LESSONS
IN
ELEMENTARY BIOLOGY
LESSONS
IN
ELEMENTARY BIOLOGY
BY
T. JEFFERY PARKER, B.Sc., F.R.S.
I'KOFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, NEW ZEALAND
WITH EIGHTY-NINE ILLUSTRATIONS
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MACMILLAN AND CO.
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he Right of Translation and Reproduction is Resetted
RICHARD CLAY AND SONS, LIMITED,
LONDON' AND BUNGAY.
I 6 SI '1
To
THOMAS H. HUXLEY
D.C.L., LL.D., F.X.S.
DEAR PROFESSOR HUXLEY,
To you I owe my scientific training, my choice of a
profession, and every advance I have made in my career.
It is therefore only fitting that J should ask you to accept,
as a slight token of affection and gratitude, the dedication
of this little book, of which I can ho7iestly say that, while
its faults are my own, any good features it may possess are
mainly due to my having had the advantage of being your
pupil.
Believe me to be,
Yours very faithfully,
T. f. PARKER.
OTAGO UNIVERSITY MUSEUM,
August 22nd, 1890.
PREFACE
J:\ his preface to the new edition of the well-known Practical
Biology, Professor Huxley gives his reasons for beginning the
study of organized nature with the higher forms of animal
life, to the abandonment of his earlier method of working
from the simpler to the more complex organisms. He says
in effect that experience has taught him the unwisdom of
taking the beginner at once into the new and strange region
of microscopic life, and the advantage of making him com-
mence his studies with a subject of which he is bound to
know something — the elementary anatomy and physiology
of a vertebrate animal.
Most teachers will probably agree with the general truth
of this opinion. The first few weeks of the beginner in
natural science are so fully occupied in mastering an
unfamiliar and difficult terminology and in acquiring the art
of using his eyes and fingers, that he is simply incapable for
a time of grasping any of the principles of the science ; and,
this being the case, the more completely his new work can
via PREFACE
be connected with any knowledge of the subject, however
vague, he may already possess, the better for his progress.
On the other hand, the advantage to logical treatment of
proceeding from the simple to the complex — of working
upwards from protists to the higher plants and animals — is
so immense that it is not to be abandoned without very
good and sufficient reasons.
In my own experience I have found that the difficulty
may be largely met by a compromise, namely, by beginning
the work of the class by a comparative study of one of the
higher plants (flowering plant or fern) and of one of the
higher animals (rabbit, frog, or crayfish). If there were no
limitations as to time, and if it were possible to avoid alto-
gether the valley of the shadow of the coming examination,
this preliminary work might be extended with advantage, and
made to include a fairly complete although elementary study
of animal physiology, with a minimum of anatomical detail,
and a somewhat extensive study of flowering plants with
special reference to their physiology and to their relations
to the rest of nature.
In any case by the time this introductory work is over,
the student of average intelligence has overcome pre-
liminary difficulties, and is ready to profit by the second
and more systematic part of the course in which organisms
are studied in the order of increasing complexity.
It is such a course of general elementary biology which
1 have attempted to give in the following Lessons, my aim
having been to provide a book which may supply in the
PREFACE ix
study the place occupied in the laboratory by " Huxley and
Martin," by giving the connected narrative which would be
.out of place in a practical handbook. I also venture to
hope that the work may be of some use to students who
have studied zoology and botany as separate subjects, as
well as to that large class of workers whose services to
English science often receive but scant recognition — I
mean amateur microscopists.
As to the general treatment of the subject I have been
guided by three principles. Firstly, that the main object of
teaching biology as part of a liberal education is to familiarize
the student not so much with the facts as with the ideas of
science. Secondly, that such ideas are best understood, at
least by beginners, when studied in connection with concrete
types of animals and plants. And, thirdly, that the types
chosen should illustrate without unnecessary complication
the particular grade of organization they are intended to
typify, and that exceptional cases are out of place in an
elementary course.
The types have therefore been selected with a view of
illustrating all the more important modifications of structure
and the chief physiological processes in plants and animals ;
and, by the occasional introduction of special lessons on
such subjects as biogenesis, evolution, £c., the entire work
is so arranged as to give a fairly-connected account of the
general principles of biology. It is in obedience to the last
of the principles just enunciated that I have described so
many of the Protozoa, omitted all but a brief reference to
x PREFACE
the development of Hydra and to the sexual process in
Penicillium, and described Nitella instead of Chara, and
Polygordius instead of the earthworm. The last-named
substitution is of course only made possible by the book
being intended for the study and not for the laboratory, but
I feel convinced that the student who masters the structure
of Polygordius, even from figures and descriptions alone,
will be in a far better position to profit by a practical study
of one of the higher worms.
Lessons XXVI I. and XXX. are mere summaries, and can
only be read profitably by those who have studied the
organisms described, or allied forms, in some detail. Such
abstracts were however necessary to the plan of the book, in
order to show how all the higher animals and plants may be
described, so to speak, in terms of Polygordius and of the fern.
For many years I have been convinced of the urgent need
for a modification of nomenclature in biology, and have now
attempted to carry out a consistent scheme, as will be seen
by referring to the definitions in the glossary. Many of
Mr. Harvey Gibson's suggestions are adopted and three new
words are introduced — phyllula, gamobium, and agamo-
bium. I expect and perhaps deserve to be criticised, or,
what is worse, let alone, for the somewhat extreme step of
using the word ovary in its zoological sense throughout the
vegetable kingdom ; and for describing as the venter of the
pistil the so-called ovary of Angiosperms. I would only
beg my critics before finally pronouncing judgment to try
and look at the book, from the point of view of the begin-
PREFACE xi
ner, as a graduated course of instruction, and to consider
the effect upon the entire scheme of using a term of funda-
mental importance in two utterly different senses.
A large proportion of the figures are copied either from
original sources or from my own drawings — the latter when
no authority is mentioned. The majority, even of those
which have previously appeared in text-books, have been
specially engraved for the work, the draughtsman being
my brother, Mr. M. P. Parker. In order to facilitate
reference the illustrations referring to each subject have, as
far as possible, been grouped together, so that the actual is
considerably larger than the nominal number of figures.
Full descriptions are given instead of mere lists of reference-
letters : these will, I hope, be found useful as abstracts of
the subjects illustrated.
I have to thank my friends, Mr. A. Dillon Bell and Pro-
fessor J. H. Scott, M.D., for constant and valuable help in
criticising the manuscript. To Dr. Paul Meyer, of the
Zoological Station, Naples, I am indebted for specimens
of Polygordius ; and to Professor Sale, of this University,
Professor Haswell, of Sydney, Professor Thomas, of Auck-
land, and Professors Howes and D. H. Scott, of South
Kensington, for important information and criticism on
special points. My brother, Professor W. Newton Parker,
has kindly promised to undertake a final revision for the
press.
DUNEDIN, N.Z.,
August, 1890.
TABLE OF CONTENTS
PAGE
PREFACE vii
LIST OF LLUSTKATIONS xix
LESSON I.
AMCEBA I
LESSON II.
H/EMATOCOCCUS 23
LESSON III.
HETEROMITA- 36
LESSON IV.
EUGLENA 44
LESSON V.
PROTOMYXA 49
THE MYCETOZOA . 52
xiv TABLE OF CONTENTS
LESSON VI.
PAGE
A COMPARISON OF THE FOREGOING ORGANISMS WITH CERTAIN
CONSTITUENT PARTS OF THE HIGHER ANIMALS AND
PLANTS ........... ...... 56
ANIMAL AND PLANT CELLS ........ 56
MINUTE STRUCTURE AND DIVISION OF CELLS AND
NUCLEI . . ........... 62
OVA OF ANIMALS AND PLANTS ....... 67
LESSON VII.
SACCHAROMYCES ........... ..... 70
LESSON VIII.
BACTERIA ................. gj
LESSON IX.
BIOGENESIS AND ABIOGENESIS ......... 93
HOMOGENESIS AND HETEROGENESIS ...... ICO
LESSON X.
PARAMCECIUM ...............
STYLONYCHIA .................
OXYTRICHA
LESSON XI.
OPALINA
LESSON XII.
VORTICELLA
124
ZOOTHAMNIUM
TABLE OF CONTENTS xv
LESSON XIII.
PAG F.
SPECIES AND THEIR ORIGIN : THE PRINCIPLES OF CLASSIFICA-
TION .................. 135
LESSON XIV.
THE FORAMIN'IFERA ............... 146
THE RADIOLARIA ........... .....
THE DIATOMACE/E ............... 153
LESSON XV.
MUCOR ....... » ........... 156
LESSON XVI.
VAUCHERIA . . . ............... 167
CAULERPA .......... ........ 173
LESSON XVII.
THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS . . 174
LESSON XVIII.
PENICILLIUM . . ...............
AGARICUS . , , ...............
LESSON XIX.
SPIROGYRA . . .............. • 192
LESSON XX
MONOSTROMA ...... ..... • • • J99
ULVA. ... ..... ..... ... 201
LAMINARIA, &C ....... ......... 2OI
xvi TABLE OF CONTENTS
LESSON XXI.
PAGE
NITELLA 204
LESSON XXTT.
HYDRA 219
LESSON XXITT.
HYDROID POLYPES 234
BOUGAINVILLEA, &C 234
DIPHYES 249
PORPITA . . 250
LESSON XXIV.
SPERMATOGENESIS AND OOGENESIS . . . 252
THE MATURATION AND IMPREGNATION OF THE OVUM . . . 255
THE CONNECTION BETWEEN UNICELLULAR AND DIPLOBLASTIC
ANIMALS 26l
LESSON XXV.
POLYGORDIUS 267
LESSON XXVI.
POLYGORDIUS (continued) 289
LESSON XXVII.
THE GENERAL CHARACTERS OF THE HIGHER ANIMALS . . . 303
THE STARFISH 305
THE CRAYFISH . ^Io
\J
THE MUSSEI .... 316
THE DOGFISH
TABLE OF CONTENTS xvii
LESSON XXVIII.
PAGE
MOSSKS 328
LESSON XXIX.
FERNS 34°
LESSON XXX.
THE GENERAL CHARACTERS OF THE HIGHER PLANTS . . . 359
EQUISETUM 362
SALVINIA • 364
SELAGINELLA 3^7
GYMNOSPERMS 369
ANGIOSPERMS ... 374
SYNOPSIS 383
INDEX AND GLOSSARY 395
b
LIST OF ILLUSTRATIONS
FIG- PAGE
1. Amoeba, various species 2
2. Protamceba pri mitiva g
3. Hitmatococcus pluvialis and H. lacustris 24
4. Heteromita rostrata 38
5. Euglena viridis ... 45
6. Protomyxa aurantiaca 50
7. Badhamia and Chondrioderma 53
8. Typical animal and plant cells 57
9. Typical animal cell 62
10. Stages in the binary fission of an animal cell ... . . 64
1 1 . Stages in the binary fission of a plant cell 66
12. Ova of Car marina and Gymnadenia . . ... 68
13. Saecharomyces cerevisia 71
14. Bacterium termo 82
15. Bacterium termo, showing flagella ... .... 83
16. Micrococcus 85
17. Bacillus subtilis 86
1 8. Vibrio serpens, Spirillum tenue, and S. volittans .... 87
19. Bacillus anthrads 89
20. Beaker with culture-tubes 98
21. Paramcecium aurelia 106
xx LIST OF ILLUSTRATIONS
FIG. PAGE
22. Paramcecium aurelia, conjugation 112
23. Stylonychia mytilus • Ir5
24. Oxytricha flava • IJ7
25. Opalina rananim . 120
26. Vorticella ... 125
27. Zoothamnium arbuscula . . 132
28. Zoothamnium, various species 136
29. Diagram illustrating the Origin of the Species of Zootham-
nium by Creation 14°
30. Diagram illustrating the Origin of the Species of Zootham-
nium by Evolution *42
31. Rotalia 147
32. Diagrams of Foraminifera ... 148
33. Alveolina quoii .... 149
34. Lithocircus annularis ... .150
35. Actinomma asteracanthion 151
36. Diagrams of a Diatom and shells of Navicula and Aulaco
discus 154
37. Alucor mucedo and M. stolonifer 15°
38. Moist chamber 161
39. Vaucheria 1 68
40. C aider pa scalpelliformis 172
41. Penicillium glaucum 184
42. Agaricus campestris 19°
43. Spirogyra ....... .... 193
44. Monostroma bullosum and J\L laceration 200
45. Laminaria claustoni and Lessoniafiitcescens 202
46. Nitclla, general structure 205
47. Nitclla, terminal bud 210
48. Nitella, spermary 213
49. Nitella, ovary 215
50. Chara, pro-embryo 217
51. Hydra viridis and H. fitsca, external form 220
52. Hydra, minute structure 224
53. Hydra viridis, ovum 232
LIST OF ILLUSTRATIONS xxi
FIG. PAGE
54. Bougainvillea ramosa 235
55. Diagrams illustrating derivation of Medtisa from Hydranth . 238
56. Eucopella campanularia, muscle-fibres and nerve-cells . . . 242
57. Laomedeaf.exuosa^\\^.Eudendriumrainosum, development. 246
58. Diphyes campanulata 248
59. For pita pacifica and P. mediterranea 250
60. Spermatogenesis in the Rat . 253
6 1. Ovum of Toxopneustes lividus 255
62. Maturation and impregnation of the animal ovum .... 256
63. The gastrula ... 261
64. JHagosphczra platiula . . 262
65. Volvox globator 263
66. Diagram illustrating the hypothetical origin of the gastrula
from a colony of unicellular zooids 264
67. Diagram illustrating the hypothetical origin of the gastrula
from a solitary multinucleate form 265
68. Polygordius neapolitanus, external form . 268
69. Polygordius neapolitanus, anatomy .... .... 270
70. Polygordius neapolitamis, nephridium 281
71. Polygordius, diagram illustrating the relations of the nervous
system 283
72. Polygordius neapolitanus ', reproductive organs 290
73. Polygordius neapolitanus, larva in the trochosphere stage . . 292
74. Diagram illustrating the origin of the trochosphere from the
gastrula 294
75. Polygordius neapolitanus, advanced trochosphere .... 296
76. Polygordiiis neapolitanus, larva in a stage intermediate be-
tween the trochosphere and the adult .... . 299
77. Starfish, diagrammatic sections .... . . 3°6
78. Crayfish, diagrammatic sections . • 312
79. Mussel, diagrammatic sections . • • 3X7
80. Dogfish, diagrammatic sections 322
81. Mosses, various genera, anatomy and histology 329
82. Funaria, reproduction and development . . . 334
83. Pteris and Aspidium, anatomy and histology . • 342
xxii LIST OF ILLUSTRATIONS
PAGE
84. Ferns, various genera, reproduction and development . . .352
85. Equisetum^ reproduction and development ...... 363
86. Salvinia, reproduction and development ....... 365
87. Selaginella, reproduction and development . . . . . . 368
88. GymnospermS) reproduction and development ..... 370
89. Angiosperms, reproduction and development ...... 375
LESSONS
IN
ELEMENTARY BIOLOGY
LESSONS
IN
ELEMENTARY BIOLOGY
LESSON I
AMCEBA
IT is hardly possible to make a better beginning of the
systematic study of Biology than by a detailed examination
of a microscopic animalcule often found adhering to weeds
and other submerged objects in stagnant water, and known
to naturalists as Amoeba.
Amoebae are mostly invisible to the naked eye, rarely
exceeding one-fourth of a millimetre (y^ inch) in dia-
meter, so that it is necessary to examine them entirely by
the aid of the microscope. They can be seen and re-
cognized under the low power of an ordinary student's
microscope which magnifies from twenty-five to fifty dia-
meters ; but for accurate examination it is necessary to
employ a far higher power, one in fact which magnifies
about 300 diameters.
Seen under this power an Amoeba appears like a little
B
AMCEBA
A
FlG. I. — A. Amoeba quartet, a living specimen, showing granular
endosarc surrounded by clear ectosarc, and several pseudopods (psd),
GENERAL CHARACTERS 3
some formed of ectosarc only, others containing a core of endosarc.
The larger bodies in the endosarc are mostly food-particles (X 300). l
B. The same species, killed and stained with carmine to show the
numerous nuclei (mt) (X 300).
C. Amceba proteus, a living specimen, showing large irregular
pseudopods, nucleus (mt), contractile vacuole (c.vac), and two food
vacuoles (f.vac), each containing a small infusor (see Lesson X.) which
has been ingested as food. The letter a to the right of the figure in-
dicates the place where two pseudopods have united to inclose the food
vacuole. The contractile vacuole in this figure is supposed to be seen
through a layer of granular protoplasm, whereas in the succeeding
figures (D, E, and G) it is seen in optical section, and therefore appears
clear.
D. An encysted Amceba, showing cell-wall or cyst (cy\ nucleus (mi),
clear contractile vacuole (c.vac), and three diatoms (see Lesson XIV. )
ingested as food.
E. Amoeba proteus i a living specimen, showing several large pseudo-
pods (psd), single nucleus (nu), and contractile vacuole (c.vac), and
numerous food particles imbedded in the granular endosarc ( X 330).
F. Nucleus of the same after staining, showing a ground substance or
achromatin, containing deeply-stained granules of chromatin, and
surrounded by a distinct membrane (X 1010).
G. Amceba vemtcosa, living specimen, showing wrinkled surface,
nucleus (mi), large contractile vacuole (c. vac) and several ingested
organisms (X 330).
H. Nucleus of the same, stained, showing the chromatin aggregated
in the centre to form a nucleolus (X 1010).
I. Amceba proteus, in the act of multiplying by binary fission
(X5oo).
(A, B, E, F, G, and H after Gruber ; c and I after Leidy ; D after
Howes.)
shapeless blob of jelly, nearly or quite colourless. The
central part of it (Fig. i, A, c, and E) is granular and semi-
transparent — something like ground glass — while surround-
ing this inner mass is a border of perfectly transparent and
colourless substance. So clear, indeed, is this outer layer
that it is easily overlooked by the beginner, who is apt to take
the granular internal substance for the whole Amoeba. If
in any way the creature can be made to turn over, or if a
number of specimens are examined in various positions,
1 A number preceded by the sign of multiplication indicates the
number of diameters to which the object is magnified.
B 2
4 AMCEBA
these two constituents will always be found to have the
same relations, whence we conclude that an Amoeba con-
sists of a granular substance the endosarc, completely
surrounded by a clear transparent layer or ectosarc.
One very noticeable thing about Amoeba is that it is never
quite the same shape for long together. Often the changes
of form are so slow as to be almost imperceptible, like the
movements of the hour-hand of a watch, but by examining
it at successive intervals the alteration becomes perfectly
obvious, and at the end of half an hour it will probably
have altered so much as to be hardly like the same thing.
In an active specimen the way in which the changes of
form are brought about is easily seen. At a particular
point the ectosarc is pushed out in the form of a small
pimple-like elevation (Fig. i. A, left side) : this increases in
size, still consisting of ectosarc only, until at last granules
from the endosarc stream into it, and the projection or
pseudopod (A, c, E, psd) comes to have the same structure
as the rest of the Amoeba. It must not be forgotten that as
the animal does not alter perceptibly in volume during the
process, every pseudopod thus protruded from one part of
the body necessitates the withdrawal of an equal volume
from some other part.
This peculiar mode of movement may be illustrated by
taking an irregular lump of clay or putty and squeezing it
between the ringers. As it is compressed in one direction
it will elongate in another, and the squeezing process may
be regulated so as to cause the protrusion of comparatively
narrow portions from the solid lump, when the resemblance
to the movements described in the preceding paragraph will
be fairly close. Only it must be borne in mind that in
Amoeba there is no external compression, the " squeezing "
being done by the animalcule itself.
•>•>
15
COMPOSITION OF PROTOPLASM 5
The occurrence of these movements is alone sufficient to
show that Amoeba is an organism or living thing, and no
mere mass of dead matter.
The jelly-like substance of which Amoeba is composed is
called protoplasm. It is shown by chemical analysis 1
to consist mainly of certain substances known as proteids,
bodies of extreme complexity in chemical constitution, the
most familiar example of which is white of egg or albumen.
They are compounds of carbon, hydrogen, oxygen, nitrogen,
and sulphur, the five elements being combined in the
following proportions : —
Carbon . . from 51*5 to 54-5 per cent.
Hydrogen . „ 6-9 „ 7-3 „
Oxygen 20-9 „ 23-5 „
Nitrogen . 15-2 „ 17-0 „
Sulphur . ,, 0*3 ,, 2*0 ,, ,,
Besides proteids, protoplasm contains small proportions
of mineral matters, especially phosphates and sulphates of
potassium, calcium, and magnesium. It also contains a
considerable quantity of water which being as essential a
constituent of it as the proteids and the mineral salts, is
called water of organization.
Protoplasm is dissolved by prolonged treatment with weak
acids or alkalies. Strong alcohol coagulates it, />., causes it
to shrink by withdrawal of water and become comparatively
hard and opaque. Coagulation is also produced by raising
the temperature to about 40° C. ; the reader will remember
how the familiar proteid white of egg is coagulated and
rendered hard and opaque by heat.
1 Accurate analyses of the protoplasm of Amoeba have not been
made, but the various micro-chemical tests which can be applied to it
leave no doubt that it agrees in all essential respects with the protoplasm
of other organisms, the composition of which is known (see p. 7).
6 AMCEBA
There is another important property of proteids which is
tested by the instrument called a dialyser. This consists
essentially of a shallow vessel, the bottom of which is made
of bladder or vegetable parchment, or some other organic
(animal or vegetable) membrane. If a solution of sugar or of
salt is placed in a dialyser and the instrument floated in a
larger vessel of distilled water, it will be found after a time that
some of the sugar or salt has passed from the dialyser into
the outer vessel through the membrane. On the other hand,
if a solution of white of egg is placed in the dialyser no
such transference to the outer vessel will take place.
The dialyser thus allows us to divide substances into
two classes : crystalloids — so called because most of them
like salt and sugar are capable of existing in the form of
crystals — which, in the state of solution, will diffuse through
an organic membrane ; and colloids or glue-like substances
which will not diffuse. Protoplasm, like the proteids of
which it is largely composed, is a colloid, that is, is non-
diffusible.
Another character of proteids is their i?istability. A
lump of salt or of sugar, a piece of wood or of chalk, may
be preserved unaltered for any length of time, but a proteid
if left to itself very soon begins to decompose ; it acquires
an offensive odour, and breaks up into simpler and simpler
compounds, the most important of which are water (H2O),
carbon dioxide or carbonic acid (CO2), ammonia (NH3),
and sulphuretted hydrogen (H2S) 1. In this character of
instability or readiness to decompose protoplasm notoriously
agrees with its constituent proteids ; any dead organism will,
1 For a more detailed account of the phenomena of putrefaction see
Lesson VIII., in which it will be seen that the above statement as to
the instability of (dead) proteids requires qualification ; as a matter of
fact they only decompose in the presence of living Bacteria.
CHARACTERS OF THE NUCLEUS 7
unless special means are taken to preserve it, undergo more
or less speedy decomposition.
Many of these properties of protoplasm can hardly be
verified in the case of Amoeba, owing to its minute size
and the difficulty of isolating it from other organisms (water-
weeds, &c.) with which it is always associated ; but there
are some tests which can be readily applied to it while
under observation beneath the microscope.
One of the most striking of these micro-chemical tests
depends upon the avidity with which protoplasm takes up
certain colouring matters. If a drop of a neutral or slightly
alkaline solution of carmine or logwood, or of some aniline
dye, or a weak solution of iodine, is added to the wTater con-
taining Amoeba, the animalcule is killed, and at the same
time becomes more or less deeply stained. The theory is
that protoplasm has a slightly acid reaction, and thus pro-
duces precipitation of the colouring matter from the neutral
or alkaline solution.
The staining is, however, not uniform. The endosarc,
owing to the granules it contains, appears darker than the
ectosarc, and there is usually to be seen, in the endosarc, a
rounded spot more brightly stained than the rest. This
structure, which can sometimes be seen in the living Amoeba
(Fig. i, c, E, and G, mi\ while frequently its presence is only
revealed by staining (comp. A and B), is called the nucleus.
But when viewed under a sufficiently high power, the
nucleus itself is seen to be unequally stained. It has lately
been shown, in many Amoebae, to be a globular body, en-
closed in a very delicate membrane, and made up of two
constituents, one of which is deeply stained by colouring
matters, and is hence called chromatin, while the other,
or achromatin, takes a lighter tint (Fig. i, F). The rela-
tive arrangement of chromatin and achromatin varies in
8 AMCEBA
different Amoebae : sometimes there are granules of chroma-
tin in an achromatic ground substance (F) : sometimes the
chromatin is collected towards the surface or periphery of
the nucleus : sometimes, again, it becomes aggregated in the
centre (G, H). In the latter case the nucleus is seen to have
a deeply-stained central portion, which is then distinguished
as the nucleolus.
When it is said that Amoebae sometimes have one kind of
nucleus and sometimes another, it must not be inferred that
the same animalcule varies in this respect. What is meant
is that there are found in stagnant water many kinds or
species of Amoeba which are distinguished from one
another, amongst other things, by the character of their
nuclei, just as the various species of Felis — the cat, lion,
tiger, lynx, &c. — are distinguished from one another, amongst
other things, by the colour and markings of their fur.
According to the method of binomial nomenclature intro-
duced into biology by Linnaeus, the same generic name
is applied to all such closely allied species, while each is
specially distinguished by a second or specific name of its
own. Thus under the genus Amoeba are included Amoeba
proteus (Fig. i, c, E, and F) with long lobed pseudopods and
a nucleus, containing evenly-disposed granules of chromatin ;
A. quarta (A and B), with short pseudopods and numerous
nuclei ; A. verrucosa (G and H) with crumpled or folded
surface, no well-marked pseudopods, and a nucleus with a
central aggregation of chromatin, or nucleolus ; and many
others.
Besides the nucleus, there is another structure frequently
visible in the living Amoeba. This is a clear, rounded space
in the ectosarc (c, E, and G, c. vac], which periodically dis-
appears with a sudden contraction and then slowly re-appears,
its movements reminding one of the beating of a minute
PROTAMCEBA
colourless heart. It is called the contractile vacuole, and
consists of a cavity in the ectosarc containing a watery
fluid.
Occasionally Amoebae — or more strictly Amoeba-like
organisms — are met with which have neither nucleus * nor
contractile vacuole, and are therefore placed in the separate
genus Protamosba (Fig. 2). They may have been looked
upon as the simplest of living things.
The preceding paragraph may be summed up by saying
that Amoeba is a mass of protoplasm produced into tempo-
rary processes or pseudopods, divisible into ectosarc and
FIG. 2 — Protamosba primitiva ; A, B, the same specimen drawn at
short intervals of time, showing changes of form.
C — E. Three stages in the process of binary fission. (After Haeckel.)
endosarc, and containing a nucleus and a contractile vacuole :
that the nucleus consists of two substances, chromatin and
achromatin, enclosed in a distinct membrane : and that the
contractile vacuole is a mere cavity in the protoplasm con-
taining fluid. All these facts come under the head of
Morphology, the division of biology which treats of form
and structure : we must now study the Physiology of our
animalcule — that is, consider the actions or functions it is
capable of performing.
1 Judging from the analogy of the Infusoria it seems very probable
that such apparently non-nucleate forms as Protamoeba contain chromatin
diffused in the form of minute granules throughout their substance (see
end of Lesson X., p. 118), or that they are forms which have lost their
nuclei.
io AMCEBA
First of all, as we have already seen, it moves, the move-
ment consisting in the slow protrusion and withdrawal of
pseudopods. This may be expressed generally by saying
that Amoeba is contractile, or that it exhibits contracti-
lity. But here it must be borne in mind that contraction
does not mean the same thing in biology as in physics.
When it is said that a red-hot bar of iron contracts on
cooling, what is meant is that there is an actual reduction in
volume, the bar becoming smaller in all dimensions. But
when it is said that an Amoeba contracts, what is meant is
that it diminishes in one dimension while increasing in
another, no perceptible alteration in volume taking place :
each time a pseudopod is protruded an equivalent volume
of protoplasm is withdrawn from some other part of the
body.
We may say then that contractility is a function of the
protoplasm of Amoeba — that is, that it is one of the actions
which the protoplasm is capable of performing.
A contraction may arise in one of two ways. In most
cases the movements of an Amoeba take place without any
obvious external cause ; they are what would be called in the
higher animals voluntary movements — movements dictated
by the will and not necessarily in response to any external
stimulus. Such movements are called automatic. On
the other hand, movements may be induced in Amoeba by
external stimuli, by a sudden shock, or by coming into
contact with an object suitable for food : such movements
are the result of irritability of the protoplasm, which is
thus both automatic and irritable — that is, its contractility
may be set in action either by internal or by external
stimuli.
Under certain circumstances an Amoeba temporarily loses
its power of movement, draws in its pseudopods, and
MODE OF FEEDING u
becomes a globular mass around which is formed a thick,
shell-like coat, called the cyst or cell-wall (Fig. i, D, cy).
The composition of this is not known ; it is certainly not
protoplasmic, and very probably consists of some nitrogenous
substance allied in composition to horn and to the chitin
which forms the external shell of Crustacea, insects, &c.
After remaining in this encysted condition for a time, the
Amoeba escapes by the rupture of its cell-wall, and resumes
its active life.
Very often an Amoeba in the course of its wanderings
comes in contact with a still smaller organism, such as a
diatom (see Lesson XIV., Fig. 36) or a small infusor (see
Lessons X. — XII.). When this happens the Amoeba may
be seen to send out pseudopods which gradually creep
round the prey, and finally unite on the far side of it, as in
Fig. i, c, a. The diatom or other organism becomes in this
way completely enclosed in a cavity or food-vacuole (f.
vac), which also contains a small quantity of water neces-
sarily included with the prey. The latter is taken in by the
Amoeba as food : so that another function performed by the
animalcule is the reception of food, the first step in the
process of nutrition. It is to be noted that the reception
of food takes place in a particular way, viz. by ingestion—
i.e. it is enclosed raw and entire in the living protoplasm. It
has been noticed that Amoeba usually ingests at its hinder
end — that is, the end directed backwards in progression.
Having thus ingested its prey, the Amoeba continues its
course, when, if carefully watched, the swallowed organism
will be found to undergo certain changes. Its protoplasm
is slowly dissolved ; if it contains chlorophyll — the green
colouring matter of plants— this is gradually turned to brown ;
and finally nothing is left but the case or cell-wall in which
many minute organisms, such as diatoms, are enclosed.
12 AMCEBA
Finally, the Amoeba as it creeps slowly on leaves this empty
cell-wall behind, and thus gets rid of what it has no further
use for. It is thus able to ingest living organisms as food ;
to dissolve or digest their protoplasm ; and to egest or
get rid of any insoluble materials they may contain. Note
that all this is done without either ingestion aperture (mouth),
digestion cavity (stomach), or egestion aperture (anus) ; the
food is simply taken in by the flowing round it of pseudopods,
digested as it lies enclosed in the protoplasm, and got rid of
by the Amoeba flowing away from it.
It has just been said that the protoplasm of the prey is
dissolved or digested : we must now consider more particu-
larly what this means.
The stomachs of the higher animals — ourselves, for
instance — produce in their interior a fluid called gastric
juice. When this fluid is brought into contact with albumen
or any other proteid a remarkable change takes place. The
proteid is dissolved and at the same time rendered diffusible,
so as to be capable, like a solution of salt or sugar, of passing
through an organic membrane (see p. 6). The diffusible
proteids thus formed by the action of gastric juice upon
ordinary proteids are called peptones : the transformation
is effected through the agency of a constituent of the gastric
juice called pepsin.
There can be little doubt that the protoplasm ot Amoeba
is able to convert that of its prey into a soluble and diffusible
form, possibly by the agency of some substance analogous
to pepsin, and that the dissolved matters diffuse through the
body of the Amoeba until the latter is, as it were, soaked
through and through with them. Under these circumstances
the Amoeba may be compared to a sponge which is allowed
to absorb water, the sponge itself representing the living
protoplasm, the water the solution of proteids which per-
GROWTH 13
meates it. It has been proved by experiment that proteids
are the only class of food which Amoeba can make use of :
it is unable to digest either starch or fat — two very important
constituents of the food of the higher animals. Mineral
matters must, however, be taken with the food in the form
of a weak watery solution, since the water in which the
animalcule lives is never absolutely pure.
The Amoeba being thus permeated, as it were, with a
nutrient solution, a very important process takes place. The
elements of the solution, hitherto arranged in the form of
peptones, mineral salts, and water, become re-arranged in
such a way as to form new particles of living protoplasm,
which are deposited among the pre-existing particles. In a
word, the food is assimilated or converted into the actual
living substance of the Amoeba.
One effect of this formation of new protoplasm is obvious :
if nothing happens to counteract it, the Amoeba must grow,
the increase in size being brought about in much the same
way as that of a heap of stones would be by continually
thrusting new pebbles into the interior of the heap. This
mode of growth — by the interposition of new particles among
old ones — is called growth by intussusception, and is very
characteristic of the growth of protoplasm. It is neces-
sary to distinguish it, because there is another mode of
growth which is characteristic of minerals and occurs also
in some organized structures. A crystal of alum, for
instance, suspended in a strong solution of the same
substance grows, but the increase is due to the deposition
of successive layers on to the surface of the original crystal,
in much the same way as a candle might be made to grow
by repeatedly dipping it into melted grease. This can be
proved by colouring the crystal with logwood or some other
dye before suspending it, when a gradually-increasing colour-
i4 AMOEBA
less layer will be deposited round the coloured crystal : if
growth took place by intussusception we should have a
gradual weakening of the tint as the crystal increased in size.
This mode of growth — by the deposition of successive layers
-is called growth by accretion.
It is probable that the cyst of Amoeba referred to above
(p. n) grows by accretion. Judging from the analogy of
other organisms it would seem that, after rounding itself off,
the surface of the sphere of protoplasm undergoes a
chemical change resulting in the formation of a thin super-
ficial layer of non-protoplasmic substance. The process is
repeated, new layers being continually deposited within the
old ones until the cell-wall attains its full thickness. The
cyst is therefore a substance separated or secreted from
the protoplasm ; it is the first instance we have met with of
a product of secretion.
From the fact that Amoeba rarely attains a greater dia-
meter than \ mm., it follows that something must happen to
counteract the constant tendency to grow, which is one of
the results of assimilation. We all know what happens in
our own case : if we take a certain amount of exercise —
walk ten miles or lift a series of heavy weights — we undergo
a loss of substance manifested by a diminution in weight
and by the sensation of hunger. Our bodies have done a
certain amount of work, and have undergone a proportional
amount of waste, just as a fire every time it blazes up
consumes a certain weight of coal.
Precisely the same thing happens on a small scale with
Amoeba. Every time it thrusts out or withdraws a pseudo-
pod, every time it contracts its vacuole it does a certain
amount of work — moves a definite weight of protoplasm
through a given space. And every movement, however
slight, is accompanied by a proportional waste of substance,
POTENTIAL OF KINETIC ENERGY 15
a certain fraction of the protoplasm becoming oxidized, or
in other words undergoing a process of low temperature
combustion.
When we say that any combustible body is burnt what we
usually mean is that it has combined with oxygen, forming
certain products of combustion due to the chemical union
of the oxygen with the substance burnt. For instance, when
carbon is burnt the product of combustion is carbon dioxide
or carbonic acid (C + O2 = CO2) : when hydrogen is burnt
water (H9 + O = H2O). The products of the slow com-
bustion which our own bodies are constantly undergoing
are these same two bodies ; carbon dioxide given off mainly
in the air breathed out ; and water given off mainly in the
form of perspiration and urine ; together with two com-
pounds containing nitrogen, urea (CH4N2O) and uric acid
(C5H4N4O3), both occurring mainly in the urine. In some
animals urea and uric acid are replaced by other com-
pounds such as guanin (C5H5N5O), but it may be taken as
proved that in all living things the products of combustion
are carbon dioxide, water, and some nitrogenous substance
of simpler constitution than proteids, and allied to the three
just mentioned.
With this breaking down of proteids the vital activities of
all organisms are invariably connected. Just as useful
mechanical work may be done by the fall of a weight from
a given height to the level of the ground, so the work done
by the organism is a result of its complex proteids falling,
so to speak, to the level of simpler substances. In both
instances potential energy or energy of position is converted
into kinetic or actual energy.
In the particular case under consideration we have to rely
upon analogy and not upon direct experiment. We may,
however, be quite sure that the products of combustion
16 AMCEBA
or waste matters of Amoeba include carbon dioxide, water
and some comparatively simple (as compared with proteids)
compound of nitrogen.
These waste matters or excretory products are given
off partly from the general surface of the body, but partly,
it would seem, through the agency of the contractile vacuole.
It appears that the water taken in with the food, together in
all probability with some of that formed by oxidation of
the protoplasm, makes its way to the vacuole, and is ex-
pelled by its contraction. We have here another function
performed by Amoeba, that of excretion, or the getting rid
of waste matters.
In this connection the reader must be warned against a
possible misunderstanding arising from the fact that the
word excretion is often used in two senses. We often hear,
for instance, of solid and liquid "excreta." In Amoeba
the solid excreta, or more correctly faces, consist of such
things as the indigestible cell-walls, starch-grains, &c., of the
organisms upon which it feeds ; but the rejection of these
is no more a process of excretion than the spitting out of
a cherry-stone, since they are simply parts of the food
which have never been assimilated — never formed part and
parcel of the organism. True excreta, on the other hand,
are invariably products of the waste or combustion of
protoplasm.
The statement just made that the protoplasm of Amoeba
constantly undergoes oxidation presupposes a constant sup-
ply of oxygen. The water in which the animalcule lives
invariably contains that gas in solution : on the other hand,
as we have seen, the protoplasm is continually forming
carbon dioxide. Now when two gases are separated from
one another by a porous partition, an interchange takes place
between them, each diffusing into the space occupied by the
EVOLUTION OF HEAT 17
other. The same process of gaseous diffusion is continually
going on between the carbon dioxide in the interior of
Amoeba and the oxygen in the surrounding water, the proto-
plasm acting as the porous partition. In this way the carbon
dioxide is got rid of, and at the same time a supply of
oxygen is obtained for further combustion.
The taking in of oxygen might be looked upon as a kind
of feeding process, the food being gaseous instead of solid
or liquid, just as we might speak of "feeding" a fire both
with coals and with air. Moreover, as we have seen, the
giving out of carbon dioxide is a process of excretion. It
is, however, usual and convenient to speak of this process
of exchange of gases as respiration or breathing, which
is therefore another function performed by the protoplasm of
Amceba.
The oxidation of protoplasm in the body of an organism,
like the combustion of wood or coal in a fire, is accompanied
by an evolution of heat. That this occurs in Amoeba can-
not be doubted although it has never been proved. The
heat thus generated is, however, constantly being lost to the
surrounding water, so that the temperature of Amoeba, if we
could but measure it, would probably be found, like that of
a frog or a fish, to be very little if at all above that of the
medium in which it lives.
We thus see that a very elaborate series of chemical pro-
cesses is constantly going on in the interior of Amoeba.
These processes are divisible into two sets : those which
begin with the digestion of food and end with the manufac-
ture of living protoplasm, and those which have to do with
the destruction of protoplasm and end with excretion.
The whole series of processes are spoken of collectively
as metabolism. We have, first of all, digested food diffused
through the protoplasm and finally converted into fresh
c
i8 AMCEBA
living protoplasm : these are processes of constructive meta-
bolism or anabolism. Next we have the protoplasm gradually
breaking down and undergoing conversion into excretory
products : this is the process of destructive metabolism or
katabolism. There can be little doubt that both are pro-
cesses of extreme complexity : it seems probable that
after the food is once dissolved there ensues the successive
formation of numerous bodies of gradually increasing
complexity (anabolic mesostates or anastates\ culminating
in protoplasm ; and that the protoplasm, when once formed,
is decomposed into a series of substances of gradually
diminishing complexity (katabolic mesostates or katastates\
the end of the series being formed by the comparatively
simple products of excretion. The granules in the endosarc
are probably to be looked upon as various mesostates im-
bedded in the protoplasm proper.
Living protoplasm is thus the most unstable of substances ;
it is never precisely the same thing for two consecutive
seconds: it "decomposes but to recompose," and recom-
poses but to decompose : its existence, like that of a water-
fall or a fountain, depends upon the constant flow of matter
into it and away from it.
It follows from what has been said that if the income of
an Amoeba, i.e., the total weight of substances taken in (food
plus oxygen plus water) is greater than its expenditure or
the total weight of substances given out (faeces plus excreta
proper plus carbon dioxide) the animacule will grow : if
less it will dwindle away : if the two are equal it will
remain of the same weight or in a state of physiological
equilibrium.
We see then that the fundamental condition of existence
of the individual Amoeba is that it should be able to form
new protoplasm out of the food supplied to it. But some-
REPRODUCTION 19
thing more than this is necessary. Amoebae are subject to
all sorts of casualties ; they may be eaten by other organ-
isms or the pool in which they live may be dried up ; in one
way or another they are constantly coming to an end.
From which it follows that if the race of Amoebae is to be pre-
served there must be some provision by which the indi-
viduals composing it are enabled to produce new individuals.
In other words Amoeba must, in addition to its other
functions, perform that of reproduction.
An Amoeba reproduces itself in a very simple way. The
nucleus first divides into two : then the whole organism
elongates, the two nuclei at the same time travelling away
from one another : next a furrow appears across the middle
of the drawn-out body between the nuclei (Fig. i, I ; fig. 2,
C, D) : the furrow deepens until finally the animalcule sepa-
rates into two separate Amoebae (Fig. 2, E) which hence-
forward lead an independent existence.
This, the simplest method of reproduction known, is
called simple fission. Notice how strikingly different it
is from the mode of multiplication with which we are
familiar in the higher animals. A fowl, for instance, multi-
plies by laying eggs at certain intervals, in each of which,
under favourable circumstances and after a definite lapse of
time, a chick is developed : moreover, the parent bird, after
continuing to produce eggs for a longer or shorter time, dies.
An Amoeba, on the other hand, simply divides into two
Amoebae, each exactly like itself, and in doing so ceases to
exist as a distinct individual. Instead of the successive
production of offspring from an ultimately dying parent, we
have the simultaneous production of offspring by the divi-
sion of the parent, which does not die, but becomes simply
merged in its progeny. There can be no better instance of
the fact that reproduction is discontinuous growth.
c 2
20 AMCEBA
From this it seems that an Amoeba, unless suffering a
violent death, is practically immortal, since it divides into
two completely organized individuals, each of which begins
life with half of the entire body of its parent, there being
therefore nothing left of the latter to die. It would appear,
however, judging from the analogy of the Infusoria (see
Lesson X.) that such organisms as Amoeba cannot go on'
multiplying indefinitely by simple fission, and that occasion-
ally two individuals come into contact and undergo complete
fission. A conjugation of this kind has been observed in
Amoeba, but has been more thoroughly studied in other
forms (see Lessons III. and X.). Whether it is a necessary
condition of continued existence in our animalcule or not,
it appears certain that " death has no place as a natural
recurrent phenomenon ': in that organism.
If an Amoeba does happen to be killed and to escape
being eaten it will undergo gradual decomposition, becoming
converted into various simple substances of which carbon
dioxide, water, and ammonia are the chief. (See p. 29.)
In conclusion, a few facts may be mentioned as to the
conditions of life of Amoeba — the circumstances under
which it will live or die, flourish or do otherwise.
In the first place it will only live within certain limits ot
temperature. In moderately warm weather the temperature
to which it is exposed may be taken as about 15° C. If
gradually warmed beyond this point the movements at first
show an increased activity, then become more and more
sluggish, and at about 30°-— 35° C. cease altogether, re-
commencing, however, when the temperature is lowered.
If the heating is continued up to about 40° C. the animalcule
is killed by the coagulation of its protoplasm (see p. 5) : it
is then said to suffer heat-rigor or death-stiffening pro-
CONDITIONS OF LIFE 21
duced by heat. Similarly when cooled below the ordinary
temperature the movements become slower and slower, and
at the freezing point (o° C.) cease entirely. But freezing,
unlike over-heating does not kill the protoplasm, but only
renders it temporarily inert ; on thawing the movements re-
commence. We may, therefore, distinguish an optimum
temperature at which the vital actions are carried on with
the greatest activity ; maximum and minimum tempera-
tures above and below which respectively they cease ; and
an ultra-maximum temperature at which death ensues.
There is no definite ultra-minimum temperature known
in the case of Amoeba.
The quantity of water present in the protoplasm — as water
of organization (see p. 5) — is another matter of importance.
The water in which Amoeba lives, although fresh, always
contains a certain percentage of salts in solution, and the
protoplasm is affected by any alteration in the density of the
surrounding medium : for instance, by replacing it by dis-
tilled water and so reducing the density, or by adding
salt and so increasing it. The addition of common salt
(sodium chloride) to the amount of 2 per cent, causes
Amoeba to withdraw its pseudopods and undergo a certain
amount of shrinkage : it is then said to pass into a con-
dition of dry-rigor. Under these circumstances it may
be restored to its normal condition by adding a sufficient
proportion of water to bring back the fluid to its original
density.
In this connection it is interesting to notice that the dele-
terious effects of an excess of salt are only produced when
the salt is added suddenly. By the very gradual addition of
sodium chloride Amoebae have been brought to live in a 4
per cent, solution, i.e., one twice as strong as would, if added
suddenly, produce dry-rigor.
22 AMCEBA
From what has been said above on the subject of respira-
tion (p. 17) it follows that free oxygen is necessary for the
existence of Amoeba. Light, on the other hand, appears to
be unnecessary, amoeboid movements having been shown to
go on actively in darkness.
LESSON II
H^EMATOCOCCUS
THE rain-water which collects in puddles, open gutters,
&c., is frequently found to have a green colour. This colour
is due to the presence of various organisms — plants or
animals — one of the commonest of which is called Hcema-
tococcus (or sometimes Protococcus or Sphcerella) pluvialis.
Like Amoeba, Hsematococcus is so small as to require a
high power for its examination. Magnified three or four
hundred diameters it has the appearance (Fig. 3, A) of an
ovoidal body, somewhat pointed at one end, and of a bright
green colour, more or less necked with equally bright red.
Like Amoeba, also, it is in constant movement, but the
character of the movement is very different in the two cases.
An active Haematococcus is seen to swim about the field of
the microscope in all directions and with considerable appa-
rent rapidity. We say apparent rapidity because the rate
of progression is magnified to the same extent as the organ-
ism itself, and what appears a racing speed under the micro-
scope is actually a very slow crawl when divided by 300.
It has been found that such organisms as Haematococcus
travel at the rate of one foot in from a quarter of an hour
to an hour : or, to express the fact in another and fairer way,
H^MATOCOCCUS
that they travel a distance equal to 2^ times their own
diameter in one second. In swimming the pointed end is
always directed forwards and the forward movement is ac-
i
200 771-772
FIG. 3. — A. Hcematococcus pluvialis, motile phase. Living speci-
men, showing protoplasm with chromatophores (chr) and pyrenoids
(//r), cell-wall (c.w) connected to cell-body by protoplasmic filaments,
and flagellay?. The scale to the left applies to Figs. A — D.
B. Resting stage of the same, showing nucleus (mi) with nucleolus
(mi'}, and thick cell-wall (c.w) in contact with protoplasm.
c. The same, showing division of the cell-body in the resting stage
into four daughter-cells.
D. The same, showing the development of flagella and detached cell-
wall by the daughter-cells before their liberation from the inclosing
mother-cell-wall.
E. Hcematococciis lacustris, showing nucleus (mi), single large
pyrenoid (//>'), and contractile vacuole (c.vac. ).
F. Diagram illustrating the movement of a flagellum : ab, its base ;
c, c', c", different positions assumed by its apex. (E, after Biitschli. )
FLAGELLA 25
companied by a rotation of the organism upon its longer
axis.
Careful watching shows that the outline of a swimming
Haematococcus does not change, so that there is evidently
no protrusion of the pseudopods, and at first the cause of
the movement appears rather mysterious. Sooner or later,
however, the little creature is sure to come to rest, and there
can then be seen projecting from the pointed end two exces-
sively delicate colourless threads (Fig. 3, A, fl\ each about
half as long again as the animalcule itself : these are called
flagella or sometimes cilia.^ In a Haematococcus which
has come to rest these can often be seen gently waving
from side to side : when this slow movement is exchanged
for a rapid one the whole organism is propelled through
the water, the flagella acting like a pair of extremely fine
and flexible fins or paddles. Thus the movement of
Hsematococcus is not amoeboid, i.e., produced by the pro-
trusion and withdrawl of pseudopods, but is ciliary, i.e.,
due to the rapid vibration of cilia or flagella.
The flagella are still more clearly seen by adding a drop
of iodine solution to the water : this immediately kills and
stains the organism, and the flagella are seen to take on a
distinct yellow tint. By this and other tests it is shown that
Haematococcus, like Amoeba, consists of protoplasm, and
that the flagella are simply filamentous processes of the
protoplasm.
It was mentioned above that in swimming the pointed end
1 The word cilium is sometimes used as a general term to include
any delicate vibratile process of protoplasm : often, however, it is used
in a restricted sense for a rhythmically vibrating thread, of which each
cell bears a considerable number (see Fig. 8, E, and Fig. 21) ; a flagel-
lum is a cilium having a whip-lash-like movement, and of which each
cell bears only a limited number — one or two, or occasionally as many
as four.
26 H^MATOCOCCUS
with the flagella went first ; this may therefore be distin-
guished as the anterior extremity, the opposite or blunt
end being posterior. So that, as compared with Amoeba,
Haematococcus exhibits a differentiation of structure : an
anterior and a posterior end can be distinguished, and a
part of the protoplasm is differentiated set apart as
flagella.
The green colour of the body is due to the presence of a
special pigment called chlorophyll, the substance to which
the colour of leaves is due. That this is something quite
distinct from the protoplasm may be seen by treatment with
alcohol, which simply kills and coagulates the protoplasm,
but completely dissolves out the chlorophyll producing a
clear green solution. The solution, although green by trans-
mitted light, is red under a strong reflected light, and is
hence fluorescent : when examined through the spectro-
scope it has the effect of absorbing the whole of the blue
and violet end of the spectrum as well as a part of the red.
The red colour which occurs in so many individuals, some-
times almost replacing the green, is due to a colouring-
matter closely allied in its properties to chlorophyll and
called h&matochrome.
At first sight the chlorophyll appears to be evenly distri-
buted over the whole body, but accurate examination under
a high power shows it to be lodged in a variable number
of irregular structures called chromatophores (Fig. 3, A,
chr.\ which together form a layer immediately beneath the
surface. Each chromatophore consists of a protoplasmic
substance impregnated with chlorophyll.
After solution of the chlorophyll with alcohol a nucleus
(B, nu.) can be made out ; like the nucleus of Amoeba it is
stained by iodine, magenta, &c. Other bodies which might
easily be mistaken for nuclei are also visible in the living
CELL-WALL 27
organism. These are small ovoidal structures (A, pyr.\
with clearly defined outlines occurring in varying numbers
in the chromatophores. When treated with iodine they
assume a deep, apparently black, but really dark blue, colour.
The assumption of a blue colour with iodine is the charac-
teristic test of the well-known substance starch, as can be
seen by letting a few drops of a weak solution of tincture or
iodine fall upon some ordinary washing starch. The bodies
in question have been found to consist of a proteid substance
covered with a layer of starch, and are called pyrenoids.
Starch itself is a definite chemical compound belonging
to the group of carbo-hydrates, i.e., bodies containing the
elements carbon, hydrogen, and oxygen : its formula is
CG Hio °5-
In Hcematococcus pluvialis there is no contractile vacuole,
but in another species, H. lacustris, this structure is pre-
sent as a minute space near the anterior or pointed end
(Fig. 3, E, c. vac.}.
There is still another characteristic structure to which no
reference has yet been made. This appears at the first view
something like a delicate haze around the green body, but
by careful focussing is seen to be really an extremely thin
globular shell (A, c.w.) composed ot some colourless trans-
parent material and separated by a space containing water
from the body, to which it is connected by very delicate
radiating strands of protoplasm. It is perforated by two
extremely minute apertures for the passage of the flagella.
Obviously we may consider this shell as a cyst or cell-
wall differing from that of an encysted Amoeba (Fig. i, D) in
not being in close contact with the protoplasm.
A more important difference, however, lies in its chemical
composition. The cyst or cell-wall of Amoeba, as stated in
the preceding lesson (p. n) is very probably nitrogenous :
28 H^MATOCOCCUS
that of Haematococcus, on the other hand, is formed of a
carbohydrate called cellulose, allied in composition to
starch, sugar, and gum, and having the formula C6H10O5.
Many vegetable substances, such as cotton, consist of
cellulose, and wood is a modification of the same com-
pound. Cellulose is stained yellow by iodine, but iodine
and sulphuric acid together turn it blue, and a similar
colour is produced by a solution of iodine and potassium
iodide in zinc chloride known as Schulze's solution. These
tests are quite easily applied to Haematococcus : the proto-
plasm stains a deep yellowish-brown, around which is seen
a sort of blue cloud due to the stained and partly-dissolved
cell-wall.
It has been stated that in stagnant water in which it has
been cultivated for a length of time Haematococcus some-
times assumes an amoeboid form. In any case, after leading
an active existence for a longer or shorter time it comes to
rest, loses its flagella, and throws around itself a thick cell-
wall of cellulose (Fig. 3, B), thus becoming encysted. So
that, as in Amoeba, there is an alternation of an active
or motile with a stationary or resting condition.
In the matter of nutrition the differences between Haema-
tococcus and Amoeba are very marked and indeed funda-
mental. As we have seen, Hsematococcus has no pseudopods,
and therefore cannot take in solid food after the manner
of Amoeba : moreover, even in its active condition it is
usually surrounded by an imperforate cell-wall, which of
course quite precludes the possibility of ingestion. As a
matter of observation, also, however long it is watched it is
never seen to feed in the ordinary sense of the word.
Nevertheless, it must take in food in some way or other, or
the oxidation of its protoplasm would soon bring it to an end.
DECOMPOSITION OF CARBON DIOXIDE 29
Haematococcus lives in rain-water. This is never pure
water, but always contains certain mineral salts, especially
nitrates, ammonia salts, and often sodium chloride or common
table salt in solution. These salts, being crystalloids, can
and do diffuse into the water of organization of the ani-
malcule, so that we may consider its protoplasm to be con-
stantly permeated by a very weak saline solution, the most
important elements contained in which are oxygen, hydro-
gen, nitrogen, potassium, sodium, calcium, sulphur, and
phosphorus.
If water containing a large quantity of Haematococcus
is exposed to sunlight, minute bubbles are found to appear
in it, and these bubbles, if connected and properly tested,
are found to consist largely of oxygen. Accurate chemical
analysis has shown that this oxygen is produced by the de-
composition of the carbon dioxide contained in solution in
rain-water, and indeed in all water exposed to the air, the
gas, which is always present in small quantities in the
atmosphere, being very soluble in water.
As the carbon dioxide is decomposed in this way, its
oxygen being given off, it is evident that its carbon must be
retained. As a matter of fact, it is retained by the organism
but not in the form of carbon : in all probability a double
decomposition takes place between the carbon dioxide ab-
sorbed and the water of organization, the result being the
liberation of oxygen in the form of gas and the simultaneous
production of some extremely simple form of carbohydrate,
/.<?., 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, pro-
30 H/EMATOCOCCUS
bably belonging to the class of amides, one of the best
known of which — asparagin— has the formula C4H8N2Or
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 anastatis (see p. 18).
But it must be noted that the steps in this process of con-
structive metabolism are widely different in the two cases.
In Amoeba we start with living protoplasm — that of the prey
-which is killed and broken up into diffusible proteids,
these being afterwards re-combined to form new molecules
of the living protoplasm of Amoeba. So that the food of
Amoeba is, to begin with, as complex as itself, and is first
broken down by digestion into simpler compounds, these
being afterwards re-combined into more complex ones. In
Haematococcus, on the other hand, we start with extremely
simple compounds, such as carbon dioxide, water, nitrates,
sulphates, &c. Nothing which can be properly called diges-
tion, i.e., a breaking up and dissolving of the food, takes
place, but its various constituents are combined into sub-
stances of gradually-increasing complexity, protoplasm, as
before, being the final result.
To express the matter in another way : Amoeba can only
make protoplasm out of proteids already formed by some
other organism : Haematococcus can form it out of simple
liquid and gaseous inorganic materials.
Speaking generally, it may be said that these two methods
DESTRUCTIVE METABOLISM 31
of nutrition are respectively characteristic of the two great
groups of living things. Animals require solid food con-
taining ready-made proteids, and cannot build up their pro-
toplasm out of simpler compounds. Green plants, i.e., all
the ordinary trees, shrubs, weeds, &c., take only liquid and
gaseous food, and build up their protoplasm out of carbon
dioxide, water, and mineral salts. The first of these methods
of nutrition is conveniently distinguished as holozoic, or
wholly-animal, the second as holophytic or wholly-vegetal.
It is important to note that only those plants or parts of
plants in which chlorophyll is present are capable of holo-
phytic nutrition. Whatever may be the precise way in which
the process is effected, it is certain that the decomposition
of carbon dioxide which characterizes this form of nutrition
is a function of chlorophyll, or to speak more accurately, of
chromatophores, since there is reason for thinking that
it is the protoplasm of these and not the actual green pigment
which is the active agent in the process.
Moreover, it must not be forgotten that the decomposition
of carbon dioxide is carried on only during daylight, so that
organisms in which holophytic nutrition obtains are depend-
ent upon the sun for their very existence. While Amoeba
derives its energy from the breaking down of the proteids in
its food (see p. 12), the food of Hsematococcus is too
simple to serve as a source of energy, and it is only by the
help of sunlight that the work of constructive metabolism
can be carried on. This may be expressed by saying that
Haematococcus, in common with other organisms containing
chlorophyll, is supplied with kinetic energy (in the form of
light or radiant energy) directly by the sun.
As in Amceba destructive metabolism is constantly going
on side by side with constructive. The protoplasm becomes
oxidized, water, carbon dioxide, and nitrogenous waste
32 H^MATOCOCCUS
matters being formed and finally got rid ot. 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 //#.$• 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 nutrition process in Haema-
tococcus is, as a whole, a process of deoxidation. In
Amoeba, on the other hand, the ingesta (food///^ respir-
atory 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 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
CILIARY MOVEMENT 33
probably formed by the conversion of a thin superficial
layer of protoplasm into cellulose, the cyst attaining its final
thickness by frequent repetition of the process (see p. 14).
The starch of the pyrenoids is apparently formed by a similar
process of decomposition or destructive metabolism of pro-
toplasm, growth taking place in both instances by accretion
and not by intussusception.
We see then that destructive metabolism may result in the
formation of (a) waste products and (b) plastic products,
the former being got rid of as of no further use, while
the latter remain an integral part of the organism.
Let us now turn once more to the movements of Haemato-
coccus, and consider in some detail the manner of their
performance.
Each flagellum (Fig. 3, A, 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. i F, abc represents it in
the position of rest, abc' will show the form assumed when
it is deflected to the left, and abc" when the bending is
towards the right. In the position abc the two sides ab, ac
are obviously equal to one another, but in the flexed
positions it is equally obvious that the concave sides ac', be"
are shorter than the convex sides be,' ac" • in other words, as
the flagellum bends to the left side ac becomes shortened,
as it bends to the right the side be.
This may be otherwise expressed by saying that, in bend-
ing to the left the side ac contracts (see p. 10), in bending to
the right the side be, or that the movement is performed
by the alternate contraction of opposite sides of the
flagellum.
