. 
 
 The mouth (mtti) leads into a short funnel-like tube, the 
 gullet (gul), 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 micronudeus : it 
 is to be considered as a second, smaller nucleus, the larger 
 body being distinguished as the meganudeus. 
 
 There are two contractile vacuoles (c. vac), one situated at 
 about a third of the entire length from the anterior end of the 
 body, the other at about the same distance from the posterior 
 end : they occur in the cortex. 
 
 The action of the contractile vacuoles is very beautifully 
 seen in a Paramoecium at rest : it is particularly striking in a 
 specimen subjected to slight pressure under a cover glass, 
 but is perfectly visible in one which has merely temporarily 
 suspended its active swimming movements. It is 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 
 
H2 PARAMCECIUM, STYLONYCHlA, 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 Paramcecium by placing in the water some finely-divided 
 carmine or indigo. When the creature comes into the 
 neighbourhood of the coloured particles, the latter are swept 
 about in various directions by the action of the cilia : some 
 of these are however certain to be swept into the neighbour- 
 hood of the buccal groove and gullet, the cilia of which all 
 work downwards, i.e. towards the inner end of the gullet. 
 The grains of carmine are thus carried into the gullet, where 
 for an instant they lie surrounded by the water of which it is 
 full : then, instantaneously, probably by the contraction of 
 the tube itself, the animalcule performs a sort of gulp, and 
 the grains with an enveloping globule of water or food-vacuole 
 are forced into the medullary protoplasm. This process is 
 repeated again and again, so that in any well-nourished 
 Paramcecium 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, trcJi) closely arranged in a single layer and perpen- 
 dicular to the surface. These are called trichocysts. 
 
 When a Paramoecium is killed, either by the addition of 
 osmic acid or some other poisonous reagent or by simple 
 pressure of the cover glass, it frequently assumes a remark- 
 able appearance. Long delicate threads suddenly appear, 
 projecting from its surface in all directions (c) and looking 
 very much as if the cilia had suddenly protruded to many 
 times their original length. But these filaments have really 
 nothing to do with the cilia ; they are contained under 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 (trcJi) 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). 
 
 Paramoecium 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. 2 1, 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. nu', mi. nu") of the remaining two one is now distin- 
 guished as the active pronudeus, the other as the stationary 
 pronudeus. 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.nu. 
 
 FlG. 21. Stages in the Conjugation of Paramcecium. 
 
 A, Commencement of conjugation : the meganuclei (mg. mi) of the 
 two gametes are almost unaltered : the micronuclei (mi. nu) are in an 
 early stage of karyokinesis. 
 
 B, The micronuclei have divided twice, each gamete now containing 
 four. 
 
 C, Two of the micronuclei (mi. nu', mi. nu")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. nu) 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. nu) 
 are enlarging to form mega-nuclei, the other two (Mi.nu) are taking on 
 the characters of micronuclei. 
 
 (After H'ortwig.) 
 
 I 2 
 
ii6 PARAMGECTUM, STYLONYCHIA, OXYTRTCHA 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 y^rnm. 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 
 
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 Paramcecium in general structure (compare 
 
 FIG. 22. A, Stylonychia mytihis, ventral aspect, showing the buccal 
 groove (buc. gr.} and mouth (mt/i), two nuclei (mi, nu), contractile 
 vacuole (c.vac), and cilia differentiated into hook-like (h. '), bristle- 
 like (b. ci), plate-like ( p. ci), and fan-like (in. ci) organs. 
 
 B, one of the plate-like cilia of the same (/. 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 (h. ci), and the various cilia of the buccal groove, including an 
 expanded fan-like organ (m. 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 (//, nu) and only 
 one contractile vacuole (c. vac]. 
 
ii8 PARAMCECIUM, STYLONYCHIA, OXYTRICHA LESS. 
 
 But it is in the character of its cilia that Stylonychia 
 is most markedly distinguished from Paramoecium : these 
 structures, instead of being all alike both in form and size, 
 are modified in a very extraordinary way. 
 
 On the dorsal surface the cilia are represented only by 
 very minute processes of the cortex (c, d. ci.) set in longi- 
 tudinal grooves and exhibiting little movement. It seems 
 probable that these are to be looked upon as vestigial or 
 rudimentary cilia, i.e., as the representatives of cilia which 
 were of the ordinary character in the ancestors of Stylo- 
 nychia, but which have undergone partial atrophy, or 
 diminution beyond the limits of usefulness, in correspond- 
 ence with the needs of an animalcule which has taken to 
 creeping on its ventral surface, instead of swimming freely 
 and so using all its cilia equally. 
 
 On the other hand, the cilia on the ventral surface have 
 undergone a corresponding enlargement or hypertrophy. 
 Near the anterior and posterior ends and about the middle are 
 three groups of cilia of comparatively immense size, shaped 
 either like hooks (h. ci.), or like flattened rods frayed at 
 their ends (p. a, and B). All these structures neither vibrate 
 rhythmically like ordinary cilia nor perform lashing move- 
 ments like flagella, but move at the base only like one- 
 jointed legs. The movement is under the animal's control, 
 so that it is able to creep about by the aid of these hooks 
 and plates in much the same way as a caterpillar by means 
 of its legs. 
 
 Notice that we have here a third form of contractility : in 
 amoeboid movement there is an irregular flowing of the pro- 
 toplasm (pp. 4 and 10) ; in ciliary movement a flexion of 
 a protoplasmic filament from side to side (p. 33) ; while 
 in the present case we have sudden contractions taking place 
 at irregular intervals. The movements of these locomotor 
 hooks and plates are therefore very similar to the muscular 
 
x DIFFERENTIATION OF CILIA 119 
 
 contractions to which the movements of the higher animals 
 are due : it cannot be said that definite muscles are present 
 in Stylonychia, but the protoplasm in certain regions of the 
 unicellular body is so modified as to be able to perform a 
 sudden contraction in a definite direction. The nature of 
 muscular contraction will be further discussed in the next 
 Lesson (see p. 130). 
 
 The remainder of the ventral surface, with the exception 
 of the buccal groove, is bare, but along each side of the 
 margin is a row of large vibratile cilia, of which three at 
 the posterior end are modified into long, stiff, bristle-like 
 processes (A, b. a). 
 
 There is also a special differentiation of the cilia of the 
 buccal groove (buc.gr.']. On its left side is a single row of 
 very large and powerful cilia (A and c, m. cf] which are the 
 chief organs for causing the food-current as well as the 
 main swimming-organs : each has the form of a triangular 
 fan-like plate (c, m. a). On the right side of the buccal 
 groove is a row of smaller but still large cilia of the ordinary 
 form, and in the interior of the gullet a row of extremely 
 delicate cilia which aid in forcing particles of food down the 
 gullet into the medulla. 
 
 In Stylonychia and allied genera intermediate forms are 
 found between these peculiar hooks, plates, bristles, and 
 fans, and ordinary cilia ; from which we may conclude that 
 these diverse appendages are to be looked upon as highly 
 modified or differentiated cilia. Probably they have been 
 evolved in the course of time from ordinary cilia, and on 
 the principle that the more complicated or specialized 
 organisms are descended from simpler or more generalized 
 forms (see Lesson XIII.), we may consider Stylonychia as 
 the highly-specialized descendant of some uniformly-ciliated 
 progenitor. 
 
120 PARAMCECIUM, STYLONYCHIA, OXYTRICHA LESS.X 
 
 A third genus of ciliated Infusoria must be just referred 
 to in concluding the present Lesson. We have seen how 
 the nucleus of a Paramoecium which has just conjugated 
 breaks up and apparently disappears (Fig. 21, K o). 
 In Oxytricha, a genus closely resembling Stylonychia, the 
 two nuclei have been found to break up into a large number 
 of minute granules (Fig. 23), which can be seen only after 
 
 FIG. 23. Oxytricha flava, killed and stained, showing the frag- 
 mentation of the nuclei. (After Gruber. ) 
 
 careful staining and by the use of high magnifying powers. 
 This process is called fragmentation of the nucleus; in 
 other cases it goes even further, and the nucleus is reduced 
 to an almost infinite number of chromatin granules only just 
 visible under the highest powers. From this it seems very 
 probable that organisms which, like Protamceba (p. 9) and 
 Protomyxa (p. 49), appear non-nucleate, are actually pro- 
 vided with a nucleus in this pulverized condition, and that 
 a nucleus in some form or other is an essential constituent 
 of the cell. 
 
UNIVERSITY 
 
 LESSON XI 
 
 OPALINA 
 
 THE large intestine of the common frog often contains 
 numbers of ciliate Infusoria belonging to two or three 
 genera. One of these parasitic animalcules, called Opalina 
 ranarum, will now be described. It is easily obtained by 
 killing a frog, opening the body, making an incision in the 
 rectum, and spreading out a little of its blackish contents in 
 a drop of water on a slide. 
 
 Opalina has a flattened body with an oval outline (Fig. 
 24, A, B), and full-sized specimens may be as much as one 
 millimetre in length. The protoplasm is divided into cortex 
 and medulla, and is covered with a cuticle, and the cilia are 
 equal-sized and uniformly arranged in longitudinal rows over 
 the whole surface (A). 
 
 On a first examination no nucleus is apparent, but after 
 staining a large number of nuclei can be seen (B, ;/#), each 
 being a globular body (c, i), consisting of a nuclear matrix 
 surrounded by a membrane and containing a coil or net- 
 work of chromatin. These nuclei multiply within the body 
 of the infusor, and in so doing pass through the various 
 changes characteristic of karyokinesis or indirect nuclear 
 
122 OPALINA LESS. 
 
 division (compare Fig. 10, p. 64, with Fig. 23) : the 
 
 FlG. 24. Opalina ranarum. 
 
 A, living specimen, surface view, showing longitudinal rows of cilia. 
 
 B, the same, stained, showing numerous nuclei (nu) in various stages 
 of division. 
 
 c, I 6, stages in nuclear division. 
 
 D, longitudinal fission. 
 
 E, transverse fission. 
 
 F, the same in a specimen reduced in size by repeated division. 
 
 G, final product of successive divisions. 
 H, encysted form. 
 
 i, uninucleate form produced fi'om cyst. 
 
 K, the same after multiplication of the nucleus has begun. 
 
 (A C, after Pfitzner ; D K, from Saville Kent after Zeller.) 
 
 chromatin breaks up (c, 2), a spindle is formed with the 
 chromosomes across its equator (3), the chromosomes pass 
 
XJ PARASITISM 123 
 
 to the poles of the spindle (4, 5), and the nucleus becomes 
 constricted (5), and finally divides into two (6). 
 
 The presence of numerous nuclei in Opalina is a fact 
 worthy of special notice. The majority of the organisms 
 we have studied are uninucleate as well as unicellular : the 
 higher animals and plants we found (Lesson VI.) to consist 
 of numerous cells each with a nucleus, so that they are 
 multicellular and multinucleate : Opalina, on the other 
 hand, is multinucleate but unicellular. An approach to 
 this condition of things is furnished by Stylonychia, which is 
 unicellular and binucleate (Fig. 24, A), but the only organisms 
 we have yet studied in which numerous nuclei of the ordi- 
 nary character occur in an undivided mass of protoplasm are 
 the Mycetozoa (p. 52), and in them the multinucleate con- 
 dition of the plasmodium is largely due to its being formed 
 by the fusion of separate cells, while in Opalina it is due, as 
 we shall see, to the repeated binary fission of an originally 
 single nucleus. 
 
 There is no contractile vacuole, and no trace of either 
 mouth or gullet, so that the ingestion of solid food is impos- 
 sible. The creature lives, as already stated, in the intestine 
 of the frog : it is therefore an internal parasite, or endo- 
 parasite, having the frog as its host. The intestine contains 
 the partially-digested food of the frog, and it is by the ab- 
 sorption of this that the Opalina is nourished. Having no 
 mouth, it feeds solely by imbibition : whether it performs 
 any kind of digestive process itself is not certainly known, 
 but the analogy of other mouthless parasites leads us to 
 expect that it simply absorbs food ready digested by its host, 
 upon which it is dependent for a constant supply of soluble 
 and diffusible nutriment. 
 
 Thus Opalina, in virtue of its parasitic mode of life, is 
 saved the performance of certain work the work of diges- 
 
124 OPALINA LESS. 
 
 tion, that work being done for it by its host. This is the 
 essence of internal parasitism : an organism exchanges a free 
 life, burdened with the necessity of finding food for itself, for 
 existence in the interior of another organism, on which, in 
 one way or another, it levies blackmail. 
 
 Note the close analogy between the nutrition of an internal 
 parasite like Opalina and the saprophytic nutrition of a 
 monad (p. 39). In both the organism absorbs proteids 
 rendered soluble and diffusible, in the one case by the 
 digestive juices of the host, in the other by the action of 
 putrefactive bacteria. 
 
 The reproduction of Opalina presents certain points of 
 interest, largely connected with its peculiar mode of life. It 
 is obvious that if the Opalinae simply went on multiplying, 
 by fission or otherwise, in the frog's intestine, the population 
 would soon outgrow the means of subsistence : moreover, 
 when the frog died there would be an end of the parasites. 
 What is wanted in this as in other internal parasites is some 
 mode of multiplication which shall serve as a means of dis- 
 persal^ or in other words, enable the progeny of the parasite 
 to find their way into the bodies of other hosts, and so start 
 new colonies instead of remaining to impoverish the mother 
 country. 
 
 Opalina multiplies by a somewhat peculiar process of 
 binary fission : an animalcule divides in an oblique direction 
 (Fig. 24, D), and then each half, instead of growing to the 
 size of the parent cell, divides again transversely (E). The 
 process is repeated again and again (F), the plane of division 
 being alternately oblique and transverse, until finally small 
 bodies are produced (G), about %$$ mm - i n length, and 
 containing two to four nuclei. 
 
 If the parent cell had divided simultaneously into a num- 
 
xi DEVELOPMENT 125 
 
 her of these little bodies the process would have been one of 
 multiple fission : as it is it forms an interesting link between 
 simple and multiple fission. 
 
 Opalina ranarum multiplies in this way in the spring i.e. 
 during the frog's breeding season. Each of the small pro- 
 ducts of division (G) becomes encysted (H), and in this 
 passive condition is passed out with the frog's excrement, 
 probably falling on to a water-weed or other aquatic object. 
 Nothing further takes place unless the cyst is swallowed by 
 a tadpole, as must frequently happen when these creatures, 
 produced in immense numbers from the frogs' eggs, browse 
 upon the water-weeds which form their chief food. 
 
 Taken into the tadpole's intestine, the cyst is burst or 
 dissolved, and its contents emerge as a lanceolate mass of 
 protoplasm (i), containing a single nucleus and covered with 
 cilia. This, as it absorbs the digested food in the intestine 
 of its host, grows, and at the same time its nucleus divides 
 repeatedly (K) in the way already described, until by the time 
 the animalcule has attained the maximum size it has also 
 acquired the large number of nuclei characteristic of the 
 genus. 
 
 Here, then, we have another interesting case of develop- 
 ment (see p. 43) : the organism begins life as a very small 
 uninucleate mass of protoplasm, and as it increases in size 
 increases also in complexity by the repeated binary fission 
 of its nucleus. 
 
LESSON XII 
 
 VORTICELLA AND ZOOTHAMNIUM 
 
 THE next organism we have to consider is a ciliated infusor 
 even commoner than those described in the two previous 
 lessons. It is hardly possible to examine the water of a 
 pond with any care without finding in it, sometimes attached 
 to weeds, sometimes to the legs of water-fleas, sometimes to 
 the sticks and stones of the bottom, numbers of exquisitely 
 beautiful little creatures, each like an inverted bell with a 
 very long handle, or a wine-glass with a very long stem. 
 These are the well-known "bell-animalcules;" the com- 
 monest among them belong to various species of the genus 
 Vorticflla. 
 
 The first thing that strikes one about Vorticella 
 (Fig. 25, A) is the fact that it is permanently fixed, 
 like a plant, the proximal or near end of the stalk 
 being always firmly fixed to some aquatic object, while to 
 the distal or far end the body proper of the animalcule is 
 attached. 
 
 But in spite of its peculiar form it presents certain very 
 obvious points of resemblance to Paramoecium, Stylonychia, 
 and Opalina. The protoplasm is divided into cortex ( Fig. 
 25, c, corf) and medulla (med), and is invested with a 
 
FlG. 25. Vorticella. 
 
 A, living specimen fully expanded, showing stalk (st) with axial fibre 
 (ax. f.), peristome (per), disc (d), mouth (mth}, gullet (gull], and 
 contractile vacuole. 
 
 B, the same, bent on its stalk and with the disc turned away from 
 the observer. 
 
 c, optical section of the same, showing cuticle (cu\ cortex (corf), 
 medulla (med), nucleus (), gullet (gull), several food-vacuoles, and 
 anus (an), as well as the structures shown in A. 
 
 D 1 , a half-retracted and D 2 a fully-retracted specimen, showing the 
 coiling of the stalk and overlapping of the disc by the peristome. 
 
128 VORTICELLA AND ZOOTHAMNIUM LESS. 
 
 E 1 , commencement of binary fission ; E 2 , completion of the process ; 
 E 3 , the barrel-shaped product of division swimming freely in the 
 direction indicated by the arrow. 
 
 F 1 , a specimen dividing into a megazooid and several microzooids (m) ; 
 F' 2 , division into one mega- and one microzooid. 
 
 G 1 , G 2 , two stages in conjugation showing the gradual absorption of 
 the microgamete (m) into the megagamete. 
 
 H 1 , multiple fission of encysted form, the nucleus dividing into nume- 
 rous masses : H 2 , spore formed by multiple fission ; H 3 H 7 , development 
 of the spore ; H 4 is undergoing binary fission. 
 
 (E H after Saville Kent.) 
 
 delicate cuticle (cu). There is a single contractile vacuole 
 (c. vac] the movements of which are very readily made out 
 owing to the ease with which the attached organism is kept 
 under observation. There is a meganucleus (nu) remarkable 
 for its elongated band-like form, and having in its neighbour- 
 hood a small rounded micronucleus. Cilia are also present, 
 but the way in which they are disposed is very peculiar and 
 characteristic. To understand it we must study the form 
 of the body a little more closely. 
 
 The conical body is attached by its apex or proximal end 
 to the stalk : its base or distal end is expanded so as to form 
 a thickened rim, the peristome (per), within which is a plate- 
 like body elevated on one side, called the disc (d), and 
 looking like the partly raised lid of a chalice. Between the 
 raised side of the disc and the peristome is a depression, the 
 mouth (mtti), leading into a conical gullet (gull). 
 
 There is reason for thinking that the whole proximal 
 region of Vorticella answers to the ventral surface of Para- 
 mcecium, and its distal surface with the peristome and 
 disc to the dorsal surface of the free-swimming genus : the 
 mouth is to the left in both. 
 
 A single row of cilia is disposed round the inner border 
 of the peristome, and continued on the one hand down the 
 gullet, and on the other round the elevated portion of the 
 
xii AXIAL FIBRE 129 
 
 disc ; the whole row of cilia thus takes a spiral direction. 
 The rest of the body is completely bare of cilia. 
 
 The movements of the cilia produce a very curious 
 optical illusion : as one watches a fully-expanded specimen 
 it is hardly possible to believe that the peristome and disc 
 are not actually revolving a state of things which would 
 imply that they were discontinuous from the rest of the 
 body. As a matter of fact the appearance is due to the 
 successive contraction of all the cilia in the same direction, 
 and is analogous to that produced by a strong wind on a 
 field of corn or long grass. The bending down of suc- 
 cessive blades of grass produces a series of waves travelling 
 across the field in the direction of the wind. If instead of 
 a field we had a large circle of grass, and if this were acted 
 upon by a cyclone, the wave would travel round the circle, 
 which would then appear to revolve. 
 
 Naturally the movement of the circlet of cilia produces a 
 small whirlpool in the neighbourhood of the Vorticella, as 
 can be seen by introducing finely-powdered carmine into 
 the water. It is through the agency of this whirlpool that 
 food particles are swept into the mouth, surrounded, as in 
 Paramcecium, by a globule of water : the food-vacuoles 
 (/ vac) thus constituted circulate in the medullary proto- 
 plasm, and the non-nutritive parts are finally egested at an 
 anal spot (an) situated near the base of the gullet. 
 
 The stalk (st) consists of a very delicate, transparent, 
 outer substance, which is continuous with the cuticle of the 
 body and contains a delicate axial fibre (ax.f.) running 
 along it from end to end in a somewhat spiral direction. 
 This fibre is a prolongation of the cortex of the body 
 (c, ax.f.) : under a very high power it appears granular or 
 delicately striated, the striae being continued into the cortex 
 of the proximal part of the body. 
 
130 VORTICELLA AND ZOOTHAMNIUM LESS. 
 
 A striking characteristic of Vorticella is its extreme 
 irritability, i.e., the readiness with which it responds to any 
 external stimulus (see p. 10). The slightest jar of the 
 microscope, the contact of some other organism, or even a 
 current of water produced by some free-swimming form like 
 Paramcecium, is felt directly by the bell-animalcule and is 
 followed by an instantaneous change in the relative position 
 of its parts. The stalk becomes coiled into a close spiral 
 (D 1 , D 2 ) so as to have a mere fraction of its original length, 
 and the body from being bell-shaped becomes globular, the 
 disc being withdrawn and the peristome closed over it 
 (Di, D*). ! 
 
 The coiling of the stalk leads us to the consideration of 
 the particular form of contractility called muscular, which 
 we have already met with in Stylonychia (p. 116). It was 
 mentioned above that while the stalk in its fully expanded 
 condition is straight, the axial fibre is not straight, but forms 
 a very open spiral, i.e., it does not lie in the centre of 
 the stalk but at any transverse section is nearer the surface 
 at one spot than elsewhere, and this point as we ascend the 
 stalk is directed successively to all points of the compass. 
 
 Now suppose that the axial fibre undergoes a sudden con- 
 traction, that is to say, a decrease in length accompanied by 
 an increase in diameter, since as we have already seen 
 (p. 10) there is no decrease in volume in protoplasmic 
 contraction. There will naturally follow a corresponding 
 shortening of the elastic cuticular substance which forms the 
 outer layer of the stalk. If the axial fibre were entirely 
 towards one side of the stalk, the result of the contraction 
 would be a flexure of the stalk towards that side, but, as its 
 direction is spiral, the stalk is bent successively in every 
 direction, that is, is thrown into a close spiral coil. 
 
 The axial fibre is therefore a portion of the protoplasm 
 
xii FISSION 131 
 
 which possesses the property of contractility in a special de- 
 gree ; in which moreover contraction takes place in a definite 
 direction the direction of the length of the fibre so that 
 its inevitable result is to shorten the fibre and consequently 
 to bring its two ends nearer together. This is the essential 
 characteristic of a muscular contraction, and the axial fibre 
 in the stalk of Vorticella is therefore to be looked upon as 
 the first instance of a clearly differentiated muscle which has 
 come under our notice. 
 
 There are some interesting features in the reproduction of 
 Vorticella. It multiplies by binary fission, dividing through 
 the long axis of the body (Fig. 25, E 1 , E 2 ). Hence it is 
 generally said that fission is longitudinal, not transverse, as 
 in Paramcecium. But on the theory (p. 107) that the peris- 
 tome and disc are dorsal and the attached end ventral, 
 fission is really transverse in this case also. 
 
 It will be seen from the figures that the process takes place 
 by a cleft appearing at the distal end (E 1 ), and gradually 
 deepening until there are produced two complete and full- 
 sized individuals upon a single stalk (E 2 ). This state of 
 things does not last long : one of the two daughter-cells takes 
 on a nearly cylindrical form, keeps its disc and peristome 
 retracted, and acquires a new circlet of cilia near its proximal 
 end (E S ) : it then detaches itself from the stalk, which it 
 leaves in the sole possession of its sister-cell, and swims about 
 freely for a time in the direction indicated by the arrow. 
 Sooner or later it settles down, becomes attached by its 
 proximal end, loses its basal circlet of cilia, and develops a 
 stalk, which ultimately attains the normal length. 
 
 The object of this arrangement is obvious. If when a 
 Vorticella divided, the plane of fission extended down the 
 stalk until two ordinary fixed forms were produced side by 
 side, the constant repetition of the process would so increase 
 
 K 2 
 
132 VORT1CELLA AND ZOOTHAMNIUM LESS. 
 
 the numbers of the species in a given spot that the food- 
 supply would inevitably run short. This is prevented by 
 one of the two sister-cells produced by fission leading a free 
 existence long enough to enable it to emigrate and settle in 
 a new locality, where the competition with its fellows will be 
 less keen. The production of these free-swimming zooids 
 is therefore a means of dispersal (see*p. 122) : contrivances 
 having this object in view are a very general characteristic 
 of fixed as of parasitic organisms. 
 
 Conjugation occasionally takes place, and presents certain 
 peculiarities. A Vorticella divides either into two unequal 
 halves (r 2 ) or into two equal halves, one of which divides 
 again into from two to eight daughter-cells (r 1 ). There are 
 thus produced from one to eight microzooids which resemble 
 the barrel-shaped form (E S ) in all but size, and like it become 
 detached and swim freely by means of a basal circlet of cilia. 
 After swimming about for a time, one of these microzooids 
 comes in contact with an ordinary form or megazooid, when 
 it attaches itself to it near the proximal end (c 1 ), and under- 
 goes gradual absorption (c 2 ), the mega- and microzooids 
 becoming completely and permanently fused. As in Para- 
 mcecium, conjugation is followed by increased activity in 
 feeding and dividing (p. 113). 
 
 Notice that in this case the conjugating bodies or gametes 
 are not of equal size and similar characters, but one, which 
 is conveniently distinguished as the microgamete ( = micro- 
 zooid) is relatively small and active, while the other or 
 megagamete ( = megazooid, or ordinary individual) is rela- 
 tively large and passive. As we shall see in a later lesson, 
 this differentiation of the gametes is precisely what we get in 
 almost all organisms with two sexes : the microgamete being 
 the male, the megagamete the female conjugating body (see 
 Lesson XVI.). 
 
xii METAMORPHOSIS 133 
 
 The result of conjugation is strikingly different in the three 
 cases already studied : in Heteromita (p. 41) the two gametes 
 unite to form a zygote, a motionless body provided with a 
 cell-wall, the protoplasm of which divides into spores : in 
 Paramcecium (p. 113) no zygote is formed, conjugation being 
 a mere temporary union : in Vorticella the zygote is an 
 actively moving and feeding body, indistinguishable from an 
 ordinary individual of the species. 
 
 Vorticella sometimes encysts itself (Fig. 25, H 1 ), and the 
 nucleus of the encysted cell has been observed to break up 
 into a number of separate masses, each doubtless surrounded 
 by a layer of protoplasm. After a time the cyst bursts, and 
 a number of small bodies or spores (n 2 ) emerge from it, each 
 containing one of the products of division of the nucleus. 
 These acquire a circlet of cilia (H S ), by means of which they 
 swim freely, and they are sometimes found to multiply by 
 simple fission (H 4 ). Finally, they settle down (n 5 ) by the 
 end at which the cilia are situated, the attached end begins 
 to elongate into a stalk (H G ), this increases in length, the 
 basal circlet of cilia is lost, and a ciliated peristome and 
 disc are formed at the free end (a 7 ). In this way the 
 ordinary form is assumed by a process of development 
 recalling what we found to occur in Heteromita (p. 42), but 
 with an important difference : the free-swimming young of 
 Vorticella (n 3 ), to which the spores formed by division of 
 the encysted protoplasm give rise, differ strikingly in form 
 and habits from the adult. This is expressed by saying 
 that development is in this case accompanied by a meta- 
 morphosis, this word, literally meaning simply a change, being 
 always used in biology to express a striking and fundamental 
 difference in form and habit between the young and the 
 adult ; as, for instance, between the tadpole and the frog, 
 or between the caterpillar and the butterfly. It is obvious 
 
134 
 
 VORTICELLA AND ZOOTHAMNtUM 
 
 LESS. 
 
 that in the present instance metamorphosis is another means 
 of ensuring dispersal. 
 
 In Vorticella, as we have seen, fission results not in the 
 
 FIG. 26. Zoothamnium arbuscula. 
 
 A, entire colony, magnified, showing nutritive (n. z) and reproductive 
 (r. 0) zooids ; ax. f axial fibre of the stem. 
 
 B, the same, natural size. 
 
 C, the same, magnified, in the condition of retraction. 
 
 D, nutritive zooid, showing nucleus () contractile vacuole (c. vac), 
 gullet, and axial fibre (ax.f). 
 
 E, reproductive zooid, showing nucleus (mi} and contractile vacuole 
 (c. vac), and absence of mouth and gullet. 
 
 F 1 , F 2 , two stages in the development of the reproductive zooid. 
 (After Saville Kent.) 
 
 production of equal and similar daughter-cells, but of one 
 stalked and one free-swimming form. It is however quite 
 possible to conceive of a Vorticella-like organism in which 
 the parent cell divides into two equal and similar products, 
 each retaining its connection with the stalk. If this process 
 were repeated again and again, and if, further, the plane of 
 
xn DIMORPHISM 135 
 
 fission were extended downwards so as to include the distal 
 end of the stalk, the result would be a branched, tree-like 
 stem with a Vorticella-like body at the end of every branch. 
 
 As a matter of fact, this process takes place not in Vorti- 
 cella itself, but in a nearly allied infusor, the beautiful 
 Zoothamnium, a common genus found mostly in sea-water 
 attached to weeds and other objects. 
 
 Zoothamnium arbuscula (Fig. 26, A) consists of a main 
 stem attached by its proximal end and giving off at its distal 
 end several branches, on each of which numerous shortly- 
 stalked bell-animalcules are borne, like foxgloves or Canter- 
 bury-bells on their stem. The entire tree is about i cm. 
 high, and so can be easily seen by the naked eye : it is shown 
 of the natural size in Fig. 26, B. 
 
 We see, then, that Zoothamnium differs from all our 
 previous types in being a compound organism. The entire 
 "tree" is called a colony or stock, and each separate 
 bell-animalcule borne thereon is an individual or zooid, 
 morphologically equivalent to a single Vorticella or 
 Paramcecium. 
 
 As in Vorticella, the stem consists of a cuticular sheath 
 with an axial muscle-fibre (ax. /), which, at the distal end 
 of the main stem, branches like the stem itself, a prolonga- 
 tion of it being traceable to each zooid (D). So that the 
 muscular system is common to the whole colony, and any 
 shock causes a general contraction, the tree-like structure 
 assuming an almost globular form (c). 
 
 It will be noticed from the figure that all the zooids of 
 the colony are not alike : the majority are bell-shaped and 
 resemble Vorticellse (A, n. z, and D), but here and there are 
 found larger bodies (A, r. z, and E) of a globular form, with- 
 out mouth, peristome, or disc, and with a basal circlet of 
 cilia. The characteristic band-like nucleus (mi) and the 
 
136 VORTICELLA AND ZOOTHAMNIUM LESS, xn 
 
 contractile vacuole (c. vac) are found in both the bell-shaped 
 and the globular zooids. 
 
 It is to these globular, mouthless zooids that the functions 
 of reproducing the whole colony and of ensuring dispersal 
 are assigned. They become detached, swim about freely 
 for a time, then settle down, develop a stalk and mouth 
 (r 1 , F 2 ), and finally, by repeated fission, give rise to the 
 adult, tree-like colony. 
 
 The Zoothamnium colony is thus dimorphic, bearing indi- 
 viduals of two kinds : nutritive zooids, which feed and add 
 to the colony by fission but are unable to give rise to a new 
 colony, and reproductive zooids, which do not feed while 
 attached, but are capable, after a period of free existence, of 
 developing a mouth and stalk, and finally producing a new 
 colony. Dimorphism is a differentiation of the individuals 
 of a colony, just as the formation of axial fibre, gullet, con- 
 tractile vacuole, and cilia are cases of differentiation of the 
 protoplasm of a single cell. 
 
LESSON XIII 
 
 SPECIES AND THEIR ORIGIN THE PRINCIPLES OF 
 
 CLASSIFICATION 
 
 MORE than once in the course of the foregoing lessons we 
 have had occasion to use the word species for instance, in 
 Lesson I. (p. 8) it was stated that there were different 
 kinds or species of Amoebae, distinguished by the characters 
 of their pseudopods, the structure of their nuclei, &c. 
 
 We must now consider a little more in detail what we 
 mean by a species, and, as in all matters of this sort, the 
 study of concrete examples is the best aid to the formation 
 of clear conceptions, we will take, by way of illustration, 
 some of the various species of Zoothamnium. 
 
 The kind described in the previous lesson is called 
 Zoothamnium arbuscula. As Fig. 26, A, shows, it consists of a 
 tolerably stout main stem, from the distal end of which 
 spring a number of slender branches diverging in a brush- 
 like manner, and bearing on short secondary branchlets the 
 separate individuals of the colony : these are of two kinds, 
 bell-shaped nutritive zooids, and globular reproductive 
 zooids, so that the colony is dimorphic. 
 
 Zoothamnium (or, for the sake of brevity, Z.) alternans 
 (Fig. 27, A) is found also in sea-water, and differs markedly 
 
SPECIES AND THEIR ORIGIN 
 
 LESS. 
 
 from Z. arbuscula in the general form of the colony. The 
 main stem is continued to the extreme distal end of the 
 colony and terminates in a zooid ; from it branches are 
 given off right and left, and on these the remaining zooids 
 are borne. To use Mr. Saville Kent's comparison, Z. arbus- 
 
 FIG. 27. Species of Zoothamnium. A, Z. alternans. B, Z. 
 dichotonmm. C, Z. simplex. D, Z. affine. E, Z. nntans. (After 
 Saville Kent.) 
 
 cula may be compared to a standard fruit tree, Z. alternans 
 to an espalier. In this species also the colony is dimorphic. 
 
 Z. dichotomum (Fig. 27, B) is also dimorphic and presents a 
 third mode of branching. The main stem divides into two, 
 and each of the secondary branches does the same, so that 
 a repeatedly forking stem is produced. The branching of 
 this species is said to be dichotomous, while that of Z. alter- 
 nans is monopodial, and that of Z. arbuscula umbellate. 
 
 Another mode of aggregation of the zooids is found in Z. 
 simplex (Fig. 27, c) in which the stem is unbranched and 
 
xni GENUS AND SPECIES 139 
 
 bears at its distal end about six zooids in a cluster. The 
 zooids are more elongated than in any of the preceding 
 species, and there are no special reproductive individuals, so 
 that the colony is homomorphic. 
 
 In Z. affine (Fig. 27, D) the stalk is dichotomous but is 
 proportionally thicker than in the preceding species, and 
 bears about four zooids, all alike. It is found in fresh water 
 attached to insects and other aquatic animals. 
 
 The last species we shall consider is Z. nutans (Fig. 27, E), 
 which is the simplest known, never bearing more than two 
 zooids, and sometimes only one. 
 
 A glance at Figs. 26 and 27 will show that these six species 
 agree with one another in the general form of the zooids, in 
 the characters of the nucleus, contractile vacuole, &c., in 
 the arrangement of the cilia, and in the fact that they are all 
 compound organisms, consisting of two or more zooids 
 attached to a common stem, the axial fibre of which branches 
 with it, i.e., is continuous throughout the colony. 
 
 On account of their possessing these important characters 
 in common, the species described are placed in the single 
 genus Zoothamnium, and the characters summarized in the 
 preceding paragraph are called generic characters. On the 
 other hand the points of difference between the various 
 species, such as the forking of the stem in Z. dichotomum, 
 the presence of only two zooids in Z. nutans, and so on, are 
 called specific characters. Similarly the name Zoothamnium, 
 which is common to all the species, is the generic name, 
 while those which are applied only to a particular species, 
 such as arbuscula, simplex, &c., are the specific names. As 
 was mentioned in the first lesson (p. 8), this method of 
 naming organisms is known as the Linnean system of 
 binomial nomenclature. 
 
 It will be seen from the foregoing account that by a 
 
140 SPECIES AND THEIR ORIGIN LESS. 
 
 species we understand an assemblage of individual or- 
 ganisms, whether simple or compound, which agree with one 
 another in all but unessential points, such as the precise 
 number of zooids in Zoothamnium, which may vary con- 
 siderably in the same species, and come, therefore, within 
 the limits of individual variation. Similarly, what we mean 
 by a genus is a group of species agreeing with one another 
 in the broad features of their organization, but differing in 
 detail, the differences being constant. 
 
 A comparison of the six species described brings out 
 several interesting relations between them. For instance, it 
 is clear that Z. arbuscula and Z. alternans are far more 
 complex /.<?., 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, afrlne, 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 
 
xiir 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 ofjlescent from a single indi- 
 
 
U2 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 
 
 yV 
 
 Z.arbuscu/a Z.alternans Z.dichotomum Z. simplex Z.affint Z.nutans 
 
 y 
 
 Ancestral Individuals 
 
 FIG. 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, c., 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 we get no absolute 
 criterion of classification : we may arrange our organisms, 
 as nearly as our knowledge allows, according to their resem- 
 blances and. differences, but the relative importance of the 
 characters relied on becomes a purely subjective matter. 
 
 According to the rival theory that of Descent or Organic 
 Evolution every species existing at the present day is 
 derived by a natural process of descent from some other 
 species which lived at a former period of the world's 
 history. If we could trace back from generation to 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 
 
 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 
 
 D , j- , . Branching Branching 
 
 ^ Branding dichotomous umbeUate monof,dial 
 
 5. J>f Z dichotomitm Z. arbuscula Z alternans\ DIMORPHIC 
 
 f HOMOMORPHIC 
 
 ....V..-J 
 
 FIG. 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 H5 
 
 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 
 
H6 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. affine 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, 
 
xni 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 fine 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 rilled 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 
 he 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 151 
 
 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 
 
 FIG. 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" 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, RADIOLARTA, 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. 
 
 SKel. 
 
 Int. caps, pi- 
 cent caps 
 
 -JExt.cqps.fr. 
 
 FIG. 33. Lithocircus annularis, one of the Radiolaria, showing 
 central capsule (cent, caps.}, intra- and extra capsular protoplasm (int. 
 caps.pr., ext. caps.pr.), nucleus (nu), pseudopods (psd), silicious skeleton, 
 (skel\ and symbiotic cells of Zooxanthella (z). (After Butschli.) 
 
 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 (int. 
 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- 
 cys tinea and should consult the plates of Haeckel's Die 
 
 FIG. 34. Skeleton of a Radiolarian (Actinommd), 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, z). 
 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- 
 thellse 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 of ready- 
 
xiv STRUCTURE OF CELL-WALL 155 
 
 digested food. It is as if a Haematococcus 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 Amoeba, 
 and the starch after conversion into sugar and the proteids 
 after being rendered diffusible would in part diffuse through 
 the cell-wall of the Hsematococcus into the surrounding 
 protoplasm of the Amoeba, 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 Diatomacetz, 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, nu) 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, RAD1OLARIA, 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 
 
 FIG. 35. A, semi-diagrammatic view of a diatom from its flat face, 
 showing cell- wall (c. zu) and protoplasm with nucleus (#), 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. w') with 
 their overlapping girdles. 
 
 c, the same in transverse section. x 
 
 D, surface view of the silicious shell of Navicnla truncata. 
 
 E, surface view of the silicious shell of Aulacodiscus sollittianus. 
 (D, after Donkin ; E, 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 Diatomaceenkunde 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 
 
Q 
 
 *-mf^ f 
 
 FIG. 36. Mucor. 
 
 A, portion of mycelium of M. mucedo (my} with two aerial hyphse 
 (#. hy\ each ending in a sporangium (spg). 
 
 B, small portion of an aerial hypha, highly magnified, showing pro- 
 toplasm (plsm)and cell-wall (c w). The scale above applies to this 
 figure only. 
 
 c 1 , immature sporangium, showing septum (sep} and undivided pro- 
 toplasm : c 2 , 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' J applies to c 1 c 2 , D 1 , and E. 
 
160 MUCOR LESS. 
 
 F, spoi-es. 
 
 G 1 , G 2 , 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 hyphae. 
 (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 hypha ; and the fila- 
 ments which grow out into the air and give the characteristic 
 fluffy appearance to the growth are aerial hypha. 
 
 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 (plsm) : 
 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 vacuoles at irregular intervals and numerous small 
 nuclei. 
 
 Thus a hypha of Mucor consists of precisely the same 
 constituents as a yeast-cell protoplasm, containing nuclei 
 and vacuoles, surrounded by cellulose. Imagine a yeast 
 cell to be pulled out as one might pull out a sphere of clay 
 or putty until it assumed the form of a long narrow cylin- 
 der, and suppose it also to be pulled out laterally at intervals 
 so as to form branches : there would be produced by such a 
 process a very good imitation of a hypha of Mucor. We 
 may therefore look upon a hypha as an elongated and 
 branched cell, so that Mucor is, like Opalina, a multinucleate 
 but unicellular organism. We shall see directly however 
 that this is strictly true of the mould only in its young state. 
 
 As stated above, the aerial hyphae 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 
 
162 MUCOR LESS. 
 
 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, bicellular : if, as is ordi- 
 narily the case, it bears numerous aerial hyphae, each with 
 its sporangium, it is multicellular. 
 
 Under unfavourable conditions of nutrition, septa fre- 
 quently appear at more or less irregular intervals in the 
 mycelial hyphae : 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\ D 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 (D 2 ). 
 
XV 
 
 GERMINATION OF SPORES 
 
 163 
 
 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- 
 
 FlG. 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 (p). 
 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 
 
 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 (c 3 ) which interlace with similar threads from 
 adjacent spores (H). The filaments are obviously hyphse, 
 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 hyphse constituting the 
 Mucor-plant are formed by the growth of spores each strictly 
 comparable to a single Saccharomyces. 
 
 It will be noticed that the growth of the mycelium is cen- 
 trifugal : each spore or group of spores serves as a centre 
 from which hyph?e 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 
 
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 hyphae 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 
 walls of the two cells then become absorbed (i 4 ) and their 
 contents mingle, forming a single mass of protoplasm 
 (i 5 , zyg), the cell- wall of which becomes greatly thickened 
 and divided into two layers, an inner delicate and trans- 
 parent, and an outer dark in colour, of considerable thick- 
 ness, and frequently ornamented with spines. 
 
 Obviously the swollen terminal cells (gam) of the short 
 lateral hyphse are gametes or conjugating bodies, and the 
 large spore-like structure (zyg) resulting from their union 
 is a zygote. The striking feature of the process is that the 
 gametes are non-motile, save in so far as their growth 
 towards one another is a mode of motion. In Heteromita 
 both gametes are active and free-swimming (p. 41) : in 
 Vorticella one is free-swimming, the other fixed but still 
 capable of active movement (p. 132) ; here both conjugating 
 bodies exhibit only the slow movement in one direction due 
 to growth. 
 
 There are equally important differences in the result of 
 the process in the three cases. In Heteromita the proto- 
 plasm of the zygote breaks up almost immediately into 
 spores ; in Vorticella the zygote is active, and the result of 
 conjugation is merely increased activity in feeding and fissive 
 
1 66 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 
 hyphae 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 between the conjugating 
 body (gamete) and the organ which produces it (gonad). See the de- 
 scription of the sexual process in Vaucheria (Lesson XVI.) and in 
 Spirogyra (Lesson XIX.). 
 
 2 An exception is formed by colonial forms such as Zoothamnium, in 
 which life is carried on from generation to generation by the reproduc- 
 tive zooids only. In all probability the colony itself, like an annual 
 plant, dies down after a longer or shorter time. Moreover the ciliate 
 infusoria are found, as already stated (p. 116), to sink into decrepitude 
 after multiplying by fission for a long series of generations. 
 
xv NUTRITION 167 
 
 tively 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 endo-parasitic animal such as Opalina or a tape- 
 worm, is no more exempted from the work of digestion 
 than a dog or a sheep : the organism upon which it lives 
 is to be looked upon rather as its prey than as its host. 
 
