LESSONS
IN
ELEMENTARY BIOLOGY
By the same Author.
WILLIAM KITCHEN PARKER, F.R.S. A
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LESSONS
IN
ELEMENTARY BIOLOGY
BY THE LATE
T. JEFFERY PARKER, D.Sc, F.R.S.
V
PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, NEW ZEALAND
WITH ONE HUNDRED AND TWENTY-SEVEN ILLUSTRATIONS
MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON
1920
=6
and S. volutaiis 88
1 8. Bacillus anthracis 9°
19. Beaker with culture tubes . . 100
20. Paramoccium caudatum 108
21. Param&cium caudatum, conjugation 115
22. Stylonychia mytilus 117
xx LIST OF ILLUSTRATIONS
FIG. PAGE
23. Oxytricha flava 120
24. Opalina ranarum 122
25. Vorticella 127
26. Zoothamnium arbuscula 134
27. Zoothamnium, various species 138
28. Diagram illustrating the Origin of the Species of Zootham-
nium by Creation . . 142
29. Diagram illustrating the Origin of the Species of Zootham-
nium by Evolution 144
30. Kotalia 149
31. Diagrams of Foraminifera . 150
32. Alveolina qttoti 151
33. Lithocircus annularis 152
34. Actinomma asteracanlhion . 153
35. Diagrams of a Diatom and shells of Navicula and Aulaco-
discus 156
36. Mucor mucedo and M. stolonifer 159
37. Moist Chamber 163
38. Vaucheria 170
39. Caulerpa scalpelliformis -. . . 175
40. Penicillium glaucum 186
41. Agaricits campestris 192
42. Spirogyra 195
43. Monostroma bullosum and M. laceratum 202
44. Nitella, general structure 204
45. Nitella, terminal bud : 209
46. Nitella, spermary '. 212
47. Nitella, ovary 214
48. Chara, pro-embryo 216
49. Hydra viridis and H. fttsca, external form 219
50. Hydra, minute structure 223
51. Hydra, nematocyst and nerve-cell 225
52. Hydra viridis, ovum 232
53. Eougainvillea ramosa 235
54. Eucopella, portion of tentacle 237
LIST OF ILLUSTRATIONS xxi
FIG. >'AGE
55. Diagrams illustrating derivation of Medusa and Hydrant h . 241
56. Eucopelia campanularia, muscle fibres and cells 243
57. Laomedeaflexuosa and Endendrhun ramosum, development. 247
58. Diphyes campanulata 250
59. Porpita pacifica and P. mediterranean 251
60. Spermatogenesis in the Mole-cricket 254
61. Ovum of Toxopneustes lividus . . 257
62. Maturation and impregnation of the animal ovum .... 258
63. Pandorina moruin 263
64. Volvox globator • 265
65. Volvox globator 266
66. Polygordius neapolitanus, external form . 269
67. Polygordius neapolita)nis, anatomy 271
68. Polygordius neapolitanus, nephridium 282
69. Polygordius^ diagram illustrating the relations of the nervous-
system 284
70. Polygordius neapolitanus, reproductive organs 291
71. Polygordius neapolitanus, larva in the trochosphere stage . 293
72. Diagram illustrating the origin of the trochosphere from the
gastrula 295
73. Polygordius neapolitamis, advanced trochosphere ...... 297
74. Polygordius neapolitanus ', larva in a stage intermediate be-
tween the trochosphere and the adult 300
75 Starfish, ventral aspect 307
76. Starfish, diagrammatic sections 309
77. Starfish, digestive organs 311
78. Starfish, water vascular system 3J3
79. Starfish, early stages in development • ... 316
80. Starfish, development of bipinnaria larva 317
81. Crayfish, side view 319
82. Crayfish, principal appendages 322
83 Crayfish, diagrammatic sections 328
84. Crayfish, action of abdominal muscles 330
85. Crayfish, leg, with muscles 331
86. Crayfish, dissection 333
xxii LIST OF ILLUSTRATIONS
FIG. PAGE
87. Crayfish, gills -356
88. Crayfish, diagram of circulation of blood 340
89. Crayfish, early development 344
90. Crayfish, early embryo in nauplius stage 345
91. Crayfish, later embryo 346
92. Mussel, side view, and shell 351
93. Mussel, diagrammatic sections 353
94. Mussel, dissection 356
95. Mussel, structure of gill 358
96. Mussel, circulatory system . 361
9^. Mussel, advanced embryo and free larva 364
98. Dogfish, side view 367
99. Dogfish, diagrammatic sections 370
100. Dogfish, skull 373
101. Dogfish, vertebrae 376
102. Dogfish, pectoral arch 378
103. Dogfish, dissection 380
104. Dogfish, vascular system 385
iO4a. Dogfish, diagram of circulation 389
105. Dogfish, brain ...... 392
106. Dogfish, early embryo 398
107. Dogfish, advanced embryo 399
108. Mosses, various genera, anatomy and histology 402
109. Funaria, reproduction and development 406
1 10. Pteris and Aspidium, anatomy and histology 414
111. Ferns, various genera, reproduction and development . . . 424
112. Equisetum^ aerial shoot and spores 435
113. Equisetum, reproduction and development . 437
114. Sa/vmia, part of plant 439
115. Salvinia, reproduction and development ........ 441
116. Selaginella., part of plant and sporangia 443
117. Selaginella, reproduction and development 445
118. Pinus, sections of stem 449
119. Gymnosperms, reproduction and development 453
1 20. Pinus, stamen 455
LIST OF ILLUSTRATIONS xxiii
FK;. TAGE
121. Pin us, carpel 456
122. /.ainia and Cycas, reproductive organs 457
123. Lily, section of stem - . 462
124. Buttercup, structure of flower 465
125. Transition from petal to stamen 467
126. Angiosperms, reproduction and development 470
[27. Helleborns, Campanula, and Ribes, flower 472
LESSONS
IN
ELEMENTARY BIOLOGY
LESSON I
AMOEBA
IT is hardly possible to make a better beginning of the
systematic study of Biology than by a detailed examination
of a microscopic animalcule often found adhering to weeds
and other submerged objects in stagnant water, and known
to naturalists as Amceba.
Amoebae are mostly invisible to the naked eye, rarely
exceeding one-fourth of a millimetre (TJ^ 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-
cognised 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
Y1G, lf — A. Anueba quarta, a living specimen, showing granular
endosarc. surrounded by clear ectosarc, and several pseudopods (psd\
I GENERAL CHARACTERS 3
some formed of ectosarc only, others containing a core of endosarc.
The larger bodies in the endosarc are mostly food-particles ( x 30x3). l
B. The same species, killed and stained with carmine to show the
numerous nuclei (nu) ( x 300).
c. Aniaba proteiis, a living specimen, showing large irregular
pseudopods, nucleus (nti\ 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 the protoplasm has united round the prey to
inclose the food vacuole. The contractile vacuole in this figure is
supposed to be seen through a layer of granular protoplasm, whereas
in the succeeding figures (D, E, and G) it is seen in optical section, and
therefore appears clear.
D. An encysted Amoeba, showing cell-wall or cyst (cy), nucleus (nu),
clear contractile vacuole, and three diatoms (see Lesson XIV.) ingested
as food.
E. Am<.rba proteus, a living specimen, showing several large pseudo-
pods (psd), single nucleus (mi), and contractile vacuole (c. vac), and
numerous food-particles embedded in the granular endosarc ( x 330).
F. Nucleus of the same after staining, showing a ground substance
or nuclear sap, containing deeply-stained granules of chromatin, and
surrounded by a distinct membrane ( x 1010).
G. Amccba verntcosa, living specimen, snowing wrinkled surface,
nucleus (nu), large contractile vacuole (c. vac), and several ingested
organisms ( x 330).
H. Nucleus of the same, stained, showing the chromatin aggregated
in the centre ( x 1010).
i. Atna'ba profeiis, in the act of multiplying by binary fission
(x 500).
(A, B, E, F, G, and H after Gruber ; c and I after Leidy ; D after
Howes.)
shapeless blob of jelly, nearly or quite colourless. 'The
central part of it (Fig. i, A, c, and E) is granular and semi-
transparent — something like ground glass — while surround-
ing this inner mass is a border of perfectly transparent and
colourless substance. So clear, indeed, is this outer layer
that it is easily overlooked by the beginner, who is apt to take
the granular internal substance for the whole Amoeba. If
in any way the creature can be made to turn over, or if a
number of specimens are examined in various positions,
these two constituents will always be found to have the
1 A number preceded by the sign of multiplication indicates the
number of diameters to which the object is magnified.
B 2
4 AMCEBA LESS.
same relations, whence we conclude that an Amoeba con-
sists of a granular substance the endosarc, completely
surrounded by a clear transparent layer or ectosarc.
One very noticeable thing about Amoeba is that it is never
of quite the same shape for long together. Often the
changes of form are so slow as to be almost imperceptible,
like the movements of the hour-hand of a watch, but by
examining it at successive intervals the alteration becomes
perfectly obvious, and at the end of half an hour it will
probably have altered so much as to be hardly like the
same thing.
In an active specimen the way in which the changes of
form are brought about is easily seen. At a particular
point the ectosarc is pushed out -in the form of a small
pimple-like elevation (Fig. i, A, left side) : this increases in
size, still consisting of ectosarc only, until at last granules
from the endosarc stream into it, and the projection or
pseudopod (A, c, E, psd) comes to have the same structure
as the rest of the Amoeba. It must not be forgotten that
the animal does not alter perceptibly in volume during
the process, every pseudopod thus protruded from one part
of the body necessitating the withdrawal of an equal volume
from some other part.
This peculiar mode of movement may be illustrated by
taking an irregular lump of clay or putty and squeezing it
between the 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.
* 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 x
to consist mainly of certain substances known as proteids,
bodies of extreme complexity in chemical constitution, the
most familiar example of which is white of egg or albumen.
They are compounds of carbon, hydrogen, oxygen, nitrogen,
and sulphur, the five elements being combined in the
following proportions : —
Carbon . . from 51*5 to 54*5 per cent.
Hydrogen . „ 6-9 „ 7-3 „ „
Oxygen . „ 20-9 „ 23-5 „ „
Nitrogen . „ 15-2 „ 17-0 „ „
Sulphur . „ 0-3 „ 2-0 „ „
Besides proteids, protoplasm contains small proportions
of mineral matters, especially phosphates and sulphates of
potassium, calcium, and magnesium. It also contains a
considerable quantity of water which, being as essential a
constituent of it as the proteids and the mineral salts, is
called water of organization.
Protoplasm is dissolved by prolonged treatment with weak
acids or alkalies. Strong alcohol coagulates it, i.e., causes it
to shrink by withdrawal of water and become comparatively
hard and opaque. Coagulation is also produced by raising
the temperature to about 40° C. ; the reader will remember
how the familiar proteid white of egg is coagulated and
rendered hard and opaque by heat.
1 Accurate analyses of the protoplasm of Amoeba have not been
made, but the various micro-chemical tests which can be applied to it
leave no doubt that it agrees in all essential respects with the protoplasm
of other organisms, the composition of which is known (see p. 7).
6 AMCEBA LESS.
There is another important property of proteids which is
tested by the instrument called a dialyser. This consists
essentially of a shallow vessel, the bottom of which is made
of bladder, or vegetable parchment, or some other organic
(animal or vegetable) membrane. If a solution of sugar or
of salt is placed in a dialyser and the instrument floated in a
larger vessel of distilled water, it will be found after a time
that some of the sugar or salt has passed from the dialyser
into the outer vessel through the membrane. On the other
hand, if a solution of white of egg is placed in the dialyser
no such transference to the outer vessel will take place.
The dialyser thus allows us to divide substances into
two classes : crystalloids— so called because most of them,
like salt and sugar, are capable of existing in the form of
crystals— which, in the state of solution, will diffuse through
an organic membrane ; and colloids or glue-like substances
which will not diffuse. Protoplasm, like the proteids of
which it is largely composed, is a colloid, that is, is non-
diffusible. It has a slightly alkaline reaction.
Another character of proteids is their instability. A
lump of salt or of sugar, a piece of wood or of chalk, may
be preserved unaltered for any length of time, but a proteid
if left to itself very soon begins to decompose ; it acquires
an offensive odour, and breaks up into simpler and simpler
compounds, the most important of which are water (H2O),
carbon dioxide or carbonic acid (CO2), ammonia (NH3),
and sulphuretted hydrogen (H^S).1 In this character of
instability or readiness to decompose protoplasm notoriously
agrees with its constituent proteids ; any dead organism will,
1 For a more detailed account of the phenomena of putrefaction see
Lesson VIII., in which it will be seen t-hat the above statement as to
the instability of (dead) proteids requires qualification ; as a matter of
fact they decompose only in the presence of living Bacteria.
i 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 Amceba, owing to its minute size and
the difficulty of isolating it from other organisms (water-
weeds, &c.) with which it is always associated ; but there
are some tests which can be readily applied to it while
under observation beneath the microscope.
One of the most striking of these micro-chemical tests
depends upon the avidity with which protoplasm takes up
certain colouring matters. If a drop of a neutral or slightly
alkaline solution of carmine or logwood, or of some aniline
dye, or a weak solution of iodine, is added to the water con-
taining Amceba, the animalcule is killed, and at the same
time becomes more or less deeply stained.
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, ««), while frequently its presence is
revealed only by staining (comp. A and B), is called the
nucleus.
But when viewed under a sufficiently high power, the
nucleus itself is seen to be unequally stained. It has lately
been shown, in many Amoebae, to be a globular body, en-
closed in a very delicate membrane, and made up of two
constituents, one of which is deeply stained by colouring
matters, and is hence called chromatin, while the other, the
nuclear sap or achromatin, takes a lighter tint (Fig. I, v\
The relative arrangement of chromatin and sap varies
in different Amoebae : sometimes there are granules of
chromatin in an achromatic ground substance (F); some-
8 AMOEBA LESS.
times the chromatin is collected towards the surface or
periphery of the nucleus ; sometimes, again, it becomes
aggregated in the centre (G, H). One or more smaller
bodies, or nuckoli, may also be present in the nucleus,
which is then distinguished as the nuckolus.
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 both in fresh and salt 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 Felts — 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 substance ; and many
others.
Besides the nucleus, there is another structure frequently
visible in the living Amoeba. This is a clear, rounded space
in the protoplasm (c, E, and G, c. vac), which periodically
disappears with a sudden contraction and then slowly
reappears, its movements reminding one of the beating of a
minute colourless heart. It is called the contractile vacuole,
and consists of a cavity containing a watery fluid.
I MORPHOLOGY AND PHYSIOLOGY 9
Occasionally Amoeba;— or more strictly Amoeba-like
organisms — are met with which show neither nucleus1 nor
contractile vacuole, and are therefore placed in the separate
genus Protamaba (Fig. 2). They may be looked upon as
the simplest of living things.
%A m «
^ B C D
FIG. 2 — Protamxba primiliva , 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.)
The preceding paragraphs may be summed up by saying
that Amoeba is a mass of protoplasm produced into tempo-
rary processes or pseudopods, divisible into ectosarc and
endosarc, and containing a nucleus and a contractile vacuole :
that the nucleus consists of two substances, chromatin and
nuclear sap, 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.
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
1 Judging from the analogy of the Infusoria it seems very probable
that such apparently non-nucleate forms as Protamoeba contain chroma-
tin diffused in the form of minute granules throughout their substance
(see end of Lesson X., p. 120), or that they are forms which have lost
their nuclei.
io AMCEBA LKSS.
that Amoeba is contractile, or that it exhibits contractility.
But here it must be borne in mind that contraction does
not mean the same thing in biology as in physics. When
it is said that a red-hot bar of iron contracts on cooling,
what is meant is that there is an actual reduction in
volume, the bar becoming smaller in all dimensions. Bui
when it is said that an Amoeba contracts, what is meant is
that it diminishes in one dimension while increasing in
another, no perceptible alteration in volume taking place
each time a pseudopod is protruded an equivalent volume
of protoplasm is withdrawn from some other part of the
body.
We may say then that contractility is a function of the
protoplasm of Amoeba — that is, that it is one of the actions
which the protoplasm is capable of performing.
A contraction may arise in one or other of two ways. Ir
most cases the movements of an Amoeba take place withou
any obvious external cause ; they are what would be callec
in the higher animals voluntary movements — movement:
dictated by the will and not necessarily in response to an>
external stimulus. Such movements are called spontamou,
or automatic. On the other hand, movements may be in
duced 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 proto
plasm, which is thus both automatic and irritable — that is
its contractility may be set in action either by internal or b]
external stimuli.
Under certain circumstances an Amoeba temporarily lose;
its power of movement, draws in its pseudopods, am
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 no
I MODE OF FEEDING 11
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
active life.
Very often an Amoeba in the course of its wanderings
comes in contact with a still smaller organism, such as a
diatom (see Lesson XIV., Fig. 35) or a small infusor (see
Lessons X. — XII.). When this happens the Amoeba may
be seen to extend itself round the lesser -organism until the
latter becomes sunk in its protoplasm in much the same way
as a marble might be pressed into a lump of clay (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 necessarily in-
cluded with the prey. The latter is taken in by the Amoeba
as food : so that another function performed by the animal-
cule is the reception of food, the first step in the process of
nutrition. It is to be noted that the reception of food takes
place in a particular way, viz. by ingestion — i.e. it is enclosed
raw and entire in the living protoplasm. It has been noticed
that Amoeba usually ingests at its hinder end — that is, the
end directed backwards in progression.
Having thus ingested its prey, the Amoeba continues its
course, when, if carefully watched, the swallowed organism
will be seen to undergo certain changes. Its protoplasm
is slowly dissolved ; if it contains chlorophyll — the green
colouring matter of plants — this is gradually turned to brown ;
and finally nothing is left but the case or cell-wall in which
many minute organisms, such as diatoms, are enclosed.
Finally, the Amoeba, as it creeps slowly on, leaves this empty
cell-wall behind, and thus gets rid of what it has no further
12 AMCEBA LESS.
use for. It is thus able to ingest living organisms as food ;
to dissolve or digest their protoplasm ; and to egest or get
rid of any insoluble materials they may contain. Note
that all this is done without either ingestive aperture (mouth),
digestive cavity (stomach), or egestive aperture (anus) ; the
food is simply taken in by the flowing round it of protoplasm,
digested as it lies enclosed in the protoplasm, and the useless
part 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
pwoteids thus formed by the action of gastric juice upon
ordinary proteids are called peptones : the transformation is
effected through the agency of a constituent of the gastric
juice called pepsin.
There can be little doubt that the protoplasm of Amoeba
is able to convert that of its prey into a soluble and diffusible
form 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 proto-
plasm, the water the solution of proteids which permeates 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
I GROWTH 13
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 rearranged 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 necessary
to distinguish it, because there is another mode of growth
which is characteristic of minerals and occurs also in some
organized structures. A crystal of alum, for instance,
suspended in a strong solution of the same substance, grows ;
but the increase is due to the deposition of successive layers
on the surface of the original crystal, in much the same way
as a candle might be made to grow by repeatedly dipping it
into melted grease. This can be proved by colouring the
crystal with logwood or some other dye before suspending
it, when a gradually-increasing colourless layer will be
deposited round the coloured crystal : if growth took place
by intussusception we should have a gradual weakening
1 4 AMCEBA LESS.
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 chemi-
cal change resulting in the formation of a thin superficial
layer of non -protoplasmic substance. The process is re-
peated,, 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 \ nun., 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 proportiona7
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, a cer-
tain fraction of the protoplasm becoming oxidized, or in other
words undergoing a process of low temperature combustion.
I POTENTIAL AND KINETIC ENERGY 15
When we say that any combustible body is burnt what we
usually mean is that it has combined with oxygen, forming
certain products of combustion due to the chemical union
of the oxygen with the substance burnt. For instance, when
carbon is burnt the product of combustion is carbon dioxide
or carbonic acid (C + O.,= CO2) : when hydrogen is burnt,
water (H2 + 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 compounds
such as guanin (C5H5Nf)O), 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
or waste matters of Amoeba include carbon dioxide, water,
and some comparatively simple (as compared with proteids)
compound of nitrogen.
16 AMCEBA LESS.
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 Amceba, 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 Amceba
the solid excreta, or more correctly faces, consist of such
things as the indigestible cell-walls, starch grains, &c., of the
organisms upon which it feeds ; but the rejection of these
is no more a process of excretion than the spitting out of
a cherry-stone, since they are simply parts of the food
which have never been assimilated — never formed part and
parcel of the organism. True excreta, on the other hand,
are invariably products of the waste or decomposition of
protoplasm.1
The statement just made that the protoplasm of Amceba
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
1 In the higher animals the distinction between excreta and faeces is
complicated by the fact that the latter always contain true excretory
products derived from the epithelium of the intestine and its glands.
i METABOLISM 17
other. The same process of gaseous diffusion is continually
going on between the carbon dioxide in the interior of
Amoeba and the oxygen in the surrounding water, the proto-
plasm acting as the porous partition. In this way the carbon
dioxide is got rid of, and at the same time a supply of
oxygen is obtained for further combustion.
The taking in of oxygen might be looked upon as a kind
of feeding process, the food being gaseous instead of solid
or liquid, just as we might speak of " feeding " a fire both
with coals and with air. Moreover, as we have seen, the
giving out of carbon dioxide is a process of excretion. It
is, however, usual and convenient to speak of this process
of exchange of gases as respiration or breathing, which is
therefore another function performed by the protoplasm of
Amoeba.
The oxidation of protoplasm in the body of an organism,
like the combustion of wood or coal in a fire, is accompanied
by an evolution of heat. That this occurs in Amoeba can-
not be doubted, although it has never been proved. The
heat thus generated is, however, constantly being lost to the
surrounding water, so that the temperature of Amoeba, if we
could but measure it, would probably be found, like that of
a frog or a fish, to be very little if at all above that of the
medium in which it lives.
We thus see that a very elaborate series of chemical pro-
cesses is constantly going on in the interior of Amoeba.
These processes are divisible into two sets : those which
begin with the digestion of food and end with the manufac-
ture of living protoplasm, and those which have to do with
the destruction of protoplasm and end with excretion.
The whole series of processes are spoken of collectively
as metabolism. We have, first of all, digested food diffused
through the protoplasm and finally converted into fresh
1 8 AMCEBA LESS.
living protoplasm : this is the process of constructive meta-
bolism or anabolism. Next we have the protoplasm, gradually
breaking down and undergoing conversion into excretory
products : tnis is the process of destructive metabolism or
katabolism. There can be little doubt that both are pro-
cesses of extreme complexity : it seems probable that
after the food is once dissolved there ensues the successive
formation of numerous bodies of gradually increasing
complexity (anabolic mesostates or anastates), culminating
in protoplasm ; and that the protoplasm, when once formed,
is decomposed into ' a series of substances of gradually
diminishing complexity (katabolic mesostates or katastates\
the end of the series being formed by the comparatively
simple products of excretion. The granules in the endosarc
are probably to be looked upon as various mesostates
imbedded in the protoplasm proper.
Living protoplasm is thus the most unstable of substances ;
it is never precisely the same thing for two consecutive
seconds: it "decomposes but to recompose," and recom-
poses but to decompose ; its existence, like that of a water-
fall or a fountain, depends upon the constant flow of matter
into it and away from it.
It follows from what has been said that if the income of
an Amoeba, i.e., the total weight of substances taken in (food
plus oxygen plus water) is greater than its expenditure or
the total weight of substances given out (fseces plus excreta
proper plus carbon dioxide) the animalcule will grow : if less
it will dwindle away : if the two are equal it will remain of
the same weight or in a state of physiological equilibrium.
We see then that the fundamental condition of existence
of the individual Amoeba is that it should be able to form
new protoplasm out of the food supplied to it. But some-
i REPRODUCTION 19
thing more than this is necessary. Aincebie are subject to
all sorts of casualties ; they may be eaten by other organ-
isms or the pool in which they live may be dried up ; in one
way or another they are constantly coming to an end.
From which it follows that if the race of Amoebae is to be
preserved there must be some provision by which the
individuals composing it are enabled to produce new in-
dividuals. In other words Amoeba must, in addition to its
other functions, perform that of reproduction.
An Amoeba reproduces itself in a very simple way. The
nucleus first divides into two : then the whole organism
elongates, the two nuclei at the same time travelling away
from one another : next a furrow appears across the middle
of the drawn out body between the nuclei (Fig. i, i ; fig. 2,
c, D) : the furrow deepens until finally the animalcule sepa-
rates into two separate Amoebae (Fig. 2, E), which hence-
forward lead an independent existence.
This, the simplest method of reproduction known, is
called simple or binary fission. Notice how strikingly dif-
ferent it is from the mode of multiplication with which we
are familiar in the higher animals. A fowl, for instance,
multiplies 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 division of the parent, which does not die, but
becomes simply merged in its progeny. There can be r.o
better instance of the fact that reproduction is discontinuous
growth.
C 2
20 AMCEBA LESS.
From this it seems that an Amoeba, unless suffering
a violent death, is practically immortal, since it divides into
two completely organised 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 is possible,
however, judging from the analogy of the Infusoria (see
Lesson X.) that such organisms as Amceba cannot go on
multiplying indefinitely by simple fission, and that occasion-
ally two individuals come into contact and undergo complete
fusion. A conjugation of this kind has been observed in
Amceba, but has been more thoroughly studied in other forms
(see Lessons III., X., XII.). 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.
• Amceba may also be propagated artificially. If a speci-
men is cut into pieces each fragment is capable of develop-
ing into a complete animalcule provided it contains a
portion of nuclear matter, but not otherwise. From this it
is obvious that the nucleus exerts an influence of the utmost
importance over the vital processes of the 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. 91.)
In conclusion, a few facts may be mentioned as to the
conditions of life of Amceba — the circumstances under
which it will live or die, flourish or otherwise.
In the first place, it will live only within certain limits of
temperature. In moderately warm weather the temperature
to which it is exposed may be taken as about 15° C. If
gradually warmed beyond this point the movements at first
I CONDITIONS OF LIFE 21
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 animal-
cule is killed by the coagulation of its protoplasm (see p. 5) :
it is then said to suffer heat-rigor or death-stiffening pro-
duced by heat. Similarly when it is cooled below the
ordinary temperature the movements become slower and
slower, and at the freezing point (o° C.) cease entirely.
But freezing, unlike over-heating, does not kill the pro-
toplasm, but only renders it temporarily inert ; on thawing,
the movements recommence. We may therefore distinguish
an optimum temperature at which the vital actions are carried
on with the greatest activity ; maximum and minimum tem-
peratures 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 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 distilled 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 two per cent, causes Amoeba to withdraw its
pseudopods and undergo a certain amount of shrinkage : it
is then said to pass into a condition of dry-rigor. Under
these circumstances it may be restored to its normal con-
dition 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-
22 AMOEBA LESS, i
terious effects of an excess of salt are produced only when
the salt is added suddenly. By the very gradual addition of
sodium chloride Amoebae have been brought to live in a four
per cent, solution, i.e., one twice as strong as would, if added
suddenly, produce dry-rigor.
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 or red colour.
The colour is due to the presence of various organisms
—plants or animals— one of the commonest of which is
called H&matococcus (or as it is sometimes called Sphczrella
or Protococcus] pluvialis.
Like Amoeba, Haamatococcus is so small as to require a
high power for its examination. Magnified three or four
hundred diameters it has the appearance (Fig. 3, A) of an
ovoidal body, somewhat pointed at one end, and of a bright
green colour, more or less flecked with equally bright red.
Like Amoeba, moreover, it is in constant movement, but
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 apparent rapidity. We say apparent rapidity
because the rate of progression is magnified to the same
extent as the organism itself, and what appears a racing
speed under the microscope is actually a very slow crawl
when divided by 300. It has been found that such
organisms as Hasmatococcus travel at the rate of one foot
in from a quarter of an hour to an hour : or, to express
H^MATOCOCCUS
LESS.
the fact in another and fairer way, that they travel a distance
equal to two and a half times their own diameter in one
second. In swimming the pointed end is always directed
FIG. 3. — A. Hamatococcus pluvialis, motile phase. Living specimen,
showing protoplasm with chromatophore (chr) and pyrenoids (pyr),
cell-wall (c. ?v) connected to cell-body by protoplasmic filaments, and
flagella (fl. ). The scale to the left applies to Figs. A — D.
B. Resting stage of the same, showing nucleus (nu) with " nucleolus "
(nu'}, and thick cell-wall (c. 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 fiagella and detached cell-
wall by the daughter-cells before their liberation from the enclosing
mother-cell-wall.
E. Hczmatococcus lacustris, showing nucleus (M(), single large
pyrenoid (py)'), and contractile vncuole (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. )
ii FLAGELLA 25
forwards and the forward movement is accompanied by a
rotation of the organism upon its longer axis.
Careful watching shows that the outline of a swimming
Hsematococcus does not change, so that there is evidently
no protrusion of pseudopods, and at first the cause of
the movement appears rather mysterious. Sooner or later,
however, the little creature is sure to come to rest, and there
can then be seen projecting from the pointed end two exces-
sively delicate colourless threads (Fig. 3, A,y?), each about
half as long again as the animalcule itself: these are called
flagella or sometimes cilia.1 In a Hsematococcus which
has come to rest these can often be seen gently waving
from side to side : when this slow movement is exchanged
for a rapid one the whole organism is propelled through
the water, the flagella acting like a pair of extremely fine
and flexible fins or paddles. Thus the movement of
Haematococcus is not amceboid, i.e., produced by the pro-
trusion and withdravral of pseudopods, but is ciliary r, 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 ciliuni 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. 20) ; a flagel-
lum is a cilium having a whip-lash-like movement, each cell bearing
only a. limited number — one or two, or occasionally as many as four.
26 H^EMATOCOCCUS LESS.
with the flagella goes first ; this may therefore be distin-
guished as the anterior extremity, the opposite or blunt
end being posterior. So that as compared with Amoeba,
Haematococcus exhibits a differentiation of structure: an
anterior and a posterior end can be distinguished, and a
part of the protoplasm is differentiated or set apart as
flagella.
The green colour of the body is due to the presence of
a special pigment called chlorophyll, the substance to which
the colour of leaves is due. That this is something quite
distinct from the protoplasm may be seen by treatment with
alcohol, which simply kills and coagulates the protoplasm,
but completely dissolves out the chlorophyll, producing a
clear green solution. The solution, although green by trans-
mitted light, is red under a strong reflected light, and is
hence fluorescent : when examined through the spectro-
scope it. has the effect of absorbing the whole of the blue
and violet end of the spectrum as well as a part of the red.
The red colour which occurs in so many individuals, some-
times entirely replacing the green, is due to a colouring
matter closely allied in its properties to chlorophyll and
called haimatochrome.
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 structure called a
chromatophore (Fig. 3, A, chr), which forms a layer immedi-
ately beneath the surface, and in this case is relatively large
and urn-shaped. It consists of a protoplasmic substance
impregnated with chlorophyll.
After solution of the chlorophyll with alcohol a nucleus
(B, nu.) can be made out ; like the nucleus of Amoeba it is
stained by iodine, magenta, &c. Other bodies which might
easily be mistaken for nuclei are also visible in the living
ii CELL-WALL 27
organism. These are small ovoidal structures (A, pyr.\
with clearly defined outlines occurring in varying numbers
in the chromatophores. When treated with iodine they
assume a deep, apparently black but really dark blue,
colour. The assumption of a blue colour with iodine is the
characteristic test of the well-known substance starch, as
can be seen by letting a few drops of a weak solution of
iodine fall upon some ordinary washing starch. The bodies
in question have been found to consist of a proteid substance
covered with a layer of starch, and are called pyrenoids.
Starch itself is a definite chemical compound belonging
to the group of carbohydrates, i.e., bodies containing the
elements carbon, hydrogen, and oxygen : its formula is
QHi(A-
In Hsematococcus pluvialis there is usually said to be no
contractile vacuole, but in another species, H. lacustris, this
structure is present 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 red or green body,
but by careful focusing is seen to be really an extremely thin
globular shell (A, c.w.) composed of some colourless trans-
parent material and separated, by a space containing water,
from the body, to which it is connected by very delicate
radiating strands of protoplasm. It is perforated by two
extremely minute apertures for the passage of the flagella.
Obviously we may consider this shell as a cyst or cell-
wall differing from that of an encysted Amoeba (Fig. i, D)
in not being in close contact with the protoplasm.
A more important difference, however, lies in its chemical
composition. The cyst or cell-wall of Amoeba, as stated in
the preceding lesson (p. n) is very probably nitrogenous :
28 H^MATOCOCCUS LESS.
that Of Haematococcus, on the other hand, is formed of a
carbohydrate called cellulose, allied in composition to
starch, sugar, and gum, and having the formula C6 H10 O,v
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, and around it is seen
a sort of blue cloud due to the stained and partly-dissolved
cell-wall.
It has been stated that in stagnant water in which it has
been cultivated for a length of time Haematococcus some-
times assumes an amoeboid form. In any case, after leading
an active existence for a longer or shorter time it comes to
rest, loses its flagella, and throws around itself a thick cell-
wall of cellulose (Fig. 3, B), thus becoming encysted. So
that, as in Amoeba, there is an alternation of an active
or motile with a stationary or resting condition.
In the matter of nutrition the differences between Haema-
tococcus and Amoeba are very marked and indeed funda-
mental. As we have seen, Haematococcus has no pseudopods,
and therefore cannot take in solid food after the manner
of 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. Never-
theless it must take in food in some way or other, or the de-
composition of its protoplasm would soon bring it to an end.
ii DECOMPOSITION OF CARBON DIOXIDE 29
Hrematococcus lives in rain-water. This is never pure
water, but always contains certain mineral salts in solution,
especially nitrates, ammonia salts, and often sodium chloride
or common table salt. These salts, being crystalloids, can
and do diffuse into the water of organization of the ani-
malcule, so that we may consider its protoplasm to be con-
stantly permeated by a very weak saline solution, the most
important elements contained in which are oxygen, hydro-
gen, nitrogen, potassium, sodium, calcium, sulphur, and
phosphorus. It must be remarked, however, that the
diffusion of these salts does not take place in the same uni-
form manner as it would through parchment or other dead
membrane. The living protoplasm has the power of
determining the extent to which each constituent of the
solution shall be absorbed.
If water containing a large quantity of Hsematococcus
is exposed to sunlight, minute bubbles are found to appear
in it, and these bubbles, if collected and properly tested,
are found to consist largely of oxygen. Accurate chemical
analysis has shown that this oxygen is produced by the de-
composition of the carbon dioxide contained in solution in
rain-water, and indeed in all water exposed to the air, the
gas, which is always present in small quantities in the
atmosphere, being very soluble in water.
As the carbon dioxide is decomposed in this way, its
oxygen being given off, it is evident that its carbon must be
retained. As a matter of fact it is retained by the organism
but not in the form of carbon ; in all probability a double
decomposition takes place between the carbon dioxide ab-
sorbed and the water of organization, the result being the
liberation of oxygen in the form of gas and the simultaneous
production of some extremely simple form of carbohydrate,
i.e. some compound of carbon, hydrogen, and oxygen,
3o H^EMATOCOCCUS LESS.
with a comparatively small number of atoms to the
molecule.
The next step seems to be that the carbohydrate thus
formed unites with the ammonia salts or the nitrates absorbed
from the surrounding water, the result being the formation
of some comparatively simple nitrogenous compound, prob-
ably belonging to the class of amides, one of the best
known of which — asparagin — has the formula C.4H8N0O3.
Then further combinations take place, substances of greater
and greater complexity are produced, sulphur from the ab-
sorbed sulphates enters into combination, and proteids are
formed. From these, finally, fresh living protoplasm
arises.
From the foregoing .account, which only aims at giving
the very briefest outline of a subject as yet imperfectly un-
derstood, it will be seen that, as in Amoeba, the final result
of the nutritive process is the manufacture of protoplasm,
and that this result is attained by the formation of various
substances of increasing complexity or anastates (see p. 18).
But it must be noted that the steps in this process of con-
structive metabolism are widely different in the two cases.
In Amoeba we start with living protoplasm — that of the prey
— which is killed and broken up into diffusible proteids,
these being afterwards re-combined to form new molecules
of the living protoplasm of Amoeba. So that the food of
Amoeba is, to begin with, as complex as itself, and is first
broken down by digestion into simpler compounds, these
being afterwards re-combined into more complex ones. In
Haematococcus, on the other hand, we start with extremely
simple compounds, such as carbon dioxide, water, nitrates,
sulphates, &c. Nothing which can be properly called diges-
tion, i.e., a breaking up and dissolving of the food, takes
place, but its various constituents are combined into sub
II NUTRITION 31
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
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 bodies 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. 15), the food of Haematococcus is too
simple to serve as a source of energy, and it is only by the
help of sunlight that the work of constructive metabolism
32 II.EMATOCOCCUS LESS.
can be carried on. This may be expressed by saying that
Haematococcus, in common with other organisms contain-
ing chlorophyll, is supplied with kinetic energy (in the form
of light or radiant energy) directly by the sun.
As in Amoeba, destructive metabolism is constantly going
on, side by side with constructive. The protoplasm becomes
oxidized, water, carbon dioxide, and nitrogenous waste
matters being formed and finally got rid of. Obviously
then,- absorption of oxygen must take place, or in other
words, respiration must be one of the functions of the pro-
toplasm of Haematococcus as of that of Amoeba. In many
green, i.e., chlorophyll-containing, plants, this has been proved
to be the case ; respiration, i.e., the taking in of oxygen and
giving out of carbon dioxide, is constantly going on, but
during daylight is obscured by the converse process — the
taking in of carbon dioxide for nutritive purposes and the
giving out of the oxygen liberated by its decomposition. In
darkness, when this latter process is in abeyance, the
occurrence of respiration is more readily ascertained.
Owing to the constant decomposition, during sunlight, of
carbon dioxide, a larger volume of oxygen than of carbon
dioxide is evolved ; and if an analysis were made of all
the ingesta of the organism (carbon dioxide plus mineral
salts plus respiratory oxygen) they would be found to con-
tain less oxygen than the egesta (oxygen from decomposition
of carbon dioxide plus water, excreted carbon dioxide and
nitrogenous waste) ; so that the nutritive process in Haema-
tococcus is, as a whole, a process of deoxidat'ion. In
Amoeba, on the other hand, the ingesta (food plus respi-
ratory oxygen) contain more oxygen than the egesta (faeces
plus carbon dioxide, water, and nitrogenous excreta), the
nutritive process being therefore on the whole one of
oxidation. This difference is, speaking broadly, character-
ii CILIARY MOVEMENT 33
istic of plants and • animals generally ; animals, as a rule,
take in more free oxygen than they give out, while green
plants always give out more than they take in.
But destructive metabolism is manifested not only in the
formation of waste products, but in that of substances,
simpler than protoplasm, which remain an integral part of
the organism, viz., cellulose and starch. The cell-wall is
probably formed by the conversion of a thin superficial
layer of protoplasm into cellulose, the cyst attaining its final
thickness by frequent repetition of the process (see p. 14).
The starch of the pyrenoids is apparently formed by a similar
process of decomposition or destructive metabolism of pro-
toplasm, growth taking place, in both instances, by accretion
and not by intussusception.
We see then that destructive metabolism may result in the
formation of (a) waste products and (b) plastic products,
the former being got rid of as of no further use, while
the latter remain an integral part of the organism.
Let us now turn once more to the movements of Hsemato-
coccus, and consider in some detail the manner of their
performance.
Each flagellum (Fig. 3, A, fl] is a thread of protoplasm of
uniform diameter except at its distal or free end where it
tapers to a point. The lashing movements are brought
about by the flagellum bending successively in different
directions ; for instance, if in Fig. 3 F, abc represents it in
the position of rest, abc' will show the form assumed when
it is deflected to the left, and abc" when the bending is
towards the right. 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
D
34 H/EMATOCOCCUS LESS.
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.
Thus the ciliary movement of Haematococcus, like the
amoeboid movement of Amoeba, is a phenomenon of con-
tractility. Imagine an Amoeba to draw in all its pseudo-
pods but two, and to protrude these two until they became
mere threads; imagine further these threads to contract
regularly and rapidly instead of irregularly and slowly ; the
result would be the substitution of pseudopods by flagella,
i.e., of temporary slow-moving processes of protoplasm by
permanent rapidly-moving ones.