D
34 H^MATOCOCCUS
Thus the ciliary movement of Hgematococcus, like the
amoeboid movement of Amoeba, is a phenomenon of con-
tractility. Imagine an Amoeba to draw in all its pseudo-
pods but two, and to protrude these two until they became
mere threads ; imagine further these threads to contract
regularly and rapidly instead of irregularly and slowly ; the
result would be the substitution of pseudopods by flagella,
i.e., of temporary slow-moving processes of protoplasm
by permanent rapidly-moving ones.
To put the matter in another way : in Amoeba the
function of contractility is performed by the whole organism;
in Hsematococcus it is discharged by a small part only, viz.,
the flagella, the rest of the protoplasm being incapable of
movement. We have therefore in Hsematococcus a dif-
ferentiation of structure accompanied by a differentiation of
function or division of physiological labour.
The expression " division of physiological labour " was
invented by the great French physiologist, Henri Milne-
Edwards, to express the fact that a sort of rough correspond-
ence exists between lowly and highly organized animals
and plants on the one hand, and lowly and highly organized
human societies on the other. In primitive communities
there is little or no division of labour : every man is his
own butcher, baker, soldier, doctor, &c., there is no distinc-
tion between "classes" and "masses," and each individual
is to a great extent independent of all the rest. Whereas in
complex civilized communities society is differentiated into
politicians, soldiers, professional men, mechanics, labourers,
and so on, each class being to a great extent dependent on
every other. This comparison of an advanced society with
a high organism is at least as old as ^Esop, who gives
expression to it in the well-known fable of " the Belly and
Members. ':
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.
Hsematococcus multiplies only in the resting condition
(p. 28, and Fig 3, B) ; like Amoeba its protoplasm undergoes
binary fission, but with the peculiarity that the process is
immediately repeated, so that four daughter- cells are pro-
duced within the single mother-cell-wall (Fig. 3 c). By the
rupture of the latter the daughter-cells are set free as the
ordinary motile form ; sometimes they acquire their flagella
and detached cell-wall before making their escape (D).
Under certain circumstances the resting form divides into
eight instead of four daughter-cells, and these when liberated
are found to be smaller than the ordinary motile form, and to
have no cell-wall. Haematococcus is therefore dimorphic,
t.e.t 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
HETEROM'ITA
*
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 Ti^ mm. (u^Vo" mcn)
in average length. It has a certain resemblance in general
form to Haematococcus, being somewhat ovoidal and pointed
at one end. Like Haematococcus also it has two flagella,
but only one of these (fl. i) proceeds from its beak-like
anterior end and is directed forwards as the creature swims ;
the other (fl. 2) springs a short distance from the beak, and
in the ordinary swimming position is trailed after the
organism as in A2 and F4. Thus in Heteromita, besides an
anterior and a posterior end, we may distinguish a ventral
GENERAL CHARACTERS 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 A1. 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 A1 shows the monad with coiled flagellum, A2 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 (mt),
and at the anterior end a contractile vacuole (c. vac}. There
is no trace of an investing membrane or cell-wall, and the
protoplasm is colourless. Also, as is invariably the case
with organisms devoid of chlorophyll, there is no starch.
In considering the nutrition of Heteromita it is necessary,
first of all, to take into consideration the precise nature of
its surroundings. It lives, as already stated, in decomposing
infusions of animal matter. Such infusions contain proteids
in solution, in part split up by the process of decomposition
into simpler compounds some of which are diffusible ; this
process is due, as we shall see hereafter (Lesson VII I. ), 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
E
FIG. 4. — Heteromita rostrata.
A1, the living organism, showing nucleus (««)» contractile vacuole
NUTRITION 39
(c. vac\ anterior flagellum (fl. i), and coiled ventral flagellum (Jl. 2)
by which the organism is anchored ; A2 shows the position at the
forward limit of the spring, the ventral flagellum being fully extended.
B' — B3, three stages in the longitudinal fission of the anchored form.
c' — c3. Three stages in the transverse fission of the same : Jl. i1,
rudiment of newly formed anterior flagellum.
D1 — D3, three stages in the fission of the free-swimming form : Jl. 21,
rudiment of the newly-formed ventral flagella.
E', free-swimming and anchored forms about to conjugate : E-, com-
mencement of conjugation : E3, E4, two stages in the development of
the zygote : E5, the fully formed zygote : E6, dehiscence of the zygote
and emission of spores.
F1 — F4, 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. 90), 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
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 only takes place 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, B1 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
B2 the process has gone further, and in B3 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 c1 — c:!, and is
seen to differ from that drsuil>ed in the preceding paragraph in the cir-
MULTIPLICATION BY FISSION 41
cumstance that the anterior flagellum of the parent form is unaffected, and
becomes without alteration the anterior flagellum of one of the daughter-
forms — that to the right in the figures. The anterior flagellum of the
other product of division— that to the left — is a new structure formed as
an outgrowth from the body : its commencement is shown in c1,^?. i'.
These two modes of fission — longitudinal and transverse — both occur
in the anchored form of Heteromita, i.e., in individuals attached by
the ventral flagellum. The free-swimming form presents a third
variety of the process. It comes to rest, loses its regular outline (D1)
becoming almost amceboicl in form and finally (D2) globular. Division
then takes place : the flagella of the parent become each the anterior
flagellum of one of the daughter cells (compare D1, D2, and D3), while
their ventral flagella are formed by the splitting of a little outgrowth of
the dividing body (D2, Jl. 2').
As in Amoeba fission is invariably preceded by division
of the nucleus.
But in Heteromita fission is not the only mode of repro-
duction. Under certain circumstances a free-swimming form
approaches an anchored form, and applies itself to it in such
a way that the posterior ends of the two are in contact (E1).
The two individuals then fuse with one another as completely
as two drops of gum on a plate unite when brought into
contact. Fusion of the nuclei also takes place, and there is
formed an irregular body (E2) with a single nucleus and
with two flagella at each end. This swims about freely, and
as it does so the last trace of distinction between the two
monads of which it is formed is lost, and a triangular form
is assumed (ES), the two pairs of cilia being situated at two
of the angles. Still later the protoplasm of this triangular
body loses all trace of nucleus, granules, &c., and becomes
perfectly clear (E4) : then it comes to rest and loses its
flagella, appearing as a clear, homogeneous, three-cornered
sac with slightly convex sides (E5). This body, formed by
the conjugation of the two monads, is called a zygote, the
two conjugating individuals being distinguished as g
42 HETEROMITA
The zygote remains quiescent for some time, and then after
undergoing wave-like movements of its surface, bursts at its
three angles (EG), its contents escaping in the form of granules
called spores, so minute as to be barely visible even under
the highest powers of the best modern microscopes. They
are formed by the protoplasm of the zygote dividing into an
immense number of separate masses, a process known as
multiple fission.
Carefully watched, these almost ultra-microscopic particles
(p1) are found to grow into clear visibility and to take on a
distinctly oval shape (p2). Still increasing in size they
develop a ventral flagellum (r3) which is at first quite
quiescent : finally, the pointed end sends out a process which
becomes an anterior flagellum (p4). The spore has now
become a Heteromita resembling the parent form in all but
size.
It will be seen that this remarkable mode of multiplication
by conjugation differs from multiplication by fission in the
fact that it requires the co-operation of two individuals which
undergo complete fusion. As we shall see more plainly
later on (Lessons XV. and XVI.) conjugation is the simplest
case of sexual reproduction , differing from the sexual repro-
duction of the higher organisms in that the two conjugating
bodies or gametes are each an entire individual, and in the
further circumstance that the gametes resemble one another
in form and size, so that there is no distinction of sex,1 but
each takes an equal and similar share in the production of
the zygote. Binary fission, on the other hand, is an example
of asexual reproduction.
It iiii^ht perhaps be allowable to consider the active, free-
\\iinuiing monad which seeks and attaches itself to the anchored form
as a male, and the passive anchored form as a female gamete
(see Lesson XII).
LIFE HISTORY
43
Notice also another important fact. The spores when
first emitted from the ruptured zygote are mere granules of
protoplasm, approaching as nearly as anything in nature to
the mathematical definition of a point, " without parts and
without magnitude." And during its growth a spore increases
not only in size but also in complexity, in other words
undergoes a progressive differentiation or development.
This is an instance of the principle known as Von Baer's
law, according to which " development is a progress from
the simple to the complex, from the general to the particular,
from the homogeneous to the heterogeneous." In Heteromita,
then, we have our first instance of development, since in
simple fission there is no development, each product of
division being from the first similar to the parent in all but
size.
Lastly, Heteromita is the first instance we have had of
an organism with a definite life-history. It multiplies
asexually by simple fission producing free-swimming and
anchored forms : these conjugate in pairs forming a zygote,
in which, by multiple fission, numerous spores are formed :
the spores develop into the adult form, asexual multiplica-
tion begins once more, and so the cycle of existence is
completed.
It must be borne in mind that further researches may
reveal the occurrence of a true sexual process in Amoeba
and Haematococcus.
LESSON IV
EUGLENA
THE] rain-water collected in puddles by the road-side, on
roofs, &c., is often found to have a bright green colour :
this is sometimes due to the presence of delicate water
weeds visible to the naked eye (Lesson XVI.), but frequently
the water when held up to the light in a glass vessel appears
uniformly green, no suspended matter being visible to the
unaided sight. Under these circumstances the green colour
is usually due to the presence of vast numbers of an organism
known as Englena viridis.
Although microscopic, Euglena is considerably larger than
either Hsematococcus or Heteromita, its length varying from
-o\- mm. to \ mm. The body is spindle-shaped, wide in the
middle and narrow at both ends (Fig. 5, A — E) : one
extremity is blunter than the other, and from it proceeds
a single long flagellum (fl} by the action of which the
organism swims with great rapidity, the flagellum being
as in Haematococcus, directed forwards. Besides its rapid
swimming movements Euglena frequently performs slow
movements of contraction and expansion, something like
those of a short worm, the body becoming broadened out
In si at l he anterior end, then in the middle, then at the
GENERAL CHARACTERS
45
posterior end, twisting to the right and left, and so on (Fig.
5, A — D). These movements are so characteristic of the
germs that the name euglenoid is applied to them.
B
c.vac
cvuc
FIG. 5. — Euglena viridis.
A — D, four views of the living organism, showing the changes of form
produced by the characteristic euglenoid movements.
E, enlarged view, showing the nucleus (mi), reservoir of the con-
tractile vacuole (c. vac), with adjacent pigment spot, and gullet with a
single flagellum springing from it.
F, enlarged view of the anterior end of E, showing pigment-spot
(pg) and reservoir (c. vac], mouth (ni), and gullet (a>s), and origin of
flagellum (/).
G, resting form after binary fission, showing cyst or cell-wall (cy),
and the nuclei (nu) and reservoirs (c. vac] of the daughter-cells.
H, active form showing contractile vacuole (c. vac], reservoir ( r],
and paramylum-bodies (/>).
(A — G, after Saville Kent : H, from Biitschli after Klebs. )
The body consists of protoplasm covered with a very
delicate skin or cuticle which is often finely striated, and
is to be looked upon as a superficial hardening of the
protoplasm. The green colour is due to the presence of
46 EUGLENA
chlorophyll which tinges all the central part of the body,
the two ends being colourless. It is difficult to make out
whether the chlorophyll is lodged in one chromatophore or
in several.
In Haematococcus we saw that chlorophyll was asso-
ciated with starch (p. 27). In Euglena there are, near the
middle of the body, a number of grains of paramylum
(H, p) a carbohydrate of the same composition as starch
(CfiH10O5), but differing from it in remaining uncoloured
by iodine.
Water containing Euglena gives off bubbles of oxygen in
sunlight : as in Hsematococcus the carbon dioxide in solution
in the water is decomposed in the presence of chlorophyll,
its oxygen evolved, and its carbon combined with the
elements of water and used in nutrition. For a long time
Euglena was thought to be nourished entirely in this way,
but there is a good deal of reason for thinking that this is
not the case.
When the anterior end of a Euglena is very highly
magnified it is found to have the form shown in Fig. 5, F.
It is produced into a blunt snout-like extremity at the base
of which is a conical depression (ees) leading into the soft
internal protoplasm : — just the sort of depression one could
make in a clay model of Euglena by thrusting one's finger or
the end of a pencil into the clay. From the bottom of this
tube the flagellum arises, and by its continual movement
gives rise to a sort of whirlpool in the neighbourhood. By
the current thus produced minute, solid food-particles are
swept down the tube and forced into the soft interna
protoplasm, where they doubtless become digested in the
same way as the substances ingested by an Amoeba. That
solid particles are so ingested by Euglena has been proved
by diffusing finely produced carmine in the water, when the
MOUTH AND GULLET 47
coloured particles were seen to be swallowed in the way
described.
The depression in question is therefore a gullet, and its
external aperture or margin (;;/) is a mouth. Euglena,
like Amoeba, takes in solid food, but instead of ingesting it
at almost any part of the body, it can only do so at one
particular point where there is a special ingestive aperture
or mouth. This is clearly a case of specialization or
differentiation of structure : in virtue of the possession of a
mouth and gullet Euglena is more highly organized than
Amoeba.
It thus appears that in Euglena nutrition is both holozoic
and holophytic : very probably it is mainly holophytic during
daylight and holozoic in darkness.
Near the centre of the body or somewhat towards the
posterior end is a nucleus (E, nu) with a well-marked
nucleolus, and at the anterior end is a clear space (c. vac)
looking very like a contractile vacuole. It has been shown,
however, that this space is in reality a non-contractile cavity
or reservoir (H, r) into which the true contractile vacuole
(c.vac) opens, and which itself discharges into the gullet.
In close relation with the reservoir is found a little bright
red speck (pg) called the pigment spot or stigma. It con-
sists of hgematochrome (see p. 26) and is curiously like an
eye in appearance, so much so that it is sometimes known
as the eye-spot. There seems, however, to be no reason for
assigning a visual function to it : indeed it has been shown
that the greatest sensitiveness to light is manifested by the
colourless anterior end of the body.
As in Haematococcus a resting condition alternates with
the motile phase : the organism loses its flagellum and
48 EUGLENA
surrounds itself with a cyst of cellulose (Fig. 5, G, cy) from
which, after a period of rest, it emerges to resume active
life.
Reproduction takes place by simple fission of the resting
form, the plane of division being always longitudinal (G).
Sometimes each product of division or daughter-cell divides
again : finally, the two or four, or sometimes even eight
daughter-cells emerge from the cyst as active Euglenae,
Recently a process of multiple fission (p. 42) has been
described, numerous minute active spores being produced
which gradually assume the ordinary form and size.
LESSON V
PROTOMYXA AND THE MYCETOZOA
WHEN Professor Haeckel was investigating the zoology of
the Canary Islands more than twenty years ago he discovered
a very remarkable organism which he named Protomyxa
aurantiaca. It was found in sea-water attached to a shell
called Spirilla, and was at once noticeable from the bright
orange colour which suggested its specific name.
In its fully developed stage Protomyxa is the largest of all
the organisms we have yet studied, being fully imm. (^- inch)
in diameter, and therefore visible to the naked eye as a
small orange speck. In general appearance (Fig. 6, A) it is
not unlike an immense Amoeba, the chief difference lying
in the fact that the pseudopods (psd) instead of being short,
blunt processes, few in number (comp. Fig. i, p. 2) are very
numerous, slender, branching threads which often unite with
one another so as to form networks. No nucleus was ob-
served x and no contractile vacuole, but it is quite possible
that a renewed examination might prove the presence of one
or both of these structures.
The figure (A) is enough to show that nutrition is holozoir
1 See p, 9, note,
£
psd
FIG. 6. — Protomyxa aurantiaca.
A, the living organism (plasmodium), showing fine branched pseudo-
pods (pscf) and several ingested organisms.
]?, the same, encysted : cy the cell-wall.
c, the protoplasm of the encysted form breaking up into spores,
i), dehiscence of the cyst and emergence of
K3 flagellulec which afterwards become converted into
F, amcubulce.
G, amcebulcC uniting to form a plasmodium. (After liacckel.)
LIFE-HISTORY 51
the specimen has ingested several minute organisms and is
in the act of capturing another.
But the main interest of Protomyxa lies in its very curious
and complicated life-history. After crawling over the Spirula
shells for a longer or shorter time it draws in its pseudopods,
comes to rest, and surrounds itself with a cyst (B, cy). The
composition of the cyst is not known, but it is apparently not
cellulose, since it is not coloured by iodine and sulphuric
acid (p. 28).
Next, the encysted protoplasm undergoes multiple fission,
dividing into a number of spores (c) : soon the cyst bursts
and its contents emerge (D) as bodies which differ utterly in
appearance from the amoeboid form from which we started.
Each spore has in fact become a little ovoid body of an
orange colour, provided with a single flagellum (E, fl] by the
lashing of which it swims through the water after the manner
of a monad.
It is convenient to have a name by which to distinguish these
flagellate bodies, just as we have special names for the young
of the higher animals such as tadpoles or kittens. From the
fact of their distinguishing character being the possession of a
flagellum they are called flagellultE ; the same name will be
applied to the flagellate young of various other organisms
which we shall study hereafter.
After swimming about actively for a time each flagellula
settles down on some convenient substratum and undergoes
a remarkable change : its movements become sluggish, its
outline irregular, and its flagellum short and thick, until it
finally takes on the form of a little Amoeba (F). For this
stage also a name is required : it is not an Amoeba but an
amoeboid phase in the life-history of a totally different
organism : it is called an amotbida.
The process just described may be taken as a practical
52 PROTOMYXA AND THE MYCETOZOA
proof of the statement made in a previous Lesson (p. 34)
that a flagellum is nothing more than a delicate and rela-
tively permanent pseudopod. In Protomyxa we have a
flagellula directly converted into an amoebula, the flagellum
of the former becoming one of the pseudopods of the
latter.
The amoebulse thus formed may simply increase in size
and send out numerous delicate pseudopods, thus becoming
converted into the ordinary Protomyxa-form. Frequently,
however, they attain this form by a very curious process :
they come together in twos and threes until they are in
actual contact with one another, when they undergo complete
and permanent fusion (G). In this case the Protomyxa-form
is produced not by the development of a single amoebula
but by the conjugation or fusion of a variable number of
amoebulae. A body formed in this way by the fusion of
amoebulae is called a plasmodium^ so that in the life-history
of Protomyxa we can distinguish an encysted, a ciliated or
flagellate, an amoeboid, and a plasmodial phase.
The nature of a plasmodium will be made clearer by a
short consideration of the strange group of organisms known
as Mycetozoa or sometimes " slime- fungi." They occur
as gelatinous masses on the bark of trees, on the surface of
tan-pits, and sometimes in water. It must be remembered
that Mycetozoa is the name not of a genus but of -a class
in which are included several genera, such as Badhamia,
Chondrioderma, &c., (see Fig. 7) : a general account of
the class is all that is necessary for our present purpose.
The Mycetozoa consist of sheets or networks of protoplasm
which may be as much as 30 cm. (ift.) in diameter, and
throughout the substance of which are found numerous
nuclei. In this condition they creep about over bark or some
THE PLASMODIUM OF BADHAMIA
53
H
FIG. 7. — A, part of the plasmodium of Badhamia (X 3i) ; £,
short pseudopod enclosing a bit of mushroom stem.
B, spore of Chondrioderma.
c, the same, undergoing dehiscence.
D, flagellulse liberated from spores of the same.
E, amoebula formed by metamorphosis of flagellula.
F, two amoebulae about to fuse : F', the same after complete union.
G, G', two stages in the formation of a three-celled plasmodium.
H, a small plasmodium.
(A, after Lister : B— H, from Sachs after Cienkowski. )
54 PROTOMYXA AND THE MYCETOZOA
other substance : and as they do so ingest solid food
(Fig. 7, A). It has been proved that they digest protoplasm :
and in one genus pepsin — the constituent of our own gastric
juice by which the digestion of proteids is effected (see p. 12)
-has been found to exist. They can also digest starch
which has been swollen by a moderate heat—as in our own
bread and rice-puddings — but are unable to make use of
raw starch.
After living in this free condition, like a gigantic terrestrial
Amoeba, for a longer or shorter time, either a part or the
whole of the protoplasm becomes encysted 1 and breaks up
into spores. These (B) consist of a globular mass of proto-
plasm covered with a wall of cellulose : the cysts are also
formed of cellulose.
By the rupture of the cell-wall of the spore (c) the protoplasm
is liberated as a flagellula (D) provided with a nucleus and a
contractile vacuole, and frequently exhibiting amoeboid as
well as ciliary movements. After a time the flagellulse lose
their cilia and pass into the condition of amcebulae (E),
which finally fuse to form the plasmodium with which
we started (F — H). In the young plasmodia (c1) the
nuclei of the constituent amcebulje are clearly visible, and
from them the nuclei of the fully developed plasmodia are
probably derived. It would seem, therefore, that in the
fusion of amcebulae to form the plasmodium of Mycetozoa the
cell-bodies (protoplasm) only coalesce, not the nuclei.
There is a suggestive analogy between this process of
plasmodium-formation and that of conjugation as seen in
Heteromita. Two Heteromitoe fuse and form a zygote the
1 The process of formation of the cyst or sporangium is a compli-
cated one, and will not be described here. See De Bary, Fungi,
Mycetozoa, and Bacteria (Oxford, 1887).
PLASMODIUM FORMATION AND CONJUGATION 55
protoplasm of which divides into spores. In Protomyxa and
the Mycetozoa not two but several Amcebulae unite to form
a plasmodium which after a time becomes encysted and
breaks up into spores. So that we might look upon the
conjugation of Heteromita as an extremely simple plasmo-
dial phase in its life-history, or upon the formation of a
plasmodium by Protomyxa and the Mycetozoa as a process
of multiple conjugation.
There is, however, an important difference between the
two cases by reason of which the analogy is far from complete.
In Heteromita the nuclei of the two gametes are no longer
visible (p. 41) : they coalesce during conjugation, and
subsequently, in all probability, break up to form the nuclei
of the spores. In the Mycetozoa neither fusion nor apparent
disappearance of the nuclei of the amoebulae has been
observed.
LESSON VI
A COMPARISON OF THE FOREGOING ORGANISMS WITH CER-
TAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS
AND PLANTS
WHEN a drop of the blood of a crayfish, lobster or crab is
examined under a high power, it is found to consist of a
nearly colourless fluid, the plasma, in which float a number
of minute solid bodies, the blood-corpuscles or leucocytes.
Each of these (Fig. 8, A) is a colourless mass of proto-
plasm, reminding one at once of an Amoeba, and if it is
watched carefully the resemblance becomes closer still, for
the corpuscle is seen to put out and withdraw pseudopods
(A1 — A4) and so gradually to alter its form completely.
Moreover the addition of iodine, logwood, or any other
suitable colouring matter reveals the presence of a large
nucleus (A5, A6, mi) \ so that, save for the absence of a con-
tractile vacuole in the leucocyte, the description of Amoeba
in Lesson I. would apply almost equally well to it
The blood of a fish, a frog (B1), a reptile, or a bird contains
quite similar leucocytes, but in addition there are found in
the blood of these red-blooded animals bodies called red
corpuscles. They are flat oval discs of protoplasm (B5, B6)
D
FIG. 8. — Typical Animal and Vegetable Cells.
A1 — A4, living leucocyte (blood corpuscle) of a crayfish showing
amoeboid movements : A5, A6, the same, killed and stained, showing
the nucleus (mi).
B1, leucocyte of the frog, nu the nucleus : B2, two leucocytes
beginning to conjugate : B3, the same after conjugation, a binucleate
plasmodium being formed : B4, a leucocyte undergoing binary fission :
B5, surface view and B6 edge view of a red corpuscle of the same,
nu, the nucleus.
c1, c2, leucocytes of the newt ; in c1 particles of vermilion, repre-
sented by black dots, have been ingested.
C3, surface view and c4 edge view of a red corpuscle of man.
D1, column or epithelium from intestine of frog : D2, a similar cell
58 EPITHELIAL CELLS
showing striated distal border from which in r>3 pseudopods are
protruded.
E1, ciliated epithelial cell from mouth of frog : E'2, E3, similar cells
from windpipe of dog.
F1, parenchyma cell from root of lily, showing nucleus («#)» vacuoles
(vac), and cell-wall : F", a similar cell from leaf of bean, showing
nucleus, vacuoles, cell-wall and chromatophores (chr}.
(B, D1 and E1, after Howes : c, E2, and E3, after Klein and Noble
Smith : D2, D3, after Wiedersheim : F1, after Sachs : F'2, after Behrens. )
coloured by a pigment called h&moglobin, and provided
each with a large nucleus (nu) which, when the corpuscle is
seen from the edge produces a bulging of its central part.
These bodies may be compared to Amoebae which have
drawn in their pseudopods, assumed a flattened form, and
become coloured with haemoglobin.
In the blood of mammals, such as the rabbit, dog, or man,
similar leucocytes occur, but their red blood corpuscles (c3, c4)
have the form of biconcave discs and are devoid of nuclei.
In many animals the leucocytes have been observed to
ingest solid particles (c1), to multiply by simple fission (B4)
and to coalesce with one another forming plasmodia (B2)
(P- 52).
The stomach and intestines of animals are lined with a
sort of soft slimy skin called mucous membrane. If a
bit of the surface of this membrane — in a frog or rabbit for
instance — is snipped off and "teased out," i.e., torn apart
with needles, it is found when examined under a high power
to be made up of an immense number of microscopic bodies
called epithelial cells, which in the living animal, lie
close to one another in the inner layer of mucous mem-
brane in something the same way as the blocks of a wood
pavement lie on the surface of a road. An epithelial cell
(D1, D2) consists of a rod-like mass of protoplasm, contain-
ing a large nucleus, and is therefore comparable to an
PARENCHYMA CELLS 59
elongated Amoeba without pseudopods. In some animals
the resemblance is still closer : the epithelial cells have been
observed to throw out pseudopods from their free surfaces
(DS), that is, from the only part where any such movement is
possible, since they are elsewhere in close contact with their
fellow cells.
The mouth of the frog and the trachea or windpipe of air-
breathing vertebrates such as reptiles, birds, and mammals,
are also lined with mucous membrane, but the epithelial
cells which constitute its inner layer differ in one important
respect from those of the stomach and intestine. If ex-
amined quite fresh each is found to bear on its free surface,
i.e., the surface which bounds the cavity of the mouth or
windpipe, a number of delicate protoplasmic threads or
cilia (E1 — E3) which are in constant vibratory movement. In
the process of teasing out the mucous membrane some of
the cells are pretty sure to become detached, and are then
seen to swim about in the containing fluid by the action
of their cilia. These ciliated epithelial cells remind one
strongly of Heteromita : except that they bear numerous
cilia in constant rhythmical movement instead of two only
— in this case distinguished as flagella — presenting an
irregular lashing movement.
Similar ciliated epithelial cells are found on the gills ot
oyster, mussel, &c., and in many other situations.
The stem or root of an ordinary herbaceous plant, such
as a geranium or sweet-pea, is found when cut across to
consist of a central mass of pith, around which is a circle
of woody substance, and around this again a soft greenish
material called the cortex. A thin section shows the latter
to be made up of innumerable polyhedral bodies called
60 PARENCHYMA CELLS
parenchyma cells, fitting closely to one another like the
bricks in a wall.
A parenchyma cell examined in detail (r1) is seen to
consist of protoplasm hollowed out internally into one or
more cavities or vacuoles (vac) containing a clear fluid.
These vacuoles differ from those of Amoeba, Heteromita, or
Euglena in being non-contractile ; they are in fact mere
cavities in the protoplasm containing a watery fluid : the
layer of protoplasm immediately surrounding them is denser
than the rest. Sometimes there is only one such space
occupying the whole interior of the cell, sometimes, as in
the example figured, there are several, separated from one
another by delicate bands or sheets of protoplasm. The cell
contains a large nucleus (nu) and is completely enclosed in
a moderately thick cell-wall composed of cellulose.
The above description applies to the cells composing the
deeper layers of the cortex, i.e., those nearest the woody
layer : in the more superficial cells, as well as in the internal
cells of a leaf, there is something else to notice. Imbedded
in the protoplasm, just within the cell-wall, are a number of
minute ovoid bodies of a bright green colour (r2, chr).
These are chromatophores or chlorophyll corpuscles ; they
consist of protoplasm coloured with chlorophyll which can
be proved experimentally to have the same properties as
the chlorophyll of Haematococcus and Euglena.
Such a green parenchyma cell is clearly comparable with
an encysted Haematococcus or Euglena, the main difference
being that in the plant cell the form is polyhedral owing to
the pressure of neighbouring cells, and the chromatophores
are relatively small and numerous. Similarly a colourless
parenchyma cell resembles an encysted Amoeba.
The pith, the epidermis or thin skin which forms the outer
surface of herbaceous plants, the greater part of the leaves,
MINUTE STRUCTURE OF CELLS 61
and other portions of the plant may be shown to consist of
an aggregation of cells agreeing in essential respects with
the above description.
We come therefore to a very remarkable result. The
higher animals and plants are built up — in part at least — of
elements which resemble in their essential features the
minute and lowly organisms studied in previous lessons.
Those elements are called by the general name of cells :
hence the higher organisms, whether plants or animals, are
multicelhdar or are to be considered as cell-aggregates,
while in the case of such beings > as Amoeba, Hsematococ-
cus, Heteromita, or Euglena, the entire organism is a
single cell, or is unicellular.
Note further that the cells of the higher animals and
plants, like entire unicellular organisms, may occur in either
the amoeboid (Fig. 8, A, B1, c1,) the ciliated (E), or the
encysted (F) condition, and that a plasmodial phase (e2) is
sometimes produced by the union of two or more amoeboid
cells.
One of the most characteristic features in the unicellullar
organisms described in the preceding lessons is the con-
stancy of the occurrence of binary fission as a mode of
multiplication. The analogy between these organisms and
the cells of the higher animals and plants becomes still
closer when we find that in the latter also simple fission is
the normal mode of multiplication, the increase in size of
growing parts being brought about by the continued division
of their constituent cells.
The process of division in animal and vegetable cells
is frequently accompanied by certain very characteristic and
62
MINUTE STRUCTURE OF CELLS
complicated changes in the nucleus to which we must now
direct our attention. First of all, however, it will be neces-
sary to describe the exact microscopic structure of cells and
their nuclei as far as it is known at present.
There seems to be a good deal of variation in the precise
structure of various animal cells, but the more recent re-
searches show that the cell-protoplasm (Fig. 9, cell-plsm)
nucl.
mem b.
nucl.plsm
chrom.
FIG. 9. — A typical animal cell, showing cell-membrane (cell-memb.),
cell-protoplasm (cell-plsni), nuclear membrane (micl-meinb.}, nuclear
protoplasm or achromatin (uncl-phm], and coil of chromatin (chrom}.
(After Carnoy. )
consists of a finely granular substance traversed by an
extremely delicate network, the constituent threads of which
are of almost inconceivable fineness, and bounded externally
by a membrane (cell-memb.) of excessive tenuity. As the
granules of the protoplasm are to be looked upon as pro-
ducts of metabolism (anastates and katastates, p. 18) it is
NUCLEAR DIVISION 63
clear that the precise appearances are sure to vary with the
state of nutrition of the cell.
The nucleus contains the same elements as in Amoeba
(see p. 7). It is bounded externally by a delicate mem-
brane (iiud. memb.} within wrhich is a granular substance
traversed by a fine network, the nuclear protoplasm or
achromatin. The chromatin or deeply-staining element
presents various appearances in different cells : sometimes
it takes the form of a network, sometimes of isolated granules
or nucleoli : but in some instances, at any rate, it consists
of a long tangled thread (ckrom.} which is said by some
observers to be in reality a tube filled with the deeply stain-
ing substance to which the name chromatin is properly
applied. It should be noticed that a coil of this kind some-
what loosely woven might easily be mistaken for a network,
and that if it were alternately constricted and dilated instead
of being regularly cylindrical it would present the appear-
ance of isolated granules or nucleoli.
The cells in the young growing parts of many plants have
much the same structure as this (Fig. n, A) except that the
delicate cell-membrane is replaced by a true cell-wall of
cellulose. In the older portions of the plant the protoplasm
is usually vacuolated (Fig. 8, F).
The precise changes which take place during the fission
of an animal cell are, like the structure of the cell itself,
subject to considerable variation. We will consider what
may probably be taken as a typical case.
First of all, when the cell is about to divide, the achro-
matic fibres of the nuclear protoplasm become arranged in
the form of a spindle (Fig. 10, A). At the same time the
chromatin filament unwinds itself, as it were, forming a loose
coil (A) : it then becomes broken up (B), and forms a series
64
CHANGE IN CELL-PROTOPLASM
of longitudinal bands of chromatin arranged along the
meridians of the nuclear spindle (c).
While this is going on the network of the cell-protoplasm
undergoes a change, some of its fibres becoming arranged in
the form of two radiating bundles of filaments one at each
pole of the spindle (B, c). And at about this stage (c) the
FIG. 10. — Stages in the binary fission ot an animal cell.
A — c, formation of the nuclear spindle and breaking up of the chro-
matin coil (black).
D, E, aggregation of the chromatin at the equator of the spindle.
F, fission of the chromatin-masses.
G, accumulation of the chromatin at the poles of the spindle and
ormation of the cell-plate.
H, reconstruction of the daughter-nuclei. (After Carnoy.)
nuclear membrane disappears so that the cell-protoplasm
mingles with the nuclear protoplasm or achromatin.
Next the longitudinal bands of chromatin gradually con-
centrate at the equator of the spindle, where they form a
ring of somewhat elongated masses (D). Each of these
undergoes a splitting, and may thus become ring-like (E) ;
CELL DIVISION 65
it then divides into two (F), the separate segments there-
upon travelling to opposite poles of the spindle where they
unite (G).
The spindle now elongates, carrying the two masses of
chromatin further away from each other (H). Around each
of them a membrane is formed (lower half of H) enclosing a
portion of protoplasm, which thus becomes the achromatin
or nuclear protoplasm of one of the two daughter-nuclei
into which the original or mother-nucleus has now com-
pletely divided. We thus get two completely formed nuclei
in a single cell.
But pari passu with this process of nuclear division,
fission of the cell-body is also going on. This may take
place by a simple process of constriction — in much the
same way as a lump of clay or dough would divide if a loop
of string were tied round its middle and then tightened — or
by the formation of what is known as a cell-plate. This
arises in a row of granules formed from the equatorial part
of the nuclear spindle (G) : the granules extend until they
form a complete equatorial plate dividing the cell-body into
two halves (H) : fission then takes place by the cell-plate
splitting into two along a plane parallel with its flat surfaces.1
In some cases the dividing nucleus instead of going
through the complicated processes just described divides
by simple constriction. We have therefore to distinguish
between direct and indirect nuclear division. To the latter very
elaborate method the name karyokinesis is often applied.
In plant cells many similar changes are gone through
during the division of the nucleus. A nuclear spindle is
1 It must not be forgotten that the cells which are necessarily repre-
sented in such diagrams as Figs. 8 — II as planes are really solid bodies,
and that consequently the cell-plate represented in the figures as a line
is actually a plane at right angles to the plane of the paper.
F
66
STAGES IN BINARY FISSION
formed (Fig. n, B) : the chromatin, at first arranged in a coil
(A), breaks up, and its segments become arranged along the
equator of the spindle (B), divide transversely (c) and travel
to the poles (D E), where they form the chromatin-coils of
the daughter-nuclei. At the same time the fibres of the
spindle give rise, across the equator of the cell, to a cell-
FIG. II. — Stages in the binary fission of a plant cell.
A, cell with resting nucleus.
B — D, formation of nuclear spindle and division of chromatin.
E, reconstruction of daughter nuclei and formation of cell-plate.
(After Strasburger).
plate (E) along which division takes place. But in the plant-
cell the cell-plate gives rise to a partition wall of cellulose
which divides the two daughter-cells from one another.1
1 The nucleoli of the plant-cell appear to be independent of the
chromatin of its nuclear thread, from which they differ markedly in
structure. They disappear during karyokinesis, and it seems probable
that they are active agents in the formation of the cell-wall.
COMPLEXITY OF CELL STRUCTURE 67
In this connection the reader will not fail to note the
extreme complexity of structure revealed in cells and their
nuclei by the highest powers of the microscope. When the
constituent cells of the higher animals and plants were
discovered, during the early years of the present century, by
Schleiden and Schwann, they were looked upon as the ultima
Thule of microscopic analysis. Now the demonstration of
the cells themselves is an easy matter, the problem is to
make out their ultimate constitution. What would be the
result if we could get microscopes as superior to those of
to-day as those of to-day are to the primitive instruments of
eighty or ninety years ago, it is impossible even to conjecture.
But of one thing we may feel confident — of the enormous
strides which our knowledge of the constitution of living
things is destined to make during the next half century.
The striking general resemblance between the cells of the
higher animals and plants and entire unicellular organisms
has been commented on as a very remarkable fact : there is
another equally significant circumstance to which we must
advert.
All the higher animals begin life as an egg, which is either
passed out of the body of the parent, as such, as in most
fishes, frogs, birds, &c., or undergoes the first stages of its
development within the body of the parent, as in sharks,
some reptiles, and nearly all mammals.
The structure of the egg is, in essential respects, the same
in all animals from the highest to the lowest. In a jelly-fish
for instance, it consists (Fig. 12, A) of a globular mass of
protoplasm (gd\ in which are deposited granules of a pro-
teinaceous substance known as yolk-spherules. Within the
protoplasm is a large clear nucleus, (g.v.\ the chromatin of
which is aggregated into a central mass or nucleolus (g. m).
F 2
68 STRUCTURE OF THE EGG
An investing membrane may or may not be present. In
other words the egg is a cell : it is convenient, for reasons
which will appear immediately, to speak of it as the ovum
or egg-cell.
The young or immature ova of all animals present this
structure, but in many cases certain modifications are under-
gone before the egg is mature, i.e., capable of development
into a new individual. For instance, the protoplasm may
throw out pseudopods, the egg becoming amoeboid (see
Fig. 53) ; or the surface of the protoplasm may secrete a thick
cell-wall (see Fig. 61). The most extraordinary modification
FIG. 12. — A, ovum of an animal (Carmarina hastata, one of the
jelly fishes), showing protoplasm (gd), nucleus (gv], and nucleolus (gm).
B, ovum of a plant (Gymnadenia conopsea, one of the orchids),
showing protoplasm (plsni), nucleus (mi), and nucleolus (mi1).
(A, from Balfour after Haeckel : B, after Marshall Ward. )
takes place in some vertebrata such as birds. In a hen's
egg, for instance, the yolk-spherules increase immensely,
swelling out the microscopic ovum until it becomes what we
know as the "yolk' of the egg: around this layers of
albumen or "white' are deposited, and finally the shell
membrane and the shell. Hence we have to distinguish
carefully in eggs of this character between the entire " egg "
in the ordinary acceptation of the term, and the ovum or
egg-cell.
But complexities of this sort do not alter the fundamental
fact that all the higher animals begin life as a single cell, or
in other words that multicellular animals, however large and
THE PLANT OVUM 69
complex they may be in their adult condition, originate as
unicellular bodies of microscopic size.
The same is the case with all the higher plants. The
pistil or seed-vessel of an ordinary flower contains one or
more little ovoidal bodies, the so-called " ovules " (more
accurately megasparangia (see Lesson XXX., and Fig. 89),
which, when the flower withers, develop into the seeds. A
section of an ovule shows it to contain a large cavity, the
embryo-sac or megaspore (see Fig. 89, F), at one end of
which is a microscopic cell (Fig. 12, B), consisting as usual
of protoplasm (plsni), nucleus («?/), and nucleolus (;///).
This is the ovum or egg-cell of the plant : from it the new
plant, which springs from the germinating seed, arises. Thus
the higher plants, like the higher animals, are, in their
earliest stage of existence, microscopic and unicellular.
LESSON VII
SACCHAROMYCES
EVERY one is familiar with the appearance of the ordinary
brewer's yeast — the light-brown, muddy, frothing substance
which is formed on the surface of the fermenting vats in
breweries — and is used in the manufacture of bread to make
the dough "rise."
Examined under the microscope yeast is seen to consist
of a fluid in which are suspended immense numbers of
minute particles, the presence of which produces the mud-
diness of the yeast. Each of these bodies is a unicellular
organism, the yeast-plant, or in botanical language Sac-
charomyces cerevisice.
Saccharomyces consists of a globular or ellipsoidal mass
of protoplasm (Fig. 13), about TJ^ mm. in diameter, and
surrounded with a delicate cell- wall of cellulose (c, c.w.).
In the protoplasm are one or more non-contractile vacuoles
(vac) — mere spaces filled with fluid and varying according to
the state of nutrition of the cell. Granules also occur in
the protoplasm which are products of metabolism, some of
them being of a proteid material, others fat globules.
Under ordinary circumstances no nucleus is to be seen ;
but recently, by the employment of a special mode of
GENERAL CHARACTERS 71
staining, a small rounded nucleus has been shown to exist
near the centre of the cell.
The cell-wall is so thin that it is difficult to be sure of its
presence unless very high powers are employed. It can
however be easily demonstrated by staining yeast with
vu
C.U'
X. x •
E
E
FIG. 13. — Saccharomyces cerevisite.
A, a group of cells under a moderately high power. The scale to
the left applies to this figure only.
B, several cells more highly magnified, showing various stages of
budding, vac, the vacuole.
C, a single cell with two buds (bd, bd') still more highly mag-
nified : c.w, cell-wall : vac, vacuole.
D, cells, crushed by pressure : c.iv, the ruptured cell- walls : flsm,
the squeezed out protoplasm.
E, E', starved cells, showing large vacuoles and fat globules (/).
F, F', formation of spores by fission of the protoplasm of a starved
cell : in F the spores are still enclosed in the mother-cell-wall, in F1
they are free.
magenta, and then applying pressure to the cover-glass so as
to crush the cell. Under this treatment the cell-walls are
burst and appear as crumpled sacs, split in various ways and
unstained by the magenta (D, c. «/), while the squeezed-out
protoplasm is seen in the form of irregular masses (pis///)
stained pink by the dye.
72 SACCHAROMYCES
The mode of multiplication of Saccharomyces is readily
made out in actively fermenting yeast, and is seen to differ
from anything we have met with hitherto. A small pimple-
like elevation (c, bd) appears on the surface of a cell and
gradually increases in size : examined under a high power
this bud is found to consist of an offshoot of the protoplasm
of the parent cell covered with a very thin layer of cellulose :
it is formed by the protoplasm growing out into an offshoot
-like a small pseudopod — which pushes the cell-wall before
it. The bud increases in size (bd'} until it forms a little
globular body touching the parent cell at one pole : then a
process of fission takes place along the plane of junction,
the protoplasm of the bud or daughter-cell becoming sepa-
rated from that of the mother-cell and a cellulose partition
being secreted between the two. Finally the bud becomes
completely detached as a separate yeast-cell.
It frequently happens that a Saccharomyces buds in
several places and each of its daughter-cells buds again,
before detachment of the buds takes place. In this way
chains or groups of cells are produced (B), such cell-
colonies consisting of two or more generations of cells, the
central one standing in relation of parent, grandparent, or
great-grandparent to the others.
It must be observed that this process ot budding or
gemmation is after all only a modification of simple
fission. In the latter the two daughter-cells are of equal
size and both smaller than the parent cell, while in gemma-
tion one — the mother-cell — is much larger than the other
daughter-cell or bud — and is of the same size as, indeed is
practically identical with, the original dividing-cell. Hence
in budding, the parent form does not, as in simple fission,
lose its individuality, becoming wholly merged in its twin
offspring, but merely undergoes separation of a small portion
MULTIPLE FISSION 73
of its substance in the form of a bud, which by assimilation
of nutriment gradually grows to the size of its parent,
the latter thus retaining its individuality and continuing to
produce fresh buds as long as it lives.
Multiplication by budding only goes on while the Sac-
charomyces is well supplied with food : if the supply of
nutriment fails, a different mode of reproduction obtains.
Yeast can be effectually starved by spreading out a thin
layer of it on a slab of plaster-of-Paris kept moist under a
bell-jar : under these circumstances the yeast is of course
supplied with nothing but water.
In a few days the yeast-cells thus circumstanced are found
to have altered in appearance : larger vacuoles appear in
them (Fig. 13, E,E') and numerous fat-globules (/) are formed.
The protoplasm has been undergoing destructive meta-
bolism, and, there being nothing to supply new material, has
diminished in quality, and at the same time been partly
converted into fat. Both in plants and animals it is found
that fatty degeneration, or the conversion of protoplasm
into fat by destructive metabolism, is a constant phenomenon
of starvation.
After a time the protoplasm collects towards the centre of
the cell and divides simultaneously into four masses arranged
like a pyramid of four billiard balls, three at the base and
one above (F). Each of these surrounds itself with a thick
cellulose coat and becomes a spore, the four spores being
sooner or later liberated by the rupture of the mother-cell
wall (F').
The spores being protected by their thick cell-walls are
able to withstand starvation and drought for a long time ;
when placed under favourable circumstances they develop
into the ordinary form of Saccharomyces. So that repro-
74 SACCHAROMYCES
duction by multiple fission appears to be, in the yeast-plant,
a last effort of the organism to withstand extinction.
The physiology of nutrition 01 Saccharomyces has been
studied with great care by several men of science and
notably by Pasteur, and is in consequence better known than
that of any other low organism. For this reason it will be
advisable to consider it somewhat in detail.
The first process in the manufacture of beer is the pre-
paration of a solution of malt called " sweet-wort." Malt
is barley which has been allowed to germinate or sprout, i.e.,
the young plant is allowed to grow to a certain extent from
the seed. During germination the starch which forms so
large a portion of the grain of barley is partly converted into
sugar : barley also contains soluble proteids and mineral
salts, so that when malt is infused in hot water the sweet-
wort formed may be looked upon as a solution of sugar,
proteid, and salts.
Into this wort a quantity of yeast is placed. Very soon
the liquid begins to froth, the quantity of yeast increasing
enormously : this means of course that the yeast-cells are
budding actively, as can be readily made out by microscopic
examination. If while this frothing is going on a lighted
candle is lowered into the vat the flame will be immediately
extinguished : if an animal were placed in the same position
it would be suffocated.
Chemical analysis shows that the extinction of the candle's
flame or of the animal's life is caused by a rapid evolution
of carbon dioxide from the fermenting wort, the frothing
being due to the escape of the gas from the liquid.
After a time the evolution of gas ceases, and the liquid
is then found to be no longer sweet but to have acquired
what we know as an alcoholic or spirituous flavour. Analysis
ALCOHOLIC FERMENTATION 75
shows that the sugar has nearly or quite disappeared, while
a new substance, alcohol, has made its appearance. The
sweet-wort has, in fact, been converted into beer.
Expressed in the form of a chemical equation what has
happened is this : —
C6H1206 = 2(C2H00) + 2(C02)
Grape sugar. Alcohol. Carbon dioxide.
One molecule of sugar has, by the action of yeast, been
split up into two molecules of alcohol which remain in the
fluid, and two of carbon dioxide which are given off as gas.
This is the process known as alcoholic fermentation.
It has been shown by accurate analysis that only about 95
per cent, of the sugar is thus converted into alcohol and
carbon dioxide : 4 per cent, is decomposed, with the for-
mation of glycerine, succinic acid, and carbon dioxide, and
i per cent, is used as nutriment by the yeast cells.
For the accurate study of fermentation the sweet-wort of
the brewer is unsuitable, being a fluid of complex and un-
certain composition, and the nature of the process, as well
as the part played in it by Saccharomyces, becomes much
clearer if we substitute the artificial wort invented by M.
Pasteur, and called after him Pasteur's solution. It is
made of the following ingredients :-
Water, H2O 8376 per cent.
Cane sugar, C12H2.2On . 15*00
Ammonium tartrate, (NH4)2C4H4O0 . 1*00
Potassium phosphate, K3PO4 . . . 0-20
Calcium phosphate, Ca3 (PO4)2 . . 0-02
Magnesium sulphate, MgSO4 . . . 0-02
11 11
11 11
11 11
11 11
11 11
lOO'OO
76 SACCHAROMYCES
The composition of this fluid is not a matter of guess-
work, but the result of careful experiments, and is deter-
mined by the following considerations.
It is obvious that if we are to study alcoholic fermentation
sugar must be present,1 since the essence of the process is
the formation of alcohol from sugar.
Then nitrogen in some form as well as carbon, oxygen,
and hydrogen must be present, since these four elements
enter into the composition of protoplasm, and all but the
first-named (nitrogen) into that of cellulose, and they are
thus required in order that the yeast should live and
multiply. The form in which nitrogen could best be assimi-
lated was found out by experiment. We saw that in the
manufacture of beer the yeast cells obtain their nitrogen
largely in the form of soluble proteids : green plants obtain
theirs largely in the simple form of nitrates. It was found
that while proteids are, so to say, an unnecessarily complex
food for Saccharomyces, nitrates are not complex enough,
and an ammonia compound is necessary, ammonium tartrate
being the most suitable. Thus while Saccharomyces can
build up the molecule of protoplasm from less complex food-
stuffs than are required by Amoeba, it cannot make use of
such comparatively simple compounds as suffice for Haema-
tococcus : moreover it appears to be indifferent whether its
nitrogen is supplied to it in the form of ammonium tartrate
or in the higher form of proteids.
Then as to the remaining ingredients of the fluid-
potassium and calcium phosphate and magnesium sulphate.
If a quantity of yeast is burnt, precisely the same thing
happens as when one of the higher animals or plants is
subjected to the same process. It first chars by the deposi-
1 It is ;i matter of indifference whether cane-sugar ur grape-sugar is
used.
EXPERIMENTS IN NUTRITION 77
tion of carbon, then as the heat is continued the carbon
is completely consumed, going off by combination with the
oxygen of the air in the form of carbon dioxide ; at the
same time the nitrogen is given off mostly as nitrogen gas,
the hydrogen by union with atmospheric oxygen as water-
vapour, and the sulphur as sulphurous acid or sulphur
dioxide (SO2). Finally, nothing is left but a small quantity
of white ash which is found by analysis to contain phos-
phoric acid, potash, lime, and magnesia ; i.e., precisely the
ingredients of the three mineral constituents of Pasteur's solu-
tion with the exception of sulphur, which, as already stated,
is given off during the process of burning as sulphur dioxide.
Thus the principle of construction of an artificial nutrient
solution such as Pasteur's is that it should contain all the
elements existing in the organism it is designed to support ;
or in other words, the substances by the combination of
which the waste of the organism due to destructive meta-
bolism may be made good.
That Pasteur's solution exactly fulfils these requirements
may be proved by omitting one or other of the constituents
from it, and finding out how the omission affects the well-
being of Saccharomyces.
If the sugar is left out the yeast-cells grow and multiply,
but with great slowness. This shows that sugar is not
necessary to the life of the organism, but only to that active
condition which accompanies fermentation. A glance at
the composition of Pasteur's solution will show that all the
necessary elements are supplied without sugar.
Omission of ammonium tartrate is fatal : without it the
cells neither grow nor multiply. This, of course, is just
what one would expect since, apart from ammonium tartrate,
the fluid contains no nitrogen without which the molecules
of protoplasm cannot be built up.
78 SACCHAROMYCES
It is somewhat curious to find that potassium and calcium
phosphates are equally necessary ; although occurring in
such minute quantities they are absolutely essential to the
well-being of the yeast-cells, and without them the organism,
although supplied with abundance of sugar and ammonium
tartrate, will not live. This may be taken as proving that
phosphorus, calcium, and magnesium form an integral part
of the protoplasm of Saccharomyces, although existing in
almost infinitesimal proportions.
Lastly, magnesium sulphate must not be omitted if the
organism is to flourish : unlike the other two mineral
constituents it is not absolutely essential to life, but without
it the vital processes are sluggish.
Thus by growing yeast in a fluid of known composition
it can be ascertained exactly what elements and combina-
tions of elements are necessary to life, what advantageous
though not absolutely essential, and what unnecessary.
The precise effect of the growth and multiplication of
yeast upon a saccharine fluid, or in other words the nature
of alcoholic fermentation, can be readily ascertained by a
simple experiment with Pasteur's solution. A quantity of
the solution with a little yeast is placed in a flask the neck
of which is fitted with a bent tube leading into a vessel of
lime-water or solution of calcium oxide. When the usual
disengagement of carbon dioxide (see p. 74) takes place the
gas passes through the tube into the lime-water and causes
an immediate precipitation of calcium carbonate as a white
powder which effervesces with acids. This proves the gas
evolved during fermentation to be carbon dioxide since no
other converts lime into carbonate. When fermentation is
complete the presence of alcohol may be proved by distil-
lation : a colourless, mobile, pungent, and inflammable
liquid being obtained.
CONDITIONS OF ALCOHOLIC FERMENTATION 79
By experimenting with several flasks of this kind it can
be proved that fermentation goes on as well in darkness as
in light, and that it is quite independent of free oxygen.
Indeed the process does not go on if free oxygen — i.e.,
oxygen in the form of dissolved gas — is present in the fluid ;
from which it would seem that Saccharomyces must be able
to obtain the oxygen, which like all other organisms it
requires for its metabolic processes, from the food supplied
to it.
The process of fermentation goes on most actively
between 28° and 34°C : at low temperature it is com-
paratively slow, and at 38°C. multiplication ceases.
If a small portion of yeast is boiled so as to kill the
cells, and then added to a flask of Pasteur's solution, no
fermentation takes place, from which it is proved that the de-
composition of sugar is effected by the living yeast-cells only.
There seems to be no doubt that the property of exciting
alcoholic fermentation is a function of the living protoplasm
of Saccharomyces. The yeast-plant is therefore known as
an organized ferment : when growing in a saccharine solu-
tion it not only performs the ordinary metabolic processes
necessary for its own existence, but induces decomposition
of the sugar present, this decomposition being unaccom-
panied by any corresponding change in the yeast-plant
itself.
It is necessary to mention in this connection that there is
an important group of not-living bodies which produce
striking chemical changes in various substances with-
out themselves undergoing any change : these are distin-
guished as unorganized ferments. A well-known example is
pepsin, which is found in the gastric juice of the higher
animals, and has the function of converting proteids into
peptones (see p. 12) : its presence has been proved in
8o SACCHAROMYCES
the Mycetozoa (p. 54), and probably it or some similar pep-
tonizing or proteolytic ferment effects this change in all
organisms which have the power of digesting proteids.
Another instance is diastase, which effects the conversion
of starch into grape sugar : it is present in germinating
barley (see p. 74), and an infinitesimal quantity of it can
convert immense quantities of starch. The ptyalin of our
own saliva has a like action, and probably some similar
diastatic or amylolytic ferment is present in the Mycetozoa
which, as we saw (p. 54), can digest cooked starch.
LESSON VIII
BACTERIA
IT is a matter of common observation that if certain moist
organic substances, such as meat, soup, milk, &c., are allowed
to stand at a moderate temperature for a few days — more or
fewer according as the weather is hot or cold — they " go
bad " or putrefy; i.e. they acquire an offensive smell, a taste
which few are willing to ascertain by direct experiment, and
often a greatly altered appearance.
One of the most convenient substances for studying the
phenomena of putrefaction is an infusion of hay, made by
pouring hot water on a handful of hay and straining the
resultant brown fluid through blotting paper. Pasteur's
solution may also be used, or mutton-broth well boiled
and filtered, or indeed almost any vegetable or animal
infusion.
If some such fluid is placed in a glass vessel covered with
a sheet of glass or paper to prevent the access of dust, the
naked-eye appearances of putrefaction will be found to
manifest themselves with great regularity. The fluid, at first
quite clear and limpid, becomes gradually dull and turbid.
The opacity increases and a scum forms on the surface :
at the same time the odour of putrefaction arises, and,
G
82 BACTERIA
especially in the case of animal infusions, quickly becomes
very strong and disagreeable.
The scum after attaining a perceptible thickness breaks up
and falls to the bottom, and after this the fluid slowly clears
again, becoming once more quite transparent and losing its
bad smell. If exposed to the light patches of green appear
in it sooner or later, due to the presence of microscopic
organisms containing chlorophyll. The fluid has acquired,
in fact, the characteristics of an ordinary stagnant pond, and
is quite incapable of further putrefaction. The whole series
of changes may occupy many months.
Microscopic examination shows that the freshly-prepared
I
f
t
FIG. 14.— Bacterium termo. A, motile stage : B, vesting stage or
zooglsea. (From Klein.)
fluid is free from organisms, and indeed, if properly filtered,
from particles of any sort. But the case is very different
when a drop of infusion in which turbidity has set in is
placed under a high power. The fluid is then seen to be
crowded with incalculable millions of minute specks, only
just visible under a power of 300 or 400 diameters, and all
in active movement. These specks are Bacteria, or as
they are sometimes called, microbes or micro-organisms ;
they belong to the particular genus and species called
Bacterium termo.
Seen under the high power of an ordinary student's
microscope Bacterium termo has the appearance shown in
BACTERIUM TERMO 83
Fig. 14, A : it is like a minute finger-biscuit, i.e. has the form
of a rod constricted in the middle. It is only by using the
very highest powers of the microscope that its form and
structure can be satisfactorily made out. It is then seen
(Fig. 15) to consist of a little double spindle, showing neither
nucleus, vacuole, nor other internal structure. It is com-
posed of a particular variety of protoplasm, and is sur-
rounded by a membrane of extreme tenuity formed of
cellulose. At each end is attached a flagellum about as
long as the cell itself.
Bacterium termo is much smaller than any organism we
have yet considered, so small in fact that, as it is always
easier to deal with whole numbers than with fractions, its
size is best expressed by taking as a standard the one-
FIG. 15. — Bacterium termo (X 4000), showing the terminal flagella.
(After Dallinger.)
thousandth of a millimetre, called a micromillimetre and
expressed by the symbol //,. The entire length of the
organism under consideration is from i'5 to 2 /z, i.e. about
the y^Q- mm. or the jo-Jo-Q- inch. In other words, its entire
length is not more than one-fourth the diameter of a yeast-
cell or of a human blood-corpuscle. The diameter of the
flagellum has been estimated by Dallinger to be about | //.
or 2 orVoir mcn> a smallness of which it is as difficult to form
any clear conception as of the distances of the fixed stars.
Some slight notion of these almost infinitely small dimen-
sions may, however, be. obtained in the following way. Fig.
15 shows a Bacterium termo magnified 4000 diameters, the
scale above the figure representing -^ mm. magnified to the
same amount. The height of this book is a little over 18 cm.;
G 2
84 BACTERI
this multiplied by 4,000 gives 72,000 cm. = 720 metres = 2362
feet. We therefore get the proportion — as 2362 feet, or
nearly six times the height of St. Paul's, is to the height of
the present volume, so the length of Fig. 15 is to that of
Bacterium termo.
It was mentioned above that at a certain stage of putre-
faction a scum forms on the surface of the fluid. This film
consists of innumerable motionless Bacteria imbedded
in a transparent gelatinous substance formed of mycoprotein
(Fig. 14, B). After continuing in the active condition for a
time the Bacteria rise to the surface, lose their flagella, and
throw out this gelatinous substance in which they lie imbedded.
The bacterial jelly thus formed is called a zooglcea. Thus
in Bacterium termo, as in so many of the organisms we have
studied, there is an alternation of an active with a resting
condition.
During the earlier stages of putrefaction Bacterium termo
is usually the only organism found in the fluid, but later on
other microbes make their appearance. Of these the com-
monest are distinguished by the generic names Micrococcus,
Bacillus, Vibrio, and Spirillum.
Micrococcus (Fig. 16) is a minute form, the cells of which
are about 2/x (5-5-^ mm.) in diameter. It differs from
Bacterium in being globular instead of spindle-shaped and
in having no motile phase. Like Bacterium it assumes the
zooglaea condition (Fig. 16, 4).