 It is a remarkable circumstance that, under certain con- 
 
1 68 MUCOR LESS, xv 
 
 ditions, Mucor is capable of exciting alcoholic fermentation 
 in a saccharine solution. When the hyphge 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 hyphae 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 Alga, a group 
 of plants which includes most of the smaller fresh-water 
 weeds, and the vast majority of sea-weeds. One of these 
 filamentous Algae, 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 (u, c.w) within which is protoplasm con- 
 taining a vacuole so large that the protoplasm has the 
 
ths mm 
 
 
 FlG. 38. Vaucheria. 
 
 A, tangled filaments of the living plant, showing mode of branching. 
 
 B, extremity of a filament, showing cell -wall (c. w) and protoplasm 
 with chromatophores (chr\ and oil-drops (o\ The scale above applies 
 to this figure only. 
 
 c 1 , immature sporangium (spg) separated from the filament by a _sep- 
 tum c 2 mature sporangium with the spore (sf>) in the act of escaping ; 
 c 3 , free-swimming spore, showing 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 (ov), 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 about to germinate : E 2 , further stage in germination. 
 
 (C 1 and c :} , after Strasburger ; C 2 and C 4 , after Sachs ; D and E, after 
 Pringsheim.) 
 
 character of a membrane lining the cellulose coat. 
 Numerous small nuclei occur in the protoplasm, as well as 
 oil-globules (0), and small, close-set, ovoid chromatophores 
 (chr) coloured with chlorophyll and containing starch. 
 
 Thus a Vaucheria-plant, like a Mucor-plant, is comparable 
 to a single multinucleate cell, extended in one dimension of 
 space so as to take on the form of a filament. 
 
 Various modes of asexual reproduction occur in different 
 species of Vaucheria : of these we need only consider that 
 which obtains in V. sessilis. In this species the end of a 
 branch swells up (c 1 ) and becomes divided off by a septum 
 (sep), forming a sporangium (spg) in principle like that of 
 Mucor, but differing in shape. The protoplasm of the 
 sporangium does not divide but separates itself from the 
 wall, and takes on the form of a single naked ovoidal spore 
 (c 3 ), formed of a colourless cortical layer containing nume- 
 rous nuclei and giving off cilia arranged in pairs, and of an 
 inner or medullary substance containing numerous chroma- 
 tophores. 
 
 The wall of the sporangium splits at its distal end (c 2 ), 
 and the contained spore (sp) escapes and swims freely in the 
 water for some time by the vibration of its cilia (c 3 ). After 
 
172 VAUCHERIA AND CAULERPA LESS. 
 
 a short active life it comes to rest, develops a cell-wall, and 
 germinates (c 4 ), i.e., gives out one or more processes which 
 extend and take on the form of ordinary Vaucheria-filaments, 
 so that in the present case, as in Mucor (p. 164), the de- 
 velopment of the plant shows it to be a single immensely 
 elongated multinucleate cell. 
 
 In its mode of sexual reproduction Vaucheria differs 
 strikingly not only from Mucor, but from all the organisms 
 we have hitherto studied. 
 
 The filaments are often found to bear small lateral pro. 
 cesses arranged in pairs (D T ), and each consisting of a little 
 bud growing from the filament and quite continuous with it. 
 These are the rudiments of the sexual reproductive organs 
 or gonads. The shorter of the two becomes swollen and 
 rounded (D 2 ), and afterwards bluntly pointed (D 3 , ovy] : its 
 protoplasm becomes divided from that of the filament, and 
 a septum (o 3 , sep') is formed between the two : the new cell 
 thus constituted is the ovary. 1 The longer of the two buds 
 undergoes further elongation and becomes bent upon itself 
 (D 2 ), its distal portion is then divided off by a septum (D B , 
 sep} forming a separate cell (spy), the spermary? 
 
 Further changes take place which are quite different in 
 the two organs. At the bluntly-pointed distal end of the 
 ovary the cell-wall becomes gelatinized and the protoplasm 
 protrudes through it as a small prominence which divides 
 off and is lost (D 4 ). The remainder of the protoplasm then 
 separates from the wall of the ovary and becomes a naked 
 cell, the ovum* or egg-cell (D 4 , ov\ which, by the gelatiniza. 
 tion and subsequent disappearance of a portion of the 
 
 1 Usually called the oogonium. 
 " Usually called the antheridiitm. 
 :{ Frequently called oosphere. 
 
xvr SEXUAL REPRODUCTION 173 
 
 wall of the ovary, is in free contact with the surrounding 
 water. 
 
 At the same time the protoplasm of the spermary under- 
 goes multiple fission, becoming converted into numerous 
 minute green bodies (D 5 ), each with two flagella, called 
 sperms.^ These are liberated by the rupture of the spermary 
 (D 7 ) at its distal end, and swim freely in the water. 
 
 Some of the sperms make their way to an ovary, and, as 
 it has been expressed, seem to grope about for the aperture, 
 which they finally pass through (D), and are then seen 
 moving actively in the space between the aperture and the 
 colourless distal end of the ovum. One of them, and pro- 
 bably only one, then attaches itself to the ovum and be- 
 comes completely united with it, forming the oosperm,- a 
 body which we must carefully distinguish from the ovum, 
 since, while agreeing with the latter in form and size, it 
 differs in having incorporated with it the substance of a 
 sperm. 
 
 Almost immediately the oosperm (D", osp) surrounds itself 
 with a cellulose wall, and numerous oil-globules are formed 
 in its interior. It becomes detached from the ovary, and, 
 after a period of rest, germinates (E 1 , E 2 ) and forms a new 
 Vaucheria plant. 
 
 It is obvious that the fusion of the sperm with the ovum 
 is a process of conjugation in which the conjugating bodies 
 differ strikingly in form and size, one the megagamete or 
 ovum being large, stationary, and more or less amoeboid : 
 the other the microgamete or sperm small, active, and 
 flagellate. In other words, we have a more obvious case of 
 sexual differentiation than was found to occur in Vorticella, 
 
 1 Often called spermatozooids or anthcrozooids : 
 
 2 Often called oospore. 
 
174 
 
 VAUCHERIA AND CAULERPA 
 
 (p. 132) : the large inactive egg-cell which furnishes by far 
 the greater portion of the material of the oosperm is the 
 female gamete ; the small active sperm-cell, the function of 
 which is probably (see Lesson XXIV.) to furnish additional 
 nuclear material, is the male gamete. 
 
 Similarly the oosperm is evidently a zygote, but a zygote 
 formed by the union of the highly differentiated gametes, 
 
 FIG. 39. Caulerpa scalpelliformis ( nat. size), showing the stem- 
 like, root-like, and leaf-like portions of the unicellular plant. (After 
 Harvey. ) 
 
 ovum and sperm, just as a zygospore (p. 164) is one formed 
 by the union of equal sized gametes. 
 
 As we shall see, this form of conjugation often distin- 
 guished as fertilization occurs in a large proportion of 
 flowerless plants, such as mosses and ferns (Lessons XXVIII. 
 and XXIX.), as well as in all animals but the very lowest. 
 From lowly water-weeds up to ferns and club mosses, and 
 from sponges and polypes up to man, the process of sexual 
 reproduction is essentially the same, consisting in the conju- 
 gation of a microgamete or sperm with a megagamete or 
 
xvi CAULERPA 175 
 
 ovum, a zygote, the oosperm or unicellular embryo, being 
 produced, which afterwards develops into an independent 
 plant or animal of the new generation. It is a truly remark- 
 able circumstance that what we may consider as the highest 
 form of the sexual process should make its appearance so 
 low down in the scale of life. 
 
 The nutrition of Vaucheria is purely holophytic ; its food 
 consists of a watery solution of mineral salts and of carbon 
 dioxide, the latter being split up, by the action of the chro- 
 matophores, into carbon and oxygen. 
 
 Mucor and Vaucheria are examples of unicellular plants 
 which attain some complexity by elongation and branching. 
 The maximum differentiation attainable in this way by a 
 unicellular plant may be illustrated by a brief description of 
 a sea-weed belonging to the genus Caulerpa. 
 
 Caulerpa (Fig. 39) is commonly found in rock-pools 
 between tide-marks, and has the form of a creeping stem 
 from which root-like fibres are given off downwards and 
 branched leaf-like organs upwards. These " leaves " may 
 attain a length of 30 cm. (i ft.) or more. So that, on a 
 superficial examination, Caulerpa appears to be as complex 
 an organism as a moss (compare Fig. 39 with Fig. 82, A). 
 But microscopical examination shows that the plant consists 
 of a single continuous mass of vacuolated protoplasm, 
 containing numerous nuclei and green chromatophores and 
 covered by a continuous cell-wall. Large and complicated 
 in form as it is, the whole plant is therefore nothing more 
 than a single branched cell, or, as it may be expressed, a 
 continuous mass of protoplasm in which no cellular structure 
 has appeared. 
 
LESSON XVII 
 
 THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS 
 
 HITHERTO the words " animal " and " plant " have been 
 either avoided altogether or Used incidentally without any 
 attempt at definition. We are now however in a position to 
 consider in some detail the precise meaning of the two words, 
 since in the last half-dozen lessons we have been dealing 
 with several organisms which can be assigned without hesi- 
 tation to one or other of the two great groups of living things. 
 No one would dream of calling Paramcecium and Stylonychia 
 plants, or Mucor and Vaucheria animals, and we may there- 
 fore use these forms as a starting-point in an attempt to form 
 a clear conception of what the names plant and animal really 
 signify, and how far it is possible to place the lowly organisms 
 described in the earlier lessons in either the vegetable or the 
 animal kingdom. 
 
 Let us consider, first of all, the chief points of resemblance 
 and of difference between the indubitable animal Paramcecium 
 on the one hand, and the two indubitable plants Mucor and 
 Vaucheria on the other. 
 
 In the first place, the essential constituents of all three 
 organisms is protoplasm, in which are contained one or more 
 nuclei. But in Paramcecium the protoplasm is invested 
 
LESS, xvn DIFFERENCES IN NUTRITION 177 
 
 only by a delicate cuticle interrupted at the mouth and anus, 
 while in Mucor and Vaucheria the outer layer is formed by 
 a firm, continuous covering of cellulose. 
 
 We thus have as the first morphological difference between 
 our selected animal and vegetable organisms the absence of 
 a cellulose cell-wall in the former and its presence in the 
 latter. This is a fundamental distinction, and applies 
 equally well to the higher forms. The constituent cells of 
 plants are in nearly all cases covered with a cellulose coat 
 (p. 60), while there is no case among the higher animals of 
 cells being so invested. 
 
 Next, let us take a physiological character. In all three 
 organisms there is constant waste of substance which has to 
 be made good by the conversion of food material into proto- 
 plasm : in other words, constructive and destructive meta- 
 bolism are continually being carried on. But when we come 
 to the nature of the food and the mode of its reception, we 
 meet at once with a very fundamental difference. In Para- 
 moecium the food consists of living organisms taken whole 
 into the interior of the body, and the digestion of this solid 
 proteinaceous food is the necessary prelude to constructive 
 metabolism. In Vaucheria the food consists of a watery 
 solution of carbon dioxide and mineral salts i.e., it is liquid 
 and inorganic, its nitrogen being in the form of nitrates or 
 of simple ammonia compounds. Mucor, like Paramoecium, 
 contains no chlorophyll, and is therefore unable to use 
 carbon dioxide as a food : like Vaucheria, it is prevented 
 by its continuous cellulose investment from ingesting solid 
 food, and is dependent upon an aqueous solution. It takes 
 its carbon in the form of sugar or some such compound, 
 while it can make use of nitrogen either in the simple form 
 of a nitrate or an ammonia salt, or in the complex form of 
 proteids or peptones. 
 
 N 
 
178 CHARACTERS OF ANIMALS AND PLANTS LESS. 
 
 In this case also our selected organisms agree with animals 
 and plants generally. Animals, with the exception of some 
 internal parasites, ingest solid food, and they must all have 
 their nitrogen supplied in the form of proteids, being unable 
 to build up their protoplasm from simpler compounds. 
 Plants take their food in the form of a watery solution ; 
 those which possess chlorophyll take their carbon in the 
 form of carbon dioxide and their nitrogen in that of a nitrate 
 or ammonia salt : those devoid of chlorophyll cannot, ex- 
 cept in the case of some bacteria, make use of carbon 
 dioxide as a food, and are able to obtain nitrogen either 
 from simple salts or from proteids. Chlorophyll-less plants 
 are therefore nourished partly like green plants, partly like 
 animals. 
 
 This difference in the character of the food is connected 
 with a morphological difference. Animals have, as a 
 rule, an ingestive aperture or mouth, and some kind of 
 digestive cavity, either permanent (stomach) or temporary 
 (food-vacuole). In plants neither of these structures 
 exists. 
 
 Another difference which was referred to at length in an 
 early lesson (p. 32), is not strictly one between plants and 
 animals, but between organisms with and organisms without 
 chlorophyll. It is that in green plants the nutritive processes 
 result in deoxidation, more oxygen being given out than is 
 taken in : while in animals and not-green plants the precise 
 contrary is the case. 
 
 There is also a difference in the method of excretion. In 
 Paramoecium there is a special structure, the contractile 
 vacuole, which collects the superfluous water taken in with 
 the food and expels it, doubtless along with nitrogenous and 
 other waste matters. In Vaucheria and Mucor there is no 
 contractile vacuole, and excretion is simply performed by 
 
xvn DEFINITIONS 179 
 
 diffusion from the general surface of the organism into the 
 surrounding medium. 
 
 This character also is of some general importance. The 
 large majority of animals possess a special organ of excretion, 
 plants have nothing of the kind. 
 
 Another difference has to do with the general form of the 
 organism. Paramoecium has a certain definite and constant 
 shape, and when once formed produces no new parts. 
 Vaucheria and Mucor are constantly forming new branches, 
 so that their shape is always changing and their growth can 
 never be said to be complete. 
 
 Finally, we have what is perhaps the most obvious and 
 striking distinction of all. Paramoecium possesses in a con- 
 spicuous degree the power of automatic movement ; in both 
 Mucor and Vaucheria the organism, as a whole, exhibits no 
 automatism but only the slow movements of growth. The 
 spores and sperms of Vaucheria are, however, actively 
 motile. 
 
 Thus, taking Paramoecium as a type of animals, and 
 Mucor and Vaucheria as types of plants, we may frame the 
 following definitions : 
 
 Animals are organisms of fixed and definite form, in which 
 the cell-body is not covered with a cellulose wall. They 
 ingest solid proteinaceous food, their nutritive processes 
 result in oxidation, they have a definite organ of excretion, 
 and are capable of automatic movement. 
 
 Plants are organisms of constantly varying form in which 
 the cell-body is surrounded by a cellulose wall ; they cannot 
 ingest solid food, but are nourished by a watery solution of 
 nutrient materials. If chlorophyll is present the carbon 
 dioxide of the air serves as a source of carbon, nitrogen is 
 obtained from simple salts, and the nutritive processes 
 
 N 2 
 
iSo CHARACTERS OF ANIMALS AND PLANTS LESS. 
 
 result in deoxidation ; if chlorophyll is absent carbon is 
 obtained from sugar or some similar compound, nitrogen 
 either from simple salts or from proteids, and the process of 
 nutrition is one of oxidation. There is no special excretory 
 organ, and, except in the case of certain reproductive bodies, 
 there is usually no locomotion. 
 
 Let us now apply these definitions to the simple forms 
 described in the first eight lessons, and see how far they 
 will help us in placing those organism in one or other of the 
 two "kingdoms" into which living things are divided. 
 
 Amoeba has a cell- wall, probably nitrogenous, in the 
 resting condition : it ingests solid proteids, its nutrition being 
 therefore holozoic : it has a contractile vacuole : and it 
 performs amoeboid movements. It may therefore be safely 
 considered as an animal. 
 
 Haematococcus has a cellulose wall : it contains chloro- 
 phyll and its nutrition is purely holophytic : a contractile 
 vacuole is present in H. lacustris but absent in H. pluvialis : 
 and its movements are ciliary. 
 
 Euglena has a cellulose wall in the encysted state : in 
 virtue of its chlorophyll it is nourished by the absorption of 
 carbon dioxide and mineral salts, but it can also ingest solid 
 food through a special mouth and gullet : it has a contractile 
 vacuole, and performs both euglenoid and ciliary move- 
 ments. 
 
 In both these organisms we evidently have conflicting 
 characters : the cellulose wall and holophytic nutrition 
 would place them both among plants, while from the con- 
 tractile vacuole and active movements of both genera and 
 from the holozoic nutrition of Euglena we should group 
 them with animals. That the difficulty is by no means 
 
xvii DOUBTFUL FORMS 181 
 
 easily overcome may be seen from the fact that both genera 
 are claimed at the present day both by zoologists and by 
 botanists. For instance, Prof. Huxley considers Haema- 
 tococcus as a plant, and expresses doubts about Euglena ; 
 Mr. Saville Kent ranks Hsematococcus as a plant and 
 Euglena as an animal ; Prof. Sachs and Mr. Thiselton 
 Dyer place both genera in the vegetable kingdom ; while 
 Profs. Ray Lankester and Biitschli group them both among 
 animals. 
 
 In Heteromita the only cell-wall is the delicate cuticle 
 which in the zygote is firm enough to hold the spores up to 
 the moment of their escape : food is taken exclusively by 
 absorption and nutrition is wholly saprophytic : there is a 
 contractile vacuole, and the movements are ciliary. 
 
 Here again the characters are conflicting : the probable 
 absence of cellulose, the contractile vacuole and the cilia 
 all have an " animal " look, but the mode of nutrition is 
 that of a fungus. 
 
 In Protomyxa there is a decided preponderance of animal 
 characteristics ingestion or living prey, and both amoeboid 
 and ciliary movements. There is no chlorophyll, and the 
 composition of the cell-wall is not known. 
 
 In the Mycetozoa, the life-history of which so closely 
 resembles that of Protomyxa, the cyst in the resting stage 
 consists of cellulose, and so does the cell-wall of the spore : 
 nutrition is holozoic, a contractile vacuole is present in the 
 flagellulre, and both amoeboid and ciliary movements are 
 performed. Here again we have a puzzling combination of 
 animal and vegetable characters, and as a consequence we 
 find these organisms included among plants under the 
 name of Myxomycetes or " slime- fungi " by Sachs and 
 Goebel, while De Bary, Biitschli, and Ray Lankester place 
 them in the animal kino-dom. 
 
i2 CHARACTERS OF ANIMALS AND PLANTS LESS. 
 
 In Saccharomyces there is a clear preponderance of 
 vegetable characters. The cell-wall consists of cellulose, 
 nutrition takes place by absorption and proteids are not essen- 
 tial, there is no contractile vacuole, and no motile phase. 
 
 Lastly, in the Bacteria the cell-wall is composed of cellu- 
 lose, nutrition is usually saprophytic, there is no contractile 
 vacuole, and the movements are ciliary. So that in all the 
 characters named, save in the presence of cellulose and the 
 absence of a contractile vacuole, the Bacteria agree with 
 Heteromita, yet they are universally except by Prof. Glaus 
 placed among plants, while Heteromita is as constantly 
 included among animals. 
 
 We see then that while it is quite easy to divide the higher 
 organisms into the two distinct groups of plants and animals, 
 any such separation is by no means easy in the case of the 
 lowest forms of life. It was in recognition of this fact that 
 Haeckel proposed, many years ago, to institute a third 
 " kingdom," called Protista, to include all unicellular organ- 
 isms. Although open to many objections in practice, there 
 is a great deal to be said for the proposal. From the strictly 
 scientific point of view it is quite as justifiable to make three 
 subdivisions of living things as two : the line between animals 
 and plants is quite as arbitrary as that between protists and 
 plants or between protists and animals, and no more so : the 
 chief objection to the change is that it doubles the difficulties 
 by making two artificial boundaries instead of one. 
 
 The important point for the student to recognize is that 
 these boundaries are artificial, and that there are no scientific 
 frontiers in Nature. As in the liquefaction of gases there is 
 a "critical point " at which the substance under experiment 
 is neither gaseous nor liquid : as in a mountainous country 
 it is impossible to say where mountain ends and valley 
 
xvir PROTISTA 183 
 
 begins : as in the development of an animal it is futile to 
 argue about the exact period when, for instance, the egg 
 becomes a tadpole or the tadpole a frog : so in the case 
 under discussion. The distinction between the higher 
 plants and animals is perfectly sharp and obvious, but when 
 the two groups are traced downwards they are found 
 gradually to merge, as it were, into an assemblage of organ- 
 isms which partake of the characters of both kingdoms, and 
 cannot without a certain violence be either included in or 
 excluded from either. Where any given " protist " has to 
 be classified the case must be decided on its individual 
 merits : the organism must be compared in detail with all 
 those which resemble it closely in structure, physiology, and 
 life-history : and then a balance must be struck and the 
 doubtful form placed in the kingdom with which it has, 
 on the whole, most points in common. 
 
 It will no doubt occur to the reader that, on the theory of 
 evolution, we may account for the fact of the animal and 
 vegetable kingdoms being related to one another like two 
 trees united at the root, by the hypothesis that the earliest 
 organisms were protists, and that from them animals and 
 plants were evolved along divergent lines of descent. And 
 in this connection the fact that some bacteria the simplest 
 organisms known and devoid of chlorophyll may flourish 
 in solutions wholly devoid of organic matter, is very 
 significant. 
 
LESSON XVIII 
 
 PENICILLIUM AND AGARICUS 
 
 ONE of the commonest and most familiar of the lower 
 organisms is the " green mould " which so quickly covers 
 with a thick sage-green growth any organic substances ex- 
 posed to damp, such as paste, jam, cheese, leather, &c. 
 This mould is a plant belonging, like Mucor, to the group 
 of Fungi, and is called Penicillium glaucum. 
 
 Examined with the naked eye a growth of Penicillium is 
 seen to have a powdery appearance, and if the finger is 
 passed over it a quantity of extremely fine dust of a sage- 
 green colour comes away. This dust consists, as we shall 
 see, of the spores of Penicillium. The best way to study 
 the plant is to sow some of the spores in a saucer of 
 Pasteur's solution by 'drawing a needle or brush over a 
 growth of the mould and stirring it round in the fluid. 
 
 It is as well to study the naked-eye 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 
 
LESS, xvni 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 
 ]iyph<z (a. hy.\ grow vertically upwards into the air, while 
 from its lower surface similar but shorter threads, the sub- 
 merged hyphce (s. hy.\ hang vertically downwards into the 
 fluid. 
 
FIG. 40. Penicilliwn glaticum. 
 
 A, Diagrammatic vertical section of a young growth ( x 5), showing 
 mycelium (my], submerged hyphae (s. hy\ and aerial hyphse (a. Ay). 
 
 B, 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 small group of germinating 
 spores. 
 
LESS, xvin MULTICELLULAR HYPILE 187 
 
 D, more advanced mycelium : the hyphae have increased in length 
 and begun to branch, and septa (sep) have appeared. 
 
 E, germinating spore (sp) very highly magnified, sending out one 
 short and one long hypha, the latter with a short lateral branch and 
 several septa (sep). Both spore and hyphae contain vacuoles (vac] in 
 their protoplasm. 
 
 F^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 (sfg). 
 
 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 a -F 5 , and F 7 x 150 : F 6 x 200 : E x 500. 
 
 As long as the growths are white or blue in colour no 
 powder can be detached by touching the aerial hyphae, 
 showing that the spores are not yet fully formed, but as soon 
 as the permanent green hue is attained the slightest touch 
 is sufficient to detach large quantities of spores. 
 
 A bit of the felt-like mycelium is easily teased out or torn 
 asunder with two needles, and is then found, like actual felt, 
 to be formed of a close interlacement of delicate threads (D). 
 These are the mycelial hyphce : they are regularly cylindrical, 
 about T Jy mm. in diameter, frequently branched, and differ 
 in an important particular from the somewhat similar hyphse 
 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), multicelhtlar^ 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- 
 
i88 .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 hyphae have the same structure, but it is 
 easier to find their actual ends than those of the mycelial 
 hyphse. 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 (r 1 , F 2 ) form secondary ones 
 (F B ), and the secondary tertiary (r 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 sterig- 
 mata (r 5 , sfg). 
 
 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). 
 
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 6 ), 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). 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 appear 
 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 2 ) : 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 hyphae (B, c), just 
 as in Mucor. But the difference between the tw.o 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 hyphge, 
 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 Eurotium 
 is described in Huxley and Martin's Elementary Biology (new edition), 
 p. 419, and figured in Howes's Atlas of Elementary Biology, pi. xix., 
 figs, xxvi and xxvii. 
 
 The nutrition of Penicillium is essentially like that of Mucor 
 (p. 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 PILEUS AND LAMELLA 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 (my), from which it springs. 
 
 On the underside of the pileus are numerous radiating 
 vertical plates or lamella (I) 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 
 
 FlG. ^i.Agaricus campestrts. 
 
 A, Diagrammatic vertical section, showing the stalk (st) springing 
 from a mycelium (my], and expanding into the pileus (/), on the under 
 side of which are the radiating lamella? (/). 
 
 B, transverse vertical section of a lamella, showing the hypha; (hy} 
 turning outwards to form the layer of club-shaped cells (a) from which 
 the sterigmata spring. 
 
 C, one of the club-shaped cells (a), highly magnified, showing its two 
 sterigmata (^), each bearing a spore (sp). 
 
 (B and C after Sachs.) 
 
 the mycelium of Penicillium, and its structure is best in- 
 vestigated by cutting thin sections of various parts and 
 examining them under a high power. 
 
XVTII HISTOLOGY OF MUSHROOM 193 
 
 Such sections show the whole mushroom to be composed 
 of immense numbers of closely interwoven, branched hyphse 
 (B) divided by numerous septa into cells. In the stalk the 
 hyphae take a longitudinal direction ; in the pileus they turn 
 outwards, passing from the centre to the circumference, and 
 finally send branches downwards to form the lamellae. Fre- 
 quently the hyphae are so closely packed as to be hardly 
 distinguishable one from another. 
 
 At the surfaces of the lamellae the hyphae turn outwards, 
 so that their ends are perpendicular to the free surfaces of 
 those plates. Their terminal cells become dilated or club- 
 shaped (B, c, 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. 
 
 
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 hyphse 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 
 
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 (phm} connected by threads with a central mass contain- 
 
 O 2 
 
196 SPIROGYRA LESS. 
 
 ing the nucleus (nu), two spiral chromatophores (chr\ and pyrenoids 
 
 (pyr\ 
 
 B 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 (gori^, gon~) 
 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 1 *) which has not yet separated from its containing 
 gonad (gon 2 ) : in C 3 the female gamete (gam 2 ) 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 (nu\ 
 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 (D 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 12 P.M.), 
 but by keeping the plant cold all night may be 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 SPIROGYRA 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 , gon 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 be dioecious, 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 
 Haematococcus 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 hyphee 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 Hsematococcus 
 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 
 
 B 
 
 1 
 
 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 
 
 FIG. 44. A, Laminaria claustoni, a young plant, showing stem with 
 branched root-like organ of attachment, and deeply-cleft leaves (about 
 th natural size). 
 
 B, Lessonia fascescens, showing tree-like form (about -faih 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 J 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. 
 
 Of THE 
 
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 J mm. in 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, rh) : the distal end is 
 
FIG. M.Nit 
 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 (rh] : 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 while 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 ; plsrn^, the quies- 
 cent outer layer of protoplasm containing chromatophores (chr) ; plsm*, 
 the inner layer, rotating in the direction indicated by the arrows, and 
 containing nuclei (nu) ; 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) y 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 (rh) 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 spermaries (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 l ) separating the two adjacent internodes from one 
 another. The leaves consist each of an elongated proximal 
 cell like an internode (D, / ; F, / *), 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 
 
 p 
 
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*) 
 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 T J^ 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, nu} 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, 7 1 ) are dis- 
 
xxr APICAL GROWTH 211 
 
 sected away, the node from which they spring (nd l ) is found 
 to give rise distally to a very short internode (int. nd 2 ), 
 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. pt), the actual distal extremity of the plant 
 the growing point m 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 
 -Q mm. in diameter. Its cell-wall is thick, and its cell-body 
 formed of dense granular protoplasm containing a large 
 rounded nucleus (nu) 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. c\ while the lower is now 
 distinguished as the sub-apical or segmental cell (s. ap. c). 
 The sub-apical cell divides again horizontally, forming two 
 cells, the uppermost of which (c, nd 4 ) almost immediately 
 becomes divided by vertical planes into several cells (D, nd^]\ 
 the lower (c, D, int. nd*) remains undivided. 
 
 The sub-apical cell is the rudiment of an entire segment ; 
 the uppermost of the two cells into which it divides is the 
 rudiment of a node, the lower of an internode. The future 
 
 ^ -.> 
 
 TT XT T \.' T7 T? v. 1 T 1 ^ 
 
int. 
 
 
 FIG. 46. Nitella : Vertical sections of the growing point at four 
 successive stages. The nodes (nd), internodes (int. nd), and leaf- 
 whorls (/) are all numbered in order from the proximal to the distal end 
 of the bud, the numbers corresponding in all the figures. The proximal 
 segment (int. nd\ nd 1 , I 1 } in these figures corresponds with the third 
 segment (int. nd 3 , / 3 ) shown in Fig. 46, F. 
 
 A, the apical cell (ap, c) is succeeded by a very rudimentary node 
 (nd 3 ) without leaves : int. nd 1 is in vertical section, showing .the proto- 
 plasm (plsm\ vacuole (vac), and two nuclei (nu). 
 
 B, the apical cell has divided transversely, forming a new apical cell 
 (ap. c) and a sub-apical cell (s. ap. c) : the leaves (/ 3 ) of nd 3 ) have 
 appeared. 
 
 c, the sub-apical cell has divided transversely into the proximally- 
 situated internode (int. nd*) and the distally-situated node (nd*) of a 
 new segment ; in the node the nucleus has divided preparatory to cell- 
 clivision. The previously formed segments have increased in size : int. 
 nd 2 has developed a vacuole (vac), and its nucleus has divided (comp. 
 int. nd 1 in A) : int. nd^ is shown in surface view with three dividing 
 nuclei (nu). 
 
 D. nd* has divided vertically, forming a transverse plate of cells, and 
 is now as far advanced as nd A in A : the nucleus of int. nd 3 is in the act 
 of dividing, while int. nd z , shown in surface view, now contains nume- 
 rous nuclei, some of them in the act of dividing. 
 
LESS, xxi MULTIPLICATION OF NUCLEUS 213 
 
 fate of the two is shown at once by the node dividing into 
 a horizontal plate of cells while the internode remains 
 unicellular. 
 
 Soon the cells of the new node begin to send out short 
 blunt processes arranged in a whorl : these increase in size, 
 undergo division, and form leaves (A D, / 2 , / 3 ). 
 
 These processes are continually being repeated ; the apical 
 cell is constantly producing new sub-apical cells, the sub- 
 apical cells dividing each into a nodal and an internodal 
 cell ; and the nodal cell dividing into a horizontal plate of 
 cells and giving off leaves, while the internodal cell remains 
 undivided. 
 
 The special characters of the fully-formed parts of the 
 plant are due to the unequal growth of the new cells. The 
 nodal cells soon cease to grow and undergo but little altera- 
 tion (comp. nd l and nd^\ whereas the internodes increase 
 immensely in length, being quite 3,000 times as long when 
 full-grown as when first separated from the sub-apical cell. 
 The leaves also, at first mere blunt projections (A, / 2 ), soon 
 increase sufficiently in length to arch over the growing point 
 and so form the characteristic terminal bud : gradually they 
 open out and assume the normal position, their successors 
 of the next younger whorl having in the meantime developed 
 sufficiently to take their place as protectors of the growing 
 point. 
 
 The multinucleate condition of the adult internodes is 
 also a result of gradual change. In its young condition an 
 internodal cell has a single rounded nucleus (A, int. nd 2 , int. 
 nd B ), but by the time it is about as long as broad the nucleus 
 has begun to divide (D, int. nd* ; c, int. nd 2 ), and when the 
 length of the cell is equal to about twice its breadth, the 
 nucleus has broken up into numerous fragments (c, int. nd l , 
 D, int. nd 2 ), many of them still in active division. This 
 
214 NITELLA LESS. 
 
 repeated fission of the nucleus reminds us of what was 
 found to occur in Opalina (p. 1 1 9). 
 
 Thus the growth of Nitella like that of Penicillium (p. 
 1 88), is apical : new cells arise only in the terminal bud, 
 and, after the first formation of nodes, internodes, and 
 leaves, the only change undergone by these parts is an in- 
 crease in size accompanied by a limited differentiation of 
 character. 
 
 A shoot arises by one of the cells in a node sending 
 off a projection distad of a leaf, te., in an axil : the process 
 separates from the parent cell and takes on the characters of 
 an apical cell of the main stem, the structure of which is in 
 this way exactly repeated by the shoot. 
 
 The leaves, unlike the branches, are strictly limited in 
 growth. At a very early period the apical cell of a leaf 
 becomes pointed and thick-walled (Fig. 45, E), and after this 
 no increase in the number of cells takes place. 
 
 The rhizoids also arise exclusively from nodal cells : they 
 consist of long filaments (Fig. 45, c), not unlike Mucor- 
 hyphse, but occasionally divided by oblique septa into linear 
 aggregates of cells, and increase in length by apical growth. 
 
 The structure of the gonads is peculiar and somewhat 
 complicated. 
 
 As we have seen, the spermary (Fig. 45, G, spy) is a 
 globular, orange-coloured body attached to a leaf by a short 
 stalk. Its wall is formed of eight pieces or shields, which 
 fit against one another by toothed edges, so that the entire 
 spermary may be compared to an orange in which an equa- 
 torial incision and two meridional incisions at right angles 
 to one another have been made through the rind dividing 
 it into eight triangular pieces. Strictly speaking, however, 
 only the four distal shields are triangular : the four proximal 
 
XXI 
 
 STRUCTURE OF SPERMARY 
 
 215 
 
 ones have each its lower angle truncated by the insertion of 
 the stalk, so that they are actually four-sided. 
 
 Each shield (Fig. 47, A and B, sh) is a single concavo~ 
 convex cell having on its inner surface numerous orange- 
 coloured chromatophores : owing to the disposition of these 
 on the inner surface only, the spermary appears to have a 
 
 FIG. 47. A, diagrammatic vertical section of the spermary of Nitella, 
 showing the stalk (stk), four of the eight shields (sh), each bearing on 
 its inner face a handle (hn), to which is attached a head-cell (hd) : each 
 head cell bears six secondary head-cells (hd 1 ), to each of which four 
 spermatic filaments (sp. f.) are attached. 
 
 B, one of the proximal shields (sh), with handle (hn), head-cell (hd), 
 secondary head-cells (hd 1 ), and spermatic filaments (sp. f.). 
 
 C, a single sperm. 
 
 D 1 , D 2 , D 3 , three stages in the development of the spermary. 
 (c, after Howes. ) 
 
 colourless transparent outer layer like an orange inclosed 
 in a close-fitting glass case. 
 
 Attached to the middle of the inner surface of each shield 
 is a cylindrical cell, the handle (Jiri), which extends towards 
 the centre of the spermary, and, like the shield itself, con- 
 tains orange chromatophores. Each of the eight handles 
 bears a colourless head-cell (hd'\ to which six secondary head 
 
216 NITELLA LESS. 
 
 cells (hd 1 ) are attached, and each of these latter bears four 
 delicate coiled filaments (sp.f.) divided by septa into small 
 cells arranged end to end, and thus not unlike the hyphae of 
 a fungus. There are therefore nearly two hundred of these 
 spermatic filaments in each spermary, coiled up in its interior 
 like a tangled mass of white cotton. 
 
 The cells of which the filaments are composed have at 
 first the ordinary character, but as the spermary arrives at 
 maturity there is produced in each a single sperm (c), having 
 the form of a spirally-coiled thread, thicker at one end than 
 the other, and bearing at its thin end two long flagella. In 
 all probability the sperm proper, i.e., the spirally-coiled body, 
 is formed from the nucleus of the cell, the flagella from its 
 protoplasm. As each of the 200 spermatic filaments con- 
 sists of from 100 to 200 cells, a single spermary gives rise 
 to between 20,000 and 40,000 sperms. 
 
 When the sperms are formed the shields separate 
 from one another and the spermatic filaments protrude 
 between them like cotton from a pod : the sperms then 
 escape from the containing cells and swim freely in the 
 water. 
 
 The ovary (Fig. 45, G, ovy, and Fig. 48 A) is ovoidal in 
 form, attached to the leaf by a short stalk (stk], and ter- 
 minated distally by a little chimney-like elevation or crown 
 (cr). It is marked externally by spiral grooves which can be 
 traced into the crown, and in young specimens its interior is 
 readily seen to be occupied by a large opaque mass (ov}. 
 Sections show that this central body is the ovum, a large cell 
 very rich in starch : it is connected with the unicellular stalk 
 by a small cell (nd} from which spring five spirally-arranged 
 cells (sp. c.) : these coil round the ovum and their free ends 
 each divided by septa into two small cells project at the 
 distal end of the organ and form the crown, enclosing a 
 
XXI 
 
 DEVELOPMENT OF GONADS 
 
 217 
 
 narrow canal which places the distal end of the ovum in free 
 communication with the surrounding water. 
 
 We saw how the various parts of the fully formed plant 
 nodal, and internodal cells, leaves, and rhizoids were all 
 formed by the modification of similar cells produced in the 
 apical bud. It is interesting to find that the same is true of 
 the diverse parts of the reproductive organs. 
 
 The spermary arises as a single stalked globular cell which 
 
 Sf.C, 
 
 FIG. 48. A, vertical section of the ovary of Nitella, showing the 
 stalk (stk], small node (nd) from which spring the five spirally-twisted 
 cells (sp. c), each ending in one of the two-celled sections of the crown 
 (cr). The ovum contains starch grains, and is represented as trans- 
 parent, the spiral cells being seen through it. 
 
 B 1 , surface view, and B 2 , section of a very young ovary : B 3 , later 
 stage in vertical section : B*, still later stage, surface view, with the 
 ovum seen through the transparent spiral cells. Letters as in A, except 
 x, small cells formed by division from the base of the ovum. (B 2 -B 4 
 after Sachs.) 
 
 becomes divided into eight octants (Fig. 47, D 1 ). Each of 
 these then divides tangentially (i.e. parallel to the surface 
 of the sphere) into two cells (D 2 ), the inner of which divides 
 again (o 3 ) so that each octant is now composed of three cells. 
 Of these the outermost forms the shield, the middle, the 
 handle, and the inner the head-cell : from the latter the 
 secondary head-cells and spermatic filaments are produced 
 
218 NITELLA LESS. 
 
 by budding. The entire spermary appears to be a modified 
 leaflet. 
 
 The ovary also arises as a single cell, but soon divides and 
 becomes differentiated into an axial row of three cells (Fig. 
 48, B 2 , 0v t nd, stk] surrounded by five others (sp. c) which arise 
 as buds from the middle cell of the axial row (nd} and are 
 at first knob-like and upright (B 1 ). The uppermost or distal 
 cell of the axial row becomes the ovum (B S , B 4 , ov\ the 
 others the stalk (stk) and intermediate cells (nd, x) : the five 
 surrounding cells elongate, and as they do so acquire a spiral 
 twist which becomes closer and closer as growth proceeds 
 (compare B 1 , B 4 , and Fig. 45, G, ovy). At the same time the 
 distal end of each develops two septa (B S ) and, projecting 
 beyond the level of the ovum, forms with its fellows the 
 chimney or crown (cr) of the ovary. There is every reason 
 to believe that the entire ovary is a highly-modified shoot : 
 the stalk representing an internode, the cell nd a node, the 
 spiral cells leaves, and the ovum an apical cell. 
 
 Thus while the ciliate Infusoria and Caulerpa furnish ex- 
 amples of cell-differentiation without cell-multiplication, and 
 Spirogyra of cell-multiplication without cell-differentiation, 
 Nitella is a simple example of an organism in which com- 
 plexity is obtained by the two processes going on hand in 
 hand. It is a solid aggregate, the constituent cells of which 
 are so arranged as to produce a well-defined external form, 
 while some of them undergo a more or less striking differen- 
 tiation according to the position they have to occupy, and 
 the function they have to perform. 
 
 Impregnation takes place in the same manner as in 
 Vaucheria (p. 173). A sperm makes its way down the 
 canal in the chimney-like crown of cells terminating the 
 
XXI 
 
 GERMINATION 
 
 219 
 
 ovary, and conjugates with the ovum converting it into an 
 oosperm. 
 
 After impregnation the ovary, with the contained oosperm, 
 becomes detached and falls to the bottom, where, after a 
 
 ap.c 
 
 term Trud 
 
 ovy 
 
 r7t 
 
 FIG. 49. Pro-embryo of Chara, showing the ovary (ovy} from the 
 oosperm in which the pro-embryo has sprung : the two nodes (nd), 
 apical cell (ap. c), rhizoids (r/i), and leaves (/) of the pro-embryo : and 
 the rudiment of the leafy plant ending in the characteristic terminal bud 
 (term. bud). (After Howes, slightly altere'd.) 
 
 period of rest, it germinates. The process of germination 
 does not appear to be known in Nitella, but has been followed 
 in detail in the closely allied genus Chara. 
 
 The oosperm sends out a filament which consists at first 
 of a single row of cells (Fig. 49) giving out a root-fibre (rh) 
 
220 NITELLA LESS. XXT 
 
 at its proximal end. Soon two nodes (nd) are formed on 
 the filament, or pro-embryo, from the lower of which rhizoids 
 (rti) proceed, while the upper gives rise to a few leaves (/), 
 not arranged in a whorl, and to a small process which is at 
 first unicellular, but, behaving like an apical cell of Nitella, 
 soon becomes a terminal bud (term, bud) and grows into the 
 ordinary leafy plant. 
 
 This is an instance of what is known as alternation of 
 generations. The Chara and presumably the Nitella 
 plant gives rise by a sexual process to a pro-embryo which in 
 turn produces, by an asexual process of budding, the Chara 
 (or Nitella) plant. No case is known of the pro-embryo 
 directly producing a pro-embryo or the leafy-plant a leafy- 
 plant. In order to complete the cycle of existence or life- 
 history of the species two generations which alternate with 
 one another are required : a sexual generation orgamobium, 
 which reproduces by the conjugation of gametes (ovum and 
 sperm), and an asexual generation or agamobium, which 
 reproduces by budding. 
 
LESSON XXII 
 
 HYDRA 
 
 WE have seen that with plants, both Fungi and Algae, the 
 next stage of morphological differentiation after the simple 
 cell is the linear aggregate. Among animals there are no 
 forms known to exist in this stage, but coming immediately 
 above the highest unicellular animals, such as the ciliate 
 Infusoria, we have true solid aggregates. The characters of 
 one of the simplest of these and the fundamental way in 
 which it differs from the plants described in the two previous 
 lessons will be made clear by a study of one of the little 
 organisms known as " fresh-water polypes " and placed 
 under the genus Hydra. 
 
 Although far from uncommon in pond-water, Hydra is not 
 always easy to find, being rarely abundant and by no means 
 conspicuous. In looking for it the best plan is to fill either 
 a clear glass bottle or beaker or a white saucer with weeds 
 and water from a pond and to let it remain undisturbed for 
 a few minutes. If the gathering is successful there will be 
 seen adhering to the sides of the glass, the bottom of the 
 saucer, or the weeds, little white, tawny, or green bodies, 
 about as thick as fine sewing cotton, and 2 6 mm. in 
 length. They adhere pretty firmly by one end, and examin- 
 

 FIG. 50. Hydra. 
 
 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 (bd 1 , bd?, bd 3 ) 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 
 (t) appearing like papilla;. 
 
 B, H. fitsca, 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, xxii 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 
 (Jiyp\ 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 (f) 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. Hydrae 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. cav} and the cavities of 
 tentacles (ent. cav). So that the wall of the body is formed 
 throughout of an outer layer of cells, the ectoderm (eci), and 
 of an inner layer, the endoderm (end\ which bounds the 
 enteric cavity. Between the two layers is a delicate trans- 
 parent membrane, the mesoglcea, 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 
 
FIG. 51. Hydra. 
 