To put the matter in another way : in Amoeba the
function of contractility is performed by the whole organism ;
in Haematococcus it is discharged by a small part only, viz.,
the flagella, the rest of the protoplasm being incapable of
movement. We have therefore in Haematococcus a dif-
ferentiation of structure accompanied by a differentiation of
function or division of physiological labour.
The expression " division of physiological labour " was
invented by the .great French physiologist, Henri Milne-
Edwards, to express the fact that a sort of rough correspond-
ence exists between lowly and highly organized animals
and plants on the one hand, and lowly and highly organized
human societies on the other. In primitive communities
there is little or no division of labour : every man is his
own butcher, baker, soldier, doctor, &c., there is no distinc-
tion between "classes" and "masses," and each individual
is to a great extent independent of all the rest. Whereas in
ir DIMORPHISM 35
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."
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
Hcematococcus its movements would be absolutely stopped.
Haematococcus multiplies only in the resting condition
(p. 28, and Fig. 3, B) ; as in Amoeba its protoplasm undergoes
simple or binary fission, but with the peculiarity that the
process is immediately repeated, so that four daughter-cells
are produced within the single mother-cell-wall (Fig. 3 c).
By the rupture of the latter the daughter-cells are set
free in 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 or even more daughter-cells, and these when liberated
are found to be smaller than the ordinary motile form, and
to have no cell-wall. Hcematococcus is therefore dimorphic,
i.e., occurs, in the motile condition, under two distinct
forms : the larger or ordinary form with detached cell-wall
is called a megazooid, the smaller form without a cell-wall a
microzooid.
LESSON III
HETEROMITA
WHEN animal or vegetable matter is placed in water and
allowed to stand at the ordinary temperature, the well known
process called decomposition sooner or later sets in, the
water becoming turbid and acquiring a bad smell. A drop
of it examined under the microscope is then found to teem
with minute organisms. To one of these, called "the
Springing Monad," or in the language of zoology, Hetero-
mita rostrata^ we must now direct our attention ; it is
found in infusion of cod's head which has been allowed to
stand for two or three months.
Heteromita (Fig. 4, A) is considerably smaller than either
Amoeba or Haematococcus, being only T^y- mm. (^V).
(A— G, after Saville Kent : H, from Biitschli after Klebs. )
The body consists of protoplasm covered with a very
delicate skin or cuticle which is often finely striated, and
is to be looked upon as a superficial hardening of the
protoplasm. The green colour is due to the presence of
46 EUGLENA LESS.
chlorophyll, which tinges all the central part of the body,
the two ends being colourless. It is difficult to make out
whether the chlorophyll is lodged in one chromatophore or
in several.
In Haematococcus we saw that chlorophyll was asso-
ciated with starch (p. 27). In Euglena there are, near the
middle of the body, a number of grains of paramylum
(H, /), a carbohydrate of the same composition as starch
(C6H]0O5), but differing from it in remaining uncoloured
by iodine.
Water containing Euglena gives off bubbles of oxygen in
sunlight : as in Haematococcus 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 (&s) leading into the soft
internal protoplasm : — just the sort of depression one could
make in a clay model of Euglena by thrusting one's finger or
the end of a pencil into the clay. From the bottom of this
tube the flagellum arises, and by its continual movement
gives rise to a sort of whirlpool in the neighbourhood. By
the current thus produced minute solid food-particles are
swept down the tube and forced into the soft internal
protoplasm, where they doubtless become digested in the
same way as the substances ingested by an Amoeba. That
solid particles are so ingested by Euglena has been proved
by diffusing finely powdered carmine in the water, when the
iv MOUTH AND GULLET 47
coloured particles were seen to be swallowed in the way
described.
The depression in question is therefore a gullet, and its
external aperture or margin (;;/) is a mouth. Euglena,
like Amoeba, takes in solid food, but instead of ingesting ft
at almost any part of the body, it can do so only at one
particular point where there is a special ingestive aperture
or mouth. This is clearly a case of specialization or
differentiation of structure : in virtue of the possession of a
mouth and gullet Euglena is more highly organized than
Amoeba.
It thus appears that in Euglena nutrition is both holozoic
and holophytic : very probably it is mainly holophytic during
daylight and holozoic in darkness.
Near the centre of the body or somewhat towards the
posterior end is a well-marked globular nucleus (E, nu\ and
at the anterior end is a clear space (c. vac] looking very like
a contractile vacuole. It has been shown, however, that
this space is in reality a non-contractile "cavity or reservoir
(H, r) into which the true contractile vacuole (c. vac) opens,
and which itself discharges into the gullet.
In close relation with the reservoir is found a little bright
red speck (pg) called the pigment spot or stigma. It con-
sists of haematochrome (see p. 26) and is curiously like an
eye in appearance, so much so that it is often known as the
eye-spot. Recent experiments seem to show that it is
specially sensitive to light and is therefore a true eye in the
sense of a light-perceiving organ although having no actual
visual function.
As in Haematococcus a resting condition alternates with
the motile phase : the organism loses its flagellum and
48 EUGLENA LESS, iv
surrounds itself with a cyst of cellulose (Fig. 5, G, cy\ from
which, after a period of rest, it emerges to resume active
life.
Reproduction takes place by simple fission of the resting
form, the plane of division being always longitudinal (G).
Sometimes each product of division or daughter-cell divides
again : finally the two, or four, or sometimes even eight
daughter-cells emerge from the cyst as active Euglenae.
A process of multiple fission (p. 42) has also been de-
scribed, numerous minute active spores being produced
which gradually assume the ordinary form and size.
LESSON V
PROTOMYXA AND THE MYCETOZOA
WHEN Professor Haeckel was investigating the zoology of
the Canary Islands more than twenty years ago he discovered
a very remarkable organism which he named Protomyxa
aurantiaca. It was found in sea- water attached to a shell
called Spirula, and was at once noticeable from the bright
orange colour which suggested its specific name. Appar-
ently no one has since been fortunate enough to find it.
In its fully developed stage Protomyxa is the largest of all
the organisms we have yet studied, being fully i mm. (^5 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,
b/unt processes, few in number (comp. Fig. i, p. 2) are very
numerous, slender, branching threads which often unite with
one another so as to form networks. No nucleus was ob-
served 1 and no contractile vacuole, but it is quite possible
that a renewed examination might prove the presence of one
or both of these structures.
The figure (A) is enough to show that nutritio'n is holozoic;
1 See p. 9, note.
E
pud
FIG. 6. — Protoinyxa aurantiaca.
A, the living organism (plasmodium), showing fine branched pseudo-
pods (psd) and several ingested organisms.
B, the same, encysted ; cy the cell- wall.
c, the protoplasm of the encysted form breaking up into spores.
D, dehiscence of the cyst and emergence of
K, flagellulse which afterwards become converted into
F, amoebuloe.
G, amoebulse uniting to form a plasmodium (After Haeckel.)
LESS, v LIFE-HISTORY 51
the specimen has ingested several minute organisms and is
in the act of capturing another.
But the main interest of Protomyxa lies in its very curious
and complicated life -history. After crawling over the Spirula
shell for a longer or shorter time it draws in its pseudopods,
comes to rest, and surrounds itself with a cyst (B, cy). The
composition of the cyst is not known, but it is apparently not
cellulose, since it is not coloured by iodine and sulphuric
acid (p. 28).
Next, the encysted protoplasm undergoes multiple fission,
dividing into a number of spores (c). Soon the cyst bursts
and its contents emerge (D) as bodies which differ utterly in
appearance from the amoeboid form from which we started.
Each spore has in fact become a little ovoid body of an
orange colour, provided with a single flagellum (E, 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 flagellultz ; 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 A mceba but an
amoeboid phase in the life-history of a totally different
organism : it is called an amcebula.
The process just described may be taken as a practical
E 2
52 PROTOMYXA AND THE MYCETOZOA LESS.
proof of the statement made in a previous Lesson (p. 34)
that a flagellum is nothing more than a delicate and rela-
tively permanent pseudopod. In Protomyxa we have a
flagellula directly converted into an amcebula, the flagellum
of the former becoming one of the pseudopods of the
latter.
The amcebulae 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 ot a single amcebula
but by the complete fusion of a variable number of
amcebulae. A body formed in this way by the fusion of
amcebulae is called a pla smodiiim, 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
brief general consideration of the strange group of organisms
known as Mycetozoa or sometimes " slime-fungi," to which
Protomyxa itself very probably belongs. The best known
members of the group occur as gelatinous masses on the
bark of trees, on dead leaves, 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, Physarum, £c.
(see Fig. 7) : a general account of the class is all that is
necessary for our present purpose.
The Mycetozoa consists of sheets or networks of proto-
plasm which may be as much as 30 cm (i ft.) in diameter,
THE PL ASM ODIUM OF BADIIAMIA
53
C'
FIG. 7.- -A, part of the plasmoclium of Badh dmia (x 3^) ; b, a short
pseudopod enclosing a bit of mushroom stem.
B, spore of Physarmn.
C, the same, undergoing dehiscence.
D, flagellulre liberated from spores of the same.
E, amoebulre formed by metamorphosis of flagelluloe.
F, two amcebulse about to fuse : F', the same .after complete union.
G, G', two stages in the formation of a three-celled plasmodium.
H, a small plasmodium.
(A, after Lister : B— H, from Sachs after Cienkowski.)
54 PROTOMYXA AND THE MYCETOZOA LESS.
and throughout the substance of which are found numerous
nuclei. In this condition they creep about over bark or
some other substance : and in doing so ingest solid food
(Fig. 7, A). It has been proved that they digest protoplasm :
and in one genus pepsin — the constituent of our own gastric
juice by which the digestion of proteids is effected (see p. 12)
— has been found. They can also digest starch which has been
swollen by a moderate heat — as in our own bread and rice-
puddings — but are unable to make use of raw starch.
After living in this free condition, like a gigantic terrestrial
Amoeba, for a longer or shorter time, either a part or the
whole of the protoplasm becomes encysted l and breaks up
into spores. These (B) consist of a globular mass of proto-
plasm covered with a wall of cellulose : the cysts are also
formed of cellulose.
By the rupture of the cell-wall of the spore (c) the proto-
plasm is liberated as a flagellula (D) provided with a nucleus
and a contractile vacuole, and frequently exhibiting amoeboid
as well as ciliary movements. After a time the flagellulae
lose their cilia and pass into the condition of amoebulae (E),
which finally fuse to form the plasmodium with which
we started (F — H). In the young plasmodia (c1) the
nuclei of the constituent amcebulae 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 amoebulae to form the plasmodium of Mycetozoa the
cell-bodies (protoplasm) alone coalesce, not the nuclei.
There is a suggestive analogy between this process of
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), and Lister, Catalogue of the
Mycetozoa (London, 1894).
v PLASMODIUM-FORMATION AND CONJUGATION 55
plasmodi urn-formation and that of conjugation as seen in
Heteromita. Two Heteromitae fuse and form a zygote the
protoplasm of which divides into spores. In Protomyxa and
the Mycetozoa not two but several amoebulse unite to form
a plasmodium which after a time becomes encysted and
breaks up into spores. So that we might look upon the
conjugation of Heteromita as an extremely simple plasmo-
dial phase in its life-history, or upon the formation of a
plasmodium by Protomyxa and the Mycetozoa as a process
of multiple conjugation.
There is, however, an important difference between the
two cases by reason of which the analogy is far from complete.
In Heteromita the nuclei of the two gametes are no longer
visible (p. 41) : they coalesce during conjugation, and
the product of their union subsequently, in all probability,
breaks up to form the nuclei of the spores. In the Myce-
tozoa neither fusion nor apparent disappearance of the
nuclei of the amcebulae has been observed.
LESSON VI
A COMPARISON OF THE FOREGOING ORGANISMS WITH CER-
TAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS
AND PLANTS
WHEN a drop of the blood of a crayfish, lobster, or crab is
examined under a high power, it is found to consist of a
nearly colourless fluid, the plasma, in which float a number
of minute solid bodies, the blood-corpuscles or leucocytes,
Each ot these (Fig. 8, -A) is a colourless mass of proto-
plasm, reminding one at once of an Amoeba, and on
careful watching the resemblance becomes closer still, for
the corpuscle is seen to put out and withdraw pseudopods
(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, nu] : so that, save for the absence of a con-
tractile vacuole in the leucocyte, the description of Amoeba
in Lesson I. would apply almost equally well to it.
The blood of a fish, a frog (p,1), 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 (e5, BG)
FIG. 8. — Typical Animal and Vegetable Cells.
A1 — A4, living leucocyte (blood corpuscle) of a crayfish showing
amceboid movements : A;'', A6, the same, killed and stained, showing
the nucleus (nu).
B1, leucocyte of the frog, nu the nucleus ; B2, two leucocytes
beginning to undergo fusion : B3, the same after fusion, a binucleate
plasmodium being formed : B4, a leucocyte, undergoing binary fission :
B5, surface view and B", edge view of a red corpuscle of the same,
tin, 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, columnar epithelial cells from intestine of frog : D2, a similar
58 EPITHELIAL CELLS LESS.
cell showing striated distal border from which in n* 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 (nu), vacuoles
(vac], and cell- wall : F3, 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 : D'-, D3, after Wiedersheim : F1, after Sachs : F'3, after Behrens.)
coloured by a pigment called k&moglobin, and provided
each with a large nucleus (nu) which, when the corpuscle is
seen from the edge (BG), 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, B3)
(p- 5')-
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
vi PARENCHYMA CELLS 59
elongated Amoeba without pseudopods. In some animals
the resemblance is still closer : the epithelial cells have been
observed to throw out pseudopods from their free surfaces
(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 for the fact that they bear
numerous cilia in constant rhythmical movement instead of
two only — in this case distinguished as flagella — presenting
an irregular lashing movement.
Similar ciliated epithelial cells are found on the gills ot
oysters, mussels, &c., and in many other situations.
The stem or root of an ordinary herbaceous plant, such
as a geranium or sweet-pea, is found when cut across to
consist of a central mass of pith, around which is a circle
of woody substance, and around this again a soft greenish
material called the cortex. A thin section shows the latter
to be made up of innumerable polyhedral bodies called
60 PARENCHYMA CELLS LESS.
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 vacuoks (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 (nit) and is 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 chromaiophores 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 differences
being that in the plant-cell the form is polyhedral owing to
the pressure of neighbouring cells and that the chromato-
phores are relatively small and numerous. Similarly a
colourless parenchyma cell resembles an encysted Amoeba.
The pith, the epidermis or. thin skin which forms the
outer surface of herbaceous plants, the greater part of the
vi MINUTE STRUCTURE OE CELLS 61
leaves, and other portions of the plant may be shown to
consist of an aggregation of cells agreeing in essential
respects with the above description.
We come therefore to a very remarkable result. The
higher animals and plants are built up — in part at least — of
elements which resemble in their essential features the
minute and lowly organisms studied in previous lessons.
Those elements are called by the general name of cells :
hence the higher organisms, whether plants or animals, are
multicellular or are to be considered as cell aggregates,
while in the case of such beings as Amceba, Haematococ-
cus, Heteromita, or Euglena, the entire organism is a
single cell, or is unicellular.
Note further that the cells of the higher animals -and
plants, like entire unicellular organisms, may occur in either
the amoeboid (Fig. 8, A, b1, c1,) the ciliated (E), or the
encysted (F) condition, and that a plasmodial phase (B2) is
sometimes produced by the union of two or more amoeboid
cells.
One of the most characteristic features in the unicellular
organisms described in the preceding lessons is the con-
stancy of the occurrence of binary fission as a mode of
multiplication. The analogy between these organisms and
the cells of the higher animals and plants becomes still
closer when we find that in the latter also simple fission is
the normal mode of multiplication, the increase in size of
growing parts being brought about by the continual division
of their constituent cells.
The process of division in animal and vegetable cells
is frequently accompanied by certain very characteristic and
complicated changes in the nucleus to which we must now
62
MINUTE STRUCTURE OF CELLS
LESS.
direct our attention. First of all, however, it will be neces-
sary to describe the exact microscopic structure of cells and
their nuclei as far as it is known at present.
chp
nu.m
FIG. 9. — A, Cell from the genital ridge of a young salamander,
showing cell- membrane (c. ///), protoplasm or cell -body (c. b} with
astrosphere (s) and centrosome (f), and nucleus with membrane
(mi, m) and irregular network of chromatin (chr), B. Cell from the
immature stamen of a lily, showing cell-wall (c. w), protoplasm, with
nucleus as in A. (The astrospheres here figured are incorrect. — IV, N, P. ).
Both figures very highly magnified.
(A, from a drawing by J. E. S. Moore ; B, after Guignard. )
There seems to be a good deal of variation in the precise
structure of various animal and plant cells, but the more
recent researches show that in the cell-body or protoplasm
(Fig. 9, c. b) two constituents may be distinguished, a clear
semi-fluid substance, traversed by a delicate sponge-work.
Now under the microscope the whole cell is not seen at
once but only an optical section of it, that is all the
parts which are in focus at one time : by altering the
focus we view the object at successive depths, each view
being practically a slice parallel to the lenses of the
instrument. This being the case, protoplasm presents the
microscopic appearance of a clear or slightly granular
vi MINUTE STRUCTURE OF NUCLEI 63
matrix traversed by a delicate network. In the epithe-
lial cells of animals the protoplasm is bounded exter-
nally by a cell-membrane (Fig. 9, A, c. m) of extreme
tenuity, in plants by a cell-wall (B, c. w) of cellulose : in
amoeboid cells the ectosarc or transparent non-granular
portion of the cell consists of clear protoplasm only, the
granular endosarc alone possessing the sponge-work. • In
the majority of full-grown plant cells (Fig. 8, F) and in
some animal cells the protoplasm is more or less exten-
sively vacuolated, but in the young growing parts as well
as in the ordinary cells of animals the foregoing description
holds good. It is quite possible that the reticular character
of the protoplasm may be merely the optical expression of
an extensive but minute vacuolation, or may be due to the
presence of innumerable minute granules developed in the
protoplasm as products of metabolism.
The nucleus is usually spherical in form : it is enclosed
in a delicate nuclear membrane (nu.m) and contains, as in
Amceba (p. 7) two constituents, the nuclear sap and the
chromatin which exhibit far more striking differences than
the two constituents of the cell-body. The nuclear sap
is a homogeneous semi-fluid substance which forms the
ground-work of the nucleus : it resembles the clear cell-
protoplasm in its general characters, amongst other things
'in being unaffected by dyes. The chromatin (chr) takes the
form of a network or sponge-work of very variable form,
and is distinguished from all other constituents of the cell
by its strong affinity for aniline and other dyes. Frequently
one or more minute globular structures, the nucleoli (B, nu'\
occur in the nucleus either connected with the network or
lying freely in its meshes : they also have a strong affinity
for dyes although they often differ considerably from the
chromatin in their micro-chemical reactions.
FIG. 10. — Diagrams illustrating the process of indirect cell division
or mitosis.
A, the resting cell : the nucleus shows a nuclear membrane (nu. ,;/),
chromatin (chr) arranged in loops united into a network (the laiter
shown on the right side only), and two nucleoli (mi') : near the nucleus
is an astrosphere (j), containing a centrosome (c) and surrounded by
radiating protoplasmic filaments. ^
B, The chromatin has resolved itself into distinct loops or chromo-
somes (chr) which have divided longitudinally : the nuclear membrane
has begun to disappear : there are two astrospheres and between them
is seen the commencement of the nuclear spindle (sp).
C, The nuclear membrane has disappeared : the chromosomes are
LESS. VI
(T.LL-DI VISION
arranged irregularly : the spindle has increased in size and is situated
definitely within the nuclear area.
D, The chromosomes are arranged round the equator of the fully
formed nuclear spindle.
K, The daughter-loops of the chromosomes are passing in opposite
directions towards the poles of the spindle, each having a spindle-fibre
attached to it.
F, I ater stage of the same process.
0, The chromosomes are now arranged in two distinct groups, one at
each pole of the spindle.
H, The daughter-cells are partly separated by constriction and the
chromosomes of each group are uniting to form the network of the
daughter-nucleus.
1, Shows the division of a plant cell by the formation of a cell-plate
(c. />/) : the daughter nuclei are fully formed.
(Altered from Flemming, Rabl, &c. )
In the body of some cells and possibly of all there is
found a globular body, surrounded by a radiating arrange-
ment of the protoplasm and called the astrosphere (s) : it
lies close to the nucleus, and contains a minute granule
known as the central particle or centre/some (c]> In many
cells two astrospheres and two or more .centrosomes have
been found in each cell (B, s).
The precise changes which take place during the fission
of a cell are, like the structure of the cell itself, subject
-to considerable variation. We will consider what may
probably be taken as a typical case (Fig. 10).
First of all, the astrosphere, with its centrosome, divides (B)
and the products of its division gradually separate from one
another (c), ultimately passing to opposite poles of the nucleus
(D). At the same time the network 'of chromatin divides
into a number of separate filaments called chromosomes (B, chr\
the number of which appears to be constant in any given
species of animal or plant, although it may vary in different
species from 2 to 168 or more. Soon after this the nuclear
membrane and the free nucleoli disappear (B, c) and the
F
66 MINUTE STRUCTURE OF CELLS LESS.
nucleus is seen to contain a spindle shaped body (sp) formed
of excessively delicate fibres which converge at each pole
to the corresponding astrosphere. The precise origin of
this nuclear spindle is uncertain : it may arise either
from the nuclear matrix or, more probably, from the
protoplasm of the cell : it is not affected by colouring
matters.
At the same time each chromosome splits along its whole
length so as to form two parallel rods or loops in close
contact with one another (B) : in this way the number of
chromosomes is doubled, each one being now represented
by a couple.
The divided chromosomes now pass to the equator of the
spindle (D) and assume the form of more or less V-shaped
loops, which arrange themselves in a radiating manner so as
to present a star-like figure when the cell is viewed in the
direction of the long axis of the spindle. Everything is now
ready for division to which all the foregoing processes are
preparatory.
The two chromosomes of each couple now gradually pass
to opposite poles of the spindle (E, F), two distinct groups
being thus produced (G) and each chromosome of each
group being the twin of one in the other group. Perhaps
the fibres of the spindle are the active agents in this
process, the chromosomes being dragged in opposite
directions by their contraction : on the other hand it is
possible that the movement is due to the contractility of the
chromosomes themselves.
After reaching the poles of the spindle the chromosomes
of each group unite with one another to form a network (H)
around which a nuclear membrane finally makes its appear-
ance (i). In this way two nuclei are produced within a
single cell, the chromosomes of the daughter-nuclei, as well
vi CELL-DIVISION 67
as their attendant astrospheres, being formed by the binary
fission of those of the mother-nucleus.
But pari passu with the process of nuclear division,
fission of the cell-body is also going on. This may take
place by a simple process of constriction (H)— in much the
same way as a lump of clay or dough would divide if a loop
of string were tied round its middle and then tightened — or
by the formation of what is known as a cell-plate. This
arises as a row of granules formed from the equatorial part
of the nuclear spindle (i) : the granules extend until they
form a complete equatorial plate divi'ding the cell-body into
two halves : fission then takes place by the cell-plate split-
ting into two along a plane parallel with its flat surfaces.1
In plants the cell-plate gives rise to a partition wall of
cellulose which divides the two daughter-cells from one
another.
In some cases the dividing nucleus, instead of going
through the complicated processes just described, divides
by simple constriction. We have therefore to distinguish
between direct and indirect nuclear division. To the latter
very elaborate method the name mitosis or karyokinesis is
applied : direct division is then distinguished as amitotic.
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
1 It must not be forgotten that the cells, which are necessarily reprt.
sented in such diagrams as Fig. 10 as planes, are really solid bodies,
and that consequently the cell-plate represented in the figures as a line
is actually a plane at right angles to the plane of the paper.
F 2
68 COMPLEXITY OF CELL STRUCTURE LESS.
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
now advert.
All the higher animals begin life as an egg, which is either
passed out of the body of the parent as such, as in most
fishes, frogs, birds, &c., or undergoes the first stages of its
development within the body of the parent, as in sharks,
some reptiles, and nearly all mammals.
The structure of the egg is, in essential respects, the same
in all animals from the highest to the lowest. In a jelly-fish,
for instance, it consists (Fig. n, A) of a globular mass of
protoplasm (gd), in which are deposited granules of a pro-
teinaceous substance known as yolk-spherules. Within the
protoplasm is a large clear nucleus (g.v) the chromatin of
which is aggregated into a central mass or nucleolus (g.w).
An investing membrane may or may not be present. In
other words the egg is a cell : it is convenient, for reasons
which will appear immediately, to speak of it as the ovum
or egg-cell.
The young or immature ova 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
vi STRUCTURE OF THE EGG 69
into a new individual. For instance, the protoplasm may
throw out pseudopods, the egg becoming amoeboid (see
Fig. 52) ; or the surface of the protoplasm may secrete a thick
cell-wall (see Fig. 61). The most extraordinary modification
takes place in some Vertebrata, such as birds. In a hen's
egg, for instance, the yolk-spherules increase immensely,
swelling out the microscopic ovum until it becomes what we
know as the " yolk " of the egg : around this layers of
albumen or " white " are deposited, and finally the shell
membrane and the shell. Hence we have to distinguish
carefully in eggs of this character between the entire " egg "
in the ordinary acceptation of the term, and the ovum or
egg-cell
But complexities of this sort do not alter the fundamental
FIG. II. — A, ovum of an animal (Carwarina Iiastata, one of the
elly fishes), showing protoplasm (gif), nucleus (gv], and nucleolus (£"<)•
B, ovum of a plant (Gytnnadei/ia conopsea, one of the orchids), showing
protoplasm (/>/sw), nucleus (////), and nucleolus (mi).
(A, from Balfour after Haeckel : B, after Marshall Ward.)
fact that all the higher animals begin life as a single cell, or
f in other words that multicellular animals, however large and
i 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 ac-
curately megasporangia — see Lesson XXXIV., and Fig. 127),
which, when the flower withers, develop into the seeds. A
70 THE PLANT OVUM LESS, vi
section of an ovule shows it to contain a large cavity, the
embryo-sac or megaspore (see Fig. 126, D), at one end of
which is a microscopic cell (ov, and Fig. n B), consisting as
usual of protoplasm (plsm], nucleus (nu). 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 stages of existence, microscopic and
unicellular.
LESSON VII
SACCHAROMYCES
EVERY one is familiar with the appearance of the ordinary
brewer's yeast — the light-brown, muddy, frothing substance
which is formed on the surface of the fermenting vats in
breweries and is used in the manufacture of bread to make
the dough "rise."
Examined under the microscope yeast is seen to consist
of a fluid in which are suspended immense numbers of
minute particles, the presence of which produces the mud-
diness of the yeast. Each of these bodies is a unicellular
organism, the yeast-plant, or, in botanical language, Sac-
char omyces cerevisia.
Saccharomyces consists of a globular or ellipsoidal mass
of protoplasm (Fig. 12), 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 in number
and size according to the state of nutrition of the cell.
Granules also occur in the protoplasm, some of them being
of a proteid material, others fat globules. Under ordinary
circumstances no nucleus is to be seen : but by the em-
ployment of a special mode of staining, a small rounded
72 SACCIIAKOMYCKS LESS.
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
FIG. 12. — SaccharoHiyccs cerevis itc .
A, a group of cells under a moderately high power. The scale to the
left applies to this figure only.
H, several cells more highly magnified, showing various stages of
budding, vac, the vacuole.
c, a single cell with tw,> buds (M, ().(') still more highly magnified :
c. ic, cell-wall : vac, vacuole.
D, cells, crushed by pressure: c. n>, the ruptured cell-walls: f>kw,
the squeezed out protoplasm.
E, E', starved cells, showing large vacuoles and fat globules (/).
F, F', formation of spores by fission of the protoplasm of a starved
cell: in F the spores are still enclosed in the mother-cell-wall, in F'
they are free.
magenta, and then applying pressure to the cover glass so as
to crush the cells. Under this treatment the cell walls are
burst and appear as crumpled sacs, split in various ways and
unstained by the magenta (D, c.w\ while the squeezed-out
protoplasm is seen in the form of irregular masses (p/sm)
stained pink by the dye.
vii . GEMMATION 73
The mode of multiplication of Saccharomyces is readily
made out in actively fermenting yeast, and is seen to differ
from anything we have met with hitherto. A small pimple-
like elevation (c, bd) appears on the surface of a cell and
gradually increases in size : examined under a high power
this bud is found to consist of an offshoot of the protoplasm
of the parent cell covered with a very thin layer of cellulose :
it is formed by the protoplasm growing out into an offshoot
—like a small pseudopod — which pushes the cell-wall before
it. While this is going on the nucleus passes to the surface
of the cell and divides, one of the products of fission remaining
in the mother-cell, the other in the bud. 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 plain of junction, the protoplasm of the bud
or daughter cell becoming separated from that of the mother-
cell and a cellulose^ partition being secreted between the
two. Finally the bud becomes completely detached as a
separate yeast-cell.
It frequently happens that a Saccharomyces buds in
several places and each of its daughter cells buds again,
before detachment of the buds takes place. In this way
chains or groups of cells are produced (B), such cell-
colonies consisting of. two or more generations of cells, the
central one standing in relation of parent, grandparent, or
great-grandparent to the others.
It must be observed that this process of budding or
gemination is after all only a modification of simple
fission. In the latter the two daughter-cells are of equal size
and are both smaller than the parent-cell, while in gemma-
tion one — the mother-cell — is much larger than the other—
the daughter-cell or bud — and is of the same size as, indeed is
practically identical with, the original dividing-cell. Hence
74 SACCHAROMYCES LESS.
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
of its substance in the form of a bud, which by assimilation
of nutriment gradually grows to the size of its parent,
the latter thus retaining its individuality and continuing to
produce fresh buds as long as it lives.
Multiplication by budding goes on only while the Sac-
charomyces is well supplied with food : if the supply of
nutriment fails, a different mode of reproduction obtains.
Yeast can be effectually starved by spreading out a thin
layer of it on a slab of plaster-of-Paris kept moist under
a bell-jar : under these circumstances the yeast is of course
supplied with nothing but water.
In a few days the yeast-cells thus circumstanced are found
to have altered in appearance : large vacuoles appear in
them (Fig. 1 2, E, E') and numerous fat-globules (/) are formed.
The protoplasm has been undergoing destructive meta-
bolism, and, there being nothing to supply new material, has
diminished in quantity, and at the same time been partly
converted into fat. Both in plants and in animals it is found
that fatty degeneration, or the conversion of protoplasm
into fat by destructive metabolism, is a constant phenomenon
of starvation.
After a time the protoplasm collects towards the centre 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
vii ALCOHOLIC FERMENTATION 75
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-
duction by multiple fission appears to be, in the yeast-plant,
a last effort of the organism to withstand extinction.
The physiology of nutrition of Saccharomyces has been
studied with great care by several men of science and
notably by Pasteur, and is in consequence better 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 the frothing is going on a lighted
candle is lowered into the vat the flame will be immediately
extinguished : if an animal were placed in the same position
it would be suffocated.
Chemical examination shows that the extinction of the
candle's flame or of the animal's life is caused by a rapid
evolution of carbon dioxide from the fermenting wort, the
frothing being due to the escape of the gas from the liquid.
After a time the evolution of gas ceases, and the liquid
76 SACCIIAKOMVCES LESS.
is then found to be no longer sweet but to have acquired'
what we know as an alcoholic or spirituous flavour. Analysis
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 : —
C0H1206 = 2(C2HG0) + 2(CO,)
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 Saccharcmyces, 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 83-76 per cent.
Cane sugar, CjgH^Ojj ....'.. 15-00 „ „
Ammonium tartrate (NH4).2C4H4O0 . rco ,, „
Potassium phosphate, K3PO4 .... 0-20 ,, ,,
Calcium phosphate, Ca3(PO4).> . . . 0-02 ,, „
Magnesium sulphate, MgSO4 .... 0*02 ,, ,,
lOO'OO
vii EXPERIMENTS IN NUTRITION 77
The composition of this fluid is not a matter of guess-
work, but is the result of careful experiments, and is deter-
mined by the following considerations.
It is obvious that if we are to study alcoholic fermentation
sugar must be present,1 since the essence of the process is
the formation of alcohol from sugar.
Then nitrogen in some form as well as carbon, oxygen,
and hydrogen must be present, since these four elements
enter into the composition of protoplasm, and all but the
fust-named (nitrogen) into that of cellulose, and they are
thus required in order that the yeast should live and
multiply. The form in which nitrogen can best be assimi-
lated was found out by experiment. We saw that in the
manufacture of beer the yeast cells obtain their nitrogen
largely in the form of soluble proteids : green plants obtain
theirs largely in the simple form of nitrates. It was found
that while proteids are, so to say, an unnecessarily complex
food for Saccharomyces, nitrates are not complex enough,
and an ammonia compound is necessary, ammonium tartrate
being the most suitable. Thus while Saccharomyces can
build up the molecule of protoplasm from less complex food-
stuffs than are required by Amoeba, it cannot make use of
such comparatively simple compounds as suffice for Hnema-
tococcus : moreover it appears to be indifferent whether its
nitrogen is supplied to it in the form of ammonium tartrate
or in the higher form of proteids.
Then as to the remaining ingredients of the fluid —
potassium and calcium phosphate and magnesium sulphate.
If a quantity of yeast is burnt, precisely the same thing
happens as when one of the higher animals or plants is
subjected to the same process. It first chars by the libera-
1 It is a matter of indifference whether cane-sugar or grape-sugar
is used.
78 SACCHAROMYCES LESS.
tion of carbon, then as the heat is continued the carbon
is completely consumed, going off by combination with the
oxygen of the air in the form of carbon dioxide ; at the
same time the nitrogen is given off mostly as nitrogen gas,
the hydrogen by union with atmospheric oxygen as water-
vapour, and the sulphur as sulphurous acid or sulphur
dioxide (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 Pastern'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, an element without which the
molecules of protoplasm cannot be built up.
vii EXPERIMENTS IN NUTRITION 79
It is somewhat curious to find that potassium and calcium
phosphates are equally necessary ; although occurring in
such minute quantities they are absolutely essential to the
well-being of the yeast cells, and without them the organism,
although supplied with abundance of sugar and ammonium
tartrate, will not live. This may be taken as proving that
phosphorus, calcium, and magnesium form an integral part
of the protoplasm of Saccharomyces, although existing in
almost infinitesimal proportions.
Lastly, magnesium sulphate must not be omitted if the
organism is to flourish : unlike the other two mineral
constituents it is not absolutely essential to life, but without
it the vital processes are sluggish.
Thus by growing yeast in a fluid of known composition
it can be ascertained exactly what elements and combina-
tions of elements are necessary to life, what advantageous
though not absolutely essential, and what unnecessary.
The precise effect of the growth and multiplication of
yeast upon a saccharine fluid, or in other words the nature
of alcoholic fermentation, can be readily ascertained by a
simple experiment with Pasteur's solution. ,A quantity of
the solution with a little yeast is placed in a flask the neck
of which is fitted with a bent tube leading into a vessel of
lime-water or solution of calcium oxide. When the usual
disengagement of carbon dioxide (see p. 75) takes place., the
gas passes through the tube into the lime-water and causes
an immediate precipitation of calcium carbonate as a white
powder which effervesces with acids. This proves the gas
evolved during fermentation to be carbon dioxide since no
other converts lime into carbonate. When fermentation is
complete the presence of alcohol may be proved by dis-
tillation : a colourless, mobile, pungent, and inflammable
liquid being obtained.
So SACCIIAROMVCKS
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 temperatures 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
decomposition 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 organised ferment : when growing in a sac-
charine solution it not only performs the ordinary metabolic
processes necessary for its own existence, but induces
decomposition of the sugar present, this decomposition
being unaccompanied 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 without
themselves undergoing any change : these are distinguished
as unorganised 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 the Mycetozoa
vii FERMENTS 81
(p. 54), and probably it or some similar peptonizing or
proteolytic ferment effects this change in all organisms
which have the power of digesting proteids. Another
instance is furnished by diastase, which effects the con-
version of starch into grape sugar : it is present in ger-
minating barley (see p. 75), 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), are able to digest
cooked starch.1
1 It has long been suspected that the so-called organised ferments
brought about their characteristic changes through the operation of
unorganised ferments. In 1897 Buchner obtained such an unorganised
ferment, now known as zymase, by vigorous triturition of yeast-cells,
under high pressure. — W.N.P.
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 01 cold — they " go
bad " or putrefy ; i.e. they acquire an offensive smell, a taste
which few are willing to ascertain by direct experiment, and
often a greatly altered appearance.
One of the most convenient substances for studying the
phenomena of putrefaction is an infusion of hay, made by
pouring hot water on a handful of hay and straining the
resultant brown fluid through blotting paper. Pasteur's
solution may also be used, or mutton-broth well boiled
and filtered, or indeed almost any vegetable or animal
infusion.
If some such fluid is placed in a glass vessel, covered with
a sheet of glass or paper to prevent the access of dust, the
naked-eye appearances of putrefaction will be found to
manifest themselves with great regularity. The fluid, at first
quite clear and limpid, becomes gradually dull and turbid.
The opacity increases and a scum forms on the surface :
at the same time the odour of putrefaction arises, and
LESS, vrii BACTERIUM TERMO 83
especially in the case of animal infusions, quickly becomes
very strong and disagreeable.
The scum after attaining a perceptible thickness breaks up
and falls to the bottom, and after this the fluid slowly clears
again, becoming once more quite transparent and losing its
bad smell. If exposed to the light, patches of green appear
in it sooner or later, due to the presence of microscopic
organisms containing chlorophyll. The fluid has acquired,
in fact, the characteristics of an ordinary stagnant pond, and
is quite incapable of further putrefaction. The whole series
of changes may occupy many months.
Microscopic examination shows that the freshly-prepared
fluid is free from organisms, and indeed, if properly filtered,
I
/
FIG. 13. — Bacterium termo. A, motile stage : B, resting stage, or
zooglcca. (From Klein.)
from particles of any sort. But the case is very different
when a drop of infusion in which turbidity has set in is
placed under a high power. The fluid is then seen to be
crowded with incalculable millions of minute specks, only
just visible under a power of 300 or 400 diameters, and all
in active movement. These specks are Bacteria, or as
they are sometimes called, microbes or micro-organisms;
they belong to the particular genus and species called
Bacterium termo.
Seen under the high power of an ordinary student's
microscope Bacterium termo has the appearance shown in
Fig. 13, A : it is like a minute finger-biscuit, i.e. has the form
G 2
84 BACTERIA LESS.
2
of a rod constricted in the middle. But it is only by using
the very highest powers of the microscope that its precise
form and structure can be satisfactorily made out. It is then
seen (Fig. 14) to consist of a little double spindle, staining
very deeply with aniline dyes. By the employment of very
high powers it has been shown that the protoplasm of the
cell contains a nucleus and is covered with a membrane of
extreme tenuity formed either of cellulose or of a proteid
material. According to Dallinger, at each end is attached
a flagellum about as long as the cell itself.
Bacterium termo is much smaller that 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
FIG 14. — Bacterittm termo ( x 4000), showing the terminal flngella.
(After Uallinger.)
size is best expressed by taking as a standard the one-
thousandth of a millimetre, called a micromillimetre and
expressed by . the symbol /x,. The entire length of the
organism under consideration is from 1-5 to 2 /x, i.e. about
the -g^y- mm. or the To J^o 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 \ /x,
or aW^nny mcni 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.
14 shows a Bacterium termo magnified 4000 diameters, the
vin BACILLUS 85
above the figure representing yj^ mm. magnified to the
same amount. The height of this book is a little over 1 8 cm. ;
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. 14 is to that of
Bacterium termo.
It was mentioned above that at a certain stage of putre-
faction a scum forms on the surface of the fluid. This film
consists of innumerable motionless Bacteria imbedded
in a transparent gelatinous substance formed of a proteid
material (Fig. 13, B). After continuing in the active con-
dition for a time the Bacteria rise to the surface, lose their
flagella, and throw out this gelatinous substance in which
they lie imbedded. The bacterial jelly thus formed is called
a zooglcea. Thus in Bacterium termo, as in so many of the
organisms we have studied, there is an alternation of an
active with a resting condition.