Bacillus is commonly found in putrescent infusions in
which the process of decay has gone on for some days : as
its numbers increase those of Bacterium termo diminish,
until Bacillus becomes the dominant form. Its cells (Fig.
17) are rod-shaped and about 6/x (Tiy- mm.) in length in the
BACILLUS 85
commonest species. Both motionless and active forms are
found, the latter having a flagellum at each end. The
zooglaea condition is often assumed, and the rods are fre-
quently found united end to end so as to form filaments.
Vibrio resembles Bacillus, but the rod-like cells (Fig. 18, A)
are wavy instead of straight. They are actively motile and
when highly magnified are found to be provided with a
flagellum at each end. Vibriones vary from 8/x to 2^ in
length.
Spirillum is at once distinguished by its spiral form, the
V
* •. •
\ ::•&:. 4
\ : :?#•&
• • •••••»••••»
% • •.?; ••'•••••
V / ••••.•;:••
:::
...... ••• :
Pft •** O •«•*
FIG. 1 6. — Micrococcus. I, single and double (dumb-bell shaped)
forms : 2 and 3, chain-forms : 4, a zooglsea.
cells resembling minute corkscrews (Fig. 18, B & c) and
being provided with a flagellum at each end (c). The
smaller species, such as S. tenue (B) are from 2 to 5/x in
length, but the larger forms, such as S. volutans (c) attain a
length of from 25 to 3o/x. In swimming Spirillum appears
on a superficial examination to undulate like a worm or a
serpent, but this is an optical illusion : the spiral is really a
permanent one, but during progression it rotates upon its
long axis, like Hsematococcus (p. 25) and this double move-
ment produces the appearance of undulation.
86
BACTERIA
Most Bacteria are colourless, but three species (Bacterium
viride, B. chlornium, and Bacillus virens] contain chlorophyll,
and several others form pigments of varying tints and often
of great intensity. For instance, there are red, yellow,
brown, blue, and violet species of Micrococcus which grow
on slices of boiled potato, hard-boiled egg, &c., forming
brilliantly coloured patches ; and the yellow colour often
FIG. 17. — Bacillus subtilis, showing various stages between single
forms and long filaments (Leptothrix).
assumed by milk after it has been allowed to stand for a
considerable time is due to the presence of Bacterium
xanthinum.
All Bacteria multiply by simple transverse fission, the
process taking place sometimes during the motile, sometimes
during the resting condition. Frequently the daughter cells
do not separate completely from one another but remain
I'.INARY FISSION
87
loosely attached, forming chains. These are very common
in some species of micrococcus (see Fig. 1 6).
Bacillus when undergoing fission behaves something like
Heteromita : the mother-cell divides transversely across the
middle, and the two halves gradually wriggle away from one
another, but remain connected for a time by a very fine thread
-MG. 1 8. — A, Vibrio. B, Spirillum temic. c, Spirillum volutaus.
(From Klein.)
of protoplasm which extends between their adjacent ends.
This is drawn out by the gradual separation of the two cells
until it attains twice the length of a flagellum when it snaps
in the middle, thus providing each daughter-cell with a new
flagellum. Bacillus may, however, divide while in the
resting condition and, under certain circumstances, the
88 BACTERIA
process is repeated again and again, and the daughter-cells
remaining in contact form a long wavy or twisted filament
called Leptothrix (Fig. 17) the separate elements of which
are usually only visible after staining.
Bacillus also multiplies by a peculiar process of spore-
formation which may take place either in the ordinary resting
form or in a leptothrix filament. A bright dot appears at
one place in the protoplasm (Fig. 1 9) : this increases in size,
the greater part of the protoplasm being used up in its
formation, and finally takes on the form of a clear oval
spore which remains for some time enclosed in the cell-wall
of the Bacillus, by the rupture of which it is finally liberated.
Spores of this kind are termed endospores, In other Bacteria
spores are formed directly from the vegetative cells, which
become thick walled (arthrospores). The spores differ from
ordinary Bacilli in being unstained by aniline dyes.
After a period of rest the spores, under favourable cir-
cumstances, germinate by growing out at one end so as to
become rod-like, and thus finally assuming the form of
ordinary Bacilli.
There are other genera often included among Bacteria for
the description of which the student is referred to the more
special treatises.1 One remark must, however, be made in
concluding the present brief account of the morphology of
the group. There is a great deal of evidence to show that
what have been spoken of as genera (Bacterium, Bacillus,
Spirillum, &c.) may merge into one another and are therefore
to be looked upon as phases in the life-history of various
microbes rather than as true and distinct genera. But this
is a point which cannot at present be considered as settled.
1 See especially De Bary, Fungi, Mycetozoa, and Bacteria (Oxford,
1887), and Klein, Micro-organisms and Disease (London, 1886).
NATURE OF GENERIC FORMS
89
The conditions of life of Bacteria are very various. Some
live in water such as that of stagnant ponds, and of these
three species as already stated (p. 86), contain chlorophyll.
The nutrition of such forms must obviously be holophytic,
and in the case of Bacterium chlorinum the giving off of
oxygen in sunlight has actually been proved.
FIG. 19. — Spore-formation in Bacillus. (From Klein.)
But this mode of nutrition is rare among the Bacteria :
nearly all of those to which reference has been made are
saprophytes, that is, live upon decomposing animal and
vegetable matters. They are, in fact, nourished in precisely
the same way as Heteromita (see p. 37). Many of these
90 BACTERIA
forms such as Bacterium termo, and species of Bacillus,
Vibrio, &c., will, however, flourish in Pasteur's solution, in
which they obtain their nitrogen in the form of ammonium
tartrate instead of decomposing proteid. It has also been
shown that some Bacteria can go further and make use of
nitrates as a source of nitrogen. But part passu with their
ordinary nutritive processes, many Bacteria exert an action
on the fluids on which they live comparable to that exerted
on a saccharine solution by the yeast-plant. Such microbes
are, in fact, organized ferments.
Every one is familiar with the turning sour of milk. This
change is due to the conversion of the milk-sugar into
lactic acid.
CGH120(! = 2(C3H60)3.
Sugar. Lactic Acid.
The transformation is brought about by the agency 01
Bacterium lactis, a microbe closely resembling B. termo.
Beer and wine are two other fluids which frequently turn
sour, there being in this case a conversion of alcohol into
acetic acid, represented by the equation —
C2H60 + 02 = H20 + C2H402,
Alcohol. Oxygen. Water. Acetic Acid.
The ferment in this instance is Bacterium aceti, often
called My coder ma aceti, or the " vinegar plant." It will
be noticed that in this case oxygen enters into the reaction :
it is a case of fermentation by oxidation.
Putrefaction itself is another instance of fermentation
induced by a microbe. Bacterium termo — the putrefactive
ferment — causes the decomposition of proteids into simpler
compounds, amongst which are such gases as ammonia
(NH.,), sulphuretted hydrogen (H2S), and ammonium
BACTERIA AS FERMENTS 91
sulphide ( (NH4)2S), the evolution of which produces the
characteristic odour of putrefaction.
Besides holophytes and saprophytes there are included
among Bacteria many parasites, that is, species which feed
not on decomposing but on living organisms. Many of the
most deadly infectious diseases, such as tuberculosis, diph-
theria, and typhoid fever, are due to the presence in the
tissues or fluids of the body of particular species of microbes,
which feed upon the parts affected and give rise to the
morbid symptoms characteristic of the disease.
Some Bacteria, like the majority of the organisms pre-
viously studied, require free oxygen for their existence, but
others, like Saccharomyces during active fermentation (see
p. 79), are quite independent of free oxygen and must there-
fore be able to take the oxygen without which their metabolic
processes could not go on, from some of the compounds
contained in the fluid in which they live. Bacteria are for
this reason divided into aerobic species which require free
oxygen, and anaerobic species which do not.
As to temperature, common observation tells us that
Bacteria only flourish within certain limits. We know for
instance that organic substances can be preserved from
putrefaction by being kept either at the freezing-point, or at
or near the boiling-point. One important branch of modern
industry, the trade in frozen meat, depends upon the fact that
the putrefactive Bacteria, like other organisms, are rendered
inactive by freezing, and every housekeeper knows how easily
putrefaction can be staved off by roasting or boiling. Simi-
larly it is a matter of common observation that a moderately
high temperature is advantageous to these organisms, the
heat of summer or of the tropics being notoriously favourable
to putrefaction. In the case of Bacterium termo, it has been
92 BACTERIA
found that the optimum temperature is from 30° to 35° C.,
but that the microbe will flourish between 5° and 40° C.
Although fully-formed Bacteria, like other organisms, are
usually killed by exposure to heat several degrees below
boiling-point, yet the spores of some species will withstand,
at any rate for a limited time, a much higher temperature — •
even as high as i3o°C. On the other hand, putrefactive
Bacteria retain their power of development after being exposed
to a temperature of — m°C., although during the time
of exposure all vital activity is of course suspended.
Bacteria also resemble other organisms in being unable
to carry on active life without a due supply of water : no
perfectly dry substance ever putrefies. The preservation for
ages of the dried bodies of animals in such countries as
Egypt and Peru depends at least as much upon the moisture-
less air as upon the antiseptics used in embalming.
For the most part Bacteria are unaffected by light, since
they grow equally well in darkness and in ordinary daylight.
Many of them, however, will not bear prolonged exposure to
direct sunlight, and it has been found possible to arrest the
putrefaction of an organic infusion by insolation, or exposure
to the direct action of the sun's rays. It has also been
proved that it is the light-rays and not the heat-rays which
are thus prejudicial to the life of micro-organisms.
LESSON IX
BIOGENESIS AND HOMOGENESIS : HOMOGENESIS AND HETERO-
GENESIS
THE study of the foregoing living things and especially of
Bacteria, the smallest and probably the simplest of all known
organisms, naturally leads us to the consideration of one of
the most important problems of biology — the problem of
the origin of life.
In all the higher organisms we know that each individual
arises in some way or other from a pre-existing individual :
no one doubts that every bird now living arose by a process
of development from an egg formed in the body of a
parent bird, and that every tree now growing took its origin
either from a seed or from a bud produced by a parent plant.
But there have always — until quite recently, at any rate-
been upholders of the view that the lower forms of life,
bacteria, monads, and the like, may under certain circum-
stances originate independently of pre-existing organisms :
that, for instance, in a flask of hay-infusion or mutton-broth,
boiled so as to kill any living things present in it, fresh
forms of life may arise de novo, may in fact be created
then and there.
We have therefore two theories of the lower organisms
94 BIOGENESIS AND HOMOGENESIS
the theory of Biogenesis, according to which each living
thing, however simple, arises by a natural process of bud-
ding, fission, spore-formation, or what not, from a parent
organism : and the theory of Abiogenesis, or as it is some-
times called Spontaneous or Equivocal Generation, accord-
ing to which fully formed living organisms sometimes
arise from not-living matter.
In former times the occurrence of abiogenesis was uni-
versally believed in. The expression that a piece of meat
has "bred maggots" ; the opinion that parasites such as the
gall-insects of plants or the tape-worms in the intestines of
animals originate where they are found ; the belief still held
in some rural districts in the occurrence of showers of frogs,
or in the transformation of horse -hairs kept in water into
.eels ; all indicate a survival of this belief.
Aristotle, one of the greatest men of science of antiquity,
explicitly teaches abiogenesis. He states that some animals
" spring from putrid matter," that certain insects " spring
from the dew which falls upon plants," that thread-worms
" originate in the mud of wells and running waters," that
fleas "originate in very small portions of corrupted matter,"
and that " bugs proceed from the moisture which collects
on the bodies of animals, lice from the flesh of other
creatures."
Little more than 200 years ago one Alexander Ross, com-
menting on Sir Thomas Browne's doubt as to " whether
mice may be bred by putrefaction," says, " so may he doubt
whether in cheese and timber worms are generated ; or 'if
beetles and wasps in cow's dung ; or if butterflies, locusts,
grasshoppers, shell- fish, snails, eels, and such like, be pro-
created of putrefied matter, which is apt to receive the form
of that creature to which it is by formative power disposed.
To question this is to question reason, sense, and experience.
THE PROBLEM LIMITED TO MICROSCOPIC FORMS 95
If he doubts of this let him go to Egypt, and there he will
find the fields swarming with mice begot of the mud of
Nylus, to the great calamity of the inhabitants."
As accurate inquiries into these matters were made, the
number of cases in which equivocal generation was sup-
posed to occur was rapidly diminished. It was a simple
matter — when once thought of — to prove, as Redi did in
1638, that no maggots were ever "bred " in meat on which
flies were prevented by wire screens from laying their eggs.
Far more difficult was the task, also begun in the seventeenth
century, of proving that parasites, such as tape-worms, arise
from eggs taken in with the food ; but gradually this pro-
position was firmly established, so that no one of any
scientific culture continued to believe in the abiogenetic
origin of the more highly organized animals any more than
in showers of frogs, or in the origin of geese from
barnacles.
But a new phase of the question opened with the inven-
tion of the microscope. In 1683, Anthony van Leuwenhoek
discovered Bacteria, and it was soon found that however
carefully meat might be protected by screens or infusions, by
being placed in well-corked or stoppered bottles, putrefaction
always set in sooner or latter, and was invariably accom-
panied by the development of myriads of bacteria, monads,
and other low organisms. It was not surprising, considering
the rapidity with which these were found to make their
appearance, that many men of science imagined them to be
produced abiogenetically.
Let us consider exactly what this implies. Suppose we
have a vessel of hay-infusion, and in it a single Bacterium.
The microbe will absorb the nutrient fluid and convert it
into fresh protoplasm : it will divide repeatedly, and its
progeny doing the same the vessel will soon contain millions
96 BIOGENESIS AND HOMOGENESIS
of Bacteria instead of one. This means, of course, that a
certain amount of fresh living protoplasm has been formed
out of the constituents of the hay-infusion, through the
agency in the first instance of a single living Bacterium.
The question naturally arises — Why may not the formation
of protoplasm take place independently of this insignificant
speck of living matter ?
It must not be thought that this question is in any way
a vain or absurd one. That living protoplasm has at some
period of the world's history originated from not-living
matter seems a necessary corollary of the doctrine of
evolution, and is obviously the very essence of the doctrine
of special creation, and there is no a priori reason why it
should be impossible to imitate the unknown conditions
under which this took place. At present, however, we have
absolutely no data towards the solution of this fundamental
problem.
But however insoluble may be the question as to how life
first dawned upon our planet, the origin of living things at
the present day is capable of investigation in the ordinary
way of observation and experiment. The problem may be
stated as follows : — any putrescible infusion, — i.e. any fluid
capable of putrefaction — will be found after a longer or
shorter exposure to swarm with bacteria and monads : do
these organisms or the spores from which they first arise reach
the infusion from without, or are they generated within it ?
And the general lines upon which an investigation into the
problem must be conducted are simple : given a vessel of any
putrescible infusion ; let this be subjected to some process
which, without rendering it incapable of supporting life, shall
kill any living things contained in it ; let it then be placed
under such circumstances that no living particles, however
small, can reach it from without. If, after these two condi-
METHOD OF FILTERING AIR 97
tions have been rigorously complied with, living organisms
appear in the fluid, such organisms have originated abio-
genetically.
To kill any microbes contained in the fluid it is usually
quite sufficient to boil it thoroughly. As we have seen,
protoplasm enters into heat-rigor at a temperature consider-
ably below the boiling-point of water, so that, with an
exception which will be referred to presently, a few minutes'
boiling suffices to sterilize all ordinary infusions, z>., to kill
any organisms they may contain.
Then as to preventing the entrance of organisms or their
spores from without. This may be done in various ways.
One way is to take a flask with the neck drawn out into
a very slender tube, to boil the fluid in it for a sufficient
time, and then, while ebullition is going on, to close the
end of the tube by melting the glass in the flame of a
Bunsen-burner or spirit-lamp, thus hermetically sealing the
flask.
By this method not only organisms and their spores are
excluded from the flask but also air. But this is obviously
unnecessary : it is evident that air may be admitted to the
fluid with perfect impunity if only it can be filtered, that is,
passed through some substance which shall retain all solid
particles however small, and therefore of course, bacteria,
monads, and their spores.
A perfectly efficient filter for this purpose is furnished by
cotton-wool. A flask or test-tube is partly filled with the
infusion : the latter is boiled, and during ebullition cotton-
wool is pushed into the mouth of the vessel until a long and
firm plug is formed (Fig. 20). When the source of heat is
removed, and, by the cooling of the fluid, the steam which
filled the upper part of the tube condenses, air passes in to
supply its place, but as it does so it is filtered of even the
H
98
BIOGENESIS AND HOMOGENESIS
smallest solid particles by having to pass through the close
meshes of the cotton-wool.
Experiments of this sort conducted with proper care have
been known for many years to give negative results in the
great majority of cases : the fluids remain perfectly sterile
for any length of time. But in certain instances, in spite of
the most careful precautions, bacteria were found to appear
FIG. 20. — A Beaker with a number of test-tubes containing putres-
cible infusions and plugged with cotton-wool. (From Klein.)
in such fluids, and for years a fierce controversy raged
between the biogenists and the abiogenists, the latter in-
sisting that the experiments in question proved the occurrence
of spontaneous generation, while the biogenists considered
that all such cases were due to defective methods — either to
imperfect sterilization of the fluid or to imperfect exclusion
of germ-containing atmospheric dust.
The matter was finally set at rest, and the biogenists
SOME SPORES NOT KILLED BY BOILING
99
proved to be in the right, by the important discovery that
the spores of bacteria and monads are not killed by a tem-
perature many degrees higher than is sufficient to destroy the
adult forms : that in fact while the fully developed organisms
are killed by a few minutes' exposure to a temperature of
70° C. the spores are frequently able to survive several
hours' boiling, and must be heated to 130° — 150° C. in
order that their destruction may be assured. It was also
shown that the more thoroughly the spores are dried the
more difficult they are to kill, just as well dried peas are
hardly affected by an amount of boiling sufficient to reduce
fresh ones to a pulp.
This discovery of the high thermal death-point or ultra-
maximum temperature of the spores of these organisms has
necessitated certain additional precautions in experiments
with putrescible infusions. In the first place the flask and
the cotton-wool should both be heated in an oven to a
temperature of 150° C., and thus effectually sterilized. The
flask being filled and plugged with cotton-wool is well boiled
and then kept for some hours at a temperature of 32°- -38° C.,
the optimum temperature for bacteria. The object of this
is to allow any spores which have not been killed by boiling
to germinate, in other words to pass into the adult con-
dition in which the temperature of boiling water is fatal.
The infusion is then boiled again, so as to destroy any such
freshly germinated forms it may contain. The same process
is repeated once or twice, the final result being that the
very driest and most indurated spores are induced to ger-
minate, and are therefore slain. It must not be forgotten
that repeated boiling does not render the fluid incapable of
supporting life, as may be seen by removing the cotton-wool
plug, when it will in a short time swarm with microbes.
Experiments conducted with these precautions all tell the
H 2
ioo HOMOGENESIS AND HETEROGENESIS
same tale : they prove conclusively that in properly sterilized
putrescible infusions, adequately protected from the entrance
of atmospheric germs, no micro-organisms ever make their
appearance. So that the last argument for abiogenesis has
been proved to be fallacious, and the doctrine of biogenesis
shown, as conclusively as observation and experiment can
show it, to be of universal application as far as existing
conditions known to us are concerned.
It is also necessary to add that the presence of microbes
in considerable quantities in our atmosphere has been
proved experimentally. By drawing air through tubes
lined with a solid nutrient material Prof. Percy Frankland
showed that the air of South Kensington contained about
thirty-five micro-organisms in every ten litres, and by ex-
posing circular discs coated with the same substance he was
further able to prove that in the same locality 279 micro-
organisms fall upon one square foot of surface in one
minute.
There is another question intimately connected with that
of Biogenesis, although strictly speaking quite independent
of it. It is a matter of common observation that, in both
animals and plants, like produces like ; that a cutting from
a willow will never give rise to an oak, nor a snake emerge
from a hen's egg. In other words, ordinary observation
teaches the general truth of the doctrine of Homogenesis.
But there has always been a residuum of belief in the
opposite doctrine of Heterogenesis^ according to which the
offspring of a germ, animal, or plant may be something
utterly different from itself, a plant giving rise to an animal
or vice versd, a lowly to a highly organized plant or animal
and so on. Perhaps the most extreme case in which hetero-
genesis was once seriously believed to occur is that of
SUPPOSED CASES OF HETEROGENESIS 101
the " barnacle-geese." Buds of a particular tree growing
near the sea were said to produce barnacles, and these
falling into the water to develop into geese. This sounds
absurd enough, but within the last twenty years two or three
men of science have described, as the result of repeated
observations, the occurrence of quite similar cases among
microscopic organisms. For instance, the blood-corpuscles
of the silkworm have been said to give rise to fungi, the
protoplasm of the green weed Nitella (see Fig. 46) to
Amoebae and Infusoria (see p. 105), Euglenas to thread-
worms, and so on.
It is proverbially difficult to prove a negative, and it might
not be easy to demonstrate, what all competent naturalists
must be firmly convinced of, that every one of these sup-
posed cases of heterogenesis is founded either upon errors
of observation or upon faulty inductions from correct
observations.
Let us take a particular case by way of example. Many
years ago Dr. Dallinger observed among a number of Vorti-
cellse or bell-animalcules (Fig. 26) one which appeared to
have become encysted upon its stalk. After watching it for
some time, there was seen to emerge from the cyst a free-
swimming ciliated Infusor called Amphileptus, not unlike a
long-necked Paramcecium (Fig. 21, p. 106). Many ob-
servers would have put this down as a clear case of hetero-
genesis : Dallinger simply recorded the observation and
waited. Two years later the occurrence was explained : he
found the same two species in a pond, and watched an
Amphileptus seize and devour a Vorticella, and, after finish-
ing its meal, become encysted upon the stalk of its victim.
It is obvious that the only way in which a case of hetero-
genesis could be proved would be by actually watching the
transformation, and this no heterogenist has ever done ; at
102 HOMOGENESIS AND HETEROGENESIS
the most, certain supposed intermediate stages between the
extreme forms have been observed — say, between a Euglena
and a thread-worm — and the rest of the process inferred.
On the other hand, innumerable observations have been
made on these and other organisms, the result being that
each species investigated has been found to go through a
definite series of changes in the course of its development,
the ultimate result being invariably an organism resembling
in all essential respects that which formed the starting-point
of the observations : Euglense always giving rise to Euglense
and nothing else, Bacteria to Bacteria and nothing else, and
so on.
There are many cases which imperfect knowledge might
class under heterogenesis, such as the origin of frogs from
tadpoles or of jelly-fishes from polypes (Lesson XXIII. Fig.
54), but in these and many other cases the apparently
anomalous transformations have been found to be part of
the normal and invariable cycle of changes undergone by
the organism in the course of its development ; the frog
always gives rise ultimately to a frog, the jelly-fish to a jelly-
fish. If a frog at one time produced a tadpole, at another a
trout, at another a worm : if jelly-fishes gave rise sometimes
to polypes, sometimes to infusoria, sometimes to cuttle-
fishes, and all without any regular sequence — that would be
heterogenesis.
It is perhaps hardly necessary to caution the reader against
the error that there is any connection between the theory of
heterogenesis and that of organic evolution. It might be
said — if, as naturalists tell us — dogs are descended from
wolves and jackals and birds from reptiles, why should not,
for instance, thread-worms spring from Euglenas or Infusoria
from Bacteria ? To this it is sufficient to answer that the
evolution of one form from another takes place by a series
EVOLUTION AND HETEROGENESIS 103
of slow, orderly progressive changes going on through a
long series of generations (see Lesson XIII.) ; whereas
heterogenesis presupposes the casual occurrence of sudden
transformations in any direction — i.e., leading to either a less
or a more highly organized form — and in the course of a
single generation.
LESSON X
PARAMOECIUM, STYLONYCHIA, AND OXYTRICHA
IT will have been noticed with regard to the simple uni-
cellular organisms hitherto considered that all are not equally
simple : that Protamceba (Fig. 2, p. 9) and Micrococcus
(Fig. 1 6, p. 85), may be considered as the lowest of all,
and that the others are raised above these forms in the scale
of being in virtue of the possession of nucleus or contractile
vacuole, or of flagella, or even, as in the case of Euglena
(Fig. 5, p. 45), of a mouth and gullet.
Thus we may speak of any of the organisms already
studied as relatively " high " or " low " with regard to the
rest : the lowest or least differentiated forms being those
which approach most nearly to the simplest conception of a
living thing — a mere lump of protoplasm : the highest or
most differentiated those in which the greatest complication
of structure has been attained. It must be remembered,
too, that this increase in structural complexity is always
accompanied by some degree of division of physiological
labour, or, in other words, that morphological and physio-
logical differentiation go hand in hand.
We have now to consider certain organisms in which this
differentiation has gone much further ; which have, in fact,
OCCURRENCE OF INFUSORIA 105
acquired many of the characteristics of the higher animals
and plants while remaining unicellular. The study of several
of these more or less highly differentiated though unicellular
forms will occupy the next seven Lessons.
It was mentioned above that, in the earlier stages of the
putrefaction of an organic infusion, bacteria only were
found, and that later, monads made their appearance. Still
later organisms much larger than monads are seen, generally
of an ovoidal form, moving about very quickly, and seen by
the use of a high power to be covered with innumerable fine
cilia. These are called dilate Infusoria, in contradistinction
to monads, which are often known as flagellate Infusoria :
many kinds are common in putrefying infusions, some occur
in the intestines of the higher animals, while others are
among the commonest inhabitants of both fresh and salt
water. Five genera of these infusoria will form the subjects
of this and the four following Lessons.
A very common ciliate infusor is the beautiful " slipper-
animalcule," Paramoedum aurelia, which from its compara-
tively large size and from the ease with which all essential
points of its organization can be made out is a very con-
venient and interesting object of study.
Compared with the majority of the organisms which have
come under our notice it may fairly be considered as gigantic
being no less than \ — J mm. (200 — 260 //,) in length : in
fact it is just visible to the naked eye as a minute whitish
speck.
Its form (Fig. 21 A) can be fairly well imitated by making
out of clay or stiff dough an elongated cylinder rounded at
one end and bluntly pointed at the other ; then giving the
broader end a slight twist ; and finally making on the side
B
A
C.TttC
, (/?:
C.TCIC.
FIG. 21. — Paramcecium aiirelia.
A, the living animal from the ventral aspect, showing the covering of
cilia, the buccal groove (to the right) ending posteriorly in the mouth
CORTEX AND MEDULLA 107
(mlh] and gullet (gul) ; several food vacuoles (/. vac], and the two
contractile vacuoles (c. vac).
B, the same in optical section, showing cuticle (cti), cortex (cort), and
medulla (med) ; buccal groove (buc. gr), mouth, and gullet (gul) ;
numerous food vacuoles (f. vac) circulating in the direction indicated
by the arrows, and containing particles of indigo, which are finally
ejected at an anal spot ; nucleus (nu) and paranucleus (pa. mi), and
trichocysts, some of which (trch) are shown with their threads ejected.
The scale to the right of this figure applies to A and B.
C, a specimen killed with osmic acid, showing the ejection of tricho-
cyst-threads, which project considerably beyond the cilia.
D, diagram of binary fission : the paranucleus (fa. mi) has already
divided, the nucleus (nu) is in the act of dividing.
(D after Lankester.)
rendered somewhat concave by the twist a wide shallow
groove beginning at the broad end and gradually narrowing
to about the middle of the body, where it ends in a tolerably
deep depression.
The groove is called the buccal groove (Fig. 21, A & B,
buc. gr.} : at the narrow end is a small aperture the mouth
(mth\ which, like the mouth of Euglena (Fig. 5) leads into
the soft internal protoplasm of the body. The surface of
the creature on which the groove is placed is distinguished
as the, ventral surface, the opposite surface being upper or
dorsal : the broad end is anterior, the narrow end posterior,
the former being directed forwards as the animalcule swims.
These descriptive terms being decided upon, it will be seen
from Fig. 21 A, that the buccal groove begins on the left side
of the body, and gradually curves over to the middle of the
ventral surface.
As the animal swims its form is seen to be permanent,
exhibiting no contractions of either an amoeboid or a
euglenoid nature. It is however distinctly flexible, often being
bent in one or other direction when passing between obstacles
such as entangled masses of weed. This permanence of
contour is due to the presence of a tolerably firm though
delicate cuticle (cii) which invests the whole surface.
io8 PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA
The protoplasm thus enclosed by the cuticle is distinctly
divisible into two portions — an external somewhat dense layer,
the cortical layer or cortex (corf], and an internal more fluid
material, the medullary substance or medulla (med}. It will be
remembered that a somewhat similar distinction of the
protoplasm into two layers is exhibited by Amoeba (p. 3), the
ectosarc being distinguished from the endosarc simply by
the absence of granules. In Paramcecium the distinction is
a far more fundamental one : the cortex is radially striated
and is comparatively firm and dense, while the medulla is
granular and semi-fluid, as may be seen from the fact that
food particles (f. vac, see below, p. no,) move freely in it,
whereas they never pass into the cortex. It has recently been
found that the medulla has a reticular structure similar to
that of the protoplasm of the ordinary animal cell (Fig. 9,
p. 62), consisting of a delicate granular network the meshes
of which are filled with a transparent material. In the
cortex the meshes of the network are closer, and so form a
comparatively dense substance. The cortex also exhibits
a superficial oblique striation, forming what is called the
my op h an layer.
The mouth (mth) leads into a short funnel-like tube, the
gullet (#«/), which is lined by cuticle and passes through the
cortex to end in the soft medulla, thus making a free com-
munication between the latter and the external water.
The cilia with which the body is covered are of approxi-
mately equal size, quite short in relation to the entire
animal, and arranged in longitudinal rows over the whole
outer surface. They consist of prolongations of the cortex,
and each passes through a minute perforation in the cuticle.
They are in constant rhythmical movement, and are thereby
distinguished from the flagella of Haematococcus, Euglena,
&c., which exhibit more or less intermittent lashing move-
CONTRACTILE VACUOLE 109
ments (see p. 25, note, and p. 59). Their rapid motion and
minute size make them somewhat difficult to see while the
Paramoecium is alive and active, but after death they are
very obvious, and look quite like a thick covering of fine
silky hairs.
Near the middle of the body, in the cortex, is a large oval
nucleus (B, nu\ which is peculiar in taking on a uniform tint
when stained, showing none of the distinction into chroma-
tin and achromatin which is so marked a feature in many
of the nuclei we have studied (see especially Fig. i, p. 2, and
Fig. 9, p. 62). It has also a further peculiarity : against one
side of it is a small oval structure (pa, mi) which is also deeply
stained by magenta or carmine. This is the paramicleus ; it
is to be considered as a kind of second, smaller nucleus.
There are two contractile vacuoles (c. vac\ one situated at
about a third of the entire length from the anterior end of the
body, the other at about the same distance from the posterior
end : they occur in the cortex.
The action of the contractile vacuoles is very beautifully
seen in a Paramoecium at rest : it is particularly striking in a
specimen subjected to slight pressure under a cover glass,
but is perfectly visible in one which has merely temporarily
suspended its active swimming movements. It is thus seen
that during the diastole, a phase of expansion of each vacuole,
a number — about six to ten — of delicate radiating, spindle-
shaped spaces filled with fluid appear round it, like the rays
of a star (upper vacuole in A & B) : the vacuole itself contracts
or performs its systole, completely disappearing from view,
and immediately afterwards the radiating canals flow together
and re-fill it, becoming themselves emptied and therefore
invisible for an instant (lower vacuole) but rapidly appearing
once more. There seems to be no doubt that the water
taken in with the food is collected into these canals, emptied
i io PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA
into the vacuole, and finally discharged into the surrounding
medium.
The process of feeding can be very conveniently studied
in Paramcecium by placing in the water some finely-divided
carmine or indigo. When the creature comes into the
neighbourhood of the coloured particles, the latter are swept
about in various directions by the action of the cilia : some
of these are however certain to be swept into the neighbour-
hood of the buccal groove and gullet, the cilia of which all
work downwards, i.e. towards the inner end of the gullet.
The grains of carmine are thus carried into the gullet, where
for an instant they lie surrounded by the water of which it is
full : then, instantaneously, probably by the contraction of
the tube itself, the animalcule performs a sort of gulp, and
the grains with an enveloping globule of water or food-vacuole
are forced into the medullary protoplasm. This process is
repeated again and again, so that in any well-nourished
Paramoecium there are to be seen numerous globular spaces
filled with water and containing particles of food — or in the
present instance of carmine or indigo. At every gulp the
newly formed food-vacuole pushes, as it were, its predecessor
before it : contraction of the medullary protoplasm also takes
place in a definite direction, and thus a circulation of food-
vacuoles is produced, as indicated in Fig. 21, B, by arrows.
After circulating in this way for some time, the water of the
food-vacuoles is gradually absorbed, being ultimately excreted
by the contractile vacuoles, so that the contained particles
come to lie in the medulla itself (refer to figure). The circu-
lation still continues, until finally the particles are brought to
a spot situated about half-way between the mouth and the
posterior end of the body : here if carefully watched they
are seen to approach the surface and then to be suddenly
ejected. The spot in question is therefore to be looked
TRICHOCYSTS in
upon as a potential anus, or aperture, for the egestion of
faeces or undigested food-matters. It is a potential and not
an actual anus, because it is not a true aperture, but only a
soft place in the cortex through which by the contractions
of the medulla solid particles are easily forced.
Of course when Paramoecium ingests, as it usually does,
not carmine but minute living organisms, the latter are
digested as they circulate through the medullary protoplasm,
and only the non-nutritious parts cast out at the anal spot.
It has been found by experiment that this infusor can digest
not only proteids but also starch and perhaps fats. The
starch is probably converted into dextrin, a carbo-hydrate
having the same formula (C6H10O5) but soluble and
diffusible. Oils or fats seem to be partly converted into
fatty acids and glycerine. The nutrition of Paramoecium is
therefore characteristically holozoic.
It was mentioned above (p. 108) that the cortex is ra-
dially striated in optical section. Careful examination with
a very high power shows that this appearance is due to the
presence in the cortex of minute spindle-shaped bodies (A
and B, trcJi) closely arranged in a single layer and perpen-
dicular to the surface. These are called trichocysts.
When a Paramoecium is killed, either by the addition of
osmic acid or some other poisonous reagent or by simple
pressure of the cover glass, it frequently assumes a remark-
able appearance. Long delicate threads suddenly appear,
projecting from its surface in all directions (c) and looking
very much as if the cilia had suddenly protruded to many
times their original length. But these filaments have really
nothing 'to do with the cilia; they are contained under
ordinary circumstances in the trichocysts, probably coiled up ;
and by the contraction of the cortex consequent upon any
sudden irritation they are projected in the way indicated.
ii2 PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA
In Fig. 21, B, a few trichocysts (trcti) are shown in the ex-
ploded condition, i.e. with the threads protruded. Most
likely these bodies are weapons of offence like the very
FIG. 22. — Stages in the conjugation of Paramcecium aurelia.
gul, gullet : n, nuclei of gametes ; p, paranuclei of gametes ; pl, /2,
products of division of paranucleus ; p. n, those products of division of
the paranucleus which afterwards unite to form the reconstituted nucleus ;
/. /, those forming the reconstituted paranucleus.
For full explanation see text below, p. 113. (After Gruber).
similar structures (nematocysts) found in polypes (see Lesson
XXII. Fig. 52).
Paramoecium multiplies by simple fission, the division of
the body being always preceded by the elongation and
subsequent division of the nucleus and paranucleus (Fig. 21,
D). As shown in the figure nuclear division is direct, there
being no formation of karyokinetic figures.
CONJUGATION 113
Conjugation also occurs, usually after multiplication by
fission has gone on for some time, but the details and the
results of the process are very different from what are found
to obtain in Heteromita (p. 62). Two Paramcecia come
into contact by their ventral faces (Fig. 22, A) the nuclei and
paranuclei break up (B — N) and are afterwards reconstituted
(o, P) : at a certain stage in the process the two conjugating
individuals or gametes separate from one another (H, i), the
union being thus a temporary one, and not followed by the
production of a zygote or by spore-formation. It is certain,
however, that conjugation has a beneficial effect, which in
all probability is due to an exchange of nuclear material.
The process is exactly comparable to that which occurs in
the sexual reproduction of multicellular animals (Lesson
XXIV.).
The conjugation of the ciliate Infusoria presents so many peculiarities
and is so difficult of observation, that very various accounts of it have
been given by skilled observers. The following abstract of Gruber's
elaborate researches on the process as it occurs in Paramoecium aurelia
may not be out of place, since the student may at any time meet with
specimens in conjugation exhibiting one or other of the complicated
phases shown in Fig. 22.
Two individuals become applied by their ventral faces (Fig. 22, A)
the paranucleus of each (/) separates from the nucleus (n}, and, after
forming the spindle characteristic of a dividing nucleus (B, /), divides
into two (c, /\ p1}. One of the products of division (pl) in each
gamete approaches the ventral face and becomes flattened out as it were
against the cuticle (D, p1} : in this way two paranuclei, one from each
gamete, are brought into intimate relations with one another, and in all
probability an exchange of nuclear material takes place between them,
although this has not been actually proved. Next, these two paranuclei
take on the form of rounded homogeneous bodies and retreat towards the
interior of the Paramoecium (E). The same process is there gone
through with the second pair of paranuclei (F, p-), so that foui homo-
geneous bodies are produced, two in each gamete (G). Each of these
takes on the spindle form (H), and divides, and at this phase the two
I
ii4 PARAMCECIUM, STYLONYCHIA, AND OXYTRICHA
gametes separate from one another (i), and are found to differ from
normal Paramcecium in having each four clear paranuclei. Division of
the latter again takes place (K), eight paranuclei being formed (L), and
at the same time the nucleus becomes band-like (K, n), and finally
breaks up into a number of separate masses (L, M). Next, four of the
eight paranuclei unite with one another, forming a single, rounded,
granular body (N, p. p), which becomes the permanent paranncleus of
the cell: at the same time the remaining four (/. n) increase in size
part passu with the gradual disappearance of the nucleus, so that at
last a Paramoecium is produced (N, o) having a single normal para-
nucleus (p, p) and four large nuclear bodies (/. «), ivhich finally unite
to form the permanent nucleus (P).
Another ciliated infusor common in stagnant water and
organic infusions is Stylonychia mytilus, an animalcule vary-
ing from yT mm. to \ mm.
Like Paramcecium it is often to be seen swimming
rapidly in the fluid, but unlike it frequently creeps about,
almost like a wood-louse or a caterpillar, on the surface
of the plants or other solid objects among which it lives.
In correspondence with this, instead of being nearly
cylindrical, it is flattened on one — the ventral — side, and
is thus irregularly plano-convex in transverse section (Fig.
23, c).
It resembles Paramoecium in general structure (compare
Fig. 23, A, with Fig. 21, A) ; but owing to the absence of
trichocysts the distinction between cortex and medulla is
less obvious : moreover, it has two nuclei («», ««) and only
one contractile vacuole (c. vac}.
But it is in the character of its cilia that Stylonychia
is most markedly distinguished from Paramoecium : these
structures, instead of being all alike both in form and size,
are modified in a very extraordinary way.
On the dorsal surface the cilia are represented only by
very minute processes of the cortex (c, d. ci.) set in longi-
tudinal grooves and exhibiting little movement. It seems
GENERAL CHARACTERS 115
probable that these are to be looked upon as vestigial or
rudimentary cilia, i.e., as the representatives of cilia which
were of the ordinary character in the ancestors of Stylo-
u
Ji.ci -
FIG. 23. — A, Stylonychia mytilus, ventral aspect, showing the buccal
groove (buc. gr.) and mouth (mth)t two nuclei (nu, mt), contractile
vacuole (c. vac), and cilia differentiated into hook-like (h. d), bristle-
like (b. ci), plate-like (/. ci), and fan-like (m. d) organs.
B, one of the plate-like cilia of the same (/. d in A), showing its
frayed extremity.
c, transverse section of Gastrostyla, an allied form to Stylonychia,
showing buccal groove (buc. gr.), small dorsal cilia (d. d), hook-like
cilium (h. d), and the various cilia of the buccal groove, including an
expanded fan-like organ (m. d). A and B after Claparede and Lach-
mann ; C after Sterki.
nychia, but which have undergone partial atrophy, or
diminution beyond the limits of usefulness, in correspond-
ence with the needs of an animalcule which has taken to
I 2
ii6 PARAMOECIUM, STYLONYCHIA, AND OXYTRICHA
creeping on its ventral surface, instead of swimming freely
and so using all its cilia equally.
On the other hand, the cilia on the ventral surface have
undergone a corresponding enlargement or hypertrophy.
Near the anterior and posterior ends and about the middle are
three groups of cilia of comparatively immense size, shaped
either like hooks (h. «'.), or like flattened rods frayed at
their ends ( p. ci. and B). All these structures neither vibrate
rhythmically like ordinary cilia nor perform lashing move-
ments like flagella, but move at the base only like one-
jointed legs. The movement is under the animal's control,
so that it is able to creep about by the aid of these hooks
and plates in much the same way as a caterpillar uses its
legs.
Notice that we have here a third form of contractility : in
amoeboid movement there is an irregular flowing of the pro-
toplasm (pp. 4 and 10) ; in ciliary movement a flexion of
a protoplasmic filament from side to side (p. 33) ; while
in the present case we have sudden contractions taking place
at irregular intervals. The movements of these locomotor
hooks and plates is therefore very similar to the muscular
contraction to which the movements of the higher animals
are due : it cannot be said that definite muscles are present
in Stylonychia, but the protoplasm in certain regions of the
unicellular body is so modified as to be able to perform a
sudden contraction in a definite direction. The nature of
muscular contraction will be further discussed in the next
Lesson (see p. 128).
The remainder of the ventral surface, with the exception
of the buccal groove is bare, but along each side of the
margin is a row of large vibratile cilia, of which three at
the posterior end are modified into long, stiff, bristle-like
processes (A, b. d ).
DIFFERENTIATION OF CILIA 117
There is also a special differentiation of the cilia of tj^e
buccal groove (buc.gr!). On its left side is a single row of
very large and powerful cilia (A and c, m a.) which are the
chief organs for causing the food-current as well as the
main swimming-organs : each has the form of a triangular
fan-like plate (c, m. a.}. On the right side of the buccal
groove is a row of smaller but still large cilia of the ordinary
form, and in the interior of the gullet a row of extremely
delicate cilia which aid in forcing particles of food down the
gullet into the medulla.
In Stylonychia and allied genera intermediate forms are
found between these peculiar hooks, plates, bristles, and
FIG. 24. — Oxytncha flava, killed and stained, showing the frag-
mentation of the nuclei. (After Gruber.)
fans and ordinary cilia ; from which we may conclude that
these diverse appendages are to be looked upon as highly
modified or differentiated cilia. Probably they have been
evolved in the course of time from ordinary cilia, and on
the principle that the more complicated or specialized
organisms are descended from simpler or more generalized
forms (see Lesson XIII.), we may consider Stylonychia as
the highly-specialized descendant of some uniformly-ciliated
progenitor.
A third series of ciliated Infusoria must be just referred
to in concluding the present Lesson. We have seen how
n8 PARAMCECIUM, STYLONYCHIA, AND OXYTRTCHA
the nucleus of a Paramoecium which has just conjugated
breaks up and apparently disappears (Fig. 22, K — o).
In Oxytricha, a genus closely resembling Stylonychia, the
two nuclei have been found to break up into a large number
of minute granules (Fig. 24), which can be seen only after
careful staining and by the use of high magnifying powers.
This process is called fragmentation of the nucleus : in
other cases it goes even further, and the nucleus is reduced
to an almost infinite number of chromatic granules only just
visible under the highest powers. From this it seems very
probable that organisms which, like Protamceba (p. 9) and
Protomyxa (p. 49), appear non-nucleate, are actually pro-
vided with a nucleus in this pulverized condition, and that
a nucleus in some form or other is an essential constituent
of the cell.
LESSON XI
OPALINA
THE large intestine of the common frog often contains
numbers of ciliate Infusoria belonging to two or three
genera. One of these parasitic animalcules, called Opalina
ranarum, will now be described. It is easily obtained by
killing a frog, , opening the body, making an incision in the
rectum, and spreading out a little of its blackish contents in
a drop of water on a slide.
Opalina has a flattened body with an oval outline (Fig.
25, A, B), and full-sized specimens may be as much as one
millimetre in length. The protoplasm is divided into cortex
and medulla, and is covered with a cuticle, and the cilia are
equal-sized and uniformly arranged in longitudinal rows -over
the whole surface (A).
On a first examination no nucleus is apparent, but after
staining a large number of nuclei can be seen (B. nu\ each
being a globular body (c, i), consisting of achromatin sur-
rounded by a membrane and containing a coil or network
of chromatin. These nuclei multiply within the body of
the infusor, and in so doing pass through the various
changes characteristic of karyokinesis or indirect nuclear
division (compare Fig. 10, p. 64, with Fig. 25, c) : the
I2O
OPALINA
chromatin breaks up (c, 2), a spindle is formed with the
chromatin across its equator (3), the chromatin passes to the
FIG. 25. — Opalina ranarum.
A, living specimen, surface view, showing longitudinal rows of cilia.
B, the same, stained, showing numerous nuclei (nu) in various stages
of division.
C, I — 6, stages in nuclear division.
D, longitudinal fission.
E, transverse fission.
F, the same in a specimen reduced in size by repeated division.
G, final product of successive divisions.
H, encysted form.
I, uninucleate form produced from cyst.
K, the same after multiplication of the nucleus has begun.
(A— c, after Pfitzner ; D— K, from Saville Kent after Zeller.)
poles of the spindle (4, 5), and the nucleus becomes con-
stricted (5), and finally divides into two (6).
PARASITIC NUTRITION 121
The presence of numerous nuclei in Opalina is a fact
worthy of special notice. The majority of the organisms
we have studied are uninucleate as well as unicellular : the
higher animals and plants we found (Lesson VI.) to consist
of numerous cells each with a nucleus, so that they are
multicellular and multinucleate : Opalina, on the other
hand, is multinucleate but unicellular. An approach to
this condition of things is furnished by Stylonychia, which is
unicellular and binucleate (Fig. 23, A), but the only organisms
we have yet studied in which numerous nuclei of the ordi-
nary character occur in an undivided mass of protoplasm are
the Mycetozoa (p. 52), and in them the multinucleate con-
dition of the plasmodium is largely due to its being formed
by the fusion of separate cells, while in Opalina it is due, as
we shall see, to the repeated binary fission of an originally
single nucleus.
There is no contractile vacuole, and no trace of either
mouth or gullet, so that the ingestion of solid food is impos-
sible. The creature lives, as already stated, in the intestine
of the frog : it is therefore an internal parasite, or endo-
parasite, having the frog as its host. The intestine contains
the partially digested food of the frog, and it is by the ab-
sorption of this that the Opalina is nourished. Having no
mouth, it feeds solely by imbibition : whether it performs
any kind of digestive process itself is not certainly known,
but the analogy of other mouthless parasites leads us to
expect that it simply absorbs food ready digested by its host,
upon which it is dependent for a constant supply of soluble
and diffusible nutriment.
Thus Opalina, in virtue of its parasitic mode of life, is
saved the performance of certain work — the work of diges-
tion, that work being done for it by its host. This is the
essence of internal parasitism : an organism exchanges a free
122 OPALINA
life, burdened with the necessity of finding food for itself, for
existence in the interior of another organism, on which in
one way or another it levies blackmail.
Note the close analogy between the nutrition of an internal
parasite like Opalina and the saprophytic nutrition of a
monad (p. 39). In both the organism absorbs proteids
rendered soluble and diffusible, in the one case by the
digestive juices of the host, in the other by the action of
putrefactive bacteria.
The reproduction of Opalina presents certain points of
interest, largely connected with its peculiar mode of life. It
is obvious that if the Opalinse simply went on multiplying,
by fission or otherwise, in the frog's intestine, the population
would soon outgrow the means of subsistence : moreover,
when the frog died there would be an end of them. What
is wanted in this as in other internal parasites is some mode of
multiplication which shall serve as a means of dispersal, or in
other words, enable the progeny of the parasite to find their
way into the bodies of other hosts, and so start new colonies
instead of remaining to impoverish the mother country.
Opalina multiplies by a somewhat peculiar process of
binary fission : an animalcule divides in an oblique direction
(Fig. 25, D), and then each half, instead of growing to the
size of the parent cell, divides again transversely (E). The
process is repeated again and again (F), the plane of division
being alternately oblique and transverse, until finally small
bodies are produced (G), about -^-"FO mm. in length, and
containing two to four nuclei.
If the parent cell had divided simultaneously into a num-
ber of these little bodies the process would have been one of
multiple fission : as it is it forms an interesting link between
simple and multiple fission.
MEANS OF DISPERSAL 123
Opalina ranarum multiplies in this way in the spring — i.e.
during the frog's breeding season. Each of the small pro-
ducts of division (G) becomes encysted (H), and in this
passive condition is passed out with the frog's excrement,
probably falling on to a water weed or other aquatic object.
Nothing further takes place unless the cyst is swallowed by
a tadpole, as must frequently happen when these creatures,
produced in immense numbers from the frogs' eggs, browse
upon the water-weeds which form their chief food.
Taken into the tadpole's intestine, the cyst is burst or
dissolved, and its contents emerge as a lanceolate mass of
protoplasm (i), containing a single nucleus and covered with
cilia. This, as it absorbs the digested food in the intestine
of its host, grows, and at the same time its nucleus divides
repeatedly (K) in the way already described, until by the time
the animalcule has attained the maximum size it has also
acquired the large number of nuclei characteristic of the
genus.
Here, then, we have another interesting case of develop-
ment (see p. 43) : the organism begins life in a very small
uninucleate mass of protoplasm, and as it increases in size
increases also in complexity by the repeated binary fission
of its nucleus.
LESSON XII
VORTICELLA AND ZOOTHAMNIUM
THE next organism we have to consider is a ciliated infusor
even commoner than those described in the two previous
lessons. It is hardly possible to examine the water of a
pond with any care without finding in it, sometimes attached
to weeds, sometimes to the legs of water-fleas, sometimes to
the sticks and stones of the bottom, numbers of exquisitely
beautiful little creatures, each like an inverted bell with a
very long handle, or a wine-glass with a very long stem.
These are the well-known " bell-animalcules ; ' the com-
monest among them belong to various species of the genus
Vorticella.
The first thing that strikes one about Vorticella
(Fig 26, A) is the fact that it is permanently fixed,
like a plant, the proximal or near end of the stalk
being always firmly fixed to some aquatic object, while to
the distal or far end the body proper of the animalcule is
attached.
But in spite of its peculiar form it presents certain very
obvious points of resemblance to Paramcecium, Stylonychia,
and Opalina. The protoplasm is divided into cortex (Fig.
26, c, corf} and medulla (ined\ and is invested with a
pe
H'
FIG. 26. — Vorticella.
A, living specimen fully expanded, showing stalk (st) with axial fibre
(ax.f), peristome (per), disc (d), mouth (mth), gullet (gull), and
contractile vacuole.
B, the same, bent on its stalk and with the disc turned away from
the observer.
c, optical section of the same, showing cuticle (CM), cortex (corf),
medulla (med), nucleus (mi), gullet (gull), several food-vacuoles, and
anus (an), as well as the stractures shown in A.
D1, a half-retracted and D2 a fully-retracted specimen, showing the
coiling of the stalk and overlapping of the disc by the peristome.
E1, commencement of binary fission ; E2, completion of the process ;
126 VORTICELLA AND ZOOTHAMNIUM
E3, the barrel-shaped product ol division swimming freely in the
direction indicated by the arrow.
F1, a specimen dividing into a megazooid and several microzooids (m) ;
F2, division into one mega- and one microzooid.
G1, Gl, two stages in conjugation showing the gradual absorption of
the microgamete (m) into the megagamete.
H1, multiple fission of encysted form, the nucleus dividing into nume-
rous masses : H2, spore formed by multiple fission ; H3 — H7, development
of the spore ; H4 is undergoing binary fission.
(E — H after Saville Kent. )
delicate cuticle (cu). There is a single contractile vacuole
(c. vac) the movements of which are very readily made out
owing to the ease with which the attached organism is kept
under observation. There is a nucleus (nu) remarkable for
its elongated band-like form, and having in its neighbour-
hood a small rounded paranucleus. Cilia are also present,
but the way in which they are disposed is very peculiar and
characteristic. To understand it we must study the form
of the body a little more closely.
The conical body is attached by its apex or proximal end
to the stalk : its base or distal end is expanded so as to form
a thickened rim, the peristome (per), within which is a plate-
like body elevated on one side, called the disc (</), 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 (mth\ 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
disc : the whole row of cilia thus takes a spiral direction
The rest of the body is completely bare of cilia.
STALK AND AXIAL FIBRE 127
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 the appearance 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 in this way that food particles are taken
in, surrounded as in Paramcecium by a globule of water :
the food-vacuoles (f. vac] thus constituted circulate in the
medullary protoplasm, and the non-nutritive parts are finally
egested at an anal spot (an) situated near the base of the
gullet.
The stalk (sf) 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.
A striking characteristic of Vorticella is its extreme
irritability, i.e., the readiness with which it responds to any
128 VORTICELLA AND ZOOTHAMNIUM
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
Paramoecium, is felt directly by the bell-animalcule and
followed by an instantaneous change in the relative position
of its parts. The stalk becomes coiled into a close spiral
(o1, D2) 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
(D1, D2).
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 con-
traction. 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
possessing the property of contractility in a special degree :
in which moreover contraction takes place in a definite
FREE-SWIMMING ZOOIDS 129
direction — the direction of 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. 26, E1, 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 (E1), and gradually
deepening until there are produced two complete and full-
sized individuals upon a single stalk (E2). 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 (E3) . 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
the numbers of the species in a given spot that the food-
supply would inevitably run short. This is prevented by
K
130 VORTICELLA AND ZOOTHAMNIUM
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 (r2) or into two equal halves, one of which divides
again into from two to eight daughter-cells (r1). 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 (c1), and under-
goes gradual absorption (c2), the mega- and microzooids
becoming completely and permanently fused. As in Para-
moecium, conjugation is followed by increased activity in
feeding and dividing (p. 113).
Notice that in this case the conjugating bodies or gametes
are not of equal size and similar characters, but one, which
is conveniently distinguished as the microgamete ( = micro-
zooid) is relatively small and active, while the other or
megagamete ( = megazooid, or ordinary individual) is rela-
tively large and passive. As we shall see in a later lesson,
this differentiation of the gametes is precisely what we get in
almost all organisms with two sexes : the microgamete being
the male, the megagamete the female conjugating body (see
Lesson XVI.).
The result of conjugation is strikingly different in the three
cases already studied : in Heteromita (p. 41) the two gametes
METAMORPHOSIS 131
unite to form a zygote, a motionless body provided with a
cell-wall, the protoplasm of which divides into spores : in
Paramcecium (p. 113) no zygote is formed, conjugation being
a mere temporary union : in Vorticella the zygote is an
actively moving and feeding body, indistinguishable from an
ordinary individual of the species.
Vorticella sometimes encysts itself (Fig. 26, H1), and the
nucleus of the encysted cell has been observed to break up
into a number of separate masses, each doubtless surrounded
by a layer of protoplasm. After a time the cyst bursts, and
a number of small bodies or spores (n2) emerge from it, each
containing one of the products of division of the nucleus.
These acquire a circlet of cilia (H3), by means of which they
swim freely, sometimes multiplying by simple fission (H4).
Finally, they settle down (H5) by the end at which the cilia
are situated, the attached end begins to elongate into a stalk
(H°), this increases in length, the basal circlet of cilia is lost,
and a ciliated peristome and disc are formed at the free end
(HT), and in this way the ordinary form is assumed by a
process of development recalling that we found to occur in
Heteromita (p. 42), but with an important difference : the
free-swimming young of Vorticella (HS), to which the spores
formed by division of the encysted protoplasm give rise,
differ strikingly in form and habits from the adult. This is
expressed by saying that development is in this case accom-
panied by a metamorphosis, this word literally meaning simply
a change, being always used in biology to express a striking
and fundamental difference in form and habit between the
young and the adult ; as, for instance, between the tadpole
and the frog, or between the caterpillar and the butterfly. It
is obvious that in the present instance metamorphosis is
another means of ensuring dispersal.
In Vorticella, as we have seen, fission results not in the
K 2
132
VORTICELLA AND ZOOTHAMNIUM
production of equal and similar daughter-cells, but of one
stalked and one free-swimming form. It is however quite
possible to conceive of a Vorticella-like organism in which
the parent cell divides into two equal and similar products,
each retaining its connection with the stalk. If this process
//.z.
FIG. 27. — Zoothamnium arlmscula.
A, entire colony, magnified, showing nutritive (n. z) and reproductive
(r. z) zooids ; (ax.f) axial fibre of the stem.
B, the same, natural size.
c, the same, magnified, in the condition of retraction.
D, nutritive zooid, showing nucleus (mi), contractile vacuole (c. vac],
gullet, and axial fibre (ax.f}.
E, reproductive zooid, showing nucleus (nit) and contractile vacuole
(c. vac), and absence of mouth and gullet.
F1, F'2, two stages in the development of the reproductive zooid.
(After Saville Kent.)
were repeated again and again, and if further the plane of
fission were extended downwards so as to include the dista-
end of the stalk, the result would be a branched, tree-like
stem with a Vorticella-like body at the end of every branch.
As a matter of fact, this process takes place not in Vorti-
DIMORPHISM 133
cella itself, but in a nearly allied infusor, the beautiful
Zoothamnium, a common genus found mostly in sea-water
attached to weeds and other objects.
Zoothamnium arbuscula (Fig. 27, A) consists of a main
stem attached by its proximal end and giving off at its distal
end several branches, on each of which numerous shortly-
stalked bell-animalcules are borne, like foxgloves or Canter-
bury-bells on their stem. The entire tree is about i cm.
high, and so can be easily seen by the naked eye : it is shown
of the natural size in Fig. 27, B.
We see, then, that Zoothamnium differs from all our
previous types in being a compound organism. The entire
" tree " is called a colony or stock, and each separate
bell animalcule borne thereon is an individual or zooid,
morphologically equivalent to a single Vorticella or
Paramcecium.
As in Vorticella the stem consists of a cuticular sheath
with an axial muscle-fibre (ax. /), which, at the distal end
of the main stem, branches like the stem itself, a prolonga-
tion of it being traceable to each zooid (D). So that the
muscular system is common to the whole colony, and any
shock causes a general contraction, the tree-like structure
assuming an almost globular form (c).
It will be noticed from the figure that all the zooids of
the colony are not alike : the majority are bell-shaped and
resemble Vorticellae (A, n. z, and D), but here and there are
found larger bodies (A, r. z, and E) of a globular form, with-
out mouth, peristome, or disc, and with a basal circlet of
cilia. The characteristic band-like nucleus (nu) and the
contractile vacuole (c. vac) are found in both the bell-shaped
and the globular zooids.
It is to these globular, mouthless zooids that the functions
of reproducing the whole colony and of ensuring dispersal
134 VORTICELLA AND ZOOTHAMNIUM
are assigned. They become detached, swim about freely
for a time, then settle down, develop a stalk and mouth
(r1, F2), and finally, by repeated fission, give rise to the
adult, tree-like colony.
The Zoothamnium colony is thus dimorphic, bearing indi-
viduals of two kinds : nutritive zooids, which feed and add to
the colony by fission, but are unable to give rise to a new
colony, and reproductive zooids, which do not feed while
attached, but are capable, after a period of free existence, of
developing a mouth and stalk, and finally producing a new
colony. Dimorphism is a differentiation of the individuals
of a colony, just as the formation of axial fibre, gullet, con-
tractile vacuole, and cilia are cases of differentiation of the
protoplasm of a single cell.