 A, Vertical section of the entire animal, showing the body- wall corny 
 posed of ectoderm (eci] and endoderm (end), enclosing an enteric cavit- 
 
LESS, xxn ECTODERM 227 
 
 (ent. 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 1 ) nematocysts : some of the endoderm cells 
 are putting out pseudopods (psd), others flagella (/?). Two buds (bd 1 , 
 bcfi) 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 (ect) and interstitial cells (in(. c) : two cnidoblasts 
 (cnbl) enclosing nematocysts (ntc), and one of them produced into a 
 cnidocil (cnc) : the layer of muscle- processed (m. pr) cut across just 
 external to the mesogloea (msgl) : endodeMi cells (end) with large 
 vacuoles and nuclei (nu), pseudopods (psd),\ and flagella (fi). 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 (), numerous 
 chromatophores (c/ir), 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, m. 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 
 
 9 2 
 
c?ib 
 
 FIG. 52. Hydra. 
 
 A, A nematocyst contained in its cnidoblast (cnb), showing the coiled 
 filament and the cnidocil (cue]. 
 
 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 Paramcecium (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, <r#/and Fig. 52), and are thus 
 in the first instance at a distance from the surface. But the 
 cnidoblasts migrate outwards, and so come to lie quite 
 superficially either in or between the large ectoderm cells. 
 On its free surface the cnidoblast is produced into a delicate 
 pointed process, the cnidocil or " trigger-hair " (cnc). In all 
 probability the slightest touch of the cnidocil causes con- 
 traction of the cnidoblast, and the nematocyst thus com- 
 pressed instantly explodes. 
 
 Nematocysts are found in the distal part of the body, but 
 are absent from the foot or proximal end, where also there 
 are no interstitial cells. They are especially abundant in the 
 tentacles, on the knob-like elevations of which due to little 
 heaps of interstitial cells they are found in great numbers. 
 Amongst these occur small nematocysts with short threads 
 and devoid of barbs (Fig. 51, A, ntc' and F). 
 
 There are sometimes found in connection with the cnido- 
 blast small irregular cells with large nuclei : they are called 
 nerve-cells (Fig. 52, c, nv. c), and constitute a rudimentary 
 nervous system, the nature of which will be more con- 
 veniently discussed in the next lesson (p. 244). 
 
 The ectoderm cells of the foot differ from those of the rest 
 of the body in being very granular (Fig. 51 A). The 
 granules are probably the material of the adhesive substance 
 by which the Hydra fixes itself, and are to be looked upon as 
 products of destructive metabolism : i.e. as being formed by 
 conversion of the protoplasm in something the same way as 
 starch-granules (p. 33). This process of formation in a cell 
 of a definite product which accumulates and is finally dis- 
 charged at the free surface of the cell is called secretion, 
 
xxli ENDODERM 231 
 
 and the cell performing the function is known as a gland 
 cell. 
 
 The endoderm consists for the most part of large cells 
 which exceed in size those of the ectoderm, and are re- 
 markable for containing one or more vacuoles, sometimes 
 so large as to reduce the protoplasm to a thin superficial 
 layer containing the nucleus (Fig. 51, A and B, end}. Then 
 again, their form is extremely variable, their free or inner 
 ends undergoing continual changes of form. This can be 
 easily made out by cutting transverse sections of a living 
 Hydra, when the endoderm cells are seen to send out long 
 blunt pseudopods (psd) into the digestive cavity, and now 
 and then to withdraw the pseudopods and send out from 
 one to three long delicate flagella (fi). Thus the endoderm 
 cells of Hydra illustrate in a very instructive manner the 
 essential similarity of flagella and pseudopods already re- 
 ferred to (p. 51). In the hypostome the endoderm is thrown 
 into longitudinal folds, so as to allow of the dilatation of 
 the mouth in swallowing. 
 
 Amongst the ordinary endoderm-cells are found long 
 narrow cells of an extremely granular character. They are 
 specially abundant in the distal part of the body, beneath 
 the origins of the tentacles, and in the hypostome, but are 
 absent in the tentacles and in the foot. There is no doubt 
 that they are gland-cells, their secretion being a fluid used 
 to aid in the digestion of the food. 
 
 In Hydra viridis the endoderm-cells (D) contain chroma- 
 tophores (chr) coloured green by chlorophyll, which performs 
 the same function as in plants, so that in this species holozoic 
 is supplemented by holophytic nutrition. There is reason 
 for believing that the chromatophores are to be regarded as 
 symbiotic algae, like those found in connection with Radio- 
 
232 HYDRA LESS. 
 
 laria (p. 154). In H. fusca bodies resembling these chromato- 
 phores are present, but are of an orange or brown colour, and 
 devoid of chlorophyll. Brown and black granules occurring 
 in the cells (B) seem to be due in part to the degeneration of 
 the chromatophores, and in part to be products of excretion. 
 
 Muscle-processes exist in connection with the endoderm 
 cells, and they are said to take a transverse or circular 
 direction, i.e., at right angles to the similar processes of 
 the ectoderm cells. 
 
 When a water-flea or other minute organism is swallowed 
 by a Hydra, it undergoes a gradual process of disintegration. 
 The process is begun by a solution of the soft parts due to 
 the action of a digestive fluid secreted by the gland-cells of 
 the endoderm : it is apparently completed by the endoderm 
 cells seizing minute particles with their pseudopods and 
 engulfing them quite after the manner of Amoebae. It is 
 often found that the protrusion of pseudopods during 
 digestion results in the almost complete obliteration of the 
 enteric cavity. 
 
 It would seem therefore that in Hydra the process of 
 digestion or solution of the food is to some extent at least 
 infra-cellular^ i.e., takes place in the interior of the cells 
 themselves, as in Amoeba or Paramcecium : it is however 
 mainly extra-cellular or enteric, i.e., is performed in a special 
 digestive cavity lined by cells. 
 
 The ectoderm cells do not take in food directly, but are 
 nourished entirely by diffusion from the endoderm. Thus 
 the two layers have different functions : the ectoderm is pro- 
 tective and sensory ; it forms the external covering of the 
 animal, and receives impressions from without : the endo- 
 derm, removed from direct communication with the outer 
 world, performs a nutrient function, its cells alone having 
 the power of digesting food. 
 
xxn INDIVIDUATION 233 
 
 The essential difference between digestion and assimilation 
 is here plainly seen : all the cells of Hydra assimilate, all 
 are constantly undergoing waste, and all must therefore form 
 new protoplasm to make good the loss. But it is the endo- 
 derm cells alone which can make use of raw or undigested 
 food : the ectoderm has to depend upon various products of 
 digestion received by osmosis from the endoderm. 
 
 It will be evident from the preceding description that 
 Hydra is comparable to a colony of Amoebae in which par- 
 ticular functions are made over to particular individuals 
 just as in a civilized community the functions of baking and 
 butchering are assigned to certain members of the commu- 
 nity, and not performed by all. Hydra is therefore an ex- 
 ample of individuation : morphologically it is equivalent 
 to an indefinite number of unicellular organisms : but, 
 these acting in concert, some taking one duty and some 
 another, form, physiologically speaking, not a colony ot 
 largely independent units, but a single multicellular in- 
 dividual. 
 
 Like so many of the organisms which have come under 
 our notice, Hydra has two distinct methods of reproduction, 
 asexual and sexual. 
 
 Asexual multiplication takes place by a process of budding. 
 A little knob appears on the body (Fig. 50, A, bd l \ and is 
 found by sections to arise from a group of ectoderm cells ; 
 soon however it takes on the character of a hollow out- 
 pushing of the wall containing a prolongation of the enteron, 
 and made up of ectoderm, mesoglcea, and endoderm. (Fig. 
 51, A, bd 1 ). In the course of a few hours this prominence 
 enlarges greatly, and near its distal end six or eight hollow 
 buds appear arranged in a whorl (Fig. 50, A, bd^ ; Fig. 51, 
 
234 HYDRA LESS. 
 
 A, bd' 2 ). These enlarge and take on the characters of ten- 
 tacles : a mouth is formed at the distal end of the bud, 
 which thus acquires the character of a small Hydra (Fig. 
 50, A, bd z \ Finally the bud becomes constricted at its base, 
 separates from the parent, and begins an independent ex- 
 istence. Sometimes, however, several buds are produced at 
 one time, and each of these buds again before becoming 
 detached : in this way temporary colonies are formed. But 
 the buds always separate sooner or later, although they 
 frequently begin to feed while still attached. 
 
 It is a curious circumstance that Hydra can also be mul- 
 tiplied by artificial division : the experiment has been tried 
 of cutting the living animal into pieces, each of which was 
 found to grow into a perfect individual. 
 
 As in Vaucheria and Nitella, the sexual organs or gonads 
 are of two kinds, spermaries and ovaries. Both are found 
 in the same individual, Hydra being, like the plants just 
 mentioned, hermaphrodite or monoecious. 
 
 The spermaries (Fig. 50, B, and Fig. 51, A, spy) are white 
 conical elevations situated near the distal end of the body : 
 as a rule not more than one or two are present at the same 
 time, but there may be as many as twenty. They are per- 
 fectly colourless, even in the green and brown species, being 
 obviously formed of ectoderm alone. 
 
 In the immature condition the spermary consists of a little 
 heap of interstitial cells covered by an investment of some- 
 what flattened cells formed by a modification of the ordinary 
 large cells of the ectoderm. When mature each of the small 
 internal cells becomes converted into a sperm (Fig. 51, G), 
 consisting of a small ovoid head formed from the nucleus of 
 the cell, and of a long vibratile tail formed from its proto- 
 plasm. By the rupture of the investing cells or wall of the 
 
SPERMARY AND OVARY 
 
 235 
 
 sperm ary the sperms are liberated and swim freely in the 
 water. 
 
 The ovaries (Fig. 50, B, and Fig. 51, A, ovy) are found 
 near the proximal end of the body, and vary in number from 
 one to eight. When ripe an ovary is larger than a spermary, 
 and of a hemispherical form. It begins, like the spermary, 
 as an aggregation of interstitial cells, so that in their earlier 
 stages the sex of the gonads is indeterminate. But while 
 
 FIG. 53. A, Ovum of Hydra viridis, showing pseudopods, nucleus 
 (gv), and numerous chromatophores and yolk spheres. 
 
 B, a single yolk sphere. (From Balfour after Kleinenberg. ) 
 
 in the spermary each cell is converted into a sperm, in the 
 ovary one cell soon begins to grow faster than the rest, 
 becomes amoeboid in form (Fig. 51, A, ov, and Fig. 53, A), 
 sending out pseudopods amongst its companions and ingest- 
 ing the fragments into which they become broken up, thus 
 continually increasing in size at their expense. Ultimately 
 the ovary comes to consist only of this single amoeboid 
 ovum, and of a layer of superficial cells forming a capsule 
 for it. As the ovum grows yolk-spheres (Fig. 53), small 
 
236 HYDRA LESS, xxn 
 
 rounded masses of proteid material, are formed in it, and in 
 Hydra viridis it also acquires green chromatophores. 
 
 When the ovary is ripe the ovum draws in its pseudopods 
 and takes on a spherical form : the investing layer then 
 bursts so as to lay bare the ovum and allow of the free access 
 to it of the sperms. One of the latter conjugates with the 
 ovum, producing an oosperm or unicellular embryo. 
 
 The oosperm divides into a number of cells, the outer- 
 most of which become changed into a hard shell or capsule. 
 The embryo, thus protected, falls to the bottom of the water, 
 and after a period of rest develops into a Hydra. As, how- 
 ever, there are certain abnormal features about the develop- 
 ment of this genus which cannot well be understood by the 
 beginner, it will not be described in detail, but the very 
 important series of changes by which the oosperm of a 
 multicellular animal becomes converted into the adult will 
 be considered in the next lesson. 
 
LESSON XXIII 
 
 HYDROID POLYPES : BOUGAINVILLEA, DIPHYES, AND PORPITA 
 
 IT was stated in the previous lesson (p. 234) that in a 
 budding Hydra the buds do not always become detached 
 at once, but may themselves bud while still in connection 
 with the parent, temporary colonies being thus produced. 
 
 Suppose this state of things to continue indefinitely : the 
 result would be a tree-like colony or compound organism 
 consisting of a stem with numerous branchlets each ending 
 in a Hydra-like zooid. Such a colony would bear much the 
 same relation to Hydra as Zoothamnium bears to Vorticella 
 (see p. 134). 
 
 As a matter of fact this is precisely what happens in a 
 great number of animals allied to Hydra and known by the 
 name of Hydroid polypes. 
 
 Every one is familiar with the common Sertularians of the 
 sea-coast, often mistaken for sea-weeds : they are delicate, 
 much-branched, semi-transparent structures of a horny con- 
 sistency, the branches beset with little cups, from each of 
 which, during life, a Hydra-like body is protruded. 
 
 A very convenient genus for our purpose is Bougainvillea, 
 a hydroid polype found as little tufts a few centimetres long 
 attached to rocks and other submarine objects. Fig. 54, A 
 
FIG. 54. Bottgainmllea ramolsa. 
 
 A, a complete living colony of the natural size, showing the branched 
 stem and root-like organ of attachment. 
 
 B, a portion of the same magnified, showing the branched stem bear- 
 ing hydranths (hyd) and medusae (mcd), one of the latter nearly mature, 
 the others undeveloped : each hydranth has a circlet of tentacles (/) 
 surrounding a hypostome (hyp), and contains an enteric cavity (ent. cav) 
 continuous with a narrow canal (ent, cav') in the stem. The stem is 
 covered by a cuticle (cu). 
 
 c, a medusa after liberation from the colony, showing the bell with 
 tentacles (/), velum (v), manubrium (rnnb), radial (rad. c) and circular 
 (fir. c) canals, and eye-spots (oc). (After Allman.) 
 
LESS, xxiii STRUCTURE OF A HYDRA^TH 239 
 
 shows a colony of the natural size, B 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 
 mesoglcea 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 b^ar 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 medusse. 
 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 
 
2 4 o IIYDROID POLYPES LESS. 
 
 the body (endo skeleton), but like the shell of a crab or lobster 
 lying altogether outside the soft parts (exoskeletonj). 
 
 As to the mode of formation of the cuticle : we saw that 
 many organisms, such as Amoeba and Hsematococcus, 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 \)f 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 proces^ 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 
 
xxni 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. 55 D 
 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 (intJi), 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, rod. 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 endoderm- 
 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, <?<:), 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 
 
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 mesogloea (msgl) black. 
 
 A, longitudinal section of a Hydra-like body, showing the tubular body 
 with enteric cavity (ent. cav), hypostome (hyp) , mouth (mtti), 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. la}, 
 produced by partial obliteration of the continuous cavity ent. '' 
 
 hypostome, and to undergo a great thickening of its meso- 
 gloea. 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 
 mesogloea : 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-/ra:6M 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 
 
NERVOUS SYSTEM 
 
 245 
 
 cells (Fig. 56, B, n. 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 
 
 n.c 
 
 FIG. 56. A, Muscle fibres from the inner face of the bell of the 
 medusa of a hydroid polype (Eucopella campannlaria], showing nucleus 
 and transverse striation. 
 
 B, portion of the nerve-ring of the same, showing two large nerve- 
 cells (n. c) and muscle-fibres (m. 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 mesoglcea : 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 
 
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 oosperms. 
 
 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 planula : it 
 
xxni DEVELOPMENT 249 
 
 swims slowly through the water by means of its cilia, the 
 
 A _ B c 
 
 FIG. 57. Stages in the development of two hydroid polypes, Lao- 
 niedea flexiwsa (A-H) and Eudendrium ramosum (l-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. 
 
 L, 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 
 with 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, a\ 
 at one end of which are attached two structures called 
 
xxni GENERAL CHARACTERS 251 
 
 swimming-bells (m, in) in form something like the bowl of a 
 German pipe, while all along the stem spring at intervals 
 groups of structures (e), one of which is shown on an enlarged 
 scale at B. 
 
 Each group contains, first, a tubular structure (B, n) with 
 an expanded, trumpet-like mouth, through which food is 
 taken : this is clearly a hydranth. From the base of the 
 hydranth proceeds a single, long, branched tentacle or 
 " grappling-line " (/), abundantly provided with nematocysts. 
 Springing from the stem near the base of the hydranth is a 
 body called a medusoid (g), very like a sort of imperfect 
 medusa, and like it containing gonads. Lastly, enclosing all 
 these structures, much as the white petaloid bract of the 
 common Arum-lily encloses the flower-stalk, is a delicate 
 folded membranous plate, to which the name bract, borrowed 
 from botany, is applied. The whole organism is propelled 
 through the water by the rhythmical contraction of the 
 swimming-bells. 
 
 Microscopic examination shows that the stem consists, like 
 that of Bougainvillea, of ectoderm, mesoglcea, and endo- 
 derm, but without a cuticle. The hydranth has a similar 
 structure to that of Bougainvillea, only differing in shape and 
 in the absence of tentacles round the mouth : the medusoids 
 are merely simplified medusae : the swimming-bells are prac- 
 tically medusae in which the manubrium is absent : and 
 both the bracts and grappling-lines are shown by com- 
 parison with allied forms to be greatly modified medusa-like 
 structures. 
 
 Diphyes is in fact a free-swimming hydroid colony which, 
 instead of being dimorphic like Bougainvillea, '^polymorphic. 
 In addition to nutritive zooids or hydranths, it possesses 
 locomotive zooids or swimming-bells, protective zooids or 
 bracts, and tentacular zooids or grappling-lines. Morpho- 
 
252 
 
 HYDROID POLYPES 
 
 LESS. 
 
 FIG. 58. Diphyes campamilata. 
 
 A, the entire colony, natural size, showing stem (a) bearing groups 01 
 zooids (e) and two swimming bells (m, m], the apertures of which ai-e 
 marked o. 
 
 B, one of the groups of zooids marked e in A, showing common stem 
 (a), hydranth (), medusoid (g), bract (/), and branched tentacle or 
 grappling line (?'). (From Gegenbaur.) 
 
XXIII 
 
 INDIVIDUATION 
 
 253 
 
 logical and physiological differentiation are thus carried 
 much further than in such a form as Bougainvillea. 
 
 FIG. 59. A, Porpita pacifica (nat. size), from beneath, showing disc- 
 like stem surrounded by tentacles (/), a single functional hydranth (hy), 
 and numerous mouthless hydranths (hy r ). 
 
 B, vertical section of- P. mediterranea, showing the relative positions 
 of the functional (hy) and mouthless (hy 1 } hydranths, the tentacles, 
 and the chambered shell (sli). (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 conv( 
 
 mvex above and concave below. 
 
 'S7 "" 
 
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, sh). 
 
 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 of 
 Diphyes or Bougainvillea. So that Porpita is not what it 
 appears at first sight, a single individual, like a Medusa or a 
 Hydra, but a colony in which the constituent zooids have 
 become so modified in accordance with an extreme division 
 of physiological labour, that the entire colony has the 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 (spermatogenesis) 
 and of ova (oogenesis\ as well as the exact steps of the pro- 
 cess by which an oosperm or unicellular embryo is formed 
 by the union of the two sexual elements. The following 
 description applies to animals : 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 
 
 B 
 
 FIG. 60. Spermatogenesis in the Mole-Cricket (Gryllotaipa). 
 
 A. Primitive sex-cell, just preparatory to division, showing twelve 
 chromosomes (chr} ; 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. 
 
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 (Toxopneustes 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 
 
FIG 62. The' Maturation and Impregnation of the Animal Ovum. 
 
 A portion of the ovum of a Round worm (Ascaris megaiocephala], 
 
 showing the sperm (sp) in the act of conjugation, and the unaltered 
 
LESS, xxiv POLAR CELLS 261 
 
 nucleus () 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 (nu) 
 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. nu. ? ) : and the sperm (sf>), 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 , Phyllirhoe], 
 
 In E 1 the male (pr. mi. <J ) and female (pr. mi. ? ) 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^F 3 , three stages in the development of the nucleus of the oosperm 
 in a Sea-urchin (Echinus microtiiberculatns) : in F 1 the nucleus contains 
 nine chromatin-fibres (chrom. 9 ) derived from the female pronucleus, 
 and a globular mass of the same (chrom, <$ ) 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 (Eupagurus 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 SfERMATOGENESIS 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 pronncleus (D, 
 pr. nu. ?). 
 
 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 pronudeus 
 (E 1 , pr. nu. $ ). 
 
 The two pronuclei, each accompanied by its directive 
 sphere and centrosome, approach one another (E ] , E 2 ) and 
 finally unite to form the single nucleus (p 1 -F 3 ) 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 (Blp) and stomach 
 (U), the outer layer of cells (Ekt) being protective and 
 sensory, the inner (Ent) having a digestive function. The 
 
 FIG. 63. A typical animal gastrula in vertical section, showing 
 ectoderm (Ekt), endoderm (Ent), enteron or digestive cavity (7), 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. 
 
FiG. 64. Panaorina mo rum. 
 
 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 
 
 ell 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 
 
 There 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 Haematococcus 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 wall (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 Hsematococcus-like 
 zooids arranged in the form of a hollow sphere containing a 
 
F JG. 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 
 
 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 (fl.) 
 
 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 (pg) and flagella (fl). 
 
 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 
 
 ,$ 
 
 ,o 
 
 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. st) ; on its ventral surface is a trans- 
 verse triangular aperture the mouth (MtJi). The rest of 
 the body is more or less distinctly marked by annular 
 grooves (D and E, gr) into body-segments or metameres 
 
FIG. 67. Polygordius neapolitanus. 
 
 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 setae (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 
 
 * E, posterior extremity from the ventral aspect, showing the last three 
 metameres (Mtmr} separated by distinct grooves (gr), the anal seg- 
 ment (An. seg] bearing the anus (An), and a circlet of papillae (/). 
 (After Fraipont.) 
 
xxv GENERAL CHARACTERS 273 
 
 (Mtmr\ 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, \beprostoinium, 
 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 (ccel). 
 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 (Gael], 
 divided into right and left portions by the dorsal (D. Mes} and ventral 
 ( V. Mes) 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 (ft. V] 
 
 Nephridia (Nphm} 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 (Br) lies in the prostomium and is connected with the 
 ventral nerve-cord ( V. Nv. Cd} by a pair of oesophageal 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 
 coelomic epithelium represented by a beaded line, and the nervous 
 system finely dotted. 
 
 The body-wall is composed of cuticle (Cu}, deric epithelium (Der> 
 Epthm}, muscle-plates (M. PI), and parietal layer of coelomic epithe- 
 lium (Cxi. Epthm}. 
 
 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 endodermal ; Ph, pharynx ; Oes, 
 oesophagus ; Int, intestine ; Ret, rectum. 
 
 The septa (Sept} are formed of muscle covered on both sides by ccelomic 
 epithelium. 
 
 Four nephridia (Nphm} 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), deric epithelium (Der. Epthm}, muscle-plates 
 (M. PI.}, and parietal layer of coelomic epithelium (Cat. 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 coelomic 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 (deric 
 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 
 cculomate 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 mesogloea. 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. Epthm\ 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. PL} 
 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 
 ccdomic epithelium (CceL Epthm\ 
 
 A transverse section of the enteric canal shows only two 
 layers. The inner consists of elongated cells (Ent. 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 (CceL Epthm'} bounding the coelome, 
 and hence called, like the innermost layer of the body-wall, 
 coelomic epithelium. We have, therefore, to distinguish 
 two layers of coelomic epithelium, an outer or parietal layer 
 (Ccel. Epthm} which lines the body-wall, and an inner or vis- 
 ceral layer (Ccel. 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. 
 
378 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 coelomic 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 ccelomic epithelium, 
 and an inner or splanchnic layer, consisting of the visceral 
 layer of ccelomic 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 ccelomic epithelium. 
 
 The relation between the diploblastic polype and the 
 triploblastic worm may therefore be expressed in a tabular 
 form as follows 
 
 Hydroid. Polygordius. 
 
 Cuticle Cuticle. 
 
 Ectoderm .... Deric epithelium or epidermis. 
 
 . Musfele-plates. 
 
 Somatic \ Coelomic epithelium 
 Mesoderm . J i ayer } (parietal layer). 
 (mdimentary) : Splanchnic f Ccelomic epithelium 
 
 layer I (visceral layer). 
 Endoderm Enteric epithelium. 
 
 i In the majority of the higher animals there is a layer of muscle 
 between the enteric and ccelomic 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 
 coelomic epithelium, the three together forming the body- 
 wall; the inner tube (enteric epithelium) is covered ex- 
 ternally by a layer of coelomic 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. Afes), which extend longitudinally 
 from the dorsal and ventral surfaces of the canal to the body 
 wall, dividing the coelome into right and left halves. The 
 structure of the mesenteries is seen in a transverse section 
 (Fig. 68, c, and Fig. 71, A) which shows that at the middle 
 
280 POLYGORDIUS LESS. 
 
 dorsal line the parietal layer of ccelomic 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 the canal, they are continued downwards as the ventral 
 mesentery, and on reaching the body-wall diverge once more 
 to join the parietal layer. Thus the mesenteries are simply 
 formed of a double layer of 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 or metameric segmen- 
 tation affects the coelome as well as the body-wall. Each 
 septum is composed of a sheet of muscle covered on both 
 sides with ccelomic 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 coelome. 
 
 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 
 ossophagus (Oes), is confined to the peristomium and is distin- 
 guished by its thick walls and comparatively great diameter ; 
 the third or intestine (Int) extends from the first metamere 
 to the last i.e., from the segment immediately following 
 the peristomium to that immediately preceding the anal 
 
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 j 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 
 cwlomic 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 coelomic 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 coelomic 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 
 
 5 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 HEMOGLOBIN 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 
 common earthworm, for instance, the blood is red, the colour 
 being due to the same pigment, hcemoglobin, which occurs 
 in our own blood and in that of other vertebrate animals. 
 
 Haemoglobin is a nitrogenous compound, containing, in 
 addition to carbon, hydrogen, nitrogen, oxygen, and sulphur, 
 a minute quantity of iron. It can be obtained pure in the 
 form of crystals which are soluble in water. Its most 
 striking and physiologically its most important property is 
 its power of entering into a loose chemical combination with 
 oxygen. If a solution of haemoglobin is brought 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 nephridia, of which each metamere possesses a pair, 
 one on either side (Fig. 68, A, B, and c, Nphm). 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. /). 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 coelome. The whole interior of the 
 tube as well as the inner face of the nephrostome is lined 
 with cilia which work outwards. 
 
 Nph.st 
 
 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. /).' (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 fOLYGORDIUS 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, JBr.} 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 cesophageal connectives, the enteric 
 canal perforates the nervous system. 
 
 It is also important to notice that the nervous system is 
 throughout in direct contact with the epidermis or ectoderm, 
 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 
 
xxv 
 
 NERVOUS SYSTEM 
 
 287 
 
 (Fig. 70, Nv. F.} interspersed with nerve-cells (Nv. C). In 
 the cord the fibres are arranged longitudinally, and the 
 nerve-cells are ventral in position, forming a layer in imme- 
 
 ter.Epthm, 
 
 FIG. 70. Diagram illustrating the relations of the nervous system oi 
 Polygordius. 
 
 The deric epithelium (Dcr. 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 (int.. muse, plex.} : the 
 latter is connected to the muscle-plates (M. PI) by efferent nerves (Ef. 
 nv). 
 
 The central nervous system consists of nerve- fibres (Nv. F) and 
 nerve-cells (Nv. C) : other nerve-cells (Nv. C) occur at intervals in 
 the inter-muscular plexus. 
 
 The muscle-plates (M. PI), one of which is entire, while only the 
 middle part of the other is shown, are invested by a delicate protoplasmic 
 network, containing nuclei (nu), 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. nv.) to the delicate proto- 
 plasmic layer covering the muscle-plates. The superficial 
 cells of both brain and cord are also, as has been said, in 
 direct connection with the overlying epidermis, and from the 
 anterior end of the brain a bundle of nerve-fibres (Fig. 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. nv.) 
 which connect the central nervous system with the epidermis, 
 and (b) motor or efferent (Ef. nv.) which connect it with the 
 muscles. 
 
 Comparing the nervous system of Polygordius with that 
 of 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 intermediatidf 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. PI.) may be made to contract by a 
 stimulus applied (a) to itself directly, (b) 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, i.e., the regulation 
 of the various movements to a common end, is lost when 
 the brain is removed. 
 
 The nervous system is thus an all-important means of 
 communication between the various parts of the organism 
 and between the organism and the external world. The 
 outer or sensory surface is by its means brought into 
 connection with the entire muscular system with such 
 perfection that the slightest touch applied to one end of the 
 body may be followed by the almost instantaneous contrac- 
 tion of muscles at the other. 
 
 In some species of Polygordius the prostomium bears a 
 pair of eye-specks, but in the majority of species the adult 
 animal is eyeless, and, save for the ciliated pits (Fig. 67, 
 B, c.p], the function of which is not known, the only definite 
 organs of sense are the tentacles, which have a tactile 
 function, their abundant nerve-supply indicating that their 
 delicacy as organs of touch far surpasses that of the general 
 surface of the body. They are beset with short, fine pro- 
 cesses of the cuticle called setce (Figs. 67 and 68, s), which 
 probably, like the whiskers of a cat, serve as conductors of 
 external stimuli to the sensitive epidermic cells. 
 
xxv PHYSIOLOGICAL DIFFERENTIATION : ORGANS 291 
 
 There are two matters of general importance in connec- 
 tion with the structure of Polygordius to which the student's 
 attention must be drawn in concluding the present lesson. 
 
 Notice in the first place how in this type, far more than in 
 any of those previously considered, we have certain definite 
 parts of the body set apart as organs for the performance of 
 particular functions. There is a mouth for the reception of 
 food, an enteric canal for its digestion, and an anus for the 
 extrusion of faeces : a coelomic fluid for the transport of the 
 products of digestion to the more distant parts of the body : 
 a system of blood-vessels for the transport of oxygen to and 
 of carbon dioxide from all parts : an epidermis as organ of 
 touch and of respiration : nephridia for getting rid of water 
 and nitrogenous waste : and a definite nervous system for 
 regulating the movements of the various parts and forming 
 a means of communication between the organism and the 
 external world. It is clear that differentiation of structure 
 and division of physiological labour play a far more obvious 
 and important part than in any of the organisms hitherto 
 studied. 
 
 Notice in the second place the vastly greater complexity 
 of microscopic structure than in any of our former types. 
 The adult organism can no longer be resolved into more or 
 less obvious cells. In the deric, enteric, and coelomic 
 epithelia we meet with nothing new, but the muscle-plates 
 are not cells, the nephridia show no cell-structure, neither do 
 the nerve-fibres nor the punctate substance of the brain. 
 The body is thus divisible into tissues or fabrics each clearly 
 distinguishable from the rest. We have epithelial tissue, 
 cuticular tissue, muscular tissue, and nervous tissue : and 
 the blood and coelomic fluid are to be looked upon as 
 liquid tissues. One result of this is that, to a far greater 
 extent that in the foregoing types, we can study the 
 
 u 3 
 
292 POLYGORDIUS LESS, xxv 
 
 morphology of Polygordius under two distinct heads : 
 anatomy, dealing with the general structure of the parts, 
 and histology, dealing with their minute or microscopic 
 structure. 
 
 One point of importance must be specially referred to in 
 connection with certain of the tissues. It has been pointed 
 out (p. 276) that the epidermis has rather the character of 
 a sheet of protoplasm with regularly-arranged nuclei than of 
 a layer of cells, and that the muscle-plates are covered with 
 a layer of protoplasm with which the ultimate nerve-fibres 
 are continuous (p. 277). Thus certain of the tissues of 
 Polygordius exhibit continuity of the protoplasm, a phenomenon 
 which appears to be of wide occurrence both in animals 
 and in plants. 
 
LESSON XXVI 
 POLYGORDIUS (Continued} 
 
 ASEXUAL reproduction is unknown in Polygordius, and 
 the organs of sexual reproduction are very simple. The 
 animal is dioecious, gonads of one sex only being found in 
 each individual. 
 
 In the species which has been most thoroughly investi- 
 gated (P. neapolitanus] the reproductive products are formed 
 in each metamere from the fourth to the last. Crossing 
 these segments obliquely are narrow bands of muscle (Fig. 
 71, A, O.M) and certain of the cells of ccelomic epithelium 
 covering these bands multiply by fission and form little 
 heaps of cells (Spy\ each of which is to be looked upon as a 
 gonad. There is thus a pair of gonads to each segment with 
 the exception of the prostomium, the peristomium, the first 
 three metameres, and the anal segment, the reproductive 
 organs exhibiting the same simple metameric arrangement 
 as the digestive, excretory, and circulatory organs. It will 
 be noticed that the primitive sex-cells, arising as they do 
 from ccelomic epithelium, are mesodermal structures, not 
 ectodermal as in hydroids (pp. 234 and 247). 
 
 In the male the primitive sex-cells divide and sub-divide, 
 the ultimate products being converted into sperms (Fig. 71, 
 
D.I 
 
 C'a 
 
 M.PI 
 
 Sff> 
 
 FiG. 71. Polygordins neapolitanus. 
 
 A, transverse section of a male specimen to show the position of the 
 immature gonads (spy) and the precise form and arrangement of the 
 various layers represented diagrammatically in Fig. 68, C. 
 
 The body-wall consists of cuticle (Cu), cleric epithelium (Der. Epthtn), 
 muscle-plates (M. Pl\ and parietal layer of ccelomic epithelium (Ccvl. 
 Epthm). The ventral nerve cord ( V. Nv. Cd) is shown to be continu- 
 ous with the deric epithelium. 
 
 The enteric canal consists of ciliated enteric epithelium (Ent. Epthm} 
 covered by the visceral layer of ccelomic epithelium (Ccel. Epthm) : 
 connecting it with the body-wall are the dorsal and ventral mesenteries 
 formed of a double layer of coelomic epithelium, and containing respec- 
 tively the dorsal (D. V) and ventral (V. V) blood-vessels. 
 
 Passing obliquely across the coelome are the oblique muscles (0. M) 
 
LESS, xxvi DEVELOPMENT 295 
 
 covered with ccelomic epithelium : by differentiation of groups of cells 
 of the latter the spermaries (Spy] are formed. 
 
 B, a single sperm, showing expanded head and delicate tail. 
 
 c, horizontal section of a sexually mature female. 
 
 The body- wall (Ctt, Der. Epthm, M. PI] has undergone partial 
 histological degeneration, and is ruptured in two places to allow of the 
 escape of the ova (ov} which still fill the ccelomic spaces enclosed between 
 the body-wall, the enteric canal (Ent. Epthm), and the septa (Se). 
 (After Fraipont.) 
 
 B : see p. 255) : in the female they enlarge immensely, and 
 take on the character of ova (c, ov). Multiplication of the 
 sexual products takes place to such an extent that the whole 
 ccelome becomes crammed full of either sperms or ova (c). 
 
 In the female the growth of the eggs takes place at the 
 expense of all other parts of the body, which undergo more 
 or less complete atrophy : the epidermis, for instance, be- 
 comes liquefied and the muscles lose their contractility. 
 Finally rupture of the body-wall takes place in each segment 
 (c), and through the slits thus formed the eggs escape. So 
 that Polygordius, like an annual plant, produces only a 
 single brood : death is the inevitable result of sexual 
 maturity. Whether or not the same dehiscence of the 
 body-wall takes place in the male is not certain : it has 
 been stated that the sperms make their escape through the 
 nephridia. 
 
 Thus while there are no specialized gonaducts, or tubes for 
 carrying off the sexual products, it is possible that the ne- 
 phridia may, in addition to their ordinary function, serve 
 the purpose of male gonaducts or spermiducts. Female gona- 
 ducts or oviducts are however entirely absent. 
 
 The ova and sperms being shed into the surrounding water, 
 impregnation takes place, and the resulting oosperm under- 
 goes segmentation or division (see p. 248), a polyplast being 
 formed. By the arrangement of its cells into two layers and 
 
296 POLYGORDIUS LESS. 
 
 the formation of an enteron or digestive cavity the polyplast 
 becomes a gastmla (see p. 265) which by further develop- 
 ment is converted into a curious free-swimming creature 
 shown in Fig. 72, A, and called a trochosphere. 
 
 FIG. 72. A, larva of Polygordius neapolitanus in the trochosphere 
 stage ; from a living specimen. 
 
 B, diagrammatic vertical section of the same : the ectoderm is dotted, 
 the endoderm radially striated, the mesoderm evenly shaded, and the 
 nervous system finely dotted. 
 
 C, transverse section through the plane ab in B. 
 
 The body -wall consists of a single layer of ectoderm cells, which, at 
 the apex of the prostomium (upper hemisphere) are modified to form the 
 brain (Br) and a pair of ocelli (oc). 
 
 The enteric canal consists of three parts : the stomodseum (St. dm], 
 opening externally by the mouth (Mth\ and lined by ectoderm ; the 
 enteron (Ent) lined by endoderm ; and the proctodseum (Prc. dm), 
 opening by the anus (An) and lined by ectoderm. 
 
 Between the body- wall and the enteric canal is the larval body-cavity 
 or blastocoele (Bl. cazl). 
 
 The mesoderm is confined to two narrow bands of cells (B and C, 
 Msd) in the blastocoele, one on either side of the proctodasum ; slender 
 mesodermal bands (Msd') are also seen in the prostomium in A. 
 
 The cilia consists of a pras-oral circlet (Pr. or. ci) above the mouth, a 
 post-oral circlet (Pt. or. ci) below the mouth, and an anal circlet (An. 
 ci) around the anus. 
 
 (A after Fraipont. ) 
 
 The trochosphere, or newly-hatched larva of Polygordius 
 (Fig. 72, A) is about \ mm. in diameter, and has something 
 the form of a top, consisting of a dome-like upper portion, 
 the prostomium^ produced into a projecting horizontal rim; 
 
xxvi THE TROCHOSPHERE 297 
 
 of an intermediate portion or peristomium, having the form 
 of an inverted hemisphere ; and of a lower somewhat conical 
 anal region. Around the projecting rim is a double circlet 
 of large cilia (Pr. or. a) by means of which the larva is 
 propelled through the water. 
 
 Beneath the edge of the ciliated rim is a rounded aperture, 
 the mouth (Mth); this leads by a short, nearly straight 
 gullet (St. dm), into a spacious stomach (Ent\ from the 
 lower side of which proceeds a short slightly curved intestine 
 (Prc. dm), opening at the extremity of the conical inferior 
 region by an anus (An). Between the body-wall and the 
 enteric canal is a space filled with fluid (Bl. cat), but, as we 
 shall see, this does not correspond with the body-cavity of 
 the adult. The body-wall and the enteric canal consist each 
 of a single layer of epithelial cells, all the tissues included in 
 the adult under the head of mesoderm (p. 278) being absent 
 or so poorly developed that they may be neglected for the 
 present. 
 
 Leaving aside all details, it will be seen that the trocho- 
 sphere of Polygordius is comparable in the general features 
 of its organization to a medusa (compare Fig. 55, p. 242), 
 consisting as it does of an outer layer of cells forming the 
 external covering of the body and of an inner layer lining 
 the digestive cavity. There are, however, two important 
 differences : the space between the two layers is occupied by 
 the mesoglcea in the medusa, while in the worm it is a cavity 
 filled with fluid ; and the digestive cavity of the trochosphere 
 has two openings instead of one. 
 
 But in order to compare more accurately the medusa 
 with the trochosphere, it is necessary to fill up, by the help 
 of other types, an important gap in our knowledge of the 
 development of Polygordius the passage from the gastrula 
 to the trochosphere. From what we know of the do^dbp- 
 
298 
 
 POLYGORDIUS 
 
 LESS. 
 
 ment of other worms, the process, in its general features, 
 is probably as follows : 
 
 The ectoderm and endoderm of the gastrula (Fig. 73, A) 
 are not in close contact with one another as in Fig. 63 
 (p. 265), but are separated by a space filled with fluid the 
 blastocozle or larval body-cavity. The mouth of the gastrula 
 closes (), the enteron (Eni), being thus converted into a 
 shut sac. At about the same time the ectoderm is tucked 
 
 FIG. 73. Diagram illustrating the origin of the trochosphere from 
 the gastrula. The ectoderm is dotted, the endoderm striated. 
 
 A, gastrula, with enteron (Ent} and gastrula-mouth (Cast. Mi/i), and 
 with the ectoderm and endoderm separated by the larval body-cavity or 
 blastocoele (Bl ccel). 
 
 B, the gastrula-mouth has closed, the enteron (Ent} becoming a shut 
 sac. 
 
 c, two ectodermal pouches, the stomodseum (St. dm) and proctodseum 
 (Prc. dm) have appeared. 
 
 D, the stomodaeum (St. dm) and proctodseum (Prc. dm) have opened 
 into the enteron (nt), forming a complete enteric canal with mouth 
 (Mth) and anus (An). 
 
 in or invaginated at two places (C), and the two little 
 pouches (St. dm, Prc. dni) thus formed grow inwards until 
 they meet with the closed enteron and finally open into it 
 (D), so that a complete enteric canal is formed formed, 
 we must not fail to notice, of three distinct parts : (i) an 
 anterior ectodermal pouch, opening externally by the mouth, 
 and distinguished as the stomodceum ; (2) the enteron, lined 
 with endoderm ; and (3) a posterior ectodermal pouch, 
 opening externally by the anus, and called the proctodaum. 
 
xxvi METAMORPHOSIS 299 
 
 In the trochosphere (Fig. 72) the gullet is derived from 
 the stomodseum, the stomach from the enteron, and the 
 intestine from the proctodaeum ; so that only the stomach of 
 the worm-larva corresponds with the digestive cavity of a 
 medusa : the gullet and intestine are structures not repre- 
 sented in the latter form. 
 
 Two or three other points in the anatomy of the trocho- 
 sphere must now be referred to. 
 
 At the apex of the dome-shaped prostomium the ecto- 
 derm is greatly thickened, forming a rounded patch of cells 
 (Figs. 72 and 74, JBr\ the rudiment of the brain. On the 
 surface of the same region and in close relation with the 
 brain is a pair of small patches of black pigment, the 
 eye-spots or ocelli (Oc). 
 
 On either side of the intestine, between its epithelium and 
 the external ectoderm, is a row of cells forming a band 
 which partly blocks up the blastoccele (B and c, Msd\ These 
 two bands are the rudiments of the whole of the meso- 
 dermal tissues of the adult muscle, coelomic epithelium, 
 &c. and hence called mesodermal bands. 
 
 Finally on either side of the lower or posterior end of the 
 stomach is a delicate tube (Fig. 74, A, Nph) opening by a 
 small aperture on to the exterior, and by a wide funnel- 
 shaped extremity into the blastocoele : it has all the relations 
 of a nephridium, and is distinguished as the head-kidney. 
 
 As the larva of Polygordius is so strikingly different from 
 the adult, it is obvious that development must, in this, as in 
 several cases which have come under our notice, be accom- 
 panied by a metamorphosis. 
 
 The first obvious change is the elongation of the conical 
 anal region of the trochosphere into a tail-like portion which 
 
3oo 
 
 POLYGORDIUS 
 
 LESS. 
 
 may be called the trunk (Fig. 74, A). The stomach 
 (enteron), which was formerly confined to the pro- and peri- 
 stomium, has now grown for a considerable distance into 
 
 Br 
 
 B 
 
 An c 
 
 FIG. 74. A, living specimen of an advanced trochosphere-larva of 
 Polygordius neapolitanus, showing the elongation of the anal region to 
 form the trunk. 
 
 B, diagrammatic vertical section of the same : the ectoderm is coarsely, 
 the nervous system finely, dotted, the endoderm radially striated, and 
 the mesoderm evenly shaded. 
 
 C, transverse section through the plane ab in B. 
 
 The pre-oral (Pr. or. ci), post-oral (Pt. or. a'.), and anal (An. ci) 
 cilia, brain (Br}, ocelli (Oc), blastocoele (BL), mouth (Mth\ stomo- 
 daeum (St. dm}, proctodseum (Prc. dm), and anus (An) as in Fig. 72, 
 A : the enteron (Ent) has extended some distance into the trunk. 
 
 In A, slender mesodermal bands (Msd. bd) in the prostomium, and the 
 branched head-kidney (NpK] are shown. 
 