During the earlier stages of putrefaction Bacterium termo
is usually the only organism found in the fluid, but later on
other microbes make their appearance. Of these the com-
monest are distinguished by the generic names Micrococcus,
Bacillus, Vibrio, and Spirillum.
Micrococcus (Fig. 15) is a minute form, the cells of which
are about 2/x (-^-^ 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. 15, 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.
86 BACTERIA LESS.
until Bacillus becomes the dominant form. Its cells (Fig.
1 6) are rod-shaped and about 6/x (T4, Z. affine. 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. 27, B) is also dimorphic and presents a
third mode of branching. The main stem divides into two,
and each of the secondary branches does the same, so that
a repeatedly forking stem is produced. The branching of
this species is said to be dichotomous, while that of Z. alter-
nans is monopodial, and that of Z. arbuscula umbellate.
Another mode of aggregation of the zooids is found in Z.
simplex (Fig. 27, c), in which the stem is unbranched and
xin GENUS AND SPECIES 139
bears at its distal end about six zooids in a cluster. The
zooids are more elongated than in any of the preceding
species, and there are no special reproductive individuals, so
that the colony is homomorphic.
In Z. affine (Fig. 27, D) the stalk is dichotomous but is
proportionally thicker than in the preceding species, and
bears about four zooids, all alike. It is found in fresh water
attached to insects and other aquatic animals.
The last species we shall consider is Z. nutans (Fig. 27, E),
which is the simplest known, never bearing more than two
zooids, and sometimes only one.
A glance at Figs. 26 and 27 will show that these six species
agree with one another in the general form of the zooids, in
the characters of the nucleus, contractile vacuole, &c., in
the arrangement of the cilia, and in the fact that they are all
compound organisms, consisting of two or more zooids
attached to a common stem, the axial fibre of which branches
with it, i.e., is continuous throughout the colony.
On account of their possessing these important characters
in common, the species described are placed in the single
genus Zoothamnium, and the characters summarized in the
preceding paragraph are called generic characters. On the
other hand the points of difference between the various
species, such as the forking of the stem in Z. dichotomum,
the presence of only two zooids in Z. nutans, and so on, are
called specific characters. Similarly the name Zoothamnium,
which is common to all the species, is the generic name,
while those which are applied only to a particular species,
such as arbuscula, simplex, &c., are the specific names. As
was mentioned in the first lesson (p. 8), this method of
naming organisms is known as the Linnean system of
binomial nomenclature.
It will be seen from the foregoing account that by a
I4o SPECIES AND THEIR ORIGIN LESS.
species we understand an assemblage of individual or-
ganisms, whether simple or compound, which agree with one
another in all but unessential points, such as the precise
number of zooids in Zoothamnium, which may vary con-
siderably in the same species, and come, therefore, within
the limits of individual variation. Similarly, what we mean
by a genus is a group of species agreeing with one another
in the broad features of their organization, but differing in
detail, the differences being constant.
A comparison of the six species described brings out
several interesting relations between them. For instance, it
is clear that Z. arbuscula and Z. alternans are far more
complex, f.e.t exhibit greater differentiation of the entire
colony, than Z. simplex, or Z. nutans ; so that, within the
limits of the one genus, we have comparatively low or
generalized, and comparatively high or specialized species.
Nevertheless, a little consideration will show that we cannot
arrange the species in a single series, beginning with the
lowest and ending with the highest, for, although we should
have no hesitation in placing Z. nutans at the bottom of
such a list, it would be impossible to say whether Z. affine
was higher or lower than Z. simplex, or Z. arbuscula than
Z. alternans.
It is, however, easy to arrange the species into groups
according to some definite system. For instance, if we take
the mode of branching as a criterion, Z. nutans, affine, and
dichotomum will all be placed together as being dichoto-
mous, and Z. simplex and arbuscula as being umbellate —
the zooids of the one and the branches of the other all
springing together from the top of the main stem : on this
system Z. alternans will stand alone on account of its mono-
podial branching. Or, we may make two groups, one of
dimorphic forms, including Z. arbuscula, alternans, and
xni CREATION AND EVOLUTION 141
dichotomum, and another of homomorphic species, including
Z. affine, simplex, and nutans. We have thus two very
obvious ways of arranging or classifying the species of
Zoothamnium, and the question arises— which of these, if
either, is the right one ? Is there any standard by which
we can judge of the accuracy of a given classification of
these or any other organisms, or does the whole thing depend
upon the fancy of the classifier, like the arrangement of
books in a library ? In other words, are all possible classi-
fications of living things more or less artificial, or is there
such a thing as a natural classification ?
Suppose we were to try and classify all the members of a
given family — parents and grandparents, uncles and aunts,
cousins, second cousins, and so on. Obviously there are a
hundred ways in which it would be possible to arrange
them — into dark and fair, tall and short, curly-haired and
straight haired and so on. But is 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 by a natural process of descent from a single indi-
142
SPECIES AND THEIR ORIGIN
LESS.
vidual, or from a pair of individuals — in each case precisely
resembling, in all essential respects, their existing descend-
ants— which came into existence by a process outside the
ordinary course of nature and known as Creation. On this
hypothesis the history of the genus Zoothamnium would be
represented by the diagram (Fig. 28) ; each of the species
being derived from a single individual which came into
Existing Individuals
Z.arbuscula Z.alternans Z.dichotomum Z. simplex
Z.affine
Z.nutans
Ancestral Individuals
FIG. 28. — Diagram illustrating the origin of the species of
Zoothamnium by creation.
existence, independently of the progenitors of all the other
species," at some distant period of the earth's history.
Notice that on this theory the various species are no more
actually related to one another than is either of them to
Vorticella, or for the matter of that to Homo. The in-
dividuals of any one species are truly related since they all
share a common descent, but there is no more relationship
between the individuals of any two independently created
species than between any two independently manufactured
xm EVOLUTION 143
chairs or tables. The words affinity, relationship, £c., as
applied to different species are, on the theory of creation,
purely metaphorical, and mean nothing more than that a
certain likeness or community of structure exists; just as
we might say that an easy chair was more nearly related to a
kitchen chair than either of them to a three-legged stool.
\Ve see therefore that on the hypothesis of creation the
varying degrees of likeness and unlikeness between the
species receive no explanation, and that we get no absolute
criterion of classification : we may arrange our organisms,
as nearly as our knowledge allows, according to their resem-
blances and differences, but the relative importance of the
characters relied on becomes a purely subjective matter.
According to the rival theory — that of Descent or Organic
Evolution — every species existing at the present day is
derived by a natural process of descent from some other
species which lived at a former period of the world's
history. If we could trace back from generation to gener-
ation the individuals of any existing species we should, on
this hypothesis, find their characters gradually change, until
finally a period was reached at which the differences were so
considerable as to necessitate the placing of the ancestral
forms in a different species from their descendants at the
present day. And in the same way if we could trace back
the species of any one genus, we should find them gradually
approach one another in structure until they finally con-
verged in a single species, differing from those now existing
but standing to all in a true parental relation.
Let us illustrate this by reference to Zoothamnium. As a
matter of fact we know nothing of the history of the genus, but
the comprehension of what is meant by the evolution of species
will be greatly facilitated by framing a working hypothesis.
Suppose that at some distant period of the world's history
SPECIES AND THEIR ORIGIN
LSSS.
there existed a Vorticella-like organism which we will call
A (Fig. 29), having the general characters of a single,
stalked zooid of Zoothamnium (compare Fig. 26, F2), and
suppose that, of the numerous descendants of this form,
represented by the lines diverging from A, there were some
in which both the zooids formed by the longitudinal division
of the body remained attached to the stalk instead of one of
them swimming off as in Vorticella. The result — it matters
Branching ctichotomous
DIMORPHIC
HOMOMORPHIC
^
FIG. 29. — Diagram illustrating the origin of the species of
Zoothamnium by evolution.
not for our present purpose how it may have been caused —
would be a simple colonial organism consisting of two zooids
attached to the end of a single undivided stalk. Let us call
this form B.
Next let us imagine that in some of the descendants of B,
represented as before by the diverging lines, the plane of
division was continued downwards so as to include the
distal end of the stalk : this would result in the production
xiii DIVERGENCE OF CHARACTER 145
of a form (c) consisting of two zooids borne on a forked
stem and resembling Z. nutans. If, in some of the descend-
ants of c, this process were repeated, each of the two zooids
again dividing into two fixed individuals and the division
as before affecting the stem, we should get a species (D) con-
sisting of four zooids on a dichotomous stem, like Z. affine.
Let the same process continue from generation to genera-
tion, the colony becoming more and more complex; we
should finally arrive at a species E, consisting of numerous
zooids on a complicated dichotomously branching stem,
and therefore resembling Z. dichotomum.
Let us further suppose that, in some of the descendants
of our hypothetical form B, repeated binary fission took
place without affecting the stem : the result would be a new
form F, consisting of numerous zooids springing in a cluster
from the end of the undivided stem, after the manner of
Z. simplex. From this a more complicated umbellate form
(G), like Z. arbuscula, may be supposed to have originated,
and again starting from B with a different mode of branch-
ing a monopodial form (H) might have arisen.
Finally, let it be assumed that while some of the descend-
ants of the forms c, D, and F became modified into more
and more complex species, others survived to the present
time with comparatively little change, forming the existing
species nutans, affine, and simplex : and that, in the similarly
surviving representatives of E, G, and H, a differentiation of
the individual zooids took place resulting in the evolution of
the dimorphic species dichotomum, arbuscula, and alternans.
It will be seen that, on this hypothesis, the relative like-
ness and unlikeness of the species of Zoothamnium are
explained as the result of their descent with greater or less
modification or divergence of character from the ancestral
form A : and that we get an arrangement or classification
L
146 SPECIES AND THEIR ORIGIN LESS.
in the form of a genealogical tree, which, on the hypothesis,
is a strictly natural one, since it shows accurately the
relationship of the various species to one another and to
the parent stock. So that, on the theory of evolution, a
natural classification of any given group of allied organisms
is simply a genealogical tree, or, as it is usually called, a
phytogeny.
It must not be forgotten that the forms A, B, c, D, E, F, G,
and H are purely hypothetical : their existence has been
assumed in order to illustrate the doctrine of descent by a
concrete example. The only way in which we could be
perfectly sure of an absolutely natural classification of the
species of Zoothamnium would be by obtaining specimens
as far back as the distant period when the genus first came
into existence ; and this is out of the question, since minute
soft-bodied organisms like these have no chance of being
preserved in the fossil state.
It will be seen that the theory of evolution has the
advantage over that of creation of offering a reasonable
explanation of certain facts. First of all the varying degrees
of likeness and unlikeness of the species are explained by
their having branched off from one another at various
periods : for instance, the greater similarity of structure
between Z. affine and Z. dichotomum than between either of
iKeTh and any other species is due to these two species
having a common ancestor in D, whereas to connect either
of them, say with Z. arbuscula, we have to go back to B.
Then again the fact that all the species, however complex in
their fully developed state, begin life as a simple zooid which
by repeated branching gradually attains the adult complexity,
is a result of the repetition by each organism, in the course
of its single life, of the series of changes passed through by
its ancestors- in the course of ages. In other words ontogeny,
xni HEREDITY AND VARIABILITY 147
or the evolution of the individual, is, in its main features, a
recapitulation si phytogeny or the evolution of the race.
One other matter must be referred to in concluding the
present lesson. It is obvious that the evolution of one
species from another presupposes the occurrence of varia-
tions in the ancestral form. As a matter of fact such
individual variation is of universal occurrence : it is a matter
of common observation that no two leaves, shells, or human
beings are precisely alike, and in our type genus Zootham-
nium the number of zooids, their precise arrangement, the
details of branching, &c., are all variables. This may be
expressed by saying that heredity, according to which the
offspring tends to resemble the parent in essentials, is
modified by variability, according to which the offspring
tends to differ from the parent in details. If from any
cause an individual variation is perpetuated there is produced
what is known as a variety of the species, and, according to
the theory of the origin of species by evolution, such a
variety may in course of time become a new species. Thus
a variety is an incipient species, and a species is a (relatively)
permanent variety.
It does not come within the scope of the present work to
discuss either the causes of variability or those which deter-
mine the elevation of a variety to the rank of a species :
both questions are far too complex to be adequately treated
except at considerable length, and anything of the nature of
a brief abstract could only be misleading. As a preliminary
to the study of Darwin's Origin of Species, the student is
recommended to read Romanes's Evidences of Organic
Evolution, in which the doctrine of Descent is expounded
as briefly as is consistent with clearness and accuracy.
L 2
LESSON XIV
• FORAMINIFERA, RADIOLARIA, AND DIATOMS
IN the four previous lessons we have learnt how a uni-
cellular organism may attain very considerable complexity
by a process of differentiation of its protoplasm. In the
present lesson we shall consider briefly certain forms of life
in which, while the protoplasm of the unicellular body un-
dergoes comparatively little differentiation, an extraordinary
variety and complexity of form is produced by the develop-
ment of a skeleton, either in the shape of a hardened cell-
wall or by the formation of hard parts within the protoplasm
itself.
The name Foraminifera is given to an extensive group of
organisms which are very common in the sea, some living
near the surface, others at various depths. They vary in
size from a sand-grain to a shilling. They consist of variously
shaped masses of protoplasm, containing nuclei, and pro-
duced into numerous pseudopods, which are extremely long
and delicate, and frequently unite with one another to form
networks, as at X in Fig. 30. The cell-body of these
organisms is therefore very simple, and may be compared
to that of a multinucleate Amoeba with fine radiating
pseudopods.
LESS. XIV
THE SHELL
149
But what gives the Foraminifera their special character is
the fact that around the protoplasm is developed a cell-wall,
sometimes membranous, but usually impregnated with cal-
cium carbonate, and so forming a shell. In some cases, as
in the genus Rotalia (Fig. 30), this is perforated by nume-
rous small holes, through which the pseudopods are pro-
truded, in others it has only one large aperture (Fig. 31),
FIG. 30.— A living Foraminifer (Kotalid], 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.
The mode of growth of Foraminifera is largely determined
by the hard and non-distensible character of the cell-wall,
150 FORAMINIFERA, RADIOLARIA, DIATOMS LESS.
which when once formed is incapable of being enlarged. In
the young condition they consist of a simple mass of proto-
plasm covered by a more or less globular shell, having at
least one aperture. But in most cases as the cell-body
grows, it protrudes through the aperture of the shell as a
mass of protoplasm, at first naked, but soon becoming
covered by the secretion around it of a second compartment
or chamber of the shell. The latter now consists of two
A
FIG. 31. — A, diagram of a Foraminifer in which new chambers are
added in a straight line : the smallest first-formed chamber is below,
the newest and largest is above and communicates with the exterior.
B, diagram of a Foraminifer in which the chambers are added in a
flat spiral : the oldest and smallest chamber is in the centre, the newest
and largest as before communicates with the exterior. (After
Carpenter.)
chambers communicating with one another by a small
aperture, and one of them — the last formed — communi-
cating with the exterior. This process may go on almost
indefinitely, the successive chambers always remaining in
communication by small apertures through which continuity
of the protoplasm is maintained, while the last formed
chamber has a terminal aperture placing its protoplasm in
free communication with the outer world.
xiv COMPLEXITY OF SHELL 151
The new chambers may be added in a straight line (Fig.
31, A) or in a gentle curve, or in a flat spiral (Fig. 31, B),
or like the segments of a Nautilus shell, or more or less
irregularly. In this way shells of great variety and beauty
FlG. 32. — Section of one of the more complicated Foraminifera
(Alveolina), showing the numerous chambers containing protoplasm
(dotted), separated by partitions of the shell (white). x 60. (From
Gegenbaur after Carpenter. )
of form are produced, often resembling the shells of Mol-
lusca, and sometimes attaining a marvellous .degree of com-
plexity (Fig. 32). The student should make a point of
examining mounted slides of some of the principal genera
and of consulting the plates in Carpenter's Introduction to
the Study of Foraminifera (Ray Society, 1862), or in Brady's
Report on the Foraminifera of the " Challenger''' Expedition,
in order to get some notion of the great amount of dif-
ferentiation attained by the shells of these extremely simple
organisms.
152 FORAMINIFERA, RADIOLARIA, DIATOMS LESS.
The Radiolaria form another group of marine animal-
cules, the numerous genera of which are, like the Foram-
inifera, amongst the most beautiful of microscopic objects.
They also (Fig. 33) consist of a mass of protoplasm giving
off numerous delicate pseudopods (psd) which usually have
a radial direction and sometimes unite to form networks.
In the centre of the protoplasmic cell-body one or more
nuclei (nu) of unusual size and complex structure are
found.
Int. caps.pr
^- cent caps
SKcl.
cqps.fr.
FIG. 33. — Lithocircus annitlans, one of the Radiolaria, showing
central capsule (cent, caps.), intra- and extra capsular protoplasm (int.
caps.pr., ext. caps. pr. ), nucleus (nit), pseudopods (psd\ silicious skeleton
(skel), and symbiotic cells of Zooxanthella (z). (After Biitschli. )
In the interior of the protoplasm, surrounding the nucleus,
is a sort of shell, called the central capsule (cent, caps.),
formed of a membranous material, and perforated by pores
which place the inclosed or intra-capsular protoplasm (int.
caps, pr.) in communication with the surrounding or extra-
capsular protoplasm (ext. caps.pr.). But besides this simple
membranous shell there is often developed, mainly in the
extra-capsular protoplasm, a skeleton (skel) formed in the
majority of cases of pure silica, and often of surpassing
beauty and complexity. One very exquisite form is shown
XIV
COMPLEXITY OF SHELL
153
in Fig. 34 : it consists of three perforated concentric spheres
connected by radiating spicules : the material of which it is
composed resembles the clearest glass.
The student should examine mounted slides of the silicious
shells of these organisms — sold under the name of Poly-
cystinece — and should consult the plates of Haeckel's Die
Radiolarien : he cannot fail to be struck with the complexity
FIG. 34. — Skeleton of a Radiolarian (Actinomma), consisting of
three concentric perforated spheres— the two outer partly broken away
to show the inner — connected by radiating spicules. (From Gegenbaur,
after Haeckel.)
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
154 FORAMINIFERA, RADIOLARIA, DIATOMS LESS.
protoplasm are found certain little rounded bodies of a
yellow colour, often known as " yellow cells " (Fig. 33, z).
Each consists of protoplasm surrounded by a cell-wall of
cellulose, and coloured by chlorophyll, with which is asso-
ciated a yellow pigment of similar character called diatomin.
For a long time these bodies were a complete puzzle to
biologists, but it has now been conclusively proved that they
are independent organisms resembling the resting condition
of Haematococcus, and called Zooxanthella nutricola.
Thus an ordinary Radiolarian, such as Lithocircus (Fig.
33), consists of two quite distinct things, the Lithocircus in
the strict sense of the word phis 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. 123), in
which one organism preys upon another, the host deriving
no benefit but only harm from the presence of the parasite.
In symbiosis, on the contrary, the two organisms are in a
condition of mutually beneficial partnership. The carbon
dioxide and nitrogenous waste given off by the Radiolarian
serve as a constant food-supply to the Zooxanthella : at the
same time the latter by decomposing the carbon dioxide
provides the Radiolarian with a constant supply of oxygen,
and at the same time with two important food-stuffs — starch
andproteids — which, after solution, diffuse from the protoplasm
of the Zooxanthella into that of the Radiolarian. The
Radiolarian may therefore be said to keep the Zooxanthellse
constantly 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
xiv MOVEMENTS OF DIATOMS 155
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 Diatoms^ as they are often called for
the sake of brevity, are a group of minute organisms, in-
cluded under a very large number of genera and species, and
so common that there is hardly a pond or stream in which
they do not occur in millions
Diatoms vary almost indefinitely in form : they may be rod-
shaped, triangular, circular, and so on. Their essential
structure is, however, very uniform : the cell-body contains a
nucleus (Fig. 35, A, nu) and vacuoles (vac), as well as two
large chromatophores (chr) of a brown or yellow colour;
these are found to contain chlorophyll, the characteristic
green tint of which is veiled, as in Zooxanthella, by diatomin.
The cell is motile, executing curious, slow, jerky or gliding
movements caused by the protrusion of delicate threads of
mucilage from between the valves of the cell-wall : the
threads are shot out at intervals in a given direction, and,
by the resistance of the water, the diatom is jerked in the
opposite direction.
1 56
FORAMINIFERA, RADIOLARIA, DIATOMS
The most interesting feature in the organisation of diatoms
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
B
FIG. 35.— A, semi-diagrammatic view of a diatom from its flat face,
showing cell-wall (c. w) and protoplasm with nucleus (nit), two vacuoles
(vac\ and two chromatophores (chr).
B, diagram of the shell of a diatom from the side, i.e,, turned on its
long axis at right angles to A, showing the two valves (c. iv, c. w') with
their overlapping girdles.
C, the same in transverse section.
D, surface view of the silicious shell of Navicitla trtmcata.
E, surface view of the silicious shell of Aulacodiscus sollittiamts.
(D, after Donkin ; E, after Norman. )
girdle, and so disposed that in the entire cell the girdle of
one valve (c. w] fits over that of the other (c. w) like the
lid of a pill-box. The cell-wall is impregnated with silica,
so that diatoms can be boiled in strong acid or exposed to
xiv MARKINGS OF DIATOMS 157
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. 35, D and E, but, in order to form some con-
ception of the extraordinary variety in form and ornamenta-
tion, specimens of the mounted cell-walls should be ex-
amined and the plates of some illustrated work consulted.
(See especially Schmidt's Atlas fur Diatomaceenkunde and
the earlier volumes of the Quarterly Journal of Micro-
scopical Science)
We see then that while Diatoms are in their essential
structure as simple as Hsematococcus, they have the power
of extracting silica from the surrounding water, and of
forming from it structures which rival in beauty of form and
intricacy of pattern the best work of the metal-worker or
the ivory-carver.
LESSON XV
MUCOR
THE five preceding lessons have shown us how complex a
cell may become either by internal differentiation of its
protoplasm, or by differentiation of its cell- wall. In this
and the following lesson we shall see how a considerable
degree of specialization may be attained by the elongation of
cells into filaments.
Mucor is the scientific name of the common white or grey
mould which every one is familiar with in the form of a
cottony deposit on damp organic substances, such as leather,
bread, jam, £c. For examination it is readily obtained by
placing a piece of damp bread or some fresh horse-dung
under an inverted tumbler or bell-jar so as to prevent evapo-
ration and consequent drying. In the course of two or
three days a number of delicate white filaments will be seen
shooting out in all directions from the bread or manure ; these
are filaments of Mucor. The species which grows on bread
is called Mucor stolonifer, that on horse-dung, M. mucedo.
The general structure and mode of growth of the mould
can be readily made out with the naked eye. It first
appears, as already stated, in the form of very fine white
threads projecting from the surface of the mouldy substance ;
and these free filaments (Fig. 36, A, a. hy) can be easily
FIG. 36. — Mucor.
A, portion of mycelium of M. mucedo (my) with two aerial hyphas
a. hy), each ending in a sporangium (sfg).
B, small portion of an aerial hypha, highly magnified, showing pro-
toplasm (plsm) and cell- wall (c, iv). The scale above B applies to this
figure only.
c1, immature sporangium, showing septum (sep) and undivided pro-
toplasm : c2, mature sporangium in which the protoplasm has divided
into spores ; the septum (sep) has become very convex dismally, forming
the columella.
i)1, mature sporangium in the act of dehiscence, showing the spores
(sp) surrounded by mucilage (g) ; D2, small portion of the same, more
highly magnified, showing spicules of calcium oxalate attached to wall.
E, a columella, left by complete dehiscence of a sporangium, showing
the attachment of the latter as a black band.
The scale above c2 and D1 applies to C1, c2, D1, and E.
160 MUCOR LESS.
F, spores.
G1, G2, G3, three stages in the germination of the spores.
H, a group of germinating spores forming a small mycelium,
ji — I5j five stages in conjugation, showing two gametes (gam) uniting
to form the zygote (zyg).
K1, K2, development of ferment cells from submerged hyphae.
(A, C2, D, E, F, G, and K, after Howes ; I, after De Bary.)
ascertained to be connected with others (my) which form a
network ramifying through the substance of the bread or
horse-dung. This network is called a mycelium ; the threads
of which it is composed are mycelial hyphce ; and the fila-
ments which grow out into the air and give the characteristic
fluffy appearance to the growth are aerial hyphce.
The aerial hyphse are somewhat thicker than those which
form the mycelium, and are at first of even diameter through-
out : they continue to grow until they attain a length, in M.
mucedo, of 6-8 cm. (two or three inches). As they grow
their ends are seen to become dilated, so that each is termi-
nated by a minute knob (A, spg) : this increases in size and
darkens in tint until it finally becomes dead black. In its
earlier stages the knobs may be touched gently without
injury, but when they .have attained their full size the
slightest touch causes them to burst and apparently to dis-
appear— their actual fate being quite invisible to the naked
eye. As we shall see, the black knobs contain spores, and
are therefore called sporangia or spore-cases.
Examined under the microscope, a hypha is found to be
a delicate, more or less branched, tube, with a clear trans-
parent wall (B, c. w) and slightly granular contents (plsm) :
its free end tapers slightly (H), and the wall is somewhat
thinner at the extremity than elsewhere. If a single hypha
could be obtained whole and unbroken, its opposite end
would be found to have much the same structure, and each
of its branches would also be seen to end in the same way.
xv ASEXUAL REPRODUCTION 161
So that the mould is simply an interlacement of branched
cylindrical filaments, each consisting of a granular substance
completely covered by a kind of thin skin of some clear
transparent material.
By the employment of the usual reagents, it can be ascer-
tained that the granular substance is protoplasm, and the
surrounding membrane cellulose. The protoplasm moreover
contains vacuoles at irregular intervals and numerous small
nuclei.
Thus a hypha of Mucor consists of precisely the same
constituents as a yeast-cell — protoplasm, containing nuclei
and vacuoles, surrounded by cellulose. Imagine a yeast-
cell to be pulled out — as one might pull out a sphere of clay
or putty — until it assumed the form of a long narrow cylin-
der, and suppose it also to be pulled out laterally at intervals
so as to form branches : there would be produced by such a
process a very good imitation of a hypha of Mucor. We
may therefore look upon a hypha as an elongated and
branched cell, so that Mucor is, like Opalina, a multinucleate
but non-cellular organism. We shall see directly, however,
that this is strictly true of the mould only in its young state.
As stated above, the aerial hyphas are at first of even
calibre, but gradually swell at their ends, forming sporangia.
Under the microscope the distal end of an aerial hypha is
found to dilate (Fig. 36, 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. 10, p. 64). The portion
thus separated is the rudiment of a sporangium.
Let us consider precisely what this process implies. Before
it takes place the protoplasm is continuous throughout the
M
162 MUCOR LESS.
whole organism, which is therefore comparable to the un-
divided plant-cell shown in Fig. 9, B. As in that case, the
protoplasm divides into two and a new layer of cellulose is
formed between the daughter-cells. Only, whereas in the
ordinary vegetable cell the products of division are of equal
size (Fig. 10, i), in Mucor they are very unequal, one being
the comparatively small sporangium, the other the rest of
the hypha.
Thus a Mucor-plant with a single aerial hypha becomes,
by the formation of a sporangium, bicellular : if. as is ordi-
narily the case, it bears numerous aerial hyphse, each with
its sporangium, it is multicellular.
Under unfavourable conditions of nutrition, septa fre-
quently appear at more or less irregular intervals in the
mycelial hyphae : the organism is then very obviously multi-
cellular, being formed of numerous cylindrical cells arranged
end to end.
The sporangium continues to grow, and, as it does so, the
septum becomes more and more convex upwards, finally
taking the form of a short, club-shaped projection, the colu-
mella, extending into the interior of the sporangium (c2) : at
the same time the protoplasm of the sporangium under-
goes multiple fission, becoming divided into numerous ovoid
masses each of which surrounds itself with a cellulose coat
and becomes a spore (o1 D-, sp). A certain amount of the
protoplasm remains unused in the formation of spores, and
is converted into a gelatinous material (g), which swells up
in water.
The original cell-wall of the sporangium is left as an
exceedingly delicate, brittle shell around the spores : minute
needle-like crystals of calcium oxalate are deposited in it,
and give it the appearance of being closely covered with
short cilia (D2).
XV
GERMINATION OF SPORES
163
In the ripe sporangium the slightest touch suffices to
rupture the brittle wall and liberate the spores, which are
dispersed by the swelling of the transparent intermediate
substance. The aerial hypha is then left terminated by the
columella (E), around the base of which is seen a narrow
black ring indicating the place of attachment of the
sporangium.
The spores (F) are clear, bright-looking, ovoidal bodies
consisting of protoplasm containing a nucleus and sur-
FlG. 37. — Moist chamber formed by cementing a ring of glass or
metal (c) on an ordinary glass slide (A), and placing over it a cover-slip
(B), on the under side of which is a hanging drop of nutrient fluid (P).
The upper figure shows the apparatus in perspective, the lower in
vertical section. (From Klein.)
rounded by a thick cell-wall. A spore is therefore an
ordinary encysted cell, quite comparable to a yeast-cell.
The development of the spores is a very instructive process,
and can be easily studied in the following way : A glass or
metal ring (Fig. 37, c) is cemented to an ordinary microscopic
slide (A) so as to form a shallow cylindrical chamber. The
top of the ring is oiled, and on it is placed a cover glass (B),
with a drop of Pasteur's solution on its under surface.
Before placing the cover-glass in position a ripe sporangium
of Mucor is touched with the point of a needle, which is
M 2
164 MUCOR LESS.
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 (c1, Fig. 36), looking
not unlike a budding Saccharomyces. The buds, however,
instead of becoming detached, increase in length until they
become filaments of a diameter slightly less than that of the
spore and somewhat bluntly pointed at the end (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. 161),
that Mucor is 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 hyphae 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
the centrifugal mode of growth, the mycelium is always
xv CONJUGATION 165
thicker at the centre than towards the circumference, since
it is the older or more central portions of the 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. 36, I1), which come into contact with
one another by their somewhat swollen free 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 hyphse are gametes or conjugating bodies, and the
large spore-like structure (zyg) resulting from their union
is a zygote. The striking feature of the process is that the
gametes are non-motile, save in so far as their growth
towards one another is a mode of motion. In Heteromita
both gametes are active and free-swimming (p. 41) : in
Vorticella one is free-swimming, the other fixed but still
capable of active movement (p. 132); here both conjugating
bodies exhibit only the slow movement in one direction due
to growth.
There are equally important differences in the result of
the process in the three cases. In Heteromita the proto-
plasm of the zygote breaks up almost immediately into
spores ; in Vorticella the zygote is active, and the result of
conjugation is merely increased activity in feeding and fissive
1 66 MUCOR
LESS.
multiplication ; in Mucor the zygote remains inactive for a
longer or shorter time, and then under favourable conditions
germinates in much the same way as an ordinary spore,
forming a mycelium from which sporangium-bearing aerial
hyphae arise. A resting zygote of this kind, formed by the
conjugation of equal-sized gametes, is often distinguished as
a zygospore.
Notice that differentiation of a very important kind is
exhibited by Mucor. In accordance with its comparatively
large size the function of reproduction is not performed by
the whole organism, as in all previously studied types, but a
certain portion of the protoplasm becomes shut off from the
rest, and to it — as spore or gamete— the office of reproduc-
ing the entire organism is assigned. So that we have for
the first time true reproductive organs, which may be of two
kinds, asexual— the sporangia, and sexual — the gametes.1
In describing the reproduction of Amceba it was pointed
out (p. 20) that as the entire organism divides into two
daughter-cells, each of which begins an independent life, an
Amceba cannot be said ever to die a natural death. The
same thing is true of the other unicellular forms we have
considered, since in the majority of them the entire organism
produces by simple fission two new individuals.2 But in
Mucor the state of things is entirely altered. A compara-
1 In Mucor no distinction can be drawn between the conjugating
body (gamete) and the organ which produces it (gonad). See the de-
scription of the sexual process in Vaucheria (Lesson XVI.) and in
bpirogyra (Lesson XIX.).
2 An exception is formed by colonial forms such as Zoothamnium, in
which life is carried on from generation to generation by the reproduc-
tive zooids only. In all probability the colony itself, like an annual
plant, dies down after a longer or shorter time. Moreover the ciliate
infusoria are said, as already stated (p. 116), to sink into decrepitude
after multiplying by fission for a long series of generations,
xv NUTRITION 167
lively small part of the organism is set apart for repro-
duction, and it is only the reproductive cells thus formed —
spores or zygote — which carry on the life of the species :
the remainder of the organism having exhausted the
available food supply and produced the largest possible
number of reproductive products, dies. That is, all vital
manifestations such as nutrition cease, and decomposition
sets in, the protoplasm becoming converted into pro-
gressively simpler compounds, the final stages being chiefly
carbon dioxide, water, and ammonia.
Mucor is able to grow either in Pasteur's or in some
similar nutrient solution, or on various organic matters such
as bread, jam, manure, &c. In the latter cases it appears to
perform some fermentative action, since food which has
become "mouldy" is found to have experienced a definite
change in appearance and flavour without actual putre-
faction. When growing on decomposing organic matter, as
it often does, the nutrition of Mucor is saprophytic, but in
some instances, as when it grows on bread, it seems to
approach very closely to the holozoic method. M. stolo-
nifer is also known to send its hyphae into the interior of
ripe fruits, causing them to rot, and thus acting as a para-
site. The parasitism in this case is, however, obviously not
quite the same thing as that of Opalina (p. 123) : 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 cor;-
i68 MUCOR LESS, xv
ditions, Mucor is capable of exciting alcoholic fermentation
in a saccharine solution. When the hyphae are submerged
in such a fluid they have been found to break up, forming
rounded cells (Fig. 36, 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 Algae, occurring in the form of dark green,
thickly-matted threads, is called Vaucheria. Besides occur-
ring in water it is often found on the surface of moist soil,
e.g., on the pots in conservatories.
Examined microscopically the organism is found to consist
of cylindrical filaments with rounded ends and occasionally
branched (Fig. 38, A). Each filament has an outer cover-
ing of cellulose (B, c.w) within which is protoplasm con-
taining a vacuole so large that the protoplasm has the
Iff. r*^5 i
\i( xtift
FlG. 38. — Vaucheria.
A, tangled filaments of the living plant, showing mode of branching.
B, extremity of a filament, showing cell-wall (c. w) and protoplasm
with chromatophores (chr], and oil-drops (o). The scale above applies
to this figure only.
C1, immature sporangium (spg) separated from the filament by a
septum (sep] ; C2, mature sporangium with the spore (sp] in the act of
escaping ; c3, free-swimming spore, showing cilia, colourless ectoplasm
LESS, xvi ASEXUAL REPRODUCTION 171
containing 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 ; D3, the fully-formed
spermary (spy) and ovary (pvy), each separated by a septum (sep, 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 fertilisation, showing the oosperm (osp] still
enclosed in the ovary and the dehisced spermary.
E1, oosperm about to germinate : E2, 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 (0), and small, close-set, ovoid chromatophores
(chr) coloured with chlorophyll and containing starch.
Thus a Vaucheria-plant, like a Mucor-plant, is non-cellular :
it 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
(c2, c3), formed of a colourless cortical layer containing
numerous nuclei and giving off cilia arranged in pairs, and
of an inner or medullary substance containing numerous
ohromatophores.
The wall of the sporangium splits at its distal end (c2),
and the contained spore (sp) escapes and swims freely in the
172 VAUCHERIA AND CAULERPA LESS.
water for some time by the vibration of its cilia (c3). After
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. 164), the de-
velopment of the plant shows it to be, to all intents and
purposes, a single immensely elongated cell, which has
become multinucleate without any corresponding division of
the protoplasm.
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 (o1), 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 (DS, 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.2
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 (o4). The remainder of the protoplasm then
separates from the wall of the ovary and becomes a naked
1 Usually called the oogonium.
2 Usually called the antkeridium.
xvr SEXUAL REPRODUCTION 173
cell, the ovtfm1 or egg-cell (D4, ov\ which, by the gelatiniza-
tion and subsequent disappearance of a portion of the 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.2 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 (D6), and are then seen
moving actively in the space between the aperture and the
colourless distal end of the ovum. One of them, and prob-
ably 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 (DT, 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
1 Frequently called oosphere.
2 Often called spertnatozooids or antherozooids.
3 Often called oo spore.
174 VAUCHERIA AND CAULERPA LESS.
flagellate. In other words, we have a more obvious case of
sexual differentiation than was found to occur in Vorticella,
(p. 132): the large inactive egg-cell which furnishes by far
the greater portion of the material of the oosperm is the
female gamete ; the small active sperm-cell, the function of
which is probably (see Lesson XXIII) 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,
ovum and sperm, just as a zygospore (p. 166) 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 XXX.
and XXXI.), 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
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 non-cellular planls
which attain some complexity by elongation and branching.
CAULERPA
ITS
The maximum differentiation attainable in this way by a
non-cellular plant may be illustrated by a brief description
of a sea-weed belonging to the genus Caulerpa.
Caulerpa (Fig. 39) is commonly found in rock-pools
between tide-marks, and has the form of a creeping stem
from .which root-like fibres are given off downwards and
branched leaf-like organs upwards. These " leaves " may
attain a length of 30 cm. (i ft.) or more. So that, on a
FIG. 39. — Caulerpa scalpelliformis (f nat. size), showing the stem-
like, root-like, and leaf-like portions of the non-cellular plant. (After
Harvey.)
superficial examination, Caulerpa appears to be as complex
an organism as a moss (compare Fig. 39 with Fig. 108, 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 continuous mass of protoplasm exhibiting no cellular
structure.
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 Paramoecium and Stylonychia
plants, or Mucor and Vaucheria animals, and we may there-
fore use these forms as a starting-point in an attempt to form
a clear conception of what the names plant and animal really
signify, and how far it is possible to place the lowly organisms
described in the earlier lessons in either the vegetable or the
animal kingdom.
Let us consider, first of all, the chief points of resemblance
and of difference between the indubitable animal Paramcecium
on the one hand, and the two indubitable plants Mucor and
Vaucheria on the other.
In the first place, the essential constituents of all three
organisms is protoplasm, in which are contained one or more
nuclei. But in Paramoecium the protoplasm is invested
LESS, xvn DIFFERENCES IN NUTRITION 177
only by a delicate cuticle interrupted at the mouth and anus,
while in Mucor and Vaucheria the outer layer is formed by
a firm, continuous covering of cellulose.
We thus have as the first morphological difference between
our selected animal and vegetable organisms the absence of
a cellulose cell-wall in the former and its presence in the
latter. This is a fundamental distinction, and applies
equally well to the higher forms. The constituent cells of
plants are in nearly all cases covered with a cellulose coat
(p. 60), while there is no case among the higher animals of
cells being so invested.
Next, let us take a physiological character. In all three
organisms there is constant waste of substance which has to
be made good by the conversion of food material into proto-
plasm : in other words, constructive and destructive meta-
bolism are continually being carried on. But when we come
to the nature of the food and the mode of its reception, we
meet at once with a very fundamental difference. In Para-
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.
N
178 CHARACTERS OF ANIMALS AND PLANTS LESS.
In this case also our selected organisms agree with animals
and plants generally. Animals, with the exception of some
internal parasites, ingest solid food, and they must all have
their nitrogen supplied in the form of proleids, being unable
to build up their protoplasm from simpler compounds.
Plants take their food in the form of a watery solution ;
those which possess chlorophyll take their carbon in the
form of carbon dioxide and their nitrogen in that of a nitrate
or ammonia salt : those devoid of chlorophyll cannot, ex-
cept in the case of some Bacteria, make use of carbon
dioxide as a food, and are able to obtain nitrogen either
from simple salts or from proteids. Chlorophyll-less plants
are therefore nourished partly like green plants, partly like
animals.
This difference in the character of the food is connected
with a morphological difference. Animals have, as a
rule, an ingestive aperture or mouth, and some kind of
digestive cavity, either permanent (stomach) or temporary
(food-vacuole). In plants neither of these structures
exists.