LESSON XIII
SPECIES AND THEIR ORIGIN — THE PRINCIPLES OF
CLASSIFICATION
MORE than once in the course of the foregoing lessons we
have had occasion to use the word species — for instance, in
Lesson I. (p. 8) it was stated that there were different
kinds or species of Amoebae, distinguished by the characters
of their pseudopods, the structure of their nuclei, &c.
We must now consider a little more in detail what we
mean by a species, and, as in all matters of this sort, the
study of concrete examples is the best aid to the formation
of clear conceptions, we will take, by way of illustration,
some of the various species of Zoothamnium.
The kind described in the previous lesson is called
Zoothamnium arbuscula. As Fig. 27, A, shows, it consists of a
tolerably stout main stem, from the distal end of which
spring a number of slender branches diverging in a brush-
like manner, and bearing on short secondary branchlets the
separate individuals of the colony : these are of two kinds,
bell-shaped nutritive zooids, and globular reproductive
zooids, so that the colony is dimorphic.
Zoothamnium (or, for the sake of brevity, Z.) alternans,
136
SPECIES AND THEIR ORIGIN
(Fig. 28, A) is found also in sea-water, and differs markedly
from Z. arbuscula in the general form of the colony. The
main stem is continued to the extreme distal end of the
colony and terminates in a zooid ; from it branches are
given off right and left, and on these the remaining zooids
are borne. To use Mr. Saville Kent's comparison, Z. arbus-
FIG. 28. — Species of Zoothamnium. A, Z, alternans. B, Z.
dichotomum. c, Z. simplex. D, Z. ajfine. E, Z. nutans. (After
Saville Kent.)
cula may be compared to a standard fruit tree, Z. alternans
to an espalier. In this species also the colony is dimorphic.
Z. dichotomum (Fig. 28, B) is also dimorphic and presents a
third mode of branching. The main stem divides into two,
and each of the secondary branches does the same, so that
a repeatedly forking stem is produced. The branching of
this species is said to be dichotomous^ while that of Z. alter-
nans is monopodial) and that of Z. arbuscula umbellate,
Another mode of aggregation of the zooids is found in Z.
GENERIC AND SPECIFIC CHARACTERS 137
simplex (Fig. 28, c) in which the stem is unbranched and
bears at its distal end about six zooids in a cluster. The
zooids are more elongated than in any of the preceding
species, and there are no special reproductive individuals, so
that the colony is homomorphic.
In Z. affine (Fig. 28, D) the stalk is dichotomous but is
proportionally thicker than in the preceding species, and
bears about four zooids all alike. It is found in fresh water
attached to insects and other aquatic animals.
The last species we shall consider is Z. nutans (Fig. 28, E),
which is the simplest known, never bearing more than two
zooids, and sometimes only one.
A glance at Figs. 27 and 28 will show that these six species
agree with one another in the general form of the zooids, in
the characters of the nucleus, contractile vacuole, £c., in
the arrangement of the cilia, and in the fact that they are all
compound organisms, consisting of two or more zooids
attached to a common stem, having an axial fibre which
branches with the stem, i.e., is continuous throughout the
colony.
On account of their possessing these important characters
in common, the species described are placed in the single
genus Zoothamnium, and the characters summarized in the
preceding paragraph are called generic characters. On the
other hand the points of difference between the various
species, such as the forking of the stem in Z. dichotomum,
the presence of only two zooids in Z. nutans, and so on, are
called specific characters. Similarly the name Zoothamnium^
which is common to all the species, is the generic name,
while those which are applied only to a particular species,
such as arbuscula, simplex, &c., are the specific names. As
was mentioned in the first lesson (p. 8), this is the Linnean
system of binomial nomenclature.
I38 SPECIES AND THEIR ORIGIN
It will be seen from the foregoing account that by a
species we understand an assemblage of individual or-
ganisms, whether simple or compound, which agree with one
another in all but unessential points, such as the precise
number of zooids in Zoothamnium, which may vary con-
siderably in the same species, and come, therefore, within
the limits of individual variation. Similarly, what we mean
by a genus is a group of species agreeing with one another
in the broad features of their organization, but differing in
detail, the differences being constant.
A comparison of the six species described brings out
several interesting relations between them. For instance, it
is clear that Z. arbuscula and Z. alternans are far more
complex, i.e., exhibit greater differentiation of the entire
colony, than Z. simplex or Z. nutans ; so that, within the
limits of the one genus, we have comparatively low or
generalized, and comparatively high or specialized species.
Nevertheless, a little consideration will show that we cannot
arrange the species in a single series, beginning with the
lowest and ending with the highest, for, although we should
have no hesitation in placing Z. nutans at the bottom of
such a list, it would be impossible to say whether Z. affine
was higher or lower than Z. simplex, or Z. arbuscula than
Z. alternans.
It is, however, easy to arrange the species into groups
according to some definite system. For instance, if we take
the mode of branching as a criterion, Z. nutans, affine, and
dichotomum will all be placed together as being dichoto-
mous, and Z. simplex and arbuscula as being umbellate -
the zooids of the one and the branches of the other all
springing together from the top of the main stem : on this
system Z. alternans will stand alone on account of its mono-
podial branching. Or, we may make two groups, one of
CREATION 139
dimorphic forms, including Z. arbuscula, alternans, and
dichotomum, and another of homomorphic species, including
Z. affine, simplex, and nutans. We have thus two very
obvious ways of arranging or classifying the species of
Zoothamnium, and the question arises — which of these, if
either, is the right one ? Is there any standard by which
we can judge of the accuracy of a given classification of
these or any other organisms, or does the whole thing depend
upon the fancy of the classifier, like the arrangement of
books in a library ? In other words, are all possible classi-
fications of living things more or less artificial, or is there
such a thing as natural classification ?
Suppose we were to try and classify all the members of a
given family — parents and grandparents, uncles and aunts,
cousins, second cousins, and so on. Obviously there are a
hundred ways in which it would be possible to arrange
them — into dark and fair, tall and short, curly-haired and
straight-haired, and so on. But it is equally obvious that all
these methods would be purely artificial, and that the only
natural way, i.e. the only way to show the real connection of
the various members of the family with one another would
be to classify them according to blood-relationship, in other
words, to let our classification take the form of a genea-
logical tree.
It may be said — what has this to do with the point under
discussion, the classification of the species of Zoothamnium ?
There are two theories which attempt to account for the
existence of the innumerable species of living things which
inhabit our earth : the theory of creation and the theory of
evolution.
According to the theory of creation, all the individuals of
every species existing at the present day- -the tens of
thousands of dogs, oak trees, amoebae, and what not — are
derived a natural process of descent from a single indi-
140
SPECIES AND THEIR ORIGIN
vidual, or from a pair of individuals, in each case precisely
resembling the existing descendants, which came into exist-
ance by a process outside the ordinary course of nature and
known as Creation. On this hypothesis the history of the
genus Zoothamnium would be represented by the diagram
(Fig. 29) ; each of the species being derived from a single
individual which came into existence independently of the
Existing Individuals
Z.arbuscula Z.alternans Z.dichotomum Z. simplex Z.affine Z.nutans
Ancestral Individuals
FIG. 29. — Diagram illustrating the origin of the species of
Zoothamnium by creation.1
progenitors of all the other species at some distant period of
the earth's history.
Notice that on this theory the various species are no more
actually related to one another than is either of them to
Vorticella or for the matter of that to Homo. The in-
dividuals of any one species are truly related since they all
share a common descent, but there is no more relationship
between the individuals of any two independently created
1 As the original drawing of the above figure could not be found just
as the book was ready for publication, the above was inserted to avoid
delay, for which the author is not responsible. — W. N. P., April 4, 1891.
ORGANIC EVOLUTION 141
species than between any two independently manufactured
chairs or tables. The words affinity, relationship, £c., as
applied to different species are, on the Creation Theory
purely metaphorical, and mean nothing more than that a
certain likeness or community of structure exists ; just as
we might say that an easy chair was more nearly related to a
kitchen chair than either of them to a three-legged stool.
We see therefore that on the hypothesis of creation the
varying degrees of likeness and unlikeness between the
species receive no explanation, and that we get no absolute
criterion of classification : we may arrange our organisms,
as nearly as our knowledge allows, according to their resem-
blances and differences, but the relative importance of the
characters relied on becomes a purely subjective matter.
According to the rival theory — that of Descent or Organic
Evolution — every species existing at the present day is
derived by a natural process of descent from some other
species which lived at a former period of the world's
history. If we could trace back from generation to gene-
ration the individuals of any existing species we should, on
this hypothesis, find their characters gradually change, until
finally a period was reached at which the differences were so
considerable as to necessitate the placing of the ancestral
forms in a different species from their descendants at the
present day. And in the same way if we could trace back
the species of any one genus, we should find them gradually
approach one another in structure until they finally con-
verged in a single species, differing from those now existing
but standing to all in a true parental relation.
Let us illustrate this by reference to Zoothamnium. As a
matter of fact we know nothing of the history of the genus, but
the comprehension of what is meant by the evolution of species
will be greatly facilitated by framing a working hypothesis.
142
SPECIES AND THEIR ORIGIN
Suppose that at some distant period of the world's history
there existed a Vorticella-like organism which we will call
A (Fig. 30), having the general characters of a single
stalked zooid of Zoothamnium (compare Fig. 27, F2), and
suppose that, of the numerous descendants of this form,
represented by the lines diverging from A, there were some
Branching dichotomous
Branching
umbellate
L^ "*-i 5?
?•*£%)
«t
<a
Branching
monopodial
Z. nut an s
Z dichoiomtim
Z. affine
Z. arbusciila
•*^
DIMORPHIC
\
HOMOMORPHfC
FIG. 30. — Diagram illustrating the origin of the species of
Zoothamnium by evolution.
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
not for our present purpose how it is 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
HYPOTHETICAL HISTORY OF ZOOTHAMNIUM 143
distal end of the stalk : this would result in the production
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, the division
as before affecting the stem, we should get a species (D) con-
sisting of four zooids on a dichotomous stem, like Z. afrme.
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 originate, and
again starting from B with a different mode of branching a
monopodial form (H) might arise.
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, afrlne, 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
144 SPECIES AND THEIR ORIGIN
form A. And that we get an arrangement or classification,
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 ad-
vantage over that of creation in 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 due to the repetition by each organism, in the course
of its single life, of the series of changes passed through by
HEREDITY AND VARIABILITY 145
its ancestors in the course of ages. In other words ontogeny,
or the evolution of the individual, is, in its main features, a
recapitulation of phylogeny 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 it gives rise to
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.
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 in 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. 31. The cell-body of these or-
ganisms is therefore very simple, and may be compared to
a multinucleate Amoeba with fine radiating pseudopods.
RELATIONS OF THE SHELL
147
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. 31), this is perforated by nume-
rous small holes, through which the pseudopods are pro-
truded, in others it has only one large aperture (Fig. 32),
VV ••;•''• . \C\\
! //I/ / / . .
/£/: ••'/ // /-
••'..'•." ." .* -
•'. • ." .•' •" «" ,**- .'•
FIG. 31. — A living Foiaminifer (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 filled internally by protoplasm.
L 2
148 FORAMINIFERA, RADIOLARIA, AND DIATOMS
The growth of Foraminifera is largely determined by the
hard and non-distensible character of the cell-wall, which
when once formed is incapable of being enlarged. In the
young condition they consist of a simple mass of proto-
plasm covered by a more or less rounded 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
FIG. 32. — 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.)
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
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
ARRANGEMENT OF CHAMBERS 149
chamber has a terminal aperture placing its protoplasm in
free contact with the outer world.
The new chambers may be added in a straight line (Fig.
32, A), or in a gentle curve, or in a flat spiral (Fig. 32, B),
or like the segments of a Nautilus shell, or more or less
irregularly. In this way shells of great variety and beauty
FIG. 33. — Section of one of the more complicated Foraminifera
(Alveolind], 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. 33). 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 For ami nif era (Ray Society, 1862), or in Brady's
Report on the Foraminifera of the " Challenger" Expedition,
150 FORAMINIFERA, RADIOLARIA, AND DIATOMS
in order to get some notion of the great amount of dif-
ferentiation attained by the shells of these extremely simple
organisms.
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. 34) consist of a mass of protoplasm giving
-Int. Caps.pr
cent caps
FIG. 34. — Lithocircus anintlaris, one of the Radiolaria, showing
central capsule (cent, caps.}, intra- and extra-capsular protoplasm (int.
caps, pr, ext.caps.pr}, nucleus (nn], pseudopods ( psd), silicious skeleton,
(skel), and symbiotic cells of Zooxanthella (2). (After Biitschli.)
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 (mi] of unusual size and complex structure are
found.
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.
SKELETON 151
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
beauty and complexity. One very exquisite form is shown
FIG. 35. — Skeleton of a Radiolarian (Actiuoinma), consisting of
three concentric perforated spheres — the two outer partly broken away
to show the inner — connected by radiating spicules. (From Gegenbaur
after Haeckel.)
in Fig. 35 : 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-
cystinece. — and should consult the plates of Haeckel's Die
152 FORAMINIFERA, RADIOLARIA, AND DIATOMS
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-
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. 34, 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.
34), consists of two quite distinct things, the Lithocircus in
the strict sense of the word plus large numbers of Zooxan-
thellae associated with it. The two organisms multiply quite
independently of one another : indeed, Zooxanthella has
been observed to multiply by fission after the death of the
associated Radiolarian.
This living together of two organisms is known as Sym-
biosis. It differs essentially from parasitism (see p. 121), in
which one organism preys upon another, the host deriving
no benefit but only harm from the presence of the parasite.
In symbiosis, on the contrary, the two organisms are in a
condition of mutually beneficial partnership. The carbon
dioxide and nitrogenous waste given off by the Radiolarian
serve as a constant food-supply to the Zooxanthella, which
by decomposing the carbon dioxide provides the Radiolarian
with a constant supply of oxygen, and at the same time with
GENERAL CHARACTERS 153
two important food-stuffs — starch and proteids. The Radio-
larian may therefore be said to keep the Zooxanthellae con-
stantly manured, while the Zooxanthellae in return supply
the Radiolarian with abundance of oxygen and of ready-
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 Haematococcus 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 Diatomacece, or as they are often called for the sake
of brevity Diatoms, are a group of minute organisms, in-
cluded under a very large number of genera and species, so
common that there is hardly a pond or stream in which they
do not occur in millions.
The general form of Diatoms is very various : they may be
rod-shaped, triangular, circular, and so on. Their essential
structure is, however, very uniform : the cell-body contains a
nucleus (Fig. 36, A, mi) and vacuoles (vac), as well as two
large chromatophores (chr) of a brown or yellow colour;
these are found to contain chlorophyll, the characteristic
green tint of which is veiled, as in Zooxanthella, by diatomin.
154 FORAMINIFERA, RADIOLARIA, AND DIATOMS
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
FIG. 36. — A, .semi-diagrammatic view of a diatom from its flat face,,
showing cell-wall (c. w] and protoplasm with nucleus (««), two vacuoles
(vac), ami 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. 10, c. 10') with
their overlapping girdles.
c, the same in transverse section.
D, surface view of the silicious shell of Navicula trimcala.
E, surface view of the silicious shell of Aulacodiscus sollitlianus.
(D, after Donkin ; E, after Norman.)
is however the structure of the cell-wall : it consists of two
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
CELL-WALLS 155
one valve (c. ?v} fits over that of the other (c. w'} like the
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
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. 36, D and E, but, in order to form some con-
ception of the extraordinary variety in form and ornamenta-
tions, 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 everyone 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 dung : 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
GENERAL CHARACTERS 157
threads projecting from the surface of the mouldy substance ;
and these free filaments (Fig. 37, A, a. hy) can be easily
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 hyplm ; and the fila-
ments which grow out into the air and give the characteristic
fluffy appearance to the growth are aerial hyp/ice.
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-S 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.
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.
FIG. 37. — Mucor.
A, portion of mycelium of M. mucedo (my) with two aerial hyphse
a. hy], each ending in a sporangium (spg).
B, small portion of an aerial hypha, highly magnified, showing pro-
toplasm (plsm) and cell-wall (c. tv). The scale above applies to this
figure only.
C1, immature sporangium, showing septum (scp) and undivided pro-
toplasm : c'2, mature sporangium in which the protoplasm has divided
into spores ; the septum (sej>) has become very convex, distally forming
the columella.
D\ mature sporangium in the act of dehiscence, showing the spores
(sp) surrounded by mucilage (g] ; D'!, small portion of the same, more
highly magnified, showing spicules of calcium oxalate attached to wall.
E, a columella, left by complete dehiscence of a sporangium, showing
the attachment of the latter as a black band.
FORMATION OF SPORANGIA 159
The scale above c- applies to c1, c2, D1, and E.
F, spores.
G1, G2, G3, three stages in the germination of the spores.
H, a group of germinating spores forming a small mycelium.
I1 — 16; five stages in conjugation, showing two gametes (gam) uniting
to form the zygote (zyg).
K1, K'2, development of ferment cells from submerged hyphse.
(A, C'J, D, E, F, G, and K, after Howes ; I, after De Bary. )
By the employment of the usual reagents, it can be ascer-
tained that the granular substance is protoplasm, and the
surrounding membrane cellulose. The protoplasm moreover
contains vacuoles at irregular intervals, and numerous small
nuclei.
Thus a hypha of Mucor consists of precisely the same
constituents as a yeast cell — protoplasm, containing nuclei
and vacuoles, surrounded by cellulose. Imagine a yeast
cell to be pulled out — as one might pull out a sphere of clay
or putty — until it assumed the form of a long, narrow cylin-
der, and suppose it also to be pulled out laterally at intervals
so as to form branches : there would be produced by such a
process a very good imitation of a hypha of Mucor. We
may therefore look upon a hypha as an elongated and
branched cell, so that Mucor is, like Opalina, a multinucleate
but unicellular organism. We shall see directly however
that this is strictly true only of the mould in its young state.
As stated above, the aerial hyphse are at first of even
calibre, but gradually swell at their ends, forming sporangia.
Under the microscope the distal end of an aerial hypha is
found to dilate (Fig. 37, c1) : immediately below the dilata-
tion the protoplasm divides at right angles to the long axis
of the hypha, the protoplasm in the dilated portion thus
becoming separated from the rest. Between the two a
cellulose partition or septum (sep) is formed, as in the ordi-
nary division of a plant-cell (Fig. n, p. 66). The portion
thus separated is the rudiment of a sporangium.
160 MUCOR
Let us consider precisely what this process implies. Before
it takes place the protoplasm is continuous throughout the
whole organism, which is therefore comparable to the un-
divided plant cell shown in Fig. n, A. As in that case, the
protoplasm divides into two and a new layer of cellulose is
formed between the daughter-cells. Only whereas in the
ordinary vegetable cell the products of division are of equal
size (Fig. n, E), in Mucor they are very unequal, one being
the comparatively small sporangium, the other the rest of
the hypha.
Thus a Mucor-plant with a single aerial hypha becomes,
by the formation of a sporangium, bicellular : if, as is ordi-
narily the case, it bears numerous aerial hyphae, each with
its sporangium, it is multicellular.
Under unfavourable conditions of nutrition, septa fre-
quently appear at more or less irregular intervals in the
mycelial hyphse : the organism is then very obviously multi-
cellular, being formed of numerous cylindrical cells arranged
end to end.
The sporangium continues to grow, and as it does so the
septum becomes more and more convex upwards, finally
taking the form of a short, club-shaped projection, the colu-
mella, extending into the interior of the sporangium (c2). At
the same time the protoplasm of the sporangium undergoes
multiple fission, becoming divided into numerous ovoid
masses, each of which surrounds itself with a cellulose coat
and becomes a spore (p1, D2, sp). A certain amount of the
protoplasm remains unused in the formation of spores, and
is converted into a gelatinous material (g), which swells up
in water.
The original cell-wall of the sporangium is left as an
exceedingly delicate, brittle shell around the spores : minute
needle-like crystals of calcium oxalate are deposited in it.
STRUCTURE OF SPORES
161
and give it the appearance of being closely covered with
short cilia (D2).
In the ripe sporangium the slightest touch suffices to
rupture the brittle wall and liberate the spores, which are
dispersed by the swelling of the transparent intermediate
substance. The aerial hypha is then left terminated by the
columella (E), around the base of which is seen a narrow
black ring indicating the place of attachment of the
sporangium.
The spores (F) are clear, bright-looking, ovoidal bodies
FIG. 38. — Moist chamber formed by cementing a ring of glass or
metal (c) on an ordinary glass slide (A), and placing over it a cover-slip
(B) on the underside of which is a hanging drop of nutrient fluid (p).
The upper figure shows the apparatus in perspective, the lower in
vertical section. (From Klein.)
consisting of protoplasm containing a nucleus and surround-
ed by a thick cell-wall. A spore is therefore an ordinary
encysted cell, quite comparable to a yeast-cell.
The development of the spores is a very instructive process,
and can be easily studied in the following way : A glass or
metal ring (Fig. 38, c) is cemented to an ordinary microscopic
slide (A) so as to form a shallow cylindrical chamber. The
top of the ring is oiled, and on it is placed a cover glass (B),
with a drop of Pasteur's solution on its under surface.
Before placing the cover-glass in position a ripe sporangium
M
162 MUCOR
of Mucor is touched with the point of a needle, which is
then stirred round in the drop of Pasteur's solution, so as to
sow it with spores. By this method the drop of nutrient
fluid is prevented from evaporating, and the changes under-
gone by the spores can be watched by examination from time
to time under a high power.
The first thing that happens to a spore under these con-
ditions is that it increases in size by imbibition of fluid, and
instead of appearing bright and clear becomes granular and
develops one or more vacuoles. Its resemblance to a
yeast-cell is now more striking than ever. Next the spore
becomes bulged out in one or more places (o1, Fig. 37) looking
not unlike a budding Saccharomyces. The buds, however,
instead of becoming detached increase in length until they
become filaments of a diameter slightly less than that of the
spore and somewhat bluntly pointed at the end (c2). These
filaments continue to grow, giving off as they do so side
branches (c3) which interlace with similar threads from
adjacent spores (H). The filaments are obviously hyphae,
and the interlacement is a mycelium.
Thus the statement made in a previous paragraph (p. 159),
that Mucor was comparable to a yeast-cell pulled out into a
filament, is seen to be fully justified by the facts of develop-
ment, which show that the branched hyph?e constituting the
Mucor-plant are formed by the growth of spores each strictly
comparable to a single Saccharomyces.
It will be noticed that the growth of the mycelium is cen-
trifugal : each spore or group of spores serves as a centre
from which hyphae radiate in all directions (H), continuing
to grow in a radial direction until, in place of one or more
spores quite invisible to the naked eye, we have a white
patch more or less circular in outline, and having the spores
from which the growth proceeded in its centre. Owing to
CONJUGATION 163
the centrifugal mode of growth the mycelium is always
thicker at the centre than towards the circumference, since
it is the older or more central portions of the hyphae which
have had most time to branch and become interlaced with
one another.
Under certain circumstances a peculiar process of con-
jugation occurs in Mucor. Two adjacent hyphae send out
short branches (Fig. 37, i1) which come into contact with
one another by their free somewhat swollen ends (i2). In
each a septum appears so as to shut off a separate terminal
cell (i3, gam) from the rest of the hypha. The opposed
walls of the two cells then become absorbed (i4) and their
contents mingle, forming a single mass of protoplasm
(i5, zyg), the cell-wall of which becomes greatly thickened
and divided into two layers, an inner delicate and trans-
parent, and an outer dark in colour, of considerable thick-
ness, and frequently ornamented with spines.
Obviously the swollen terminal cells (gam) of the short
lateral hyphae are gametes or conjugating bodies, and the
large spore-like structure (zyg) resulting from their union is
a zygote. The striking feature of the process is that the
gametes are non-motile, save in so far as their growth
towards one another is a mode of motion. In Heteromita
both gametes are active and free-swimming (p. 41) : in
Vorticella one is free-swimming, the other fixed but still
capable of active movement (p. 130) ; here both conjugating
bodies only exhibit the slow movement in one direction due
to growth.
There are equally important differences in the result of
the process in the three cases. In Heteromita the proto-
plasm of the zygote breaks up almost immediately into
spores ; in Vorticella the zygote is active and the result of
M 2
1 64 MUCOR
conjugation is merely increased activity in feeding and fissive
multiplication ; in Mucor the zygote remains inactive for a
longer or shorter time, and then under favourable conditions
germinates in much the same way as an ordinary spore?
forming a mycelium from which sporangium-bearing aerial
hyphse arise. A resting zygote of this kind, formed by the
conjugation of equal-sized gametes, is often distinguished as
•»
a zygospore.
Notice that differentiation of a very important kind is
exhibited by Mucor. In relation with its comparatively
large size the function of reproduction is not performed by
the whole organism, as in all previously studied types, but a
certain portion of the protoplasm becomes shut off from the
rest, and to it — as spore or gamete — the office of reproduc-
ing the entire organism is assigned. So that we have for
the first time true reproductive organs, which may be of two
kinds, asexual — the sporangia, and sexual — the gametes.1
In describing the reproduction of Amoeba it was pointed
out (p. 20) that as the entire organism divided into two
daughter-cells, each of which began an independent life, an
Amoeba could not be said ever to die a natural death. The
same thing is true of the other unicellular forms we have
considered in the majority of which the entire organism
produces by simple fission two new individuals.2 But in
Mucor the state of things is entirely altered. A compara-
tively small part of the organism is set apart for repro-
1 In Mucor no distinction can be drawn between the conjugating
body (gamete) and the organ which produces it (gonad). See the de-
scription of the sexual process in Vaucheria (Lesson XVI.) and in
Spirogyra (Lesson XIX.).
2 An exception is formed by colonial orms such as Zoothamnium, in
which life is carried on from generation to generation by the reproduc-
tive zooids only. In all probability the colony itself, like an annual
plant, dies down after a longer or shorter time.
FERMENT CELLS 165
duction, and it is only the reproductive cells thus formed-
spores or zygote — which carry on the life of the species :
the remainder of the organism, having exhausted the
available food supply and produced the largest possible
number of reproductive products, dies. That is, all vital
manifestations, such as nutrition, cease and decomposition
sets in, the protoplasm becoming converted into pro-
gressively simpler compounds, the final stages being chiefly
carbon dioxide, water, and ammonia.
Mucor is able to grow either in Pasteur's or some similar
nutrient solution, or on various organic matters such as
bread, jam, manure, &c. In the latter cases it appears to
perform some fermentative action, since food which has
become " mouldy " is found to have experienced a definite
change in appearance and flavour without actual putre-
faction. When growing on decomposing organic matter, as
it often does, the nutrition of Mucor is saprophytic, but in
some instances, as when it grows on bread, it seems to
approach very closely to the holozoic method. M. stolo-
nifer is also known to send its hyphae into the interior of
ripe fruits, causing them to rot, and thus acting as a para-
site. The parasitism in this case is, however, obviously not
quite the same thing as that of Opalina (p. 121) : the Mucor
feeds not upon the ready digested food of its host but upon
its actual living substance which it digests by the action of
its own ferments. Thus a parasitic fungus such as Mucor,
unlike an endo-parasitic animal such as Opalina or a tape-
worm, is no more exempted from the work of digestion
than a dog or a sheep : the organism upon which it lives
is to be looked upon rather as its prey than as its host.
It is a remarkable circumstance that, under certain con-
ditions, Mucor is capable of exciting alcoholic fermentation
1 66 MUCOR
in a saccharine solution. When the hyphse are submerged
in such a fluid they have been found to break up, forming
rounded cells (Fig. 37, K1, K2), which not only resemble
yeast-cells in appearance but are able like them to set up
alcoholic fermentation.
The aerial hyphse of Mucor exhibit in an interesting way
what is known as heliotropism, i.e., a tendency to turn to-
wards the light. This is very marked if a growth of the
fungus is placed in a room lighted from one side : the long
aerial hyphae all bend towards the window. This is due to
the fact that growth is more rapid on the side of each hypha
turned away from the light than on the more strongly
illuminated aspect.
LESSON XVI
VAUCHERIA AND CAULERPA
STAGNANT ponds, puddles, and other pieces of still, fresh
water usually contain a quantity of green scum which in the
undisturbed condition shows no distinction of parts to the
naked eye, but appears like a homogeneous slime full of
bubbles if exposed to sunlight. If a little of the scum
is spread out in a saucer of water, it is seen to be com-
posed of great numbers of loosely interwoven green
filaments.
There are many organisms which have this general naked-
eye character, all of them belonging to the Algce, a group
of plants which includes most of the smaller fresh-water
weeds, and the vast majority of sea-weeds. One of these
filamentous Alg^e, occurring in the form of dark-green
thickly-matted threads is called Vaucheria. Besides occur-
ring in water it is often found on the surface of moist soil,
e.g., on the pots in conservatories.
Examined microscopically the organism is found to consist
of cylindrical filaments with rounded ends and occasionally
branched (Fig. 39, A). Each filament has an outer cover-
ing of cellulose (B, c.w} within which is protoplasm con-
taining a vacuole so large that the protoplasm has the
ths mm
FIG. 39. — Vauckeria.
A, tangled filaments of the living plant, showing mode of branching.
B, extremity of a filament, showing cell-wall (c. w] and protoplasm
with chromatophores (chr), and oil-drops (o). The scale above applies
to this figure only.
c1, immature sporangium (.<•/><;•) separated from the filament by a sep-
tum ; C-, mature sporangium with the spore (.«/>) in the act of escaping ;
c:;, free-swimming spore, showing cilia, colourless ectoplasm containing
ASEXUAL REPRODUCTION 169
nuclei, and endoplasm containing the green chromatophores ; c4, the
same at the commencement of germination.
D1, early, and D2, later stages in the development of the gonads, the
spermary to the left, the ovary to the right ; Da, the fully-formed
spermary (spy) and ovary (ovy], each separated by a septum (sep) from
the filament.
D4, the ovary after dehiscence, showing the ovum (ov), with small
detached portion of protoplasm ; D5, sperms ; D6, distal end of ripe
ovary, showing sperms (sp] passing through the aperture towards the
ovum (ov).
D7, the gonads after fertilization, showing the oosperm (osp) still
inclosed in the ovary and the dehisced spermary.
E1, oosperm about to germinate : E", further stage in germination.
(c1 and C3, after Strasburger ; c2 and C4, after Sachs ; D and E, after
Pringsheim.)
character of a membrane lining the cellulose coat.
Numerous small nuclei occur in the protoplasm, as well as
oil-globules (o), and small, close-set, ovoid chromatophores
(chr) coloured with chlorophyll and containing starch.
Thus a Vaucheria-plant, like a Mucor-plant, is comparable
to a single multinucleate cell, extended in one dimension of
space so as to take on the form of a filament.
Various modes of asexual reproduction occur in different
species of Vaucheria : of these we need only consider that
which obtains in V. sessilis. In this species the end of a
branch swells up (c1) and becomes divided off by a septum
(sep), forming a sporangium (spg) in principle like that of
Mucor, but differing in shape. The protoplasm of the
sporangium does not divide, but separates itself from -the
wall, and takes on the form of a single naked ovoidal spore
(c3), formed of a colourless cortical layer containing nume-
rous nuclei and giving off cilia arranged in pairs, and of an
inner or medullary substance containing numerous chroma-
tophores.
The wall of the sporangium splits at its distal end (c2),
and the contained spore (sp) escapes and swims freely in the
water for some time by the vibration of its cilia (c3). After
170 VAUCHERIA AND CAULERPA
a short active life it comes to rest, develops a cell-wall, and
germinates (c4), i.e., gives out one or more processes which
extend and take on the form of ordinary Vaucheria-filaments.
so that in the present case, as in Mucor (p. 162), the de-
velopment of the plant shows it to be a single immensely
elongated multinucleate cell.
In its mode of sexual reproduction Vaucheria differs
strikingly not only from Mucor, but from all the organisms
we have hitherto studied.
The filaments are often found to bear small lateral pro-
cesses arranged in pairs (D1), and each consisting of a little
bud growing from the filament and quite continuous with it.
These are the rudiments of the sexual reproductive organs
or gonads. The shorter of the two becomes swollen and
rounded (D2), and afterwards bluntly pointed (DS, ovy) : its
protoplasm becomes divided from that of the filament, and
a septum (o3, sep'} is formed between the two : the new cell
thus constituted is the ovary.1 The longer of the two buds
undergoes further elongation and becomes bent upon itself
(o2) its distal portion is then divided off by a septum (DS,
sep) forming a separate cell (spy), the spermary?
Further changes take place which are quite different in
the two organs. At the bluntly-pointed distal end of the
ovary the cell-wall becomes gelatinized and the protoplasm
protrudes through it as a small prominence which divides
off and is lost (D4). The remainder of the protoplasm then
separates from the wall of the ovary and becomes a naked
cell, the ovum '•'' or egg-cell (D4, ov), which, by the gelatiniza-
tion and subsequent disappearance of a portion of the
1 Usually called the oogoninm.
Usually called the anthcriiliiun.
I'Yequciitly called oosfhcrc.
OOSPERM 171
wall of the ovary, is in free contact with the surrounding
water.
At the same time the protoplasm of the spermary under-
goes multiple fission, becoming converted into numerous
minute green bodies (D5), each with two flagella, called
sperms}- These are liberated by the rupture of the spermary
(DT) at its distal end, and swim freely in the water.
Some of the sperms make their way to an ovary, and, as
it has been expressed, seem to grope about for the aperture,
which they finally pass through (DG), and are then seen
moving actively in the space between the aperture and the
colourless distal end of the ovum. One of them, and pro-
bably only one, then attaches itself to the ovum and be-
comes completely united with it, forming the oosperm^ a
body which we must carefully distinguish from the ovum,
since, while agreeing with the latter in form, and size, it
differs in having incorporated with it the substance of a
sperm.
Almost immediately the oosperm (D", osp) surrounds itself
with a cellulose wall, and numerous oil-globules are formed
in its interior. It becomes detached from the ovary, and,
after a period of rest, germinates (E1, E2) and forms a new
Vaucheria plant.
It is obvious that the fusion of the sperm with the ovum
is a process of conjugation in which the conjugating bodies
differ strikingly in form and size, one — the megagamete or
ovum — being large, stationary, and more or less amoeboid ;
the other — the microgamete or sperm — small, active, and
flagellate. In other words, we have a more obvious case of
sexual differentiation than was found to occur in Vorticella
(p. 130) : the large inactive egg-cell which furnishes by far
1 Often called spermatozooids or antherozooids.
• Often called oospore.
172
VAUCHERIA AND CAULERPA
the greater portion of the material of the oosperm is the
female gamete ; the small active sperm-cell, the function of
which is probably (see Lesson XXIV.) to furnish additional
nuclear material, is the male gamete.
Similarly the oosperm is evidently a zygote, but a zygote
formed by the union of the highly differentiated gametes,
FlG. 40. — Caulerpa scalpelliformis (§ nat. size), showing the stem-
like, root-like, and leaf-like portions of the unicellular plant. (After
Hervey.)
ovum and sperm, just as a zygospore (p. 164) is one formed
by the union of equal-sized gametes.
As we shall see, this form of conjugation — often distin-
guished as fertilization — occurs in a large proportion of
flowerless plants, such as mosses and ferns (Lessons XXVIII.
and XXIX.), as well as in all animals but the very lowest.
From lowly water-weeds up to ferns and club-mosses, and
from sponges and polypes up to man, the process of sexual
reproduction is essentially the same, consisting in the conju-
gation of a microgamete or sperm with a megagamete or
ovum, a zygote, the oosperm or unicellular embryo, being
MAXIMUM DIFFERENTIATION 173
produced, which afterwards develops into an independent
plant or animal of the new generation. It is a truly remark-
able circumstance that what we may consider as the highest
form of the sexual process should make its appearance so
low down in the scale of life.
The nutrition of Vaucheria is purely holophytic : its food
consists of a watery solution of mineral salts and of carbon
dioxide, the latter being split up, by the action of the chro-
matophores, into carbon and oxygen.
Mucor and Vaucheria are examples of unicellular plants
which attain some complexity by elongation and branching.
The maximum differentiation attainable in this way by a
unicellular plant may be illustrated by a brief description of
a sea-weed belonging to the genus Caulerpa.
Caulerpa (Fig. 40) is commonly found in rock-pools
between tide-marks, and has the form of a creeping stem
from which root-like fibres are given off downwards and
branched leaf-like organs upwards. These " leaves " may
attain a length of 30 cm. (i ft.) or more. So that, on a
superficial examination, Caulerpa appears to be as complex
an organism as a moss (compare Fig. 40 with Fig. 81, A).
But microscopical examination shows that the plant consists
of a single continuous mass of vacuolated protoplasm,
containing numerous nuclei and green chromatophores and
covered by a continuous cell-wall. Large and complicated
in form as it is, the whole plant is therefore nothing more
than a single branched cell, or, as it may be expressed, a
continuous mass of protoplasm in which no cellular structure
has appeared.
LESSON XVII
THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS
HITHERTO the words "animal' and "plant" have been
either avoided altogether or used incidentally without any
attempt at definition. We are now however in a position to
consider in some detail the precise meaning of the two words,
since in the last half-dozen lessons we have been dealing
with several organisms which can be assigned without hesi-
tation to one or other of the two great groups of living things.
No one would dream of calling Paramcecium and Stylonychia
plants, or Mucor and Vaucheria animals, and we may there-
fore use these forms as a starting-point in an attempt to form
a clear conception of what the words plant and animal really
signify, and how far it is possible to place the lowly organisms
described in the earlier lessons in either the vegetable or the
animal kingdom.
Let us consider, first of all, the chief points of resemblance
and of difference between the indubitable animal Paramcecium
on the one hand, and the two indubitable plants Mucor and
Vaucheria on the other.
In the first place, the essential constituents of all three
organisms is protoplasm, in which is contained one or more
nuclei. But in Paramcecium the protoplasm is invested
COMPARISON OF TYPICAL FORMS 175
only by a delicate cuticle interrupted at the mouth and anus,
while in Mucor and Vaucheria the outer layer is formed by
a firm, continuous covering of cellulose.
We thus have as the first morphological difference between
our selected animal and vegetable organisms the absence of
a cellulose cell-wall in the former and its presence in the
latter. This is a fundamental distinction, and applies
equally well to the higher forms. The constituent cells of
plants are in nearly all cases covered with a cellulose coat
(p. 60), while there is no case among the higher animals of
cells being so invested.
Next, let us take a physiological character. In all three
organisms there is constant waste of substance which has to
be made good by the conversion of food material into proto-
plasm : in other words, constructive and destructive meta-
bolism are continually being carried on. But when we come
to the nature of the food and the mode of its reception, we
meet at once with a very fundamental difference. In Para-
mcecium the food consists of living organisms taken whole
into the interior of the body, and the digestion of this solid
proteinaceous food is the necessary prelude to constructive
metabolism. In Vaucheria the food consists of a watery
solution of carbon dioxide and mineral salts — i.e. it is liquid
and inorganic, its nitrogen being in the form of nitrates or
of simple ammonia compounds. Mucor, like Paramcecium,
contains no chlorophyll, and is therefore unable to use
carbon dioxide as a food : like Vaucheria, it is prevented
by its continuous cellulose investment from ingesting solid
food, and is dependent upon an aqueous solution. It takes
its carbon in the form of sugar or some such compound,
while it can make use of nitrogen either in the simple form
of a nitrate or an ammonia salt, or in the complex form of
proteids or peptones.
1 76 CHARACTERS OF ANIMALS AND PLANTS
In this case also our selected organisms agree with animals
and plants generally. Animals, with the exception of some
internal parasites, ingest solid food, and they must all have
their nitrogen supplied in the form of proteids, being unable
to build up their protoplasm from simpler compounds.
Plants take their food in the form of a watery solution :
those which possess chlorophyll take their carbon in the
form of carbon dioxide and their nitrogen in that of a nitrate
or ammonia salt : those devoid of chlorophyll cannot make
use of carbon dioxide as a food, and are able to obtain
nitrogen either from simple salts or from proteids. Chloro-
phyll-less plants are therefore nourished partly like green
plants, partly like animals.
This difference in the character of the food is connected
with a morphological difference. Animals have, as a rule,
an ingestive aperture or mouth, and some kind of digestive
cavity, either permanent (stomach) or temporary (food-
vacuole). In plants neither of these structures exists.
Another difference which was referred to at length in an
early lesson (p. 32), is not strictly one between plants and
animals, but between organisms with and organisms without
chlorophyll. It is that in green plants the nutritive processes
result in deoxidation, more oxygen being given out than is
taken in : while in animals and not-green plants the precise
contrary is the case.
There is also a difference in the method of excretion. In
Paramcecium there is a special structure, the contractile
vacuole, which collects the superfluous water taken in with
the food and expels it, doubtless along with nitrogenous and
other waste matters. In Vaucheria and Mucor there is no
contractile vacuole, and excretion is simply performed by
diffusion from the general surface of the organism into the
surrounding medium.
DEFINITION 177
This character also is of some general importance. The
large majority of animals possess a special organ of excre-
tion, plants have nothing of the kind.
Another difference has to do with the general form of the
organism. Paramoecium has a certain definite and constant
shape, and when once formed produces no new parts.
Vaucheria and Mucor are constantly forming new branches,
so that their shape is always changing and their growth can
never be said to be complete.
Finally, we have what is perhaps the most obvious and
striking distinction of all. Paramoecium possesses in a con-
spicuous degree the power of automatic movement ; in both
Mucor and Vaucheria the organism, as a whole, exhibits no
automatism but only the slow movements of growth. The
spores and sperms of Vaucheria are, however, actively
motile.
%
Thus, taking Paramcecium as a type of animals, and
Mucor and Vaucheria as types of plants, we may frame the
following definitions : —
Animals are organisms of fixed and definite form, in
which the cell-body is not covered with a cellulose wall.
They ingest solid proteinaceous food, their nutritive pro-
cesses result in oxidation, they have a definite organ of
excretion, and are capable of automatic movement.
Plants are organisms of constantly varying form in which
the cell-body is surrounded by a cellulose wall ; they cannot
ingest solid food, but are nourished by a watery solution of
nutrient materials. If chlorophyll is present the carbon
dioxide of the air serves as a source of carbon, nitrogen is
obtained from simple salts, and the nutritive processes
result in deoxidation ; if chlorophyll is absent carbon is
obtained from sugar or some similar compound, nitrogen,
N
178 CHARACTERS OF ANIMALS AND PLANTS
either from simple salts or from proteids, and the process of
nutrition is one of oxidation. There is no special excretory
organ, and, except in the case of certain reproductive bodies
there is no locomotion as a rule.
Let us now apply these definitions to the simple forms
described in the first eight lessons, and see how far they
will help us in placing those organisms in one or other of
the two " kingdoms " into which living things are divided.
Amoeba has a cell-wall, probably nitrogenous, in the
resting condition : it ingests solid proteids, its nutrition being
therefore holozoic : it has a contractile vacuole : and it
performs amoeboid movements. It may therefore be safely
considered as an animal.
Haematococcus has a cellulose wall : it contains chloro-
phyll and its nutrition is purely holophytic : a contractile
vacuole is present in H. lacustris but absent in H. pluvialis :
and its movements are ciliary.
Euglena has a cellulose wall in the encysted state : in
virtue of its chlorophyll it is nourished by the absorption of
carbon dioxide and mineral salts, but it can also ingest solid
food through a special mouth and gullet : it has a contractile
vacuole, and performs both euglenoid and ciliary move-
ments.
In both these organisms we evidently have conflicting
characters : the cellulose wall and holophytic nutrition
would place them both among plants, while from the con-
tractile vacuole and active movements of both genera and
from the holozoic nutrition of Euglena we should group
them with animals. That the difficulty is by no means
easily overcome may be seen from the fact that both genera
are claimed at the present day both by zoologists and by
DISCUSSION OF DOUBTFUL FORMS 179
botanists. For instance, Prof. Huxley considers Hasma-
tococcus as a plant, and expresses doubts about Euglena :
Mr. Saville Kent ranks Haematococcus as a plant and
Euglena as an animal ; Prof. Sachs and Mr. Thiselton
Dyer place both genera in the vegetable kingdom : while
Profs. Ray Lankester and Biitschli group them both among
animals.
In Heteromita the only cell-wall is the delicate cuticle
wrhich in the zygote is firm enough to hold the spores up to
the moment of their escape : food is taken exclusively by
absorption and nutrition is wholly saprophytic : there is a
contractile vacuole, and the movements are ciliary.
Here again the characters are conflicting : the probable
absence of cellulose, the contractile vacuole and the cilia
all have an "animal" look, but the mode of nutrition is
that of a fungus.
In Protomyxa there is a decided preponderance of animal
characteristics — ingestion 01 living prey, and both amoeboid
and ciliary movements. There is no chlorophyll, and the
composition of the cell-wall is not known.
In the Mycetozoa, the life history of which so closely
resembles that of Protomyxa, the cyst in the resting stage
consists of cellulose, and so does the cell-wall of the spore :
nutrition is holozoic, a contractile vacuole is present in the
flagellulae, and both amoeboid and ciliary movements are
performed. Here again we have a puzzling combination of
animal and vegetable characters, and as a consequence we
find these organisms included among plants — under the
name of Myxomycetes or " slime-fungi ' -by Sachs and
Goebel, while De Bary, Biitschli, and Ray Lankester place
them in the animal kingdom.
In Saccharomyces there is a clear preponderance of
vegetable characters. The cell-wall consists of cellulose,
N 2
i8o CHARACTERS OF ANIMALS AND PLANTS
nutrition takes place by absorption and proteids are not essen-
tial, there is no contractile vacuole, and no motile phase.
Lastly, in the Bacteria the cell-wall is composed of cellu-
lose, nutrition is saprophytic, there is no contractile vacuole,
and the movements are ciliary. So that in all the characters
named, save in the presence of cellulose and the absence of
a contractile vacuole, the Bacteria agree with Heteromita,
yet they are universally — except by Prof. Glaus — placed
among plants, while Heteromita is as constantly included
among animals.
We see then that while it is quite easy to divide the higher
organisms into the two distinct groups of plants and animals,
any such separation is by no means easy in the case of the
lowest forms of life. It was in recognition of this fact that
Haeckel proposed many years ago to institute a third
"kingdom," called Protista, to include all unicellular organ-
isms. Although open to many objections in practice, there
is a great deal to be said for the proposal. From the strictly
scientific point of view it is quite as justifiable to make three
subdivisions of living things as two : the line between animals
and plants is quite as arbitrary as that between protists and
plants or between protists and animals, and no more so : the
chief objection to the change is that it doubles the difficulties
by making two artificial boundaries instead of one.
The important point for the student to recognize is that
these boundaries are artificial, and that there are no scientific
frontiers in Nature. As in the liquefaction of gases there is
a " critical point " at which the substance under experiment
is neither gaseous nor liquid : as in a mountainous country
it is impossible to say where mountain ends and valley
begins : as in the development of an animal it is futile to
argue about the exact period when, for instance, the egg
EVOLUTION OF THE TWO KINGDOMS 181
becomes a tadpole or the tadpole a frog : so in the case
under discussion. The distinction between the higher
plants and animals is perfectly sharp and obvious, but when
the two groups are traced downwards they are found
gradually to merge, as it were, into an assemblage of organ-
isms which partake of the characters of both kingdoms, and
cannot without a certain violence be either included in or
excluded from either. Where any given " protist " has to
be classified the case must be decided on its individual
merits : the organism must be compared in detail with all
those which resemble it closely in structure, physiology, and
life-history : and then a balance must be struck and the
doubtful form placed in the kingdom with which it has, on
the whole, most points in common.
It will no doubt occur to the reader that, on the theory of
evolution, we may account for the fact of the animal and
vegetable kingdoms being related to one another like two
trees united at the root, by the hypothesis that the earliest
organisms were protists, and that from them animals and
plants were evolved along divergent lines of descent.
LESSON XVIII
PENICILLIUM AND AGARICUS
ONE of the commonest and most familiar of the lower
organisms is the " green mould ': which so quickly covers
with a thick sage-green growth any organic substances ex-
posed to damp, such as paste, jam, cheese, leather, &c.
This mould is a plant belonging, like Mucor, to the group
of Fungi, and is called Penicillium glaucum.
Examined with the naked eye a growth of Penicillium is
seen to have a powdery appearance, and if the finger is
passed over it a quantity of extremely fine dust of a sage-
green colour comes away. This dust consists, as we shall
see, of the spores of Penicillium. The best way to study
the plant is to sow some of the spores in a saucer of
Pasteur's solution by drawing a needle or brush over a
growth of the mould and stirring it round in the fluid.
It is as well to study the naked-eye appearance first. If
the quantity of spores taken is not too large and they are
sufficiently well diffused through the fluid, little or no trace
of them will be apparent to the naked eye. After a few
days, however, extremely small white dots appear on the
surface of the fluid : these increase in size and are seen,
especially by the aid of a hand-magnifier, to consist of little
NAKED-EYE CHARACTERS 183
•
discs, circular or nearly so in outline, and distinctly thicker
in the centre than towards the edge : they float on the fluid
so that their upper surfaces are dry. Each of these patches
is a young Penicillium-growth, formed, as will be seen
hereafter, by the germination of a group of spores.
As the growths are examined day by day they are found
to increase steadily in size, and as they do so to become
thicker and thicker in the middle : their growth is evidently
centrifugal. The thicker central portion acquires a fluffy
appearance, and, by the time the growth has attained a
diameter of about 4 or 5 mm., a further conspicuous change
takes place : the centre of the patch acquires a pale blue
tint, the circumference still remaining pure white. When
the diameter has increased to about 6-10 mm. the colour of
the centre gradually changes to dull sage-green : around this
is a ring of light blue, and finally an outer circle of white.
In all probability some of the growths, several of which will
most likely occur in the saucer, will by this time be found
to have come together by their edges : they then become
completely interwoven, their original boundaries remaining
evident for some time by their wrhite tint. Sooner or later,
however, the white is replaced by blue and the blue by sage-
green, until the whole surface of the fluid is covered by a
single growth of a uniform green colour.
Even when they are not more than 2 — 3 mm. in diameter
the growths are strong enough to be lifted up from the fluid,
and are easily seen under a low power to be formed of a
tough, felt-like substance, the mycelium, Fig. 41 A, (my), from
the upper surface of which delicate threads, the aerial
hyphcz (a. hy.) grow vertically upwards into the air, while
from its lower surface similar but shorter threads, the sub-
merged hyphen (s. /iy.) hang vertically downwards into the
fluid.
B
FIG. 41. — Penicillium glaucitm.
A Diagrammatic vertical section of a young growth (X 5)> showing
mycelium (my], submerged hyphae (s. hy], and aerial hyphse (a. hy).
B, group of spores : I, before commencement of germination ; 2, after
inhibition of fluid : the remaining three have begun to germinate.
C, very young mycelium formed by a small group of germinating
spores.
A LINEAR AGGREGATE 185
D, more advanced mycelium : the hypha; have increased in length
and begun to branch, and septa (sep) have appeared.
E, germinating spore (sp) very highly magnified, sending out one
short and one long hypha, the latter with a short lateral branch and
several septa (sep). Both spore and hyphae contain vacuoles (vac] in
their protoplasm.
F^F4, development of the spore-bearing brushes by repeated branch-
ing of an aerial hypha : the short terminal branches or sterigmata are
already being constricted to form spores.
F5, a fully-developed brush with a row of spores developed from each
sterigma (stg).
F6, a single sterigma (stg) with its spores (sp).
F7, an over-ripe brush in which the structure is obscured by spores
which have dropped from the sterigmata.
B-D, F^F5, and F7 X 150 : F6 X 200 : E X 500.
As long as the growths are white or blue in colour no
powder can be detached by touching the aerial hyphae,
showing that the spores are not yet fully formed, but as soon
as the permanent green hue is attained the slightest touch
is sufficient to detach large quantities of spores.
A bit of the felt-like mycelium is easily teased out or torn
asunder with two needles, and is then found, like actual felt,
to be formed of a close interlacement of delicate threads (D).
These are the mycelial hyp/ice : they are regularly cylindrical,
about TjQ- mm. in diameter, frequently branched, and differ
in an important particular from the somewhat similar hyphae
of Mucor (p. 159). The protoplasm is not continuous, but is
interrupted at regular intervals by transverse partitions or
septa (D, E, sep). In other words, a hypha of Penicillium
is normally, what a hypha of Mucor becomes under un-
favourable conditions (p. 160), multicellular^ the septa dividing
it into separate portions, each of which is morphologically
comparable to a single yeast-cell.
Penicillium shows therefore a very important advance in
structure over the organisms hitherto considered. While in
these latter the entire organism is a single cell ; in Peni-
1 86 PENICILLIUM AND AGARTCUS
cellium it is a cell-aggregate — an accumulation of numerous
cells all in organic connection with one another. As the
cells are arranged in a single longitudinal series, Penicillium
is an example of a linear aggregate.
Each cell is surrounded, as already described, by a wall
of cellulose : its protoplasm is more or less vacuolated (E, vac),
sometimes so much so as to form a mere thin layer within
the cell-wall, the whole interior of the cell being occupied by
one large vacuole. Recently, by staining with logwood,
numerous nuclei have been found, so that the Penicillium
cell, like an Oxytricha (p. 118), or a filament of Mucor or
Vaucheria, is multinucleate.
The submerged hyphas have the same structure, but it is
easier to find their actual ends than those of the mycelial
hyphae. The free extremity tapers to a blunt point where
the cellulose wall is thinner than elsewhere (see E). «
The aerial hyphae from the youngest (white) part of a
growth consist of unbranched filaments, but taken from a
part which is just beginning to turn blue they are found to
have a very characteristic appearance (r1 — F4). Each sends
off from a distal or upper end a larger or smaller number of
branches which remain short and grow parallel to one
another : the primary branches (p1, F2) form secondary ones
(FS), and the secondary tertiary (F4), so that the hypha finally
assumes the appearance of a little brush or pencil, or more
accurately, of a minute cactus, with thick-set forking branches.
The ultimate or distal branches are short cells called sterig-
mata (r5, stg).
Next, the ends of the sterigmata become constricted,
exactly as if a thread were tied round them and gradually
tightened (p1 F6), the result being to separate the distal end
of the sterigma as a globular daughter-cell, in very much the
same way as a bud is separated in Saccharomyces (p. 72).
GERMINATION OF SPORES 187
In this way a spore is produced. The process is repeated,
the end of the sterigma is constricted again and a new spore
formed, the old one being pushed further onwards. By a
continual repetition of the same process a longitudinal row
of spores is formed (r5, F°), of which the proximal or lower
one is the youngest, the distal or upper one the eldest. The
spores grow for some time after their formation, and are
therefore found to become larger and larger in passing from
the proximal to the distal end of the chain (FG). Sooner or
later they lose their connection with each other, become
detached, and fall, covering the whole growth with a fine
dust which readily adheres to all parts owing to the some-
what sticky character of the spores. In this stage it is by
no means easy to make out the structure of the brushes,
since they are quite obscured by the number of spores
adhering to them (FT).
It is at the period of complete formation of the spores that
the growth turns green. The colour is not due to the presence
of chlorophyll. Under a high power the spores appear quite
colourless, whereas a cell of the same size covered with
chlorophyll would appear bright green.
The germination of the spores can be readily studied by
sowing them in a drop of Pasteur's solution in a moist chamber
(Fig. 38, p. 161). The spores, several of which usually adhere
together, are at first clear and bright (B1) : soon they swell
considerably, and the protoplasm becomes granular and
vacuolated (B2) : in this stage they are hardly distinguishable
from yeast-cells (compare Fig. 13, p. 71). Then one or more
buds spring from each and elongate into hyphse (B, c), just
as in Mucor. But the difference between the two moulds is
soon apparent : by the time a hypha has grown to a length
equal to about six or eight times its own diameter, the pro-
toplasm in it divides transversely and a cellulose septum is
i88 PENICILLIUM AND AGARICUS
formed (D, E, sef) dividing the young hypha into two cells
(compare Fig. 37, H, p. 158). The distal cell then elongates
and divides again, and in this way the hyphse are, almost from
the first, divided into cells of approximately equal length.
The mode of growth of the distal or apical cell of a hypha
is probably as follows : — the free end tapers slightly (E) and
the cellulose wall thins out as it approaches the apex. The
protoplasm performing constructive more rapidly than de-
structive metabolism increases in volume, and its tendency is
to grow in all directions : as, however, the cellulose mem-
brane surrounding it is thinner at the apex than elsewhere,
it naturally, on the principle of least resistance, extends in
that direction, thus increasing the length of the cell without
adding to its thickness. Thus the growth of a hypha of
Penicillium is apical, i.e. takes place only at the distal end, the
cells once formed ceasing to grow. Thus also the oldest cells
are those nearest the original spore from which the hypha
sprang, the youngest those further removed from it.
A process which has been described as sexual, sometimes, but appa-
rently very rarely, occurs in Penicillium, and is said to consist essentially
in the conjugation of two gametes having the form of twisted hyphse,
and the subsequent development of spores in the resulting branched
zygote. But as the details of the prdcess are complicated and its sexual
character is doubtful, it is considered best to do no more than call
attention to it. The student is referred to Brefeld's original account of
the process in the Quarterly Journal of Microscopical Science, vol. xv.,
p. 342. The so-called sexual reproduction of the closely-allied Eurotium
is described in Huxley and Martin's Elementary Biology (new edition),
p. 419, and figured in Howes's Atlas of Elementary Biology ; pi. xix.,
figs, xxvi and xxvii.
The nutrition of Penicillium is essentially like that of Mucor
(p. 165). But, as it has been remarked, "it is often content
with the poorest food which would be too bad for higher
fungi. It lives in the human ear : it does not shun cast-off
GENERAL FEATURES 189
clothes, damp boots, or dried-up ink. Sometimes it contents
itself with a solution of sugar with a very little [nitrogenous]
organic matter, at other times it appears as if it preferred the
purest solution of a salt with only a trace of organic matter.
It will even tolerate the hurtful influence of poisonous
solutions of copper and arsenious acid." It flourishes best
in a solution of peptones and sugar.
This eclecticism in matters of diet is one obvious ex-
planation of the universal occurrence of Penicillium ; another
is the extraordinary vitality of the spores. They will ger-
minate at any temperature between 1-5° and 43° C., the
optimum being about 22° C. They are not killed by a dry
heat of 1 08° C., and some will even survive a temperature
of 120°. And lastly, they will germinate after being kept
for two years.
We have seen that the form of a Penicillium growth is ir-
regular, and is determined by the surface on which it grows.
There are, however, certain fungi which are quite constant
and determinate both in form and size, and are yet found
on analysis to be formed exclusively of interlaced hyphae,
that is, to belong to the type of linear aggregates. Among
the most striking of these are the mushrooms and toad-
stools.
A mushroom (Agancus) consists of a stout vertical stalk
(Fig. 42, A, st\ on the upper or distal end of which is borne
an umbrella-like disc or p ileus (p). The lower or proximal
end of the stalk is in connection with an underground
mycelium (my), from which it springs.
On the underside of the pileus are numerous radiating
vertical plates or lamellae (I) extending a part or the whole
of the distance from the circumference of the pileus to the
stalk. In the common edible mushroom (Agariats cam-
PENICILLIUM AND AGARICUS
pestris) these lamellae are pink in young specimens, and
afterwards become dark brown.
The mushroom is too tough to be readily teased out like
FIG. 42. — Agarictts cainpestris.
A, Diagrammatic vertical section, showing the stalk (sf) springing
from a mycelium (my), and expanding into the pileus (/), on the under
side of which are the radiating lamella.
B, transverse vertical section of a lamella, showing the hyphce (hy)
turning outwards to form the layer of club-shaped cells (a) from which
the sterigmata spring.
c, one of the club-shaped cells (a), highly magnified, showing its two
sterigmata (sty), each bearing a spore (sp).