 In B and C the mesoderm (Msd) is seen to have obliterated the blasto- 
 ccele in the trunk-region : the ectoderm has undergone a thickening, 
 forming the ventral nerve -cord ( V. Nv. Cd). 
 
 (A after Fraipont. ) 
 
 the trunk (B, enf), so that the proctodaeum (Prc. dm) 
 occupies only the portion in proximity to the anus. 
 
 Important internal changes have also taken place. The 
 deric epithelium or external ectoderm is for the most part 
 composed, as in the preceding stage, of a single layer of 
 
xxvi DEVELOPMENT OF METAMERES 301 
 
 cells ; but on that aspect of the trunk which lies on the same 
 side as the mouth i.e., 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 increased to such an extent as to surround com- 
 pletely the enteron and obliterate the blastocoele (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 acoelomate. 
 
 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 proctodseum (Prc. dm) 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. d) round the posterior end of the trunk. 
 
 The internal changes undergone during the assumption of 
 the present form are very striking. In every fully formed 
 metamere the mesoderm solid, it will be remembered, 
 in the previous stage has become divided into two layers, 
 a somatic layer (B and c, Msd (soni) ) in contact with the 
 ectoderm and a splanchnic layer (Msd (spl) ) in contact 
 with the endoderm. The space between the two layers 
 (Cxi] 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 
 blastocoele, 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 
 
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 (Pt. or. ci), and anal (An. ci) 
 cilia, the blastocoele (Bl. ccel), stomodseum (6V. dm], and proctodseum 
 (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 (sotn) ) 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 
 coelome 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. Epthm) : 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. Epthm). 
 
 (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 (D\ Msd 
 (Som}\ but before long each cell splits up in a radial 
 direction (D 2 ) from without inwards i.e., from the ectoderm 
 (Der. Epthni) towards the ccelome finally taking on the 
 form of a book with four or more slightly separated leaves 
 directed outwards or towards the surface of the body, and 
 with its back the undivided portion of the cell bounding 
 the ccelome. The cells being arranged in longitudinal series, 
 we have a number of such books placed end to end in 
 a row with the corresponding leaves in contact page one 
 of the first book being followed by page one of the second, 
 third, fourth, &c., page two by page two, and so on through 
 one or more segments of the trunk. Next, what we have 
 compared with the leaves of the books the divided 
 portions of the cells become separated from the backs 
 the undivided portions (D S ) and each leaf (M~. PI) fuses 
 with the corresponding leaves of a certain number of books 
 in the same longitudinal series. The final result is that the 
 undivided portions of the cells (backs of the books, Ca>l. 
 Epthni) become the parietal layer of ccelomic epithelium, the 
 
xxvi SIGNIFICANCE OF DEVELOPMENTAL STAGES 305 
 
 longitudinal bands formed by the union of the leaves 
 (M. PI) becoming the muscle-plates, which are thus cell- 
 fusions^ each being formed by the union of portions of 
 a series of longitudinally arranged cells. 
 
 At the same time the cells of the splanchnic layer 
 of mesoderm thin out and become the visceral layer of 
 ccelomic epithelium. 
 
 We see then that by the time the larva has reached the 
 stage shown in Fig. 75, it is no longer a mere aggregate of 
 simple cells arranged in certain layers. The cells them- 
 selves have undergone differentiation, some becoming modi- 
 fied into nerve-fibres, others by division and subsequent 
 fusion with their neighbours forming muscle-plates, while 
 others, such as the epithelial cells, remain almost unaltered. 
 
 Thus, in the course of the development of Polygordius, 
 cell-multiplication and cell-differentiation go hand in hand, 
 the result being the formation of those complex tissues the 
 presence of which forms so striking a difference between the 
 worm and the simpler types previously studied. 
 
 It is important to notice that this comparatively complex 
 animal is in one stage of its existence the oosperm as 
 simple as an Amoeba ; in another the polyplast it is com- 
 parable to a Pandorina or a Volvox; in a third the 
 gastrula it corresponds in general features with a Hydra ; 
 while in a fourth the trochosphere it resembles in many 
 respects a Medusa. As in other cases we have met with, 
 the comparatively highly-organized form passes through 
 stages in the course of its individual development similar in 
 general characters to those which, on the theory of evolution, 
 its ancestors may be considered to have passed through in 
 their gradual ascent from a lower to a higher stage of 
 organization. 
 
 x 
 
306 POLYGORDIUS LESS, xxvi 
 
 The rest of the development of Polygordius may be 
 summarized very briefly. The trunk grows so much faster 
 than the head (pro-flhts peri-stomium) that the latter under- 
 goes a relative diminution in size, finally becoming of equal 
 diameter with the trunk, as in the adult. The ciliated rings 
 are lost, the tentacles grow to their full size, the eye-spots 
 atrophy, and thus the adult form is assumed. 
 
LESSON XXVII 1 
 
 THE GENERAL CHARACTERS OF THE HIGHER ANIMALS 
 
 THE student who has once thoroughly grasped the facts of 
 structure of such typical unicellular animals as Amoeba and 
 the Infusoria, of such typical diploblastic animals as Hydra 
 and Bougainvillea, and of such a typical triploblastic animal 
 as Polygordius, ought to have no difficulty in understanding 
 the general features of the organization of any other members 
 of the animal kingdom. When once the notions of a cell, a 
 cell-layer, a tissue, body-wall, enteron, stomodaeum, procto- 
 dseum, coelome, somatic and splanchnic mesoderm, are fairly 
 understood, all other points of structure become hardly more 
 than matters of detail. 
 
 If we turn to any text-book of Zoology we shall find that 
 the animal kingdom is divisible into seven primary sub- 
 divisions, called sub-kingdoms, types, or phyla. These are 
 as follows : 
 
 Protozoa. Ccelenterata. 
 
 Venues. Echinodermata. 
 
 Arthropoda. Mollusca. 
 Vertebrata 
 
 1 Readers who have not studied zoology, or at least examined a series 
 of selected animal types, should omit this lesson and go on to the next. 
 
 X 2 
 
3o8 CHARACTERS OF THE HIGHER ANIMALS LESS. 
 
 With a few exceptions, the discussion of which would be out 
 of place here, the vast number of animals known to us may 
 be arranged in one or other of these groups. 
 
 The Protozoa are the unicellular animals : they have been 
 represented in previous lessons by Amoeba and Protamoeba, 
 Hsematococcus, Heteromita, Euglena, the Mycetozoa, Para- 
 mcecium, Stylonychia, Oxytricha, Opalina, Vorticella, Zooth- 
 amnium, the Foraminifera, and the Radiolaria. According to 
 many authors, Pandorina and Volvox are also included in 
 this group. The reader will therefore have no difficulty in 
 grasping the general features of this phylum. 
 
 The Cxlenterata are the diploblastic animals, and have 
 also been well represented in the foregoing pages, namely, 
 by Hydra, Bougainvillea, Diphyes, and Porpita. The sea- 
 anemones, corals, and sponges also belong to this phylum. 
 
 The Vermes, or Worms, are a very heterogeneous assem- 
 blage. They are all triploblastic, but while some are 
 ccelomate, others have no body-cavity; some, again, are 
 segmented, others not. Still, if the structure of Polygordius 
 is thoroughly understood, there will be little difficulty in 
 understanding that of a fluke, a tape-worm, a round-worm, 
 an earthworm, or one of the ordinary marine worms. 
 
 Of the remaining four sub-kingdoms we have, so far, 
 studied no example, but a brief description of a single 
 typical form of each will show how they all conform to the 
 general plan of organization of Polygordius, being all triplo- 
 blastic and ccelomate. 
 
 Under the Echinodermata are included the various kinds 
 of starfishes sand-stars, brittle-stars, and feather-stars, as 
 well as sea-urchins, sea-cucumbers, &c. A starfish will serve 
 as an example of the group. 
 
 The phylum Arthropoda includes crayfishes, lobsters, 
 crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions, 
 
GENERAL STRUCTURE 39 
 
 spiders, and mites ; centipedes and millipedes ; and all 
 kinds of insects, such as cockroaches, beetles, flies, ants, 
 bees, butterflies, and moths. A crayfish forms a very fair 
 type of the group. 
 
 In the phylum Mollusca are included the ordinary bi- 
 valves, such as mussels and oysters ; snails, slugs, and other 
 univalves or one-shelled forms ; sea-butterflies ; and cuttle- 
 fish, squids, and Octopi. An account of a fresh-water 
 mussel will serve to give a general notion of the character 
 of this group. 
 
 Finally, under the head of Vertebrata are included all the 
 backboned animals : the lampreys and hags ; true fishes, 
 such as the shark, skate, sturgeon, cod, perch, trout, &c. ; 
 amphibians, such as frogs, toads, newts, and salamanders ; 
 true reptiles, such as lizards, crocodiles, snakes, and tor- 
 toises ; birds ; and mammals, or creatures with a hairy skin 
 which suckle their young, such as the ordinary hairy 
 quadrupeds, whales and porpoises, apes, and man. The 
 essential structure of a vertebrate animal will be understood 
 from a brief description of a dog-fish. 
 
 THE STARFISH. 1 
 
 A common starfish consists of a central disc-like portion, 
 from which radiate five arms or rays. It crawls over the 
 rocks with its ventral surface downwards, its dorsal surface 
 upwards. It can move in any direction, so that, in the 
 ordinary sense of the words, anterior and posterior extremi- 
 ties cannot be distinguished. Radial symmetry such as this, 
 i.e., the division of the body into similar parts radiating from 
 a common centre, is characteristic of the Echinodermata 
 generally. 
 
 1 For a detailed description of a Starfish, see Rolleston and Hatchett 
 Jackson, Forms of Animal Life (Oxford, 1888), pp. 190 and 311. 
 
Tire, fa 
 
 FIG. 76. "Diagrammatic sections of a Starfish. 
 
 A, vertical section passing on the right through a radius on the left 
 through an inter-radius. The off side of the ambulacral groove with 
 the tube feet ( T, F) and ampullae (Amp) are shown in perspective. 
 
 B, transverse section through an arm. 
 
 The ectoderm is coarsely dotted, the nervous system finely dotted, the 
 endoderm radially striated, the mesoderm evenly shaded, the ossicles of 
 the skeleton black, and the ccelomic epithelium represented by a beaded 
 line. 
 
 The body-wall consists of deric epithelium (Der. Eptkm), derails 
 (Derm), and the parietal layer of ccelomic epithelium (Ccel. Epthni). 
 
 To the body-wall are attached pedicellariae (Ped), and the end of the 
 arm bears a tentacle (/) with an ocellus (oc) at its base. 
 
 The skeleton consists of ossicles (as) imbedded in the derm : large 
 ambulacral ossicles (Amb. os) bound the ambulacral grooves on the 
 ventral surfaces of the arms. 
 
 The mouth (Mth) leads by a short gullet into a stomach (St), which 
 gives off a cardiac caecum (Cd. ca>) and a pair cf pyloric caeca (Pyl. ccc) 
 to each arm, and passes into an intestine (hit} which gives off intestinal 
 caeca (Int. cce) to the inter-radii, and ends in the anus (An). The 
 pyloric caeca are connected to the dorsal body- wall by mesenteries 
 (Mes. in B). The wall of the enteric canal consists of enteric epithelium 
 covered by the visceral layer of coelomic epithelium (Ccel. Epthm'}. 
 
 From .the ccelome are given off respiratory caeca (Resp. CCE), which 
 project through the body-wall : the latter contains peri-haemal spaces 
 (p. h) derived from the ccelome. 
 
LESS, xxvn TUBE-FEET 3 11 
 
 The circular blood-vessel (C. B. V) surrounds the gullet and gives 
 off radial vessels (Rad. B. V} to the arms and an inter-radial plexus 
 connected with a pentagonal ring round the intestine. 
 
 The circular ambulacral vessel (C. Amb. V} gives off radial vessels 
 (Rad. Amb. V} to the arms connected with the ampulla; (Amp} and 
 tube-feet (T. F) : it is also connected with the stone-canal (St. Q, which 
 opens externally by the madreporite (Mdpr). 
 
 The nerve-ring (Nv. R] gives off radial nerves (Rad. Nv) to the 
 arms. 
 
 The ovary (Ovy] is inter-radial, and opens by a dorsal oviduct (Ovd]. 
 
 In the centre of the disc on the ventral surface is the large 
 mouth (Fig. 76, A, Mth\ and from it radiate five grooves, 
 one along the ventral surface of each arm (A and B). In the 
 living animal numerous delicate semi-transparent cylinders, 
 the ttibefeet (T. F), are protruded from these grooves; they 
 are very extensible and each ends in a sucker. It is by 
 moving these structures in various directions, protruding 
 some and withdrawing others, that the starfish is able to 
 move along either a horizontal or a vertical surface, and 
 even to turn itself over when placed with the ventral side 
 upwards. 
 
 Near the middle of the disc, on the dorsal surface, is the 
 very minute anus (A, An) ; it is situated on a line drawn 
 from the centre of the disc to the re-entering angle between 
 two of the rays, and is therefore said to be inter-radial in 
 position. Near the anus, and also inter-radially situated, is 
 a circular calcareous plate, the madreporite (Mdpr), per- 
 forated by numerous microscopic apertures. Innumerable 
 other calcareous plates, or ossicles (os), are embedded in 
 the body-wall, and constitute a skeleton, to which the firm 
 and resistant character of the starfish is due. 
 
 Sections show that there is a well-marked ccelome, separ- 
 ating the body-wall from the enteric canal and containing 
 the gonads, blood-vessels, &c. The body-wall consists ex- 
 ternally of a very thin cuticle, then of a layer of deric 
 
312 THE STARFISH LESS. 
 
 epithelium or epidermis (Der. EptJwi], then of a thick 
 fibrous layer (Derm) the dermis or deep layer of the skin, 
 then of a thin and interrupted layer of muscle, and finally, 
 of a layer of coelomic epithelium (CoeL Epthui] bounding 
 the body cavity. 
 
 The dermis is formed of connective tissue, a substance not 
 met with in Polygordius, formed by the elongation of meso- 
 derm cells into wavy fibres. The ossicles of the skeleton 
 (as) are formed by deposits of calcium carbonate in the 
 dermis ; the skeleton is therefore a dermal exoskeleton. 
 The large ambulacral ossicles (Amb. os), however, which 
 bound the ambulacral grooves, lie internal to the vessels 
 (Rad. B. V., Rad. Amb. V.) and have an endoskeletal 
 character. 
 
 The enteric canal passes vertically from mouth (A, Mth) 
 to anus (An), and is divisible into gullet, stomach (St\ and 
 intestine (Int). The stomach gives off five wide pouches 
 (Cd. cce), one extending into the base of each arm, and 
 above these five other pouches (Pyl. cce), each of which 
 divides into two (B, PyL CCE) and extends to the extremity 
 of the corresponding arm. The intestine gives off smaller 
 pouches (Int, cce) which are inter-radial in position. Thus 
 the enteric canal, like the body as a whole, exhibits radial 
 symmetry. The canal is lined by enteric epithelium, mostly 
 endodermal, and is covered externally by coelomic epithelium 
 (Ccel. Epthm'}. 
 
 Respiration is affected by blind, finger-like offshoots of the 
 coelome, the respiratory cczca (Resp. coe], which pass between 
 the ossicles of the skeleton and project on the surface of the 
 body, thus bringing the coelomic fluid into close relation 
 with the surrounding water. 
 
 The blood-system consists of a circular vessel (A, C. B. V) 
 round the gullet, connected with a pentagonal vessel round 
 
xxvii NERVOUS SYSTEM, ETC. 313 
 
 the intestine by an elongated network or plexus of vessels. 
 From the circular vessel five radiating trunks (Rad. B. V] 
 pass to the arms. 
 
 Parallel with and above the circular blood-vessel is a 
 similar but larger structure, the ambulacral ring ( C. Amb. V) 
 which also sends off five radiating vessels (Rad. Amb. V] to 
 the arms. These give off a branchlet to each tube-foot 
 (B, T.F.\ the branchlet having a sac or ampulla (Amp) at 
 its base. From the ambulacral ring a tube with calcareous 
 walls, the stone-canal (St. C} passes upwards and ends 
 in the madreporite (Mdpr\ by the apertures in which the 
 fluid filling the whole of the ambtilacral system of vessels is 
 placed in communication with the surrounding water. 
 
 The function of the ambulacral system is mainly locomo- 
 tive. By the contraction of the ampullae fluid is forced into 
 the tube feet, and by the action of the muscles of the tube- 
 feet it is sent back into the ampullae, and in this way the 
 tube-feet are protruded and retracted at the will of the 
 animal. The system, which is peculiar to the Echinodermata, 
 is lined with epithelium, continuous, in the larva, with the 
 ccelomic epithelium. It has been compared to a gigantic 
 and greatly modified nephridium. 
 
 The nervous system is very simple. It consists of a 
 pentagonal ring (A, Nv." R] round the mouth giving off 
 five radial nerves (A and B, Rad. Nv) which pass along the 
 ambulacral grooves, below the blood-vessels, to the ex- 
 tremities of the arms, where each is connected with an eye- 
 spot. Both nerve-ring and radial nerves are mere thicken- 
 ings of the deric epithelium. 
 
 The gonads (A, Ovy) are branched organs, five in num- 
 ber, which lie inter-radially near the bases of the arms, and 
 open by gonaducts (Ovd) on the dorsal surface of the disc. 
 The sexes are lodged in distinct individuals. 
 
3H THE CRAYFISH LESS. 
 
 Both eggs and sperms are shed into the water, and after 
 impregnation the oosperm becomes a gastrula, which is con- 
 verted into a peculiar free-swimming larva ; this undergoes 
 metamorphosis and is converted into the adult form. 
 
 THE CRAYFISH. 1 
 
 In a crayfish or lobster the body is bilaterally symmetrical 
 and is distinctly segmented, consisting of a prostomium and 
 of nineteen metameres. The anterior twelve metameres are 
 united with one another and with the prostomium to form an 
 unjointed portion of the body, the cephalothorax (Fig. 77, 
 A, C. Th.) : the seven posterior segments are free and con- 
 stitute the abdomen (Abd. Seg. i, Abd. Seg. 7). It is very 
 generally characteristic of Arthropods to have the meta- 
 meres limited and constant in number, and for more or 
 fewer of them to undergo concrescence. 
 
 Another distinctive arthropod character illustrated fc by 
 the Crayfish is the possession of lateral appendages of the 
 body. These are given off from the ventral region, two pairs 
 being borne by the prostomium and one by each of the 
 metameres, except the last. Moreover the appendages 
 themselves are segmented, being, divided into freely arti- 
 culated limb-segments m podomeres. 
 
 In the Crayfish there is a marked differentiation of the 
 appendages. Those of the prostomium are a pair of eye- 
 stalks, and one of small feelers or antennules which perform 
 
 1 For detailed descriptions of the Crayfish see Huxley, The Crayfish 
 (London, 1880) : Huxley and Martin, Elementary Biology, new ed. 
 (London, 1888), p. 173: Rolleston and Jackson, Forms of Animal 
 Life (Oxford, 1888), pp. 162 and 307 : Marshall and Hurst, Practical 
 Zoology, 3rd. ed. (London, 1892), p. 130: and Parker, The Skeleton of 
 the New Zealand Crayfishes (Wellington, N.Z., 1889). 
 
xxvii STRUCTURE OF BODY-WALL 315 
 
 an olfactory function and also contain the organ of hearing. 1 
 The metameres of the cephalothorax bear one pair of tactile 
 appendages or antennae, six pairs acting as jaws (mandibles, 
 first and second maxillae, and first, second, and third max- 
 illipedes), and five pairs of legs, the first of which are in 
 the fresh-water crayfishes and in lobsters much larger than 
 the rest. The abdomen bears small fin-like swimmerets on 
 its first five metameres, the sixth bearing larger appendages 
 which, together with the seventh segment or telson, con- 
 stitute the tail-fin. 
 
 Sections show the body-wall to consist of a layer of deric 
 epithelium (Der. Epthm) secreting a thick cuticle (Cu\ a 
 layer of connective tissue forming the Dermis (Derm), and 
 a very thick layer of large and complicated muscles (M\ 
 which fill up a great part of the interior of the body. 
 
 The cuticle (Cu) is of great thickness, and except at the 
 joints between the various segments of the body and limbs, 
 is impregnated with lime salts so as to form a hard, jointed 
 armour. It thus constitutes a skeleton which, unlike that 
 of the starfish (p. 312), is a cuticular exoskeleton, forming a 
 continuous investment over the whole body but discon- 
 tinuously calcified. 
 
 The mouth (MtK) is on the ventral surface of the head, 
 in the segments of the mandibles or first pair of jaws. It 
 has therefore, as compared with the mouth of Polygordius, 
 undergone a backward shifting, the appendages of the first 
 metamere (antennae) being altogether in front of it. The 
 enteric canal consists of a short gullet (Gut), a large 
 stomach (St), and a straight intestine divisible into a short 
 anterior division or small intestine (6*. Jnt) and a long 
 posterior division or large intestine (L. Int) : the latter 
 
 1 The antennules are frequently considered as belonging to the first 
 metamere, the number of segments being then reckoned as twenty. 
 
LESS, xxvii GENERAL CHARACTERS 317 
 
 The body is divided into a head (Hd) and thorax (T/i), together 
 constituting the cephalothorax (C. Th), and seven free abdominal 
 segments (Abd. seg. i, Abd. seg. 7) : the head is produced in front into 
 a rostrum. 
 
 The body- wall consists of cuticle (Cu), partly calcified to form the 
 exoskeleton, deric epithelium (Der. Epthm), dermis (Derm}, and a 
 very thick layer of muscle (M) which in the abdomen is distinctly 
 segmented. 
 
 The mouth (Mth) leads by a short gullet (Gul) into a large stomach 
 (St), from which a short small intestine (S. Inf) leads into a large in- 
 testine (L. Int\ ending in the anus (An). Opening into the small 
 intestine are the digestive glands (D. 67). The epithelium of the small 
 intestine and digestive glands is endodermal, that of the rest of the canal 
 is ectodermal and secretes a cuticle : the outer layer throughout is 
 mesodermal (connective tissue and muscle). 
 
 The cavity (B. S) between the enteric canal and the body-muscles is 
 a blood-sinus. 
 
 The heart (Ht) is enclosed in the pericardial sinus (Per. S) : the 
 chief ventral blood-vessel or sternal artery (St. A) is shown in B. 
 
 The gills (B, Gill) are enclosed in a cavity formed by a fold of the 
 thoracic body-wall called the branchiostegite (Brstg) : they are formed 
 of the same layers as the body-wall, of which they are offshoots. 
 
 The kidneys (A, K) are situated in the head. 
 
 The brain (Br) lies in the prostomium : the ventral nerve-cord ( V. 
 Nv. Cd) consists of a chain of ganglia (Gn) united by connectives. 
 
 The ovary (ovy) is a hollow organ opening by an oviduct (B, ovd) on 
 the base of one of the legs (Leg). 
 
 opens by an anus (An) on the ventral surface of the last 
 segment. The study of development shows that the only 
 part of the canal derived from the enteron of the embryo is 
 the small intestine : the gullet and stomach arise from the 
 stomodseum, the large intestine from the proctodseum. 
 Thus the only portion of the enteric epithelium which 
 is endodermal is that of the small intestine : the epithelium 
 of gullet, stomach, and large intestine is ectodermal, and 
 like the deric epithelium secretes a cuticle. The outer 
 layer of the whole enteric canal consists of connective 
 tissue and muscle : there is no ccelomic epithelium. 
 
 On each side of the small intestine is a large organ, the 
 digestive gland (D. Gl) \ it consists of numberless glove- 
 finger-like processes or caeca which open by a short tube or 
 
3i8 THE CRAYFISH LESS. 
 
 duct into the small intestine (B, D. Gl). Both caeca 
 and duct are lined with epithelium derived from the endo- 
 derm, and the whole digestive gland is to be looked upon 
 as a branched lateral outgrowth of the enteron. The 
 secretion of digestive juice is performed exclusively by the 
 epithelium of the digestive glands. 
 
 Between the enteric canal and the body-wall are a series 
 of spaces (B.S) containing blood and having the general 
 relations of a coelome, but very probably only representing 
 a number of enlarged blood-spaces or sinuses. 
 
 Respiration is performed by special organs, the gills 
 (B, Gill) see p. 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 (Ht\ and the rest of the vessels, now 
 called arteries (B, St. A], instead of forming by themselves 
 a closed system, ramify extensively over the body, their ulti- 
 mate branches opening into larger cavities or sinuses between 
 the muscles. One of these cavities the pericardial sinus 
 Pcd. S) surrounds the heart. The heart, arteries, and 
 sinuses together form a closed system through which the 
 blood is propelled in a definite direction by the contractions 
 of the heart. 
 
 Renal excretion is performed by a pair of glandular 
 bodies, the kidneys (A, K\ situated in the front part of the 
 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 deferentia; on the bases of the fifth 
 pair of legs. There is some reason for thinking that the 
 gonaducts represent modified nephridia, and the cavities 
 of the hollow gonads a greatly reduced 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 (Br] united 
 by cesophageal connectives to a ventral nerve-cord (V. 
 Nv.CcT). 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, Mant\ 
 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 calcined shell (B., S/i). 
 The ventral region of the body is produced into a laterally 
 compressed muscular structure, ihefoot (A and B, Foot], by 
 the contraction of which the animal can move slowly 
 through the sand or mud in which it lives partly buried. 
 
 The possession of a mantle formed as a prolongation of 
 the dorsal region, of a calcareous shell secreted by the 
 mantle, and of a muscular foot formed as an unpaired 
 prolongation of the ventral region, are the most characteristic 
 features of the Mollusca generally. 
 
 Posteriorly the edges of the mantle-lobes are greatly 
 thickened, and are connected with one another in such a 
 way as to form two apertures, a large ventral inhalent (Ink. 
 Ap\ and a small dorsal exhalent aperture (Exh. Ap). By 
 means of the cilia of the gills (see below) a current of water 
 is produced which enters at the inhalent aperture, carrying 
 abundant oxygen and the minute organisms used as food, 
 and makes its escape at the exhalent aperture, taking with it 
 the various products of excretion and faecal matter. 
 
 The mouth (Mth) is anterior and ventral, lying just in 
 front of the foot : it is bounded on either side by a pair of 
 
 1 For detailed descriptions of the fresh-water Mussel see Rolleston 
 and Jackson, Forms of Animal Life, pp. 124 and 285 : Huxley and 
 Martin, Elementary Biology ; p. 305 : and Marshall and Hurst, Practical 
 Zoology, p. 80. 
 
Der. 
 
 Ccel .Epthm. 
 'CceLFpthm- 
 Sh 
 Intf 
 JJcr.Eptlnn 
 
 FIG. 78. Diagrammatic sections of the Fresh-water Mussel. 
 
 A, longitudinal section : the right mantle-lobe (Mant) and gills (/. G, 
 O. G) are shown in perspective. 
 
 B, transverse section. 
 
 The cuticular shell (Sh}, shown only in B, is black, the ectoderm 
 dotted, the nervous system finely dotted, the endoderm radially striated, 
 the mesoderm evenly shaded, and the ccelomic epithelium represented 
 by a beaded line. 
 
 The dorsal region is produced into the right and left mantle-lobes 
 (Afant), attached to which are the valves of the shell (Sh) joined dorsally 
 by an elastic ligament (tig). 
 
 The mantle-lobes are partly united so as to form the inhalent (Ink. 
 Ap) and exhalent (Exh. Ap) apertures at the posterior end. 
 
 The body is produced ventrally into the foot (Foot), on each side of 
 which are the gills, an inner (/. G) and an outer (O. G), each formed 
 of an inner and an outer lamella. 
 
 The body is covered externally by cleric epithelium (Der. Epthm), 
 within which is mesoderm (Msd) largely differentiated into muscles, of 
 which the anterior (A. Ad) and posterior (P. Ad) adductors are indi- 
 cated in A. 
 
 The mouth (Mth) leads by the short gullet ( Gul) into the stomach 
 (St), from which proceeds the coiled intestine (//), ending in the anus 
 
322 THE FRESH-WATER MUSSEL LESS. 
 
 (An) : the enteric epithelium is mostly endodermal. The digestive gland 
 (D. 67) surrounds the stomach. The ccelome (Ccel) is reduced to a 
 small dorsal chamber enclosing part of the intestine and the heart ; the 
 parietal (Cal. Epthm) and visceral (Cael. EpthnP] layers of ccelomic 
 epithelium are shown. 
 
 The heart consists of a median ventricle ( Vent], enclosing part of the 
 intestine, and of paired auricles (Aur). 
 
 The paired nephridia (Nphm) open by apertures into the coelome 
 (Nph. st) and on the exterior (Nph. p}. 
 
 The gonads (Gon) are imbedded in the solid mesoderm, and open on 
 the exterior by gonaducts (Gnd). 
 
 The nervous system consists of a pair of cerebro-pleural ganglia 
 (C. P. Gn) above the gullet, a pair of pedal ganglia (Pd. Gn) in the 
 foot, and a pair of visceral ganglia ( V. Gn} below the posterior adductor 
 muscle. 
 
 triangular bodies, the labial palpi, and leads by a short 
 gullet (Gut) into a stomach (St) from which proceeds a 
 long, coiled intestine (Int) : this makes several turns in the 
 ventral region of the trunk, then passes to the dorsal region, 
 and finally backwards in the median plane to open by an 
 anus (An) at the posterior end of the body, just within the 
 exhalent aperture. The enteric canal is formed almost 
 exclusively from the enteron, the stomodseum and procto- 
 daeum being both insignificant : hence the enteric epithelium 
 is almost wholly endodermal. There is a large digestive 
 gland (D. Gl) surrounding the stomach and opening into 
 it by several ducts. 
 
 The coelome (Cod) is a small cavity in the dorsal region 
 containing a portion of the intestine : the rest of the enteric 
 canal is embedded in solid mesoderm. 
 
 The mesoderm, as usual, is largely differentiated into 
 muscle. There are numerous muscles connected with the 
 foot, and two very large ones (A. Ad, P. Ad) pass trans- 
 versely from valve to valve of the shell, one immediately 
 above the gullet, the other immediately below the anal end 
 of the intestine ; these latter are called adductors, and serve 
 to close the shell. 
 
xxvn GENERAL CHARACTERS 323 
 
 On either side of the body, between the trunk and the 
 mantle, are two gills (/. G, O. G), each having the form of a 
 double plate (B) nearly as long as the body. They serve, in 
 conjunction with the mantle, as respiratory organs, but their 
 main function is to produce the current of water referred to 
 above by means of the cilia with which they are covered. 
 
 There is an extensive system of blood-vessels. The heart 
 lies in the coelome, and consists of three chambers, a median 
 ventricle ( Ve?it\ which surrounds the intestine, and paired 
 auricles (Aur). 
 
 Excretion is performed by a single pair of nephridia 
 (Nphm) which open at one end (Nph. st) into the coelome 
 and at the other (Nph. p) on to the exterior. 
 
 The nervous system consists of three pairs of ganglia, the 
 two ganglia of each pair being united by transverse com- 
 missures. The cerebro-pleural ganglia ( C. P. Gri) lie above 
 the gullet, and represent, in a general way, the brain of 
 Polygordius and the crayfish ; they are united by longitu- 
 dinal connectives with the pedal ganglia (P. Gn}, which lie 
 in the foot and may be taken as representing the ventral 
 nerve-cord of worms and arthropods, and with the visceral 
 ganglia (V. Gn) which are placed beneath the posterior 
 adductor muscle. 
 
 The gonads (Goti) are large irregular organs, very similar 
 in appearance in the two sexes, situated among the coils of 
 the intestine and opening by a duct (Gnd) on either side of 
 the trunk, close to the nephridiopore. The impregnated 
 eggs are passed into the cavity of the outer gill of the 
 female, where they undergo the early stages of their develop- 
 ment. The larva of the fresh-water mussel is a peculiar 
 bivalved form, very unlike the adult, and called %.glochidium ; 
 but in the more typical molluscs the embryo leaves the egg 
 as a trochosphere, closely resembling that of Polygordius. 
 
 Y 2 
 
324 THE DOG-FISH LESS. 
 
 THE Doc-FiSH. 1 
 
 A dog-fish is bilaterally symmetrical, the nearly cylin- 
 drical body (Fig. 79, A) terminating in front in a blunt 
 snout and behind passing insensibly into an upturned tail. 
 Externally there is no appearance of segmentation. 
 
 The mouth (MtJi) is on the ventral surface of the head 
 or anterior region of the body ; it is transversely elongated, 
 and is supported by jaws which are respectively anterior 
 (upper) and posterior (lower). They thus differ funda- 
 mentally from the jaws of arthropods, which are modified 
 appendages and are therefore disposed right and left. 
 
 A short distance behind the mouth are five vertical slits 
 (B, Ext. br. ap) arranged in a longitudinal series, the 
 external branchial* apertures or gill-clefts. The vent, or 
 cloacal aperture (An) is situated on the ventral surface a 
 considerable distance from the end of the tail. That part 
 of the body lying in front of the last gill-cleft is counted as 
 the head, all behind the vent as the tail, the intermediate 
 portion as the trunk. 
 
 Appendages are present, but in a very different form from 
 those of the crayfish. They consist of flat processes of the 
 body-wall called fins. Two of them (D.F^,D. F*) are 
 situated in the middle line of the back (dorsal fins) : one 
 (V.F) in the middle ventral line behind the cloacal aperture 
 (ventral fin\ and one (C.F) is attached to the up-turned end 
 of the tail (caudal fin) : all these being unpaired structures or 
 median fins. Then there is a pair of pectoral fins situated 
 
 1 For a detailed description of a dog-fish see Marshall and Hurst, 
 Practical Zoology (London, 1892), p. 206. For descriptions of other 
 fishes, equally suitable in some respects as types of Vertebrata, see 
 Rolleston and Jackson, Forms of Animal Life (Oxford, 1888), pp. 83 
 and 273 : and Parker, Zoototny (London, 1884), pp. I, 27, 86. 
 
xxvn GENERAL CHARACTERS 325 
 
 one on each side just behind the last gill-cleft, and a pair of 
 pelvic fins placed one on either side of the vent : these are 
 the lateral or paired fins. It is characteristic of Vertebrata 
 that the number of lateral appendages never exceeds two 
 pairs. 
 
 The skin or external layer of the body-wall consists of an 
 outer epidermis (Der. Epthni) composed of several layers of 
 cells, and of an inner connective tissue layer or dermis 
 (Derm). In the latter are found innumerable bony scales 
 (Derm. Sp) constituting a dermal exoskeleton. The muscular 
 layer of the body-wall ( M) is of great thickness, especially 
 in the dorsal region, and is distinctly segmented, indicating 
 that the body of the dog-fish, like that of Polygordius and 
 the crayfish, is divisible into metameres, although there is no 
 indication of them externally. 
 
 The large ccelome (Cat) is confined to the trunk : it is 
 characteristic of vertebrates that both head and tail are 
 acoelomate in the adult. The coelomic epithelium (Cal. 
 Epthm, Ccel. Mpthm') is underlaid by a distinct layer of 
 connective tissue, the two together forming the peritoneum. 
 
 Another important vertebrate character is that the dorsal 
 region of the body-wall contains a median longitudinal 
 canal (C. Sp. Cav.) extending from shortly behind the snout 
 to near the end of the tail. This is the cerebro-spinal cavity 
 and contains the central nervous system. 
 
 Still another characteristic feature is the presence, in 
 addition to the dermal exoskeleton, of an endoskeleton, or 
 system of internal supporting structures. Between the 
 cerebro-spinal cavity above and the ccelome below is a 
 longitudinal series of biconcave discs or vertebral centra 
 (V. Cent) : they are formed of a peculiar tissue called 
 cartilage or gristle, and are strongly impregnated with lime- 
 salts : in the young condition their place is occupied by a 
 
LESS, xxvii GENERAL CHARACTERS 327 
 
 A, longitudinal vertical section. 
 
 B, horizontal section through the pharynx and gills, 
 c, transverse section through the trunk. 
 
 The ectoderm is dotted, the nervous system finely dotted, the endo- 
 derm radially striated, the mesoderm evenly shaded, the ccelomic 
 epithelium represented by a beaded line, and all skeletal structures 
 black. 
 
 The body gives origin to the dorsal (D. F 1 , D. F\ ventral (V. F), 
 and caudal (C. F} fins ; the paired fins are not shown. 
 
 The body- wall consists of deric epithelium (Der. Epthm}, dermis 
 Derm}, and muscle (M} : the latter is metamerically segmented and is 
 very thick, especially dorsally, where it forms half the total vertical 
 height (c). 
 
 The exoskeleton consists of calcified dermal spines {Derm. Sp} in the 
 dermis, and of dermal fin-rays (Derm. F. R} in the fins. 
 
 The endoskeleton consists of a row of vertebral centra ( V. Cent} below 
 the spinal cord (Sp. Cd}, giving rise to neural arches (N. A), which enclose 
 the cord, and in the caudal regions to haemal arches (ff. A.) : a cranium 
 (Cr} enclosing the brain (Br} : upper and lower jaws : branchial arches 
 (Br. A) and rays (Br. J?, Br. R'), shown only in B, supporting the 
 gills : shoulder (Sh. G) and pelvic (Pelv. G) girdles : and pterygiosphores 
 (Ptgph} supporting the fins. 
 
 The mouth (Mth} leads into the oral cavity (Or. cav}, from which the 
 pharynx (Ph} and gullet (Gul) lead to the stomach (St} : this is con- 
 nected with a short intestine (Tnt} opening into a cloaca (C!) which 
 communicates with the exterior by the vent (An}. The oral cavity and 
 cloaca are the only parts of the canal lined by ectoderm. 
 
 Connected with the enteric canal are the liver (Lr) with the gall- 
 bladder (G, Bl) and bile-duct (B. D), the pancreas (/*), and the spleen 
 (Spl). The mouth is bounded above and below by teeth (T). 
 
 The respiratory organs consist of pouches (shown in B) communicating 
 with the pharynx by internal (Int. br. ap} and with the exterior by 
 external (Ext. br. ap} branchial apertures, and lined by mucous mem- 
 brane raised into branchial filaments (Br. Fil}. 
 
 The heart (Ht} is ventral and anterior, and is situated in a special 
 compartment of the ccelome (Pcd}. Six of the most important blood- 
 vessels, the dorsal vessel (dorsal aorta, D. Ao}, the cardinal veins 
 (Card. V), the lateral vessels (lateral veins, Lat. F), and the ventral 
 vessel (intra-intestinal vein, /. int. V} are shown in C. 
 
 The whole ccelome is lined by epithelium, showing parietal (Ccel. 
 Epthm} and visceral (Ccel. Epthm'} layers. 
 
 The ovaries (Ovy} are connected with the dorsal body-wall : the 
 oviducts ( Ovd} open anteriorly into the ccelome (ovd'} and posteriorly 
 into the cloaca. 
 
 The kidneys (K} are made up of nephridia (Nph} and open by ureters 
 ( Ur} into the cloaca. 
 
 The nervous system is lodged in the cerebro-spinal cavity ( C. Sp. Cav} 
 hollowed out in the dorsal body- wall : it consists of brain (Br} and 
 spinal cord (Sp. Cd}, and contains a continuous cavity, the neuroccele 
 n. cce}. 
 
328 THE DOG-FISH LESS. 
 
 gelatinous rod, the notochord. The centra, which alternate 
 with the muscle-segments, are connected with a series of 
 cartilaginous arches (N.A), which extend over the cerebro- 
 spinal cavity and with the centra constitute the vertebral 
 column. In the tail there is also a ventral series of arches 
 (ff.A.} enclosing a space (ff. C) which indicates a backward 
 extension of the ccelome in the embryo. 
 
 Anteriorly the vertebral column is continued into a 
 cartilaginous box, the cranium ( Cr) which encloses the brain 
 and the organs of smell and hearing. The jaws, referred to 
 above, are cartilaginous rods which bound the mouth above 
 and below. The gills are supported by a complicated 
 system of cartilages (Br. A, Br. R., Br. R'\ and both 
 median and paired fins by parallel rods of the same 
 material (Ptgpli). All these cartilages are strengthened 
 by a more or less extensive superficial deposit of bony 
 matter. 
 
 The mouth (Mtti) leads into a large oral cavity (Or. cav) 
 which passes insensibly into a wide throat or pharynx (P/i) : 
 from this a short gullet (Gul) leads into a large U-shaped 
 stomach (St), whence is continued a short wide intestine 
 (Int) opening on to the exterior through the intermediation 
 of a small chamber, the cloaca (C!). From the gullet 
 backwards the enteric canal is contained in the ccelome. 
 The greater part of the enteric epithelium is endodermal : 
 only the oral cavity arises from the stomodaeum and the 
 cloaca from the proctodaeum. 
 
 In the skin covering the jaws dermal ossicles of unusual 
 size are developed and constitute the teeth (T). The chief 
 digestive glands are two in number, an immense liver (Lr) 
 occupying the whole anterior and ventral region of the 
 coelome, and a small pancreas (Pn), attached to the anterior 
 end of the intestine. The ducts of both glands open into 
 
GILLS AND HEART 329 
 
 the intestine, and their secreting cells are, as in former cases, 
 endodermal. Gland-cells are also found in the walls of 
 the stomach and intestine. 
 
 The respiratory organs or gills (B) consist of five pairs of 
 pouches opening on the one hand into the pharynx (Ph) 
 and on the other to the exterior by the branchial clefts 
 already noticed : they have their walls raised into ridges, 
 the branchial filaments (Br. Fit), which are covered with 
 epithelium (Resp. Epthni] and are abundantly supplied 
 with blood-vessels. The gills are developed as offshoots of 
 the pharynx, and the respiratory epithelium is therefore 
 endodermal, not ectodermal as in the crayfish and mussel. 
 
 The heart (H) lies below the pharynx in a separate 
 anterior compartment of the coelome, the pericardial cavity. 
 It is composed of four chambers arranged in a single longi- 
 tudinal series (sinus venosus, auricle, ventricle, and conus 
 arteriosus), and is to be looked upon as a muscular dilatation 
 of a ventral blood-vessel. The blood is propelled by the 
 heart from the conus arteriosus into a paired series of 
 hoop-like vessels (aortic arches) resembling the transverse 
 commissures of Polygordius (Fig. 69, A, p. 282), which take 
 it through the gills and pour it, in a purified condition, into 
 the dorsal vessel (dorsal aorta, D. Ao) whence it is taken to 
 all parts of the body to be finally returned by thin-walled 
 vessels, called veins, to the sinus venosus. The ventral 
 position of the heart and the fact that the blood is sent 
 directly from the heart to the respiratory organs are 
 characteristic vertebrate features : so also is the circumstance 
 that the blood from the stomach, intestine, &c., is taken by 
 a specially modified portion of the ventral vessel (portal 
 vein) through the liver on its way to the heart. The blood 
 is red, containing, in addition to leucocytes, oval corpuscles 
 coloured by haemoglobin (see p. 58). 
 
330 THE DOG-FISH LESS. 
 
 The excretory organs are a pair of kidneys (K} situated 
 at the posterior end of the dorsal region of the coelome, and 
 opening by ducts, the ureters (Ur\ into the cloaca. De- 
 velopment shows that they consist of an aggregation of 
 nephridia (Nph\ the nephrostomes of which open in the 
 young and sometimes throughout life, into the ccelome, 
 while the nephridiopores discharge not directly on the 
 exterior, but into a common tube. 
 
 The gonads (ovaries, Ovy, or spermaries) are situated in 
 the anterior part of the ccelome, attached by peritoneum 
 to its dorsal wall. The sex-cells are differentiated from 
 ccelomic epithelium. The gonaducts of both sexes 
 (Ovd} are developed from the nephridial system of 
 the embryo. 
 
 As already stated, the central nervous system is contained 
 in a cavity (C. Sp. Cav) of the dorsal body-wall, and is 
 therefore far removed from the ectoderm from which it 
 originates. It consists of a long cylindrical rod, the spinal 
 cord(Sp. Cd} which is continued in front into a complicated 
 brain (r). It has the further peculiarity of being hollow, 
 a more or less cylindrical cavity, the neurocoele (n. cce) ex- 
 tending through its whole length. 
 