Another difference which was referred to at length in an
early lesson (p. 32), is not strictly one between plants and
animals, but between organisms with and organisms without
chlorophyll. It is that in green plants the nutritive processes
result in deoxidation, more oxygen being given out than is
taken in ; while in animals and not-green plants the precise
contrary is the case.
There is also a difference in the method of excretion. In
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
DEFINITIONS 179
diffusion from the general surface of the organism into the
surrounding medium.
This character also is of some general inportance. The
large majority of animals possess a special organ of excretion,
plants have nothing of the kind.
Another difference has to do with the general form of the
organism. Paramcecium 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. Paramcecium 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 processes
result in oxidation, they have a definite organ of excretion,
and are capable of automatic movement."
Plants are organisms of constantly varying form in which
the cell-body is surrounded by a cellulose wall ; they cannot
ingest solid food, but are nourished by a watery solution of
nutrient materials. If chlorophyll is present the carbon
dioxide of the air serves as a source of carbon, nitrogen is
obtained from simple salts, and the nutritive processes
N 2
i8o CHARACTERS OF ANIMALS AND PLANTS LESS.
result in deoxidation ; if chlorophyll is absent carbon is
obtained from sugar or some similar compound, nitrogen
either from simple salts or from proteids, and the process of
nutrition is one of oxidation. There is no special excretory
organ, and, except in the case of certain reproductive bodies,
there is usually no locomotion.
Let us now apply these definitions to the simple forms
described in the first eight lessons, and see how far they
will help us in placing those 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 : 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
chem with animals. That the difficulty is by no means
xvii DOUBTFUL FORMS 181
easily overcome may be seen from the fact that both genera
are claimed at the present day both by zoologists and by
botanists. For instance, Prof. Huxley considered Haema-
tococcus as a plant, and expressed 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,
which in the zygote is firm enough to hold the spores up to
the moment of their escape : food is taken exclusively by
absorption, and nutrition is wholly saprophytic : there is a
contractile vacuole, and the movements are ciliary.
Here again the characters are conflicting : the probable
absence of cellulose, the contractile vacuole, and the cilia
all have an " animal " look, but the mode of nutrition is
that of a fungus.
In Protomyxa there is a decided preponderance of animal
characteristics— ingestion of 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.
;82 CHARACTERS OF ANIMALS AND PLANTS LESS.
.In Saccharomyces there is a clear preponderance of
vegetable characters. The cell-wall consists of cellulose,
nutrition takes place by absorption and proteids are not essen-
tial, there is no contractile vacuole, and no motile phase.
Lastly, in the Bacteria the cell-wall is composed of cellu-
lose, nutrition is usually saprophytic, there is no contractile
vacuole, and the movements are ciliary. So that in all the
characters named, save in the presence of cellulose and the
absence of a contractile vacuole, the Bacteria agree with
Heteromita, yet they are universally — except by Prof. Claus
— 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 recognise 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
xvii PROTISTA 183
begins : as in the development of an animal it is futile to
argue about the exact period when, for instance, the egg
becomes a tadpole or the tadpole a frog : so in the case
under discussion. The distinction between the higher
plants and animals is perfectly sharp and obvious, but when
the two groups are traced downwards they are found
gradually to merge, as it were, into an assemblage of organ-
isms which partake of the characters of both kingdoms, and
cannot without a certain violence be either included in or
excluded from either. When 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 roots, by the hypothesis that the earliest
organisms were protists, and that from them animals and
plants were evolved along divergent lines of descent. And
in this connection the fact that some bacteria — the simplest
organisms known and devoid of chlorophyll — may flourish
in solutions wholly devoid of organic matter, is very
significant.
LESSON XVIII
PENICILLIUM AND AGARICUS
ONE of the commonest and most familiar of the lower
organisms is the " green mould " which so quickly covers
with a thick sage-green growth any organic substances ex-
posed to damp, such as paste, jam, cheese, leather, &c.
This mould is a plant belonging, like Mucor, to the group
of Fungi, and is called Penicillium glaucum.
Examined with the naked eye a growth of Penicillium is
seen to have a powdery appearance, and, if the finger is
passed over it, a quantity of extremely fine dust of a sage-
green colour comes away. This dust consists, as we shall
see, of the spores of Penicillium. The best way to study
the plant is to sow some of the spores in a saucer of
Pasteur's solution by drawing a needle or brush over a
growth of the mould and stirring it round in the fluid.
It is as well to study the naked eye appearances first. If
the quantity of spores taken is not too large and they are
sufficiently well diffused through the fluid, little or no trace
of them will be apparent to the naked eye. After a few
days, however, extremely small white dots appear on the
surface of the fluid ; these increase in size and are seen,
especially by the aid of a hand-magnifier, to consist of little
LESS, xvni MYCELIUM 185
discs, circular or nearly so in outline, and distinctly thicker
in the centre than towards the edge : they float on the fluid
so that their upper surfaces are dry. Each of these patches
is a young Penicillium-growth, formed, as will be seen
hereafter, by the germination of a group of spores.
As the growths are examined day by day they are found
to increase steadily in size, and as they do so to become
thicker and thicker in the middle : their growth is evidently
centrifugal. The thicker central portion acquires a fluffy
appearance, and, by the time the growth has attained a
diameter of about 4 or 5 mm., a further conspicuous change
takes place : the centre of the patch acquires a pale blue
tint, the circumference still remaining pure white. When
the diameter has increased to about 6-10 mm. the colour of
the centre gradually changes to dull sage-green : around this
is a ring of light blue, and finally an outer circle of white.
In all probability some of the growths, several of which will
most likely occur in the saucer, will by this time be found
to have come together by their edges : they then become
completely interwoven, their original boundaries remaining
evident for some time by their white tint. Sooner or later,
however, the white is replaced by blue and the blue by sage-
green, until the whole surface of the fluid is covered by a
single growth of a uniform green colour.
Even when they are -not more than 2-3 mm. in diameter
the growths are strong enough to be lifted up from the fluid,
and are easily seen under a low power to be .formed of a
tough, felt-like substance, the mycelium (Fig, 40, A my\ from
the upper surface of which delicate threads, the aerial
hyph(z (a. /ty.), grow vertically upwards into the air, while
from its lower surface similar but shorter threads, the sub-
merged hyfhce (s. hy.\ hang vertically downwards into the
fluid.
FIG. 40. — Penicillium glaucum.
A, Diagrammatic vertical section of a young growth (x 5), showing
mycelium (my), submerged hyphse (s. hy\ and aerial hyphee (a. hy\
B, group of spores : I, before commencement of germination ; 2, after
imbibition of fluid : the remaining three have begun to germinate.
C, very young mycelium formed by a small group of germinating
spores.
LESS, xvm MULTICELLULAR HYPKLE 187
D, more advanced mycelium : the hyphse 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 hyphse 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).
F8, a single sterigma (stg} with its spores (sp).
F", an over-ripe brush in which the structure is obscured by spores
which have dropped from the sterigmata.
B-D, F1-?5, 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 hyphce : they are regularly cylindrical,
about yj^- mm. in diameter, frequently branched, and differ
in an important particular from the somewhat similar hyphae
of Mucor (p. 1 60). The protoplasm is not continuous, but
is interrupted at regular intervals by transverse partitions or
septa (D, E, sep). In other words, a hypha of Penicillium
is normally what a hypha of Mucor becomes under un-
favourable conditions (p. 162), 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 either a single cell in
i88 PENICILLIUM AND AGARICUS LESS.
the strict sense, or a continuous multinucleate mass of
protoplasm not divided into cells ; in Penicillium 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 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 Opalina (p. 123) or a filament of Mucor or Vau-
cheria, is multinucleate.
The submerged hyphae 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 (F1— F4). Each sends
off from its distal or upper end a larger or smaller number of
branches which remain short and grow parallel to one
another : the primary branches (r1, 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 (F5, sfg).
Next, the ends of the sterigmata become constricted,
exactly as if a thread were tied round them and gradually
tightened (r1, F6), the result being to separate the distal end
xvin GERMINATION OF SPORES 189
of the sterigma as a globular daughter-cell, in very much the
same way as a bud is separated in Saccharomyces (p. 73).
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 (K5, FG), of which the proximal or lower
one is the youngest, the distal or upper one the oldest. The
spores grow for some time after their formation, and are
therefore found to become larger and larger in passing from
the proximal to the distal end of the chain (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 (r7).
It is at the period of complete formation of the spores that
the growth turns green. The colour is not due to the pres-
ence of chlorophyll. Under a high power the spores appear
quite colourless, whereas a cell of the same size coloured
with chlorophyll would appear bright green.
The germination of the spores can be readily studied by
sowing them in a drop of Pasteur's solution in a moist chamber
(Fig. 37, p. 163). The spores, several of which usually adhere
together, are at first clear and bright (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. 12, p. 72). 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
190 PENICILLIUM AND AGARICUS LKsS.
equal to about six or eight times its own diameter, the pro-
toplasm in it divides transversely and a cellulose septum is
formed (D, E, sef) dividing the young hypha into two cells
'(compare Fig. 36, H, p. 159). The distal cell then elongates
and divides again, and in this way the hyphae are, almost from
the first, divided into cells of approximately equal length.
The mode of growth of the distal or apical cell of a hypha
is probably as follows. The free end tapers slightly (E) and
the cellulose wall thins out as it approaches the apex. The
protoplasm performing constructive more rapidly than de-
structive metabolism increases in volume, and its tendency is
to grow in all directions : as, however, the cellulose mem-
brane surrounding it is thinner at the apex than elsewhere,
it naturally, on the principle of least resistance, extends in
that direction, thus increasing the length of the cell without
adding to its thickness. Thus the growth of a hypha of
Penicillium is apical^ i.e. takes place only at the distal end, the
cells once formed ceasing to grow. Thus also the oldest cells
are those nearest the original spore from which the hypha
sprang, the youngest those furthest removed from it.
A process which has been described as sexual, sometimes, but appa-
rently very rarely, occurs in Penicillium, and is said to consist essentially
in the conjugation of two gametes having the form of twisted hypha?,
and the subsequent development of spores in the resulting branched
zygote. But as the details of the process are complicated and its sexual
character is doubtful, it is considered best to do no more than call
attention to it. The student is referred to Brefeld's original account of
the process in the Quarterly Journal of Microscopical Science, vol. xv.,
p. 342. The so-called sexual reproduction of the closely-allied Eitrotiuin
is described in Huxley and Martin's Elementary Biology (new edition),
p. 419, and figured in Howes's Atlas of Elementary Biology, pi. xix.,
figs. xxvi. and xxvii.
The nutrition of Penicillium is essentially like that of Mucor
(p. 167). But, as it has been remarked, "it is often content
PILEUS AND LAMELLA 191
with the poorest food, which would be too bad for higher
fungi. It lives in the human ear ; it does not shun cast-off
clothes, damp boots, or dried-up ink. Sometimes it contents
itself with a solution of sugar with a very little [nitrogenous]
organic matter, at other times it appears as if it preferred the
purest solution of a salt with only a trace of organic matter.
It will even tolerate the hurtful influence of poisonous
solutions of copper and arsenious acid." It flourishes best
in a solution of peptones and sugar.
This eclecticism in matters of diet is one obvious ex-
planation of the universal occurrence of Penicillium ; another
is the extraordinary vitality of the spores. They will ger-
minate at any temperature between 1-5° and 43° C, the
optimum being about 22° C. They are not killed by a dry
heat of 1 08° C., and some will even survive a temperature
of 120°. And lastly, they will germinate after being kept
for two years.
We have seen that the form of a Penicillium growth is ir
regular, and is determined by the surface on which it grows.
There are, however, certain fungi which are quite constant
and determinate both in form and size, and are yet found
on analysis to be formed exclusively of interlaced hyphae,
that is, to belong to the type of linear aggregates. Among
the most striking of these are the mushrooms and toad-
stools.
A mushroom (Agaricus) consists of a stout vertical stalk
(Fig. 41, A, st\ on the upper or distal end of which is borne
an umbrella-like disc or p ileus (/). rl he lower or proximal
end of the stalk is in connection with an underground
mycelium (niy\ from which it springs.
On the under side of the pileus are numerous radiating
vertical plates or lamellae. (/) extending a part or the whole
192
PENICILLIUM AND AGARICUS
LESS.
of the distance from the circumference of the pileus to the
stalk. In the common edible mushroom (Agaricus cam-
pestris) these lamellae are pink in young specimens, and
afterwards become dark brown.
FIG. 41.— Agaricus campestris.
A, Diagrammatic vertical section, showing the stalk (sf) springing
from a mycelium (my}y and expanding into the pileus (p), on the under
side of which are the radiating lamellae (/).
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 (stg), each bearing a spore (sp).
(B and c after Sachs.)
The mushroom is too tough to be readily teased out like
the mycelium of Penicillium, and its structure is best in-
xvni HISTOLOGY OF MUSHROOM 193
vestigated by cutting tHin sections of various parts and
examining them under a high power.
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, a), and give off two small branches or sterig-
mata (c, ^), the ends of which swell up and become
constricted off as spores (sp). l These fall on the ground and
germinate, forming a mycelium from which more or fewer
mushrooms are in due course produced.
Thus in point of structure a mushroom bears much the
same relation to Penicillium as Caulerpa (p. 175) bears to
Vaucheria. Caulerpa shows the extreme development of
which a branched non-cellular organism is capable, the
mushroom how complicated in structure and definite in
form a simple linear aggregate may become.
1 Fusion of a pair of nuclei in the young club-shaped cell or basidium
precedes the nuclear division which provides a single nucleus for each
spore. — W.N. P.
LESSON XIX
SPIROGYRA
AMONGST the numerous weeds which form a green scum
in stagnant ponds and slowly flowing streams, one, called
Spirogyra, is perhaps the commonest. It is recognised at
once under a low power by the long delicate green filaments
of which it is composed being marked with a regular green
spiral band.
Examined under the microscope the filaments are seen to
be, like the hyphae of Penicillium, linear aggregates, that is,
to be composed of a single row of cells arranged end to
end. But in Penicillium the hyphae are frequently branched,
and it is always possible in an entire hypha to distinguish
the slightly tapering distal end from the proximal end
which springs either from another hypha or from a spore.
In Spirogyra the filaments do not branch, and there is no
distinction between their opposite ends.
The cells of which the filaments are composed (Fig. 42, A)
are cylindrical, covered with a .cellulose cell-wall (c, w), and
separated from adjacent cells by septa (sep) of the same
substance. The protoplasmic cell-body presents certain
characteristic peculiarities.
It has been noticed in more than one instance that in the
FIG. 42. — Spirogyra.
A, small portion of a living filament, showing a single cell, with cell-
wall (c. w), septa (sep) separating it from adjacent cells, peripheral layer
of protoplasm (plsm] connected by threads with a central mass contain-
O 2
196 SPIROGYRA LESS.
ing the nucleus (nu}> 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 (jW*\ goifi}
are connected by short processes of their adjacent sides : in c2 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 (gam*) which has not yet separated from its containing
gonad (gori-} : in c1 the female gamete (ganF) has undergone separation,
and the male gamete (gam1} 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 D1 the adjacent cells
(gonads) have sent out conjugating processes (a} : in Dw> 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 (z\g).
E, parthenogenetic formation of zygotes.
F, fully developed zygote (zygospore).
G, early stage in the germination of the zygote.
(B after Sachs : c after Strasburger : F and G from Sachs after
Pringsheim. )
larger cells of plants the development of vacuoles is so ex-
tensive that the protoplasm is reduced to a thin layer in
contact with the cell-wall (see pp. 169 and 188). This state
of things is carried to excess in Spirogyra : the central vacuole
is so large that the protoplasm (A, phni] 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.
xi k INTERSTITIAL GROWTH 197
confined to the primordial utricle ; towards the centre of the
vacuole is a small irregular mass of protoplasm connected to
the peripheral layer by extremely delicate protoplasmic
strands. Imbedded in this central mass is the nucleus (nu\
which has the form of a biconvex lens and contains a distinct
nucleolus.
The chromatophores differ from anything we have yet
considered, having the form of green spiral bands (chr], of
which each cell may contain one (o1) or two coiled in oppo-
site directions (A). Imbedded in the chromatophores are
numerous pyrenoids (pyr, see p. 27), to which the strands
of protoplasm proceeding from the central nucleus-containing
mass can be traced.
The process of growth in Spirogyra is brought about by
the binary fission of its constituent cells. It takes place
under ordinary circumstances during the night (i i — 1 2 P.M.),
but by keeping the plant cold all night may be delayed until
morning.
The nucleus divides by the complicated process (mitosis)
already described in general terms (p. 67), so that two nuclei
are formed at equal distances from the centre of the cell.
The cell-body with its chromatophores then begins to
divide across the middle (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.
198 SPIROGYRA LESS.
Any of the cells of a Spirogyra-filament may divide in this
way, so that the filament grows by the intercalation of new
cells between the old ones. This is an example of interstitial
growth. Note its difference from the apical growth which
was found to take place in Penicillium (p. 190), a difference
which explains the fact mentioned above (p. 194) that there is
no distinction between the two ends of a filament of Spicogyra,
while in Penicillium the proximal and distal ends can always
be distinguished in a complete hypha. »
The sexual reproduction of Spirogyra is interesting, as
being intermediate between the very different processes which
were found to obtain in Mucor (p. 165) and in Vaucheria
(p. 172).
In summer or autumn adjoining filaments become arranged
parallel to one another and the opposite cells of each send
out short rounded processes which meet (Fig. 42, 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, gonz). 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. 166, note i) : 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,
%aml\ Then the gamete which is ready first (gam1) passes
through the connecting canal (c3) and conjugates with the
other (gam2), 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.
xix , CONJUGATION 199
Thus, as in Mucor, the gametes are similar and equal-
sized, and the result of the process is a resting zygote or
zygospore. But while in Mucor each gamete meets the other
half way, so that there is absolutely no sexual differentiation,
in Spirogyra, as in Vaucheria, one gamete remains passive,
and conjugation is effected by the activity of the other. So
that we have here the very simplest case of sexual differen-
tiation : the gametes, although of equal size and similar ap-
pearance, are divisible into an active or male cell, correspond-
ing with the sperm of Vaucheria, and a passive or female
cell corresponding with the ovum. It will be seen that in
Spirogyra the whole of the protoplasm of each gonad is used
up in the formation of a single gamete, whereas in Vaucheria,
while this is the case with the ovary, numerous gametes
(sperms) are formed from the protoplasm of the spermary.
In some forms of Spirogyra conjugation takes place not
between opposite cells of distinct filaments, but between
adjacent cells of the same filament. Each of the gonads
sends out a short process • (D1, a) \vhich abuts against a
corresponding process from the adjoining cell : the two
processes are placed in communication with one another by
a small aperture (D2) through which the male gamete makes
its way in order to conjugate with the female gamete and
form a zygote (zyg).
In the ordinary ladder-like method of conjugation the
conjugating filaments appear to be of opposite sexes, one
producing only male, the other only female gametes : the plant
in this case is said to be dioecious, i.e. has the sexes lodged in
distinct individuals, and conjugation is a process of cross-
fertilization. But in the method described in the preceding
paragraph the individual filaments are monoecious, i.e. produce
both male and female cells, and conjugation is a process of
self-fertilization .
200 SPIROGYRA LESS, xix
Sometimes filaments are found in which the protoplasm of
certain cells separates from the wall, and surrounds itself
with a thick coat of cellulose forming a body which is quite
indistinguishable from a zygote (E). There seems to be
some doubt as to whether such cells ever germinate, but they
have all the appearance of female cells which for some
reason have developed into zygote-like bodies without 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
pa rthenogenesis.
When the zygote is fully developed (F) its cell wall is
divided into three layers, the middle one undergoing a
peculiar change which renders it waterproof: at the same
time the starch in its protoplasm is replaced by oil. In this
condition it undergoes a long period of rest, its structure
enabling it to offer great resistance to drought, frost, &c.
Finally it germinates : the two outer coats are ruptured, and
the protoplasm covered by the inner coat protrudes as a
club-shaped process (G) which gradually takes on the form
of an ordinary Spirogyra filament, dividing as it does so into
numerous cells.
Thus in the present case, as in Penicillium and the
mushroom, the multicellular adult organism is originally
unicellular.
The nutrition of Spirogyra is purely holophytic : like
Haematococcus and Vaucheria it lives upon the carbon
dioxide and mineral salts dissolved in the surrounding
water. Like these organisms also it decomposes carbon
dioxide and forms starch only under the influence of
sunlight.
LESSON XX
MONOSTROMA, ULVA, AND NITELLA
IT was pointed out in a previous lesson (p. 193) that the
highest and most complicated fungi, such as the mushrooms,
are found on analysis to be built up of linear aggregates of
cells — to consist of hyphae so interwoven as to form struc-
tures often of considerable size and of definite and regular
form.
This is not the case with the Algae or lower green plants —
the group to which Vaucheria, Caulerpa, Spirogyra, the
diatoms, and, in the view of some authors, Haematococcus
and Euglena, belong. These agree with fungi in the fact
that the lowest among them (e.g. Zooxanthella) are unicellu-
lar, and others (e.g. Spirogyra) simple linear aggregates ; but
the higher forms, such as the majority of sea-weeds, have,
as it were, gone beyond the fungi in point of structure and
attained a distinctly higher stage of morphological differen-
tiation. This will be made clear by a study of three typical
genera.
Amongst the immense variety of sea- weeds found in rock-
pools between high and low water-marks are several kinds
having the form of flat irregular expansions or of bladder-
MONOSTROMA, ULVA, AND NITELLA
LESS.
like masses, of a bright green colour and very transparent.
One of these is the genus Monostroma, of which M. bullosum
is a fresh-water species.
Examined microscopically the plant (Fig. 43) is found to
consist of a single layer of close-set, green cells, the cell-walls
of which are in close approximation, so that the cell-bodies
appear as if embedded in a continuous layer of transparent
cellulose. Thus Monostroma, like Spirogyra, is only one
B
FIG. 43. — Monostroma.
A, surface view of M. bullosum, showing the cells embedded in a
common layer of cellulose : many of them are in various stages of
division.
B, vertical section of M. laceratum, showing the arrangement of the
cells in a single layer.
(A after Reinke : B after Cooke.)
cell thick (B), but unlike that genus it is not one but many
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
xx SOLID AGGREGATES 203
at right angles to the long axis of the filament, so that growth
takes place in one dimension of space only, namely in length.
In Monostroma the plane of division may be inclined in any
direction provided it is perpendicular to the surface of the
plant, so that growth goes on in two dimensions of space,
namely in length and breadth.
Another of the flat, leaf-like, green sea-weeds is the very
common genus Ulva, sometimes called "sea-lettuce." It
consists of irregular, more or less lobed expansions with
crinkled edges, and under the microscope closely resembles
Monostroma, with one important difference : it is formed
not of one but of two layers of cells, and is therefore not a
superficial but a solid aggregate. To return to the geometrical
analogy used above it is to be compared not to a plane but
to a solid body.
As in Monostroma growth takes place by the binary
fission of the cells. But these divide not only along variously
inclined planes at right angles to the surface of the plant
but also along a plane parallel to the surface, so that growth
takes place in all three dimensions of space — in, length,
breadth, and thickness.
Ulva may be looked upon as the simplest example of a
solid aggregate, being built up of similar cells, and therefore
exhibiting no cell-differentiation.
We shall now make a detailed study of a solid aggregate
in which the constituent cells differ very considerably from
one another in form and size, the result being a degree of
complexity far beyond anything we have hitherto met with.
Nitella (Fig. 44, A) is a not uncommon fresh-water weed,
found in ponds and water-races, and distinguished at once
FIG. w.—Nitella>.
A, the entire plant (nat. size), showing the segmented stem, each seg-
1 This and the following figures are taken from a New Zealand
species closely allied to, if not identical with, the British N. flexilis.
LESS, xx EXTERNAL CHARACTERS 205
ment (seg) consisting of a proximal internode (int. nd) and distal node
(nd) : the leaves (/) arranged in whorls and ending in leaflets (/') : the
rhizoids (rk} : 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 ; plsm1, the quies-
cent outer layer of protoplasm containing chromatophores (chr) ; plstir,
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).
from such low Algae as Vaucheria and Spirogyra by its ex-
ternal resemblance to one of the higher plants, since it
presents structures which may be distinguished as stem,
branches, leaves, &c.
A Nitella plant consists of a slender cylindrical stem,
some 15-20 cm. and upwards in length, but not more than
about J mm. in diameter. The proximal end is loosely
rooted to the mud at the bottom of the stream or pond by
delicate root-filaments or rhizoids (A, rh) : the distal end is
free. Springing from it at intervals are circlets or whorls of
delicate, pointed leaves (/).
Owing to the regular arrangement of the leaves the stem
is divisible into successive sections or segments (seg), each
consisting of a very short distal division or node (nd} from
which the leaves spring, and pf 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
2o6 MONOSTROMA, ULVA, AND NITELLA . LESS.
towards the distal end the internodes become rapidly shorter
and the whorls consequently closer together, until, at the
actual distal end, a whorl is found the leaves of which, in-
stead of spreading outwards like the rest, are curled upwards
so that their points are in contact. In this way is formed
the terminal bud (term. bud\ by which the uninjured stem
is always terminated distally.
The angle between the stem and a leaf, above (distal to)
the attachment of the latter, is called the axil of the leaf.
There is frequently found springing from the axil of one of
the leaves in a whorl a branch or shoot (br) which repeats
the structure of the main stem, i.e. consists of an axis from
which spring whorls of leaves, the whole ending in a ter-
minal bud. The axis or stem of a shoot is called a second-
ary axis, the main stem of the plant being the primary axis.
It is important to notice that both primary and secondary
axes always end in terminal buds, and thus differ from the
leaves which have pointed extremities.
The rhizoids or root-filaments (rJi) arise, like the leaves
and branches, exclusively from nodes.
In the autumn the more distal leaves present a peculiar
appearance, owing to the development on them of the gonads
or sexual reproductive organs (Fig. 44, 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^) 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
xx HISTOLOGY 207
cells (ndl) separating the two adjacent internodes from one
another. The leaves consist each of an elongated proximal
cell like an internode (D, /; F, /*), then of a few small cells
having the character of a node, and finally of two or three
leaflets (D, G, /'), each consisting usually of three cells, the
distal one of which is small and pointed.
Thus the Nitella plant is a solid aggregate in which
the cells have a very definite and characteristic arrange-
ment.
The details of structure of a single cell are readily made
out by examining a leaflet under a high power. The cell is
surrounded by a wall of cellulose (E, c.w] of considerable
thickness. Within this is a layer of protoplasm (primordial
utricle, p. 196), enclosing a large central vacuole (vac), and
clearly divisible into two layers, an outer (pls/nl) in im-
mediate contact with the cell- wall, and an inner (ptsw2)
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 TJ^ mm.
long, and arranged in obliquely longitudinal rows (D). On
opposite sides of the cylindrical cell are two narrow ob-
lique 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
208 MONOSTROMA, ULVA, AND NITELLA LESS, xx
common in vegetable cells in which, owing to the confining
cell- wall, no freer movement is possible.
The numerous nuclei (E, nu) are rod-like and often
curved : they can be seen to advantage only after staining
(Fig. 45). 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. 44, A,
term, bud) formed of a whorl of leaves with their apices
curved towards one another. If these leaves (F, I1) are dis-
sected away, the node from which they spring (ndl) is found
to give rise distally to a very short internode (int. nd^\
above which is a node (nd2) 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. nd3) 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 visible only 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 or punctum vegetationis.
The structure of the growing point and the mode of
growth of the whole plant is readily made out by examining
vertical sections of the terminal bud in numerous specimens
(Fig- 45)-
The growing point is formed of a single cell, the apical
cell (A, ap. c\ approximately hemispherical in form and about
gV 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
nd?
FIG. 45. — Nitella : Vertical sections of the growing point at four
successive stages. The nodes (nd), internodes (int. nd), and leaf-
whorls (/) are all numbered in order from the proximal to the distal end
of the bud, the numbers corresponding in all the figures. The proximal
segment (int. ndl, nd1, I1} in these figures corresponds with the third
segment (int. nd3, I3) shown in Fig. 44, F.
In A, the apical cell (ap. c] is succeeded by a very rudimentary node
(nd3) without leaves : int. nd1 is in vertical section, showing the proto-
plasm (plsm], vacuole (vac\ and two nuclei (mi}.
In 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 nd3 have
appeared.
In c, the sub-apical cell has divided transversely into the proximally-
situated internode (int. nd*) and the distally-situated node (nd*) of a
new segment ; in the node the nucleus has divided preparatory to cell-
division. The previously formed segments have increased in size : int.
net1 has developed a vacuole (vac\ and its nucleus has divided (comp.
int. nd2 in A) : int. nd1 is shown in surface view with three dividing
nuclei (nit}.
In D, nd1 has divided vertically, forming a transverse plate of cells,
and is now as far advanced as nd3 in A : the nucleus of int. nd3 is in the
act of dividing, while int. nd2, shown in surface view, now contains
numerous nuclei, some of them in the act of dividing.
210 MONOSTROMA, ULVA, AND NITELLA LESS.
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 segmenfal cell (s. ap. c).
The sub-apical cell divides again horizontally, forming two
cells, the uppermost of which (c, nd*) almost immediately
becomes divided by vertical planes into several cells (D, nd 4) ;
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
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. ?idl and ;/^3), 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
xx MULTIPLICATION OF NUCLEUS 211
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. nd2, int.
nd3), but by the time it is about as long as broad the nucleus
has begun to divide (D, int. nd 3 ; c, int. nd 2), and when the
length of the cell is equal to about twice its breadth, the
nucleus has broken up into numerous fragments (c, int. nd'1,
D, int. nd2), many of them still in active (amitotic) division.
This repeated fission of the nucleus reminds us of what
was found to occur in Opalina (p. 123).
Thus the growth of Nitella like that of Penicillium (p.
190), 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 distal to a leaf, />., in an axil : the process
separates from the parent cell and takes on the characters of
the apical ceil 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. 44, 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. 44, 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.
212
MONOSTROMA, ULVA, AND NITELLA LESS.
As we have seen, the spermary (Fig. 44, G, spy) is a
globular, orange-coloured body attached to a leaf by a short
stalk. Its wall is formed of eight pieces or shields, which
fit against one another by toothed edges, so that the entire
spermary may be compared to an orange in which an equa-
torial incision and two meridional incisions at right angles
to one another have been made through the rind, dividing
6/1
FIG. 46. — A, diagrammatic vertical section of the spermary of Nitella,
showing the stalk (stk\ four of the eight shields (sh\ each bearing on
its inner face a handle (An], to which is attached a head-cell (hd) : each
head-cell bears six secondary head-cells (hd'}, to each of which four
spermatic filaments (sp. f.) are attached.
B, one of the proximal shields (sh), with handle (hn\ head-cell (hd),
secondary head-cells (hd'), and spermatic filaments (sp. f.).
c, a single sperm.
D1, D2, D3, three stages in the development of the spermary.
(c, after Howes.)
it into eight triangular pieces. Strictly speaking, however,
only the four distal shields are triangular : the four proximal
ones have each its lower angle truncated by the insertion of
the stalk, so that they are actually four-sided.
Each shield (Fig. 46, A and B, sK) is a single concavo-
convex cell having on its inner surface numerous orange-
coloured chromatophores : owing to the disposition of these
xx STRUCTURE OF SPERMARY 213
on the inner surface only, the spermary appears to have a
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 (hd), to which six secondary head
cells (hd') 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. 44, G, ovy, and Fig. 47 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
214
MONOSTROMA, ULVA, AND NITELLA LESS.
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
nil
FIG. 47. — 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 ovary : 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. )
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
xx DEVELOPMENT OF GONADS 215
becomes divided into eight octants (Fig. 46, 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 (DS), 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
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.
47, 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 (s3, 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. 44, 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,
216
MONOSTROMA, ULVA, AND NITELLA LESS.
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.
ap.c
nd
rfi
FIG. 48. — Embryo of Chara, an ally of Nitella, showing the ovary
(ovy)t from the oosperm in which the embryo has sprung : the two
nodes (nd), apical cell (ap. c), rhizoids (rh), and leaves (/) of the
embryo : and the rudiment of the leafy plant (shaded) ending in the
characteristic terminal bud (term. bud). (After Howes, slightly altered. )
Impregnation takes place in the same manner as in
Vaucheria (p. 173). A sperm makes its way down the
canal in the chimney-like crown of cells terminating the
ovary, and conjugates with the ovum converting it into an
oosperm.
After impregnation the ovary, with the contained oosperm,
becomes detached and falls to the bottom, where, after a
xx GERMINATION 217
period of rest, it germinates. The process begins by the
division of the oosperm into two cells, a small one nearest
the crown and composed almost wholly of protoplasm, and
a larger one full of starch granules. The larger cell serves
simply as a store of nutriment to the growing plant which
is itself developed exclusively from the small cell. The
latter divides into two cells one of which grows downwards
as a root-fibre, the other upwards as a shoot, consisting at
first of a single row of cells (Fig. 48). Soon two nodes (net)
are formed on the filament, or embryo, from the lower of
which rhizoids (rh] proceed, while the upper gives rise to a
few leaves (/), and to a small process which is at first uni-
cellular, but, behaving like an apical cell of Nitella, soon
becomes a terminal bud (term, bud] and grows into the
adult plant.
It will be seen that the development of Nitella is remark-
able for the facts that the adult plant is not formed directly
from the oosperm but that the latter gives rise to an embryo,
quite different from the adult in structure, and that, from
the embryo, the adult is finally developed as a lateral bud.
LESSON XXI
HYDRA.
WE have seen that with plants, both Fungi and Algae, the
next stage of morphological differentiation after the simple
unicellular or non-cellular organism is the linear aggregate .
Among animals there are no forms known to exist in this
stage, but coming immediately above the highest unicellular
animals, such as the ciliate Infusoria, we have true solid
aggregates. The characters of one of the simplest of these
and the fundamental way in which it differs from the plants
described in the two previous lessons will be made clear by
a study of one of the little organisms known as " fresh-water
polypes " and placed under the genus Hydra.
Although far from uncommon in pond-water, Hydra is not
always easy to find, being rarely abundant and by no means
conspicuous. In looking for it the best plan is to fill either
a clear glass bottle or beaker or a white saucer with weeds
and water from a pond and to let it remain undisturbed for
a few minutes. If the gathering is successful there will be
seen adhering to the sides of the glass, the bottom of the
saucer, or the weeds, little white, tawny, or green bodies,
about as thick as fine sewing cotton, and 2 — 6 mm. in
length. They adhere pretty firmly by one end, and examin-
FIG. 49.— Hydra.
A, Two living specimens of H. viridis attached to a bit of weed.
The larger specimen is fully expanded, and shows the elongated body
ending distally in the hypostome (hyp), surrounded by tentacles (/), and
three buds (bd{, bd1, bd'A) in different stages of development : a small
water-flea (a) has been captured by one tentacle. The smaller specimen
(to the right and above) is in a state of complete retraction, the tentacles
(t) appearing like papillae.
B, H. fttsca, showing the mouth (mth) at the end of the hypostome
(hyp], the circlet of tentacles (/), two spermaries (spy], and an ovary
(ovy).
c, a Hydra creeping on a flat surface by looping movements,
D, a specimen crawling on its tentacles,
(c and D after W. Marshall.)
220 HYDRA LESS.
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. 49, 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
(hyp\ at the apex of which is a circular aperture, the mouth
(mtti). 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 enter on (Fig. 50, 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. vulgartSj 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. 49, 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. 49, 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.
xxi MOVEMENTS 221
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.
49, c) : the body is bent round until the distal end touches
the surface : then the base is detached and moved nearer the
distal end, which is again moved forward, and so on. It has
also been observed to crawl like a cuttle fish (D) by means of
its tentacles, the body being kept nearly vertical.
It is also possible to watch a Hydra feed. It is a very
voracious creature, and to see it catch and devour its prey is
a curious and interesting sight. In the water in which it
lives are always to be found numbers of " water-fleas," minute
animals from about a millimetre downwards in length,
belonging to the class Crustacea, a group which includes
lobsters, crabs, shrimps, &c.
Water-fleas swim very rapidly, and occasionally one may be
seen to come in contact with a Hydra's tentacle. Instantly
its hitherto active movements stop dead, and it remains
adhering in an apparently mysterious manner to the tentacle.
If the Hydra is not hungry it usually liberates its prey after
a time, and the water-flea may then be seen to drop through
the water like a stone for a short distance, but finally to
expand its limbs and swim off. If however the Hydra has
not eaten recently it gradually contracts the tentacle until
the prey is brought near the mouth, the other tentacles being
also used to aid in the process. The water-flea is thus forced
against the apex of the hypostome, the mouth expands
widely and seizes it, and it is finally passed down into the
digestive cavity. Hydrae can often be seen with their bodies
bulged out in one or more places by recently swallowed
water-fleas.
The precise structure of Hydra is best made out by cutting
222 HYDRA LESS. XXI
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. 50.
The whole animal is seen to be built up of cells, each
consisting of protoplasm with a large nucleus (B, c, nu), and
with or without vacuoles. As in the case of most animal
cells, there is no cell-wall. Hydra is therefore a solid aggre-
gate : but the way in which its constituent cells are arranged
is highly characteristic and distinguishes it at once from a
plant.
The essential feature in the arrangement of the cells is
that they are disposed in two layers round the central
digestive cavity or enteron (A, ent. cav) and the cavities of
tentacles (ent. cav'}. So that the wall of the body is formed
throughout of an outer layer of cells, the ectoderm (set), 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 air'mal, and
may be compared to a chimney built of two layers of radially
arranged bricks with a space between the layers filled with
mortar or concrete.
Accurate examination of thin sections, and of specimens
teased out or torn into minute fragments with needles, shows
that the structure is really much more complicated than the
foregoing brief description would indicate.
The ectoderm cells are of two kinds. The first and most
obvious (B, ect and c), are large cells of a conical form, the
bases of the cones being external, their apices internal. Spaces
FIG. 50.— Hydra.
A, Vertical section of the entire animal, showing the body-wall com-
posed of ectoderm (ect) and endoderm (end), enclosing an enteric cavity
224 HYDRA LESS, xxi
(ent. cav), which, as well as the two layers, is continued (ent. cav') into
the tentacles, and opens externally by the mouth (mtk) at the apex of
the hypostome (hyp}. Between the ectoderm and endoderm is the
mesoglcea (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 (fl\ Two buds (&/',
bd?) in different stages of development are shown on the left side, and
on the right a spermary (spy) and an ovary (ovy) containing a single
ovum (ov).
B, portion of a transverse section more highly magnified, showing the
large ectoderm cells (ect) and interstitial cells (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 mesoglcea (msgl) : endoderm cells (end) with large
vacuoles and nuclei (mi), pseudopods (psd), and flagella (y?). 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 (mi) and muscle-
process (m. pr).
D, an endoderm cell of H. viridis, showing nucleus (nil), 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. /r), 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. 129), a high degree of contractility, the
almost instantaneous shortening of the body being due, in
great measure at least, to their rapid and simultaneous
contraction. It is probably correct to say that, while the
ectoderm cells are both contractile and irritable, a special
FIG. 51. — Hydra.
A, A nematocyst contained in its cnidoblast (cub], showing the coiled
filament and the cnidocil (cnc\
B, The same after extrusion of the thread, showing the larger and
smaller barbs at the base of the thread, nu, the nucleus of the
cnidoblast.
c, A cnidoblast, with its contained nemalocyst, connected with one
of the processes of a nerve-cell (nv. c}.
(After Schneider.)
Q
226 HYDRA LESS.
degree of contractility is assigned to the muscle-processes
while the cells themselves are eminently irritable, the slightest
stimulus applied to them being followed by an immediate
contraction of the whole body.
Imbedded in some of the large ectoderm cells are found
clear, oval sacs (A and B, ntc\ with very well-defined walls
and called nematocysts. Both in the living specimen and in
sections they ordinarily present the appearance shown in
Fig. 50, B. ntc, and Fig. 51 A, but are frequently met with
in the condition shown in Fig. 50 E, and Fig. 51 B: that
is, with a short conical tube protruding from the mouth of
the sac, armed near its distal end with three recurved
barbs besides several similar processes of smaller size,
and giving rise distally to a long, delicate, flexible fila-
ment.