(B and c after Sachs.)
the mycelium of Penicillium, and its structure is best in-
vestigated by cutting thin sections of various parts and
examining them under a high power.
FORMATION OF SPORES 191
Such sections show the whole mushroom to be composed
of immense numbers of closely interwoven branched hyphae
(B), divided by numerous septa into cells. In the stalk the
hyphae take a longitudinal direction ; in the pileus they turn
outwards, passing from the centre to the circumference, and
finally send branches downwards to form the lamellae. Fre-
quently the hyphae are so closely packed as to be hardly
distinguishable one from another.
At the surfaces of the lamellae the hyphae turn outwards,
so that their ends are perpendicular to the free surfaces of
those plates. Their terminal cells become dilated or club-
shaped (B, c, #), and give off two small branches or sterig-
mata (c, stg\ the ends of which swell up and become
constricted off as spores (sp). These fall on the ground and
germinate, forming a mycelium from which more or fewer
mushrooms are in due course produced.
Thus in point of structure a mushroom bears much the
same relation to Penicillium as Caulerpa (p. 173) bears to
Vaucheria. Caulerpa shows the extreme development of
which a single branched cell is capable, the mushroom how
complicated in structure and definite in form a simple linear
aggregate may become.
LESSON XIX
SPIROGYRA
AMONGST the numerous weeds which form a green scum in
stagnant ponds and slowly-flowing streams, one, called Spiro-
gyra, is perhaps the commonest. It is recognized at once
under a low power by the long delicate green filaments of
which it is composed being marked with a regular green
spiral band.
Examined under the microscope the filaments are seen to
be, like the hyphae of Penicillium, linear aggregates, that is,
to be composed of a single row of cells arranged end to
end. But in Penicillium the hyphae are frequently branched,
and it is always possible in an entire hypha to distinguish
the slightly tapering distal end from the proximal end, which
springs either from another hypha or from a spore. In
Spirogyra the filaments do not branch, and there is no
distinction between their opposite ends.
The cells of which the filaments are composed (Fig. 43, A)
are cylindrical, covered with a cellulose cell-wall (c. iv), and
separated from adjacent cells by septa (sep} of the same
substance. The protoplasmic cell-body presents certain
characteristic peculiarities.
It has been noticed in more than one instance that in the
tram;*-
c3
FIG. 43. — Spirogyra.
A, small portion of a living filament, showing a single cell, with cell-
wall (c. TV), septa (sep) separating it from adjacent cells, peripheral layer
of protoplasm (plsm) connected by threads with a central mass contain-
O
194 SPIROGYRA
ing the nucleus (««), two spiral chromatophores (chr), and pyrenoids
(pyr).
B1, B2. middle portion of a cell, showing two stages in binary
fission.
c, four stages in dioecious conjugation : in c1 the gonads (gon1, gon^}
are connected by short processes of their adjacent sides : in c- the active
or male gamete (gam1} has separated from the wall of the gonad (gon1)
preparatory to passing across the connecting bridge to the stationary or
female gamete (gam1), which has not yet separated from its containing
gonad (gon-) : in c3 the female gamete (g<im'-) has undergone separa-
tion, and the male gamete (gam^) is in the act of conjugating with it : in
c4 the two have united to form a zygote (zyg) lying in the female gonad.
D, two stages in monoecious conjugation : in C1 the adjacent cells
(gonads) have sent out conjugating processes (a) : in D2 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 con-
tact with the cell- wall (see pp. 167 and 186). This state of
things is carried to excess in Spirogyra : the central vacuole
is so large that the protoplasm (A, plsvi} 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,
INTERSTITIAL GROWTH 195
confined to the primordial utricle ; towards the centre of the
vacuole is a small irregular mass of protoplasm connected to
the peripheral layer by extremely delicate protoplasmic
strands. Imbedded in this central mass is the nucleus (;«/),
which has the form of a biconvex lens and contains a distinct
nucleolus.
The chromatophores differ from anything we have yet
considered, having the form of green spiral bands (chr\ of
which each cell may contain one (D1) 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 (n--i2 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. 63), 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 (B1), 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 (B2). 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.
o 2
196 SPIROGYRA
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. 188), a difference
which explains the fact mentioned above (p. 192) 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. 163) and in Vaucheria
(p. 170).
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, c1), and
finally become united by the absorption of the adjacent walls,
thus forming a free communication between the two connected
cells or gonads (gon1, gvn2). As several pairs of cells on the
same two filaments unite simultaneously a ladder-like ap-
pearance is produced.
The protoplasmic cell-bodies (c2, gam1, gam^} of the two
gonads become rounded off and form gametes or conjugating
bodies (see p. 164, note x) : it is observable that this process
of separation from the wall of the gonad always takes place
earlier in one gamete (c2, gam1) than in the other (c2, c3,
gam**). Then the gamete which is ready first (gam1) passes
through the connecting canal (c3) and conjugates with the
other (gamz) forming a zygote (c4, 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.
MONCECIOUS CONJUGATION 197
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 first indication 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 proces (o1, 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 (o2) 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.
198 SPIROGYRA
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 fertili-
zation. 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
Hsematococcus 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
i
MONOSTROMA, ULVA, LAMINARIA, &C.
IT was pointed out in a previous lesson (p. 189) 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 hyphas so interwoven as to form struc-
tures often of considerable size and of definite and regular
form.
This is not the case with the Algas or lower green plants-
the group to which Vaucheria, Caulerpa, Spirogyra, the
diatoms, and in the view of some authors Haematococcus
and Euglena, belong. These agree with fungi in the fact
that the lowest among them (e.g. Zooxanthella) are unicellu-
lar, and others (e.g. Spirogyra) simple linear aggregates, but
the higher forms, such as the majority of sea-weeds, have
as it were gone beyond the fungi in point of structure and
attained a distinctly higher stage of morphological differen-
tiation. This will be made clear by a study of three typical
genera.
Amongst the immense variety of sea-weeds found in rock-
pools between high and low water-marks are several kinds
having the form of flat irregular expansions, of a bright green
200 MONOSTROMA, ULVA, LAMINARIA, ETC.
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. 44) 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 imbedded 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
FlG. 44. — 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 Haematococcus 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
A SOLID AGGREGATE 201
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 not only divide 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. 45, 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,
2O2
MONOSTROMA, ULVA, LAMINARIA, ETC.
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. 45, 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. 45. — A, Laminaria clciTistoni, a young plant, showing stem with
branched root-like organ of attachment, and deeply-cleft leaves (about
~th natural size).
B, Lessonia fuscescens, showing tree-like form (about -^ih 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 -I metres (over
i X ft.) in six months.
I'.ut in spite of their immense size these olive sea-weeds
are comparatively simple solid aggregates of cells. Even
STRUCTURE 203
without the microscope the difference between one of
them 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. But it must
be noted that these sea-weeds often reach a high degree
of differentiation, reminding one of the structure seen in
the higher plants.
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-multiplication
accompanied 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 con-
siderably from one another in form and size.
Nitella (Fig. 46, 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
1- 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, r/i) : the distal end is free.
FIG. ifi.—Nitetta?
AJ 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.
206 NITELLA
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 white line. The arrows indicate the direction of rota-
tion of the protoplasm.
E, distal end of a leaflet, showing the general structure of a typical
cell of Nitella in optical section : c. w, the cell-wall ; plsni1, the quies-
cent outer layer of protoplasm containing chromatophores (chr) ; plsni2,
the inner layer, rotating in the direction indicated by the arrows, and
containing nuclei (mi) ; vac, the large vacuole.
F, terminal bud, partly dissected, showing the nodes (nd), internodes
(int. nd), and leaf-whorls (/), numbered from I to 4, starting from the
proximal end ; gr. pt, growing point.
G, distal end of a leaf (/) with two leaflets (/'), at the base of which
are attached a spermary (spy) and two ovaries (ovy).
Springing from it at intervals are circlets or whorls of
delicate, pointed leaves (I).
Owing to the regular arrangement of the leaves the stem
is divisible into successive sections or segments (seg), each
consisting of a very short distal division or node (nd} from
which the leaves spring, and of an elongated proximal
division or internode (int. nd\ which bears no leaves.
Throughout the greater part of the stem the whorls of
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,
instead 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.
NODES AND INTERNODES : GONADS 207
There is frequently found springing from the axil of one of
the leaves in a whorl a brancii 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 terminal
bud. The axis or stem, if a shoot, is called a secondary axis,
the main stem of the plant being the primary axis. It is
important to notice that both primary and secondary axes
always end in terminal buds, and thus differ from the leaves
which have pointed extremities.
The rhizoids or root-filaments (rJi] arise, like the leaves
and branches, exclusively from nodes.
In the autumn the more distal leaves present a peculiar
appearance, owing to the development on them of the gonads
or sexual reproductive organs (Fig. 46, 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 (nd1} separating the two adjacent internodes from one
another. The leaves consist each of an elongated proximal
cell like an internode (D, / ; F, 71), 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
208 NITELLA
out by examining a leaflet under a high power. The cell is
surrounded by a wall of cellulose (E, c.iv) of considerable
thickness. Within this is a layer of protoplasm (primordial
utricle, p. 194), enclosing a large central vacuole (vac), and
clearly divisible into two layers, an outer (plsm1) in im-
mediate contact with the cell- wall, and an inner (plsm2)
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 -^^ 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. 211). If the tem-
perature is not too low this layer is seen to be in active
rotating movement, streaming up one side of the cell and
down the other (E), the boundary between the upward and
downward currents being marked by the colourless bands
just mentioned, along which no movement takes place (D).
This rotation of protoplasm is a form of contractility very
common in vegetable cells in which, owing to the confining
cell-wall, no freer movement is possible.
The numerous nuclei (E, mi) are rod-like and often curved :
they can only be seen to advantage after staining (Fig. 47).
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. 46, A,
term, bud), formed of a whorl of leaves with their apices
curved towards one another. If these leaves (F, 71) are dis-
STRUCTURE OF TERMINAL BUD 209
sected away, the node from which they spring (ndl) is found
to give rise distally to a very short internode (inf. nd^),
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. nas), 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 Q\ 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. 47)-
The growing point is formed of a single cell, the apical
cell (A, ap. c), approximately hemispherical in form and about
-^o 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, ?id^) 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
p
nu,
D
nd*
int.ndf
Is
ap.c
B
s.ap.c
niv
' } &"*&''•'' i •'•' )'-1'
' •- *-,?/'*" i " ~~ '
— int. nd.1
FIG. 47. — Nitella : Vertical sections of the growing point at four
successive stages. The nodes (nd), internodes (int. net), 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. nd1, nd1, /J) in these figures corresponds with the third
segment (int. ndz, /3) shown in Fig. 46, F.
A, the apical cell (ap. c) is succeeded by a very rudimentary node
(nd'A) without leaves : int. nd1 is in vertical section, showing the proto-
plasm (plsm), vacuole (vac), and two nuclei (;/?/).
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'A have
appeared.
C, the sub-apical cell has divided transversely into the proximally-
situated internode (int. nd*) and the distally-situated node (nd4) of a
new segment : in the node the nucleus has divided preparatory to cell-
division. The previously formed segments have increased in size : int.
nd2 has developed a vacuole (vac), and its nucleus has divided (comp.
int. nd'2 in A) : int. nd1 is shown in surface view with three dividing
nuclei (nu).
D, nd4 has divided vertically, forming a transverse plate of cells, and
is now as far advanced as nds in A : the nucleus of int. nds is in the act
of dividing, while int. nd'*, shown in surface view, now contains nume-
rous nuclei, some of them in the act of dividing.
GROWTH OF PARTS IN THE TERMINAL BUD 211
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 1 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*}, but by the time it is about as long as broad the nucleus
has begun to divide (D, int. ndz ; 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. ndr] ;
D, int. nd'2}, many of them still in active division. This
P 2
212 NITELLA
repeated fission of the nucleus reminds us of what was
found to occur in Opalina (p. 119).
Then*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, i.e., 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. 46, 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. 46, c), not unlike Mucor-
hyphae, 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. 46, 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
O O
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
STRUCTURE OF SPERMARY
21
ones have each its lower angle truncated by the insertion of
the stalk, so that they are actually four-sided.
Each shield (Fig. 48, A and B, s/i) 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
B
FIG. 48. — A, diagrammatic vertical section of the spermary of Nitella,
showing the stalk (stk), four of the eight shields (s/i), each bearing on
its inner face a handle (/in), to which is attached a head-cell (/id) : each
head-cell bears six secondary head-cells (hd1), to each of which four
spermatic filaments (sp. f, ) are attached.
B, one of the proximal shields (s/i), with handle (/in), head-cell (/id),
secondary head-cells (/id'), and spermatic filaments (sp. f.).
C, a single sperm.
D1, D2, D3, 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 (/in), 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 (1id'\ to which six secondary head-
214 NITELLA
cells (/id') 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 hyphse 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. 46, G, ovy, and Fig. 49 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
DEVELOPMENT OF THE GONADS
215
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
sp.c
ov-
FIG. 49. — 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.
B1, surface view, and B2, section of a very young ovaiy : B3, later
stage in vertical section : B4, 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. (B2-B4
after Sachs.)
becomes divided into eight octants (Fig. 48 D1). Each of
these then divides tangentially (i.e. parallel to the surface
of the sphere) into two cells (D2), the inner of which divides
again (o1) 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
216 NITELLA
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.
49, B2, ov, 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 (B1). The uppermost or distal
cell of the axial row becomes the ovum (BS, B4, 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 B1, B4, and Fig. 46, G, ovy). At the same time the
distal end of each develops two septa (B3) 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. 170). A sperm makes its way down the
canal in the chimney-like crown of cells terminating the
PRO-EMBRYO
217
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, when, after a
ap.c
term ~bvd
rTi
FIG. 50. — 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 altered.)
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. 50) giving out a root-fibre (rh)
218 NITELLA
at its proximal end. Soon two nodes (nd) are formed on
the filament, or pro-embryo, from the lower of which rhizoids
(r/i) 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 one 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 or gamobium,
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 generic name 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-
B
FIG. 51. — 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 clistally in the hypostome (hyp), surrounded by tentacles (t], and
three buds (bdl, bd-, bd*} 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 papillae.
B, H.fusca, showing the mouth (mth) at the end of the hypostome
(hyp], the circlet of tentacles (t}, two spermaries (spy], and an ovary
(pvy}.
C, a Hydra creeping on a flat surface by looping movements.
D, a specimen crawling on its tentacles.
(C and D after W. Marshall.)
GENERAL CHARACTERS 221
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. 51, 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
(;;////.). At the junction of the hypostome with the body
proper are given off from six to eight long delicate ten-
tacles (/) arranged in a circlet or whorl. A longitudinal
section shows that the body is hollow, containing a spacious
cavity, the enteron (Fig. 52, 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. vulgar is ) 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. 51, 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. 51, 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.
222 HYDRA
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.
51, c) : the body is bent round until the distal end touches
the surface ; then the base is detached and moved nearer the
distal end which is again moved forward, and so on. It has
also been observed to crawl like a cuttle-fish (D) by means of
its tentacles, the body being kept nearly vertical.
It is also possible to watch a Hydra feed. It is a very
voracious creature, and to see it catch and devour its prey is
a curious and interesting sight. In the water in which it
lives are always to be found numbers of " water-fleas," minute
animals from about a millimetre downwards in length,
belonging to the class Crustacea, a group which includes
lobsters, crabs, shrimps, &c.
Water-fleas swim very rapidly, and occasionally one may be
seen to come in contact with a Hydra's tentacle. Instantly
its hitherto active movements stop dead, and it remains
adhering in an apparently mysterious manner to the tentacle.
If the Hydra is not hungry it usually liberates its prey after a
time, and the water-flea may then be seen to drop through
the water like a stone for a short distance, but finally to
expand its limbs and swim off. If however the Hydra has
not eaten recently it gradually contracts the tentacle until
the prey is brought near the mouth, the other tentacles being
also used to aid in the process. The water-flea is thus forced
against the apex of the hypostome, the mouth expands
widely and seizes it, and it is finally passed down into the
digestive cavity. Hydrse can often be seen with their bodies
bulged out in one or more places by recently swallowed
water-fleas.
The precise structure of Hydra is best made out by cutting
MINUTE STRUCTURE 223
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. 52.
The whole animal is seen to be built up of cells, each
consisting of protoplasm with a large nucleus (B, 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 wray 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 (ect\ and
of an inner layer the endoderm (end), which bounds the
enteric cavity. Between the two layers is a delicate trans-
parent membrane, the 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
_
~ioa~ —
FIG. 52. — Hydra.
A, Vertical section of the entire animal, showing the body- wall com-
posed of ectoderm (ect} and endoderm (end], enclosing an enteric cavity
ECTODERM AND ENDODERM 225
(ent. can], which, as well as the two layers, is continued (ent. cav^} into
the tentacles, and opens externally by the mouth (mtli) 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 (ntc1) nematocysts : some of the endoderm cells
are putting out pseudopods (psd), others flagella (JJ). Two buds (bdl,
bd'2) 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 (oi1}.
B, portion of a transverse section more highly magnified, showing the
large ectoderm cells (ect) and interstitial cells (int. c) : two cnidoblasts
(cnbl} enclosing nematocysts (ntc), and one of them produced into a
cnidocil (cnc) : the layer of muscle-processes (m. pr) cut across just
external to the mesogloea (msgl) : endoderm cells (end) with large
vacuoles and nuclei (mi), pseudopods (psd), and flagella (fl). The
endoderm cell to the right has ingested a diatom (a), and all enclose
minute black granules.
C, two of the large ectoderm cells, showing nucleus (nu) and muscle-
process (m. pr}.
D, an endoderm cell of H. viridis, showing nucleus (mi), numerous
chromatophores (chr), and an ingested nematocyst (ntc).
E, one of the larger nematocysts with extruded thread barbed at the
base.
F, one of the smaller nematocysts.
G, a single sperm.
(D after Lankester : F and G after Howes.)
are necessarily left between their inner or narrow ends, and
these are filled up with the second kind of cells (int. c), small
rounded bodies which lie closely packed between their larger
companions and are distinguished as interstitial cells.
The inner ends of the large ectoderm cells are continued
into narrow, pointed prolongations (c, m. pr} placed at right
angles to the cells themselves and parallel to the long axis of
the body. There is thus a layer of these longitudinally-
arranged muscle-processes lying immediately external to the
mesoglcea (B, m. pr). They appear to possess, like the axial
fibre of Vorticella (p. 128), 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
Q
226 HYDRA
ectoderm cells are both contractile and irritable, a special
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 denned walls,
and called mmatocysts. Both in the living specimen and in
sections they ordinarily present the appearance shown in
Fig. 52, B, ntc, but are frequently met with in the condition
shown in E, 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
filament.
The examination of nematocysts in animals allied to, but
larger than Hydra, shows that the structure of these curious
bodies is as follows : Each consists of a tough sac, 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, 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. in) should be
noted — that the Hydra is enabled to paralyze its prey. Pro-
bably 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
SECRETION : GLAND CELLS 227
produce an effect on the human skin quite like the sting of
a nettle.
The nematocysts are formed in special interstitial cells
called cnidoblasts (B, cnbl} 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 contraction of the cnidoblast
and the nematocyst thus compressed 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 (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, and constitute a rudimentary nervous system, the
nature of which will be more conveniently discussed in the
next lesson (p. 241).
The ectoderm cells of the foot differ from those of the rest
of the body in being very granular (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 discharged
at the free surface of the cell is called secretion, and the cell
performing the function is known as a gland-cell.
Q 2
228 HYDRA
The endoderm cells (A and B, end.} are of two kinds-
larger and smaller. The larger cells exceed in size those
of the ectoderm, and are remarkable for containing one
or more vacuoles, sometimes so large as to reduce the
protoplasm to a thin superficial layer containing the
nucleus. 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 trans-
verse 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 (fl\ Thus the endoderm cells of Hydra illustrate
in a very instructive manner the essential similarity of flagella
and pseudopods already referred to (52).
In the hypostome the endoderm is thrown into longitu-
dinal folds, so as to allow of the dilatation of the mouth in
swallowing. The smaller cells (A) are long and narrow, and
have the character of gland-cells : the secretion in this case
is probably 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. In H. fusca bodies
resembling these chromatophores 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 great measure at least, to the degeneration of the
chromatophores.
Muscle-processes exist in connection with the endoderm
cells, and they are said to take a transverse or circular
direction, /.£., at right angles to the similar processes of
the ectoderm cells.
DIGESTION AND ASSIMILATION 229
When a water-flea or other minute organism is swallowed
by a Hydra, it undergoes a gradual process of disintegration.
The process is probably begun by a partial solution of the
soft parts due to the action of a digestive fluid secreted by
the gland-cells of the endoderm : it is certainly 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 pseudo-
pods 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 partly infra-cellular^ i.e.,
takes place in the interior of the cells themselves, as in
Amoeba or Paramcecium : and partly 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.
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-
230 HYDRA
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,
they form, physiologically speaking, not a colony of largely
independent units, but a single multicellular individual.
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. 51, A, bdl\ and is
found by sections (Fig. 52, A, bdl) to be a hollow out-
pushing of the wall containing a prolongation of the
enteron, and made up of ectoderm, mesogloea, and endo-
derm. In the course of a few hours this prominence en-
larges greatly, and near its distal end six or eight hollow
buds appear arranged in a whorl (Fig. 51, A, bd^ • Fig. 52,
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.
51, A, M3). 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
SPERMARY AND OVARY 231
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. 51, B, and Fig. 52, 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. 52, 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 the sperms are liberated and swim freely in the
water.
The ovaries (Fig. 51, B, and Fig. 52, 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
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. 52, A, ov, and Fig. 53, A),
sending out pseudopods amongst its companions and ab-
232
HYDRA
sorbing nutriment from them, 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
v oik-spheres (Fig. 53), small 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
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.)
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,
DEVELOPMENT 233
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. 230) 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. 133).
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
consistency, 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. — Bougainvillea ramosa.
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 (meet], one of the latter nearly mature,
the others undeveloped : each hydranth has a circlet of tentacles (t)
surrounding a hypostome (hyp], and contains an enteric cavity (ent. cav)
continuous with a narrow canal (ent. cav1) 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 (mnb), radial (rad. c), and circular
(dr. c) canals, and eye-spots (oc}. (After Allman.)
236 HYDROID POLYPES
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
(t). 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 digestion 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. cav1.) continuous with those of the hydranths, and
thus the structure of a hydroid polype is, so far, simply
that of a Hydra in which the process of budding has
gone on to an indefinite extent and without separation of
the buds.
There is however an additional layer added in the stem
for protective and strengthening purposes. It is evident
that a colony of the size shown in Fig. 54, A would, if formed
only of soft ectodermal and endodermal cells, be so weak as
to be hardly able to bear its own weight even in water. To
remedy this a layer of transparent, yellowish substance of
horny consistency, called the cuticle, is developed outside the
ectoderm of the stem, extending on to the branches and only
stopping at the bases of the hydranths and medusae. It is
this layer which, when the organism dies and decays, is left
as a semi-transparent branched structure resembling the
living colony in all but the absence of hydranths and medusae.
The cuticle is therefore a supporting organ or skeleton, not
like our own bones formed in the interior of the body (endo
STRUCTURE OF A MEDUSA 237
skeleton) but like the shell of a crab or lobster lying altogether
outside the soft parts (exoskeletori).
As to the mode of formation of the cuticle : — we saw that
many organisms, such as Amoeba and Haematococcus, form,
on entering into the resting condition, a cyst or cell-wall by
secreting or separating from the surface of their protoplasm a
succession of layers either of cellulose or of a transparent
horn-like substance. But Amoeba and Haematococcus are
unicellular, and are therefore free to form this protective
layer at all parts of their surface. The ectoderm cells of
Bousainvillea on the other hand are in close contact with
O
their neighbours on all sides and with the mesoglcea at their
inner ends, so that it is not surprising to find the secretion
of skeletal substance taking place only at their outer ends.
As the process takes place simultaneously in adjacent cells,
the result is a continuous layer common to the whole
ectoderm instead of a capsule to each individual cell. It is
to an exo-skeletal 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 strictly applied in multicellular
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 a medusa must now be described in some
detail. The bell (c) is formed of a gelatinous substance
(Fig. 55, p. 238 : D msgl) covered on both its inner and
eel
•msyl
B
eel
FIG. 55. — Diagrams illustrating the derivation of the medusa from
the hydranth. In the whole series of figures the ectoderm (ect) is dotted,
the endoderm (end) striated, and the mesogloea (msgl) black.
A, longitudinal section of a Hydra-like body, snowing the tubular body
with enteric cavity (ent. cav), hypostome (hyp), mouth (mth), and
tentacles (/).
DERIVATION OF MEDUSA FROM HYDRANTH 239
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. cav1) :
the hypostome now forms a manubrium (mnb}.
c', transverse section of the same through the plane a b, showing the
continuous cavity (ent. cav1} in the bell.
D, fully formed medusa : the cavity in the bell is reduced to the
radiating (rad. c) and circular (cir. c] canals, the velum (v) is formed,
and a double nerve-ring (nv, nv1) is produced from the ectoderm.
D', transverse section of the same through the plane a b, showing the
four radiating canals (rad. c] united by the endoderm-lamella (end. lam),
produced by partial obliteration of the continuous cavity ent. cav1 in c1.
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 (mt/i), used by the medusa for
the ingestion of food. At its upper (attached or proximal)
end the cavity of the manubrium is continued into four
narrow, radial canals (Fig. 54, c, rad. c', and Fig. 55, D and
D' rad) which extend though the gelatinous substance of the
bell at equal distances from one another, like four meridians,
and finally open into a circular canal (cir. 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
substance of the bell, is a delicate sheet of cells, the endoderm-
lamella (D', end. lam).
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 (ot), the ocellus or eye-spot. Lastly,
240 HYDROID POLYPES
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
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. c) 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. lam)— indicat-
ing 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,
PARTIAL TRIPLOBLASTIC CONDITION 241
like Zoothamnium (p. 134), is dimorphic, bearing zooids of
two kinds.
The ectoderm cells of the hydranth bear muscle-processes
like those of Hydra (p. 225, Fig. 52, 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-firvtess 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. 223), it shows a tendency towards the as-
sumption of a three-layered or triploblastic condition.1 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 ot
the medusa which has not been referred to. At the junction
1 This intermediate layer is not, however, exactly comparable to the
mesoderm of higher animals (p. 274).
R
242
HYDROID POLYPES
of the velum with the edge of the bell there lies, imme-
diately beneath the ectoderm, a layer of peculiar branched
cells (Fig. 56, B, n. c\ containing large nuclei and produced
into long fibre-like processes. These nerve-cells (see p. 227)
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
B
771. C
FIG. 56. — A, Muscle fibres from the inner face of the bell of the
medusa of a hydroid polype (Eiicopella campamilaria), 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.)
the velum. An irregular network of similar cells and fibres
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
FUNCTION OF THE NERVOUS SYSTEM 243
the nervous system with the external world : others, in some
instances at least, are probably directly connected with
muscle-fibres.
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 paralyzed.
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
R 2
244 HYDROID POLYPES
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
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. 133).
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 producing
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
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
DEVELOPMENT: POLYPLAST STAGE 245
Bougainvillea, however, the reproductive products are said
to originate in the manubrium.
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
rate, they arise in the ectoderm, but migrate into the endoderm at a
very early stage.
246
HYDROID POLYPES
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
A
B
FIG. 57. — Stages in the development of two hydroid polypes, Eao-
medea flexuosa (A-H) and Eudendrium ramosum (I-M).
A, oosperm.
B, two-celled, and c, four-celled stage.
D, E, polyplast.
F, G, formation of planula by differentiation of ectoderm and
endoderm.
In A-G the embryo is embedded in the maternal tissues.
H, free-swimming planula, showing ciliated ectodei'm, 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 rapture of the cuticle and liberation of the tentacles.
(After Allman.)
ALTERNATION OF FORMATIONS 247
cavity. In this stage the embryo is called a plamda : it
swims slowly through the water by means of its cilia, the
broader end being directed forwards in progression. It then
loses its cilia and settles down on a rock, shell, sea-weed, or
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. 217), 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.
248
HYDROID POLYPES
FIG. 58. — Diphyes campanulata.
A, the entire colony, natural size, showing stem (a) bearing groups of
zooids (e) and two swimming bells (m, ml), the apertures of which are
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 (z). (From Gegenbaur. )
GENERAL CHARACTERS 249
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
swimming-bells (;;/, m') 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, ?/) 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
those of Bougainvillea, of ectoderm, mesogloea, and endo-
derm, but without a cuticle. The hydranth has the same
structure as 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 practi-
cally medusae in which the manubrium is absent : and both
the bracts and grappling-lines are shown by comparison 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, is polymorphic.
250
HYDROID POLYPES
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-
^Jri'Tb 's * -^ - . * v-\ -• '-
^ Vo^X'.V-:''. .
' ".• >V *?*..•' ^'-V^^ . .- -
^$'%$'h$$l'{i $ '> i'.'f ^ '• ;,i) Hi &ife\!^.' : ' • •:•";,''
^llil; &yW
--,;/., • ;
FIG. 59. — A, Porpita pacified (nat. size), from beneath, showing disc-
like stem surrounded by tentacles (/), a single functional hydranth (/$j),
and numerous mouthless hydranths (/y1).
B, vertical section of P. mediterranea, showing the relative positions
of the functional (hy) and mouthless (Ay1) hydranths, the tentacles (/),
and the chambered shell. (A after Duperrey ; B from Huxley after
Kolliker.)
logical and physiological differentiation are thus carried
much further than in such a form as Bougainvillea.
Porpita is another free-swimming Hydroid, presenting at
INDIVIDUATION 251
first sight no resemblance whatever to Diphyes. It has much
the appearance of a flattened medusa (Fig. 59), consisting
of a circular disc, slightly convex above and concave below,
bearing round its edge a number of close-set tentacles, and
on its under side a central tubular organ (hy) with a ter-
minal mouth, like the manubrium of a medusa, surrounded
by a great number of structures like hollow tentacles (hy).
The discoid body is supported by a sort of shell having the
consistency of cartilage and divided into chambers which
contain air (B).
Accurate examination shows that the manubrium-like
body (hy) on the under surface is a hydranth, that the short,
hollow, tentacle-like bodies (hy1) surrounding it are mouthless
hydranths, and that the disc represents the common stem of
Diphyes or Bougainvillea. So that Porpita is not what it
appears at first sight, a single individual, like a Medusa or a
Hydra, but a colony in which the constituent zooids have
become so modified in accordance with an extreme division
of physiological labour, that the entire colony has the character
of a single physiological individual.
It was pointed out in the previous lesson (p. 229) that
Hydra, while metaphorically the equivalent of an indefinite
number of unicellular organisms, was yet physiologically a
single individual, its constituent cells being so differentiated
and combined as to form one whole. A further stage in this
same process of individuation is seen in Porpita, in which not
cells but zooids, each the morphological equivalent of an
entire Hydra, are combined and differentiated so as to form
a colony which, from the physiological point of view, has
the characters of a single individual.
LESSON XXIV
SPERMATOGENESIS AND OOGENESIS. THE MATURATION AND
IMPREGNATION OF THE OVUM. THE CONNECTION BE-
TWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS
IN the preceding lessons it has more than once been stated
that sperms arise from ordinary undifferentiated cells in the
spermary, and that ova are produced by the enlargement of
similar cells in the ovary. Fertilization has also been de-
scribed as the conjugation or fusion of ovum and sperm. We
have now to consider in greater detail what is known as to
the precise mode of development of sperms (spermatogenesis)
and of ova (oogenesis), as well as the exact steps of the pro-
cess by which an oosperm or unicellular embryo is formed
by the union of the two sexual elements. The following
description applies to animals in which all these processes
have been worked out with greater detail than in plants.
Both ovary and spermary are at first composed of cells of
the ordinary kind, the primitive sex-cells, and it is only by
the further development of these that the sex of the gonad
is determined.
In the spermary the sex-cells undergo repeated fission,
forming a group of small cells, each of which (Fig. 60, A)
ORIGIN OF HEAD AND TAIL OF SPERM
253
becomes differentiated into a sperm in the following way.
A delicate filament of protoplasm (B, /) grows out on one
side, and at the same time the nucleus (nit) retreats to the
opposite extremity (c — F). Next the protoplasm draws itself
away, as it were, from the nucleus and forms a kind of swel-
ling (G and H, x) round the base of the filament, which in-
creases considerably in length. The filament is now recog-
FIG. 60. — Spermatogenesis in the Rat.
A, Sperm-cell : nu, the nucleus.
B, first indication of the tail (/) as a protoplasmic filament.
C-F, further growth of the tail, and retreat of the nucleus (mt) to
the opposite end of the cell to form the head.
G, H, separation of protoplasmic globule (it), afterwards cast off.
I, fully formed sperm. (After H. H. Brown.)
nizable as the flagellum or tail of the sperm, the nucleus
as its head. The lump of protoplasm (x} at the junction
of head and tail assumes the form of a globule attached
only by one side (H) : this gradually separates itself from
the now fully-developed sperm, and is finally completely
detached (i).
Thus a sperm is a true cell, the nucleus being represented
by the head, and the cell-body by the tail. But it is to be
254 SPERMATOGENESIS AND OOGENESIS
noted that the whole cell is not used up in the formation
of the sperm, a part of it being cast off in the form of a
protoplasmic globule as the mature form is assumed.
As already stated, the ova arise from primitive sex-cells,
precisely resembling those which give rise to sperms. Very
soon however the behaviour of the sex-cells in the ovary
distinguishes them from the similar cells of the spermary.
Instead of actively dividing they remain passive and increase,
often enormously, in size, by the absorption of nutriment
from surrounding parts. Sometimes this nutriment is simply
taken in by osmosis, in other cases the growing ovum actually
ingests neighbouring cells after the manner of an Amoeba.
Thus in the developing egg the processes of constructive are
vastly in excess of those of destructive metabolism.
We saw in the second lesson (p. 33) that the products of
destructive metabolism might take the form either of waste
products which are got rid of, or of plastic products which
are stored up as an integral part of the organism. In the
developing egg, in addition to increase in the bulk of the
protoplasm itself, a formation of plastic products usually
goes on to an immense extent. In plants the stored-up
materials may take the form of starch, as in Nitella (p. 214),
of oil, or of proteid substance : in animals it consists of
rounded or angular grains of proteid material, known as
yolk-granules. These being deposited, like plums in a
pudding, in the protoplasm, have the effect of rendering the
fully-framed egg opaque, so that its structure can often be
made out only in sections. When the quantity of yolk is
very great the ovum may attain a comparatively enormous
size, as for instance in birds, in which as already mentioned
(p. 69) the " yolk " is simply an immense egg-cell.
When fully formed the typical animal ovum (Fig. 61)
STRUCTURE OF THE OVUM 255
consists of a more or less globular mass of protoplasm,
generally exhibiting a reticular structure and enclosing a
larger or smaller quantity of yolk-granules. Surrounding the
cell-body is usually a cell-wall or cuticle, often of considerable
thickness and known as the vitelline membrane. The nucleus
is large and has the usual constituents (p. 62), nuclear mem-
brane, achromatin, and chromatin. As a rule the chromatin
or a portion of it is aggregated in the form of a very definite
FIG. 6 1. — Ovum of a Sea-urchin (Toxopneustes lividtts], showing the
radially-striated cell-wall (vitelline membrane), the protoplasm contain-
ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its
protoplasmic network, and a large nucleolus (germinal spot). (From
Balfour after Hertwig.)
nucleolus, which is often known as the germinal spot, the
entire nucleus being called the germinal vesicle.
Such a fully-formed ovum is, however, incapable of being
fertilized or of developing into an embryo : before it is ripe for
conjugation with a sperm or able to undergo the first stages
of yolk division it has to go through a process known as the
maturation of the egg.
B
fiAro7/i-.<J*
FIG. 62. — The Maturation and Impregnation of the Animal Ovum.
A, portion of the ovum of a Round worm (Ascaris megalocephala),
showing the sperm (sp) in the act of conjugation, and the unaltered
POLAR CELLS 257
nucleus (mi) of the egg, Ascaris being an animal in which the conjuga-
tion of ovum and sperm takes place before the maturation of the former.
In the nucleus the nuclear membrane, achromatin, and 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 (nn]
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.
o
C, formation of the first polar cell (/. 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 (/. c. 2) to the surface of the protoplasm : the female pronucleus
(pr. nu. ? ) : and the sperm (sp), which has penetrated into the cell-
protoplasm, but has not yet become converted into the male pro-
nucleus.
E1, E2, two stages in the conjugation of the pronuclei in Molluscs
(E1, Pterotrachea, E'2, Phyllirhoe}.
In E1 the male (pr. mi. 6 ) and female (pr. nu. 9 ) pronuclei are
separated : in E2 they are applied by their flattened adjacent faces : in
connection with each the cell-protoplasm is arranged in the characteristic
"sun "-form : the polar cells (/. c.i, p. c.2) are shown.
F:-F3, three stages in the development of the nucleus of the oosperm
in a Sea-urchin (Echinus microtuberculatus) : in F1 the nucleus contains
nine chromatin-fibres (chrom. ? ) derived from the female pronucleus,
and a globular mass of the same (chrom. 6 ) derived from the male pro-
nucleus : the two "suns" are now situated one at each end of the
nucleus. In F~ the male chromatin (chrom. S ) has begun to unwind
itself : in F3 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 prideauxii],
showing the conjugation of the pronuclei. The male and female chro-
matin-networks are fused along the plane of union. The pronuclei are
surrounded by finely-granular protoplasm devoid of yolk-spheres.
(A-F after Boveri ; G after Weismann and Ischikawa. )
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
form of an ordinary nuclear spindle (B, nu, see p. 63). 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
s
258 SPERMATOGENESIS AND OOGENESIS
division of the protoplasm, and the bud becomes separated
as a small cell distinguished as the first polar cell (c — E,
p.c. i).
It was mentioned in a previous lesson (p. 198) that in
some case 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 only
takes place 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 pronuclens (D,
pr. mi. $ ).
Shortly after, or in some cases before maturation the
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
FUSION OF PRONUCLEI 259
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
(E1, pr. nu. £ )
The two pronuclei, each surrounded, in some cases at any
rate, with an investment of protoplasm, approach one
another (E1, E2) and finally unite to form the single nucleus
(p1 — F3) 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 the two parents.
In connection with each pronucleus is a sun-like figure
(E1, E2) produced by part of the cell-protoplasm becoming
arranged in the form of threads radiating from a common
centre as in the ordinary dividing cell (p. 64) : after the
fusion of the pronuclei one of these " suns " is found at each
end of the resultant oosperm-nucleus (r1). The radiating
threads appear to have an important action in connection
with the characteristic movements of the chromatin during
karyokinesis (Fig. 10, p. 64).
The precise mode of union of the two pronuclei is even
now hardly certain : some observations seem to show that it
consists in an interweaving of the two chromatin filaments
(p1 — p3) : others appear to indicate that an actual fusion of
male and female chromatin takes place (G).
Fertilization being thus effected the process of segmentation
or division of the oosperm takes place as described in the
preceding lesson (p. 245).
s 2
260 SPERMATOGENESIS AND OOGENESIS
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. 245), 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 multi cellular 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
not by a mouthless planula but by a two-layered embryo
with a mouth at one end, called a 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 (13 lp} and stomach
(£/), the outer layer of cells (Ekt) being protective and
MAGOSPH^RA
261
sensory, the inner (Ent] having a digestive function. The
planula of a hydroid may be looked upon as a gastrula in
which the mouth has not yet appeared.
What we want then is to be able to form some notion of
the steps by which the gastrula may be derived from a
unicellular form. There are two interesting organisms which
serve to indicate one way in which the gulf may be bridged.
Sip
Ent
_
FIG. 63. — A typical animal gastrula in vertical section, showing
ectoderm (Ekt), endoderm (Ent), enteron or digestive cavity (U), and
mouth (B //). (From Wiedersheim's Anatomy.)
Haeckel has given the generic name of Magosphara 1
(Fig. 64, A, B) to a minute spherical organism about
0*07 mm. in diameter, consisting of a number of conical
cells radiating from a common centre and imbedded in
a clear gelatinous substance. Each cell has a nucleus and
contractile vacuole, and its outer or free surface is beset with
cilia by means of which the entire sphere is propelled through
1 Unfortunately nobody has since seen this organism.
262
SPERMATOGENESIS AND OOGENESIS
the water. After a time the sphere breaks up, the constituent
zooids (c) swimming about independently, and finally losing
their cilia and becoming amoeboid (D). The amoebulae, in all
probability, become encysted (E), forming egg-like bodies
which by repeated division of their protoplasm (F) produce
the spherical colony.
It is obvious that Magosphaera resembles the polyplast
stage of an embryo : moreover it is produced by the repeated
fission of an amoebula just as the polyplast is formed by the
repeated fission of an oosperm.
FIG. 64. — Magosphcera planula.
A, the entire colony, surface view, showing the numerous ciliated
unicellular zooids.
B, the same in vertical section, showing the zooids imbedded in a
gelatinous intermediate substance.
C, a single free-swimming zooid.
D, the same after assuming an amoeboid form.
E, the same encysted.
F, the encysted form with its protoplasm dividing to form a new
colony. (After Haeckel.)
The beautiful Volvox (Fig, 65), one of the favourite
studies of microscopists, is a colony of Haematococcus-like
zooids arranged in the form of a hollow sphere. Each cell
(c) has a nucleus, a contractile vacuole, a large green chroma-
tophore, a small red pigment-spot like that of Euglena (p. 47)
and two flagella : by the combined movement of all the
flagella a rotating movement is given to the entire colony.
VOLVOX GLOBATOR
263
ovy
H
FIG. 65. — Volvox globator.
A, the entire colony, surface view, showing the biflagellate zooids, and
several daughter-colonies swimming freely in the interior : the latter are
produced by the repeated fission of non-flagellate reproductive zooids
(a).
B, the same during sexual maturity, showing spermaries from the
surface (spy], in profile (spy'} and after complete formation of sperms
(spy"} : and ovaries from the surface (ovy, ovy", ovy'"} and in profile
(ovy'}.
C, four zooids in optical section, showing cell-wall, nucleus, contractile
vacuole, with adjacent pigment-spot, and flagella.
D^D5, stages in the formation of a colony by the repeated binary
fission of a sexual reproductive zooid.
E, a ripe spermary.
F, a single sperm, showing pigment-spot (pg) and flagella (ft).
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.)
264
SPERMATOGENESIS AND OOGENESIS
Asexual reproduction takes place by certain of the zooids,
which are not ciliated, undergoing a process very like the
segmentation of the hydroid egg (p. 245), dividing into 2, 4,
8, 1 6, &c. cells (A, a, and D1 — D5), and so forming a daughter
FIG, 66. — Diagram illustrating the hypothetical origin of the gastrula
from a colony of unicellular zooids.
1, simple unicellular form, showing nucleus, ectosarc (Ec), and endo-
sarc (En). The black specks represent food-particles.
2, solid colony of unicellular forms : letters as before except E and F,
food particles.
3, hollow colony of unicellular forms, with central digestive cavity
containing food-particles (F).
4, gastrula formed by the division of each zooid (z) into an ecto-
clermal (Ec) and an endodermal (En] cell, and by the formation of a
mouth (M) placing the digestive cavity in communication with the ex-
terior. (After Lankester. )
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'
ORIGIN OF GASTRULA
265
ovy", ovy'"} the protoplasm of each forming a single ovum :
the protoplasm of others divides repeatedly and forms aggre-
gations of sperms (spy, spy', spy'1}. 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
presents the further resemblance to the higher or multicellular
MtTi
fvac
--Ecb
nu
FIG. 67. — Diagram illustrating the hypothetical origin of the gastrula
from a simple multinucleate form.
A, infusor-like animal with mouth (Mtk) and gullet, and numerous
nuclei (mi) and food vacuoles (f. vac).
B, similar form in which the food vacuoles have coalesced into a
single digestive cavity (f. vac') communicating with the gullet, and the
nuclei (nu) are regularly arranged in two layers.
C, gastrula formed by the division of the protoplasm of B into as
many cells as there are nuclei : the cells form ectoderm (Ect) and
endoderm (End), and the digestive cavity is a well-defined enteron
(Ent).
animals that certain of its cells are differentiated to form
true sexual products.
Starting from these two forms we may suppose an Amoeba-
like organism (Fig. 66, B1) to have given rise to a solid
spherical colony (2), something like Magosphsera. From
this, by accumulation of fluid in the interior we get a hollow
one-layered sphere (z), like a greatly simplified Volvox, and
from this by division of each of its cells into two, an inner
266 SPERMATOGENESIS AND OOGENESIS
(4, En), and an outer (Ef), and the separation of the cells at
one pole to form a mouth (M) a gastrula is produced.
But the same result might be arrived at in another way.
We have seen that an ordinary Mucor-hypha becomes under
certain conditions, multicellular (p. 160), that is, a single
multinucleate cell becomes divided into numerous cells after
its final form is attained. We have also seen that some of
the ciliate Infusoria, such as Opalina and Oxytricha, are
multinucleate (pp. 119, 118).
Let us suppose an infusor-like animal (Fig. 67, A) with a
mouth (MtJi) and gullet, numerous nuclei (««) and the
usual large number of food-vacuoles (f. vac). Imagine the
food-vacuoles to unite into a single central cavity (B, f. vac')
opening externally by the gullet, and the nuclei (nu) to
arrange themselves in two layers around the cavity. It is
obvious that all that would be necessary to convert such a
form into a gastrula (c) would be for its protoplasm to divide
into as many cells as there were nuclei.
It is thus quite conceivable that diploblastic animals may
have arisen either from colonies of unicellular zooids or from
solitary unicellular but multinucleate forms. At present
there is not sufficient evidence to allow of any decision being
arrived at on this point, but as the continuity of the animal
series cannot be grasped without imagining some such
intermediate stages as those described, it is thought advis-
able to put these two hypotheses before the reader simply as
hypotheses, either or both of which may any day be over-
thrown by the inexorable logic of facts.
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. 68, 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. si] 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 (Mth). The rest of
the body is more or less distinctly marked by annular
grooves (D and E, gr) into body-segments or metameres
268
POLYGORDIUS
/•
FIG. 68. — 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 setre (s) and ciliated pit (c. p).
C, ventral aspect of the same : letters as before except Mth, mouth.
D, portion of body showing metameres (Mtmr) separated by grooves
(gr).
E, posterior extremity from the ventral aspect, showing the last three
metameres (Mtmr} separated by distinct grooves (gr), and the anal seg-
ment (An. seg) bearing the anus (An) and a circlet of papillae (p).
(After Fraipont.)
(Mfmr), the number of which varies considerably. Poly-
gordius is thus the first instance we have met with of a trans-
GENERAL CHARACTERS 269
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 (/) ; it
is separated from the preceding segment by a deep groove,
and bears at its posterior end a small circular aperture, the
anus (Ait).
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, the prostomium,
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. 69) 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 (coil).
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
the holes a narrow tube of the same length as the wide one.
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GENERAL CHARACTERS 271
Between the enteric canal and the body-wall is the ccelome (Ccel),
divided into right and left portions by the dorsal (D. Mes) and ventral
( V. Mes) mesenteries, and into segmental compartments by the septa
(sep\
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 (./?. 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. st), 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 cesophageal connectives
((Es. com}.
B, diagrammatic longitudinal section showing the cell-layers.
The cuticle is represented by a black line, the ectoderm is dotted,
the endoderm radially striated, the muscle plates evenly shaded, the
ccelomic epithelium represented by a beaded line, and the nervous
system finely dotted.
The body-wall is composed of cuticle (Cu), deric epithelium (Der.
Epthm), muscle-plates (M. PI), and parietal layer of ccelomic epithe-
lium (Ccel. Epthm).
The enteric canal is formed of enteric epithelium (Ent. Epthm)
covered by the visceral layer of ccelomic epithelium (Gael. Epthm) : in
the neighbourhood of the mouth (Mth) and anus (An) the enteric epithe-
lium is ectodermal : elsewhere it is endodermal : Ph, pharynx ; Oes,
oesophagus ; int, intestine ; ret, rectum.
The septa (sep) are formed of muscle covered on both sides by ccelomic
epithelium.
Two 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), and parietal
layer of ccelomic epithelium (Ccel. Epthm}, forming the body-wall ; and
the enteric epithelium (Ent. Epthm) and visceral layer of ccelomic
epithelium (Ccel. Epthm1) 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. 72, A (p. 290), should be compared with this figure, as it is an
accurate representation of the parts here shown diagrammatically.
272 POLYGORDIUS
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
cozlomate 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. 69, 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, p. 238, A'),
the two layers being however in contact or only separated
by the thin mesoglsea. 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. 69, c, and Fig. 72, A, cu) showing
no structure beyond a delicate striation. Next comes a
layer of epithelial cells (Der. Epthm^, their long axis 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]
ENTERIC EPITHELIUM 273
having the form of long flat spindles (Fig. 71, p. 283, M. PL]
exhibiting a delicate longitudinal striation and covered on
their free surfaces 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. 72,
M. PL\ It is by the contraction of the muscle-plates that
the movements of the body, which resemble those of an
earthworm, are produced. Finally, within the muscular
layer and lining the ccelome is a very thin layer of cells, the
ccelomic epithelium (CoeL Epthui).
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 (CoeL Ept/im') bounding the ccelome,
and hence called, like the innermost layer of the body-wall,
ccelomic epithelium. We have, therefore, to distinguish
two layers of ccelomic epithelium, an outer or parietal layer
(Ca'L 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, A1, and Fig.
69, 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. 236). 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 mesoglaea.
274 POLYGORDIUS
But it will be remembered that in Medusae there is some-
times found a layer of separate muscle-fibres between the
ectoderm and the mesoglaea, and it was pointed out (p. 241)
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-
f Muscle-plates.
Somatic \ /
-> Ccelomic epithelium
n "S. \^-/ V^*^ A V_/ J. J. i J. ^^ V_. \J J. L JL 1 V^ -1 J. V-l
( (parietal layer).
Splanchnic ( Ccelomic epithelium
Mesoderm . .
(rudimentary)
layer ( (visceral layer).
Endoderm Enteric epithelium.
1 In the majority of the higher animals there is a layer of muscle
between the enteric and coelomic epithelia : in such cases the body-wall
and enteric canal consist of the same layers but in reverse order, the
coloemic epithelium being internal in the one, external in the other.
GENERAL STRUCTURE 275
Strictly speaking, this comparison does not hold good of
the anterior and posterior ends of the worm : at both mouth
and anus the deric passes insensibly into the enteric epithe-
lium, and the study of development shows (p. 294) that the
cells lining both the anterior and posterior ends of the canal
are, as indicated in the diagram (Fig. 69, B), ectodermal. For
this reason the terms deric and enteric epithelium are not
mere synomyms of ectoderm and endoderm respectively.
It is important that the student should, before reading
further, understand clearly the general composition of a
triploblastic animal as typified by Polygordius, which may
be summarized as follows : It consists of two tubes formed
of epithelial cells, one within and parallel to the other, the
two being continuous at either end of the body where the
inner tube (enteric epithelium) is in free communication
with the exterior ; the outer tube (deric epithelium) is lined
by a layer of muscle plates within which is a thin layer of
ccelornic epithelium, the three together forming the body-
wall ; the inner tube (enteric epithelium) is covered ex-
ternally by a layer of ccelomic epithelium which forms with
it the enteric canal ; lastly, the body-wall and enteric canal
are separated by a considerable space, the ccelome.
The enteric canal is not, as might be supposed from the
foregoing description, connected with the body-wall only at
the mouth and anus, but is supported in a peculiar and
somewhat complicated way. In the first place there are
thin vertical plates, the dorsal and ventral mesenteries (Fig.
69, A and c, D. Mes, V. Mes\ which extend longitudinally
from the dorsal and ventral surfaces of the canal to the body
wall, dividing the coelome into right and left halves. The
structure of the mesenteries is seen in a transverse section
(Fig. 69, c, and Fig. 72, A) which shows that at the middle
T 2
276 POLYGORDIUS
dorsal line the parietal layer of coelomic epithelium becomes
deflected downwards forming a two-layered membrane, the
dorsal mesentery : the two layers of this on reaching the
enteric canal diverge and pass one on each side of it, form-
ing the visceral layer of coelomic epithelium : uniting again
below the canal they are continued downwards as the ventral
mesentery, and on reaching the body-wall diverge once more
to join the parietal layer. Thus the mesenteries are simply
formed of a double layer of ccelomic epithelium, continuous
on the one hand with the parietal and on the other with the
visceral layer of that membrane.
Besides the mesenteries the canal is supported by trans-
verse vertical partitions or septa (Fig. 69, A and B, sef) which
extend right across the body-cavity each being perforated by
the canal. The septa are regularly arranged and correspond
with the external grooves by which the body is divided into
metameres. Thus the transverse or metameric segmen-
tation affects the coelome as well as the body-wall. Each
septum is composed of a sheet of muscle covered on both
sides with coelomic epithelium (B sep).
Where the septa come in contact with the enteric canal
the latter is more or less definitely constricted so as to pre-
sent a beaded appearance (A and B) : thus we have segmen-
tation of the canal as well as of the body-wall and ccelome.
The digestive canal, moreover, is not a simple tube of
even calibre throughout, but is divisible into four portions.
The first or pharynx (PJi) is very short, and can be pro-
truded during feeding : the second, called the gullet or
oesophagus (ats\ is confined to the peristomium and is distin-
guished by its thick walls and comparatively great diameter :
the third or intestine (Int) extends from the first metamere
to the last — i.e., from the segment immediately following
the peristomium to that immediately preceding the anal
DIGESTION 277
segment ; it is laterally compressed so as to have an
elongated form in cross section (c, and Fig. 72, A) : the
fourth portion or rectum (Ret) is confined to the anal seg-
ment ; it is somewhat dilated and is not laterally compressed.
The epithelium of the intestine is as indicated in the
diagram (B) endodermal ; that of the remaining division of
the canal is ectodermal. The large majority of the cells in
all parts of the canal are ciliated.
The cells of the enteric canal and especially those of the
gullet are very granular, and like the endoderm cells of the
hypstome of Hydra (p. 228) are to be considered as gland
cells. They doubtless secrete a digestive juice which,
mixing with the various substances taken in by the mouth,
dissolves the proteids and other digestible parts, so as to
allow of their absorption. There is no evidence of intra-
cellular digestion such as occurs in Hydra (p. 229), and it is
very probable that the process is purely extra-cellular or
enteric, the food being dissolved and rendered diffusible
entirely in the cavity of the canal. By the movements of
the canal — caused partly by the general movements of the
body and partly by the contraction of the muscles of the
septa — aided by the action of the cilia, the contents are
gradually forced backwards and the sand and other indi-
gestible matters are expelled at the anus.
The coelome is filled with a colourless, transparent
ccslomic fluid in which are suspended minute, irregular,
colourless bodies, as well as oval bodies containing yellow
granules. From the analogy of the higher animals one
would expect these to be leucocytes (p. 56), but their
cellular nature has not been proved.
The function of the ccelomic fluid is probably to distribute
the digested food in the enteric canal to all parts of the
278 POLYGORDIUS
body. In Hydra, where the lining wall of the digestive
cavity is in direct contact with the simple wall of the body,
the products of digestion can pass at once by diffusion from
endoderm to ectoderm, but in the present case a means of
communication is wanted between the enteric epithelium
and the comparatively complex and distant body-wall. The
peptones and other products of digestion diffuse through
the enteric epithelium into the ccelomic fluid and by the con-
tinual movement of the latter — due to the contractions of
the body-wall — are distributed to all parts. Thus the
external epithelium and the muscles, as well as the nervous
system and reproductive organs not yet described, are
wholly dependent upon the enteric epithelium for their
supply of nutriment.
We have now to deal with structures which we find for the
first time in Polygordius, namely blood-vessels. Lying in
the thickness of the dorsal mesentery is a delicate tube (Fig.
69, A and c, D. V] passing along almost the whole length of
the body : this is the dorsal vessel. A similar ventral vessel
(V.V) is contained in the ventral mesentery1 and the two are
placed in communication with one another in every segment
by a pair of commissural vessels (A, com.v.} which spring right
and left from the dorsal trunk, pass downwards in or close
behind the corresponding septum, following the contour of
body-wall, and finally open into the ventral vessel. Each
commissural vessel, at about the middle of its length, gives
off a recurrent vessel (R.V.) which passes backwards and
1 The statement that the dorsal and ventral vessels lie in the thickness
of the mesenteries requires qualification. As a matter of fact, these
vessels are simply spaces formed by the divergence of the two layers of
epithelium composing the mesentery (Fig. 69, C, and Fig. 72, A) : only
their anterior ends have proper walls.
HEMOGLOBIN 279
ends blindly. The anterior pair of commissural vessels lie
in the peristomium and have an oblique direction, one on
each side of the gullet. The whole of these vessels form a
single closed vascular system, there being no communication
between them and any of the remaining cavities of the
body.
The vascular system contains a fluid, the blood, which
varies in colour in the different species of Polygordius, being
either colourless, red, green, or yellow. In one species cor-
puscles (? leucocytes) have been found in it.
The function of the blood has not been actually proved
in Polygordius, but is well known in other worms. In the
common earthworm, for instance, the blood is red, the colour
being due to the same pigment, hcemoglobin, which occurs
in our own blood and in that of other vertebrate animals.
Haemoglobin is a nitrogenous compound containing in
addition to carbon, hydrogen, nitrogen, oxygen, and sulphur,
a minute quantity of iron. It can be obtained pure in the
form of crystals which are soluble in water. Its most
striking and physiologically its most important property is
its power of entering into a loose chemical combination with
oxygen. If a solution of haemoglobin is brought in contact
with oxygen it acquires a bright scarlet colour : the solution
is then found to have a characteristic spectrum distinguished
by two absorption-bands, one in the yellow, another in the
green. Loss of oxygen changes the colour from scarlet to
purple, and the spectrum then presents a single broad
absorption-band intermediate in position between the two of
the oxygenated solution.
This property is of use in the following way. All parts
of the organism are constantly undergoing destructive meta-
bolism and giving off carbon dioxide : this gas is absorbed
by the blood, and at the same time the haemoglobin gives up
28o POLYGORDIUS
its oxygen to the tissues. On the other hand whenever the
blood is brought sufficiently near the external air — or water
in the case of an aquatic animal — the opposite process takes
place, oxygen being absorbed and carbon dioxide given off.
Haemoglobin is therefore to be looked upon as a respiratory
or oxygen-carrying pigment : its function is to provide the
various parts of the body with a constant supply of oxygen,
while the carbon dioxide formed by their oxidation is given
up to the blood. The particular part of the body in which
the carbon dioxide accumulated in the blood is exchanged
for the oxygen is called a respiratory organ : in Polygordius,
as in the earthworm and many others of the lower animals
there is no specialized respiratory organ — lung or gill — but
the necessary exchange of gases is performed by the entire
surface of the body,
In discussing in a previous lesson the differences between
plants and animals we found (p. 176) that, in the unicellular
organisms previously studied, the presence of an excretory
organ in the form of a contractile vacuole was a characteristic
feature of such undoubted animals as the ciliate infusoria,
but was absent in such undoubted plants as Vaucheria and
Mucor. But the reader will have noticed that Hydra and its
allies have no specialized excretory organ, waste products
being apparently discharged from any part of the surface.
In Polygordius we meet once more with an animal in which
excretory organs are present, although, in correspondence
with the complexity of the animal itself, they are very
different from the simple contractile vacuoles of Paramce-
cium or Vorticella.