 The possession of a hollow nervous system lying altogether 
 dorsal to the enteric canal and ccelome, of either a noto- 
 chord or a chain of vertebral centra below the nervous 
 system, and of pharyngeal pouches communicating with the 
 exterior, are the three most characteristic features of the 
 vertebrate phylum. 
 
 The organs of sense are highly developed, and consist of 
 paired olfactory sacs, eyes, and auditory sacs situated in the 
 head, together with an extensive system of integumentary 
 organs. Their sensory cells are in every case ectodermal. 
 
 The eggs are very large, and are impregnated within the. 
 
xxvii DEVELOPMENT 331 
 
 body of the female. In the common Dog-fish (Scy Ilium) 
 they are laid shortly after impregnation, each enclosed in a 
 horny egg-shell : in the Piked Dog-fish (Acanthias) and the 
 Smooth Hound (Mustelus) they are retained in the oviduct 
 until the adult form is assumed. 
 
LESSON XXVIII 
 
 MOSSES. 
 
 IN the four previous lessons we have traced the advance 
 in organization of animals from the simple diploblastic 
 Hydra to the complicated triploblastic forms which con- 
 stitute the five higher phyla of the animal kingdom. We 
 have now to follow in the same way the advance in structure 
 of plants. The last member of the vegetable kingdom with 
 which we were concerned was Nitella (p. 206), a solid 
 aggregate, exhibiting a certain differentiation of form and 
 structure, but yet composed of what were clearly recognizable 
 as cells, there being, as in Hydra, none of that formation of 
 well-marked tissues which is so noticeable a feature in 
 Polygordius as in other animals above the Ccelenterata. 
 
 Taking Nitella as a starting point, we shall see that among 
 plants, as among animals, there is an increasing differentiation 
 in structure and in function as we ascend the series. The 
 first steps in the process are well illustrated by a considera- 
 tion of that very abundant and beautiful group of plants, the 
 Mosses. In spite of the variations in detail met with in 
 different genera of the group, the essential features of their 
 organization are so constant that the following description 
 will be found to apply to any of the common forms. 
 
FIG. 80. The Anatomy and Histology of Mosses. 
 
 A, Entire plant of Funaria hygrometrica, showing stem (sf), leaves 
 (/), and rhizoids (rh). (x 6.) 
 
 B, leaf of the same, showing midrib (tnd. r) and lateral portions, 
 (x 25.) 
 
334 MOSSES LESS. 
 
 C, semi-diagrammatic vertical section of a moss, showing the arrange- 
 ment of the tissues. The stem is formed externally of sclerenchyma 
 (set), and contains an axial bundle (ax. b) : in some of the leaves (/) 
 the section passes through the midrib, in others (/') through the lateral 
 portion : the stem ends distally in an apical cell (ap. <r), from which 
 segmental cells (seg. c] are separated. 
 
 D, transverse section of the stem of Brytun roseum, showing scleren- 
 chyma (scl), axial bundle (ax. b}, and rhizoids (rh). ( x 60.) 
 
 E, transverse section of a leaf of Funaria, showing the midrib (md. r) 
 formed of several layers of cells, and the lateral portions one cell thick, 
 (x 150.) 
 
 F, small portion of the lateral region of the same, showing the form 
 of the cells and the chromatophores (chr). (x 150.) 
 
 G, distal end of the stern of Fontinalis antipyretica in vertical section, 
 showing the apical cell (ap. c} giving rise to segmental cells (seg. c), 
 which by subsequent division form the segments of the stem with the 
 leaves : the thick lines show the boundaries of the segments. 
 
 H, diagram of the apical cell of a moss in the form of a tetrahedron 
 with rounded base abc and three flat sides abd, bcd> acd. 
 (D after Sachs ; G after Leitgeb. ) 
 
 The plant consists of a short slender stem (Fig. 80, A, st), 
 from which are given off structures of two kinds, rhizoids or 
 root-hairs (rh), which pass downwards into the soil, and leaves 
 (/), which are closely set on the stem and its branches. As 
 in Nitella (p. 208) the portion of the stem from which a leaf 
 arises is called a node, and the part intervening between any 
 two nodes an internode, while the name segment is applied 
 to a node with the internode next below it. At the upper or 
 distal end of the stem the leaves are crowded, forming a 
 terminal bud. 
 
 Owing to the opacity of the stem, its structure can only be 
 made out by the examination of thin sections (c and D). It is 
 a solid aggregate of close-set cells which are not all alike, but 
 exhibit a certain amount of differentiation. In the outer 
 two or three rows the cells (scl) are elongated in the direction 
 of the length of the stem, so as to have a spindle-shape, and 
 their walls are greatly thickened and of a reddish colour. 
 They thus form a protective and supporting tissue, to which 
 the name sclerenchyma is applied. Running longitudinally 
 
xxviii TERMINAL BUD 335 
 
 through the centre of the stem is a mass of tissue (ax. b] 
 distinguished by its small, thin-walled cells, and constituting 
 the axial bundle. 
 
 The leaves (B) are shaped like a spear-head, pointed 
 distally, and attached proximally by a broad base to the 
 stem. The axial portion (B and E, md. r., c. /) consists of 
 several layers of somewhat elongated cells and is called the 
 midrib : the lateral portions (E and F : c, /') are formed of a 
 single layer of short cells. Thus the leaf has, for the most 
 part, the character of a superficial aggregate. The cells 
 contain oval chromatophores (F, chr). 
 
 The rhizoids (c and D, rh) are linear aggregates, being 
 formed of elongated cells, devoid of chlorophyll, arranged 
 end to end. 
 
 In the terminal bud the leaves, as in Nitella (pp. 208 and 
 210), arch over the growing point of the stem, which in this 
 case also is formed of a single apical cell (c and G, ap. c). 
 But in correspondence with the increased complexity of the 
 plant, the apical cell is not a hemisphere from which new 
 segments are cut off parallel to its flat base, but has the form 
 (H) of an inverted, three-sided pyramid or tetrahedron, the 
 rounded base of which (abc) forms the apex of the stem 
 while segments (seg. c) are cut off from each of its three 
 triangular sides in succession. 
 
 The best way to understand the apical growth of a moss 
 is to cut a tetrahedron with rounded base out of a carrot or 
 turnip : this represents the apical cell (H) : then cut off a 
 slice parallel to the side abd, a second parallel to bed, and a 
 third parallel to acd : these represent three successively 
 formed segments. Now imagine that after every division 
 the tetrahedron grows to its original size, and a very fair 
 notion will be obtained of the way in which the successive 
 segments of the moss-stem are formed by the fission in three 
 
336 MOSSES LESS. 
 
 planes of the apical cell. Each segment (c and G, seg. c) 
 immediately after its separation divides and subdivides, pro- 
 ducing a mass of cells from which a projection grows out 
 forming a leaf, and in this way the stem increases in length 
 and the leaves in number. 
 
 Asexual reproduction takes place in various ways : all of 
 them are, however, varieties of budding, and the buds always 
 arise in the form of a linear aggregate of cells called a 
 protonema : from this the moss-plant develops in the same 
 way as from the protonema arising from a spore (p. 339). 
 
 The gonads are developed at the extremity of the main 
 stem or one of its branches, and are enclosed in a tuft of 
 leaves often of a reddish colour the terminal bud of the 
 fertile shoot or so-called "flower" of the moss. 
 
 The spermary (Fig. 81, A 1 , A 2 ) is an elongated club-shaped 
 body consisting of a solid mass of cells, the outermost of 
 which form the wall of the organ, while the inner (A S ) become 
 converted into sperms. The latter (A 4 ) are spirally coiled 
 and provided with two cilia : they are liberated by the 
 rupture of the wall of the spermary at its distal end (A 2 ). 
 
 The ovaries 1 (see Preface, p. x, and p. 381) (s 1 , B 2 ) may 
 or may not occur on the same plant as the spermaries, some 
 mosses being monoecious, others dioecious. Like the sperm- 
 aries, they consist at first of a solid mass of cells which 
 assumes the form of a flask, having a rounded basal portion 
 or venter (v) and a long neck (). The outer layer of cells 
 in the neck and the two outer layers in the venter form the 
 wall of the ovary, the internal cells are arranged in a single 
 axial row at first similar to those of the wall. As the ovary 
 develops, the proximal or lowermost cell of the axial row 
 
 1 The ovary of mosses, ferns, &c., is usually called an archegonium : 
 the spermary, as in the lower plants, an antheridium. 
 
DEVELOPMENT OF SPOROGONIUM 337 
 
 takes on the character of an ovum (e 2 , ov] ; the others, called 
 canal cells (en. c) are converted into mucilage, which by its 
 expansion forces open the mouth of the flask and thus makes 
 a clear passage from the exterior to the ovum (B S ). 
 
 Through the passage thus formed a sperm makes it way 
 and conjugates with the ovum, producing as usual ah 
 oosperm or unicellular embryo. 
 
 The development of the embryo is at first remarkably 
 like what we have found to take place in Hydroids (p. 248). 
 The oosperm divided into two cells by a wall at right angles 
 to the long axis of the ovary : each of these cells divides 
 again repeatedly, and there is produced a solid multicellular 
 embryo or polyplast (c 1 , spgnm). 
 
 Very early, however, the moss-polyplast exhibits a striking 
 difference from the animal polyplast or morula : one of its 
 cells that nearest the neck of the ovary takes on the 
 character of an apical cell, and begins to form fresh seg- 
 ments like the apical cell of the stem. Thus the plant 
 embryo differs almost from the first from the animal embryo. 
 In the animal there is no apical cell : all the cells of the 
 polyplast divide and take their share in the formation of the 
 permanent tissues. In the plant one cell is at a very early 
 period differentiated into an apical cell, and from it all cells 
 thereafter produced are, directly or indirectly, derived. 
 
 The embryo continues to grow, forming a long rod-like 
 body (c 2 , spgnni) the base of which becomes sunk in the 
 tissue of the moss-stem, while its distal end projects vertically 
 upwards, covered by the distended venter (v) of the ovary. 
 Gradually it elongates more and more and its distal end 
 dilates : the embryo has now become a sporogonium, con- 
 sisting of a slender stalk (c 4 , sf] bearing a vase-like capsule 
 or urn (u) at its distal end. In the meantime the elonga- 
 tion of the stalk has caused the rupture of the enveloping 
 
FIG. 81. Reproduction and Development of Mosses. 
 A 1 , A spermary of Funaria in optical section, showing the wall en- 
 closing a central mass of sperm-cells : A 2 , the same from the surface 
 discharging its sperms. ( x 300. ) 
 
LESS, xxvni PROTONEMA 339 
 
 A 3 , a sperm-cell with enclosed sperm : A 4 , a free-swimming sperm. 
 (x 800.) 
 
 B 1 , an ovary of Funaria, surface view, showing venter (v) and neck 
 (n) : B-, the same in optical section, showing ovum (ov) and canal cells 
 (en. c) : B 3 , the same after disappearance of the canal cells : the neck is 
 freely open, and the ovum (ov} exposed. ( x 200. ) 
 
 C 1 , ovary with withered neck containing an embryo (spgnni) in the 
 polyplast stage ( x 200) : in C 2 the ovary, consisting of swollen venter (v) 
 and shrivelled neck (n), encloses a young sporogonium (spgnni) ; the 
 distal end of the stem is shown with bases of leaves (/) ; in c 3 the venter 
 has ruptured, forming a proximal portion or sheath and a distal portion 
 or calyptra which is carried up by the growth of the sporogonium. 
 (x 10.) 
 
 C 4 , a small plant of Funaria with ripe sporogonium consisting of seta 
 (st), with urn () and lid (/) covered by the calyptra (c}. 
 
 C 5 , diagrammatic vertical section of urn (), showing lid (/), air spaces 
 (a), and spores (sp). 
 
 D 1 , a germinating spore of Funaria, showing ruptured outer coat (sp) 
 and young protonema (pr) with rhizoid (rh). ( x 550.) 
 
 D 2 , portion of protonema of the same, showing lateral bud (bd), from 
 which the leafy plant arises. ( x 90. ) 
 
 (A and D after Sachs ; B, c 1 , and C 5 altered from Sachs.) 
 
 venter of the ovary (c 3 ) : its proximal part remains as a sort 
 of sheath round the base of the stalk, while its distal portion, 
 with the shrivelled remains of the neck (n), is carried up by 
 the elongation of the sporogonium and forms an extinguisher- 
 like cap or calyptra (c 4 , c} over the urn. 
 
 As development goes on, the distal end of the urn be- 
 comes separated in the form of a lid (c 4 , c 5 , /), and certain 
 of the cells in its interior, called spore-mother cells, divide 
 each into four daughter cells, which acquire a double cell- 
 wall and constitute the spores (c 5 , sp) of the moss. 
 
 When the spores are ripe the calyptra falls off or is blown 
 away by the wind, the lid separates from the urn, and the 
 spores are scattered. 
 
 In germination, the protoplasm of the spore covered by 
 the inner layer of the cell-wall protrudes through a split in 
 the outer layer (D 1 , sp) and grows into a long filament, the 
 protonema (pr.}, divided by oblique septa into a row of cells. 
 The protonema which it will be observed is a simple linear 
 
 z 2 
 
340 MOSSES LESS. 
 
 aggregate branches, and may form a closely- matted mass 
 of filaments. Sooner or later small lateral buds (o 2 , bd) 
 appear at various places on the protonema : each of these 
 takes on the form of a three-sided pyramidal apical cell, 
 which then proceeds to divide in the characteristic way 
 (p. 335), forming three rows of segments from which leaves 
 spring. In this way each lateral bud of the protonema gives 
 rise to a moss-plant. 
 
 Obviously we have here a somewhat complicated case of 
 alternation of generations (see p. 220). The gamobium or 
 sexual generation is represented by the moss-plant, which 
 originates by budding and produces the sexual organs, while 
 the agamobium consists of the sporogonium, developed from 
 the oosperm and reproducing by means of spores. The 
 protonema, arising from a spore and producing the leafy 
 plant by budding, is merely a stage of the gamobium. 
 
 The nutrition of mosses is holophytic ; but there is a 
 striking differentiation of function correlated with terrestrial 
 habits. In Nitella the entire organism is submerged in 
 water and all the cells contain chlorophyll, so that decom- 
 position of carbon dioxide and absorption of an aqueous 
 solution of salts are performed by all parts alike, every 
 cell being nourished independently of the rest. In the 
 moss, on the other hand, the rootlets are removed from 
 the influence of light and contain no chlorophyll : hence 
 they cannot decompose carbon dioxide ; but, being sur- 
 jounded by moist soil, are in the most favourable position 
 for absorbing water and mineral salts. The stem, again, is 
 converted into an organ of support : the thickness of its 
 external cells prevents absorption and it contains no 
 chlorophyll. Hence the function of decomposing carbon 
 dioxide is confined to the leaves. 
 
xxvin DISTRIBUTION OF FOOD-MATERIALS 341 
 
 We have thus as an important fact in the nutrition of an 
 ordinary terrestrial plant that its carbon is taken in at one 
 place, its water, nitrogen, sulphur, potassium, &c., at another. 
 But as all parts of the plant require all these substances it is 
 evident that there must be some means by which the root 
 can obtain a supply of carbon, and the leaves a supply of 
 elements other than carbon. In other words, we find for 
 the first time in the ascending series of plants, just as we 
 did in ascending from the simple Hydra to the complex 
 Polygordius (p. 281) the need for some contrivance for the 
 distribution of food-materials. 
 
 The way in which this distributing process is performed 
 has been studied chiefly in the higher plants, but its essential 
 features are probably the same for mosses. 
 
 Water is continually evaporating from the surface of the 
 leaves, its place being as constantly supplied by water with 
 salts in solution taken in by the rhizoids. This trans- 
 piration, or the giving off of water from the leaves, is one 
 important factor in the process under consideration, since 
 it ensures a constant upward current of water, or, more 
 accurately, of an aqueous solution of mineral salts. The 
 withering of a plucked moss-plant is of course due to the 
 fact that when the roots are not embedded in moist soil or 
 in water, transpiration is no longer balanced by absorption. 1 
 In the higher plants it has been found that the root-hairs 
 have an absorbent action independent of transpiration, so 
 that water may be absorbed in the absence of leaves. 
 
 By the transpiration current, then, the leaves are kept 
 constantly supplied with a solution of mineral salts derived 
 from the soil, and are thus nourished like any of the aquatic 
 green plants considered in previous lessons : by the double 
 
 1 Mosses, however, unlike most higher plants, can absorb water by 
 their leaves. 
 
342 MOSSES LESS. 
 
 decomposition of water and carbon dioxide a carbo-hydrate 
 is formed : this, by further combination with the nitrogen 
 of the absorbed ammonium salts or nitrates, forms simple 
 nitrogenous compounds, and from these, probably through 
 a long series of mesostates or intermediate products, proto- 
 plasm is finally manufactured. 
 
 In this way the food supply of the green cells of the 
 leaves is accounted for, but we have still to consider that of 
 the colourless cells of the stem and rhizoids, which, as we 
 have seen, are supplied by the transpiration current with 
 everything they require except carbon, and this, owing to 
 their possessing no chlorophyll, they are unable to take in 
 in the form of carbon dioxide. 
 
 As a matter of fact the chlorophyll-containing cells of the 
 leaves have to provide not only their own food, but also 
 that of their not-green fellows. In addition to making good 
 the waste of their own protoplasm they produce large 
 quantities of plastic products (see p. 33) such as grape 
 sugar, and simple nitrogenous compounds like asparagin, 
 and these pass by diffusion from cell to cell until they reach 
 the uttermost parts of the plant, such as the centre of the 
 stem and the extremities of the rhizoids. The colourless 
 cells are in this way provided not only with the salts 
 contained in the ascending transpiration current, but with 
 carbo-hydrates and nitrogenous compounds. From these 
 they derive their nutriment, living therefore like yeast-cells 
 in Pasteur's solution, or like Bacteria in an organic 
 infusion. 
 
 We see then that the colourless cells of the stem and 
 rhizoids are dependent upon the green cells of the leaves 
 for their supplies. Like other cells devoid of chlorophyll 
 they are unable to make use of carbon dioxide as a source 
 of carbon, but require ready-made carbo-hydrates, the 
 
xxvni DISTRIBUTION OF FOOD MATERIALS 343 
 
 manufacture of which is continually going on, during 
 daylight, in the chlorophyll-containing cells of the leaves. 
 This striking division of labour is the most important 
 physiological difference between mosses and the more lowly 
 organized green plants described in previous lessons. 
 
LESSON XXIX 
 
 FERNS 
 
 WE saw in the previous lesson that in mosses there is a 
 certain though small amount of histological differentiation, 
 some cells being modified to form sclerenchyma, others to 
 form axial bundles. We have now to consider a group of 
 plants which may be considered to be, in this respect, on 
 much the same morphological level as Polygordius, the 
 adult organism being composed not of a mere aggregate of 
 simple cells, but of various well-marked tissues. 
 
 A fern-plant has a strong stem which in some forms, such 
 as the common Bracken (Pteris aquilina) is a horizontal 
 underground structure, and is hence often incorrectly con- 
 sidered as a root : in others it creeps over the trunks of 
 trees or over rocks : in others again, such as the tree-ferns, 
 it is vertical, and may attain a height of three or four metres. 
 From the stem are given off structures of two kinds the 
 leaves, which present an almost infinite variety of form in 
 the various species, and the numerous slender roots. In 
 some cases, such as the tree-ferns and the common Male 
 Shield-fern (Aspidium filix-mas\ the plant ends distallyin a 
 terminal bud, consisting, as in Nitella and mosses, of the 
 growing end of the stem over-arched by leaves : in others 
 
LESS, xxix . TISSUES OF THE STEM 345 
 
 such as Pteris, the stem ends in a blunt, knob-like extremity 
 quite uncovered by leaves. On the proximal portion of the 
 stem are usually found the withered remains of the leaves 
 of previous seasons, or the scars left by their fall. The 
 roots are given off from the whole surface of the stem, 
 often covering it with a closely-matted mass of dark brown 
 fibres. 
 
 When the stem is cut across transversely (Fig. 82, A) it 
 is seen, even with the naked eye, to consist of three well 
 marked tissues. The main mass of it is formed of a whitish 
 substance, soft and rather sticky to the touch, and called 
 ground-parenchyma (par) : this is covered by an external 
 layer of very hard tissue, dark brown or black in colour, the 
 hypodermis (hyp} : bands of a similar hard brown substance 
 are variously distributed through the parenchyma, and con- 
 stitute the sclerenchyma (scl) : and interspersed with these 
 are rounded or oval patches of a yellowish colour (V.B] 
 harder than the parenchyma, but not so hard as the 
 sclerenchyma, and called vascular bundles. 
 
 The general distribution of these tissues can be made out 
 by making longitudinal sections of the stem in various 
 planes or by cutting away the hypodermis, and then scraping 
 the parenchyma from the vascular bundles and bands of 
 sclerenchyma. The hypodermis is found to form a more or 
 less complete hard sheath or shell to the stem, while the 
 internal sclerenchyma and vascular bundles form longi- 
 tudinal bands and rods imbedded in the parenchyma, and 
 serve as a sort of supporting framework or skeleton. 
 
 The minute structure of the stem can be made out by 
 the examination either of very thin longitudinal and trans- 
 verse sections, or of a bit of stem which has been reduced 
 to a pulp by boiling in nitric acid with the addition of a few 
 crystals of potassium chlorate : by this process the various 
 
T.S 
 
 4CV 
 
 cp.o 
 
 FIG. 82. Anatomy and Histology of Ferns. 
 
LESS, xxix GENERAL CHARACTERS 347 
 
 A, Transverse section of the stem of Pteris aqnilina, showing hypo- 
 dermis (hyp], ground parenchyma (par], sclerenchyma (set), and vascular 
 bundles ( V. B). (x 2.) 
 
 B, transverse section of a vascular bundle, showing bundle-sheath 
 (b. sh\ sieve-tubes (sv. f), scalariform vessels (sc. v), and spiral vessels 
 (sp. v}. (x 6.) 
 
 C, semi-diagrammatic vertical section of the growing point of the 
 stem, showing apical cell (op. c), segmental cells (seg. c), and apical 
 meristem (ap. mer) passing into permanent tissue consisting of epidermis 
 (ep\ hypodermis (hyp], ground parenchyma (par], sclerenchyma (scl), 
 and vascular bundles in which the sheath (b. s/i), sieve-tubes (sv. /), 
 scalariform vessels (sc. v), and spiral vessels (sp. v) are indicated. 
 
 D, a single parenchyma cell, showing nucleus (), and vacuole 
 (vac]. 
 
 E, cell of hypodermis. 
 
 F, portion of a sieve-tube, showing sieve-plates (sv. pi). 
 
 G, portion of a spiral vessel with the spiral fibre partly unrolled at the 
 lower end. 
 
 H, fibre-like cell of sclerenchyma. 
 
 I, portion of a scalariform vessel, part of the wall being supposed to 
 be removed. 
 
 K, vertical section of a leaf of Pteris, showing upper and lower epi- 
 dermis (ep\ mesophyll cells (ms. ph), with intercellular spaces (i. c. sp), 
 a stoma (sf) in the lower epidermis, and hairs (A). 
 
 L, surface view of epidermis of leaf of Aspidium, showing two stomata 
 (sf) with their guard-cells (gd. c). 
 
 M, vertical section of the end of a root, showing apical cell (ap. c}, 
 segmental cells (seg. c), and root-cap (r. cp} with its youngest cap-cells 
 marked cp. c. 
 
 (A, B, and D-K after Howes ; M from Sachs, slightly altered.) 
 
 tissue elements are separated from one another, and can be 
 readily examined under a" high power. 
 
 By combining these two methods of sectioning and 
 dissociation, the parenchyma is found to consist of an 
 aggregate of polyhedral cells (D) considerably longer than 
 broad, their long axes being parallel with that of the stem 
 itself. The cells are to be considered as right cylinders 
 which have been converted into polyhedra by mutual pres- 
 sure. They have the usual structure, and their protoplasm is 
 frequently loaded with large starch-grains. They do not fit 
 quite closely together, but spaces are left between them, 
 especially at the angles, called intercellular spaces. 
 
348 FERNS LESS. 
 
 The cells of the hypodermis (E) are proportionally longer 
 than those of the parenchyma, and are pointed at each end : 
 they contain no starch. Their walls are greatly thickened, 
 and are composed not of cellulose but of lignin, a carbo- 
 hydrate allied in composition to cellulose, but containing a 
 larger proportion of carbon Schulze's solution, which, as 
 we have seen, stains cellulose blue, imparts a yellow colour 
 to lignin. 
 
 Outside the hypodermis is a single layer of cells (c, ep) 
 not distinguishable by the naked eye and forming the actual 
 external layer of the stem : the cells have slightly thickened, 
 yellowish-brown walls, and constitute the epidermis. From 
 many of them are given off delicate filamentous processes con- 
 sisting each of a single row of cells : these are called hairs. 
 
 In the sclerenchyma the cells (H) are greatly elongated' 
 and pointed at both ends, so as to have the character rather 
 of fibres than of cells. Their walls are immensely thickened 
 and lignified, and present at intervals oblique markings due to 
 narrow but deep clefts : these are produced by the deposition 
 of lignin from the surface of the protoplasm (see p. 32) being 
 interrupted here and there, instead of going on continuously 
 as in the case of a cell-wall of uniform thickness. 
 
 The vascular bundles have in transverse section (B) the 
 appearance of a very complicated network, with meshes of 
 varying diameter. In longitudinal sections (c) and in dis- 
 sociated specimens they are found to be partly composed of 
 cells, but to contain besides structures which cannot be 
 called cells at all. 
 
 In the centre of the bundle are a few narrow cylindrical 
 tubes (B and c, sp. v.) characterized at once by a spiral 
 marking, and hence called spiral vessels. Accurate exam- 
 ination shows that their walls (G) are for the most part thin, 
 but are thickened by a spiral fibre, just as a paper tube 
 
xxix XYLEM AND PHLOEM 349 
 
 might be strengthened by gumming a spiral strip of paste- 
 board to its inner surface. These vessels are of considerable 
 length, and are open at both ends : moreover they contain 
 no protoplasm, but are filled with either air or water : they 
 have therefore none of the characteristics of cells. They 
 are shown, by treatment with Schulze's solution, to be com- 
 posed of lignin. 
 
 Surrounding the group of spiral vessels, and forming the 
 large polygonal meshes so obvious in a transverse section, 
 are wide tubes (B and c, sc. v) pointed at both ends and 
 fitting against one another in longitudinal series by their 
 oblique extremities. They have transverse markings like 
 the rungs of a ladder, and are hence called scalariform 
 vessels. The markings (i) are due to wide transverse pits 
 in the otherwise thick lignified walls : in the oblique ends 
 by which the vessels fit against one another the pits are 
 frequently replaced by actual slits, so that a longitudinal 
 series of such vessels forms a continuous tube containing, 
 like the spiral vessels, air or water, but no protoplasm. In 
 most ferns the terminal walls are not thus perforated, and 
 the elements are then called tracheides. 
 
 The presence of these vessels spiral and scalariform 
 is the most important histological character separating ferns 
 and mosses. Arhe latter group and all plants below them are 
 composed exclusively of cells : ferns and all plants above 
 them contain vessels in addition, and are hence called vas- 
 cular plants. 
 
 The vessels, together with small parenchyma-cells inter- 
 spersed among them, make up the central portion of the 
 vascular bundle, called the wood or xylem. The peripheral 
 portion is formed of several layers of cells composing the bast 
 or phloem, and surrounding the whole is a single layer of 
 small cells, the bundle-sheath (b. sh). 
 
35 FERNS LESS. 
 
 The cells of the phloem are for the most part parenchy- 
 matous, but amongst them are some to which special 
 attention must be drawn. These (B and c, sv. /), are many 
 times as long as they are broad, and have on their walls 
 irregular patches or sieve-plates (F, sv. pi.) composed of groups 
 of minute holes through which the protoplasm of the cell is 
 continuous with that of an adjacent cell. The transverse or 
 oblique partitions between the cells of a longitudinal series 
 are also perforated, so that a row of such cells forms a sieve- 
 tube in which the protoplasm is continuous from end to end. 
 We have here, therefore, as striking an instance of proto- 
 plasmic continuity as in the deric epithelium and certain other 
 tissues of Polygordius (see p. 276). 
 
 The distal or growing end of the stem terminates in a blunt 
 apical cone or punctum vegetationis (c), surrounded by the 
 leaves of the terminal bud in the case of vertical stems, or 
 sunk in a depression and protected by close-set hairs in the 
 underground stem of the bracken. A rough longitudinal 
 section shows that, at a short distance from the apical cone, 
 the various tissues of the stem epidermis, parenchyma, 
 sclerenchyma, and vascular bundles merge insensibly into 
 a whitish substance, resembling parenchyma to the naked 
 eye, and called apical merislem (ap. mer). 
 
 Thin sections show that the summit of the apical cone is 
 occupied by a wedge-shaped apical cell (ap. c} which in 
 vertical stems is three-sided like that of mosses (Fig. 80, H, 
 p. 335), while in the horizontal stem of Pteris it is two-sided. 
 As in mosses, segmental cells (seg. c] are cut off from the three 
 (or two) sides of the apical cell in succession, and by further 
 division form the apical meristem (ap. mer), which consists 
 of small, close-set cells without intercellular spaces. As the 
 base of the apical cone is reached, the meristem is found to 
 
xxix APICAL GROWTH 351 
 
 pass insensibly into the permanent tissues, the cells near the 
 surface gradually merging into epidermis and hypodermis, 
 those towards the central region into sclerenchyma and the 
 various constituents of the vascular bundles, and those of 
 the intermediate regions into parenchyma. 
 
 The examination of the growing end of the stem shows us 
 how the process of apical growth is carried on in a compli- 
 cated plant like the fern. The apical cell is continually 
 undergoing fission, forming a succession of segmental cells ; 
 these divide and form the apical meristem, which is thus 
 being constantly added to at the growing end by the formation 
 and subsequent fission of new segmental cells : in this way the 
 apex of the stem is continually growing upwards or forwards. 
 But at the same time the meristem cells farthest from the 
 apex begin to differentiate : some elongate but slightly, 
 increasing greatly in size, and become parenchyma cells : 
 others by elongation in the direction of length of the stem 
 and by thickening and lignification of the cell-wall become 
 sclerenchyma cells : others again elongate greatly, become 
 arranged end to end in longitudinal rows, and, by the loss 
 of their protoplasm and of the transverse partitions between 
 the cells of each row, are converted into vessels spiral or 
 scalariform according to the character of their walls. Thus 
 while the epidermis, parenchyma, and sclerenchyma are 
 formed of cells, the spiral and scalariform vessels are cell- 
 fusions, or more accurately cell-wall-fusions, being formed by 
 the union in a longitudinal series of a greater or less number 
 of cell- walls. It will be remembered that the muscle-plates 
 of Polygordius are proved by the study of development to be 
 cell-fusions (p. 305). 
 
 We thus see that every cell in the stem of the fern was once 
 a cell in the apical meristem, that every vessel has arisen by 
 the concrescence of a number of such cells, and that the 
 
352 FERNS LESS. 
 
 meristem cells themselves are all derived, by the ordinary 
 process of binary fission, from the apical cell. In this way 
 the concurrent processes of cell-division, cell-differentiation, 
 and cell-fusion result in the production of the various and 
 complex tissues of the fully-formed stem. 
 
 The leaves vary greatly in form in the numerous genera 
 and species of ferns : they may consist of an unbranched 
 stalk bearing a single expanded green blade: or the stalk 
 may be more or less branched, its ramifications bearing the 
 numerous subdivisions of the blade, ox pinnce. 
 
 The anatomy of the leaf, like that of the stem, can be 
 readily made ou,t by a rough dissection. The leaf-stalk and 
 its branches have the same general structure as the stem, 
 consisting of parenchyma coated externally with epidermis 
 and strengthened internally by vascular bundles, which are 
 continuous with those of the stem. But the blade, or in the 
 case of a compound leaf, the pinna, has a different and quite 
 peculiar structure. It is invested by a layer of epidermis 
 which can be readily stripped off as an extremely thin, colour- 
 less membrane, exposing a soft, green substance, the leaf 
 parenchyma or mesophylL The leaf is marked externally by 
 a network of delicate ridges, the veins ; these are shown by 
 dissection to be due to the presence of fine white threads 
 which ramify through the mesophyll, and can be proved by 
 tracing them into the leaf-stalk to spring from its vascular 
 bundles, of which they are in effect the greatly branched 
 distal ends. 
 
 Microscopic examination shows the epidermis of the leaf 
 (K, ep and L) to consist of flattened, colourless cells of very 
 irregular outline and fitting closely to one another like the 
 parts of a child's puzzle. Amongst them are found at 
 intervals pairs of sausage -shaped cells (gd. c) placed with 
 
xxix LEAVES AND ROOTS 353 
 
 their concavities towards one another so as to bound a 
 narrow slit-like aperture (st). These apertures, which are 
 the only intercellular spaces in the epidermis, are called 
 stomates : the cells bounding them are the guard-cells, and 
 are distinguished from the remaining epidermic cells by 
 the possession of a few chromatophores. 
 
 The mesophyll, which as we have seen occupies the whole 
 space between the upper and lower epidermis, is formed of 
 thin-walled cells loaded with chromatophores (K, ms.pli) and 
 therefore of a deep green colour. The cells in contact with 
 the upper epidermis are cylindrical, and are arranged verti- 
 cally in a single row : those towards the lower surface are 
 very irregular both in form and arrangement. Large inter- 
 cellular spaces (/. c. sp] occur between the mesophyll-cells 
 and communicate with the outer air through the stomates. 
 
 The leaves arise as outgrowths of the distal or growing 
 end of the stem, each originating from a single segmental 
 cell of the apical cone. 
 
 The fern is the first plant we have yet considered which 
 possesses true roots, the structures so-called differing funda- 
 mentally from the simple rhizoids of Nitella and the mosses. 
 Instead of being mere linear aggregates of cells, they agree 
 in general structure with the stem from which they spring, 
 consisting of an outer layer of epidermis within which is 
 parenchyma strengthened by bands of selerenchyma and by 
 a single vascular bundle in the centre. The epidermic cells 
 give rise to unicellular prominences, the root- hairs. 
 
 The apex of the root, like that of the stem, is formed of 
 a mass of meristem in which a single wedge-shaped apical 
 cell (Fig. 82, M, ap. c] can be distinguished. But instead 
 of the base of this cell forming the actual distal extremity, 
 as in the stem (compare c), it is covered by several lavers of : 
 
 t>*' 
 
354 FERNS LESS. 
 
 cells which constitute the root-cap (r.cp). In fact the apical 
 cell of the root divides not only by planes parallel to its 
 three sides, but also by a plane parallel to its base, and in 
 this way produces not only three series of segmental cells 
 (seg. c) which afterwards subdivide to form the apical 
 meristem, but also a series of cap-cells (cp. c] which form a 
 protective sheath over the tender growing end of the root as 
 it forces its way through the soil. 
 
 Roots are also peculiar in their development. Instead ot 
 being, like leaves, prominences of the superficial tissues of 
 the stem, they arise from a layer of cells immediately ex- 
 ternal to the vascular bundles, and in growing force their 
 way through the superficial portion of the stem, through 
 a fissure from which they finally emerge. They are thus said 
 to be endogenous in origin while leaves are exogenous. 
 
 The nutrition of ferns is carried on in much the same 
 way as in mosses (see p. 340). Judging from the analogy of 
 flowering plants it would seem that the ascending current of 
 water from the roots passes mainly through the xylem of the 
 vascular bundles, while the descending current of nitrogenous 
 and other nutrient matters for the supply of the colourless 
 cells of the stem and roots passes chiefly through the phloem 
 and especially through the sieve-tubes. The absorption of 
 water is effected by the root-hairs. 
 
 In the autumn there are found on the under surfaces of 
 the leaves brown patches called sori, differing greatly in 
 form and arrangement in the various genera, and formed of 
 innumerable, minute, seed-like, bodies, the sporangia (Fig. 
 83, A), just visible to the naked eye. Each sorus or group 
 of sporangia is covered by a fold of the epidermis of the 
 leaf, called the indusiwn. 
 
xxix REPRODUCTION 355 
 
 A sporangium is attached to the leaf by a multicellular 
 stalk (st), and consists of a sac resembling two watch-glasses 
 placed with their concave surfaces towards one another and 
 their edges united by a thick rim (an). The sides are 
 formed by thin flattened cells with irregular outlines, the 
 rim or annulus of peculiarly shaped cells which are thin and 
 broad at one edge (to the left in A), but on the other (to the 
 right) are thick, strongly lignified, and of a yellowish-brown 
 colour. The whole internal cavity is filled with spores 
 (B, sp) having the form of tetrahedra with rounded edges, 
 and'each consisting of protoplasm containing a nucleus, and 
 surrounded by a double wall of cellulose. A spore is there- 
 fore, as in mosses, a single cell. 
 
 Each sporangium arises from a single epidermic cell of 
 the leaf. This divides repeatedly so as to form a solid mass 
 of cells, of which the outermost become the wall of the 
 sporangium while the inner are the spore-mother-cells. The 
 latter divide each into four spores, as in mosses (p. 339). 
 
 As the spores ripen, the wall of the sporangium dries, and 
 as it does so the thickened part of the annulus straightens 
 out, tearing the thin cells and producing a great rent through ' 
 which the spores escape (B). 
 
 When the spores are sown on moist earth they germinate, 
 by the protoplasm, covered by the inner coat, protruding 
 through the ruptured outer coat (c, sp) in the form of a 
 short filament. This divides transversely, forming two cells, 
 the proximal of which sends off a short rhizoid (rh). The 
 resemblance of this stage to the young protonema of a moss 
 is sufficiently obvious (see Fig. 81, D 1 ., p. 338). 
 
 Further cell-division takes place, and before long the 
 distal cells divide longitudinally, a leaf-like body being 
 produced, which is called the prothallus (D). This is at first 
 
 A A 2 
 
T-7l. 
 
 FiG. 83. Reproduction and Development of Ferns. 
 
 A, Sporangium of Pteris> external view, showing stalk (s) and 
 annulus (an). 
 
 B, the same, during dehiscence, the spores (sp) escaping. 
 
 C, a germinating spore, showing the ruptured outer coat (.?/), and a 
 
LESS, xxix THE PROTHALLUS 357 
 
 rhizoid (rJi) springing from the proximal cell of the rudimentary (two- 
 celled) prothallus. 
 
 D, a young prothallus, showing spore, rhizoid (rJi), apical cell 
 (ap. c), and segmental cells (seg. c). 
 
 E, an advanced prothallus, from beneath, showing rhizoids (rh}, 
 ovaries (ovy), and spermaries (spy}. 
 
 F, a mature spermary of Pteris, inverted (i.e. with its distal end 
 directed upwards) so as to compare with Fig. 82, A. 
 
 G, a single sperm, showing coiled body and numerous cilia. 
 
 H, a mature ovary of Aspidium, inverted so as to compare with Fig. 
 82, B 2 , showing venter (v), neck (#}, ovum (ov), and canal cells (en. c). 
 
 I, small portion of a prothallus of Asplenium in vertical section, 
 showing the venter {v) and part of the neck (n) of a single ovary after 
 fertilization. The venter contains an embryo just passing from the 
 polyplast into the phyllula stage, and divided into four groups of cells, 
 the rudiments respectively of the foot (ft], stem (st), root (rt), and 
 cotyledon (ct}. 
 
 K, vertical section of a prothallus (prtJi) of Nephrolepis, bearing 
 rhizoids (r/i), and a single ovary with greatly dilated venter (v) and 
 withered neck (). The venter contains an embryo in the phyllula 
 stage, consisting of foot (ft}, rudiments of stem (.$/), and root (rt}, and 
 cotyledon (ct} beginning to grow upwards. 
 
 L, prothallus (prth} with rhizoids (r/i), bearing a young fern plant, 
 consisting of foot (//), rudiment of stem (st), first root (rt), cotyledon 
 (ct), and first ordinary leaf (/). (After Howes.) 
 
 only one layer of cells thick, but it gradually increases in 
 size, becoming more or less kidney-shaped (E), and as it does 
 so its cells divide parallel to the surface, making it two and 
 finally several cells in thickness. Thus the prothallus is 
 at first a linear, then a superficial, and ultimately a solid 
 aggregate. Root-hairs (rh) are produced in great number 
 from its lower surface, and penetrating into the soil serve 
 for the absorption of nutriment. At an early period a two- 
 sided apical cell (D, ap. c) is differentiated, and gives off 
 segmental cells (seg. c) in the usual way : an abundant forma- 
 tion of chromatophores also takes place at a very early period 
 in the cells of the prothallus, which therefore resembles both 
 in structure and in habit some very simple form of moss. 
 
 On the lower surface of the prothallus gonads (E, spy, ovy) 
 are developed, resembling in their essential features those of 
 
358 FERNS LESS. 
 
 mosses. The spermaries (spy] make their appearance first, 
 being frequently found on very young prothalli. One of the 
 lower cells forms a projection which becomes divided off by 
 a septum : further division takes place, resulting in the 
 differentiation (F) of an outer layer of cells forming the wall 
 of the spermary, and of an internal mass of sperm-mother-cells 
 in each of which a sperm is produced. The sperm (G) is a 
 corkscrew-like body, probably formed from the nucleus of the 
 cell, bearing at its narrow end a number of cilia which 
 appear to originate from the protoplasm. To the thick end 
 is often attached a globular body, also arising from the 
 protoplasm of the mother-cell ; this is finally detached. 
 
 The ovaries (E and H, ovy) are not usually formed until the 
 prothallus has attained a considerable size. Each arises, like 
 a spermary, from a single cell cut off by a septum from one 
 of the lower cells of the prothallus : the cell divides and 
 forms a structure resembling in general characters the ovary 
 of a moss (see Fig. 81, B, p. 338), except that the venter (H, 
 v) is sunk in the prothallus, and is therefore a less distinct 
 structure than in the lower type. As in mosses, also, an 
 axial row of cells is early distinguished from those forming 
 the wall of the ovary : the proximal of these becomes the 
 ovum (ov), the others are the canal cells (en. <:), which are 
 converted into mucilage, and by their expansion force open 
 the neck and make a clear passage for the sperm. 
 
 The sperms swarm round the aperture of the ovary and 
 make their way down the canal, one of them finally conju- 
 gating with the ovum and converting it into an oosperm. 
 
 The early stages in the development of the embryo 
 remind us, in their general features, of what we found to 
 occur in mosses (p. 337). The oosperm first divides by a 
 plane parallel to the neck of the ovary, forming two cells, an 
 anterior nearest the growing or distal end of the prothallus, 
 
POLYPLAST AND PHYLLULA 359 
 
 and a posterior towards its proximal end. Each of these 
 divides again by a plane at right angles to the first, there 
 being now an upper and a lower anterior, and an upper and 
 a lower posterior cell : the lower in each case being that 
 towards the downwardly directed neck of the ovary. Each 
 of the four cells undergoes fission, the embryo then consisting 
 of eight cells, two upper anterior (right and left), two lower 
 anterior, two upper posterior, and two lower posterior. We 
 thus get a multicellular but undifferentiated stage, the 
 polyplast. 
 
 It will be remembered that in mosses the polyplast forms 
 an apical cell, and develops directly into the sporogonium 
 (P- 337)- I n tne f ern the later stages are more complex. 
 One of the upper anterior cells remains undeveloped, the 
 other (Fig. 83, i and K, st) takes on the form of a wedge- 
 shaped apical cell, and, dividing in the usual way, forms a 
 structure like the apex of the fern-stem, of which it is in fact 
 the rudiment. The two upper posterior cells divide and 
 subdivide, and form a multicellular mass called the/w/(//), 
 which becomes embedded in the prothailus, and serves the 
 growing embryo for the absorption of nutriment. One of the 
 lower posterior cells remains undeveloped, the other (rt) 
 takes on the form of the apical cell of a root, /.<?., of a wedge- 
 shaped cell, which not only produces three sets of segmental 
 cells from its sides but also cap-cells from its base (p. 354) : 
 division of this cell goes on very rapidly, and a root is pro- 
 duced which at once grows downwards into the soil. Finally 
 the two lower anterior cells undergo rapid fission, and 
 develop into the first leaf of the embryo or cotyledon (<:/), 
 which soon begins to grow upwards towards the light. 
 