Accurate examination of the nematocysts shows that the
structure of these curious bodies is as follows. Each con-
sists of a tough sac (Fig. 51, A), one end of which is turned
in as a hollow pouch : the free end of the latter is continued
into a hollow coiled filament, and from its inner surface
project the barbs. The whole space between the wall of
the sac and the contained pouch and thread is tensely filled
with fluid. When pressure is brought to bear on the outside
of the sac the whole apparatus goes off like a harpoon-gun
(B), the compression of the fluid forcing out first the barbed
pouch and then the filament, until finally both are turned
inside out.
It is by means of the nematocysts — the resemblance of
which to the trichocysts of Paramcecium (p. 113) should be
noted — that the Hydra is enabled to paralyze its prey. Prob-
ably some specific poison is formed and ejected into the
wound with the thread : in the larger members of the group
to which Hydra belongs, such as jelly-fishes, the nematocysts
xxi NEMATOCYSTS 227
produce an effect on the human skin quite like the sting of
a nettle.
The nematocysts are formed in special interstitial cells
called cnidob lasts (Fig. 50, B, cnbl and Fig. 51), and are thus
in the first instance at a distance from the surface. But the
cnidoblasts migrate outwards, and so come to lie quite
superficially either in or between the large ectoderm cells.
On its free surface the cnidoblast is produced into a delicate
pointed process, the cnidocil or " trigger-hair " (cnc}. In all
probability the slightest touch of the cnidocil causes con-
traction of the cnidoblast, and the nematocyst, thus com-
pressed, instantly explodes.
Nematocysts are found in the distal part of the body, but
are absent from the foot or proximal end, where also there
are no interstitial cells. They are especially abundant in the
tentacles, on the knob-like elevations of which — due to little
heaps of interstitial cells — they are found in great numbers.
Amongst these occur small nematocysts with short threads
and devoid of barbs (Fig. 50, A, ntc and F).
There are sometimes found in connection with the cnido-
blasts small irregular cells with large nuclei : they are called
nerve-cells (Fig. 51, c, nv. c\ and constitute a rudimentary
nervous system, the nature of which will be more con-
veniently discussed in the next lesson (p. 242).
The ectoderm cells of the foot differ from those of the rest
of the body in being very granular (Fig. 50 A). The
granules are probably the material of the adhesive substance
by which the Hydra fixes itself, and are to be looked upon as
products of destructive metabolism : i.e. as being formed by
conversion of the protoplasm in something the same way as
starch granules (p. 33). This process of formation in a cell
of a definite product which accumulates and is finally dis-
charged at the free surface of the cell is called secretion,
Q 2
228 HYDRA LESS.
and the cell performing the function is known as a gland-
cell.
The endoderm consists for the most part of large cells
which exceed in size those of the ectoderm, and are re-
markable for containing one or more vacuoles, sometimes
so large as to reduce the protoplasm to a thin superficial
layer containing the nucleus (Fig. 50, A and B, end}. Then
again, their form is extremely variable, their free or inner
ends undergoing continual changes of form. This can be
easily made out by cutting transverse sections of a living
Hydra, when the endoderm cells are seen to send out long
blunt pseudopods (psd) into the digestive cavity, and now
and then to withdraw the pseudopods and send out from
one to three long delicate flagella (fl). Thus the endoderm
cells of Hydra illustrate in a very instructive manner the
essential similarity of flagella and pseudopods already re-
ferred to (p. 52). In the hypostome the endoderm is thrown
into longitudinal folds, so as to allow of the dilatation of
the mouth in swallowing.
Amongst the ordinary endoderm cells are found long
narrow cells of an extremely granular character. They are
specially abundant in the distal part of the body, beneath
the origins of the tentacles, and in the hypostome, but are
absent in the tentacles and in the foot. There is no doubtf
that they are gland-cells, their secretion being a fluid used
to aid in the digestion of the food.
In Hydra viridis the endoderm-cells (D) contain chroma-
tophores (chr) coloured green by chlorophyll, which performs
the same function as in plants, so that in this species holozoic
is supplemented by holophytic nutrition. There is reason
for believing that the chromatophores are to be regarded as
symbiotic algae, like those found in connection with Radio-
xxi DIGESTION 229
laria (p. 154). In H. fusca bodies resembling these chromato-
phores are present, but are of an orange or brown colour, and
devoid of chlorophyll. Brown and black granules occurring
in the cells (B) seem to be due in part to the degeneration of
the chromatophores, and in part to be products of excretion.
Muscle-processes exist in connection with the endoderm
cells, and they are said to take a transverse or circular
direction, i.e., at right angles to the similar processes of
the ectoderm cells.
When a water-flea or other minute organism is swallowed
by a Hydra, it undergoes a gradual process of disintegration.
The process is begun by a solution of the soft parts due to
the action of a digestive fluid secreted by the gland-cells of
the endoderm ; it is apparently completed by the endoderm
cells seizing minute particles with their pseudopods and
engulfing them quite after the manner of Amoebae. It is
often found that the protrusion of pseudopods during
digestion results in the almost complete obliteration of the
enteric cavity.
It would seem therefore that in Hydra the process of
digestion or solution of the food is to some extent at least
infra-cellular, i.e., takes place in the interior of the cells
themselves, as in Amoeba or Paramcecium : it is however
mainly extra-cellular or enteric i.e., is performed in a special
digestive cavity lined by cells.
The ectoderm cells do not take in food directly, but are
nourished entirely by diffusion from the endoderm. Thus
the two layers have different functions : the ectoderm is pro-
tective and sensory ; it forms the external covering of the
animal, and receives impressions from without ; the endo-
derm, removed from direct communication with the outer
world, performs a nutrient function, its cells alone having
the power of digesting food.
230 HYDRA LESS.
The essential difference between digestion and assimilation
is here plainly seen : all the cells of Hydra assimilate, all
are constantly undergoing waste, and all must therefore form
new protoplasm to make good the loss. But it is the endo-
derm cells alone which can make use of raw or undigested
food : the ectoderm has to depend upon various products of
digestion received by osmosis from the endoderm.
It will be evident from the preceding description that
Hydra is comparable to a colony of Amoebae in which par-
ticular functions are made over to particular individuals —
just as in a civilized community the functions of baking and
butchering are assigned to certain members of the commu-
nity, and not performed by all. Hydra is therefore an ex-
ample of individuation : morphologically it is equivalent
to an indefinite number of unicellular organisms : but,
these acting in concert, some taking one duty and some
another, form, physiologically speaking, not a colony of
largely independent units, but a single multicellular in-
dividual.
Like 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. 49, A, b;sts (ntc). In the middle part the ectoderm is removed, and
the muscle-fibres (w.f) and nerve-cells (nv. c) are exposed. In the
upper part the muscular and nervous layer is removed, and parts of two
endoderm cells (end) are shown ; n-n, nucleus.
(From Parker and Haswell, after von Lendenfeld.)
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. 53, A, would, if formed
238 HYDROID POLYPES LESS.
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 skeleton}, but like the shell of a crab
or lobster lying altogether outside the soft parts (exo-
skeleton).
As to the mode of formation of the cuticle : — we saw that
many organisms, such as Amceba and Hsematococcus, form,
on entering into the resting condition, a cyst or cell- wall, by
secreting or separating from the surface of their protoplasm
a succession of layers either of cellulose or of a transparent
horn-like substance. But Amceba and Haematococcus are
unicellular, and are therefore free to form this protective
layer at all parts of their surface. The* ectoderm cells of
Bougainvillea on the other hand are in close contact with
their neighbours on all side's 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 exoskeletal structure formed in this way, i.e. by the
secretion of successive layers from the free faces of adjacent
cells, that the name cuticle is in strictness applied in multi-
cellular organisms.
xxn STRUCTURE OF A MEDUSA 239
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 or umbrella (c) is formed of a gela-
tinous substance (Fig. 55, D, insgl) covered on both its inner
surface or sub-umbrella and on its outer surface or ex-umbrella
by a thin layer of delicate cells (ect). The clapper-like
organ or manubrium (Fig. 53, 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
mesogloea; it is hollow, its cavity (Fig. 55, D, ent. cav) open-
ing 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. 53, c, rad. c, and Fig. 54, D and D' rad) which extend
through the gelatinous substance of the umbrella at equal
distances from one another, like four meridians, and finally
open into a circular canal (dr. c} which runs round the edge
of the umbrella. 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
umbrella is a delicate sheet of cells, the endoderm-lamella
(D', end. Id).
From the edge of the umbrella four pairs of tentacles
(Fig. 53, c and Fig. 55, D, t) are given off, one pair corres-
240 HYDROID POLYPES LESS.
ponding to each radial canal, and close to the base of each
tentacle is a little speck of pigment (Fig. 53, oc\ the ocellus
or eye-spot. Lastly, the margin of the umbrella 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 simple polype 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-
glcea. A form would be produced like c, i.e. a medusa-like
body with umbrella and manubrium, but with a continuous
cavity (c', ent. cav) in the thickness of the umbrella instead- of
four radial canals. Finally, suppose the inner and outer walls
of this cavity to grow towards one another and meet, thus
obliterating the cavity, except along four narrow radial areas
(D, rad) and a circular area near the edge of the umbrella
(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. Id] — indi-
cating the former extension of the cavity.
It follows from this that the inner and outer layers of the
manubrium are respectively endoderm and ectoderm : that
the gelatinous tissue of the umbrella is an immensely
thickened mesoglcea : that the layer of cells covering both
inner and outer surfaces of the umbrella is ectodermal : and
that the layer of cells lining the system of canals, together
with the endoderm-lamella. is endodermal,
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 simple polype, showing the tubular body
with enteric cavity (ent. cav), hypostome (hyp), mouth (nith\ and
tentacles (/).
R
242 HYDROID POLYPES LESS.
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 umbrella
(ent. cav') : the hypostome now forms a manubrium (mtib).
c', transverse section of the same through the plane a b, showing the
continuous cavity (ent. cav') in the umbrella.
D, fully formed medusa : the cavity in the umbrella is reduced to the
radiating (rad) and circular (cir. c) canals, the velum (v) is formed, and
a double nerve-ring (nv, nv) is produced from the ectoderm.
D', transverse section of the same through the plane a &, showing the
four radiating canals (rad) united by the endoderm-lamella (end. /a),
produced by partial obliteration of the continuous cavity ent. cav' in C'
Thus the medusa and the hydranth are similarly con-
structed or homologous structures, and the hydroid colony,
like Zoothamnium (p. 136), is dimorphic, bearing zooids of
two kinds.
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 umbrella
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 pro-
pelled through the water by a series of jerks. The movement
is performed by means of the muscle-processes and muscle-
fibres of the sub-umbrella and velum, both of which differ
from the similar structures in the hydranth in exhibiting a
delicate transverse striation (Fig. 57).
There is still another important matter in the structure of
the medusa which has not been referred to. At the junction
of the velum with the edge of the bell there lies, imme-
diately beneath the ectoderm, a layer of peculiar branched
cells (Fig. 56, B, n. c), containing large nuclei and produced
into long fibre-like processes. These nerve-cells (see p. 227)
XXII
NERVOUS SYSTEM
243
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 (nv1} immediately below the insertion of the
velum. An irregular network of similar cells and fibres occurs
on the inner or concave face of the umbrella, between the
ectoderm and the layer of muscle-fibres. The whole consti-
FiG. 56. — A, Muscle fibres from the inner face of the bell of the
medusa of a hydroicl polype (Eucopel/a campanidaria), showing nucleus
and transverse striation.
B, portion of the nerve- ring of the same, showing two large nerve-
cells (;/. c) and muscle-fibres (in. c] on either side. (After von Len-
denfeld.).
tutes the nervous system of the medusa ; the double nerve-ring
is the central, the network the peripheral nervous system.
Some of the processes of the nerve-cells are connected
with ordinary ectoderm -cells, which thus as it were connect
the nervous system with the external world : others, in some
instances at least, are probably directly connected with
muscle-fibres.
We thus see that while the manubrium of a medusa has
the same simple structure as a hydranth, or what comes to
R 2
244 HYDROID POLYPES LESS.
the same thing, as a Hydra, the umbrella has undergone a very
remarkable differentiation of its tissues. Its ordinary ecto-
derm cells, instead of being large and eminently contractile,
form little more than a thin cellular skin or epithelium over
the gelatinous mesogloea : they have largely given up the
function of contractility to the muscle processes or fibres,
and have taken on the functions of a protective and sensitive
layer.
Similarly the function of automatism, possessed by the
whole body of Hydra, is made over to the group of specially
modified ectodermal cells which constitute the central
nervous system. If a Hydra is cut into any number of
pieces, each of them is able to perform the ordinary move-
ments of expansion and contraction, but if the nerve-ring
of a medusa is removed by cutting away the edge of the
umbrella, the rhythmical swimming movements stop dead :
the bell is in fact permanently paralysed.
It is not, however, rendered incapable of movement, for
a sharp pinch, i.e. an external stimulus, causes a single con-
traction, showing that the muscles still retain their irritability.
But no movement takes place without such external stimulus,
each stimulus giving rise infallibly to one single contraction :
the power possessed by the entire animal of independently
originating movement, i.e. of supplying its own stimuli, is
lost with the central nervous system.
Another instance of morphological and physiological
differentiation is furnished by the pigment spots or ocelli
(Fig- 53) c> oc) situated at the bases of the tentacles. They
consist of groups of ectoderm cells in which are deposited
granules of deep red pigment. Their function is proved by
the following experiment.
If a number of medusae are placed in a glass vessel of
water in a dark room, and a beam of light from a lantern is
xxii GONADS 245
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. 136)0
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 pro-
ducing 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 arise in the first in-
stance from the ectoderm of the stem o the hydroid
colony, afterwards migrating, while still small and im-
mature, to their permanent situation where they undergo
their final development. In 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.
246 HYDROID POLYPES LESS.
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 polyp last.
So far all the cells of the polyplast are alike — globular
nucleated masses of protoplasm squeezed into a polyhedral
form by mutual pressure. But before long the cells lying
next the surface alter their form, becoming cylindrical, with
their long axes disposed radially (F). In this way a superficial
layer of cells, or ectoderm, is differentiated from an internal
mass, or endoderm.
The embryo now assumes an elongated form (G) and
begins to exhibit slow, worm-like movements, finally escaping
from the parent and beginning a free existence (H). The
ectoderm cells are now found to be ciliated, and before long
a cavity appears in the previously solid mass of endoderm
cells : this is the first appearance of the enteron or digestive
cavity. In this stage the embryo is called a planula : it
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-
xxii DEVELOPMENT 247
merit, a dilatation is formed a short distance from the free or
A ^ B _ C _ Dx^ni^N. E
FIG. 57, — Stages in the development of two hydroid polypes, Lao-
niedea flexuosa (A-H) and Eudendriiim ramosum (I-M).
A, oosperm.
B, two-celled, and c, four-celled stage,
u, E, polyplast.
F, G, formation of planula by differentiation of ectoderm and
endoderm.
In A--G the embryo is embedded in the maternal tissues.
H, free swimming planula, showing ciliated ectoderm, and endoderm
enclosing a narrow enteric cavity.
I, planula, after loss of its cilia, about to affix itself.
K, the same after fixation.
L, Hydra-like stage, still enclosed in cuticle.
M, the same after rupture of the cuticle and liberation of the tentacles.
(After Allman.)
distal end, and a thin cuticle is secreted from the whole
surface of the ectoderm (K). From the dilated portion
248 HYDROID POLYPES LESS.
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. We have
what is called an alternation of generations, the asexual genera-
tion or agamobium (hydroid colony) giving rise by budding
to the sexual generation QI gamobium (medusa), which in its
turn produces the agamobium by a sexual process, i.e. by
the conjugation of ovum and sperm.
Two other Hydroids must be briefly referred to in con-
cluding the present lesson.
Floating on the surface of the ocean in many parts of the
world is found a beautiful transparent organism called
Diphyes. It consists of a long, slender stem (Fig. 58, A, a\
at one end of which are attached two structures called
swimming-bells (m, 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, n) with
an expanded, trumpet-like mouth, through which food is
taken : this is clearly a hydranth. From the base of the
hydranth proceeds a single, long, branched tentacle or
xxii DIPHYES AND PORPITA 249
" grappling-line " (/), abundantly provided with nematocysts.
Springing from the stem near the base of the hydranth is a
body called a medusoid (§), very like a sort of imperfect
medusa, and like it, containing gonads. Lastly, enclosing all
these structures, much as the white petal oid bract of the
common Arum-lily encloses the flower-stalk, is a delicate
folded membranous plate (t\ to which the name bract.
borrowed from botany, is applied. The whole organism is
propelled through the water by the rhythmical contraction
of the swimming-bells.
Microscopic examination shows that the stem consists, like
that of Bougainvillea, of ectoderm, mesoglcea, and endo-
derm, but without a cuticle. The hydranth has a similar
structure to that of Bougainvillea, only differing in shape
and in the absence of tentacles round the mouth : the grap-
pling lines are formed on the polype-type : the medusoids are
merely simplified medusae : the swimming-bells are practic-
ally medusae in which the manubrium is absent : and the
bracts 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.
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-
logical and physiological differentiation are thus carried
much further than in such a form as Bougainvillea.
Porpita is another free-swimming Hydroid, presenting at
first sight no resemblance whatever to Diphyes. It has much
the appearance of a flattened medusa (Fig. 59), consisting
of a circular disc, slightly convex above and concave below,
bearing, round its edge a number of close-set tentacles, and
on its under side a central tubular organ (hy) with a ter-
250
HYDROID POLYPES
LESS.
FlG. 58. — Diphyes campanulata.
A, the entire colony, natural size, showing stem (a) bearing groups of
zooids (") and two swimming bells (m, m), 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 (*). (From Gegenbaur.)
XXII
DIPHYES AND PORPITA
251
minal mouth, like the manubrium of a medusa, surrounded
by a great number of structures like hollow tentacles (hy1).
FIG. 59. — A, Porpita pacifica (nat. size), from beneath, showing disc-
like stem surrounded by tentacles (t\ a single functional hydranth (hy),
and numerous mouthless hydranths (hy'\
B, vertical section of P, mediterranea, showing the relative positions
of the functional (hy) and mouthless (hy") hydranths, the tentacles,
and the chambered shell (sh\ (A after Duperrey ; B from Huxley, after
Kolliker.)
The discoid body is supported by a sort of shell having the
consistency of cartilage and divided into chambers which
contain air (B, s/i).
Accurate examination shows that the manubrium-like
252 HYDROID POLYPES LESS, xxn
body (/(r) on the under surface is a hydranth, that the short,
hollow, tentacle-like bodies (///) surrounding it are mouthless
hydranths, and that the disc represents the common stem of
Diphyes or Bougainvillea. So that Porpita is not what it
appears at first sight, a single individual, like a Medusa or a
Hydra, but a colony in which the constituent zooids have
become so modified in accordance with an extreme division
of physiological labour, that the entire colony has the char-
acter of a single physiological individual.
It was pointed out in the previous lesson (p. 230) that
Hydra, while morphologically the equivalent of an indefinite
number of unicellular organisms, was yet physiologically a
single individual,- its constituent cells being so differentiated
and combined as to form one whole. A further stage in this
same process of individuation is seen in Porpita, in which not
cells but zooids, each the morphological equivalent of an
entire Hydra, are combined and differentiated so as to form
a colony which, from the physiological point of view, has
the characters of a single individual.
LESSON XXIII
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. Fertilisation 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 (pogenesis\ as well as the exact steps of the pro-
cess by which an oosperm or unicellular embryo is formed
by the union of the two sexual elements. The following
description applies to animals : recent researches show that
essentially similar processes take place in plants.
Both ovary and spermary are at first composed of cells of
the ordinary kind, the primitive sex-cells, and it is only by
the further development of these that the sex of the gonad
is determined.
In the spermary the sex-cells (Fig.6o, A) undergo repeated
fission, forming what are known as the sperm-mother-cells
(B). These have been found in several instances to be
LESS.
254 SPERMATOGENESIS AND OOGENESIS
distinguished by a peculiar condition of the nucleus. We
saw (p. 65) that the number of chromosomes is constant in
B
cJir
-chr
D
FlG. 60. — Spermatogenesis in the Mole-Cricket {Gryllotalpd}.
A. Primitive sex-cell, just preparatory to division, showing twelve
chromosomes (chr) ; c, the centrosome.
B. Sperm-mother-cell, formed by the division of A, and containing
twenty-four chromosomes. The centrosome has divided into two.
C. The sperm-mother-cell has divided into two by a reducing division,
each daughter-cell containing twelve chromosomes.
D. Each daughter-cell has divided again in the same manner, a group
of four sperm-cells being produced, each with six chromosomes.
E. A single sperm-cell about to elongate to form a sperm.
F. Immature sperm ; the six chromosomes are still visible in the
head.
G. Fully formed sperm.
(After von Rath. )
xxin REDUCING DIVISION 255
any given animal, though varying greatly in different species.
In the formation of the sperm-mother-cells from the primitive
sex-cells the number becomes doubled : in the case of
the mole-cricket, for instance, shown in Fig. 61, while the
ordinary cells of the body, including the primitive sex-
cells, contain twelves chromosomes, the sperm-mother-cells
contain twenty-four.
The sperm-mother-cell now divides (c), but instead of its
chromosomes splitting in the ordinary way (p. 64 and Fig. 10)
half of their total number — in the present instance twelve —
passes into each daughter cell : in this way two cells are
produced having the normal number of chromosomes. The
process of division is immediately repeated in the same
peculiar way (D), the result being that each sperm-mother-
cell gives rise to a group of four cells having half the normal
number of chromosomes — in the present instance six. The
four cells thus produced are the immature sperms (E) : in
the majority of cases the protoplasm of each undergoes a
great elongation, being converted into a long vibratile thread,
the tail of the sperm (F, G), while the nucleus becomes its
more or less spindle-shaped head and the centrosome takes
the form of a small intermediate piece at the junction of
head and tail.
Thus the sperm or male gamete is a true cell, specially
modified in most cases for active movement : its head,
representing the nucleus, is directed forwards in progres-
sion, its long tail, formed from the protoplasm, backwards.
The direction of movement is thus the precise opposite of
that of a monad (p. 36) to which a sperm presents a certain
resemblance. This actively motile tailed form is, however,
by no means essential : in many animals the sperms are
non-motile and in some they resemble ordinary cells.
The peculiar variety of mitosis described above, by which
256 SPERMATOGENESIS AND OOGENESIS LESS.
the number of chromosomes in the sperm-mother-cells is
reduced by one-half, is known as a reducing division.
As already stated, the ova arise from primitive sex-cells,
precisely resembling those which give rise to sperms. These
divide and give rise to the egg-mother-cells in which, as in
the sperm-mother-cells, the number of chromosomes is
doubled. The egg-mother-cells do not immediately undergo
division but remain passive and increase, often enormously,
in size, by the absorption of nutriment from surrounding
parts : in this way each egg-mother-cell becomes an ovum.
Sometimes this nutriment is simply taken in by osmosis,
in other cases the growing ovum actually ingests neigh-
bouring cells after the manner of an Amoeba. Thus in the
developing egg the processes of constructive are vastly
in excess of those of destructive metabolism.
We saw in the second lesson (p. 33) that the products of
destructive metabolism might take the form either of waste
products which are got rid of, or of plastic products which
are stored up as an integral part of the organism. In the
developing egg, in addition to increase in the bulk of the
protoplasm itself, a formation of' plastic products usually
goes on to an immense extent. In plants the stored-up
materials may take the form of starch, as in Nitella (p. 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-formed egg opaque, so that its structure can often be
made out only in sections. When the quantity of yolk is
very great the ovum may attain a comparatively enormous
size, as for instance in birds, in which, as already mentioned
(p. 69), the " yolk " is simply an immense egg-cell.
When fully formed, the typical animal ovum (Fig. 61)
STRUCTURE OF THE OVUM 257
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 con-
siderable thickness and known as the vitelline membrane :
frequently it is perforated at one pole by an aperture, the
micropyle (fig. 62, microp). The nucleus is large and has
FIG. 61. — Ovum of a Sea-urchin (Toxopneiistes lividits\ showing the
radially- striated cell-wall (vitelline membrane), the protoplasm contain-
ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its
network of cliromaiin, and a large nucleolus (germinal spot). (From
Balfour after Hertwig.)
the usual constituents (p. 63) — nuclear membrane, nuclear
sap, and chromatin. As a rule there is a very definite nucle-
olus, 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 segmentation it has to go through a process known
as the maturation of the egg.
s
B
mem,
2 prort
FIG. 62.— The Maturation and Impregnation of the Animal Ovum.
A, the ovum, surrounded by the vitelline membrane (mew), in the
act of forming the first polar cell (pot) : ? cent, centrosome.
B, both polar cells (pol) are formed, the female pronucleus ( 9 frv*}
lies near the centre of the ovum, and one of several sperms is shown
making its way into the ovum at the micropyle (microp}.
LESS, xxiii POLAR CELLS 259
C, the head of the sperm has become the male pronucleus ( 6 pron),
its intermediate piece the male centrosome ( ) ; it
is separated from the preceding segment by a deep groove
and bears at its posterior end a small circular aperture, the
anus (An).
Eolygordius 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 and are distinguished by
special characters ; while between the peristomium 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. 67) show
that this cavity is not a mere space excavated in the interior
of the body, but a definite tube, the enteric canal (A, B),
which passes in a straight line from mouth to anus, and is
separated in its whole extent from the walls of the body
(A, B. W) by a wide space, the body cavity or ccelome (Ccel).
So that the general structure of Polygordius might be imi-
tated by taking a wide tube, stopping the ends of it with
?S
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). (After Fraipont. )
A nephridium may therefore be defined as a ciliated tube,
lying in the thickness of the body-wall and opening at one
end into the coelome and at the other on the exterior of
the body.
In the higher worms, such as the earthworm, the nephridia
are lined in part by gland-cells, and are abundantly supplied
with blood-vessels. Water and nitrogenous waste from all
parts of the body pass by diffusion into the blood and are
conveyed to the nephridia, the gland-cells of which withdraw
the waste products and pass them into the cavities of the
tubes, whence they are finally discharged into the surround-
ing medium. In all probability some such process as this
takes place in Polygordius
XXTV NERVOUS SYSTEM 283
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. 243) 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-
organised 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. 67, 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 frpm 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 (Ksophageal connectives (CEs. Co?i.} which pass respectively
right and left of the gullet.
It is to be noted that one division of the central nervous
system — the brain — lies altogether above and in front of the
enteric canal, the other division — the ventral nerve-cord —
altogether beneath it, and that, in virtue of the union of the
two divisions by the cesophageal connectives, the enteric
canal perforates the nervous system.
It is also important to notice that the nervous system is
throughout in direct contact with the epidermis or ectoderm,
the ventral cord appearing in sections (Fig. 67, c, and Fig.
70, A) as a mere thickening of the latter.
Both brain and cord are composed of delicate nerve-fibres
284
POLYGORDIUS
LESS.
(Fig. 69, 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-
Ejjthm
FIG. 69. — Diagram illustrating the relations of the nervous system of
Polygordius.
The deric epithelium (Der. Epthni) is either in direct contact with the
central nervous system (lower part of figure), or is connected by afferent
nerves (of. nv) with the inter-muscular plexus (int. tmtsc. plx) : the
latter is connected with 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. )
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
xxiv NERVOUS SYSTEM 285
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, plx.)
with nerve-cells (Nv. (?) at intervals, the inter-muscular
plexus. Some of the branches of this plexus are traceable
to nerve-cells in the central nervous system, others (af. nv.)
to epidermic cells, others (Ef. nv.} to the delicate proto-
plasmic layer covering the muscle-plates. The superficial
cells of both brain and cord are also, as has been said, in
direct connection with the overlying epidermis, and from the
anterior end of the brain a bundle of nerve-fibres (Fig 67, B,
Nv.) is given off on each side to the corresponding tentacle,
constituting the nerve of that organ, to the epidermic cells of
which its fibres are distributed.
We see then that, apart from the direct connection of
nerve-cells with the epidermis, the central nervous system is
connected, through the intermediation of nerve-fibres (a)
with the sensitive cells of the deric epithelium and (b) with
the contractile muscle-plates. And we can thus distinguish
two sets of nerve-fibres, (a) sensory or afferent (af. nv.)
which connect the central nervous system with the epidermis,
and (b) motor or efferent (Ef. nv.) which connect it with the
muscles.
Comparing the nervous system of Polygordius with that
of a medusa (p. 243) there are two chief points to be noticed.
Firstly, the concentration of the central nervous system in
the higher type, and the special concentration at the anterior
end of the body to form a brain. Secondly, the important
fact that the inter-muscular plexus is not, like the peripheral
nervous system of a medusa which it resembles, situated
immediately beneath the epidermis (ectoderm) but lies in the
muscular layer, or, in other words, has sunk into the
mesoderm.
286 POLYGORDIUS LESS.
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 (ther-
mal stimulus), or an electric current (electrical 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 then, as it
were, reflected along a motor nerve to a muscle, is called a
reflex action ; the essence of the arrangement is the inter-
position of nerve-cells between sensory or afferent nerves
connected with sensory cells, and motor or efferent nerves
connected with muscles.
The diagram (Fig. 69) serves to illustrate this matter.
The muscle-plate (M. PL) may be made to contract by a
ORGANS OF SENSE 287
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 (af. nv.), or (e) to the
epidermic cells (Der. Epthni}.
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 writh 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. 66,
B, c.p\ the function of which is not known, the only definite
organs of sense are the tentacles, which have a tactile
function, their abundant nerve-supply indicating that their
delicacy as organs of touch far surpasses that of the general
surface of the body. They are beset with short, fine pro-
cesses of the cuticle called seta (Figs. 66 and 67, s\ which
probably, like the whiskers of a cat, serve as conductors of
external stimuli to the sensitive epidermic cells.
288 POLYGORDIUS LESS.
There are two matters of general importance in connec-
tion with the structure of Polygordius to which the student's
attention must be drawn in concluding the present lesson.
Notice in the first place how in this type, far more than in
any of those previously considered, we have certain definite
parts of the body set apart as organs for the performance of
particular functions. There is a mouth for the reception of
food, an enteric canal for its digestion, and an anus for the
extrusion of faeces : a ccelomic 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 ccelomic
epithelia we meet with nothing new, but the muscle-plates
are not cells, the nephridia show no cell-structure, neither do
the nerve-fibres nor the punctate substance of the brain.
The body is thus divisible into tissues or fabrics each clearly
distinguishable from the rest. We have epithelial tissue,
cuticular tissue, muscular tissue, and nervous tissue : and
the blood and 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 :
xxiv ANATOMY AND HISTOLOGY 289
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. 273) 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. 274). Thus certain of the tissues of
Polygordius are multinucleate but non-cellular. They are
comparable in minute structure to an Opalina or to the
plasmodium of a Mycotozoon, and must therefore be dis-
tinguished from such definitely cellular tissues as the enteric
epithelium. ,
LESSON XXV
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.
70, A, O.M) and certain of the cells of codomic 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 245).
In the male the primitive sex-cells divide and sub-divide.
the ultimate products being converted into sperms (Fig. 70
D.V
t'u
M.Pl
FIG. 70. — 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. 67, c
The body-wall consists of cuticle (Ctt), deric epithelium (Der. Epthm},
muscle-plates (.17. /"/). and parietal layer of coelomic epithelium (Ca~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 coelomic epithelium (Cal. Epthm'} :
connecting it with the body-wall are the dorsal and ventral mesenteries
formed of a double layer of coelomic epithelium, and containing respec-
tively the dorsal (D. V} and ventral ( V. V) blood-vessels.
Passing obliquely across the ccelome are the oblique muscles ( 0. M}
U 2
292 POLYGORDIUS LESS.
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. Epthm, A/. 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. Epthm], and the septa (Sep).
(After Fraipont.)
B: see p. 255) : in the female they enlarge immensely, and
take on the character of ova (c, ov). Multiplication of the
sexual products takes place to such an extent that the whole
ccelome becomes crammed full of either sperms or ova (c).
In the female the growth of the eggs takes place at the
expense of all other parts of the body, which undergo more
or less complete atrophy : the epidermis for instance, be-
comes liquefied and the muscles lose their contractility.
Finally rupture of the body-wall takes place in each segment
(c), and through the slits thus formed the eggs escape. So
that Polygordius, like an annual plant, produces only a
single brood ; death is the inevitable result of sexual
maturity. Whether or not the same dehiscence of the body-
wall takes place in the male is not certain : it has been stated
that the sperms make their escape through the nephridia.
Thus while there are no specialized gonoducts, or tubes for
carrying off the sexual products, it is possible that the ne-
phridia may, in -addition to their ordinary function, serve
the purpose of male gonoducts or spermiducts. Female gono-
ducts or oviducts are however entirely absent.
The ova and sperms being shed into the surrounding
water, impregnation takes place, and the resulting oosperm
undergoes segmentation or division (see p. 246), a polyplast
being formed. The cells of the polyplast become differen-
tiated, an enteron or digestive cavity is formed, and the
XXV
THE TROCHOSPHERE
293
embryo is gradually converted into a curious free-swimming
creature shown in Fig. 7 1 , A, and called a trochosphere.
The trochosphere, or newly-hatched larva of Polygordius
(Fig. 71, A) is about J mm. in diameter, and has something
A
FIG. 71.— A, larva of Polygordius neapolitamis 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 (Sf. 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 cat).
The mesoderm is confined to two narrow bands of cells (B and C,
Msd] in the blastocoele, one on either side of the proctodaeum ; slender
mesodermal bands (Msd'} are also seen in the prostomium in A.
The cilia consist of a prae-oral circlet (Pr. or. ci) above the mouth, a
post-oral circlet (Pt. or. ci) below the mouth, and an anal circlet (An.
ci) around the anus.
(A after Fraipont.)
the form of a top, consisting of a dome-like upper portion,
the prostomium, produced into a projecting horizontal rim ;
of an intermediate portion or peristomium, having the form
of an inverted hemisphere ; and of a lower somewhat conical
294 POLVGORDIUS LESS.
anal region. Around the projecting rim is a double circlet
of large cilia (Pr. or. a) by means of which the larva is
propelled through the water.
Beneath the edge of the ciliated rim is a rounded aperture,
the mouth (Mfh) ; 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. dni), 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. 275) being absent
or so poorly developed that they may be neglected for the
present.
Leaving aside ail 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. 241),
consisting as it does of an outer layer of cells forming the
external covering of the body and of an inner layer lining
the digestive cavity. There are, however, two important
differences : the space between the two layers is occupied by
the mesoglcea in the medusa, while in the worm it is a cavity
filled with fluid ; and the digestive cavity of the trochosphere
has two openings instead of one.
But in order to compare more accurately the medusa
with the trochosphere, it is necessary to fill up, by the help
of other types, an important gap in our knowledge of the
development of Polygordius— the passage from the polyplast
to the trochosphere. From what we know of the develop-
ment of other worms, the process, in its general features,
is probably as follows : —
The polyplast is converted, by the accumulation of fluid
XXV
FORMATION OF TROCHOSPHERE
295
in its interior, into a hollow sphere, bounded by a single
layer of cells and containing a cavity, the blastoccele : this
stage of development is called the blastula. Next, one side
of the blastula becomes tucked in or invaginated so as to
convert the embryo from a single-layered sphere into a
double-layered cup (Fig. 72, A). This process can be
sufficiently well imitated by pushing in one side of
a hollow india-rubber ball. The resulting embryonic stage
FIG. 72. — Diagram illustrating the origin of the trochosphere from
the gastrula. The ectoderm is dotted, the endoderm striated.
A, gastrula, with enteron (Ent) and gastrula-mouth (Cast. Mth\ and
with the ectoderm and endoderm separated by the larval body-cavity or
blastocoele (Bl. cccl}.
B, the gastrula-mouth has closed, the enteron (Ent) becoming a shut
sac.
C, two ectodermal pouches, the stomodseum (St. dm] and proctodseum
(Prc. dm) have appeared.
D, the stomodaeum (St. dm) and proctodseum (Prc. dm) have opened
into the enteron (Ent), forming a complete enteric canal with mouth
(Mth) and anus (An).
is known as the gastmla : its cavity is the enteron (Eni) and
is bounded by the invaginated cells which now con-
stitute the endoderm, the remaining cells, forming the outer
wall of the gastrula, being the ectoderm. The two layers
are continuous at the aperture of the cup, the gastrula-
mouth or blastopore (Gast. Mtti). Between the ectoderm
and endoderm is a space, the greatly diminished blastoccele.
The resemblance of the gastrula to a simplified Hydra,
devoid of tentacles, will be at once apparent
296 POLYGORDIUS LESS.
Before long 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 in or invaginated at two places
(C)f 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 (D\ so that a complete enteric canal is
formed — formed, we must not fail to notice, of three distinct
parts : (i) an anterior ectodermal pouch, opening externally
by the mouth, and distinguished as the stomodaum ; (2) the
enteron, lined with endoderm ; and (3) a posterior ectoder-
mal pouch, opening externally by the anus, and called the
proctodaiim.
In the trochosphere (Fig. 71) 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 ecto-
derm is greatly thickened, forming a rounded patch of cells
(Figs. 71 and 73, 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 either side of the intestine, between its epithelium and
the external ectoderm, is a row of cells forming a band
which partly blocks up the blastocoele (B and c, Afsd). These
two bands are the rudiments of the whole of the meso-
dermal tissues of the adult — muscle, coelomic epithelium,
&c. — and are hence called mesodermal bands.
THE TROCHOSPHERE
297
Finally on either side of the lower or posterior end of the
stomach is a delicate tube (Fig. 73, A, NpK) opening by a small
aperture on to the exterior, and by a wide funnel-shaped
Br
An
FIG. 73. — 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 (PL or. «'), and anal (An. ci]
cilia, brain (Br), ocelli (Oc), blastoccele (/?/), mouth (Mth\ stomo-
daeum (St. dm], proctodseum (Prc. dm\ and anus (An] as in Fig. 71,
the enteron (Ent) has extended some distance into the trunk.
In A, slender mesodermal bands (Msd. bd] in the prostomium, and the
branched head-nephridium (NpK} are shown.
In B and c the mesoderm (Msd) is seen to have obliterated the blasto-
ccele in the trunk-region r the ectoderm has undergone a thickening,
forming the ventral nerve-cord ( V. JNv. Cd).
(A after Fraipont.)
extremity into, the blastoccele : it has all the relations of a
nephridium, and is distinguished as the head-nephdridium.
As the larva of Polygordius is so strikingly different from
the adult, it is obvious that development must, in this, as in
298 POLYGORDIUS LESS.
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 may be called the trunk (Fig. 73, 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 procto-
dseum (Prc. dm} occupies only the portion in proximity to
the anus.
Important internal changes have also taken place. The
deric epithelium or external ectoderm is for the most part
composed, as in the preceding stage, of a single layer of
cells ; but on that aspect of the trunk which lies on the same
side as the mouth — i.e., to the left in Fig. 73, 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. 71, B and c, Msd}.
have now increased to such an extent as completely to sur-
round the enteron and obliterate the blastocoele (Fig. 73, B
and c, 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.
xxv METAMORPHOSIS 299
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. 74, 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. 66, D, p. 269). 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, t.e.t the seg-
ment next the peristomium is the oldest, and new ones are
continually being added between the last formed and the
extremity of the trunk, or what may now be called the anal
segment. By this process the larva has assumed the appear-
ance of a worm with an immense head and a very slender
trunk.
The original larval stomach (enteron) has extended, with
the formation of the metameres, so as to form the greater
portion of the intestine : the proctodaeum (Prc. dvi) is
confined to the anal segment.
Two other obvious changes are the appearance of a pair
of small slender processes (A, /) — the rudiments of the
tentacles — on the apex of the prostomium, and of a circlet
of cilia (Pr. an. a) round the posterior end of the trunk.
The internal changes undergone during the assumption of
the present form are very striking. In every fully formed
metamere the mesoderm — solid, it will be remembered,
in the previous stage — has become divided into two layers,
a somatic layer (B and c, Msd (som) ) in contact with the
ectoderm and a splanchnic layer (Msd (spl) ) in contact
with the endoderm. The space between the two layers
(Cat) is the permanent body-cavity or ccelome, which is
FIG. 74. — A, larva of Polygordius neapolitanus in a condition inter-
mediate between the trochosphere and the adult worm, the trunk-region
being elongated and divided into metameres.