The excretory organs of Polygordius consist of little tubes
called mphridia, of which each metamere possesses a pair,
one on each side (Fig. 69, A, B, and c, Nphui). Each
NEPHRIDIA 281
nephridium (Fig. 70) is an extremely delicate tube consisting
of two divisions bent at right angles. The outer division is
placed vertically, lies in the thickness of the body-wall, and
opens externally by a minute aperture, the nephridiopore
(Figs. 69 and 70, Nph. f). The inner division is horizontal
and lies in the ccelomic epithelium : passing forward it pierces
the septum which bounds the segment in front (Fig. 69,
A and B), and then dilates into a funnel-shaped extremity or
nephrostome (Nph. si) which places its cavity in free com-
munication with the ccelome. The whole interior of the
Np~h.sb
FIG. 70. — A nephridium of Polygordius, showing the cilia lining the
tube, the ciliated funnel or nephrostome (Nph. st), and the external
aperture or nephridospore (Nph. p). (After Fraipont.)
tube as well as the inner face of the nephrostome is lined
with cilia wrhich work outwards.
A nephridium may therefore be defined as a ciliated tube,
lying in the thickness of the body-wall and opening at one
end into the ccelome and at the other on the exterior of the
body.
In the higher worms, such as the earthworm, the nephridia
are lined in part by gland-cells, and are abundantly supplied
with blood-vessels. Water and nitrogenous waste from all
parts of the body pass by diffusion into the blood and are
conveyed to the nephridia, the gland-cells of which withdraw
the waste products and pass them into the cavities of the
tubes, whence they are finally discharged into the surround-
282 POLYGORDIUS
ing medium. In all probability some such process as this
takes place in Polygordius.
In discussing the hydroid polypes we found that one of
the most important points of difference between the loco-
motive medusa and the fixed hydranth was the presence in
the former of a well-developed nervous system (p. 242) con-
sisting of an arrangement of peculiarly modified cells, to
which the function of automatism was assigned. It is
natural to expect in such an active and otherwise highly
organized animal as Polygordius a nervous system of a
considerably higher degree of complexity than that of a
medusa.
The central nervous system consists of two parts, the
brain and the ventral nerve-cord. The brain (Fig. 69, A and
B, Br^] is a rounded mass occupying the whole interior of
the prostomium and divided by a transverse groove into two
lobes, the anterior of which is again marked by a longitu-
dinal groove. The ventral nerve cord (V. Nv. Cd.) is a
longitudinal band extending along the whole middle ventral
line of the body from the peristomium to the anal segment.
The posterior lobe of the brain is connected with the anterior
end of the ventral nerve-cord by a pair of nervous bands,
the cesophageal connectives (CES. con.} which pass respectively
right and left of the gullet.
It is to be noted that one division of the central nervous
system — the brain — lies altogether above and in front of the
enteric canal, the other division — the ventral nerve-cord-
altogether beneath it, and that, in virtue of the union of the
two divisions by the cesophageal connectives, the enteric
canal perforates the nervous system.
It is also important to notice that the nervous system is
throughout in direct contact with the epidermis or ectoderm,
PERIPHERAL NERVOUS SYSTEM
283
the ventral cord appearing in sections (Fig. 69, c, and Fig.
72, A) as a mere thickening of the latter.
Both brain and cord are composed of delicate nerve-fibres
Drr
aer. Ep th m-
FIG. 71. — Diagram illustrating the relations of the nervous system of
Polygordius.
The deric epithelium (Der. Epthm} is either in direct contact with the
central nervous system (lower part of figure), or is connected by afferent
nerves (af. nv.} with the inter-muscular plexus (int. muse, pi ex.} : the
latter is connected to the muscle-plates (M. PI] by efferent nerves (Ef.
nv).
The central nervous system consists of nerve-fibres (Nv. F) and
nerve-cells (Nv. C) : other nerve-cells (Nv. C') occur at intervals in
the inter-muscular plexus.
The muscle-plates (M. PI], one of which is entire, while only the
middle part of the other is shown, are invested by a delicate protoplasmic
network, containing nuclei (««), to which the efferent nerves can be
traced. (The details copied from Fraipont.)
(Fig. 71, Nv. F.} interspersed with nerve-cells (Nv. C.}. In
the cord the fibres are arranged longitudinally, and the
nerve-cells are ventral in position forming a layer in imme-
284 POLYGORDIUS
diate contact with the deric epithelium. In the posterior
lobe of the brain the nerve-cells are superficial and the
central part of the organ is formed of a finely punctate
substance in which neither cells nor fibres can be made out.
Ramifying through the entire muscular layer of the body-
wall is a network of delicate nerve-fibres (Int. Muse. Fix.}
with nerve-cells (Nv. C') at intervals, the inter-muscular
plexus. Some of the branches of this plexus are traceable
to nerve-cells in the central nervous system, others (of. Nv.)
to epidermic cells, others (Ef. Nv.) to the delicate proto-
plasmic layer covering the muscle-plates. The superficial
cells of both brain and cord are also, as has been said, in
direct connection with the overlying epidermis, and from the
anterior end of the brain a bundle of nerve-fibres (Fig. 69, B,
t. nv.) is given off on each side to the corresponding tentacle,
constituting the nerve of that organ, to the epidermic cells of
which its fibres are distributed.
We see then that, apart from the direct connection of
nerve-cells with the epidermis, the central nervous system is
connected, through the intermediation of nerve fibres (a)
with the sensitive cells of the deric epithelium and (b) with
the contractile muscle-plates. And we can thus distinguish
two sets of nerve-fibres, (a) sensory or afferent (of. Nv.)
which connect the central nervous system with the epidermis,
and (b) motor or efferent (Ef. Nv.) which connect it with the
muscles.
Comparing the nervous system of Polygordius with that
of medusa (p. 242) there are two chief points to be noticed.
Firstly, the concentration of the central nervous system in
the higher type, and the special concentration at the anterior
end of the body to form a brain. Secondly, the important
fact that the intermuscular plexus is not, like the peripheral
nervous system of a medusa which it resembles, situated
FUNCTIONS OF NERVOUS SYSTEM 285
immediately beneath the epidermis (ectoderm) but lies in the
muscular layer, or in other words, has sunk into the
mesoderm.
It is obvious that direct experiments on the nervous system
would be a very difficult matter in so small an animal as
Polygordius. But numerous experiments on a large number
of other animals, both higher and lower, allow us to infer
with considerable confidence the functions of the various
parts in this particular case.
If a muscle be laid bare or removed from the body in a
living animal it may be made to contract by the application
of various stimuli, such as a smart tap (mechanical stimulus),
a drop of acid or alkali (chemical stimulus), a hot wire
(thermal stimulus), or an electric current (electric stimulus).
If the motor nerve of the muscle is left intact the application
to it of any of these stimuli produces the same effect as its
direct application to the muscle, the stimulus being con-
ducted along the eminently irritable but non-contractile
nerve.
Further, if the motor nerve is left in connection with the
central nervous system, /.&, with one or more nerve-cells,
direct stimulation of these is followed by a contraction, and
not only so, but stimulation of a sensory nerve connected
with such cells produces a similar result. And finally,
stimulation of an ectoderm cell connected, either directly
or through the intermediation of a sensory nerve, with the
nerve-cells, is also followed by muscular contraction. An
action of this kind, in which a stimulus applied to the free
sensitive surface of the body is transmitted along a sensory
nerve to a nerve-cell or group of such cells and is thus, as it
were, reflected along a motor nerve to a muscle, is called a
reflex action; the essence of the arrangement is the inter-
286 POLYGORDIUS
position of nerve-cells between sensory or afferent nerves
connected with sensory cells, and motor or efferent nerves
connected with muscles.
The diagram (Fig. 71) serves to illustrate this matter.
The muscle plate (M. PL) may be made to contract by a
stimulus applied (a) to itself directly, (b) to the motor-fibre
(Ef. Nv.), (c) to the nerve-cells (Nv. C.) in the central
nervous system, or to those (Nv. C) in the inter-muscular
plexus, (d) to the sensory fibre (of. Nv.), or (e) to the
epidermic cells (Der. JEpthm.).
In all probability the whole central nervous system of
Polygordius is capable of automatic action. It is a well-
known fact that if the body of an earthworm is cut into
several pieces each performs independent movements ; in
other words, the whole body is not, as in the higher animals,
paralysed by removal of the brain. There can, however, be
little doubt that complete co-ordination, i.e., the regulation
of the various movements to a common end, is lost when
the brain is removed.
The nervous system is thus an all-important means of
communication between the various parts of the organism
and between the organism and the external world. The
outer or sensory surface is by its means brought into
connection with the entire muscular system with such
perfection that the slightest touch applied to one end of the
body may be followed by the almost instantaneous contrac-
tion of muscles at the other.
In some species of Polygordius the prostomium bears a
pair of eye-specks, but in the majority of species the adult
animal is eyeless, and, save for the ciliated pits (Fig. 68,
B, c.p) the function of which is not known, the only definite
organs of sense are the tentacles which have a tactile
PHYSIOLOGICAL DIFFERENTIATION: ORGANS 287
function, their abundant nerve-supply indicating that their
delicacy as organs of touch far surpasses that of the general
surface of the body. They are beset with short, fine pro-
cesses of the cuticle called setce. (Fig. 68 and 69, s) which
probably, like the whiskers of a cat, serve as conductors of
external stimuli to the sensitive epidermic cells.
There are two matters of general importance in connec-
tion with the structure of Polygordius to which the student's
attention must be drawn in concluding the present lesson.
Notice in the first place how in this type, far more than in
any of those previously considered, we have certain definite
parts of the body set apart as organs for the performance of
particular functions. There is a mouth for the reception of
food, an enteric canal for its digestion, and an anus for the
extrusion of fasces : a coelomic fluid for the transport of the
products of digestion to the more distant parts of the body :
a system of blood-vessels for the transport of oxygen to and
of carbon dioxide from all parts : an epidermis as organ of
touch and of respiration : nephridia for getting rid of water
and nitrogenous waste : and a definite nervous system for
regulating the movements of the various parts and forming
a means of communication between the organism and the
external world. It is clear that differentiation of structure
and division of physiological labour play a far more obvious
and important part than in any of the organisms hitherto
studied.
Notice in the second place the vastly greater complexity
of microscopic structure than in any of our former types.
The adult organism can no longer be resolved into more or
less obvious cells. In the deric, enteric, and coelomic
epithelia we meet with nothing new, but the muscle plates
are not cells, the nephridia show no cell-structure, neither do
288 POLYGORDIUS
the nerve-fibres nor the punctate substance of the brain.
The body is thus divisible into tissues or fabrics each clearly
distinguishable from the rest. We have epithelial tissue,
cuticular tissue, muscular tissue, and nervous tissue : and
the blood and ccelomic fluid are to be looked upon as
liquid tissues. One result of this is that, to a far greater
extent than in the foregoing types, we can study the
morphology of Polygordius under two distinct heads :
anatomy, dealing with the general structure of the parts,
and histology, dealing with their minute or microscopic
structure.
One point of importance must be specially referred to in
connection with certain of the tissues. It has been pointed
out (p. 272) that the epidermis has rather the character of
a sheet of protoplasm with regularly arranged nuclei than of
a layer of cells, and that the muscle-plates are covered with
a layer of protoplasm with which the ultimate nerve-fibres
are continuous (p. 273). Thus certain of the tissues of
Polygordius exhibit continuity of the protoplasm, a fact of
considerable interest in connection with the question of the
origin of multicellular animals discussed in the previous
lesson (p. 266).
LESSON XXVI
POLYGORDIUS (Continued]
ASEXUAL reproduction is unknown in Polygordius, and
the organs of sexual reproduction are very simple. The
animal is dioecious, gonads of one sex only being found in
each individual.
In the species which has been most thoroughly investi-
gated (P. neapolitanus) the reproductive products are formed
in each metamere from the fourth to the last. Crossing
these segments obliquely are narrow bands of muscle (Fig.
72, A, O.M) and certain of the cells of coelomic epithelium
covering these bands multiply by fission and form little
heaps of cells (spy) each of which is to be looked upon as a
gonad. There is thus a pair of gonads to each segment with
the exception of the prostomium, the peristomium, the first
three metameres, and the anal segment, the reproductive
organs exhibiting the same simple metameric arrangement
as the digestive, excretory, and circulatory organs. It will
be noticed that the primitive sex-cells, arising as they do
from ccelomic epithelium, are mesodermal structures, not
ectodermal as in hydroids (pp. 231 and 244).
In the male the primitive sex-cells divide and sub-divide,
the ultimate products being converted into sperms (Fig. 72,
u
D.V
DerEpthm
B
M.Pl
V.Nv.Cd Cosl.Epknm.
i«
FIG. 72- — Polygordius neapolitanus.
A, transverse section of a male specimen to show the position of the
immature gonads (spy} and the precise form and arrangement of the
various layers represented diagrammatically in Fig. 69, C.
The body-wall consists of cuticle (Cu), deric epithelium (Der. Epthm),
muscle-plates (M. PI), and parietal layer of ccelomic epithelium (C&l.
Epthm). The ventral nerve-cord ( V. Nv. Cd) is shown to be continu-
ous with the deric epithelium.
The enteric canal consists of ciliated enteric epithelium (Ent. Epthm}
covered by the visceral layer of ccelomic epithelium (Ccel. Epthm') :
connecting it with the body-wall are the dorsal and ventral mesenteries
formed of a double layer of ccelomic epithelium, and containing respec-
tively the dorsal (D. V) and ventral (V. V] blood-vessels.
Passing obliquely across the ccelome are the oblique muscles (0. M)
DEVELOPMENT 291
covered with coelomic epithelium : by differentiation of groups of cells
of the latter the spermaries (spy] are formed.
B, a single sperm, showing expanded head and delicate tail.
C, horizontal section of a sexually mature female.
The body- wall (Cu, Der. Eptkm, M. PI} has undergone partial
histological degeneration, and is ruptured in two places to allow of the
escape of the ova (ov) which still fill the coelomic spaces enclosed between
the body-wall, the enteric canal (Ent. Epthni), and the septa (sep).
(After Fraipont.)
B : see p. 252) : in the female they enlarge immensely, and
take on the character of ova (c, ov). Multiplication of the
sexual products takes place to such an extent that the whole
ccelome becomes crammed full of either sperms or ova (c).
In the female the growth of the eggs takes place at the
expense of all other parts of the body, which undergo more
or less complete atrophy : the epidermis, for instance, be-
comes liquefied and the muscles lose their contractility.
Finally rupture of the body-wall takes place in each segment
(c), and through the slits thus formed the eggs escape. So
that Polygordius, like an annual plant, produces only a
single brood : death is the inevitable result of sexual
maturity. Whether or not the same dehiscence of the
body-wall takes place in the male is not certain : it has
been stated that the sperms make their escape through the
nephridia.
Thus while there are no specialized gonaducts, or tubes for
carrying off the sexual products, it is possible that the neph-
ridia may, in addition to their ordinary function, serve the
purpose of male gonaducts or spermiducts. Female gona-
ducts or oviducts are however entirely absent.
The ova and sperms being shed into the surrounding water
impregnation takes place, and the resulting oosperm under-
goes segmentation or division (see p. 245), a polyplast being
formed. By the arrangement of its cells into two layers and
U 2
POLYGORDIUS
the formation of an enteron or digestive cavity the polyplast
becomes a gastrula (see p. 260) which by further develop-
ment is converted into a curious, free-swimming creature
shown in Fig. 73, A, and called a trochosphere.
oo
A
•and
Pt-.or.CL
dm
FIG. 73. — A, larva of Polygordius neapolitanus in the trochosphere
stage : from a living specimen.
B, diagrammatic vertical section of the same : the ectoderm is dotted,
the endoderm radially striated, the mesoderm evenly shaded, and the
nervous system finely dotted.
C, transverse section through the plane ab in B.
The body-wall consists of a single layer of ectoderm cells, which, at
the apex of the prostomium (upper hemisphere) are modified to form the
brain (Br) and a pair of ocelli (oc).
The enteric canal consists of three parts : the stomodaeum (st. dm],
opening externally by the mouth (Mth), and lined by ectoderm ; the
enteron (Ent) lined by endoderm ; and the proctodseum (Prc. dm],
opening by the anus (An) and lined by ectoderm.
Between the body-wall and the enteric canal is the larval body-cavity
or blastoccele (BL cccl).
The mesoderm is confined to two narrow bands of cells (B and C,
Msd) in the blastoccele, one on either side of the proctodaeum ; slender
mesodermal bands (Msd' ) are also seen in the prostomium in A.
The cilia consists of a prse-oral circlet (Pr. or. ci) above the mouth, a
post-oral circlet (PL or. ci) below the mouth, and an anal circlet (An.
ci} around the anus.
(A after Fraipont.)
The trochosphere, or newly-hatched larva of Polygordius
(Fig. 73, A) is about \ mm. in diameter, and has something
the form of a top, consisting of a dome-like upper portion,
the prostomium, produced into a projecting horizontal rim ;
THE TROCHOSPHERE 293
of an intermediate portion or peristomium^ having the form
of an inverted hemisphere ; and of a lower somewhat conical
anal region. Around the projecting rim is a double circlet
of large cilia (Pr. or. d.) by means of which the larva is
propelled through the water.
Beneath the edge of the ciliated rim is a rounded aperture,
the mouth (Mt/i.) ; this leads by a short, nearly straight
gullet (St. dm.) into a spacious stomach (Ent.\ from the
lower side of which proceeds a short slightly curved intestine
(Prc. dm.), opening at the extremity of the conical inferior
region by an anus (An.). Between the body-wall and the
enteric canal is a space filled with fluid (Bl. cceL), but, as we
shall see, this does not correspond with the body-cavity of
the adult. The body-wall and the enteric canal consist each
of a single layer of epithelial cells, all the tissues included in
the adult under the head of mesoderm (p. 274) being absent
or so poorly developed that they may be neglected for the
present.
Leaving aside all details it will be seen that the trocho-
sphere of Polygordius is comparable in the general features
of its organization to a medusa (compare Fig. 55, p. 238),
consisting as it does of an outer layer of cells forming the
external covering of the body and of an inner layer lining
the digestive cavity. There are, however, two important
differences : the space between the two layers is occupied by
the mesoglcea in the medusa, while in the worm it is a cavity
filled with fluid ; and the digestive cavity of the trochosphere
has two openings instead of one.
But still, in order to compare the medusa with the trocho-
sphere, it is necessary to fill up, by the help of other types,
an important gap in our knowledge of the development of
Polygordius — the passage from the gastrula to the trocho-
sphere. From what we know of the development of other
294
POLYGORDIUS
worms, the process, in its general features, is probably as
follows : —
.The ectoderm and endoderm of the gastrula (Fig. 74, A)
are not in close contact with one another as in Fig. 63
(p. 261), but are separated by a space filled with fluid — the
blastoccele or larval body-cavity. The mouth of the gastrula
closes (B) the enteron (Ent\ being thus converted into a
shut sac. At about the same time the ectoderm is tucked
GaslMlh.
FIG. 74. — Diagram illustrating the origin of the trochosphere from
the gastrula. The ectoderm is dotted, the endoderm striated.
A, gastrula, with enteron (Ent) and gastrula-mouth (G. Mth), and
with the ectoderm and endoderm separated by the larval body-cavity or
blastoccele (Bl. ccel).
B, the gastrula-mouth has closed, the enteron (Ent) becoming a shut
sac.
c, two ectodermal pouches, the stomodseum (St. dm) and proctodseum
(Prc. dm) have appeared.
D, the stomodseum (St. dm) and proctodaeum (Prc. dm) have opened
into the enteron (Ent), forming a complete enteric canal with mouth
(Mth) and anus (An).
in or invaginated at two places (C), and the two little
pouches (St. dm, Prc. dm) thus formed grow inwards until
they meet with the closed enteron and finally open into it
(Z>), so that a complete enteric canal is formed — formed,
we must not fail to notice, of three distinct parts : (i) an
anterior ectodermal pouch, opening externally by the mouth,
and distinguished as the stomodaum ; (2) the enteron, lined
with endoderm; and (3) a posterior ectodermal pouch,
opening externally by the anus, and called the proctodaum.
METAMORPHOSIS 295
In the trochosphere (Fig. 73) the gullet is derived from
the stomodaeum, the stomach from the enteron, and the
intestine from the proctodaeum ; so that only the stomach of
the worm-larva corresponds with the digestive cavity of a
medusa : the gullet and intestine are structures not repre-
sented in the latter form.
Two or three other points in the anatomy of the trocho-
sphere must now be referred to.
At the apex of the dome-shaped prostomium the dcto-
derm is greatly thickened forming a rounded patch of cells
(Figs. 73 and 75, Br\ the rudiment of the brain. On the
surface of the same region and in close relation with the
brain is a pair of small patches of black pigment, the
eye-spots or ocelli (Oc).
On each side of the intestine between its epithelium and
the external ectoderm is a row of cells forming a band
which partly blocks up the blastoccele (B and c, Msd). These
two bands are the rudiments of the whole of the meso-
dermal tissues of the adult — muscle, coelomic epithelium,
&c. — and hence called mesodermal bands.
Finally on each side of the lower or posterior end of the
stomach is a delicate tube (Fig. 75, A, Np]i] opening by a
small aperture on to the exterior, and by a wide funnel-
shaped extremity into the blastoccele : it has all the relations
of a nephridium, and is distinguished as the head-kidney.
As the larva of Polygordius is so strikingly different from
the adult, it is obvious that development must, in this as in
several cases which have come under our notice, be accom-
panied by a metamorphosis.
The first obvious change is the elongation of the conical
anal region of the trochosphere into a tail-like portion which
296
POLYGORDIUS
may be called the trunk (Fig. 75, A). The stomach
(enteron), which was formerly confined to the pro- and peri-
stomium, has now grown for a considerable distance into
the trunk (B, ent) so that the proctodseum (Prc. dm}
occupies only the portion in proximity to the anus.
Br
B
An ci
FIG. 75. — A, living specimen of an advanced trochosphere-larva of
Polygordius neapolitanus, showing the elongation of the anal region to
form the trunk.
B, diagrammatic vertical section of the same : the ectoderm is coarsely,
the nervous system finely dotted, the endoderm radially striated, and
the mesoderm evenly shaded.
C, transverse section through the plane ab in B.
The pre-oral (Pr. or. «'), post-oral (Pt. or. ci], and anal (An. ci)
cilia, brain (£r), ocelli (oc), blastoccele (Bl. ccel). mouth (Mth}> stomo-
dseum (St. dm), proctodaeum (Prc. dm], and anus (An) as in Fig. 73,
A : the enteron (Ent} has extended some distance into the trunk.
In A, slender mesodermal bands (Msd. bd) in the prostomium and the
branched head-kidney (Nph) are shown.
In B and C the mesoderm (Msd} is seen to have obliterated the blasto-
ccele in the trunk-region : the ectoderm has undergone a thickening,
forming the ventral nerve-cord ( V. Nv. Cd}.
(A after Fraipont.)
Important internal changes have also taken place. The
deric epithelium or external ectoderm is for the most part
composed, as in the preceding stage, of a single layer of
DEVELOPMENT OF METAMERES 297
cells, but on that aspect of the trunk which lies on the same
side as the mouth — i.e., to the left in Fig. 75, A and B — this
layer has undergone a notable thickening, being now com-
posed of several layers of cells. This ectodermal thickening
is the rudiment of the ventral nerve-cord ( V. Nv. Cd\ and
the side of the trunk on which it appears is now definitely
marked out as the ventral aspect of the future worm, the
opposite aspect — that to the right in the figures — being
dorsal. At a later stage two ectodermal cords, the cesopha-
geal connectives, are formed connecting the anterior end of
the ventral nerve-cord with the brain. Note that the two
divisions of the central nervous system are originally quite
distinct.
The mesodermal bands which were small and quite
separate in the preceding stage (Fig. 73, B and c, Msd)
have now increased to such an extent as to surround com-
pletely the enteron and obliterate the blastoccele (Fig. 75,
B and B, Msd). At this stage therefore there is no body-
cavity in the trunk, but the space between the deric and
enteric epithelia is occupied by a solid mass of mesoderm.
In a word, the larva is at present, as far as the trunk is con-
cerned, triploblastic but acotlomate.
Development continues, and the larva assumes the form
shown in Fig. 76, A. The trunk has undergone a great
increase in length and at the same time has become divided,
by a series of annular grooves, into segments or metameres
like those of the adult worm but more distinct (compare
Fig. 68, D, p. 268). By following the growth of the larva
from the preceding to the present stage it is seen that these
segments are formed from before backwards, i.e., the seg-
ment next the peristomium is the oldest, and new ones are
continually being added between the last formed and the
298 POLYGORDIUS
extremity of the trunk or what may now be called the anal
segment. By this process the larva'has assumed the appear-
ance of a worm with an immense head and a very slender
trunk.
The original larval stomach (enteron) has extended with
the formation of the metameres so as to form the greater
portion of the intestine : the proctodseum (Prc. dni) is
confined to the anal segment.
Two other obvious changes are the appearance of a pair
of small slender processes (A, /) — the rudiments of the
tentacles — on the apex of the prostomium, and of a circlet
of cilia (Pr. An. ci) round the posterior end of the trunk.
The internal changes undergone during the assumption of
the present form are very striking. In every fully formed
metamere the mesoderm — solid, it will be remembered,
in the previous stage — has become divided into two layers,
a somatic layer (B and c, Msd (soni) ) in contact with the
ectoderm and a splanchnic layer (Msd (spl) ) in contact
with the endoderm. The space between the two layers
(Ccel) is the permanent body-cavity or ccelome, which is
thus quite a different thing from the larval body-cavity
or blastoccele, being formed, not as a space between
ectoderm and endoderm, but by the splitting of an
originally solid mesoderm.
The division of the mesoderm does not however extend
quite to the middle dorsal and middle ventral lines : in both
these situations a layer of undivided mesoderm is left (c)
and in this way the dorsal and ventral mesenteries are
formed. Spaces in these, apparently the remains of the
blastoccele, form the dorsal and ventral blood-vessels. More-
over the splitting process takes place independently in each
segment and a transverse vertical layer of undivided
mesoderm (B, Sep) is left separating each segment from the
Msd (fern)
v.m
fr.an.ci-
An.ci
FIG. 76. — A, larva of Polygordius neapolitanus in a condition inter-
mediate between the trochosphere and the adult worm, the trunk-region
having elongated and divided into metameres.
B, diagrammatic vertical section of the same : the ectoderm is coarsely,
the nervous system finely dotted, the endoderm radially striated, and
the mesoderm evenly shaded.
c, transverse section along the plane ab in B.
The pre-oral (Pr. or. ci), post-oral (PL or. cz), and anal (An. ci)
cilia, the blastoccele (Bl. cosl}, stomodaeum (St. dm], and proctodaeum
(Prc. dm] are as in Fig. 73, A and B : the enteron now extends through-
out the segmented region of the trunk.
A pair of tentacles (t] has appeared on the prostomium near the ocelli
(oc), and a pre-anal circlet of cilia (Pr. an. ci} is developed.
The mesoderm has divided into somatic (J\hd (som) ) and splanchnic
(Msd (spl) ) layers with the ccelome (Ccet) between : the septa (sep) are
formed by undivided plates of mesoderm separating the segments of the
ccelome from one another.
D1-D3, three stages in the development of the somatic mesoderm. In
D1 it (Msd (som} } consists of a single layer of cells in contact with the
deric epithelium (Der. Epthm} : in D2 the cells have begun to split up
in a radial direction : in D3 each has divided into a number of radially
arranged sections of muscle-plates (M. PI} and a single cell of ccelome
epithelium (Ccel. Epthm}.
(A after Fraipont.)
300 POLYGORDIUS
adjacent ones before and behind : in this way the septa
arise.
The nephridia appear to have a double origin, the super-
ficial portion of each being formed from ectoderm, the
deep portion including the nephrostome from the somatic
layer of mesoderm.
In the ventral nerve cord the cells lying nearest the outer
surface have enlarged and formed nerve-cells, while those on
the dorsal aspect of the cord have elongated longitudinally
and become converted into nerve fibres. This process has
already begun in the preceding stage.
But the most striking histological changes are those which
gradually take place in the somatic layer of mesoderm. At
first this layer consists of ordinary nucleated cells (o1, Msd
(soni) ), but before long each cell splits up in a radial
direction (D2) from without inwards — i.e.t from the ectoderm
(Der. Epthm} towards the coelome — finally taking on the
form of a book with four or more slightly separated leaves
directed outwards or towards the surface of the body, and
with its back — the undivided portion of the cell — bounding
the coelome. The cells being arranged in longitudinal series
we have a number of such books placed end to end in
a row with the corresponding leaves in contact — page one
of the first book being followed by page one of the second,
third, fourth, £c., page two by page two, and so on through
one or more segments of the trunk. Next what we have
compared with the leaves of the books — the divided
portions of the cells — become separated from the backs-
the undivided portions (DS) — and each leaf (M. PI) fuses
with the corresponding leaves of a certain number of books
in the same longitudinal series. The final result is that the
undivided portions of the cells (backs of the books, CceL
Epthni) become the parietal layer of coelomic epithelium, the
SIGNIFICANCE OF DEVELOPMENTAL STAGES 301
longitudinal bands formed by the union of the leaves
(M. PI] becoming the muscle-plates which are thus, cell-
fusions, each being formed by the union of portions of
a series of longitudinally arranged cells.
At the same time the cells of the splanchnic layer
of mesoderm thin out and become the visceral layer of
ccelomic epithelium.
We see then that by the time the larva has reached the
stage shown in Fig. 76, it is no longer a mere aggregate of
simple cells arranged in certain layers. The cells them-
selves have undergone differentiation, some becoming modi-
fied into nerve-fibres, others by division and subsequent
fusion with their neighbours forming muscle-plates, while
others, such as the epithelial cells, remain almost unaltered.
Thus, in the course of the development of Polygordius,
cell-multiplication and cell-differentiation go hand in hand,
the result being the formation of those complex tissues the
presence of which forms so striking a difference between the
worm and the simpler types previously studied.
It is important to notice that this comparatively complex
animal is in one stage of its existence — the oosperm — as
simple as an Amoeba ; in another — the polyplast — it is com-
parable to a Magosphsera or a Volvox ; in a third — the
gastrula — it corresponds in general features with a Hydra ;
while in a fourth — the trochosphere — it resembles in many
respects a Medusa. As in other cases we have met with,
the comparatively highly organized form passes through
stages in the course of its individual development similar in
general characters to those which, on the theory of evolution,
its ancestors may be considered to have passed through in
their gradual ascent from a lower to a higher stage of
organization.
302 POLYGORDIUS
The rest of the development of Polygordius may be
summarized very briefly. The trunk grows so much faster
than the head (pro- plus peri-stomium) that the latter under-
goes a relative diminution in size, finally becoming of equal
diameter with the trunk as in the adult. The ciliated rings
are lost, the tentacles grow to their full size, the eye-spots
atrophy, and thus the adult form is assumed.
LESSON XXVII1
THE GENERAL CHARACTERS OF THE HIGHER ANIMALS
THE student who has once thoroughly grasped the facts of
structure of such typical unicellular animals as Amoeba and
the Infusoria, of such typical diploblastic animals as Hydra
and Bougainvillea, and of such a typical triploblastic animal
as Polygordius, ought to have no difficulty in understanding
the general features of the organization of any other members
of the animal kingdom. When once the notions of a cell, a
cell-layer, a tissue, body-wall, enteron, stomodaeum, procto-
daeum, ccelome, somatic and splanchnic mesoderm are fairly
understood, all other points of structure become hardly more
than matters of detail.
If we turn to any text-book of Zoology we shall find that
the animal kingdom is divisible into seven primary sub-
divisions, called sub-kingdoms, types, or phyla. These are
as follows :—
Protozoa. Coslenterata.
Verities. Echinodermata.
A rth ropoda. Mollusca.
Vertebrata.
1 Readers who have not studied zoology, or at least examined a series
of selected animal types, should omit this lesson and go on to the next.
304 GENERAL CHARACTERS OF THE HIGHER ANIMALS
With a few exceptions, the discussion of which would be out
of place here, the vast number of animals known to us can
be arranged in one or other of these groups.
The Protozoa are the unicellular animals : they have been
represented in previous lessons by Amoeba and Protamceba,
Hsematococcus, Heteromita, Euglena, the Mycetozoa, Para-
moecium, Stylonychia, Oxytricha, Opalina, Vorticella, Zooth-
amnium, the Foraminifera, the Radiolaria, Magosphaera, and
Volvox. The reader will therefore have no difficulty in
grasping the general features of this phylum.
The Coeltnterata are the diploblastic animals, and have
also been well represented in the foregoing pages, namely,
by Hydra, Bougainvillea, Diphyes, and Porpita. The sea-
anemones, corals, and sponges also belong to this phylum.
The Vermes, or Worms, are a very heterogeneous assem-
blage. They are all triploblastic, but while some are
ccelomate, others have no body-cavity ; some, again, are
segmented, others not. Still, if the structure of Polygordius
is thoroughly understood, there will be little difficulty in
understanding that of a fluke, a tape-worm, a round-worm,
an earthworm, or one of the ordinary marine worms.
Of the remaining four sub-kingdoms we have, so far,
studied no example, but a brief description of a single
typical form of each will show how they all conform to the
general plan of organization of Polygordius, being all triplo-
blastic and ccelomate.
Under the Echinodermata are included the various kinds
of starfishes — sand-stars, brittle-stars, and feather-stars, as
well as sea-urchins, sea-cucumbers, &c. A starfish will serve
as an example of the group.
The phylum Arthropoda includes crayfishes, lobsters,
crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions,
spiders, and mites ; centipedes and millipedes ; and all
GENERAL STRUCTURE 305
kinds of insects, such as cockroaches, beetles, flies, ants,
bees, butterflies, and moths. A crayfish forms a very fair
type of the group.
In the phylum Mollusca are included the ordinary bi-
valves, such as mussels and oysters ; snails, slugs, and other
univalves or one-shelled forms ; sea-butterflies ; and cuttlefish,
squids, and Octopi. An account of a fresh-water mussel
will serve to give a general notion of the character of this
group.
Finally, under the head of Vertebrata are included all the
backboned animals : the lampreys and hags ; true fishes,
such as the shark, skate, sturgeon, cod, perch, trout, &c. ;
amphibians, such as frogs, toads, newts, and salamanders ;
true reptiles, such as lizards, crocodiles, snakes, and tor-
toises ; birds ; and mammals, or creatures with a hairy skin
which suckle their young, such as the ordinary hairy
quadrupeds, whales and porpoises, apes, and man. The
essential structure of a vertebrate animal will be understood
from a brief description of a dog-fish.
THE STARFISH.1
A common starfish consists of a central disc-like portion,
from which radiate five arms or rays. It crawls over the
rocks with its ventral surface downwards, its dorsal surface
upwards. It can move in any direction, so that, in the
ordinary sense of the words, anterior and posterior extremi-
ties cannot be distinguished. Radial symmetry such as this,
/.<?., 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.
X
TnC.CtR
M&V^ T.F
^Itad.Amb. V.
FIG. 77. — 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. Epthm), dermis
(Derm], and the parietal layer of ccelomic epithelium (Ccel. Epthm}.
To the body-wall are attached pedicellariae (Fed), and the end of the
arm bears a tentacle (t) with an ocellus (oc) at its base.
The skeleton consists of ossicles (Os) imbedded in the derm : large
ambulacral ossicles (Amb. os] bound the ambulacral grooves on the
ventral surfaces of the arms.
The mouth (Ml/i) leads by a short gullet into a stomach (St), which
gives off a cardiac ccecum (Cd. cce} and a pair of pyloric cceca (Pyl. coe]
to each arm, and passes into an intestine (hit] which gives off intestinal
cceca (Int. ccc] to the inter-radii, and ends in the anus (An). The
pyloric cceca 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 ccelomic epithelium (Ccel. Eplhm').
From the ccelome are given off respiratory cceca (Resp. cce), which
TUBE-FEET 307
project through the body-wall : the latter contains peri-haemal spaces
(/. h) derived from the coelome.
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 ampullae (Amp) and
tube-feet (T. F): it is also connected with the stone-canal (St. c), which
opens externally by the madreporite (Mdpr).
The nerve-ring (Nv. R) gives off radial nerves (Rd. 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. 77, A, MtJi), 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 tube-feet (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 coelome separ-
ating the body-wall from the enteric canal and containing
the gonads, blood-vessels, &c. The body-wall consists ex-
X 2
3o8 THE STARFISH
ternally of a very thin cuticle, then of a layer of deric
epithelium or epidermis (Der. Epthni), 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 ccelomic epithelium (Cxi. Epthm) 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
(Os) 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. K, 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 (6V), and
intestine (/«/). The stomach gives off five wide pouches
(Cd. Cce), one extending into the base of each arm, and
above these five other pouches (Pyl. Cos) 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 ccelomic epithelium
(Cod. EptH).
Respiration is effected by blind, finger-like offshoots of the
ccelome, the respiratory cceca (Resp. cce.), which pass between
the ossicles of the skeleton and project on the surface of the
body, thus bringing the ccelomic fluid into close relation
with the surrounding water.
The blood-system consists of a circular vessel (A, C. B. V)
NERVOUS SYSTEM, ETC. 309
round the gullet connected with a pentagonal vessel round
the intestine by an elongated network or plexus of vessels.
From the circular vessel five radiating trunks (Rad. J3. 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 ambulacral 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
310 THE CRAYFISH
open by gonaducts (Ovd} on the dorsal surfaces of the disc.
The sexes are lodged in distinct individuals.
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 cephalo-thorax (Fig. 78,
A, C. T/i.) : 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 metameres
limited and constant in number, and for more or fewer
/
of them to undergo concrescence.
Another distinctive arthropod character illustrated 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 or podomeres.
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 (London, 1888), p. 125 : and Parker, The Skeleton of the New
Zealand Crayfishes (Wellington, N.Z., 1889).
STRUCTURE OF BODY- WALL 311
In the Crayfish there is a marked differentiation of the
appendages. Those of the prostomium are a pair of eye-
stalks, and one of the small feelers or antennules which
perform 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 maxillipedes), and five pairs of legs, the first of
which are — in the fresh-water crayfishes and 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, constitute the tail-fin.
Sections show the body-wall to consist of a layer of deric
epithelium (Der. EptJuri) secreting a thick cuticle (Cu), a
layer of connective tissue forming the Dermis (Derm), and
a very thick layer of large and complicated muscles (J/),
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. 308), is a cuticular exoskeleton, forming a
continuous investment over the whole body but discon-
tinuously calcified.
The mouth (Mfh) is on the ventral surface of the head,
in the segment 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 (Gul\ a large
1 The antennules are frequently considered as belonging to the first
metamere, the number of segments being then reckoned as twenty.
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GENERAL CHARACTERS 313
The body is divided into a head (Hd] and thorax ( Th), together
constituting the cephalo-thorax (C. Th}, and seven free abdominal
segments (Abd. seg. I, Abd. seg. 7) : the head is produced in front into
a rostrum.
The body- wall consists of cuticle (Cu), partly calcified to form the
exoskeleton, deric epithelium (Der. Epthni), dermis (Derm.}, and a
very thick layer of muscle (M) which in the abdomen is distinctly
segmented.
The mouth (Mth} leads by a short gullet (Gul} into a large stomach
(6Y), from which a short small intestine (S. Ini} leads into a large in-
testine (L. Itit}, ending in the anus (An}. Opening into the small
intestine are the digestive glands (D. Gl}. The epithelium of the small
intestine and digestive glands is endodermal, that of the rest of the canal
is ectodermal and secretes a cuticle : the outer layer throughout is meso-
dermal (connective tissue and muscle).
The cavity (B. S) between the enteric canal and the body-muscles is
a blood- sinus.
The heart (Hi} is enclosed in the pericardial sinus (Per. S) : the
chief ventral blood-vessel or sternal artery (St. A} is shown in B.
The gills (B, Gill} are enclosed in a cavity formed by a fold of the
thoracic body-wall called the branchiostigite (Brstg] : they are formed
of the same layers as the body-wall, of which they are offshoots.
The kidneys (A, K] are situated in the head.
The brain (Br) lies in the prostomium : the ventral nerve-cord ( V.
Nv. Cd} consists of a chain of ganglia (Gn) united by connectives.
The ovary (ovy] is a hollow organ opening by an oviduct (B, ovd} on
the base of one of the legs (Leg).
stomach (67), and a straight intestine divisible into a short
anterior division or small intestine (S. Int.] and a long
posterior division or larger intestine (L. Int.] : the latter
opens by an anus (An) on the ventral surface of the last
segment. The study of development shows that the only
part of the canal derived from the enteron of the embryo is
the small intestine : the gullet and stomach arise from the
stomodseum, the large intestine from the proctodaeum.
Thus the only portion of the enteric epithelium which
is endodermal is that of the small intestine : the epithelium
of gullet, stomach, and large intestine is ectodermal, and,
like the deric epithelium secretes a cuticle. The outer
layer of the whole enteric canal consists of connective
tissue and muscle : there is no coelomic epithelium.
3H THE CRAYFISH
On each side of the small intestine is a large organ, the
digestive gland (D. Gl) : it consists of numberless glove-
finger-like processes or cozca which open by a short tube or
duct into the small intestine (B, D. Gl}. Both coeca
and duct are lined with epithelium derived from the endo-
derm, and the whole digestive gland is to be looked upon
as a branched lateral outgrowth of the enteron. The
secretion of digestive juice is performed exclusively by the
epithelium of the digestive glands.
Between the enteric canal and the body-wall are a series
of spaces (jB.S) containing blood and having the general
relations of a coelome, but very probably only representing
a number of enlarged blood-spaces or sinuses.
Respiration is performed by special organs, the gills
(B, Gill, see p. 313) developed in the thoracic region as out-
growths of the body-wall, and containing the same layers
(cuticle, epithelium, and connective tissue) as the latter.
They have a brush-like form and are protected by a fold of
the body-wall (Brstg).
The blood-system is constructed on the same general
lines as that of Polygordius but is greatly modified. A
portion of the dorsal vessel is enlarged to form a muscular
dilatation, the heart (Ht\ and the rest of the vessels, now
called arteries (B, St. A), instead of forming by themselves
a closed system, ramify extensively over the body, their ulti-
mate branches opening into larger cavities or sinuses between
the muscles. One of these cavities — the pericardial sinus
(Pcd. S) — surrounds the heart. The heart, arteries, and
sinuses together form a closed system through which the
blood is propelled in a definite direction by the contractions
of the heart.
Renal excretion is performed by a pair of glandular
bodies, the kidneys (A, K}, situated in the front part of the
ABSENCE OF CILIA 315
head and opening by ducts on the bases of antennae. They
consist of convoluted tubes lined by epithelium, and are
probably to be looked upon as greatly modified nephridia.
The Crayfish is dioecious. The ovaries (Ovy) are a pair
of hollow organs, united in the middle line in some genera,
situated in the thorax, and opening by oviducts (B, Ovd] on
the bases of the third pair of legs. The spermaries (testes)
are also frequently united in the middle line and open
by spermiducts (vasa deferentia) on the bases of the fifth
pair of legs. There is some reason for thinking that the
gonaducts represent modified nephridia, and the cavities
of the hollow gonads a greatly reduced coslome from the
epithelium of which the sex-cells are produced.
The nervous system is formed on quite the same plan as
that of Polygordius, consisting of a dorsal brain (Br) united
by oesophageal connectives to a ventral nerve-cord (V.
Nv. Cd). In the cord, however, the nerve-cells, instead of
being evenly distributed, are aggregated into little enlarge-
ments or ganglia (Gn), of which there is primitively a pair
to each metamere, the number being reduced in the adult
by concrescence. The portions of the ventral nerve-cord
between the ganglia consist of nerve-fibres only, and are
called connectives. In the embryo the nervous system is,
as in Polygordius, in direct connection with the epidermis,
but in the adult it has sunk inwards so as to be entirely
surrounded by mesoderm.
A striking feature in the histology of the Crayfish, and
one in which it agrees with the vast majority of Arthropoda,
is the entire absence of cilia. Another peculiarity — also
shared by the greater part of the phylum — is that the sperms
are non-motile.
The laid eggs become attached to the swimmerets of the
mother, and in this situation undergo their development. In
316 THE FRESH-WATER MUSSEL
the fresh-water crayfish the young is hatched in a condition
closely resembling the adult, but in the lobster and the sea-
crayfish there is a metamorphosis.
THE FRESH-WATER MussEL.1
*
The body is bilaterally symmetrical, and is greatly com-
pressed from side to side. Its dorsal margin is produced
into paired flaps, the mantle-lobes (Fig. 79, A and B, Mant\
which pass downwards one on each side of the body.
Closely applied to the outer surface of the mantle-lobes, and
formed as a cuticular secretion of their deric epithelium, are
the two valves of the bivalved strongly calcified shell (J3., Sh).
The ventral region of the body is produced into a laterally
compressed muscular structure, the foot (A and B, foot), by
the contraction of which the animal can move slowly
through the sand or mud in which it lives partly buried.
The possession of a mantle formed as a prolongation of
the dorsal region, of a calcareous shell secreted by the
mantle, and of a muscular foot formed as an impaired
prolongation of the ventral region, are the most characteristic
features of the Mollusca generally.
Posteriorly the edges of the mantle-lobes are greatly
thickened and are united to one another in such a way as to
form two apertures, a large ventral inhalent (Ink. Ap\ and
a small dorsal exhalent aperture (Exh. Ap). By means of
the cilia of the gills (see below) a current of water is pro-
duced which enters at the inhalent aperture carrying
1 For detailed descriptions of the fresh-water Mussel see Rolleston
and Jackson, Forms of Animal Life, pp. 124 and 285 : Huxley and
Martin, Elementary Biology, p. 305 : and Marshall and Hurst, Practical
Zoology, p. 76.
l'a.t.£]>llnn CaLEnthm* -K,., L
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CP.Un
FIG. 79. — Diagrammatic sections of the Fresh-water Mussel.
A, longitudinal section : the right mantle-lobe (Mant) and gills (/. G.
O. G) are shown in perspective.
B, transverse section.
The cuticular shell (S/i), shown only in B, is black, the ectoderm
dotted, the nervous system finely dotted, the endoderm radially striated,
the mesoderm evenly shaded, and the ccelornic epithelium represented
by a beaded line.
'The dorsal region is produced into the right and left mantle-lobes
(Mant), attached to which are the valves of the shell (Sh) joined dorsally
by an elastic ligament (lig).
The mantle-lobes are partly united so as to form the inhalent (Ink.
Ap) and exhalent (Exk. Ap) apertures at the posterior end.
The body is produced ventrally into the foot (Foot), on each side of
which are the gills, an inner (/. G) and an outer (0. G), each formed
of an inner and an outer lamella.
The body is covered externally by deric epithelium (Der. Epthin],
within which is mesoderm (Msd) largely differentiated into muscles, of
which the anterior (A. Ad) and posterior (P. Ad) adductors are indi-
cated in A.
The mouth (Mth) leads by the short gullet (Gtil) into the stomach
(St), from which proceeds the coiled intestine (Int), ending in the anus
318 THE FRESH-WATER MUSSEL
(An): the enteric epithelium is mostly endodermal. The digestive gland
(D. Gl) surrounds the stomach. The ccelome (C&l) is reduced to a
small dorsal chamber enclosing part of the intestine and the heart : the
parietal (Ccel. Epthni) and visceral (Ccel. Epthn^} layers of ccelomic
epithelium are shown.
The heart consists of a median ventricle ( Vent), enclosing part of the
intestine, and of paired auricles (Aur).
The paired nephridia (Nphni) open by apertures into the ccelome
(Nph. si] and on the exterior (Nph. p).
The gonads (Gon) are imbedded in the solid mesoderm, and open on
the exterior by gonaducts (Gnd).
The nervous system consists of a pair of cerebro-pleural ganglia
(C. P. Gn) above the gullet, a pair of pedal ganglia (Pd. Gn) in the
foot, and a pair of visceral ganglia ( V. Gn} below the posterior adductor
muscle.
abundant oxygen and the minute organisms used as food,
and makes its escape at the exhalent aperture, taking with it
the various products of excretion and faecal matter.
The mouth (MtJi) is anterior and ventral, lying just in
front of the foot : it is bounded on each side by a pair of
triangular bodies, the labial palpi, and leads by a short
gullet (Gul) into a stomach (St) from which proceeds a
long coiled intestine (Int) : this makes several turns in the
ventral region of the trunk, then passes to the dorsal region,
and finally backwards in the median plane to open by an
anus (An) at the posterior end of the body just within the
exhalent aperture. The enteric canal is formed almost
exclusively from the enteron, the stomodaeum and procto-
daeum being both insignificant, hence the enteric epithelium
is almost wholly endodermal. There is a large digestive
gland (D. Gl) surrounding the stomach and opening into
it by several ducts.
The coelome (Ccel) is a small cavity in the dorsal region
containing a portion of the intestine : the rest of the enteric
canal is embedded in solid mesoderm.
The mesoderm, as usual, is largely differentiated into
muscle, There are numerous muscles connected with the
REPRODUCTION AND DEVELOPMENT 319
foot, and two very large ones (A. Ad, P. Ad) pass trans-
versely from valve to valve of the shell, one immediately
above the gullet, the other immediately below the anal end
of the intestine ; these latter are called adductors, and serve
to close the shell.
On either side of the body, between the trunk and the
mantle, are two gills (/. G, O. G\ each having the form of
a double plate (B) nearly as long as the body. They serve,
in conjunction with the mantle, as respiratory organs, but
their main function is to produce the current of water re-
ferred to above by means of the cilia with which they are
covered.
There is an extensive system of blood-vessels. The heart
lies in the ccelome, and consists of three chambers, a median
ventricle (Vent), which surrounds the intestine, and paired
auricles (Aur).
Excretion is performed by a single pair of nephridia
(Nphm) which open at one end (Nph. st] into the ccelome
and at the other (Nph. p) on to the exterior.
The nervous system consists of three pairs of ganglia, the
two ganglia of each pair being united by transverse com-
missures. The cerebro-pleural ganglia (C. P. Gn) lie above
the gullet, and represent, in a general way, the brain of
Polygordius and the crayfish ; they are united by longitu-
dinal connectives with the pedal ganglia (P. Gn} which lie
in the foot, and may be taken as representing the ventral
nerve-cord of worms and arthropods, and with the visceral
ganglia ( V. Gn) which are placed beneath the posterior
adductor muscle.
The gonads (Gon) are large irregular organs, very similar
in appearance in the two sexes, situated among the coils of
the intestine and opening by a duct (Gnd) on each side of
the trunk, close to the nephridiopore. The impregnated
320 THE DOG-FISH
eggs are passed into the cavity of the outer gill of the
female, where they undergo the early stages of their develop-
ment. The larva of the fresh-water mussel is a peculiar
bivalved form, very unlike the adult, and called a glochidium^
but in the more typical molluscs the embryo leaves the egg
as a trochosphere, closely resembling that of Polygordius.
THE Doc-FisH.1
A dog-fish is bilaterally symmetrical, the nearly cylin-
drical body (Fig. 80, A) terminating in front in a blunt
snout and behind passing insensibly into an upturned tail.
Externally there is no appearance of segmentation.
The mouth (MtK) is on the ventral surface of the head
or anterior region of the body ; it is transversely elongated,
and is supported by jaws which are respectively anterior
(upper) and posterior (lower). They thus differ funda-
mentally from the jaws of arthropods, which are modified
appendages and are therefore disposed right and left.
A short distance behind the mouth are five vertical slits
(B, Ext. br. ap) arranged in a longitudinal series, the
external branchial apertures or gill-clefts. The vent, or
cloacal aperture (An) is situated on the ventral surface a
considerable distance from the end of the tail. That part
of the body lying in front of the last gill-cleft is counted as
the head, all behind the vent as the tail, the intermediate
portion as the trunk.
1 For a detailed description of a dog-fish see Marshall and Hurst,
Practical Zoology (London, 1888), p. 196. For descriptions of other
fishes, equally suitable in some respects as types of Vertebrata, see
Rolleston and Jackson, Forms of Animal Life (Oxford, 1888), pp. 83
and 273 : and Parker, Zootomy (London, 1884), pp. I, 27, 86.
APPENDAGES 321
Appendages are present, but in a very different form from
those .of the crayfish. They consist of flat processes of the
body- wall called fins. Two of them (D.F1, D.F") are
situated in the middle line of the back (dorsal fins] : one
( V.F} in the middle ventral line behind the anus (ventral
fin\ and one (C.J?) is attached to the up-turned end of the
tail (caudal fiii) : all these being unpaired structures or
median fins. Then there is a pair of pectoral fins situated
one on each side just behind the last gill-cleft, and a pair of
pelvic fins placed one on each side of the vent : these are
the lateral or paired fins. It is characteristic of Vertebrata
that the number of lateral appendages never exceeds two
pairs.
The skin or external layer of the body-wall consists of an
outer epidermis (Der. Rpthm) composed of several layers of
cells, and of an inner connective tissue layer or dermis
(Demi}. In the latter are found innumerable bony scales
(Derm. Sp) constituting a dermal exoskeleton. The muscular
layer of the body-wall (M) is of great thickness, especially
in the dorsal region, and is distinctly segmented, indicating
that the body of the dog-fish, like that of Polygordius and
the crayfish, is divisible into metameres, although there is no
indication of these externally.
The large coelome (cceV) is confined to the trunk : it is
characteristic of vertebrates that both head and tail are
accelomate in the adult. The coelomic epithelium (Ccel.
Epthm, Ccvl. Epthm1) is underlaid by a distinct layer of
connective tissue, the two together forming the peritoneum.
Another important vertebrate character is that the dorsal
region of the body-wall contains a median longitudinal
canal (C. Sp. cav.) extending from shortly behind the snout
to near the end of the tail. This is the cerebro-spinal cavity
and contains the central nervous system.
Y
APPENDAGES 323
A, longitudinal vertical section.
B, horizontal section through the pharynx and gills.
C, transverse section through the trunk.
The ectoderm is dotted, the nervous system finely dotted, the endo-
derm radially striated, the mesoderm evenly shaded, the coelomic
epithelium represented by a beaded line, and all skeletal structures
black.
The body gives origin to the dorsal (D. F1, D. F-), ventral ( V. F),
and caudal (C. F) fins : the paired fins are not shown.
The body-wall consists of deric epithelium (Der. Epthm}, dermis
(Derm], and muscle (M) : the latter is metamerically segmented and is
very thick, especially dorsally, where it forms half the total vertical
height (C).
The exoskeleton consists of calcified dermal spines (Derm, sp} in the
dermis, and of dermal fin-rays (Derm. F. R) in the fins.
The endoskeleton consists of a row of vertebral centra ( V. Cent} below
the spinal cord (sp. cd), giving rise to neural arches (n. a), which enclose
the cord, and in the caudal regions to haemal arches (h. a) : a cranium
(Cr) enclosing the brain (Br) : upper and lower jaws : branchial arches
(Br. A) and rays (Br. R, Br. Rl), shown only in B, supporting the
gills : shoulder (Sh. G) and pelvic (Pelv. G] girdles : and pterygiosphores
(Ptgph] supporting the fins.
The mouth (Mtfi) leads into the oral cavity (Or. cav], from which the
pharynx (P/i) and gullet (Gttl} lead to the stomach (St) : this is con-
nected with a short intestine (Int) opening into a cloaca (C!) which
communicates with the extei'ior by the vent (An). The oral cavity and
cloaca are the only parts of the canal lined by ectoderm.
Connected with the enteric canal are the liver (Lr) with the gall-
bladder (G. Bl) and bile-duct (B. D}, the pancreas (/"«), and the spleen
(Spl). The mouth is bounded above and below by teeth ( T).
The respiratory organs consist of pouches (shown in B) communicating
with the pharynx by internal (Int. br. ap} and with the exterior by
external (Ext. br. ap} branchial apertures, and lined by mucous mem-
brane raised into branchial filaments (br. fil).
1'he heart (Ht) is ventral and anterior, and is situated in a special
compartment of the ccelome (Ped). Six of the most important blood-
vessels, the dorsal vessel (dorsal aorta, D. Ao}, the cardinal veins
(Card. V), the lateral vessels (lateral veins, Lat. F), and the ventral
vessel (intra-intestinal vein, /. int. V] are shown in c.
The whole ccelome is lined by epithelium, showing parietal (Cat.
Epthni) and visceral (Cal. Epthm1} layers.
The ovaries (Ovy) are connected with the dorsal body- wall : the
oviducts (ovd) open anteriorly into the ccelome (ovd'} and posteriorly
into the cloaca.
The kidneys (K] are made up of nephridia (NpJi] and open by ureters
( Ur) into the cloaca.
The nervous system is lodged in the cerebro-spinal cavity ( C. Sp. cav}
hollowed out in the dorsal body-wall : it consists of brain (Br} and
spinal cord (Sp. cd}, and contains a continuous cavity, the neuroccele
(n. cce).
Y 2
324 THE DOG-FISH
Still another characteristic feature is the presence, in
addition to the dermal exoskeleton, of an endoskeleton^ or
system of internal supporting structures. Between the
cerebro-spinal cavity above and the coelome below is a
longitudinal series of biconcave discs or vertebral centra
( V. Cent) : they are formed of a peculiar tissue called
cartilage or gristle, and are strongly impregnated with lime-
salts : in the young condition their place is occupied by a
gelatinous rod, the notochord. The centra, which alternate
with the muscle-segments, are connected with a series of
cartilaginous arches (n.a) which extend over the cerebro- '
spinal cavity and with the centra constitute the vertebral
column. In the tail there is also a ventral series of arches
(h.a.) enclosing a space (H.C] which indicates a backward
extension of the ccelome in the embryo.
Anteriorly the vertebral column is continued into a
cartilaginous box, the cranium (Cr) which encloses the brain
and the organs of smell and hearing. The jaws, referred to
above, are cartilaginous rods which bound the mouth above
and below. The gills are supported by a complicated
system of cartilages (Br. A, Br. R, Br. JR.') and both
median and paired fins by parallel rods of the same
material (Ptgph). All these cartilages are strengthened
by a more or less extensive superficial deposit of bony
matter.
The mouth (MtJi) leads into a large oral cavity (Or. cav)
which passes insensibly into a wide throat or pharynx (P/i) :
from this a short gullet (Gut) leads into a large U-shaped
stomach (St\ whence is continued a short wide intestine
(Int) opening on to the exterior through the intermediation
of a small chamber, the cloaca (6V). From the gullet
backwards the enteric canal is contained in the coelome.
The greater part of the enteric epithelium is endodermal :
GILLS AND HEART 325
only the oral cavity arises from the stomodaeum and the
cloaca from the proctodaeum.
In the skin covering the jaws dermal ossicles of unusual
size are developed and constitute the teeth (T). The chief
digestive glands are two in number, an immense liver (Lr)
occupying the whole anterior and ventral region of the
ccelome, and a small pancreas (Pn), attached to the anterior
end of the intestine. The ducts of both glands open into
the intestine, and their secreting cells are, as in former cases,
endodermal. Gland-cells are also found in the walls of
the stomach and intestine.
The respiratory organs or gills (B) consist of five pairs of
pouches opening on the one hand into the pharynx (P/i)
and on the other to the exterior by the branchial clefts
already noticed : they have their walls raised into ridges,
the branchial filaments (Br. fil) which are covered with
epithelium and are abundantly supplied with blood-vessels.
The gills are developed as offshoots of the pharynx, and the
respiratory epithelium is therefore endodermal, not ecto-
dermal as in the crayfish and mussel.
The heart (fit) lies below the pharynx in a separate
anterior compartment of the ccelome, the pericardial cavity.