 Thus at a comparatively early stage of its development 
 the fern-embryo has attained a degree of differentiation far 
 beyond anything which occurs in the moss-embryo. The 
 
360 FERNS LESS. 
 
 scarcely differentiated polyplast has passed into a stage 
 which may be called the phyllula, distinguished by the 
 possession of those two characteristic organs of the higher 
 plants, the leaf and root. 
 
 Notice how early in development the essential features of 
 animal or plant manifest themselves. In Polygordius the 
 polyplast is succeeded by a gastrula distinguished by the 
 possession of a digestive cavity : in the fern no such cavity 
 is formed, but the polyplast is succeeded by a stage dis- 
 tinguished by the possession of a leaf and root. In the 
 one case the characteristic organ for holozoic, in the other 
 the characteristic organs for holophytic nutrition make their 
 appearance, and so mark the embryo at once as an animal 
 or plant. We may say then that while the oosperm and 
 the polyplast stages of the embryo are common to the 
 higher plants and the higher animals, the correspond- 
 ence goes no further, the next step being the formation 
 in the animal of an enteron, in the plant of a leaf and 
 root. In other words the phyllula is the correlative of 
 the gastrula. 
 
 The cotyledon increases rapidly in size, and emerges 
 between the lobes of the kidney-shaped prothallus (L) : the 
 root at the same time grows to a considerable length, the 
 result being that the phyllula becomes a very obvious 
 structure in close connection with the prothallus, and indeed 
 appearing to be part of it. The two are actually, however, 
 quite distinct, their union depending merely upon the fact 
 that the foot of the phyllula is embedded in the tissue of 
 the prothallus like a root in the soil. Hence the phyllula 
 is related to the prothallus in precisely the same way as the 
 sporogonium to the moss plant (compare Fig. 83, K, with 
 Fig. 81, c 2 , and Fig. 83, L, with Fig. 81, c 4 ). 
 
 The rudiment of the stem (L, sf) continues to grow by the 
 
xxix GAMOBIUM AND AGAMOBIUM 361 
 
 production of fresh segments from its apical cell : leaves (/) are 
 developed from the segments, and grow upwards parallel with 
 the cotyledon. The leaves first formed are small and 
 simple in structure, but those arising later become succes- 
 sively larger and more complicated, until they finally attain 
 the size and complexity of the ordinary leaves of the fern. 
 In the meantime new roots are formed ; the cotyledon, the 
 foot, and the prothallus wither, and thus the phyllula, by the 
 successive formation of new parts from its constantly growing 
 stem, becomes a fern-plant. 
 
 We see that the life-history of the fern resembles in 
 essentials that of the moss. In both, alternation of genera- 
 tion occurs, a gamobium or sexual generation giving rise, by 
 the conjugation of ovum and sperm, to an agamobium or 
 asexual generation, which, by an asexual process of spore- 
 formation, produces the gamobium. But in the relative 
 proportions of the two generations the difference is very great. 
 What we know as the moss plant is the gamobium, and the 
 agamobium is a mere spore-producing structure, never getting 
 beyond the stage of a highly differentiated polyplast, and 
 dependent throughout its existence upon the gamobium, to 
 which it is permanently attached. What we know as the 
 fern plant is the agamobium, a large and complex structure 
 dependent only for a brief period of its early life upon the 
 small and insignificant gamobium. Thus while the gamobium 
 is the dominant phase in the life-history of mosses, the 
 agamobium appearing like a mere organ, in ferns the 
 positions are more than reversed the agamobium may 
 assume the proportions of a tree, while the gamobium is so 
 small that its very existence is unknown to a large propor- 
 tion of fern-collectors. 
 
 It follows from what has just been said that the various 
 organs of a fern do not severally correspond with those of a 
 
362 FERNS LESS, xxix 
 
 moss. The leaves of a moss are not homologous with those 
 of a fern, but are rather comparable to lobes of the pro- 
 thallus : in the same way the rhizoids of a moss correspond 
 not with the complicated roots of the fern but with the 
 rhizoids of the prothallus. 
 
LESSON XXX 1 
 
 THE GENERAL CHARACTERS OF THE HIGHER PLANTS 
 
 IN the 2yth Lesson (p. 307) it was pointed out that a 
 thorough comprehension of the structure and development 
 of Polygordius would enable the student to understand the 
 main features of the organization of all the higher animals. 
 
 In the same way the study of the fern paves the way to 
 that of the higher groups of plants, all of which indeed, differ 
 far less from the fern than do the various animal types con- 
 sidered in Lesson XXVII from Polygordius. We saw that 
 the differences between these included matters of such im- 
 portance as the presence or absence of segmentation and of 
 lateral appendages, the characters of the skeleton, and the 
 structure and position of the nervous system. In the higher 
 plants, on the other hand, the essential organs root, stem, 
 and leaves are, save in details of form, size, &c., practically 
 the same in all : the tissues always consist of epidermis, 
 ground-parenchyma, and vascular bundles, the latter being 
 divisible into phloem and xylem : the growing point both of 
 stem and of root is formed of meristem, from which the per- 
 manent tissues arise ; and the growing point of the root is 
 
 1 Readers who have not studied botany, or at least examined types 
 of the chief groups of plants, will derive little benefit from this lesson. 
 
364 CHARACTERS OF THE HIGHER PLANTS LESS. 
 
 always protected by a root-cap, that of the stem being simply 
 over-arched by leaves. Moreover an alternation of genera- 
 tions can be traced in all cases. 
 
 Plants may be conveniently divided into the following 
 chief groups or phyla : 
 
 Alga. 
 
 Fungi. 
 
 Muscinea. 
 
 Vascular Cryptogams. 
 
 Filicinae. 
 
 Equisetaceae. 
 
 Lycopodineae. 
 Phanerogams. 
 
 Gymnosperms. 
 
 Angiosperms. 
 
 The Alga are the lower green plants. They may be 
 unicellular, or may take the form of linear, superficial, 
 or solid aggregates : they never exhibit more than a 
 limited amount of cell-differentiation. This group has been 
 represented in the foregoing pages by Zooxanthella, diatoms, 
 Vaucheria, Caulerpa, Monostroma, Ulva, Laminaria, and 
 Nitella. 1 
 
 The Fungi are the lower plants devoid of chlorophyll : 
 some are unicellular, others are linear aggregates : in none 
 is there any cell-differentiation worth mentioning. Saccharo- 
 myces, Mucor, Penicillium, and the mushroom belong to 
 this group. 
 
 The position of some of the lower forms which have come 
 under our notice is still doubtful. Bacteria, for instance, 
 are considered by some authors to be Fungi, by others Algae, 
 
 1 By some authors Nitella is placed near the Muscinea:. 
 
xxx CHARACTERS OF THE PHYLA 365 
 
 while others place them in a group apart. Diatoms also are 
 sometimes placed in a distinct group. It must, moreover, 
 be remembered that most botanists include Haematococcus 
 and Volvox among Algae, and place the Mycetozoa either 
 among Fungi or in a separate group of chlorophyll-less 
 plants (p. 181). 
 
 The Mustinetz are the mosses and liverworts, the former 
 of which were fully described in Lesson XXVIII. 
 
 The Vascular Cryptogams are flowerless plants in which 
 vascular bundles are present. Together with the Phanero- 
 gams they constitute what are known as vascular plants, in 
 contradistinction to the non-vascular Algae, Fungi, and 
 Muscineae, in which no formation of vessels takes place. The 
 group contains three subdivisions. 
 
 The first division of Vascular Cryptogams, the Filicince, 
 includes the ferns, an account of which has been given in 
 the previous lesson. It will be necessary, however, to devote 
 some attention to an aquatic form, called Salvinia, which 
 differs in certain important particulars from the more familiar 
 members of the group. 
 
 The Equisetacece include the common horsetails (genus 
 Equisctum), a brief account of which will be given, as 
 they form an interesting link in their reproductive processes 
 between the ordinary ferns and Salvinia. 
 
 The Lycopodine&i or club-mosses, are the highest of the 
 Cryptogams or flowerless plants. A short description of one 
 of them, the genus Selaginella, will illustrate the most 
 striking peculiarities of the group. 
 
 The Phanerogams, or flowering plants, are so called from 
 the fact that their reproductive organs take the form of 
 specially modified shoots, called cones or flowers. They are 
 sometimes called by the more appropriate name of Sperma- 
 phytes, or seed-plants, from the fact that they alone among 
 
366 EQUISETUM LESS. 
 
 plants, reproduce by means of seeds, structures which differ 
 from spores in the fact that each contains an embryo plant 
 in the phyllula stage. 
 
 The Gymnosperms, or naked-seeded Phanerogams, include 
 the cone-bearing trees, such as pines, larches, cypresses, &c., 
 as well as cycads and some other less familiar forms. A 
 general account of this group will be given. 
 
 The Angiosperms, or covered-seeded Phanerogams, include 
 all the ordinary flowering plants, as well as such trees as 
 oaks, elms, poplars, chestnuts, &c. A brief description of 
 the general features of this group will conclude the lesson. 
 
 EQUISETUM 
 
 A horsetail consists of an underground creeping stem 
 from which vertical shoots are given off. Some of these 
 bear only leaves and branches, others are peculiarly modified 
 and produce sporangia. 
 
 A fertile or sporangium-bearing shoot terminates distally 
 in a conical body (Fig. 84, A), formed of closely-fitting 
 hexagonal scales (sp.pti). Each scale (B, sp.ph) is attached 
 by a stalk to the axis of the shoot, and bears on its inner 
 surface a number of sporangia (spg). The scales are 
 modified leaves, and since they alone produce sporangia 
 they are distinguished from the ordinary foliage-leaves as 
 sporophylls. 
 
 The spores, which have the same general structure as those 
 of ferns, are liberated by the bursting of the sporangia, and 
 germinate, giving rise to prothalli. But instead of the 
 prothalli being all alike in form and size and all monoecious, 
 some (c) remain small and simple, and produce only 
 spermaries (spy}-, others (D) attain a complicated form and 
 a length of over a centimetre, and produced only ovaries 
 
XXX 
 
 DIMORPHISM OF THE GAMOBIUM 
 
 367 
 
 (ovy). Thus although there is no difference in the spores, 
 the prothalli produced from them are of two distinct kinds, 
 the smaller being exclusively male, the larger female. 
 
 FIG. 84. Reproduction and Development of Equisetuin. 
 
 A, distal end of a fertile shoot, showing two leaf-sheaths (/. sh\ and 
 the cone formed of hexagonal sporophylls (sp. ph}. (Nat. size.) 
 
 B, diagrammatic vertical section of a portion of the cone, showing the 
 sporophylls (sp. p/t) attached by short stalks to the axis of the cone, and 
 bearing sporangia (spg) on their inner surfaces. 
 
 c, a male prothallus bearing three s'permaries (spy), (x 100.) 
 
 D, portion of a female prothallus bearing three ovaries (ovy}, those to 
 
 the right and left containing ova, that in the middle a polyplast ; rh, 
 
 rhizoids. ( x 30.) 
 
 (A, after Le Maout and Decaisne ; C and D, after Hofmeister. ) 
 
 The oosperm develops in much the same way as in 
 ferns : it divides and forms a polyplast, which, by formation 
 of a stem, root, foot, and two cotyledons, becomes a 
 phyllula and grows into the adult plant, 
 
368 SALVINIA LESS. 
 
 As in the fern, the Equisetum plant, reproducing as it 
 does by asexual spores, is the agamobium, the gamobium 
 being represented by the prothallus. The peculiarity in the 
 present case is that the gamobium is sexually dimorphic, 
 some prothalli producing only male, others only female 
 gonads. 
 
 SALVINIA 
 
 Salvinia is a fresh-water plant, consisting of a long floating 
 stem bearing at intervals whorls of leaves. Of these some 
 have the ordinary character while others hang downwards 
 into the water and have the form and function of roots. 
 True roots are absent. 
 
 The sori or groups of sporangia (Fig. 85, A) are borne on 
 the proximal ends of the submerged leaves, each being en- 
 closed in a globular case corresponding to the indusium of 
 ordinary ferns. They differ from the sori of the typical 
 ferns in being dimorphic, some containing a comparatively 
 small number of large sporangia (mg. spg) others a much 
 larger number of small ones (mi. spg). The larger kind, 
 distinguished as megasporangia, contain each a single large 
 spore, or megaspore : the smaller kind, or microsporangia, 
 contain a large number of minute spores, like those of an 
 ordinary fern, and called microspores. It is this striking 
 dimorphism of the sori, sporangia, and spores which forms 
 the chief distinction between Salvinia and its allies and the 
 true ferns. 
 
 The microspore germinates (B), while still enclosed in its 
 sporangium, by sending out a filament, the end of which (spy] 
 becomes separated off by a septum and then divided into 
 two cells. The protoplasm of each of these divides into 
 four sperm-mother-cells, and from these, spirally-twisted 
 sperms are produced in the usual manner. It is obvious 
 
XXX 
 
 REDUCTION OF THE GAMOBIUM 
 
 369 
 
 that the two cells in which the sperms are developed repre- 
 sent a greatly simplified sperm ary : the single proximal cell 
 
 mi. 
 
 FIG. 85. Reproduction and Development of Salvinia. 
 
 A, portion of a submerged leaf, showing three sori in vertical section, 
 two containing microsporangia (mi. spg} and one megasporangia (mg. 
 spg}. (x 10.) 
 
 B, a germinating microspore (mi. ste\ showing the vestigial prothallus 
 (prth} and spermary (spy}. ( x 150.) 
 
 c, diagrammatic vertical section of a germinating megaspore, showing 
 the outer (mg. sp} and inner (nig. sp'} coats of the spore, and its cavity 
 (c) containing plastic products, separated by a septum (d] from the pro- 
 thallus (prth}, in which two ovaries (ovy} are shown, that to the left 
 containing an ovum, that to the right a polyplast. ( x 50.) 
 
 D, megaspore (mg. sp} with prothallus (prth} and phyllula just begin- 
 ning to develop into the leafy plant : st, stem ; cf, cotyledon ; and /, 
 outermost leaf of the terminal bud. ( x 20.) 
 
 (A and B, after Sachs ; D, after Pringsheim.) 
 
 (prth) of the filament arising from the microspore, a still 
 more simplified prothallus. Both prothallus and spermary 
 are vestigial structures ; the prothallus is microscopic and 
 unicellular instead of being a solid aggregate of considerable 
 size, as in the two preceding types ; the spermary is bicellular 
 
 B B 
 
370 SALVINIA LESS. 
 
 instead of being formed of a distinct wall and an internal 
 mass of cells ; and the number of sperms is reduced to 
 eight. 
 
 The contents of the megaspore are divisible into a com- 
 paratively small mass of protoplasm at one end and of starch 
 grains, oil-globules, and proteid bodies, which fill up the rest 
 (c, c} of the spore. The megaspore has in fact attained its 
 large size by the accumulation of great quantities of plastic 
 products, which serve as nutriment to the future prothallus 
 and embryo. 
 
 The protoplasm of the megaspore (c) divides and forms a 
 prothallus (prtti} in the form of a three-sided multicellular 
 mass projecting from the spore, which it slightly exceeds in 
 size. Several ovaries (oi>y] are formed on it, having much 
 the same structure as in ordinary ferns. Thus the reduction 
 of the prothallus produced from the megaspore, although 
 obvious, is far less than in the case of that arising from the 
 microspore. 
 
 We see that sexual dimorphism has gone a step further in 
 Salvinia than in Equisetum : not only are the prothalli 
 differentiated into male and female, but also the spores 
 from which they arise. 
 
 Impregnation takes place in the usual way, and the 
 oosperm divides to form a polyplast, which, by differentiation 
 of a stem-rudiment, a cotyledon, and a foot, passes into the 
 phyllula stage : no root is developed in Salvinia. By the 
 gradual elongation of the stem (D, sf] and the successive 
 
 formation of whorls of leaves (/), the adult form is 
 
 assumed. 
 
 Thus the life-history of Salvinia resembles that of the 
 
 fern, but with two important differences : the spores are 
 
 dimorphic, and the gamobium, represented by the male and 
 
 female prothalli, is greatly reduced. 
 
xxx SELAGINELLA 371 
 
 SELAGINELLA 
 
 Selaginella, one of the club-mosses, consists of a long 
 branching stem bearing numerous close-set leaves. It thus 
 resembles in external appearance a moss, but the essential 
 difference between the two is seen from a study of their 
 histology, Selaginella having a distinct epidermis and 
 vascular bundles like the other Vascular Cryptogams. 
 
 The branches terminate in cones (Fig. 86, A) formed of 
 small leaves (sp. p/i) which overlap in somewhat the same 
 way as the scales of a pine-cone. Each of these leaves is a 
 sporophyll, and bears on its upper or distal side, near the 
 base, a globular sporangium. The sporangia are fairly 
 uniform in size, but some are megasporangia (mg. spg) and 
 contain usually four megaspores, others are microsporangia 
 (mi. spg) containing numerous microspores. 
 
 The microspore (B) cannot be said to germinate at all. Its 
 protoplasm divides, forming a small cell (prtti), which repre- 
 sents a vestigial prothallus, and a large cell, the representative 
 of a spermary. The latter (spy) undergoes further division, 
 forming six to eight cells in which numerous sperm-mother- 
 cells are developed. 
 
 A similar but less complete reduction of the prothallus is 
 seen in the case of the megaspore (c). Its contents are 
 divided, as in Salvinia, into a small mass of protoplasm at 
 one end, and a large quantity of plastic products rilling up 
 the rest of its cavity. The protoplasm divides and forms a 
 small prothallus ( prtti), and a process of division also takes 
 place in the remaining contents (prth 1 ) of the spore, pro- 
 ducing a large-celled tissue, the secondary prothallus. 
 
 By the rupture of the double cell-wall of the megaspore 
 the prothallus is exposed to the air, but it never protrudes 
 through the opening thus made, and is therefore, Uklt$hfe\ " 
 
 s nv 
 
372 
 
 SELAGINELLA 
 
 LESS. 
 
 corresponding male structure, purely endogenous. A few 
 ovaries (ovy) are formed on it, each consisting of a short 
 neck, an ovum, and two canal-cells afterwards converted into 
 
 spar 
 spsr 
 
 FIG. 86. Reproduction and Development of Selaginella. 
 
 A, diagrammatic vertical section of a cone, consisting of an axis bear- 
 ing close-set sporophylls (sp. ph}, on the bases of which microsporangia 
 (mi. spg} and megasporangia (mg. spg) are borne. 
 
 B, section of a microspore, showing the outer coat (mi. sp}, prothallial 
 cell (prt)i) y and multicellular spermary (spy). 
 
 C, vertical section of a megaspore, the wall of which (mg. sp} has been 
 burst by the growth of the prothallus (prtfi) : its cavity (prth'} contains 
 a large-celled tissue, the secondary prothallus : in the prothallus are 
 three ovaries (ovy}, that to the left containing an ovum, that to the right 
 an embryo (emb} in the polyplast stage, and that in the centre an embryo 
 in the phyllula stage, showing stem-rudiment (st}> foot (/), and two 
 cotyledons (ct} : both embryos are provided with suspensors (dotted) 
 (spsr), and have sunk into the secondary prothallus. 
 
 (Altered from Sachs. ) 
 
 mucilage : there is no venter, and the neck consists of only 
 two tiers of cells. 
 
 The oosperm divides by a plane at right angles to the 
 neck of the ovary, forming the earliest or two-celled stage of 
 
LESS, xxx EMBRYO AND SUSPENSOR 373 
 
 the polyplast. The upper cell undergoes further division, 
 forming an elongated structure, the suspensor (dotted in c) : 
 the lower or embryo proper (emb) is forced downwards into 
 the secondary prothallus by the elongation of the suspensor, 
 and soon passes into the phyllula stage by the differentiation 
 of a stem-rudiment (si), two cotyledons (<:/), a foot, (/) and 
 subsequently of a root. 
 
 A further reduction of the gamobium is seen in Selagi- 
 nella : both male and female prothalli are quite vestigial, 
 never emerging from the spores : and the spermary and 
 ovary are greatly simplified in structure. 
 
 GYMNOSPERMS 
 
 Such common Gymnosperms as the pines and larches 
 have the character of forest trees, the stem being a strong, 
 woody trunk. The numerous, close-set branches bear small, 
 needle-like leaves, and the root is large and extensively 
 branched. 
 
 On the branches are borne structures of two kinds, the 
 male and female cones or flowers (Fig. 87, A and c). Both 
 ar.e to be considered as abbreviated shoots consisting of an 
 axis bearing numerous sporophylls (up. p/i). Frequently, as 
 in the pines, several male cones are aggregated together, 
 forming an inflorescence, or group of flowers. 
 
 In the male cone (A) the sporophylls (stamens, sp. ph. $ ) 
 are more or less leaf-like structures, each bearing on. its 
 under or proximal side two or more microsporangia (pollen- 
 sacs, mi. spg). The mother-cells of these divide each into 
 four microspores (pollen-grains), which are liberated by the 
 rupture of the microsporangia in immense quantities. The 
 microspore (B) is at first an ordinary cell consisting of proto- 
 plasm with a nucleus and a double cell-wall, but upon being 
 
FIG. 87. Reproduction and Development of Gymnosperms. 
 A, diagrammatic vertical section of male cone, showing axis with male 
 sporophylls (sp. ph. 6 ) bearing microsporangia (mi. s_/>g) : per, scale-like 
 leaves forming a rudimentary perianth. 
 
LESS, xxx SPOROrilYLLS . 375 
 
 B, a single microspore, showing bladder-like processes of outer coat, 
 and contents divided into small prothallial cell (a} and large cell (/'), 
 from which the pollen-tube arises. 
 
 c, diagrammatic vertical section of female cone, showing axis with 
 female sporophylls (sp. ph. ? ) bearing megasporangia (ing. spg), each of 
 which contains a single megaspore (ing. sp): per, the scale-like perianth 
 leaves. 
 
 D, diagrammatic vertical section of a megasporangium, showing 
 cellular coat (t}> and nucellus (ncl}, micropyle (inpy}, and megaspore 
 (ing. sp} : the latter contains the prothallus (prth} in which are two 
 ovaries, that to the left showing a large ovum (ov} and neck-cells, while 
 that to the right has given rise to an embryo (emb] which is in the 
 phyllula stage, and has sunk into the tissue of the prothallus by the 
 elongation of the long suspensor (spsr). 
 
 A microspore (mi. sp} is seen in the micropyle sending off a pollen - 
 tube (/. t}, the end of which is applied to the necks of the two ovaries. 
 
 E, diagrammatic vertical section of a seed, showing coat (/), micro- 
 pyle (inpy}, and endosperm (end], in which is embedded an embryo in 
 the phyllula stage, consisting of stem-rudiment (st), cotyledons (ct}, and 
 root (r). 
 
 (A and B, altered from Strasburger ; D and E, altered from Sachs. ) 
 
 liberated the protoplasm divides, as in Selaginella, into two 
 cells, a small one (a) the vestige of the male prothallus, and 
 a large one (b) which does not develop sperms, but under 
 favourable circumstances undergoes changes which will be 
 described presently. 
 
 In the female cone (c) each sporophyll (carpel, sp. ph, 2 ) 
 bears on its upper or distal side two megasporangia (so-called 
 ovules, mg. spg) the structure of which is peculiar. Each 
 consists of a solid mass of small cells called the nucellus (v,ncl\ 
 attached by its proximal end to the sporophyll, and sur- 
 rounded by a wall or integument (/) also formed of a small- 
 celled tissue. The integument is in close contact with the 
 nucellus, but is perforated distally by an aperture, the 
 micropyle (mpy\ through which a small area of the nucellus 
 is exposed. 
 
 Each megasporangium contains only a single megaspore 
 (embryo sac, c and D, mg. sp) in the form of a large ovoidal 
 body embedded in the tissue of the nucellus. It has at 
 
376 GYMNOSPERMS LESS. 
 
 first the characters of a single cell, but afterwards, by division 
 of its protoplasm, becomes filled with small cells representing 
 a prothallus (prth\ As in Vascular Cryptogams, single 
 superficial cells of the prothallus are converted into ovaries 
 which are extremely simple in structure, each consisting 
 of a large ovum (ov), and of a variable number of neck- 
 cells. 
 
 The pollen, liberated by the rupture of the microsporangia, 
 is carried to considerable distances by the wind, some of it 
 falling on the female cones of the same or another tree. In 
 this way single microspores (pollen-grains) find their way 
 into the micropyle of a megasporangium (D, mi. sp). This 
 is the process known as pollination, and is the necessary 
 antecedent of fertilization. 
 
 The microspore now germinates : the outer coat bursts, 
 and the larger of the two cells (B, b) protrudes in the form 
 of a filament resembling a hypha of Mucor, and called a 
 pollen-tube (D, p.t). This forces its way into the tissue of 
 the nucellus, like a root making its way through the soil, 
 and finally reaches the megaspore in the immediate neigh- 
 bourhood of an ovary. A process then grows out from the 
 end of the tube, passes between the neck-cells, and comes in 
 contact with the ovum. 
 
 In the meantime the nucleus of the large cell (b) of the 
 microspore that from which the pollen-tube grows has 
 travelled to the end of the pollen-tube and divided into two. 
 Protoplasm collects round each of the daughter nuclei, con- 
 verting them into cells, one of which remains undivided, 
 while the other divides, and its substance passes from the 
 pollen-tube into the ovum, where it forms a cell-like body, 
 to which the name of male pronucleus (see p. 263) has 
 been applied. This conjugates with the nucleus of the 
 ovum, or female pronucleus, and thus effects the process 
 
xxx FORMATION OF THE SEED 377 
 
 of fertilization, or the conversion of the ovum into the 
 oosperm. 
 
 The mode of formation of cells described in the preceding 
 paragraph should be specially noted. Instead of the ordin- 
 ary process of fission hitherto met with, the products of 
 division of a nucleus become surrounded by protoplasm, 
 cells being produced which lie freely in the interior of the 
 mother-cell. This is called free cell-formation. 
 
 The development of the oosperm is a very complicated 
 process, and results in the formation not of a single polyplast 
 but of four, each at the end of a long suspensor (D, spsr\ 
 in the form of a linear aggregate of cells, which by its elonga- 
 tion carries the embryo (emb) down into the tissue of the 
 prothallus. As a rule only one of these embryos comes to 
 maturity : it develops a rudimentary stem, root, and four 
 or more cotyledons, and so becomes a phyllula. 
 
 While these processes are going on the female cone 
 increases greatly in size and becomes woody. The mega- 
 sporangia also become much larger, their integuments (E, /), 
 becoming brown and hard, and the megaspore in each 
 enlarges so much as to displace the nucellus : at the same 
 time the cells of the prothallus filling the megaspore develop 
 large quantities of plastic products, such as fat and albumin- 
 ous substances, to be used in the nutrition of the embryo : 
 the tissue thus formed is the endosperm (end). The mega- 
 sporangium is now called a seed (see p. 365). 
 
 Under favourable circumstances the seed germinates. 
 By absorption of moisture its contents swell and burst the 
 seed-coat, and the root of the phyllula (r) emerges, followed 
 before long by the stem (j/) and cotyledons (ct). The 
 phyllula thus becomes the seedling plant, and by further 
 growth and the successive formation of new parts is converted 
 into the adult 
 
378 ANGIOSPERMS LESS, xxx 
 
 In Gymnosperms we se an even more striking reduction 
 of the gamobium than in Selaginella. The female prothallus 
 is permanently inclosed in the megaspore, and the megaspore 
 in the megasporangium : the ovaries also are greatly simplified. 
 The male prothallus is represented by the smaller cell of the 
 microspore, and no formation of sperms takes place, fertiliza- 
 tion being effected by cells developed in the extremity of a 
 tubular prolongation of the larger cell of the microspore, and 
 resulting from a modification of its nucleus. 
 
 It is worthy of notice that Phanerogams alone among 
 the higher organisms, have abandoned the ordinary method 
 of fertilization by the conjugation of ovum and sperm. In 
 this respect they are the most specialized of living things, 
 
 ANGIOSPERMS 
 
 In this group the general relations of the main parts of 
 the plant stem, leaves, roots, &c. are the same as in 
 Gymnosperms. 
 
 The flowers, in which, as in Gymnosperms, the organs of 
 reproduction are contained, have a very characteristic struc- 
 ture, which, although presenting almost infinite variety in 
 detail, is the same in its essential features throughout the 
 group. 
 
 A typical angiospermous flower (Fig. 88, A) is a greatly 
 abbreviated shoot, consisting of a short axis (fl. r) of limited 
 growth bearing four whorls of leaves, of which those of the 
 two distal whorls are sporophylls. 
 
 The axis of the floral shoot (A. fl. r) is usually broad and 
 more or less conical in form and is called the floral 'receptacle. 
 The leaves of the lower or proximal whorl (pcr^\ usually 
 from three to five in number, are small green bodies which 
 cover the other parts in the unopened flower : they are called 
 sepals and together constitute the calyx. 
 
ntZ 
 
 FIG. 88. Reproduction and Development of Angiosperms. 
 A, diagrammatic vertical section of a flower consisting of an abbreviated 
 axis or floral receptacle (ft. r) bearing a proximal (per 1 } and a distal 
 (per"} whorl of perianth leaves (sepals and petals), a whorl of male 
 
380 ANGIOS PERMS LESS. 
 
 sporophylls or stamens (sp. ph. cJ ), and one of female sporophylls or 
 carpels (sp. ph. 9 ). 
 
 The male sporophyll bears microsporangia (mi. spg) containing 
 microspores (mi. sp). 
 
 The female sporophyll consists of a solid style (st) terminated by a 
 stigma (sfg), and of a hollow venter (v) containing a megasporangium 
 (nig. spg) in which is a single megaspore (nig. sp}. 
 
 On the right side a microspore is shown on the stigma, and has sent 
 off a pollen-tube (p. t) through the tissue of the style to the micropyle 
 of the megasporangium. 
 
 B 1 , diagram of a female sporophyll from the distal aspect, and B 2 , the 
 same in transverse section, showing the folding in of its edges to form 
 the cavity or venter in which the megasporangia (mg. spg) are enclosed : 
 m.r, the midrib. 
 
 C 1 , a microspore, showing the two cells (a and b) into which its 
 contents divide. 
 
 C 2 , the same, sending out a pollen-tube (p. t) : nu, mt l t the two nuclei. 
 
 D, diagrammatic vertical section of a megasporangium, showing the 
 double integument C/ 1 ,/ 2 ), nucellus (ncl), micropyle (m.py), and mega- 
 spore (mg. sp) : the latter contains the secondary nucleus (nu) in the 
 centre, three antipodal cells (ant) at the proximal end, and two syner- 
 gidse (sng) and an ovum (ov) at the distal end. 
 
 A pollen-tube (p. t) is shown with its end in contact with the 
 synergidae. 
 
 E, semi-diagrammatic section of the megaspore of a young seed, 
 showing an embryo (emb) in the polyplast stage with its suspensor 
 (spsr) : also numerous vacuoles (vac) and nuclei (nu). 
 
 F, diagrammatic vertical section of a ripe seed, showing the seed-coat 
 (/), mieropyle (mpy\ perisperm (per) derived from the tissue of the 
 nucellus, and endosperm (end} formed in the megaspore and containing 
 an embryo in the phyllula stage with stem-rudiment (st), cotyledons (ct), 
 and root (r). 
 
 (B 1 after Behrens ; c 1 , c 2 , and E altered from Howes.) 
 
 Above the sepals comes a whorl of leaves (per*), usually 
 of large size and bright colour, forming in fact the most 
 obvious part of the flower. These are the petals and together 
 constitute the corolla. The calyx and corolla together are 
 conveniently called the perianth, because they inclose the 
 sporophylls or essential part of the flower. The presence of 
 a well-marked perianth is characteristic of the majority of 
 Angiosperms, and distinguishes them from Gymnosperms, in 
 which this part of the flower is quite rudimentary (see Fig. 
 87, A. and P., per}. 
 
xxx SPOROPHYLLS 381 
 
 The third whorl is called collectively the &ndrad*m, and 
 consists of a variable number of stamens or male sporo- 
 phylls(-y/./>/2. $ ). Each stamen is along narrow leaf bearing at 
 its distal end four microsporangia (pollen sacs, mi. spg) united 
 into a lobed knob-like body, the anther. The microspores 
 (c l ) are at first simple cells with double cell-walls, but sub- 
 sequently the protoplasm becomes divided into two cells, 
 as in Gymnosperms, a smaller (a) and a larger (fr). The two 
 are not, however, separated by a firm septum of cellulose, 
 and the smaller cell frequently comes to lie freely in the 
 protoplasm of the larger. Moreover it appears that the 
 nucleus of the smaller is the active agent in fertilization, and 
 that the larger must therefore be considered as representing 
 the vestigial prothallus. 
 
 The fourth or distal whorl of the flower is called collec- 
 tively the gyncecium or pistil, and consists of one or more 
 carpels or female sporophylls (sp. ph. $), which are modified 
 in a characteristic manner. In some cases each carpel (B 1 , 
 B 2 ) becomes folded longitudinally along its midrib (m.r), and 
 its two edges, thus brought into contact, unite so as to 
 inclose a cavity. Concrescence only affects the proximal 
 part of the carpel, which thus becomes a hollow capsule, the 
 venter (so-called ovary, A, v) : its distal portion usually takes 
 the form of a slender rod-like body, the style (j/), terminated 
 by an enlarged extremity, the stigma (stg) which is covered 
 with hairs and is frequently sticky. In some flowers, on the 
 other hand, all the carpels of the gynaecium unite with one 
 another by their adjacent edges, so as to inclose a cavity 
 common to all : in this case also the hollow portion or venter 
 is formed by the proximal part only of the carpels, their 
 distal portions forming a simple or multiple style and 
 stigma. 
 
 The megasporangia (ovules, A and B, mg. spg) are usually 
 
382 CHARACTERS OF THE HIGHER PLANTS LESS. 
 
 borne on the edges of the carpels, and, owing to the union 
 of the latter, become inclosed in the cavity of the venter, 
 and are thus completely shut off from all direct communica- 
 tion with the external world. It is this inclosure of the 
 megasporangia in a cavity formed by the sporophylls on 
 which they are borne which constitutes the chief character 
 distinguishing Angiosperms from Gymnosperms. 
 
 The megasporangia (D) differ from those of Gymnosperms 
 chiefly in having a double integument : both coats (t l , / 2 ) as 
 well as the nucellus (nd\ or central mass of tissue, are com- 
 posed of small cells : and the megaspore (embryo-sac, mg. 
 sp) is a single cell of great size embedded in the nucellus. 
 
 No prothallus is formed in the megaspore, but its nucleus 
 divides, the products of division pass to opposite ends of the 
 spore, and each divides again and then again, so that four 
 nuclei are produced at each extremity. Three of the nuclei 
 at the proximal end that furthest from the micropyle 
 become surrounded by protoplasm and take on the character 
 of cells (D, ant] : the fourth remains unchanged. Similarly 
 of the four nuclei at the distal or micropylar end, one 
 remains unchanged and three assume the form of cells by 
 becoming invested with protoplasm (see p. 376). Of these 
 three, two lie near the wall of the megaspore and are called 
 synergidce (sng) : the third, more deeply placed, is the ovum 
 (ov). The two unaltered nuclei now travel to the centre of 
 the megaspore and unite with one another, forming the 
 secondary nucleus (nu) of the spore. 
 
 There is thus a single ovum produced in each megaspore, 
 but no ovary and no prothallus : the female portion of the 
 gamobium is reduced to its simplest expression. 
 
 Pollination may take place, as in Gymnosperms, by the 
 agency of the wind, but usually the microspores are carried 
 by insects, which visit the flowers for the sake of obtaining 
 
xxx POLLINATION AND FERTILIZATION 383 
 
 nectar, a saccharine fluid* secreted by certain parts. The 
 microspores are deposited on the stigma (A), where they 
 germinate, each sending off a pollen-tube (A and c 2 , /. /), 
 which grows downwards through the tissue of the stigma and 
 style to the cavity of the venter, where it reaches a megaspo- 
 rangium, and entering at the micropyle (D, p. /), continues 
 its course through the nucellus, finally applying itself to the 
 distal end of the megaspore in the immediate neighbourhood 
 of the synergidae. 
 
 In the meantime the nuclei of the microspore (c 2 , nu, 
 iiu*) have passed into the end of the pollen-tube. The 
 nucleus of the larger cell undergoes degeneration, becoming 
 shrivelled and unaffected by dyes ; that of the smaller cell 
 divides by karyokinesis. One of the two daughter-nuclei 
 thus formed also degenerates, the other, accompanied by 
 its directive spheres, passes through the softened cell-wall of 
 the swollen end of the pollen-tube and enters the ovum, 
 uniting with its nucleus in the usual way. 
 
 The ovum is thus converted into an oosperm or unicellu- 
 lar embryo : it acquires a cell-wall and almost immediately 
 divides into two cells, of which that nearest the micropyle 
 becomes the suspensor (E, spsr\ the other, or embryo 
 proper (emb\ forming a solid aggregate of cells, the 
 polyplast. By further differentiation rudiments of a stem 
 (F, st), a root (r) and either one or two cotyledons (<:/) 
 are formed, and the embryo passes into the phyllula stage. 
 
 While the early development of the embryo is going on, 
 the secondary nucleus of the megaspore divides repeatedly, 
 and the products of division (E, nu] becoming surrounded 
 by protoplasm, a number of cells are produced, which, by 
 further multiplication, fill up all that part of the megaspore 
 which is not occupied by the embryo. The tissue thus 
 formed is called the endosperm (F. end), and occupies pre- 
 
384 ANGIOSPERMS 
 
 LESS. 
 
 cisely the position of the vestigial prothallus of Gymnosperms 
 (Fig, 87, p. 374, n,prth, and E, end: and p. 376), differing 
 from it in the fact that it is only formed after fertilization. 
 We. have here a case of retarded development : the degenera- 
 tion of the prothallus has gone so far that it arises, by free 
 cell-formation, long after the formation of the ovum which, 
 in both Gymnosperms and Vascular Cryptogams, is a 
 specially modified prothallial cell. 
 
 The phyllula continues to grow and remains inclosed in 
 the megasporangium, which undergoes a corresponding in- 
 crease in size and becomes the seed. One or more seeds 
 also remain inclosed in the venter of the pistil, which grows 
 considerably and constitutes \\\Q fruit. Finally the seeds are 
 liberated, the phyllula protrudes first its root, and then its 
 stem and cotyledons through the ruptured seed-coat, and 
 becomes the seedling plant. 
 
 We learn from the present lesson that there is a far greater 
 uniformity of organization among the higher plants than 
 among the higher animals, not only in anatomical and 
 histological structure, but also in the fact that alternation of 
 generations is universal from Nitella and the mosses up to 
 the highest flowering plants. But as we ascend the series, 
 the gamobium sinks from the position of a conspicuous 
 leafy plant to that of a small and insignificant prothallus, 
 becoming finally so reduced as to be only recognizable as 
 such by comparison with the lower forms. 
 
SYNOPSIS 
 
 A. AN ACCOUNT OF THE STRUCTURE, PHYSIOLOGY, 
 AND LIFE-HISTORY OF A SERIES OF TYPICAL 
 ORGANISMS IN THE ORDER OF INCREASING 
 COMPLEXITY. 
 
 I. THE SIMPLER UNICELLULAR ORGANISMS. 
 
 I'AGR 
 
 Cell-body amoeboid or encysted : cell-wall nitrogenous (?): 
 nutrition holozoic : reproduction by simple or binary 
 fission ...................... |! i 
 
 2. Htcmatococcus. 
 
 Cell-body ciliated or encysted : cell-wall of cellulose : 
 nutrition holophytic : reproduction by binary fission . . 23 
 
 3. Heteromita. 
 
 Cell-body ciliated : nutrition saprophytic : asexual repro- 
 duction by binary fission : sexual reproduction by conju- 
 gation of equal and similar gametes followed by multiple 
 fission of the protoplasm of the zygote, forming spores . 36 
 
 4. Euglena. 
 
 Cell-body ciliated or encysted : cell-wall of cellulose 
 mouth and gullet present : nutrition holophytic and 
 holozoic : reproduction by binary and multiple fission . .' 44 
 
 5. Protomyxa. 
 
 Cell-body amoeboid, ciliated or encysted : plasmodia 
 formed by concrescence of amcebulse : cell-wall nitro- 
 genous (?) : nutrition holozoic : reproduction by multiple 
 fission of encysted plasmodium .......... 49 
 
 C C 
 
386 SYNOPSIS 
 
 PAGE 
 
 6. Mycetozoa. 
 
 Like Protomyxa, but owing to the presence of nuclei the 
 relation of the individual cell-bodies to the plasmodium 
 is more clearly seen : cell wall of cellulose 5 2 
 
 o 7- Saccharomyces. 
 
 Cell-body encysted : cell-wall of cellulose : nutrition 
 saprophytic : reproduction by gemmation or by internal 
 fission : acts as an organized ferment 7 
 
 - 8. Bacteria. 
 
 Cell-body ciliated or encysted : cell-wall of cellulose : 
 nutrition saprophytic : reproduction by binary fission or 
 by spore-formation : act as organized ferments : the 
 simplest and most abundant of organisms 82 
 
 II. UNICELLULAR ORGANISMS IN WHICH THERE is CONSIDERABLE 
 COMPLEXITY OF STRUCTURE ACCOMPANIED BY PHYSIOLOGICAL 
 DIFFERENTIATION. 
 
 a. Complexity attained by differentiation of cell-body. 
 
 9. Parani(Kcium . 
 
 Medulla, cortex, and cuticle : trichocysts : complex con- 
 tractile vacuoles : nucleus and paranucleus : mouth, 
 gullet, and anal spot : conjugation temporary, no zygote 
 being formed, but interchange of nuclear material during 
 temporary union 106 
 
 10. Stylonychia. 
 
 Extreme differentiation or heteromorphism of cilia . . . 116 
 
 1 1 . Oxytricha. 
 
 Fragmentation of nucleus 120 
 
 1 2 Opalina. 
 
 Multiplication of nuclei ; parasitism and its results ; 
 necessity for special means of dispersal of an internal 
 parasite 121 
 
 13. Vorticella. 
 
 A stationary organism : limitation of cilia to defined 
 regions : muscle- fibre in stalk : necessity for means of 
 dispersal in a fixed organism : conjugation between free- 
 swimming micro- and fixed mega-gamete : zygote indis- 
 tinguishable from a zooid of the ordinary kind .... 126 
 
 14. Zoothamnium. 
 
 A compound organism or colony with dimorphic (nutri- 
 tive and reproductive) zooids : begins life as a single 
 zooid " 135 
 
SYNOPSIS 387 
 
 PAGE 
 
 />. Complexity attained by differentiation of cell-wall or by forma- 
 tion of skeletal structures in the protoplasm. 
 
 1 5 Foraminifera . 
 
 Calcareous shells (cell-walls) of various and complicated 
 form 148 
 
 1 6. Radiolaria. 
 
 Membranous perforated shell (cell-wall) and external 
 silicious skeleton often of great complexity : symbiotic 
 relations with Zooxanthella 152 
 
 17. Diatoms. 
 
 Silicious, two-valved, highly-ornamented shells .... 155 
 
 c. Complexity attained by simple elongation and branching of the 
 cell. 
 
 1 8. Mucor. 
 