B, diagrammatic vertical section of the same: the ectoderm is coarsely,
the nervous system finely, dotted, the endoderm radially striated, and
the mesoderm evenly shaded.
C, transverse section along the plane ab in B.
The pre-oral (Pi: or. ci), post-oral (Pt. or. ci}, and anal (An. ci)
cilia, the blastocoele (BI. cat), stomodaeum (St. dm], and proctodaeum
(Prc. dm] are as in Fig. 71, A and B : the enteron now extends through-
out the segmented region of the trunk.
A pair of tentacles (/) has appeared on the prostomium near the ocelli
(o), and a pre-anal circlet of cilia (Pr. an. ci} is developed.
The mesoderm has divided into somatic (Msd (som] ) and splanchnic
(Msd(spl] ) layers with the coelome (Ccel] between : the septa (Sep) are
formed by undivided plates of mesoderm separating the segments of the
coelome from one an ^ther.
D^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. Epthni) : 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 ccelomic
epithelium (Ccel. Epthm).
(A after Fraipont. )
xxv METAMORPHOSIS 301
thus quite a different thing from the larval body-cavity
or blastocoele, 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
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 (D1, Msd
Som\ but before long each cell splits up in a radial
direction (D2) from without inwards — i.e.^ from the ectoderm
(Der. Epthni] towards the ccelome — finally taking on the
form of a book with four or more slightly separated leaves
directed outwards or towards the surface of the body, and
with its back — the undivided portion of the cell — bounding
302 POLYGORDIUS LESS.
the ccelome. The cells being arranged in longitudinal series,
we have a number of such books placed end to end in
a row with the corresponding leaves in contact — page one
of the first book being followed by page one of the second,
third, fourth, &c., page two by page two, and so on through
one or more segments of the trunk. Next, what we have
compared with the leaves of the books — the divided
portions of the cells — become separated from the backs —
the undivided portions (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, CouL
Epthni) become the parietal layer of ccelomic epithelium, the
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. 74, 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.
xxv SIGNIFICANCE OF DEVELOPMENTAL STAGES 303
It is important to notice that this comparatively complex
animal is in one stage of its existence — the oosperm — as
simple as an Amoeba ; in another — the polyplast — it is com-
parable to a Pandorina, and in a third — the blastula — to a
Volvox ; in a fourth — the gastrula — it corresponds in general
features with a Hydra ; while in a fifth — the trochosphere —
it resembles in many respects a Medusa. As in other cases
we have met. with, the comparatively highly-organised form
passes through stages in the course of its individual develop-
ment 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.
The rest of the development of Polygordius may be
summarized very briefly. The trunk grows so much faster
than the head (pro-pltts 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 XXVI
THE CHIEF DIVISIONS OF THE ANIMAL KINGDOM : THE
STARFISH
THE student who has once thoroughly grasped the facts of
structure of such typical unicellular animals as Amceba 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, an organ, body-wall, enteron, stomodseum,
proctodaeum, coelome, somatic and splanchnic mesoderm,
are fairly understood, all other points of structure become
hardly more than matters ot detail.
If we turn to a text-book of Zoology we shall find that
the animal kingdom is roughly divisible into eight primary
sub-divisions, called sub-kingdoms, types, or phyla. These
are as follows : —
Protozoa, Echinodermata.
Porifera. Arthropoda.
Ccelenterata. Mollusca.
" Vermes." Vertebraia.
LESS, xxvi GENERAL STRUCTURE 305
With a few exceptions, the discussion of which would be out
of place here, the vast number of animals known to us may
be arranged in one or other of these groups.
The Protozoa are animals which are either unicellular in
the strict sense, or non-cellular, or colonies of unicellular
zooids : they have been represented in previous lessons by
Amoeba and Protamoeba, Haematococcus, Heteromita,
Euglena, the Mycetozoa, Paramoecium, Stylonychia, Oxy-
tricha, Opalina, Vorticella, Zoothamnium, the Foraminifera,
the Radiolaria, Pandorina, and Volvox. The reader will
therefore have no difficulty in grasping the general features
of this phylum.
The Coslenterata are the diploblastic animals, and have
also been well represented in the foregoing pages, namely
by Hydra, Bougainvillea, Diphyes, and Porpita. The sea-
anemones and corals also belong to this phylum, in which
also the Porifera or sponges were formerly included.
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
example of each will show how they all conform to the
general plan of organisation 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.
x
306 THE STARFISH LESS.
The phylum Arthropoda includes crayfishes, lobsters,
crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions,
spiders, and mites ; centipedes and millipedes ; and all
kinds of insects, such as cockroaches, beetles, flies, ants,
bees, butterflies, and moths. A crayfish forms a very fair
example of the aquatic kinds (Crustacea).
In the phylum Mollusca are included the ordinary bi-
valves, such as mussels and oysters ; snails, slugs, and other
univalves or one-shelled forms ; and cuttle-fishes, 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.
The commonest British starfish is Asterias rubens^ but
the main features of the following description will apply to
any species. The starfish consists of a central disc-like
portion, from which radiate five arms or rays. The animal
crawls over the rocks with its flat, light-coloured ventral
surface downwards, and with its darker, convex, dorsal
surface upwards. It can move in any direction, so that, in
the ordinary sense of the words, anterior and posterior ex-
tremities cannot be distinguished. Radial symmetry such
xxvi TUBE-FEET 307
as this, i.e., the division of the body into similar parts
radiating from a common centre, is characteristic of the
Echinodermata generally.
In the centre of the disc on the ventral surface is a five-
sided depression, at the bottom of which is the large mouth
(Fig. 75 and Fig. 76, A, Mtfi). From it radiate five grooves
FIG. 75. — A Starfish, from the ventral aspect, showing the disc and
arms, the central mouth, and the numerous tube-feet. (From Parker
and Haswell's Zoology, after Leuckart and Nitsche. )
called the ambulacral grooves, one along the ventral surface
of each arm (Fig. 76, A and B). In the living animal numerous
delicate semi-transparent cylinders, the tube-feet (Fig. 7 5 and
Fig. 76, T, JF), 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
X 2
3o8 THE STARFISH LESS, xxvi
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 (Fig. 76 A, Ati] ; 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. The presence
of this structure disturbs the radial symmetry of the starfish
and gives rise to a bilateral symmetry, since the animal can
be divided into two truly equal halves by a single plane
only, viz., the plane passing through the middle of the
madreporite and of the arm opposite to it.
The body, though flexible, is tolerably firm and resistant,
owing to the fact that immediately beneath the soft, slimy
skin there is a layer of little irregular calcareous bodies, the
ossicles (Fig. 76, os), forming a kind of scale armour. Many
of them give attachment to spines, and between them are
minute apertures, the dermal pores, through which, during
the life of the animal, are protruded delicate, glove-finger-
like processes, the dermal gills or respiratory cceca (Resp.
cce). Both on the dorsal and the ventral surfaces are found
curious and characteristic organs called pedicellarice, (Ped).
These are minute forceps-like structures, consisting of a
basal piece or stalk and of two jaws, each supported by a
calcareous plate : the jaws are worked by muscles, and
apparently serve to remove faecal matter, foreign bodies, &c.,
from the surface of the animal.
The tube-feet, already referred to, are arranged symme-
trically on either side of each ambulacral groove. At the
extremity of the groove is a single structure (t) like a tube-
foot without the terminal sucker : it is called the tentacle,
Ovd.
FIG. 76.— Diagrammatic sections of a Starfish.
A, vertical section passing on the right through a radius, on the left
through an inter-radius. The off-side of the ambulacral groove, with
the tube feet ( T. F) and ampullae (Amp), is 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 (Cat. Epthm).
To the body-wall are attached pedicellarias (Ped), and the end of the
arm bears a tentacle (/) with an ocellus (oc) at its base.
The skeleton consists of ossicles (os) imbedded in the dermis : large
ambulacral ossicles (Amb. os) bound the ambulacral grooves on the
ventral surfaces of the arms.
The mouth (Mth} leads by a short gullet into a stomach (St), which
gives off a cardiac caecum (Cd. c\ already referred to, communicate
with the ccelome, and are, in fact, hollow outpushings of
the body-wall. They serve to bring the ccelomic fluid into
close relation with the surrounding water, and are therefore
to be looked upon as organs of respiration.
One of the most characteristic structures in the anatomy
of the starfish is a peculiar system of vessels called the
water-vascular or ambulacral system: it is of great func
tional importance, being connected with the working of the;
tube-feet.
XXVI
AMBULACRAL SYSTEM
313
The central part of the ambulacral system is a pentagonal
tube (Fig. 78, c ; Fig. 76, C. Amb. V) which surrounds the
gullet, and is called the ambulacral ring-vessel. From each
angle of the pentagon is given off a radial ambulacral vessel
FIG. 78. — The water vascular system of a Starfish (diagramatic).
The ring-vessel (c} gives off five radial vessels (r), lateral off-shoots of
which (/) are connected with the tube-feet (/) and ampullae (a}.
Inter-radially the ring-vessels give off Polian vesicles (
O. G) are shown in perspective.
B, transverse section.
The cuticular shell (Sh), shown only in B, is black, the ectoderm
dotted, the nervous system finely dotted, the endoderm radially striated,
the mesoderm evenly shaded, and the coelomic 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 inhalant (/;///.
Ap) and exhalant (Exh. Ap} apertures at the posterior end.
The body is produced ventrally into the foot (Foot), on each side of
which are the gills, an inner (/. G) and an outer (O. G), each formed
of an inner and an outer lamella.
The body is covered externally by deric epithelium (Der. Epthni),
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 (Gut) into the stomach
(St), from which proceeds the coiled intestine (Int), ending in the anus
A A
354 THE FRESH-WATER MUSSEL LESS.
(An) : the enteric epithelium is mostly endodermal. The digestive gland
(D. Gl) surrounds the stomach. The ccelome (Cat) is reduced to a
small dorsal chamber enclosing part of the intestine and the heart ; the
parietal (Ccel. Epthm) and visceral (CceL Epthrn^-) layers of coelomic
epithelium are shown.
The heart consists of a median ventricle ( Vent}> enclosing part of the
intestine, and of paired auricles (Aitr).
The paired nephridia (Nphm) open by apertures into the coelome
(Nph. st) and on the exterior (Nph. p).
The gonads (Gon) are imbedded in the solid mesoderm, and open on
the exterior by gonoducts (Gnd).
The nervous system consists of a pair of cerebro-pleural ganglia
(C. P. Gri) 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.
the dorso-lateral region of the body of its own side, together
with the corresponding mantle-lobe, and it is from the epi-
thelium (Fig. 93, Der. Epthm) covering these parts that the
shell is formed as a cuticular secretion. The whole space
between the two mantle-lobes, containing the gills, visceral
mass, and foot is called the mantle-cavity.
A single layer of epithelial cells, the deric epithelium or
epidermis (Der. Epthni), covers the whole external surface,
i.e., the body proper, both surfaces of the mantle, the gills,
and foot ; that of the gills and the inner surface of the
mantle is ciliated. Beneath the epidermis come connective
and muscular tissue, which occupy nearly the whole of the
interior of the body not taken up by the viscera, the ccelome
being, as we shall see, much reduced. The muscles are all
unstriped, and are arranged in distinct bands or sheets,
many of them very large and conspicuous. The largest are
the anterior and posterior adductors (Figs. 92, 93, and 94,
a. ad, p. ad\ great cylindrical muscles which pass trans-
versely across the body and are inserted at either end into
the valves of the shell, which are approximated by their
contraction. Two muscles of much smaller size pass from
the foot to the shell, which they serve to draw back : they
xxvni DIGESTIVE ORGANS 355
are called the anterior (a, r) and posterior (p. r) retractors.
A third muscle (prc) is inserted into the shell close
to the anterior adductor, and has its fibres spread fan-wise
over the visceral mass which it serves to compress, thus
forcing out the foot and acting as a protractor of that organ.
The substance of the foot itself consists of a complex mass
of fibres, the intrinsic muscles of the foot, many of which
also act as protractors. Lastly, all along the border of the
mantle is a row of delicate pallial muscles (Fig. 92, B, pi.
m), which, by their insertion into the shell, give rise to the
pallial line already seen.
The ccelome is reduced to a single ovoidal chamber, the
pericardium (Fig. 93, Cosl ; Fig. 94, >r), lying in the dorsal
region of the body and containing the heart and part of the
intestine : it is lined by ccelomic epithelium ( Ccel. Epthm\
and does not correspond with the pericardial sinus of ths
crayfish, which is a blood-space. In the remainder of the
body the space between the ectoderm and the viscera is
filled by the muscles and connective tissue.
The mouth (Fig. 94, mtJi] lies in the middle line, just
below the anterior adductor. On each side of it are two
triangular flaps, the internal and external labial palps ; the
external palps unite with one another in front of the mouth,
forming an upper lip ; the internal are similarly united
behind the mouth, forming a lower lip : both are ciliated
externally. The mouth leads by a short gullet (Fig. 94,
gut) into a large stomach (st\ which receives the ducts of a
pair of irregular, dark-brown digestive glands (d. gl). The
intestine (int) goes off from the posterior end of the stomach,
descends into the visceral mass, where it is coiled upon
itself, then ascends parallel to its first portion, turns sharply
backwards, and proceeds, as the rectum (ret), through the
pericardium, where it traverses the ventricle of the heart,
A A 2
356
THE FRESH-WATER MUSSEL
II
FIG. 94. — Dissection of Anodonta, made by removing the mantle -
lobe, inner and outer gills, wall of pericardium, and auricle of the left
side, and dissecting away the skin, muscles, £c. of the same side down
to the level of the enteric canal, kidney, nervous system, &c. Part of
the enteric canal is laid open, as also are the kidney (kd} and bladder
(bl). The connection between the cerebro-pleural (c. t>L gn) and
visceral (v. gn} ganglia is indicated by a dotted line.
a, anus ; a. ad, anterior adductor ; a. ao, anterior aorta ; a. v. ap,
auriculo-ventricular aperture ; bl, urinary bladder ; c. pL gn, cerebro-
pleural ganglion : d. d, duct of digestive gland ; d. gl, digestive gland ;
d.p. a, dorsal pallial aperture ; ex. sph, exhalant siphon ; ft, foot ; g. ap,
genital aperture ; gon, gonad ; gut, gullet ; i. I. j, inter-lamellar junc-
tion ; in. sph, inhalant siphon ; int, intestine ; kd, kidney ; m, mantle ;
mth, mouth ; p. ao, posterior aorta ; p. ad, posterior adductor ;pc, peri-
cardium ; pd. gn, pedal ganglion ; r. ap, renal aperture ; r. an, right
auricle ; ret, rectum ; r. p. a, reno-pericardial aperture ; st, stomach ;
ty, typhlosole ; v, ventricle ; v. gn, visceral ganglion ; w. t, water-
tubes ; x, aperture between right and left urinary bladders.
(From Parker and Haswell's Zoology. )
and above the posterior adductor, finally discharging by the
anus (a) into the exhalant siphon, or cloaca. The wall of
xxvm GILLS 357
the rectum is produced into a longitudinal ridge, or typhlosole
(ty\ and two similar ridges begin in the stomach and are
continued into the first portion of the intestine. The
stomach contains at certain seasons of the year a gelatinous
rod, the crystalline style.
The gills consist, as we have seen, of two plate-like bodies
on each side between the visceral mass and the mantle : we
have thus a right and a left outer (Fig. 93, B, O. G), and a
right and a left inner gill (I. G). Seen from the surface
(Fig. 94), each gill presents a delicate double striation,
being marked by faint lines running parallel with, and by
more pronounced lines running at right angles to, the long
axis of the organ. Moreover, each gill is double, being
formed of two similar plates, the inner and outer lamella,
united with one another along the anterior, ventral, and
posterior edges of the gill, but free dorsally. The gill has
thus the form of a long and extremely shallow bag open above
(Figs. 94 and 95) : its cavity is subdivided by vertical plates
of tissue, the inter-lamellar junctions (Fig. 95, i. /. /), which
extend between the two lamellae and divide the intervening
space into distinct compartments or water-tubes (w. t\
closed ventrally, but freely open along the dorsal edge of
the gill. The vertical striation of the gill is due to the fact
that each lamella is made up of a number of close-set gill-
filaments (/) : the longitudinal striation to the circumstance
that these filaments are connected by horizontal bars, the
inter-filamentar junctions (t.f.f). At the thin free or ventral
edge of the gill the filaments of the two lamellae are con-
tinuous with one another, so that each gill has actually a
single set of V-shaped filaments, the outer limbs of which
go to form the outer lamella, their inner limbs the inner
lamella. Between the filaments, and bounded above and
below by the inter-filamentar junctions are minute apertures,
358
THE FRESH-WATER MUSSEL
or ostia (os), which lead from the mantle-cavity through a
more or less irregular series of cavities into the interior of
the water-tubes. The filaments themselves are supported
by chitinous rods, and covered with ciliated epithelium, the
large cilia of which produce a current running from the
exterior through the ostia into the water-tubes, and finally
IV.t
FIG. 95.— Diagram of the structure of the gill of Anodonta.
The gill is made up of V-shaped gill-filaments (/) arranged in longi-
tudinal series and bound together by horizontal inter-filamentar junctions
(t./.j) which cross them at right angles, forming a kind of basket-work
with apertures, the ostia (0s), leading from the outside and opening (of)
into the cavity of the gill. The latter is divided by vertical partitions,
the inter-lamellar junctions (/. /./), into compartments or water-tubes
(«/. /) which open also into the supra-branchial chamber ; b. v, blood-
vessels.
(From Parker and HaswelFs Zoology.}
escaping by the wide dorsal apertures of the latter. The
whole organ is traversed by blood-vessels (b. v).
The mode of attachment of the gills presents certain
features of importance. The outer lamella of the outer gill
is attached along its whole length to the mantle : the inner
xxvin EXCRETORY ORGANS 359
lamella of the outer, and the outer lamella of the inner gill
are attached together to the sides of the visceral mass a
little below the origin of the mantle : the inner lamella of
the inner gill is also attached to the visceral mass in front,
but is free further back. The gills are longer than the
visceral mass, and project behind it, below the posterior
adductor (Fig. 94), as far as the posterior edge of the
mantle : in this region the inner lamellae of the inner gills
are united with one another, and the dorsal edges of all
four gills constitute a horizontal partition between the pallial
cavity below and the exhalant chamber or cloaca above.
Owing to this arrangement it will be seen that the water-
tubes all open dorsally into a supra-branchial chamber, con-
tinuous posteriorly with the cloaca and thus opening on
the exterior by the exhalant siphon.
The physiological importance of the gills will now be
obvious. ' By the action of their cilia a current is produced
which sets in through the inhalant siphon into the pallial
cavity, through the ostia into the water-tubes, thence into the
supra-branchial chamber, and out at the exhalant siphon.
The in-going current carries with it not only oxygen for the
aeration of the blood, but also diatoms, infusoria, and other
microscopic organisms, which are swept into the mouth by
the cilia covering the labial palps. The out-going current
carries with it the various products of excretion and the
faeces passed into the cloaca. The action of the gills in
producing the food-current is of more importance than their
respiratory function, which they share with the mantle.
The excretory organs are a single pair of curiously-modified
nephridia, situated one on each side of the body just below
the pericardium. Each nephridium consists of two parts, a
brown spongy glandular portion or kidney (Fig. 94, kd\ and
a thin-walled non-glandular part or bladder (bl\ The two
360 THE FRESH-WATER MUSSEL LESS.
parts lie parallel to one another, the bladder being placed
dorsally and immediately below the floor of the pericardium :
they communicate with one another posteriorly, while in
front the kidney opens into the pericardium (r. p. ap), and
the bladder on the exterior by a minute aperture (r. ap),
situated between the inner gill and the visceral mass. Thus
the whole organ (Fig. 93, Nphm\ often called after its dis-
coverer, the organ of Bojanus, is simply a tube bent upon
itself, opening at one end into the ccelome (Nph. st\ and at
the other on the external surface of the body (Nph. p) : it
has therefore the normal relations of a nephridium. The
epithelium of the bladder is ciliated, and produces an
outward current.
It seems probable that an excretory function is also dis-
charged by a large glandular mass of reddish-brown colour,
called the pericardial gland or Keber's organ (Fig. 92, B,
k. o). It lies in the anterior region of the body just in front
of the pericardium, into which it discharges.
The circulatory system is well developed. The heart lies
in the pericardium, and consists of a single ventricle (Fig. 93,
Vent, and Figs. 94 and 96, v) and of right and left auricles
(ait). The ventricle is a muscular chamber which has the
peculiarity of surrounding the rectum (Figs. 93 and 94) :
the auricles are thin-walled chambers communicating with
the ventricle by valvular apertures opening towards the
latter. From each end of the ventricle an artery is given
off, the anterior aorta (Fig. 94, a. ao) passing above, the
posterior aorta (p. ao) below the rectum. From the aortre
the blood passes into arteries (Fig. 96, art.,1 arfl) which
ramify all over the body, finally forming an extensive net-
work of vessels, many of which are devoid of proper walls
and have therefore the nature of sinuses. The returning
blood passes into a large longitudinal vein, the vena cava
CIRCULATORY SYSTEM 361
(v. c), placed between the nephridia, whence it is taken to
the kidneys themselves (nph. v\ thence by afferent branchial
art./
a.r-12
FIG. 96. — Diagram of the Circulatory System of Anodonta.
The blood received from the auricles (att) is pumped by the ventricle
(v) into the aorta (ao) and thence passes to the mantle (art.1) and to
the body generally (art.2).
The blood which has circulated through the mantle is returned
directly to the auricle : that from the body generally is collected into
the vena cava (v. c), passes by nephridial veins (nph) to the kidneys,
thence by afferent branchial veins (of. br. v) to the gills, and is returned
by efferent branchial veins (ef. br. v} to the auricles ; pc, pericardium.
(From Parker and Haswell's Zoology.
veins (af. br. v) to the gills, a*nd is finally returned by efferent
branchial veins (ef. br. v) to the auricles. The mantle has a
very extensive blood -supply, and probably acts as the chief
362 THE FRESH-WATER MUSSEL LESS.
respiratory organ : its blood (art1) is returned directly to the
auricles without passing through either the kidneys or the
gills. The blood is colourless and contains leucocytes.
There is no communication between the blood-system and
the pericardium.
The nervous system is formed on a type quite different
from anything we have yet met with. On each side of the
gullet is a small cerebro-pleural ganglion (Fig. 94, c. pL gn},
united with its fellow of the opposite side by a nerve-cord,
the cerebral commissure, passing above the gullet. Each
cerebro-pleural ganglion also gives off a cord, the cerebro-
pedal connective, which passes downwards and backwards to
a pedal ganglion (pd. gn) situated at the junction of the
visceral mass with the foot : the two pedal ganglia are so
closely united as to form a single bilobed mass. From each
cerebro-pleural ganglion there further proceeds a long cerelro-
visceral connective, which passes directly backwards through
the kidney, and ends in a visceral ganglion (v. gn) placed on
the ventral side of the posterior adductor muscle. The
visceral, like the pedal ganglia, are fused together. The
cerebro-pleural ganglia supply the labial palps and the
anterior part of the mantle; the pedal, the foot and its
muscles ; the visceral, the enteric canal, heart, gills, and
posterior portion of the mantle.
It will be seen that the cerebral commissures and cerebro-
pedal connectives, together with the cerebro-pleural and
pedal ganglia, form a nerve-ring which surrounds the gullet :
the cerebro-pleural ganglia may be looked upon as a supra-
cesophageal nerve mass corresponding with the brain
of Polygordius and the Crayfish, and the pedal ganglia
as an infra-cesophageal mass representing the ventral nerve
cord.
Sensory organs are poorly developed, as might be ex-
xxviii DEVELOPMENT 363
pected in an animal of such sedentary habits. In connec-
tion with each visceral ganglion is a patch of sensory
epithelium forming the so-called olfactory organ or, better,
osphradium^ the function of which is apparently to test the
purity of the water entering by the respiratory current.
Close to the pedal ganglion is a minute otocyst or statocyst,
the nerve of which is said to spring from the cerebro-pedal
connective, being probably derived from the cerebral gang-
lion. Sensory cells, probably tactile, also occur round the
edge of the mantle, and especially on the fimbrise of the in-
halant siphon.
The sexes are separate. The gonads (Figs. 93 and 94,
gon) are large, paired, racemose bodies, occupying a con-
siderable portion of the visceral mass amongst the coils of
the intestine : the spermary is white, the ovary reddish. The
gonad of each side has a short duct which opens (g. ap] on
the surface of the visceral mass, just in front of the renal
aperture.
In the breeding season the eggs, extruded from the genital
aperture, pass into the supra-branchial chamber, and so to
the cloaca. There, in all probability, they are impregnated
by sperms introduced with the respiratory current. The
oosperms are then passed into the cavities of the outer gills,
which they distend enormously. Thus the outer gills act as
brood-pouches, and in them the embryo develops into the
peculiar larval form presently to be described.
The segmentation of the oosperm is remarkable for the
fact that the cells of the polyplast are of two sizes, small
cells composed entirely of protoplasm, and large cells loaded
with yolk -granules. In the formation of the gastrula the
large are invaginated into the small cells, but the enteron
thus formed is very small and quite unimportant during
early larval life, the young mussels being nourished, after
364 THE FRESH-WATER MUSSEL LESS.
the manner of parasites, by a secretion from the gills of the
parent.
The dorsal surface of the embryo is soon marked out by
the appearance of a deep depression, the shell-gland, which
secretes, in the first place, a single median shell. This is,
however, soon replaced by a bivalved larval shell (Pig. 97,
s), of triangular form, the ventral angles being produced into
hooks (sh). The body at the same time becomes cleft from
FIG. 97. — A, advanced embryo of Anodonta enclosed in the egg-mem-
brane. B, free larva or glochidium.
f, -byssus ; g, lateral pits ; sy shell ; s/i, hooks ; .?;//, adductor muscle ;
so, sensory hairs ; w, ciliated area.
(From Korschelt and Heider. )
below upwards (A), forming the right and left mantle-lobes.
On the ventral surface, between the lobes of the mantle,
is formed a glandular pouch, which secretes a bunch of
silky threads, the byssrts (/). The larva is now called a
glochidium.
The glochidia, entangled together by means of their
byssal threads, escape from the gills of the parent by the
xxviii METAMORPHOSIS 365
exhalant siphon, and eventually attach themselves, by their
hooked valves, to the body of a passing fish, such as a
stickleback. Here they live for a time as external parasites,
gradually undergoing metamorphosis ; and finally drop from
the host and assume the sedentary habits of the adult.
LESSON XXIX
THE DOGFISH
THE animals studied in the three previous Lessons have
served to illustrate three widely different types of organiza-
tion. The starfish is radially symmetrical, with an under-
lying bilateral symmetry, and no indication of metamerism :
the crayfish is bilaterally symmetrical, metamerically seg-
mented, and provided with numerous limbs, both trunk and
limbs being covered with a hard, jointed armour or exo-
skeleton : the mussel is likewise bilaterally symmetrical,
covered with a shell formed of paired pieces, and having no
indication of metamerism, and no trace of limbs. We have
now to consider, in the dogfish, an animal belonging to the
great group of Vertebrata, in which the bilaterally symme-
trical body is definitely divided into metameres, although
there is no indication of the fact externally. There are only
two pairs of limbs or paired appendages, and the main sup-
porting structures aje a complicated internal system of
articulated hard parts, forming the endoskekton or internal
skeleton.
The commonest British dogfishes are the Rough Hound
368 THE DOGFISH LESS.
(Scyllium canicula), the Lesser Spotted Dogfish (S. catulus\
the Piked Dogfish (Acanthias vulgaris\ and the Smooth
Hound (Mustelus vulgaris). The following description,
though referring mainly to Scyllium^ will apply, in essential
respects, to any of these.
The dogfish has a spindle-shaped body (Fig. 98), ending
in front in a bluntly-pointed snout or cut-water, and behind
tapering off into an upturned tail. On the ventral surface
of the head is the large, transversely elongated mouth (inth\
supported by a pair of jaws which work in a vertical, and
not, like those of the crayfish, in a transverse plane, and
are, in fact, portions of the skull, having nothing to do with
limbs. They are covered with teeth which vary in form in
the different species. In front of the mouth, on the ventral
surface of the snout, are the paired nostrils (na\ each lead-
ing into a cup-like nasal sac. The eyes (e) are also two in
number and are placed one on each side of the head, above
the mouth. Behind the mouth are five slit-like apertures
(ex. br. ap\ arranged in a longitudinal series : these are the
gill-clefts or external branchial apertures. Just behind the
eye is a small aperture, the spiracle (sp) : like the gill-clefts,
it communicates with the pharynx, and it is found by de-
velopment to be actually the functionless first gill-cleft.
On the ventral surface of the body, about half-way
between its two ends, is the anus or cloacal aperture (an\
and on either side of it a small hole, the abdominal pore,
opening into the ccelome. From the end of the snout to
the last gill-cleft is considered as the head of the fish ; from
the last gill-cleft to the anus as the trunk ; and the rest as
the tail.
A longitudinal streak (/. /) on each side of the body, con-
nected in front with a series of branching lines on the head
and continued backwards to the tail, is known as the lateral
xxix EXOSKELETON 369
line. The whole apparatus, together with other canals in the
head, is really a system of tubes sunk in the skin, and con-
stitutes an important, but imperfectly understood, sensory
organ.
Springing from the body are a number of flattened folds,
called the fins, divisible into median and paired. The
median folds are two dorsal fins (d.f. i, d.f. 2) along the
middle line of the back, a caudal fin (cd. f) lying mostly
along the ventral edge of the upturned tail, and a ventral
fin (v. /) behind the anus. The paired folds are the pectoral
fins (ptf.f), situated one on each side of the trunk just
behind the last gill-cleft, and the pelvic fins, one on each
side of the anus. The pectoral and pelvic fins are the
paired appendages or limbs of the dogfish : as in other
Vertebrates there are only two pairs, the pectorals corre-
sponding with the fore-limbs, the pelvics with the hind
limbs of the higher forms.
The fish swims by vigorous strokes of the tail : the
pectoral fins are used chiefly for steering, and the dorsal
and ventral fins serve, like the keel of a boat, to maintain
equilibrium.
The skin or external layer of the body-wall consists, as
usual, of two layers, an outer layer of deric epithelium
(Fig. 99, Der. Epthm) differing from that of previous types
in being formed of several layers of cells, and an inner
layer of connective tissue, the dermis. In the dermis are
innumerable close-set calcareous bodies (Fig. 99, Derm. Sp\
each consisting of a little irregular plate of bone produced
into a short enamelled spine, which projects through the
epidermis and gives a rough, sand-paper-like character to
the skin. These placoid scales or dermal teeth together
constitute the exoskeleton of the dogfish : it is a discon-
tinuous dermal exoskeleton like that of the starfish.
Beneath the dermis is the muscular layer in which we
R B
LESS, xxix GENERAL STRUCTURE 371
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. F], D. F-), ventral ( V. F),
and caudal (C. F) fins ; the paired fins are not shown.
The body-wall consists of deric epithelium (Der. Epthni), 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. .A5) 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 hsemal arches (H. A] : a cranium
(Cr) enclosing the brain (Br) : upper and lower jaws : branchial arches
(Br. A] and rays (Br. A', Br. A"), shown only in B, supporting the
gills : shoulder (Sh. G) and pelvic (Pelv. G) girdles : and pterygiophores
(Ptgph) supporting the fins.
The mouth (Mth) leads into the oral cavity (Or. cav), from which the
pharynx (Ph) and gullet (Gut) lead to the stomach (St) : this is con-
nected with a short intestine (Int) opening into a cloaca (Cl) which
communicates with the exterior by the vent (An). The oral cavity and
cloaca are the only parts of the canal lined by ectoderm.
Connected with the enteric canal are the liver (Lr) with the gall-
bladder (G. Bl) and bile-duct (B. D}, the pancreas (Pn), and the spleen
(SpZ). The mouth is bounded above and below by teeth ( 7').
The respiratory organs consist of pouches (shown in B) communicating
with the pharynx by internal (/;//. br. ap) and with the exterior by
external (Ext. br. ap) branchial apertures, and lined by mucous mem-
brane raised into branchial filaments (Br. Fil).
The heart (fft) is ventral and anterior, and is situated in a special
compartment of the coelome (Pcd). Six of the most important blood-
vessels, the dorsal vessel (dorsal aorta, D. Ao), the cardinal veins
(Card. F), 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 (Cxi.
Epthni) and visceral (Ccel. Epthni'} layers.
The ovaries (Ovy) are connected with the dorsal body-wall : the
oviducts (Ovd) open anteriorly into the coelome (ovd'} and posteriorly
into the cloaca.
The kidneys (K} are made up of nephridia (Nph) and open by ureters
( Ur) into the cloaca.
The nervous system is lodged in the cerebro-spinal cavity ( C. Sp. Cav]
hollowed out in the dorsal body-wall : it consists of brain (Br) and
spinal cord (Sp. Cd), and contains a continuous cavity, the neuroccele
(;/. frt').
R B 2
372 THE DOGFISH LESS, xxix
meet, for the first time in our present subject, with distinct
metameric segmentation. The muscles are divided into
segments or myomeres (Fig. 98, mym) following one another
from before backwards, and having a zigzag disposition.
The fibres composing them are longitudinal, and are inserted
at either end into fibrous partitions or myocommas (myc\
which separate the myomeres from one another. The mus-
cular layer is of great thickness, especially its dorsal portion
(Fig. 99, c). The fibres of all the body muscles are of the
striped kind.
There is a large ccelome (Fig. 99, Co?/), remarkable for being
confined to the trunk, both head and tail being, in the adult,
accelomate. The cavity is divisible into two parts : a large
abdominal cavity, containing most of the viscera, and a small
anterior and ventral compartment, fat pericardial cavity (Pcd\
containing the heart. Both are lined by coelomic epithelium
(Ccel. Epthm\ underlain by a layer of connective tissue,
a strong lining membrane being thus produced, called peri-
toneum in the abdominal, pericardium in the pericardial cavity.
Another very characteristic feature is that the dorsal body-
wall is tunnelled, from end to end, by a median longitudinal
neural cavity, in which the central nervous system is con-
tained. The greater part of the cavity is narrow and cylin-
drical, and contains the spinal cord : its anterior or cerebral
portion is dilated, and contains the brain.
Imbedded in the body-wall and extending into the fins
are the various parts of the endoskeleton. This characteristic
supporting framework is formed of a tough elastic tissue
called cartilage or gristle, more or less impregnated with
lime salts, so as to have, in part, the appearance of bone.
As, however, the hard parts of the dogfish's skeleton have a
different microscopic structure from the bones of the higher
vertebrates, they are best described as calcified cartilage.
374 THE DOGFISH LESS.
The entire skeleton consists of separate pieces of cartilage,
calcified or not, and connected with one another by sheets
or bands of connective tissue called ligaments : it is divisible
into the following parts : —
A. The skull or skeleton of the head, consisting of —
1. The cranium or brain-case, enclosing the brain
and the chief sense-organs.
2. The upper and loiver jaws.
3. The visceral arches, a series of cartilaginous hoops
supporting the gills.
B. The vertebral column or " backbone," a jointed axis ex-
tending from the cranium to the end of the tail,
and enclosing the spinal cord,
c. The skeleton of the median fins.
D. The skeleton of the paired fins, consisting of—
1. The shoulder-girdle or pectoral arch, to which are
attached
2. The pectoral fi ns.
3. The hip-girdle or pelvic arch, to which are at-
tached
4. The pelvic fins.
The cranium (Fig. 100, Cr) is an irregular cartilaginous
box containing a spacious cavity for the brain, and pro-
duced into two pairs of outstanding projections : a posterior
pair, called the auditory capsules (aud. cp\ for the lodgment
•of the organs of hearing, and an anterior pair, the olfactory
capsules (olf. cp\ for the organs of smell. Between the
olfactory and auditory capsules, on each sidej the cranium
is hollowed out into an orbit (or) for the reception of the
eye. In front the cranium is produced into three cartila-
ginous rods (r), which support the snout. On its posterior
xxix BRANCHIAL ARCHES 375
face is a large aperture, the foramen magnum, through which
the brain joins the spinal cord, and on each side of the
foramen is an oval elevation or condyle for articulation with
the first vertebra.
In the human and other higher vertebrate skulls the
upper jaw is firmly united to the cranium, and the lower
alone is free. But in the dogfish both jaws (up. /, /. j) are
connected with the cranium by ligament (/£-, Ig) only, and
each consists of strong paired (right and left) moieties,
united with one another by fibrous tissue. The posterior
end of the upper jaw presents a rounded surface, on which
fits a corresponding concavity on the lower jaw, so that a
free articulation is produced, the lower jaw working up and
down in the vertical plane, not from side to side like the
jaws of the crayfish.
The visceral arches consist of six pairs of cartilaginous
half-hoops, lying in the walls of the pharynx (Fig. 99, B,
Br. A\ and united with one another below so as to form a
basket-like apparatus supporting the gills. The first of these
arches is distinguished as the hyoid, and is situated imme-
diately behind the jaws. It consists of two parts, a strong,
rod-like hyomandibular (Fig. 100, hy. m), which articulates
above with the auditory capsule, and is connected below by
fibrous tissue with the jaws, thus helping to suspend them
to the cranium : and a hyoid cornu, which curves forwards
inside the lower jaw, and is connected with its fellow of the
opposite side by a median plate which supports the tongue.
The remaining five arches (br. a. i — br. #.5) are called the
branchial arches. Each is formed of several separate pieces-,
movably united by fibrous tissue so as to render possible the
distension of the throat during swallowing. Both they and
the hyoid give attachment to delicate cartilaginous branchial
rays (br. r, br. r : Fig. 99, Br^ R] which support the gills.
376
THE DOGFISH
LESS.
The vertebral column has the general charactei of a
jointed tube surrounding the spinal portion of the neural
canal. Lying beneath this cavity, i.e., between it and the
ccelome, is a longitudinal row of biconcave discs, the ver-
tr.jbr
FIG. 101. — A, Three trunk vertebrae of Scyllium from the side.
B, a single trunk vertebra viewed from one end.
c, three caudal vertebrae from the side.
D, a single caudal vertebra from one end.
f, centrum ; //. a, haemal arch ; n. #, neural arch ; tr. pr. transverse
process.
(After Hasse.)
tebral centra (Fig. 101, c ; Fig. 99, V. Cent) : they are
formed of cartilage, but have their anterior and posterior
faces strongly calcified. The biconcave intervals between
them (Fig. 99, A) are filled with a gelatinous matter or inter-
VERTEBRAL COLUMN 377
vertebral substance. The centra are united by ligament, so
that the whole chain of discs is very flexible. Connected
with the dorsal aspect of the series of centra is a cartila-
ginous tunnel, arching over the spinal cord : it is divided
into segments, corresponding with, but usually twice as
numerous as the centra, and called the neural arches
(Fig. 101, n. a; Fig. 99, N. A).
In the anterior part of the vertebral column the centra
give off paired outstanding processes (Fig. 101, A and B,
tr. pr) called transverse processes, to the end of each of
which is articulated a short cartilaginous rod, the rib.
Further back the transverse processes are directed down-
wards, instead of outwards, and in the whole caudal or tail
region they unite below, forming haemal arches (Fig. 101, c
and D, h. a ; Fig. 99, A, H. A), which together constitute a
kind of inverted tunnel in which lie the artery and vein of
the tail. In the region of the caudal fin the haemal arches are
produced into strong median hamal spines (Fig. 99, A, H. A
to the right), which act as supports to the fin. A centrum,
together with the corresponding neural arch and transverse
processes or haemal arch, forms a vertebra or single segment
of the vertebral column.
It should be noticed that in the vertebral column we
have another instance of the metameric segmentation of the
vertebrate body. The vertebrae do not, however, correspond
with the myomeres, but alternate with them. The myo-
commas are attached to the middle of the vertebrae, so that
each myomere acts upon two vertebrae and thus produces
the lateral flexion of the backbone.