It is composed of four chambers arranged in a single longi-
tudinal series (sinus venosus, auricle, ventricle, and conus
arteriosus), and is to be looked upon as a muscular dilatation
of a ventral blood-vessel. The blood is propelled by the
heart from the conus arteriosus into a paired series of
hoop-like vessels (aortic arches) resembling the transverse
commissures of Polygordius (Fig. 69, A, p. 270), which take
it through the gills and pour it, in a purified condition, into
the dorsal vessel (dorsal aorta, D. Ao) whence it is taken to
all parts of the body to be finally returned by thin-walled
vessels, called veins, to the sinus venosus. The ventral
326 THE DOG-FISH
position of the heart and the fact that the blood is sent
directly from the heart to the respiratory organs are
characteristic vertebrate features : so also is the circumstance
that the blood from the stomach, intestine, &c., is taken by
a specially modified portion of the ventral vessel (portal
vein) through the liver on its way to the heart. The blood
is red, containing, in addition to leucocytes, oval corpuscles
coloured by haemoglobin (see p. 56).
The excretory organs are a pair of kidneys (K) situated
at the posterior end of the dorsal region of the co3lome, and
opening by ducts, the ureters (Ur), into the cloaca. De-
velopment shows that they consist of an aggregation of
nephridia (Nph\ the nephrostomes of which open in the
young and sometimes throughout life, into the ccelome,
while the nephridiopores discharge not directly on the
exterior, but into a common tube.
The gonads (ovaries, Ovy, or spermaries) are situated in
the anterior part of the ccelome attached by peritoneum to
its dorsal wall. The sex-cells are differentiated from ccelomic
epithelium. The gonaducts of both sexes (Ovd) are de-
veloped from the nephridial system of the embryo.
As already stated the central nervous system is contained
in a cavity ( C. Sp. cav) of the dorsal body-wall, and is
therefore far removed from the ectoderm from which it
originates. It consists of a long cylindrical rod, the spinal
cord (Sp. cd) which is continued in front into a complicated
brain (Br). It has the further peculiarity of being hollow,
a more or less cylindrical cavity, the neurocosle (n. cce) ex-
tending through its whole length.
The possession of a hollow nervous system lying altogether
dorsal to the enteric canal and ccelome, of either a noto-
chord or a chain of vertebral centra below the nervous
system, and of pharyngeal pouches communicating with the
DEVELOPMENT 327
exterior, are the three most characteristic features of the
vertebrate phylum.
The organs of sense are highly developed, and consist of
paired olfactory sacs, eyes, and auditory sacs situated in the
head, together with an extensive system of integumentary
organs. Their sensory cells are in every case ectodermal.
The eggs are very large, and are impregnated within the
body of the female. In the common Dog-fish (Scyllium)
they are laid shortly after impregnation, each enclosed in a
horny egg-shell : in the Piked Dog-fish (Acanthias] and the
Smooth Hound (Mustelus) they are retained in the oviduct
until the adult form is assumed.
LESSON XXVIII
MOSSES
IN the four previous lessons we have traced the advance
in organization of animals from the simple diploblastic
Hydra to the complicated triploblastic forms which con-
stitute the five higher phyla of the animal kingdom. We
have now to follow in the same way the advance in structure
of plants. The last member of the vegetable kingdom with
which we were concerned was Nitella (p. 204), a solid
aggregate, exhibiting a certain differentiation of form and
structure, but yet composed of what were clearly recognizable
as cells, there being, as in Hydra, none of that formation of
well-marked tissues which is so noticeable a feature in
Polygordius as in other animals above the Ccelenterata.
Taking Nitella as a starting point, we shall see that among
plants as among animals there is an increasing differentiation
in structure and in function as we ascend the series. The
first steps in the process are well illustrated by a considera-
tion of that very abundant and beautiful group of plants, the
Mosses. In spite of the variations in detail met with in
different genera of the group the essential features of their
organization are so constant that the following description
will be found to apply to any of the common forms.
B
te°.w>?
^Q^L.ma.r
FIG. 81. — The Anatomy and Histology of Mosses.
A, Entire plant of Funaria hygrometrica, showing stem (st), leaves
(/), and rhizoids (r/i). (X 6.)
B, leaf of the same, showing midrib (md. r) and lateral portions.
(X 25.)
C, semi-diagrammatic vertical section of a moss, showing the arrange-
330 MOSSES
ment of the tissues. The stem is formed externally of sclerenchyma
(scl), and contains an axial bundle (ax. b) : in some of the leaves (/)
the section passes through the midrib, in others (71) through the lateral
portion : the stem ends distally in an apical cell (ap. c), from which
segmental cells (seg. c) are separated.
D, transverse section of the stem of Bryum rosettm, showing scleren-
chyma (scl), axial bundle (ax. b). and rhizoids (r/i). (X 60.)
E, transverse section of a leaf of Funaria, showing the midrib (md. r)
formed of several layers of cells, and the lateral portions one cell thick.
(X 150.)
F, small portion of the lateral region of the same, showing the form
of the cells and the chromatophores (chr). (X 150.)
G, distal end of the stem of Fontinalis antipyretica in vertical section,
showing the apical cell (ap. c) giving rise to segmental cells (seg. c),
which by subsequent division form the segments of the stem with the
leaves : the thick lines show the boundaries of the segments.
H, diagram of the apical cell of a moss in the form of a tetrahedron
with rounded base abc and three flat sides abd, bed, acd.
(D after Sachs ; G after Leitgeb. )
The plant consists of a short slender stem (Fig. 81, A, st\
from which are given off structures of two kinds, rhizoids or
root-hairs (r/i), which pass downwards into the soil, and leaves
(/), which are closely set on the stem and its branches. As
in Nitella (p. 206) the portion of the stem from which a leaf
arises is called a node and the part intervening between any
two nodes an internode, while the name segment is applied
to a node with the internode next below it. At the upper or
distal end of the stem the leaves are crowded, forming a
terminal bud.
Owing to the opacity of the stem its structure can only be
made out by the examination of thin sections (c and D). It is
a solid aggregate of close-set cells which are not all alike but
exhibit a certain amount of differentiation. In the outer
two or three rows the cells (sd} are elongated in the direction
of length of the stem, so as to have a spindle-shape, and their
walls are greatly thickened and of a reddish colour. They
thus form a protective and supporting tissue to which the
name sclerenchyma is applied. Running longitudinally
STRUCTURE OF LEAF 331
through the centre of the stem is a mass of tissue (ax. b)
distinguished by its small, thin-walled cells, and constituting
the axial bundle.
The leaves (B) are shaped like a spear-head, pointed
distally, and attached proximally by a broad base to the
stem. The axial portion (B and E, md. r., c. /) consists of
several layers of somewhat elongated cells and is called the
midrib : the lateral portions (E and F : c, I1} are formed of a
single layer of short cells. Thus the leaf has, for the most
part, the character of a superficial aggregate. The cells
contain oval chromatophores (F, chr).
The rhizoids (c and D, rh) are linear aggregates, being
formed of elongated cells, devoid of chlorophyll, arranged,
end to end.
In the terminal bud the leaves, as in Nitella (pp. 206 and
208), arch over the growing point of the stem, which in this
case also is formed of a single apical cell (c and G, ap. c).
But in correspondence with the increased complexity of the
plant the apical cell is not a hemisphere from which new
segments are cut off parallel to its flat base, but has the form
(H) of an inverted, three-sided pyramid or tetrahedron, the
rounded base of which (a be} forms the apex of the stem
while segments (seg. c} are cut off from each of its three
triangular sides in succession.
The best way to understand the apical growth of a moss
is to cut a tetrahedron with rounded base out of a carrot or
turnip : this represents the apical cell (H) : then cut off a
slice parallel to the side abd^ a second parallel to bed, and a
third parallel to acd : these represent three successively
formed segments. Now imagine that after every division
the tetrahedron grows to its original size, and a very fair
notion will be obtained of the way in which the successive
segments of the moss-stem are formed by the fission in three
332 MOSSES
planes of the apical cell. Each segment (c and G, seg. c]
immediately after its separation divides and subdivides, pro-
ducing a mass of cells from which a projection grows out
forming a leaf, and in this way the stem increases in length
and the leaves in number.
Asexual reproduction takes place in various ways : all of
them are, however, varieties of budding, and the buds always
arise in the form of a linear aggregate of cells called a
protonema : from this the moss-plant develops in the same
way as from the protonema arising from a spore (p. 335).
The gonads are developed at the extremity of the main
stem or of one of its branches, and are enclosed in a tuft of
leaves often of a reddish colour — the terminal bud of the
fertile shoot or so-called " flower " of the moss.
The spermary (Fig. 82, A1, A2) is an elongated club-shaped
body consisting of a solid mass of cells, the outermost of
which form the wall of the organ, while the inner (AS) become
converted into sperms. The latter (A4) are spirally coiled
and provided with two cilia : they are liberated by the
rupture of the wall of the spermary at its distal end (A2).
The ovaries1 (see Preface, p. x, and p. 377) (B1, B2) may
or may not occur on the same plant as the spermaries, some
mosses being monoecious, others dioecious. Like the sperm-
aries they consist at first of a solid mass of cells which
assumes the form of a flask, having a rounded basal portion
or venter (v) and a long neck (n). The outer layer of cells
in the neck and the two outer layers in the venter form the
wall of the ovary, the internal cells are arranged in a single
axial row at first similar to those of the wall. As the ovary
develops the proximal or lowermost cell of the axial row
1 The ovary of mosses, ferns, &c., is usually called an archegonium :
the spermary, as in the lower plants, an antheridium.
DEVELOPMENT OF SPOROGONIUM 333
takes on the character of an ovum (B2, ov\ the others, called
canal-cells (en. c) are converted into mucilage, which by its
expansion forces open the mouth of the flask and thus makes
a clear passage from the exterior to the ovum (BS).
Through the passage thus formed a sperm makes it way
and conjugates with the ovum, producing as usual an
oosperm or unicellular embryo.
The development of the embryo is at first remarkably
like what we have found to take place in Hydroids (p. 245).
The oosperm divides into two cells by a wall at right angles
to the long axis of the ovary : each of these cells divides
again repeatedly, and there is produced a solid multicellular
embryo or polyplast (c1 spgnm).
Very early, however, the moss-polyplast exhibits a striking
difference from the animal polyplast or morula : one of its
cells— that nearest the neck of the ovary — takes on the
character of an apical cell, and begins to form fresh seg-
ments like the apical cell of the stem. Thus the plant
embryo differs almost from the first from the animal embryo.
In the animal there is no apical cell : all the cells of the
polyplast divide and take their share in the formation of the
permanent tissues. In the plant one cell is at a very early
period differentiated into an apical cell, and from it all cells
thereafter produced are, directly or indirectly, derived.
The embryo continues to grow, forming a long rod-like
body (c2, spgnm) the base of which becomes sunk in the
tissue of the moss-stem, while its distal end projects vertically
upwards, covered by the distended venter (v) of the ovary.
Gradually it elongates more and more and its distal end
dilates : the embryo has now become a sporogonium^ con-
sisting of a slender stalk (c4, st) bearing a vase-like capsule
or urn (?/) at its distal end. In the meantime the elonga-
tion of the stalk has caused the rupture of the enveloping
A*
FIG. 82. — Reproduction and Development of Mosses.
A1, A spermary of Funaria in optical section, showing the wall en-
closing a central mass of sperm-cells : A2, the same from the surface
discharging its sperms. (X 300.)
PROTONEMA 335
A3, a sperm-cell with enclosed sperm : A4, a free-swimming sperm.
(X 800.)
B1, an ovary of Funaria, surface view, showing venter (v) and neck
(n) : B2, the same in optical section, showing ovum (ov) and canal cells
(en. c) : B3, the same after disappearance of the canal cells ; the neck is
freely open, and the ovum (ov} exposed. (X 200.)
C1, ovary with withered neck containing an embryo (spgnm) in the
polyplast stage ( X 200) : in c2 the ovary, consisting of swollen venter (v)
and shrivelled neck (n), encloses a young sporogonium (spgnm) ; the
distal end of the stem is shown with bases of leaves (/) ; in c3 the venter
has ruptured, forming a proximal portion or sheath and a distal portion
or calyptra which is carried up by the growth of the sporogonium. (X 10.)
C4, a small plant of Funaria with ripe sporogonium consisting of seta
(si], with urn (u) and lid (/) covered by the calyptra (c}.
C5, diagrammatic vertical section of urn (it,}, showing lid (/), airspaces
(a), and spores (sp}.
D1, a germinating spore of Funaria, showing ruptured outer coat (sp)
and young protonema (pr) with rhizoid (;-//). (X 550.)
D2, portion of protonema of the same, showing lateral bud (bd), from
which the leafy plant arises. (X 90.)
(A and D after Sachs ; B, c1, and C5 altered from Sachs.)
venter of the ovary (c3 ) : its proximal part remains as a sort
of sheath round the base of the stalk, while its distal portion,
with the shrivelled remains of the neck (n) is carried up by
the elongation of the sporogonium and forms an extinguisher-
like cap or calyptra (c4, c) over the urn.
As development goes on the distal end of the urn be-
comes separated in the form of a lid (c4, c5, /) and certain
of the cells in its interior, called spore-mother-cells, divide
each into four daughter cells, which acquire a double cell-
wall and constitute the spores (c5, sp) of the moss.
When the spores are ripe the calyptra falls off or is blown
away by the wind, the lid separates from the urn, and the
spores are scattered.
In germination the protoplasm of the spore covered by
the inner layer of the cell-wall protrudes through a split in
the outer layer (D1, sp.) and grows into a long filament, the
protonema (pr.), divided by oblique septa into a row of cells.
The protonema — which it will be observed is a simple linear
336 MOSSES
aggregate — branches, and may form a closely-matted mass
of filaments. Sooner or later small lateral buds (D2, bd)
appear at various places on the protonema : each of these
takes on the form of a three-sided pyramidal apical cell,
which then proceeds to divide in the characteristic way (p.
331) forming three rows of segments from which leaves
spring. In this way each lateral bud of the protonema gives
rise to a moss-plant.
Obviously we have here a somewhat complicated case of
alternation of generations (see p. 218). The gamobium or
sexual generation is represented by the moss-plant, which
originates by budding and produces the sexual organs, while
the agamobium consists of the sporogonium, developed from
the oosperm and reproducing by means of spores. The
protonema arising from a spore and producing the leafy
plant by budding, is merely a stage of the gamobiurn.
The nutrition of mosses is holophytic ; but there is a
striking differentiation of function correlated with terrestrial
habits. In Nitella the entire organism is submerged in
water and all the cells contain chlorophyll, so that decom-
position of carbon dioxide and absorption of an aqueous
solution of salts is performed by all parts alike, every
cell being nourished independently of the rest. In the
moss, on the other hand, the rootlets are removed from
the influence of light and contain no chlorophyll : hence
they cannot decompose carbon dioxide ; but, being sur-
rounded by moist soil, are in the most favourable position
for absorbing water and mineral salts. The stem, again, is
converted into an organ of support : the thickness of its
external cells prevents absorption and it contains no chlo-
rophyll. Hence the function of decomposing carbon
dioxide is confined to the leaves.
DISTRIBUTION OF FOOD MATERIALS 337
We have thus as an important fact in the nutrition of an
ordinary terrestrial plant that its carbon is taken in at one
place, its water, nitrogen, sulphur, potassium, &c., at another.
But as all parts of the plant require all these substances it is
evident that there must be some means by which the root
can obtain a supply of carbon, and the leaves a supply of
elements other than carbon. Tn other words, we find for
the first time in the ascending series of plants, just as we
did in ascending from the simple Hydra to the complex
Polygordius (p. 277) the need for some contrivance for the
distribution of food-materials.
The way in which this distributing process is performed
has been studied chiefly in the higher plants, but its essential
features are probably the same for mosses.
Water is continually evaporating from the surface of the
leaves, its place being as constantly supplied by water — with
salts in solution — taken in by the rhizoids. This trans-
piration, or the giving off of water from the leaves, is one
important factor in the process under consideration, since it
ensures a constant upward current of water, or, more ac-
curately, of an aqueous solution of mineral salts. The
withering of a plucked moss-plant is of course due to the
fact that where the roots are not imbedded in moist soil or
in water, transpiration is no longer balanced by absorption.1
In the higher plants it has been found that the root-hairs
have an absorbent action independent of transpiration, so
that water may be absorbed in the absence of leaves.
By the transpiration current, then, the leaves are kept
constantly supplied with a solution of mineral salts derived
from the soil, and are thus nourished like any of the aquatic
green plants considered in previous lessons : by the double
1 Mosses, however unlike most higher plants, can absorb water by
their leaves.
Z
338 MOSSES
decomposition of water and carbon dioxide a carbo-hydrate
is formed : this, by further combination with the nitrogen
of the absorbed ammonium salts or nitrates, forms simple
nitrogenous compounds, and from these, probably through
a long series of mesostates or intermediate products, proto-
plasm is finally manufactured.
In this way the food supply of the green cells of the
leaves is accounted for, but we have still to consider that of
the colourless cells of the stem and rhizoids, which, as we
have seen, are supplied by the transpiration current with
everything they require except carbon, and this, owing to
their possessing no chlorophyll, they are unable to take in
the form of carbon dioxide.
As a matter of fact the chlorophyll-containing cells of the
leaves have to provide not only their own food, but also
that of their not-green fellows. In addition to making good
the waste of their own protoplasm they produce large
quantities of plastic products (see p. 337) such as grape
sugar, and simple nitrogenous compounds like asparagin,
and these pass by diffusion from cell to cell until they reach
the uttermost parts of the plant, such as the centre of the
stem and the extremities of the rhizoids. The colourless
cells are in this way provided not only with the salts
contained in the ascending transpiration current, but with
carbo-hydrates and nitrogenous compounds. From these
they derive their nutriment, living therefore like yeast-cells
in Pasteur's solution, or like Bacteria in an organic
infusion.
We see then that the colourless cells of the stem and
rhizoids are dependent upon the green cells of the leaves
for their supplies. Like other cells devoid of chlorophyll
they are unable to make use of carbon dioxide as a source
of carbon but require ready-made carbo-hydrates, the
DISTRIBUTION OF FOOD MATERIALS 339
manufacture of which is continually going on, during
daylight, in the chlorophyll-containing cells of the leaves.
This striking division of labour is the most important
physiological difference between mosses and the more lowly
organized green plants described in previous lessons.
Z 2
LESSON XXIX
FERNS
WE saw in the previous lesson that in mosses there is a
certain though small amount of histological differentation,
some cells being modified to form sclerenchyma, others to
form axial bundles. We have now to consider a group of
plants which may be considered to be, in this respect, on
much the same morphological level as Polygordius, the
adult organism being composed not of a mere aggregate of
simple cells, but of various well-marked tissues.
A fern-plant has a strong stem which in some forms, such
as the common Bracken (Ptei'is aquilina) is a horizontal
underground structure, and is hence often incorrectly con-
sidered as a root : in others it creeps over the trunks of
trees or over rocks : in others again, such as the tree-ferns,
it is vertical, and may attain a height of three or four metres.
From the stem are given off structures of two kinds, the
leaves, which present an almost infinite variety of form in
the various species, and the numerous slender roots. In
some cases, such as the tree-ferns and the common Male
Shield-fern (Aspidium filix-mas\ the plant ends distally in a
terminal bud, consisting, as in Nitella and mosses, of the
growing end of the stem over-arched by leaves : in others
TISSUES OF THE STEM 341
such as Pteris, the stem ends in a blunt, knob-like extremity
quite uncovered by leaves. On the proximal portion of the
stem are usually found the withered remains of the leaves
of previous seasons, or the scars left by their fall. The
roots are given off from the whole surface of the stem,
often covering it with a closely-matted mass of dark brown
fibres.
When the stem is cut across transversely (Fig. 83, A) it
is seen, even with the naked eye, to consist of three well-
marked tissues. The main mass of it is formed of a wrhitish
substance, soft and rather sticky to the touch, and called
ground-parenchyma (par) : this is covered by an external
layer of very hard tissue, dark brown or black in colour, the
hypodermis (hyp) : bands of a similar hard brown substance
are variously distributed through the parenchyma, and con-
stitute the sdereHchyma (set] : and interspersed with these
are rounded or oval patches of a yellowish colour (V.B)
harder than the parenchyma, but not so hard as the
sclerenchyma, and called vascular bundles,
The general distribution of these tissues can be made out
O
by making longitudinal sections of the stem in various
planes or by cutting awray the hypodermis, and then scraping
the parenchyma from the vascular bundles and bands of
sclerenchyma. The hypodermis is found to form a more or
less complete hard sheath or shell to the stem, while the
internal sclerenchyma and vascular bundles form longi-
tudinal bands and rods imbedded in the parenchyma, and
serve as a sort of supporting framework or skeleton.
The minute structure of the stem can be made out by
the examination either of very thin longitudinal and trans-
verse sections, or of a bit of stem which has been reduced
to a pulp by boiling in nitric acid with the addition of a few
crystals of potassium chlorate : by this process the various
1/.B
par
st
FIG. 83.— Anatomy and Histology of Ferns.
GENERAL CHARACTERS 343
A, Transverse section of the stem of Pteris aquilina, showing hypo-
dermis (hyp], ground parenchyma (par), sclerenchyma (scl), and vascular
bundles (V. B). (X 2.)
B, transverse section of a vascular bundle, showing bundle-sheath
(b. s/i), sieve-tubes (sv. t). scalariform vessels (sc. v), and spiral vessels
(sp.v). (X 6.)
C, semi-diagrammatic vertical section of the growing point of the
stem, showing apical cell (ap. c), segmental cells (seg. c), and apical
meristem (ap. mer) passing into permanent tissue consisting of epidermis
(ep), hypodermis (hyp), ground parenchyma (par), sclerenchyma (scl),
and vascular bundles in which the sheath (b. sh), sieve-tubes (sv. t),
scalariform vessels (sc. v), and spiral vessels (sp. v) are indicated.
D, a single parenchyma cell, showing nucleus (mi), and vacuole
(vac).
E, cell of hypodermis.
F, portion of a sieve-tube, showing sieve-plates (sv. pi).
G, portion of a spiral vessel with the spiral fibre partly unrolled at the
lower end.
H, fibre-like cell of sclerenchyma.
I, portion of a scalariform vessel, part of the wall being supposed to
be removed.
K, vertical section of a leaf of Pteris, showing upper and lower epi-
dermis (ep), mesophyll cells (msph), with intercellular spaces (i. c. sp),
a stoma (st) in the lower epidermis, and hairs (h).
L, surface view of epidermis of leaf of Aspidium, showing two stomata
(st) with their guard-cells (gd. c).
M, vertical section of the end of a root, showing apical cell (ap. c),
segmental cells (seg. c), and root-cap (r. cp) with its youngest cap-cells
marked cp. c.
(A, B, and D-K after Howes ; M from Sachs, slightly altered.)
tissue elements are separated from one another, and can be
readily examined under a high power.
By combining these two methods of sectioning and
dissociation the parenchyma is found to consist of an
aggregate of polyhedral cells (D) considerably longer than
broad, their long axes being parallel with that of the stem
itself. The cells are to be considered as right cylinders
which have been converted into polyhedra by mutual pres-
sure. They have the usual structure, and their protoplasm is
frequently loaded with large starch-grains. They do not fit
quite closely together, but spaces are left between them,
especially at the angles, called inter-cellular spaces.
344 FERNS
The cells of the hypodermis (E) are proportionally longer
than those of the parenchyma, and are pointed at each end :
they contain no starch. Their walls are greatly thickened,
and are composed not of cellulose but of lignin, a carbo-
hydrate allied in composition to cellulose, but containing a
larger proportion of carbon. Schulze's solution, which, as
we have seen, stains cellulose blue, imparts a yellow colour
to lignin.
Outside the hypodermis is a single layer of cells (c, ep)
not distinguishable by the naked eye and forming the actual
external layer of the stem : the cells have slightly thickened,
yellowish-brown walls, and constitute the epidermis. From
many of them are given off delicate filamentous processes con-
sisting each of a single row of cells : these are called hairs.
In the sclerenchyma the cells (H) are greatly elongated
and pointed at both ends, so as to have the character rather
of fibres than of cells. Their walls are immensely thickened
and lignified, and present at intervals oblique markings due to
narrow but deep clefts : these are produced by the deposition
of lignin from the surface of the protoplasm (see p. 32) being
interrupted here and there instead of going on continually
as is the case of a cell-wall of uniform thickness.
The vascular bundles have in transverse section (B) the
appearance of a very complicated network with meshes of
varying diameter. In longitudinal sections (c) and in dis-
sociated specimens they are found to be partly composed of
cells, but to contain besides structures which cannot be
called cells at all.
In the centre of the bundle are a few narrow cylindrical
tubes (B and c, sp. v.) characterized at once by a spiral
marking, and hence called spiral vessels. Accurate exam-
ination shows that their walls (G) are for the most part thin,
but are thickened by a spiral fibre, just as a paper tube
SIEVE-TUBES 345
might be strengthened by gumming a spiral strip of paste-
board to its inner surface. These vessels are of considerable
length, and are open at both ends : moreover they contain
no protoplasm, but are filled with either air or water : they
have therefore none of the characteristics of cells. They
are showrn, by treatment with Schulze's solution, to be com-
posed of lignin.
Surrounding the group of spiral vessels, and forming the
large polygonal meshes so obvious in a transverse section,
are wide tubes (B and c, sc. v) pointed at both ends and
fitting against one another in longitudinal series by their
oblique extremities. They have transverse markings like
the rungs of a ladder, and are hence called scalariform
vessels. The markings (i) are due to wide transverse pits
in the otherwise thick lignified walls : in the oblique ends
by which the vessels fit against one another, the pits are
frequently replaced by actual slits, so that a longitudinal
series of such vessels forms a continuous tube containing,
like the spiral vessels, air or water, but no protoplasm. In
most ferns the terminal walls are not thus perforated, and
the elements are then called tracheides.
The presence of these vessels — spiral and scalariform-
is the most important histological character separating ferns
and mosses. The latter group and all plants below them are
composed exclusively of cells : ferns and all plants above
them contain vessels in addition, and are hence called vas-
cular plants.
The vessels, together with small parenchyma-cells inter-
spersed among them, make up the central portion of the
vascular bundle, called the wood or xylem. The peripheral
portion is formed of several layers of cells composing the bast
or phloem, and surrounding the whole is a single layer of
small cells, the bundle-sheath (p. s/i).
346 FERNS
The cells of the phloem are for the most part parenchy-
matous, but amongst them are some to which special
attention must be drawn. These (B and c, sv. t\ are many
times as long as they are broad, and have on their walls
irregular patches or sieve-plates (F, sv.pl.) composed of groups
of minute holes through which the protoplasm of the cell is
continuous with that of an adjacent cell. The transverse or
oblique partitions between the cells of a longitudinal series
are also perforated, so that a row of such cells forms a sieve-
tube in which the protoplasm is continuous from end to end.
We have here, therefore, as striking an instance of proto-
plasmic continuity as in the deric epithelium and certain other
tissues of Polygordius (see p. 272).
The distal or growing end of the stem terminates in a blunt
apical cone or punctum vegetationis (c), surrounded by the
leaves of the terminal bud in the case of vertical stems, or
sunk in a depression and protected by close-set hairs in the
underground stem of the bracken. A rough longitudinal
section shows that, at a short distance from the apical cone,
the various tissues of the stem — epidermis, parenchyma,
sclerenchyma, and vascular bundles — merge insensibly into
a whitish substance, resembling parenchyma to the naked
eye, and called apical meristem (ap. mer).
Thin sections show that the summit of the apical cone is
occupied by a wedge-shaped apical cell (ap. c) which in
vertical stems is three-sided like that of mosses (Fig. 81, H,
p. 329), while in the horizontal stem of Pteris it is two-sided.
As in mosses segmental cells (seg. c) are cut off from the three
(or two) sides of the apical cell in succession and by further
division form the apical meristem (ap. mer\ which consists
of small, close-set cells without intercellular spaces. As the
base of the apical cone is reached the meristem is found to
APICAL GROWTH 347
pass insensibly into the permanent tissues, the cells near the
surface gradually merging into epidermis and hypodermis,
those towards the central region into sclerenchyma and the
various constituents of the vascular bundles, and those of
the intermediate regions into parenchyma.
The examination of the growing end of the stem shows us
how the process of apical growth is carried on in a compli-
cated plant like the fern. The apical cell is continually
undergoing fission, forming a succession of segmental cells :
these divide and form the apical meristem, which is thus
being constantly added to at the growing end by the formation
and subsequent fission of new segmental cells : in this way the
apex of the stem is continually growing upwards or forwards.
But at the same time the meristem cells farthest from the
apex begin to differentiate : some elongate but slightly,
increasing greatly in size, and become parenchyma cells :
others by elongation in the direction of length of the stem
and by thickening and lignification of the cell-wall become
sclerenchyma cells : others again elongate greatly, become
arranged end to end in longitudinal rows, and, by the loss
of their protoplasm and of the transverse partitions between
the cells of each row, are converted into vessels — spiral or
scalariform according to the character of their walls. Thus
while the epidermis, parenchyma, and sclerenchyma are
formed of cells, the spiral and scalariform vessels are cell-
fusions, or more accurately cell-wall-fusions, being formed by
the union in a longitudinal series of a greater or less number
of cell-walls. It will be remembered that the muscle-plates
of Polygordius are proved by the study of development to be
cell-fusions (p. 300).
We thus see that every cell in the stem of the fern was once
a cell in the apical meristem, that every vessel has arisen by
the concrescence of a number of such cells, and that the
348 FERNS
meristem cells themselves are all derived, by the ordinary
process of binary fission, from the apical cell. In this way
the concurrent processes of cell-division, cell-differentiation,
and cell-fusion result in the production of the various and
complex tissues of the fully-formed stem.
The leaves vary greatly in form in the numerous genera
and species of ferns : they may consist of an unbranched
stalk bearing a single expanded green blade : or the stalk
may be more or less branched, its ramifications bearing the
numerous subdivisions of the blade <yc pinnce.
The anatomy of the leaf, like that of the stem, can be
readily made out 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
TISSUES OF THE ROOT 349
their concavities towards one another so as to bound a
narrow slit-like aperture (st). These apertures, which are
the only inter-cellullar 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, mspJi) 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 sclerenchyma 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. 83, 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 layers of
350 FERNS
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 of
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 in 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. 336). 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 son, differing greatly in
form and arrangement in the various genera, and formed of
innumerable, minute, seed-like bodies, the sporangia (Fig.
84, 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 indusium.
TISSUES OF THE LEAF 351
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 wrall 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. 335).
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 (r/i). The
resemblance of this stage to the young protonema of a moss
is sufficiently obvious (see Fig. 82, D1, p. 334).
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). It is at first
FIG. 84. — Reproduction and Development of Ferns.
A, Sporangium of Pleris, external view, showing stalk (st) and
annulus (an}.
B, the same, during dehiscence, the spores (sp) escaping.
C, a germinating spore, showing the ruptured outer coat (sp), and a
THE PROTHALLUS 353
rhizoid (rh} springing from the proximal cell of the rudimentary (two-
celled) prothallus.
D, a young prothallus, showing spore (sp), rhizoid (rh), 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 Aspidi-um, inverted so as to compare with Fig.
82, E2, showing venter (v), neck (n), 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 (//), stem (st), root (rt), and
cotyledon (ct).
K, vertical section of a prothallus (prth) of Nephrolepis, bearing
rhizoids (rh) and a single ovary with greatly dilated venter (v) and
withered neck (n). The venter contains an embryo in the phyllula
stage, consisting of foot (//), rudiments of stem (st), and root (rt), and
cotyledon (ct) beginning to grow upwards.
L, prothallus (prth) with rhizoids (rh), bearing a young fern plant,
consisting of foot (ft), 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
A A
354 FERNS
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 narrower 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, like
the somewhat similar structure in the animal sperm (Fig. 60,
x, p. 252).
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. 82, B, p. 334), 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 (w), the others are the canal-cells (en. c), 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. 333). The oosperm first divides by a
POLYPLAST AND PHYLLULA 355
plane parallel to the neck of the ovary, forming two cells, an
anterior nearest the growing or distal end of the prothallus,
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- 333)- In the fern the later stages are more complex.
One of the upper anterior cells remains undeveloped, the
other (Fig. 84, 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 \hzfoot (ft),
which becomes embedded in the prothallus, and serves the
growing embryo for the absorption of nutriment. One of the
lower posterior cells remains undeveloped, the other (rf]
takes on the form of the apical cell of a root, i.e. 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. 349) :
division of this cell goes on very rapidly, and a root is produced
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 (ct\ which soon begins
to grow upwards towards the light.
Thus at a comparatively early stage of its development
A A 2
356 FERNS
the fern-embryo has attained a degree of differentiation far
beyond anything which occurs in the moss-embryo. The
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 distinguished
by the possession of a leaf and root. In the one case the
characteristic organ for holozoic, in the other the character-
istic organs for holophytic nutrition, in the higher organisms,
make their appearance, and so mark the embryo at once as
as animal or plant. We may say then that while the oosperm
and polyplast stages of the embryo are common to the
higher plants and the higher animals, the correspondence
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. 84, K, with
Fig. 82, c2, and Fig. 84, L, with Fig. 82, c4).
GAMOBIUM AND AGAMOBIUM 357
The rudiment of the stem (L, sf) continues to grow by the
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-
tions 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
358 FERNS
organs of a fern do not severally correspond with those of a
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 i
THE GENERAL CHARACTERS OF THE HIGHER PLANTS
IN the 2yth Lesson (p. 303) 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 or xylem : the growing point both of
stem and 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.
360 GENERAL CHARACTERS OF THE HIGHER PLANTS
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 :
Algce.
Fungi.
MuscinecE.
Vascular Cryptogams.
Filicinae.
Equisetaceae.
Lycopodineae.
Phanerogams.
Gymnosperms.
Angiosperms.
The Alg<z are the lower green plants. They may be
unicellular, or may take the form either of linear, superfi-
cial, 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 Muscineae.
CHARACTERS OF THE PHYLA 361
while others place them in a group apart. Diatoms also are
sometimes placed in a distinct group. It must be remem-
bered also that most botanists include Hsematococcus and
Volvox among Algae (p. 178), and place the Mycetozoa
either among Fungi or in a separate group of chlorophyll-
less plants (p. 179).
The MuscinecR 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 Filicincz,
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 EquisetacecB include the common horsetails (genus
Equisetuni], 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
362 EQUISETUM
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 horse-tail 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. 85, A), formed of closely-fitting
hexagonal scales (sp. pli). Each scale (B, sp.pJi) 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
DIMORPHISM OF THE GAMOBIUM
363
a length of over a centimetre, and produce only ovaries
(ovy). Thus although there is no difference in the spores,
FIG. 85. — Reproduction and Development of Equisetum.
A, distal end of a fertile shoot, showing two leaf-sheaths (/. s/i), 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.pk) attached by short stalks to the axis of the cone, and
bearing sporangia (spg) on their inner surfaces.
C, a male prothallus bearing three spermaries (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 prothalli produced from them are of two distinct kinds,
the smaller being exclusively male, the larger female.
The oosperm develops in much the same way as in
364 SALVINIA
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.
As in the fern, the Equisetimi 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. 86, 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. sfig), 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
REDUCTION OF THE GAMOBIUM
365
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
. mi
-rm.sp
FlG. 86. — 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 (nig.
spg). (X 10.)
B, a germinating microspore (mi. spg), showing the vestigial prothallus
(prlh) and spermary (spy). (X 150.)
C, diagrammatic vertical section of a germinating megaspore, showing
the outer (mg. sp) and inner (mg. sp') coats of the spore, and its cavity
(c) containing plastic products, separated by a septum (</) 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 (prtJi) and phyllula just begin-
ning to develop into the leafy plant : sf, stem ; ct, cotyledon ; and /,
outermost leaf of the terminal bud. (X 20. )
(A and B, after Sachs ; D, after Pringsheim.)
four sperm-mother-cells, and from these spirally-twisted
sperms are produced in the usual manner. It is obvious
that the two cells in which the sperms are developed repre-
sent a greatly simplified spermary : the single proximal cell
366 SALVINIA
(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
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 (prth) in the form of a three-sided multicellular
mass projecting from the spore, which it slightly exceeds in
size. Several ovaries (ovy) 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, st) and the successive
formation of whorls of leaves (/), the adult form is assumed.
ENDOGENOUS PROTHALLUS 367
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.
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. 87, A) formed of
small leaves (sp. pJi) which overlap in something 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 (ing. 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 (prtli), 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 filling up
the rest of its cavity. The protoplasm divides and forms a
368
SELAGINELLA
small prothallus (prth), and a process of division also takes
place in the remaining contents (c) of the spore, producing a
large-celled tissue, the secondary prothallus.
By the rupture of the double cell-wall of the megaspore,
mi
-*--•• «. FIG. 87. — 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 (prth), 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 (prth) : its cavity (c) 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) and two cotyledons
(ct) : both embryos are provided with suspensors (dotted) (spst), and
have sunk into the secondary prothallus.
(Altered from Sachs. )
the prothallus is exposed to the air, but it never protrudes
through the opening thus made, and is therefore, like the
SECONDARY PROTHALLUS : SUSPENSOR 369
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
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
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 (.?/), 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. 88, A and c). Both
are to be considered as abbreviated shoots consisting of an
axis bearing numerous sporophylls (sp. pJi). Frequently, as
in the pines, several male cones are aggregated together,
forming an inflorescence^ or group of flowers.
B B
FlG. 88. — Reproduction and Development of Gymnosperms.
A, diagrammatic vertical section of male cone, showing axis with male
sporophylls (sp. ph. $ ) bearing microsporangia (mi. sfg) : per, scale-like
leaves forming a rudimentary perianth.
STRUCTURE OF MICROSPORE 371
B, a single microspore, showing bladder-like processes of outer coat,
and contents divided into small prothallial cell (a) and large cell (b),
from which the pollen-tube arises.
C, diagrammatic vertical section of female cone, showing axis with
female sporophylls (sp. ph. 9 ) bearing megasporangia (nig. spg), each of
which contains a single megaspore (mg. sp) : per, the scale-like perianth
leaves.
D, diagrammatic vertical section of a megasporangium, showing
cellular coat (t), and nucellus (ncl), micropyle (mpy), and megaspore
(mg. sp) : the latter contains the prothallus (prtli) 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 (emU) 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 (/. /), the end of which is applied to the necks of the two ovaries.
E, diagrammatic vertical section of a seed, showing coat (t), micro-
pyle (mpy), 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.)
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 microspcres (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
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. $ )
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 micellus(£>, ncl\
attached by its proximal end to the sporophyll, and sur-
B B 2
372 GYMNOSPERMS
rounded by a wall or integument (/) also formed of a small-
celled tissue. The integument is in close contact with the
nucellus, but is perforated distally by an aperture, the
micropyle (mpy\ through which a small area of the nucellus
is exposed.
Each megasporangium contains only a single megaspore
(embryo sac, c and D, mg. sp) in the form of a large ovoidal
body embedded in the tissue of the nucellus. It has at
first the characters of a single cell, but afterwards, by division
of its protoplasm, becomes filled with small cells representing
a prothallus (prt/i). As in Vascular Cryptogams, single
superficial cells of the prothallus are converted into ovaries
which are extremely simple in structure, each consisting of
a large ovum (ov\ and of a variable number of neck-cells.
The pollen, liberated by the rupture of the microsporangia,
is carried to considerable distances by the wind, some of it
falling on the female cones of the same or another tree. In
this way single microspores (pollen-grains) find their way
into the micropyle of a megasporangium (D, mi. sp). This
is the process known as pollination, and is the necessary
antecedent of fertilization.
The microspore now germinates : the outer coat bursts,
and the larger of the two cells (B, b) protrudes in the form
of a filament resembling a hypha of Mucor, and called a
pollen-tube (D, p.f). This forces its way into the tissue of
the nucellus, like a root making its way through the soil,
and finally reaches the megaspore in the immediate neigh-
bourhood of an ovary. A process then grows out from the
end of the tube, passes between the neck-cells, and comes in
contact with the ovum.
In the meantime the nucleus of the large cell (b) of the
microspore — that from which the pollen-tube grows — has
travelled to the end of the pollen-tube and divided into two
FORMATION OF THE SEED 373
Protoplasm collects round each of the daughter nuclei, con-
verting them into cells, one of which remains undivided,
while the other divides, and its substance, in some way not
understood, passes from the pollen-tube into the ovum, where
it forms a cell-like body, to which the name of male pronucleus
(see p. 259) has been applied. This conjugates with the
nucleus of the ovum, or female pronucleus, and thus effects
the process of fertilization, or the conversion of the ovum
into the oosperm.
The mode of formation of cells described in the preceding
paragraph should be specially noted. Instead of the ordin-
ary process of fission hitherto met with, the products of
division of a nucleus become surrounded by protoplasm,
cells being produced which lie freely in the interior of the
mother-cell. This is called free cell-formation.
The development of the oosperm is a very complicated
process, and results in the formation not of a single polyplast
but of four, each at the end of a long suspensor (D, spsr\
in the form of a linear aggregate of cells, which by its elonga-
tion carries the embryo (emb) down into the tissue of the
prothallus. As a rule only one of these embryos comes to
maturity : it develops a rudimentary stem and root and
four or more cotyledons, and so becomes a phyllula.
While these processes are going on the female cone
increases greatly in size and becomes woody. The mega-
sporangia also become much larger, their integuments (E, /),
becoming brown and hard, and the megaspore in each
enlarges so much as to displace the nucellus : at the same
time the cells of the prothallus filling the megaspore develop
large quantities of plastic products, such as fat and albumin-
ous substances to be used in the nutrition of the embryo :
the tissue thus formed is the endosperm (end}. The mega-
sporangium is now called a seed (see p. 361).
374 ANGIOSPERMS
Under favourable circumstances the seed germinates.
By absorption of moisture its contents swell and burst the
seed-coat, and the root of the phyllula (r) emerges, followed
before long by the stem (st) and cotyledons (<tf). The
phyllula thus becomes the seedling plant, and by further
growth and the successive formation of new parts is converted
into the adult.
In Gymnosperms we see an even more striking reduction
of the gamobium than in Selaginella. The female prothallus
is permanently inclosed in the megaspore, and the megaspore
in themegasporangium : 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. 89, A) is a greatly
FIG. 89. — 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^) and a distal
(per1} whorl of perianth leaves (sepals and petals), a whorl of male
376 ANGIOSPERMS
sporophylls or stamens (sp. ph. <$ ), and one of female sporophylls or
carpels (sp. ph. ? ).
The male sporophyll bears microsporangia (mi. spg) containing
microspores (mi. sp).
The female sporophyll consists of a solid style (st) terminated by a
stigma (stg), and of a hollow venter (v) containing a megasporangium
(mg. spg] in which is a single megaspore (mg. sp).
On the right side a microspore is shown on the stigma, and has sent
ofif a pollen-tube (/. /) through the tissue of the style to the micropyle
of the megasporangium.
B1, diagram of a female sporophyll from the distal aspect, and B2, 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 :
mr, the midrib.
c1, a microspore, showing the two cells (a and b) into which its
contents divide.
c'2, the same sending out a pollen-tube (/. t) : nu1, nu1, the two
nuclei.
D, diagrammatic vertical section of a megasporangium, showing the
double integument (tl, f2), nucellus (ncl), micropyle (mpy), 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-
gidae (sng) and an ovum (ov) at the distal end.
A pollen tube (/. /) is shown with its end in contact with the
synergidae.
E, semi-diagrammatic section of the megaspore (mg. sp) 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
(t), micropyle (inpy), 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).
(B1 after Behrens ; c1, C2, and E altered from Howes.)
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 (per1), 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.
STRUCTURE OF MICROSPORE 377
Above the sepals comes a whorl of leaves (per2), 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.
88, A and B, per).
The third whorl is called collectively the andrcecium and
consists of a variable number of stamens or male sporo-
phylls (sp.ph. $ ). Each stamen is a long narrow leaf bearing at
its distal end four microsporangia (pollen sacs, mi. spg) united
into a lobed knob-like body, the anther. . The microspores
(c1) are at first simple cells with double cell-walls, but
subsequently the protoplasm becomes divided into two cells,
as in Gymnosperms, a smaller (a) and a larger ($). 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 (B1,
B2) becomes folded longitudinally along its midrib (mr), 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
378 ANGIOSPERMS
the form of a slender rod-like body, the style (st), 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 gynsecium 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
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 (/a, t2) 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 imbedded 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. 373). Of these
three, two lie near the wall of the megaspore and are called
POLLINATION AND FERTILIZATION 379
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 (mi) 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
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 c2, /. /),
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, /. /), 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 (c2, nu,
nu'} have passed into the end of the pollen tube. They do
not become converted into cells as in Gymnosperms (p. 373)
but one usually remains undivided, while the other divides
by karyokinesis.
Soon after this the ovum is found to contain two nuclei
instead of one, the second being the male pronucleus, derived
apparently from the nucleus of the smaller cell (c1, a) of the
microspore. It has been shown that one of the synergidae
performs the function of transmitting the male nuclear
material from the pollen tube to the ovum, but the precise
steps of the process are still uncertain.
The two nuclei (male and female pronuclei) unite, and the
ovum, acquiring a cell-wall, becomes the oosperm or uni-
380 ANGIOSPERMS
cellular embryo. This 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, si), a root (r) and either one or two cotyledons
(ct) 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-
cisely the position of the vestigial prothallus of Gymnosperms
(Fig 88, p. 370, D, prth) and E, end : and p. 372), 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 the/ra/V. 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
REDUCTION OF PROTHALLUS 381
among the higher animals, not only in anatomical and
histological structure, but 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, becom-
ing 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.
1. Amoeba.
PAGE
Cell- body amoeboid or encysted : cell-wall nitro-
genous (?) : nutrition holozoic : reproduction by simple
or binary fission I
2. Hczmatococcus.
Cell-body ciliated or encysted : cell-wall of cellulose :
nutrition holophytic : reproduction by binary fission . . 23
3. Heleromita.
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 amoebulas : cell-wall nitro-
genous (?) : nutrition holozoic : reproduction by multiple
fission of encysted plasmodium 49
384 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 52
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 .... 81
II. — UNICELLULAR ORGANISMS IN WHICH THERE is CONSIDERABLE
COMPLEXITY OF STRUCTURE, ACCOMPANIED BY PHYSIOLOGICAL
DIFFERENTIATION.
a. Complexity attained by differentiation of cell-body.
9. Paramcecium.
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 104
10. Stylonychia.
Extreme differentiation or heteromorphism of cilia . . 114
11. Oxytricha.
Fragmentation of nucleus .... 117
1 2. Opalina.
Multiplication of nuclei : parasitism and its results :
necessity for special means of dispersal of an internal
parasite . . 119
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 . . . 124
14. Zootharnniiim.
A compound organism or colony with dimorphic (nutri-
tive and reproductive) zooids : begins life as a single
zooid 133
SYNOPSIS 385
PAGE
/'. Complexity attained by differentiation of cell-wall or by forma-
tion of skeletal structures in the protoplasm.
15. Foraminifera.
Calcareous shells (cell-walls) of various and complicated
form 146
1 6. Radiolaria.
Membranous perforated shell (cell-wall) and external
silicious skeleton often of great complexity : symbiotic
relations with Zooxanthella ... 150
1 7. Diatoms.
Silicious, two-valved, highly-ornamented shells . . 153
c. Complexity attained by simple elongation and branching of the
cell.
1 8. Miicor
A branched 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 156
1 9. I 'aucheria.
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 and oosperm ... . . 167
20. Caulerpa.
Illustrates maximum differentiation of a unicellular plant :
stem-like, leaf-like, and root-like parts . ... 173
III.— ORGANISMS IN WHICH COMPLEXITY is ATTAINED BY CELL-
MULTIPLICATION, ACCOMPANIED BY NO OR BUT LITTLE CELL-
DlFFERENTIATION.
a. Linear aggregates.
21. Penicillinm.
A multicellular, filamentous, branched fungus : mycelial,
submerged, and aerial hyphre : apical growth : abundant
production of spores by constriction of aerial hyphre . 182
22. Agaricus.
Complexity attained by interweaving of hyphoe in a de-
finite form : illustrates maximum complexity of a linear
aggregate 189
C C
386 SYNOPSIS
PAGE
23. Spirogyra.
A multicellular filamentous unbranched alga : interstitial
growth : gonads equal and similar, b.ut gametes show
first indication of sexual differentiation 192
b. Superficial aggregates.
24. Monostroma.
Cell-division takes place in two dimensions .... 199
c. Solid aggregates.
25. Ulva.
Like Monostroma, but cell-division takes place in three
dimensions 201
26. Laminaria.
Illustrates maximum size and complexity of compara-
tively slightly differentiated cells ... .... 201
IV. — SOLID AGGREGATES IN WHICH COMPLEXITY is INCREASED
BY A LIMITED AMOUNT OF CELL-DIFFERENTIATION.
27. Nitella.
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) 204
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 . . 219
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) 234
SYNOPSIS 387
PAGE
30. Diphyes.
A free-swimming colony with polymorphic (nutritive,
reproductive, protective, and natatory) zooids .... 259
31. For pita.
Extreme polymorphism of zooids giving the colony the
character of a single physiological individual .... 250
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 tripoblastic, coslomate animal with metameric seg-
mentation : prostomium, peristomium, metameres, and
anal segment : besides ecto- and endoderm there is a
well developed mesoderrn divided into somatic and
splanchnic layers separated by the ccelome : 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 : — circulatory, re-
spiratory, 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 stomodaeum and proctodreum), late
trochosphere (tripoblastic but accelomate) 267
33. A fosses.
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 328
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) .... .... 340
C C 2
388 SYNOPSIS
VI. — BRIEF DESCRIPTIONS OF TYPES OF THE HIGHER GROUPS OF
ANIMALS AND PLANTS IN TERMS OF POLYGORDIUS AND OF
THE FERN RESPECTIVELY.
PAGE
a. Animals.
All are tripoblastic and coelomate.
35. Starfish.
Radially symmetrical : discontinuous dermal exo-skele-
ton : characteristic organs of locomotion (tube feet) in
connection with ambulacral system of vessels .... 305
36. Crayfish.
Metamerically segmented : segmented lateral append-
ages : differentiation of metameres and appendages :
continuous cuticular exo-skeleton discontinuously calci-
fied : gills as paired lateral offshoots of the body-wall :
heart as muscular dilatation of dorsal vessel : coelome
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 . . . . 310
37. Mussel.
Non-segmented : mantle formed as paired lateral out-
growths of dorsal region : foot as unpaired median out-
growth of ventral region : cuticular exo-skeleton in the
form of a calcified bivalved shell : gills as paired lateral
outgrowth of body-wall : heart as muscular dilatation of
dorsal vessel : coelome reduced to pericardium : nervous
system consists of three pairs of ganglia sunk in the
mesoderm . 316
38. Dogfish.
Metamerically segmented : differentiated into head,
trunk, and tail : trunk alone coelomate in adult : ap-
pendages as median (dorsal, ventral, and caudal) and
paired (pectoral and pelvic) fins : discontinuous dermal
exo-skeleton and extensive endo-skeleton of partially
calcified cartilage, including a chain of vertebral centra
below the nervous system replacing an embryonic noto-
chord : gills as pouches of pharynx opening on exterior :
heart as muscular dilatation of ventral vessel : hollow
dorsal nervous system not perforated by enteric canal 320
/>. Plants. All exhibit alternation of generations and the series
shows the gradual subordination of the gamobium to the
agamobium.
SYNOPSIS 389
PAGE
39. Equisetum.
Sporangia borne on sporophylls arranged in cones :
spores homomorphic : prothalli dimorphic (male and
female) ... 362
40. Salvinia.
Spores dimorphic : microspore produces vestigial male
prothallus : megaspore produces greatly reduced female
prothallus 364
41. Selaginella.
Microspore produces unicellular prothallus and multi-
cellular spermary, both endogenously : female prothallus
formed in megaspore and is almost endogenous : embryo
provided with suspensor . 367
42. Gymnospenns.
Cones dimorphic (male and female), with rudimentary
perianth : no sperms formed but microspore gives rise to
pollen tube, nuclei in \vhich are the active agents in fer-
tilization : single megaspore permanently inclosed in
each megasporangium : female prothallus purely endo-
genous : embryo (phyllula) remains inclosed in mega-
sporangium which becomes a seed . 369
43. Angiosperms.
Cone modified into flower by differentiation of sporo-
phylls and perianth : female sporophyll forms closed
cavity in which megasporangia are contained : mega-
spore produces a single ovary represented simply by an
ovum and two synergidae : formation of prothallus re-
tarded until after fertilization . . 374
B.— SUBJECTS OF GENERAL IMPORTANCE DISCUSSED
IN SPECIAL LESSONS.
I. — CELLS AND NUCLEI
a. The higher plants and animals contain cells similar in struc-
ture to entire unicellular organism, and like them exist-
ing in either the amoeboid, ciliated, encysted, or plas
modial condition 56
b. Minute structure of cells : — cell-protoplasm, cell-membrane,
nuclear membrane, achromatin, chromatin 6 1
c. Direct and indirect nuclear division .... ... 63
d. The higher plants and animals begin life as a single cell, the
ovum 67
390 SYNOPSIS
II. — BIOGENESIS.
PAGE
a. Definition of biogenesis and abiogenesis : brief history of
the controversy .... 93
b. Crucial experiment with putrescible infusions : sterilization :
germ-filters : occurrence of abiogenesis disproved under
known existing conditions ... 96
III. HOMOGENESIS.
Definition of homogenesis and heterogenesis : truth of the
former firmly established .... 100
IV. — ORIGIN OF SPECIES.
a. Meaning of the term Species : the question illustrated by a
consideration of certain species of Zoothamnium . . . 135
b. Definition of Creation and Evolution : hypothetical histories
of Zoothamnium in accordance with the two theories . 139
c. The principles of Classification : natural and artificial
classifications ... . .138
d. The connection between ontogeny and phylogeny . .143
V. — PLANTS AND ANIMALS.
a. Attempt to define the words plant and animal, and to place
the previously considered types in one or other king-
dom ... . . 174
b. Significance of the " third kingdom," Protista 180
VI. — SPERMATOGENESIS AND OOGENESIS.
Origin of sperms and ova from primitive sex-cells : differences
in structure and development of the sexual elements . 252
VII. — MATURATION AND IMPREGNATION.
a. Formation of first and second polar ceils and of female
pronucleus ... 257
b. Entrance of sperm and formation of male pronucleus . . 258
c. Conjugation of pronuclei . . 259
SYNOPSIS 391
VIII. — UNICELLULAR AND DIPLOBLASTIC ANIMALS.
PAGE
a. In plants there is a clear transition from unicellular forms to
solid aggregates, but in animals the connection of the
gastrula with unicellular forms is uncertain 260
b. Two hypotheses of the origin of the gastrula : (i) from a
colony of unicellular zooids ; (2) from a solitary multi-
nucleate but unicellular form . 265
C. — Other matters of general importance, such as the composition
and properties of protoplasm, cellulose, chlorophyll, starch, &c. :
metabolism : holozoic, holophytic, and saprophytic nutrition : intra- and
extra-cellular digestion : amoeboid, ciliary, and muscular movements :
the elementary physiology of muscle and nerve : parasitism and sym-
biosis : asexual and sexual generation : and the elements of embryology
— are discussed under the various types and will be most conveniently
referred to by consulting the Index.
INDEX AND GLOSSARY
INDEX AND GLOSSARY
AbiOgen'esiS (a, not : |3io?, life : -yeVeo-i?,
origin), the origin of organisms from
not-living matter : former belief in, 94
Absorption by root-hairs, 337
Accre'tion (ad, to : cresco, to grow), in-
crease by addition of successive layers,
*4
Acnrom'atin (a, not : xp^>Ma> colour), the
constituent of the nucleus which is un-
affected or but slightly affected by dyes,
7, 62
Acoelom'ate (a, not : Kot'Aw/xa, a hollow),
having no ccelome (<?.?'.) : 297
Adduct'or muscles, Mussel, 319
Aerobic (a»?p, air : /3u>9, life), applied to
those microbes to which free oxygen is
unnecessary, 91
Agamob'ium (a, not : -ya^o?, marriage :
/Si'o?, life), the asexual generation in or-
ganisms exhibiting alternation of gene-
rations (g.i>.)
AGAR'ICUS (mushroom) :— Figure, 190 :
general characters, 189 : microscopic
structure, 189, 190 : spore-formation,
191
Algae (alga, sea-weed), 167
Alternation of Generations, meaning of
the phrase explained under Nitella, 218 :
Bougainvillea, 247 : Moss, 336 : Fern,
357 : Equisetum, 363, 364 : Salvinia, 366 :
Selaginella, 369 : Gymnosperms, 374 :
Angiosperms, 380, 381
Ambula'cral (ambulacrujii), a walking
place) system, starfish, 309
AMCEB'A (<x/xoi/36s, changing) : — Figure,
2 : occurrence and general characters, i :
movements, 4, 10 : species of, 8 : resting
condition, 10, n : nutrition, 11 : growth,
13: respiration, 17 : metabolism, 17 : re-
production, 19 : immortality, 20 : conju-
gation, 20 : death, 20, 21 : conditions of
life, 21 : animal or plant ? 178
Amceb'Oid. movements, 4
Amoeb'ula (diminutive of Amoeba), the
amoeboid germ of one of the lower or-
ganisms, 51-54
Anab'olism
that which is
thrown up). See Metabolism, construc-
tive.
Anaerob'ie (a, not : di?p, air : jSio?, life),
applied to those microbes to which free
oxygen is unnecessary, 91
An'al (anus, the vent) segment, Poly-
gordius, 269
An'al spot, Paramcecium, in
An'astates (<xi/acrra.T05, from ai/aorrji'flu.,
to rise up, 18. See Mesostates, anabolic.
Anatomy (di/are'/xvoj, to cut up), the study
of the structure of organisms as made
out by dissection
Andrce'cium (dvyp, a male : OIKOS, a
dwelling), the collective name for the
male sporophylls in the flower of Angio-
sperms, 377
AN'GIOSPERMS (ayyeiov, a vessel:
cTTre'pjiia, seed): — Figure, 375'. general
characters, 374-377 : structure of flower,
374 : reduction of gamobium, 377, 378 :
pollination and fertilization, 379 : forma-
tion of fruit and seed, and development
of the leafy plant. 380, 381
Animal, definition of, 174
Animals, classification of, 303
Animals and Plants, comparison of type
forms, 174, 175 : discussion of doubtful
forms, 178
Animals, Protists, and Plants, boun-
daries between artificial, 179-181
An'ther, 377
Antherid'ium. See Spermary.
Antherozo'id. See Sperm.
Antip'odal cells, 378, 379
An 'TIS (anus, the vent), the posterior aper-
ture of the enteric canal, 269
Apical cell:— Penicillium, 188 : Nitella,
209 : Moss, 331 : stem of Fern, 346 : root
of Fern, 349 : prothallus of Fern. 353
Ap'ical cone, Fern, 346
396
INDEX AND GLOSSARY
A'pical growth, 188, 347
A'pical mer'istem, a mass of meristem
(g.v.) at the apex of a stem or root, 346
Appen'dages, lateral : — crayfish, 310 : dog-
fish, 321
Archegonlum (apx*?, beginning : yoi/o?,
production), the name usually given to
the ovary of the higher plants
Aristotle, abiogenesis taught by, 94
Arteries, in the crayfish, 314
Arthropoda, the, 304
Arthrospore (apOpov, a joint : crTropa, a
seed), in Bacteria, 88
Artificial reproduction of Hydra, 230
Asexual generation. See Agamobium.
Asexual reproduction. See Fission,
Budding, Spore.
Assimila'tion (assimilo, to make like), the
conversion of food materials into living
protoplasm, 13
At'rophy (a, without : rpo^rj, nourish-
ment), a wasting away, 115
Aur'icle. See Heart.