 A branching filamentous fungus : necessity for special 
 reproductive organs in such an organism : they may be 
 sporangia producing asexual spores, or equal and similar 
 gametes producing a resting zygote 158 
 
 19. Vaiicheria. 
 
 A branched filamentous alga : clear distinction between 
 the gametes or conjugating bodies and the sexual repro- 
 ductive organs or gonads in which they are produced : 
 gonads differentiated into male (spermary) and female 
 (ovary) : gametes differentiated into male (sperm) and 
 female (ovum) : zygote an oosperm 169 
 
 20. Caulerpa. 
 
 Illustrates maximum differentiation ol a unicellular 
 plant : stem-like, leaf-like, and root-like parts 175 
 
 III. ORGANISMS IN WHICH COMPLEXITY is ATTAINED BY CELL- 
 MULTIPLICATION, ACCOMPANIED BY NO OR BUT LITTLE CELL- 
 DlFFERENTIATION. 
 
 a. Linear aggregates. 
 
 21. Penicillium. 
 
 A multicellular, filamentous, branched fungus : mycelial, 
 submerged, and aerial hyphae : apical growth : abundant 
 production of spores by constriction of aerial hyphse . . 184 
 
 22. Agaricus. 
 
 Complexity attained by interweaving of hyphae in a de- 
 finite form : illustrates maximum complexity of a linear 
 
 a gg re g ate > 
 
 C C 2 
 
388 SYNOPSIS 
 
 PAGE 
 
 23. Spirogyra. 
 
 A multicellular filamentous unbranched alga : interstitial 
 growth : gonads equal and similar, but gametes show 
 first indication of sexual differentiation 194 
 
 /'. Superficial aggregate. 
 
 24. Moiwstroma. 
 
 Cell-division takes place in two dimensions 202 
 
 c. Solid aggregates. 
 
 25. Ulva. 
 
 Like Monostroma, but cell -division takes place in three 
 dimensions 203 
 
 26. Laminaria. 
 
 Illustrates maximum size and complexity of a solid 
 aggregate of comparatively slightly differentiated cells . 203 
 
 IV. SOLID AGGREGATES IN WHICH COMPLEXITY is INCREASED 
 
 BY A LIMITED AMOUNT OF CELL-DlFFERENTIATION. 
 
 27. Nite.Ua. 
 
 Segmented axis : nodes and internodes : appendages 
 leaves and rhizoids : apical growth by binary fission of 
 apical cell accompanied by immediate division and dif- 
 ferentiation of newly-formed segmental cells : complex 
 gonads (ovaries and spermaries) : alternation of genera- 
 tions, a gamobium or sexual generation (the leafy plant) 
 alternating with an agamobium or asexual generation 
 (the pro-embryo) 206 
 
 28. Hydra. 
 
 Example of a simple diploblastic animal : cells arranged 
 in two layers (ecto- and endoderm) inclosing an enteron 
 which opens externally by the mouth : combination of 
 intra-cellular with extra-cellular or enteric digestion . . 221 
 
 29. Bougainvillea. 
 
 Example of a colony with diploblastic zooids which are 
 nutritive (hydranths) and reproductive (medusae) : differ- 
 entiation of a rudimentary mesoderm producing imper- 
 fect tripoblastic condition : central and peripheral nervous 
 system : alternation of generations, a gamobium (the 
 medusa) alternating with an agamobium (the hydroid 
 colony) ; significance of developmental stages oosperm 
 (unicellular), polyplast (multicellular but undifferentiated), 
 and planula (diploblastic) 237 
 
SYNOPSIS 389 
 
 PAGE 
 
 30. Dipkyes. 
 
 A free-swimming colony with polymorphic (nutritive, 
 reproductive, protective, and natatory) zooids 250 
 
 31. For pita. 
 
 Extreme polymorphism of zooids giving the colony the 
 character of a single physiological individual 253 
 
 V. SOLID AGGREGATES IN WHICH CELL-DIFFERENTIATION, AC- 
 COMPANIED BY CELL FUSION, TAKES AN IMPORTANT PART IN 
 PRODUCING GREAT COMPLEXITY IN THE ADULT ORGANISM. 
 
 32. Polygordius. 
 
 A triploblastic, coelomate animal with metameric seg- 
 mentation : prostomium, peristomium, metameres, and 
 anal segment : besides ecto- and endoderm there is a 
 well developed mesoderm divided into somatic and 
 splanchnic layers separated by the coelome : differentia- 
 tion of cells into fibres, &c. : muscle-plates formed as 
 cell-fusions : necessity for distributing system for supply 
 of food to parts of the body other than the enteric canal, 
 and for the removal of waste matters : circulatorv, res- 
 piratory, and excretory systems : high development of 
 nervous system brain and ventral cord, afferent and 
 efferent nerves : characteristic developmental stages 
 oosperm, polyplast, gastrula (diploblastic), trochosphere 
 (diploblastic with stomodseum and proctodeeum), late 
 trochosphere (triploblastic but accelomate) 271 
 
 33. Mosses. 
 
 Cell-differentiation very slight, but the type necessary to 
 lead up to ferns : sclerenchyma and axial bundle : dis- 
 tributing system rendered necessary by carbon dioxide 
 being taken in by the leaves, water and mineral salts by 
 the rhizoids : alternation of generations the leafy plant 
 is the gamobium, the agamobium being represented by 
 the spore-producing sporogonium : developmental stages 
 oosperm and polyplast, the latter becoming highly diffe- 
 rentiated to form the sporogonium 332 
 
 34. Ferns. 
 
 Extensive cell-differentiation : formation of fibres (elon- 
 gated cells) and vessels (cell-fusions) ; general differentia- 
 tion of tissues into epidermis, ground-parenchyma, and 
 vascular bundles : presence of true roots : the leafy plant 
 is the amagobium and produces spores from which the 
 gamobium, in the form of a small prothallus, arises : 
 developmental stages oosperm, polyplast, and phyllula 
 (leaf- and root-bearing stage) , 344 
 
390 SYNOPSIS 
 
 VI. BRIEF DESCRIPTIONS OF TYPES OF THE HIGHER GROUPS OF 
 ANIMALS AND PLANTS IN TERMS OF POLYGORDIUS AND OF 
 THE FERN RESPECTIVELY. 
 
 a. Animals. 
 
 All are triploblastic and ccelomate. 
 
 35. Starfish. 
 
 Radially symmetrical : discontinuous dermal exoskele- 
 ton : characteristic organs of locomotion (tube feet) in 
 connection with ambulacral system of vessels 309 
 
 36. Crayfish. 
 
 Metamerically segmented : segmented lateral append- 
 ages : differentiation of metameres and appendages : 
 continuous cuticular exoskeleton discontinuously calci- 
 fied : gills as paired lateral offshoots of the body-wall : 
 heart as muscular dilatation of dorsal vessel : ccelome 
 greatly reduced and its place taken by an extensive series 
 of blood-spaces : nervous system sunk in the mesoderm 
 and consisting of brain and ventral nerve-cord '. .... 314 
 
 37. Mussel. 
 
 Non-segmented : mantle formed as paired lateral out- 
 growths of dorsal region : foot as unpaired median out- 
 growth of ventral region : cuticular exoskeleton in the 
 form of a calcified bivalved shel.l : gills as paired lateral 
 outgrowth of body-wall : heart as muscular dilatation of 
 dorsal vessel : coslome reduced to pericardium : nervous 
 system consists of three pairs of ganglia sunk in the 
 mesoderm 320 
 
 38. Dogfish. 
 
 Metamerically segmented : differentiated into head, trunk, 
 and tail : trunk alone ccelomate in adult : appendages as 
 median (dorsal, ventral, and caudal) and paired (pectoral 
 and pelvic) fins : discontinuous dermal exoskeleton and 
 extensive endoskeleton of partially calcified cartilage, 
 including a chain of vertebral centra below the nervous 
 system replacing an embryonic notochord : gills as 
 pouches of pharynx opening on exterior : heart as 
 muscular dilatation of ventral vessel : hollow dorsal 
 nervous system not perforated by enteric canal 324 
 
 b. Plants. 
 
 All exhibit alternation of generations and the series 
 shows the gradual subordination of the gamobium to the 
 agamobium. 
 
SYNOPSIS 91 
 
 PAGE 
 
 39. Equisetum. 
 
 Sporangia borne on sporophylls arranged in cones : 
 spores homomorphic : prothalli dimorphic (male and 
 female) 366 
 
 40. Salvinia. 
 
 Spores dimorphic ; microspore produces vestigial male 
 prothallus : megaspore produces greatly reduced female 
 prothallus 368 
 
 41. Selaginella. 
 
 Microspore produces unicellular prothallus and multi- 
 cellular spermary, both endogenously : female prothallus 
 formed in megaspore and is almost endogenous : embryo 
 provided with suspensor 371 
 
 42. Gymnosperms. 
 
 Cones dimorphic (male and female), with rudimentary 
 perianth : no sperms formed but microspore gives rise to 
 pollen tube, nuclei in which are the active agents in fer- 
 tilization : single megaspore permanently inclosed in each 
 megasporangium : female prothallus purely endogenous : 
 embryo (phyllula) remains inclosed in megasporangium 
 which becomes a seed . . . . 373 
 
 43. Angio sperms. 
 
 Cone modified into flower by differentiation of sporo- 
 phylls and perianth : female sporophyll forms closed 
 cavity in which megasporangia are contained : mega- 
 spore produces a -single ovary represented simply by an 
 ovum and two synergidse : formation of prothallus re- 
 tarded until after fertilization 378 
 
 B. SUBJECTS OF GENERAL IMPORTANCE DISCUSSED 
 IN SPECIAL LESSONS. 
 
 I. CELLS AND NUCLEI. 
 
 a. The higher plants and animals contain cells similar in struc- 
 
 ture to entire unicellular organisms, and like them exist- 
 ing in either the amoeboid, ciliated, encysted, or plas- 
 modial condition 56 
 
 b. Minute structure of cells : cell-protoplasm, cell-membrane, 
 
 nuclear membrane, achromatin, chromatin 62 
 
 c. Direct and indirect nuclear division 65 
 
 d. The higher plants and animals begin life as a single cell, the 
 
 ovum 68 
 
392 SYNOPSIS 
 
 II. BIOGENESIS. 
 
 PAGE 
 
 a. Definition of biogenesis and abiogenesis : brief history of 
 
 the controversy 95 
 
 b. Crucial experiment with putrescible infusions : sterilization : 
 
 germ-filters : occurrence of abiogenesis disproved under 
 known existing conditions 98 
 
 III. HOMOGENESIS. 
 
 Definition of homogenesis and heterogenesis ; truth of the 
 
 former firmly established 102 
 
 IV. ORIGIN OF SPECIES. 
 
 a. Meaning of the term Species : the question illustrated by a 
 
 consideration of certain species of Zoothamnium .... 137 
 
 b. Definition of Creation and Evolution : hypothetical histories 
 
 of Zoothamnium in accordance with the two theories . . 141 
 
 c. The principles of Classification : natural and artificial 
 
 classifications 140 
 
 d. The connection between ontogeny and phylogeny 146 
 
 V. PLANTS AND ANIMALS. 
 
 a. Attempt to define the words plant and animal, and to place 
 
 the previously considered types in one or other king- 
 dom 176 
 
 b. Significance of the "third kingdom," Protista 182 
 
 VI. SPERMATOGENESIS AND OOGENESIS. 
 
 Origin of sperms and ova from primitive sex-cells ; differences 
 
 in structure and development of the sexual elements . . 255 
 
 VII. MATURATION AND IMPREGNATION. 
 
 a. Formation of first and second polar cells and of female 
 
 pronucleus 259 
 
 b. Entrance of sperm and formation of male pronucleus . . 263 
 
 c. Conjugation of pronuclei ....... 263 
 
SYNOPSIS 393 
 
 VIII. UNICELLULAR AND DIPLOBLASTIC ANIMALS. 
 
 PACK 
 
 a. In plants there is a clear transition from unicellular forms to 
 solid aggregates, but in animals the connection of the 
 gastrula with unicellular forms is uncertain 264 
 
 l>. Hypothesis of the origin of multicellular forms from a colony 
 
 of unicellular zooids 265 
 
 C. Other matters of general importance, such as the composition 
 and properties of protoplasm, cellulose, chlorophyll, starch, &c. : meta- 
 bolism : holozoic, holophytic, and saprophytic nutrition : intra- and 
 extra-cellular digestion : amoeboid, ciliary, and muscular movements : 
 the elementary physiology of muscle and nerve : parasitism and sym- 
 biosis : asexual and sexual generation : and the elements of embryology 
 are discussed under the various types, and will be most conveniently 
 referred to by consulting the Index. 
 
INDEX AND GLOSSARY 
 
 AbiOgenesiS (a, not: /3i'os, life: yeVeo-is, 
 origin), the origin of organisms from 
 not-living matter : former belief in, 96 
 Absorption by root-hairs, 341 
 Accre'tion (ad, to : cresco, to grow), in- 
 crease by addition of successive layers, 
 J 4 
 
 Achrom'atin (a, not : xpwjaa, colour), the 
 constituent of the nucleus which is un- 
 affected or but slightly affected by dyes, 
 7, 63 
 Accel om'ate (a, not : KotA.<o/u.a, a hollow), 
 
 having no coelome (q.v.) : 301 
 AddUCt'or muscles, Mussel, 322 
 Aerob'iC (cbjp, air : /3t'os, life), applied to 
 those microbes to which free oxygen is 
 unnecessary, 93 
 
 Aganiob'ium. (a, not : ya/oto?, marriage : 
 /3ios, life), the asexual generation in or- 
 ganisms exhibiting alternation of gene- 
 rations (q.v.) 
 
 AGAB'ICUS (mushroom) '.Figure, 192 : 
 general characters, 191 : microscopic 
 structure, 193 : spore-formation, 193 
 Algae (a/ga, sea-weed), 169 
 Alternation of Generations, meaning of 
 the phrase explained under Nitella, 220 : 
 Bougainvillea, 250 : Moss, 340 : Fern, 
 361 : Equisetum, 367, 368 : Salvinia, 370 : 
 Selaginella, 373 : Gymnosperms, 377 : 
 Angiosperms, 383, 384 
 Ambula'cral (ambulacrum^ a walking 
 
 place) system, starfish, 313 
 AMCEB'A (aju.oij36s, changing) : Figure, 
 2 : occurrence and general characters, i : 
 movements, 4, to : species of, 8 : resting 
 condition, 10, n : nutrition, n : growth, 
 13: respiration, 17 : metabolism, 17 , re- 
 production, 19 : immortality, 20 : conju- 
 gation, 20 : death, 20, 21 : conditions of 
 life, 21 : animal or plant? 180 
 Amoeb'oid movements, 4 
 Amceb'ula (diminutive of Amoeba), the 
 
 amoeboid germ of one of the lower or 
 ganisms, 51-54 
 
 Anab'olism (ai>a/3oArj, that which is 
 thrown up). See Metabolism, construc- 
 tive. 
 
 Anaerobic (a, not : a??p, air : /3t'os, life), 
 applied to those microbes to which free 
 oxygen is unnecessary, 93 
 
 An'al (anus, the vent) segment, Poly- 
 gordius, 273 
 
 An'al spot, Paramoecium, 113 
 
 An'astates (avao-Taros, from ai/aar^i/at, 
 to rise up, 18. See Mesostates, anabolic. 
 
 Anatomy (ivareVi/w, to cut up), the study 
 of the structure of organisms as made 
 out by dissection. 
 
 Androe'cium (ai/ijp, a male : OIKOS, a 
 dwelling), the collective name for the 
 male sporophylls in the flower of Angio- 
 sperms, 381 
 
 AN'GIOSPERMS (iy-yeZoi/, a vessel: 
 oWpjua, seed) : Figure, 379 : general 
 characters, 378-381 : structure of flower, 
 378 : reduction of gamobium, 381, 382 : 
 pollination and fertilization, 382, 383 : 
 formation of fruit and seed, and develop- 
 ment of the leafy plant, 383, 384 
 
 Animal, definition of, 176 
 
 Animals, classification of, 307 
 
 Animals and Plants, comparison of type 
 forms, 176, 177 : discussion of doubtful 
 forms, 1 80 
 
 Animals, Protists, and Plants, boun- 
 daries between artificial, 181-183 
 
 Anther, ^81 
 
 Antherid'ium. See Spermary. 
 
 Antherozo'id See Sperm. 
 
 Antip'odal cells, 382 
 
 An'US (anus, the vent), the posterior aper- 
 ture of the enteric canal, 273 
 
 Ap'ical cell : Penicillium, 190: Nitella, 
 21 1 : Moss, 335 : stem of Fern 350 : n ot 
 of Fern, 353 : prothallus of Fern. 357 
 
 Ap'ical cone, Fern, 350 
 
 A'pical growth, 190,351 
 
396 
 
 INDEX AND GLOSSARY 
 
 A'pical mer'istem, a mass of meristem 
 (g.v.) at the apex of a stem or root, 350 
 
 Appen'dages, lateral : crayfish, 314 : dog- 
 fish, 324 
 
 Archegonlum (apx?, beginning : -yoi/os, 
 production), the name usually given to 
 the ovary of the higher plants 
 
 Aristotle, abiogenesis taught by, 96 
 
 Arteries, in the crayfish, 318 
 
 Arthropoda, the, 308 
 
 Arthosppre (apOpov, a joint : a-nopa, a 
 seed), in Bacteria, 89 
 
 Artificial reproduction of Hydra, 234 
 
 Asexual generation. See Agamobium. 
 
 Asexual reproduction. See Fission, 
 Budding, Spore. 
 
 Asparagin, 338 
 
 Assimila,'tion(assimi70, to make like), the 
 conversion of food materials into living 
 protoplasm, 13 
 
 At rophy (a, without : rpo^, nourish- 
 ment), a wasting away, 118 
 
 Auricle. See Heart. 
 
 Autom'atism (avrdjuaTO?, acting of one's 
 own will), 10, 246 
 
 Axial bundle, Moss, 335 
 Axial fibre, Vorticella,,^ 
 Axil (axilla, the :irm-pit), 208 
 Axis, primary and secondary, 209 
 
 B 
 
 BACIL'LUS (Imcillum, a little staff), 85 _ 
 Figure, 87 
 
 BACTERIA (/3a/fTrjptoi/, a little staff) or 
 MICROBES (juic POS, small : /3 t 'os. life) : 
 occurrence. 82 : structure of chief genera, 
 84-87 : reproduction, 87-89 : nutrition, 
 90 : ferment-action, 91 : parasitism, 92 : 
 conditions of life, 92-94 : presence in at- 
 mosphere, 101-102: animals or plants? 
 182 
 
 BACTERIUM termo (Figures) 83, 84 
 
 Baer, von, Law of Development, 43 
 
 Barnacle-geese, supposed heterogenetic 
 production of, 103 
 
 Bast. See Phloem. 
 
 Binomial nomenclature, 8, 139 
 
 Biogen'esis (jSt'os, life : yeVeais, origin), 
 the origin of organisms from pre-existing 
 organisms, 96 : early experiments on, 96, 
 97 : crucial experiment on, 97-100 
 
 Biol'ogy (|3ios, life : Ao-yos, a discussion), 
 the science which treats of living things 
 
 Blast'OCCele (/SAacrro?, a bud ; /coiAo^, a 
 hollow), the larval body-cavity, 298 
 
 Blood, Polygordius, 283 
 
 Blood-corpuscles : colourless, see Leuco- 
 cytes : red, 56 : Figures, 57 
 
 Blood-vessels, Polygordius, 282: develop- 
 ment of, 302 
 
 Body-cavity. See Blastocccleand Ccclome. 
 
 Body-segments. See Metameres. 
 
 BOUGAINVILLEA (after L. A. de Bou- 
 gainville, the French navigator : 
 Figures, 238, 241 : occurrence and gene- 
 ral characters, 237 : microscopic struc 
 ture, 239 : structure of medusa, 240 
 structure and functions of nervous sys 
 tern, 245 : organs of sight, 246 : repro 
 duction and development, 247, 248 . 
 alternation of generations, 250 
 
 Bract (bractea, a thin plate), 251 
 
 Brain: Polygordius. 286: trochosphere, 
 299 : Crayfish, 319 : Dogfish, 330 
 
 Branch, Nitella, 209 
 
 Branchial (Ppdyxia., branchiae, gills) 
 
 apertures, Dogfish, 324, 329 
 
 Browne, Sir Thomas, on abiogenetic origin 
 of mice, 96 
 
 Buc'cal (bucca, the cheek) groove, Para- 
 mcecium, 109 
 
 Bud, budding, Saccharomyces, 73 : com- 
 parison of with fission. 73 : Hydra, 233 
 
 Bundle-sheath, 349 
 
 Calyp'tra (/caAvTrrpa. a veil), 339 
 Cal'yx (caAu, the cup of a "flower), the 
 
 outer or proximal whorl of the perianth 
 
 in the flower of Angiosperms, 378 
 Canals, radial and circular, medusa, 241 
 Canal-cells of ovary, 337, 358 
 Cap-cells of roots, 354 
 Carbon dioxide, decomposition of by 
 
 chlorophyll bodies, 29 
 Car'pel (Kap-n-os, fruit), a female sporophyll. 
 
 381 
 Cartilage, 324 
 
 Cauler'pa (/covAd? a stem : e'p7rw, to 
 
 creep), 174 (Figure) 
 Cell (cella, a closet or hut, from the first 
 
 conception of a cell having been derived 
 
 from the walled plant-cell) : meaning of 
 
 term, 60 : minute structure of (Figure), 
 
 62 : varieties of (Figure), 57 
 Cell-aggregate, meaning of term, 188 
 Cell-colony : temporary, Saccharomyces. 
 
 73: permanent, Zoothamnium, 135, 136 
 Cell-division, 64-67 
 Cell-fusion 305, 351 
 Cell-layer, 277 
 
 Cell-membrane or wall, u, 27, 28, 63 
 Cell-multiplication and differentiation, 
 
 218 : Polygordius, 305 : Fern, 351 
 Cell-plate, 67 
 Cell-protoplasm, 60 
 
 Cell'ulOSe, composition and properties of, 
 
 28 
 
 Central capsule, Radiolaria, 152 
 Central particle or Centrosome (KtWpor. 
 
 centre : (ru>ju.a, the body), 65, 256 (Fig- 
 ure). See also Directive sphere. 
 
 Ceph'alothor'ax, Crayfish, 314 
 Cerebral ganglion. See Brain. 
 Cerebro-pleural ganglion, Mussel, 323 
 
INDEX AND GLOSSARY 
 
 397 
 
 Cerebro-spinal cavity, Dogfish, 325 
 
 CHARA (\apa, delight), development and 
 alternation of generations, 219, 220 
 
 Chlor'ophyll (\Au>p6?, green : <}>v\\oi>, a 
 leaf), the green colouring matter of 
 plants, properties of, 26 : occurrence in 
 Bacteria, 87 : in Hydra, 231 
 
 Chrpm'atin (xpw/xa, a colour), the con- 
 stituent of the nucleus which is deeply 
 stained by dyes, 7, 63 : male and female 
 in nucleus of oosperm, 263 
 
 Chrom'atophore (xpw^a, colour : <e'pw, 
 to bear), a mass of proteid material im- 
 pregnated with chlorophyll or some other 
 colouring matter, 26, 46, 197, 215, 231 
 
 Chromosome (^pio/na, colour : orco/ua, 
 body), 65, 66, 257, 262, 263 
 
 Cil'ium (cilimn, an eye-lash), defined, 
 25 : comparisons of with pseudopod, 34, 
 52 : absence of cilia in Arthropoda, 319 
 
 Ciliary movement, 25 : a form of con- 
 tractility, 34 
 
 Cillate Infusoria, 107 
 
 Classification, natural and artificial, 141 : 
 natural, a genealogical tree, 142 
 
 Cnid Oblast (Kclfy a nettle : j3A<xcn-o's, a 
 bud), the cell in which a nematocyst 
 (q.v.) is developed, 230 
 
 Cnid'OCil (wiSy and c ilium), the "trigger- 
 hair" of a cnidoblast, 230 
 
 Coelenterata, the, 308 
 
 Coelome (/cot'Aw/xa, a hollow), the body- 
 cavity : Polygordius, 273 : Starfish, 311: 
 Crayfish, 319 : Mussel, 322 : Dogfish, 
 325 : development of, Polygordius, 302 
 
 CoBlom'ate, provided with a coelome, 276 
 
 Ccelomic epithelium. See Epithelium. 
 
 Coelomic fluid, Polygordius, 281 
 
 Colloids (KoAAa, glue : el6o?, form), pro- 
 perties of, 6 
 
 Colony, Colonial organism, meaning of 
 
 term, 135, 247 : formation of temporary 
 
 colonies, Hydra, 234 
 Columel'la (a little column), 162 
 Com'miSSUre (commissura, a band), 282 
 Compound organism. See Colony. 
 Concres'cence (cum, together : cresco, to 
 
 grow), the union of parts during growth 
 Cone, an axis bearing sporophylls : Equi- 
 
 setum, 366: Selaginella, 371 : Gymno- 
 
 sperms. 373 
 
 Conjuga'tion (conjug&tio, a coupling), the 
 
 union of two cells, in sexual reproduc- 
 tion : Amoeba, 20 : Heteromita, 41, 42 : 
 Paramoecium, 114116: Vorticella, 132 : 
 Alucor, 165 : Spirogyra, 198 : of ovum 
 and sperm, 263 : monoecious and dioe- 
 cious, 199 : comparison with plasmodiiun- 
 formation, 54 
 
 Connective, oasophageal, 286 
 
 Connective tissue, 312 
 
 Contractile vac'uole (vacuus, empty) : 
 Amoeba, 8, 9 : Euglena, 47 : Paramoe- 
 cium, in 
 
 Contractility (contracts, a drawing to 
 gether), nature of, 10, 34 : muscular, 130 
 
 Contraction, physical and biological, 10 
 
 Corolla (corolla, a little wreath), the 
 inner or distal whorl of the perianth in 
 the flower of Angiosperms, 380 
 
 Corpuscles. See Blood-corpuscles, and 
 Leucocytes 
 
 Cortex, cor'tical layer (cortex, bark), 
 constitution of, 59 : Infusoria, no 
 
 Cotton-WOOl as a germ-fitter, 99 
 
 Cotyle'don (KoruArjSwv, a cup or socket), 
 the first leaf or leaves of the phyllula 
 (q.v.) in vascular plants, 359 
 
 Cranium (/cpdi/iW, the skull), 328 
 
 CRAYFISH : Figure, 316: general charac- 
 ters, 314, 315 : limited number and con- 
 crescence of metameres, 314 : append- 
 dages, 314 : exoskeleton, 311 : enteric 
 canal, 315 : gills, 318 : blood-system, 
 318 : kidney, 318 : nervous system, 319 
 
 Creation (creo, to produce), definition of, 
 141 : illustrated in connection with species 
 of Zoothamnium (Diagram), 142 
 
 Cross-fertilization : applied to the sexual 
 process when the gametes spring from 
 different individuals, 199 
 
 Cryst'alloids (KpuoraAAos, crystal : etSos. 
 form), properties of, 6 
 
 Cuticle (cnticnla, the outer skin), nature 
 of in unicellular animals, 45, 109 : in 
 multicellular animals, 239 
 
 Cyst (KVCTTIS, a bag), used for cell-wall in 
 many cases, n, 54 
 
 Dallinger, Dr. W. H., observations on an. 
 apparent case of heterogenesis, 103 
 
 Daughter-cells, cells formed by the fission 
 or gemmation of a mother-cell, 35, 67 
 
 Death, phenomena attending, 20, 21, 166. 
 167 
 
 Decomposition, nature of, 6 
 
 Dennis (Se'p/xa, skin), the deep or connec- 
 tive tissue layer of the skin, 312 
 
 Descent, doctrine of. See Evolution. 
 
 Development, meaning of the term, 43. ' 
 For development of the various types 
 see under their names 
 
 Dextrin, 113 
 
 Diastase, 81 
 
 Diast'Ole (fitao-TeAAco, to separate), the 
 phase of dilatation of a heart, contractile 
 vacuole, &c., in 
 
 DIATOMA'CE.33 (fiiare'javto, to cut across, 
 because of the division of the shell into 
 two valves), 155 : Figure, 156 
 
 Diat'omin, the characteristic yellow colour- 
 ing matter of diatoms, 155 
 
 Dichot'omous (SixoTo/aew, to cut in two), 
 applied to branching in which the stem 
 divides into two axes of equal value, 138 
 
398 
 
 INDEX AND GLOSSARY 
 
 Differentia'tion (differo, to carry different 
 ways), explained and illustrated, 34, 119 
 
 Diges'tion (digero, to arrange or digest), 
 the process by which food is rendered fit 
 for absorption, 12, 13: intra- and extra- 
 cellular, 232 : contrasted with assimila- 
 tion, 233 
 
 Digestive gland, 317 
 
 Dimorpb/ism, dimorph'ic (Si's, twice ' 
 /u.op0rj, form), existing under two forms, 
 35, 136, 243, 368, 370 
 
 Dioe'cious (8i's, twice : ol/co?, a dwelling), 
 applied to organisms in which the male 
 and female organs occur in different in- 
 dividuals, 199 
 
 DIPH'YES (St<|>vij?, double) : Figure, 252 : 
 occurrence and general characters, 250 : 
 polymorphism, 251 
 
 Diploblast'iC (SiTrAoo?, double : /3Aao-Tos, a 
 bud), two-layered : applied to animals in 
 which the body consists of ectoderm and 
 endoderm, 244 : derivation of diploblas- 
 tic from unicellular animals (Figures), 
 266, 268, 269 
 
 Directive sphere, 65, 261, 263. See also 
 Centrosome. 
 
 DISC, Vorticella, 128 
 
 Dispersal, means of : in internal parasite, 
 124 : in h'xed organisms, 133-136 
 
 Distal, the end furthest from the point ot 
 attachment or organic base, 126 
 
 Distribution of food-materials : in a 
 complex animal, 281 : in a complex 
 plant, 341 
 
 Divergence of character, 145 
 
 Division of physiological labour, 34 
 
 DOGFISH '.Figure, 326 : general charac- 
 ters, 324 : fins, 324 : exoskeleton, 325 : 
 endoskeleton, 325 : enteric canal, 328 t 
 gills, 329 : blood-system, 329 : kidney, 
 330 : gonads, 330 : nervous system, 330 
 
 Dry-rigor, stiffening of protoplasm due to 
 abstraction of water, 21 
 
 Bchinodermata, the, 308 
 
 Ect'oderm (CKTOS, outside : Sep/xa, skin), 
 the outer cell-layer of diploblastic and 
 triploblastic animals, 225-230, 278 
 
 Ect'osarc (CKTOS, outside : <rap, flesh), the 
 outer layer of protoplasm in the lower 
 unicellular organisms, distinguished by 
 freedom from granules, 4 
 
 Egest'ion (egero, to expel), the expulsion 
 of waste matters, 12 
 
 Egg-cell. See Ovum. 
 
 Em'bryo (e/ajSpvov, an embryo or foetus), 
 the young of an organism before the 
 commencement of free existence. 
 
 Em'bryo-SaC. See Megaspore. 
 
 Encysta'tion, being enclosed in a cyst 
 
 
 
 End'oderm 
 
 , within : Sep/ita, skin 
 
 the inner cell-layer of diploblastic and 
 triploblastic animals, 225, 231, 278 
 
 End'oderm-lamella, Medusa, 241 
 
 Endogenous (ZvSov, within : yiyi'oju.ai, to 
 come into being), arising from within, 
 e.g. the roots of vascular plants, 354 
 
 End OSarc (ei/Soi/, within : cr<xp, flesh), 
 the inner, granular protoplasm of the 
 lower unicellular organisms, 4 
 
 Endoskel'eton (evSov, within, and skeleton, 
 from ovce'AAw, to dry), the internal skele- 
 ton of animals, 325 
 
 End'OSperm (evSov, within : o-7rep|u,a, seed), 
 nutrient tissue formed in the megaspore 
 of Phanerogams, 377, 383 
 
 Endospore (evSov, within : (TTropa, a seed), 
 a spore formed within a vegetative cell, 
 89 
 
 Energy, conversion of potential into 
 kinetic, 15 : source of, in chlorophyll- 
 containing organisms, 31 
 
 Enteric (ej/repov, intestine), Canal, the 
 entire food-tube from mouth to anus : 
 Polygordius, 273, 279 : Starfish, 312 : 
 Crayfish, 315 : Mussel, 322 : Dogfish, 328 
 
 Ent'eron or Enteric cavity, the simple 
 digestive chamber of diploblastic ani- 
 mals, 225 
 
 Epidermis (CTU upon : Sep^a, the skin) : 
 in animals synonymous with deric epi- 
 thelium (q.v., under Epithelium) : in vas- 
 cular plants a single external layer of 
 cells, 348, 352, 353 
 
 Epithelial cells : columnar, 58 : ciliated, 
 
 Epithelium (en-i, upon : flrjA^, the nipple), 
 a cellular membrane bounding a free 
 surface, 246 : coelomic, 277, 304 : deric, 
 272 : enteric, 276, 278 
 
 Equatorial plate, 67 
 
 EQUISETUM (eqnus, a horse : seta, a 
 bristle : Figure, 367 : general charac- 
 ters, 366 : cone and sporophylls, 366 : 
 male and female prothalli, 366, 367 : al- 
 ternation of generations, 367, 368 
 
 Equiv'ocal generation. See Abiogenesis. 
 
 EUGLEN'A ( euyAiji/o?, bright-eyed) : 
 Figure, 45 : occurrence and general 
 characters, 44 : movements, 44 : struc- 
 ture, 45 : nutrition, 46 : resting stage, 
 47 : reproduction, 48 : animal or plant ? 
 178 
 
 Euglen'oid movements, 45 
 
 Ev'olution (evolvo, to roll out), organic : 
 definition, 143 : illustration of in connec- 
 tion with species of Zoothamnium (Dia- 
 gram), 144 
 
 Excre'tion (excerno, to separate), the 
 separation of waste matters derived from 
 the destructive metabolism of the or- 
 ganism, 16, 284 
 
 Exogenous (e, put of: yCyvo/j-ai., to come 
 into being), arising from the exterior, 
 t.g. leaves, 354 
 
INDEX AND GLOSSARY 
 
 399 
 
 Exoskel'eton (e'w, outside, and skeleton, 
 Irom ovce'AAw to dry), the external or 
 skin-skeleton : CUtiCUlar, 239, 277-279 : 
 cuticular and calcified, 315, 320: epi- 
 dermal (hair and nails): dermal, 312, 325 
 
 Eye-spots or Ocelli : Medusa, 241 : 
 , 290, 299 
 
 Faeces (faex, dregs), solid excrement, 
 consisting of the undigested portions of 
 the food, 16 
 
 Ferm'ent (fermentum, yeast, from, for- 
 veo, to boil or ferment), a substance 
 which induces fermenta'tion, i.e. a 
 definite chemical change, in certain sub- 
 stances with which it is brought into 
 contact, without itself undergoing 
 change : unorganized and organized 
 ferments 80 : alcoholic, 76-81 : ace- 
 tous, 91 : diastatic or amylolytic, 81 : 
 lactic, 91 : peptonizing or proteolytic, 
 8 1 : putrefactive, 91 : ferment-cells of 
 Mucor, 168 
 
 FERNS : Figures, 346, 356 : general 
 characters 344, 345 : histology of stem, 
 leaf, and root, 344-354 : nutrition, 354 : 
 spore-formation, 355 : prothallus and 
 gonads, 357-358 : development, 359 : al- 
 ternation of generations, 361 
 
 Fertiliza'tion (fertilis, bearing fruit) ; 
 the process of conjugation of a sperm or 
 sperm-nucleus with an ovum, whereby 
 the latter is rendered capable of develop- 
 ment : a special case of conjugation 
 (g.v.), 199 : details of process, 263 : in 
 Vaucheria, 173 : in Gynmosperms, 376, 
 in Angiosperms, 383 
 
 Filtering air, method of, 99 
 
 Fins, Dogfish 324 
 
 Fission (Jissio, a cleaving), Simple or 
 binary, the division of a mother- cell 
 into two daughter-cells : in Amoeba, 19 ; 
 Heteromita, 40 : animal- and plant-cells 
 generally, 65-67 : Paramcecium, 114 : 
 Vorticella 131 
 
 Fission, multiple, the division of a 
 mother-cell into numerous daughter- 
 cells : in Heteromita, 42 : Protomyxa, 
 51 : Saccharomyces, 74 
 
 Fission, process intermediate between 
 simple and multiple, Opalina, 124 
 
 Flagella. See Cilium. 
 
 Flag'ellate Infusoria, 107 
 
 Flagell'ula (diminutive oiflagelluni), the 
 flagellate germ of one of the lower 
 organisms (often called zoospores, 51, 54 
 
 FlageU'um (fiagellum, a whip) : defined! 
 25 : transition to pseudopod, 52, 231 
 
 Floral receptacle, the abbreviated axis of 
 an angiospermous flower 378 
 
 Flower, a specially modified cone (<;.?'.), 
 
 having a shortened axis, which bears 
 perianth-leaves as well as sporophylls, 
 378 : often applied to the cone of Gymno- 
 sperms, 373 
 
 Food-current, Mussel 320, 322 
 
 Food-vacuole, a temporary space in the 
 protoplasm of a cell containing water 
 and food-particles, n, 112 
 
 Foot : of Mussel, 320 : of phyllula of fern 
 
 FORAMINIF'ERA (foramen, a hole \fero 
 to bear), 148 : Figures, 149, 150, 151 
 
 Fragmenta'tion of the nucleus, 120 
 
 Free cell formation, 377 
 
 Fruit of Angiosperms, 384 
 
 Func'tion (functio, a performing), mean- 
 ing of the term, 9 
 
 Gam'ete (yaixew, to marry), a conjugating 
 cell, whether of indeterniinate or deter- 
 minate sex : Heteromita, 41 : Mucor, 
 165 : Spirogyra, 198 : Vaucheria 173 
 
 Gamoblum (yd/no?, marriage : 0t'ps, life), 
 the sexual generation in organisms ex- 
 hibiting alternation of generations (.q.v.) : 
 progressive subordination of, to agamp- 
 bium in vascular plants, 361, 384 
 
 Ganglion (yayyAiov, a tumour), a swelliffi| 
 on a nerve-cord in which nerve-cells ^j| 
 accumulated, 319, 323 
 
 Gastric juice (yao-njp, the stomach), pr 
 perties of, 12 
 
 Gast'rula (diminutive of yaonjp, the 
 stomach), the diploblastic stage of the 
 animal embryo in which there is a diges- 
 tive cavity with an external opening : 
 characters and Figure of, 265 : contrasted 
 with phyllula,, 360 
 
 Gemma 'tion (gemma, a bud). See Bud- 
 ding. 
 
 Genera'tion, asexual, See Agamobium. 
 Sexual. See Gamobium. 
 
 Generations, Alternation of. See Al- 
 ternation of generations. 
 
 Generalized, meaning of term, 140 
 
 Ge'nus (genus, a race), generic name, 
 generic characters, 8, 139 
 
 Germ-filter, 99 
 
 Ger'minal spot, the nucleolus of the 
 ovum, 259 
 
 Germina'tion (germinatio, a budding), 
 the sprouting of a spore, zygote, or 
 oosperm to form the adult plant : for 
 germination of the various types see 
 under their names. 
 
 Gill, an aquatic respiratory organ. 318, 323, 
 324, 329 
 
 Gland .(glows, an acorn), an organ of se- 
 cretion (q.v.) '. gland-cells, 231, 232, 285, 
 
 Glochid'ium, 323 
 
 Gon'ad (yot/os, offspring, seed), the essen- 
 
400 
 
 INDEX AND GLOSSARY 
 
 tial organ of sexual reproduction, 
 whether of indeterminate or determinate 
 sex, i.e. an organ producing either un- 
 differentiated gametes, ova, or sperms ; 
 see under the various types, and espe- 
 cially 172, 198, 209, 214 
 
 Gon'aduct (gonad. and ittlco, to lead), a 
 tube carrying the ova or sperms from the 
 gonad to the exterior, 295, 313, 319, 323, 
 3 o 
 
 Grapping-lines, Diphyes, 251 
 
 Growing point : Nitella, 211, Moss, 535 : 
 Fern, 350 
 
 Growth, 13 
 
 Guard-cells of stomates, 353 
 
 Gullet, the simple food-tube of Infusoria, 
 47, no : or part of the enteric canal of 
 the higher animals, 280 
 
 GYM'NOSPERMS (yu/ou/os, naked : oWpjoia 
 seed) : Figure, 37^: general characters, 
 373 : structure oflBkies and sporophylls, 
 
 373, 375, : reductron of gamobium (pro- 
 thalli and gonads), 376, : pollination and 
 fertilization, 376, 377 : formation of seed 
 and development of leafy plant, 377 
 Gyncec'ium (yvv^, a female : ot/cos, a 
 dwelling), the collective name for the 
 female sporophylls in the flower of 
 ngiosperms, 381 
 
 H 
 
 m'atochrome (al^a, blood : xp^M a > 
 lour), a red colouring matter allied to 
 chlorophyll, 26 
 
 H^EMATOCOp'CUS (aI M a, blood : KOKKOS 
 a berry): Figure, 24: general characters, 
 23 : rate of progression, 23 : ciliary move- 
 ments, 25, 33 : colouring matters, 26 : 
 motile and stationary phrases. 28 : nutri- 
 tion, 28 : source of energy, 30 ; reproduc- 
 tion, 35 : dimorphism, 35 : animal or 
 plant ? 180 
 
 Haemoglobin (al/ua, blood : globus, a 
 round body, from the circular red cor- 
 puscles of human blood), 58 : properties 
 and functions of, 283 
 
 Head-kidney : trochosphere, 299 
 
 Heart : Crayfish, 318 : Mussel, 323 : 
 Dogfish, 329 
 
 Heat, evolution of, by oxidation of proto- 
 plasm, 17 
 
 Heat-rigor (rigor, stiffness), heat-stiffen- 
 ing, 21 
 
 Heliotropism, 168 
 
 Heredity (hereditas, heirship), 147 
 
 Hermaph'rodite (epju.a<J>p6StTo?, from 
 Hermes and Aphrodite). See Monoecious. 
 
 Heterpgen'esiS (erepos, different : -yeVecri?, 
 origin), meaning of term, 102 : supposed 
 cases of, 103 : not to be confounded with 
 metamorphosis or with evolution, 104 
 
 HETEROM'ITA (eVepos, different : /uuVos, 
 
 a thread) : Figure 38 : occurrence and 
 general characters, 36 ; movements, 37 : 
 nutrition, 37 : asexual reproduction, 40 : 
 conjugation, 41 : development and life- 
 history, 42, 43 : animal or plant ? 181 
 
 High and low organisms, 106 
 
 Higher (triploblastic) animals, uniformity 
 in general structure of, 307 
 
 Higher (vascular) plants, uniformity in 
 general structure of, 363 
 
 Histol'Ogy (tcrrtW, a thing woven : Aoyo?, 
 a discussion), minute or microscopic 
 anatomy. 
 
 Holophyt'ic (6Aos, whole : ^UTOI', a plant), 
 nutrition, defined, 31 
 
 Holozo'ic (oAo?, whole : <Jwoi', an animal), 
 nutrition, defined, 31 
 
 Homegen'esis (6/ot6s, the same : yeVco-t? 
 origin), meaning of the term, 102 
 
 Homol'OgOUS (ojaoAoyos, agreeing), applied 
 to parts which have had a common 
 origin, 243 
 
 Homomorph'ism homomorph'ic (6/u6? 
 
 the same : ju.op</j, form), existing under 
 
 a single form, 139 
 Host, term applied to the organism upon 
 
 which a parasite preys, 123 
 HYDRA (vSpa, a water-serpent) : Figures, 
 
 222, 226, 228, 235 : occurrence and general 
 characters, 221 : species, 223 : move- 
 ments, 223, 224 : mode of feeding, 224 : 
 microscopic structure, 225 : digestion, 
 232 : asexual, artificial, and sexual re- 
 production 233, 235 : development, 236 
 
 Hydr'anth (vSpa, a water-serpent : avdos 
 a flower), the nutritive zooid of a hydroid 
 polype, 239, 243 
 
 Hydroid (vSpa, a water-serpent : elfio? 
 form) Polypes (rroAvTrov?, many-footed), 
 compound organisms, the zooids of which 
 have a general resemblance to Hydra, 
 237 
 
 Hyper 'trophy (iiTrep, over : rpo<j>rf, nourish- 
 ment), an increase in size beyond the 
 usual limits, 118 
 
 Hyph'a (u(J>cuVco, to weave) applied to the 
 separate filaments of a fungus : they 
 may be mycelial (see mycelium), sub- 
 merged, or aerial : Mucor, 160, 168, 
 Penicillium, 185, 188 
 
 Hyp'odermis ( VTTO, under : 8e'pju.a, skin), 
 Fern, 345, 348 
 
 Hyp'OStome (UTTO, under : erro/xa, mouth), 
 
 223, 239 
 
 Immortality, virtual, of lower organisms, 
 
 21 
 Income and expenditure of protoplasm, 
 
 18 
 
 Individual. See Zooid. 
 Individuation, meaning of the term. 233. 
 