In the embryo, before the development of the vertebral
column, an unsegmented gelatinous rod, the notochord, lies
beneath the neural cavity in the position occupied in the
adult by the line of centra, by the development of which it
378 THE DOGFISH LESS.
is largely replaced. Much of it, however, remains as the
gelatinous intervertebral substance. The notochord is one
of the most characteristic organs of the Vertebrata.
The skeleton of the median fins consists of a series of
parallel cartilaginous rods, the fin-rays or pterygiophores
FIG. 1 02. — Ventral view of pectoral arch of Scyllium with right
pectoral fin.
The pectoral arch is divisible into dorsal (pet. g) and ventral (pet. g]
portions separated by the articular facets (art. f) for the fin.
The pectoral fin is formed of three basal cartilages (bs. 1-3) and
numerous radials (rod) ; its free edge is supported by dermal rays (d.f. r).
(Modified from Marshall and Hurst.)
(Fig. 99, Ptgpti), the proximal ends of which are more or
less fused together to form basal cartilages or basalia. The
free edges of the fins are supported by a double series of
delicate horn-like fibres, the dermal fin-rays (Derm. F. R}.
The shoulder-girdle (Fig. 102) is a strong, inverted arch of
xxix TEETH 379
calcified cartilage, situated just behind the last branchial
arch (Fig. 99, A, Sh. G). On each side of its outer surface
it presents three elevations or articular facets (Fig. 102, art.f]
for the pectoral fin ; the presence of these allows of the divi-
sion of each side of the arch into a narrow, pointed dorsal
portion (pct.g), and a broader ventral portion (pcf.g) united
in the middle line with its fellow of the opposite side. The
pectoral fin is formed of pterygiophores (rad), fused proxim-
ally to form basals (JBs. i. — 3), which are three in number,
and very large and strong.
The pelvic girdle is a transverse bar of cartilage situated
just in front of the vent (Fig. 99, A, Pelv. G\ and present-
ing on its posterior edge facets for the pelvic fin. The latter
has the same general structure as the other fins, but has a.
single very targe basal cartilage, and its first or anterior
radial is also much enlarged. The free edges of both
pectoral and pelvic fins are supported by horn-like dermal
rays (Fig. 102, d.f.r).
It will be noticed that while the skeleton of the crayfish
is a series of articulated tubes with the muscles inside
them, that of the dogfish is a series of articulated rods with
the muscles outside. The joints, formed by two rods
applied at their ends and bound together by ligament, are
not confined to movement in one plane, like the hinge-
joints of the crayfish, but are capable of more or less
rotatory movement.
The mouth, as we have seen, is a transverse aperture
bounded by the upper and lower jaws. In the mucous
membrane covering the jaws are imbedded large numbers
of teeth, (Fig. 99, T) bony conical bodies, with enamelled
tips, arranged in transverse rows. They are to be looked
upon as special developments of the placoid scales or dermal
teeth, enlarged for the purpose of seizing prey.
., 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. 422) :
division of this cell goes on very rapidly, and a primary 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, called the
cotyledon (ct), which soon begins to grow upwards towards
the light.
428 FERNS LESS.
Thus at a comparatively early stage of its development
the fern -embryo has attained a degree of differentiation far
beyond anything which occurs in the moss-embryo^ The
scarcely differentiated polyplast has passed into a stage
which may be called the phyllula, distinguished by the
possession of those two characteristic organs of the higher
plants, the leaf and root.
Notice how early in development the essential features of
animal or plant manifest themselves. In Polygordius the
polyplast is succeeded by a gastrula distinguished by the
possession of a digestive cavity : in the fern no such cavity
is formed, but the polyplast is succeeded by a stage dis-
tinguished by the possession of a leaf and root. In the
one case the characteristic organ for holozoic, in the other
the characteristic organs for holophytic nutrition make their
appearance, and so mark the embryo at once as animal
or plant. We may say then that while the oosperm and
the polyplast stages of the embryo are common to the
higher plants and the higher animals, the correspond-
ence goes no further, the next step being the formation
in the animal of an enteron, in the plant of a leaf and
root. In other words the phyllula is the correlative of the
gastrula.
The cotyledon increases rapidly in size, and emerges
between the lobes of the kidney-shaped prothallus (L) : the
root at the same time grows to a considerable length, the
result being that the phyllula becomes a very obvious
structure in close connection with the prothallus, and indeed
appearing to be part of it. The two are actually, however,
quite distinct, their union depending merely upon the fact
that the foot of the phyllula is embedded in the tissue of
the prothallus like a root in the soil. Hence the phyllula
is related to the prothallus in precisely the same way as the
xxxi GAMOBIUM AND AGAMOBIUM 429
sporogonium to the moss plant (compare Fig. in, K, with
Fig. 109, c2, and Fig. in, L, with Fig. 109, c4).
The rudiment of the stem (L, .r/) 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 and the primary root
ceases to be distinguishable ; the cotyledon, the foot, and
the prothallus wither, and thus the phyllula, by the successive
formation of new parts from its constantly growing stem,
becomes a fern-plant.
We see that the life-history of the fern resembles in
essentials that of the moss. In both, alternation of genera-
tion occurs, a gamobium or sexual generation giving rise, by
the conjugation of ovum and sperm, to an agamobium or
asexual generation, which, by an asexual process of spore-
formation, produces the gamobium. But in the relative
proportions of the two generations the difference is very great.
What we know as the moss plant is the gamobium, and the
agamobium is a mere spore-producing structure, never getting
beyond the stage of a highly differentiated polyplast, and
dependent throughout its existence upon the gamobium, to
which it is permanently attached. What we know as the
fern plant is the agamobium, a large and complex structure
dependent only for a brief period of its early life upon the
small and insignificant gamobium. Thus while the gamobium
is the dominant phase in the life-history of mosses, the
agamobium appearing like a mere organ, in ferns the
positions are more than reversed — the agamobium may
assume the proportions of a tree, while the gamobium is so
430 FERNS LESS, xxxi
small that its very existence is unknown to a large propor-
tion of fern-collectors.
It follows from what has just been said that the various
organs of a fern do not severally correspond with those of a
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 XXXII
THE CHIEF DIVISIONS OF THE VEGETABLE KINGDOM :
EQUISETUM : SALVINIA : SELAGINELLA
IN the XXVIth Lesson (p. 304) 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 organisation 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 forms con-
sidered in Lessons XXVI— XXIX from Polygordius. We
saw that the differences between these included matters of
such importance as the presence or absence of segmentation
and of lateral appendages, the characters of the skeleton,
and the structure and position of the nervous system. In
the higher plants, on the other hand, the essential organs —
root, stem, and leaves — are, save in details of form, size, &c.,
practically the same in all : the tissues always consist of
epidermis, ground-parenchyma, and vascular bundles, the
latter being divisible into phloem and xylem : the growing
point both of stem and of foot is formed of meristem, from
which the permanent tissues arise ; and the growing point of
432 CHARACTERS OF THE HIGHER PLANTS LESS.
the root is always protected by a root-cap, that of the stem
being simply over-arched by leaves. Moreover, an alterna-
tion of generations can be traced in all cases.
Plants may be 'conveniently divided into the following
chief groups or phyla :
1. Alga.
2. Fungi.
3. Muscinece.
4. Vascular Cryptogams.
a. Filicinae.
b. Equisetaceae.
c. Lycopodinese.
5. Phanerogams.
a. Gymnosperms.
b. Angiosperms.
The Algce are the lower green plants. They may be
unicellular, or may take the form of linear, superficial, or
solid aggregates : they never exhibit more than a limited
amount of cell-differentiation. This group has been repre-
sented in the foregoing pages by Zooxanthella, Diatoms,
Vaucheria, Caulerpa, Monostroma, Ulva, and Nitella.
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,
while others place them in a group apart. Diatoms also are
xxxii CHARACTERS OF THE PHYLA 433
sometimes placed in a distinct group. It must, moreover,
be remembered that most botanists include Haematococcus,
Pandorina, and Volvox among Algae, and place the Myce-
tozoa either among Fungi or in a separate group of chloro-
phyll-less plants (p. 181).
The MusdnecB are the mosses and liverworts, the former
of which were fully described in Lesson XXX.
The Vascular Cryptogams are flowerless plants in which
vascular bundles are present. Together with the Phanero-
gams they constitute what are known as vascular plants, in
contradistinction to the non-vascular Algae, Fungi, and
Muscineae, in which no formation of vessels takes place. The
group contains three subdivisions.
The first division of Vascular Cryptogams, the FilicincE,
includes the ferns, an account of which has been given in
the previous lesson. It will be necessary, however, to devote
some attention to an aquatic form, called Salvinia, which
differs in certain important particulars from the more familiar
members of the group.
The Equisetacea include the common horsetails (genus
Equisetum\ 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 Lycopodinece, 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
plants reproduce by means of seeds structures which differ
F F
434 EQUISETUM LESS.
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 Lessons.
EQUISETUM
The horsetails are common British plants found usually
in moist or marshy situations, and reaching a height of
i to 3 feet.
The plant consists of a branched underground stem or
rhizome, lateral branches of which grow vertically upwards,
and constitute the aerial shoots. Both stem and branches
have a very characteristic appearance : they are distinctly
segmented or divided into nodes and internodes, and from
each node springs a crown-like structure or leaf-sheath
(Fig. 112, A, and Fig. 113, A, /. sk\ formed by a whorl of
leaves united into a continuous structure. In some cases
the aerial shoots also give rise to secondary shoots (Fig. 112,
A, sh\ arranged in whorls and apparently arising below the
leaves : actually, however, they originate in axillary buds, as
in Nitella, but, instead of growing out between the stem and
the leaf, perforate the base of the latter.
The internodes of both rhizome and aerial shoots are
hollow, each having a large axial air-cavity (Fig. 112, B, cl)
extending throughout its whole length, and formed by the
disintegration of the central parenchyma-cells of the young
XXXII
STRUCTURE
435
stem. At each node is a transverse partition separating the
internodal spaces from one another. Around the central
cavity, and corresponding with the longitudinal ribs with
which the stem is marked, is a series of smaller air-cavities
(c2), arranged in a circle, and alternating with these, between
FIG. 112. — A, portion of aerial shoot of Equisetum, showing a node
(nd) from which arise a leaf-sheath (/. s/i) and a whorl of secondary
shoots (sh). (Nat. size. )
B, transverse section of aerial shoot, showing central (cl) and peri-
pheral (c'2) air-cavities, and ring of vascular bundles with smaller air-
cavities (c'A). ( x 2.)
C, a single sporophyll (sp. pk) with stalk (s/) and sporangia (spg).
(x 10.)
D, a single spore showing coiled elater (el).
E, the same, with elater (el) expanded.
(A-c, after G-oebel ; t> and E, after Le Maout and Decaisne. )
them and the central cavity, are the vascular bundles (v. b\
each with a small air-cavity (^3) in its inner or central
portion.
The microscopic structure of the plant agrees in essential
respects with that of the fern, though differing in many
details to which no further reference need be made here.
F F 2
436 EQUISETUM LESS.
Each axis— rhizome and shoots — terminates in a tetrahedral
apical cell.
As in ferns, there is no primary root in the adult, but
numerous roots spring from the nodes of the rhizome, and
agree in all essential points of structure and development
with those of ferns.
Some of the aerial shoots bear only leaf-sheaths and
branches, and are hence called sterile shoots: others, the
fertile shoots, terminate in a cone-like structure (Fig. 113, A),
formed of hexagonal scales (sp. ph\ at first closely applied
to one another at their edges, 'but afterwards becoming
separated. Each scale (Fig. 112, c, and Fig. 113, P., sp.ph)
is a mushroom-like body, springing from the axis of the cone
by a stalk (st) attached to the centre of the inner surface of
its expanded portion. Around the point of attachment of
the stalk spring from five to ten elongated sacs, the sporangia
The structure and development of these mushroom-like
bodies or scales of the cone show them to be peculiarly
modified leaves, developed in whorls like the ordinary leaves
of the stem, but not cohering into sheaths, and assuming
the characteristic form just described in relation with their
special function of bearing the sporangia. We have there-
fore to distinguish, in Equisetum, between ordinary or
foliage-leaves and spore-bearing leaves or sporophylls.
The spores are developed in the same way as in mosses
and ferns, but have a very distinctive structure. Outside
the usual double cell-wall is a third coat, which, as develop-
ment proceeds, becomes split up into four bands (Fig. 1 1 2,
D, E, £/), wound spirally round the spore and attached to it
by one end, the opposite expanded end being free. These
bands or elaters are hygroscopic : when moist they are coiled
round the spore (D), when dry they straighten themselves
DIMORPHISM OF THE GAMOBIUM
437
and stand out separately from its surface (E). The spores
become entangled by their elaters, by the coiling and un-
coiling of which they are able to execute slight movements.
FIG. 113. — Reproduction and Development of Equisetum.
A, distal end of a fertile shoot, showing two leaf-sheaths (/. sh}, and
the cone formed of hexagonal sporophylls (sp. pk). (Nat. size.)
B, diagrammatic vertical section of a portion of the cone, showing the
sporophylls (sp. ph) 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 ; rht
rhizoids. ( x 30. )
(A, after Le Maout and Decaisne ; c and»D, after Hofmeister.)
The spores 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.
438 SALVINIA LESS.
some (c) remain small and simple, and produce only
spermaries (spy) ; others (D) attain a complicated form and
a length of over a centimetre, and produce only ovaries
(ovy). Thus although there is no difference in the spores,
the prothalli produced from them are of two distinct kinds,
the smaller being usually exclusively male, the larger female.
The oosperm develops in much the same way as in ferns :
it divides and forms a polyplast, which, by formation of a
stem, root, foot, and two cotyledons, becomes a phyllula
and grows into the adult plant.
As in the fern, the Equisetum plant, reproducing as it
does by asexual spores, is the agamobium, the gamobium
being represented by the prothallus. The peculiarity in the
present case is that the gamobium is sexually dimorphic,
some prothalli producing only male, others only female
gonads.
SALVINIA
Salvinia is a small fresh-water plant, found floating, like
duckweed, on the surface of still water.
The stem (Fig. 114, st) is an elongated slender rhizome
floating at or near the surface, and distinctly divided into
nodes and internodes. Each node gives off three appen-
dages, two broad, flat foliage-leaves (f. /. 1-3 ; f. 1. i'— 3'),
which lie above the surface of the water, and a branched
structure (s. I. 1-3) which has all the appearance of a root,
its thread-like branches hanging down into the water and
being covered with hairs. The study of their development
shows, however, that these organs arise exogenously from
the node and have no root-cap : they are, in fact, not roots,
but submerged leaves, performing the function of roots.
XXXII
STRUCTURE
439
The latter organs are, quite exceptionally among the higher
plants, wholly absent.
The stem ends distally in a terminal bud (/. bd\ the
S.l.f
FIG. 114. — Distal portion of a Salvinia plant seen obliquely from
below.
The stern (st) ends in a terminal bud (/. bd), and the part figured
contains three nodes, each bearing a pair of foliage-leaves (f. I. 1-3,
/. /. i'-3'), and a much-divided root-like submerged leaf (s. I. 1-3).
On the bases of the submerged leaves are borne groups of sori (so),
containing sporangia. (Slightly enlarged.)
(From Vines, after Sachs.)
growing point of which is formed by a two-sided apical cell :
it is traversed by a single vascular bundle, which sends
branches into the leaves.
440 SALVINIA LESS.
Springing from the bases of the submerged leaves are
numerous globular capsules (so), each containing a number
of sporangia. The wall of the capsule (Fig. 115, A) corre-
sponds with the indusium of a fern, and the contained group
of sporangia with a sorus. But the sori of Salvinia, unlike
those of ordinary ferns, are dimorphic, some containing a
comparatively small number of large sporangia (mg. spg\
others a much larger number of small ones (mi. spg). The
larger kind, distinguished as mcgasporangia, contain each a
single large spore, or megaspore : the smaller kind, or micro-
sporangia, 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.
When ripe the sporangia become detached and float on
the surface of the water. The microspores germinate (B),
while still enclosed in their sporangium : each sends out a
filament, which protrudes through the wall of the micro-
sporangium, its extremity (spy) becoming separated off by a
septum and then divided into two cells. The protoplasm
of each of these divides into four sperm-mother-cells, and
from these spirally-twisted sperms are produced in the usual
manner. It is obvious that the two cells in which the
sperms are developed represent greatly simplified spermaries :
the single proximal cell (prtK) of the filament arising from
the microspore, a still more simplified prothallus. Both
prothallus and spermaries are vestigial structures ; the pro-
thallus is microscopic and unicellular instead of being a
solid aggregate of considerable size, as in the two preceding
types ; each spermary forms only four sperm-mother-cells,
and the total number of sperms is therefore reduced to
eight.
XXXII
REDUCTION OF THE GAMOBIUM
441
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
mi.sp
FIG. 115 — Reproduction and Development of Saivtnia.
A, portion of a submerged leaf, showing three sori in vertical section,
two containing microsporangia (mi. spg) and one megasporangia (mg.
spg). (x io.)
B, a germinating microspore (mi. spg), showing the vestigial prothallus
(prth) and its two spermaries (spy). ( x 150.)
c, diagrammatic vertical section of a germinating megaspore, showing
the outer (mg. sp) and inner (mg. sp1) coats of the spore, and its cavity
(c) containing plastic products, separated by a septum (d) from the pro-
thallus (prth), in which two ovaries (ovy) are shown, that to the left
containing an ovum, that to the right a polyplast. ( x 50.)
D, megaspore (mg. sp] with prothallus (prth) and phyllula just begin-
ning to develop into the leafy plant : st, stem ; cti cotyledon ; and /,
outermost leaf of the terminal bud. ( x 20. )
(A and B, after Sachs ; D, after Pringsheim.)
(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
442 SELAGINELLA LESS.
and embryo, after the manner of the yolk in the eggs of the
crayfish and dogfish.
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. Three ovaries (pvy) are formed on it, having much
the same structure as in ordinary ferns : if neither of these
should be fertilised others are developed subsequently.
Thus the reduction of the prothallus produced from the
megaspore, although obvious, is far less than in the case of
that arising from the microspore.
We see that sexual dimorphism has gone a step further in
Salvinia than in Equisetum : not only are the prothalli
differentiated into male and female, but also the spores from
which they arise.
Impregnation takes place in the usual way, and the
oosperm divides to form a polyplast, which, by differentiation
of a stem-rudiment, a cotyledon, and a foot, passes into the
phyllula stage : no root is developed in Salvinia. By the
gradual elongation of the stem (D, sf) and the successive
formation of whorls of leaves (/), the adult form is assumed.
Thus the life-history of Salvinia resembles that of the
fern, but with two important differences : the spores are
dimorphic, and the gamobium, represented by the male and
female prothalli, is greatly reduced.
SELAGINELLA
Selaginella, one of the club-mosses, is common on hill-
sides in many parts of the world. In the commoner species
there is a creeping stem which forks repeatedly in the hori-
zontal plane, and bears numerous small, close-set leaves,
giving the whole plant much the appearance of a moss.
XXXII
STRUCTURE
443
The leaves (Fig. 116, A) arise in four longitudinal rows,
but, owing to the horizontal position of the plant, the two
rows belonging to the lower side (/2) project laterally, and
FIG. 116. — A, distal end of a shoot of Selaginella, showing the two
rows of small dorsal leaves (/a), the two laterally placed rows of ventral
leaves (/2), and the terminal cone (c). (Nat. size.)
B, a microsporangium bursting to allow of the escape of the micro-
spores (mi. sp).
C, a megasporangium, with four megaspores (mg. sp\
(A, after Sachs ; B and C, after Le Maout and Decaisne.)
are many times larger than the two upper rows (71). Each
leaf bears on its upper or distal surface, near the base, a
small process called a ligule.
The stem usually ends in a two- or three-sided apical
cell, from which segments are cut off to form the apical
444 SELAGINELLA LESS.
meristem, but in some species no apical cell can be distin-
guished. There are from one to three vascular bundles
running through the stem, each surrounded by a ring of
small air-cavities : from them a single bundle is given off to
each leaf. The presence of vascular bundles and of a well-
marked epidermis is enough to distinguish our present type
from the mosses, to which it bears a superficial resemblance.
The peculiar forked branching is due to the development
of lateral branches alternately on each side of the stem. The
roots arise from peculiar leafless branches, sometimes mis-
taken for true roots.
The branches terminate in cones (Fig. 116, A, c, and Fig.
117, A) formed of small leaves (sp. ph), which overlap in
somewhat the same way as the scales of a pine-cone. Each
of these leaves is a sporophyll, and bears on its upper or distal
side, near the base, a globular sporangium. The sporangia
are fairly uniform in size, but some are megasporangia
(Fig. 1 1 6, c, and Fig. 117, A, mg. spg\ and contain usually
four megaspores ; others are microsporangia (Fig. 116, B,
and Fig. 117, A, mi. spg\ containing numerous microspores.
.The microspore (Fig. 117, B) cannot be said to germinate at
all. Its protoplasm divides, forming a small cell (prth), which
represents a vestigial prothallus, and a large cell, the repre-
sentative of a spermary. The latter (spy) undergoes further
division, forming six to eight cells in which numerous sperm-
mother-cells are developed. The sperms are finally liberated
by the rupture of the coats of the microspore.
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
, small prothallus (prth), and a process of division also takes
XXXII
PROTHALLUS
445
place in the remaining contents (prth1} of the spore, pro-
ducing a large-celled tissue, the secondary prothallus.
By the rupture of the double cell-wall of the megaspore
spsr
FIG. 117. — 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 (prthl\ contains
a large-celled tissue, the secondary prothallus : in the prothallus are
three ovaries (ovy}> that to the left containing an ovum, that to the right
an embryo (emb) in the polyplast stage, and that in the centre an embryo
in the phyllula stage, showing stem-rudiment (st), foot (/"), and two
cotyledons (ct} : both embryos are provided with suspensors (dotted)
(spsr), and have sunk into the secondary prothallus.
(Altered from Sachs. )
the prothallus is exposed to the air, but it never protrudes
through the opening thus made, and is, therefore, like the
corresponding male structure, purely endogenous. One or
446 SELAGINELLA LESS, xxxil
more ovaries (pvy) are formed on it, each consisting of a
short neck, an ovum, and two canal-cells afterwards con-
verted into mucilage : there is no venter, and the neck con-
sists 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 (spsr) : 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 (st), 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.
LESSON XXXIII
GYMNOSPERMS
THE commonest Gymnosperms are the evergreen cone-
bearing trees such as pines, spruces, larches, cypresses, and
yews. They all have a primary axis or trunk from which
branches arise in a monopodial manner, i.e., the oldest are
near the proximal, the youngest near the distal end. The
branches give off, in successive seasons, branches of a higher
order, so that the older or lower branches are always them-
selves more or less extensively ramified, and the whole plant
tends to assume a conical form, the base of the cone being
formed by the oldest secondary axes springing from the
base of the trunk, the apex by the distal end of the primary
axis.
The branches are all axillary, each arising from the axil
of a leaf, and, like the main stem, ending distally in a
terminal bud. The foliage-leaves differ greatly in the various
genera of Gymnosperms : in the pines they are long, needle-
like structures, borne in pairs on short axillary branches or
dwarf-shoots.
In correspondence with the size attained by the aerial
portion of the plant, the root attains far greater relative
dimensions than in any case we have previously studied.
448 GYMNOSPERMS LESS.
The trunk is continued downwards by a great primary root,
from which secondary roots arise in regular order, and, these
branching again and again, there is produced a root-system
of immense size and complexity, extending into the soil to a
sufficient depth to resist the strain to which the aerial part or
the tree is subjected by the wind.
One remarkable feature about the pines and their allies as
compared with the plants previously studied, is their practi-
cally unlimited growth. In mosses, ferns, &c., the stem
after attaining a certain diameter ceases to grow in thick-
ness, so that even in the tallest tree-ferns the stem is always
slender. But in pines the trunk, the branches, and the
roots continue to increase in thickness for an indefinite
period, the trunk in the common Scotch Fir (Pimts
sylvestris) attaining a circumference of four or five metres
or even more, and the other parts in proportion. The tree
may survive for hundreds of years.
The changes undergone during this remarkable process of
growth are best studied, in the first instance, by a series of
rough transverse sections of branches of different ages. In a
first year's branch the middle is occupied by an axial strand
of soft tissue, the pith or medulla (Fig. 118, A and B, med) ;
outside this comes a ring of wood (xy), divided into radially
arranged wedge-shaped masses; and this in turn is sur-
rounded by the bark or cortex (cor\ which can be readily
stripped off the wood, and which 'contains numerous resin-
canals (r. c) appearing in the section as rounded apertures
with drops of resin oozing from them. In a somewhat older
branch the layer of wood is seen to have increased greatly
in thickness, and has a well-marked concentric and radial
striation (c) : the cortex also has thickened though to a less
extent, while the pith is unaltered. The bark, moreover, is
clearly divisible into an inner light coloured layer, the bast
STRUCTURE OF STEM
449
or phloem (phi), a middle green layer of cortical parenchyma
(cor) containing resin-canals, and an outer brown layer, the
cork (ck). Lastly, in the trunk and larger branches the wood
forms by far the greater part of the whole section, the bark
being a comparatively thin layer, easily stripped off, with no
ck
FIG. 118. — Diagrammatic transverse sections of three branches of
Pinus of different ages.
A, very young axis, showing epidermis \ep), cortex (cor] with resin-
canals (r. c}, medulla (med), and ring of vascular bundles, separated by
medullary rays (med. r), and each consisting of xylem (xy), cambium
(cb\ and phloem (phi).
B, older axis, in which the cambium forms a complete cylinder, owing
to the formation of interfascicular cambium (cb') between the bundles.
c, Axis of the third year, showing xylem of first (xy1), second
(xyz), and third (xy3) year's growth ; cork (ck) ; and cork -cambium
(ck. cb.)
cortical parenchyma, and with its corky outer layer much
thickened, gnarled, and wrinkled.
The wood has been stated to exhibit both concentric and
radial striations. The radial markings are called medullary
rays (Fig. 118, c, med. r) and follow the "grain" of the
G G
450 GYMNOSPERMS LESS.
wood. The concentric markings, which are against the
grain, are the annual rings (xy1, xy'2, xy3), and owe their
existence to the fact that the wood formed in summer and
autumn is denser than that formed in spring, while in winter
there is a cessation of wood-production. Thus, by counting
the annual rings of the main trunk, the age of the tree may
be estimated. The wood, it will be observed, grows from
within outwards, a new layer being added each year outside
the old.
The power of indefinite increase in diameter, which is so
striking a feature in the pines and their allies, is connected
with a peculiarity in the structure and arrangement of the
vascular bundles. In the very young condition, i.e., in the
terminal bud, the vascular bundles of the stem (Fig. 118, A)
are wedge-shaped in transverse section and are arranged in
a circle, the apex of each being turned towards the axis of
the stem, the base towards its periphery. Actually, of course,
as in the fern, the bundles are longitudinal strands with pro-
longation into the leaves.
The arrangement of the tissues in the vascular bundles
differs in an important respect from the condition we are
familiar with in the fern. Instead of the xylem occupying
the centre of the bundle and being surrounded by phloem,
the xylem (Fig. 118, A, xy) forms the whole of the in-turned
side, i.e., the narrow portion of the wedge in transverse sec-
tions, the phloem (phi) the outer portion or broad end of
the wedge. In a word, the bundles are not concentric as in
the fern, but collateral. Moreover, the phloem and xylem
are separated by a layer of small thin-walled cells, called the
cambium layer (cb).
By this arrangement of the vascular bundles the ground-
parenchyma of the stem is divisible into three portions, an
external layer, the cortex (cor), between the epidermis (ep),
xxxiii GROWTH IN THICKNESS 451
and the phloem bundles, an axial cylinder, the ///// or
medulla (med), internal to the xylem bundles, and a series
of radial plates, the primary medullary rays (med. r) separat-
ing the bundles from one another.
As development proceeds the parenchyma-cells connecting
the cambium of adjacent bundles take on the characters of
cambium-cells, the result being the formation of a closed
cambium-cylinder, or, in transverse section, cambium-ring
(B, cb, cb'). In this a distinction is to be drawn between
the fascicular cambium (cb) or original cambium of the
bundles and inter-fascicular cambium (cb') formed by con-
version of cells of the medullary rays.
The cambium-cells now begin to divide in a tangential
direction, i.e., along a plane parallel to the surface of the
stem. If this process went on alone the result would be
simply an increase in the thickness of the cambium layer,
but as it proceeds the products of division of the cells
on the inner face or* the cambium-cylinder become con-
verted into new xylem-elements, those on its outer face
into new phloem-elements. We have thus a formation of
secondary wood and secondary bast, which, being formed
from the whole of the cambium-cylinder, show no division
into bundles but form a continuous cylinder (c, xy, phi) of
constantly increasing thickness. The phloem now forms
the inner layer of the bark, which, as we have seen, can be
readily stripped from the wood owing to the delicate
cambium-cells being easily torn apart.
At the same time a layer of cells of the cortical parenchymr
begins to divide tangentially so as to form a cylinder, or in
transverse section a ring, of cork-cambium (Fig. 118, c, ck.
cb), from the outer face of which layer after layer of cork-
cells (ck) is formed. In the cork-cells the protoplasm dis-
appears and the cell-walls undergo a peculiar change by
o G 2
452 GYMNOSPERMS LESS, xxxm
which they become waterproof : this process, besides pro-
tecting the interior of the stem from external moisture,
prevents the access of nutrient matters to the epidermis
and outer layers of cortical parenchyma. Both these layers
consequently die and peel off, the outer surface coming to
be formed by the cork itself.
The wood of pines contains no vessels, i.e., cells joined end
to end so as to form a continuous tube, but only tracheides,
z>., elongated spindle-shaped cells with lignified walls and
devoid of protoplasm (p. 417). Radial bands of cells
mostly parenchymatous, are formed between the tracheides
of the secondary wood, and give rise to the secondary
medullary rays (c, med. r) to which the radial striation of
the wood is due : they increase in number with the increase
in thickness of the wood. The tracheides formed in
autumn have smaller cavities and thicker walls than those
formed in spring and summer : hence the formation of
annual rings. The tracheides are not scalariform like those
of ferns, but their walls have at intervals circular depressions
perforated in the centre and called bordered pits. The
tracheides of the primary xylem bundles have spirally
thickened walls, like the spiral vessels of ferns. The
phloem, both primary and secondary, consists of sieve-
tubes and parenchyma.
The growing point of Gymnosperms presents a striking
difference to that of ferns and other flowerless plants. It
consists simply of a mass of meristem cells among which no
apical cell is to be distinguished.
Pines, like horsetails and club-mosses, reproduce by
means of cones or floivers. These are of two kinds, male
and female, so that sexual differentiation is carried a step
further than in Selaginella, in which sporangia of both sexes
FIG. 119. — Reproduction and Development of Gymnospermt.
A, diagrammatic vertical section of male cone, showing axis with maie
sporophylls (sp. ph. & ) bearing microsporangia (mi. spg) : £er, scale-like
leaves forming a rudimentary perianth.
454 GYMNOSPERMS LESS.
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 (nig. 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 (prth) in which are two
ovaries, that to the left showing a large ovum (ov) and neck-cells, while
that to the right has given rise - to an embryo (emb) which is in the
phyllula stage, and has sunk into the tissue of the prothallus by the
elongation of the long suspensor (sfsr).
A microspore (mi. sp) is seen in the micropyle sending off a pollen-
tube (p. /), the end of which is applied to the necks of the two ovaries.
E, diagrammatic vertical section of a seed, showing coat (/), micro-
pyle (mpy), and endosperm (end), in which is imbedde I 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. )
are borne on the same cone. In the pines and their allies
both male and female cones are usually borne on the same
tree, so that the plant is monoecious: many Gymnosperms,
on the other hand, are dioecious^ each tree bearing either
male or female cones only.
The male cones (Fig. 119, A) are borne in clusters or
inflorescences near the distal ends of the branches. P^ach
cone consists, as in Equisetum and Selaginella, of an axis
bearing a large number of sporophylls (sp. ph. g ) : it springs
from the axil of a leaf and is to be looked upon as an
abbreviated and peculiarly modified shoot.
The sporophylls or stamens as they are commonly called
(Figs. 119, sp.ph. £ and Fig. 120), are more or less leaf-like
structures, each consisting of a short stalk or filament and an
expanded portion or anther, the latter bearing on its under or
proximal side two microsporangia or pollen-sacs (mi. spg).
The mother-cells of these divide each into four microspores
or pollen-grains, which are liberated by the rupture of the
REPRODUCTIVE ORGANS 455
microsporangia in immense quantities, in the form of clouds
of light yellow powder called pollen. The microspore (B)
is at first an ordinary cell consisting of protoplasm with a
nucleus and a double cell-wall, but eventually the proto-
plasm divides into two cells; a small one (a\ the vestige of
the male prothallus, which soon divides again forming two
or more cells, one of which is distinguished as the generative
cell; and a large one (b\ the vegetative cell. Under favour-
able circumstances these cells undergo changes which will
be described presently.
The structure of the female cone is best made out in the
FIG. 1 20. — A single stamen or male sporophyll of the pine, showing
:he two microsporangia or pollen-sacs.
larch. It also consists (Fig. 119, c) of an axis bearing
sporophylls (sp. ph. ? ), or, as they are usually called in
Phanerogams, carpels. Each carpel is a crimson leaf with a
green midrib produced distally into a projecting point, and
bears on its upper or distal surface a little flattened body,
the placental scale, on the upper surface of which are two
peculiarly modified megasporangia (mg. spg.), commonly
known as ovules. In the pine the placental scales (Fig. 121)
are larger than the carpels, and their thickened distal ends
form the rhomboid areas into which the surface of the cone
is divided.
456 GYMNOSPERMS LESS.
The comparison of the reproductive organs of the pine
and larch with those of Vascular Cryptogams and of
Angiosperms will be facilitated by a consideration of two
exotic genera of palm-like Gymnosperms. In Zamia both
male (Fig. 122, A) and female (B) cones bear a close
external resemblance to those of Equisetum, the sporophylls
(sp. ph. $ , sp. ph. 9 ) being stalked hexagonal scales on the
inner surfaces of which the pollen-sacs (B, mi. spg) or ovules
(D, mg. spg} are borne. In the female Cycas the carpels
(E, sp. ph. $ ) are not arranged in a cone, but form a whorl
FIG. I2i. — A single carpel or female sporophyll of pine, with pla-
cental scale bearing two megasporangia or ovules.
of leaf-like bodies obviously homologous with foliage leaves.
Each carpel is, in fact, a leaf 20-30 cm. long, and deeply
lobed at its edge : in the distal portion the lobes are long
and slender, but proximally they take the form of ovoidal
bodies (mg. spg), about the size of plums, the ovules or
megasporangia.
The ovules differ strikingly in structure from the megaspor-
angia of Cryptogams. Each consists of a solid mass of small
cells called the nucellus (Fig. 119, D, net), attached by its
proximal end to the sporophyll, and surrounded by a wall
or integument (/) also formed of a small-celled tissue. The
REPRODUCTIVE ORGANS
457
FlG. 122. — A, male cone of Zamia, showing the hexagonal sporo-
phylls (sp. ph. :. Conjugation of pronuclei 260
SYNOPSIS 485
PAGE
VIII.— UNICELLULAR AND DIPLOBLASTIC ANIMALS.
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 261
C. — Other matters of general importance, such as the composition
and properties of protoplasm, cellulose, chlorophyll, starch, &c. : meta-
bolism : holozoic, holophytic, and saprophytic nutrition : intra- and
extra-cellular digestion : amoeboid, ciliary, and muscular movements :
the elementary physiology of muscle and nerve : parasitism and sym-
biosis : asexual and sexual generation : and the elements of embryology
— are discussed under the various types, and will be most conveniently
referred to by consulting the Index.
INDEX AND GLOSSARY
INDEX AND GLOSSARY
Abdomen, Crayfish, 318
Abiogenesis (a, not : /Si'os, life: y«'i/«/xa, colour), the
constituent of the nucleus which is un-
affected or but slightly affected by dyes.
See nuclear sap
ACGBlom'ate (a, not : Koi'Awjxa, a hollow),
having no coelome (q.v.) '. 299
Adduct or muscles, Mussel, 354
Aerob'iC (arjp, air : /Si'os, life), applied to
those microbes to which free oxygen is
unnecessary, 93
Agamob ium (a, not : ya/uo?, marriage :
/3i'os, life), the asexual generation in or-
ganisms exhibiting alternation of gene-
rations (g.v.)
AGAR'ICUS (mushroom) '.—Figure, 192 :
general characters, 191 : microscopic
structure, 193'. spore-formation, 193
Algae (alga, sea-weed), 169, 432
Alternation of Generations, meaning of
the phrase explained under Bougain-
villea, 248 : Moss, 408 : Fern, 429 :
Equisetum, 438 : Salvinia, 442 : Selagin-
ella, 446 : Gymnosperms, 460 : Angio-
sperms, 476
Ambula cral (ambulacrum, a walking
place) groove, 307 : ossicles, 308 : System,
starfish, 309 — 313
AMCEB'A (aju.j, that which is
thrown up). See Metabolism, construc-
tive.
Anaerobic (a, not : aijp, air : /Sios, life),
applied to those microbes to which free
oxygen is unnecessary, 93
An'al (antes, the vent) segment, Poly-
gordius, 270
An'al spot, Paramoecium, 113
An astates (ai/ao-TOTO?, from ai/aonji/ai,
to rise up). See Mesostates, anabolic.
Anatomy (avare^vnt, to cut up), the study
of the structure of organisms as made
out by dissection, 289
Androe ciurn. (avrip, a male : ot/co
gills), 337
Anthropoda, the, 306
Artificial reproduction of Hydra, 231
Asexual generation. See Agamobium.
Asexual reproduction. See Fission,
Budding, Spore.
Asparagin, 410
Assimila'tion (assiinilo, to make like), the
conversion of food materials into living
protoplasm, 13
Ast'acus. See Crayfish.
Asterias. See Starfish.
Astrosphere, 65
At'rophy (a, without : rpo, a little staff ) or
MICROBES GUUKPOS, small : /Sios, life):—
occurrence, 82 : structure of chief genera,
84 : reproduction, 87 : nutrition, 89 :
ferment-action, 91 ; parasitism, 92 : con-
ditions of life, 92 : presence in atmos-
phere, 102 : animals or plants? 182
BACTERIUM termo (Figures) 83, 84
Baer, von, Law of Development, 43
Barnacle-geese, supposed heterogenetic
production of, 103
Bark. See Cortex.
Bast. See Phloem.
Bionomlnal menclature, 8, 139
BiOgen'eslS, (/3t'o;, life : yeWo-is, origin),
Calyp'tra (KoAvTrrpa, a veil), 407
Cal'yx (KaAvf, the cup of a flower), the
outer or proximal whorl of the perianth in
the flower of Angiosperms, 464, 471, 473
Cambium, 450
Canals, radial and circular, medusa, 239
Canal-cells of ovary, 405. 426
Cap-cells of roots, 422
Carapace, Crayfish, ?i8
Carbon dioxide, decomposition of by
chlorophyll bodies, 29
ardiacdivi
Cardiac division. See Stomach.
Car'pelOcapTro?, fruit), a female sporophyll.
455, 456, 464, 467
Car tilage, 372
CAULER'PA OcouAo?, a stem : epn-w, to
creep), 175 (Figure)
Cell (cella, a closet or hut, from the first
INDEX AND GLOSSARY
491
conception of a cell having been derived
from the walled plant-cell, '.—meaning of
term, 61 : minute structure of (Figure),
62 : varieties of (Figure), 57
Cell-aggregate, meaning of term, 188
Cell-COlony : — temporary, Saccharomyces,
73 : permanent, Zoothamnium, 135
Cell-division, 65 (Figure)
Cell-fusion 302, 419
Cell-layer, 222, 273
Cell-membrane or wall, 10, 27, 63
Cell-multiplication and differentiation,
215 : Polygordius, 302 : Fern, 419
Cell-plate, 67
Cell-protoplasm, 62
Cell'ulose, composition and properties of,
28
Central capsule, Radiolaria, 152
Central particle or Centrosome (itevrpov,
centre : aCo^a, the body), 65, 261 (Fig-
ure)
Ceph'alOthor'aX (*eaArj, head : 0u>paf ,
breast-plate), Crayfish, 318
Cerebral ganglion. See Brain.