Autom'atism (avro'/aaros, acting of one's
own will), 10, 243
Axial bundle, Moss, 331
Axial fibre, Vorticella, 127
Axil (axilla, the arm-pit), 206
Axis, primary and secondary, 207
B
BACIL'LUS (bacillum, a little staff), 84 :
Figure, 86
BACTER'IA (/3aKTr?pioi/, a little staff, or
MICROBES Campos, small : jSi'os, life) :—
occurrence, 81 : structure of chief genera,
83, 85, 86: reproduction, 87, 88: nutrition,
89 : ferment-action, 90 : parasitism, 91 :
conditions of life, 91, 92 : presence in at-
mosphere, -99, 100 '.animals or plants? 180
BACTERIUM termo (Figures), 82, 83
Barnacle-geese, supposed heterogenetic
production of, 101
Bast. See Phloem.
Binomial nomenclature, 8, 137
BiOgen'esiS (/3i'os, life : yeVeo-is, origin),
the origin of organisms from pre-existing
organisms, 93 : early experiments on, 94,
95 : crucial experiment on, 95-98
Biol'Ogy OSt'os, life : Ao'yos, a discussion),
the science which tieats of living things
Blast'OCOele (/SAaaro's, a bud : /coiAoi/, a
hollow), the larval body-cavity, 294
Blood, Polygordius, 279
Blood-corpuscles : colourless, see Leuco-
cytes : red, 56 : Figures, 57
Blood-vessels, Polygordius, 278 : develop-
ment of, 298
Body-cavity. See Blastocoele and Coelome.
Body-segments. See Metameres.
BOUGAINVILLEA (after L. A. de Bou-
gainville, the French navigator : —
Figures, 235, 238 : occurrence and gene-
ral characters, 234 : microscopic struc-
ture, 236 : structure of medusa, 237 :
structure and functions of nervous sys-
tem, 242, 243 : organs of sight, 243 : re-
production and development, 244, 245 :
alternation of generations, 247
Bract (bractea, a thin plate), 249
Brain : — Polygordius, 282 : trochosphere,
295 : Crayfish, 315 : Dogfish, 326
Branch, Nitella, 207
Branchial (Ppdyx<.a., branchice, gills)
apertures, Dogfish, 320, 325
Browne, Sir Thomas, on abiogenetic origin
of mice, 94
Buc'cal (bncca, the cheek) groove, Para-
moecium, 107
Bud, budding, Saccharomyces, 72 : com-
parison of with fission, 72 : Hydra, 230
Bundle-sheath, 345
Calyp'tra (KaAvVrpa, a veil), 335
Cal'yx (/cdAv£, the cup of a flower), the
outer or proximal whorl of the perianth
in the flower of Angiosperms, 376
Canals, radial and circular, medusa, 239
Canal-cells of ovary, 333, 354
Cap-cells of root, 350
Carbon dioxide, decomposition of by
chlorophyll bodies, 29
Car'pel (Kapiros, fruit, a female sporophyll,
Car'tilage, 324
Cauler'pa (/cavAo's, a stem : e'pTrto, to
creep), 172 (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, 186
Cell-COlony : — temporary, Saccharomyces,
72 : permanent, Zoothamnium, 132, 133
Cell-fusion, 301, 347
Cell-layer, 273
Cell-membrane or wall, n, 27, 28, 62, 63
Cell-multiplication and differentiation,
216: Polygordius, 301, Fern, 347
Cell-plate, 65
Cell-protoplasm, 60
Cell'ulose, composition and properties of,
28
Central capsule, Radiolaria, 150
Ceph'alo-thor'ax, Crayfish, 310
Cerebral ganglion. See Brain.
Cerebro-pleural ganglion, Mussel, 319
Cerebro-spinal cavity, Dogfish, 321
CHARA (x«pa), delight), development and
alternation of generations, 217, 218
Chlor'ophyll (xAwpo's, green : 0vAAoi/, a
leaf), the green colouring matter of
INDEX AND GLOSSARY
397
plants, properties of, 26 : occurrence in
Bacteria, 86 : in Hydra, 228
Chrpm'atin (xpo>ju.<x, a colour), the con-
stituent of the nucleus which is deeply
stained by dyes, 7, 63 : male and female
in nucleus of oosperm, 259
Chrom'atophore (xpi^a, colour : <f>epaj,
to bear), a mass of proteid material im-
pregnated with chlorophyll or some other
colouring matter, 26, 46, 195, 213, 228
Cil'ium (ciliuin, an eye-lash), defined,
25» : comparisons of with pseudopod, 34,
52 : absence of cilia in Arthropoda, 315
Ciliary movement, 25 : a form of con-
tractility, 34
Ciliate Infusoria, 105
Classification, natural and artificial, 139 :
natural, a genealogical tree, 142
Cnid'Ofolast (wiSy, a nettle : j8A.aoro'?, a
bud), the cell in which a nematocyst
(g.v.) is developed, 227
Cnid'OCil (wiSr) and ciliiiui), the "trigger-
hair " of a cnidoblast, 227
Ccelenterata, the, 304
Ccelome (/coiAu>ju.a, a hollow), the body-
cavity : — Polygordius, 269 : Starfish, 307 :
Crayfish, 314 : Mussel, 318 : Dogfish,
321 : development of, Polygordius, 298
Ccelom'ate, provided with a coelome, 272
Cffilomic epithelium. See Epithelium.
Ccelomic fluid, Polygordius, 277
Colloids (/co'AAa, glue : elSo?, form), pro-
perties of, 6
Colony, Colonial organism, meaning of
term, 133, 241 : formation of temporary
colonies, Hydra, 230
Columel'la (a little column), 160
Com'missure (commissura, a band), 278
Compound organism. See Colony.
Concres'cence (cum, together : cresco, to
grow), the union of parts during growth
Cone, an axis bearing sporophylls : — Equi-
setum, 362 : Selaginella, 367 : Gymno-
sperms, 369
Conjuga'tion (conjugfitio, a coupling), the
union of two cells in sexual reproduc-
tion : — Amoeba, 20 : Heteromita, 41, 42 :
Paramoscium, 112, 113: Vorticella, 130:
Mucor, 163 : Spirogyra, 196 : of ovum
and sperm, 259 : monoecious and dioe-
cious, 197 : comparison with plasmodium-
formation. 54
Connective, ossophageal, 282
Connective tissue, 308
Contractile vac'uole (ziaciiiis, empty) :—
Amoeba, 8, 9 : Euglena, 47 : Para-
moecium, 109
Contractility (contracts, a drawing to-
gether), nature of, 10, 34 : muscular, 128
Contraction, physical and biological, 10
Corolla (corolla, a little wreath), the
inner or distal whorl of the perianth in
the flower of Angiosperms, 377
Corpuscles. See Blood-corpuscles, and
Leucocytes.
Cortex, cor'tical layer (cortex, bark),
constitution of, 59 : Infusoria, 108
Cotton- wool as a germ-fitter, 97
Cotyle'don (/cOTuATjSioV, a cup or socket),
the first leaf or leaves of the phyllula
(q.t>.) in vascular plants, 355
Cranium (xpd.vi.ov, the skull), 324
CRAYFISH : — Figure, 312: general charac-
ters, 310, 311 : limited number and con-
crescence of metameres, 310 : appen-
dages, 310: exoskeleton, 311: enteric
canal, 311: gills, 314: blood-system,
314 kidney, 314 : nervous system, 315
Creation (creo, to produce), definition of,
1 39 illustrated in connection with species
of Zoothamnium (Diagram), 140
Cross-fertilization : applied to the sexual
process when the gametes spring from
different individuals, 197
Cryst'alloids (Kpuo-raAAo?, crystal : elSos.
form), properties of, 6
Cuticle (cuticula, the outer skin), nature
of in unicellular animals, 45, 107 : in
multicellular animals, 236
Cyst ( KU'CTTIS, a bag), used for cell-wall in
many cases, n, 54
D
Dallinger, Dr. W. H., observations on an
apparent case of heterogenesis, 101
Daughter-cells, cells formed by the fission
or gemmation of a mother-cell, 35, 65
Death, phenomena attending, 20, 21, 164,
165
Decomposition, nature of, 6
DermiS (6e'p/xa, skin), the deep or connec-
tive tissue layer of the skin, 308
Descent, doctrine of. See Evolution.
Development, meaning of the term, 43.
For development of the various types
see under their names.
Dextrin, m
Diastase, 80
Diast'Ole (SiourreAAoj, to separate), the
phase of dilatation of a heart, contractile
vacuole, £c. , 109
DIATOMA'CE-S (5taTe>i/w, to cut across,
because of the division of the shell into
two valves), 153: Figure^, 154
Diat'omin, the characteristic yellow colour-
ing matter of diatoms, 152
DichOt'OmOUS (Sixoro/aeo), to cut in two),
applied to branching in which the stem
divides into two axes of equal value, 136
Differentiation (diffiro, to carry different
ways), explained and illustrated, 34, 117
Diges'tion (digero, to arrange or digest),
the process by which food is rendered fit
for absorption, 12, 13 : intra- and extra-
398
INDEX AND GLOSSARY
cellular, 229 : contrasted with assimila-
tion, 229
Digestive gland, 314
Dimorphism, dimorphic (<3is, twice :
popart, form), existing under two forms,
35, 134, 241, 364, 366
Dice'ClOUS (<5i's, twice : ot*o?, a dwelling),
applied to organisms in which the male
and female organs occur in different in-
dividuals, 197
DIPH'YES (Si^v-n?, double) : Figure, 248 :
occurrence and general characters, 249 :
polymorphism, 249
Diploblastic (6177X005, double : £Aao-Tos, a
bud), two-layered : applied to animals in
which the body consists of ectoderm and
endoderm, 241 : derivation of diploblas-
tic from unicellular animals (Figures).
264, 265
Disc, Vorticella, 126
Dispersal, means 6f : in internal parasite,
122 : in fixed organisms, 131-134
Distal, the end furthest from the point of
attachment or organic base, 124
Distribution of food-materials :— in a
complex animal, 277 : in a complex
plant, 337
Divergence of character, 143
Division of physiological labour, 34
DOGFISH : — Figure, 322 : general charac-
ters, 320 : fins, 321 : exoskeleton, 321 :
endoskeleton, 324 : enteric canal, 324 :
gills, 325 : blood-system, 325 : kidney,
326 : gonads, 326 : nervous system, 326
Dry-rigor, stiffening of protoplasm due to
abstraction of water, 21
E
Echinodermata, the, 304
Ect'Oderm (e/cro'?, outside : Septet, skin),
the outer cell-layer of diploblastic and
triploblastic animals, 223, 227. 274
Ect'OSarC (e/cros, outside : crap£, flesh), the
outer layer of protoplasm in the lower
unicellular organisms, distinguished by
freedom from granules, 4
Egestlon (egero, to expel), the expulsion
of waste matters, 12
Egg-cell. See Ovum.
Em'bryo (e'/u./Spvoi', an embryo or foetus),
the young of an organism before the
commencement of free existence.
Em'bryo-sac. See Megaspore.
Encysta'tion, being enclosed in a cyst
(?.,*-)
End'oderm (evSov, within : Sep^a, skin),
the inner cell-layer of diploblastic and
triploblastic animals, 223, 228. 274
End'oderm-lamella, Medusa, 239
EndOg'enOUS (evSov, within : yiyi/o/xai, to
come into being), arising from within,
e.g. the roots, of vascular plants, 350
End'OSarc (evBov, within: o-ap£, flesh),
the inner, granular protoplasm of the
lower unicellular organisms, 4
Endoskel'eton(eV5ov, within, and skeleton,
from o-Ke'AAto, to dry), the internal skele-
ton of animals, 324
End'osperm (evSov, within : 0-irepiJ.a, seed),
nutrient tissue formed in the megaspore
of Phanerogams, 373, 380
Endospore (iivSov, within : CTTropd, a seed),
a spore formed within a vegetative cell,
OQ
OO
Energy, conversion of potential into
kinetic, 15 : source of, in chlorophyll-
containing organisms, 31
Enteric (eWepoi/, intestine) canal, the
entire food-tube from mouth to anus : —
Polygordius, 269, 275 ; Starfish, 307 ;
Crayfish, 311 ; Mussel, 318 ; Dogfish, 324
Ent'eron or Enteric cavity, the simple
digestive chamber of diploblastic ani-
mals, 221
Epidermis (eTu, upon : Sep^a, the skin) :
in animals synonymous with deric epi-
thelium (g.v., under Epithelium) : in vas-
cular plants a single external layer of
cells, 344, 348, 349
Epithelial cells : columnar, 58 : ciliated,
59
Epithelium (en-t, upon : tfrjA.^ the nipple),
a cellular membrane bounding a free
surface, 243 : ccelomic, 273, 300 : deric,
272 : enteric, 273, 275
Equatorial plate, 65
EQUISETUM (egnits, a horse: seta, a
bristle) : — Figure, 363 : general charac-
ters, 362 : cone and sporophylls, 362 :
male and female prothalli, 363, 364 : al-
ternation of generations, 363, 364
Equiv'ocal generation. See Abiogenesis.
EUGLEN'A (euyArjvo?, bright-eyed) :—
Figure, 45 : occurrence and general
characters, 44 : movements, 44 : struc-
ture, 45 : nutrition, 46 : resting stage,
47 : reproduction, 48 : animal or plant ?
178
Euglen'oid movements, 45
Ev'olution (evolvo, to roll out), organic :
definition, 139 : illustrated in connection
with species of Zoothamnium (Diagram),
142
Excre'tion (cxcerno, to separate), the
separation of waste matters derived from
the destructive metabolism of the or-
ganism, 1 6, 280
Exogenous (ef, out of: -yiy^o^at, to come
into being), arising from the exterior,
e.g. leaves, 350
Exoskel'eton (e£w, outside, and skeleton,
from o-KeAAw, to dry), the external or
skin-skeleton : cuticular, 237, 273-275 :
cuticular and calcined, 311, 316 : epi-
dermal (hair and nails) : dermal, 308, 321
INDEX AND GLOSSARY
399
Eye-spots or Ocelli :— Medusa, 239
Polygordius, 286, 295
Faeces (faex, dregs), solid excrement,
consisting of the undigested portions of
the food, 1 6
Ferm'ent (fermentum, yeast, from fer-
veo, to boil or ferment), a substance
which induces fermentation, i.e. a
definite chemical change, in certain sub-
stances with which it is brought into
contact, without itself undergoing
change : unorganized and organized
ferments, 79 : alcoholic, 75-80 : ace-
tous, 90 : diastatic or amylolytic, 80 :
lactic, 90 : peptonizing or proteolytic,
80 : putrefactive, 90 : ferment-cells of
Mucor, 165
FERNS : — Figures, 342, 352 : general
characters, 340, 341 : histology of stem,
leaf, and root, 340-350 : nutrition, 350 :
spore-formation, 351 : prothallus and
gonads, 351-354 : development, 355 : al-
ternation of generations, 357
Fertilization (fertilis, bearing fruit) :
the process of conjugation of a sperm or
sperm-nucleus with an ovum whereby
the latter is rendered capable of develop-
ment : a special case of conjugation
(q.v.), 197 : details of process, 259 : in
Vaucheria, 171 : in Gymnosperms, 372,
373 : in Angiosperms, 379
Filtering air, method of, 97
Fins, Dogfish, 321
FiSS'ion (fissio, a cleaving), Simple or
binary), the division of a mother-cell
into two daughter-cells : in Amoeba, 19 :
Heteromita, 40 : animal cells generally,
63-65 : plant-cells generally, 65-67 : Para-
moecium, 112 : Vorticella, 129
Fission, multiple, the division of a
mother-cell into numerous daughter-
cells : — in Heteromita, 42 : Protomyxa,
51 : Saccharomyces, 73
Fission, process intermediate between
simple and multiple, Opalina, 122
Flagella. See Cilium.
Flag'ellate Infusoria, 105
Flagell'ula (diminutive of flagelluni), the
flagellate germ of one of the lower
organisms (often called zoospores, 51, 54
Flagell'um (flagellum, a whip) : defined,
25 : transition to pseudopod, 52, 228
Floral receptacle, the abbreviated axis of
an angiospermous flower, 376
Flower, a specially modified cone (g.v.),
having a shortened axis which bears
perianth-leaves as well as sporophylls,
374 : often applied to the cone of Gymno-
sperms, 369
Food-current, Mussel, 316, 318
Food-vacuole, a temporary space in the
protoplasm of a cell containing water
and food-particles, n, no
Foot : of Mussel, 316 : of phyllula of fern
FORAMINIF ERA (foramen,* hole \fero,
to bear), 146 : Figures, 147, 148, 149
Fragmentation of the nucleus, n8
Free cell formation, 373
Fruit of Angiosperms, 380
Function (fttnctio, a performing), mean-
ing of the term, 9
Gam'ete (ya/ae'co, to marry), a conjugating
cell, whether of indeterminate or deter-
minate sex : — Heteromita, 41 : Mucor,
163 : Spirogyra, 196 : Vaucheria, 171
Gamob'ium (ya/aos, marriage : /St'os, life),
the sexual generation in organisms ex-
hibiting alternation of generations (q.v.) :
progressive subordination of, to agamo-
bium in vascular plants, 357, 381
Ganglion (yayyAiov, a tumour), a swelling
on a nerve-cord in which nerve-cells are
accumulated, 315, 319
Gastric juice (yaa-nfip, the stomach), pro-
perties of, 12
Gast'mla (diminutive of yacrrxjp, the
stomach), the diploblastic stage of the
animal embryo in which there is a diges-
tive cavity with an external opening :
characters and Figure of, 261 : two
hypotheses of origin of (Figures), 264,
265 : contrasted with phyllula, 356
Gemmation (gemma, a bud). See Bud-
ding.
Generation, asexual, See Agamobium.
Sexual. See Gamobium.
Generations, Alternation of. See Al-
ternation of generations.
Generalized, meaning of term, 138
Gen'US (genus, a race), generic name,
generic characters, 8, 137
Germ-filter, 97
Germinal spot, the nucleolus 01 the
ovum, 255
Germination (germinatio, a budding),
the sprouting of a spore, zygote, or
oosperm to form the adult plant : for
germination of the various types see
under their names.
Gill, an aquatic respiratory organ, 314, 319,
32o> 325
Gland (glans, an acorn), an organ of se-
cretion (q.v .) : gland-cells, 227, 229, 281
Glochid'ium, 320
Gon'ad (yo'i/o?, offspring, seed), the essen-
tial organ of sexual reproduction,
whether of indeterminate or determinate
sex, i.e. an organ producing either un-
4OO
INDEX AND GLOSSARY
differentiated gametes, ova, or sperms :
see under the various types, and espe-
cially 170, 196, 207, 212
Gon'aduct (gonad, and dftco, to lead), a
tube carrying the ova or sperms from the
gonad to the exterior, 291, 310, 315, 319,
326
Grappling-lines, Diphyes, 249
Growing point : Nitella, 209 : Moss, 331 :
Fern, 346
Growth, 13
Guard-cells of stomates, 349
Gullet, the simple food-tube of Infusoria,
47, 108 : or part of the enteric canal of
the higher animals, 276
GYM'NOSPERMS (yufxi/o'?,naked: a-nep^a,
seed) : Figure, 370 : general characters,
369 : structure of cones and sporophylls,
369, 371 : reduction of gamobium (pro-
thalli and gonads), 372 : pollination and
fertilization, 372, 373 : formation of seed
and development of leafy plant, 373,
374
Gynoec'ium (yvvr/, a female : <KKO?, a
dwelling), the collective name for the
female sporophylls in the flower of
Angiosperms, 377
H
Haem'atochrome (at/jia, blood :
colour), a red colouring matter allied to
chlorophyll, 26
HJEMATOCOC'CUS (aljua, blood : KOKKOS,
a berry) : — Figure, 24 : general charac-
ters, 23 : rate of progression, 23 : ciliary
movements, 25, 33 : colouring matters,
26 : motile and stationary phases, 28 :
nutrition, 28 : source of energy, 30 : re-
production, 35 : dimorphism, 35 : animal
or plant ? 178
Haemoglobin (al/u.a, blood : globns, a
round body, from the circular red cor-
puscles of human blood), 58 : properties
and functions of, 279
Head-kidney, trochosphere, 295
Heart : — Crayfish, 314 : Mussel, 319 :
Dogfish, 325
Heat, evolution of, by oxidation of proto-
plasm, 17
Heat-rigor (rigor, stiffness), heat-stiffen-
ing, 21
Heredity (heredttas, heirship), 145
Hermaph'rodite (epju.a$pti6ZTo?, from
Hermes and Aphrodite). SeeMoncecious.
Heterogen'esiS (eVepos, different : yeVecri?,
origin), meaning of term, 100 : supposed
cases of, 101 : not to be confounded with
metamorphosis or with evolution, 102
HETEROM'ITA (erepo?, different : JUI'TO?,
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? 179
High and low organisms, 104
Higher (triploblastic) animals, uniformity
in general structure of, 303
Higher (vascular) plants, uniformity in
general structure of, 359
HistOl'Ogy (iarioi/, a thing woven : Ao'-yos,
a discussion), minute or microscopic
anatomy.
Holophyt'ic (0X05, whole : </>uroV, a plant)
nutrition, defined, 31
Holozo'ic (oAos, whole : £o>oi/, an animal),
nutrition, defined, 31
Homogen'esis (6/x6s, the same : yeVeo-is,
origin), meaning of the term, 100
Homol'ogOUS (6/xoAo-yos, agreeing), applied
to parts which have had a common
origin, 240
Homomorph'ism, homomorph'ic (6/ixos,
the same : /u.op<£ij, form), existing under
a single form, 137
Host, term applied to the organism upon
which a parasite preys, 121
HYDRA (v5pa, a water-serpent) : Figures,
220, 224, 232 : occurrence and general
characters, 219 : species, 221 : move-
ments, 221, 222 : mode of feeding, 222 :
microscopic structure, 223 : digestion,
229 : asexual, artificial, and sexual re-
production, 230, 231 : development, 232,
Oil
Hydr'anth (iiSpa, a water-serpent : avOos,
a flower), the nutritive zooid of a hydroid
polype, 236, 240
Hydroid (v6pa, a water-serpent : elSos,
form) Polypes (TroAuVov?, many-footed),
compound organisms, the zooids of
which have a general resemblance to
Hydra, 234
Hyper'trophy (vTre'p, over: rpo^-tj, nourish-
ment), an increase in size beyond the
usual limits, 116
Hyph'a (v^atVw, to weave), applied to the
separate filaments of a fungus : they
may be mycelial (see mycelium), sub-
merged, or aerial : Mucor, 157, 166
Penicillium, 183, 186
Hyp'odermiS (UTTO, under : Se'p/ia, skin),
Fern, 341, 344 ^
Hyp'ostome (vno, under : 0-To/xa, mouth),
221, 236
I
Immortality, virtual, of lower organisms,
21
Income and expenditure of protoplasm, 18
Individual. See Zooid.
Individuation, meaning of the term, 230
251
Indus'ium (indusz'um, an under-garment),
35°
INDEX AND GLOSSARY
401
Inflores'cence (floresco, to begin to
flower), an aggregation of cones or
flowers, 369
Infusoria (so called because of their fre-
quent occurrence in infusions), 105
Ingesta (ingero, to put into) and Egesta
(egero, to expel), balance of, 32
Ingestion (ingero, to put into), the taking
in of solid food, n, 58
Insola'tion (insolo, to place in the sun),
exposure to direct sunlight, 92
Integ'ument (integiimen.tum, a covering)
of megaspore : Gymnosperms, 372 :
Angiosperms, 378
Inter-cellular spaces, 343
Inter-muscular plexus (n-Ae'icw, to twine),
284
Internode (inter, between : nodus, a
knot), the portion of stem intervening
between two nodes, 206
Interstitial (attersttfium, a space be-
tween) cells. Hydra, 225 : growth,
Spirogyra, 196
Intestine (intcsttnns, internal), part of
the enteric canal of the higher animals,
276
IntUS-SUSCep'tion (in/us, into : suscipio,
to take up), addition of new matter to
the interior, 13
Iodine, test for starch, 27
Irritability (irritabllis, irritable), the
property of responding to an external
stimulus, 10
Jaws: — Crayfish, 311 : Dogfish, 325
K
Karyokinesls (napvov, a kernel _or_ nu-
cleus : /ciVrjcri?, a movement), indirect
nuclear division, 65
Katab'olism (/cara^o^, a laying down),
18. See Metabolism, destructive.
Kat'astateS ((caTa.on-TJi'ai, to sink down),
1 8. See Mesostates, katabolic.
Kidney : — Crayfish, 314 : Dogfish, 326
LAMINARTA (lamina, a plate), 201
(Figure), 202
Labial palps, Mussel, 318
Larva, the free-living young of an animal
in which development is accompanied by
a metamorphosis, 295
Larval stages, significance of, Polygor-
dius, 297
Leaf, structure of:— Nitella. 206, 207,
212 : Moss, 331 : Fern, 340, 348 : limited
growth of, 207
Leaflet, Nitella, 207
Lept'othrix(A.e7TT09, slender : 0pif, a hair),
filamentous condition of Bacillus, 88 :
Figure, 86
LESSONIA (after Lesson, the French
naturalist), 202 (Figure)
Leuc'OCyte (Aev/<o's, white : KVTOS, a hollow
vessel, cell), a colourless blood corpuscle :
— structure of. in various animals
(Figures), 57 : ingestion of solid par-
ticles by, 58 : fission of, 58 : formation
of plasmodia by, 58
Leuwenhoek, Anthony van, discoverer
of Bacteria, 95
Life, origin of. See Biogenesis.
Life-history, meaning of the term, 43
Lignin (lignum, wood), composition and
properties of, 345
Linear aggregate, an aggregate of cells
arranged in a single longitudinal series, 1 86
Linnaeus, C., introducer of binomial no-
menclature, 8, 137
Liver, Dogfish, 325
M
MACROCYST'IS (/aa*po'?, long : KU'OTIS, a
bladder), 202
Mad'reporite (from its similarity to a
jiiadrepore or stone-coral), 307
MAGOSPHJER'A (nayos, magical :
o-<£aipa, a ball), 262 (Figure)
Mantle, Mussel, 316
Manub'rium (manubrium, a handle) of
Medusa, 239
Matura'tion of ovum, 255, 257
Maximum temperature of amoeboid
movements, 21
Medulla or medullary substance (me-
dulla, marrow) : in Infusoria, 108
Medus'a (Me'§ou<ra, name of one of the
Gorgons), the free-swimming reproduc-
tive zooid of a hydroid polype, 237-240 :
derivation of a, from hydranth (Figure),
238
Medus'oid, a reproductive zooid having
the form of an imperfect Medusa,
Diphyes, 249
Meg'agam'ete (^eyas, large : -yayaew, to
marry), a female gamete (q.v.) distin-
guished by its greater size from the male
or microgamete, 130
Meg'asporan'gium (fj.>iyag, large : an-opa,
seed : ayyelov, a vessel), the female
sporangium in plants with sexually di-
morphic sporangia, usually distinguished
by its greater size frorn the male or
micro-sporangium : — Salvinia, 364 : Sela-
ginella, 367 : Gymnosperms, 372 : Angio-
sperms, 378
D D
402
INDEX AND GLOSSARY
Meg'aspore (/xeyas, large : erTropa, a seed),
the female spore in plants with sexually
dimorphic spores, always distinguished
by its large size from the male or micro-
spore : — Salvinia, 364 : Selaginella, 367 :
Gymnosperms, 372 : Angiosperms, 378
Megazo'oid (jae'yas, large : £uov, animal :
et6os, form), the larger zooid in unicel-
lular organisms with dimorphic zooids,
35, 130
Mer'istem (juepurr^a, formed from
jaept^io, to divide), indifferent tissue of
plants from which permanent tissues are
differentiated, 346
Mes'entery (jae'cros, middle : eVrepoi/, in-
testine), a membrane connecting the en-
teric canal with the body-wall, 275 : de-
velopment of, 300
Mes'oderm (/ueVo?, middle : 5e'p/u.a, skin),
the middle cell-layer of triploblastic
animals, n, 241 : Polygordius, 274 : de-
velopment of, 295 : splitting of to form
somatic and splanchnic layers, 298
Mesoglce'a (/ueVo?, middle : y\oia, glue),
a transparent layer between the ecto- and
endo-derm of Ccelenterates : — in Hydra,
223 : in Bougainvillea, 241
Mes'ophyll (/aeVo?, middle : (f>v\\ov, a
leaf), the parenchyma of leaves, 348
Mes'OStates (/o.eo-0?, middle : oTTJi'ai, to
stand), intermediate products formed
during metabolism (g.v.) and divisible
into (a) anabolic mesostates or ana-
states, products formed during the con-
version of food-materials into proto-
plasm : and (li) katabolic mesostates
or katastates, products formed during
the breaking down of protoplasm, 18
Metab'Olism (/aera/SoAry, a change), the
entire series of processes connected with
the manufacture of protoplasm, and
divisible into («) constructive meta-
bolism or anabolism, the processes by
which the substances taken as food are
converted into protoplasm, and (&) de-
£ structive metabolism or katabolism,
the processes by which the protoplasm
i breaks down into simpler products, ex-
cretory or plastic, 17
Met'amere OueYa, after : ju.e'pos, a part), a
body-segment in a transversely seg-
mented animal such as Polygordius, 267,
269 : development of, 297 : limited num-
ber and concrescence of in Crayfish,
310
MetamorphOS'iS (/xeTa/xop^wcris), a trans-
formation applied to the striking change
of form undergone by certain organisms
in the course of development after the
commencement of free existence : — Vor-
ticella, 131 : Polygordius, 302
Mic'robe Campos, small : /St'os, life). See
Bacteria.
MICROCOC'CUS (jal/cpo?, small : KOKKOS,
a berry) (Figure), 85
Microgam'ete (^I/epos, small : ya/xew. to
marry), a male gamete (q.v.\ distin-
guished by its smaller size from the
female or megagamete, 130
Micro-millimetre, the one-thousandth of
a millimetre, or i-25,oooth of an inch, 83
Micro-organism. See Bacteria.
Micropyle (/xt/cpo?, small : irv\ri, an en-
trance), 372
Micro-sporan'gium (jul/cpo?, small : a-iropd.
a seed : ayyelov., a vessel), the male
sporangium in plants with sexually di-
morphic sporangia, usually distinguished
by its smaller size from the female or
mega-sporangium : — Salvinia, 364 : Se-
laginella, 367 : Gymnosperms, 371 : An-
giosperms, 377
Mic'rospore (julKpos, small : crnopd, a
seed), the male spore in plants with
sexually dimorphic spores, always dis-
tinguished by its small size from the
female or mega-spore : — Salvinia, 364 :
Selaginella, 367 : Gymnosperms, 371 '.
Angiosperms, 377
Microzo Oid (ju-Z/cpo?, small : giaov, an ani-
mal : eiSos, form), the smaller zooid in
unicellular organisms with dimorphic
zooids, 35, 130
Midrib of leaf, Moss, 331
Minimum temperature for amoeboid move-
ments, 21
Mollusca, the, 305
Moncec'iOUS (/j.oVo?, single : oucos, a
house), applied to organisms in which
the male and female organs occur in the
same individual, 197, 231
Monopod'ial Guoi'os, single : TTOV'?, a foot),
applied to branching in which the main
axis continues to grow in a straight line
and sends off secondary axes to the
sides, 1^6
MONOSTROMA OAO^OS, single :
anything spread out), 200 (Figure)
MorphOl'ogy (/xop^, form : Aoyo?, a dis-
cussion), the department of biology
which treats of form and structure, 9
Mor'ula (diminutive of indnnii, a mul-
berry). See Polyplast.
MOSSES: — Figures, 329, 334: general
characters, 328 : structure of stem, 330 :
leaf, 331 : rhizoids, 331 : terminal bud.
331 : reproduction, 332 : development
of sporogonium, 333 : of leafy plant,
335, 336 : alternation of generations,
336 : nutrition, 336, 337
Mouth : — Euglena, 47 : Paramoecium, 108 :
Hydra, 222 : Medusa, 239 : Polygordius.
267 : backward shifting of in Crayfish,
311
MUCOR (uincor, mould) :— Figure, 158:
occurrence and general characters, 156 :
INDEX AND GLOSSARY
403
mycelium and aerial hyphae, 157, 159 '.
sporangia and spores, 157-159 : transition
from uni- to multi-cellular condition,
160 : development of spores, 161 : con-
jugation, 163 : death, 165 : nutrition,
165 : parasitism, 165 : ferment-cells, 166
Mucous membrane, 58
Multicellular, formed of many cells, 61,
1 60
Muscle (miescielns,a. little mouse,a muscle),
nature of, 128, 129
Muscle-fibre, Bougainvillea, 241
Muscle-plate, Polygordius, development
of, 301
Muscle-process. Hydra, 225, 228
Mushroom. See Agaricus.
MUSSEL (same root as muscle), Fresh-
water : — Figure, 317 : general characters,
316 : mantle, shell, and foot, 316 : food-
current, 316, 318 : enteric canal, 318 :
gills and blood-system, 319 : nephridia,
gonads, and nervous system, 319
Mycelial Iiypha3, the hyphee interwoven
to form a mycelium.
Mycelium (JUVKT/?, a fungus), a more or
less felt-like mass formed of interwoven
hyphae : — Mucor, 157 : Penicillium, 185
MYCET'OZOA OXV'/CTJ?, a fungus : &ov,
an animal) : — Figure, 53 : occurrence
and general characters, 52 : nutrition,
54 : reproduction and life-history, 54,
55 : animals or plants? 179
My'ophan (M^S, mouse, muscle : 4>aii/ci>. to
appear), 108
Myxomyce'tes (MV£<X, mucus :
fungus). See Mycetozoa.
NITELL'A (nit io, to shine) :— Figures,
205, 210, 213, 215, 217 : occurrence and
general characters, 204 : microscopic
structure, 207 : terminal bud, 208 : struc-
ture and development of gonads, 207,
212 : development, 214 : alternation of
generations, 218
Node (nodus, a knot), the portion of a
stem which gives rise to leaves, 206
Not'ochord (I/WTO?, the back : \op§ri, a
string), 324 _
Nucel'lus (diminutive of nucleus, the
name formerly applied), 371, 379
Nuclear division, indirect : in animal
cell, 64 (Figure) : in plant cell, 65, 66
(Figure) : direct, 65
Nuclear membrane, 62, 63
Nuclear protoplasm. See Achromatin.
Nuclear spindle, 64, 65
Nucle'olus (diminutive of nucleus), 8
Nu'cleus (nrtclens, a kernel), minute struc-
ture of, 63 : Amoeba, 7, 8 : Paramcecium,
109, 113 : Opalina, 121 : Vorticella,
126 : Nitella, 208, 214 : fragmentation
of, 118
Nucleus, secondary, of megaspore, An-
giosperms, 379
Nutrient solution, artificial, principles of
construction of, 78
Nutrition :— Amoeba (holozoic), IT : Hx-
matococcus (holophytic), 28 : Hetero-
mita (saprophytic), 37 : Opalina (type of
internal parasite), 122 : Mucor 165 :
Penicillium, 188 : Polygordius (type of
higher animals), 269, 277 ; Moss (type
of higher plants), 336
N
O
Nem'atOCyst (fTJ/ua, a thread : KVO-TIS, a
bag), 226
Nephrid'iopore (ve^pos, a kidney : Tropo?,
a passage), the external opening of a
nephridium, 281
Nepnrid'ium (ve^po?, a kidney), structure
of, Polygordius, 281 (Figure) : develop-
ment of, 300 ; Mussel, 319 : Dogfish,
326
Neph'rOStome (ve<}>p6s, a kidney : crroiJ-a,
a mouth), the internal or ccelomic aper-
ture of a nephridium, 281
Nerve, afferent and efferent, functions of,
284
Nerve-cell, 227, 242
Nervous system, central and peripheral :
— Medusa, 242 : Polygordius, 282 : func-
tions of, 285 : Starfish, 309 : Crayfish,
315 : Mussel, 319 : Dogfish, 326
Neur'OCCele (vevpov, a nerve : KOI'ATJ, a
hollow), the central cavity of the verte-
brate nervous system, 326
Ocellus (ocellus, a little eye), structure
and functions of, Medusa, 239, 243
CEsoph'agUS (ot<ro(f)dvo?, the gullet). See
Gullet.
OntOg'eny (OVTO?, being : yere'p-is, origin),
the development of the individual :
a recapitulation of phylogeny (<7.7-.),
145
Oogen'esiS (<i>6v, an egg : yefe'cris, origin),
the development of an ovum from a
primitive sex-cell, 252, 254
Oogon ium (u>ov, egg : yovo?, produc-
tion), the name usually given to the
ovary of many of the lower plants.
Oosperm (tioV, egg : o-n-ep/aa, seed), a
zygote (q.v.) formed by the ovum and
sperm : a unicellular embryo, 171 :
origin of nucleus of, 257
Oosphere (uov, an egg : o-<£oupa, a sphere),
a name frequently given to the ovum of
plants.
Oospore (u>6v, an egg : a-rropd, a seed), a
404
INDEX AND GLOSSARY
name frequently applied to the oosperm
of plants.
OPALIN'A (from its opalescent appear-
ance) : — Figure, 120: occurrence and
general characters, 119-121 : structure
and division of nuclei, 119: parasitic
nutrition, 121 : reproduction, 122 : means
of dispersal, 122 : development, 123
Optimum (pptimus, best) temperature for
amoeboid movements, 21 : for sapro-
phytic monads, 40
Organ (opyavov, an instrument), a portion
of the body set apart for the perform-
ance of a particular function, 287
Or'ganism, any living thing, whether
animal or plant, 5
Ossicle (diminutive of os, a bone), 308
Ov'ary (dvmii), an egg), the female gonad
or ovum-producing organ ; see under the
various types and especially Vaucheria,
170 : atrophy of, in Angiosperms, 378.
The name is also incorrectly applied to
the venter of the pistil of Angiosperms,
377
Ovi'duct (ovuni, an egg : duco, to lead),
a tube conveying the ova from the ovary
to the exterior, 291
Ov'um (ovum, an egg), the female or
megagamete in its highest stage of dif-
ferentiation : general structure of, 68,
69 : minute structure and maturation of,
254> 255 : see also under the various
types and especially Vaucheria, 170 :
formation of, in Angiosperms, 379
Ov'ule (diminutive of ovum\ the name
usually applied to the megasporangium
of Phanerogams.
Oxidation of protoplasm, 15
OXYTRICH A (ofus, sharp : 0pi£, a hair),
1 17 (Figure)
Pancreas (Trayxpeas, sweetbread), 325
Param'ylum (Trapa, beside : a^vAoi/, fine
meal, starch), 46
PARAM(E'CIUM :— Figures, 106, 112:
structure, 104: mode of feeding, no:
asexual reproduction, 112: conjugation,
112
Para'nucleus (beside the nucleus), 109
Par'asite, parasitism (Trapao-Zros, one
who lives at another's table) : — Opalina,
121 : Bacteria, 91 : Mucor, 165
Paren'chyma (Trape'yxuMa.anything poured
in beside, a word originally used to de-
scribe the substance of the lungs, liver,
and other soft internal organs), applied
to the cells of plants the length of which
does not greatly exceed their bread tli
and which have soft non-liguified walls,
60 : ground-parenchyma, 343. 344
Pari'etal (paries, a wall), applied to the
layer of ccelomic epithelium lining the
body-wall, 273, 274
Parthenogen'esis (TrapfleVo?, a virgin :
yeVecri?, origin), development from an
unfertilized ovum or other female
gamete, 198
Parthenogenet'ic ova, characteristics of,
258
Pasteur, Louis, researches on yeast, 77-
79
Pasteur's solution, composition of, 77-79
Pedal (pes, the foot) ganglion, Mussel,
3i9
PENICILL'IUM (pcnicillnm, a painter's
brush from the form of the fully-deve-
loped aerial hypha;) : — Figure, 184: oc-
currence and general characters, 182:
mode of growth, 183: microscopic
structure, 183 : formation and germina-
tion of spores, 187, 188 : sexual repro-
duction, 188 : nutrition, 188 : vitality of
spores, 189
Pepsin (TreTTTw, to digest), the proteolytic
or peptonizing ferment of the gastric
juice, 12, 79
Peptones, 12
Perianth (n-ept, around : av9os, a flower),
the proximal infertile leaves of a flower,
37.7
Perisperm (rrepi, around : o-rrep/^a, seed),
nutrient tissue developed in the nucleus
of the seed, 376 (description of figure)
Peristom'e (irept, around ; oro/ma, the
mouth), Vorticella, 126
Peristomlum (jrepi, around : oro/ouoi/, a
little mouth), the mouth-bearing segment
of worms, 267, 293
Peritone'um (Tre'pn-ovcuoi/), the membrane
covering the viscera, 321
Pet'alS (fl-e'TaAof, a leaf), the inner or dis-
tal perianth leaves in the flower of
Angiosperms, 377
Phar'ynx (</>apu-y£, the throat) :— Poly-
gordius, 276 : Dogfish, 324
Phloem ($A.<HOS, bark or bast), the outer
portion of a vascular bundle, 345
Phyla ((f>OA.ov, a tribe) of the animal king-
dom, 303 : of the vegetable kingdom,
360
Phyll'ula (diminutive of <frv\\ov, a leaf),
the stage in the embryo of vascular
plants at which the first leaf and root
have appeared, 356 : contrasted with
gastrula, 356
PhylOg'eny (<f>vXoi^, a race : ytVto-ts,
origin), the development of the race,
144
Physiol'Ogy ($u<ns, the nature or property
of a thing : Aoyos, a discussion), the de-
partment of biology which treats of
function, 9 ct seq.
Pigment-spot, Euglena, 47 .
INDEX AND GLOSSARY
405
PileUS (ptlcus, a cap), Agaricus, 189
Pinna (pinna, a feather), of leaf, 348
Pistil (pistilliiw, a pestle, from pinso, to
pound). See Gynoecium.
Plan'ula (diminutive of irAaros, a wander-
ing about), the mouthless diploblastic
larva of a hydroid, 247
Plant, definition of, 174
Plants, classification of, 174
Plas'ma (Tr\da-y.a, anything moulded), of
blood, 56
Plasmod'ium (7rA.acr/u.a, any thing moulded),
52-55 : comparison of with zygote, 54
Plastic (TrAao-Ttxos, formed by moulding)
products, products of katabolism which
remain an integral part of the organism,
Pod'omere (TTOU?, a foot : juepos, a part), a
limb-segment, 310
Polar cells, formation of, 258
Pollen grain (pollen, fine flour), a name
given to the microspore of Phanero-
gams.
Pollen-sac, a name given to the microspo-
rangium of Phanerogams.
Pollen-tube, 373, 379
Pollina'tion, 373, 379
POLYGORD'IUS (TTOA.V?, many : TopSio?,
King of Phrygia, inventor of the Gordian
knot) : — Figures, 268, 270, 281, 283, 290,
292, 294, 296, 299 : occurrence and gene-
ral characters, 267, 269 : metameric seg-
mentation, 267-269 : mode of feeding,
269 : enteric canal, 269, 273 : cell-layers,
272-274 : ccelome, 269, 273 : distribution
of food, 277 : blood-system, 278 : nephri-
dia, 280 : nervous system, 282 : differen-
tiation of definite organs, 287 : and tissues,
288 : reproduction, 289 : development and
metamorphosis, 295-302
Polymorphism (TTOA.V?, many : fj-optfrri,
form), existing under many forms,
249
Pol'yplast (TTOA.VS, many: irAaaTog, formed,
modelled), the multicellular stage of the
embryo before the differentiation of cell-
layers or organs : — Hydroids, 245 : Moss,
333 : Fern, 355
PORPITA (TTOPTTTJ, a brooch), 250 : (Figure),
251
Primor'dial utricle, 194, 208
Proctodae'um (n-pwKTos, the anus : 6Sai6s,
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, 294
Pro-embryo, chara, 217 (Figure)
Pro-nucleus, female, 258 : male, 259 :
conjugation of male and female, 259
Pl'OStom'ium (wpo, before : OTOJAKH-, a little
mouth), the first or pre-oral segment in
worms, &c., 267, 292
PROTAMCEBA (jrpwTos, first : a/
changing), 9 (Figure).
Prothal'lus (Trpo, before : 0aXA6s, a twig),
the gamobium of vascular plants : — Fern,
351 : dimorphism of in Equisetum, 363 :
reduction of in Salvinia, 366 ; Selagi-
nella, 367, 368, and Gymnosperms, 372,
373 : retarded development of in Angio-
sperms, 377
Prothallus, secondary, Selaginella, 368
Prot'eids( 7rpo>T09, first), composition of, 5
ProtiSt'a (TTPCOTICTTO?, the first of all), the
lowest organisms intermediate between
the lowest undoubted animals and plants,
1 80
ProtOCOC'CUS (TTpwros, first : KOK/CO?, berry).
See Hffimatococcus.
PROTOMYXA (TrpdjTos, first :/xi!fa, mucus):
Figure, 50 : occurrence and general cha-
acters, 49 : life-history, 51 : animal or
plant? 179
Protonem'a(7rpu)ro?, first : frj/xa, a thread),
Moss, 332, 335 _
Prot'oplasm (Trpwros, first : n\dcriJ.a, any-
thing moulded), composition of, 5 : pro-
perties of, 5-7 : micro-chemical tests for,
7, 8 : minute structure of, 62, 63 : con-
tinuity of in Fern, 346 : in Polygordius,
288_ : intra- and extra-capsular, Radio-
laria, 151
Protozoa, the, 304
Prox'imal (proximus, nearest), the end
nearest the point of attachment or or-
ganic base, e.g. in the stalk of Vorticella,
124
Pseud 'opod ((/>ev6^?, false : 7rou5, foot),
described, 4 : comparison of with cilium,
34, 52 : in columnar epithelium, 59 : in
endoderm cells of Hydra, 228
PteriS. See Ferns.
Punctum vegetationis. See Growing
point.
Putrefac'tion (putrefacio, to make rotten),
nature of, 81 : a process of fermentation,
90 : conditions of temperature, moisture,
&c., 91, 92
Putres'cent (putresco, to grow rotten)
solution, characters of, 37, 81
Putres'cible infusion, sterilization of, 96-
100
Pyren'oid (nvpriv, the stone of stone-fruit :
elfio?, form), a small mass of proteid
material invested by starch, 27
Radial symmetry, starfish, 305
RADIOLAR IA (radiits, a spoke or ray):—
Figures, 150, 151 : occurrence and general
characters, 150 : central capsule, 150 :
intra- and extra-capsular protoplasm.
406
INDEX AND GLOSSARY
151 : silicious skeleton, 151 '. symbiotic
relations with Zooxanthella, 152
Rect'um (intestinum rectum, the straight
gut), the posterior or anal division of the
enteric canal, 277
Redi, Francisco (Italian savant), experi-
ments on biogenesis, 95
Reflex action, 285
Reproduction, necessity for, 19
Reproductive organ. See Gonad.
Reservoir of contractile vacuole, Euglena,
47
Respiration: — Amoeba, 17: Polygordius,
280
Respiratory caeca, Starfish, 308
Rhiz'oid (pufc, root : elfios, form):— Nitella,
204, 212 : Moss, 331 : prothallusof Fern,
35i
Root, Fern, 340, 349
Root-cap, 350
Root-hairs, 349, 353
ROSS, Alexander, on abiogenetic origin 01
mice, insects, &c., 94
Rotation of protoplasm, 208
Rudiment, rudimentary (rudimentum, a
beginning), the early stage of a part or
organ : often used for a structure which
has undergone partial atrophy, but in
such cases the word vestige (ff-v.) is
more suitable.
Segment'al cell :— Nitella, 209 : Moss,
331 : Fern, 347
Segmentation, metameric. See Meta-
mere.
SACCHAROMY CES (a-dKXa.pov, sugar :
/XV'KTJS, fungus): — Figure, 71 : occurrence,
70 : structure, 70 : budding, 72, 73 : in-
ternal fission, 73 : nutrition, 74 : alco-
holic fermentation caused by, 74, 78, 79 :
experiments on nutrition of, 77-79 : ani-
mal or plant? 179
SALVIN'IA : — Figure, 365 : general cha-
racters, 364 : mega- and micro-sporangia
and spores, 364 : male and female pro-
thalli and gonads, 366 : development and
alternation of generations, 366
Saprophyt'iC (craTrpo?, putrid : (^VTOI/, a
plant) nutrition, defined, 39
Schulze's solution, test for cellulose, 28 :
for lignin, 344, 345
Scleren'chyma (o-/cATjp6?, hard : e-yx^M-a,
infusion): — Moss, 330: Fern, 344, 347
Secre'tion (secrctus, separate), nature of,
227 : formation of cell-wall a process of,
-, I4
Seed, formation of, 373, 374, 380 : germi-
nation of, 374
Seg'ment (scg»icntu)>i, a piece cut off), in
plants a node together with the next
proximal internode, 206 : in animals the
name is variously applied. See Meta-
mere, Podomere.
SELAGINELL'A (treAa-yew, to shine):—
Figure, 368 : general characters, 367: cone,
sporangia, and spores, 367 : prothalli and
gonads, 367 : development and alterna-
tion of generations, 369
Self-fertilization, applied to the sexual
process when the gametes spring from
the same individual, 197
Sep'alS (separ, separate), the outer or
proximal perianth-leaves in the flower of
Angiosperms, 377
Sep'tum (septitm, a barrier) : — In plants,
185 : in Polygordius, 276 : development
of, 298
Set'a (seta, a bristle), 287
Sex-cells, primitive, 252 : origin of in
Hydroids, 244 : in Polygordius, 289
Sexual differentiation, illustrated by
Vaucheria, 170: by Spirogyra, 197
Sexual generation. See Gamobium.
Sexual reproduction, nature of, 42
Shell, Mussel, 316
Shoot, in plants, an axis of the second or
any higher order with its leaves, 207
Sieve-tubes and plates, 346
Sinus (shins, a hollow), a spacious cavity,
Skeleton. See Endo- and Exo-skeleton.
Slime-fungi. See Mycetozoa.
Solid aggregate, 201
Somat'iC (o-w/xa, the body), applied to the
layer of mesoderm which is in contact
with the ectoderm and with it forms the
body-wall, 274
Sor'us (crajpo?, a heap), an aggregation of
sporangia, 350, 364
Species (species, a kind), meaning of term
illustrated, 8, 135 : definition of, 137 :
origin of, 139-142
Specific characters, specific name, 8.
137
Specialized, meaning of, 138
Sperm (a-irepfj-a, seed), the male or micro-
gamete in its highest stage of differentia-
tion : structure and development of, 252 :
see also under the various types, and
especially Vaucheria, 170, 171
Spermatozo'id, spermatozo'on (o-Trep/ua.
seed : ftoof, animal, from the actively
moving sperms of animals having been
supposed to be parasites), synonyms of
sperm.
Spermary (o-Tre'p/xa, seed), the male gonad
or sperm-producing organ : see under the
various types, and especially Vaucheria,
170
Sperm 'idUCt (a-n-ep^a, seed : duco, to lead),
a tube conveying the sperm from the
spermary to the exterior, 291
INDEX AND GLOSSARY
407
SpermatOgen'esiS (crTrep/xa, seed : yeVecrts,
origin), the development of a sperm from
a primitive sex-cell, 252, 253 (Figure).
Spinal cord, Dogfish, 326
Spiral vessel. See Vessel.
SPIRILL'UM (spira, a coil) (Figure) 86, 87
Spirogyra (spira, a coil : gyms, a revolu-
tion) : — Figure, 193 : occurrence and
general characters, 192 : microscopic
structure, 192 : growth, 195 : conjugation,
196 : development, 198 : nutrition, 198
' Splanch/niC (crn\6.yxvov> intestine or vis-
cus), applied to the layer of mesoderm
which is in contact with the endoderm
and with it forms the enteric canal, 274
Spontaneous generation. See Abio-
genesis.
Sporan'gium (<nropd, seed : ayyeiW, a
vessel), a spore-case : — Mucor, 157 : Vau-
cheria, 169 : Fern, 351. See also Mega-
and Micro-sporangium.
Spore (crTropd, a seed), an asexual repro-
ductive cell : see under the various types
and especially Heteromita, 42 : Saccha-
romyces, 73 : Bacteria, 88 : vitality of
in Bacteria, 96, 97 : Penicillium, 187 :
Moss, 335: Fern, 351. See also Mega-
and Micro-spore.
Sporogon'ium (o-Ti-opa, seed: 761/0?, pro-
duction), the agamobium of a moss, 333
Spor'ophyll (a-nopd, seed : <f>v\\ov, leaf),
a sporangium-bearing leaf : — Equisetum,
362 : Selaginella, 367 : Gymnosperms,
369, 372 : Angiosperms, 377
Stamen (stamen, a. thread), a male sporo-
phyll. 371, 377 _
Starch, composition and properties of, 27
STARFISH :— Figure, 306 : general cha-
racters, 305-307 : radial symmetry, 305 :
tube-feet and ambulacral system, 307,
309 : exoskeleton, 308
Stem, structure of: — Moss, 330 ; Fern, 341
Sterig'ma (crT^pty^a, a support : — Penicil-
lium, 186 ; Agaricus, 190
Sterilization of putrescible infusions, 96-
IOO
Stigma (o-Ttyjua, a spot), the receptive ex-
tremity of the style, 378
Stimulus, various kinds of, 285
Stock. See Colony.
Stom'ate (o-ro/xa, mouth), 349
Stomodae'um (o-To/aa, mouth : oSaios, be-
longing to a way), an ectodermal pouch
which unites with the enteron and forms
the anterior end of the enteric canal, its
aperture being the permanent mouth,
294
Stone-canal, Starfish, 309
Style (stylus, a column), the distal solid
portion of the female sporophyll or of the
entire gyncecium in Angiosperms, 378
STYLONYCH'IA (arvAos, a column : ow£ ,
a claw), Figure, 115: occurrence and
general characters, 1 14 *. polymorphism
of cilia, 114-116
Sub-apical cell. See Segmental cell.
Superficial aggregate, 200
Supporting lamella. See Mesogloca.
Suspensor :— Selaginella, 369 : Gymno-
sperms, 373 : Angiosperms, 380
Sweet Wort, composition of, 74
Swimming-bell, Diphyes, 249
Symbio'sis (crv/a/Stajo-t?, a living with), an
intimate and mutually advantageous
association between two organisms, 152
Syner'gidas (crwepyos, a fellow worker),
379
Sys'tole (crvoToAvj, a drawing together,
contraction), the phase of contraction of
a heart, contractile vacuole, £c., 109
Teeth, Dogfish, 325
Temperature, effects of on protoplasmic
movements, 20, 21
Tentacles : — Hydra, 222 : Bougainvillea,
236 : Polygordius, 266 : development of,
298
Terminal bud :— Nitella, 206, 208 : Moss,
331
TestiS (the Latin word), generally used for
the spermary in animals.
Thermal death-point. See Ultra-maxi-
mum temperature.
Tissues, differentiation of: — Polygordius,
288 : Fern, 349
Tracheides (rpaxv's, rough : elSos, form).
See Vessels of Plants, 345
Transpiration, the giving off of water
from the leaves of plants, 337
Trich'ocyst (Qpi£, a hair : KUCTTIS, a bag),
in
Triploblast'iC (rpiTrAoos, triple : /SAacrros,
a bud), three-layered : applied to ani-
mals in which the body consists of ecto-
derm, mesoderm, and endoderm, 241,
274
Troch'OSphere (rpoxo?, a wheel, in reference
to the circlet of cilia : cn/>aipa, a sphere),
the free -swimming larva of Polygordius.
&c. : — characters of, 292 (Figure) ; origin
of from gastrula, 293, 294 ; metamorpho-
sis of, 295
Tube-feet, Starfish, 307, 309
U
Ultra-maximum temperature, for amoe-
boid movements, 21 ; for monads, 40 ; for
Bacteria, 92
ULVA (nh'ci, an aquatic plant), 201
Umbell'ate (nmbella, a sun-shade, um-
brella), applied to branching in which
the primary axis is of limited growth and
408
INDEX AND GLOSSARY
sends off a number of secondary axes
from its distal end, 136
Unicell'ular, formed of a single cell, 61 ;
connection of uni- with multi-cellular
organisms, 260-265
Ureter (ovprjnjp, the Greek name), the
duct of the kidney, 326
VORTICELLA (diminutive of vortex, an
eddy) : — Figure, 125 : occurrence and
general characters, 124 : structure, 126 :
asexual reproduction, 129 : conjugation,
130: means of dispersal, 130-134: encys-
tation, spore-formation, development,
and metamorphosis, 131
Vac'uole (vacuus, empty), contractile, n,
109: non-contractile, 70
Variability, 145
Variation, individual, 138, 145
Variety, an incipient species, 145
Vasc'ular (vasculum, a small vessel)
bundles, 341, 344
Vascular plants, 361
VAUCHERIA (after J. P. E. Vaucher, a
Swiss botanist) : — Figure, 168 : occur-
rence and general characters, 167: minute
structure, 167 : asexual reproduction, 169:
sexual reproduction, 170 : nutrition, 173
Veins of Dogfish, 325 : of leaves, 348
Vel'um (velum, a veil) of medusa, 240
Vent, the aperture of the cloaca, 320
Venter (venter, the belly), of ovary o.
Moss, 332, and Fern, 354 : of the female
sporophyll or of the entire gynoecium of
Angiosperms (so-called ovary), 377
Ventral nerve-cord :— Polygordius, 282 :
development of, 297 : Crayfish, 315
Ventricle. See Heart.
Venues, the, 304
Ver'tebral (vertebra, a joint) centra and
column, Dogfish, 324
Vertebrata, the, 305
Vessels : — of plants, spiral and scalariform,
344, 347 : of animals, see Blood-vessels.
Vestige, vestigial (vestigium, a trace),
applied to any structure which has become
atrophied or undergone reduction beyond
the limits of usefulness, 115
Vib'rio (vibro, to vibrate), 85, (Figure) 87
Visc'eral (viscits, an internal organ), ap-
plied to the layer of ccelomic epithelium,
or of petitoneum, covering the intestine
and other internal organs, 273
Visceral ganglion, Mussel, 319
Vitelline (vitellus, yolk) membrane, the
cell-membrane of the ovum, 255
VolVQX (volvo, to roll), 263 (Figure)
W
Waste-products, 33
Water of organization, 5, 29
Whorl of leaves, 206
Wood. See Xylem.
Work and Waste, 14
X
Xylem (£u/\oi', wood), the inner portion ot
vascular bundle, 345
Yeast, 70
Yeast-plant. See Saccharomyces.
YellOW-cells of Radiolaria, 152
Yolk-granules or spheres, 68, 232, 254
Zoogl03'a (£<aov, an animal : y\oia, glue).
84
ZOOid (£<I>of, an animal ; eifios, form), a
single individual of a compound organism.
138, 234
Zootham'nium (£u>ov, an animal : Od^vcs.
a bush) : — Figures, 132, 136 : occurrence
and general characters, 133 : dimorphism
of zooids, 133 : means of dispersal, 133 :
characters and mutual relations of species.
I*"1 *"* T ° A
JJ! Ij4
Zooxanthell'a (£u>ov, an animal : £ai/0u?,
yellow), 152
Zyg'ospore (£vyoi>, a yoke : cnropd, a seed),
applied to a resting zygote formed by the
conjugation of similar gametes, 164
Zygote (^vycoTo?, yoked), the products of
conjugation of two gametes : — Hetero-
mita, 41 : Vorticella, 131 : Mucor, 164 :
Vaucheria, 197 : Spirogyra, 198.
THE END
K1C1IAKD CLAY AND SONS, LIMITED. LONDON AND BUNGAY
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