 254 
 
INDEX AND GLOSSARY 
 
 401 
 
 Indus ium (indusium, an under-garment), 
 
 Inflores'cence (floresco, to begin to 
 
 flower), an aggregation of cones or 
 
 flowers, 373 
 Infusoria (so called because of their fre- 
 
 quent occurrence in infusions), 107 
 Ingesta (ingcro, to put into) and Egesta 
 
 (e^cro, to expel), balance of, 32 
 Ingestion (ingcro, to put into), the taking 
 
 in of solid food, n, 58 
 Insola'tiOU (insolo, to place in the sun), 
 
 exposure to direct sunlight, 94 
 Integ'ument (integii mention, a covering) 
 
 of megaspore : Gymnosperms, 375 : 
 
 Angiosperms, 382 
 
 Inter-cellular spaces, 347 
 Inter-muscular plexus (TrAe/cw, to twine), 
 
 Leaf, structure of: Nitella, 208, 209 
 213 : Moss, 335 : Fern, 344, 352 : limited 
 growth of, 214 
 
 Leaflet, Nitella, 209 
 
 Lept'othrix (ACTTTOS, slender ; 0pif , a hair), 
 filamentous condition of Bacillus, 89 : 
 
 288 
 Internode (inter, between : nodus, a 
 
 knot), the portion' of stem intervening 
 
 between two nodes, 208 
 Intersti'tial (interstltiiiin, a space be- 
 
 tween) cells, Hydra, 227 : growth, 
 
 Spirogyra, 198 
 Intestine (intestlnus, internal), part of 
 
 the enteric canal of the higher animals, 
 
 280 
 Intus-suscep'tion (intus, into : snscipio, 
 
 to take up), addition of new matter to 
 
 the interior, 13 
 Iodine, test for starch, 27 
 Irritability (irritabllis, irritable), the 
 
 property of responding to an external 
 
 stimulus, 10 
 
 Jaws : Crayfish, 315 : Dogfish, 324, 328 
 
 K 
 
 Karyokines'is (/capuof, a kernel or nu- 
 cleus : KiVijo-is, a movement), indirect 
 nuclear division, 67 
 
 Katab'olism (Kara/SoAr), a laying down), 
 18. See Metabolism, destructive. 
 
 Kat'astates (*<xTacrrTJi/ai, to sink down), 
 1 8. See Mesostates, katabolic. 
 
 Kidney : Crayfish, 318 : Dogfish, 330 
 
 LAMINARIA (lamina, a plate), 203 
 C Figure), 204 
 
 Labial palps, Mussel, 322 
 
 Larva, the free living young of an animal 
 
 in which development is accompanied by 
 
 a metamorphosis, 299 
 Larval stages, significance of, Polygor- 
 
 dius, 301 
 
 (after Lesson, the French 
 naturalist), 204 (Figure) 
 
 Leuc'ocyte (Aeu/cos, white : KV'TOS, a hollow 
 vessel, cell), a colourlessblood corpuscle : 
 structure of, in various animals 
 (Figures), 57 : ingestion of solid par- 
 ticles by, 58 : fission of, 58 : formation 
 of plasmodia by, 58 
 
 Leuwennoek, Anthony van, discoverer 
 of Bacteria, 97 
 
 Life, origin of. See Biogenesis. 
 
 Life-history, meanin^of the term, 43 
 
 Lignin (lignum, wo^), composition and 
 properties of, 348 
 
 Linear aggregate, an aggregate of cells 
 arranged in a single longitudinal series, 
 
 LinnaBUS, C., introducer of binomial no- 
 menclature, 8, 139 
 Liver, Dogfish, 328 
 
 M 
 
 MACROCYSTIS (ju,a*p6s, long : KV 
 bladder), 204 
 
 Mad'reporite (from its similarity to a 
 madrepore or stone-coral), 311 
 
 Mantle, Mussel, 320 
 
 Manub'rium (mauubrium, a handle) of 
 Medusa, 241 
 
 Matura'tion of ovum, 259 
 
 Maximum temperature of amoeboid 
 movements. 21 
 
 Medulla or medullary substance (me- 
 dulla, marrow): in Infusoria, no 
 
 MedUS'a (Me'Sovo-a, name of one of the 
 Gorgons), the free-swimming reproduc- 
 tive zooid of a hydroid polype, 239-243 : 
 derivation of a, from hydranth (Figure), 
 241 
 
 Medus'oid, a reproductive zooid having 
 the form of an imperfect Medusa, 
 Dijjhyes, 251 
 
 Meg agam'ete (fxeya?, large : ya/xew, to 
 marry), a female gamete (q.v.) distin- 
 guished by its greater size from the male 
 or microgamete, 132 
 
 Meg'anucleus (/xe'yas, large: nucleus, a 
 kernel), in, 128 
 
 Meg'asporan'gium (/u.eya? large : '<nropd, 
 seed : (iyyeioi/, a vessel), the female 
 sporangium in plants with sexually di- 
 morphic sporangia, usually distinguished 
 by its greater size from the male ur 
 micro-sporangium : Salvinia, 368 : Sela- 
 D D 
 
402 
 
 INDEX AND GLOSSARY 
 
 ginella, 371 : Gymnosperms, 375 : Angio- 
 sperms, 381. 
 
 Meg aspore (jj.eya.s, large : <riropd, a seed), 
 the female spore in plants with sexually 
 dimorphic spores, al-ways distinguished 
 by its large size from the male or micro- 
 spore : Salvinia, 368: Selaginella, 371 : 
 Gymnosperms, 375 : Angiosperms, 382 
 
 Megazo'oid (/At'ya? large : ro>oi/, animal : 
 eldos, form), the larger zooid in unicel- 
 lular organisms with dimorphic zooids, 
 35> 132 
 
 Mer istem. (/u.epumj|u.a, formed from 
 /aepujou, to divide), indifferent tissue of 
 plants from which permanent tissues are 
 differentiated, 350 
 
 Mes'entery (ju.e'<ros, middle : evrepov, in- 
 testine), a membrane connecting the en- 
 teric canal with the body-wall, 279 : de- 
 velopment of, 302 
 
 Mes'oderm (/meo-os, middle : Sepfia, skin), 
 the middle cell-layer of triploblastic 
 animals : Polygordius, 278 . develop- 
 ment of, 299: splitting of to form somatic 
 and splanchnic layers, 302 
 
 Mesoglce'a (ju.eVos, middle : y\oia, glue), 
 a^transparent layer between the ecto- and 
 endo-derm of Coelenterates : in Hydra, 
 225 : in Bougainvillea, 244 
 
 Mes'ophyll (ju,e'pos, middle : <j>v\\ov, a 
 leai), the parenchyma of leaves, 352 
 
 Mes'ostates (ju,e'cro?, middle : trrrfvai, to 
 stand), intermediate products formed 
 during metabolism (q.v.) and divisible 
 into (a) anabolic mesostates or ana- 
 states, products formed during the con- 
 version of food-materials into proto- 
 plasm ; and (b) katabolic mesostates 
 or katastates, products formed during 
 the breaking down of protoplasm, 18 
 
 Metab'olism Oxera/SoArj, a change), the 
 entire series of processes connected with 
 the manufacture of protoplasm, and 
 divisible into (a.) constructive meta- 
 bolism or anabolism, the processes by 
 which the substances taken as food are 
 converted into protoplasm, and (b) de- 
 structive metabolism or katabolism, 
 the processes by which the protoplasm 
 breaks down into simpler products, ex- 
 cretory or plastic, 17 
 
 Met'amere (/u.eYa, after : /mepos, a part), a 
 body-segment in a transversely seg- 
 mented animal such as Polygordius, 271, 
 273 : development of, 301 : limited num- 
 ber and concrescence of in Crayfish, '314 
 
 Metamorphosis (/mtTa^op^oxris), a trans- 
 formation applied to the striking change 
 of form undergone by certain organisms 
 in the course of development after the 
 commencement of free existeuce : Vor- 
 ticella, 133 : Polygordius, 306 
 
 Mic robe (/ui/cpos, small : /Si'os, life). See 
 
 MICROCOC'CUS (jai/cpos, small : KOKKOS, 
 a berry) (Figure), 86 
 
 Mlcrogam'ete (/uuxpos, small : -ya^e'cu, to 
 marry), a male gamete (g.v.), distin- 
 guished by its smaller size from the 
 female or megagamete, 132 
 
 Micro-millimetre, the one-thousandth of 
 a millimetre, or i-25,oooth of an inch, 84 
 
 Micro-organism. See Bacteria. 
 
 Micronucleus ( juu*p6s, small : micleits, a 
 kernel), in, 128 
 
 Micropyle (jul/cpoj, small : irvArj, an en- 
 trance), 375 
 
 Micro-sporan'giumGu/cpos, small : <rnopd, 
 a seed : dyyeiov, a vessel), the male 
 sporangium in plants with sexually di- 
 morphic sporangia, usually distinguished 
 by its smaller size from the female or 
 mega-sporangium : Salvinia, 368 : Se- 
 laginella, 371 : Gymnosperms, 376 : An- 
 giosperms, 381 
 
 Mic'rospore (juuKpo?, small t OTropa, a 
 seed), the male spore in plants with 
 sexually dimorphic spores, always dis- 
 tinguished by its small size from the 
 female or mega-spore : Salvinia, 368 : 
 Selaginella, 371 : Gymnosperms, 376 : 
 Angiosperms. 381 
 
 Microzo'oid (jul^po?, small : ^Voov, an ani- 
 mal : clSo?, form), the smaller zooid in 
 unicellalar organisms with dimorphic 
 zooids, 35, 132 
 
 Midrib of leaf, Moss, 335 
 
 Minimum temperature for amoeboid move- 
 ments, 21 
 
 Mollusca, the, 309 
 
 Monoecious (/uoyo?, single : OIKOS, a 
 house), applied to organisms in which 
 the male and female organs occur in the 
 same individual, 199, 234 
 
 Monopod'ial (/xo^os, single : TTOVS, a foot), 
 applied to branching in which the main 
 axis continues to grow in a straight line 
 and sends off secondary axes to the 
 sides. 138 
 
 MONOSTROMA (/xdvos, single : o-rpw/xa, 
 anything spread out), 202 (Figure) 
 
 Morphol'Ogy (juop^, form : Adyos a dis- 
 cussion), the department of biology 
 which treats of form and structure, 9 
 
 Mor'ula (diminutive of vionnn, a mul- 
 berry) See Polyplast. 
 
 MOSSES : Figures, 333, 338 : general 
 characters, 332 : structure of stem, 334 : 
 leaf, 335 : rhizoids, 335 : terminal bud, 
 335 1 reproduction, 336 : development 
 of sporogonium, 337 : of leafy plant, 
 339, 340 : alternation of generations, 
 340 : nutrition, 340, 341 
 
 Mouth : Kuglena, 47 : Paramoccium, no: 
 Hydra, 223 : Medusa, 241 : Polygordius, 
 271: backward shifting of in Crayfish, 
 
 MUCOR (mncor, mould) '.Figure, 159: 
 
INDEX AND GLOSSARY 
 
 403 
 
 .- 5 
 Multicellular, formed of many cells, 61, 
 
 occurrence and general characters, 158 : 
 mycelium and aerial hyphse, 160-163 : 
 sporangia and spores, 160-162-165 ; transi- 
 tion from uni- to multi-cellular condi- 
 tion, 162 : development of spores, 163 ; 
 conjugation, 165 : death, 166 : nutrition, 
 167 ; parasitism, 167 : ferment-cells, 168 
 Mucous membrane 
 [ulti 
 162 
 Muscle (ww-r^/w-s-ja little mouse,a muscle), 
 
 nature of. 130, 131 
 Muscle-fibres, Bougainvillea, 244 
 Muscle-plate, Polygordius, development 
 
 of, 305 
 
 Muscle-process, Hydra, 227, 232 
 Mushroom. See Agaricus. 
 MUSSEL (same root as muscle), Fresh- 
 water : Figure, 321 : general characters, 
 320 : mantle, shell, and foot, 320 : food- 
 current, 320 : enteric canal, 322 : gills 
 and blood-system, 323 : nephridia, gon- 
 ads, and nervous system, 323 
 Mycelial hyphse, the hyphae interwoven 
 
 to form a mycelium. 
 
 Mycel'ium (/IXVKTJS, a fungus), a more or 
 less felt-like mass formed of interwoven 
 hyphae : Mucor, 160 : Penicillium, 185 
 YCET'OZOA (MV'KTJS, a fungus : faov, 
 
 MY MVKTJS, a ungus 
 
 an animal) : Figure, 53 : occurrence 
 and general characters, 52 : nutrition, 
 
 54 : reproduction and life-history, 54, 
 
 55 : animals or plants? 181 
 My'ophan (/u.us, mouse, muscle : <f>aiVco. to 
 
 appear), no 
 
 Myxomyce'tes (nvt-a, mucus : nvxys, a 
 fungus). See Mycetozoa. 
 
 N 
 
 Norn atocyst (I^/IAO., a thread : KVOTIS, a 
 
 bag), 229 
 
 Nephrid'iopore (i/e$po?, a kidney : Tropos, 
 a passage), the external opening of a 
 nephridium, 285 
 
 Nephrld'ium (i/e^pos, a kidney), structure 
 of, Polygordius, 285 (Figure) : develop- 
 ment of, 304 : Mussel, 323 : Dogfish, 
 33 
 
 Neph'rostome (ve<f>po?, a kidney : 0-TOju.a, 
 a mouth), the internal or ccelomic aper- 
 ture of a nephridium, 285 
 Nerve, afferent and efferent, functions of, 
 
 288 
 
 Nerve-cell, 230, 245 
 
 Nervous system, central and peripheral : 
 Medusa, 245 : Polygordius, 286 : func- 
 tions of, 288 : Starfish, 313 : Crayfish, 
 319 : Mussel, 323 : Dogfish, 330 
 Neur'OCOele (vevpov, a nerve: KOI'ATJ, a 
 hollow), the central cavity of the verte- 
 brate nervous system, 330 
 NITELL'A (niteo, to shine) : Figures, 
 
 207, 212, 215, 217, 219 : occurrence and 
 general characters, 206 : microscopic 
 structure, 209 : terminal bud, 211 : struc- 
 ture and development of gonads, 209, 
 214: development, 219: alternation of 
 generations, 220 
 Node (nodus, a knot), the portion of a 
 
 stem which gives rise to leaves, 208 
 Not'OChord (VWTOS, the back: xP^> a 
 
 string), 328 
 Nucel'lus (diminutive of nucleus, the 
 
 name formerly applied), 375, 382 
 Nuclear division, indirect : 64 (Figure) : 
 
 65, 67 : direct, 67 
 Nuclear membrane, 62, 
 Nuclear protoplasm. See Achromatin. 
 Nuclear spindle, 65, 66 
 Nucle'olUS (diminutive of nucleus), 8 
 Nu'cleus (nucleus, a kernel), minute struc- 
 ture of, 63 ; Amoeba, 7, 8 : Paramcecium, 
 in, 114: Opalina, 121: Vorticella, 
 128 : Nitella, 210, 213 : fragmentation 
 of, 1 20 
 Nucleus, secondary, of megaspore, An- 
 
 giosperms, 382 
 Nutrient solution, artificial, principles of 
 
 construction of, 78, 
 
 Nutrition : Amoeba (holozoic), n : Hse- 
 matococcus (holophytic), 28 : Hetero- 
 mita (saprophytic), 37 : Opalina (type of 
 internal parasite), 123 : Mucor 167 : 
 Penicillium, 190 : Polygordius (type of 
 higher animals), 273, 281 : Moss (type 
 of higher plants), 340 
 
 Ocellus (ocellus, a little eye), structure 
 and functions of, Medusa, 241, 246 
 
 CEsoph'agUS (oicro4>avos, the gullet). See 
 Gullet. 
 
 Ontog'eny (O^TOS, being : yeyeerts, origin), 
 the development of the individual : 
 a recapitulation of phylogeny (y.v.), 
 146 
 
 Oogen'esis (u>6v, an egg : yei/e'<ris, origin), 
 the development of an ovum from a 
 primitive sex-cell, 255, 258 
 
 Oogon ium (J>oV, egg : yo^o?. produc- 
 tion), the name usually given to the 
 ovary of many of the lower plants. 
 
 Oosperm (u>oV, egg : <77repju.a, seed), a 
 zygote (q.v). formed by the ovum and 
 sperm: a unicellular embryo, 173: 
 origin of nucleus of, 263 
 
 Oosphere (wov, an egg : cr<cupa, a sphere), 
 a name frequently given to the ovum of 
 plants. 
 
 Oospore (taov, an egg : criropa, a seed), a 
 name frequently applied to the oosperm 
 of plants. 
 
 OPALIN'A (from its opalescent appear- 
 ance) : Figure, 122 : occurrence and 
 general characters, 121-123 structure 
 
 fl 
 
 i 
 
404 
 
 INDEX AND GLOSSARY 
 
 and division of nuclei, 121 : parasitic 
 nutrition, 123 : reproduction, 124 : means 
 of dispersal, 124 : development, 125 
 
 Opt'inium (pptimus, best) temperature for 
 amoeboid movements, 21 : for sapro- 
 phytic monads, 40 
 
 Organ (opyavov, an instrument), a portion 
 of the body set apart for the perform- 
 ance of a particular function, 291 
 
 Or'ganism, any living thing, whether 
 animal or plant, 5 
 
 Ossicle (diminutive of <>s, a bone), 311 
 
 Ov'ary (ovum, an egg), the female gonad 
 or ovum-producing organ ; see under the 
 various types and especially Vaucheria, 
 172 : atrophy of, in Angiosperms, 382. 
 The name is also incorrectly applied to 
 the venter of the pistil of Angiosperms, 
 381 
 
 Ovi'duct (ovum, an egg ' ditco, to lead), 
 a tube conveying the ova from the ovary 
 to the exterior, 295 
 
 Ovum (ovum, an egg), the female or 
 megagamete in its highest stage of dif- 
 ferentiation : general structure of, 68, 
 69 : minute structure and maturation of, 
 258, 259 : see also under the various 
 types and especially Vaucheria, 172 : 
 formation of, in Angiosperms, 382 
 
 Ov'ule (diminutive of cnntni), the name 
 usually applied to the megasporangium 
 of Phanerogams. 
 
 Oxidation of protoplasm, 15 
 
 OXYTRICH'A (6vs, sharp : 0p<:, a hair), 
 1 20 (Figure) 
 
 Pancreas (rra-yicpeVr, sweetbread), 328 
 
 Pandorina, 266 (Figure), 267 
 
 Param'ylum (napd, beside : aju.vA.oi/, fine 
 meal, starch), 46 
 
 PARAMCE'CIUM : Figures, 108, 115: 
 structure, 107: mode of feeding, 112: 
 asexual reproduction, 114 : conjugation, 
 114 
 
 Par'asite, parasitism (Trapao-tro?, one 
 who lives at another's table) : --Opalina, 
 123 : Bacteria, 92 : Mucor, 167 
 
 Paren'chyma ( napeyxv^a, anything 
 poured in beside, a word originally used 
 to describe the substance of the lungs, 
 liver, and other soft internal organs), 
 applied to the cells of plants the length 
 of which does not greatly exceed their 
 breadth and which have soft non-lignified 
 walls, 6c : ground-parenchyma, 345, 347 
 
 Pari'etal (paries, a wall), applied to the 
 layer of ccelomic epithelium lining the 
 body-wall, 277, 278 
 
 Parthenogenesis (Trapflt'cos, a virgin : 
 
 yeVeeris, origin), development from an 
 unfertilized ovum or other female 
 gamete, 200 
 
 Parthenogenet'ic ova, characteristics ot, 
 
 262 
 Pasteur, Louis, researches on yeast, 78- 
 
 80 
 
 Pasteur's solution, composition of, 76 
 Pedal (pes, the foot) ganglion, Mussel, 
 
 PENICILL'IUM (pcnicillnin, a painter's 
 brush, from the form of the fully-deve- 
 loped aerial hyphae) : Figure, 186 : oc- 
 currence and general characters, 184: 
 mode of growth, 185 : microscopic 
 structure, 185 : formation and germina- 
 tion of spores, 189 : sexual reproduction, 
 190: nutrition, 190: vitality of spores, 
 191 
 
 Pepsin (TreVrco, to digest), the proteolytic 
 or pepsonizing ferment of the gastric 
 juice, 12, 80 
 
 Peptones, 12 
 
 Perianth (irepi, around : ai'flos, a flower), 
 the proximal infertile leaves of a flower, 
 380 
 
 Perisperm (rrepi, around : <r;repju,a, seed), 
 nutrient tissue developed in the nucleus 
 of the seed, 380 (description of figure) 
 
 Peristom'e (rrepi, around : <rr6juia., the 
 mouth), Vorticella, 128 
 
 PeriStom'ium (nepi, around : O-TO/XIOI/, a 
 little mouth), the mouth-bearing segment 
 of worms, 273, 207 
 
 Peritone'um (Trepiroi/aioi/), the membrane 
 covering the viscera, 325 
 
 Pet'alS (TreTaXov, a leaf), the inner or dis- 
 tal perianth leaves in the flower of 
 Angiosperms, 380 
 
 Phar'ynx ($apvy, the throat) : Poly- 
 gordius, 280 : Dogfish, 328 
 
 Phloem (0Aoi6s, bark or bast), the outer 
 portion of a vascular bundle, 349 
 
 Phyla (</>CAoi', a tribe) of the animal king- 
 dom, 307 : of the vegetable kingdom, 
 364 
 
 Phyll'ula (diminutive of $vAAov, a leaf), 
 the stage in the embryo of vascular 
 plants at which the first leaf and root 
 have appeared, 360 : contrasted with 
 gastrula, 360 
 
 Phylog eny (<t>v\oi>, a race : 
 
 origin), the development of the race, 
 
 Physiol'ogy (<(>vcns, the nature or proper 
 of a thing : Ao-j/ps, a discussion), the < 
 partment of biology which treats 
 function, 9 ct seq. 
 
 Pigment-spot, Euglena, 
 
 PileUS (pllcus, a cap), AgaHCtlS, 191 
 Pinna (pinna, a feather), of leaf, 352 
 Pistil (pistillutx, a pestle, from pinso, to 
 
 pound.) See Gyncecium. 
 Plan'ula (diminutive of 7rAai/os, a wander- 
 ing about), the mouthless diploblastic 
 larva of a hydroid, 248 
 Plant, definition of, 176 
 
INDEX AND GLOSSARY 
 
 45 
 
 Plants, classification of, 364 
 
 Plas'ma (TrAacr/xa, anything moulded), of 
 
 blood, 56 
 Plasmo'dium (TrAao-yaa, any thing moulded), 
 
 52-55 : comparison of with zygote, 54 
 Plastic (TrAeun-iAcos, formed by moulding) 
 
 products, products of katabolism which 
 
 remain an integral part of the organism, 
 
 Pod'omere (TTOV'S, a foot : /ixepos, a part), a 
 limb-segment, 314 
 
 Polar cells, formation of, 262 
 
 Pollen grain (pollen, fine flour), a name 
 given to the microscope of Phanero- 
 gams. 
 
 Pollen-sac, a name given to the microspo- 
 rangium of Phanerogams. 
 
 Pollen-tube, 376, 383 
 
 Pollina'tion, 376, 383 
 
 POLYGORD'IUS (TroAvs, many : TopSios, 
 King of Phrygia, inventor of the Gordian 
 knot) : Figures, 272, 274, 285, 287, 294, 
 296, 298, 300, 303 : occurrence and gene- 
 ral characters, 271, 274 : metameric seg- 
 mentation, 271-273 : mode of feeding, 
 273 : enteric canal, 273, 277 ; cell-layers, 
 276-278 ; ccclome, 273, 277 : distribution 
 of food, 281 ; blood-system, 282 : nephri- 
 dia, 284 : nervous system, 286 : differen- 
 tiation of definite organs and tissues, 
 291 : reproduction, 293 : development and 
 metamorphosis, 299-306 
 
 Polymorphism (TroAvs, many : MP<W, 
 form), existing under many forms, 251 
 
 Pol'yplast (rroAi)?, many: TrAaerros, formed, 
 modelled), the multicellular stage of the 
 embyro before the differentiation of cell- 
 layers or organs : Hydroids, 248 : Moss, 
 337 : Fern, 359 
 
 PORPITA (TTOPTTYJ, a brooch), 253 : (Figure), 
 253 
 
 Primor'dial utricle, 196, 210 
 
 ProctodaB'um (Trpco/cros, the anus : 6Scuo?, 
 belonging to a way), an ectodermal 
 pouch which unites with the enteron and 
 forms the posterior end of the enteric 
 canal, its external aperture being the 
 permanent anus, 298 
 
 Pro-embryo, chara, 219 (Figure) 
 
 Pro-nucleus, female, 262 : male, 263 : 
 
 conjugation of male and female, 263 
 Pl'OStom'ium (Trpo, before : (TTOjuiov, a little 
 mouth), the first or pre-oral segment in 
 worms, Sic., 271, 296 
 
 PROT'AMCEBA (n-pwros, first : a/uoi/Sos, 
 changing), 9 (Figure). 
 
 Prothal'lus (n-po, before : 0aAA6?, a twig), 
 the gamobium of vascular plants : Fern 
 355 : dimorphism of in Equisetum, 367 
 reduction of in Salvinia, 369 ; Selagi 
 nella, 371, and Gymnosperms, 376, 378 
 retarded development of in Angiosperms 
 
 ProthallUS, secondary, Selaginella, 371 
 
 Prot'eidS (n-pwro?, first), composition of, 5 
 ProtiSt'a (7rpum<rros, the first of all), the 
 lowest organisms intermediate between 
 the lowest undoubted animals and plants, 
 182 
 ProtOCOC'CUS (7rpo)TO?, first : KOKKOS, berry). 
 
 See Haematococcus. 
 
 PROTOMYX'A OU.PCOTOS, first :/nva, mucus): 
 
 Figure, 50 : occurrence and general cha- 
 
 acters, 49 : life-history, 51 ; animal or 
 
 plant? 181 
 
 Protonem'a(7rpaTOS, first : i>rj/u,a, a thread), 
 
 Moss, 336, 339 
 
 Prot'oplasm (Trpwros, first : TrAacr/ixa, any- 
 thing moulded), composition of, 5 : pro- 
 perties of, 5-7 : micro-chemical tests for, 
 7, 8 : minute structure of, 62, 63 : con- 
 tinuity of in Fern. 350 : in Polygordius, 
 292 : intra- and extra-capsular, Kadio- 
 laria, 152 
 
 Protozoa, the, 308 
 
 Proximal (proximus. nearest), the end 
 nearest the point of attachment or or- 
 ganic base, e.g. in the stalk of Vorticella, 
 126 
 
 Pseud'opod 1 (vl/evSrjs, false : TTOVS, foot), 
 described, 4: comparison of with cilium, 
 34, 52 : in columnar epithelium, 59 : in 
 endoderm cells of Hydra, 231 
 Pteris. See Ferns. 
 Punctum vegetationis. See Growing 
 
 point. 
 
 Putrefac'tidn(/?/6-/Y7, to make rotten) 
 nature of, 82 : a process of fermentation, 
 91 : conditions of temperature, moisture, 
 &c., 93, 94 
 
 Putres'cent (putresco, to grow rotten) 
 Solution, characters of, 37, 82 
 
 Putres'cible infusion, sterilization of, 99- 
 
 IO2 
 
 Pyren'oid (n-upryi/, the stone of stone-fruit : 
 elSos, form), a small mass of pr^teid 
 material invested by starch, 27 
 
 Radial symmetry, starfish, 309 
 
 RADIOLAR'IA (radius, a spoke or ray): 
 Figures, 152, 153 : occurrence and general 
 characters, 152 : central capsule, 152 : 
 intra- and extra-capsular protoplasm, 
 152 : silicious skeleton, 152 : symbiotic 
 relations with Zooxanthella, 154 
 
 Rect'UUl (intestinum rectum, the straight 
 gut), the posterior or anal division of the 
 enteric canal, 281 
 
 Redi, Francisco (Italian savant}, experi- 
 ments on biogenesis, 97 
 
 Reducing division, 257, 262 
 Reflex action, 289 
 Reproduction, necessity for, 19 
 Reproductive organ. See Gonad. 
 
406 
 
 INDEX AND GLOSSARY 
 
 Reservoir of contractile vacuole, Euglena, 
 47 
 
 Respiration : Amoeba, 17 : Polygordius, 
 
 284 
 Respiratory caeca, Starfish, 312 
 
 Rhiz'oid(pia, root telSos, form): Nitella, 
 206, 214 : Moss, 335 : prothallus of Fern, 
 
 Root, Fern, 344, 353 
 
 Root-cap, 354 
 
 Root-hairs, 353 , 357 
 
 ROSS, Alexander, on abiogenetic origin of 
 mice, insects, &c., 96 
 
 Rotation of protoplasm, 210 
 
 Rudiment, rudimentary (mdimentum, a 
 beginning), the early stage of a part or 
 organ : often used for a structure which 
 has undergone partial atrophy, but in 
 such cases the word vestige (q. v.) is 
 more suitable. 
 
 s 
 
 SACCHAROMY'CES (<ra.Kxapov, sugar: 
 /ixu'/crjs, fungus): Figure, 72 : occurrence, 
 71 : structure, 71 : budding, 73 : in- 
 ternal fission, 74 : nutrition, 75 : alco- 
 holic fermentation caused by, 75, 79, 80 : 
 experiments on nutrition of, 78-80 : ani- 
 mal or plant? 182 
 
 SALVIN'IA : Figure, 369 : general cha- 
 racters, 368 : mega- and micro-sporangia 
 and spores, 368 : male and female pro- 
 thalli and gonads, 369, 370 : development 
 and alternation of generations, 370 
 
 Saprophyt'ic (o-a.7rp6s, putrid : fyvrov, 3. 
 plant) nutrition, defined, 39 
 
 Schulze's solution, test for cellulose, 28 : 
 for lignin, 348. 349 
 
 Scleren'chyma (ovcAijpo?, hard : e-yxv/u-a, 
 infusion) : Moss, 334 : Fern, 345, 348, 
 
 Secre'tion (secrctus, separate), nature of, 
 231 : formation of cell- wall a process of, 
 *4 
 
 Seed, formation of, 377, 384 : germination 
 of, 377 
 
 Seg'ment (segmentwm, a piece cut off), in 
 plants a node together with the next 
 proximal internode, 208 : in animals the 
 name is variously applied. See Meta- 
 mere, Podomere. 
 
 Segment'al cell: Nitella, 211: Moss, 
 335 : Fern, 350 
 
 Segmentation, metameric. See Meta- 
 mere, 
 
 SELAGINELL'A (o-eAa-yeou, to shine): 
 Figure, 372 ; general characters, 371 : 
 cone, sporangia, and spores, 371 : pro- 
 thalli and gonads, 371 : development and 
 alternation of generations, 372, 373 
 
 Self-fertilization, applied to the sexual 
 
 process when the gametes spring from 
 the same individual, 199 
 Sep'alS (separ, separate), the outer or 
 proximal perianth-leaves in the flower of 
 Angiosperms, 378 
 
 Sep'tum (septttm, a barrier) : In plants 
 187 : in Polygordius, 280 : development 
 o f, 302 
 
 Set'a (seta, a bristle), 290 
 Sex-cells, primitive, 255 : origin of in 
 
 Hydroids, 247 : in Polygordius, 293 
 Sexual differentiation, illustrated by 
 
 Vaucheria, 172 : by Spirogyra, 199 
 Sexual generation. See Gamobium. 
 Sexual reproduction, nature of, 42 
 Shell, Mussel, 320 
 Shoot, in plants, an axis of the second or 
 
 any higher order with its leaves, 209 
 Sieve-tubes and plates, 350 
 Sinus (sinus, a hollow), a spacious cavity 
 
 3i8 
 
 Skeleton. See Endo- and Exo-skeleton. 
 Slime-fungi, See.Mycetozoa. 
 Solid aggregate, 203 
 
 Somatic (o-w/u.a, the body), applied to the 
 
 layer of mesoderm which is in contact 
 
 with the ectoderm and with it forms the 
 
 body-wall, 278 
 
 Sor'us (eriopo?, a heap), an aggregation of 
 
 sporangia, 354, 368 
 
 Species (species, a kind), meaning of term 
 illustrated, 8, 137 ; definition of, 139 : 
 origin of, 141, 144 
 Specific characters, specific name, 8, 
 
 139 
 
 Specialized, meaning of, 140 
 Sperm (o-irep/ota, seed), the male or micro- 
 gamete in its highest stage of differentia- 
 tion : structure and development of, 255 : 
 see also under the various types, and 
 especially Vaucheria, 172, 173 
 Spermatozo id, spermatozo'on (o-7rep/u.a, 
 seed : a>ov, animal, from the actively 
 moving sperms of animals having been 
 supposed to be parasites), synonyms of 
 sperm. 
 
 Spermary (orn-ep/ota, seed), the male gonad 
 or sperm-producing organ : see under the 
 various types, and especially Vaucheria, 
 172 
 
 Sperm'iduct (<T7re'pju.a, seed : duco, to lead), 
 a tube conveying the sperm from the 
 ' spermary to the exterior, 295 
 SpermatOgen'esiS (o-Trep/u-a, seed ; yeVecrts, 
 origin), the development of a sperm from 
 a primitive sex-cell, 255, 256 (Figure). 
 Spinal cord, Dogfish, 330 
 Spiral vessel. See Vessel. 
 SPIRILL'UM O^Vrt, a coil) 86, 88 (Figure) 
 SPIROGYRA (spira, a coil : gyms, a revo- 
 tion) : Figure, 195 : occurrence and 
 general characters, 194 : microscopic 
 structure, 194 : growth, 197 : conjugation, 
 198 : development, 200 : nutrition, 200 
 
INDEX AND GLOSSARY 
 
 407 
 
 Splanch'niC (<rn \dyxvov, intestine or vis- 
 cus), applied to the layer of mesoderm 
 which is in contact with the endoderm 
 and with it forms the enteric canal, 278 
 
 Spontaneous generation. See Abio- 
 genesis. 
 
 Sporan'gium (a-iropd, seed : dyyelov, a 
 vessel), a spore-case: Mucor, 160 : Vau- 
 cheria, 171: Fern, 354. See also Mega- 
 and Micro-sporangium. 
 
 Spore (o-Tropd, a seed), an asexual repro- 
 ductive cell : see under the various types 
 and especially Heteromita, 42 : Saccha- 
 romyces, 74 : Bacteria, 89 : vitality of 
 in Bacteria, 99, 101 : Pemcillium, 189 : 
 Moss, 339 : Fern, 355. See also Mega- 
 and Micro-spore. 
 
 Sporogon'ium a-nopd seed : yovos, pro- 
 duction), the agamobium of a moss, 337 
 
 Spor'ophyll (a-nopd, seed : <j>v\\ov, leaf), 
 a sporangium-bearing leaf: Equisetum, 
 366 : Selaginella, 371 : Gymnosperms, 
 373i 375 ' Angiosperms, 381 
 
 Stamen (stamen, a thread), a male sporo- 
 phyll, 373, 381 ^ 
 
 Starch, composition and properties of, 27 
 
 STARFISH ; Figure, 310: general cha- 
 racters, 309-311 : radial symmetry, 309: 
 tube-feet and ambulacral system, 311, 
 313 : exoskeleton, 312 
 
 Stem, structure of: Moss, 334 ; Fern. 345 
 
 Sterig'ma (o"rjpry^.a, a support) : Penicil- 
 lium, 188 ; Agaricus, 193 
 
 Sterilization of putrescible infusions, 99- 
 
 IO2 
 
 Stigma (o-Tiy/ma, a spot), the receptive ex- 
 tremity of the style, 381 
 
 Stimulus, various kinds of, 289 
 
 Stock. See Colony. 
 
 Stom'ate (crro^ia, mouth), 353 
 
 Stomodse'um (a-ro/xa, mouth : ofiaios, be- 
 longing to a way), an ectodermal pouch 
 which unites with the enteron and forms 
 the anterior end of the enteric canal, its 
 aperture being the permanent mouth, 
 298 
 
 Stone-canal, Starfish, 313 
 
 Style (stylus, a column), the distal solid 
 portion of the female sporophyll or of the 
 entire gyncecium in Angiosperms, 381 
 
 QT VT nxrvr'iTT'T A / ~ \ ' ., i. 
 
 01 iJ-iUJN iul lA(<7TvAos, a column : ow, 
 
 a claw), Figure, 117: occurrence and 
 general characters, 116 : polymorphism 
 of cilia, 118-119 
 
 Sub-apical cell. See Segmental cell. 
 
 Superficial aggregate. 202 
 
 Supporting lamella. See Mesoglcea. 
 
 Suspensor: Selaginella, 373: Gymno- 
 sperms, 377 : Angiosperms. 383 
 
 Sweet Wort, composition of, 75 
 
 Swimming-bell, Diphyes, 251 
 
 Symbio'siS (cru/x/3t'ojcrt?, a living with), an 
 intimate and mutually advantageous 
 association between two organisms, 154 
 
 Syner'gidffi (ovvepyos, a fellow worker), 
 382 
 
 Sys'tole (cruo-ToATj, a drawing together, 
 contraction), the phase of contraction of 
 a heart, contractile vacuole, &c. } in 
 
 Teeth, Dogfish, 328 
 
 Temperature, effects of on protoplasmic 
 
 movements, 20, 21 
 Tentacles :Hydra, 223 : Bougainvillea, 
 
 239 ; Polygordius, 271 : development of, 
 
 302 
 Terminal bud : Nitella, 208, 210: Moss 
 
 TestiS (the Latin word), generally used for 
 the spermary in animals. 
 
 Thermal death-point. See Ultra-maxi- 
 mum temperature. 
 
 Tissues, differentiation of: Polygordius, 
 291 : Fern, 353 
 
 Tracheides (rpa^vs, rough : eiSos, form). 
 See Vessels of Plants, 349 
 
 Transpiration, the giving off of water 
 from the leaves of plants, 341 
 
 Trich'ocyst (0pif, a hair : KVO-TIS, a bag), 
 JI 3 
 
 TliploblaSt'iC (rpiTrAoos, triple : /SAao-ros, 
 a bud), three-layered : applied to ani- 
 mals in which the body consists of ecto- 
 derm, mesoderm, and endoderm, 244, 
 278 
 
 TrOCh'OSphere Orpoxos,a wheel, in reference 
 to the circlet of cilia : o^cupa, a sphere), 
 the free-swimming larva of Polygordius, 
 &c. : characters of, 296 (Figure) ; origin 
 of from gastrula, 297, 298 ; metamorpho- 
 sis of, 299 
 
 Tube-feet, Starfish, 311, 313 
 
 U 
 
 Ultra-maximum temperature, for amoe- 
 boid movements, 21 ; for monads, 40 ; for 
 Bacteria, 93 
 
 ULVA (jilva, an aquatic plant), 203 
 
 Umbell'ate (inubella, a sun-shade, um- 
 brella) applied to branching in which 
 the primary axis is of limited growth and 
 sends off a number of secondary axes 
 from its distal end, 138 
 
 Unicell'ular, formed of a single cell, 61 ; 
 connection of uni- with multi-cellular 
 organisms, 264-270 
 
 Ureter (ovprjr^p, the Greek name), the 
 duct of the kidney, 330 
 
 Vac'uole (vacuus, empty), contractile, 13 
 
 in : non-contractile, 71 
 Variability, 147 
 Variation, individual, 140, 147 
 
408 
 
 INDEX AND GLOSSARY 
 
 Variety, an incipient species, 147 
 
 Vasc'ular (vasciiltun, a small vessel) 
 bundles, 345, 348 
 
 Vascular plants, 365 
 
 VAUCHERIA (after J. P. E. Vaucher, a 
 Swiss botanist) : Figure, 170 : occur- 
 rence and general characters, 169 : minute 
 structure, 169 : asexual reproduction, 171: 
 sexual reproduction, 172 : nutrition, 175 
 
 Veins of Dogfish, 329 : of leaves, 352 
 
 Vel'um (velum, a veil) of medusa, 241 
 
 Vent, the aperture of the cloaca, 324 
 
 Venter (venter, the belly), of ovary of 
 Moss, 336, and Fern, 358 : of the female 
 sporophyll or of the entire gynoecium of 
 Angiosperms (so-called ovary) 381 
 
 Ventral nerve-cord : Polygordius, 286 : 
 development of, 301 : Crayfish, 319 
 
 Ventricle. See Heart. 
 
 Vermes, the, 308 
 
 Ver'tebral^T^ra, a joint) centra and 
 column, Dogfish, 328 
 
 Vertebrata, the, 309 
 
 Vessels : of plants, spiral and scalariform, 
 348, 349 : of animals, see Blood-vessels. 
 
 Vestige, vestigial (vestigium, a trace), 
 applied to any structure which has be- 
 come atrophied or undergone reduction 
 beyond the limits of usefulness, 118 
 
 Vib'riO (vibro, to vibrate), 86, 88, (Figure) 
 
 ViSC'eral (viscns, an internal organ), ap- 
 plied to the layer of coelomic epithelium, 
 or of peritoneum, covering the intestine 
 and other internal organs, 277 
 Visceral ganglion, Mussel, 323 
 
 Vitelline (vitellus, yolk) membrane, the 
 
 cell-membrane of the ovum, 259 
 Volvox (volvo, to roll), 267, 268, 269. 
 
 (Figure*) 
 
 VORTIOELLA (diminutive of vortex, 
 eddy) : Figure, 127 : occurrence .. 
 general characters, 126 : structure, 126 : 
 asexual reproduction, 131 : conjugation, 
 132 : means of dispersal, 132-136 : ency-<- 
 tadon, spore-formation, development, 
 and metamorphosis, 133 
 
 W 
 
 Waste-products, 33 
 Water of organisation, 5, 29 
 Whorl of leaves, 208 
 Wood. See Xylem. 
 Work and Waste, 14 
 
 Xylem (uAoi/, wood), the inner portion o. 
 vascular bundle, 349 
 
 Yeast, 7 r 
 
 Yeast-plant. See Saccharomyces. 
 YellOW-cells of Radiolaria, 154 
 Yolk-granules or spheres, 68, 235, 
 
 Zoogloe'a (<aov, an animal : -y/Aoi'a, glue), 
 85 
 
 Zooid (a>oc, an animal* etSos, form), a 
 single individual of a compound organism, 
 137, 237 
 
 Zootham'nium (^(aov, an animal : 0t.,xi;o?, 
 a bush): Figures, 134, 138: OCL -rence 
 and general characters, 135 -.dimorphism 
 ofzooids, 135: means of dispersal, 136: 
 characters arid mutual relations of species, 
 
 Zooxanthell'a (fo>oi> an animal : ai'06?, 
 yellow), 154 
 
 Zyg'ospore(c;iryoi>, a yoke : <ntopa, a seed), 
 applied to a resting zygote formed by the 
 conjugation of similar garnet , 166 
 
 ZygOte (vywTos, yoked), the p.oducts of 
 conjugation of two gametes : Hetero- 
 mita, 41 : Vorticella, 133 : Mucor, 165 : 
 Vaucheria, 174 : Spirogyra, 198-200. 
 
 THE END 
 
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