Cerebro-pleural ganglion, Mussel, 362
Cheliped (xy^y, claws : TTOVS, foot), 323
Chlor'ophyll (xAu>pos, green : <£vAAov, a
leaf), the green colouring matler of
plants, properties of, 26, 31 : occurrence
in Bacteria, 87 : in Hydra, 228
Chrpm'atin (xpw/u.a, a colour), the con-
stituent of the nucleus which is deeply
stained by dyes, 7, 63 : male and female
in nucleus of oosperm, 261
Chrom'atophore (xp£>na, colour : <£epw
to bear), a mass of proteid material im-
pregnated with chlorophyll or some other
colouring matter, 26, 46, 197, 207, 228
Chromosome (XPWM<*, colour : o-w/ua body),
65, 66, 261
Cil'ium (czY/'w/H, an eye-lash), defined,
•2$note : comparison of with pseudopod,
34, 52 : absence of cilia in Arthropoda,
Ciliary movement, 25 : a form of con-
tractility, 33
Cil'iate Infusoria, 107
Circulatory organs, Polygordius, 282 :
Crayfish, 337 : Mussel, 360 : Dogfish,
Classification, natural and artificial, 141 :
natural, a genealogical tree, 145
Cnid'oblast (KI/I^TJ, a nettle : /SAourros, a
bud), the cell in which a nematocyst
(?.z>.)is developed, 227
Cnid'OCil (xi/iSr/and ciliuni), the " trigger-
hair" of a cnidoblast, 227
Ccelenterata, the, 305
COBlome (KoiAwjaa, a hollow), the body-
cavity : — Polygordius, 270 : Starfish,
306 : Crayfish, 335, 343 : Mussel, 355 :
Dogfish, 372 : development of, Poly-
gordius. 299
CkBlom'ate, provided with a ocelome, 273
CcelomiC epithelium. See Epithelium.
Coelomic fluid, Polygordius, 278
Colloids (xoAAa, glue : elSos, form), pro-
perties of 6
Colony, Colonial organism, meaning of
term, 135, 234 : formation of temporary
colonies, Hydra, 231
Columel la (a little column), 162
Com'miSSUre (comnissura, a band), 279
Compound organism. See Colony.
Concres'cence (c«m, together : cresco, to
grow), the un on of parts during growth
Cone, an axis bearing sporophylls : — Equi-
setum, 436 : Selaginella, 444 : Gymno-
sperms, 452
Conjugation (conjugdtio, a coupling), the
union of two cells, in sexual reproduc-
tion : — Amceba, 20 : Hettromita, 41 :
Paramcecium, 114: Vorticella, 132:
Mucor, 165 : Spirogyra, 198 : of ovum
and sperm, 260 : monoecious and dioe-
cious, 199 : comparison with plasmodium-
formation, 54
Connective, cesophageal, 283, 341
Connective tissue, 329. 369
Contractile vac'uole (vacnns, empty) :--
Amceba, 8, 16 : Euglena, 47 : Paramoe-
cium, in
Contractility (contracts, a drawing to-
gether), nature of, 10, 34 : muscular,
130
Contraction, physical and biological, 10
Conus artinosus, 384
Cork, 449
Cork-cambium, 451
Corolla (corolla, a little wreath), the
inner or distal whorl of the perianth in
the flower of Angiosperms, 464, 468, 471,
Corpuscles. See Blood-corpuscles, and
Leucocytes.
Cortex, cor'tical layer (cortex, bark),
Flowering plants, 59, 448 : Infusoria,
no, 126
Cotton-WOOl as a germ-fitter, 99
Cotyle'don (wrvXtfuav, a cup or socket),
the first leaf or leaves of the phyllula
(q.i>!) in vascular plants, 427
Cranium (xpaviov, the skull), 374
CRAYFISH :— Figure. 319: general charac-
ters. 314, 315 : limited number and con-
crescence of metameres (Figure), 320: ap-
pendages (Figure), 332 : exoskeleton,
319 : enteric canal (Figures), 322 : gills
(Figure), 318 : blood-system (Figure.)
335) 337 : kidney, 337 : nervous system,
319 : Muscles (Figures), 327 : reproduc-
tive organs, 343 : development, 343
Creation (creo, to produce), definition of,
141 : illustrated in connecton with species
of Zootha-mnium (Diagram}, 142
Cross-fertilization : applied to the sexual
process when the gametes spring from
different individuals, 199
492
INDEX AND GLOSSARY
Cryst'alloidS (*puo-TaAAos, crystal : eZSos
form), properties of, 6
Cuticle (ciittciila, the outer skin), nature
of in unicellular animals, 45, 109 : in
multicellular animals, 238
Cyst (KVCTTI?, a bag), used for cell-wall in
many cases, 10, 51
Dallinger, Dr. W. H , observations on an
apparent case of heterogenesis, 103
Daughter-cells, cells formed by the fission
or gemmation of a mother-cell, 35, 67
Death, phenomena attending, 20, 21, 166
Decomposition, nature of, 6, 91
Dermal gills. See Respiratory Coeca.
Dennis (8e'p/xa, skin), the deep or connec-
tive tissue layer of the skin, 326
Descent, doctrine of. See Evolution.
Development, meaning of the term, 43.
For development of the various types
see under their names
Dextrin, 113
Diastase, 81
Diast'Ole (SiaTreAAco, to separate), the
phase of dilatation of a heart, contractile
vacuole, &c., in
DIATOMA CE,ffi (5taTe>j/w, to cut across,
because of the division of the shell into
two valves), 155 : Figure, 156
Diat'omin, the characteristic yellow colour-
ing matter of diatoms, 154. 155
Dichot'omous (8txorojuteu>, to cut in two),
applied to branching in which the stem
divides into two axes of equal value, 318
Differentiation (differo, to carry different
ways), explained and illustrated, 34, 119
Diges tion (digero, to arrange or digest),
the process by which food is rendered fit
for absorption, 12, : intra- and extra-
cellular, 229: contrasted with assimila-
tion, 230
Digestive gland, 335, 355, 382
Dimorph'ism, dimorph'ic (fit's, twice:
xj, form), existing under two forms,
35, 136, 242, 438, 442
Dice'cipUS (Si's, twice : OIKOS, a dwelling),
applied to organisms in which the male
and female organs occur in different in-
dividuals, 199
DIPH'YES (6i(/>v7js, double) : Figure, 250 :
occurrence and general characters, 248 :
polymorphism, 249 .
DiplOblaSt'iC (Sin-Aoo?, double : /SAacrro?, a
bud), two-layered : applied to animals in
which the body consists of ectoderm and
endoderm, 236 : derivation of diploblas-
tic from unicellular animals, 261
Directive sphere, see also Astrosphere
Disc, Vorticella, 128
Dispersal, means of: in internal parasite,
124: in fixed organisms, 132, 134
Distal, the end furthest from the point of
attachment or organic base, 126
Distribution of food-materials :— in a
complex animal, 278 : in a complex
plant, 409
Divergence of character, 145
Division of physiological labour, 34
DOGFISH '.— Figure, 367 : general charac-
ters, 368 : exoskeleton, 369 : endo-
skeleton (Figures). 372 : enteric canal
(Figures), 381: gills, 383: blood-system
(Figures), 384 : kidney, 396 : gonads,
396 : nervous system and sense organs
(Figure), 391 : development (Figure),
Dry-rigor, stiffening of protoplasm due to
abstraction of water, 21
Ecdysis (e^Sucm, a slipping out), 325
Echinodermata, the. 305
Ect'oderm (e/crds, outside : 5e'p/xa, skin),
the outer cell-layer of diploblastic and
triploblastic animals, 222, 275
Ect'OSarC (e/crd?, outside : o-os, bright-eyed) : —
Figure, 45 : occurrence and general
characters, 44 : movements, 44 : struc-
ture, 45: nutrition, 46: resting stage,
47 : reproduction, 48 : animal or plant ?
1 80
Euglen'oid movements, 45
Ev'olution (evolvo, to roll out), organic :
definition, 143 : illustration of in connec-
tion with species of Zoothamnium (Dia-
gram), 144
Excre'tion (excerno, to separate), the
separation of waste matters derived from
the destructive metabolism of the or-
ganism, 16, 281
Exogenous (ef, out of: -yiyi/o/ouu, to come
into being), arising from the exterior,
*.g. leaves, 422
Exopodite (e'£w, outside : TTOU'S, foot), 323
Exoskel eton (e£co, outside, and skeleton,
from o-Ke'AAio to dry), fhe external or
skin-skeleton : cuticular, 238, 273 : der-
mal, 308, 327, 350, 369
Eye, Crayfish, 342 : Dogfish, 394 :
Eye-spots or Ocelli :— Medusa, 244 :
Polygordius, 296
Eye-stalks, 321
F
Faeces (faex, dregs), solid excrement,
consisting of the undigested portions of
the food, 16
Perm'ent (fermentum., yeast, from/er-
veo, to boil or ferment), a substance
which induces fermenta'tion, i.e. a
definite chemical change, in certain sub-
stances with which it is brought into
contact, without itself undergoing
change : unorganized and organized
ferments 80 : alcoholic, 76 : ace-
tous, 91 : diastatic or amylolytic, 81 :
lactic, 91 : peptonizing or proteolytic,
8 1 : putrefactive, 91 : ferment-cells of
Mucor, 168
FERNS :— Figures, 414, 424 : general
characters 412 : histology of stem, leaf,
and root, 415 : nutrition, 422 : spore-
formation, 422 : prothallus and gonads,
425 : development, 426 : alternation of
generations, 429
Fertiliza'tion (fertilis. bearing fruit) ;
the process of conjugation of a sperm or
sperm-nucleus with an ovum, whereby
the latter is rendered capable of develop-
ment : a special case of conjugation
(q.v.), 199 : details of process, 260 : in
Vaucheria, 173 : in Gynmosperms, 458,
in Angiosperms, 475
Filtering air, method of, 99
Fins, Dogfish 369
Fission (Jissio, a cleaving), Simple or
binary, the division of a mother cell
into two daughter-cells : in Amoeba, 19 ;
Heteromita. 40 : animal- and plant-cells
generally, 65 : Paramoecium, 114 :
Vorticella 131
Fission, multiple, the division of a
mother cell into numerous daughter-
cells :— in Heteromita, 42 : Protomyxa,
51 : Saccharomyces, 74
Fission, process intermediate between
simple and multiple, Opalina, 124
Flagellum. See Cilium.
Flag'ellate Infusoria, 107
Flagell'ula (diminutive oiJJagellnjii), the
flagellate germ of one of the lower
organisms (often called zoospores, 51, S4
Flageirum (JJagellnm, a whip) : defineds
25 : transition to pseudopod. 52, 228
Floral receptacle, the abbreviated axis of
an.angiospermous flower 464, 471, 473
Flower, a specially modified cone (ff.-v,),
having a shortened axis, which bears
perianth-leaves as well as sporophylls,
463 : often applied to the cone of Gymno-
sperms. 452
Food-current, Mussel, 359
Food-vacuole, a temporary space in the
protoplasm of a cell containing water
and food-particles, n, 112
Foot : of Mussel, 349 : of phyllula of fern,
FORAMINIF'ERA (>r«;^«, a hole :fero
to bear), 148 : Figures, 149, 150, 151
Fragmentation of the nucleus, 120
Fruit of Angiosperms, 476
Func'tion (functio, a performing), mean-
ing of the term, 9
Gam'ete (ya(te(a, to marry), a conjugating
cell, whether of indeterminate or deter-
494
INDEX AND GLOSSARY
pr
bi
minate sex : — Heteromita, 41 : Mucor,
156 : Spirogyra, 198 : Vaucheria 173
Gamob'ium (yd/uos, marriage : /Sips, life),
the sexual generation in organisms ex-
hibiting alternation of generations (q.v.) '.
rogressive subordination of, to agamo-
ium in vascular plants, 429, 440, 444,
Ganglion (yayyAiov, a tumour), a swelling
on a nerve-cord in which nerve-cells are
accumulated, 341
Gastric juice (yaa-rrjp, the stomach), pro-
perties Of, 12
Gastric mill, 334
Gastrolith (yaorijp, stomach : Ai'0o?,
stone), 334
Gast'rula (diminutive of yaoTTjp, the
stomach), the diploblastic stage of the
animal embryo in %vhich there is a diges-
tive cavity with an external opening : f
characters and Figure of, 295 : contrasted '
with phyllula, 428
Gemma 'tion (gemma, a bud). See Bud-
ding.
Generation, asexual, See Agamobium.
Generation, sexual. See Gamobium.
Generations, Alternation of. See Al-
ternation of generations.
Generative cell, 455
Generative nucleus, 474
Generalized, meaning of term, 140
Ge'nus (genus, a race), generic name,
generic characters, 8, 139
Germ-filter, 99
Ger'minal spot, the nucleolus of the
ovum. 257
Germina'tion (germinatio, a budding),
the sprouting of a spore, zygote, or
oosperm to form the adult plant : for
germination of the various types see
under their names.
Gill, an aquatic respiratory organ. 335, 357,
383
Gland (glans, an acorn), an organ of
secretion ($•&•) 1 gland-cells, 228,
278
Glochid'ium, larva of Mussel, 365
Gon'ad (761/05, offspring, seed), the essen-
tial organ of sexual reproduction,
whether of indeterminate or determinate
sex, i.e. an organ producing either un-
differentiated gametes, ova, or sperms ;
see under the various types, and espe-
cially 172, 198, 211, 290
Gon'odUCt (gonad and duco, to lead), a
tube carrying the ova or sperms from the
gonad to the exterior, 292
Grapping-lines, Diphyes, 249
Green gland See Kidney
Growing point : Nitella, 208 : Moss, 403 :
Fern, 418: Gymnosperms, 452
Growth, 13
Guard-cells of stomates, 421
Gullet, the simple food-tube of Infusoria,
47, no : or part of the enteric canal of
the higher animals, 277
GYM'NOSPERMS(yu/a«/6s, naked :p6StTos, from
Hermes and Aphrodite). See Monoecious.
Heterogen'esis (eVepos, different : yeVeeris,
origin), meaning of term, 102 : supposed
cases of, 103 : not to be confounded with
metamorphosis or with evolution, 104
HETEROMITA (erepos, different : niros,
a thread):- Figure, 38: occurrence and
general characters, 36 : movements, 37 :
nutrition, 37 : asexual reproduction, 40 :
conjugation, 41 : development and life-
history, 42, 43 : animal or plant? 181
High and low organisms, 106
Higher (triploblastic) animals, uniformity
in general structure of 304
Higher (vascular) plants, uniformity in
general structure of, 431
HistOl'Ogy (itrriov, a thing woven : Adyos,
a discussion), minute or microscopic
anatomy, 289
INDEX AND GLOSSARY
495
HolOphytlC (6Aos, whole : fyvrov, a plant),
nutrition, defined, 31
Holozo'ic (6Aos. whole : ^wov, an animal),
nutrition, defined, 31
Homogen esis (6/U.6? . the same : yeVeo-t?
origin), meaning of the term, 102
Homol'OgOUS (ofj.6\oyo<;, agreeing), applied
to parts which have had a common
origin, 242
Homomorph ism homomorph'ic (6/0165,
the same : fiop^, form), existing under
a single form, i ^9
Host, term applied to the organism upon
which a parasite preys, 123
HYDRA (vSpa, a water-serpent) : Figures,
219, 223, 225, 232: occurrence and general
characters, 218 : species, 220 : move-
ments, 220 : mode of feeding, 221 : m.cro-
scopic structure, 222 : digestion, 229 :
asexual, artificial, and sexual reproduc-
tion 230 : development, 233
Hydr'anth (vSpa, a water-serpent : avOos,
a flower), the nutritive zooid of a hydroid
polype, 236
Hydroid (vfipa, a water- serpent : eu5o?,
form) Polypes (n-oAvTrov?. many footed),
compound organisms, the zooids of which
have a general resemblance to Hydra,
234
Hypertrophy (uTre'p, over : rpo^, nourish-
ment), an increase in, size beyond the
usual limits, 118
Hyph'a (v0aiVo>, to weave) applied to the
separate filaments of a fungus : they
may be mycelial (see mycelium), sub-
merged, or aerial : Mucor, 160 : Peni-
cillium, 185
Hyp'odermis (wo, under : Se'p/xa , skin),
Fern, 413, 416
Hypostome (viro, under : o-ro/xa, mouth),
220, 236
Insolation (insolo, to place in the sun),
exposure to direct sunlight, 94
Integ'ument (integuntentum, a covering)
of megaspore : Gymnosperms, 456 :
Angiosperms, 466
Inter-cellular spaces, 415
Inter-muscular plexus (TrAeVcw, to twine),
285
Internode (inter, between : nodus, a
knot), the portion of stem intervening
between two nodes, 205
Interstitial (interstltlum, a. space be-
tween) cells, Hydra, 224: growth,
Spirogyra, 198
Intestine (intestlnus, internal), part of
the enteric canal of the higher animals,
277
Intus-SUSCeption (intus, into: suscipio,
to take up), addition of new matter to
the interior, 13
Iodine, test for starch, 27
Irritability (irritabilis, irritable, the
property of responding to an external
stimulus, 10
Jaws : Crayfish, 324 : Dogfish. 368, 375
Karyokines is (napvov, a kernel or nu-
cleus : /ciVrjo-is, a movement), indirect
nuclear division, 67
Katab Olism (/cara^oATJ, a laying down),
18. See Metabolism, destructive.
Kat'astates (Karao-TTJi/ai, to sink down),
18. See Mesostates, katabolic.
Kidney :— Crayfish, 337 : Mussel, 359 :
Dogfish, 396
Immortality, virtual, of lower organisms,
21
Income and expenditure of protoplasm,
18
Individual See Zooid.
Individuation, meaning of the term, 230,
252
IndUS'ium (indusium, an under-garment,
423
Inflores'cence (floresco. to begin to
flower), an aggregation of cones or
flowers, 454
Infusoria (so called because of their fre-
quent occurrence in infusions), 107
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
Labial palps, Mussel, 355
Larva, the free-living young of an animal
in which development is accompanied by
a metamorphosis, 293
Larval Stages, significance of, Polygor-
dius, 303
Leaf, structure of: — Nitella, 205, 207 :
Moss, 403 : Fern, 420 : limited growth
of. 211
Leaflet, Nitella, 207
Leg, Crayfish. 324
Lept'othrix aeirrds, slender : flpi'f , a hair),
filamentous condition of Bacillus, 89 :
Figure, 87
LeUC'OCyte (AevKos white : KV'TOS a hollow
vessel, cell), a colourless blood corpuscle :
— structure of, in various animals
(Figures), 57 : ingastion of solid par-
496
INDEX AND GLOSSARY
tides by, 58 : fission of, 58 : formation
of plasmodia by, 58
Leuwenhoek, Anthony van, discoverer
of Bacteria, 97
Life, origin of. See Biogenesis.
Life-hiStorj, meaning of the term, 43
Lignin (li&nnm, wood), composition and
properties of, 416
Ligule, 443
Linear aggregate, an aggregate of cells
arranged in a single longitudinal series,
188
Linnaeus, C., introducer of binomial no-
menclature, 8, 139
Liver, Dogfish. 382
Lymphatics, Dogfish, 391
M
Mad'reporite (from its similarity to a
madrepore or stone-coral), 308
Mandible, 324
Mantle, Mussel, 348
Manub rium (mannbrinm, a handle) of
Medusa, 239
Maturation of ovum, 259
Maxilla, 324
Maxilliped (maxilla, jaw ; pes, foot),
323
Maximum temperature of amoeboid
movements. 21
Medulla or Medullary substance (me-
dulla, marrow): in Infusoria, not in
Gymnosnerms, 448
Medullary rays, 449, 452
Medus'a (Mefiovo-a, name of one of the
Gorgons), the free-swimming reproduc-
tive zooid of a hydroid polype, 239:
derivation of a, from hydranth (Figure),
240
Medus'oid, a reproductive zooid having
the form of an imperfect Medusa,
Diphyes, 249
Meg'agam'Ote (/ae'yas large : ya/u.ew to
marry), a female gamete (?.v.) distin-
guished by its greater size from the male
or microgamete, 132
Meg anucleus (n-fya.<; large ; nucleus, a
kernel), in, 128
Meg asporan'gium (/aeya? large : v\\ov, a
leaf), the parenchyma of leaves, 420
MBS estates (/u.ew e existence : — Vor-
ticella, 133 t Polygordius, 298 t Mussel,
365
MiC robe Oxi/cpbs. small : £tos, life). See
Bacteria
MICROCOC'CUS (|ou*cpbs, small t KOKKOS,
a berry) (Figure), 86
Microgam'ete OiiKpbs, small t •yaiu.e'w, to
marry), a male gamete (q.v.), distin
guished by its smaller size from the
female or megagamete, 132
Micro-millimetre, the one-thousandth of
IXDKX AXD GLOSSARY
497
a millimetre, or i-25,oooth of an inch,
S4
Micro-Orfijanism. See Hacteria.
Micronvu.ieus ( /x'*po«, sn-all : nucleus, a
kernel), in, 128
Micropyle (/xiKpos, small : TruArj, an en-
trance), 457, 466
Micro-sporan'gium (jut/epos, small : o-n-opa,
a seed : ayyeiov, 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, 440 : Se-
laginella, 441 : Gymnosperms, 455 : An
giosperms, 466
Mic'rospore (^i/cpos, small : vnopd, 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, 440 :
Selaginella, 441 : Gymnosperms, 455 :
Angiosperms, 466
Microzo Old (/ai/cpos. small : ^wor, an ani-
mal : etfios, form), the smaller zooid in
unicellular organisms with dimorphic
zooids, 35, 132
Midrib of leaf, Moss, 403
Minimum temperature for amoeboid mov-e-
ments, 21
Mollusca, the, 306
MonOBC'lOUS (/aofos, single : OIKO;, a
house), applied to organisms in which
the male and female organs occur in the
same individual, 199
Monopod ial (^wos, single : wovs a foot),
applied to branching in which the main
axis continues to grow in a straight line
and sends off secondary axe.-: to the
sides, 138
MONOSTROMA (/aocos, single : 0-rpw/ixa,
anything spread out), 202 (Figure)
Morphol'Ogy OAopr/, form : Aovos a dis-
cussion), the department of biology
which treats of form and structure, 9
Mor'ula (diminutive of ittdrntii, a mul-
berry) See Polyplast.
MOSSES : — Figures. 401, 406 : general
characters, 402 : structure of stem, 402 :
leaf, 403 .' rhizoids, 403 : terminal bud,
403 : reproduction, 404 : development
of sporogonium, 405 : of leafy plant,
408 : alternation of generations, 340 :
nutrition, 408
Mouth :— Euglena, 47 : Paramcecium, 109 :
Hydra, 220 : Medusa, 239 : Polygordius,
968
MUCOR (mncor, mould) :— Figure, 159 :
occurrence and general characters, 158 :
mycelium and aerial hyphae, 160 :
sporangia and spores, 160 ; transition
from uni- to multi-cellular condition.
162 : development of spores, 16^ ; con-
jugation, 165 : death, 166 : nutrition. 167 :
parasitism. 167 : ferment-cells, 168
Mucous membrane, 58
Multicellular, formed of many cells, 61,
162
Muscle (itiiisculits.a. little mouse, a muscle)
nature of. 130
Muscle-fibres, Bougainvillea, 236
Muscle-plate, Polygordius, 273 : develop-
ment of, 301
Muscle-process, Hydra, 224, 232
Muscular System, Crayfish, 32*7: Mussel,
Mushroom. See Agaricus.
MUSSEL (same root as muscle), Fresh-
water -.— Figures, 351, 353, 356, 358,
361, 364 : general characters, 348 :
mantle, shell, and foot, 348 : food-
current, 359 : enteric canal, 355 : gills,
357 : blood-system, 360 : muscles, 354 :
nephridia, 359 : gonads, 363 : nervous
system, 362
Mycelial hyphae, the hyphae interwoven
to form a mycelium.
Mycelium (JUVKT)?, a fungus), a more or
less felt-like mass formed of interwoven
hyphae :— Mucor, 160 : Penicillium, 185
MYCET'OZOA (MU/CTJS, a fungus : &ov,
an animal) : — Figure, 53 : occurrence
and general characters. 52 : nutrition,
54 : reproduction and life-history, 54 :
animals or plants ''. 181
Myomere OAVS, mouse, muscle : /oiepos, a
part), a muscle-segment, 327
My'ophan (MUS, mouse, muscle : <}>a.ii>ia. to
appear), 1 10
Myxomyce tes (>tu£a, slime : /UVKTJS, a
fungus). See Myceto/oa.
X
NaUpliUS, embryo of Crayfish, 346
Nem atOCyst (i>rftJ.a, a thread :" (cuoTis, a
bag), Figure, 226
Nephrid'iopore (repo?, a kidney : Tropo?,
a passage), the external opening of a
nephridium, 282
Nephrid ium (cep6s, a kidney), structure
of, Polygordius, 281 (Figure) : develop-
ment of, 301 : Mussel, 359: Dogfish,
396
NephTOStome (i>ep6s, a kidney : o-rofxa,
a mouth), the internal or ccelomic aper-
ture of a nephridium, 282
Nerve, afferent and efferent, functions of,
286
Nerve-cell, 227, 242
Nervous system, central and peripheral :
— Medusa, 243 : Polygordius, 283 : func-
tions of, 287: Starfish, 315: Crayfish,
241 : Mussel. 362 : Dogfish,. 391
Neur'OC03le (vevpav, a nerve : KOI'ATJ, a
hollow), the central cavity of the verte-
brate nervous system. 391
NITELL'A (niteo, to shine) -.—Figures,
498
INDEX AND GLOSSARY
204, 209, 212, 214 : occurrence and
general characters, 203 : microscopic
structure, 206 : terminal bud, 208 : struc-
ture and development of gonads, 206,
2it : development, 216: alternation of
generations, 217
Node (nodus, a knot), the portion of a
stem which gives rise to leaves, 205
Not'OChord (VUTOV, the back: x°P5>7> a
string), 377
Nucel'lUS (diminutive of nucleus, the
name formerly applied), 456, 466
Nuclear division, indirect : 64 (Figure) :
direct, 67
Nuclear membrane, 63
Nuclear sap, 7, 63
Nuclear spindle, 66
Nucle'olUS (diminutive of nucleus), 8, 63
Nu'cleus (nucleus, a kernel), minute struc-
ture of, 63 ; Amoeba, 7, 8 : Paramcecium,
in, 114: Opalina, 121: Vorticella,
128 : Nitella, 208, 211 : fragmentation
of, 1 20
Nucleus, secondary, of megaspore, An-
giosperms, 474
Nutrient solution, artificial, principles of
construction of, 77,
Nutrition '.—Amoeba (holozoic), n : Hae-
matococcus (holophytic), 28 : Hetero-
mita (saprophytic), 37 : Opalina (type of
internal parasite), 123 : Mucor 167 :
Penidl'ium, 190 : Polygordius (type of
higher animals), 270, 279 : Moss (type
of higher plants), 408
Ocellus (ocellus, a little ey«), structure
and functions of, Medusa, 240, 244
(Esoph'agUS (oio-yos, the gullet). See
Gullet.
Olfactory organ, Crayfish, 342 : Mussel,
363: Dogfish, 394
Ommatideum (dim. of oft/ma, eye), 342
OntOg'eny (OVTOS, being : yeVeo-is, origin),
the development of the individual :
a recapitulation of phylogeny (g.v.),
146
Oogen'esiS (<&6v, an egg : -yeVeo-is, origin),
the development of an ovum from a
primitive sex-cell, 256
Oogon'ium (woV, egg : yovo<>. produc-
tion), the name usually given to the
ovary (q.v.) of many of the lower plants.
Oosperm (oV, egg: oV, an egg: ox/>cupa, a sphere),
a name frequently given to the ovum
(g.v.) of plants.
Oospore (u>6v, an egg : apvy£, the throat) : — Poly-
gordius, 277 : Dogfish, 381
Phloem (4>Aoi6?, bark or bast), the outer
portion of a vascular bundle, 417
Phyla (<£i)Aoi/. a tribe) of the animal king-
dom, 304 : of the vegetable kingdom,
432
Phyll'ula (diminutive of vAAoj/, a leaf),
the stage in the embryo of vascular
plants at which the first leaf and root
have appeared, 360 : contrasted with
gastrula, 428
PhylOg'eny ({)Aov, a race : yeVeo-is,
origin), the development of the race, 147
Physiol'Ogy (4>vcris, the nature or property
of a thing : \6yos, a discussion), the de-
partment of biology which treats of
function, 9 et seq.
Pigment-spot, Euglena, 47
Pileus (/z^7«, a cap\ Agaricus, 191
Pinnule (dim. oi pinna, a feather), of leaf,
420
Pistil (pistillum, a pestle, from pinso, to
pound.) See Gynoecium.
Pith. See Medulla.
Placoid scale, 369
Plan'ula (diminutive of TrAayos, a wander-
ing about), the mouthless diploblastic
larva of a hydroid, 246
Plant, definition of, 179
Plants, classification of, 434
Plas'ma (TrAaTos, first: v^a, a thread),
Moss, 404, 408
Prot'oplasm (Trpwros, first : 7rAaa>xa, any-
thing moulded), composition of, 5 : pro-
perties of, 5 : micro-chemical tests for,
7 : minute structure of, 62 : continuity
of in Fern, 418: in Polygordius, 280:
intra- and extra-capsular, Radiolaria,
!52
Protopodite (TTPU>T//ev6rjs, false : JTOV?, 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.
Putrefaction (/>ntref,>n\>. to make rotten)
nature of, 82 : a process of fermentation,
91 : conditions of temperature, moisture,
&c., 93
Putre scent (flnt>-esco, to gro\\- n.itfn)
solution, characters of, 37, 82
Putrescible infusion, sterilization <.f. 99
Pyloric division. S.-e Stomach.
Pyren'Oid (n-upr/f, the stone of stone-fruit :
eiSo? form), a small mass of proteid
material invested by starch, 27
Radial symmetry, starfish. 306
RADIOLAR IA (radius, a spoke or ray):—
Figures, 152, 153: occurrence and
general characters, 152 : central capsule,
152 : intra- and extra-capsular pro-
toplasm, 152 : silicious skeleton, 152 :
symbiotic relations with Zooxanthella.
Rect'um (intestinum rectum, the straiglit
gut), the posterior or anal division of the
enteric canal, 278
Redi, Francisco (Italian savant), experi*
ments on biogenesis, 97
Reducing division, 256, 260
Reflex action, 286
Reproduction, necessity for. 19
Reproductive organ. See Gomel.
Reservoir of contractile vacuole, Euglena,
47
Respiration :— Amoeba, 17 : Polygordius.
280
Respiratory caeca, Starfish, 308
Rhiz'0id(pi£a, root :el£os, form): — Nitella,
205, 2ii : Moss, 403 : prothallus of Fern,
423. 43°
Root, Fern, 421, 427 : Gynmosperms, 448
Root-cap, 422
Root-hairs, 421
ROSS, Alexander, on abiogenetic origin of
mice, insects, &c., 96
Rostrum (rostrum, beak), 321
Rotation of protoplasm, 207
Rudiment, rudimentary (riidintcntnin. 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 (if. v.) is
more suitable.
SACCHAROMY CES (, sugar :
IU.UK17S, fungus): — Figure, 72 : occurrence,
71: structure, 71: budding, 73: in-
ternal fission, 74 : nutrition, 75 : alco-
holic fermentation caused by, 75 : experi-
ments on nutrition of. 78, 80 : animal
or plant? 182
SALVIN'IA : — Figures. 439, 441: general
characters, 438 : mega- and micro-spor-
angia and spores, 440 : male and female
prothalli and gonads, 442 : development
and alternation of generations, 442
Saprophyt'ic (/u.a. the body), applied to the
layer of mesoderm which is in contact
with the ectoderm and with it forms the
body-wall, 275
Sor'us (
Specialized, meaning of, 140
Sperm (a-rrepfj-a., seed), the male or micro-
gamete in its highest stage of differentia-
tion ; structure and development of, 255 :
see also under the various types, and
especially Vaucheria, 173
Spermatozo id, spermatbzo on (fxa,
seed : (Jwoi/, animal, from the actively
moving sperms of animals having been
supposed to be parasites), synonyms of
sperm.
Spermary ((rirepna., seed), the male gonad
or sperm-producing organ : see under the
various types, and especially Vaucheria,
172
Sperm iduct (cnrc'pjua, seed : duco, to lead),
a tube conveying the sperm from the
spermary to the exterior, 292
SpermatOgen'esiS (arrep/u-a, seed ; yeVeo-is,
origin), the development of a sperm from
a primitive sex-cell, 254 (Figure)
Spinal cord, Dogfish, 391
Spiral vessel See Vessel.
SPIRILL'UM (spira, a coil) 86, 88 (Figure)
SPIROGYRA (spiia, a coil : gyrus. a revo-
tion) : — Figure, 195 : occurrence and
general characters, 194 : microscopic
structure, 194 : growth, 197 : conjugation,
198 : development, 200 : nutrition, 200
Splanch'nic (&ir\dyxvov, intestine or vis-
cus), applied to the layer of mesoderm
which is in contact with the endoderm
and with it forms the enteric canal, 275
Spontaneous generation. See Abio-
genesis.
Sporan gium (a"iropd, seed : dyyelov, a
vessel), a spore-case: — Mucor, 160 : Vau-
cheria, 171 : Fern, 422. See also Mega-
and Micro-sporangium.
Spore ((TTropa, a seed), an asexual repro-
ductive cell : see under the various types
and especially Heteromita, 42 : Saccha-
romyces, 74 : Bacteria, 89 : vitality of
in Bacteria, 99, 101 : Pemcillium, 189 :
Moss, 407 : Fern, 423. See also Mega-
and Micro-spore.
Sporogon'ium a-nopd seed : yovos, pro-
duction), the agamobiumofa moss, 407
Spor'ophyll (a-iropd, seed : (|>uAAor, leaf),
a sporangium-bearing leaf: — Equisetum,
436 : Selaginella, 444 : Gymnosperms,
454^ 455 '• Angiosperms, 466, 471
Stamen (stamen, a thread), a male sporo-
phyll, 454, 456, 466
Starch, composition and properties of, 27
STARFISH :— Figures, 307-317 : general
characters, 305-310 . radial symmetry,
306 : tube-feet and ambulacral system,
307, 313: exoskeleton, 310: nervous
system, 315 : reproduction and develop-
ment, 315-317
Stem, structure of : — Moss, 402 ; Fern. 413 :
Gymnosperms. 448 : Monocotyledons,
462
Sterig'ma (a-Trjpiy^a, a support): Pcni.il
Hum. 188 ; Agaricus, 193
Sterilization of putrescible infusions, 99-
502
INDEX AND GLOSSARY
Sternum (crrepvov, the breast), 320
Stigma (ori'-yua, a spot), the receptive ex-
tremity of the style, 465
Stimulus, various kinds of, 286
Stock. See Colony.
Stomach, Starfish, 310 : Crayfish, 332 :
Mussel, 355 : Dogfish, 381.
Stom'ate (erro/na, mouth), 421
Stomodae um (aTo/u.a, mouth : ofiatos, 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,
296
Stone-canal, Starfish, 314
Style (stylus, a column), the distal solid
portion of the female sporophyll or of the
entire gynoecium in Angiosperms, 467
STYLONYCH'IA(o-TvAos, a column : owl;,
a claw), Figure, 117 : occurrence and
general characters, i ID : polymorphism
of cilia, 118
Sub-apical cell. See Segmental cell.
Superficial aggregate, 202
Supporting lamella. See Mesogloea.
Suspensor : Selagiriella, 446 : Gymno-
spenns, 459 : Angiosperms. 475
Sweet Wort, composition of, 75
Swimming-bell, Diphyes, 248
Symbio'siS (o-v/u/Buocris, a living with), an
intimate and mutually advantageous
association between two organisms, 154
Syner'gid.86 (o-uvepyos, a fellow worker),
Sys'tole (aipa, a sphere),
the free-swimming larva of Polygordius,
&c. : — characters of, 293 (Figure) : origin
of from gastrula, 295 : metamorphosis of,
298
Tube-feet, Starfish, 307, 314
u
Ultra-maximum temperature, for amoe-
boid movements, 21 : for monads, 40 ; for
Bacteria, 93
ULVA (ulva, an aquatic plant), 203
Umbell'ate (umbella, a sun-shade, um-
brella) applied to branching in which
the primary axis is of limited growth and
sends off a number of secondary axes
from its distal end, 138
Unicell'ular, formed of a single cell, 61 :
connection of uni- with multi-cellular
organisms, 261
UniO. See Mussel.
Ureter (ovprjTrjp, the Greek name), the
duct of the kidney, 396
Uropod (ovpa, tail : TTOVS, foot), 323
Vac'uole (vacuus, empty), contractile, n,
in : non-contractile, 71
Variability, 147
Variation, individual, 140, 147
Variety, an incipient species, 147
Vasc'ular (vasculum, a small vessel)
bundles. 413, 416, 450, 462
Vascular plants, 417
VAUCHERIA (after J. P. E. Vaucher, a
Swiss botanist) : — Figure, 870 : occur-
rence and general characters, 169 : minute
structure, 169: asexual reproduction, 171 :
sexual reproduction, 172 : nutrition, 174
Vegetative cell, 455
Vegetative nucleus, 474
Veins, of Crayfish, 338 : of Mussel, 362 :
of Dogfish, 387 : of leaves, 420
Vel'um (velum, a veil) of medusa, 240
Venter (venter, the belly), of ovary of
Moss, 404, and Fern, 426 : of the female
sporophyll or of the entire gynoecium of
Angiosperms (so-called ovary) 465, 473
Ventral nerve-cord : — Polygordius, 283 :
development of, 298 : Crayfish, 341
Ventricle. J-ee Heart.
Vennes. the 305
Ver'tebral (vertebra, a joint) centra and
column. Dogfish, 376
Vertebrata, the, 306
Vessels :— of plants spiral and scalariform
416, 419 : of animals, see Blood-vessels.
INDEX AND GLOSSARY
503
Vestige, vestigial (vestigium, a trace),
applied to any structure which has be-
come atrophied or undergone reduction
beyond the limits of usefulness, 118
VWrio(vibro, to vibrate), 86, 88, (Figure)
ViSC'eral (viscns, an internal organ), ap-
plied to the layer of ccelomic epithelium,
or of peritoneum, covering the intestine
and other internal organs, 274
Visceral ganglion, Mussel, 362
Vitelline (vi tellies, yolk) membrane, the
cell-membrane of the ovum, 257
VolVQX (volvo, to roll), 264 (Figures)
VORTICELLA (diminutive of -vortex, an
eddy): — Figure, 127: occurrence and
general characters, 126 : structure, 126 :
asexual reproduction, 131 : conjugation,
132 : means of dispersal, 132, 134 : encys-
tation, spore-formation, development,
and metamorphosis, 133
W
Waste-products, 33
Water of organization. 5. 29
Whorl of leaves, 205
Wood. See Xylem.
Work and Waste, 14
Yeast, 71
Yeast-plant. See Saccharomyces.
Yellow-cells of Radiolaria, 154
Yolk-granules or spheres. 68, 233, 256
Zoogloe'a (£d>oi'. an animal : yAoca, glue),
85
Zooid (tjaov, an animal : elSo?, form\ a
single individualof a compound organism,
135. 234.
DOthE
v, wood), the inner portion of
a vascular bundle, 417, 450, 463
Zootham'nium (&ov, an animal : 006<;,
yellow), is4
Zyg'ospore (fwydi/, a yoke : trnopd, a seed\
applied to a resting zygote formed by the
conjugation of similar gametes, 166
Zygote (fvywros, yoked), the products of
conjugation of two gametes :— Hi tero-
mita, 41 : Vorticella, 133 : Mucor, 165 :
Vaucheria, 174 : Spirogyra. 198
THE END
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